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Performance evaluation of an integrated solar water heater as an option for building energy conservation
Energy and Buildings 38 (2006) 214–219 www.elsevier.com/locate/enbuild
Performance evaluation of an integrated solar water heater as an option for building energy conservation C. Dharuman *, J.H. Arakeri, K. Srinivasan Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India Received 8 December 2004; received in revised form 10 May 2005; accepted 22 May 2005
Abstract Since a majority of residential and industrial building hot water needs are around 50 8C, an integrated solar water heater could provide a bulk source that blends collection and storage into one unit. This paper describes the design, construction and performance test results of one such water-heating device. The test unit has an absorber area of 1.3 m2 and can hold 170 l of water, of which extractable volume per day is 100 l. Its performance was evaluated under various typical operating conditions. Every morning at about 7:00 a.m., 100 l of hot water were drawn from the sump and replaced with cold water from the mains. Although, during most of the days, the peak temperatures of water obtained are between 50 and 60 8C, the next morning temperatures were lower at 45–50 8C. Daytime collection efficiencies of about 60% and overall efficiencies of about 40% were obtained. Tests were conducted with and without stratification. Night radiation losses were reduced by use of a screen insulation. # 2005 Published by Elsevier B.V. Keywords: Solar energy; Convection; Screen insulation; Integrated solar water heater; Energy storage
1. Introduction Residential, commercial and industrial buildings often have hot water requirements at <60 8C. Bathing, laundry and cleaning operations in the domestic sector need hot water at about 50 8C. In industrial buildings, hot water needs could vary depending on the type of activity. Sanitary water needs such as for cleaning of containers in the food industry, for increasing the solubility of salts in solutions in electroplating industry, dissolving dyes in aqueous medium in the paint industry are examples of low grade hot water needs in industrial environment. In particular, for animal houses, preparation of animal feed requires hot water around these temperatures. Preheated process water at about 50– 60 8C could be a preferred heat source over ambient air for heat pumps, which are increasingly becoming popular as energy conservation devices in various process industries. Although, conventional flat plate solar water heaters have been the first choice, they face the problem of space * Corresponding author. Tel.: +91 80 22932302; fax: +91 80 23600648. E-mail address: [email protected] (C. Dharuman). 0378-7788/$ – see front matter # 2005 Published by Elsevier B.V. doi:10.1016/j.enbuild.2005.05.007
requirements as adequate separate footprint areas need to be allocated for collectors and storage devices. Further, the efficiency of conversion drops considerably with increasing collection temperatures. In addition, costs arising out of complex manufacturing issues have deterred their widespread use. It will be prudent to consider integrated solar water heaters as cost effective but efficient devices for low temperature hot water needs in built environment which will also enable conserving the net space requirements. The advantages of integrated collectors over conventional flat plate collectors are simplicity of design and construction and lower cost [1,2]. The problems associated with thermosyphon circulation, a dominant mechanism of circulation in the latter are avoided in the former. The fabrication costs could be drastically reduced because integrating the pipes and fins can be avoided. A source of long-term performance degradation due to scaling in water flow passages of a flat plate collector can be avoided. There have been a number of experimental and theoretical studies on integrated solar water heaters [1,3–8]. Yet, the level of fruition in integrated water heating flat plate collectors is well short of that in conventional ones. The main disadvantage of an integrated solar water
C. Dharuman et al. / Energy and Buildings 38 (2006) 214–219
Nomenclature A area (m2) Cp specific heat of water (J/kg K) Eincident total incident energy (J) I solar insolation (W/m2) m mass of the water (kg) Q total thermal energy (J) t time (s) T water temperature (K) TE total thermal energy in the water (J) Ul overall loss coefficient (W/K) h efficiency Subscripts 0 reference 6 a.m. morning 6 8 p.m. evening 8 abs absorber avg average day daytime total total
heater is that since the absorber plate is not insulated, there is large nighttime temperatures drop of stored water. Other possible drawbacks are the bulkiness, stratification of water and low heat transfer due to lack of flow. The study reported herein complements the on going efforts by combating large nighttime loss by providing: (a) a screen insulation, which can be drawn in at night and (b) a large volume of water to increase the thermal inertia. The impetus for this study is derived from the need for hot water at <55 8C for numerous built environment sectors mentioned earlier, in tandem with the need to reduce the costs of such a service.
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and the absorber plate. Fig. 1 shows the main components of the integrated solar water heater. 2.1. Insulation screen: daytime mixing Various techniques have been employed in the past to reduce heat losses in integrated solar collectors during day and night [1,5,9–13], one of them being convection suppression using transparent honeycomb structures [14– 17]. The method adopted in the present study is to provide nighttime insulation. This is in the form of a screen made of very thin material, which covers the collector during the evening and night. The screen has 100 pleats and is located between the two glass covers as shown in Fig. 1. The mechanism to draw the screen is a galvanized iron threaded rod 10 mm in diameter and 1100 mm long fixed in-between the two glass covers and operated by a 12 V DC wiper motor. At an input voltage of 12 V to the motor, the insulation can be opened or closed in about 10 min. When fully deployed, the height and width of each pleat are about 45 and 10 mm, respectively. When the screen uncovers the absorber plate the pleats are tightly drawn together to a thickness of 150 mm. Two types of insulation materials were tried for the screen insulation, namely, polyethylene terrypthalate (type 1) and aluminized Mylar (type 2). When it was used, the screen insulation was drawn in at around 4:30 p.m. and opened the next day at about 7:00 a.m. During the daytime, a stable stratification is set up in water with the hottest layer of water at the top and coldest layer of water at the bottom. To study the effects of this stratification on the collector performances, stratification was disturbed periodically by mixing the water. This was achieved by running a 1/4 HP mono block pump every hour such that it draws water from the top layers and discharges at the bottom of the storage unit. 3. Data reduction
2. Description of the water heater The collector and storage assembly consists of a stainless-steel tray (1.45 m 0.7 m 0.2 m) with the sides and bottom insulated with 100 mm thick rock wool covered with a selectively coated copper absorber plate. Silicone rubber sealant is used as a gasket and an adhesive to fix the absorber plate to the stainless steel tank. The size of the absorber plate is 1.45 m 0.70 m and is about 0.6 mm thick. The collector assembly has a double-glazing of 4.4 mm thick toughened glass plate (1.63 m 1.08 m) mounted in an aluminum frame. The gap between the two glass plates is 70 mm. Neoprene rubber gaskets were used to seal the air gap between the aluminum frame and the glass covers. The glazing is mounted above the collector at an angle of 288 facing south to be commensurate with the latitude of Bangalore (138 N). Aluminized Mylar reflector sheets were fixed on the sidewalls in-between the glass cover
There do not appear to be standards for testing the type of collectors described here. However, the measurement standards prescribed for instrumentation for other waterheating collectors wherever applicable were followed. Temperatures were measured with calibrated T type (copper-constantan) multi-strand thermocouples (TC Ltd., UK) whose measurement uncertainty is less than 0.5 8C. Temperature of the absorber plate, of water at various depths and the ambient were logged using a PC-based data acquisition system which has a built-in reference junction compensation. Solar insolation was measured on some of the days. At any given time, the amount of thermal energy in the water in the collector was calculated using the water temperatures recorded at the different positions. To calculate the energy, the following equation has been used. Z Q ¼ Cp ðT T0 Þ dm (1)
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Fig. 1. Integrated solar water heater. (1) Glass covers, (2) absorber plates, (3) screen insulation, (4) motor with pully, (5) SS tank, (6) side insulation, (7) heater, (8) feed water tank, (9) inlet valve, (10) outlet, (11) mirror and (12) float.
where Q is the total thermal energy in the water, m the mass of the water, Cp the specific heat of water, T the water temperature and T0 is the reference temperature, which is taken as 25 8C. The total mass of the water in the storage tank is about 170 kg. Based on the thermocouple positions in the storage tank shown in Table 1, the mass of water is apportioned into different layers. The total thermal energy was calculated by summing the energies in the different layers. The average temperature of water in the collector is given by: Tavg ¼
Q þ T0 mCp
(2)
where subscripts 6 a.m. and 8 p.m. refer to the times of the day. Tamb is the average ambient temperature between 6 a.m. and 8 p.m., Tabs is the average absorber plate temperature and t is the time between 6 a.m. and 8 p.m. Two efficiencies are defined, one related to the water temperature at 4 p.m. on the same day and the other related to the water temperature at 6 a.m. the next day: hday ¼
Q4 p:m: Q9 a:m: total solar energy input
(4)
htotal ¼
Q6 a:m: next day Q9 a:m: total solar energy input
(5)
3.1. Heat loss coefficients and efficiency The overall loss coefficient (Ul) between the water heater and the ambient during nighttime was determined from the experimental data as follows: Ul ¼ mCp
dTavg =dt T6 a:m: T8 p:m: mCp Tabs Tamb tðTabs Tamb Þ
(3)
Table 1 Thermocouple positions in the storage tank Thermocouples
Position from bottom (in mm)
T1 T2 T3 T4 T5 T6 T7 T8
0.0 30 60 90 120 150 160 170
Fig. 2. Incident solar radiation on 27th March 2001.
C. Dharuman et al. / Energy and Buildings 38 (2006) 214–219
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Fig. 5. Temperature profiles in the water at different times during the day. Experiment type is without insulation and without mixing (NINM). Legend: (^) 7:15; (&) 9:15; (~) 11:15; () 13:15; (*) 15:15; (*) 17:15. Fig. 3. Incident solar radiation on 22nd March 2001.
3.2. Solar insolation The total incident solar energy (Eincident) was calculated from the measured solar insolation (I). Z (6) Eincident ¼ IAabs dt Solar insolation measured on a typical day (27 March 2001) is shown in Fig. 2, which was a sunny day. Data were taken from 9 a.m., at which time the insolation is about 500 W/m2. The peak insolation of 1000 W/m2 occured at around 12:30 p.m. and there is no useful radiation after about 4:30 p.m. Fig. 3 shows the insolation measured on another day (22 March 2001), which was a partially cloudy day. 4. Results and discussion Forty-seven experiments were carried out during February, March and April of 2001, during which both
Fig. 4. Temperature variations with time at different heights in the water. Experiment type is without insulation and without mixing (NINM). Legend: (^) T1; (&) T2; (~) T3; () T4; (*) T5; (*) T6; (+) T7; () T8.
clear and cloudy days were obtained. Experiments were done with and without the screen insulation drawn at night, and with and without mixing of the water in the daytime. Thus, four combinations of experimental conditions as specified below were investigated: (a) without insulation and without mixing (NINM), (b) without insulation and with mixing using the circulating pump (NIM), (c) with insulation and without mixing (INM) and (d) with insulation and mixing (IM). Every morning at about 7 a.m., 100 l of hot water were drawn from the collector and replaced with cold water from the mains. During most of the days, the peak water temperatures were 55–65 8C and the next morning temperatures were 47– 50 8C. Collection efficiencies (during daytime) of about 60% and total efficiencies of about 40% were obtained. The overall heat loss coefficient (Ul) between absorber and ambient is 3–5 W/K. The collector performance was satisfactory even during partially cloudy days. The temperature profiles in water at different times during the day and night on 26 March 2001 for an NINM type experiment are shown Figs. 4 and 5. Between 7 and
Fig. 6. Temperatures variation with time at different heights in the water. Experiment type is with insulation and mixing (IM). Legend: (^) T1; (&) T2; (~) T3; () T4; (*) T5; (*) T6; (+) T7; () T8.
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C. Dharuman et al. / Energy and Buildings 38 (2006) 214–219
Fig. 7. Temperature profiles in the water at different times during the day. Experiment type is with insulation and mixing (IM). Mixing with pump was done at 8.30, 10.30, 12.30, 2.30, 4.30 and 5.30 h. Legend: (^) 6:15; (&) 8:15; (~) 10:15; () 12:15; (*) 14:15; (*) 16:15; (+) 18:15.
9 a.m., due to simultaneous removal of hot water and introduction of cold water, the collector water temperature drops from about 50 to about 35 8C. From about 9 a.m., the absorption of solar radiation increases the temperatures in the upper layers and the temperature near the top reaches a maximum at about 3 p.m. At 9:30 a.m. already, the effect of water heating is seen. At 11:13 a.m., there exists a steep temperature gradient extending down to about 90 mm from the top. The water temperature at the top is about 55 8C, the temperature in the lowest 60 mm of water has been unaltered from the value at 9:30 a.m. Beyond about noon, heat is conducted to the lower water levels. The temperature of water at the top after reaching a maximum of 75 8C at 3:12 p.m. reduces monotonically thereafter. During the night, due to free convection mixing, the stratification is destroyed.
Fig. 8. Temperatures of the water in the collector 9 a.m. (shaded bars), at 4 p.m. (darkened bars) and next day morning at 6 a.m. (clear bars) for the month of March 2001 experiments.
Fig. 9. Temperatures of the water in the collector at 9 a.m. (shaded bars), at 4 p.m. (blackened bars) and next day morning at 6 a.m. (clear bars) for the month of April experiments.
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The experiments with mixing (IM and NIM types) were done to reduce the stratification during the day, and thus reduce the average absorber temperature. The idea is to reduce the heat loss to ambient, which is proportional to the difference between the absorber plate temperature and the ambient temperature. The temperature profiles in the water for an IM type experiment are shown in Figs. 6 and 7. From about 8:15 a.m. water warms up and a stable stratification is set up. At 8:30 a.m., the first mixing is done resulting in a nearly uniform temperature. Just about 15 min before next mixing (10:14 a.m.; Fig. 6) the top layers are heated and a stratification is setup. The stratification is destroyed at the next mixing at about 10:30 a.m. Continual mixings reduce the difference between the maximum water temperature (near the absorber plate) and the minimum water temperature. The insulation is expected to affect the night loss of heat, in particular the resistance between the bottom glass and top glass should increase. During the daytime, the behaviour of the collector will be similar to the cases without screen insulation. Both mixing and the insulation screen had only a marginal influence on the performance. Figs. 8 and 9 show histograms of water temperatures at three instances for the experiments in March and April. The instances are about 9 a.m., just after the introduction of cold water, about 4 p.m., approximately when the peak temperature is obtained, and about 6 a.m. the next day.
5. Conclusions The present model of the integrated solar water heater can supply 100 l of hot water at 45–50 8C in the early morning, which is quite adequate for bathing and cleaning in the domestic sector. Higher water temperatures are obtained during daytime, which could be gainfully exploited in the industrial and hospitality sectors. It is simple in design and is estimated to be 30% less expensive compared to conventional flat plate collectors of equivalent storage capacity. However, it appears that the screen insulation during nighttime and mixing to remove stratification only marginally improved the performance of collector. There was no tangible effect of the material of screen insulation. The
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overall heat loss coefficient between absorber and ambient is between 3 and 5 W/K.
References [1] H.P. Garg, Year round performance studies on a built-in storage type solar water heater at Jodhpur, Solar Energy 17 (1975) 167–172. [2] R.S. Chauhan, V. Kadambi, Performance of a collector-cum-storage type of solar water heater, Solar Energy 18 (1976) 327–335. [3] H.P. Garg, R. Usha, Theoretical and experimental studies on collector/ storage type solar water heater, Solar Energy 29 (1982) 467–478. [4] A comparative performance rating for an integrated solar collector/ storage vessel with inner sleeves to increase heat retention, Solar Energy 66 (1999) 291–202. [5] M. Smyth, P.C. Eames, B. Norton, Annual performance of heat retaining integrated collector/solar water heaters in a northern maritime climate, Solar Energy 70 (2001) 391–401. [6] D. Faiman, H. Haim, Idolaufer, Reducing the heat loss at night from solar water heaters of the integrated collector-storage variety, Solar Energy 71 (2001) 87–93. [7] S.M. Mousa, A.A. Bilal, On integrated solar water heating system, Solar Energy 29 (2002) 135–140. [8] Y. Tripanagnostopoulos, M. Souliotis, Th. Nousia, CPC type integrated collector storage systems, Solar Energy 72 (2002) 327–335. [9] I. Catton, D.K. Edwards, Effect of side walls on natural convection between horizontal plates heated from below, Transactions of the ASME Journal of Heat Transfer (1967) 295–299. [10] P.B.L. Chaurasia, J. Twidell, Collector cum storage solar water heater with and without transparent insulation material, Solar Energy 70 (2001) 403–416. [11] W.L. Heitz, J.W. Westwater, Critical Rayleigh numbers for natural convection of water confined in square cells with l/d from 0.5 to 8, Transactions of the ASME Journal of Heat Transfer 93 (2) (1971) 188– 196. [12] A.A. Mohamad, Integrated solar collector-storage tank system with thermal diode, Solar Energy 61 (1997) 211–218. [13] Y. Tripanagnostopoulos, P. Yianouslis, Integrated collector-storage systems with suppressed thermal losses, Solar Energy 48 (1992) 31– 43. [14] W.J. Platzer, Directional—hemispherical solar transmittance data for plastic honeycomb type structures, Solar Energy 49 (1992) 359–369. [15] J.G. Symons, The solar transmittance of some convection suppression devices for solar energy Application: An experimental study, Journal of Solar Energy Engineering 104 (1982) 251–256. [16] K.G.T. Hollands, Natural convection in horizontal thin-walled honeycomb panels, Transactions of the ASME Journal of Heat Transfer 95 (1982) 439–444. [17] H. Buchberg, K. Edwards, Design considerations for solar collectors with cylindrical glass honeycombs, Solar Energy 18 (1976) 193–203.
Performance evaluation of an integrated solar water heater as an option for building energy conservation C. Dharuman *, J.H. Arakeri, K. Srinivasan Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560012, India Received 8 December 2004; received in revised form 10 May 2005; accepted 22 May 2005
Abstract Since a majority of residential and industrial building hot water needs are around 50 8C, an integrated solar water heater could provide a bulk source that blends collection and storage into one unit. This paper describes the design, construction and performance test results of one such water-heating device. The test unit has an absorber area of 1.3 m2 and can hold 170 l of water, of which extractable volume per day is 100 l. Its performance was evaluated under various typical operating conditions. Every morning at about 7:00 a.m., 100 l of hot water were drawn from the sump and replaced with cold water from the mains. Although, during most of the days, the peak temperatures of water obtained are between 50 and 60 8C, the next morning temperatures were lower at 45–50 8C. Daytime collection efficiencies of about 60% and overall efficiencies of about 40% were obtained. Tests were conducted with and without stratification. Night radiation losses were reduced by use of a screen insulation. # 2005 Published by Elsevier B.V. Keywords: Solar energy; Convection; Screen insulation; Integrated solar water heater; Energy storage
1. Introduction Residential, commercial and industrial buildings often have hot water requirements at <60 8C. Bathing, laundry and cleaning operations in the domestic sector need hot water at about 50 8C. In industrial buildings, hot water needs could vary depending on the type of activity. Sanitary water needs such as for cleaning of containers in the food industry, for increasing the solubility of salts in solutions in electroplating industry, dissolving dyes in aqueous medium in the paint industry are examples of low grade hot water needs in industrial environment. In particular, for animal houses, preparation of animal feed requires hot water around these temperatures. Preheated process water at about 50– 60 8C could be a preferred heat source over ambient air for heat pumps, which are increasingly becoming popular as energy conservation devices in various process industries. Although, conventional flat plate solar water heaters have been the first choice, they face the problem of space * Corresponding author. Tel.: +91 80 22932302; fax: +91 80 23600648. E-mail address: [email protected] (C. Dharuman). 0378-7788/$ – see front matter # 2005 Published by Elsevier B.V. doi:10.1016/j.enbuild.2005.05.007
requirements as adequate separate footprint areas need to be allocated for collectors and storage devices. Further, the efficiency of conversion drops considerably with increasing collection temperatures. In addition, costs arising out of complex manufacturing issues have deterred their widespread use. It will be prudent to consider integrated solar water heaters as cost effective but efficient devices for low temperature hot water needs in built environment which will also enable conserving the net space requirements. The advantages of integrated collectors over conventional flat plate collectors are simplicity of design and construction and lower cost [1,2]. The problems associated with thermosyphon circulation, a dominant mechanism of circulation in the latter are avoided in the former. The fabrication costs could be drastically reduced because integrating the pipes and fins can be avoided. A source of long-term performance degradation due to scaling in water flow passages of a flat plate collector can be avoided. There have been a number of experimental and theoretical studies on integrated solar water heaters [1,3–8]. Yet, the level of fruition in integrated water heating flat plate collectors is well short of that in conventional ones. The main disadvantage of an integrated solar water
C. Dharuman et al. / Energy and Buildings 38 (2006) 214–219
Nomenclature A area (m2) Cp specific heat of water (J/kg K) Eincident total incident energy (J) I solar insolation (W/m2) m mass of the water (kg) Q total thermal energy (J) t time (s) T water temperature (K) TE total thermal energy in the water (J) Ul overall loss coefficient (W/K) h efficiency Subscripts 0 reference 6 a.m. morning 6 8 p.m. evening 8 abs absorber avg average day daytime total total
heater is that since the absorber plate is not insulated, there is large nighttime temperatures drop of stored water. Other possible drawbacks are the bulkiness, stratification of water and low heat transfer due to lack of flow. The study reported herein complements the on going efforts by combating large nighttime loss by providing: (a) a screen insulation, which can be drawn in at night and (b) a large volume of water to increase the thermal inertia. The impetus for this study is derived from the need for hot water at <55 8C for numerous built environment sectors mentioned earlier, in tandem with the need to reduce the costs of such a service.
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and the absorber plate. Fig. 1 shows the main components of the integrated solar water heater. 2.1. Insulation screen: daytime mixing Various techniques have been employed in the past to reduce heat losses in integrated solar collectors during day and night [1,5,9–13], one of them being convection suppression using transparent honeycomb structures [14– 17]. The method adopted in the present study is to provide nighttime insulation. This is in the form of a screen made of very thin material, which covers the collector during the evening and night. The screen has 100 pleats and is located between the two glass covers as shown in Fig. 1. The mechanism to draw the screen is a galvanized iron threaded rod 10 mm in diameter and 1100 mm long fixed in-between the two glass covers and operated by a 12 V DC wiper motor. At an input voltage of 12 V to the motor, the insulation can be opened or closed in about 10 min. When fully deployed, the height and width of each pleat are about 45 and 10 mm, respectively. When the screen uncovers the absorber plate the pleats are tightly drawn together to a thickness of 150 mm. Two types of insulation materials were tried for the screen insulation, namely, polyethylene terrypthalate (type 1) and aluminized Mylar (type 2). When it was used, the screen insulation was drawn in at around 4:30 p.m. and opened the next day at about 7:00 a.m. During the daytime, a stable stratification is set up in water with the hottest layer of water at the top and coldest layer of water at the bottom. To study the effects of this stratification on the collector performances, stratification was disturbed periodically by mixing the water. This was achieved by running a 1/4 HP mono block pump every hour such that it draws water from the top layers and discharges at the bottom of the storage unit. 3. Data reduction
2. Description of the water heater The collector and storage assembly consists of a stainless-steel tray (1.45 m 0.7 m 0.2 m) with the sides and bottom insulated with 100 mm thick rock wool covered with a selectively coated copper absorber plate. Silicone rubber sealant is used as a gasket and an adhesive to fix the absorber plate to the stainless steel tank. The size of the absorber plate is 1.45 m 0.70 m and is about 0.6 mm thick. The collector assembly has a double-glazing of 4.4 mm thick toughened glass plate (1.63 m 1.08 m) mounted in an aluminum frame. The gap between the two glass plates is 70 mm. Neoprene rubber gaskets were used to seal the air gap between the aluminum frame and the glass covers. The glazing is mounted above the collector at an angle of 288 facing south to be commensurate with the latitude of Bangalore (138 N). Aluminized Mylar reflector sheets were fixed on the sidewalls in-between the glass cover
There do not appear to be standards for testing the type of collectors described here. However, the measurement standards prescribed for instrumentation for other waterheating collectors wherever applicable were followed. Temperatures were measured with calibrated T type (copper-constantan) multi-strand thermocouples (TC Ltd., UK) whose measurement uncertainty is less than 0.5 8C. Temperature of the absorber plate, of water at various depths and the ambient were logged using a PC-based data acquisition system which has a built-in reference junction compensation. Solar insolation was measured on some of the days. At any given time, the amount of thermal energy in the water in the collector was calculated using the water temperatures recorded at the different positions. To calculate the energy, the following equation has been used. Z Q ¼ Cp ðT T0 Þ dm (1)
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Fig. 1. Integrated solar water heater. (1) Glass covers, (2) absorber plates, (3) screen insulation, (4) motor with pully, (5) SS tank, (6) side insulation, (7) heater, (8) feed water tank, (9) inlet valve, (10) outlet, (11) mirror and (12) float.
where Q is the total thermal energy in the water, m the mass of the water, Cp the specific heat of water, T the water temperature and T0 is the reference temperature, which is taken as 25 8C. The total mass of the water in the storage tank is about 170 kg. Based on the thermocouple positions in the storage tank shown in Table 1, the mass of water is apportioned into different layers. The total thermal energy was calculated by summing the energies in the different layers. The average temperature of water in the collector is given by: Tavg ¼
Q þ T0 mCp
(2)
where subscripts 6 a.m. and 8 p.m. refer to the times of the day. Tamb is the average ambient temperature between 6 a.m. and 8 p.m., Tabs is the average absorber plate temperature and t is the time between 6 a.m. and 8 p.m. Two efficiencies are defined, one related to the water temperature at 4 p.m. on the same day and the other related to the water temperature at 6 a.m. the next day: hday ¼
Q4 p:m: Q9 a:m: total solar energy input
(4)
htotal ¼
Q6 a:m: next day Q9 a:m: total solar energy input
(5)
3.1. Heat loss coefficients and efficiency The overall loss coefficient (Ul) between the water heater and the ambient during nighttime was determined from the experimental data as follows: Ul ¼ mCp
dTavg =dt T6 a:m: T8 p:m: mCp Tabs Tamb tðTabs Tamb Þ
(3)
Table 1 Thermocouple positions in the storage tank Thermocouples
Position from bottom (in mm)
T1 T2 T3 T4 T5 T6 T7 T8
0.0 30 60 90 120 150 160 170
Fig. 2. Incident solar radiation on 27th March 2001.
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Fig. 5. Temperature profiles in the water at different times during the day. Experiment type is without insulation and without mixing (NINM). Legend: (^) 7:15; (&) 9:15; (~) 11:15; () 13:15; (*) 15:15; (*) 17:15. Fig. 3. Incident solar radiation on 22nd March 2001.
3.2. Solar insolation The total incident solar energy (Eincident) was calculated from the measured solar insolation (I). Z (6) Eincident ¼ IAabs dt Solar insolation measured on a typical day (27 March 2001) is shown in Fig. 2, which was a sunny day. Data were taken from 9 a.m., at which time the insolation is about 500 W/m2. The peak insolation of 1000 W/m2 occured at around 12:30 p.m. and there is no useful radiation after about 4:30 p.m. Fig. 3 shows the insolation measured on another day (22 March 2001), which was a partially cloudy day. 4. Results and discussion Forty-seven experiments were carried out during February, March and April of 2001, during which both
Fig. 4. Temperature variations with time at different heights in the water. Experiment type is without insulation and without mixing (NINM). Legend: (^) T1; (&) T2; (~) T3; () T4; (*) T5; (*) T6; (+) T7; () T8.
clear and cloudy days were obtained. Experiments were done with and without the screen insulation drawn at night, and with and without mixing of the water in the daytime. Thus, four combinations of experimental conditions as specified below were investigated: (a) without insulation and without mixing (NINM), (b) without insulation and with mixing using the circulating pump (NIM), (c) with insulation and without mixing (INM) and (d) with insulation and mixing (IM). Every morning at about 7 a.m., 100 l of hot water were drawn from the collector and replaced with cold water from the mains. During most of the days, the peak water temperatures were 55–65 8C and the next morning temperatures were 47– 50 8C. Collection efficiencies (during daytime) of about 60% and total efficiencies of about 40% were obtained. The overall heat loss coefficient (Ul) between absorber and ambient is 3–5 W/K. The collector performance was satisfactory even during partially cloudy days. The temperature profiles in water at different times during the day and night on 26 March 2001 for an NINM type experiment are shown Figs. 4 and 5. Between 7 and
Fig. 6. Temperatures variation with time at different heights in the water. Experiment type is with insulation and mixing (IM). Legend: (^) T1; (&) T2; (~) T3; () T4; (*) T5; (*) T6; (+) T7; () T8.
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Fig. 7. Temperature profiles in the water at different times during the day. Experiment type is with insulation and mixing (IM). Mixing with pump was done at 8.30, 10.30, 12.30, 2.30, 4.30 and 5.30 h. Legend: (^) 6:15; (&) 8:15; (~) 10:15; () 12:15; (*) 14:15; (*) 16:15; (+) 18:15.
9 a.m., due to simultaneous removal of hot water and introduction of cold water, the collector water temperature drops from about 50 to about 35 8C. From about 9 a.m., the absorption of solar radiation increases the temperatures in the upper layers and the temperature near the top reaches a maximum at about 3 p.m. At 9:30 a.m. already, the effect of water heating is seen. At 11:13 a.m., there exists a steep temperature gradient extending down to about 90 mm from the top. The water temperature at the top is about 55 8C, the temperature in the lowest 60 mm of water has been unaltered from the value at 9:30 a.m. Beyond about noon, heat is conducted to the lower water levels. The temperature of water at the top after reaching a maximum of 75 8C at 3:12 p.m. reduces monotonically thereafter. During the night, due to free convection mixing, the stratification is destroyed.
Fig. 8. Temperatures of the water in the collector 9 a.m. (shaded bars), at 4 p.m. (darkened bars) and next day morning at 6 a.m. (clear bars) for the month of March 2001 experiments.
Fig. 9. Temperatures of the water in the collector at 9 a.m. (shaded bars), at 4 p.m. (blackened bars) and next day morning at 6 a.m. (clear bars) for the month of April experiments.
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The experiments with mixing (IM and NIM types) were done to reduce the stratification during the day, and thus reduce the average absorber temperature. The idea is to reduce the heat loss to ambient, which is proportional to the difference between the absorber plate temperature and the ambient temperature. The temperature profiles in the water for an IM type experiment are shown in Figs. 6 and 7. From about 8:15 a.m. water warms up and a stable stratification is set up. At 8:30 a.m., the first mixing is done resulting in a nearly uniform temperature. Just about 15 min before next mixing (10:14 a.m.; Fig. 6) the top layers are heated and a stratification is setup. The stratification is destroyed at the next mixing at about 10:30 a.m. Continual mixings reduce the difference between the maximum water temperature (near the absorber plate) and the minimum water temperature. The insulation is expected to affect the night loss of heat, in particular the resistance between the bottom glass and top glass should increase. During the daytime, the behaviour of the collector will be similar to the cases without screen insulation. Both mixing and the insulation screen had only a marginal influence on the performance. Figs. 8 and 9 show histograms of water temperatures at three instances for the experiments in March and April. The instances are about 9 a.m., just after the introduction of cold water, about 4 p.m., approximately when the peak temperature is obtained, and about 6 a.m. the next day.
5. Conclusions The present model of the integrated solar water heater can supply 100 l of hot water at 45–50 8C in the early morning, which is quite adequate for bathing and cleaning in the domestic sector. Higher water temperatures are obtained during daytime, which could be gainfully exploited in the industrial and hospitality sectors. It is simple in design and is estimated to be 30% less expensive compared to conventional flat plate collectors of equivalent storage capacity. However, it appears that the screen insulation during nighttime and mixing to remove stratification only marginally improved the performance of collector. There was no tangible effect of the material of screen insulation. The
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overall heat loss coefficient between absorber and ambient is between 3 and 5 W/K.
References [1] H.P. Garg, Year round performance studies on a built-in storage type solar water heater at Jodhpur, Solar Energy 17 (1975) 167–172. [2] R.S. Chauhan, V. Kadambi, Performance of a collector-cum-storage type of solar water heater, Solar Energy 18 (1976) 327–335. [3] H.P. Garg, R. Usha, Theoretical and experimental studies on collector/ storage type solar water heater, Solar Energy 29 (1982) 467–478. [4] A comparative performance rating for an integrated solar collector/ storage vessel with inner sleeves to increase heat retention, Solar Energy 66 (1999) 291–202. [5] M. Smyth, P.C. Eames, B. Norton, Annual performance of heat retaining integrated collector/solar water heaters in a northern maritime climate, Solar Energy 70 (2001) 391–401. [6] D. Faiman, H. Haim, Idolaufer, Reducing the heat loss at night from solar water heaters of the integrated collector-storage variety, Solar Energy 71 (2001) 87–93. [7] S.M. Mousa, A.A. Bilal, On integrated solar water heating system, Solar Energy 29 (2002) 135–140. [8] Y. Tripanagnostopoulos, M. Souliotis, Th. Nousia, CPC type integrated collector storage systems, Solar Energy 72 (2002) 327–335. [9] I. Catton, D.K. Edwards, Effect of side walls on natural convection between horizontal plates heated from below, Transactions of the ASME Journal of Heat Transfer (1967) 295–299. [10] P.B.L. Chaurasia, J. Twidell, Collector cum storage solar water heater with and without transparent insulation material, Solar Energy 70 (2001) 403–416. [11] W.L. Heitz, J.W. Westwater, Critical Rayleigh numbers for natural convection of water confined in square cells with l/d from 0.5 to 8, Transactions of the ASME Journal of Heat Transfer 93 (2) (1971) 188– 196. [12] A.A. Mohamad, Integrated solar collector-storage tank system with thermal diode, Solar Energy 61 (1997) 211–218. [13] Y. Tripanagnostopoulos, P. Yianouslis, Integrated collector-storage systems with suppressed thermal losses, Solar Energy 48 (1992) 31– 43. [14] W.J. Platzer, Directional—hemispherical solar transmittance data for plastic honeycomb type structures, Solar Energy 49 (1992) 359–369. [15] J.G. Symons, The solar transmittance of some convection suppression devices for solar energy Application: An experimental study, Journal of Solar Energy Engineering 104 (1982) 251–256. [16] K.G.T. Hollands, Natural convection in horizontal thin-walled honeycomb panels, Transactions of the ASME Journal of Heat Transfer 95 (1982) 439–444. [17] H. Buchberg, K. Edwards, Design considerations for solar collectors with cylindrical glass honeycombs, Solar Energy 18 (1976) 193–203.