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Analysis of thermal and electrical performance of semi-transparent photovoltaic (PV) module
Energy 35 (2010) 2681–2687
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Analysis of thermal and electrical performance of semi-transparent photovoltaic (PV) module K.E. Park a, b, *, G.H. Kang b, H.I. Kim b, G.J. Yu b, J.T. Kim a a b
Department of Architectural Engineering, Kongju National University (KNU), P.O. Box 275, Budae-dong, Cheonan, Republic of Korea Photovoltaic Research Group, Korea Institute of Energy Research (KIER), P.O. Box 102, Gajeong-ro, Yuseong-gu, Daejeon, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2008 Received in revised form 7 July 2009 Accepted 11 July 2009 Available online 15 August 2009
Building-integrated PhotoVoltaic (BIPV) is one of the most fascinating PV application technologies these days. To apply PV modules in buildings, various factors should be considered, such as the installation angle and orientation of PV module, shading, and temperature. The temperature of PV modules that are attached to building surfaces especially is one of the most important factors, as it affects both the electrical efficiency of a PV module and the energy load in a building. This study investigates the electrical and thermal performance of a semi-transparent PV module that was designed as a glazing component. The study evaluates the effects of the PV module’s thermal characteristics on its electrical generation performance. The experiment was performed under both Standard Test Condition (STC) and outdoor conditions. The results showed that the power decreased about 0.48% (in STC with the exception of the temperature condition) and 0.52%(in outdoor conditions, under 500 W/m2) per the 1 C increase of the PV module temperature. It was also found that the property of the glass used for the module affected the PV module temperature followed by its electrical performance. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Building-Integrated PhotoVoltaic(BIPV) Semi-transparent PV module Temperature variation Electrical performance
1. Introduction Ever since the PhotoVoltaic (PV) industry started boom, the PV system has been advancing both in technological aspects and in economical aspects. Because of this trend, the installation capacity of various application types has been expanding worldwide. BIPV especially is becoming a popular option among the various PV application technologies. Recently, various PV modules are starting to be used in buildings. In particular, semi-transparent PV modules play multifunctional roles as electricity producers, building envelope components, and glazing components. In order for the BIPV systems to achieve multifunctional roles, various factors need to be taken into account, such as the PV’s module temperature, shading, installation angle, and orientation. Among these factors, the irradiance and PV module temperature should be regarded as one of the most important factors, since it affects both the electrical efficiency of the BIPV system and the energy performance of buildings where BIPV systems are installed. The results of basic studies regarding irradiance and energy output of PV system have been
* Corresponding author at: Department of Architectural Engineering, Kongju National University (KNU), P.O. Box 275, Budae-dong, Cheonan, Republic of Korea. Tel.: þ82 10 2478 3158; fax: þ82 42 860 3692. E-mail address: [email protected] (K.E. Park). 0360-5442/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2009.07.019
reported by some researchers [1–3], while there have been other studies regarding the temperature and generation performance of PV modules [4–6]. Some studies focused on the semi-transparent PV module that maintains space between the PV cells in order to transmit light. In particular, recent studies performed by Wong [7], Fung [8] and Boer [9] dealt with BIPV modules and their performance modelling using the measured data of the PV module. They also analyzed the total building energy consumption, which included heating, cooling, and artificial lighting loads. Li [10] also carried out the economical analysis on the application of PV modules in buildings. As the application potential of PV modules as building façade components has been increasing, there have been other studies about PV building façades. Vartiainen [11], Infield [12], Khedari [13] and Alzoubi [14] have studied ventilated PV façades, or PV sunscreens, as blinds. Most of the studies about BIPV modules considered the PV module as building envelope material that has typical module configuration. It is necessary to develop more diverse BIPV modules, like semi-transparent ones, with various configurations and designs. These developments will provide more options for architects and building industries on how to apply PVs in buildings. Based on this background, this paper aims to analyze the electrical characteristics of semi-transparent PV modules in relation to their temperature variations. The study analyzed the effects of solar radiation, ambient temperature, and glass used for the PV laminate
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K.E. Park et al. / Energy 35 (2010) 2681–2687
Nomenclature I sc Imp Voc Vmp Pmax FF
Short Circuit Current(A) Current at Maximum Power(A) Open Circuit Voltage(V) Voltage at Maximum Power(V) Maximum Power(W) Fill Factor(%)
on the temperature of the semi-transparent PV modules and their electrical performance. The experiment was conducted under the standard test conditions and in outdoor settings. Monitoring began in May 2007 and continued November 2008. Fig. 2. Results of the PV cell tests.
2. Semi-transparent PV module
2.2. PV cell
Modern buildings are constructed in various ways and finished with various materials, PV modules can be used as one of the special finishing materials for old buildings, as well as new ones. It is not difficult to find buildings where the whole envelope is covered with glass. A semi-transparent PV module, which is the subject of this paper, is suitable for those buildings. For this research, opaque crystalline PV cells were used for the semitransparent PV module that had a transparent area to allow for light penetration. This section will describe the module in detail.
Before manufacturing the PV module for the experiment, PV cell tests were performed. Fig. 2 shows the test results. The 5 5 inch polycrystalline Silicon (p-Si) PV cells had the electrical generation performance from the maximum power of 2.5 W to the minimum power of 2.3 W. The 500 cells also had the average output power of 2.4 W and a uniformity of 2.9%. They were sorted into categories based on the test results and were used for manufacturing the semi-transparent PV laminates.
2.3. Glass 2.1. Structure of semi-transparent PV module The semi-transparent PV module used in this study had a PV laminate with polycrystalline Silicon (p-Si) PV cells that were spaced so that some portion of light passed through the glass area. The PV module consisted of several layers: glass, encapsulation material (Ethylene Vinyl Acetate (EAV) Sheet), PV cells, encapsulation material (EVA Sheet), glass, air gap with spacer and glass. Fig. 1 shows the layers of the PV module. In order to apply a semitransparent PV module as glazing material for buildings, it should have a certain level of thermal resistance. For this reason, one more glass was used to form the air gap for the PV laminate.
Three glass sheets were used for this PV module as Fig. 1 shows. A low-iron tempered glass was used for the front cover and various glasses were used for the supporting back cover. The types of glass used included clear, coloured, and reflected. The last glass was also low iron tempered which was similar to the first glass. 2.4. Arrangement and connection of PV cells The prototype PV module used in this study consisted of 32 p-Si cells that were connected serially. This PV module includes bypass diodes that protected the PV module from the hot spot caused by
Fig. 1. Layer structure of the PV module.
K.E. Park et al. / Energy 35 (2010) 2681–2687
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Fig. 3. Configuration of the circuit of the PV module.
partial shading and other partial damages. Fig. 3 shows the circuit of the semi-transparent PV module. 3. Description of experiment
that is better than 1% on the test field of 3 m 3 m, and its tighter control of spectral matches, has a better performance of at least by a factor of 2 against International Electrotechnical Commission (IEC) class A specifications [18].
3.1. Temperature sensor setting
3.3. Installation
In order to analyze the heat transfer and temperature variation between the layers in more detail, we installed temperature sensors in each layer (front side, back side, air gap, and inside) as shown in Fig. 4. In order to measure each layer’s temperature, thermocouples (Type T) for each surface and Resistance Temperature Detector (RTD) for air gap were used.
PV modules were applied as glazing components in a sunroom and covered balcony, as Fig. 5 shows. The PV modules were installed on the top and in the front as glazing materials. Two pyranometers were installed at the same plane in order to measure global radiation. In this paper, the vertically installed PV modules were only studied, and all of the modules faced south.
3.2. Equipment
4. Results
The solar simulator used in this experiment was Pasan IIIb (Balval, Switzerland), which has been previously used in domestic certificate tests of PV modules in Korea. This solar simulator, with its light pulse duration of 10 ms that can stabilize to better than 1%, its uniformity
4.1. Initial electrical performance of the PV modules in standard test conditions In this experiment, various glass materials, such as normal clear, green (normal and reflection), blue (reflection), and bronze glass were used as rear materials for the PV module. The electrical characteristics of these modules, such as current, voltage and power were similar under the standard test conditions (STC) of 1000 W/m2 solar irradiance and 25 C PV module temperature. Fig. 6 shows the initial I-V curve and the temperature difference of the PV module that had coloured glass. According to these results, we found that the color of the glass on the backside had little effect on the electrical generation of the PV module under STC. We also confirmed the output power uniformity of the manufactured PV modules that were used for this study [15]. 4.2. Results of the temperature measurement of the PV module in outdoor conditions
Fig. 4. Position of temperature sensors.
4.2.1. Temperature variation of a standard PV module In this section, we will show the results of the experiment performed outdoors. First, we analyzed the temperature variation of a standard PV module that had clear glass on the front cover.
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Fig. 5. Overview of PV module installation in a sunroom.
The monitoring of the PV modules’ temperature started in May 2007. Fig. 7 shows the monitoring results of irradiance, outdoor temperature, and temperature variation of semi-transparent PV modules during spring (May) and during summer (August). The temperature variation of each layer appeared to be similar. As time passed, however, the difference of temperature in each layer had increased: the largest difference in temperature appeared between noon and 3 p.m. After that time to sunset, the difference in temperature decreased, while after sunset the temperature of all layers became similar. The air temperature of air space was the highest at all the time, followed by the temperature of the PV cell’s supporting glass. In this study, we assumed that the surface temperature of the PV cell supporting glass is regarded as the temperature of
PV module which has an effect on the electrical generation of the PV module. As the air gap did not contact the PV cell directly, the glass layer that maintained the air gap from the PV laminate seemed to be influenced more by the indoor environments, such as the indoor air temperature, and heating or cooling sources. The variation pattern of the layers’ temperature appeared to be similar at all seasons. The temperature variation range varied according to the outdoor temperature, irradiance gain, generation level, and characteristics of materials. 4.2.2. Analysis of module temperature In order to analyze the variation of the PV module temperature according to the PV cell supporting glass type, two modules were
Fig. 6. I-V curve of modules with various glasses under STC.
K.E. Park et al. / Energy 35 (2010) 2681–2687
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Fig. 7. Temperature variation of PV module during one particular day (May 2007).
compared. One was the module with clear glass and the other was the module with bronze glass. Fig. 8 shows the variation of ambient temperature that occurs during a typical day in summer and winter. For this research, we selected the days near the summer solstice and the winter solstice for monitoring. As shown in the figure, the average ambient temperature measured during the summer was about 21.1 C and was about 6.4 C in winter. But, the measured module temperature appeared to be the opposite of the ambient temperature. The PV module temperature in the winter was higher than in the summer. Fig. 9 shows the temperature variation of the semi-transparent PV module. It is believed that the variation was related to the seasonal difference in the radiation gains of the PV modules rather than the ambient temperature. As the PV module was applied on the vertical surface, the amount of solar radiation on the PV module differed according to the season. The module gained less radiation in the summer when the altitude of the sun was the highest. In addition, the summer rainy season in Korea could be one of many factors that decreased the radiation gain in summer. As Fig. 10(a) shows the variation of daily solar radiation that was measured during the monitoring period, there was a certain difference of solar radiation gains between the winter and summer. Fig. 10(b) shows the solar radiation measured on the vertical surfaces on the same day, with Fig. 9 showing the PV module temperatures being measured on a day in the winter and another day in the summer. The results show that the solar radiation of a clear day in winter was larger than in summer.
Fig. 8. Comparison of ambient temperature.
Fig. 9. Comparison of PV module temperature (a) Winter (December 2007) (b) Summer (June 2008).
From Fig. 9, we could see that the glass type that supports the PV cells also affects the PV module temperature. During a winter day, for instance, the PV module with the bronze glass has a higher temperature compared to the module with clear glass. This is due to the fact that the solar radiation that reaches the PV module was absorbed more by the bronze glass than clear glass. These effects on the temperature can also be explained by the different properties of the glasses, such as solar heat gain and light transmittance of the glass, as shown in Table 1. It is obvious that the bronze glass’ absorption of solar energy was much higher than of the clear glass. With an absorption rate of 49%, the bronze glass will contribute to raise the temperature of the PV module when exposed to solar radiation for a certain period of time. On the other hand, the clear glass used at the back side of the PV module will provide more daylight inside the building as its light transmittance is 92 %, compared to the bronze glass’ rate of 43%. 4.2.3. Analysis of PV electrical generation In this section, we analyze the characteristics of the power generation by the PV module according to the variations of the PV module temperature. Fig. 11 shows the difference of electrical performance of those PV modules according to season and glass type. It indicates that the PV module produced more electricity in the winter than in the summer, and the PV module with clear glass outperformed the one made of bronze glass. This fact indicates that the electrical performance of PV modules is related to their temperature variation: a PV module that has a higher temperature can decrease its electricity generation. In our study, the PV module with bronze glass
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Fig. 10. Daily radiation (a) Daily radiation during monitoring period. (b) Comparison of irradiance in winter and summer.
appeared to have a higher temperature than the module with clear glass. In order to analyze the electrical characteristics of the semitransparent PV module according to the variation of the module temperature, its electrical generation was measured in both STC and outdoor settings. The experiments were performed with a prototype of a semi-transparent PV module with clear glass. Fig. 12 shows the measuring results in STC, with the exception of the PV module temperature that changed from 25 C (at the standard test conditions) to 65 C. The results show that the voltage reduced about 0.49% per 1 C increase of the PV module temperature and the current raised about 0.01% per its 1 C increase. The results also show that the PV module temperature affected its electrical performance: about 0.48% of power generation decreased as of the PV module temperature increase 1 C. Fig. 13 shows other experimental results with the same module measured in outdoor settings. We analyzed the data measured in 500 W/m2 of irradiance, as it was difficult to satisfy the STC in outdoor conditions due to the fact that the PV module was installed
Fig. 11. Comparison of output of PV module (a) Winter (December 2007) (b) Summer (June 2008).
on a vertical surface. As shown in Fig. 13, the characteristics of the voltage and current from the PV module seem to be similar to the results in STC. The power loss was also similar to the results in STC. The power decreased about 0.52% per 1 C increase of the PV module temperature. These results are in the power reduction range of about 0.4 w 0.6% per 1 C which was defined by other studies [16,17]. For the other types of semi-transparent modules with bronze module, the relation of the PV module temperature and its electrical performance was found to be similar when the coloured glass was used at the backside of PV cells.
Table 1 Properties of glass.
Visible Spectrum Solar radiation
Transmittance (%) Reflectance (%) Transmittance (%) Reflectance (%) Absorptance (%)
Clear
Bronze
92 8 90 8 2
43 5 46 5 49
Fig. 12. Electrical performance of PV module with temperature variation in STC.
K.E. Park et al. / Energy 35 (2010) 2681–2687
Fig. 13. Electrical performance of PV module with temperature variation in outdoor settings.
5. Conclusion This paper reported on an experimental investigation on the electrical generation of a semi-transparent PV module that can be used for glazing in buildings. The analysis was performed with the results being obtained from data measured under standard test condition and in outdoor conditions. The temperature of an air gap among layers of the PV module was the highest and was followed by that of the PV cell supporting glass. With the assumption that the surface temperature of the backside glass is equivalent to the PV module temperature, the temperature of the PV module rose by 55 C on clear day. This study shows that the temperature of the PV module influences its electrical generation. Thus, we confirm that the power generation of PV module decreased about 0.48% per 1 C increase in the indoor test(standard test conditions except temperature) and decreased approximately 0.52% per 1 C increase in the outdoor test (under 500 W/m2 of irradiance). In this study, it was found that under the standard test conditions, the type of glass supporting PV cells at the semi-transparent module hardly affected the PV module temperature or the generation performance of the PV module. On the other hand, the characteristics of the glass influenced solar heat gain and heat transfer on the layer that could in turn determine the PV module temperature. As the module temperature is related to the PV’s electrical performance, it is important to consider the proper glass type for this kind of BIPV module. One should also consider the design of the PV modules and buildings that will receive with the modules, because it affects the amount of daylight gained and the amount of heat gained in interior space, as well as its electrical performance. It is also necessary to analyze the effects of semi-transparent PV modules on thermal and visual environments in a building and on the overall energy performance of a whole building. This is due to the fact that the light transmission characteristics, as well as the temperature variation of the PV modules, influence not only its electrical generation, but also the building’s cooling and heating loads. References [1] Rahman S, Khallat MA, Salameh ZM. Characterization of insolation data for use in photovoltaic system analysis models. Energy 1988;13(1):63–72.
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[2] Celik AN. Long-term energy output estimation for photovoltaic energy systems using synthetic solar irradiation data. Energy 2003;28(5):479–93. [3] E.W. Smiley, Low irradiance performance modelling for building integrated photovoltaics, In: 17th European PV solar energy conference, Munich, Germany; 2001. [4] Mattei M, Notton G, Cristofari C, Muselli M, Poggi P. Calculation of the polycrystalline PV module temperature using a simple method of energy balance. Renewable Energy 2006;31:553–67. [5] Carr AJ, Pryor TL. A comparison of the performance of different PV module types in temperate climates. Solar Energy 2004;76:285–94. [6] Chenni R, Makhlouf M, Kerbache T, Bouzid A. A detailed modeling method for photovoltaic cells. Energy 2007;32(9):1724–30. [7] Wong PW, Shimoda Y, Nonaka M, Inoue M, Mizuno M. Semi-transparent PV: thermal performance, power generation, daylight modelling and energy saving potential in a residential application. Renewable Energy 2008;33: 1024–36. [8] Fung TYY, Yang H. Study on thermal performance of semi-transparent building-integrated photovoltaic glazings. Energy and Buildings 2008;40:341–50. [9] B.J. de Boer, W.G.J. van Helden. PV MOBI-PV modules optimized for building integration, In: 9th International conference on solar energy in high latitudes. Northsun Leiden, Netherlands; 2001. [10] Li DHW, Lam TNT, Chan WWH, Mak AHL. Energy and cost analysis of semi-transparent photovoltaic in office buildings. Applied Energy 2009;86(5): 722–9. [11] Varitiainen E. Daylight modelling and optimization of solar facades, doctoral dissertation. Finland: Helsinki University of Technology; 2001. [12] Infield D, Eicker U, Fux V, Mei L, Schumacher J. A simplified approach to thermal performance calculation for building integrated mechanically ventilated PV facades. Building and Environment 2006;41:893–901. [13] Khedari J, Waewsak J, Supheng W, Hirunlabh J. Experimental investigation of performance of a multi-purpose PV-slat window. Solar Energy Materials & Solar Cells 2004;82:431–45. [14] Alzoubi HH. Optimizing lighting, thermal performance, and energy production of building facades by using automated blinds and PV cells. U.S.: University of Michigan, Doctoral Dissertation, University of Michigan; 2005. [15] G.J. Yu, G.H.Kang, K.E.Park, H.I.Kim. Development of BIPV modules and manufacturing technology for environmental friendly building, report no. 2004-N-PV12-P-06, Seoul, Korea: Ministry of Commerce, Industry and Energy (MOCIE), 2007. [16] Skoplaki E, Palyvos JA. On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations. Solar Energy 2009;83(5):614–24. [17] The German Energy Society. Planning and installing photovoltaic systems: a guide for installers architects and engineers. 2nd edition. London: Earthscan; 2008. [18] The Pasan. Specifications of Pasan sun simulator b, Products. See also:. Neuchaˆtel: Pasan http://www.pasan.ch/website/en/products/sun-simulators/ pasan-sun-simulator-iiic/; 2008.
Further reading [1] Aschehoug O, Bell D. BP solar skin: a façade concept for a sustainable future, report no. STF22 A03510, NO-7465 trondheim. Norway: SINTEF Civil and Environmental Engineering; 2003. [2] F.P. Baumgartner, D. Sutter. The solar roof window, In: 14th European Photovoltaic Solar Energy Conference, Barcelona, Spain; 1997, p. 1883–6. [3] Miyazaki T, Akisawa A, Kashiwagi T. Energy saving of office buildings by the use of semi-transparent solar cells for windows. Renewable Energy 2005;30: 281–304. [4] Kim JT, Lee KL, Oh MT, Park KE, Kim JH. The performance and energy saving effect of a 2 kW proof-integrated photovoltaic system. Solar Energy (Korea) 2006;26(1):13–9. [5] K.E. Park, G.H. Kang, J.H. So, G.J. Yu, H.I. Kim, K.S. Kim, et-al. The shading state and characteristic analysis of building integrated photovoltaic, In: 21st European Photovoltaic Solar Energy Conference Dresden, Germany; 2006, p. 210–2. [6] K.E. Park, J.T. Kim, G.H. Kang, H.I. Kim, G.J. Yu. Comparison analysis of temperature variation and generation characteristics according to structural elements of BIPV modules, In: Conference of Korean Institute of Architectural Environment and Building Systems, Seoul, Korea; 2007, p. 13–9. [7] G.H. Kang, K.E. Park, H.I. Kim, G.J. Yu, J.T. Kim. Analysis of temperature and power generation characteristic of building integrated photovoltaic semitransparent module, In: 22nd European Photovoltaic Solar Energy Conference, Milan, Italy, 2007, p.3334–6.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Analysis of thermal and electrical performance of semi-transparent photovoltaic (PV) module K.E. Park a, b, *, G.H. Kang b, H.I. Kim b, G.J. Yu b, J.T. Kim a a b
Department of Architectural Engineering, Kongju National University (KNU), P.O. Box 275, Budae-dong, Cheonan, Republic of Korea Photovoltaic Research Group, Korea Institute of Energy Research (KIER), P.O. Box 102, Gajeong-ro, Yuseong-gu, Daejeon, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2008 Received in revised form 7 July 2009 Accepted 11 July 2009 Available online 15 August 2009
Building-integrated PhotoVoltaic (BIPV) is one of the most fascinating PV application technologies these days. To apply PV modules in buildings, various factors should be considered, such as the installation angle and orientation of PV module, shading, and temperature. The temperature of PV modules that are attached to building surfaces especially is one of the most important factors, as it affects both the electrical efficiency of a PV module and the energy load in a building. This study investigates the electrical and thermal performance of a semi-transparent PV module that was designed as a glazing component. The study evaluates the effects of the PV module’s thermal characteristics on its electrical generation performance. The experiment was performed under both Standard Test Condition (STC) and outdoor conditions. The results showed that the power decreased about 0.48% (in STC with the exception of the temperature condition) and 0.52%(in outdoor conditions, under 500 W/m2) per the 1 C increase of the PV module temperature. It was also found that the property of the glass used for the module affected the PV module temperature followed by its electrical performance. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Building-Integrated PhotoVoltaic(BIPV) Semi-transparent PV module Temperature variation Electrical performance
1. Introduction Ever since the PhotoVoltaic (PV) industry started boom, the PV system has been advancing both in technological aspects and in economical aspects. Because of this trend, the installation capacity of various application types has been expanding worldwide. BIPV especially is becoming a popular option among the various PV application technologies. Recently, various PV modules are starting to be used in buildings. In particular, semi-transparent PV modules play multifunctional roles as electricity producers, building envelope components, and glazing components. In order for the BIPV systems to achieve multifunctional roles, various factors need to be taken into account, such as the PV’s module temperature, shading, installation angle, and orientation. Among these factors, the irradiance and PV module temperature should be regarded as one of the most important factors, since it affects both the electrical efficiency of the BIPV system and the energy performance of buildings where BIPV systems are installed. The results of basic studies regarding irradiance and energy output of PV system have been
* Corresponding author at: Department of Architectural Engineering, Kongju National University (KNU), P.O. Box 275, Budae-dong, Cheonan, Republic of Korea. Tel.: þ82 10 2478 3158; fax: þ82 42 860 3692. E-mail address: [email protected] (K.E. Park). 0360-5442/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2009.07.019
reported by some researchers [1–3], while there have been other studies regarding the temperature and generation performance of PV modules [4–6]. Some studies focused on the semi-transparent PV module that maintains space between the PV cells in order to transmit light. In particular, recent studies performed by Wong [7], Fung [8] and Boer [9] dealt with BIPV modules and their performance modelling using the measured data of the PV module. They also analyzed the total building energy consumption, which included heating, cooling, and artificial lighting loads. Li [10] also carried out the economical analysis on the application of PV modules in buildings. As the application potential of PV modules as building façade components has been increasing, there have been other studies about PV building façades. Vartiainen [11], Infield [12], Khedari [13] and Alzoubi [14] have studied ventilated PV façades, or PV sunscreens, as blinds. Most of the studies about BIPV modules considered the PV module as building envelope material that has typical module configuration. It is necessary to develop more diverse BIPV modules, like semi-transparent ones, with various configurations and designs. These developments will provide more options for architects and building industries on how to apply PVs in buildings. Based on this background, this paper aims to analyze the electrical characteristics of semi-transparent PV modules in relation to their temperature variations. The study analyzed the effects of solar radiation, ambient temperature, and glass used for the PV laminate
2682
K.E. Park et al. / Energy 35 (2010) 2681–2687
Nomenclature I sc Imp Voc Vmp Pmax FF
Short Circuit Current(A) Current at Maximum Power(A) Open Circuit Voltage(V) Voltage at Maximum Power(V) Maximum Power(W) Fill Factor(%)
on the temperature of the semi-transparent PV modules and their electrical performance. The experiment was conducted under the standard test conditions and in outdoor settings. Monitoring began in May 2007 and continued November 2008. Fig. 2. Results of the PV cell tests.
2. Semi-transparent PV module
2.2. PV cell
Modern buildings are constructed in various ways and finished with various materials, PV modules can be used as one of the special finishing materials for old buildings, as well as new ones. It is not difficult to find buildings where the whole envelope is covered with glass. A semi-transparent PV module, which is the subject of this paper, is suitable for those buildings. For this research, opaque crystalline PV cells were used for the semitransparent PV module that had a transparent area to allow for light penetration. This section will describe the module in detail.
Before manufacturing the PV module for the experiment, PV cell tests were performed. Fig. 2 shows the test results. The 5 5 inch polycrystalline Silicon (p-Si) PV cells had the electrical generation performance from the maximum power of 2.5 W to the minimum power of 2.3 W. The 500 cells also had the average output power of 2.4 W and a uniformity of 2.9%. They were sorted into categories based on the test results and were used for manufacturing the semi-transparent PV laminates.
2.3. Glass 2.1. Structure of semi-transparent PV module The semi-transparent PV module used in this study had a PV laminate with polycrystalline Silicon (p-Si) PV cells that were spaced so that some portion of light passed through the glass area. The PV module consisted of several layers: glass, encapsulation material (Ethylene Vinyl Acetate (EAV) Sheet), PV cells, encapsulation material (EVA Sheet), glass, air gap with spacer and glass. Fig. 1 shows the layers of the PV module. In order to apply a semitransparent PV module as glazing material for buildings, it should have a certain level of thermal resistance. For this reason, one more glass was used to form the air gap for the PV laminate.
Three glass sheets were used for this PV module as Fig. 1 shows. A low-iron tempered glass was used for the front cover and various glasses were used for the supporting back cover. The types of glass used included clear, coloured, and reflected. The last glass was also low iron tempered which was similar to the first glass. 2.4. Arrangement and connection of PV cells The prototype PV module used in this study consisted of 32 p-Si cells that were connected serially. This PV module includes bypass diodes that protected the PV module from the hot spot caused by
Fig. 1. Layer structure of the PV module.
K.E. Park et al. / Energy 35 (2010) 2681–2687
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Fig. 3. Configuration of the circuit of the PV module.
partial shading and other partial damages. Fig. 3 shows the circuit of the semi-transparent PV module. 3. Description of experiment
that is better than 1% on the test field of 3 m 3 m, and its tighter control of spectral matches, has a better performance of at least by a factor of 2 against International Electrotechnical Commission (IEC) class A specifications [18].
3.1. Temperature sensor setting
3.3. Installation
In order to analyze the heat transfer and temperature variation between the layers in more detail, we installed temperature sensors in each layer (front side, back side, air gap, and inside) as shown in Fig. 4. In order to measure each layer’s temperature, thermocouples (Type T) for each surface and Resistance Temperature Detector (RTD) for air gap were used.
PV modules were applied as glazing components in a sunroom and covered balcony, as Fig. 5 shows. The PV modules were installed on the top and in the front as glazing materials. Two pyranometers were installed at the same plane in order to measure global radiation. In this paper, the vertically installed PV modules were only studied, and all of the modules faced south.
3.2. Equipment
4. Results
The solar simulator used in this experiment was Pasan IIIb (Balval, Switzerland), which has been previously used in domestic certificate tests of PV modules in Korea. This solar simulator, with its light pulse duration of 10 ms that can stabilize to better than 1%, its uniformity
4.1. Initial electrical performance of the PV modules in standard test conditions In this experiment, various glass materials, such as normal clear, green (normal and reflection), blue (reflection), and bronze glass were used as rear materials for the PV module. The electrical characteristics of these modules, such as current, voltage and power were similar under the standard test conditions (STC) of 1000 W/m2 solar irradiance and 25 C PV module temperature. Fig. 6 shows the initial I-V curve and the temperature difference of the PV module that had coloured glass. According to these results, we found that the color of the glass on the backside had little effect on the electrical generation of the PV module under STC. We also confirmed the output power uniformity of the manufactured PV modules that were used for this study [15]. 4.2. Results of the temperature measurement of the PV module in outdoor conditions
Fig. 4. Position of temperature sensors.
4.2.1. Temperature variation of a standard PV module In this section, we will show the results of the experiment performed outdoors. First, we analyzed the temperature variation of a standard PV module that had clear glass on the front cover.
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Fig. 5. Overview of PV module installation in a sunroom.
The monitoring of the PV modules’ temperature started in May 2007. Fig. 7 shows the monitoring results of irradiance, outdoor temperature, and temperature variation of semi-transparent PV modules during spring (May) and during summer (August). The temperature variation of each layer appeared to be similar. As time passed, however, the difference of temperature in each layer had increased: the largest difference in temperature appeared between noon and 3 p.m. After that time to sunset, the difference in temperature decreased, while after sunset the temperature of all layers became similar. The air temperature of air space was the highest at all the time, followed by the temperature of the PV cell’s supporting glass. In this study, we assumed that the surface temperature of the PV cell supporting glass is regarded as the temperature of
PV module which has an effect on the electrical generation of the PV module. As the air gap did not contact the PV cell directly, the glass layer that maintained the air gap from the PV laminate seemed to be influenced more by the indoor environments, such as the indoor air temperature, and heating or cooling sources. The variation pattern of the layers’ temperature appeared to be similar at all seasons. The temperature variation range varied according to the outdoor temperature, irradiance gain, generation level, and characteristics of materials. 4.2.2. Analysis of module temperature In order to analyze the variation of the PV module temperature according to the PV cell supporting glass type, two modules were
Fig. 6. I-V curve of modules with various glasses under STC.
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Fig. 7. Temperature variation of PV module during one particular day (May 2007).
compared. One was the module with clear glass and the other was the module with bronze glass. Fig. 8 shows the variation of ambient temperature that occurs during a typical day in summer and winter. For this research, we selected the days near the summer solstice and the winter solstice for monitoring. As shown in the figure, the average ambient temperature measured during the summer was about 21.1 C and was about 6.4 C in winter. But, the measured module temperature appeared to be the opposite of the ambient temperature. The PV module temperature in the winter was higher than in the summer. Fig. 9 shows the temperature variation of the semi-transparent PV module. It is believed that the variation was related to the seasonal difference in the radiation gains of the PV modules rather than the ambient temperature. As the PV module was applied on the vertical surface, the amount of solar radiation on the PV module differed according to the season. The module gained less radiation in the summer when the altitude of the sun was the highest. In addition, the summer rainy season in Korea could be one of many factors that decreased the radiation gain in summer. As Fig. 10(a) shows the variation of daily solar radiation that was measured during the monitoring period, there was a certain difference of solar radiation gains between the winter and summer. Fig. 10(b) shows the solar radiation measured on the vertical surfaces on the same day, with Fig. 9 showing the PV module temperatures being measured on a day in the winter and another day in the summer. The results show that the solar radiation of a clear day in winter was larger than in summer.
Fig. 8. Comparison of ambient temperature.
Fig. 9. Comparison of PV module temperature (a) Winter (December 2007) (b) Summer (June 2008).
From Fig. 9, we could see that the glass type that supports the PV cells also affects the PV module temperature. During a winter day, for instance, the PV module with the bronze glass has a higher temperature compared to the module with clear glass. This is due to the fact that the solar radiation that reaches the PV module was absorbed more by the bronze glass than clear glass. These effects on the temperature can also be explained by the different properties of the glasses, such as solar heat gain and light transmittance of the glass, as shown in Table 1. It is obvious that the bronze glass’ absorption of solar energy was much higher than of the clear glass. With an absorption rate of 49%, the bronze glass will contribute to raise the temperature of the PV module when exposed to solar radiation for a certain period of time. On the other hand, the clear glass used at the back side of the PV module will provide more daylight inside the building as its light transmittance is 92 %, compared to the bronze glass’ rate of 43%. 4.2.3. Analysis of PV electrical generation In this section, we analyze the characteristics of the power generation by the PV module according to the variations of the PV module temperature. Fig. 11 shows the difference of electrical performance of those PV modules according to season and glass type. It indicates that the PV module produced more electricity in the winter than in the summer, and the PV module with clear glass outperformed the one made of bronze glass. This fact indicates that the electrical performance of PV modules is related to their temperature variation: a PV module that has a higher temperature can decrease its electricity generation. In our study, the PV module with bronze glass
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Fig. 10. Daily radiation (a) Daily radiation during monitoring period. (b) Comparison of irradiance in winter and summer.
appeared to have a higher temperature than the module with clear glass. In order to analyze the electrical characteristics of the semitransparent PV module according to the variation of the module temperature, its electrical generation was measured in both STC and outdoor settings. The experiments were performed with a prototype of a semi-transparent PV module with clear glass. Fig. 12 shows the measuring results in STC, with the exception of the PV module temperature that changed from 25 C (at the standard test conditions) to 65 C. The results show that the voltage reduced about 0.49% per 1 C increase of the PV module temperature and the current raised about 0.01% per its 1 C increase. The results also show that the PV module temperature affected its electrical performance: about 0.48% of power generation decreased as of the PV module temperature increase 1 C. Fig. 13 shows other experimental results with the same module measured in outdoor settings. We analyzed the data measured in 500 W/m2 of irradiance, as it was difficult to satisfy the STC in outdoor conditions due to the fact that the PV module was installed
Fig. 11. Comparison of output of PV module (a) Winter (December 2007) (b) Summer (June 2008).
on a vertical surface. As shown in Fig. 13, the characteristics of the voltage and current from the PV module seem to be similar to the results in STC. The power loss was also similar to the results in STC. The power decreased about 0.52% per 1 C increase of the PV module temperature. These results are in the power reduction range of about 0.4 w 0.6% per 1 C which was defined by other studies [16,17]. For the other types of semi-transparent modules with bronze module, the relation of the PV module temperature and its electrical performance was found to be similar when the coloured glass was used at the backside of PV cells.
Table 1 Properties of glass.
Visible Spectrum Solar radiation
Transmittance (%) Reflectance (%) Transmittance (%) Reflectance (%) Absorptance (%)
Clear
Bronze
92 8 90 8 2
43 5 46 5 49
Fig. 12. Electrical performance of PV module with temperature variation in STC.
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Fig. 13. Electrical performance of PV module with temperature variation in outdoor settings.
5. Conclusion This paper reported on an experimental investigation on the electrical generation of a semi-transparent PV module that can be used for glazing in buildings. The analysis was performed with the results being obtained from data measured under standard test condition and in outdoor conditions. The temperature of an air gap among layers of the PV module was the highest and was followed by that of the PV cell supporting glass. With the assumption that the surface temperature of the backside glass is equivalent to the PV module temperature, the temperature of the PV module rose by 55 C on clear day. This study shows that the temperature of the PV module influences its electrical generation. Thus, we confirm that the power generation of PV module decreased about 0.48% per 1 C increase in the indoor test(standard test conditions except temperature) and decreased approximately 0.52% per 1 C increase in the outdoor test (under 500 W/m2 of irradiance). In this study, it was found that under the standard test conditions, the type of glass supporting PV cells at the semi-transparent module hardly affected the PV module temperature or the generation performance of the PV module. On the other hand, the characteristics of the glass influenced solar heat gain and heat transfer on the layer that could in turn determine the PV module temperature. As the module temperature is related to the PV’s electrical performance, it is important to consider the proper glass type for this kind of BIPV module. One should also consider the design of the PV modules and buildings that will receive with the modules, because it affects the amount of daylight gained and the amount of heat gained in interior space, as well as its electrical performance. It is also necessary to analyze the effects of semi-transparent PV modules on thermal and visual environments in a building and on the overall energy performance of a whole building. This is due to the fact that the light transmission characteristics, as well as the temperature variation of the PV modules, influence not only its electrical generation, but also the building’s cooling and heating loads. References [1] Rahman S, Khallat MA, Salameh ZM. Characterization of insolation data for use in photovoltaic system analysis models. Energy 1988;13(1):63–72.
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[2] Celik AN. Long-term energy output estimation for photovoltaic energy systems using synthetic solar irradiation data. Energy 2003;28(5):479–93. [3] E.W. Smiley, Low irradiance performance modelling for building integrated photovoltaics, In: 17th European PV solar energy conference, Munich, Germany; 2001. [4] Mattei M, Notton G, Cristofari C, Muselli M, Poggi P. Calculation of the polycrystalline PV module temperature using a simple method of energy balance. Renewable Energy 2006;31:553–67. [5] Carr AJ, Pryor TL. A comparison of the performance of different PV module types in temperate climates. Solar Energy 2004;76:285–94. [6] Chenni R, Makhlouf M, Kerbache T, Bouzid A. A detailed modeling method for photovoltaic cells. Energy 2007;32(9):1724–30. [7] Wong PW, Shimoda Y, Nonaka M, Inoue M, Mizuno M. Semi-transparent PV: thermal performance, power generation, daylight modelling and energy saving potential in a residential application. Renewable Energy 2008;33: 1024–36. [8] Fung TYY, Yang H. Study on thermal performance of semi-transparent building-integrated photovoltaic glazings. Energy and Buildings 2008;40:341–50. [9] B.J. de Boer, W.G.J. van Helden. PV MOBI-PV modules optimized for building integration, In: 9th International conference on solar energy in high latitudes. Northsun Leiden, Netherlands; 2001. [10] Li DHW, Lam TNT, Chan WWH, Mak AHL. Energy and cost analysis of semi-transparent photovoltaic in office buildings. Applied Energy 2009;86(5): 722–9. [11] Varitiainen E. Daylight modelling and optimization of solar facades, doctoral dissertation. Finland: Helsinki University of Technology; 2001. [12] Infield D, Eicker U, Fux V, Mei L, Schumacher J. A simplified approach to thermal performance calculation for building integrated mechanically ventilated PV facades. Building and Environment 2006;41:893–901. [13] Khedari J, Waewsak J, Supheng W, Hirunlabh J. Experimental investigation of performance of a multi-purpose PV-slat window. Solar Energy Materials & Solar Cells 2004;82:431–45. [14] Alzoubi HH. Optimizing lighting, thermal performance, and energy production of building facades by using automated blinds and PV cells. U.S.: University of Michigan, Doctoral Dissertation, University of Michigan; 2005. [15] G.J. Yu, G.H.Kang, K.E.Park, H.I.Kim. Development of BIPV modules and manufacturing technology for environmental friendly building, report no. 2004-N-PV12-P-06, Seoul, Korea: Ministry of Commerce, Industry and Energy (MOCIE), 2007. [16] Skoplaki E, Palyvos JA. On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations. Solar Energy 2009;83(5):614–24. [17] The German Energy Society. Planning and installing photovoltaic systems: a guide for installers architects and engineers. 2nd edition. London: Earthscan; 2008. [18] The Pasan. Specifications of Pasan sun simulator b, Products. See also:. Neuchaˆtel: Pasan http://www.pasan.ch/website/en/products/sun-simulators/ pasan-sun-simulator-iiic/; 2008.
Further reading [1] Aschehoug O, Bell D. BP solar skin: a façade concept for a sustainable future, report no. STF22 A03510, NO-7465 trondheim. Norway: SINTEF Civil and Environmental Engineering; 2003. [2] F.P. Baumgartner, D. Sutter. The solar roof window, In: 14th European Photovoltaic Solar Energy Conference, Barcelona, Spain; 1997, p. 1883–6. [3] Miyazaki T, Akisawa A, Kashiwagi T. Energy saving of office buildings by the use of semi-transparent solar cells for windows. Renewable Energy 2005;30: 281–304. [4] Kim JT, Lee KL, Oh MT, Park KE, Kim JH. The performance and energy saving effect of a 2 kW proof-integrated photovoltaic system. Solar Energy (Korea) 2006;26(1):13–9. [5] K.E. Park, G.H. Kang, J.H. So, G.J. Yu, H.I. Kim, K.S. Kim, et-al. The shading state and characteristic analysis of building integrated photovoltaic, In: 21st European Photovoltaic Solar Energy Conference Dresden, Germany; 2006, p. 210–2. [6] K.E. Park, J.T. Kim, G.H. Kang, H.I. Kim, G.J. Yu. Comparison analysis of temperature variation and generation characteristics according to structural elements of BIPV modules, In: Conference of Korean Institute of Architectural Environment and Building Systems, Seoul, Korea; 2007, p. 13–9. [7] G.H. Kang, K.E. Park, H.I. Kim, G.J. Yu, J.T. Kim. Analysis of temperature and power generation characteristic of building integrated photovoltaic semitransparent module, In: 22nd European Photovoltaic Solar Energy Conference, Milan, Italy, 2007, p.3334–6.