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Pergamon 0960-1481(94)0005!-4
Renewahh, Enery)', Vol. 4, No. 8, p p 897 905, 1994 Copyright ¢ 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0960 1481/94 $7.1}0+0,00
E X P E R I M E N T A L S T U D Y OF THE P E R F O R M A N C E OF A S O L A R T H E R M A L - P H O T O V O L T A I C I N T E G R A T E D SYSTEM M. A. HAMMAD University of Jordan, Faculty of Engineering and Technology, P.O. Box 13240, Amman, Jordan
(Received 30 July 1993" accepted 10 June 1994) Abstraet~In this work a self-contained solar heating forced water cooling unit was selected and assembled. It consists of three flat-plate solar collectors, each of area 1.2 m 2, a d.c. pump, a photovoltaic module and a storage tank. The electrical power produced by the photovoltaic module operates the d.c. pump, which circulates the cooling water through the solar collectors to transfer the heat to the storage tank. The electrical voltage and current, the water rate of flow, and water temperature at inlet and outlet of the collector were all measured. Solar irradiation, wind speed and ambient temperature were also measured. Daily distribution of electrical current, water mass rate of flow, module efficiency and collector efficiency were plotted in figures. Module efficiency, pump efficiency and collector efficiency were taken as dependent variables, while the solar irradiation, ambient temperature and time were the independent variables. Optimum values were graphically indicated and related to each other in a clear discussion. An economic study and comparison of three different systems were carried out : a common thermosyphon system ; an a.c. pump circulating system ; and this system, which is a d.c. pump circulating system. Results revealed that collector efficiency reached a daily average value of 47% due to d.c. pump installation powered by photovoltaic electric output.
INTRODUCTION
duces variable water mass flow rate, a n d hence variable efficiency. Daily distribution p e r f o r m a n c e a n d solar intensity effect were studied, and o p t i m u m working parameters were indicated. Experiments were carried out in A m m a n , J o r d a n , during the m o n t h s of April a n d May.
W a t e r heating accounts for a b o u t 2 0 % of the total energy c o n s u m p t i o n o f residential buildings, a n d a b o u t 7 % of t h a t of business buildings [1]. W a t e r solar heated by a fiat-plate collector is considered one of the c o m m o n l y used, reliable a n d m a i n t e n a n c e free sources of domestic hot water in most of the countries. The c o m m o n design practice for such systems is to install a storage tank a b o v e the level o f the collectors to create natural circulation p r o d u c e d by the t h e r m o syphon effect. This m e t h o d creates a slow circulation flow away fi'om o p t i m u m efficiency conditions, d e p e n d i n g on the water t e m p e r a t u r e a n d the head difference between the tank a n d the collectors. As the water flow increases, the heat collection efficiency increases [3]. As the rate of flow increases, the heat removal efficiency a n d the collector efficiency increase [2, 4-6]. Circulation p u m p s are needed to reach better efficiencies a n d o p t i m u m circulation conditions. A d.c. p u m p powered by the electrical o u t p u t of a p h o t o voltaic m o d u l e was used in this work to circulate the cooling water t h r o u g h a unit of three fiat-plate collectors, of 3.6 m 2 total collecting area. A n o r m a l storage tank was used in the system, as s h o w n in Figs 1 a n d 2. The variable current p r o d u c e d by the m o d u l e pro-
UNIT DESCRIPTION AND EXPERIMENTAL PROCEDURE A closed-circuit fiat-plate solar collector unit consisting o f three fiat plates, each of 1.2 m -~area, with a storage t a n k a n d a circulating p u m p , was constructed [7]. The fiat-plate collectors were m a d e by a local c o m p a n y , each h a v i n g seven copper pipes of 0.5 in. diameter, a n d a copper fiat plate. The storage tank was a 0.25 m 3 capacity steel tank. The p u m p was selected to be the smallest d.c. p u m p that can be f o u n d in the market, because of the small power required to circulate the water, which was only a b o u t 5 W. T h e p u m p used was of 14 1 s ~m a x i m u m flow rate, a n d of 5 m water m a x i m u m head. Consulting Ref. [8], a 7 W peak load module was used, consisting of 36 a m o r p h o u s p h o t o v o l t a i c cells, each of 7.5 x 7.5 cm area a n d of a total area 72.5 x 3l cm, fitted with a m o v a b l e base to select suitable inclinations a n d orientations, con897
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M.A. HAMMAD
Fig. 1. Photograph showing the system.
Storagetank Solarmodule 0~.OOQ[~Electrieal . wires
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Solar intensity
nected to a control panel containing a voltmeter and an ammeter, to measure the output. The following quantities were measured.
A solar integrator pyranometer was used to measure the solar intensity each quarter of an hour, in W m -2.
Temperature
Water rate of flow
A digital microprocessor was used with multithermocouple connections to measure the followings temperatures :
The cooling water rate of flow was measured using a positive displacement rotameter, with a range of 20100 g s -I.
water temperature inlet to the collector, T~°C, water temperature outlet of the collector, T2°C, ambient temperature, T~°C.
Wind velocity An anemometer with a digital liquid crystal display (LCD) was used to give the wind velocity in m s-~.
899
Solar thermal photovoltaic integrated system Voltage
A voltmeter within the module control panel was used to measure the output voltage. CHFI'cnI An ammeter, also as a part of the control panel, was used to measure the output current of the module. The flat-plate collectors and the module were fixed at 3 2 to the horizontal plane, the pyranometer was installed parallel, all were directed to the South. All above-mentioned parameters were recorded each 15 min of the day-long experiments, which began usually at 9.00 a.m. and ended at 4.30 p.m.
THE MATHEMATICAL ANALYSIS Quantities required for the analysis and to construct the figures shown within this work were extracted from the~collected data using the following equations. The water pumping power (Pw) in watts was calculated using the commonly known equation [9] : (1)
P,~=pxgxQxH,
where p is the water density (kg m-3), g is the gravity acceleration, Q is the volumetric rate of flow (m 3 s- ~) and H is the head of flow in metres. Equation (1) can be put in the form P,~ = AP × Q,
sure drop diagrams in Ref. [10] and the rate of flow quantities measured by the experiments. The power input to the pump and motor unit (Pp) in watts was also calculated from a known equation :
Pp ~ / x V,
(3)
where 1 is the current in amperes (A) and V is the voltage in volts (V). The pump and motor unit effeciency qr, is qp = A P x Q/(I x v).
(4)
It is a c o m m o n known fact that, as the size of the pump decreases, the efficiency decreases. This is why, in addition to other factors which will be discussed later, the values of qp ranged between 10% and 13%. The photovoltaic module efficiency ~/m will be calculated using the following equation : (5)
~M = I x V/([,XAm),
where/~ is the solar intensity in W m 2 and Am is the module area. The solar pumping efficiency q~ is the multiplication of both the module efficiency and the pump efficiency : ~ = ~/px r/re.
(6)
The flat-plate collector efficiency qf is calculated also using the following equation :
(2)
where AP is the pressure head of the pump in Pascal (Pa). The pressure drop can be found using the pres-
(7)
qr=M~xCx(T2-T,)/(LxAc),
where Mw is the mass rate of flow of the water (in kg
1.0
0.8
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Fig. 3. ~ V curve.
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M. A. HAMMAD
900 1100 1000 900 v >I,Z LU I-Z I-= he"
800
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TIME OF DAY ( h r ) Fig. 4. Solar intensity vs time. 0.9
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TIME OF DAY ( h r ) Fig. 5. Module current vs time.
s i), C is the specific heat of the water and Ac is the total collector area. DISCUSSION The performance of the thermal-photovoltaic integrated unit was studied as daily distribution of output current, water mass rate of flow, module efficiency and collector efficiency. Also, dependence of all mentioned variables on the solar intensity was studied, in addition to pump and solar pumping efficiencies. Experimental results are illustrated in Figs 3-12 ; these figures represent typical experimental results for April. Figure 3 shows the characteristic ~ V curves for both
the used module (amorphous 36 cells of silicon film) and the used d.c. pump. Curve (1) represents the p u m p - m o t o r unit performance as a straight line. Curve (2) represents t h e / - V curve of the module. The maximum power point, Pm is shown where, under load, maximum power can be obtained. The fill factor was found in this experiment to equal 0.713, which is considered a good performance of the module. The point of intersection of curves (1) and (2) is the working point (P). Both points Pm and P are identical on curve (3), which was plotted for an Is of 970 W m -2. This case represents the optimum performance. It is not easy to maintain such performance, because all parameters are changing with time, a condition first
Solar thermal photovoltaic integrated system
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Fig. 6. Mass flow rate vs time.
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TIME OF DAY ( h r )
Fig. 7. Module efficiency vs time.
discussed by Le Guerre and Ganibil [11], where they powered a water pumping unit by a photovoltaic module.
Daily distribution of parameters Figures 4~8 show the daily distribution of the measured and calculated parameters as follows. Figure 4 shows the solar intensity daily distribution, which shows a maximum value of about 1060 W m -2 normal to the collector plane, at about 12:30 noon. The sinusoidal form shown was repeated in Figs 5 and 6 for the module current and water mass rate of flow, respectively, but both had a slow change at the peak due to equipment thermal capacity and surrounding thermal effect. The current output reached a
maximum value of 0.82 A, while the voltage output remained constant at all conditions at 7.5 V. The mass rate of flow pattern followed the current pattern with flow rate ranging from 55 g s t up to 87 g s ~. The module panel efficiency increases with solar intensity to about 500 W m 2 Then it stays constant, as will be shown in Fig. 11, but it decreases with module temperature increase. Both effects were combined to form the daily distribution of efficiency shown in Fig. 7. It reached a low value in the middle of the day, and higher values in both the morning and the afternoon. Efficiency increased in Fig. 8 and reached the value of 55% at about 9:30 a.m. Then it decreased slowly to reach a value of about 30% at 16:00 h. The daily average efficiency was calculated
902
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I 12
I 13
I 14
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o
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0.5
0-4
400
/ I 500
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I 1000
1100
SOLAR INTENSITY (Wlrn=) Fig. 9. Module current vs solar intensity.
to equal 47%, which is higher than that of natural circulation, reported by literature to equal 40% [12].
iour, which was caused by the fact that the pump output is proportional to the power input, as shown in Fig. 10.
Solar intensity effect Figures 9-11 illustrate the effect of solar intensity Is on parameters such as current, mass rate of flow and efficiency. It is shown in Fig. 9 that the module output current increased with solar intensity increase. The rate was very high at low solar intensity values, but it decreased at high values of solar intensity. The same trend was followed by the cooling water mass rate o f flow for the same reasons Ol ~ ,,ly distribution behav-
Optimum efficiencies Three different efficiencies are shown in Fig. 1 I. The pump efficiency ranged from 6% to 13%. These low values are due to the fact that it is a very small pump and it was working away from its design range (24.0 V and 2.0 A maximum). The module efficiency ranged from 2.5% to 3.9%. With maximum value reached at 500 W m -2 solar
Solar thermal-photovoltaic integrated system
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80 75
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I
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600
700
800
900
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1100
SOLAR INTENSITY ( W / m z)
Fig. 10. Mass flow rate vs solar intensity.
14
12
A
10
o°
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,' MODULE " PUMP o TOTAL x10
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2 0 400
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500
600
700
800
900
1000
1100
SOLAR INTENSITY ( W l m 2)
Fig. 11. Efficiency vs solar intensity.
intensity, the efficiencies are reasonable since the cells are amorphous silicon films. This performance compares to literature results of Refs [8, 12]. The solar pumping efficiency qs reached a value of 0.4%. The optimum collector efficiency occured at about 62 g s- ~water rate of flow, as shown in Fig. 12. This result confirms the values of optimum flow in Refs [2, 4-6]. This optimum flow occurred at the solar intensity value of 500 W m z which also gave optimum performance of the module.
Economic review
The addition of the pump and the module to the c o m m o n house-size three-flat-plate-collector unit will increase efficiency. At the same time an increase in cost will be encountered. Table 1 shows the cost increase and daily average efficiency, from this the payback period was calculated assuming 8% interest rate, the a.c.-driven pump system was added to the table for the sake of comparison, taking the results from the literature [14].
904
M.A. HAMMAD 80
75 70 65 Z la.I 1.1_ U.. ILl
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60 55
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60 65 70 75 MASS FLOW RATE ( g / s )
I
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80
85
90
Fig. 12. Collector efficiency vs mass flow rate.
Table 1. Cost analysis for different systems System used Common, natural circulation a.c. electrical mains pump d.c. PV pump, this project
Costs increase JD* US$ 0.00 150.0 + 14 year i 200.0
0.00 215.0 +20 year i 286.0
Average daily efficiency
Payback period
0.4 [13] 0.49 [14] 0.47
0.00 18 years 16.7 years
*Jordanian Dinars.
The modification costs included the p u m p prices a n d installation costs. Yearly electrical costs to drive the a.c. p u m p are s h o w n to equal 14 J o r d a n i a n D i n a r s per year ; these values were calculated at 1992 prices.
2.
RESULTS
4.
T h e average daily efficiency of the flat-plate watercooled collectors reached 47%, by modifying it with a circulating d.c. p u m p powered by a p h o t o v o l t a i c module. This unit was built to be completely indep e n d e n t o f electrical m a i n s input. The unit showed a n acceptable reasonable performance, but showed a very long p a y b a c k period o f a b o u t 16.7 years. These kinds o f units need to be studied with the changes o f m a r k e t prices o f b o t h fuel a n d the p h o t o v o l t a i c cells, to reach prices which lower the p a y b a c k period. REFERENCES
1. E. Vine, D. Crawly and P. Centolella (eds) Energy
3.
5.
6. 7. 8. 9. 10.
efficiency and the environment. American Council for an Energy-Efficient Economy (ACE) (1991). W. B. Stine and R. W. Harrigan, Solar Energy Fundamentals and Design. Wiley, New York (1985). J. Duffle and W. Beckman, Solar Engineering of Thermal Processes. Wiley, New York (1980). Method of Testing to Determine Thermal Performance of Solar Collectors. ASHRAE Standards 93-77. American Society for Heating, Refrigeration and Air Conditioning Engineers, New York (1977). P. B. Howells, Effect of some design factors on the performance of domestic solar hot water heating systems. Solar World Forum, Vol. 1, 1981, Pergamon Press, 4(~ 44. C. W. J. Van Koppen, Active Heating in Buildings. In Solar World Forum, Vol. 1. Pergamon Press, pp. 8-19. M. Ashhab, N. Nawayseh and A. Haddadin, Thermal photovoltaic solar integrated system. Report, University of Jordan (May 1993). S. Robert, Solar Electricity. Prentice-Hall, New York (1991). N. Streeter and E. Wyliey, Fluid Mechanics. McGrawHill, New York (1975). ASHRAE, 85 Hand Book of Fudamentals. Atlanta, GA.
Solar thermal photovoltaic integrated system I I. J. R. Le Querre and C. Ganibal, Theoretical and Experimental AnaO,sis o[" a Solar Pump. Intersol 85, Vol. 3. Pergamon Press, Oxford, 1684-1689. 12. M. Mahmoud, 1. Nabhan and H, Akjian, Outdoor testing of photovoltaic module and systems. Joint Research Project between EEC (ISPRA) and The Royal Scientific Society, Report, p. 57 (1986).
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13. M. Alsaad, Annual performance of Thermo-syphon solar domestic hot water system DIRASA T XlV (9), 191 210 (1987). 14. G. Faidi, H. Ma'ayeh and 1. Halasah, Desalination of sea water using flash vapor utilizing solar energy. Report, University of Jordan, pp. 1-40 (1992).