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Performance of a solar water pump with ethyl ether as working fluid
Renewable Energy 22 (2001) 389±394 www.elsevier.com/locate/renene
Performance of a solar water pump with ethyl ether as working ¯uid Y.W. Wong, K. Sumathy* Department of Mechanical Engineering, University of Hong Kong, Hong Kong
Abstract This paper presents the description and operation of a solar-powered thermal water pump with ethyl ether as the working ¯uid. A simple ¯at-plate collector with an exposed area of 1 m2 is employed to pump water ranging from 700±1400 l/day depending upon the lift (6±10 m). Such a pumping system could achieve eciency of about 0.42±0.34%. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction An unconventional pump has long been recognized as a convenient device for operation, particularly in rural areas which are not electri®ed. In this pump, the vapour of a low boiling point liquid (so called working ¯uid) generated in a system of ¯at plate collectors provides motive power for lifting water, while its condensation and consequent decrease in pressure provides suction. Based on this principle, numerous solar thermal water pumping systems have been developed and studied [1±3]. The eciency of such a pump is low, being in the order of 0.1%. The merit of such pumps, however, lies in the fact that there are no moving parts and that they do not require electricity for their operation. These features make such units attractive for use in rural areas, despite their poor eciency. The present work focuses attention on such a pump with ethyl ether as the working * Corresponding author. Tel. +852-2859-2632; fax: +852-2858-5415. E-mail address: [email protected] (K. Sumathy). 0960-1481/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 1 4 8 1 ( 0 0 ) 0 0 0 6 5 - 3
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Y.W. Wong, K. Sumathy / Renewable Energy 22 (2001) 389±394
¯uid. For an assumed set of parameters such as the intensity of solar radiation, meteorological data, collector characteristics and discharge head, the number of cycles per day, quantity of water lifted per day and the overall eciency of the pump are evaluated from the thermodynamic analysis of the system for three discharge heads namely 6, 8 and 10 m. 2. System description Fig. 1 shows the schematic of the solar thermal water pump considered in this work. Liquid ethyl ether, set in motion by thermosiphon action, is heated in the ¯at-plate collector. The saturated ethyl ether vapour separating in tank S is stored in the vapour storage tank N. When vapour pressure in tank N reaches a predetermined value, it is isolated from tank S by closing valve 1. Valve 2 is simultaneously opened so that some vapour from N moves quickly to vessel A, which initially contains brine at atmospheric conditions (Ethyl ether is immiscible with aqueous salt solutions [4]). As a consequence, brine in vessel A is displaced into vessel B, which initially contains air at atmospheric conditions. The rising column of brine in B compresses the air in it and the compressed air in turn pushes the water in vessel C to the overhead tank D and this constitutes the pumping operation. At the end of pumping, valve 2 is closed and valve 1 is opened so that tank N is replenished with ethyl ether vapour from the collection system. Simultaneously, water from the overhead tank D is allowed to ¯ow through the cooling coils in vessel A (before the water goes for end use) to
Fig. 1. Schematic of the solar thermal water pump.
Y.W. Wong, K. Sumathy / Renewable Energy 22 (2001) 389±394
391
condense the spent ethyl ether vapour in it. Because of condensation the pressure in vessel A decreases, as a result of which the brine in vessel B returns to vessel A. During this period, the pressure of air in vessel B returns to its initial value. Consequently, the well water is sucked into vessel C through the one-way valve 5. One cycle of operation is thus completed and the pump is now ready for the next cycle.
3. System analysis The thermodynamic analysis of the pump is carried out [5], with the help of the collector analysis as suggested by Venkatesh and Sriramulu [6], with suitable modi®cations. For an assumed set of parameters such as intensity of solar radiation, meteorological data, collector characteristics and discharge head, the number of cycles per day, amount of water lifted per day and the overall eciency of the pump have been predicted. It is assumed to start with, that the collector and the separation tank together contain m kg of working ¯uid at the ambient temperature ta. Valve 1 is kept open so that the vapour in N and liquid in the collector system are in equilibrium and the system pressure is equal to the saturation pressure corresponding to the initial temperature of working ¯uid in the collector. Thus, in the heat transfer analysis, the system is assumed to have the working ¯uid as a mixture of liquid and vapour at a certain dryness fraction which can be calculated at any instant. Solar radiation incident on the collector heats the liquid ethyl ether gradually, until the pressure reaches a value which could pump 15 l of water per cycle to the assumed discharge head. The ®gure of 15 l is used in the analysis because, the proposed system operating with a ¯at-plate collector of area 1 m2, is designed for this value. However, the analysis proposed is general and is valid for any other quantity of water to be pumped as well. The minimum pressure is determined based on the work done (W1) by the vapour in compressing the air (assumed to be isothermal) in vessel B to raise the pressure to the discharge pressure, and the work done (W2) in lifting of 15 l of water to the assumed discharge head. In such case, W1 P1b V1b ln V2b =V1b :
1
where P1b and V1b are the initial pressure and volume of air in vessel B and V2b is its ®nal volume, after air is being compressed to the discharge pressure. The work done in lifting the water is given by, W2 Vw rw 9:81 head;
2
where, Vw and rw are the volume of water pumped and the density of water respectively. Once the minimum pressure is reached, the ®rst cycle of operation is commenced. The overall eciency Z0, is de®ned as the ratio of hydraulic work done by the
392
Y.W. Wong, K. Sumathy / Renewable Energy 22 (2001) 389±394
pump in a day to the total solar radiation incident on the collector during the period of operation of the pump. Z0 NWh =Htot
3
where N is the number of cycles per day and Wh is the hydraulic work per cycle given by, Wh Vc rw gh
4
where Vc is the volume of vessel C (theoretically, Vc is equal to the volume of water pumped per cycle) and h is the discharge head. The analysis is carried out for dierent discharge heads to study its eect on the performance of the pump. 4. Results and discussion The results of the thermodynamic analysis are presented to highlight the eect of discharge head on the performance of the system. In the analysis it is assumed that the solar radiation intensity varies sinusoidally from sunrise to sunset with a maximum of 1000 W/m2 at solar noon, and that the ambient temperature is 308C. The model is simulated based on the data corresponding to the thermodynamics and physical transport properties of ethyl ether (b.p. 348C) [7,8]. Fig. 2 shows the eect of discharge head on the number of cycles the pump can work and the amount of water that can be pumped in a day. It can be seen that as the discharge head increases, the number of cycles per day decreases. The major reason for this is that the pump requires a higher starting pressure to pump water at higher discharge heads. This in turn has an impact in the starting time of the ®rst cycle. As the head increases the pump starts working at a later time. This reduces the period of operation of the pump thereby resulting in a decreased number of cycles. The other reason is, as the discharge head increases, the air in B
Fig. 2. Eect of discharge head on number of cycles per day.
Y.W. Wong, K. Sumathy / Renewable Energy 22 (2001) 389±394
393
Fig. 3. Eect of discharge head on overall eciency.
has to be compressed to a higher pressure. This obviously requires an increased mass of ethyl ether vapour in vessel A in each cycle. The requirement of this increased ethyl ether mass per cycle results in a longer time for its generation and subsequent condensation; that is, increased condensation time increases the time required for each cycle. This in turn reduces the number of cycles and hence decreases the amount of water pumped per day. It can be seen from Fig. 2 that, while the pump can lift nearly 1.4 m3 water per day at a discharge head of 6 m, it can lift only 0.68 m3 at a discharge head of 10 m. Fig. 3 shows the variation in the overall eciency with discharge head. The eciency decreases with an increase in discharge head. The eciency is de®ned as the ratio of work done by the pump per day (product of work required to lift the water per cycle and the number of cycles per day) to the solar radiation incident on the collector during the period of operation of the pump. Although the work required to lift water per cycle increases with increase in discharge head, the number of cycles decrease drastically with increase in discharge head. Hence, for a given solar radiation intensity, the drastic decrease in the number of cycles with the increase in discharge head results in lower eciency. The overall eciency is around 0.42% at a discharge head of 6 m, and it is around 0.34% at 10 m.
5. Conclusion It is possible to predict the performance of a solar thermal water pump of the kind discussed in this paper from the thermodynamic analysis. The overall eciency of the pump is found to be around 0.34% at a discharge head of 10 m. Despite the poor eciency, the merit of such pumps, however, lie in the fact that there is no moving part and that they do not require electricity for their operation. Such pumps are of special signi®cance to countries where the farming communities are scattered over large and distant areas and where electrical power is not readily available.
394
Y.W. Wong, K. Sumathy / Renewable Energy 22 (2001) 389±394
References [1] Hariprakash Rao R. Theoretical and experimental investigations of a solar thermal water pump. PhD thesis, Dept. of Mechanical Engng, Indian Institute of Technology, Madras, India, 1990. [2] Kishore VVN, Gandhi MR, Pathak N, Gomkale SD, Rao KS. Development of a solar (thermal) water pump prototype Ð an Indo-Swiss experience. Solar Energy 1986;36(3):257±65. [3] Kwant KW, Rao DP, Srivastava AK. Experimental studies of a solar water pump. In: Proc. Int. Solar Energy Congress, Brighton, 1981. p. 1172±6. [4] Xie WH, Shiu WY, Mackay D. A review of the eect of salts on the solubility of organic compounds in seawater. Marine Environmental Research 1997;44(4):429±44. [5] Sumathy K, Venkatesh A, Sriramulu V. Eect of discharge head on the performance of a solar water pump. Int J Energy Research 1994;18:623±9. [6] Venkatesh A, Sriramulu V. Analysis of a ¯at-plate collector serving as a generator in an intermittent solar refrigeration system. J Energy 1989;14:23±8. [7] Yaws CL. Handbook of thermodynamic diagrams. Houston: Gulf Pub. Co, 1996. [8] Yaws CL. Thermodynamic and physical property data. Houston: Gulf Pub. Co, 1992.
Performance of a solar water pump with ethyl ether as working ¯uid Y.W. Wong, K. Sumathy* Department of Mechanical Engineering, University of Hong Kong, Hong Kong
Abstract This paper presents the description and operation of a solar-powered thermal water pump with ethyl ether as the working ¯uid. A simple ¯at-plate collector with an exposed area of 1 m2 is employed to pump water ranging from 700±1400 l/day depending upon the lift (6±10 m). Such a pumping system could achieve eciency of about 0.42±0.34%. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction An unconventional pump has long been recognized as a convenient device for operation, particularly in rural areas which are not electri®ed. In this pump, the vapour of a low boiling point liquid (so called working ¯uid) generated in a system of ¯at plate collectors provides motive power for lifting water, while its condensation and consequent decrease in pressure provides suction. Based on this principle, numerous solar thermal water pumping systems have been developed and studied [1±3]. The eciency of such a pump is low, being in the order of 0.1%. The merit of such pumps, however, lies in the fact that there are no moving parts and that they do not require electricity for their operation. These features make such units attractive for use in rural areas, despite their poor eciency. The present work focuses attention on such a pump with ethyl ether as the working * Corresponding author. Tel. +852-2859-2632; fax: +852-2858-5415. E-mail address: [email protected] (K. Sumathy). 0960-1481/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 1 4 8 1 ( 0 0 ) 0 0 0 6 5 - 3
390
Y.W. Wong, K. Sumathy / Renewable Energy 22 (2001) 389±394
¯uid. For an assumed set of parameters such as the intensity of solar radiation, meteorological data, collector characteristics and discharge head, the number of cycles per day, quantity of water lifted per day and the overall eciency of the pump are evaluated from the thermodynamic analysis of the system for three discharge heads namely 6, 8 and 10 m. 2. System description Fig. 1 shows the schematic of the solar thermal water pump considered in this work. Liquid ethyl ether, set in motion by thermosiphon action, is heated in the ¯at-plate collector. The saturated ethyl ether vapour separating in tank S is stored in the vapour storage tank N. When vapour pressure in tank N reaches a predetermined value, it is isolated from tank S by closing valve 1. Valve 2 is simultaneously opened so that some vapour from N moves quickly to vessel A, which initially contains brine at atmospheric conditions (Ethyl ether is immiscible with aqueous salt solutions [4]). As a consequence, brine in vessel A is displaced into vessel B, which initially contains air at atmospheric conditions. The rising column of brine in B compresses the air in it and the compressed air in turn pushes the water in vessel C to the overhead tank D and this constitutes the pumping operation. At the end of pumping, valve 2 is closed and valve 1 is opened so that tank N is replenished with ethyl ether vapour from the collection system. Simultaneously, water from the overhead tank D is allowed to ¯ow through the cooling coils in vessel A (before the water goes for end use) to
Fig. 1. Schematic of the solar thermal water pump.
Y.W. Wong, K. Sumathy / Renewable Energy 22 (2001) 389±394
391
condense the spent ethyl ether vapour in it. Because of condensation the pressure in vessel A decreases, as a result of which the brine in vessel B returns to vessel A. During this period, the pressure of air in vessel B returns to its initial value. Consequently, the well water is sucked into vessel C through the one-way valve 5. One cycle of operation is thus completed and the pump is now ready for the next cycle.
3. System analysis The thermodynamic analysis of the pump is carried out [5], with the help of the collector analysis as suggested by Venkatesh and Sriramulu [6], with suitable modi®cations. For an assumed set of parameters such as intensity of solar radiation, meteorological data, collector characteristics and discharge head, the number of cycles per day, amount of water lifted per day and the overall eciency of the pump have been predicted. It is assumed to start with, that the collector and the separation tank together contain m kg of working ¯uid at the ambient temperature ta. Valve 1 is kept open so that the vapour in N and liquid in the collector system are in equilibrium and the system pressure is equal to the saturation pressure corresponding to the initial temperature of working ¯uid in the collector. Thus, in the heat transfer analysis, the system is assumed to have the working ¯uid as a mixture of liquid and vapour at a certain dryness fraction which can be calculated at any instant. Solar radiation incident on the collector heats the liquid ethyl ether gradually, until the pressure reaches a value which could pump 15 l of water per cycle to the assumed discharge head. The ®gure of 15 l is used in the analysis because, the proposed system operating with a ¯at-plate collector of area 1 m2, is designed for this value. However, the analysis proposed is general and is valid for any other quantity of water to be pumped as well. The minimum pressure is determined based on the work done (W1) by the vapour in compressing the air (assumed to be isothermal) in vessel B to raise the pressure to the discharge pressure, and the work done (W2) in lifting of 15 l of water to the assumed discharge head. In such case, W1 P1b V1b ln V2b =V1b :
1
where P1b and V1b are the initial pressure and volume of air in vessel B and V2b is its ®nal volume, after air is being compressed to the discharge pressure. The work done in lifting the water is given by, W2 Vw rw 9:81 head;
2
where, Vw and rw are the volume of water pumped and the density of water respectively. Once the minimum pressure is reached, the ®rst cycle of operation is commenced. The overall eciency Z0, is de®ned as the ratio of hydraulic work done by the
392
Y.W. Wong, K. Sumathy / Renewable Energy 22 (2001) 389±394
pump in a day to the total solar radiation incident on the collector during the period of operation of the pump. Z0 NWh =Htot
3
where N is the number of cycles per day and Wh is the hydraulic work per cycle given by, Wh Vc rw gh
4
where Vc is the volume of vessel C (theoretically, Vc is equal to the volume of water pumped per cycle) and h is the discharge head. The analysis is carried out for dierent discharge heads to study its eect on the performance of the pump. 4. Results and discussion The results of the thermodynamic analysis are presented to highlight the eect of discharge head on the performance of the system. In the analysis it is assumed that the solar radiation intensity varies sinusoidally from sunrise to sunset with a maximum of 1000 W/m2 at solar noon, and that the ambient temperature is 308C. The model is simulated based on the data corresponding to the thermodynamics and physical transport properties of ethyl ether (b.p. 348C) [7,8]. Fig. 2 shows the eect of discharge head on the number of cycles the pump can work and the amount of water that can be pumped in a day. It can be seen that as the discharge head increases, the number of cycles per day decreases. The major reason for this is that the pump requires a higher starting pressure to pump water at higher discharge heads. This in turn has an impact in the starting time of the ®rst cycle. As the head increases the pump starts working at a later time. This reduces the period of operation of the pump thereby resulting in a decreased number of cycles. The other reason is, as the discharge head increases, the air in B
Fig. 2. Eect of discharge head on number of cycles per day.
Y.W. Wong, K. Sumathy / Renewable Energy 22 (2001) 389±394
393
Fig. 3. Eect of discharge head on overall eciency.
has to be compressed to a higher pressure. This obviously requires an increased mass of ethyl ether vapour in vessel A in each cycle. The requirement of this increased ethyl ether mass per cycle results in a longer time for its generation and subsequent condensation; that is, increased condensation time increases the time required for each cycle. This in turn reduces the number of cycles and hence decreases the amount of water pumped per day. It can be seen from Fig. 2 that, while the pump can lift nearly 1.4 m3 water per day at a discharge head of 6 m, it can lift only 0.68 m3 at a discharge head of 10 m. Fig. 3 shows the variation in the overall eciency with discharge head. The eciency decreases with an increase in discharge head. The eciency is de®ned as the ratio of work done by the pump per day (product of work required to lift the water per cycle and the number of cycles per day) to the solar radiation incident on the collector during the period of operation of the pump. Although the work required to lift water per cycle increases with increase in discharge head, the number of cycles decrease drastically with increase in discharge head. Hence, for a given solar radiation intensity, the drastic decrease in the number of cycles with the increase in discharge head results in lower eciency. The overall eciency is around 0.42% at a discharge head of 6 m, and it is around 0.34% at 10 m.
5. Conclusion It is possible to predict the performance of a solar thermal water pump of the kind discussed in this paper from the thermodynamic analysis. The overall eciency of the pump is found to be around 0.34% at a discharge head of 10 m. Despite the poor eciency, the merit of such pumps, however, lie in the fact that there is no moving part and that they do not require electricity for their operation. Such pumps are of special signi®cance to countries where the farming communities are scattered over large and distant areas and where electrical power is not readily available.
394
Y.W. Wong, K. Sumathy / Renewable Energy 22 (2001) 389±394
References [1] Hariprakash Rao R. Theoretical and experimental investigations of a solar thermal water pump. PhD thesis, Dept. of Mechanical Engng, Indian Institute of Technology, Madras, India, 1990. [2] Kishore VVN, Gandhi MR, Pathak N, Gomkale SD, Rao KS. Development of a solar (thermal) water pump prototype Ð an Indo-Swiss experience. Solar Energy 1986;36(3):257±65. [3] Kwant KW, Rao DP, Srivastava AK. Experimental studies of a solar water pump. In: Proc. Int. Solar Energy Congress, Brighton, 1981. p. 1172±6. [4] Xie WH, Shiu WY, Mackay D. A review of the eect of salts on the solubility of organic compounds in seawater. Marine Environmental Research 1997;44(4):429±44. [5] Sumathy K, Venkatesh A, Sriramulu V. Eect of discharge head on the performance of a solar water pump. Int J Energy Research 1994;18:623±9. [6] Venkatesh A, Sriramulu V. Analysis of a ¯at-plate collector serving as a generator in an intermittent solar refrigeration system. J Energy 1989;14:23±8. [7] Yaws CL. Handbook of thermodynamic diagrams. Houston: Gulf Pub. Co, 1996. [8] Yaws CL. Thermodynamic and physical property data. Houston: Gulf Pub. Co, 1992.