Commercial Solar Water Heating Systems

Transcription

1 Commercial Solar Water Heating Systems Design review of two of Ireland s largest commercial Solar Water Heating Systems By Dissertation submitted to Mary Doyle-Kent The School of Engineering Waterford Institute of Technology In partial fulfilment of the requirement for The Degree of Master of Science In Sustainable Energy Engineering October 2012

2 Declaration I declare that this dissertation which I now submit for examination in partial fulfilment for the award of Master of Science, is entirely my own work and has not been taken from the work of other, save and to the extent that such work has been cited and acknowledged within the text of my work. This dissertation was prepared according to the regulations for postgraduate study in Waterford Institute of Technology and has not been submitted in whole or in part for another award in any other Institute. The work reported on in this dissertation conforms to the principles and requirements of the Institute s guidelines for ethics in research. The institute has permission to keep, lend or copy this thesis in whole or in part on condition that any such use of the material of the dissertation is duly acknowledged Signature Date i

3 Abstract Renewable sources of energy are anticipated to play a significant role in energy generation in Ireland in the future. Ireland has a lack of indigenous fossil fuel therefore making the production of energy expensive and environmentally unfriendly. One such renewable source of energy that can fulfil this role is commercial solar water heating. Solar water heating has not been popular in Ireland to date and this research aims to establish if the software used in the design of the systems is a contributing factor to this. An extensive literature review was firstly carried out to establish the main design issues of solar water heating systems. Results of questionnaires sent to designers and installers of the systems indicated that Tsol and Polysun were the main simulation software s used in Ireland. The research concentrates on running simulations on two case studies, Cloughjordan Eco Village in Co. Tipperary and Bewleys Hotel Dublin Airport. The results of the simulations indicate that the tilt angle of the collectors in both case studies was not installed at the optimum angle. The results also indicate for when the space heat demand is greatest in Cloughjordan Eco Village an increased angle to 60 would provide a higher output in the heating season of April to October. The two simulation software s used in the research were compared and analysed. Polysun proved to be a more complete package with an option to create custom hydraulic diagrams to exactly match the system being simulated. The weather file used in Polysun also interpolates the location of the system rather than using nearby weather station data. It was concluded that some simulation software s such as Polysun do offer an optimised design solution for solar water heating systems in Ireland but careful consideration must be given to select one that does. ii

4 Acknowledgements The author would like to thank Mary Doyle Kent for her excellent support and guidance during her role as supervisor of this dissertation. The author would like to thank Duncan J. Martin of the Cloughjordan Eco Village Management team for his assistance in providing information for the case studies in this dissertation. The author would also like to thank Vela Solaris AG for providing a student licence of Polysun and Dr. Valentin Energie software GmbH for providing a student licence for Tsol. The author would like to thank his family and girlfriend in particular whose support and help has guided them through this dissertation. iii

5 Word Count Chapter 1 - Introduction 1,650 Chapter 2 - Literature Review 9,050 Chapter 3 - Overview of Case Studies 1,186 Chapter 4 - Methodology 2,203 Chapter 5 - Research Findings & Discussion 6,516 Chapter 6 - Conclusions & Recommendations 2,496 Total Word Count 23,101 iv

6 Table of Contents Declaration... i Abstract... ii Acknowledgements... iii Word Count... iv Table of Contents... v List of Figures... ix List of Tables... xii List of Abbreviations... xiii Chapter 1: Introduction Background Hypothesis Research Objectives Justification for Research Structure of the Study... 6 Chapter 2: Literature Review Introduction Solar Water Heating Systems Open and closed systems Direct and Indirect systems Drain back systems Design Issues Solar Energy Collectors Flat Plate Collectors Evacuated Tube Collectors v

7 2.3.3 Tracking Collectors Parabolic Concentrating Collectors Solar Combined Heating Systems Solar Assisted District Heating Solar Storage Solar Store Stratification Seasonal Storage Solar Radiation Direct & Diffuse Radiation Tilt & Orientation Estimating Solar Radiation Collector Performance Efficiency of Collectors Collector Certification Incident Angle Modifier Modelling Solar Water Heating Systems Summary Chapter 3: Overview of Case Studies Introduction Bewley s Hotel Case Study Drain-back System Cloughjordan Eco Village District Heating System Chapter 4: Methodology Introduction Case Study Research vi

8 4.3 Simulation Research Interviews Research Objectives Methodologies Predicted Contribution Solar Output Temperature Calculation of the Effect of Tilt and Orientation Variation in Cloud Cover Evacuated Tubes Comparison Limitations of the Research Chapter 5: Research Findings and Discussion Introduction Case Study 1 Cloughjordan Eco Village Predicted Contribution Polysun Predicted Contribution TSOL Calculation of the Effect of Tilt and Orientation Evacuated Tubes Comparison Case Study 2 - Bewleys Hotel Dublin Airport Predicted Contribution Polysun Predicted Contribution TSOL Calculation of the effect of tilt and orientation Evacuated Tubes Comparison Variation in Cloud Cover Discussion of Findings Predicted Output Simulations Solar Output Temperature Optimum Orientation and Tilt Angle Evacuated Tubes Comparison vii

9 5.5.5 Variation in Cloud Cover Chapter 6: Conclusions & Recommendations Introduction Summary of Dissertation Research Aim & Objectives Conclusions Recommendations Further Research References Appendix A: Details of Solar Collectors Appendix B: Results of Simulations Appendix C: Software Details Appendix D: Interviews & Questionnaires Appendix E: Photographic survey viii

10 List of Figures Fig. 1.1 Irish Dependency on Imported Fossil Fuels compared to EU average... 1 Fig. 1.2 Total installed capacity of solar collectors in ten leading countries... 2 Fig. 1.3 Renewable Thermal Energy as a share of Total Thermal Energy... 3 Fig. 2.1 Schematic Diagram of Natural Circulation System... 7 Fig. 2.2 Drain back system Fig. 2.3 Flat Plate Collector Fig. 2.4 Flat Plate Collector Exploded View Fig. 2.5 Direct Flow Evacuated Tube Fig. 2.6 Heat Pipe Evacuated Tubes Fig. 2.7 Parabolic Concentrating Collectors Fig. 2.8 Combined Solar store and DHW store Fig. 2.9 Separate solar store preheating DHW store Fig Stratification of solar warm water Fig Seasonal Storage tank Fig Borehole Seasonal Thermal Storage Fig Daily average solar radiation (kwh/m 2 ) Fig Comparison of daily solar radiation levels Fig Direct & Diffuse Irradiation levels Dublin Airport Fig Variation of annual irradiation according to tilt and orientation Fig Variation of the optimum tilt angle Fig Efficiency and temperature ranges Fig Expected and measured solar output ix

11 Fig Diagram of the absorption of radiation on an inclined collector Fig 2.21 Comparison of how solar radiation is absorbed in flat plat and evacuated tube collectors Fig Comparison of IAM values between flat plat collectors and evacuated tube flask collectors Fig Collector used in the study Fig Results of the TRNSYS simulation compared to actual figures Fig. 3.1 Solar Panel Installation Bewley s Hotel Fig. 3.2 Hydraulic diagram of Drain-back system Fig. 3.3 Cloughjordan Eco Village Master Plan Fig. 3.4 Ground Mounted Solar Array Fig. 5.2 Comparison of Solar Thermal Energy versus Irradiation Fig. 5.3 Daily Maximum temperature of Collector outflow ( C) Fig. 5.4 Cumulative Frequency Distribution Fig. 5.6 Comparison of results from Tsol & Polysun Fig. 5.7 Daily Maximum Temperature Tsol Fig. 5.8 South Orientation Collector Inclination Comparison Fig. 5.9 Comparison of all orientations and inclinations Fig Tracking Tilt Orientation Comparison Fig Comparison of tilt 30 facing south to azimuth tracking and biaxial tracking 62 Fig Heating Season Contribution Fig South Orientation Tilt angle comparison Fig Comparison of Flat Plate Collector to Evacuated Tube Collector Fig Comparison of IAM for flat plate collector x

12 Fig Comparison of IAM for evacuated tube collector Fig Solar Fraction of Bewley s Hotel Dublin Airport Fig Energy Flow Diagram Fig Frequency Distribution of Collector Outflow Temperatures Fig Predicted Contribution comparison of Tsol & Polysun Fig Inclination Comparison South Orientation Fig Orientation and inclination comparison Bewley s Hotel Dublin Airport Fig Comparison of Collector Outflow Temperatures East & West Fig Yield comparison of evacuated tube collector Fig Comparison of the effect of the IAM Zen 56P Fig Comparison of the effect of the IAM SP Fig Frequency of cloud cover Dublin Fig Plot of cloud cover versus direct and diffuse radiation Fig Frequency of cloud cover 8 oktas xi

13 List of Tables Table 5.1 Polysun Simulation Results Table 5.5 Tsol Simulation Results Table 5.14 Total Contribution above 58 C Tilt Table 5.15 Total Contribution above 58 C Tilt Table 5.19 Polysun Simulation Results Table 5.23 Predicted Contribution TSOL Table 5.34 Comparison to previous research xii

14 List of Abbreviations SEAI GWth kwth IEI IAM kwhr CIBSE DHW Wm 2 k -1 TRNSYS SERVE DHS Sustainable Energy Authority of Ireland Gigawatt Thermal Kilowatt Thermal Institute of Engineers Ireland Incident Angle Modifier Kilowatt Hour Chartered Institute of Building Services Engineers Domestic Hot Water Watts per Square Metre Kelvin Transient System Simulation Tool Sustainable Energy for the Rural Village Environment District Heating System xiii

15 Chapter 1: Introduction 1.1 Background The need for renewable energy has been recognised globally as populations swell and the demand for fossil fuels increase. The depleting resources of fossil fuels and increased costs have led to the development of new and technologically advanced renewable energy sources. With the government committed to targets set by the E.U. renewable sources of energy will slowly replace the use of some fossil fuels. Ireland has a lack of indigenous fossil fuel therefore making energy production expensive and environmentally unfriendly. The country as a whole is over reliant on imported gas, coal and oil. Solar water heating production has become more appealing in recent years on a global level; however it has been less popular in Ireland to date on commercial projects. With the over reliance on fossil fuels for energy production, the Irish Government have realised the need to research and develop this area further. The National Renewable Energy Action Plan sets a target that 16% of Irish energy sources will be from renewable sources by This strategy also includes a renewable heat market penetration target of 12% by 2020 (Government White Paper, 2007). Fig. 1.1 Irish Dependency on Imported Fossil Fuels compared to EU average (SEAI, 2008) 1

16 With Ireland s dependency on fossil fuel in mind, solar water heating, where water is heated directly from the energy of the sun is becoming an increasingly attractive option. The amount of solar thermal projects installed grew by 9% around the world in The total output worldwide reached 88,845 GWh, which resulted in the prevention of 39.3 million tons of CO 2 emissions escaping into the atmosphere. Statistics at the end of 2010 indicated that the total solar thermal collector capacity in operation worldwide was GWth (gigawatt thermal), which equalled to million m 2 of collectors. At the end of 2011 the capacity had increased by 25%, to 245 GWth. Making up this installed capacity, 88.3% comprised of flat-plate collectors and evacuated tube collectors, 11% unglazed water collectors and 0.7% air collectors. China accounts for the majority of glazed and unglazed water and air collectors in operation with GWth, Europe accounts for 36.0 GWth and the United States & Canada has 16.0 GWth, which are mostly unglazed collectors. All of these countries and continents together account for 86.6% of total installed capacity in the world (Weiss & Mauthner, 2012). Fig. 1.2 Total installed capacity of solar collectors in ten leading countries (Weiss & Mauthner, 2012) The figures above indicate that solar thermal technology has expanded and grown significantly over the past few years, particularly in Europe and China. The market in Europe for the technology tripled between 2002 and 2008 (Arizu & Balaya, 2011). In Ireland at present approximately 158,429 m 2 (110,900 kwth) of solar collectors had been installed for the year ending The capacity installed annually in 2009, 2010 and 2011 was 32,211, 24,918 and 27,000 m 2 respectively. These figures represent a growth of 8.4 % from (ESTIF, 2012) Fig. 1.3 published by the Sustainable 2

17 Energy Authority Ireland shows the small percentage of solar energy of the total renewable thermal energy usage in Ireland. Thermal energy produced from renewable sources is currently dominated by biomass. Fig. 1.3 Renewable Thermal Energy as a share of Total Thermal Energy (SEAI, 2012) Furthermore the figures above include the residential sector which accounts for the largest share of final thermal energy use (47% in 2011), followed by industry 32%, services 17% and agriculture 5% (SEAI, 2012) 1.2 Hypothesis With Commercial Solar Water Heating Systems being largely underused in Ireland at present this dissertation intends to investigate if the design of the systems is suitable for the Irish climate. In order to state the position of the argument the following hypothesis was derived: Currently available simulation software programs do offer an optimised design solution for Commercial Solar Water Heating design in the Irish context 3

18 1.3 Research Objectives The overall aim of the dissertation is to To assess the suitability of different Commercial Solar Water Heating Systems in the Irish climate and evaluate the accuracy of the software used at the design stage. The following objectives have been established to achieve this aim: 1. Calculate the predicted contribution of the solar array on the chosen case studies using two simulation software s and compare the results of both. 2. Calculate the outflow temperature of the systems and the suitability to the end use. 3. Calculate the effect that tilt and orientation has on the chosen studies using simulation software. 4. Calculate the heat output if evacuated tubes had been fitted as opposed to the flat plate collectors currently installed on both projects. 5. Quantify the impact that the Incident Angle Modifier has on the efficiency of collectors 6. Examine weather records for patterns in diurnal variation of cloud cover in both case studies 1.4 Justification for Research In the current economic climate Ireland is focused on reducing costs, reducing energy usage and increasing sustainability. For these reasons the accurate and economical design of solar water heating systems is imperative to reducing energy usage and ensuring a sustainable future. The Author of this dissertation is a member of the Institute of Engineers Ireland (IEI) and in keeping with their code of ethics is carrying out this research to that effect. Members of the IEI strive at all times to be conscious of the effects of their work on the health and safety of all individuals and on the general wellbeing of society. Members also endeavour to eliminate risk during all stages of projects. They also design and execute all project work with minimal impact on the natural environment. 4

19 In fostering the principals and practices of sustainable development the Author justifies the need to investigate solar water heating installations in Irish commercial buildings. Efficient design of these systems will support the development of sustainability in Ireland for present and future generations. The code of ethics states that members must try to accomplish the objectives of their work through the most efficient consumption of natural resources including the maximum reduction in energy usage. The Irish climate has proven to be a renewable, effective, practical and above all economical resource. In investigating the influence the Irish climate may have on sustainable energy there is a huge drive to reduce energy, waste and pollution in Ireland (IEI, 2012) With regards to the research published there has been no examination into the operations of solar water heating installations in commercial buildings in Ireland. The best example of similar research was conducted by Hernandez et al (2011). Hernandez focused his exploration on domestic solar water heating installations across Ireland which is predominantly a more simplified design than those of commercial buildings. He explored the net energy output and analysed the performance of six such installations. Another area lacking review is research undertaken by Gargan, (2012) on the Incident Angle Modifier (IAM) with regards to evacuated tube collectors. Aside from this study there is little or no other research into comparing the IAM in different solar collectors. The research compared a standard Irish made flat plate collector to a Chinese evacuated tube flask collector. This research indicated that European test standards for solar collectors did not take into account the performance of a collector during a whole day and only accounted for the midday sun while it was directly above the collector. He concluded that either side of midday that the evacuated tube flask collector increased in efficiency while the flat plate collector actually dropped in efficiency due to the IAM. While any exploration of a topic is a positive approach it is important that research is contributed to by relevant parties so a more progressive overview can be gained. Due to the lack of exploration into commercial building solar water heating installations and absence of research into the effects of the IAM on evacuated tubes and how the Irish climate influences these tubes efficiency, there lies a gap in up to date literature thereby justifying the need for this dissertation to bridge a growing gap. 5

20 The author also learned that the solar assisted district heating system in Cloughjordan, which forms one of the case studies in this dissertation, has had operational problems with the solar array. The solar array has been installed for two years but has not contributed to the district heating system at all. In light of this revelation the design of these systems will be analysed in this dissertation along with the simulation software used to design them. 1.5 Structure of the Study The study is presented in six chapters: Chapter one of this dissertation gives an overview and background to the area of commercial solar water heating. Included in this chapter is the aim hypothesis, objectives and the structure of the study. Chapter two presents a review of the literature on commercial solar water heating. The chapter outlines the different types of systems and collectors and their uses and limitations. Research conducted in the areas of types of radiation, collector tilt, collector orientation, and efficiencies are also critically analysed. Finally different types of simulation software are detailed and research papers utilising them are analysed. Chapter three gives an overview of both the case studies that were analysed, Bewley s Hotel Dublin Airport and Cloughjordan Eco Village. The published literature on the case studies is detailed along with the system design and layout. Chapter four describes the methodology for the primary research conducted. It outlines the reasons for choosing those methods and the limitations from their use. It also details the case studies examined and why they were chosen. Chapter five presents the findings of primary research in diagrammatic form and addresses the research objectives set out in Chapter one. It also discusses the findings of the primary research and reservations are clearly stated. Chapter six is the conclusion to the whole dissertation and recommendations for future research are also included. 6

21 Chapter 2: Literature Review 2.1 Introduction The purpose of this chapter is to review the current literature published on large scale commercial solar water heating systems. Reference will be made to the different types of collectors available, the different types of systems used and examples of successful schemes in Europe will be detailed. The literature on the simulation software used to model the solar water heating systems will be analysed so that the chosen software for the primary research can be critically analysed. 2.2 Solar Water Heating Systems Presently there are two main types of solar water heating system designs available, natural circulation and forced circulation. Natural circulation systems are very simple and the cost of manufacturing is low, however these systems are only suitable for warm climates as freezing can occur in cold climates. Forced circulation systems are more suitable for climates where freezing conditions occur and are at present the Irish system of choice (Goswani et al, 2000). Natural circulation systems are also known as thermosyphon systems and a typical schematic of the system can be seen in Fig.2.1 below. Fig. 2.1 Schematic Diagram of Natural Circulation System (Goswani et al, 2000) 7

22 There are many disadvantages associated with thermosyphon solar water heating systems and these lie in the fact that they are relatively tall units, which can make them very unsightly on buildings and prone to damage in windy conditions. The design usually incorporates a cold water storage tank which is installed on top of the solar collector. This system supplies both the hot water cylinder and the cold water feed to the house, which in turn makes the complete unit taller and even less aesthetically pleasing. The schematic of the system and a photo of an installed system are shown in Fig. 2.1 on the previous page (Kalogirou, 2009) Open and closed systems The two main types of systems above can be broken down into a number of different setups. Open systems have an open container at the highest point of the solar loop, which absorbs the volumetric expansion of the liquid caused by the temperature changes. The pressure in open systems consequently corresponds at its maximum to the static pressure of the liquid column. Closed (sealed) systems are designed to operate with a higher pressure ( bar), which in turn influences the physical properties such as the evaporation temperature of the solar liquid. Closed systems therefore require special safety devices (German Solar Energy Society, 2007) Direct and Indirect systems Direct systems operate by the water continuously circulating from the solar storage tank to the collector and then back again. Direct solar systems can be installed in various different formats. The system is often connected to a separate pre-heat cylinder or to a combined cylinder with dedicated solar storage volume in the bottom. This system can also be found connected to an existing conventional hot water storage vessel with a traditional heat source, e.g. a boiler (German Solar Energy Society, 2007). Indirect systems have two separate circuits, the solar circuit and the cold mains water circuit. This particular system is the UK and Ireland s most prevalent form of solar thermal system being used at present. The operation of this system involves the heat transfer fluid passing through the solar collector. This heat transfer fluid is isolated by a heat exchanger from the consumed water. The separating of the heat transfer fluid allows for the use of antifreeze and anti-corrosion inhibitors such a glycol. 8

23 This system also has the added benefit of preventing the introduction of contaminants from the incoming cold mains such as limescale and sludge which can diminish the efficiency of the collector (CIBSE, 2007). The risks associated with bacterial growth and scalding is significantly reduced with an indirect circuit. However, a major disadvantage occurs in the process of heat exchange where a reduction in efficiency occurs because the temperature difference between the primary and secondary circuit is increased to permit heat transfer. The solar loop consists of the collectors, the ascending pipes, the solar pump with safety equipment and a heat exchanger. A mixture of water and an antifreeze agent such as glycol can be used as the heat transfer fluid (CIBSE, 2007) Drain back systems The durability and reliability of solar water collector systems are influenced by the behaviour of the collector circuit/loop. As has been detailed previously with indirect systems the collector circuit usually has an antifreeze-water mixture as the heat transfer fluid. A heat exchanger is therefore essential for heat transfer to the buffer tank. There are exceptions to this setup which include drain back systems. With drain-back systems, (Fig 2.2) both overheating and freezing of fluid in the solar collector loop can be avoided. When the collector circuit is not in operation, the system can work using plain water without antifreeze additives due to the drain-back of the collector fluid. This system operates by draining the water from the collectors and outdoor collector feed pipes using gravity and in turn replacing the drained liquid with air from the top. This process of replacing the collector fluid with air means that ice cannot form in freezing conditions and damage to the system can be avoided. The system also has the added benefit that if the heat store is fully charged it will also drain back, consequently avoiding situations where the water can boil and high pressures can develop inside the system. If a power cut happens while the system is operational the system will automatically drain (Fanniger, 2012). 9

24 Fig. 2.2 Drain back system (Fanniger, 2012) According to (Fanniger, 2012) the following are the main advantages of using a drain back system: The water only fluid used in this system does not have to face the drawbacks of using glycol as an additive for instance, which over time can change in material properties and cause possible corrosion of the collector loop. Water has superior heat transfer properties in both heat capacity and viscosity, and can outperform other comparable heat transfer fluids. Water is the cheapest of all collector fluids and is readily available. The design of collector circuit means that high overpressures do not occur, which in turn means the system is generally safer requiring less safety devices. The degree of maintenance required for drain-back systems is much lower. The disadvantages of using drainback systems are as follows: There is less flexibility with the type of collector that can be used. The design stage and installation are critical which special attention required for the loop design. 10

25 2.2.4 Design Issues Over the course of the design history of solar water heating systems, significant improvement has occurred in performance and production volumes as well as reductions in installation costs due to economies of scale, as well as learning and technological developments. An example of this progress is improved methods of production and the use of surfaces with increased absorption capability. Modularisation has facilitated production optimisation while improved pump designs have been adapted to allow for different flow regimes. Development in control devices have also contributed to system improvements. Reductions in collector and hot water tank heat losses have also been achieved due to improving insulating materials. Auxiliary heating systems have been integrated into storage tanks while new tank designs with enhanced stratification mechanisms have been developed, thus reducing system losses (Duffie & Beckman, 1991). Solar primary systems are however subject to extreme temperatures and this remains a concern in design. Collector temperatures can vary from the extreme cold of 20 C in winter to over 200 C for flat plate collectors and 350 C for evacuated tube collectors if there is continuous high solar radiation without heat consumption (i.e. stagnation temperatures when there is no fluid flow). Situations can occur where a high performance collector can absorb enough heat and radiation to change the transfer fluid to steam under significant pressure. The design of solar water heating systems must incorporate safety devices to allow for these conditions (Rawlings, 2009) High Temperature Conditions Although the peak power of the sun, at 1 kw/m 2, may initially appear to be modest compared to power outputs from fossil fuel heating appliances, it should not be forgotten that solar energy is a renewable energy source, which is largely uncontrolled, can in some extreme situations absorb enough radiation in collectors to change the transfer fluid to steam under significant pressure. Designers of solar water heating systems need to acknowledge that these extreme situations do occur, so all solar water heating systems should be designed for this scenario from the outset. (CIBSE, 2007). Sometimes a high temperature situation arises during a circulation failure, i.e. a faulty pump, power cut, or even as part of normal operation under thermostatic primary or 11

26 storage safety controls. Regardless of the cause of this failure, both the primary and secondary system may overheat. The state in which there is no net heat extraction from the collector is described as stagnation. If the collector is not designed for the stagnation temperature, it can be considered to be over-heated and potentially unsafe. Paragraph of BS EN describes how modern solar water heating systems can be designed and installed to automatically and safely resume normal operation after an excess temperature event as specified by meeting the following criteria: There should be no release of any high temperature fluid (vapour or liquid) in any conditions while the collector is in operation. There must be auto-resumption of normal operation after stagnation occurs, without intervention of the operator. In research conducted by CIBSE, (2007) it was acknowledged that a liquid based primary circulation system meeting these criteria is termed hydraulically secure and this can be achieved with sealed, indirect primary systems that contain a vessel that is capable of holding the transfer fluid of the collector at all the permitted pressures in the system. This vessel can be of the membrane expansion type or of the drainback airpocket type. These assertions made by CIBSE are further researched by Rawlings, (2009) who states that all the materials used in the system must be capable of withstanding the full range of temperatures to which they will be exposed. This means that all the materials close to the solar collectors must be able to withstand the collector stagnation temperature and that the other materials in an indirect system can withstand temperatures up to 150 C Freezing Conditions To prevent freezing an indirect solar primary circuit, antifreeze (usually polypropylene glycol) is added to the fluid, along with corrosion inhibitors. Drainback systems also occasionally use an antifreeze additive for extra security even though the fluid can be drained from the collectors and other external components by switching-off the pump. Direct systems, however cannot make use of this antifreeze and it is imperative that the collectors and hydraulics of the system can withstand freezing or offer sufficient 12

27 protection against freezing. Some systems use freeze-tolerant materials that can accommodate the expansion caused by freezing (Rawlings, 2009). 2.3 Solar Energy Collectors Solar energy collectors work in a similar manner to heat exchangers in that they transform one form of energy, solar radiation to another in the form of hot water. The component that allows this exchange of energy is a solar collector. The solar collector absorbs radiation and converts it into heat. This heat is then transfered to a fluid which is usually water or a glycol mixture, flowing through the collector (Kalogirou, 2004). The energy that is collected from the process just described is then transferred from the fluid either directly to where it is required or to a solar water heating storage tank from which it can be used when necessary. There are two ways that solar collectors can be mounted; stationary or tracking. If the collector is to be mounted in a stationary position calculations are carried out at the design stage as to the optimum inclination of the panels for location and usage. The collectors will then remain fixed to this tilt angle all year round and for the lifespan of the system. In a tracking position the collector s inclination will change as the sun s angle in the sky changes hour to hour and day to day in order to receive the optimum amount of radiation. (Kalogirou, 2004) Flat Plate Collectors Flat plate collectors currently are currently manufactured in two different forms. Firstly collectors using liquid with no glazing are manufactured using a black absorbent polymer coating without an insulated backing. The manufacturing costs of these particular collectors is extremely low but with this comes a negative as they also have high heat losses to the environment making them inefficient. Such collectors are not suitable when used in low temperature applications such as swimming pools and industrial heating (Sabonnadiere, 2009). 13

28 Fig.2.3 Flat Plate Collector (Solar Server, 2011) Fig. 2.4 Flat Plate Collector Exploded View (Sabonnadiere, 2009) The second form of flat plate collector uses glazing (Fig. 2.3 & 2.4) and makes use of an absorber plate that absorbs the solar radiation and in turn heats the copper tubes that contain the transfer liquid (Sabonnadiere, 2009). The side of the casing and underside of the absorber plate are heavily insulated to reduce conduction losses while in operation. The liquid tubes are sometimes welded to the absorbing plate, or they can be manufactured as part of the plate. These tubes are then connected at both ends by large diameter header tubes. These collectors also utilise a transparent cover to reduce the 14

29 convection losses from the absorber plate by trapping a layer of stagnant air between the absorber plate and the glass (Kalogirou, 2004) Evacuated Tube Collectors Evacuated tube collectors are in most cases more efficient than most flat plate collectors, but as a result of this increased efficiency are also more costly due to their complex design. Due to the absorber being mounted in an evacuated and pressure-proof glass tube, conductive and convective losses are minimised increasing efficiency. According to Rawlings (2009 p.10) Evacuated tubes work efficiently at low radiation levels with high absorber temperatures and can provide higher output temperatures than flat plate collectors and they can be used in applications where the demand temperature is C or in colder climates. There are currently two principal types of evacuated tube collectors on the current market, direct flow and heat pipe. The first type is known as a direct flow evacuated tube and as shown in Fig. 2.5 below where the heat transfer liquid is pumped in the tubes. Fig. 2.5 Direct Flow Evacuated Tube (Darling, 2012) The second type of collector shown in Fig. 2.6 utilises heat pipes inside vacuum sealed glass tubes with a reflector also used to further increase the ability to absorb radiation. The collector operates by vapour rising to the heat exchanger shown on the left of Fig. 2.6, where heat is then transferred to the systems primary circuit and condensed fluid flows back down the heat pipe. Choosing the correct collector can depend on the temperature of hot water required in the system and the climate where the system is 15

30 installed. The suitability of a collector to a system therefore depends on the rated efficiency of the panel and suitability to the application (Zambolin & Del Col, 2009) Fig. 2.6 Heat Pipe Evacuated Tubes (Solar Panel Plus, 2012) The vacuum envelopes in evacuated tube collectors reduce conduction and convection losses, which enables the collectors to operate at higher temperatures than flat plate collectors. This advantage means that these collectors are always used for high temperature applications. They also have the ability to absorb both direct and diffuse radiation like flat plate collectors but at lower incident angles their efficiency is greater. According to Kalogirou (2004 p.246) This effect tends to give Evacuated Tube Collectors an advantage over Flat Plate Collectors in day-long performance Tracking Collectors For a solar collector to receive the maximum amount of radiation per square metre it is essential that the tilt of the collector is at the optimum angle to the direction of direct radiation. For maximum efficiency and energy extraction the collector surface must be kept perpendicular to the incident solar radiation throughout the day. In a tracking mode the collector can be mounted on a motorised support which tracks the (azimuth) angle of the sun as it changes throughout the day and also the year. It is possible to collect

31 times more energy from the collector with a tracking collector as opposed to a static setup each day (Sen, 2008) Parabolic Concentrating Collectors Parabolic concentrators shown below in Fig. 2.7 are rarely used in the European climate but they are very useful for high temperature applications from C, where the efficiency of the collector outperforms that of vacuum tube collectors. In very hot countries where solar cooling systems are used, temperatures levels of 150 C or higher are easily achievable. Fig. 2.7 Parabolic Concentrating Collectors (Solar Thermal, 2012) Some feasibility studies and concept designs exist where parabolic concentrators can be used on commercial buildings however to date they have not been used in that scenario. Parabolic collectors rely heavily on direct irradiance and climates with a high direct irradiance proportion and little cloud cover are best suited for their use. According to Eicker (2003 p.48) The investment costs of parabolic collectors are about 300 /m². The solar heat costs can be as low as /kwh on a Turkish site with 1900 kwh/m² direct normal irradiance and 0.11 /kwh on a southern German site with 890 kwh/m². In research conducted by (Sen, 2008) he notes a few disadvantages to concentrating collectors. In the most common type of collector on the market; the flat plate collector, diffuse solar radiation can be absorbed by the collector as well as direct radiation. Parabolic concentrating collectors however focus the incident sunlight on the collector 17

32 surface which excludes the contribution of diffuse radiation. Another disadvantage noted is that these types of collectors must be fitted in a tracking mode so collector can tilt accordingly to match the movements of the sun throughout the day These collectors can achieve a temperature of up to C in the heater which makes their lifespan shorter than other types of collectors. They must be accurately aligned to ensure that the focus coincides with the collector face. The fact that these collectors must be tracking makes them more costly with expensive motors, pivoting framing and sensors required (Sen, 2008). 2.4 Solar Combined Heating Systems At suitable locations, solar-supported space heating systems (Solar Combisystems) can be considered for low-energy buildings, and District heating systems. Efficient operating solar-combined heating systems require high building insulation as well as low-temperature heat distribution. At the design stage the storage volume and area of collectors as well as the strategy of storing the hot water are the key elements. In cases where the solar water heating system works in combination with a space heating system and some of the contribution from solar heating system is used for heating purposes, the area of the collectors along with the storage volume must be increased. In cases of a combined system there exists phases where hot water from the solar water heating system is unused in the period where space heating demand is low. Alternative and efficient uses of this excess hot water can be utilised if an extra heat demand exists on site in the summer months. Some examples of these alternative uses include low temperature hot water for a heated outdoor swimming pool or the use of a ground-coupled heat pump system which operates by heating the surrounding soil (Fanniger, 2012) Solar Assisted District Heating Solar assisted district heating can be defined as central heating supply topped up by solar energy to large residential or industrial estates. Solar assisted district heating networks can consist of a central heating systems with a buffer tank and a boiler for auxiliary heating. The network also includes a hot water distribution network, building transfer stations and the solar array. As well as the short-term buffer tank in the central 18

33 heating installation, excess heat from the summer period can be stored till the winter by means of a long-term heat store. This long term heat store actually increases the amount of useful solar hot water in the system (Eicker, 2003). These systems often vary from tens to hundreds of square metres of collectors and have recently showed to represent an important share in some European markets such as France and Germany. Solar Energy can be an attractive option for district heating systems, where typical working temperatures range from 30 C to around 100 C for water storage (IEA, 2012). 2.5 Solar Storage In an efficient solar domestic hot water (DHW) system the solar hot water needs to be stored in a solar storage tank to allow the build-up of heat slowly during the day. Most vessels used for solar storage use water although there is research being conducted into the use of alternative fluids and the possibility of even using solids is being explored. Due to the pattern of solar gain, it is in most cases a given that a back-up heat source will also have to be incorporated into the system. At the design stage a strategic choice is made as to whether to combine the outputs of both back-up and solar heat sources into one storage vessel. Using a single vessel, as shown below in Fig 2.8, is the most common solution adopted in the UK and Ireland where a cylinder is fitted with two indirect coils, one heated by solar, the other heated by a boiler to heat the domestic hot water (DHW) (CIBSE, 2007). Fig. 2.8 Combined Solar store and DHW store (CIBSE, 2007) The other frequently adopted alternative is to use a solar storage vessel that is without any input from the back-up heat source. This vessel stands alone as a store that can then 19

34 be used to pre-heat the water feeding the downstream heat store connected to the boiler, shown in Fig. 2.9 on the next page. Fig. 2.9 Separate solar store preheating DHW store (CIBSE, 2007) Solar Store Stratification The stratification of water at various temperature levels in a solar store is vital for the high efficiency of the solar primary circuit. Stratification arises from the natural layering of water at different densities, where the hottest water rises to the top and the coolest descends to the bottom, shown in Fig below. Fig Stratification of solar warm water (CIBSE, 2007) 20

35 This layering forms as the solar circuit heats the store and it is advantageous that the return of the primary circuit remains cool for as long as possible throughout the day. With a cool return, the collector will have lower heat losses and will readily absorb more heat from the sun s radiation. In a well designed system, the return temperature of the solar circuit will only rise above 50 C once the whole store has heated from top to bottom. As water is drawn off through the secondary system, the cold feed will assist the stratification by bringing in more cool water, but only if a baffle plate is located to prevent the inrush from causing unwanted turbulence. Similarly unwanted turbulence from secondary circulation pumps, boiler coils or powerful immersion heaters can all serve to disrupt stratification (CIBSE, 2007) Seasonal Storage Over the past thirty years many concepts have been turned into reality in the areas of long term thermal storage; however the difficulties of such systems still remains. In research conducted by Boyle (1996 p. 122) he states that the volume of hot water storage needed to supply just one house is almost the same size as the house itself. In addition to this the tank would need to very well insulated. A standard domestic solar water cylinder would require insulation four metres thick to retain its heat from summer to winter. Consequently with these conclusions it makes financial sense to make storage volumes as big as possible as it reduces the ratio of surface area to volume. Three different types of storage have been commonly used in Europe which include earth reservoirs, earth probe storage systems and aquifer storage systems (Ochs et al, 2008) Earth Reservoir Storage Tanks This type of system is generally constructed using concrete as containment and is either partially or fully submerged in the earth. The tank is lined to seal against vapour diffusion and is insulated to prevent thermal losses from the hot water (Earthscan, 2010). This type of seasonal storage (Fig. 2.11) is very expensive with most of the costs occurring with ensuring the long-term impermeability of the liner to water and vapour, and the resistance of other materials to high temperatures(lottner et al, 2000) 21

36 Fig Seasonal Storage tank (Lottner et al, 2000) Borehole Thermal Energy Store For this type of storage system, heat exchanger pipes are laid horizontally in the earth or vertically into drilled holes and are thermally insulated up to the surface. The surrounding soil is used as the storage medium and heats up or cools down. Practically any size of storage volume is possible but the soils characteristics play a major role (Earthscan, 2010). Fig Borehole Seasonal Thermal Storage (DLSC, 2012) 22

37 Aquifer Thermal Energy Storage Aquifer Thermal Energy Storage (ATES) works in a similar nature to Borehole Thermal Energy Storage (BTES). This form of energy storage utilises a natural occurring underground layer (e.g. a sand, sandstone, or chalk layer) as a storage medium for the short-term storage of heat or cold. This storage system operates by transferring the thermal energy by the extraction of groundwater from the naturally occurring layer with being re-injected at an altered temperature elsewhere (IEA, 2012). 2.6 Solar Radiation The levels of solar radiation in Ireland is much lower than other warmer countries such as India, Australia, Italy or Cyprus, but with radiation levels of approximately 1000 kwh of per square meter over a year, the levels are comparable to apparently sunnier countries in central Europe such as France (Fig ) (Hernandez & Kenny, 2012). Fig Daily average solar radiation (kwh/m 2 ) (Hernandez & Kenny, 2012) According to Wilson (2007 p.37) in his research of solar radiation in Ireland he states that Although it is often claimed that a good solar heating system will provide up to 20 percent of wintertime domestic hot water requirements, this is not the case. The extremely low levels of solar radiation from mid November to early February will mean in a number of solar water heating setups an almost total dependency on conventional methods of heating water. In general, it can be assumed that on days per 23

38 annum, the output from the average solar heating system will be close to zero. Wilson s claim is indeed interesting and he also backs it up with figures. Met Eireann carried out an analysis of solar radiation levels at Dublin Airport. The analysis concluded that solar radiation exceeded a useful level of 3kWh/m 2 on a total of 124 days, but out of these days only one occurred between Sept 21st and March 21st. During the summer months however, solar radiation exceeded 7kWh/m 2 on a number of days. Fig below shows the low levels of daily solar radiation measures in December in four locations across Ireland and four other locations across the world. Fig Comparison of daily solar radiation levels (Wilson, 2007) Direct & Diffuse Radiation Global radiation is comprised of two components, direct radiation and diffuse radiation. Direct radiation arrives in the form of sunshine that comes in a straight line from the sun through the atmosphere. Diffuse radiation is the element of global radiation that arrives at the ground after being reflected of clouds or the ground and scattered by dust, water vapour or air molecules in the atmosphere. The high amount of cloud cover and in particular weather variations means that Ireland and Britain have an Atlantic maritime climate. This climate on a high number of occasions produces high levels of diffuse 24

39 radiation, predominately on cloudy overcast days. This climate can also deliver a number of days with broken sunshine, when mainly cloudy conditions are replaced by largely sunny conditions, and global radiation values can exceed 1200 W/m². The summers in Ireland are now often accompanied by rain but on a positive note these conditions do not affect the efficiency or performance of solar collectors. Weather data files and studies indicate that the average percentage of diffuse to direct radiance has been found to be between 50% and 60%, with higher values of diffuse radiation in the winter due to the cloudy conditions. The graph below (Fig. 2.15) shows a sample of this weather data with the average daily Global radiation kwhr/m² comprising of diffuse and direct irradiation measured in Dublin Airport in 2005 (Solarbook, 2012). Fig Direct & Diffuse Irradiation levels Dublin Airport (Solarbook, 2012) As well as the above consideration, the sun s energy passes into the atmosphere in both the visible, narrow, and non-visible wide band form. The entire spectrum falls onto a collector and so it can be said that collectors can work with daylight as well as sunlight. It is the sum of both direct and diffuse solar energy that permits solar systems to achieve a reasonable annual performance (CIBSE, 2007). 25

40 2.6.2 Tilt & Orientation Tilt angle and orientation are the two contributing factors that contribute to the amount of radiation that is absorbed by a solar collector surface. The definition of a tilt angle is the angle (inclination) between the solar collector surface and the horizontal plane. The orientation of the solar collector can also be defined by its relationship with the horizontal plane. The orientation can be defined as an angle between the line due south and the projection of the solar collector to the surface on the horizontal plane (Armstrong & Hurley 2010). In research conducted by Sukhatme (1996 p.139) it is noted that It is now widely acknowledged that the optimum orientation for a solar collector, in the northern hemisphere, is facing due south. This assertion is also backed up by (CIBSE, 2007) who conclude that the optimum irradiation available from the sun lies between at an orientation between South East and South West from horizontal. This conclusion is explained in diagrammatic form in Fig below. Fig Variation of annual irradiation according to tilt and orientation (CIBSE, 2007) 26

41 The legend on the left in Fig is the annual percentage of irradiation the collector receives with red being 100% down to blue which is 70%. The concentric circles form the tilt angles from 10 to 90 and the graph can be read by picking an orientation and picking the point where the concentric circles cross (tilt angles) cross this orientation. For example for an orientation facing south east and a tilt angle of 40, 90% of the optimum amount of annual irradiation would fall on the collector. To select the choice of the optimum tilt angle, however, there are some differing proposals in the published research. The proposals analysed can be separated into two groups, the first is calculating the tilt angle by location latitude angle and the second is maximising the amount of solar radiation falling on the solar collector similar to CIBSE s chart above. A point worth noting is that the accuracy of these proposals may not apply to Ireland as they have been validated in sunny climates where the beam portion of global radiation dominates. In climates such as Ireland where the weather is frequently overcast even in the summer the beam radiation is lessened and the diffuse radiation is prominent (Armstrong & Hurley 2010). An example of research carried out on the first proposal by calculating the tilt angle by location latitude is Kern & Harris, (1975). Their research concluded that for four locations in South Africa that the optimum tilt angle was indeed equal to the latitude location of South Africa. The validity of this research has to be questioned however as it has not taken into account what the end use of the solar energy is. Indeed Sukatme (1996 p.143) states that For an application like space heating, the demand may be high in the winter months of December, January and February. Similarly if the solar energy was used for running an absorption refrigeration plant, the demand would be highest in months like April, May & June. He further concludes that in such cases it would be beneficial to use a tilt angle greater than the location latitude angle and it would be less in the case of a summer application. It is recommended for space heating in winter to increase the tilt angle by 10 or 15 above the latitude location angle and for a summer application to decrease this angle by 10 or 15. Research in Turkey was carried out by (Gunerhan & Hepsbasli, 2005) in determining the optimum tilt angle of solar collectors. As mentioned previously the optimum tilt angle would be taken to be equal to the locations latitude in order to utilise solar energy throughout the year. Results of the research (Fig. 2.17) show that for approximately two 27

42 months of the year March (day 75 in Fig. 2.17) and September (day 258) the optimum tilt angle is equal to the latitude of the site of in Izmir Turkey. The results show how the optimum tilt angle changed as the sun changed angles through the seasons. Fig Variation of the optimum tilt angle (Gunnerhan & Hepsbasli, 2005) Estimating Solar Radiation The amount of global solar radiation that a location receives and the amount of diffuse and direct irradiation that it comprises are the most important figures needed for designing solar energy systems. Adequate knowledge of these parameters is required for the accurate prediction of the system efficiency at a particular location. There are currently three main methods of estimating solar radiation levels on a plane each with its own drawbacks particularly for the Irish climate Angstrom Sunshine Hours Method At present solar irradiation intensities are only measured in a small number of sites throughout the world due to the latitude, altitude and many meteorological factors. The sites that do measure solar irradiation require the use of pyranometers. The cost of operating and maintaining these instruments is very expensive and far exceeds smaller budget restrictions of some of the smaller local meteorological stations. It has therefore become necessary to make objective interpolations of measurements sites without pyranometers using sunshine duration data to estimate the levels of global irradiation. 28

43 Sunshine duration data is measured at nearly every all weather stations in the world and is currently the most widely available factor for the solar irradiation estimations. The relationship between sunshine hour s duration and global solar irradiation was first determined by Angstrom in 1924 using a simple linear model (Sen, 2001). The drawbacks of this method include: The duration of sunshine hours measured can on occasion lead to an overestimate of cloudy conditions. The variables a and b are gathered from global radiation data which is dependent on local conditions. These variables are also interpolated from long term data fitting, therefore limiting the area of application to a specific location (Linacre & Geerts, 2005) Satellite Based Methods Satellite based methods involve the global horizontal radiation firstly being estimated by looking at the pixel value of the satellite image. The next step involves calculating the clear sky radiation and a cloud index is interpolated from the pixel value of the satellite images. Subtracting the clear sky radiation from the cloud cover obtains the global radiation at ground level (Hammer et al, 2003). METEOSAT which is a geostationary satellite can assist with obtaining information on solar irradiance over a large area with a temporal resolution of up to 30 min and a spatial resolution of up to 2.5 km. The values from this method of measuring global radiation have in comparison been as accurate as ground station measurements at a distance of 25km (Zelenka et al, 1999). The drawbacks of this method include: Errors can occur on days in winter due to difficulties in distinguishing between clouds, fog, frost or snow from the satellite image (Vignola et al, 2007). Estimates of diffuse radiation are not accurate under thin and scattered clouds due to the solar radiation that reflects back off the ground not being taken into account by the satellite imagery (Armstrong & Hurley, 2010). 29

44 Temperature Based Methods A simple equation to estimate daily global radiation uses the difference between maximum and minimum temperature to give global radiation as follows: H = ahoe (Tmax Tmin )+c Where: Tmax = Tmin = maximum temperature ( C) minimum temperature ( C) a & c = empirical constants (Hargreaves et al, 1985) The drawbacks of this method include: This method is unacceptable to use on wet climates in Northern Europe. The climate in Ireland is unsuitable for this method because variations in temperature are from the result of cloud effects. The Gulf Stream is a more dominant temperature moderator and can mask the effects of cloud cover (Supit & Kappel, 1998). 2.7 Collector Performance Collector performance can be characterised by means of two experimentally determined constants: Conversion factor: The collector efficiency when the ambient air temperature equals the collector temperature. Heat loss coefficient: The mean heat loss of the collector per aperture area for a measured temperature difference between the collector and the ambient air temperature in W m 2 K 1. These collector constants are determined under defined conditions (global radiation intensity, angle of incidence, air temperature, wind velocity, etc.) (Fanniger, 2012). The heat balance of a collector will have three components as follows: Absorbed Heat Lost Heat = Removed Heat by the transfer fluid. It is possible to define the heat loss coefficient for the collector as follows: Heat loss coefficient = (absorbed heat lost heat) / incident solar radiation (Sen, 2008). 30

45 2.7.1 Efficiency of Collectors The performance of the collector is usually is denoted by η, and is a calculation of the ratio of heat taken from the manifold by the transfer liquid and the amount of irradiation striking the collector absorber. Riffat et al.(2005 p.910) notes that This measure of performance (η) will vary with a number of external parameters, including global solar irradiation I n, ambient temperature t a, as well as cooling fluid inlet temperature t 0 and mass flow rate m. These parameters may be grouped by a specially-defined parameter, termed (t mean - t a ) / I n, whereby t mean is the average temperature of the cooling fluid and may be written as η and is usually expressed as the function of (t mean - t a ) / I n as follows: η = η 0 χ 1 Where η 0 and χ 1 are the collector parameters. Riffat et al.( 2005) With regards to the two main types of collectors used in Ireland, the published low efficiencies of flat plate collectors can be explained by the fact that heat loss occurs at the cover surface as a result of conduction and convection in the collector. The quoted efficiencies of collectors can vary with flat plate collectors achieving efficiencies of 50% or less while evacuated tube collectors can have efficiencies of about 50-80% (Riffat et al, 2005). The desired temperature range of the application will have a big bearing on the type of collector chosen. The amount of radiation expected, weather conditions and the space available will also influence collector choice. Flat plate collectors are more commonly used than evacuated tube collectors. They are usually cheaper but, as the efficiency is lower, a larger collector area is needed to provide equivalent output. Fig below shows the efficiency and temperature range of various collectors (Rawlings, 2009). 31

46 Fig Efficiency and temperature ranges (Rawlings, 2009) In relation to the efficiency of collectors in the Irish climate research was carried out by (Hernandez & Kenny, 2012) on the performance of six solar water heating systems in Ireland. The results of the research (Fig. 2.19) demonstrated that when solar systems are properly sized, installed and operated, the performance is as expected. The results show that this is the case in system no. 1 and no. 5 in the study with an actual increase in the output of more than what was expected by 9% and 12% respectively. The results also indicate that the payback of the systems analysed can be below 3 years. The flip side of this is that four of the six installations analysed proved to be far below predicted outputs. The systems that performed poorly were in some cases oversised in relation to the hot water needs or in other cases combined with relatively efficient auxiliary heating systems such as ground source heat pumps. In these cases the measured performance of the installations was worse than predicted due to defects in the installation or control. 32

47 Fig Expected and measured solar output (Hernandez & Kenny, 2012) Collector Certification In respect of durability, reliability and performance, solar collectors should be fully tested and independently certified to BS EN This standard outlines the procedures required to test the reliability and durability of the collectors when under extreme conditions, and can also provide an indication of the performance. Collectors that fall within the scope of the Pressure Equipment Regulations and they are tested by a notified body, then the CE mark can be shown on the collector. The tests just described are on occasion undertaken by independent specialists that are accredited to carry out tests and certify the performance of the collector. This permits the use of the Solar Keymark logo (CIBSE, 2007). As well as the raw efficiency of the collector, the BS EN test also gives an indication of the following variables: Dynamic fluid pressure drop Thermal inertia - response rate to variable conditions Power output - rate of energy from collector Insulation of non-glazed areas via a U-value These results above can be used in solar water heating simulation software s and other calculation methods that can assess the monthly or annual performance of systems (CIBSE, 2007). 33

48 2.8 Incident Angle Modifier The measured thermal efficiency of a collector will largely depend on the angle of incidence of the solar irradiance on it. The incidence angle modifier, Kθ b (θ), is a measure used to account for this angular dependence (Rojas et al, 2008). In the diagram shown below in Fig the sun is directly overhead of the collector. In a scenario where a solar collector is installed along the line of the hypotenuse the concentrated radiation onto the collector will consequently be spread out or shared over a larger surface area. In principle the shadow in the diagram is equal to the adjacent side and the corresponding energy of this is spread over the larger hypotenuse surface. A formula to describe this scenario is as follows: Radiation on Panel = Cosine of Angle (A) multiplied by intensity of radiation. In the case of Fig below If A = 35 Cosine 35 = so 819 watts will be absorbed by the panel. If A = 50, Cosine 50 = 0.643, the incident radiation will be 643 watts (Solarbook, 2012). Fig Diagram of the absorption of radiation on an inclined collector (Solarbook, 2012) The Incidence Angle Modifier values allow for a performance factor, so that the efficiency of the panel can be calculated when the radiation from the sun is not perpendicular to the collector. The modification of the cosine as described above in Fig is not taken into account in this case. For example an IAM value of 1, at a certain incident angle, the output is proportional to the cosine of the incident angle multiplied by the full radiation intensity. In comparison, when the sun moves through the sky and the solar irradiation is at different angles, different solar collectors will reflect differing 34

49 amounts of radiation. Flat plate collectors which are the most common type of collector perform in a manner that the more radiation that is reflected from the surface of the collector the more inclined the incident angle is, resulting in the new IAM being less than 1.0. In most vacuum tube collectors, particularly the double walled type, there are gaps between the tubes. When the sun moves across the sky, the radiation will still reach a full tube so the output stays constant, even though the theory suggests that the output should fall by the Cosine factor of the incident angle. In the case of evacuated tube collectors the IAM will essentially be greater than 1.0 (Solarbook, 2012). Research into this area in Ireland was conducted by (Gargan, 2010). In his research it is noted how European test standards measure zero loss efficiency at noon when the sun is directly overhead and ignore the fact when the sun moves through the sky either side of noon when it is facing the tubes at different directions. Rojas et al (2008 p. 751) describes that The incident angle modifier test consists of measuring the collector efficiency at a fixed inlet temperature at steady-state conditions with different incidence angles. The different incidence angles are obtained by varying the azimuthal angle of the collector. The IAM is included in the European test results however the values need to be multiplied by the zero loss efficiency to get the true efficiency of the collector. Fig 2.21 Comparison of how solar radiation is absorbed in flat plat and evacuated tube collectors (Gargan, 2010) 35

50 Gargan also details how Chinese evacuated tube flask panels suffer in the quoted efficiency from the zero loss efficiency being measured at noon. Most flat plates and evacuated tube panels show efficiencies of 75 to 80% and conversely Chinese vacuum flask panels show efficiencies in the mid 60 s or less. However if the IAM is taken into account the efficiency of flat plate and evacuated tube collectors falls slightly either side of noon in comparison to the vacuum flasks where it actually increases. The reasons for this increase lie in the fact that the curved surface of the tubes can track the movements of the sun. At noon when the sun is directly above the collector it shines between the tubes, but as the sun moves either side of noon, the reflection from tube to tube increases the absorption capabilities. This increase in efficiency after noon can be seen in Fig on the next page with a comparison between a flat plate collector and a vacuum flask collector. The perpendicular axis is the angle of the sun in degrees and the vertical axis is the efficiency. The graph shows that the output of the flat plate collector would be reduced by 10% with the sun at an angle of 60 degrees. At the same point the evacuated tube flask s efficiency is actually increased by 44%. (Gargan 2010). Fig Comparison of IAM values between flat plat collectors and evacuated tube flask collectors (Gargan, 2010) In support of Gargan s article regarding the importance of the IAM, a journal published in 2000 compared the incident angle modifier of two types of collector s glazed and unglazed flat plate. The research describes how in unglazed absorbers the values of b 0 and c depend on the type of absorbers used. In glazed absorbers the exponent c approaches 36

51 unity irrespective of the absorber type. Results of this study indicate that the glass cover has a huge impact on the optical performance of the collector at higher angles of incidence (Tesfamichael & Wackelgard, 2000). 2.9 Modelling Solar Water Heating Systems There are several calculation methods and simulation software s available to size and estimate the performance of solar water heating systems. One such software commonly used in research articles is called TRNSYS and many different studies have been carried out using this software including: (Hobbi & Siddiqui, 2009) who researched the optimum design of forced circulation systems in a cold climate, (Sweet & McClesky, 2012) investigated if seasonal storage would be suitable for a single house, (Raab et al, 2005) simulated the thermal behaviour of the water in seasonal storage of solar assisted district heating systems. Other software packages that are used by solar water heating designers include Polysun and Tsol. In these software packages the hydraulic layout can be input and the performance of the systems calculated accurately for a yearly output, including energy performance and efficiencies at hourly intervals. The minimum information requirements for calculation methods are typically the following: Solar irradiation on the collection plane: Daily and monthly horizontal global irradiation values are required from local weather stations. The global irradiation values can then be used to calculate the irradiation values on the collection plane based on the orientation and slope of the solar collectors. Collector loop efficiency: The efficiency of the collector loop is required as an input in simulation software s with the level of detail depending on the method. In most cases, a database of collectors is provided in the software with all the parameters and variables input already, obtained from test certificates for the collectors. Storage tank size and losses: The size of the storage or buffer tank is required as an input in all software s. Some methods and software s allow for inputs such as the position of the tank (horizontal or vertical), insulation levels and controls. 37

52 Circulation system: Solar water heating systems can be thermosyphon or forced circulation. A library of solar water heating systems can be selected with some simulation software s allowing for the hydraulic diagram to be designed to match the system being simulated Hot water demand profile: The amount and usage of the DHW demand is a critical element in calculation methods and software s. The domestic hot water demand can sometimes be estimated by the building size and layout and the expected number of occupants (Hernandez & Kenny, 2012). Research was carried out by (Ayompe et al, 2011) using TRNSYS to model a flat plate collector system and evacuated tube system. The results of the simulations were then compared to actual measurements from installed systems. The collectors (Fig ) used in this study were installed on a flat roof of the Focas Institute building, Dublin Institute of Technology, Dublin, Ireland. Both were installed south facing and at an inclination of 53 equal to the local latitude of the location. The hot water cylinders were installed nearby in the building s plant room. Fig Collector used in the study (Ayompe et al, 2011) The software underestimated the collector outlet fluid temperature by 9.6% and overestimated the heat collected and heat delivered to load by 7.6% and 6.9% for the Flat Plate Collector system. The model overestimated all three parameters by 13.7%, 12.4% and 7.6% for the Evacuated Tube Collector. The results of the study can be seen in diagrammatic form in Fig below with the evacuated tube collector graph on the top and the flat plate collector on the bottom. 38

53 Fig Results of the TRNSYS simulation compared to actual figures The validated TRNSYS software can be used to: To give an accurate prediction of the performance of solar water heating systems in different locations Simulate accurate performances of systems under different climates and conditions. Optimise solar water heating system sizes to match different load profiles (Ayompe et al, 2011). 39

54 2.10 Summary Conducting the literature review has identified some areas that can be explored in the case of solar water heating systems and the simulation software used to design them. Research by (Kaligirou, 2004) indicated that evacuated tube collectors are more efficient than flat plate collectors. The literature also indicates that tracking collectors will produce up to 1.5 times more energy per day than fixed collectors (Sen, 2008). Wilson in his research on radiation levels indicated that a solar water heating system in Ireland would be effectively useless for days of the year. In research conducted on the effect of inclination and orientation (Sukhatme, 1996) concludes that the optimum orientation of a collector in the northern hemisphere is due south. In regards to the tilt he notes that the end use of the solar energy is important for the optimum tilt angle selection. For a system that generates DHW for the building all year round an inclination equal to the locations latitude is the optimum tilt angle. However if space heating is required in winter and most of the energy demand occurs in this season a tilt angle of 10 to 15 more than the locations latitude angle would be better suited. Similarly if the highest demand is in summer a tilt angle of 10 to 15 less than this angle is correct. Research conducted by (Gargan, 2010) explains how the IAM is not taken into account in the efficiency figures quoted in test certificates. The IAM will have a dramatic effect on the on the performance of different types of collectors. The efficiency measured in the test certificates is the zero-loss efficiency and is measured at noon when the sun is directly above the collector. As the sun moves through the sky and the angle of the sun changes the IAM will actually increase the efficiency of evacuated tube collector due to the gaps between the tubes and the curved surface of the tubes passively tracking the sun. The efficiency of flat plate collectors actually decreases during these times. Having conducted the literature review there is a gap in the literature relating to commercial solar water heating systems in Ireland. The research that most resembles this area was conducted by (Herandez & Kenny, 2012) who analysed the performance of six domestic solar water heating systems in Ireland and also simulated the predicted output beforehand. Domestic solar water heating systems are much smaller and have fewer complexities than commercial system nevertheless the results of the study are interesting. Of the six systems studied only two actually equalled/exceeded the 40

55 predicted output. The other four were well below their predicted targets with Hernandez and Kenny citing poor design and installation as the cause of the poor results. This study indicates that the design of the systems in not sufficient and that this may be a concern on commercial system considering that the installers and suppliers work in both sectors. 41

56 Chapter 3: Overview of Case Studies 3.1 Introduction This chapter gives an overview of all the case studies that formed the basis of the primary research. Details of the building type where system is used are explained along with the solar water heating system type, system layout and total costs. 3.2 Bewley s Hotel Case Study The Bewley s Hotel near Dublin Airport is the newest hotel in the Bewley s Hotel Group and was opened in June The designers of the hotel and the client want to put energy management first and foremost in the design of the hotel. It was estimated at the design stage that energy consumption for hot water, for baths, showers, kitchen etc, would contribute to approximately 50% of the hotel s total energy use. Fig. 3.1 Solar Panel Installation Bewley s Hotel This discovery prompted the design team to incorporate systems that could lower the costs of the hotel s hot water production. The most popular and feasible option was to install a large scale solar water heating system which was the largest in Ireland at the 42

57 time of construction. The large scale system includes 56 solar panels or 308m 2 of total collector area on the roof of the hotel (SEAI, 2007) Drain-back System The system designed by Zen Renewables supplies part (30-40%) of the hot water demand of Bewleys Hotel. The system is designed to be a pre-heater system with two 5000 litre solar storage cylinders. The first cylinder on the loop is a combined drainback / solar heat storage and is joined in a closed drain-back configuration with the collector field on top of the hotel. The water in this primary collector circuit is pure water in a closed circuit and does not need treatment like other systems which contain glycol. In the second cylinder water is fed through a heat exchanger which contains preheated water from the mains supply (ZEN Renewables, 2008). Fig.3.2 Hydraulic diagram of Drain-back system The pre-heated hot water is supplied to 3 x 5000 litre indirect gas heated cylinders in order to guarantee the hot water supply at all times for the hotels occupants. The collector field is mounted on the 7 th story roof of the hotel. With the tall height of the hotel the steel frame support construction and collectors are engineered for the maximum wind loads for the height and location. The advantages of the drain-back concept as detailed in Chapter 2 is that there are no anti-freeze additives required and 43

58 the system is inherently safe for overheating situations or power failures. Furthermore the drain-back concept has minimum maintenance requirements, whilst the system performance is still very high (Zen Renewables, 2008). The Bewley s hotel building at Dublin Airport faces slightly east of pure south. As the effect on performance was minimal, the solar collectors were oriented parallel to the façade of the building and not directly south, for visual reasons. The estimated cost of the solar system was 210,000 approximately 25% of this was obtained from SEAI for the purchase and installation of the solar panels. The SEAI estimated in a case study on the project that the system would contribute 198,000 kwh of solar energy to the heat demand of 466,000 kwh. This represents a CO 2 emissions saving of 46 tonnes per year. This solar energy contribution equates to 15,000 of annual fuel cost savings in gas with a payback of 10 years (SEAI, 2007). These figures will be evaluated in the findings chapter where a simulation of the system is run. 44

59 3.3 Cloughjordan Eco Village Cloughjordan eco village in North Tipperary is Ireland s first and only sustainable community. As of July 2012 the 67 acre site currently has over fifty low energy homes and a few work units currently occupied. It is envisaged that when the site is full that there will be 114 low energy homes and 16 work units constructed and occupied. The site is currently owned and run by Sustainable Projects Ireland Ltd (SPIL) and provides its potential house buyers with fully serviced sites which have either outline planning permission, or full planning permission for certain buildings. The buildings are built in accordance with SPIL s overall master plan design, and with their Ecological Charter specifications. The master plan which was devised by Solearth ecological architecture is shown below in Fig 3.3. Fig. 3.3 Cloughjordan Eco Village Master Plan 45

60 3.3.1 District Heating System In Cloughjordan Eco-Village, all space heating and hot water are provided by a solar assisted district heating system, owned and run by the not-for-profit Cloughjordan Ecovillage Service Company. Two high-efficiency 500kW boilers fired with wood-chip (waste from a Midlands sawmill) are the main heat source. The biomass boilers are also backed up by 512 sq metres of ground mounted solar panels. Both sources supply hot water into well lagged distribution pipes, which provide a metered supply of heat to a heat storage tank in every home. This tank gives the householder complete control over the distribution of heat. The system has received substantial grant funding from the SERVE project (section 4.6.4), and SEAI s House of Tomorrow programme. The plant was first fired up in October 2009 however the ground-mounted solar array has not been functioning correctly since then (Comhar, 2011). Fig 3.4 Ground Mounted Solar Array The 512 m 2 of ground mounted flat plate solar collectors were supplied by Carey Glass. The collectors (Fig. 3.4) are arranged in nine rows, with five rows of eight number 46

61 panels, one row of seven panels, two rows of five panels and one row of four panels. The solar array is orientated due south with an inclination of 30. The collectors feed a 17,000 litre buffer tank through a heat exchanger with a minimum return temperature of 58 C. If the temperature does not equal or exceed this figure the solar output will not be useful to the district heating circuit. Origen Energy Ltd in conjunction with Polytherm Heating Systems Ltd designed and supplied a complete solar piping system, to remove the heat from the solar field and transport it back to the energy centre. This was done through the utilisation of a special preinsulated piping system, supplied from Brugg piping systems, known as Casaflex. Casaflex is made from corrugated stainless steel and has a very high temperature and pressure resistance. It can operate up to 180 C and has a maximum operating pressure of 25 bar (Origen, 2012). The district heating network piping consists of 2,500m of network pipe in total. 700m of this is 100mm diameter steel district heating piping, with 100mm insulation and strong PVC covering. The remainder of the piping varies between 60 and 40mm distribution piping. The total cost of the district heating system was approximately 1,700,000 with the solar array costing 227,150. The solar array is currently the largest in Ireland at present and the first to be used in a district heating system (SPIL, 2012). Research was carried out by Monaghan, (2011) on the whole district heating system with a particular focus on the biomass boilers. Simulations were carried out to predict the overall output of the solar array with a result of 228,999 kwhrs total output. 47

62 Chapter 4: Methodology 4.1 Introduction This chapter outlines the research methods used for the completion of this dissertation. The tools and techniques used in this research will be detailed along with the rationale for the methods chosen. The research methods chosen for this dissertation involve both quantitative and qualitative methods of research. The quantitative methods included simulation research and case study research. The qualitative methods included interviews with two representatives involved with the design and maintenance of the chosen case studies and a survey sent to solar water heating system installers asking which design software they used in their company. 4.2 Case Study Research According to Fellows & Liu (2008 p.111) Case study approaches facilitate in-depth investigation of particular instances of a phenomenon. Those instances may be selected in a number of ways to be representative of general cases, bespoke cases and random cases. Normally because only a small number of cases are studied and the studies are in-depth, the purpose is to secure theoretical validity, rather than the statistical validity required of surveys. The case studies chosen for this research are the two largest commercial solar water heating systems in Ireland, Cloughjordan Eco Village and Bewleys Hotel Dublin Airport. The scale of these systems should provide a good indication of the performance of the commercial systems in the Irish climate and secure theoretical validity for the hypothesis. 4.3 Simulation Research According to Martinez & Ioannou (1999 p.267) Quantitative simulation research is founded upon mathematics, probability and statistics. Simulation studies attempt to derive a semantic content from models which represent actual systems. Morgan (1984) suggests a variety of purposes for simulation: Explicitly mimic the behaviour of a model Examine the performance of alternative techniques Check complex mathematical/analytic models 48

63 Evaluate the behaviour of complex random variables, the precise distributions of which are unknown The results of the survey sent to the solar water heating system installers will denote which software packages will be used in this research. The system installers will be sent a questionnaire asking which software they use in their professional practice and what were the reasons for choosing that software package. The questionnaire will also ask what the respondents feel are the limitations of the software which will help in identifying if the software is suitable to achieve the objectives of the dissertation. Two software packages will be chosen from the results of this questionnaire and will be used for the simulation research in this dissertation. The performance of software packages will also be analysed and compared whilst carrying out the objectives. 4.4 Interviews Two informal interviews were conducted for this research with representatives from the chosen case studies, to obtain information on the relevant systems and acquire design information. The first meeting that took place was in March 2012 was with a member of the management team from Cloughjordan Eco Village. The meeting included a tour of the site including the biomass boilers, plant room, woodchip storage and solar array. The complex system was explained in detail and all the systems capabilities and shortcomings were explained. The second interview took place in August 2012 with a representative from Bewleys Hotel. The interview was also coupled with a site visit and a tour of the system was given including the collectors on the seventh storey of the hotel, solar storage tanks, gas fired boilers and the control operating system. Minutes of both meeting are included in Appendix D. 49

64 4.5 Research Objectives Methodologies The methodologies chosen to carry out each objective are detailed below. Simulation research is predominately used on the case studies outlined in Chapter three Predicted Contribution The methodology used for calculating the predicted contribution of the system in both case studies involved using software to simulate the output. Firstly the hydraulic diagram of the system had to be compiled in the software. Drawings were obtained of the actual system and it was replicated on the software diagram generator. Specifications for the collectors are entered including orientation and tilt angle. The orientation on the software is denoted as South (0 ), South East (45 ), East (90 ), South West (-45 ) and West (-90 ). Both simulation software packages contain a database of collectors to choose from with technical specifications already input. The collectors used in both case studies were only contained in one of the software packages so the technical specification had to be input manually. The details input include, collector type (flat plate or evacuated tube), gross absorber area, aperture area, collector efficiency, first order of heat loss co-efficient (a1), second order of heat loss co-efficient (a2) and the volume of the collector. Further specifications for the system were then input including pump design, solar storage volume and hot water demand. The software requires a location to be set using co-ordinates and a weather file is then applied. The nearest weather station to the site in Cloughjordan is Birr, Co. Offaly and the nearest to Bewleys Hotel is Dublin Airport. The weather file gives daily average averages for mean temperatures, solar irradiance and cloud cover among other various data to give an accurate reflection of the climate in that area. The predicted output will be measured in kwhrs and simulations will be run in both Tsol and Polysun to compare results of both. 50

65 4.5.2 Solar Output Temperature The results of the simulation on the predicted output were used to establish the predicted temperature of the collector outflow. In the case of Cloughjordan the return temperature of the District Heating System is 58 C and the output from the solar array must equal or exceed this temperature to contribute to the buffer tank. The results of the simulation give the predicted temperature of the collector for each hour of the year. The percentage of time the output of the collector is useful is then quantified and representative figure is given for the actual output to the system. In the Bewleys Hotel case study the optimum temperature of the DHW is 65 C without any top-up of heat from the gas fired heating cylinders. A frequency table will be compiled from the results of the simulation to quantify how often this temperature and above is achieved Calculation of the Effect of Tilt and Orientation This calculation will be carried out using simulation software to quantify the effect that both the tilt of the collector and orientation of the collector have on the output of the system. The collectors in the Bewleys Hotel case study have an orientation of South and a tilt of 30, further simulations will be carried out with the collector at inclinations of 0 up to 90 C in increments of 10. Simulations will be carried out with the collector facing South, South East, West and East. Each of these orientations will then have simulations carried out with the collector in different inclinations as described earlier. Three additional simulations will be run with the collector tracking inclination, tracking azimuth and biaxial tracking Heating Season Contribution In the Cloughjordan Eco-Village case study the collectors are currently installed at an orientation facing directly south with an inclination of 30. As described previously simulations will be carried out changing the orientation and the inclination for each. It should be noted that the solar assisted district heating system in Cloughjordan provides space heating for the houses in addition to domestic hot water. This fact equates to there being a higher heat demand in the winter than the summer due to the cold winter in the Irish climate. This research will establish if a different inclination would be suitable for the higher demand in the winter than the current configuration. The collector temperature will also be analysed giving the percentage of time that the temperature 51

66 exceeds 58 C for each orientation and inclination. It was also established from the Literature Review that it is widely accepted that the optimum inclination of the collector should be equal to the latitude of the location the collector is situated. This research will investigate if that is true for the case studies chosen Variation in Cloud Cover In the literature review cloud cover was mentioned as having a big bearing on the performance of a solar water heating system. With Ireland having a high degree of cloud cover all year round an analysis of its variation may prove beneficial in the design of a system. Simulation software called IES Virtual Environment (IES VE) was used to estimate the cloud cover year round on the chosen case studies. Cloud cover is measured in a unit called an okta. In meteorology, an okta is a unit of measurement used to describe the amount of cloud cover at any given location such as a weather station. Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging from 0 oktas (completely clear sky) through to 8 oktas (completely overcast). The data from the IES VE weather data was analysed and a percentage of time that the cloud cover reached 8 oktas is shown in a diagrammatic form using a frequency distribution table Evacuated Tubes Comparison Both the projects in Cloughjordan and Bewleys hotel have used flat plate collectors as opposed to evacuated tubes. The reasons for this selection may be primarily to do with cost and huge the scale of both systems. Simulations will be carried out on both case studies to establish if evacuated tube collectors would provide more or less output than the flat plate collectors that are currently installed. Results of the simulation will detail the temperature and output of both at different times of the day. The evacuated tube collectors chosen for this comparison are manufactured by Surface Power. This company who are based in Ireland claim to have the best solar collector on the world market with an output of 783 kwhrs/m 2 /yr based on a minimum hot water temperature of 54 C. This claim will be examined using simulation software on the systems in both Cloughjordan and Bewleys Hotel. The overall output of the proposed evacuated tubes will be compared to the flat plate panels in both cases and the amount 52

67 of time that a useful temperature of 58 C is supplied in Cloughjordan will also be examined Incident Angle Modifier The results of the above simulation will also quantify the effect that the incident angle modifier has on the efficiency of the collectors in both Cloughjordan and Bewleys Hotel. The global irradiation onto the collectors will be quantified and compared to the irradiation after the IAM on both flat plate collectors in the case studies. The simulations run in the evacuated tube comparison will then be utilised to establish the effect that the IAM has on the collector. These results will then be compared to the flat plate collector s results to quantify if there is an increase in the irradiation absorbed onto the evacuated tube collector. As was noted in the literature review evacuated tubes can improve in efficiency either side of noon with the incident angle modifier. This research will establish using simulation if this does occur. 4.6 Limitations of the Research The research carried out for this dissertation has some limitations that must be noted. The selection of the case studies was decided on because they are the two largest commercial solar water heating systems in Ireland. The time frame of the dissertation also meant that with the scale of these projects that analysing two case studies would only be achievable. To gain a more complete picture of the Irish climate and design of systems at least six systems would need to be analysed spread out across locations in Ireland. That objective in itself would perhaps be problematic as the other commercial solar water heating systems are very small in size ranging from 30 m 2 to 60 m 2. For this reason a comparison with Cloughjordan Eco Village- 512 m 2 and Bewleys Hotel Dublin Airport 308 m 2 would not be appropriate. When contact was made with the both representatives from Cloughjordan Eco Village and Bewleys Hotel Dublin Airport data of the actual output from the solar water heating systems was promised to be made available. In the case of Cloughjordan Eco Village the solar array was not operational when first contact was made in February 2012 however it was meant to be operational in March and approximately six months of data would have been made available. The money required to get the array operational was not provided and so the data would not be available. 53

68 The contact established with Zen Renewables on the Bewleys Hotel Dublin Airport case study maintained that they would have no issue sharing the performance data of the system for this dissertation. After contact was made several times to obtain this data the company declined to share the data. The proposed objective to compare the actual data from both case studies in this dissertation to the simulated output could have given a clearer indication of the complete accuracy of both Polysun and Tsol. The overall aim of this dissertation can still be achieved with the objectives set out as the simulation software s are used at the design stage and it will aim to assess the performance of them at this stage. 54

69 Chapter 5: Research Findings and Discussion 5.1 Introduction This chapter will detail all the results obtained from data and simulations run as detailed in the previous chapter. The data is then analysed and presented in the form of figures and tables. The layout of this chapter will be divided into each case study Cloughjordan eco village and Bewleys Hotel Dublin airport then follow the list of objectives set out at the start of the dissertation presenting the findings from each. 5.2 Case Study 1 Cloughjordan Eco Village Predicted Contribution Polysun To estimate the overall contribution of the system simulations were carried out using TSOL and Polysun. The first simulation carried out using Polysun on the predicted solar thermal output resulted in the solar array contributing 238,156 kwh (Table. 5.1) to the District Heating System in the Eco Village. Year: 2012 Month Energy to System (kwhrs) January 8012 February March April May June July August September October November 9187 December 5977 Total Table 5.1 Polysun Simulation Results The efficiency of the collector estimated by Polysun is 46.1 % with a yield relating to gross area of 470 kwh/m 2 /Year. A summary of these results can be seen in Appendix B. 55

70 The amount of irradiation received at this location versus the thermal energy received is shown below in Fig Solar Thermal Energy vs Irradiation 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Energy to System (kwhrs) Irradiation onto collector (kwhrs) Fig. 5.2 Comparison of Solar Thermal Energy versus Irradiation Solar Output Temperature The system in Cloughjordan Eco Village requires a return temperature from the solar array of 58 C. The results of the simulation above were analysed to see when the temperature exceeded or equalled this temperature. Fig. 5.3 Daily Maximum temperature of Collector outflow ( C) 56

71 The maximum temperature that the collector achieved each day of the year is shown in Fig Note how the solar array does not even achieve a maximum temperature of 58 C for the first and last two months of the year. The results were further analysed to examine how often each temperature was achieved by the collector. A cumulative frequency table Fig. 5.4 was devised to see how often the collector supplied useful hot water to the system Cumulative Frequency of collector temperature Cumulative Frequency (hrs) Collector Outflow Temp ( C) Fig.5.4 Cumulative Frequency Distribution It must be noted that the cumulative frequency distribution plotted above is for the whole year (8761 hrs) and includes hours when the collector is not operational i.e. at night. The collector provides a useful contribution to the district heating system (DHS) of equal to or above 58 C for 475 hours of this period. The number of hours that the solar array is operational in a year is 4,583 hrs. This means the array provides a useful contribution for 10% of the time it is operational. 57

72 5.2.2 Predicted Contribution TSOL Details of the system were also entered into TSOL simulation software and a simulation was run with data from a nearby weather station in Birr. Table 5.5 below shows the results of the simulation with the predicted output over a year of 227,794 kwhrs with the system in its current installed state of 30 tilt orientated south. The total output simulated by TSOL is 10,363 kwhrs less than the output of POLYSUN which is 238,157kWhrs. Year: 2012 Month Energy to System (kwhrs) January 5,170 February 8,796 March 17,090 April 27,058 May 34,524 June 30,180 July 30,283 August 25,933 September 23,051 October 13,589 November 7,911 December 4,209 Total 227,794 Table 5.5 Tsol Simulation Results The results of TSOL are compared to the results from POLYSUN in Fig. 5.6 on the next page. The simulated output in the summer months is very similar with TSOL predicting a greater contribution in May of 2,842 kwhrs. The software also simulated a reduced output in August compared to POLYSUN of 2,690 kwhrs. POLYSUN predicted an increased output of approximately 2,000 kwhrs compared to TSOL for all of the summer months. 58

73 Tsol vs Polysun 35,000 30,000 25,000 20,000 15,000 10,000 5,000 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Energy to System TSOL (kwhrs) Energy to System POLYSUN (kwhrs) Fig. 5.6 Comparison of results from Tsol & Polysun The daily maximum temperature of the collector is shown below in Fig The maximum temperature of the collector rarely exceeds the required 58 C except in the summer months when the space heating demand is lower. The temperatures simulated are a little different to the temperatures simulated in Polysun in that the temperature achieved by the collector only reaches a temperature of 58 C from June to September. The predicted maximum temperatures in Polysun exceeded 58 C from March to October which can be seen in Fig Fig. 5.7 Daily Maximum Temperature Tsol 59

74 5.2.3 Calculation of the Effect of Tilt and Orientation To establish the results of this objective, simulations were carried out using Polysun with the collector in its current configuration and then with changes to the tilt and orientation as detailed in the methodologies chapter. The first orientation simulated was facing south (Fig. 5.8) as this is the current orientation of the solar array at present. Note how optimum tilt and inclination combination is facing South with a tilt of 40. The results of the simulation indicate that at this tilt an increase in the irradiation onto the collector area of 10 kwhrs/m 2 /yr compared to a tilt of 30. This represents a total increase of 5,120 kwhrs of irradiation for the total collector area. The increase in irradiation consequently increases the yield by 6 kwhrs/m 2 /yr and 3,072 kwhrs for the total collector area of 512m Collector Inclination Comparison 1000 kwhr/m2/yr Collector Yield Irradiation Fig. 5.8 South Orientation Collector Inclination Comparison Simulations were also carried out in the other orientations, South East, South West, East and West. Fig. 5.9 on the next page shows the overall effect that orientation has on the yield of the system on all orientations and inclinations. The results of the simulations with the collectors facing East and West result in a similar yield for all inclination angles. This is also the case with the collectors facing South East and South West. The 60

75 optimum tilt is 40 and the highest yields for facing South, South East and South West occur at this tilt angle. There is a decrease in yield for the year from a collector facing South at this tilt angle to a collector facing South East or South West of approximately 6%. This compares with a decrease of yield of 22% with the collector facing East and West. The optimum tilt angle for collectors facing East and West is in fact 0 with a yield of 194,600 kwhrs. Collector Yield (kwhrs) 250, , , , , , , , , , , , , ,000 Orientation & Inclination West South West South South East East Fig. 5.9 Comparison of all orientations and inclinations Many solar collectors in Europe are mounted on a pivot so that the tilt angle can track the sun as it changes angle during the year and seasons. Simulations were run with the collector in this configuration on each of the orientations West, South West, South, South East and East. The results of these simulations are shown in Fig on the next page. Note how the irradiation value for the south orientation represents the highest value however the highest yield resulted in the East and West orientation. 61

76 Tracking Tilt Orientation Comparison Collector Yield (kwh/m2/yr) Irradiation(kWh/m2/yr) West South West South South East East Fig Tracking Tilt Orientation Comparison The literature review also details how the collectors can be mounted on a horizontal pivot tracking the azimuth (orientation) of the sun. The collectors can also be mounted with biaxial tracking which tracks in both directions (tilt & azimuth). Fig below shows that the overall collector yield results of this simulation compared to the currently installed case of non tracking South orientation and 30 tilt angle. Comparison to azimuth and biaxial tracking Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Energy to System 30 (kwhrs) Energy to System Tracking Azmiuth (kwhrs) Energy to System Biaxial Tracking (kwhrs) Fig Comparison of tilt 30 facing south to azimuth tracking and biaxial tracking 62

77 With the system in azimuth tracking mode the total output of the system is 302,468 kwhrs compared to 238,157 kwhrs in the standard configuration. This shows an increase of 27% and 64,311 kwhrs. With the collectors mounted for biaxial tracking the total output is 337,206 kwhrs. This equals an increase in output from the standard configuration of 41% and 99,409 kwhrs Heating Season Contribution The district heating system in Cloughjordan Eco Village has the dual purpose of providing domestic hot water and space heating. With the higher demand in the heating season the results of the above simulation were analysed to establish which orientation and inclination combination would contribute the highest yield in the heating season in Ireland running from the 1 st of October to the 30 th of April. South Orientation - Heating Season Contribution Collector Yield (kwhr) Series Fig Heating Season Contribution The results of the simulations (Fig. 5.12) indicate that at a tilt angle of 60 the system produces an increase in output during the heating season of 10,625 kwhrs compared to the currently installed tilt angle of 30. A comparative graph of the output between both tilt angles along with a table of the results is shown in below in Fig

78 Tilt Angle Comparison Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Energy to System 30 (kwhrs) Energy to System 60 (kwhrs) South Orientation (0 ) Inclination Irradiation Irradiation Total Collector Collector Yield onto onto Collector Efficiency per gross area collector collector Yield (%) (kwh/m 2 /year) area area (kwh) (kwh) (kwh/m2) , , , , Fig South Orientation Tilt angle comparison With the increased output in the winter using an inclination of 60 a distinct advantage is achieved for space heating on most solar water heating systems. The system in Cloughjordan however requires a return temperature of 58 C before the solar array can contribute. The results of the simulations on both tilt angles were analysed to quantify how often the system equals or exceeds this temperature. Using the results of global irradiance from the simulation it was calculated that Cloughjordan gets 4,583 hours of irradiation on average per year. The table on the next page (Fig. 5.14) shows the useful contribution and number of hours of irradiation for each month. 64

79 South 30 No. of hours Year: contribution No of hours of 2012 above or equal irradiation Month to 58 C Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Table 5.14 Total Contribution above 58 C Tilt 30 The figures above (Table. 5.14) show that the solar array can contribute in its current tilt and orientation to the District Heating System for approximately 10% of the time that it gets irradiation on to the collectors. The winter months with the tilt at 30 have a very poor amount of contribution with January and December producing zero hours above 58 C. The summer months produce an even amount of contribution of about 13% of the equivalent amount of hours of irradiation. 65

80 South 60 Year: 2012 Month No. of hours contribution above or equal No of hours of irradiation to 58 C Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Table 5.15 Total Contribution above 58 C Tilt 60 The amount that the system contributes with a tilt angle of 60 facing south (Table. 5.15) is also approximately 10% of the time its gets irradiation onto the collectors. The winter months show a large improvement compared to the tilt angle of 30 (Table. 5.14) with three times the contribution in November and twice the contribution in February. The summer months do not achieve as much contribution but the decrease is minimal and with the extra demand of space heating in winter in Cloughjordan a higher output is advantageous. 66

81 5.2.4 Evacuated Tubes Comparison To compare the output of the system if evacuated tubes were used a collector that is supplied by an Irish based company called Surface Power was selected. The collector is quoted as having a superior performance to that of a flat plate collector and indeed other evacuated tube collectors. The results of the simulations with the yield of the current collectors were compared to the yield of this proposed collector. The results of these simulations are show in Fig 5.16 below. 35,000 30,000 Yield Comparison of CGSFP 2.0 TO SP501 Collector Yield (kwhr) 25,000 20,000 15,000 10,000 5,000 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec SP501 10,469 12,884 20,872 27,455 32,675 31,705 30,982 28,998 23,702 17,372 11,954 8,089. CGSFP 8, ,271 19,512 26,396 31,265 30,103 29,947 28,576 22,899 15,604 9,866. 6,525. Fig Comparison of Flat Plate Collector to Evacuated Tube Collector The SP501 evacuated tube collector has a superior performance to the CGSFP 2.0 all year round. The total simulated yield from the SP501 is 257,162 kwhrs and for the CGSFP ,646 kwhrs. The increase in output from the SP501 over the year is 16,516 kwhrs. The yield from the SP501 collectors shows the biggest increase in yield in the winter months with an increase of approximately 2,000 kwhrs per month. 67

82 Incident Angle Modifier To analyse if the IAM has an effect on the irradiation absorbed by a collector, the measured irradiation for the year from the results of the simulation was plotted against the irradiation after the IAM took effect. Fig 5.17 below shows that the IAM has a greater effect on the flat plate collector with a decrease of approximately 4,000 kwhr in the summer months. 80, , , IAM Flat Plate Panel CGSFP 2.0 Carey Glass Irraduation (kwhr) 50, , , , , Irradiation onto collector area Global irradiation after IAM 0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fig Comparison of IAM for flat plate collector A similar methodology was applied to compare the effects of the IAM on a proposed evacuated tube collector. The evacuated tube collector manufactured by Surface Power is quoted as having a superior performance to flat plate collectors. Fig 5.18 on the next page shows how the IAM actually increases the amount of irradiation that the collector absorbs by approximately 24,000 kwh in the summer months and approximately 14,000 kwh in the winter months. 68

83 IAM Evacuated Tube - SP501 Surface Power 120, , Irraduation (kwh0 80, , , , Irradiation onto collector area Global irradiation after IAM 0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fig Comparison of IAM for evacuated tube collector 69

84 5.3 Case Study 2 - Bewleys Hotel Dublin Airport Predicted Contribution Polysun The system details for Bewleys Hotel Dublin Airport were input to the Polysun simulation software and Table shows the results for each month of the year. The system contributed 174,627 kwhrs in total to the hotels DHW supply. Energy to Year: 2012 Month System (kwhrs) Jan 6,657 Feb 8,409 Mar 14,681 Apr 18,720 May 23,021 Jun 21,585 Jul 22,087 Aug 20,345 Sep 16,224 Oct 11,997 Nov 6,825 Dec 4,076 Total 174,627 Table 5.19 Polysun Simulation Results The total daily demand of hot water in the hotel is 22,000 litres. The software calculated that the overall energy demand for the year to supply this hot water demand is 535,438 kwhrs. With the solar contribution of 174,627 kwhrs that gives an overall solar fraction for the year of 33%. The rest of the energy demand for the hot water is met by the indirect gas fired cylinder heaters. Fig shows the solar fraction of the system in diagrammatic form. 70

85 Solar Fraction Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total Energy Consumption (kwhrs) Solar Energy to System (kwhrs) Fig Solar Fraction of Bewley s Hotel Dublin Airport Even though the overall solar fraction of the system is 33% the solar contribution to the system in the summer months is approximately 50%. The total solar contribution of 174,627 kwhrs equates to a saving of 18,479 m 3 of gas and approximate reduction of 44,936 kg s of CO 2. Fig below shows the energy flow of the system including the losses to surroundings and pump energy consumption. Fig Energy Flow Diagram 71

86 Solar Output Temperature The contribution of the drain back dual solar store system in Bewleys Hotel is different to the other case study in Cloughjordan in that even low temperatures from the collectors are useful to the Heating System used in the Hotel. The optimum temperature for hot water in the hotel is 65 C. The data from the simulations was therefore analysed to quantify how often the collectors would supply this temperature without the need for the indirect gas fired cylinders to bring the temperature to this required temperature. The result of this analysis is shown in Fig below. The collector achieves an outflow temperature of 65 C for 278 hours out of the full year. This figure equates to the heating system not needing any input or top up of heat from the gas heaters for 3.2% of the year. Frequency of Collector Temperatures Frequency (hrs) Collector Outflow Temperature ( C) Fig Frequency Distribution of Collector Outflow Temperatures 72