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Design, development and performance monitoring of a photovoltaic-thermal (PVT) air collector
ARTICLE IN PRESS
Renewable Energy 33 (2008) 914–927 www.elsevier.com/locate/renene
Design, development and performance monitoring of a photovoltaic-thermal (PVT) air collector Niccolo` Aste, Giancarlo Chiesa, Francesco Verri Department of Building Environment Sciences and Technology (BEST), Politecnico di Milano, Via Garofalo 39, 20097 Milan, Italy Received 23 January 2007; accepted 13 June 2007 Available online 14 September 2007
Abstract The photovoltaic-thermal (PVT) systems allow the enhancement of the energy performance of photovoltaics, by removing thermal energy and subsequently decreasing the operating temperature of the cells. The possibility of the utilization of heat for climatization makes them attractive for the building integration. In order to diffuse this kind of solar systems it is necessary to translate the basic concepts into efficient and functional technological components and associated performance should be evaluated in a reliable manner. This paper presents the experimental and theoretical results of a research and development program carried out at the Politecnico di Milano on the design, development and performance monitoring of a hybrid PVT air collector. One of the main products of the research consists of a simulation model for performance prediction of the system. This R&D program led to the development of the TIS (tetto integrale solarizzato, i.e. integrated solar roof), an innovative technological system for building integration of hybrid PVT air collectors. The successful commercial application of the TIS in a research center building is also shown as a case study. r 2007 Elsevier Ltd. All rights reserved. Keywords: Photovoltaic-thermal air collector; TIS-integrated solar roof; Simulation model; Measured data; Thermal and electrical performance; Case study
1. Introduction In recent years, building integration of photovoltaic modules has become more and more popular in the industrialized part of the world, where national support programs have accelerated the installation and dissemination of grid-connected PV systems [1]. For these systems, a number of studies related to the utilization of waste heat from photovoltaic modules exist and are well documented in the literature [2]. It is well known that the hybrid photovoltaic-thermal (PVT) systems allow better energy performances of the PV technology. In fact, the heat accumulated in the solar cells is recovered in the form of low temperature thermal energy, resulting improvements in the electrical conversion efficiency of PV modules. Over the last few years, different PVT systems, based on air and water as heat carrying fluid, have been studied, developed and reported in the literature Corresponding author. Tel.: +39 02 2399 9466; fax: +39 02 2399 9467.
E-mail address: [email protected] (N. Aste). 0960-1481/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2007.06.022
[3–6]. Among various PVT collectors, the hybrid air collectors are the most common, even if their applications remain yet to experimental level and the industrial products are relatively few. The studies conducted by different researchers on the hybrid air collectors are numerous and show the validity of the concept. However, most of the related studies published in the literature, are focused more on the thermal characterization of the components in comparison to the electrical one. There are a number of factors that influence the PV efficiency, and a detailed evaluation is necessary to predict accurately the combined (thermal as well as electrical) performance of PVT systems. The present work describes the research and development program carried out at the Politecnico di Milano on the design, development and performance monitoring of a hybrid PVT air collector. The results of this R&D program led to the development of the tetto integrale solarizzato, i.e. integrated solar roof (TIS), an innovative technological system for building integration of hybrid PVT air collectors.
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Nomenclature AMOD front module surface area exposed to the Sun, m2 CP volume air heat capacity at constant pressure, Wh m3 1C1 F sky view factor of collector hePV external radiative+convective heat transfer coefficient related to the PV sandwich, W m2 1C1 hV external convective heat transfer coefficient, W m2 1C1 hic convective heat transfer coefficient in the air gap, W m2 1C1 heG external radiative+convective heat transfer coefficient related to the glazed surface, W m2 1C1 hrG–P radiative heat transfer coefficient between the part of sandwich without PV cells and the absorber plate, W m2 1C1 hrG–sky external radiative heat transfer coefficient the part of sandwich without PV cells and the sky, W m2 1C1 hrPV–P radiative heat transfer coefficient between PV cells and absorber plate, W m2 1C1 hrPV–sky external radiative coefficient between PV cells and sky, W m2 1C1 It solar irradiance on the module, W m2 ka absorption correction factor of PV efficiency kg temperature correction factor of PV efficiency ky optical correction factor of PV efficiency kl spectrum correction factor of PV efficiency L thickness of the air gap, m mair volume air flow rate inside the gap, m3 h1 N sky cloud coverage in octaves Nu Nusselt number PMOD power generated by the module, W SG area of the glazed part of the sandwich without PV cells, m2 SP area of the absorber plate, m2 SPV area of the part of sandwich with embedded PV cells, m2
tb tPV
te tm tP tG tout tin TGP TPV–P TPV–sky Ub we aP aG aPVy aPVn eG eG–P eP ePV ePV–P g Za ZMOD Zn Z* lair s tGq tGn
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temperature of the rear adjacent ambient, 1C temperature of PV cells and of the part of sandwich with embedded the cells (considered the same), 1C ambient air temperature, 1C average temperature of the air in the gap, 1C temperature of the absorber plate, 1C temperature of the glazed part of the sandwich without PV cells, 1C collector outlet air temperature, 1C collector inlet air temperature, 1C average glass–absorber temperature, K average PV cells–absorber temperature, K average PV–sky temperature, K backside heat loss coefficient of the collector, W m2 1C1 wind velocity, m s1 solar absorbance of the absorber plate solar absorbance of the part of sandwich without PV cells solar absorbance of PV cells dependent on incident angle solar absorbance of PV cells at normal incident angle near infrared emissivity of the glass equivalent glass–absorber infrared emissivity near infrared emissivity of the absorber plate near infrared emissivity of the PV cells equivalent PV cells–absorber near infrared emissivity temperature power coefficient of the PV cells actual efficiency of the PV module PV module efficiency nominal efficiency of PV the module thermal–spectral efficiency of the PV module air thermal conductivity, W m1 1C1 Stefan–Boltzmann constant, W m2 K4 solar transmittance of the glass of the sandwich, dependent on incident angle solar transmittance of the glass of the sandwich, at normal incident angle
2. The TIS (integrated solar roof) project at the Politecnico di Milano
systems for building application. The program is completed in 2005 and is consisted of the following phases [7–10]:
In the framework of the research activities conducted at the Politecnico di Milano for the utilization of solar energy systems, in the year 2000 the Department of Energetica and Department of Building Environment Sciences and Technology started a program of research on the experimentation and development of a hybrid PVT air component. The program was supported and funded by the industry SeccoSistemi S.p.A. that has a long tradition in the production of solar energy
preliminary project on the component design with the variation in technology and dimension, based on the cooling requirements of the photovoltaic laminates to recover a part of thermal energy accumulated and consequently to increase the electrical conversion efficiency of PV; development of a simulation model for the prediction of energy performances through the elaboration of
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a number of algorithms capable to calculate the electrical and thermal productivity of the component as a function of inputs related to the climatic parameters and operating conditions; performance estimation for different solutions in terms of electrical and thermal efficiencies achievable from different system configurations; identification of better solution, resulted as a direct flow front cover PV collector; realization of an experimental prototype suitable for testing in the open-field conditions and to verify the functioning of simulation model; organization of an experimental campaign conducted during 2002–2004 for data acquisition on the prototype for the long-period operation in different climatic conditions; validation of the simulation model through the comparison between the experimental monitored data with the calculated one and the refinement of some model parameters; performance estimation in different design and operational conditions using the improved simulation model; designing of commercial product on the basis of performance optimization.
It should be mentioned that the program, in reality, had not a logical course but passed through different moments of interaction among various phases, according to the process learning by doing that led to the necessity to verify and update the results obtained each time. 3. Design of the PVT collector The hybrid collector has been developed as an ‘‘upgrade’’ of a conventional solar air collector, i.e. a front cover direct flow PV/T collector, organized in a modular structure.
The module has been designed in order to be integrated in common sloped roofs or vertical facades, replacing the external covering, waterproof and insulation layers. The schematics of the PVT collector are shown in Fig. 1. The upper cover is represented by a glass sandwich that includes PV cells. The cell area can cover the entire glazed surface or can be distributed in a grid where the spacing between adjacent columns and rows can allow a direct gain of solar radiation to the backward absorber plate. The glass sandwich looks like a chessboard composed of squares with or without PV cells embedded. Different configurations of PVT collector can be created changing the cell area density in order to balance electricity and thermal energy output of the system. The airflow into the gap (between the sandwich and the absorber plate) can be achieved by the forced flow using a fan or natural flow through buoyancy effect. Fresh air removes the heat absorbed by the collector, cooling in the same time the rear of the PV cells with the effect of an increment in the electrical conversion efficiency.
4. Considerations about the PV efficiency The most important parameter considered for the performance evaluation of the PVT collector is the PV-effective conversion efficiency in operative conditions, which affects the electricity generation and thus the most valuable product of the component. The conversion efficiency of a PV module is given by the ratio between the generated electrical power and the incident solar radiation, according to the following expression: ZMOD ¼
PMOD , I t AMOD
(1)
where ZMOD is the module efficiency; PMOD is the power generated by the module (W); It is the solar irradiance on
Fig. 1. Preliminary sketches of the collector.
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the module (W m2); AMOD is the front module surface exposed to the Sun (m2). The nominal efficiency of a PV module (Zn) is that measured in standard test conditions (STC), i.e. a cell temperature of 25 1C, an irradiance on the front side of the module of 1.000 W m2 with normal incidence on its plan and a solar spectrum corresponding to air mass (AM) 1.5. In real operating conditions, e.g. in open field with real meteorological parameters, the actual efficiency Za of the same module can differ significantly from its nominal value Zn, due to the deviation of the operating conditions from STC. The value of Za depends mainly on type and materials of solar cells, operational temperature of the cells, incidence angle of solar irradiation on the module and composition of solar spectrum. It is possible to define its value by the following correlation: Za ¼ Zn k g k y k a k l ,
(2)
where kg is the temperature correction factor, function of the PV power temperature coefficient g, given by: 100 gðtPV 25Þ . (3) 100 ky is the optical correction factor, and is defined as the ratio between the light transmittance factor corresponding to the actual incident angle of the solar radiation on the front surface of the module and that corresponding to the normal incident angle measured in STC, given by: ty ky ¼ . (4) tn kg ¼
ka is the absorption correction factor, and is defined as the ratio between the light absorbance factor of PV cells corresponding to the actual incident angle of the solar radiation on the collector surface and that corresponding to the normal incident angle measured in STC, given by: ay ka ¼ . (5) an kl is the spectrum correction factor of PV efficiency, function of the actual spectrum of the solar radiation on the collector surface in respect of that measured in STC. It can be calculated using an empiric formula that relates the changes of the incident solar light spectrum to AM to characterize the influence of the spectral variation on the production of the PV module [11]: 1 , FS where kl ¼
(6)
FS ¼ 0:928 þ 0:06796 AM 0:01507 AM2 þ 0:001587 AM3 0:00006377 AM4 . A useful parameter for the simulation model further considered is the actual thermal–spectral efficiency, which represents the variation of Zn in operating conditions due to actual temperature of the cells and solar spectrum only. It can
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be expressed as follows: Z ¼ Zn k g k l .
(7)
5. Simulation model The developed simulation model is based on the energy balances of the different components of the PVT collector (PV cells, glass panes, air gap, absorber plate). As described above, the simulation model has been updated and optimized many times during the course of research study and the final version is being presented here. The model is based on 6 equations of energy balance of the collector sub-systems and 5 equations for the dynamic calculation of convective and/or radiative coefficients. The energy balance equations are organized in a numerical matrix, whereas the radiative coefficient equations are solved by means of an iterative procedure. Energy balance equations for different components of PVT air collector can be written as follows. 5.1. Thermal balance of the PV cells For defining the thermal balance, it is considered that the part of sandwich with the cells embedded is at the same temperature of the cells. This hypothesis is reasonable, as the glass cover has a very small thickness. Z I t S PV tGy aPVy 1 tGn aPVn ¼ hePV S PV ðtPV te Þ þ hic SPV ðtPV tm Þ þ hrPVP SPV ðtPV tP Þ,
ð8Þ 2
where It is solar irradiation on the collector (W m ); SPV is the area of the part of sandwich with embedded PV cells (m2); tGy is the solar transmittance of the glass of the sandwich, dependent on incident angle; aPVy is the solar absorbance of PV cells solar, dependent on incident angle; Z* is the thermal–spectral efficiency of the PV module; tGn is the solar transmittance of the glass of the sandwich, at normal incident angle; aPVn is the solar absorbance of PV cells at normal incident angle; hePV is the external radiative+convective heat transfer coefficient related to the PV sandwich (W m2 1C1); tPV is the temperature of PV cells and of the part of sandwich with embedded the cells (considered the same) (1C); te is the ambient air temperature (1C); hic is the convective heat transfer coefficient in the air gap (W m2 1C1); tm is the average temperature of the air in the gap (1C); hrPVP is the radiative heat transfer coefficient between PV cells and absorber plate (W m2 1C1); and tP is the temperature of the absorber plate (1C).
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5.2. Thermal equilibrium of the glass (part of the sandwich without PV cells inside) I t S G aG ¼ heG S G ðtG te Þ þ hic S G ðtG tm Þ þ hrGP S G ðtG tP Þ,
ð9Þ
where SG is the area of the glazed part of the sandwich without PV cells (m2); aG is the solar absorbance of the part of sandwich without PV cells; heG is the external radiative+convective heat transfer coefficient related to the glazed surface (W m2 1C1); tG is the temperature of the glazed part of the sandwich without PV cells (1C); and hrGP is the radiative heat transfer coefficient between the part of sandwich without PV cells and the absorber plate (W m2 1C1).
The external convective and radiative heat transfer coefficients depend on the average temperatures, TPV–sky, TG–sky, TPV–P and TG–P, described below. For the determination of the relevant heat transfer coefficients, the simulation model employs an iterative procedure to obtain the values of these temperatures. The external convective and radiative coefficient of PV sandwich is calculated by the equation: hePV ¼ hv þ hrPVsky ,
(14)
where hv is the external convective coefficient and can be expressed as follows: hv ¼ 2:8 þ 3we . hrPV–sky is the external radiative coefficient between the PV sandwich and the sky and can be expressed as follows:
5.3. Thermal equilibrium in the air gap mair C p ðtout tin Þ ¼ hic SPV ðtPV tm Þ þ hic S G ðtG tm Þ þ hic S P ðtP tm Þ,
5.7. Thermal exchange coefficients
hrPVsky ¼ F 4 PV s T 3PVsky . ð10Þ
where mair is the volume airflow rate inside the gap (m3 h1); CP is the volume air heat capacity at constant pressure (Wh m3 1C1); tout is the outlet air temperature (1C); tin is the inlet air temperature (1C); and SP is the area of the absorber plate (m2). 5.4. Thermal equilibrium of the absorber plate
TPV–sky is the average PV–sky temperature and can be expressed as T PVsky ¼
T PV þ T sky . 2
Tsky is the equivalent radiative temperature of the sky and can be expressed as follows: T sky ¼ 0:0552 T 1:5 e þ 2:625 N.
I t S G tGy aP þ hrPVP S PV ðtPV tP Þ þ hrGP S G ðtG tP Þ ¼ hic S P ðtP tm Þ þ U b SP ðtP tb Þ,
ð11Þ
where aP is the solar absorbance of the absorber plate; Ub is the backside heat loss coefficient of the collector (W m2 1C1); and tb is the temperature of the rear adjacent ambient (1C). 5.5. PV thermal–spectral actual efficiency 100 gðtPV 25Þ , (12) 100 FS where g is the temperature power coefficient of the PV cells and FS is the spectrum correction factor of PV efficiency, as described above. Z ¼ Zn
The external convective and radiative coefficient of glazed surface without PV cells embedded is calculated in a similar way by means of the parameter average glass–sky temperature (TG–sky) and can be written as follows: heG ¼ hv þ hrG2sky .
(15)
The glass–absorber radiative coefficient is calculated by the following equation: hrGP ¼ 4 GP s T 3GP ,
(16)
where eGP is the equivalent glass–absorber infrared emissivity and can be written as GP ¼
1 , 1=G þ 1=P 1
5.6. Average temperature of the air in the gap tin þ tout , (13) 2 where tin is the inlet air temperature (1C) and tout is the outlet air temperature (1C). tm ¼
and TGP is the average PV glass-absorber temperature in K. The PV cells–absorber radiative coefficient is calculated in a similar way to the previous: hrPVP ¼ 4 PVP s T 3PVP ,
(17)
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where, ePVP is the equivalent PV cells–absorber near infrared emissivity and can be expressed as follows: 1 PVP ¼ 1=PV þ 1=P 1 and TPVP is the average PV cells-absorber temperature in K.
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The convective coefficient related to the internal surfaces of the collector is given by the following expression: hic ¼
Nu lair , L
Fig. 2. (A and B): prototype PVT collector installed at the experimental site Parco Lambro and meteorological station.
(18)
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Table 1 Dimensions of the collector Length of the collector Width of the collector Thickness of the collector Air gap section dimensions Tilt angle of the collector Orientation of the collector
2000 mm 1200 mm 120 mm 1065 75 mm 301 South
where Nu is the Nusselt number; lair is the air thermal conductivity (W m1 1C1); and L is thickness of the air gap (m). 6. Prototype installation and data monitoring On the basis of the design concept, a prototype PVT air heating system has been fabricated and installed at the
Fig. 3. (A and B): data monitoring system and computer display.
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experimental site Parco Lambro of Politecnico di Milano (Fig. 2A) and connected to a meteorological station, placed on the roof of the near two-storey building (Fig. 2B). The proto-type PV sub-system consists of two BP SX 60 photovoltaic module of 60 Wp in series connected to an inverter and one BP SX 120 photovoltaic module of 120 Wp connected to another inverter. This scheme allows for evaluating the performance of two different kinds of modules separately. The support on which the PVT collector is fixed allows to changing the tilt of the collector. The air duct between the PV sandwich and the insulation box is of 75 mm in width and 1065 mm in length. The upper
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side of the insulation box is painted black to act as an absorber plate. The dimensions of the PVT prototype are shown in Table 1. The outside air enters into the collector through a grid at the lower side. The airflow can be varied and controlled by means of an exhaust fan along with a regulator connected to the upper side of the collector by a flexible duct. A computer-based real time monitoring system has been connected to measure the thermal and electrical performance of the system. The data monitoring system consists of the following equipments:
Fig. 4. (A and B): measured and calculated performance (23 March).
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1 solar radiation probe installed at the tilt and orientation of collector for measuring the solar radiation over the modules; 1 manual hot-wire anemometer; 6 surface probes of type PT-100 installed at different positions to measure the temperature of PV laminate and secondary absorber; 4 probes of type PT-100 to measure the temperature of the air at inlet, inside and outlet of the collector; 1 meteorological station installed near the system to measure other climatic parameters.
Electric outputs are measured at the inverter poles.
A data logger supported by a personal computer is used for data monitoring and storage. Data have been acquired at every 10 min time interval. Fig. 3A and B show the layout of the monitoring system and a sample of the time series of climatic and operating data acquired in one day of the test campaign. 7. Results and discussion During the course of research program, a detailed data monitoring on prototype PVT system has been carried out in successive periods to verify the operation of the prototype and to calibrate the simulation model. In this
Fig. 5. (A and B): measured and calculated performance (14 March).
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Fig. 6. (A and B): measured and calculated performance (19 August).
paper, the results are presented in respect of last experimental campaign for 7 sample days, carried out during March to August. It should be mentioned that the available instrumentation does not allow to measure both the direct and diffuse components of solar radiation, necessary for the calculation of ty. Therefore, the days with clear sky conditions are selected to obtain, with a good approximation, the distribution of direct and diffuse solar radiation [12]. The experimental data are compared with those obtained from simulation model, providing input data as the real operational conditions. Instead of using the nominal value
of PV module efficiency in the simulation model, it was preferred to introduce a more reliable value, which also takes into account the effect of the maximum power point tracker of inverter. For this purpose, some measurements are carried out in the conditions similar to STC and corrections have been applied to the factors described in the Section 4. The value adopted for Zn is 9.5%. During the experimental measurements, the ventilation inside the module was obtained by forced circulation using the fan. A fixed value of air velocity of 0.5 m s1 is maintained with the volumetric flow rate of about 127 m3 h1.
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Fig. 7. (A and B): validation of simulation model.
Figs. 4–6 show the performance results of PVT collector (module BP SX 120) for 3 sample days (23 March, 14 April and 22 June). These figures show the comparison between the experimental results and those calculated from simulation model for two important performance parameters: the electrical power produced from the photovoltaic module (Figs. 4A, 5A, 6A) and the outlet air temperature of the PV collector (Figs. 4B, 5B, 6B). It can be seen that the thermal performance of PVT collector increases moving towards the warm season with the outlet air temperatures of collector reaching more than 40 1C. However, the photovoltaic performance increases during mid-seasons and reduces in the warmest months, when the effect of the temperature becomes more significant and compensates the higher incident solar radiation. It can be observed from the figures that there is a quite good agreement between measured values and those
calculated by the simulation model. Fig. 7 shows that the slopes of the calculated/measured regression lines are very close to 1 with R2 higher than 0.99. 8. Commercial product (TIS) and application case study On the basis of the results obtained in R&D program, the concept of the hybrid PVT collector described above was implemented in the project of an industrial product specifically designed for the application of solar energy in architecture. The developed system is called TIS and allows the realization of facades and roofs with the traditional functions of building envelope and integrates entirely the hybrid modules (Fig. 8). Fig. 9 shows the hybrid photovoltaic-thermal fac- ade of the Fiat Research Centre (CRF) at Orbassano, Italy, which represents the first commercial realization of the TIS
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Fig. 8. TIS details of installation.
Fig. 9. TIS-based CRF hybrid PVT fac- ade.
system, currently largest in Italy and among the largest ones in the world. The photovoltaic plant has a total power of 20 kWp and the hot air produced by the system is used for air-conditioning of the office building in winter (preheating) as well as in summer (desiccant cooling). The sizing of the PVT plant and the preliminary study on the electrical and thermal performance of the system have been carried out using the developed simulation model presented in this paper [13]. This integrated solar fac- ade is being monitored on-line by a highly sophisticated monitoring system and the data measured until now confirm the system performance predicted using the developed simula-
tion model. Fig. 10 shows the estimated annual energy performance of CRF hybrid PVT fac- ade. The experimental results on the thermal and electrical efficiencies of hybrid fac- ade for the summer (18 May) and winter (15 January) days are shown in Figs. 11 and 12. It can be seen from the figures that the thermal efficiency varies in average from 20% to 40% during the day (higher values are obtained in the early hours of morning and late hours in the evening). However, the average electrical efficiency obtained is around 9–10% during the day. It should be mentioned here that the electrical efficiency shown in the figures also include the BOS efficiency.
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Fig. 10. CRF solar fac- ade—estimated monthly thermal and electrical energy production.
Fig. 11. CRF solar fac- ade—thermal and electrical efficiency (summer day).
9. Conclusions The R&D program on PVT air collectors led to the development of an innovative building component TIS. The presented case study of CRF facade is the first commercial application of the TIS component. This type of realization, based on the interaction between university and industry, provides the possibilities to demonstrate the feasibility and to disclose the relative knowledge to the technological innovation and its application potential. The simulation model, developed under this program predicts quite well the thermal and electrical performance of a PVT collector. The model, in general, can be utilized
for any set of design and operational parameters for evaluating the performance of front cover direct flow PVT air collector, semitransparent with different solar cell density (i.e. the ratio between the area of the cells and the total laminate surface) or completely opaque (e.g. standard PV laminate like those employed in the experimental campaign presented). Acknowledgments Special thanks to Dr. R.S. Adhikari for fruitful discussions and suggestions. The authors are also grateful to Prof. Federico Butera, Prof. Marco Beccali, Prof. Giulio
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Fig. 12. CRF solar fac- ade—thermal and electrical efficiency (winter day).
Solaini and Eng. Alberto Agostini for their support and cooperation during the research works. References [1] IEA (International Energy Agency). Photovoltaics in buildings—a design handbook for architects. Sick F, Erge T, editors. London: James & James; 1996. [2] IEA (International Energy Agency). Photovoltaic power systems programme: task 7. Photovoltaics/thermal solar energy systems, status of the technology and roadmap for future development. Report IEA PVPS T7-10; 2002. [3] Bazilian MD, Leenders F, Van Der Ree BGC, Prasad D. Photovoltaic cogeneration in the built environment. Sol Energy 2000;71(1):57–69. [4] Zondag HA, De Vries DW, Van Helden WG, Van Zolingen RJC, Van Steenhoven AA. The yield of different combined PV-thermal collector designs. Sol Energy 2003;74:253–79. [5] Garg HP, Adhikari RS. Transient simulation of conventional hybrid photovoltaic/thermal air heating collectors. Int J Energy Res 1998;22:547–62. [6] Tripanagnostopoulos Y, Nousia Th, Souliotis M, Yianoulis P. Hybrid photovoltaic/thermal solar systems. Sol Energy 2002;72(3):217–34.
[7] Aste N, Beccali M, Solaini G. A theoretical study on the performance of some PV-thermal system configuration. Fourth international congress ‘‘Energy, environment and technological innovation,’’ Rome, Italy, 20–24 September; 1999. [8] Aste N, Beccali M, Chiesa G. Experimental evaluation of the performance of a proto-type hybrid solar photovoltaicthermal (PV/T) air collector for the integration in sloped roof. Conference ‘‘EPIC 2002 AIVC,’’ Lyon, France, 23–26 October; 2002. [9] Aste N, Beccali M, Solaini G. Experimental validation of a simulation model for a PV/T collector. ISES solar world congress 2003, Go¨teborg, Sweden, 14–19 June; 2003. [10] Aste N, Del Pero C, Simulation model for forecast of the energy performance of PV plants. ASME ATI conference, Milano, Italy, 14–17 May; 2006. [11] Hecktheuer LA, Krenzinger A. The effects on the photovoltaic system response of reflection, spectrum, voltage drop and temperature. 17th EPVSEC European photovoltaic energy conference, Munich, Germany; 2001. [12] Iqbal M. An introduction to solar radiation. London, UK: Academic Press; 1983. [13] Butera F, Adhikari RS, Aste N, Bracco R. Building integrated solar roof: CRF hybrid facade case study. SESI J 2005;15:1–9.
Renewable Energy 33 (2008) 914–927 www.elsevier.com/locate/renene
Design, development and performance monitoring of a photovoltaic-thermal (PVT) air collector Niccolo` Aste, Giancarlo Chiesa, Francesco Verri Department of Building Environment Sciences and Technology (BEST), Politecnico di Milano, Via Garofalo 39, 20097 Milan, Italy Received 23 January 2007; accepted 13 June 2007 Available online 14 September 2007
Abstract The photovoltaic-thermal (PVT) systems allow the enhancement of the energy performance of photovoltaics, by removing thermal energy and subsequently decreasing the operating temperature of the cells. The possibility of the utilization of heat for climatization makes them attractive for the building integration. In order to diffuse this kind of solar systems it is necessary to translate the basic concepts into efficient and functional technological components and associated performance should be evaluated in a reliable manner. This paper presents the experimental and theoretical results of a research and development program carried out at the Politecnico di Milano on the design, development and performance monitoring of a hybrid PVT air collector. One of the main products of the research consists of a simulation model for performance prediction of the system. This R&D program led to the development of the TIS (tetto integrale solarizzato, i.e. integrated solar roof), an innovative technological system for building integration of hybrid PVT air collectors. The successful commercial application of the TIS in a research center building is also shown as a case study. r 2007 Elsevier Ltd. All rights reserved. Keywords: Photovoltaic-thermal air collector; TIS-integrated solar roof; Simulation model; Measured data; Thermal and electrical performance; Case study
1. Introduction In recent years, building integration of photovoltaic modules has become more and more popular in the industrialized part of the world, where national support programs have accelerated the installation and dissemination of grid-connected PV systems [1]. For these systems, a number of studies related to the utilization of waste heat from photovoltaic modules exist and are well documented in the literature [2]. It is well known that the hybrid photovoltaic-thermal (PVT) systems allow better energy performances of the PV technology. In fact, the heat accumulated in the solar cells is recovered in the form of low temperature thermal energy, resulting improvements in the electrical conversion efficiency of PV modules. Over the last few years, different PVT systems, based on air and water as heat carrying fluid, have been studied, developed and reported in the literature Corresponding author. Tel.: +39 02 2399 9466; fax: +39 02 2399 9467.
E-mail address: [email protected] (N. Aste). 0960-1481/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2007.06.022
[3–6]. Among various PVT collectors, the hybrid air collectors are the most common, even if their applications remain yet to experimental level and the industrial products are relatively few. The studies conducted by different researchers on the hybrid air collectors are numerous and show the validity of the concept. However, most of the related studies published in the literature, are focused more on the thermal characterization of the components in comparison to the electrical one. There are a number of factors that influence the PV efficiency, and a detailed evaluation is necessary to predict accurately the combined (thermal as well as electrical) performance of PVT systems. The present work describes the research and development program carried out at the Politecnico di Milano on the design, development and performance monitoring of a hybrid PVT air collector. The results of this R&D program led to the development of the tetto integrale solarizzato, i.e. integrated solar roof (TIS), an innovative technological system for building integration of hybrid PVT air collectors.
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Nomenclature AMOD front module surface area exposed to the Sun, m2 CP volume air heat capacity at constant pressure, Wh m3 1C1 F sky view factor of collector hePV external radiative+convective heat transfer coefficient related to the PV sandwich, W m2 1C1 hV external convective heat transfer coefficient, W m2 1C1 hic convective heat transfer coefficient in the air gap, W m2 1C1 heG external radiative+convective heat transfer coefficient related to the glazed surface, W m2 1C1 hrG–P radiative heat transfer coefficient between the part of sandwich without PV cells and the absorber plate, W m2 1C1 hrG–sky external radiative heat transfer coefficient the part of sandwich without PV cells and the sky, W m2 1C1 hrPV–P radiative heat transfer coefficient between PV cells and absorber plate, W m2 1C1 hrPV–sky external radiative coefficient between PV cells and sky, W m2 1C1 It solar irradiance on the module, W m2 ka absorption correction factor of PV efficiency kg temperature correction factor of PV efficiency ky optical correction factor of PV efficiency kl spectrum correction factor of PV efficiency L thickness of the air gap, m mair volume air flow rate inside the gap, m3 h1 N sky cloud coverage in octaves Nu Nusselt number PMOD power generated by the module, W SG area of the glazed part of the sandwich without PV cells, m2 SP area of the absorber plate, m2 SPV area of the part of sandwich with embedded PV cells, m2
tb tPV
te tm tP tG tout tin TGP TPV–P TPV–sky Ub we aP aG aPVy aPVn eG eG–P eP ePV ePV–P g Za ZMOD Zn Z* lair s tGq tGn
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temperature of the rear adjacent ambient, 1C temperature of PV cells and of the part of sandwich with embedded the cells (considered the same), 1C ambient air temperature, 1C average temperature of the air in the gap, 1C temperature of the absorber plate, 1C temperature of the glazed part of the sandwich without PV cells, 1C collector outlet air temperature, 1C collector inlet air temperature, 1C average glass–absorber temperature, K average PV cells–absorber temperature, K average PV–sky temperature, K backside heat loss coefficient of the collector, W m2 1C1 wind velocity, m s1 solar absorbance of the absorber plate solar absorbance of the part of sandwich without PV cells solar absorbance of PV cells dependent on incident angle solar absorbance of PV cells at normal incident angle near infrared emissivity of the glass equivalent glass–absorber infrared emissivity near infrared emissivity of the absorber plate near infrared emissivity of the PV cells equivalent PV cells–absorber near infrared emissivity temperature power coefficient of the PV cells actual efficiency of the PV module PV module efficiency nominal efficiency of PV the module thermal–spectral efficiency of the PV module air thermal conductivity, W m1 1C1 Stefan–Boltzmann constant, W m2 K4 solar transmittance of the glass of the sandwich, dependent on incident angle solar transmittance of the glass of the sandwich, at normal incident angle
2. The TIS (integrated solar roof) project at the Politecnico di Milano
systems for building application. The program is completed in 2005 and is consisted of the following phases [7–10]:
In the framework of the research activities conducted at the Politecnico di Milano for the utilization of solar energy systems, in the year 2000 the Department of Energetica and Department of Building Environment Sciences and Technology started a program of research on the experimentation and development of a hybrid PVT air component. The program was supported and funded by the industry SeccoSistemi S.p.A. that has a long tradition in the production of solar energy
preliminary project on the component design with the variation in technology and dimension, based on the cooling requirements of the photovoltaic laminates to recover a part of thermal energy accumulated and consequently to increase the electrical conversion efficiency of PV; development of a simulation model for the prediction of energy performances through the elaboration of
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a number of algorithms capable to calculate the electrical and thermal productivity of the component as a function of inputs related to the climatic parameters and operating conditions; performance estimation for different solutions in terms of electrical and thermal efficiencies achievable from different system configurations; identification of better solution, resulted as a direct flow front cover PV collector; realization of an experimental prototype suitable for testing in the open-field conditions and to verify the functioning of simulation model; organization of an experimental campaign conducted during 2002–2004 for data acquisition on the prototype for the long-period operation in different climatic conditions; validation of the simulation model through the comparison between the experimental monitored data with the calculated one and the refinement of some model parameters; performance estimation in different design and operational conditions using the improved simulation model; designing of commercial product on the basis of performance optimization.
It should be mentioned that the program, in reality, had not a logical course but passed through different moments of interaction among various phases, according to the process learning by doing that led to the necessity to verify and update the results obtained each time. 3. Design of the PVT collector The hybrid collector has been developed as an ‘‘upgrade’’ of a conventional solar air collector, i.e. a front cover direct flow PV/T collector, organized in a modular structure.
The module has been designed in order to be integrated in common sloped roofs or vertical facades, replacing the external covering, waterproof and insulation layers. The schematics of the PVT collector are shown in Fig. 1. The upper cover is represented by a glass sandwich that includes PV cells. The cell area can cover the entire glazed surface or can be distributed in a grid where the spacing between adjacent columns and rows can allow a direct gain of solar radiation to the backward absorber plate. The glass sandwich looks like a chessboard composed of squares with or without PV cells embedded. Different configurations of PVT collector can be created changing the cell area density in order to balance electricity and thermal energy output of the system. The airflow into the gap (between the sandwich and the absorber plate) can be achieved by the forced flow using a fan or natural flow through buoyancy effect. Fresh air removes the heat absorbed by the collector, cooling in the same time the rear of the PV cells with the effect of an increment in the electrical conversion efficiency.
4. Considerations about the PV efficiency The most important parameter considered for the performance evaluation of the PVT collector is the PV-effective conversion efficiency in operative conditions, which affects the electricity generation and thus the most valuable product of the component. The conversion efficiency of a PV module is given by the ratio between the generated electrical power and the incident solar radiation, according to the following expression: ZMOD ¼
PMOD , I t AMOD
(1)
where ZMOD is the module efficiency; PMOD is the power generated by the module (W); It is the solar irradiance on
Fig. 1. Preliminary sketches of the collector.
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the module (W m2); AMOD is the front module surface exposed to the Sun (m2). The nominal efficiency of a PV module (Zn) is that measured in standard test conditions (STC), i.e. a cell temperature of 25 1C, an irradiance on the front side of the module of 1.000 W m2 with normal incidence on its plan and a solar spectrum corresponding to air mass (AM) 1.5. In real operating conditions, e.g. in open field with real meteorological parameters, the actual efficiency Za of the same module can differ significantly from its nominal value Zn, due to the deviation of the operating conditions from STC. The value of Za depends mainly on type and materials of solar cells, operational temperature of the cells, incidence angle of solar irradiation on the module and composition of solar spectrum. It is possible to define its value by the following correlation: Za ¼ Zn k g k y k a k l ,
(2)
where kg is the temperature correction factor, function of the PV power temperature coefficient g, given by: 100 gðtPV 25Þ . (3) 100 ky is the optical correction factor, and is defined as the ratio between the light transmittance factor corresponding to the actual incident angle of the solar radiation on the front surface of the module and that corresponding to the normal incident angle measured in STC, given by: ty ky ¼ . (4) tn kg ¼
ka is the absorption correction factor, and is defined as the ratio between the light absorbance factor of PV cells corresponding to the actual incident angle of the solar radiation on the collector surface and that corresponding to the normal incident angle measured in STC, given by: ay ka ¼ . (5) an kl is the spectrum correction factor of PV efficiency, function of the actual spectrum of the solar radiation on the collector surface in respect of that measured in STC. It can be calculated using an empiric formula that relates the changes of the incident solar light spectrum to AM to characterize the influence of the spectral variation on the production of the PV module [11]: 1 , FS where kl ¼
(6)
FS ¼ 0:928 þ 0:06796 AM 0:01507 AM2 þ 0:001587 AM3 0:00006377 AM4 . A useful parameter for the simulation model further considered is the actual thermal–spectral efficiency, which represents the variation of Zn in operating conditions due to actual temperature of the cells and solar spectrum only. It can
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be expressed as follows: Z ¼ Zn k g k l .
(7)
5. Simulation model The developed simulation model is based on the energy balances of the different components of the PVT collector (PV cells, glass panes, air gap, absorber plate). As described above, the simulation model has been updated and optimized many times during the course of research study and the final version is being presented here. The model is based on 6 equations of energy balance of the collector sub-systems and 5 equations for the dynamic calculation of convective and/or radiative coefficients. The energy balance equations are organized in a numerical matrix, whereas the radiative coefficient equations are solved by means of an iterative procedure. Energy balance equations for different components of PVT air collector can be written as follows. 5.1. Thermal balance of the PV cells For defining the thermal balance, it is considered that the part of sandwich with the cells embedded is at the same temperature of the cells. This hypothesis is reasonable, as the glass cover has a very small thickness. Z I t S PV tGy aPVy 1 tGn aPVn ¼ hePV S PV ðtPV te Þ þ hic SPV ðtPV tm Þ þ hrPVP SPV ðtPV tP Þ,
ð8Þ 2
where It is solar irradiation on the collector (W m ); SPV is the area of the part of sandwich with embedded PV cells (m2); tGy is the solar transmittance of the glass of the sandwich, dependent on incident angle; aPVy is the solar absorbance of PV cells solar, dependent on incident angle; Z* is the thermal–spectral efficiency of the PV module; tGn is the solar transmittance of the glass of the sandwich, at normal incident angle; aPVn is the solar absorbance of PV cells at normal incident angle; hePV is the external radiative+convective heat transfer coefficient related to the PV sandwich (W m2 1C1); tPV is the temperature of PV cells and of the part of sandwich with embedded the cells (considered the same) (1C); te is the ambient air temperature (1C); hic is the convective heat transfer coefficient in the air gap (W m2 1C1); tm is the average temperature of the air in the gap (1C); hrPVP is the radiative heat transfer coefficient between PV cells and absorber plate (W m2 1C1); and tP is the temperature of the absorber plate (1C).
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5.2. Thermal equilibrium of the glass (part of the sandwich without PV cells inside) I t S G aG ¼ heG S G ðtG te Þ þ hic S G ðtG tm Þ þ hrGP S G ðtG tP Þ,
ð9Þ
where SG is the area of the glazed part of the sandwich without PV cells (m2); aG is the solar absorbance of the part of sandwich without PV cells; heG is the external radiative+convective heat transfer coefficient related to the glazed surface (W m2 1C1); tG is the temperature of the glazed part of the sandwich without PV cells (1C); and hrGP is the radiative heat transfer coefficient between the part of sandwich without PV cells and the absorber plate (W m2 1C1).
The external convective and radiative heat transfer coefficients depend on the average temperatures, TPV–sky, TG–sky, TPV–P and TG–P, described below. For the determination of the relevant heat transfer coefficients, the simulation model employs an iterative procedure to obtain the values of these temperatures. The external convective and radiative coefficient of PV sandwich is calculated by the equation: hePV ¼ hv þ hrPVsky ,
(14)
where hv is the external convective coefficient and can be expressed as follows: hv ¼ 2:8 þ 3we . hrPV–sky is the external radiative coefficient between the PV sandwich and the sky and can be expressed as follows:
5.3. Thermal equilibrium in the air gap mair C p ðtout tin Þ ¼ hic SPV ðtPV tm Þ þ hic S G ðtG tm Þ þ hic S P ðtP tm Þ,
5.7. Thermal exchange coefficients
hrPVsky ¼ F 4 PV s T 3PVsky . ð10Þ
where mair is the volume airflow rate inside the gap (m3 h1); CP is the volume air heat capacity at constant pressure (Wh m3 1C1); tout is the outlet air temperature (1C); tin is the inlet air temperature (1C); and SP is the area of the absorber plate (m2). 5.4. Thermal equilibrium of the absorber plate
TPV–sky is the average PV–sky temperature and can be expressed as T PVsky ¼
T PV þ T sky . 2
Tsky is the equivalent radiative temperature of the sky and can be expressed as follows: T sky ¼ 0:0552 T 1:5 e þ 2:625 N.
I t S G tGy aP þ hrPVP S PV ðtPV tP Þ þ hrGP S G ðtG tP Þ ¼ hic S P ðtP tm Þ þ U b SP ðtP tb Þ,
ð11Þ
where aP is the solar absorbance of the absorber plate; Ub is the backside heat loss coefficient of the collector (W m2 1C1); and tb is the temperature of the rear adjacent ambient (1C). 5.5. PV thermal–spectral actual efficiency 100 gðtPV 25Þ , (12) 100 FS where g is the temperature power coefficient of the PV cells and FS is the spectrum correction factor of PV efficiency, as described above. Z ¼ Zn
The external convective and radiative coefficient of glazed surface without PV cells embedded is calculated in a similar way by means of the parameter average glass–sky temperature (TG–sky) and can be written as follows: heG ¼ hv þ hrG2sky .
(15)
The glass–absorber radiative coefficient is calculated by the following equation: hrGP ¼ 4 GP s T 3GP ,
(16)
where eGP is the equivalent glass–absorber infrared emissivity and can be written as GP ¼
1 , 1=G þ 1=P 1
5.6. Average temperature of the air in the gap tin þ tout , (13) 2 where tin is the inlet air temperature (1C) and tout is the outlet air temperature (1C). tm ¼
and TGP is the average PV glass-absorber temperature in K. The PV cells–absorber radiative coefficient is calculated in a similar way to the previous: hrPVP ¼ 4 PVP s T 3PVP ,
(17)
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where, ePVP is the equivalent PV cells–absorber near infrared emissivity and can be expressed as follows: 1 PVP ¼ 1=PV þ 1=P 1 and TPVP is the average PV cells-absorber temperature in K.
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The convective coefficient related to the internal surfaces of the collector is given by the following expression: hic ¼
Nu lair , L
Fig. 2. (A and B): prototype PVT collector installed at the experimental site Parco Lambro and meteorological station.
(18)
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Table 1 Dimensions of the collector Length of the collector Width of the collector Thickness of the collector Air gap section dimensions Tilt angle of the collector Orientation of the collector
2000 mm 1200 mm 120 mm 1065 75 mm 301 South
where Nu is the Nusselt number; lair is the air thermal conductivity (W m1 1C1); and L is thickness of the air gap (m). 6. Prototype installation and data monitoring On the basis of the design concept, a prototype PVT air heating system has been fabricated and installed at the
Fig. 3. (A and B): data monitoring system and computer display.
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experimental site Parco Lambro of Politecnico di Milano (Fig. 2A) and connected to a meteorological station, placed on the roof of the near two-storey building (Fig. 2B). The proto-type PV sub-system consists of two BP SX 60 photovoltaic module of 60 Wp in series connected to an inverter and one BP SX 120 photovoltaic module of 120 Wp connected to another inverter. This scheme allows for evaluating the performance of two different kinds of modules separately. The support on which the PVT collector is fixed allows to changing the tilt of the collector. The air duct between the PV sandwich and the insulation box is of 75 mm in width and 1065 mm in length. The upper
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side of the insulation box is painted black to act as an absorber plate. The dimensions of the PVT prototype are shown in Table 1. The outside air enters into the collector through a grid at the lower side. The airflow can be varied and controlled by means of an exhaust fan along with a regulator connected to the upper side of the collector by a flexible duct. A computer-based real time monitoring system has been connected to measure the thermal and electrical performance of the system. The data monitoring system consists of the following equipments:
Fig. 4. (A and B): measured and calculated performance (23 March).
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1 solar radiation probe installed at the tilt and orientation of collector for measuring the solar radiation over the modules; 1 manual hot-wire anemometer; 6 surface probes of type PT-100 installed at different positions to measure the temperature of PV laminate and secondary absorber; 4 probes of type PT-100 to measure the temperature of the air at inlet, inside and outlet of the collector; 1 meteorological station installed near the system to measure other climatic parameters.
Electric outputs are measured at the inverter poles.
A data logger supported by a personal computer is used for data monitoring and storage. Data have been acquired at every 10 min time interval. Fig. 3A and B show the layout of the monitoring system and a sample of the time series of climatic and operating data acquired in one day of the test campaign. 7. Results and discussion During the course of research program, a detailed data monitoring on prototype PVT system has been carried out in successive periods to verify the operation of the prototype and to calibrate the simulation model. In this
Fig. 5. (A and B): measured and calculated performance (14 March).
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Fig. 6. (A and B): measured and calculated performance (19 August).
paper, the results are presented in respect of last experimental campaign for 7 sample days, carried out during March to August. It should be mentioned that the available instrumentation does not allow to measure both the direct and diffuse components of solar radiation, necessary for the calculation of ty. Therefore, the days with clear sky conditions are selected to obtain, with a good approximation, the distribution of direct and diffuse solar radiation [12]. The experimental data are compared with those obtained from simulation model, providing input data as the real operational conditions. Instead of using the nominal value
of PV module efficiency in the simulation model, it was preferred to introduce a more reliable value, which also takes into account the effect of the maximum power point tracker of inverter. For this purpose, some measurements are carried out in the conditions similar to STC and corrections have been applied to the factors described in the Section 4. The value adopted for Zn is 9.5%. During the experimental measurements, the ventilation inside the module was obtained by forced circulation using the fan. A fixed value of air velocity of 0.5 m s1 is maintained with the volumetric flow rate of about 127 m3 h1.
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Fig. 7. (A and B): validation of simulation model.
Figs. 4–6 show the performance results of PVT collector (module BP SX 120) for 3 sample days (23 March, 14 April and 22 June). These figures show the comparison between the experimental results and those calculated from simulation model for two important performance parameters: the electrical power produced from the photovoltaic module (Figs. 4A, 5A, 6A) and the outlet air temperature of the PV collector (Figs. 4B, 5B, 6B). It can be seen that the thermal performance of PVT collector increases moving towards the warm season with the outlet air temperatures of collector reaching more than 40 1C. However, the photovoltaic performance increases during mid-seasons and reduces in the warmest months, when the effect of the temperature becomes more significant and compensates the higher incident solar radiation. It can be observed from the figures that there is a quite good agreement between measured values and those
calculated by the simulation model. Fig. 7 shows that the slopes of the calculated/measured regression lines are very close to 1 with R2 higher than 0.99. 8. Commercial product (TIS) and application case study On the basis of the results obtained in R&D program, the concept of the hybrid PVT collector described above was implemented in the project of an industrial product specifically designed for the application of solar energy in architecture. The developed system is called TIS and allows the realization of facades and roofs with the traditional functions of building envelope and integrates entirely the hybrid modules (Fig. 8). Fig. 9 shows the hybrid photovoltaic-thermal fac- ade of the Fiat Research Centre (CRF) at Orbassano, Italy, which represents the first commercial realization of the TIS
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Fig. 8. TIS details of installation.
Fig. 9. TIS-based CRF hybrid PVT fac- ade.
system, currently largest in Italy and among the largest ones in the world. The photovoltaic plant has a total power of 20 kWp and the hot air produced by the system is used for air-conditioning of the office building in winter (preheating) as well as in summer (desiccant cooling). The sizing of the PVT plant and the preliminary study on the electrical and thermal performance of the system have been carried out using the developed simulation model presented in this paper [13]. This integrated solar fac- ade is being monitored on-line by a highly sophisticated monitoring system and the data measured until now confirm the system performance predicted using the developed simula-
tion model. Fig. 10 shows the estimated annual energy performance of CRF hybrid PVT fac- ade. The experimental results on the thermal and electrical efficiencies of hybrid fac- ade for the summer (18 May) and winter (15 January) days are shown in Figs. 11 and 12. It can be seen from the figures that the thermal efficiency varies in average from 20% to 40% during the day (higher values are obtained in the early hours of morning and late hours in the evening). However, the average electrical efficiency obtained is around 9–10% during the day. It should be mentioned here that the electrical efficiency shown in the figures also include the BOS efficiency.
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Fig. 10. CRF solar fac- ade—estimated monthly thermal and electrical energy production.
Fig. 11. CRF solar fac- ade—thermal and electrical efficiency (summer day).
9. Conclusions The R&D program on PVT air collectors led to the development of an innovative building component TIS. The presented case study of CRF facade is the first commercial application of the TIS component. This type of realization, based on the interaction between university and industry, provides the possibilities to demonstrate the feasibility and to disclose the relative knowledge to the technological innovation and its application potential. The simulation model, developed under this program predicts quite well the thermal and electrical performance of a PVT collector. The model, in general, can be utilized
for any set of design and operational parameters for evaluating the performance of front cover direct flow PVT air collector, semitransparent with different solar cell density (i.e. the ratio between the area of the cells and the total laminate surface) or completely opaque (e.g. standard PV laminate like those employed in the experimental campaign presented). Acknowledgments Special thanks to Dr. R.S. Adhikari for fruitful discussions and suggestions. The authors are also grateful to Prof. Federico Butera, Prof. Marco Beccali, Prof. Giulio
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Fig. 12. CRF solar fac- ade—thermal and electrical efficiency (winter day).
Solaini and Eng. Alberto Agostini for their support and cooperation during the research works. References [1] IEA (International Energy Agency). Photovoltaics in buildings—a design handbook for architects. Sick F, Erge T, editors. London: James & James; 1996. [2] IEA (International Energy Agency). Photovoltaic power systems programme: task 7. Photovoltaics/thermal solar energy systems, status of the technology and roadmap for future development. Report IEA PVPS T7-10; 2002. [3] Bazilian MD, Leenders F, Van Der Ree BGC, Prasad D. Photovoltaic cogeneration in the built environment. Sol Energy 2000;71(1):57–69. [4] Zondag HA, De Vries DW, Van Helden WG, Van Zolingen RJC, Van Steenhoven AA. The yield of different combined PV-thermal collector designs. Sol Energy 2003;74:253–79. [5] Garg HP, Adhikari RS. Transient simulation of conventional hybrid photovoltaic/thermal air heating collectors. Int J Energy Res 1998;22:547–62. [6] Tripanagnostopoulos Y, Nousia Th, Souliotis M, Yianoulis P. Hybrid photovoltaic/thermal solar systems. Sol Energy 2002;72(3):217–34.
[7] Aste N, Beccali M, Solaini G. A theoretical study on the performance of some PV-thermal system configuration. Fourth international congress ‘‘Energy, environment and technological innovation,’’ Rome, Italy, 20–24 September; 1999. [8] Aste N, Beccali M, Chiesa G. Experimental evaluation of the performance of a proto-type hybrid solar photovoltaicthermal (PV/T) air collector for the integration in sloped roof. Conference ‘‘EPIC 2002 AIVC,’’ Lyon, France, 23–26 October; 2002. [9] Aste N, Beccali M, Solaini G. Experimental validation of a simulation model for a PV/T collector. ISES solar world congress 2003, Go¨teborg, Sweden, 14–19 June; 2003. [10] Aste N, Del Pero C, Simulation model for forecast of the energy performance of PV plants. ASME ATI conference, Milano, Italy, 14–17 May; 2006. [11] Hecktheuer LA, Krenzinger A. The effects on the photovoltaic system response of reflection, spectrum, voltage drop and temperature. 17th EPVSEC European photovoltaic energy conference, Munich, Germany; 2001. [12] Iqbal M. An introduction to solar radiation. London, UK: Academic Press; 1983. [13] Butera F, Adhikari RS, Aste N, Bracco R. Building integrated solar roof: CRF hybrid facade case study. SESI J 2005;15:1–9.