Domestic Water Heating with Flat Collector

Transcription

1 Domestic Water Heating with Flat Collector

2 Dr. K.Boedecker This manual must be kept by the unit. Before operating the unit: - Read this manual. - All participants must be instructed on handling of the unit and, where appropriate, on the necessary safety precautions. Version 2.2 Subject to technical alterations i

3 Table of Contents 1 Introduction Energy & Environment Key learning area: solar thermal energy Safety Intended use Structure of safety instructions Safety instructions Risks to equipment and function Ambient conditions for the operating and storage location Description of the device Process schematic Process description Device components Flat collector Controller Solar station Expansion vessel Measurement data acquisition Positioning and connection Commissioning Refilling the solar circuit Filling the secondary circuit Operation, care and maintenance Decommissioning, storage and disposal Artificial Light Source iii

4 4 Basic principles Solar energy Solar thermal vs. photovoltaics Radiation spectrum of the Sun Availability of solar radiation Directional properties of global radiation Measuring solar radiation Energy balance of a solar thermal collector Determining the efficiency Optical losses Thermal losses Heat radiation and selective absorber Heat conduction and convection Combination of the types of loss Efficiency curves Comparison between flat plate and evacuated tube collectors Collector types Simple absorber Flat collector Vacuum tube collector based on the heat pipe principle Vacuum tube collector in the thermos flask principle Heat storage Storage tank models Design of solar thermal systems Simulation example of a domestic hot water system Tasks Solutions iv

5 7 Experiments Experiment 1: Heating up the content of the storage tank Objective of the experiment Preparation for the experiment Conducting the experiment Measurement results and analysis Evaluation Experiment 2: Determining the efficiency curve Objective of the experiment Preparation for the experiment Conducting the experiment Measured values Analysis of the experiment Experiment 3: Use of the solar regulator Objective of the experiment Preparation for the experiment Conducting the experiment Measured values Analysis of the experiment Appendix Technical data List of abbreviations List of formula symbols and units List of symbols in the process schematic Tables and graphs Worksheets Controller UVR61-3 with D-LOGG Data logger Index v

6 1 Introduction 1.1 Energy & Environment The / Domestic Water Heating with Flat Collector trainer is part of the 2E - ENERGY & ENVIRONMENT product range. In the context of limited resources and increasing environmental pollution, the 2E range is based around an integrated approach to engineering education. Sustainable processes and systems are part of this concept. This provides an important guide for aspiring engineers and professionals in the fields of energy and environment and for the acquisition of interdisciplinary knowledge. In this respect, the 2E range is designed to provide practical experience through a specially developed series of equipment. All devices allow complex subject matter to be demonstrated in a clear and practical manner. ENERGY Generation, conversion, transportation and efficient use are partial steps of how we work with energy. Many trend-setting solutions in the field of energy efficiency are based on interdisciplinary approaches, some of which deviate significantly from the traditional structure of the disciplines involved. Among renewable energies, the largest proportion currently in use can be attributed directly or indirectly to the effect of solar energy. This includes wind energy as well as the majority 1 Introduction 1

7 of hydropower, which are both caused by climate processes driven by the Sun. Recently, the direct use of absorbed solar radiation has been gaining increasing importance. In addition to the generation of warmth for heating and domestic hot water production discussed in this manual, solar power generation is also achieving a breakthrough in economic terms. Both photovoltaics and commercial solar thermal power generation are becoming much more important. The formation of biomass may be considered as the oldest process for converting solar energy. Other primary sources of renewable energies are provided by the moon's tidal forces and by geothermal processes. ENVIRONMENT Contaminants are transported and transformed in the hydrosphere (water), atmosphere (air) and pedosphere (soil). Water, soil and air are described as environmental compartments and are connected to each other by the global water cycle. Additionally, the ENVIRONMENT area includes the training area of waste. 2 1 Introduction

8 Target group The trainer is designed for the education of students and apprentices with previous technical knowledge. The possible fields of expertise include: Energy technology Sanitation and heating systems Building services engineering Plumbers Solar installation engineers Energy and environmental technology Engineering physics Energy management 4 1 Introduction

9 Learning objectives The following educational content is the focus for use in the classroom: Commissioning and use of solar thermal systems Characteristics of a flat collector Effect of collector temperature Influence of flow rate, illuminance and inclination angle Measuring characteristic curves Establishing energy balances Determining efficiencies Components from real-world solar thermal systems Functions of the solar regulator Construction of solar thermal systems Working through this content using the extensive section on general principles represents a wellfounded contribution to the promotion of fundamental technical knowledge about alternative energy forms. 1 Introduction 5

10 Didactic notes for teachers These materials are intended to be used to help you prepare your lessons. You can compile parts of the materials as information for students for use in the classroom. In the materials you will also find prepared exercise sheets for the students along with the corresponding solutions. We also provide you with these materials in PDF format on a CD to support your lessons. We grant you unlimited reproduction rights for use within the context of your teaching duties. We hope that you enjoy using this trainer from the 2E range and wish you success in your important task of introducing students to the fundamentals of technology. Should you have any comments about the trainer or the instructional material, please do not hesitate to contact us. 6 1 Introduction

11 2 Safety 2.1 Intended use The unit is to be used only for teaching purposes. 2.2 Structure of safety instructions Signal word DANGER WARNING CAUTION NOTICE The signal words DANGER, WARNING or CAUTION indicate the probability and potential severity of injury. An additional symbol indicates the nature of the hazard or a required action. Explanation Indicates a situation which, if not avoided, will result in death or serious injury. Indicates a situation which, if not avoided, may result in death or serious injury. Indicates a situation which, if not avoided, may result in minor or moderately serious injury. Indicates a situation which may result in damage to equipment, or provides instructions on operation of the equipment. 2 Safety 7

12 3.1 Process schematic A1 A2 c 1 Flat collector T1 Collector inlet thermometer 2 Heat exchanger T2 Collector outlet thermometer 3 Expansion vessel T3 Return thermometer 4 Pressure relief valve T4 Inlet thermometer 5 Storage tank T5 Ambient air thermometer 6 Controller P1 Pressure 7 Data Logger 8 Illuminance sensor S1-S6 Sensors connected to controller A1 Solar circuit pump R Illuminance A2 Secondary circuit pump F Flow rate Fig. 3.1 process schematic 14 3 Description of the device

13 3.3 Device components Fig. 3.2 : components 16 3 Description of the device

14 1 Storage tank 13 Collector inlet thermometer T1 2 Storage tank temperature sensor TS3 14 Bleed valves 3 Heat exchanger 15 Collector outlet temperature sensor TS1 4 Secondary circuit pump A2 16 Illuminance sensor R1 5 Solar circuit valve 17 Collector outlet thermometer T2 6 Solar circuit temperature sensor TS2 18 Controller 7 Expansion vessel 19 Data Logger with USB Connector 8 Solar circuit station 20 Main switch 9 Pressure indicator P1 21 Flow meter F1 10 Pressure relief valve 22 Storage tank overflow 11 Ambient air thermometer T5 23 Storage tank fresh water supply 12 Tilt axis Flat collector Tab. 3.1 : components from Fig. 3.2 The flat collector is the main component of the trainer. The collector contains a glass cover made of low-iron solar glass. This provides improved transmission of the incident light compared to normal window glass. An absorber is situated below the glass cover. The absorber consists of an aluminium sheet with selective coating (cf. Chapter , Page 46). A meandering copper tube is mounted to the rear of this sheet, through which the heat transfer fluid can flow. At the head and foot of the collector the thinner copper tubes lead into thicker collecting pipes, which are led to the outer side of the collector housing. The cold liquid enters at the foot end and the heated liquid exits through the head end. Fig. 3.3 Design of the flat collector 3 Description of the device 17

15 Solar circuit: unfilled 1 Nitrogen gas 2 Membrane 3 Heat transfer fluid Filled solar circuit at room temperature Maximum pressure in the solar circuit at maximum temperature of the heat transfer fluid Fig. 3.7 Function of the expansion vessel in the solar circuit When heated, the heat transfer fluid expands and presses against the membrane. Since the heat transfer fluid is incompressible, unlike the gas, the membrane moves into the gas cushion. In this way, the heat transfer fluid can expand without causing a noticeable pressure increase in the system Description of the device

16 3.7 Operation, care and maintenance NOTICE During operation, make sure that pumps are never operated without water or heat transfer fluid under any circumstances. Doing so risks causing damage to the pump. The pressure in the solar circuit and the level in the storage tank must be checked at regular intervals. NOTICE If poor heat transport is observed in the solar circuit, or air bubbles can be seen in the flow meter, the corresponding lines should be bled immediately. (Refill with fluid if necessary). 3.8 Decommissioning, storage and disposal NOTICE If the trainer is not going to be used for a longer period, all tanks and lines should be thoroughly drained to decommission the unit. NOTICE If there is a risk of frost at the storage location, icing of residual water in particular may cause cracks in lines or tanks Description of the device

17 . specific spectral irradiance in W/(m 2 nm) Fig. 4.6 Wavelength in nm A Spectrum of the black body radiator (T = 5777K) B Solar spectrum outside the Earth's atmosphere (AM 0) C Solar spectrum at ground level (AM 1,5) UV Ultraviolet radiation VIS Visible radiation IR Infrared radiation Specific spectral radiation intensity influenced by the Earth's atmosphere The gaps in the real radiation spectrum of the Sun are due to absorption by water vapour and other gases in the atmosphere. 4 Basic principles 35

18 4.1.3 Availability of solar radiation Due to the inclination of the Earth's axis with respect to Earth's orbit around the Sun, the usability of sunlight on the Earth's surface is determined by the geographical location. The resulting astronomical constraints are shown in figure 4.6 using the example of the Sun's path in the sky during different seasons for the location of Berlin. A Zenith B Summer solstice C Autumn/spring D Winter solstice Fig. 4.7 The path of the Sun in the sky at different seasons (source: German Section of the International Society for Solar Energy) 36 4 Basic principles

19 4.2 Energy balance of a solar thermal collector The function of a thermal solar collector is to use the available solar radiation in an optimal manner for generating heat and to provide the heat for later consumption. Fig details the most important energy flows of a typical flat collector. 1 Solar radiation 4 Losses via radiation 2 Losses via reflection 5 Useful power 3 Losses through convection Fig Energy balance on the collector Below we have developed some formulae to determine the efficiency from direct measurements and to be able to describe the relationship between the measured efficiency and various losses. 4 Basic principles 43

20 4.2.1 Determining the efficiency A collector's efficiency is defined as the ratio of useful power P N to solar input power P in col = P N P in (4.2) The solar input power can be described as a product of the measured illuminance R and collector surface area A col. P in = R A col (4.3) The usable thermal output P N can be determined from the measured temperature difference between the collector return flow and feed flow (T 2 -T 1 ) and the flow rate F through the collector. The density and the specific thermal capacity c p of the heat transfer fluid used must be known. (e.g. water 995g/L, c p = 4,18kJ/(kg*K) P N = F c p T 2 T 1 (4.4) When using Formula (4.4) we also need to take into account the temperature dependence of and c p for the mixture of heat transfer fluid used. We can usually refer to tabulated values here Basic principles

21 emissivity is high in the solar light spectrum and low in the heat radiation range. Normalised emission/absorption without units Wavelength in µm A B C D Emission spectrum at 5777K (solar spectrum) Emission spectrum at 300K (emission of the absorber) Absorption spectrum of the selective absorber Absorption spectrum of the uncoated absorber Fig Normalised absorption and emission coefficients as a function of wavelength In summary: A selective absorber absorbs just as well as a non-selective absorber. However, the selective absorber radiates less heat Basic principles

22 approach is used, which describes the losses at a collector sufficiently well. For thermal power loss P th it is assumed: P th = k 1 A col m col + k 2 A col m col 2 (4.11) In conjunction with Formula (4.5), Page 45 for optical efficiency and Formula (4.6), Page 46, we get the collector's overall efficiency to: k col 1 m col k m col 2 = R R (4.12) Where k 1 denotes the linear loss coefficient (unit: W/(m 2 K)) and k 2 the quadratic loss coefficient (unit: W/(m 2 K 2 )). This approach provides good results up to a temperature difference of about 120 K. At much higher temperatures, loss is primarily through heat radiation. The error of the quadratic component increases with increasing collector temperatures, since according to the theory (cf. Formula (4.7), Page 46), the temperature is considered to the fourth power in the radiant power Basic principles

23 4.3 Efficiency curves Fig plots the course of a typical efficiency curve (A) as a function of temperature difference between absorber and environment. The chart also includes different marked areas to illustrate the composition and the temperature behaviour of the loss components. Efficiency in % T St Temperature difference between collector and environment in C 1 Optical losses 2 Losses through heat radiation 3 Losses through convection and heat conduction A Measured characteristic curve of a typical collector (Viessmann Vitosol 300 T) B Calculated characteristic curve for losses by heat radiation T St Collector stagnation temperature Fig Efficiency and loss components as a function of temperature difference between collector and environment. 4 Basic principles 51

24 4.4 Collector types Considering the loss mechanisms of collectors shows that there may be significant differences in capacity caused by design. Each type of collector has its strengths and is optimised for a particular application. 20 C-30 C 20 C 40 C 60 C 80 C 100 C Swimming pool water heating Fig Absorber (plastic) Absorber (stainless steel) Classification of collector types Domestic water heating, domestic water heating with heater support Flat collector Vacuum flat collector Storage collector Vacuum collector Vacuum tube collector Direct throughflow with reflector no reflector Heat pipe dry connection wet connection Fig lists different types of thermal solar collectors from left to right in ascending order by their temperature application range. We will look at a few collector types in more detail below. 4 Basic principles 55

25 5 Tasks 1. Complete the labels for the components shown of the following energy flow diagram: Losses through convection Losses via reflection Solar radiation Useful power Losses via radiation Fig. 5.1 "Energy flow diagram of a flat collector": solution 5 Tasks 73

26 7. What losses do not occur in vacuum tube collectors? There are no convection losses in vacuum tube collectors. 8. What is particularly important when designing auxiliary heating for solar thermal systems? When designing auxiliary heating, the typical weather data for the proposed location must be considered. In the winter months there is often less than 20% of the global radiation of the summer months available. 9. What characteristic of a thermal solar system is described by its system efficiency? The system efficiency of a solar thermal system describes the ratio of the actually used solar heat yield to the maximum possible heat yield of the system. 10.What temperatures are crucial for the efficiency of a solar thermal collector? The efficiency of a solar thermal collector depends on the average temperature of the absorber and the ambient temperature T a. The average absorber temperature can be estimated byformula (4.10), Page 49 as the average of the temperatures at the collector flow T 1 and the collector return flow T 2 as follows: T 2 + T 1 m col = T 2 a 6 Solutions 79

27 7 Experiments Experim ent Tab. 7.1 The selection of experiments makes no claims of completeness but is intended to be used as a stimulus for your own experiments. The results shown are intended as a guide only. Depending on the construction of the individual components, experimental skills and environmental conditions, deviations may occur in the experiments. Nevertheless, the laws can be clearly demonstrated.. Description Learning objective Duration Section 1 Heating up the storage tank How temperature affects thermal output and efficiency 2 Change in the flow rate How the flow rate affects thermal output and efficiency 3 Regular operation with controller when changing the lighting and extracting hot water Overview of the experiments Familiarisation with the operating behaviour, influence of controller parameters approx. 4h Chapter 7.1, Page 82 2,5h Chapter 7.2, Page 87 5h Chapter 7.3, Page 93 7 Experiments 81

28 7.1 Experiment 1: Heating up the content of the storage tank Objective of the experiment Preparation for the experiment The aim of the experiment is to heat up the contents of the storage tank using the flat collector. In doing so we shall observe the curves obtained for the temperatures at the different measuring points and the thermal power of the collector. This experiment can be performed with natural sunlight or with the artificial light source. The artificial light source is preferred on especially cloudy days. The distance between the.01 Artificial Light Source and the collector should be about 1,6m. At this distance there is an illuminance of approximately 430W/m 2. CAUTION The housing of the halogen spotlights gets very hot during operation. Contact can cause burns Never touch the halogen spotlights when switched on without wearing suitable protective gloves 82 7 Experiments

29 7.2 Experiment 2: Determining the efficiency curve Objective of the experiment The aim of the experiment is to determine the efficiency (cf. Formula (4.2), Page 44) of the collector at different temperatures. Fig. 7.2 with Preparation for the experiment To do this, the incident power of the artificial light source and the thermal net power output of the collector are measured at various collector temperatures. At constant illuminance we get difference collector temperatures when the flow rate is changed. To avoid errors in the energy balance, it is important to take these measurements under steady-state conditions. This means that a measured value can only be recorded when the system is in a steady state and no more changes to the measured values can be observed. This condition is only achieved very slowly when heating the storage tank is allowed. It is more advantageous to keep the storage tank at a constant temperature where possible. Under these conditions, the storage tank almost represents an ideal heat sink. This experiment is conducted with the.01 Artificial Light Source. The distance to the collector is 1m. If necessary, the spotlights on HL must be aligned. The homogeneity of the lighting should be checked using the illuminance sensor directly in front of the flat collector's glass cover. If there are still inhomogeneities present, it may be necessary to calculate the illuminance for 7 Experiments 87

30 7.2.3 Conducting the experiment The smallest measurable flow rate (approx. 20L/h) is set after switching on the pumps, the flushing for the storage tank and the artificial light source. To do this, the shut-off valve below the rotameter is closed until the desired flow rate has established itself. Fig. 7.4 Adjusting the flow rate Measured values Now, all displayed temperatures together with the illuminance and current time are recorded (see worksheet in the appendix). These values are recorded again after approximately 2 minutes. The actual measured values can be recorded for the current flow rate as soon as the temperature at the collector outlet stops changing. Then the next value for the flow rate can be set via the shut-off valve. In this case, an increase of 10 L/h each time is appropriate. For each measuring point, we need to take into account a waiting time until a stable temperature at the collector outlet is reached. The measurement is repeated until the maximum flow rate is reached. In the chart below we can clearly see the curve of the experiment. 7 Experiments 89

31 Efficiency Temperature difference in C Fig. 7.6 Efficiency as a function of the temperature difference In addition to the calculated efficiencies (measuring points) it also shows a comparison curve in accordance with Formula (4.12), Page 50. The curve shown is obtained for the following parameters. 0 = 0,81 k 1 = 4,5W/(m 2 K) k 2 = 0,18W/(m 2 K 2 ) This results in a significant decrease in efficiency with increasing temperature difference. As shown in Chapter 4.2, Page 43, this effect can be explained by the increase of convection and radiation heat losses Experiments

32 7.3 Experiment 3: Use of the solar regulator Objective of the experiment Preparation for the experiment The aim of the experiment is to study the behaviour of the system during normal operation and to learn about the functions of the solar regulator. To this end, we shall investigate different operating conditions. Fig. 7.7 Water extraction at the storage tank This experiment can also be performed with natural sunlight or with the artificial light source. If the artificial light source is used, we must ensure that there is sufficient space available in front of the collector. To change the illuminance we have to increase the distance between the light source and the collector to up to 3 m. In addition, in this experiment heated domestic water is taken from the storage tank. To do this, fresh water from a laboratory connection is added to the storage tank via a hose connection into the bottom inlet. Heated water exits at the top outlet when a sufficient level has been reached. 7 Experiments 93

33 7.3.4 Measured values The following diagram shows the main variables and operations of the experiment. Pump A1on Pump A1off Pump A2on Pump A2off T S1 Temperature in C T S3 R 1 HW Illuminance in W/m² Duration of the experiment in h:mm A1 A2 T S1 T S3 R 1 HW Arrow Solar circuit pump Secondary circuit pump Collector outlet temperature Storage tank temperature Illuminance Hot water extraction Change in temperature difference Fig. 7.8 Chart of experiment Experiments

34 8.4 List of symbols in the process schematic Symbol Description Heat exchanger Shut-off valve Safety valve Expansion vessel Circulation pump Illuminance sensor Controller connection for illuminance sensor Flow rate sensor with controller connection Appendix

35 9 Index A B Absorbance Absorber , 56 Atmosphere Availability Black body , 47 Buffer storage C D E Collector design , 61 Collector stagnation temperature , 52 Collector temperature Colour temperature Convection , 49 Density Design Domestic hot water tank Efficiency , 43, 44, 46, 50 Efficiency curve , 53 Emissivity Energy balance , 43 Energy efficiency Energy flows Energy supply , 32 Expansion vessel F Flat collector Flow rate Index 115