Model Based Design and Performance Analysis of Solar Absorption Cooling and Heating System

Transcription

1 Model Based Design and Performance Analysis of Solar Absorption ooling and Heating System Ming Qu arnegie Mellon University School of Architecture Ph.D. ommittee Prof. Volker Hartkopf, Ph.D. (hair) Prof. David Archer, Ph.D. Prof. Khee Poh Lam, Ph.D.

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4 opyright Declaration I hereby declare that I am the sole author of this thesis. I authorize arnegie Mellon University, Pittsburgh, Pennsylvania to lend this thesis to other institutions or individuals for the purpose of scholarly research. I authorize arnegie Mellon University, Pittsburgh, Pennsylvania to reproduce this thesis by photo copying or by other means, in total or in part, at the request of other institutions or individuals for the purpose of scholarly research. opyright 2008 by Ming Qu i

5 Acknowledgement I wish to express my gratitude to my advisor, Dr. Volker Hartkopf, for his invaluable vision, support, and encouragement. His enthusiasm and inspiration were essential to the success of this research. Surely, I would like to extend my sincere appreciation and profound gratitude to Dr. David Archer who has played a pivotal role in this thesis. He has far exceeded his duty as an advisor. He gave me a deep understanding of mechanical engineering; he taught me how to develop critical thinking and work effectively. He has been an ever-present source of guidance and encouragement throughout my doctoral program. It gives me great pleasure to thank Dr. Khee Poh Lam for providing valuable suggestions and carefully reviewing and constructively critiquing of my work. I owe many thanks to my dear colleague and husband, Hongxi Yin, who gave me continuous support and took care of our babies, Ryan and David, who fill us with joy every day. This thesis is dedicated to my parents in their confidence, their high expectations, and their hearty blessing. ii

6 Model Based Design and Performance Analysis of Solar Absorption ooling and Heating System iii

7 Table of ontents 1 Introduction Background and motivation Solar receivers Absorption ycle Solar collector coupled with absorption chillers urrent studies on solar absorption cooling and heating systems Research objective Research approach The planning of the test system The development of solar collector model The development of annual system performance simulation The installation of the test system The test program and experimental data gathering Data analyses, model validation and simulation evaluation hapter overview Solar absorption cooling and heating test system and program Parabolic trough solar collector Device description Major components and characteristic Absorption chiller Device description Major components and characteristics Solar absorption cooling and heating test system System description The solar collection loop The load loop Instrumentation, control and data acquisition system The test program The results of PTS test at the transient states The results of PTS test at a steady state iv

8 2.4.3 The results of solar absorption cooling / heating daily test The results of solar heating daily test by using heat exchanger The interpretation of PTS performance data The energy balance of the PTS The selection of experimental data PTS performance predicted by statistic tool Discussion of test program Solar collector performance model PTS model assumption Energy balance analysis Heat transfer analysis alculation procedure Solar irradiation absorption Direct normal solar radiation Incident angle and incident modifier End-loss Shadow-loss onclusion Model-based experimental data analysis of PTS Analytical method Model validation Model-based PTS performance analysis Temperature distribution in the receiver pipe Thermal losses PTS efficiency and solar radiation PTS efficiency and incident angle of solar beam PTS efficiency and wind speed PTS efficiency and fluid type PTS efficiency and flow rate PTS efficiency and air in the annular space PTS efficiency and glass envelope Recommendations on the PTS s design Bellow design v

9 4.3.2 Glass cover Diameter of the glass envelope Diameter of the absorber pipe Solar absorption cooling and heating system simulation Model approach Model assumptions Weather Assumptions in the model of solar energy supply system System components and operation controls omponents in the solar heating base-case Operational controls in the solar heating base-case omponents and operation controls in the solar cooling base-case Simulation evaluation Base-case result of solar cooling and heating simulation Building simulation results Solar energy system simulation results Simulation-based design and performance analysis on solar cooling and heating Orientation of PTS Orientation of the PTS for increased, effective solar energy recovery Orientation, tracking limitation, and solar beam irradiation on the PTS Orientation and overall system performance System operation and control onstant-flow or constant-outlet temperature control of the PTS Storage tank requirements The volume of the storage tank Storage used for shifting energy for later use in solar heating Storage used for shifting energy for later use in solar cooling Storage used for preheating Auxiliary heater for preheating in the solar collection loop The length and diameter of collection loop pipe and solar system performance The area of solar collector and storage tank Guidelines for design and operation of solar cooling and heating system vi

10 7 ontributions and areas of future research ontributions Areas of future research Improving the tracking system of the PTS Extending the operational controls of the PTS, the absorption chiller, and the heat recovery exchanger Integrate thermal storage in the cooling/heating system ost model References vii

11 List of Figures Figure 1-1 Simplified system arrangement of solar absorption cooling and heating system... 2 Figure 1-2 Two types of solar collector... 4 Figure 1-3 Electric chiller and absorption chiller... 4 Figure 1-4 Solar collector efficiency and operating temperature required by absorption chiller... 6 Figure 1-5 Research approach schematic chart Figure 1-6 Process and instrumentation diagram of the test solar absorption cooling and heating system Figure 2-1 The PTS s installed on the IW Figure 2-2 Broad BJ16A parabolic trough solar collectors and the receiver tube Figure 2-3 Absorption chiller Figure 2-4 Absorption chiller in cooling cycle Figure 2-5 Absorption chiller in heating cycle Figure 2-6 Broad pump and control package Figure 2-7 Overall solar absorption cooling and heating test system Figure 2-8 Structure of control system...24 Figure 2-9 Interface of the WebTRL Figure 2-10 PTS test diagram at transient state Figure 2-11 Solar absorption cooling /heating system daily test Figure 2-12 Solar heating daily test by using heat exchanger Figure 2-13 Operating temperatures of the PTS test at transient state on 29 March Figure 2-14 Energy flows of the PTS test at the transient state on 29 March Figure 2-15 Operating temperatures of the PTS test at steady state on 22 April Figure 2-16 Energy flows of the PTS test at steady state on 22 April Figure 2-17 Operating temperatures of solar cooling test on 31 July Figure 2-18 Operating temperatures of solar cooling test on 16 July Figure 2-19 ooling capacity of solar cooling system on 31 July Figure 2-20 ooling capacity of solar cooling system on 16 July Figure 2-21 Operating temperatures of solar absorption heating test on 9 March Figure 2-22 Heating capacity of solar absorption heating system on 9 March Figure 2-23 Operating temperatures of HX based solar heating system on 2 March Figure 2-24 Heating apacity of HX based solar heating system on 2 March Figure 2-25 Scatter plot of I*Aa*cos(theta) and m*(po*to-pi*ti) viii

12 Figure 2-26 Scatter plot of average operation temperature and m*(po*to-pi*ti) Figure 3-1 Energy flow in the PTS Figure 3-2 The thermal network Figure 3-3 The connection between the PTSs Figure 3-4 Incident angle of the PTS Figure 3-5 Incident angle modifier and incident angle Figure 3-6 End-loss of the PTS Figure 3-7 The length of the end-loss in the solar field Figure 3-8 Shadow loss from the adjacent solar collector array Figure 4-1 Measured temperature distribution of the glass envelope Figure 4-2 omparison between the measured data and calculation solutions Figure 4-3 Temperature distribution in the receiver pipe Figure 4-4 Thermal losses through the receiver pipe Figure 4-5 PTS s efficiency and direct normal solar radiation at 0 incident angle Figure 4-6 PTS s efficiency and incident angle Figure 4-7 PTS s efficiency and direct normal solar radiation at 15 incident angle Figure 4-8 PTS s efficiency and wind speed Figure 4-9 PTS s efficiency and fluid type Figure 4-10 PTS s efficiency & flow rate Figure 4-11 PTS s efficiency and air in the annual space Figure 4-12 Thermal losses with Sun or No-sun Figure 4-13 PTS s efficiency and glass cover Figure 4-14 New bellow design from SOLEL Figure 5-1 Information flow of TRNSYS simulation Figure 5-2 Monthly average dry bulb temperature of Pittsburgh Figure 5-3 Direct normal solar radiation in Pittsburgh Figure 5-4 Daily average solar radiation throughout a year in Pittsburgh Figure 5-5 TRNSYS information flow diagram of solar cooling base case Figure 5-6 TRNSYS information flow diagram of solar heating base case Figure 5-7 Boiling temperature and pressure of aqueous propylene glycol solutions Figure 5-8 BROAD PTS tracking range Figure 5-9 Operation temperature comparison between solar heating evaluation simulation and experiment ix

13 Figure 5-10 Energy flow comparison between solar heating evaluation simulation and experiment Figure 5-11 Operating temperature comparison between cooling evaluation simulation and experiement Figure 5-12 Energy flow comparison between solar cooling evaluation simulation and experiment Figure 5-13 IW building heating and cooling load estimated by building simulation Figure 5-14 Useful solar energy and IW sensible heating load in January Figure 5-15 Useful solar energy and IW sensible cooling on 30 December Figure 5-16 Useful solar energy, cooling load and energy provided by chiller in August Figure 5-17 Useful solar energy, cooling load and energy provided by chiller on 09 August Figure 6-1 Twelve orientations in the simulation Figure 6-2 Orientation and solar beam irradiation on a PTS on 10 June in Pittsburgh Figure 6-3 Orientation and solar beam irradiation on a PTS on 2 December in Pittsburgh Figure 6-4 Orientation and solar beam irradiation on a PTS in summer of Pittsburgh Figure 6-5 Orientation and solar beam irradiation on a PTS in winter of Pittsburgh Figure 6-6 Annual solar beam irradiation on PTS with different orientations in Pittsburgh Figure 6-7 Tracking angle and orientation of the PTS on 21Jun Figure 6-8 Tracking angle and orientation of the PTS on 21 Dec Figure 6-9 Solar beam irradiation and orientation of the PTS in Pittsburgh Figure 6-10 System performance comparison of alternate controls on 9 August Figure 6-11 System operating temperature comparison of alternate controls on 9 August Figure 6-12 Trnsys information flow diagram of solar heating system with storage Figure 6-13 Solar energy collected, heating load, and energy provided on 14, 15 November Figure 6-14 Trnsys information flow diagram of solar cooling system with storage for shifting energy Figure 6-15 Operating temperature of solar cooling system with and without storage on 9 August Figure 6-16 Trnsys information flow diagram for solar cooling with storage for preheating Figure 6-17 Effect of a heater on energy flow for solar cooling on 09 August Figure 6-18 Effect of a heater on operating temperature for solar cooling on 9 August Figure 6-19 Solar fraction and pipe size under two control strategies Figure 6-20 System energy performance and pipe size on 9 August x

14 Figure 6-21 Operating temperature and pipe size on 9 August Figure 6-22 Effect of PTS area and storage volume on the solar fraction in IW cooling and heating Figure 6-23 Idealized IW solar cooling/heating system performance and system sensitivity analysis xi

15 List of Tables Table 2-1 Specifications of the parabolic trough solar collector installed Table 2-2 Specifications of the absorption chiller installed Table 2-3 Instrumentation of IW solar cooling and heating system Table 2-4 Four types of tests conducted Table 2-5 Heat capacity of the solar collection loop Table 2-6 Operating condition in the PTS performance tests Table 2-7 Heating system performance comparison between HX based and absorption chiller based Table 3-1 Heat transfer correlations used in the PTS model Table 3-2 Parameters and values used in the PTS model Table 4-1 Glass temperature measurements in the test Table 4-2 omparison between measured values and model calculations Table 5-1 ontrol mode in the base-case simulation of solar heating system Table 5-2 ontrol mode in the base-case simulation of solar cooling system Table 5-3 System performance estimated by IW solar heating system base-case simulation Table 5-4 System performance estimated by IW solar cooling system base-case simulation Table 6-1 Effect of PTS s orientation on overall system performance Table 6-2 Effect of flow controls on overall system performance Table 6-3 Effect of storage volume on solar heating system performance Table 6-4 Effect of storage volume on solar cooling system performance Table 6-5 Effect of Preheat storage tank volume on solar cooling performance Table 6-6 Effect of collection loop volume on solar heating system performance Table 6-7 Effect of collection loop volume on solar cooling system performance xii

16 Abstract The work presented in this thesis deals with the question of how solar energy might most effectively and efficiently be used in supplying energy for the operation of a building. The approach to dealing with this question has involved a specific building space, arnegie Mellon s Intelligent Workplace; a specific solar system, parabolic trough solar thermal receivers, Parabolic Trough Solar ollector s; and a specific building energy use, space cooling and heating. The work has involved the design, installation, and test of a system incorporating PTS s, an absorption chiller, a heat recovery exchanger, auxiliary equipment, instrumentation and controls. Mathematical models based on fundamental scientific and engineering principles have been developed and programmed for both the PTS s and the overall IW cooling and heating system, These models have been improved and validated through comparisons of predicted and measured PTS and IW cooling and heating system performance. The work reported in this thesis has developed suggestions and methods for the effective design and evaluation of PTS s and also for the optimized design and operation of solar absorption cooling and heating systems, so that the system is able to reduce building energy consumption, and achieve environmental benefits in the operation of buildings by the use of renewable, solar energy. xiii

17 1 Introduction HVA systems are the major users of electricity in commercial buildings. In the United States, commercial air conditioning makes up 40% of the summer time peak electrical demand (Kulkarni 1994). In recent years, the increasing power demand of building HVA system, and the increasing costs of energy have caused people to seek alternative cheaper, renewable energy sources for building cooling and heating. In addition, the environmental issues such as global warming, ozone depletion, and energy conservation, are other important factors, impelling people to look for space cooling and heating without involving gas or electricity. The use of solar energy for building cooling and heating can potentially provide the solution to these economic and environmental problems. An important motivation for research and development in solar cooling is the coincidence of comfort cooling demand and the availability of solar radiation. Building cooling systems which use solar thermal energy can make use of absorption cycles, desiccant cycles, on solar-mechanical processes. ompared to solar desiccant cycles and solar-mechanical processes, solar absorption cycle technology is more developed; solar thermally driven systems can provide reliable and quiet cooling. In addition, combining solar heating and domestic hot water production with cooling can improve economic performance of the system, compared to solar heating or solar cooling alone. Solar absorption cooling was a subject of significant research interest from 1970 to 1980, when a number of demonstration projects were conducted in the United States. However, these systems failed to establish a significant global market for cooling systems due to their high initial cost, lack of commercial hot water driven absorption chillers, and scarcity of demonstrations and impartial assessments by reputable institutions (Kulkarni 1994). This thesis investigates the technical and energy efficient aspects of using high temperature solar thermal receivers with a two stage absorption chiller to cool and heat a building space; it reassesses the feasibility of commercializing solar absorption cooling technology by considering the recent improvements in solar collection technologies, severe electric shortages, and the environmental problems. This research contributes in depth knowledge and methods for the design and operation of solar absorption cooling systems that reduce energy consumption, decrease operational costs, and minimize air quality problems. 1

18 The work reported in this thesis has developed, validated and applied a numerical performance model, a parabolic trough solar collector (PTS) model; this model provides a tool for the analysis of performance data and for the design and operation of a solar absorption cooling and heating system for a building. Additionally, the work developed a comprehensive simulation and applied it to system optimization and sensitivity analyses. This thesis provides generic guidelines on the design and operation of a solar absorption cooling and heating system in order to reduce energy consumption and operation costs, as well as benefit the environment. 1.1 Background and motivation Solar heating systems have been studied for almost 70 years since the Massachusetts Institute of Technology began their studies in 1938 (Beckman 1980). urrently solar heating is relatively mature. However, despite substantial research and development efforts on solar cooling, most have focused on simulation and isolated experiments. Few involved the current state of hot water driven, two stage absorption chillers and well designed solar chillers with effective control systems. The requirements for effective operation and maintenance practices have not been explored. A typical solar absorption cooling and heating system is comprised primarily of the solar receivers and the absorption chiller. The solar receivers convert solar radiation to thermal energy in a heated fluid; the absorption chiller then uses this energy in summer to generate chilled water. In the cooling cycle, the system acts as a heat pump; it gets the heat from chilled water and from the sun and rejects heat by cooling water. In winter, instead of using cooling water, the system directly transfer heat to the loop providing space heating. A simplified schematic of a solar absorption cooling and heating system is illustrated in Figure 1-1. solar collectors absorption chiller Hot water building cooling water hilled water heating cycle cooling cycle Figure 1-1 Simplified system arrangement of solar absorption cooling and heating system 2

19 1.1.1 Solar receivers Solar receivers are normally are classified in two groups: flat plate collectors and concentrating collectors as shown in Figure 1-2. A flat plate collector can use both direct and diffuse solar irradiation on a fixed receiving plate, while a concentrating device can only use the direct solar irradiation since the diffuse solar irradiation from various directions focused by the reflector away its focal line where the receiver pipe located. oncentrating collectors, with a relatively small absorption area, can heat the heat transfer fluid to temperatures, far above those attainable by flat plate collectors. Flat plate collectors are normally used in applications that require only low temperatures, less than 100 ; and concentrating collectors are utilized in medium or high temperature applications, up to Absorption ycle Absorption chillers can be thermally driven by using heat from the sun, from engine exhaust gases, or from other variety of sources, to provide reliable and quiet cooling. They do not use atmosphere harming halogenated refrigerants, and they can be used to reduce summer electric peak demand. To illustrate how an absorption chiller works, a comparison between an absorption chiller and an electrically driven vapor compression chiller is shown in Figure 1-3. In the electrically driven vapor compression chiller, on the left of Figure 1-3, a refrigerant vapor is compressed to a higher pressure by a compressor and condensed by rejecting heat to the ambient in the condenser. The refrigerant liquid then flows through the expansion valve to an evaporator maintained at a low pressure mixture of liquid and vapor. This liquid refrigerant initial flushes into a mixture of vapor and liquid. The liquid then vaporizes in the evaporator as it absorbs heat from the water to be cooled thermally producing the cooling effect. Instead of using a compressor, an absorption chiller, on the right of Figure 1-3, produces the same compression effect, raising the pressure of water vapor by absorbing vapor at low pressure in the absorber and consequently desorbing this vapor at a high pressure in the regenerator. The water vapor is absorbed at a low pressure by the concentrated sorbent solution in the absorber. A solution pump then pumps up the diluted solution to a higher pressure and temperature in the 3

20 Flat-plate collector Parabolic trough solar collector (PTS) (one axis tracking) Evacuated-tube collector Integrated compound parabolic collector (fixed in a flat assembly) Flat-plate collector oncentrating collector Figure 1-2 Two types of solar collector Rejected heat ondenser Rejected heat ondenser Heat input Regenerator Refrigerant expansion valve P ompressor Work P Refrigerant expansion valve Solution pump Solution expansion valve Evaporator Evaporator Absorber Heat absorbed from chilled water T Heat absorbed from chilled water T Rejected heat Electric chiller Absorption chiller Figure 1-3 Electric chiller and absorption chiller 4

21 regenerator. The water vapor is boiled off from the diluted sorbent solution in the regenerator by adding thermal energy, and the water vapor is condensed by rejecting heat to cooling water in the condenser. The other processes in the absorption chiller, the vaporization of the water refrigerant and the removal of heat from the chilled water in the evaporator, operate similarly to the ones in the electric chiller. If the temperature of the refrigerant water vapor (steam) produced in the regenerator, is high enough, then it can be used to produce more refrigerant vapor from the weak solution. In return, this vapor is then used in a chiller, which then serves as a double effect absorption chiller. From the thermodynamic point of view, the absorption chiller is a combination of a heat engine and heat pump. The heat engine absorbs heat at a high temperature, rejects heat at a lower temperature, and produces work. The work drives a heat pump that absorbs heat at a low temperature and rejects it to the ambient at a higher temperature Solar collector coupled with absorption chillers All the varieties of solar collectors can be used for cooling, given the broad operating temperature ranges of absorption chillers. Figure 1-4 plots the efficiency of the four types of solar collectors - - flat plate, evacuated tube without (ET) and with concentration (P), and linear parabolic trough (PTS) depending on their operating temperature. Typically, a single effect LiBr/H 2 O absorption chiller requires a heat source; and a double effect one requires a heat source to generate chilled water. Normally flat plat solar collectors are coupled with single effect chillers due to their relatively high efficiency at low operating temperature; and evacuated tube and parabolic trough collectors are used to provide a high temperature heat source for double effect absorption chillers. This thesis is focused on the solar absorption cooling system, comprised of the PTS s and the double effect absorption chiller as indicated by the rectangular outline in Figure urrent studies on solar absorption cooling and heating systems Substantial research and development efforts on solar absorption cooling are in progress; and the technology is evolving. Most of the research focuses on simulation analyses of solar driven single 5

22 effect absorption cooling systems. In general, it has been concluded that this approach is the most economic configuration with the highest system performance. Although theoretically ET or P collectors can heat fluid to 150~160, the required temperature by the double effect absorption chiller, there are few successful studies showing that these two types of solar receivers can be successfully incorporated with double effect absorption chillers. Duff (Duff 2004) has reported that m 2 of integrated compound parabolic concentrating collectors (IP) have been operated at to serve a 70kW (20 ton) hot water driven double effect McQuay/Sanyo chiller to serve a commercial building in Sacramento, alifornia. At the beginning of the operation in 1998, daily collection efficiencies were nearly 50% in the operating range of While later, the highest operating temperature was at 110 with 55% daily collection efficiencies, due to the lower operating temperature with less heat loss. Figure 1-4 Solar collector efficiency and operating temperature required by absorption chiller 6

23 At U.S. Army s Yuma Proving Ground in Arizona a 1245 m 2 of Hexel PTS s solar system supplying heated water to a 160 ton LiBr/H2O double effect absorption cooling system has been successfully operated for nearly 14 years since its installation in 1979 (Hewett 1995). Hewett discussed the technical and economic performances of this project and drew the conclusion that the economics of solar absorption cooling systems are unattractive compared to conventional alternatives (in 1995). The primary rationale included the high capital and operating costs of solar collectors with absorption chillers, compared to those of electrically driven vapor compression chillers. Supplementing experimental research, many simulation based studies have been performed on solar driven double effect absorption cooling systems. Lokurlu carried out system simulation analyses in TRNSYS (Lokurlu 2002). He concluded that the combination of PTS and a double effect absorption chiller is promising for cooling systems with a load of at least 100 kw; Since the PTSs were not available for the study, this work remained in the simulation and preliminary design stage. Wardono developed a computer model of the double effect LiBr/H2O absorption cooling system coupled with tilted flat plate solar collectors for an application in Albuquerque, NM (Wardono 1996). He calculated that the total solar energy input depends upon the ambient temperature, sky clearness index, and system design; and he indicated that the solar contribution for supplying energy to the double effect LiBr/H2O cooling system did not significantly vary for various latitudes. 1.3 Research objective The primary purpose of this research program is the development of systems which reduce the energy requirements for the operation of buildings by a factor of two or greater, and the provision of techniques and tools for the design and evaluation of such systems. This thesis re-assesses the technical aspect of a solar driven double effect absorption cooling system by means of experimental equipment, modeling and system simulation. The aim of this research is to develop methods and tools for the effective design and optimization of a solar absorption cooling and heating system that reduces energy consumption, decreases 7

24 capital, operation, and maintenance costs, and also benefits the environment. The special objectives of this thesis are: the establishment of a unique experimental set up and procedures for investing a solar absorption cooling and heating system the carrying out a test program on both a parabolic trough solar collector and an absorption cooling and heating system the construction of a computerized performance model of a parabolic trough solar collector based on energy balances and heat transfer correlations and validation of the model with data obtained from the test program the development of annual overall solar cooling and heating system simulation, the use of this simulation to design the system and to optimize its performance; and validation of the simulation by data obtained from the test program the analysis of the experimental data, refinement of the model, and improvement of the design and operation on the basis of the simulations The solar collector model and overall solar system simulations developed are now being used as tools to adapt parabolic trough solar collectors and double effect absorption chillers to various climate zones, building applications, and system configurations in order to provide optimized design and operation guidelines. In both its practical and theoretical aspects, this study contributes important knowledge for the application of the solar absorption cooling and heating systems. The practical observation and operation of the particular solar cooling and heating system has laid the groundwork for improvement of the control and integration of solar collectors and double effect absorption chillers; thus the analytical methods provide a platform to analyze and improve systems. 1.4 Research approach To achieve the thesis objectives, this research is focused on solar collector model development, system performance simulation, system installation and test of a high temperature based solar absorption cooling and heating system as illustrated in Figure

25 In this research, mathematical modeling, system simulation, equipment testing, system testing, and data analyses were combined to provide a deeper understanding of the system, to discover the possible improvements in the solar collector design, to optimize the system design and operation, and to provide a framework to design and evaluate solar absorption cooling and heating systems. The research has been carried out by the following steps The planning of the test system Prior to the design and installation of the solar cooling and heating system, studies were performed regarding the solar field location, the structure for supporting the solar collectors, system energy balances, the piping, and other engineering issues. In the mean time, a comprehensive parabolic trough solar collector model and annual system performance simulations were developed to assist the system design and operation The development of solar collector model The performance model of the solar collector is focused on a coated absorber pipe enclosed in an evacuated glass envelope: the receiver of the parabolic trough solar collector (PTS). This steady state, single dimensional model is based on fundamental material and energy balances together with heat transfer correlations programmed in the Engineering Equation Solver (EES). Incident solar energy on the solar collector is distributed among useful energy gain, optical losses, and thermal losses. To represent different optical losses, coefficients have been introduced in the model. This model deals with the thermal losses resulting from conduction, convection and radiation heat transfer to the surroundings, from the receiver. This PTS model is based on energy balance relations for the absorber pipe and the glass envelope together with heat transfer correlations for the various the energy streams among them and the surroundings. It has 192 variables and 160 equations. This model predicts how the efficiency of the PTS is influenced by direct normal solar radiation, the incidence angle, collector dimensions, material properties, the operating temperature, the presence of air in the annular space, the wind speed, the type of the fluid, and the operating flow rate. This model was used to select the proper operating conditions, to detect the possible problems in the operation of the collectors such as incipient boiling at the tube surface, and to select a circulation pump. Additionally the estimated performance of PTS has been used to optimize design parameters and operating conditions of the solar absorption cooling and heating system. 9

26 1.4.3 The development of annual system performance simulation The system performance simulations were modeled in the TRNSYS transient simulation program by two sections; the building simulation and the solar energy system simulation. The sophisticated building simulation calculates the building heating and cooling loads based on the inputs of the configuration of the building; weather conditions; the schedules for occupancy, lighting, equipment; and set points for temperature and humidity in the test building space. The thermal system simulations estimated the required energy input to meet the calculated building loads, either as available solar radiation or as natural gas auxiliary fuel. The thermal system simulation included all of major system components and operation strategies. Most of the major components in the test solar energy supply system are available in the simulation library. Only two new components, the solar receiver and the system controls, were written to integrate controls for PTS, chiller, pumps and fan coils. Figure 1-5 Research approach schematic chart 10

27 The system simulation has been used as a generic system model to optimize system design and operation, and to assess the impact of these parameters on the system performance: the orientation of the PTS; the location and volume of the storage tank; the piping diameter, length, thickness, and the insulation; as well as the operating strategy The installation of the test system The solar thermal system was designed to cool or heat the south zone of the Robert L. Preger Intelligent Workplace (IW), an office space for multiple uses class rooms, laboratories, meeting spaces, offices for faculty and students, at arnegie Mellon University. To meet the cooling and heating loads of this space, a 16 kw double effect absorption chiller was selected and installed. Figure 1-6 Process and instrumentation diagram of the test solar absorption cooling and heating system This chiller is driven either by hot water or by natural gas to provide cooling in the summer and heating in the winter. The chiller switches between the cooling mode and heating mode by adjusting a two-position valve. This chiller incorporates a cooling tower to reject heat from its operation as required in the cooling cycle. To satisfy the requirement of the double effect absorption chiller, 52 m2 of linear parabolic trough solar thermal receivers, (PTS) were installed, including a circulating propylene glycol water mixture, instrumentation for flow, temperature, pressure and direct normal solar radiation; circulation pumps, an expansion/pressure tank; and a drain/ filling apparatus. A web-based automation system was also installed to operate 11

28 the solar collector, heat exchanger and the absorption chiller with their auxiliary system, monitor the overall system status, and collect experimental data. In addition, there is a heat exchanger installed parallel to the absorption chiller to compare the system performance between the solar heating systems using an absorption chiller and the heat exchanger. The process instrumentation diagram of the test solar absorption cooling and heating system is shown in Figure The test program and experimental data gathering The experiments provided significant knowledge and understanding of the PTS s and the solar absorption cooling and heating system. The test program characterized the equipment and the systems, validated the mathematic model of the solar collector, and the system simulation for evaluating the annual system performance. The experiments in the test program are classified into three groups: Solar collector performance testing the solar collectors operated at a steady state, the inlet temperature of heat transfer fluid (HTF) entering the solar collector receiver arrays and the direct normal solar radiation are constant; testing the solar collection at a transient state, when the operation temperature of the solar collectors increases with time until an elevated temperature is researched. Solar heating in the morning, circulating the heat transfer fluid through the bypass until the desired temperature is reached, and then diverting the HTF to the absorption chiller/ heat exchanger to produce hot water for space heating. The building load, heat exchanger HX-1 maintains the hot water from the chiller / HX-2 in the reasonable range by the available flow of chilled water. Ultimately, when the solar energy is no longer adequate to operate chiller / HX-2 due to the heat loss from the system and the reduction of the direct normal solar radiation, the HTF is then diverted through the bypass. Solar cooling in the morning, circulating the HTF through bypass until the temperature required by the absorption chiller is reached, then diverting the HTF flow through the absorption chiller to produce chilled water for space cooling. Ultimately, when the amount of solar energy supply is no longer adequate to operate the absorption chiller, the HTF is then diverted through the bypass. During these tests, as indicated in Figure 1.6, data on temperature, pressure, flow, and direct normal solar radiation were gathered by the data acquisition system for further data analyses. 12

29 1.4.6 Data analyses, model validation and simulation evaluation After the data were collected, they were analyzed using the basic steady state heat balance of the receiver and the balance of the system to determine fundamental characteristics of the system such as the solar collector efficiency, the collector and system heat capacities, and the heat losses from the system making use of statistical and other mathematical tools. The data then were used to validate the PTS model and to evaluate system simulation by comparisons with the solutions of the solar collector model or system simulations. There were two observations from the comparison. First, the initial collector model predicted higher collector efficiency than the measurements. This discrepancy proved to be due to the absorptivity of the glass envelope, a significant parameter impacting on the collector efficiency. This absorptivity was apparently much higher than the value given by Broad, the equipment manufacturer. After the properties of the glass envelope in the initial collector model were adjusted, the deviation between the experimental data and the model solution was minimized. Second, initial system performance simulation estimated much shorter preheat time than those observed at the beginning of each day. This discrepancy proved to be due to the pipe component in the TRNSYS modeled as an empty pipe without the heat capacity. A small storage tank, inserted in the system simulation, added appropriate heat capacity to the piping. As a consequence, the experimental data and the system simulation solution came into agreement. 1.5 hapter overview This thesis contains seven chapters followed by references, appendixes, and nomenclature. hapter 1, Introduction introduces the background and motivation of this dissertation and summarizes the research objectives and approach. hapter 2, Solar absorption cooling and heating test system and program introduces the overall experimental system setups of solar absorption cooling and heating test system. It presents detailed information on the system devices, instrumentation and control. The test program and data acquisition are introduced. The experimental data are analyzed. hapter 3, Solar collector performance model addresses the model assumptions, energy balance and heat transfer and calculation procedure. It also introduces the facts of the absorption of solar irradiation impact on the solar collector efficiency. 13

30 hapter 4, Experimental data analysis on PTS model assesses and validates the solar model by using experimental data. The model is used to analyze the PTS performance under various weather and operational condition. hapter 5, Solar absorption cooling and heating system simulation introduces the simulation assumptions and in depth description of operation and control in the base case of the solar cooling and heating simulation. This chapter also includes the assessment and evaluation of the system performance model by using the experimental daily data. hapter 6, Simulation-based design and performance analysis on absorption cooling and heating system presents the system optimization and system sensitivity analysis by serial system comparison simulations. The guidelines of design and operation for solar cooling and heating system are provided. hapter 7, onclusions and recommendations summarizes the finding and contributions of this thesis and suggests future areas for research and the issues involved, including: the thermal storage equipment, an advanced control system, and the integration of cooling and heating devices. 14

31 2 Solar absorption cooling and heating test system and program A solar absorption cooling and heating test system has been designed, installed, and tested in the IW at arnegie Mellon University of Pittsburgh. As shown in Figure 1-6, the system consists of 52 m 2 of parabolic trough solar collectors, PTS s; a 16 kw double effect absorption chiller; a heat recovery exchanger; and a variable, simulated building load exchanger to measure the performance of the solar collector and the overall solar cooling and heating system. A web-based data acquisition and control system was developed and installed to operate the solar system while storing and displaying the test measurement data. The PTS was tested at various operating conditions: direct solar irradiation, wind load, heat transfer fluid, flow rate, and temperature. Daily tests on the solar cooling and heating were conducted at various weather conditions: clear day, mostly sunny day, mostly cloudy and overcast in winter and summer, respectively. In the future, this solar absorption cooling and heating test system will be integrated with the cooling and heating units of the IW and incorporated in the campus cooling and heating grids. 2.1 Parabolic trough solar collector Device description The solar collectors installed in the IW are single axis tracking solar concentrators: parabolic trough solar collectors (PTSs). They track the altitude of the sun as it travels from east to west during the day to ensure that the radiation from the sun is continuously focused on the linear receiver. These PTS s, provided by Broad Air onditioning o., have a 52m 2 aperture surface, which is the total open cross sectional area of four modules of parabolic reflectors. Figure 2-1 shows two arrays of PTS s installed in series in the two valleys of the IW saw-tooth roof. They are connected by the supply and return lines of the heat transfer fluid (HTF), an aqueous solution containing 50% propylene glycol, with the absorption chiller installed on the southeast platform adjacent to the IW. The tracking axes of the PTS s are oriented 15 east of true north because this orientation minimizes the collector height, wind loading, and structural requirements of the installation. 15

32 The PTSs The IW Pipelines Figure 2-1 The PTS s installed on the IW Parabolic trough reflector (Stine 1987) Support structure Receiver tube Tracking mechanism Figure 2-2 Broad BJ16A parabolic trough solar collectors and the receiver tube 16

33 2.1.2 Major components and characteristic The installed PTS, shown in Figure 2-2, comprises a parabolic trough reflector mirror; a receiver tube, a surface treated absorber pipe at its focal line surrounded by an evacuated transparent tube; supporting structure; and a tracking mechanism. A module of the Broad PTS receiver weighs 200 kg. It is designed to withstand a 31 m/sec wind load. The m 2 aperture area and 0.68 m 2 receiver area corresponds to a 19.6 concentration ratio. The parabolic reflector trough, according to Broad, has a reflectance of 0.8. Most of solar radiation on the reflector is focused on the receiver tube, after it impinges on this mirror. The receiver tube consists of an absorber pipe at its center surrounded by a glass envelope as shown in the right bottom of Figure 2-2. The absorber pipe is coated with selective blackened nickel, which has a high absorptivity of 0.96 for short wave length solar radiation and a low emissivity of 0.14 at 100 for long wave length heat radiation. The glass envelope plays an important role in reducing convective and the radiative losses from the receiver tube to the atmosphere. Its surface temperature is much lower than that of the absorber pipe, and it is opaque to thermal radiation from that tube. In addition, the annular space between the absorber pipe and the glass envelope is evacuated in order to decrease the conduction and convection between them. The Broad PTS tracking drive is a large semi circular gear engaged with a small gear powered by a 24V servo motor. PTS s typically have a higher efficiency than plate solar collectors such as a flat solar collector or an evacuated-tube collector when the operation temperature is high. The characteristics of the PTS installed in the IW are addressed in Table Absorption chiller Device description The absorption chiller installed in IW is a dual fired two-stage, water-libr chiller with a cooling tower. This chiller, also fabricated by Broad Air onditioning o., has a 16 kw rated cooling / heating capacity driven by either hot water or natural gas. Figure 2-3 is the absorption chiller installed on the southeast platform of the IW. It is connected with solar collection loop, and its chilled / hot water supply and return lines. Its working schematic flow diagrams 1 are illustrated in Figure 2-4 and Figure From the absorption chiller brochure of Broad Air onditioning o. 17

34 Table 2-1 Specifications of the parabolic trough solar collector installed Parabolic trough solar collector features (BJ16A) Manufacturer: Operating temperature: Module size: Module operating weight: Drive group size: Delta-T loop size: Rim angle, 2 : Reflectors: Focal length: Receiver: Sun tracking Tracking drive System Wind loads Broad Air onditioning Broad Town, hang Sha, Hu Nan, HINA * 5.75 m; m kg 2 modules; m 2 2 drive groups; 52.9 m 2 73 Typical reflectance cm Absorber OD: 3.8 cm Base material: Stainless steel 304L oating: Black nickel Typical absorptivity: 0.96 Typical emittance: 0.14@100 Pyrex glass cover OD: 10.2 cm Transmissivity: 0.91 Vacuum in the annular space Single-axis elevation tracking based on the calculated sun position 24 V powered Servo motor Small gear, big gear. 16 m/sec (tracking) 31 m/sec (stowed) Major components and characteristics The absorption chiller consists of five major and minor heat transfer components, three pumps, a cooling tower, and other associated valves and pipe fittings in Figure 2-4. The five major components include: an evaporator (marked 4), an absorber (marked 5), a hightemperature regenerator (marked 2), a low-temperature regenerator (marked 1), and a condenser (marked 3). The three minor heat transfer components are: a high temperature heat interchanger 2 Rim angle is the angle between the line from vertex to focus and the line from focus to the parabola ridge point. 18