- Email: [email protected]
Performance of a solar water pump with n-pentane and ethyl ether as working fluids
Energy Conversion & Management 41 (2000) 915±927
www.elsevier.com/locate/enconman
Performance of a solar water pump with n-pentane and ethyl ether as working ¯uids Y.W. Wong, K. Sumathy* Department of Mechanical Engineering, University of Hong Kong, Pokfulam Road, Hong Kong Received 16 March 1999; accepted 20 September 1999
Abstract In this study, the performance of a solar thermal water pump with two dierent working ¯uids such as n-pentane and ethyl ether, is reported. The theoretical comparison is made in terms of the number of cycles the pump can work per day, the amount of water that can be pumped per day and the overall eciency of the system. It is found that the eciency of the pump with ethyl ether is about 17% higher than that with n-pentane at a discharge head of 6 m. It is shown that ethyl ether seems to be a better choice for operating a non-conventional solar thermal water pump in terms of eciency and economics. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Solar water pump; Flat plate collector; n-Pentane; Ethyl ether; Thermodynamic analysis
1. Introduction Solar 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 [1]. In the recent past, several solar thermal water pumping systems have been studied. Most of these systems have the advantages of being simple, inexpensive, maintenance free, easily assembled and non-mechanical. Most of the solar thermal water pumps (STWP) make use of the fact that there is a change in the volume of a liquid on vapourization and also when the vapour is condensed. The volume increase at a given pressure may be utilized to displace water to a * Corresponding author. Tel.: +852-2859-2632; fax: +852-2858-5415. E-mail address: [email protected] (K. Sumathy). 0196-8904/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 9 9 ) 0 0 1 6 7 - 3
916
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
Nomenclature dm g Hmax Htot h m m1 m2 N P1b Pap pi p1 p2 pd ta tp tw t1 U1 U2 U'2 ui V V1b V2b VC VN Vw v1 v2 W1 W2 Wh WT
mass of working ¯uid entering vessel A to eect pumping (kg) acceleration due to gravity (m/s2) maximum solar intensity at noon (W/m2) total solar radiation incident on collector during period of operation of pump (J) discharge head (m) initial mass of working ¯uid in collector system (kg) mass of working ¯uid in tank N before pumping action (kg) mass of working ¯uid remaining in tank N after pumping action (kg) number of pumping cycles per day initial air pressure in tank B (Pa) minimum pressure required for actuating pump (Pa) arbitrary assumed pressure (Pa) pressure of working ¯uid in tank N before pumping (Pa) pressure of working ¯uid in tank N after pumping (Pa) discharge pressure (Pa) ambient temperature (K) temperature of absorber plate (K) temperature of tube wall of risers (K) temperature of working ¯uid in collector and tank S and N when valve 1 is open (K) initial internal energy of working ¯uid in tank N before pumping (J) ®nal internal energy of working ¯uid in tank N and vessel A after pumping at discharge pressure (J) ®nal internal energy of working ¯uid in tank N and vessel A after pumping at discharge pressure required to lift 15 l of water (J) initial speci®c internal energy of working ¯uid in tank N before pumping (J/kg) volume occupied by vapour in tank A (m3) initial volume of air in tank B (m3) ®nal volume of air in tank B (m3) volume of tank C (m3)=volume of water pumped per cycle volume of tank N (m3) volume of water pumped in each cycle (m3) speci®c volume of vapour in tank N before pumping (m3/kg) speci®c volume of vapour in tank N after pumping (m3/kg) work done in compressing air in vessel B (J) work done in lifting 15 l of water to assumed head (J) hydraulic work per cycle (J)=W2 total work done required in each cycle (J)
Greek Letters rw density of water (kg/m3)
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
g tc th Z0
917
ratio of speci®c heat capacities time for condensation of spent vapour (s) time for heating adequate vapour for pumping (s) overall eciency of pump (%)
higher elevation, while the volume reduction at a lower pressure is used for suction of water for the next cycle of pumping action. The volume of water displaced is proportional to the change in volume of the ¯uid undergoing the phase change. n-Pentane is one of the most widely used working ¯uids, as it possesses the desirable properties required to operate the above said non-conventional pump [2±4]. Since the npentane is expensive, ethyl ether is chosen as one of the potential substitutes. Ethyl ether possesses most of the desirable properties from the view point of thermodynamics and the heat transfer required by an STWP. Also, it is available at a much lower price. Hence, in the present study, a comparative study is made on the performance of an STWP using ethyl ether and n-pentane as the working ¯uids.
Fig. 1. Schematic of the solar thermal water pump.
918
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
2. System description Fig. 1 shows the schematic of the solar thermal water pump considered in this work. The working ¯uid, set in motion by thermosiphon action, is heated in the ¯at plate collector. The saturated vapour separated in tank S is stored in the vapour storage tank N. When the 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 water at atmospheric conditions. As a consequence, the water in vessel A is displaced into vessel B, which initially contains air at atmospheric conditions. The rising column of water in B compresses the air in it, and the compressed air, in turn, pushes the water in vessel C to the overhead tank D, constituting the pumping operation. At the end of pumping, valve 2 is closed and valve 1 is opened so that tank N is replenished with vapour of the working ¯uid 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 condense the spent vapour in it. Because of condensation, the pressure in vessel A decreases, as a result of which the water 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. The process undergone by the working ¯uid during this ®rst cycle of operation is clearly shown in the p±v diagram of Fig. 2, by 1±2±3±4±5±6. Here, 1±2 represents heating of the working ¯uid in the collector. Working ¯uid vapourized at state 3, in tank N, expands to state 4 (when vapour in tank N is suddenly led to vessel A) to reach the pressure corresponding to the discharge head. 4±5 and 5±6 show the idealized processes of cooling and condensation of
Fig. 2. p±v diagram.
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
919
the vapour. The time interval between the ®rst and the second cycle is determined by the time taken for condensation of the vapour in the ®rst cycle. The faster the condensation, the quicker will be the return of water to vessel A from vessel B, since the pressure is set back to the value that existed at the start of the cycle. During the period of condensation, the working ¯uid in the collector is being continuously heated by solar energy. Hence, at the start of the second cycle of pumping water, the temperature and hence the pressure of the working ¯uid will normally be higher than those at the beginning of the ®rst cycle. The new pressure is shown as the one that corresponds to states 8 and 9 in Fig. 2. The process of lifting water and the subsequent condensation of the working ¯uid in tank A follows the same pattern as before. The process undergone by the working ¯uid during this cycle is shown as 7±8±9±10±11±6. Similar sets of events occur in the subsequent cycles, and the pump continues to operate as long as the working ¯uid in the collector can get heated. The total quantity of water the pump can lift in a day is equal to the product of the number of cycles per day and the volume of water pumped in each cycle. Theoretically, the latter is equal to the volume of vessel C. 3. Thermodynamic analysis The thermodynamic analysis of the pump is performed [5] with the help of the collector analysis suggested by Venkatesh and Sriramulu [6], with suitable modi®cations. 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, amount of water lifted per day and the overall eciency of the pump have been predicted. It is assumed to start, 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 the 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 working ¯uid 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 15 l of water to the assumed discharge head. In such a 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 the air has been compressed to the discharge pressure. The work done in lifting the water is given by,
920
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
W2 Vw rw 9:81 head
2
where Vw and rw are the volume of water pumped and the density of the water, respectively. Since the discharge of vapour from tank N is assumed to be rapid, the process can be considered to be adiabatic. The initial internal energy U1 of m1 kg of vapour in tank N is given by U1 m1 ui
3
where ui is the speci®c internal energy of saturated vapour at an arbitrarily assumed pressure pi, which could allow the pump to lift 15 l of water. Suppose U2 is the ®nal internal energy of the same mass of vapour after it expands into vessel A to cause the air in B to get compressed and then lift 15 l of water from C to D, then from the ®rst law of thermodynamics, U2 U1 ÿ WT :
4
The work done by the vapour WT is given by, WT W1 W2
5
U2 corresponds to the condition when a part of the m1 kg of vapour is in tank N and the rest is in vessel A. It is also a fact that the ®nal pressure in the tank N and in vessel A will be equal to the discharge pressure (Pd). The ®nal speci®c volume of this mass of vapour is given by, v2 Vn V =m1 ;
6
Here, Vn is the volume of tank N and V is the volume occupied by the working ¯uid vapour in tank A. With the help of the property relations, the ®nal internal energy U'2 can now be evaluated in terms of pd and v2. If the assumed value of pi is such that the pump can lift 15 l of water, the values of U'2 and U2 must be the same. If not, a new value is assumed for pi, and a similar analysis is made. This iterative procedure is continued, using a computer, until the unique value of pi that makes U'2=U2 is obtained. In the p±v diagram, this pressure is equal to the pressure corresponding to states 2 and 3. Let this pressure be pap. When the temperature of the working ¯uid in the collector rises and the saturated vapour in tank N reaches the value pap, valve 1 is closed, and valve 2 is opened. A part of the working ¯uid vapour in tank N enters vessel A and displaces water in it to vessel B, thereby increasing the pressure of the air in B to the discharge pressure so as to eect the required pumping. An indirect method is used to determine the mass of working ¯uid, dm, that enters vessel A to eect the pumping of water from vessel C to the overhead tank D. It is assumed that the m2 kg of vapour remaining in tank N has undergone a reversible adiabatic process. Assuming that the vapour behaves like a perfect gas, p1 vg1 p2 vg2
7
where states 1 and 2 correspond to the equilibrium conditions in tank N before opening valve 2 and after closing it, respectively. In this equation, the unknowns are p2 and v2, since the mass
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
921
remaining in the vessel is not known. Hence, two equations of constraint are needed to evaluate the values of p2 and v2. One is the above equation (Eq. (7)) and the other is the energy equation. For an assumed pressure p2, v2 can be evaluated and thus m2 can be calculated as it is equal to VN/v2. The mass of vapour transferred to vessel A is then equal to (m1ÿm2). If the energy equation, using this mass, is not satis®ed, then a new p2 is assumed until the energy equation is satis®ed, and real value of m2 is obtained. At the end of pumping, vessels A, B and C are at a pressure corresponding to the discharge head. The water from the overhead tank is allowed to pass through the cooling coils in vessel A. As the working ¯uid vapour condenses, the pressure in vessel A decreases, and it enables the water to be sucked through the one way valve 5 into vessel C, making the system ready for the next cycle of pumping. During the period of condensation of the spent vapour in the ®rst cycle of operation, valve 1 is opened, keeping valve 2 closed to charge the tank N with saturated vapour. As the collectors are continuously exposed to the sun, the temperature of the remaining working ¯uid (liquid+vapour) in the system will rise. The temperature increase is predicted by equating the heat transfer during the period of heating to the dierence in internal energy of the working ¯uid in the collection system. The analysis is continued until such time that the working ¯uid in the collection system can continuously get heated. Heat transfer analysis suggests that the pump can operate until around 2.00 p.m. The overall eciency Z0, is de®ned as the ratio of the hydraulic work done by the pump in a day to the total solar radiation incident on the collector during the period of operation of the pump. Z0 NWh =Htot
8
where N is the number of cycles per day and Wh is the hydraulic work per cycle given by Wh VC rw gh
9
where VC is the volume of water pumped and h is the discharge head. The analysis is performed for dierent discharge heads to study its eect on the performance of the pump.
4. Selection of working ¯uids For successful operation of the non-conventional solar thermal water pump discussed, careful selection of the working ¯uid is essential. The selected ¯uid must have most of the desirable properties from the viewpoint of thermodynamics and heat transfer, such as [7]: . . . . . .
normal boiling point should be slightly higher than the local average ambient temperature; low enthalpy of vapourization in the working temperature range of 300±400 K; immiscible with the liquid in vessel A (i.e. water); (a) high thermal conductivity (b) low viscosity (c) high speci®c heat
922
. . . . .
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
(d) high density of the liquid, but should be lighter than the liquid in vessel A; non-toxic; non-in¯ammable; non-corrosive; chemically stable in the working temperature range.
The choice of working ¯uid, to a large extent, depends upon the boiling point. In the tropics, where the ambient temperature ranges between 20 and 458C, the ¯uid chosen should have its
Fig. 3. Flow chart of the simulation computer program.
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
923
normal boiling point in this range. If the normal boiling point is high, it may either take a long time for the pressure to increase to the required actuating pressure or it may not be possible to achieve this pressure at all when heated in a ¯at plate collector. On the other hand, if the normal boiling point is less than the ambient temperature, condensation of the vapour will be hampered because the medium used for cooling is water at the ambient temperature. If condensation is not possible, the pressure in vessel A will not decrease at the end of each cycle, and so, the suction of water through the one way valve into vessel C becomes impossible, and hence, the pump ceases to operate. It is for this reason that it is always desirable to have a working ¯uid whose normal boiling point is around the ambient temperature, preferably, a little higher than the ambient temperature. n-Pentane is being used extensively as the working ¯uid in solar thermal water pumps. It possesses most of the desirable characteristics for water pumping systems, such as thermal and chemical stability, thermodynamic suitability, non-toxic, non-corrosive and appropriate boiling point, but it is relatively expensive (1US$25/l), which does not support the economic viability of such kind of pumping systems. On the other hand, ethyl ether seems to be an appropriate alternative to n-pentane for such solar water pumps. It not only possesses the desirable characteristics required but also, it is available at a much lower price (1US$4.5/l). From the thermodynamic and heat transfer points of view, ethyl ether and n-pentane are equally good working substances, as they have almost identical properties. The only dierence is that npentane is immiscible in water while ethyl ether is slightly miscible in the same [8]. Nevertheless, the solubility of ethyl ether in water can be eliminated by replacing the water in vessel A by brine. It has long been established that organic compounds are generally less soluble in aqueous salt solutions [9]. This property is the `salting-out eect', in which the solubility of a non-electrolyte (ethyl ether) in water is decreased when the electrolyte (salt, e.g., sodium chloride) is added. A simple explanation is that the ions attract the water dipoles into their hydration sheaths, thereby reducing the eective concentration of free water. Increasing the degree of structure within the solvent makes the aqueous environment enthalpically or entropically even less favourable for insertion of hydrocarbon groups (ethyl ether) [10]. Therefore, in the present work, both n-pentane and ethyl ether are chosen as working ¯uids to study their performance for a STWP. Fig. 3 is the ¯ow chart which details the study of the thermodynamic performances of these working ¯uids used in the pumping system. The foregoing analysis brings out clearly the fact that the performance of the system depends greatly on N, the number of cycles per day. It may also be discerned that N, in turn, depends upon the discharge head, maximum solar intensity at noon (Hmax), ambient temperature (ta) and the mass of working ¯uid initially present in the system (m ). The results of the thermodynamic analysis performed in conjunction with the heat transfer analysis of the collector are presented below.
5. Results and discussion The results of the thermodynamic analysis are presented to highlight the eect of discharge head on the performance of the system for the two working ¯uids considered in this work. In
924
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
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 thermodynamic and physical transport properties of ethyl ether (b.p. 348C) and n-pentane (b.p. 368C) [11,12]. Fig. 4 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 for the two working ¯uids. 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 on 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 has to be compressed to a higher pressure. This obviously requires an increased mass of ethyl ether/n-pentane vapour in vessel A in each cycle. The requirement of this increased 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. Also shown in Fig. 4 is the variation in the quantity of water pumped per day with discharge head. As the discharge head increases, the quantity of water lifted per day decreases. The quantity of water lifted per cycle, theoretically, is equal to the volume of water in vessel C. Hence, it is the decrease in the number of cycles that decreases the water pumped per day. It
Fig. 4. Eect of discharge head on number of cycles and water pumped per day.
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
925
can be seen from Fig. 4 that in the case of ethyl ether, 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. From Fig. 4, it is to be noted that when ethyl ether is used as the working ¯uid in a STWP, the number of cycles and the amount of water pumped increase, compared to n-pentane. This can be explained by two reasons. The ®rst reason is that ethyl ether (b.p. 348C) has a lower normal boiling point compared to that of n-pentane (b.p. 368C). Therefore, the ®rst cycle can be operated at a much earlier time which, in turn, increases the period of operation. This is re¯ected in the increased number of cycles. The second reason is the enthalpy of vapourization (hfg) of ethyl ether (350 kJ/kg) is marginally less than that of n-pentane (358 kJ/kg). Therefore, when ethyl ether is employed as the working ¯uid, the heating time and condensation time of the vapour is relatively shorter. Hence, the time required for each pumping cycle is shorter and the number of cycles the pump can work per day increases. It can be seen from Fig. 4 that while the pump can work 92 cycles at a discharge head of 6 m when ethyl ether is used, it can work only 80 cycles (i.e., 12 cycles less) at the same discharge head with n-pentane. Fig. 5 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 the work done by the pump per day (product of the 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 the water per cycle increases with
Fig. 5. Eect of discharge head on eciency.
926
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
increase in the discharge head, the number of cycles decreases drastically with increase in the 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. When ethyl ether is used, the overall eciency is around 0.42% at a discharge head of 6 m, and it is around 0.34% at 10 m. As shown in Fig. 5, using ethyl ether results in a better eciency of the pumping system, when compared to n-pentane. The reason is, as mentioned earlier, when ethyl ether is employed, the starting time of the ®rst cycle of pumping and the time required for each pumping cycle is shorter than for n-pentane. This increases the number of cycles the pump can work per day and, consequently, increases the eciency of the system for any given discharge head. Also, the heat loss with ethyl ether as the working ¯uid was much less compared to npentane. In addition to this, the `heating time' required for ethyl ether was quicker. Hence, at any given discharge head, the performance of a pump with ethyl ether was better than with npentane. As shown in Fig. 5, for a discharge head at 6 m, the overall eciency is 0.42% when ethyl ether is used, while it is only around 0.36% with n-pentane as the working ¯uid.
6. Conclusions A theoretical analysis has been made to study the thermal performance of a solar water pump with n-pentane and ethyl ether as the working ¯uids. A performance comparison was performed in terms of the number of cycles the pump can work per day, the amount of water pumped per day and the overall eciency. It was found that the eciency of the pump with ethyl ether is about 17% higher than that with n-pentane at a discharge head of 6 m. The comparative evaluation suggests ethyl ether is a better choice as the working ¯uid for the solar thermal water pump. However, since ethyl ether is slightly soluble in water, the water in vessel A should be replaced by brine when ethyl ether is used.
Acknowledgements The authors wish to express their thanks to the Committee on Research and Conference Grants (CRCG), the University of Hong Kong, Hong Kong, for its ®nancial supports.
References [1] Murlidhar HP. A review of solar pumps and their principles. In: Proceedings of the International Solar Energy Society Congress, New Delhi, India, 1978. p. 2129. [2] Sumathy K, Venkatesh A, Sriramulu V. Thermodynamic analysis of a solar thermal water pump. Solar Energy 1996;57(2):155±61. [3] Sudhakar K, Murali Krishna M, Rao DP, Soin RS. Anaylsis and simulation of a solar water pump for lift irrigation. Solar Energy 1980;24:71±82. [4] Rao DP, Rao KS. Solar water pump for lift irrigation. Solar Energy 1976;18:405±11.
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
927
[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] Sumathy K. Studies on a solar thermal water pump. Ph.D. thesis in Mechanical Engineering. Indian Institute of Technology, India, 1994. [8] Martha Windholz. The Merck index. Merck Co. Inc., 10th ed., 1983. [9] 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. [10] Grant DJW, Higuchi T. In: Solubility behaviour of organic compounds. New York: Wiley, 1990. p. 398. [11] Yaws CL. In: Handbook of thermodynamic diagrams. Houston: Gulf Pub. Co, 1996. p. 72 and 296. [12] Yaws CL. Thermodynamic and physical property data. Houston: Gulf Pub. Co, 1992.
www.elsevier.com/locate/enconman
Performance of a solar water pump with n-pentane and ethyl ether as working ¯uids Y.W. Wong, K. Sumathy* Department of Mechanical Engineering, University of Hong Kong, Pokfulam Road, Hong Kong Received 16 March 1999; accepted 20 September 1999
Abstract In this study, the performance of a solar thermal water pump with two dierent working ¯uids such as n-pentane and ethyl ether, is reported. The theoretical comparison is made in terms of the number of cycles the pump can work per day, the amount of water that can be pumped per day and the overall eciency of the system. It is found that the eciency of the pump with ethyl ether is about 17% higher than that with n-pentane at a discharge head of 6 m. It is shown that ethyl ether seems to be a better choice for operating a non-conventional solar thermal water pump in terms of eciency and economics. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Solar water pump; Flat plate collector; n-Pentane; Ethyl ether; Thermodynamic analysis
1. Introduction Solar 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 [1]. In the recent past, several solar thermal water pumping systems have been studied. Most of these systems have the advantages of being simple, inexpensive, maintenance free, easily assembled and non-mechanical. Most of the solar thermal water pumps (STWP) make use of the fact that there is a change in the volume of a liquid on vapourization and also when the vapour is condensed. The volume increase at a given pressure may be utilized to displace water to a * Corresponding author. Tel.: +852-2859-2632; fax: +852-2858-5415. E-mail address: [email protected] (K. Sumathy). 0196-8904/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 6 - 8 9 0 4 ( 9 9 ) 0 0 1 6 7 - 3
916
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
Nomenclature dm g Hmax Htot h m m1 m2 N P1b Pap pi p1 p2 pd ta tp tw t1 U1 U2 U'2 ui V V1b V2b VC VN Vw v1 v2 W1 W2 Wh WT
mass of working ¯uid entering vessel A to eect pumping (kg) acceleration due to gravity (m/s2) maximum solar intensity at noon (W/m2) total solar radiation incident on collector during period of operation of pump (J) discharge head (m) initial mass of working ¯uid in collector system (kg) mass of working ¯uid in tank N before pumping action (kg) mass of working ¯uid remaining in tank N after pumping action (kg) number of pumping cycles per day initial air pressure in tank B (Pa) minimum pressure required for actuating pump (Pa) arbitrary assumed pressure (Pa) pressure of working ¯uid in tank N before pumping (Pa) pressure of working ¯uid in tank N after pumping (Pa) discharge pressure (Pa) ambient temperature (K) temperature of absorber plate (K) temperature of tube wall of risers (K) temperature of working ¯uid in collector and tank S and N when valve 1 is open (K) initial internal energy of working ¯uid in tank N before pumping (J) ®nal internal energy of working ¯uid in tank N and vessel A after pumping at discharge pressure (J) ®nal internal energy of working ¯uid in tank N and vessel A after pumping at discharge pressure required to lift 15 l of water (J) initial speci®c internal energy of working ¯uid in tank N before pumping (J/kg) volume occupied by vapour in tank A (m3) initial volume of air in tank B (m3) ®nal volume of air in tank B (m3) volume of tank C (m3)=volume of water pumped per cycle volume of tank N (m3) volume of water pumped in each cycle (m3) speci®c volume of vapour in tank N before pumping (m3/kg) speci®c volume of vapour in tank N after pumping (m3/kg) work done in compressing air in vessel B (J) work done in lifting 15 l of water to assumed head (J) hydraulic work per cycle (J)=W2 total work done required in each cycle (J)
Greek Letters rw density of water (kg/m3)
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
g tc th Z0
917
ratio of speci®c heat capacities time for condensation of spent vapour (s) time for heating adequate vapour for pumping (s) overall eciency of pump (%)
higher elevation, while the volume reduction at a lower pressure is used for suction of water for the next cycle of pumping action. The volume of water displaced is proportional to the change in volume of the ¯uid undergoing the phase change. n-Pentane is one of the most widely used working ¯uids, as it possesses the desirable properties required to operate the above said non-conventional pump [2±4]. Since the npentane is expensive, ethyl ether is chosen as one of the potential substitutes. Ethyl ether possesses most of the desirable properties from the view point of thermodynamics and the heat transfer required by an STWP. Also, it is available at a much lower price. Hence, in the present study, a comparative study is made on the performance of an STWP using ethyl ether and n-pentane as the working ¯uids.
Fig. 1. Schematic of the solar thermal water pump.
918
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
2. System description Fig. 1 shows the schematic of the solar thermal water pump considered in this work. The working ¯uid, set in motion by thermosiphon action, is heated in the ¯at plate collector. The saturated vapour separated in tank S is stored in the vapour storage tank N. When the 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 water at atmospheric conditions. As a consequence, the water in vessel A is displaced into vessel B, which initially contains air at atmospheric conditions. The rising column of water in B compresses the air in it, and the compressed air, in turn, pushes the water in vessel C to the overhead tank D, constituting the pumping operation. At the end of pumping, valve 2 is closed and valve 1 is opened so that tank N is replenished with vapour of the working ¯uid 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 condense the spent vapour in it. Because of condensation, the pressure in vessel A decreases, as a result of which the water 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. The process undergone by the working ¯uid during this ®rst cycle of operation is clearly shown in the p±v diagram of Fig. 2, by 1±2±3±4±5±6. Here, 1±2 represents heating of the working ¯uid in the collector. Working ¯uid vapourized at state 3, in tank N, expands to state 4 (when vapour in tank N is suddenly led to vessel A) to reach the pressure corresponding to the discharge head. 4±5 and 5±6 show the idealized processes of cooling and condensation of
Fig. 2. p±v diagram.
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
919
the vapour. The time interval between the ®rst and the second cycle is determined by the time taken for condensation of the vapour in the ®rst cycle. The faster the condensation, the quicker will be the return of water to vessel A from vessel B, since the pressure is set back to the value that existed at the start of the cycle. During the period of condensation, the working ¯uid in the collector is being continuously heated by solar energy. Hence, at the start of the second cycle of pumping water, the temperature and hence the pressure of the working ¯uid will normally be higher than those at the beginning of the ®rst cycle. The new pressure is shown as the one that corresponds to states 8 and 9 in Fig. 2. The process of lifting water and the subsequent condensation of the working ¯uid in tank A follows the same pattern as before. The process undergone by the working ¯uid during this cycle is shown as 7±8±9±10±11±6. Similar sets of events occur in the subsequent cycles, and the pump continues to operate as long as the working ¯uid in the collector can get heated. The total quantity of water the pump can lift in a day is equal to the product of the number of cycles per day and the volume of water pumped in each cycle. Theoretically, the latter is equal to the volume of vessel C. 3. Thermodynamic analysis The thermodynamic analysis of the pump is performed [5] with the help of the collector analysis suggested by Venkatesh and Sriramulu [6], with suitable modi®cations. 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, amount of water lifted per day and the overall eciency of the pump have been predicted. It is assumed to start, 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 the 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 working ¯uid 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 15 l of water to the assumed discharge head. In such a 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 the air has been compressed to the discharge pressure. The work done in lifting the water is given by,
920
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
W2 Vw rw 9:81 head
2
where Vw and rw are the volume of water pumped and the density of the water, respectively. Since the discharge of vapour from tank N is assumed to be rapid, the process can be considered to be adiabatic. The initial internal energy U1 of m1 kg of vapour in tank N is given by U1 m1 ui
3
where ui is the speci®c internal energy of saturated vapour at an arbitrarily assumed pressure pi, which could allow the pump to lift 15 l of water. Suppose U2 is the ®nal internal energy of the same mass of vapour after it expands into vessel A to cause the air in B to get compressed and then lift 15 l of water from C to D, then from the ®rst law of thermodynamics, U2 U1 ÿ WT :
4
The work done by the vapour WT is given by, WT W1 W2
5
U2 corresponds to the condition when a part of the m1 kg of vapour is in tank N and the rest is in vessel A. It is also a fact that the ®nal pressure in the tank N and in vessel A will be equal to the discharge pressure (Pd). The ®nal speci®c volume of this mass of vapour is given by, v2 Vn V =m1 ;
6
Here, Vn is the volume of tank N and V is the volume occupied by the working ¯uid vapour in tank A. With the help of the property relations, the ®nal internal energy U'2 can now be evaluated in terms of pd and v2. If the assumed value of pi is such that the pump can lift 15 l of water, the values of U'2 and U2 must be the same. If not, a new value is assumed for pi, and a similar analysis is made. This iterative procedure is continued, using a computer, until the unique value of pi that makes U'2=U2 is obtained. In the p±v diagram, this pressure is equal to the pressure corresponding to states 2 and 3. Let this pressure be pap. When the temperature of the working ¯uid in the collector rises and the saturated vapour in tank N reaches the value pap, valve 1 is closed, and valve 2 is opened. A part of the working ¯uid vapour in tank N enters vessel A and displaces water in it to vessel B, thereby increasing the pressure of the air in B to the discharge pressure so as to eect the required pumping. An indirect method is used to determine the mass of working ¯uid, dm, that enters vessel A to eect the pumping of water from vessel C to the overhead tank D. It is assumed that the m2 kg of vapour remaining in tank N has undergone a reversible adiabatic process. Assuming that the vapour behaves like a perfect gas, p1 vg1 p2 vg2
7
where states 1 and 2 correspond to the equilibrium conditions in tank N before opening valve 2 and after closing it, respectively. In this equation, the unknowns are p2 and v2, since the mass
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
921
remaining in the vessel is not known. Hence, two equations of constraint are needed to evaluate the values of p2 and v2. One is the above equation (Eq. (7)) and the other is the energy equation. For an assumed pressure p2, v2 can be evaluated and thus m2 can be calculated as it is equal to VN/v2. The mass of vapour transferred to vessel A is then equal to (m1ÿm2). If the energy equation, using this mass, is not satis®ed, then a new p2 is assumed until the energy equation is satis®ed, and real value of m2 is obtained. At the end of pumping, vessels A, B and C are at a pressure corresponding to the discharge head. The water from the overhead tank is allowed to pass through the cooling coils in vessel A. As the working ¯uid vapour condenses, the pressure in vessel A decreases, and it enables the water to be sucked through the one way valve 5 into vessel C, making the system ready for the next cycle of pumping. During the period of condensation of the spent vapour in the ®rst cycle of operation, valve 1 is opened, keeping valve 2 closed to charge the tank N with saturated vapour. As the collectors are continuously exposed to the sun, the temperature of the remaining working ¯uid (liquid+vapour) in the system will rise. The temperature increase is predicted by equating the heat transfer during the period of heating to the dierence in internal energy of the working ¯uid in the collection system. The analysis is continued until such time that the working ¯uid in the collection system can continuously get heated. Heat transfer analysis suggests that the pump can operate until around 2.00 p.m. The overall eciency Z0, is de®ned as the ratio of the hydraulic work done by the pump in a day to the total solar radiation incident on the collector during the period of operation of the pump. Z0 NWh =Htot
8
where N is the number of cycles per day and Wh is the hydraulic work per cycle given by Wh VC rw gh
9
where VC is the volume of water pumped and h is the discharge head. The analysis is performed for dierent discharge heads to study its eect on the performance of the pump.
4. Selection of working ¯uids For successful operation of the non-conventional solar thermal water pump discussed, careful selection of the working ¯uid is essential. The selected ¯uid must have most of the desirable properties from the viewpoint of thermodynamics and heat transfer, such as [7]: . . . . . .
normal boiling point should be slightly higher than the local average ambient temperature; low enthalpy of vapourization in the working temperature range of 300±400 K; immiscible with the liquid in vessel A (i.e. water); (a) high thermal conductivity (b) low viscosity (c) high speci®c heat
922
. . . . .
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
(d) high density of the liquid, but should be lighter than the liquid in vessel A; non-toxic; non-in¯ammable; non-corrosive; chemically stable in the working temperature range.
The choice of working ¯uid, to a large extent, depends upon the boiling point. In the tropics, where the ambient temperature ranges between 20 and 458C, the ¯uid chosen should have its
Fig. 3. Flow chart of the simulation computer program.
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
923
normal boiling point in this range. If the normal boiling point is high, it may either take a long time for the pressure to increase to the required actuating pressure or it may not be possible to achieve this pressure at all when heated in a ¯at plate collector. On the other hand, if the normal boiling point is less than the ambient temperature, condensation of the vapour will be hampered because the medium used for cooling is water at the ambient temperature. If condensation is not possible, the pressure in vessel A will not decrease at the end of each cycle, and so, the suction of water through the one way valve into vessel C becomes impossible, and hence, the pump ceases to operate. It is for this reason that it is always desirable to have a working ¯uid whose normal boiling point is around the ambient temperature, preferably, a little higher than the ambient temperature. n-Pentane is being used extensively as the working ¯uid in solar thermal water pumps. It possesses most of the desirable characteristics for water pumping systems, such as thermal and chemical stability, thermodynamic suitability, non-toxic, non-corrosive and appropriate boiling point, but it is relatively expensive (1US$25/l), which does not support the economic viability of such kind of pumping systems. On the other hand, ethyl ether seems to be an appropriate alternative to n-pentane for such solar water pumps. It not only possesses the desirable characteristics required but also, it is available at a much lower price (1US$4.5/l). From the thermodynamic and heat transfer points of view, ethyl ether and n-pentane are equally good working substances, as they have almost identical properties. The only dierence is that npentane is immiscible in water while ethyl ether is slightly miscible in the same [8]. Nevertheless, the solubility of ethyl ether in water can be eliminated by replacing the water in vessel A by brine. It has long been established that organic compounds are generally less soluble in aqueous salt solutions [9]. This property is the `salting-out eect', in which the solubility of a non-electrolyte (ethyl ether) in water is decreased when the electrolyte (salt, e.g., sodium chloride) is added. A simple explanation is that the ions attract the water dipoles into their hydration sheaths, thereby reducing the eective concentration of free water. Increasing the degree of structure within the solvent makes the aqueous environment enthalpically or entropically even less favourable for insertion of hydrocarbon groups (ethyl ether) [10]. Therefore, in the present work, both n-pentane and ethyl ether are chosen as working ¯uids to study their performance for a STWP. Fig. 3 is the ¯ow chart which details the study of the thermodynamic performances of these working ¯uids used in the pumping system. The foregoing analysis brings out clearly the fact that the performance of the system depends greatly on N, the number of cycles per day. It may also be discerned that N, in turn, depends upon the discharge head, maximum solar intensity at noon (Hmax), ambient temperature (ta) and the mass of working ¯uid initially present in the system (m ). The results of the thermodynamic analysis performed in conjunction with the heat transfer analysis of the collector are presented below.
5. Results and discussion The results of the thermodynamic analysis are presented to highlight the eect of discharge head on the performance of the system for the two working ¯uids considered in this work. In
924
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
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 thermodynamic and physical transport properties of ethyl ether (b.p. 348C) and n-pentane (b.p. 368C) [11,12]. Fig. 4 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 for the two working ¯uids. 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 on 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 has to be compressed to a higher pressure. This obviously requires an increased mass of ethyl ether/n-pentane vapour in vessel A in each cycle. The requirement of this increased 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. Also shown in Fig. 4 is the variation in the quantity of water pumped per day with discharge head. As the discharge head increases, the quantity of water lifted per day decreases. The quantity of water lifted per cycle, theoretically, is equal to the volume of water in vessel C. Hence, it is the decrease in the number of cycles that decreases the water pumped per day. It
Fig. 4. Eect of discharge head on number of cycles and water pumped per day.
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
925
can be seen from Fig. 4 that in the case of ethyl ether, 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. From Fig. 4, it is to be noted that when ethyl ether is used as the working ¯uid in a STWP, the number of cycles and the amount of water pumped increase, compared to n-pentane. This can be explained by two reasons. The ®rst reason is that ethyl ether (b.p. 348C) has a lower normal boiling point compared to that of n-pentane (b.p. 368C). Therefore, the ®rst cycle can be operated at a much earlier time which, in turn, increases the period of operation. This is re¯ected in the increased number of cycles. The second reason is the enthalpy of vapourization (hfg) of ethyl ether (350 kJ/kg) is marginally less than that of n-pentane (358 kJ/kg). Therefore, when ethyl ether is employed as the working ¯uid, the heating time and condensation time of the vapour is relatively shorter. Hence, the time required for each pumping cycle is shorter and the number of cycles the pump can work per day increases. It can be seen from Fig. 4 that while the pump can work 92 cycles at a discharge head of 6 m when ethyl ether is used, it can work only 80 cycles (i.e., 12 cycles less) at the same discharge head with n-pentane. Fig. 5 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 the work done by the pump per day (product of the 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 the water per cycle increases with
Fig. 5. Eect of discharge head on eciency.
926
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
increase in the discharge head, the number of cycles decreases drastically with increase in the 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. When ethyl ether is used, the overall eciency is around 0.42% at a discharge head of 6 m, and it is around 0.34% at 10 m. As shown in Fig. 5, using ethyl ether results in a better eciency of the pumping system, when compared to n-pentane. The reason is, as mentioned earlier, when ethyl ether is employed, the starting time of the ®rst cycle of pumping and the time required for each pumping cycle is shorter than for n-pentane. This increases the number of cycles the pump can work per day and, consequently, increases the eciency of the system for any given discharge head. Also, the heat loss with ethyl ether as the working ¯uid was much less compared to npentane. In addition to this, the `heating time' required for ethyl ether was quicker. Hence, at any given discharge head, the performance of a pump with ethyl ether was better than with npentane. As shown in Fig. 5, for a discharge head at 6 m, the overall eciency is 0.42% when ethyl ether is used, while it is only around 0.36% with n-pentane as the working ¯uid.
6. Conclusions A theoretical analysis has been made to study the thermal performance of a solar water pump with n-pentane and ethyl ether as the working ¯uids. A performance comparison was performed in terms of the number of cycles the pump can work per day, the amount of water pumped per day and the overall eciency. It was found that the eciency of the pump with ethyl ether is about 17% higher than that with n-pentane at a discharge head of 6 m. The comparative evaluation suggests ethyl ether is a better choice as the working ¯uid for the solar thermal water pump. However, since ethyl ether is slightly soluble in water, the water in vessel A should be replaced by brine when ethyl ether is used.
Acknowledgements The authors wish to express their thanks to the Committee on Research and Conference Grants (CRCG), the University of Hong Kong, Hong Kong, for its ®nancial supports.
References [1] Murlidhar HP. A review of solar pumps and their principles. In: Proceedings of the International Solar Energy Society Congress, New Delhi, India, 1978. p. 2129. [2] Sumathy K, Venkatesh A, Sriramulu V. Thermodynamic analysis of a solar thermal water pump. Solar Energy 1996;57(2):155±61. [3] Sudhakar K, Murali Krishna M, Rao DP, Soin RS. Anaylsis and simulation of a solar water pump for lift irrigation. Solar Energy 1980;24:71±82. [4] Rao DP, Rao KS. Solar water pump for lift irrigation. Solar Energy 1976;18:405±11.
Y.W. Wong, K. Sumathy / Energy Conversion & Management 41 (2000) 915±927
927
[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] Sumathy K. Studies on a solar thermal water pump. Ph.D. thesis in Mechanical Engineering. Indian Institute of Technology, India, 1994. [8] Martha Windholz. The Merck index. Merck Co. Inc., 10th ed., 1983. [9] 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. [10] Grant DJW, Higuchi T. In: Solubility behaviour of organic compounds. New York: Wiley, 1990. p. 398. [11] Yaws CL. In: Handbook of thermodynamic diagrams. Houston: Gulf Pub. Co, 1996. p. 72 and 296. [12] Yaws CL. Thermodynamic and physical property data. Houston: Gulf Pub. Co, 1992.