experimental and modeling comparison of modular

i

EXPERIMENTAL AND MODELING COMPARISON OF MODULAR PHOTOVOLTAIC-THERMAL SOLAR PANELS by NICOLE C. ANNIS A THESIS Presented to the Faculty of the Graduate School of the MISSOURI UNIVERSITY OF SCIENCE AND TECHNOLOGY In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE IN CIVIL ENGINEERING August 2010

Approved by: Dr. Stuart W. Baur, Advisor Dr. Katie Grantham Dr. Kelly O. Homan Dr. Jeffery S. Volz

ii

 2010 Nicole C. Annis All Rights Reserved

iii

ABSTRACT Three different prototype photovoltaic-thermal panels using water as the cooling fluid were tested simultaneously. The first two panels (Panel A and B) consisted of a highly conductive thermal sheeting and different sized copper tubing. The third panel (Panel C) consisted of copper tubing with an aluminum fin. Thermal images were used to verify the heat transfer across the panels and compare the amount of heat radiating off the back of the photovoltaic-thermal panels versus the standard photovoltaic panel. Three A type panels were thermally connected in series. Three photovoltaic panels were also tested for an electrical comparison. The purpose of this experiment was to create a modular photovoltaic-thermal panel, which would be easily implemented and maintained by the average consumer. A TRNSYS model was created for both setups to gather year-round efficiency approximates. The thermal efficiencies of photovoltaic-thermal panels A, B and C at 0.5 gallon per minute (gpm) were 33.6%, 26.4% and 28.7%, respectively. The overall thermal efficiencies of photovoltaic-thermal panels A1-3 in series at 0.5, 1.0 and 1.5 gpm were 51.0%, 40.3% and 59.2%, respectively, and electrical efficiencies of 11.6% for 0.5 gpm and 11.2% for 1.0 and 1.5 gpm. Panels A1-3 at 0.5, 1.0 and 1.5 gpm had thermal gain plus electrical output equivalents of 931.9, 1281.2 and 1496.8 watts, respectively. Overall, the individual panels in series were modeled more accurately then the models for the entire system. The modeling precision also increased as the fluid flow rate increased but the electrical models were the least representative of the experimental data.

iv

ACKNOWLEDGMENTS As with most projects, this project and subsequence thesis would never be possible without support from others around me. These few lines and thank you can never equal the time and support you have all given me in the past two years. First to my advisor, Dr Baur, thank you for taking me on as a graduate student; I know it could not have always been easy. Also to your wife, Martina Baur, for never minding my endless phone calls and the solar panels in the back yard! To Joel Lamson, thank you for your ‘crash course’ in solar panels and for coming to my rescue when I was really in trouble. To Art Boyt, your decades of experience in solar panels and eagerness to help have been an invaluable resource to me. To Mike Chiles, thank you for thinking outside of the box and seeing the potential in this project. To Dr. Elmore, thank you and your students for helping with our missing weather data. Also, thank you to the Missouri Office of Administration Division of Facilities Management, Design, and Construction for sponsoring and giving Dr. Elmore and his team a completely functioning weather station. To my advising committee, Dr. Grantham, Dr. Homan and Dr. Volz, thank you for your enthusiasm and willingness to read this thesis and give me really great advice. Dr. Katie Grantham, I truly appreciate your support during the EPA P3 convention in Washington, D.C. I felt like you had my back the whole time! Dr. Kelly Homan, thank you for answering my never ending list of questions, and teaching me the in’s and out’s of thermal dynamics. Dr. Jeffery Volz, thank you for your cheerful nature; you brighten up my day every time I see you. Finally thank you to my parents; in particularly to my father. Dad, you pushed me to be a better person, encouraged my desire for knowledge and was never bothered by my endless assault of questions. Whenever I had one of my crazy ideas, you were the first one to say okay and ask if I needed any help. Thank you!

v

TABLE OF CONTENTS ABSTRACT ............................................................................................................................iii ACKNOWLEDGMENTS .........................................................................................................iv LIST OF ILLUSTRATIONS..................................................................................................... viii LIST OF TABLES .....................................................................................................................x NOMENCLATURE................................................................................................................. xi SECTION 1. INTRODUCTION ................................................................................................... 1 1.1. OVERVIEW .................................................................................................. 1 1.2. REVIEW OF LITERATURE ............................................................................. 4 2. PANEL CONFIGURATIONS ................................................................................... 7 2.1. PANEL D...................................................................................................... 7 2.2. PANEL C ...................................................................................................... 7 2.3. PANEL B ...................................................................................................... 9 2.4. PANEL A .................................................................................................... 11 3. EXPERIMENTATION ........................................................................................... 14 3.1. OVERVIEW ................................................................................................ 14 3.2. TESTING PROCEDURES ............................................................................. 16 3.4. SETUP #1 .................................................................................................. 18 3.4.1. Configuration .................................................................................. 18 3.4.2. Thermal ........................................................................................... 20 3.4.3. Electrical .......................................................................................... 23

vi

3.5. SETUP #2 .................................................................................................. 25 3.5.1. Configuration .................................................................................. 25 3.5.2. Thermal ........................................................................................... 26 3.5.3. Electrical .......................................................................................... 28 3.6. DATA ANALYSIS AND RESULTS ................................................................. 30 3.6.1. Methodology .................................................................................. 30 3.6.2. Setup #1 Data Results .................................................................... 33 3.6.3. Setup #1 Thermal Images ............................................................... 34 3.6.4. Setup #2 Thermal Data Results ....................................................... 37 3.6.5. Setup #2 Thermal Images ............................................................... 41 3.6.6. Setup #2 Electrical Data Results ..................................................... 43 4. MODELING ........................................................................................................ 48 4.1. INTRODUCTION ........................................................................................ 48 4.2. METHODOLOGY ...................................................................................... 48 4.3. MODELS.................................................................................................... 53 4.4. RESULTS.................................................................................................... 55 5. EXPERIMENTAL AND MODELING COMPARISON .............................................. 58 5.1. OVERVIEW ................................................................................................ 58 5.2. THERMAL.................................................................................................. 58 5.3. ELECTRICAL............................................................................................... 69 6. CONCLUSIONS ................................................................................................... 72 7. FUTURE WORK .................................................................................................. 74

vii

APPENDICES A. SETUP PICTURES AND SCHEMATICS ................................................................. 75 B. PANEL PICTURES & SPECIFICATIONS ................................................................ 84 C. EXPERIMENTAL DATA GRAPHS ......................................................................... 92 D. EXPERIMENTAL TESTING: THERMAL IMAGES ................................................ 104 E. TRNSYS MODELING GRAPHS ........................................................................... 135 BIBLIOGRAPHY ................................................................................................................ 180 VITA ................................................................................................................................. 183

viii

LIST OF ILLUSTRATIONS

Figure:

Page #:

2.1. Panel C Partial Cross Section View with Material List ................................................. 8 2.2. Panel B Partial Cross Section with Material List ........................................................ 10 2.3. Panel A Partial Cross Section with Material List ........................................................ 12 3.1. Panel Angle on Surface of Earth ................................................................................ 15 3.2. Setup #1 Panel Layout and Placement ...................................................................... 18 3.3. Yearly Irradiation Values on Titled and Horizontal Surfaces ..................................... 19 3.4. Thermal Layout and Water Flow for Setup #1 ........................................................... 21 3.5. Picture of Diverting PVT Inlet Flow ............................................................................ 22 3.6. Schematic Electrical Layout for Setup #1................................................................... 24 3.7. Setup #2 Panel Layout and Placement ...................................................................... 25 3.8. Thermal Layout and Water Flow for Setup #2 ........................................................... 27 3.9. Schematic Electrical Layout for Setup #2................................................................... 30 3.10. Setup #1 Thermal Efficiency Curves Summary ........................................................ 34 3.11. Thermal Images of the Front of Panels D, C, B and A (from left to right) ............... 36 3.12. Thermal Images of the Back of Panels A, B, C and D (from left to right) ................. 37 3.13. Setup #2 Thermal Efficiency Curves Summary at 0.5 gpm ...................................... 38 3.14. Setup #2 Thermal Efficiency Curves Summary at 1.0 gpm ...................................... 38 3.15. Setup #2 Thermal Efficiency Curves Summary at 1.5 gpm ...................................... 39

ix

3.16. Thermal Images of the Front (Left) and Back (Right) of Panels D3 and A1 at 0.5 gpm .................................................................................. 42 3.17. Thermal Images of the Front (Left) and Back (Right) of Panels D3 and A1 at 1.5 gpm ................................................................................. 43 3.18. Setup #2 Electrical Efficiency Curves Summary at 0.5, 1.0 and 1.5 gpm ................. 44 3.19. Setup #2 Electrical Power and Power Due to Thermal Gain at 0.5, 1.0, and 1.5 gpm ................................................................................... 47 4.1. Single Photovoltaic-Thermal Panel TRNSYS Model Configuration ............................ 54 4.2. Multiple Photovoltaic-Thermal Panel TRNSYS Model Configuration ........................ 54 5.1. Modeling and Experimental Thermal Efficiency for Panel A, B and C at 0.5 gpm ...................................................................................... 60 5.2. Modeling and Experimental Thermal Efficiency for Panel A1 at 0.5, 1.0 and 1.5gpm ............................................................................... 62 5.3. Modeling and Experimental Thermal Efficiency for Panel A2 at 0.5, 1.0 and 1.5gpm ............................................................................... 64 5.4. Modeling and Experimental Thermal Efficiency for Panel A3 at 0.5, 1.0 and 1.5gpm ............................................................................... 66 5.5. Modeling and Experimental Thermal Efficiency for Panel A1-3 at 0.5, 1.0 and 1.5 gpm .......................................................................... 68 5.6. Modeling and Experimental Electrical Efficiency for Panel A1-3 at 0.5, 1.0 and 1.5 gpm .......................................................................... 70

x

LIST OF TABLES

Table:

Page #:

4.1. TRNSYS Model Inputs and Parameters ...................................................................... 49 4.2. TRNSYS Modeling Parameters with Values Used ...................................................... 55 5.1. Thermal Modeling Percent Error from Experimental Graph for Panel A, B and C at 0.5 gpm ...................................................................................... 61 5.2. Thermal Modeling Percent Error from Experimental Graph for Panel A1 of Series at 0.5, 1.0 and 1.5 gpm ................................................................ 63 5.3. Thermal Modeling Percent Error from Experimental Graph for Panel A2 of Series at 0.5, 1.0 and 1.5 gpm ................................................................ 65 5.4. Thermal Modeling Percent Error from Experimental Graph for Panel A3 of Series at 0.5, 1.0 and 1.5 gpm ................................................................ 67 5.5. Thermal Modeling Percent Error from Experimental Graph for Panels A1-3 in Series at 0.5, 1.0 and 1.5 gpm ........................................................... 69 5.6. Electrical Modeling Percent Error from Experimental Graph for Panels A1-3 in Series ................................................................................................. 71

xi

NOMENCLATURE Aback : Area of foam board back (m2) Ac : Area of collector (m2) Aedge : Area of edge (m2) Cb : Bond conductance Cp : Fluid specific heat (J/kg·°C) D : Outside diameter of pipes (m) Di : Inside pipe diameter (m) F : Fine efficiency (%) F’ : Thermal collector efficiency (%) G : Solar irradiation (Watts/m2) hfi : Heat transfer coefficient between fluid and tube wall (m) hw : Wind heat transfer coefficient (W/m2·°C) IMPP : Current at max power point (Amp) k : Thermal conductivity (W/m2·°C) L : Thickness (inches) : Mass flow rate (kg/sec) Ta : Ambient temperature (°C) teq : Equivalent foam thickness (m) Tin : Inlet temperature (°C) Tout : Outlet temperature (°C) Tpm : Mean plate temperature (°C)

xii

Ub : Bottom loss coefficient (W/m2·°C) Ue : Edge loss coefficient (W/m2·°C) Uedge : Heat transfer coefficient of edge material (W/m2·°C) UL : Collector overall loss coefficient (W/m2·°C) Ut : Top loss coefficient (W/m2·°C) ∀actual : Actual volume of foam (m3) VMPP : Voltage at max power point (Volts) W : Center-to-center pipe spacing (m) β : Collector tilt (degrees) δ : Thickness of plate (m) g

: Emittance of glass

p

: Emittance of plate

elec

: Electrical efficiency (%)

them

: Thermal efficiency (%)

1

1. INTRODUCTION 1.1. OVERVIEW In 2005, the United States of America consumed approximately 21.7% of the world’s total energy usage. This amount translated to 100.1 quadrillion (10^15) Btu or 4.51 billion tons of coal or 17.0 billion barrels of crude oil. In 2006, 72.4% of the total U.S. electricity consumption was used by buildings (36.9% for residential and 35.5% for commercial buildings), which 12.5% of this was used to heat water for domestic usage. Water heating in 2006 cost the American people approximately $30.5 billion and released 147.6 million metric tons of carbon emissions into the air. (1) As the countries around the world discover their impact on the Earth’s finite resources, many governments have enacted programs which help to lessen the global strain. The Building America Program by the U.S. Department of Energy (2) is intended to improve the overall building efficiency of standard American homes. Two of the program’s main objectives are to reduce average whole-house energy use by 30% - 90% and to integrate clean onsite power systems, such as electric and thermal solar panels. Solar electric panels, also known as photovoltaic panels (PV), are comprised of smaller photovoltaic cells, which act like electrical diodes. The cells only have current flowing through the cell as long as the cell is illuminated, and since the cells are similar to diodes, the current can only go in one direction. As the sun illuminates the photovoltaic panel, only a small portion (less than 45% for crystalline silicon cells) is actually used by the cell to produce current (3). Most of the extra 55% of the solar radiation is converted to heat, which radiates off of the panel. (4)

2

There are several different types of photovoltaic cells. The most common photovoltaic material is silicon. Silicon has the highest efficiency rates without the extremely high production costs of other materials. The main cell structure configurations are monocrystalline, polycrystalline and thin film. Monocrystalline cells are made of a single large crystal, and are approximately 15% - 18% efficient (5). Polycrystalline cells are made up of small crystals that are formed into a cell and are approximately 12% - 14% efficient (5). Thin-film cells are made up of a thin, flexible laminate sheet that has the silicon mixture applied to the surface. The thin-film manufacturing process uses approximately 1/300th of the material that monocrystalline cells use, but the thin-film cells are only approximately 5% - 6% efficient (6). Solar thermal panels (T) cover a fairly large collection of panels, but all use fluid (usually water), air or a combination of the two to remove heat and use it for domestic applications. Typically, the heat is used as preheating for hot water (in systems with water) or building heating (in systems with air). The most common thermal panel is a flat plate collector, which is an enclosed insulated metal box with a dark-color absorber plate. Solar thermal panels are relatively inexpensive to produce and are comprised of common building materials. According to the U.S. Department of Energy (7), “a typical residential solar water-heating system reduces the need for conventional water heating by about two-thirds.” Solar photovoltaic-thermal panels (PVT) are hybrid panels, which have photovoltaic cells on top of a typical thermal flat plate collector. This combined unit has several advantages over the separate photovoltaic and thermal panels. Since more

3

than half of the solar radiation is wasted by the photovoltaic panel as excess heat, the thermal panel fluid helps to remove the heat from behind the photovoltaic cells. This is most advantageous since photovoltaic cells decrease in electrical efficiency and overall life expectancy as the temperature goes above the standard operating range for an extended amount of time. PVTs are a more efficient use of roof space, and they typically need half as much mounting equipment. They also require less material to construct the panels because the metal frame that holds the photovoltaic panel can also contain the pipes and insulation for the thermal panel. According to studies conducted by U.S. Department of Energy and the EU Coordination Action PV-Catapult in Europe (8), further development in new materials and methods of assembly is still needed. In September 2007, the International Energy Agency Task 35 Solar Heating and Cooling (Task 35-SHC) concluded a three year international study on the research and development of Photovoltaic/Thermal (PVT) systems (9). The group stated, “the technology is promising as the system develops to a potentially lower production and installation cost.” The purpose and objective of this research was to design and build a modularized solar thermal electric panel system that would be aligned with Task 35-SHC goals of increasing system efficiency and lowering assembly costs. The scope of the project included three steps. Step one was to create several prototype photovoltaic-thermal panels and test them. Step two was to take the most efficient PVT panel from Step one and test it in series with multiples of the same panel. Step three was to model all panels and configurations in TRNSYS 16 to gather year-round data for all prototypes.

4

1.2 REVIEW OF LITERATURE There have been documents to help the research and development and the market introduction of photovoltaic-thermal technology, such as ‘PVT Roadmap,’ which was released within Europe (10). Another document from the same project was the ‘PVT Performance Measurement Guidelines,’ which attempts to set PVT standards testing (11). The reports outline potential problem areas and when certain tests should be performed, annual energy predictions, measurement of various collector characteristics and efficiency measurements. Other documents to help standardize photovoltaic-thermal panel and system designs and modeling were created by the International Energy Agency (IEA) and Solar Heating and Cooling Programme (HCP) – Task 35 committee in 2008 (12) (13) (14). In 2001, S.A. Kalorgirou used TRNSYS models to simulate photovoltaic-thermal panels in Cyprus (15). He concluded that optimum water flow rate for the system was 25 l/hr, and the hybrid system increased the mean annual efficiency of the PV solar system from 2.8% to 7.7%. In a study completed by C.D. Corbin and Z.J. Zhai (16) in 2009 at the University of Colorado – Boulder, experimental data was used to help validate a computational fluid dynamics (CFD) model of a building integrated photovoltaic-thermal collector. The team concluded that the cell efficiency could be raised by 5.3% and that water temperatures suitable for domestic usage was possible. Their thermal and electrical efficiencies reached 19% and 15.9%, respectively. The team also developed a correlation between electrical efficiency and inputs such as inlet temperature, ambient air temperature and isolation.

5

A study out of the Netherlands (2000) by H. Zondag, D. Vries, W. Helden, R. Zolingen and A. Steenhoven (17), simulated numerical models for photovoltaic-thermal system under four different configurations: 3D dynamical, 3D steady state, 2D steady state and 1D steady state. The group concluded that models all fell within a 5% accuracy from the experiments, and the simple 1D model preformed as well as the more complex 2D or 3D models. In a study on PVT domestic systems (18), H. Zondag and W. Helden concluded that photovoltaic-thermal panels that use water perform better than those cooled by air. Also, covered panels in a closed loop systems perform considerably better than uncovered panels and open loop systems. In another article by H. Zondag and W. Helden with the aid of M. Jong (19), the group concluded that the covered photovoltaicthermal have much higher thermal efficiencies, but the uncovered panels have higher electrical efficiencies. H. Zondag carried on with his work, with the aid of D. Vries, W. Helden, R. Zonlingen and A. Steenhoven (20), to produce a comparison between photovoltaic-thermal panels with different thermal panel configurations. They studied combinations of restricted and unrestricted flows with water-air combination cooling. The primary conclusion was that the PV on-sheet-and-tube design was only 2% in thermal efficiency, but was much easier to manufacture than the more complex configurations. In a study conducted in Cyprus and Greece by S.A. Kalogirou and Y. Tripanagnostopoulos, a comparison was made between the efficiency and cost of larger systems versus smaller systems (21). They also found that the typical photovoltaic

6

panels without heat extraction units (i.e. thermal panels) produce about 38% more electrical energy, but all excess heat is lost from the panels. The team also concluded that the photovoltaic-thermal units became more economically viable with larger arrays and areas with higher available solar radiation. Earlier in 2000 Y. Tripanagnostopoulos with the help of T. Nousia, M. Souliotis and P. Yianoulis studied various photovoltaicthermal panel configurations in Greece (22). They concluded that the use of an additional layer of glazing helped to increase thermal output and a booster diffuse reflector increased electrical and thermal output. In a Chinese study by W. He, T. Chow, J. Jie, J. Lu, G. Pei and L. Chan in 2004 (23), the team concluded that daily thermal efficiencies could reach around 40% when the fluid inlet and air ambient temperature were the same . According to M. Bakker, M. Jong and K. Strootman (24), the market is ready for cost-effective photovoltaic-thermals. Their study (consisting of 25 m2) concluded that the photovoltaic-thermal system’s cost and payback period was two-thirds that of separate photovoltaic and thermal systems of the same size. In a study completed by E. Erdil, M. Iklan an F. Egelioglu, the team found that the addition of a thermal system to a standard size (10 m2) photovoltaic array in Cyprus added approximately 2.8 KWh thermal energy per day (25). This reduced the electrical output by about 11.5%, but the pay-back period for the modification was less than 2 years.

7

2. PANEL CONFIGURATIONS The subsequent selections describe each panel design, which include materials used and their dimensions. Each panel contained one modification from the previous panel. This minimized the number of variables and factors to consider when comparing the performance of the panels. The schematic drawings were used to develop input parameters for the TRNSYS models.

2.1. PANEL D Panel D was the only photovoltaic stand alone panel that was tested. It was a BP, 175 watt panel with approximately 14.7% efficiency. The panel was tested side-byside with the other photovoltaic panels so there would a comparison panel for electrical output and thermal gradation across the panel. The panel was setup with nothing attached to the back side of it to how air flow behind the panel.

2.2. PANEL C Panel C was a photovoltaic-thermal panel, which was comprised of threequarters inch (0.75” or 19.05 mm) copper tubes with a seven and a half inch (7.5” or 19.05 cm) aluminum fin. This panel was first designed and tested by Joel Lamson and Dr. Stuart Baur in 2006 at University of Missouri – Rolla. This panel was chosen to be a comparison to the other prototypes because of its design as a photovoltaic-thermal panel. This allowed observers to have a baseline when comparing the two different experiments.

8

The thermal pipe of Panel C was made up of three copper pipes continuing four and half feet (4.5’) up the length of the back of the photovoltaic panel. The pipes were spaced eight and three-quarters inches (8.75” or 22.23 cm) on center apart from each other. Two copper pipes connected the longitudinal pipes laterally on either end. The lateral pipes also served as the inlet/outlet for the thermal panel. The extruded aluminum fin had a rounded portion, which fit firmly around the copper pipes, and a flat portion, which was attached to the back of the photovoltaic panel and conducted heat toward the copper pipes. The fins were attached only on the vertical pipes, but each pipe had one continuous fin that continued the entire length of the pipe. The figure below (Figure 2.1) is a partial cross section of Panel C. A full cross sectional view and pictures of the panel are located in Appendix B.

Figure 2.1. Panel C Partial Cross Section View with Material List

Once the aluminum fins and copper pipes were assembled, the thermal panel was adhered to the back of the photovoltaic panel using a thin layer of silicone caulk. Next a sheet of three-quarters inch (0.75” or 19.05 mm) thick extruded polystyrene

9

foam board was cut down so that it fit tightly in the aluminum photovoltaic frame. Two wood strips, approximately half inch (0.5” or 12.7 mm) thick, was cut to the width of the panel frame and holes were drilled in them, so they matched up with the holes that were predrilled in the frame. Then the wood strips were bolted to the photovoltaic frame and wooden shims were added so that a slight pressure was added to the foam board and the thermal panel. This ensured a solid contact between the photovoltaic panel and the aluminum fins. The entire assembly was designed to minimize any change to the photovoltaic panel and allow it to be added easily to any standard size panel.

2.3. PANEL B Panel B was a photovoltaic-thermal panel, which was comprised of three-quarter inch (0.75” or 19.05 mm) copper pipes and a highly conductive thermal sheet. The panel was made up of three vertical lines running four and half feet (4.5’ or 1.37 m) up the length of the back of the photovoltaic panel. The pipes were spaced eight and three-quarters inches (8.75” or 22.23 cm) on center apart from each other. Two copper pipes connected the longitudinal pipes laterally on either end. The lateral pipes also served as the inlet/outlet for the thermal panel. The pipe size and layout was kept the same as Panel C so that the only difference in the panels was the fin materials. The thermal sheet was used to conduct heat across the photovoltaic panel and into the copper pipes. The sheet came in a roll, which was eighteen inches (18” or 45.72 cm) wide and had an adhesive backing on one side. One sheet of inch and a half (1.5” or

10

3.81 cm) extruded polystyrene foam board was cut to the size of the photovoltaic frame and grooves were scored for the thermal panel pipes. Strips of the thermal sheeting were cut to the width of the panel with several extra inches so the sheeting could be partially wrapped around the copper pipes. Each thermal sheet was tightly fit into the three grooves across the panel and then kept in place with its adhesive backing. The cooper pipe assembly was then tapped into the foam with the conductive thermal sheets. Extra strips of the thermal sheeting were cut a few inches wide and attached to the top of the copper pipes which were not in contact with the foam. These small strips helped to create a thermal bridge across the back of the photovoltaic panel and created a consistent flow of heat. The figure below (Figure 2.2) shows a partial cross sectional view of Panel B. A full cross sectional view and pictures of the panel are located in Appendix B.

Figure 2.2. Panel B Partial Cross Section with Material List

Next the constructed thermal panel was attached to the photovoltaic panel. The thermal panel was placed into the back aluminum frame of the photovoltaic panel with

11

the copper pipes and thermal sheet facing the back of the photovoltaic panel. Two wood strips, approximately half inch (0.5” or 12.7 mm) thick, was cut to the width of the panel frame, and holes were drilled to match up with the holes that were predrilled in the frame. The wood strips were then bolted to the photovoltaic frame and wooden shims were added so that a slight pressure was added to the foam board and the thermal panel. This ensured a solid contact between the photovoltaic and thermal panels. The entire assembly was designed to minimize any change to the photovoltaic panel and allow it to be added easily to any standard size panel.

2.4. PANEL A Panel A was a photovoltaic-thermal panel, which was comprised of half inch (0.5”) diameter copper pipes and a highly conductive thermal sheet. Panel A was designed to be the same as Panel B with the exception that the pipes in Panel A were slightly smaller than the pipes within Panel B. The thermal panel of Panel A was made up of three vertical lines running four and half feet (4.5’) up the length of the back of the photovoltaic panel. The pipes were spaced eight and three-quarters inches (8.75” or 22.23 cm) on center apart from each other. Two copper pipes connected the longitudinal pipes laterally on either end. The lateral pipes also served as the inlet/outlet for the thermal panel. The thermal sheet was used to conduct heat across the photovoltaic panel and into the copper pipes. The sheet came in a roll, which was eighteen inches (18” or 45.72 cm) wide and had an adhesive backing on one side. One sheet of inch and a half (1.5” or

12

3.81 cm) rigid thermal insulation board was cut to the size of the photovoltaic frame and grooves were scored for the thermal panel pipes. The insulation board used for Panel A is a polyisocyanurate foam core board that is bonded to a glass fiber aluminum foil facer. The rigid board provided structural stability to the pipes and thermal sheeting, so the entire assembly was could be easily moved and set in place by only one person. Strips of the thermal sheeting were cut to the width of the panel with several extra inches so that the sheeting could be partially wrapped around the copper pipes. Each thermal sheet was tightly fitted into the three grooves across the panel and then kept in place with its adhesive backing. From there the cooper pipe assembly was tapped into the foam with the conductive thermal sheets. Extra strips of the thermal sheeting were cut a few inches wide and attached to the top of the copper pipes, which were not in contact with the foam. These small strips helped to create a thermal bridge across the back of the photovoltaic panel and created a consistent flow of heat. The figure below (Figure 2.3) shows a partial cross section view of Panel A. A full cross sectional view and pictures of the panel are located in Appendix B.

Figure 2.3. Panel A Partial Cross Section with Material List

13

Next the constructed thermal panel was attached to the photovoltaic panel. The thermal panel was placed into the back aluminum frame of the photovoltaic panel with the copper pipes and thermal sheet facing the back of the photovoltaic panel. Two wood strips, approximately half inch (0.5” or 12.7 mm) thick, were cut to the width of the panel frame, and holes were drilled to match up with the holes that were predrilled in the frame. The wood strips were then bolted to the photovoltaic frame, and wooden shims were added so that a slight pressure was added to the foam board and the thermal panel. This ensured a solid contact between the photovoltaic and thermal panels. The entire assembly was designed to minimize any change to the photovoltaic panel. It could also be easily added to any standard size panel and later removed if needed.

14

3. EXPERIMENTAION 3.1. OVERVIEW The experimentation of this project included two different testing configurations and setups. The intent of the Setup #1 was to test different photovoltaic-thermal panels and determine which one was the most efficient. The Setup #2 was designed to take the most efficient photovoltaic-thermal panel for Setup #1 and test several of the same panels in series. The purpose of this setup was to determine the change in thermal efficiency, if any, when multiple photovoltaic-thermal panels were connected in series. In typical residential photovoltaic-thermal systems, water would not flow through just one panel, but would continue through a series of panels to further heat the water for domestic usage. The typical residential photovoltaic-thermal systems consisted of panels connected directly to a water-holding tanking or through the use of a heat exchanger. The initial system allows the thermal energy from the sun to be transferred directly to the domestic water supply. Whereas the latter configuration allows the use of ethylene glycol mixture in regions where overnight freezing maybe an issue. Also the system requires less fluid to be circulating through the system, which minimizes maintenance issues. For this experiment, water storage and heat exchange was not a major concern. The system was tested for thermal gain as the fluid passed once through the system, which allowed for consistent testing conditions and calculation of the maximal thermal efficiency.

15

Placement of the entire system was a key issue and concern, which had to be addressed before the frames and panels could be setup. For Rolla, Missouri, and all of the Northern Hemisphere, solar panels work best when placed facing south. This results from the curvature and tilt of the Earth’s surface. The figure below (Figure 3.1) is a drawing of a panel angled on the Earth’s surface at the Rolla, Missouri latitude (37.95°).

Figure 3.1. Panel Angle on Surface of Earth

Another consideration for the site selection was the potential shading from objects over the course of the day and the testing period. If a photovoltaic cell within a panel is shaded, it dramatically affects the performance of all other cells within that

16

panel. The photovoltaic cells work like a diode, which means that current (i.e. electrons) is only allowed to travel in one direction. If one cell was shaded and the current of the cell was reduced, then the current through all of the remaining cells would be reduced by the same amount. For this reason, the site chosen for the experiment had to be clear of all objects, which could potentially shade the panel.

3.2. TESTING PROCEDURES The testing procedures were the same for both Setup #1 and #2, which included setting framing systems, positioning photovoltaic panels, connecting wires and pipes, calibrating sensors, setting water flow and collecting testing data. Testing was done over the course of several days to obtain more reliable data and remove abnormal weather conditions. After the testing was complete, the data was complied, analyzed, and graphed so that it could be interpreted and compared. Before the panels were placed on the framing system, the frame had to be completely leveled. A level frame and panel was crucial for the water to flow evenly through the photovoltaic-thermal panels. If the panel was uneven, flow would be diverted through one or two of the vertical pipes behind the photovoltaic panel. This would result in hot spots and a decrease in electrical output by the photovoltaic panels and lower thermal gain by the thermal panel. Once the framing system was leveled, the panels were placed on the frames. The wires for the panels and sensors and hoses for the thermal panels were connected. Thermal couple sensor wires were measured to the same length, so the difference in

17

metal expansion would not be affected by varying wire lengths. The thermal couples measuring water temperatures were hung horizontally so that the water would fill the pipe, and air would not affect the temperature readings. A pyranometer, which measured the available solar radiation, was attached to the framing system at the same angle as the panels. Before the start of each testing day, the photovoltaic panels were cleaned to remove any pollen, dust or organic matter from the surface. These materials would have a minimal impact of electrical efficiency, but over time it would build up and dramatically reduce the available power output. According to a previous study conducted in Egypt, the accumulation of dust on a photovoltaic panel can reduce transmittance by approximately 52.54–12.38%, which would cause a 17.4% decrease in power output (26). After the panels were set up and sensors, wires and hoses were connected, the inlet temperature would be set and the flow calibrated. Periodically the flow was checked to verify that it was still at the original rate. Inlet temperatures were changed a few times during the course of the day or kept consistence during the entire day to see the affect changing irradiation had on thermal efficiency. The data logger recorded all data points for the inlet/outlet temperatures, photovoltaic panel temperatures, voltage, amperes and irradiation. The weather station recorded ambient temperature, which was later added to the data spreadsheet.

18

3.4. SETUP #1 3.4.1. Configuration. The initial setup consisted of one (1) panel for each of the three (3) prototype photovoltaic-thermal panels (Type A, B and C) and one (1) photovoltaic panel (Type D). All four (4) panels were tested side-by-side to decrease the number of weather-based variables. The simultaneous testing also allowed for inlet temperatures to be consistent for all the panels, which enabled a direct comparison between the photovoltaic-thermal panels. The schematic below (Figure 3.2) shows all four (4) panels setup in the framing system. Additional site photos and schematic drawings are located in Appendix A.

Figure 3.2. Setup #1 Panel Layout and Placement

The panels were placed on a wooden framing system, which sat directly on the ground. The frame was set to the Rolla, Missouri latitude (37.95°). The latitude was used because it optimized the system for year-round use and not just for one season. One of the project objectives was to design a panel and system, which would be easily

19

implemented and maintained by the average consumer. Most residential photovoltaic and/or photovoltaic-thermal systems do not allow for easy modifications (i.e. changing the framing angle) from season to season. As a result, the research team decided to use the most optimal angle for the entire experiment. Figure 3.3 below shows the yearly irradiation values for horizontal surfaces (i.e. the Earth’s surface) shown in green and a panel set to the latitude angle (37.94 degrees) shown in blue. This graph was generated from the TRNSYS model, which will be discussed about later in this paper.

1200 Titled (37.95 Deg) Panel Earth (Horizontal) Surface

1000

(Watts/m2)

Irradiation

800

600

400

200

0 1/1

1/29 2/26 3/26 4/23 5/21 6/18 7/16 8/13 9/10 10/8 11/5 12/3 12/31

Day of Year Figure 3.3. Yearly Irradiation Values on Titled and Horizontal Surfaces

20

3.4.2. Thermal. The thermal portion of the photovoltaic-thermal system included all water pipes, tank(s), pumps, valves and the copper manifold, which was enclosed behind the photovoltaic panel. One of the challenges that arose with the testing of the photovoltaic-thermal panels was that the panel response to a moderate change in the thermal system (i.e. inlet temperature or inlet flow) was not instantaneous. Although the sensor readings were being recorded at regular intervals (every 15 seconds), they could not take into effect the time delay required for an alteration to travel through the entire thermal system and the panels to equalize. The open-loop system was selected to help minimize this effect. An open-loop system means that once the water passes through the panels, it was not reused within the panels again. The benefit to the open-loop system was that the inlet temperatures could be kept at a consistent temperature for long periods of time, which resulted in more statistically reliable data. The following figure (Figure 3.4) shows a schematic of the open loop system. Additional schematic views are located in Appendix A. The main water supply used during testing was the local groundwater sources, which had an average temperature of approximately 55-60 degrees Fahrenheit. To obtain data for various inlet temperatures, a hot water heater was used in conjunction with the groundwater source. The two sources were connected to a mixing value, which allowed the desired inlet temperature to be set and changed when needed.

21

Figure 3.4. Thermal Layout and Water Flow for Setup #1

After the inlet temperature was established, the flow continued until just before the photovoltaic-thermals panel at which point the flow was diverted (look at Figure 3.5) into three equal flows which supplied water for each panel. The flow was controlled individually with separate mechanical flow meters on each water line. The flow meters were carefully monitored during the experiment since they worked off of pressure in the water lines to stay consistent. In addition to the mechanical flow values, the flow was also verified with a measuring container and a stop watch. Both methods

22

helped to ensure that the water flow was identical for each of the three (3) photovoltaic-thermal panels.

Figure 3.5. Picture of Diverting PVT Inlet Flow

Calibrated temperature sensors were vital to collecting reliable data. The temperature sensors used were Type-J/K/T/E thermal couples manufactured by OMEGA®. Thermal couples worked by measuring the difference in expanding and contracting of two metals within the thermal couple. As a result, it was necessary for all thermal couples to be completely in contact with the surface it was measuring and for all of the sensor wires to be the same length. The sensors were placed on the inlet and outlet flow water line and on each photovoltaic panel. All of the thermal couples measuring water temperatures were securely adhered to a highly conductive copper pipes and covered with insulation. For the back of the photovoltaic panels, the thermal couple were adhered directly to the back of the panel. The ambient temperature was

23

measured by air sensor, which was part of a weather station mounted to the back of the panel framing system out of direct sunlight.

3.4.3. Electrical. The electrical portion of the photovoltaic-thermal system consisted of the electrical wiring, batteries, lights, shunts and the photovoltaic panel. Although the electrical output was a critical part of the system, it was not used to determine which photovoltaic-thermal prototype would be chosen for the Setup #2. Solar cells, even highly efficient mono-crystalline cells, vary slightly for cell to cell. As a result, photovoltaic panels vary slightly in efficiency and electrical output from one panel to the next. Since the Setup #1 was testing only one (1) panel for each of the three (3) prototype photovoltaic-thermal panels, the variation in electrical output is too small to definitely conclude what caused the difference. The photovoltaic panel used was a BP 4175 model, which is made up of 72 mono-crystalline cells with anti-reflective Silicon Nitride coating. The panel had a rated power of 175 watts and panel efficiency of approximately 14.6 %. The overall panel dimensions were 62.7 inches (1.16 meters) by 31.1 inches (0.79 meter). Panel specifications are located in Appendix B. To determine the amperes, which would be used to calculate the wattage for each panel, a shunt was connected to each negative electrical line leaving the photovoltaic-thermal panel. An electrical shunt uses a small but extremely calibrated resistance to enable the data logger to determine the amperes for each panel. The voltage of the system was kept consistent by connecting two (2) twelve (12) volt

24

batteries together as a load. To keep the batteries from over-charging, car head lights were hooked to the batteries. The lights were rated for one hundred watts and twelve volts each. Two (2) of the lights were hooked together in series for each panel being tested, and then all of the coupled lights, four (4) groups in total, were connected in parallel. The batteries were fully recharged after each test day. The figure below (Figure 3.6) is a schematic of the electrical layout. Additional schematic views are located in Appendix A.

Figure 3.6. Schematic Electrical Layout for Setup #1

25

3.5. SETUP #2 3.5.1. Configuration. The Setup #2 consisted of three (3) photovoltaic panels (Type D) and three (3) prototype photovoltaic-thermal panel (Type A), which was selected based on its performance during the Setup #1. The three photovoltaic panels were used as an electrical baseline and in thermal images to see typical heat radiation in standard photovoltaic panels. The schematic below (Figure 3.7) shows all six (6) panels setup in the framing system.

Figure 3.7. Setup #2 Panel Layout and Placement

The panels were placed on a wooden framing system, which were used in the Setup #1. The frame was set to the Rolla, Missouri latitude (37.95°), and placed directly on the ground. The latitude was used because it optimized the system for year-round and not just for one season. Additional site photos and schematic drawings are located in Appendix A.

26

3.5.2. Thermal. The thermal portion of the photovoltaic-thermal system included all water pipes, tank(s), pumps, valves and the copper manifolds, which was enclosed behind the photovoltaic panels. During the Setup #1, one of the problems which emerged was the photovoltaic-thermal panel response was not instantaneous to moderate changes in the thermal system (i.e. inlet temperature or inlet flow). Although the sensor readings were being recorded at regular intervals (every 15 seconds), they could not take into effect the time delay required for an alteration to travel through the entire thermal system and the panels to equalize out. As a result, an open-loop system was selected for the Setup #1 and was continued for the Setup #2. An open-loop system means that once the water passes through the panels it was not reused within the panels again. The benefit to the open-loop system was that the inlet temperatures could be kept at a consistent temperature for long periods of time, which resulted in more statistically reliable data. The following figure (Figure 3.8), shows a schematic of the open loop system for the Setup #2. Additional schematic views are located in Appendix A. As with the Setup #1, a hot water heater was used in conjunction with the main groundwater source to obtain various inlet temperatures. The two sources were connected to a mixing/flow value, which allowed the desired inlet temperature and flow to be set and changed when needed. The flow meter was carefully monitored during the experiment since it worked off of pressure in the water lines to stay consistent. In addition to the mechanical flow value, the flow was also verified with a measuring container and a stop watch.

27

Figure 3.8. Thermal Layout and Water Flow for Setup #2

The water flowed from the groundwater/hot water mixing value to the first photovoltaic-thermal panel (Panel A1). The other two (2) photovoltaic-thermal panels were linked to the first in the series, so the fluid passed through all three (3) panels before exiting the system. The tubing connected one panel to the next was only as long as required, and contained a thermal sensor in the middle of the tubing. This allowed the outlet temperature of one panel to be used as the inlet of the next panel in thermal efficiency calculations. The same temperature sensors were used in both setups (Setup #1 and #2). The sensors were calibrated at the beginning of the experiment, and extension wires were

28

cut to the same length. Having the same wiring lengths were important, but the thermal couples worked by measuring the difference in expanding and contracting of two metals within the thermal couple. All of the thermal couples that measured water temperatures had to be securely adhered to a highly conductive copper pipes and covered with insulation. The ambient temperature was measured by an air sensor, which was part of a weather station and was mounted to the back of the panel framing system out of direct sunlight. During a few of the testing days, the weather station did fail to record data, so data from another weather station on the Missouri University of Science and Technology (Missouri S&T) campus was used. The weather station was part of an experimentation conducted by Dr. Elmore and his students, and was donated by the Missouri Office of Administration Division of Facilities Management, Design, and Construction.

3.5.3. Electrical. The electrical portion of the photovoltaic-thermal system consisted of the electrical wiring, batteries, lights, shunts and the photovoltaic panel. The electrical output and electrical efficiency of the three (3) photovoltaic panels were directly compared to the electrical output and electrical efficiency of the three (3) photovoltaic-thermal panels in the Setup #2. Whereas, the electrical output of the four (4) individual panels in Setup #1 was determine to be unreliable due to minor variations in photovoltaic panel efficiencies. Setup #2 used the same photovoltaic panels as were used in Setup #1. The photovoltaic panels were BP 4175 model, which is made up of 72 mono-crystalline cells

29

with anti-reflective Silicon Nitride coating. The panel had a rated power of 175 watts and panel efficiency of approximately 14.6 %. The overall panel dimensions were 62.7 inches (1.16 meters) by 31.1 inches (0.79 meter). Panel specifications are located in Appendix B. To determine the amperes, which would later be used to calculate the wattage for the three (3) photovoltaic panels and the three (3) photovoltaic-thermal panels, the three negative leads were connected using a busbar and a shunt, which was connected to wiring leaving the busbar. The negative lead of the wiring was then connected to the batteries. An electrical shunt uses a small but extremely calibrated resistance to enable the data logger to determine the amperes for each panel. The voltage of the system was kept consistent by connecting two (2) twelve (12) volt batteries together as a load. To keep the batteries from over-charging, car head lights were hooked to the batteries. The lights were rated for one hundred watts and twelve volts each. Two (2) of the lights were hooked together in series for each panel being tested, and then all of the coupled lights, six (6) groups in total, were connected in parallel. The batteries were fully recharged after each test day. Figure 3.9 is a schematic of the electrical layout. Additional thermal schematic views are located in Appendix A.

30

Figure 3.9. Schematic Electrical Layout for Setup #2

3.6. DATA ANALYSIS AND RESULTS 3.6.1. Methodology. Data points were collected from all temperature, electrical and pyranometer sensors every fifteen (15) seconds during each testing day. After all of the data was collected, the information was transferred to a spreadsheet for analysis. Once the data was combined with the weather information, a third-order statistical analysis was performed on the thermal efficiencies calculated from the testing

31

data. Each order used a plus/minus two (2) standard deviations to determine data outliers. A third-order analysis was required because the photovoltaic-thermal panels took several seconds to equalize after a change was made to the system. This problem was seen throughout the experiment and resulted from the distance the fluid was required to travel through the thermal panels. When the data logger would record sensor readings it would record electrical output, solar irradiation, fluid inlet temperature and fluid outlet temperature at the same moment. The fluid took longer to travel through the thermal panels than the fifteen (15) second recording intervals. As a result, the fluid outlet temperatures would be off from the other readings. After the statistical analysis, graphs were generated from the data. Thermal efficiency and electrical efficiency curves were generated for each photovoltaic-thermal panel for both setups (Setup #1 and #2). The equations for thermal efficiency (3.1) and electrical efficiency (3.2) can be seen below. The x-axis, which is most commonly used, takes the difference in inlet and ambient temperatures and divides it by the product of available solar radiation and panel area. The x-axis equation used can be found below (3.3). After the data points were graphed, a line/curve to best fit the data was plotted and line equation(s) were generated. These curves helped to determine potential efficiencies under similar weather conditions but at different inlet temperatures. The efficiency graphs also helped make some comparison between the various photovoltaicthermal panels.

32

𝜂𝑡ℎ𝑒𝑟𝑚 =

[𝑚̇ ∙ 𝐶𝑝 ∙ (𝑇𝑜𝑢𝑡 − 𝑇𝑖𝑛 )] (𝐴 ∙ 𝐺)

Where: them = Thermal efficiency (%) = Mass flow rate (kg/sec) Cp = Fluid specific heat (J/kg·°C) Tout = Outlet temperature (°C) Tin = Inlet temperature (°C) A = Area of collector (m2) G = Solar irradiation (Watts/m2)

(3.1)

𝜂𝑒𝑙𝑒𝑐 =

(𝐼 ∙ 𝑉)𝑀𝑃𝑃 (𝐴 ∙ 𝐺)

Where: elec = Electrical efficiency (%) IMPP = Current at max power point (Amp) VMPP = Voltage at max power point (Volts) A = Area of collector (m2) G = Solar irradiation (Watts/m2)

Equation 3.3. 𝑋𝑎𝑥𝑖𝑠 ∶ Where: Tin = Inlet temperature (°C) Ta = Ambient temperature (°C) A = Area of collector (m2) G = Solar irradiation (Watts/m2)

(3.2)

[𝑇𝑖𝑛 − 𝑇𝑎 ] (𝐴 ∙ 𝐺) (3.3)

33

3.6.2. Setup #1 Data Results. Most thermal efficiency graphs contained a single linear line to represent the data. The Fraunhofer-Institute for Solar Energy Systems (ISE), created a report, which outlines a standard form for thermal efficiency curves (27), but once the data was plotted for this experiment, it formed a curved line. As a result, exponential and linear curves were plotted and line equations were created. Figure 3.10 summarized the experimental thermal efficiency curves for Setup #1. The experimental line was a combination of exponential and linear lines. From -0.02 and less, the linear line best represented the data, and from -0.02 and greater, the exponential line best represented the data. Individual graphs with daily data points, curves, and line equations are located in Appendix C.

Figure 3.10. Setup #1 Thermal Efficiency Curves Summary

34

During a standard late summer day (August through Early September) [Solar Radiation = 1000 watts/m2, Inlet Temperature = 55°F, Ambient Temperature = 85°F – 90°F] the average thermal efficiencies for Panels A, B and C were as such: 31.2% – 33.6%, 25.0% – 26.4% and 28.1% – 28.7% respectively. The randomness of Panel C efficiency made the efficiency curves flatter, which caused the panel to be less reliable. The efficiency values for Panels A and B were tighter to each other and resulted in a smooth curved line. During cooler days (when ambient temperature was less than the inlet temperature), Panels A and B performed equally well. However, during warmer days (when ambient temperature was greater than the inlet temperature), Panel A performed considerably better than Panel B. This difference was far more important for warmer day since photovoltaic-thermal panels usually only function during warm to very hot days. During warm to hot days, the cold fluid in thermal panel helped to keep the photovoltaic panel cooler, which resulted in higher electrical output and a greater thermal gain in the fluid once it left the photovoltaic-thermal panels.

3.6.3. Setup #1 Thermal Images. During the experimentation, thermal images were taken periodically to show the temperature gradation across each of the panels. The images helped to determine if there were any stagnation (when the fluid does not evenly flow through the thermal panel) spots and how well the thermal sheeting and aluminum fins were conducting the heat toward the copper fluid tubes. The images below show the fronts (Figure 3.11) and the backs (Figure 3.12) of each of the three (3) photovoltaic-thermal panels and the one (1) photovoltaic panel. These

35

images were taken on a ‘standard’ late summer day [Date: 08.12.09, Time: 1:30pm, Flow: 0.5 gpm, Ambient Temperature: 93.9°F, Inlet Temperature: 65.3°F, Pyranometer: 1031 Watts/m2, Clouds: Minimal]. Look at Appendix D for larger photos on various days. The use of thermal images was the main reason why a standard photovoltaic panel was used in the Setup #1 since the electrical data was not used as a comparison between all of the panels. A typical photovoltaic panel predominately radiates excess heat off the back of the panel and into the air. This is further illustrated in the thermal images as the back of the photovoltaic panel was much hotter than the front of the panel. The insulation within the photovoltaic-thermal panels helped to prevent the heat from escaping from the back, and the thermal sheeting or aluminum fin conducted the heat towards the copper tubes. In the front view of the photovoltaic-thermal panels (Figure 3.11), the panels are outlined in the black dashed line, and the approximate pipe locations were indicated with solid black lines. In the front image of the experimental panels (Figure 3.9), the photovoltaic panel (PV D) had a uniform temperature gradation across the panel with an average temperature around 130°F (54.4°C). Photovoltaic-thermal panel C (PVT C), which contained three-quarter inch copper tubing with aluminum fins, had a server temperature difference across the front of the panel. The temperatures ranged from around 100°F (37.8°C) where the tubes where located to over 140°F (60°C) at the edge of the panel. Photovoltaic-thermal panel B (PVT B), which contained three-quarter inch copper tubing with a thermal sheet, had less temperature difference across the panel as compared to PVT C, but a higher base temperature than PVT C. PVT B showed an

36

approximate temperature at the pipe locations of 126°F (52.2°C) and approximately 138°F (58.9°C) between the copper pipes. Photovoltaic-thermal panel A (PVT A), which contained half inch copper tubing with a thermal sheet, performed much like PVT B with slightly higher temperatures. At the pipe locations, the average temperature was approximately 123°F (50.6°C), and between the pipes and at the edge of the pipes, the temperature was around 140°F (60°C) In the back image (Figure 3.12) of the photovoltaic-thermal panels A, B and C (PVT A, B,C), the temperature gradation was minimal. The average back temperatures were only about 55% of the max front temperature. The temperatures ranged from about 75°F (23.9°C) to 80°F (26.7°C). The back of the photovoltaic panel (PV D) was fairly uniform in temperature, which was approximately 140°F – 145°F (60°C – 62.8°C). As a result of using insulation behind the photovoltaic-thermal panel, most of the heat was collected in the thermal panel or radiated off the front of the photovoltaic panel as excess heat.

PV D

PVT C

PVT B

PVT A

Figure 3.11. Thermal Images of the Front of Panels D, C, B and A (from left to right)

37

PVT A

PVT B

PVT C

PV D

Figure 3.12. Thermal Images of the Back of Panels A, B, C and D (from left to right)

3.6.4. Setup #2 Thermal Data Results. The individual photovoltaic-thermal panels in Setup #2 acted much as the single panels in Setup #1 in that the thermal efficiencies formed a curved line of best fit instead of the typical linear line. However the efficiency of the entire system (Panel A1-A3) constantly formed a linear line for all fluid flows tested. Figures 3.13-15 show the photovoltaic-thermal panel exponential and linear data curves for 0.5, 1.0 and 1.5 gallon per minute flows. The experimental line was either the exponential and linear line that best represented the data. For A1, 2 and 3 at 0.5 and 1.0 gallon per minute flow, the exponential line was used, and for 1.5 gallon per minute the linear line was used. For the total system, linear lines were used throughout. Individual graphs with daily data points, curves, and line equations are located in Appendix C. During a standard fall day (October) [Solar Radiation = 1000 watts/m2, Inlet Temperature = 55°F, Ambient Temperature = 70°F – 75°F] the average system thermal efficiencies where 43.7% - 51.0% at 0.5 gallon per minute. The graph for Setup #2 at 0.5

38

Figure 3.13. Setup #2 Thermal Efficiency Curves Summary at 0.5 gpm

Figure 3.14. Setup #2 Thermal Efficiency Curves Summary at 1.0 gpm

39

Figure 3.15. Setup #2 Thermal Efficiency Curves Summary at 1.5 gpm

gallon per minute showed similar characteristics to the graph for Setup #1 at 0.5 gallon per minute in that the data points formed a curved trendline rather than the typical linear line. There was no noticeable difference in precision between Panels A1, A2 and A3, but the thermal efficiency of Panel A2 dropped 31.4% from Panel A1 when inlet equaled ambient. Panel A3 dropped 14.5% from A1 when inlet equaled ambient, but was less than the drop from Panel A1 to Panel A2. The system thermal efficiency line had no obvious curvature in the data points. The total system data points formed a highly slopped linear line. This translated to a large change in the system’s thermal efficiency with a small change in inlet and ambient temperatures.

40

During a standard fall day (October) [Solar Radiation = 1000 watts/m2, Inlet Temperature = 55°F, Ambient Temperature = 70°F – 75°F] the average system thermal efficiencies where 37.3% - 40.3% at 1.0 gallon per minute. The graph for Setup #2 at 1.0 gallon per minute showed slightly different characteristics from the setup at 0.5 gallon per minute. The data was less curved than previously seen and formed a more linear trendline. The total system efficiencies had a high level of accuracy. There was also less of a slope on the system trendline, which suggested that the system would be less affected by small changes in the inlet and ambient temperatures. The efficiencies from Panel A1 and A3 were almost identical in Setup #2 at 1.0 gallon per minute when inlet temperature approximately equaled the ambient temperature. As seen in Setup #2 at 0.5 gallon per minute, Panel A2 showed a 24.8% decrease in efficiency from Panel A1 when the inlet temperature equaled the ambient temperature. During a standard fall day (October) [Solar Radiation = 1000 watts/m2, Inlet Temperature = 55°F, Ambient Temperature = 70°F - 75°F] the average system thermal efficiencies where 58.0% - 64.4% at 1.5 gallon per minute. The graph for Setup #2 at 1.5 gallon per minute showed a noticeable different characteristic from those in Setup #2 at 0.5 and 1.0 gallon per minute. The scatter data points were more random than any of the previous setups. A best fit curved or linear trendline yielded lower system efficiencies as compared to the pervious setups with lower flow rates. The lower efficiency may have been the result of the graph containing only three days worth of data rather than the normal five days, which were typical for the other setups. One commonality between all Setup #2 graphs was that there was a drop in thermal

41

efficiency from Panel A1 to Panel A2. In Setup #2 at 1.5 gallon per minute, there was a 25% drop from Panel A1 to A2 and a 11.3% from Panel A1 to A3 when inlet and ambient temperatures equaled.

3.6.5. Setup #2 Thermal Images. As with Setup #1, thermal images were taken periodically during Setup #2 to show the temperature gradation across each of the panels. The images helped to determine if there were any stagnation (when the fluid does not evenly flow through the thermal panel) spots and if and how the gradation changed as the fluid flowed from one photovoltaic-thermal panel to the next. The images show the fronts (Figure 3.16) and the backs (Figure 3.17) of one (1) photovoltaic-thermal panel and the one (1) photovoltaic panel 0.5 and 1.5 gallon per minute flow. The images below were taken two separate fall days [Date: 11.03.09, Time: 11:20am, Flow: 0.5 gpm, Ambient Temperature: 49.8°F, Inlet Temperature: 65.2°F, Pyranometer: ~950 Watts/m2, Clouds: Minimal] and [Date: 11.02.09, Time: 1:00pm, Flow: 1.5 gpm, Ambient Temperature: 60.3°F, Inlet Temperature: 68.6°F, Pyranometer: ~600 - 1000 Watts/m2, Clouds: Some]. Additional photos are located in Appendix D. The gradation difference between the photovoltaic panel and photovoltaicthermal panel are similar to the thermal images that were taken for Setup #1. The thermal gradation difference between the photovoltaic-thermal panels at 0.5 gallon per minute and 1.5 gallon per minute was inconclusive. Although the ambient temperatures were different (approximately 10°F difference) the main obstacle was the

42

cloud coverage during the 1.5 gallon per minute images. The periodical clouds kept the panels from heating as much as if it was a sunny day, so determining the cause of the gradation difference between the two flows is unlikely with the photos taken during the experimentation.

PV D3

PVT A1

PVT A1

PV D3

Figure 3.16. Thermal Images of the Front (Left) and Back (Right) of Panels D3 and A1 at 0.5 gpm

PV D3

PVT A1

PVT A1

PV D3

Figure 3.17. Thermal Images of the Front (Left) and Back (Right) of Panels D3 and A1 at 1.5 gpm

43

3.6.6. Setup #2 Electrical Data Results. Setup #2 compared six identical photovoltaic panels; three where used as they were while the other three where modified into photovoltaic-thermal panels. Figure 3.18-19 show the electrical data for 0.5, 1.0 and 1.5 gallon per minute flow through the photovoltaic-thermal panels. Additional individual electrical data graphs with data points for each flow tested are located in Appendix C. The equation used for the x-axis in the electrical efficiency curves was the same equation used in the thermal efficiency curves. The equation for the x-axis allowed for a comparison to be made which included the inlet and ambient temperatures, as well as the available solar radiation at the angle of the panels. All of the factors played a vital role in the total performance for both the photovoltaic and the photovoltaic-thermal panels. Since the photovoltaic-thermal (PVT A1-3) and photovoltaic (PV D1-3) panels were tested simultaneously, the fluid inlet temperature of the photovoltaic-thermal panels were used for both series. This allowed for a comparison of electrical efficiencies to be made between the prototype photovoltaic-thermal panels (PVT A1-3) and the same panels (PV D1-3) with no modifications. Figure 3.18 shows the photovoltaicthermal data as solid lines and the photovoltaic panels as dashed lines. Typically, the electrical efficiency would decrease as the temperature of the photovoltaic cell increases due to the material properties of the photovoltaic cells, but Graph 3.6 did not completely follow generalization. The graphs in Appendix C (Figure C.17, 19 and 21), which contain data points, gave a better depiction of the electrical performance for both photovoltaic-thermal and photovoltaic panels. At 0.5 gallon per

44

Figure 3.18. Setup #2 Electrical Efficiency Curves Summary at 0.5, 1.0 and 1.5 gpm

minute the linear regression provided the best fit representation of the data based on percentage of error. At zero on the x-axis (when inlet and ambient temperatures equaled) the photovoltaic-thermal panels (A1-3) and photovoltaic panels (PV D1-3) experienced electrical efficiencies of approximately 11.6% and 11.7% respectfully, which was approximately a 0.86% difference. At 1.0 and 1.5 gallon per minute the linear regressions are misrepresentative of the actual panel performance due to irregular data upon further examination and review. There was no identifiable cause for the fluctuation in the data. During much of the time the photovoltaic-thermal panels were being tested at 1.0 gallon per minute,

45

the electrical output (and subsequently the electrical efficiency) by the photovoltaic panels (PV D1-3) were higher than the electrical output of the photovoltaic-thermal panels (PVT A1-3). Only a small amount of the PV D1-3 electrical output data was lower than the PVT A1-3; however, the data collected established a linear trendline. Based on the linear regression, the electrical efficiencies of PVT A1-3 and PV D1-3 were both around 11.2% at x equals zero. The PV D1-3 efficiencies were approximately 3.1% higher than the efficiencies of PVT A1-3. At 1.5 gallon per minute, the linear regression for PVT A1-3 and PV D1-3 was characteristic of the data points with the PV D1-3 yielding a greater slope. The electrical efficiency for PV D1-3 was consistently 32.5% lower than PVT A1-3. Typically in residential photovoltaic systems, the panels are used to produce electricity, which is then used to heat water. Photovoltaic-thermal panels, however, use the excess solar radiation, which was not converted into power to heat water directly. The rise in temperature, also known as thermal gain, was converted into terms of electrical power. This allowed a direct comparison of the total system power output to be made for both the photovoltaic and photovoltaic-thermal panels. Figure 3.19 illustrates the difference in actual electrical output (shown as dashed lines), thermal gain in terms of power (shown as dash-dot lines) and the combined system power output (shown as solid lines) for 0.5, 1.0 and 1.5 gallon per minute flows. For the conversion between thermal gain and electrical power the specific heat of water (4186 J/kg·°C) was used along with the assumption that the conversion was ideal. This assumption made the total system output rather conservative since most electrical hot

46

water heaters were quite inefficient and would have required a large amount of power to create the same rise in water temperature. At 0.5 gallon per minute and at 1000 watt per square meter, the electrical output for the photovoltaic (PV D1-3) and photovoltaic-thermal (PVT A1-3) panels were approximately 365.8 and 388.2 watts respectively. Even without the addition of the thermal gain power, the PVT A1-3 out preformed the PV D1-3 (approximately 6.1% increase). The thermal gain power was approximately 562.9 watts (a 53.9% increase from PV D1-3) and based on linear regression produced a total system output of approximately 931.9 watts (a 154.7% increase for PV D1-3). At 1.0 gallon per minute and at 1000 watt per square meter, the electrical output for the photovoltaic (PV D1-3) and photovoltaic-thermal (PVT A1-3) panels were approximately 354.6 and 373.9 watts respectively. Even without the addition of the thermal gain power, the PVT A1-3 out preformed the PV D1-3 (approximately 5.4% increase). The thermal gain power was approximately 897.7 watts (a 153.2% increase from PV D1-3) and based on linear regression produced a total system output of approximately 1281.2 watts (a 261.4% increase for PV D1-3). At 1.5 gallon per minute and at 1000 watt per square meter, the electrical output for the photovoltaic (PV D1-3) and photovoltaic-thermal (PVT A1-3) panels were approximately 294.0 and 366.8 watts respectively. Even without the addition of the thermal gain power, the PVT A1-3 out preformed the PV D1-3 (approximately 5.4% increase). The thermal gain power was approximately 1145.9 watts (a 289.8% increase

47

from PV D1-3) and based on linear regression produced a total system output of approximately 1496.8 watts (a 409.1% increase for PV D1-3).

Figure 3.19. Setup #2 Electrical Power and Power Due to Thermal Gain at 0.5, 1.0, and 1.5 gpm

48

4. MODELING 4.1. INTRODUCTION The modeling program used in this project was TRNSYS 16, which was a transient systems simulation program with a modular style structure. The modular structure allowed for customizable components to be used. The model component type used to simulate the photovoltaic-thermal panel was Type 250. The Type 250 was created by Michael Collins and team at the University of Waterloo in Waterloo, Canada from the original TRNSYS Type 50 component in the Trademark Electronic Search System (TESS) library. The Type 250 and Type 50 were photovoltaic-thermal panels which were cooled by forced liquid.

4.2. METHODOLOGY To model each photovoltaic-thermal panel as accurately as possible, the parameters were unique for each panel. The required parameters are listed below in Table 4.1. Equations came from ‘Solar Engineering of Thermal Processes’ by J. Duffie and W. Beckman (28) and ‘Photovoltaic Systems Engineering’ by R. Messenger and J. Ventre (29). The collector efficiency factor (Equation 4.1) referred to the efficiency of the thermal panel, which included the copper pipes, insulation and thermal sheeting or aluminum fin. To calculate the collector efficiency, first the fin efficiency of the thermal sheet or aluminum fin was determined (Equation 4.2). Next, the front, back and edge loss coefficients were determined using Equation 4.3-6. The loss coefficients

49

Table 4.1. TRNSYS Model Inputs and Parameters

Inputs

Parameters

Items Collector Area Collector Efficiency Factor Fluid Heat Capacity Number of Glass Covers KL Product Back and Edge Loss Coefficient PV Absorptivity PV Emissivity PV Efficiency PV Temperature Coefficient PV Ref Temperature PV Packing Factor Fluid Inlet Temperature Collector Specific Flowrate Collector Slope Columbia, MO Weather Data - Ambient Temperature - Wind Speed - Total Radiation on Horizontal - Total Radiation on Panel

required a consistent material thickness, or equivalent thickness, across the width of the panel cross section. To obtain these values, the actual material volume was divided by the panel back area (Equation 4.7). The KL Product referred to the thermal conductivity of the PV cells. One article written by Sark, Meijerink, Schropp, Roosmalen and Lysen, proposed the following comparison (Equation 4.8) between cell properties and the KL product of the photovoltaic cells (30). The PV emissivity was a unitless ratio of a surfaces ability to emit energy by radiation/heat. The PV absorptivity was a unitless ratio of the photovoltaic cell’s ability to absorb the solar radiation; that is to say that a

50

higher absorptivity ratio will result in higher cell efficiency. The PV efficiency was based on Equation 4.9, which is a ration of the voltage and current at the maximum power point divided by the area of the panel times an average solar radiation. The reference solar radiation for the photovoltaic panel, given in the BP 4175 specifications, was 1000 watts per square meter. The PV packing factor referred to the percentage of the photovoltaic panel which was covered by photovoltaic cells (Equation 4.10).

F′ =

1⁄UL W∙[

1 1 1 +C + ] π ∙ Di ∙ hfi UL ∙ [D + (W − D) ∙ F] b

Where: F’ = Thermal collector efficiency (%) UL = Collector overall loss coefficient (W/m2·°C) D = Outside diameter of pipes (m) W = Center-to-center pipe spacing (m) F = Fine efficiency (%) Cb = Bond conductance (assumed: ∞) Di = Inside pipe diameter (m) hfi = Heat transfer coefficient between fluid and tube wall (W/m2·°C)

F=

(4.1)

tanh[m ∙ (W − D)/2] m ∙ (W − D)/2

Where: F = Fin efficiency (%) 0.5 m = (𝑈𝐿 ⁄(𝑘 ∙ 𝛿)𝑝𝑙𝑎𝑡𝑒 ) k = Thermal conductivity of plate (W/m2·°C) δ = Thickness of plate (m) W = Center-to-center pipe spacing (m) D = Outside diameter of pipes (m)

(4.2)

51

UL = Ut + Ub + Ue Where: UL = Collector overall loss coefficient (W/m2·°C) Ut = Top loss coefficient (W/m2·°C) Ub = Bottom loss coefficient (W/m2·°C) Ue = Edge loss coefficient (W/m2·°C)

(4.3)

−1

Ut =

N

e+

(Tpm − Ta ) C ∙[ ] (N + f) (Tpm

1 hw

+ )

2 σ ∙ (Tpm + Ta ) ∙ (Tpm + Ta2 ) 2N + f − 1 + 0.133εp 1 + −N εg (εp + 0.00591 ∙ N ∙ hw )

Where: Ut = Top loss coefficient (W/m2·°C) N = Number of glass covers f = (1+0.089·hw – 0.1166·hw· p)(1+0.07866·N) C = 520(1 - 0.0000512·β2) for 0°< β