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New proposal for photovoltaic-thermal solar energy utilization method
Solar Energy,Vol. 52. No. 3, pp. 241-245. 1994 Copyright0 1994ElsevierScienceLtd Printed in the USA.All rightsreserved
Pergamon
0038-092X/94 $6.00+ .OO
NEW PROPOSAL FOR PHOTOVOLTAIC-THERMAL ENERGY UTILIZATION METHOD
SOLAR
TAKUMI TAKASHIMA,* TADAYOSHITANAKA,* TAKUYA Dor, * JUNJI KAMOSHIDA,** TATSUO TANI, *** and TAKASHI HORIGOME**** *Electrotechnical Laboratory, I-l-4 Umezono Tsukuba, Ibaraki, Japan, **Shibaura Institute of Technology, 307 Tameihara-Fukasaku Ohmiya, Saitama, Japan, ***Science University of Tokyo, l-3 Kagurazaka Shinjuku, Tokyo, Japan, and ****Tokyo University of Agriculture and Technology. 2-24-16 Nakamachi Koganei, Tokyo, Japan Abstract-One of the most effective methods of utilizing solar energy is to use the sunlight and solar thermal energy such as a photovohaic-thermal panel (PV/T panel) simultaneously. From such a viewpoint, systems using various kinds of PV panels were constructed in the world. In these panels, solar cells are set up at an absorber collecting solar thermal energy. Therefore, temperature of solar cell increases up to the prescribed temperature of thermal energy use, although it is lower than the cell temperature when using only solar cell panel. For maintaining cell conversion efficiency at the standard conditions, it is necessary to keep the cell at lower temperature. In this paper, electric and thermal energy obtained from a PV/T panel is evaluated in terms of energy. Based on this evaluation, the method of not to decrease cell conversion efficiency with collecting solar thermal energy was proposed.
1. INTRODUCIION Environmental pollutions, such as warming atmosphere with gas effluents or forest damage by acid rain caused by mass consumption of fossil fuel from the industrial development, are at present serious global environmental problems. Under the circumstances, to keep the growth of economics in the future for us who are poor in energy resources, it is important to develop and utilize nonpolluting clean energy resources. From the viewpoint of environmental conservation and energy security in our country, solar energy is one of the clean and self-sufficient energy resources. Effective utilization of solar energy in our country, which does not depend on energy resources imported from overseas, will make a significant contribution to preserve our global environment. Technical researches on photovoltaic conversion and solar thermal energy had already been developed in Japan. Now it seems that basic technologies have almost reached the stage of practical application. Therefore, we think it is possible to utilize solar energy more effectively by the integration of those technologies. In general, the most important energies we use are electricity and heat. If we simultaneously obtain both energy forms from the sun, it is very convenient to use for industry and domesticity. Therefore, as a “solar cogeneration system,” a solar total energy system with solar heat was studied by Tanaka et al. [ 11, and a photovoltaic-thermal hybrid system (PV/T system) using solar cells was reported by Loferski et al.[ 2 1. A solar
total energy system obtains high temperature heat from the sun, and utilizes them for generating electric power and for heating. This system can conveniently supply electric power and heat according to demand. Since the solar condition in Japan is not so suitable, we cannot get high temperature heat constantly and effectively. Therefore, further development of this system has not been promoted. On the other hand, a PV/T system has been developed as a method to increase the use of a photovoltaic cell (PV cell). In this system, a PV/T panel is constructed to obtain both electricity and heat, and a PV cell is usually constructed in a flat solar collector as reported by Loferski et al.[ 21. Therefore, electrical power and thermal energy are supplied separately, but they cannot be supplied according to the demands as in a solar total energy system. Furthermore, if we need to obtain high temperature heat from the PV/T panel, the temperature of the PV cell becomes high and its output power falls. Thus, it seems that ordinary construction of a PV/T panel makes the output of the PV cell down. If electricity and thermal energy are each obtained by an isolated unit or component, it is easy to convert electricity into thermal energy, but its inverse conversion is difficult because quality of both energy are different. From the above reason, it is necessary to develop a new method to get electricity and thermal energy effectively from the PV/T system. To attain this purpose, we evaluated the quality of electricity and thermal energy by using the concept of exergy, and proposed the method to obtain them without rising cell temperature.
241
T. TAKASHIMA etal.
242 2. EXERCY
EVALUATION
OF SOLAR
ENERGY
When electricity and thermal energy are compared quantitatively, we often feel that electricity and thermal energy is the same kind of energy because we use the same unit to describe their amounts. Actually, we cannot get effective thermal energy when temperature difference between two materials does not exist, even if their temperatures are high. However, electricity does not have such a restriction. Therefore, in the case of evaluating the quality of electricity and thermal energy obtained in a PV/T panel, we believe that it is reasonable to discuss their quality in terms of exergy. The basic evaluation of solar energy in terms of exergy has already been reported by Oshida [ 31, Suzuki et al.[ 41, Fujiwara [ 5 1, and so on. But we are unable to locate studies on a PV/T system. In this paper, we evaluated a PV/T system in terms of exergy. A system chosen as the subject of study is a PV/T collector, which is generally composed of a flat plate collector including PV cells like the collector proposed by Loferski et al. [ 21. In order to evaluate solar energy in terms of exergy, we assume that the sun is the heat reservoir. Then, Ts and Q0 denote the sun temperature and the incident energy flux from the sun, respectively. The exergy of solar energy, EO, is expressed as follows; Eo = (1 - To/Ts)Qo
(1)
where, To is the ambient temperature. By assuming that T, is much larger than To, it is possible to simplify eqn ( 1) as follows; Eo = Qo
(2)
temperature, the exergy of electricity obtained in PV panel, Es, becomes Es = Qs
The exergy efficiency of PV cell, &, is calculated byeqns(2)and(7). 4s = EslEo = QslQo = mdl
nc= QlQo = rloc-
Uc(Tc-
To)
(3)
where, 71~ and UC are optical efficiency and heat loss coefficient of the collector respectively. Q and Tc are the collected heat by the PV /T panel and average heat collecting temperature. Thus, EC, the exergy of Q, is expressed as EC = (1 - To/ TclQ
(4)
We define the exergy efficiency of the heat, &-, as follows; Fc = EclEo = ( 1 - To/Tc)Q/Qo
(5)
Substitute eqn ( 3) into eqn (5) gives: tc = (7-c - To){I~,
- Uc(Tc - To)}/Tc
(6)
As electricity, Q.,-, is not affected by environmental
- UdT-
To)} (8)
where pros, Us, and Tare the conversion efficiency on standard condition, temperature coefficient, and temperature of PV panel. We can obtain eqn (8) by assuming that PV output power depends only on its temperature and that T = Tc. Thus, the condition which the solar thermal exergy is larger than the PV electrical exergy is fc > Es. From eqns (6) and (8), the following relations can be obtained TCl <‘Tc<7c2
(9)
7cr,2 = {B + (B2 - 4C)“2}/2 B = (90~ - ~0s + ToUs)l(Uc
- Us),
C = r~osTol( UC - Us) where, 7c = Tc - To. To calculate the relations of eqn (9), we assume the characteristics of solar collector and PV panel can be written in terms of first order functions of temperature as follows: solar collector PV panel
On the other hand, collector efficiency, qc, is defined by the following equation.
(7)
nc = 0.8 - 0.004~~
(10)
nsI = O.l( 1 - 0.005~~)
( 1 la)
qs2 = 0.2( I - 0.005~~)
( 1 lb)
The characteristic of the solar collector is appropriate to that commercially available in Japan. Those of PV panel in eqns ( 11) are present and predicted values, and their variation with temperature are both 0.5%/“C. Figure 1 shows the relations rearranged by substituting eqns ( 10) and ( 11) into eqn (9). The vertical axis represents energy efficiency and exergy efficiency, and the horizontal axis, the temperature difference, rc, between average heat collecting temperature and environmental temperature. From Fig. 1 or eqns ( 10 ) to ( 1 lb), it is clear that the changes in performance of the collector and the panel are linear, and the collector efficiency is significantly larger than PV cells. Moreover, the collector exergy has the maximum point. The temperature region where the solar thermal exergy is larger than the PV cells is calculated by eqn (9). As indicated in Fig. 1, the maximum heat cohecting temperature, 7c2, is 200°C in spite of the difference in cell characteristics, since rc is equal to 200°C when qsl = qs2 = 0 from eqns ( 1 la) and ( 1 lb). The minimum
243
New photovoltaic-thermal solar energy utilization method
account both the electrical and thermal energy effects from the sun. In the next section, we propose a new construction of a PV/T panel to effectively produce both heat and electricity. 3. NEW METHOD TO UTILIZE SUNLIGHT AND SOLAR THERMAL ENERGY
0
150 r $!2)=42.8
1 100
150 T (c)2c0
r =87.3 m
c
Fig. I. Energy and exergy efficiencies of a PV cell and heat collector.
temperature, rcl, depends on it. Its value is 42.8”C and 1OO’C in Sl and in S2, respectively. This means that exergy of collected heat is higher than that of the electrical power generated by PV cell when the present cell is used and the collecting temperature is above 42.8”C. On the other hand, if cell efficiency becomes better than 20%, 7c rises high. But the collector efficiency falls down. In Fig. 1, T, is the maximum exergy temperature calculated using eqn (6).
7,
=
To{(l + me/w-0)“*
- 1)
(12)
Substituting the collector characteristic value of eqn (10) into eqn (12), we obtain 7, = 87.3’C. When we evaluate PV panel in terms of exergy, it is the most desirable to set the heat-collecting temperature at T, for obtaining the effective energy by collector. Under this condition, the cell temperature is so high that PV electrical output decmases to about one-half compared to the standard conditions. When the conversion efficiency of PV cell is low, as expressed by eqn ( 1 la), the exergy of the heat obtained by the colkector is higher than that of PV cell, even if the temperature is lower than T, . Thus, it is necezzuy to collect the solar thermal energy in lower temperature because we can use the PV cell under favorable condition. When the efficiency of PV cell is high as eqn ( 11b), the heat-collecting temperature can be higher than rm. This requires that the heat collector should be operated around the maximum exergy point, without increasing the cell temperature, which would be accompanied by the decrease in cell efficiency. In the conventional PV/T panel consisting of a heat collector and including the PV cell, the cell temperature rises and its efficiency fails when we obtain high exergy solar thermal energy. Therefore, we do not think that this conventional model is good enough to take into
PV panels, being researched through the world as well as in Japan, are combined solar collectors and PV cells. As mentioned before, the cell performance is restricted by the heat-collecting temperature. We examined the new model in which solar collector does not affect the output of PV cell and produces electricity and heat effectively. Figure 2 shows the proposed model of a PV/T panel. As shown in this figure, the PV panel is placed in parallel to the solar collector and separated by a gap. In order for the sunlight to reach the solar collector, the PV cell should be packed with transparent materials. When the solar heat is absorbed by the solar collector, free convection is caused between the solar collector and the PV panel. Consequently, fresh air comes from the lower inlet of the gap, and the air flow cools the PV cell. In this model, solar thermal energy is collected without decreasing the efficiency of the PV cell. Based on such a configuration of the solar collector and the PV panel, we analyzedfie effect of cooling of the PV panel by natural convection. To analyze this model, we make the following assumptions. The rising flow of the air heating up by the collector does not transfer the heat to the PV panel. Outlet temperature of the air flow is equal to the temperature of the PV panel. The air flow removes the heat from the PV panel and from the collector. The surface temperatures of the collector and the PV panel are uniform and is in a steady state without the effect of outer wind. Under the above assumptions, the heat adsorbed by the PV panel, qs, is qs=as(l
-R)Qo
(13)
where, (Yeis the absorptivity and R is the transparent ratio defined by a ratio of transparent area to the whole area of the PV panel.
Olaf Colkctor cr.3
Fig. 2. Proposed model of a PV/T panel.
T.
244
TAKASHIMA ef al
Table I. Constants of analysis
The heat transmitted from the solar collector to the air flow, qc, is 4c = ssR( 1 - w)Qo
(14)
where, 7~ is transmissibility of the PV panel. The pressure difference between inlet and outlet in the gap, Ap, which is induced by buoyancy effect, is expressed as follows; Ap = gLpj3( Tc - To)sin 0 = gLpflr&in
0
(16)
where, p is the viscosity and S is the width of the gap between the collector and the PV panel, as described by Knudsen and Katz [ 61. From eqns ( 15) and ( 16), mean-velocity, U,,,, is given by Or, = J’garcsin
tI/ 12v
(17)
where, v, the kinematic viscosity (=~/p). From eqn ( 17 ) , we can decide the mass flow rate, and the heat gain of the air flow is calculated from its rise in temperature. That is, SWU,,,C,,p(T-
To)sinO = WL(qs+qc)
Q. (kW/m*)
1.102 17.86 300
7s % R L(m)
1.0
e (0)
(18)
where, Wand C, are the width of the PV panel and specific heat at constant pressure, respectively. Substituting eqns ( 13) and (14) into eqn ( 18), the temperature of the PV panel becomes
0.8 0.5 0.3 2 20, 30
7 = 12vLQo{ as( 1 - R) + T,JR( 1 - qoc + UCT~}/
(S3gflC,prcsin20)
(15)
where, g, L, p, fl, and 0 are the acceleration of gravity, flow length, density of the air, expansion ratio, and the angle between horizontal line and the solar collector, respectively. When this pressure difference causes the air flow, the relation between pressure difference and mean velocity of the laminar Bow in the gap, U,,,, becomes Ap = 12pU,,,LfS2
1.008
C, W/kgK) P (k/m’) v (mm*/s) To 6)
where, 7 = T-
(19)
To.
The mean velocity and the panel temperature calculated from eqns ( 17) and ( 19), together with the values in Table 1, are displayed in Fig. 3. In Table 1, properties of the air are all at 320 K. The PV panel is made of plastics, and the absorptivity is derived from the loss factors of the PV cell. The vertical axes of Fig. 3 are the temperature difference between the PV panel and the environment, 7, and the mean flow velocity, U,,,. Horizontal axis is temperature difference between the heat-collecting temperature and the environment temperature, 7~. In the parentheses in this figure, the left element is the angle of the panel and another is the width of the gap. As shown in eqn ( 17), the mean flow velocity is in proportion to rc. When the angle becomes large, buoyancy effect also becomes large and the flow velocity increases. As the width increases, the flow velocity increases substantially. On the other hand, the temperature of a PV panel, 7, decreases inversely with increasing in temperature. That is, as 7c increases, the panel temperature decreases rapidly. When the angle is 30”, the width is 5 cm and heat-collecting temperature is 40°C higher than ambient, the rise of the panel temperature is less than 4°C. At temperatures above T,, which is the maximum exergy efficiency temperature of the heat collector, the rise of the panel temperature is relatively small. Figure 4 displays the improvement of our proposed model. The vertical axes are the total energy efficiency,
20.0
15.0
G 10.0 -S YE 3
5.0
0.0 0
20
40
60
80
100
r p)
Fig. 3. Relation of PV cell temperature, flow velocity, and heat collecting temperature.
Fig. 4. Comparison of a conventional and proposed panel.
New photovoltaic-thermal solar energy utilization method v,, and the total exergy efficiency, .&, which are the sums of both efficiencies of the collector and the PV panel. The energy efficiencies are calculated by eqns ( IO) and ( 11b). On our proposed model, we assumed that the gap between the panel and the collector is 5 cm and the angle of the panel 30”. Figure 4 shows that the higher 7c becomes, the larger becomes the difference of the energy efficiencies. Since both energy efficiencies of conventional and proposed collectors are calculated by eqn (lo), they take the same value. Therefore, the difference of energy efficiencies is caused by the difference of PV output power. As to the exergy efficiencies, there is significant difference between the conventional and proposed model. The maximum value corresponding to the conventional model is given at 7c. = 46.4”C, while that of the proposed model is given at 7c = 88.3”C. Even if the solar collector temperature becomes high, the PV cell temperature is still low because of the cooling by natural convection. Therefore, the cell efficiency does not decrease and the total exergy efficiency is kept high. Though the cell temperature, 7, draws a curve in Fig. 3, the energy efficiency draws a line in Fig. 4. In comparison with the energy efficiency of the solar collector and the PV panel, that of the cell is smaller. Thus, even if the temperature of the cell changes like the curve shown in Fig. 3, the effect of temperature on efficiency is so little that the change of the total efficiency is small. Therefore, the efficiency changes linearly. In this analysis, according to eqn ( 15), the flow due to the buoyancy effect disappears when TV = 0. Thus, the point T<.= 0 becomes a singularity, and we cannot calculate the rise ofthe cell temperature using eqn ( 19). Placing the PV panel packed with transparent materials on solar collector with a gap results in the cooling of the PV cell, and we can generate both heat and electricity effectively at the same time. We can operate the PV/T panel under favorable condition. Nawata [ 7 ] and Yoshimi [ 8 ] reported when the PV panel is independently used on a roof without the solar collector, the rise of the cell temperature results in a drastic decrease of output power. Therefore, if we use the PV panel made of transparent materials, natural convection due to the roof prevents the rise of PV cell temperature, and the PV panel would supply the maximum power under desirable conditions.
245
4. CONCLUSIONS
Studies of a PV/T hybrid panel have been undertaken in Japan. Based on the results of those studies, we evaluated the energy quality ofa solar collector and PV cell in terms of exergy, and discussed a system placing the PV cell on the solar collector to generate electricity and heat effectively from the sum. These results lead to following conclusions. (a) By placing a PV panel on a solar collector with a gap, performance of the PV cell is higher than that of conventional type of a PV/T panel. (b) The cooling effect by natural convection is effective to prevent the rise of the PV cell temperature. PV panels composed of PV cells packed with transparent materials have been fabricated as a trial. These panels seem to be suitable for integration of PV in buildings. We believe that incorporating a PV/T panel such as indicated in this paper would be a significant contribution to spreading the solar energy utilization technology. Acknowledgments-The
authors wish to thank Dr. K. Sugisaki and Dr. K. Kurokawa on Electrotechnical Laboratory and Dr. T. Kajikawa on Shonan Institute of Technology for their helpful advice. REFERENCES T. Tanaka , I. Tsuda, and T. Tani, Development of solar total energy system in Japan, Solur Energy 37( I ). 55 (1986). 2. J. J. Loferski, C. Case, D. Doodlesack, B. Roessler, R. Dobbins, T. Russell, J. Beall, and J. Krikorian, Design and construction of a hybrid photovoltaic( 3kWp)-thermal solar energy system for a residential/commercial building, Cot$ Rec. IEEE Photovoltaic Spec. Cor$ 16,
I.
188 (1982).
3. I. Oshida, Lectures of exergy, Solar Energy Laboratory, Tokyo (1986). 4. A. Suzuki, H. Okamura, and I. Oshida, Application of exergy concept to the analysis ofoptimum operating conditions of solar heat collectors, Trans. qfASME., J. Solar Energy Eng. 109,337
(1987).
5. M. Fujiwara, Studies on the evaluation and the improvement of performance of solar thermal systems, Researches of the Electrotechnical Laboratory, 9 I9 ( 1990).
6. J. G. Knudsen and D. L. Katz, Fluid dynamiu and heat transfer. McGraw-Hill, New York (1958). 7. Y. Nawata, Evaluation of solar cooling and heating system aided by a photovoltaic array, 2nd ASME-JSES-JSME Int. Solar Energy Conf 491 ( I99 I ) . 8. T. Yoshimi, Basis and application of photovoltaic power generation system, J. JSES 16( I ), I7 ( l990),
Pergamon
0038-092X/94 $6.00+ .OO
NEW PROPOSAL FOR PHOTOVOLTAIC-THERMAL ENERGY UTILIZATION METHOD
SOLAR
TAKUMI TAKASHIMA,* TADAYOSHITANAKA,* TAKUYA Dor, * JUNJI KAMOSHIDA,** TATSUO TANI, *** and TAKASHI HORIGOME**** *Electrotechnical Laboratory, I-l-4 Umezono Tsukuba, Ibaraki, Japan, **Shibaura Institute of Technology, 307 Tameihara-Fukasaku Ohmiya, Saitama, Japan, ***Science University of Tokyo, l-3 Kagurazaka Shinjuku, Tokyo, Japan, and ****Tokyo University of Agriculture and Technology. 2-24-16 Nakamachi Koganei, Tokyo, Japan Abstract-One of the most effective methods of utilizing solar energy is to use the sunlight and solar thermal energy such as a photovohaic-thermal panel (PV/T panel) simultaneously. From such a viewpoint, systems using various kinds of PV panels were constructed in the world. In these panels, solar cells are set up at an absorber collecting solar thermal energy. Therefore, temperature of solar cell increases up to the prescribed temperature of thermal energy use, although it is lower than the cell temperature when using only solar cell panel. For maintaining cell conversion efficiency at the standard conditions, it is necessary to keep the cell at lower temperature. In this paper, electric and thermal energy obtained from a PV/T panel is evaluated in terms of energy. Based on this evaluation, the method of not to decrease cell conversion efficiency with collecting solar thermal energy was proposed.
1. INTRODUCIION Environmental pollutions, such as warming atmosphere with gas effluents or forest damage by acid rain caused by mass consumption of fossil fuel from the industrial development, are at present serious global environmental problems. Under the circumstances, to keep the growth of economics in the future for us who are poor in energy resources, it is important to develop and utilize nonpolluting clean energy resources. From the viewpoint of environmental conservation and energy security in our country, solar energy is one of the clean and self-sufficient energy resources. Effective utilization of solar energy in our country, which does not depend on energy resources imported from overseas, will make a significant contribution to preserve our global environment. Technical researches on photovoltaic conversion and solar thermal energy had already been developed in Japan. Now it seems that basic technologies have almost reached the stage of practical application. Therefore, we think it is possible to utilize solar energy more effectively by the integration of those technologies. In general, the most important energies we use are electricity and heat. If we simultaneously obtain both energy forms from the sun, it is very convenient to use for industry and domesticity. Therefore, as a “solar cogeneration system,” a solar total energy system with solar heat was studied by Tanaka et al. [ 11, and a photovoltaic-thermal hybrid system (PV/T system) using solar cells was reported by Loferski et al.[ 2 1. A solar
total energy system obtains high temperature heat from the sun, and utilizes them for generating electric power and for heating. This system can conveniently supply electric power and heat according to demand. Since the solar condition in Japan is not so suitable, we cannot get high temperature heat constantly and effectively. Therefore, further development of this system has not been promoted. On the other hand, a PV/T system has been developed as a method to increase the use of a photovoltaic cell (PV cell). In this system, a PV/T panel is constructed to obtain both electricity and heat, and a PV cell is usually constructed in a flat solar collector as reported by Loferski et al.[ 21. Therefore, electrical power and thermal energy are supplied separately, but they cannot be supplied according to the demands as in a solar total energy system. Furthermore, if we need to obtain high temperature heat from the PV/T panel, the temperature of the PV cell becomes high and its output power falls. Thus, it seems that ordinary construction of a PV/T panel makes the output of the PV cell down. If electricity and thermal energy are each obtained by an isolated unit or component, it is easy to convert electricity into thermal energy, but its inverse conversion is difficult because quality of both energy are different. From the above reason, it is necessary to develop a new method to get electricity and thermal energy effectively from the PV/T system. To attain this purpose, we evaluated the quality of electricity and thermal energy by using the concept of exergy, and proposed the method to obtain them without rising cell temperature.
241
T. TAKASHIMA etal.
242 2. EXERCY
EVALUATION
OF SOLAR
ENERGY
When electricity and thermal energy are compared quantitatively, we often feel that electricity and thermal energy is the same kind of energy because we use the same unit to describe their amounts. Actually, we cannot get effective thermal energy when temperature difference between two materials does not exist, even if their temperatures are high. However, electricity does not have such a restriction. Therefore, in the case of evaluating the quality of electricity and thermal energy obtained in a PV/T panel, we believe that it is reasonable to discuss their quality in terms of exergy. The basic evaluation of solar energy in terms of exergy has already been reported by Oshida [ 31, Suzuki et al.[ 41, Fujiwara [ 5 1, and so on. But we are unable to locate studies on a PV/T system. In this paper, we evaluated a PV/T system in terms of exergy. A system chosen as the subject of study is a PV/T collector, which is generally composed of a flat plate collector including PV cells like the collector proposed by Loferski et al. [ 21. In order to evaluate solar energy in terms of exergy, we assume that the sun is the heat reservoir. Then, Ts and Q0 denote the sun temperature and the incident energy flux from the sun, respectively. The exergy of solar energy, EO, is expressed as follows; Eo = (1 - To/Ts)Qo
(1)
where, To is the ambient temperature. By assuming that T, is much larger than To, it is possible to simplify eqn ( 1) as follows; Eo = Qo
(2)
temperature, the exergy of electricity obtained in PV panel, Es, becomes Es = Qs
The exergy efficiency of PV cell, &, is calculated byeqns(2)and(7). 4s = EslEo = QslQo = mdl
nc= QlQo = rloc-
Uc(Tc-
To)
(3)
where, 71~ and UC are optical efficiency and heat loss coefficient of the collector respectively. Q and Tc are the collected heat by the PV /T panel and average heat collecting temperature. Thus, EC, the exergy of Q, is expressed as EC = (1 - To/ TclQ
(4)
We define the exergy efficiency of the heat, &-, as follows; Fc = EclEo = ( 1 - To/Tc)Q/Qo
(5)
Substitute eqn ( 3) into eqn (5) gives: tc = (7-c - To){I~,
- Uc(Tc - To)}/Tc
(6)
As electricity, Q.,-, is not affected by environmental
- UdT-
To)} (8)
where pros, Us, and Tare the conversion efficiency on standard condition, temperature coefficient, and temperature of PV panel. We can obtain eqn (8) by assuming that PV output power depends only on its temperature and that T = Tc. Thus, the condition which the solar thermal exergy is larger than the PV electrical exergy is fc > Es. From eqns (6) and (8), the following relations can be obtained TCl <‘Tc<7c2
(9)
7cr,2 = {B + (B2 - 4C)“2}/2 B = (90~ - ~0s + ToUs)l(Uc
- Us),
C = r~osTol( UC - Us) where, 7c = Tc - To. To calculate the relations of eqn (9), we assume the characteristics of solar collector and PV panel can be written in terms of first order functions of temperature as follows: solar collector PV panel
On the other hand, collector efficiency, qc, is defined by the following equation.
(7)
nc = 0.8 - 0.004~~
(10)
nsI = O.l( 1 - 0.005~~)
( 1 la)
qs2 = 0.2( I - 0.005~~)
( 1 lb)
The characteristic of the solar collector is appropriate to that commercially available in Japan. Those of PV panel in eqns ( 11) are present and predicted values, and their variation with temperature are both 0.5%/“C. Figure 1 shows the relations rearranged by substituting eqns ( 10) and ( 11) into eqn (9). The vertical axis represents energy efficiency and exergy efficiency, and the horizontal axis, the temperature difference, rc, between average heat collecting temperature and environmental temperature. From Fig. 1 or eqns ( 10 ) to ( 1 lb), it is clear that the changes in performance of the collector and the panel are linear, and the collector efficiency is significantly larger than PV cells. Moreover, the collector exergy has the maximum point. The temperature region where the solar thermal exergy is larger than the PV cells is calculated by eqn (9). As indicated in Fig. 1, the maximum heat cohecting temperature, 7c2, is 200°C in spite of the difference in cell characteristics, since rc is equal to 200°C when qsl = qs2 = 0 from eqns ( 1 la) and ( 1 lb). The minimum
243
New photovoltaic-thermal solar energy utilization method
account both the electrical and thermal energy effects from the sun. In the next section, we propose a new construction of a PV/T panel to effectively produce both heat and electricity. 3. NEW METHOD TO UTILIZE SUNLIGHT AND SOLAR THERMAL ENERGY
0
150 r $!2)=42.8
1 100
150 T (c)2c0
r =87.3 m
c
Fig. I. Energy and exergy efficiencies of a PV cell and heat collector.
temperature, rcl, depends on it. Its value is 42.8”C and 1OO’C in Sl and in S2, respectively. This means that exergy of collected heat is higher than that of the electrical power generated by PV cell when the present cell is used and the collecting temperature is above 42.8”C. On the other hand, if cell efficiency becomes better than 20%, 7c rises high. But the collector efficiency falls down. In Fig. 1, T, is the maximum exergy temperature calculated using eqn (6).
7,
=
To{(l + me/w-0)“*
- 1)
(12)
Substituting the collector characteristic value of eqn (10) into eqn (12), we obtain 7, = 87.3’C. When we evaluate PV panel in terms of exergy, it is the most desirable to set the heat-collecting temperature at T, for obtaining the effective energy by collector. Under this condition, the cell temperature is so high that PV electrical output decmases to about one-half compared to the standard conditions. When the conversion efficiency of PV cell is low, as expressed by eqn ( 1 la), the exergy of the heat obtained by the colkector is higher than that of PV cell, even if the temperature is lower than T, . Thus, it is necezzuy to collect the solar thermal energy in lower temperature because we can use the PV cell under favorable condition. When the efficiency of PV cell is high as eqn ( 11b), the heat-collecting temperature can be higher than rm. This requires that the heat collector should be operated around the maximum exergy point, without increasing the cell temperature, which would be accompanied by the decrease in cell efficiency. In the conventional PV/T panel consisting of a heat collector and including the PV cell, the cell temperature rises and its efficiency fails when we obtain high exergy solar thermal energy. Therefore, we do not think that this conventional model is good enough to take into
PV panels, being researched through the world as well as in Japan, are combined solar collectors and PV cells. As mentioned before, the cell performance is restricted by the heat-collecting temperature. We examined the new model in which solar collector does not affect the output of PV cell and produces electricity and heat effectively. Figure 2 shows the proposed model of a PV/T panel. As shown in this figure, the PV panel is placed in parallel to the solar collector and separated by a gap. In order for the sunlight to reach the solar collector, the PV cell should be packed with transparent materials. When the solar heat is absorbed by the solar collector, free convection is caused between the solar collector and the PV panel. Consequently, fresh air comes from the lower inlet of the gap, and the air flow cools the PV cell. In this model, solar thermal energy is collected without decreasing the efficiency of the PV cell. Based on such a configuration of the solar collector and the PV panel, we analyzedfie effect of cooling of the PV panel by natural convection. To analyze this model, we make the following assumptions. The rising flow of the air heating up by the collector does not transfer the heat to the PV panel. Outlet temperature of the air flow is equal to the temperature of the PV panel. The air flow removes the heat from the PV panel and from the collector. The surface temperatures of the collector and the PV panel are uniform and is in a steady state without the effect of outer wind. Under the above assumptions, the heat adsorbed by the PV panel, qs, is qs=as(l
-R)Qo
(13)
where, (Yeis the absorptivity and R is the transparent ratio defined by a ratio of transparent area to the whole area of the PV panel.
Olaf Colkctor cr.3
Fig. 2. Proposed model of a PV/T panel.
T.
244
TAKASHIMA ef al
Table I. Constants of analysis
The heat transmitted from the solar collector to the air flow, qc, is 4c = ssR( 1 - w)Qo
(14)
where, 7~ is transmissibility of the PV panel. The pressure difference between inlet and outlet in the gap, Ap, which is induced by buoyancy effect, is expressed as follows; Ap = gLpj3( Tc - To)sin 0 = gLpflr&in
0
(16)
where, p is the viscosity and S is the width of the gap between the collector and the PV panel, as described by Knudsen and Katz [ 61. From eqns ( 15) and ( 16), mean-velocity, U,,,, is given by Or, = J’garcsin
tI/ 12v
(17)
where, v, the kinematic viscosity (=~/p). From eqn ( 17 ) , we can decide the mass flow rate, and the heat gain of the air flow is calculated from its rise in temperature. That is, SWU,,,C,,p(T-
To)sinO = WL(qs+qc)
Q. (kW/m*)
1.102 17.86 300
7s % R L(m)
1.0
e (0)
(18)
where, Wand C, are the width of the PV panel and specific heat at constant pressure, respectively. Substituting eqns ( 13) and (14) into eqn ( 18), the temperature of the PV panel becomes
0.8 0.5 0.3 2 20, 30
7 = 12vLQo{ as( 1 - R) + T,JR( 1 - qoc + UCT~}/
(S3gflC,prcsin20)
(15)
where, g, L, p, fl, and 0 are the acceleration of gravity, flow length, density of the air, expansion ratio, and the angle between horizontal line and the solar collector, respectively. When this pressure difference causes the air flow, the relation between pressure difference and mean velocity of the laminar Bow in the gap, U,,,, becomes Ap = 12pU,,,LfS2
1.008
C, W/kgK) P (k/m’) v (mm*/s) To 6)
where, 7 = T-
(19)
To.
The mean velocity and the panel temperature calculated from eqns ( 17) and ( 19), together with the values in Table 1, are displayed in Fig. 3. In Table 1, properties of the air are all at 320 K. The PV panel is made of plastics, and the absorptivity is derived from the loss factors of the PV cell. The vertical axes of Fig. 3 are the temperature difference between the PV panel and the environment, 7, and the mean flow velocity, U,,,. Horizontal axis is temperature difference between the heat-collecting temperature and the environment temperature, 7~. In the parentheses in this figure, the left element is the angle of the panel and another is the width of the gap. As shown in eqn ( 17), the mean flow velocity is in proportion to rc. When the angle becomes large, buoyancy effect also becomes large and the flow velocity increases. As the width increases, the flow velocity increases substantially. On the other hand, the temperature of a PV panel, 7, decreases inversely with increasing in temperature. That is, as 7c increases, the panel temperature decreases rapidly. When the angle is 30”, the width is 5 cm and heat-collecting temperature is 40°C higher than ambient, the rise of the panel temperature is less than 4°C. At temperatures above T,, which is the maximum exergy efficiency temperature of the heat collector, the rise of the panel temperature is relatively small. Figure 4 displays the improvement of our proposed model. The vertical axes are the total energy efficiency,
20.0
15.0
G 10.0 -S YE 3
5.0
0.0 0
20
40
60
80
100
r p)
Fig. 3. Relation of PV cell temperature, flow velocity, and heat collecting temperature.
Fig. 4. Comparison of a conventional and proposed panel.
New photovoltaic-thermal solar energy utilization method v,, and the total exergy efficiency, .&, which are the sums of both efficiencies of the collector and the PV panel. The energy efficiencies are calculated by eqns ( IO) and ( 11b). On our proposed model, we assumed that the gap between the panel and the collector is 5 cm and the angle of the panel 30”. Figure 4 shows that the higher 7c becomes, the larger becomes the difference of the energy efficiencies. Since both energy efficiencies of conventional and proposed collectors are calculated by eqn (lo), they take the same value. Therefore, the difference of energy efficiencies is caused by the difference of PV output power. As to the exergy efficiencies, there is significant difference between the conventional and proposed model. The maximum value corresponding to the conventional model is given at 7c. = 46.4”C, while that of the proposed model is given at 7c = 88.3”C. Even if the solar collector temperature becomes high, the PV cell temperature is still low because of the cooling by natural convection. Therefore, the cell efficiency does not decrease and the total exergy efficiency is kept high. Though the cell temperature, 7, draws a curve in Fig. 3, the energy efficiency draws a line in Fig. 4. In comparison with the energy efficiency of the solar collector and the PV panel, that of the cell is smaller. Thus, even if the temperature of the cell changes like the curve shown in Fig. 3, the effect of temperature on efficiency is so little that the change of the total efficiency is small. Therefore, the efficiency changes linearly. In this analysis, according to eqn ( 15), the flow due to the buoyancy effect disappears when TV = 0. Thus, the point T<.= 0 becomes a singularity, and we cannot calculate the rise ofthe cell temperature using eqn ( 19). Placing the PV panel packed with transparent materials on solar collector with a gap results in the cooling of the PV cell, and we can generate both heat and electricity effectively at the same time. We can operate the PV/T panel under favorable condition. Nawata [ 7 ] and Yoshimi [ 8 ] reported when the PV panel is independently used on a roof without the solar collector, the rise of the cell temperature results in a drastic decrease of output power. Therefore, if we use the PV panel made of transparent materials, natural convection due to the roof prevents the rise of PV cell temperature, and the PV panel would supply the maximum power under desirable conditions.
245
4. CONCLUSIONS
Studies of a PV/T hybrid panel have been undertaken in Japan. Based on the results of those studies, we evaluated the energy quality ofa solar collector and PV cell in terms of exergy, and discussed a system placing the PV cell on the solar collector to generate electricity and heat effectively from the sum. These results lead to following conclusions. (a) By placing a PV panel on a solar collector with a gap, performance of the PV cell is higher than that of conventional type of a PV/T panel. (b) The cooling effect by natural convection is effective to prevent the rise of the PV cell temperature. PV panels composed of PV cells packed with transparent materials have been fabricated as a trial. These panels seem to be suitable for integration of PV in buildings. We believe that incorporating a PV/T panel such as indicated in this paper would be a significant contribution to spreading the solar energy utilization technology. Acknowledgments-The
authors wish to thank Dr. K. Sugisaki and Dr. K. Kurokawa on Electrotechnical Laboratory and Dr. T. Kajikawa on Shonan Institute of Technology for their helpful advice. REFERENCES T. Tanaka , I. Tsuda, and T. Tani, Development of solar total energy system in Japan, Solur Energy 37( I ). 55 (1986). 2. J. J. Loferski, C. Case, D. Doodlesack, B. Roessler, R. Dobbins, T. Russell, J. Beall, and J. Krikorian, Design and construction of a hybrid photovoltaic( 3kWp)-thermal solar energy system for a residential/commercial building, Cot$ Rec. IEEE Photovoltaic Spec. Cor$ 16,
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