- Email: [email protected]
Hybrid solar still by heat pump compression
Desalination 250 (2010) 444–449
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Hybrid solar still by heat pump compression☆ Khaoula Hidouri ⁎, Romdhane Ben Slama, Slimanne Gabsi Analysis Laboratory of the Processes, ENIG, Road of Medenine 6029 GABES Tunisia
a r t i c l e
i n f o
Available online 21 October 2009 Keywords: Solar still Hybrid solar still Heat pump Efficiency Convective Evaporative Heat transfer
a b s t r a c t Our study is related to an experimental work and modelling of a simple solar still (SSS), green house type, asymmetrical and a hybrid system of a solar still connected to a heat pump (SSSHP). Simple solar stills have in general very low efficiency and our study aims in improving that incorporation by heat pump. This will increase vapor condensation, improve efficiency and consequently the output per m2 of still surface area. Data obtained from our experimental research are used to determine convective and evaporative heat transfer coefficients such as the experimental and theoretical efficiencies. The nom of the hybrid system is HSSHP. Daily output increased from 2 l/m2 for the SSS up to 12 l/m2 for the HSSHP and average efficiency increased from 20% to 80%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Potable water demand is increasing due to rapid population increase and also due to uncontrolled pollution of the fresh water resources. For some places, desalination became the only solution to the water quality problem. Conventional thermal desalination methods, as Multi-Stage-Flash (MSF), Multiple-Effect (MED) and minor Thermal and/or Mechanical Vapor Compression found application for large capacity installations. Reverse Osmosis, a membrane operating method which functions by electricity, is used as well in small or large capacity plants. All these methods found application worldwide, but especially in places with a total lack of fresh water and more or less dense population. They are operated by conventional energy sources as fuels. For small or remote communities where there is lack of water but also of electrical grid the only solution is the use of renewable energies, as solar, wind etc., in connection either to small capacity conventional desalination units but better to use solar energy with solar stills. The use of solar energy for desalination and/or for solar distillation has been investigated by many scientists. Tzen and Morris [1] proposed the use of renewable energy sources (RES) coupled with existing technologies as one method, whereby, the need of conventional energy can be reduced, desalination costs lowered in the long run and does not result in environmental degradations. Hilal et al. [2] have studied the solar model for double purpose and triple effect while determining the mathematical equations, which are used for, calculating the production of water. The method for the double effect can reach a value of about 6.1 l. Khedim [3] determined the production of drinking water of using simple solar still. By recuperating and ☆ Presented at the 1st Conference on Environmental Management, Engineering, Planning and Economics (CEMEPE), Skiathos, Greece, 7-10 December 2007. ⁎ Corresponding author. E-mail address: [email protected] (K. Hidouri). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.09.075
reusing the same quantity of thermal energy in several stages, it has been possible to reduce the total energy needed to the extent that solar energy can now be seen as an acceptable energy source for water desalination (COP: 3.0–4.0). Esteban et al. [4] have study the performance of two solar stills: solar still coupled with a collector and solar still without collector. They found that production is 36.5 l for the first whereas it does not exceed 21.5 l for the second. Badran et al. [5] carried out installations assisted with a solar heat collector that is determining the temperatures in the condenser, the evaporator and the solar intensities. They found a relation between the production of water, which reaches 45.1 l, by using a collector at the end of 24 h where the effectiveness is equal to 24.57%. The active of distillation system can be incorporated with electrically operated fan and condensing chamber to increase the distillate output [6]. Fig. 1 shows a hybrid solar distillation system in which single slope basin-type still is attached with active components [7]. The active components (electrical blower and condenser) can be attached with a distillation unit having a collector panel. Hybrid signifies that to run the active components of the system one can use photovoltaic panel, wind tower, hydropower and other sources of electricity. The application of heat pump in water desalination has been receiving attention in terms of its potential for cost reduction. Experimental work on heat pump assisted water purification has been carried out in Mexico since 1981, where much of works were carried out on the use of heat pumps for the desalination of water. Siqueiros and Holland [8] noted that the production cost of drinking water for cities was comparable with that to connect to a thermal factory finely of a heat pump of desalination, which could increase significantly and economically. Selesarenko [9] found that, connecting a heat pump to a thermal desalination plant could increase significantly the economic viability. Tripathi et al. [10] studied the heat and mass transfer coefficients for passive and active solar distillation systems. It is a wellknown fact that the distillate output (the yield) decreases significantly
K. Hidouri et al. / Desalination 250 (2010) 444–449
445
Fig. 1. Hybrid solar distillation system [7].
with the increase of water depth in the basin of the solar still. It is also known that more yields is obtained in the case of active solar distillation system as compared to passive solar still due to higher temperature difference between the water and inner glass cover temperatures in the active mode. The results showed that the heat transfer coefficient per evaporation in the active mode increases with the depth of the basin (0.15 m) during 24 h, and equal to 2.79 W/m2 °C, that of coefficient of transfer convective and equalize 2.6 W/m2 °C in the same mode. Hawlader et al. [11] shows that the performance ratio and the coefficient of performance (COP) have been evaluated. The performance ratio (PR) obtained from the experiments ranges from 0.77 to 1.15 and COP of the system was found to vary between 5.0 and 7.0. Kaabi [12] shows that the resolution of equations, based on method of finite differences, is better for the efficiency that is obtained in maximum temperature difference, as well as we can obtain this latter by a low glass thickness, a gradient (angle of inclination) closer to that of the area latitude, in which our solar still is placed, a low thickness of the solution to be distilled and a high wind velocity. Results issued from this study show clearly the importance of a cooled condensation surface and a hotter evaporation surface. Ghosal et al. [13] show the effect of mass flow rate on the overall thermal efficiency of solar still which indicates that with the increase of mass flow rate the efficiency decreases. It is due to the fact that rate of evaporation decreases with increased mass flow rate resulting in lower output of fresh water. In this paper, analytical expressions for various parameters have been derived for SSS as well as HSSHP [14] Experimental validation has also been carried out by using the following measured climatic parameters: solar intensity on the glass cover and ambient air temperature. The temperature gradient is the driving force of the phenomena that are developed inside the still. On one hand, temperature differences cause natural circulation of the humid air. The proposed model predicts the theoretical values of water temperature, glass temperature, and the yield at hourly intervals. Hence there is a strong need to have a basic knowledge of heat and mass transfer coefficients in detail for SSS and HSSHP. In this communication, an attempt has been made
to study the effect of heat pump in a solar still on the heat and mass transfer coefficients for the SSS as well as HSSHP models and the variation of the efficiency. 2. Experimental set up In our experimental work, two models are used. The first one is called the SSS (simple solar still) model, in which the water output is simply obtained by purely solar energy. This model works only in the day. The second one is named the HSSHP (Hybrid Simple Solar still-Heat Pump system). In this model, a heat pump was used in order to increase the quantity of water output. This works by using both purely solar energy and heat pump, consequently it was used on day and night. It should be noticed that, the condenser will contribute to the heating of water and thus its evaporation especially in the morning and accordingly in midday to compensate for missing the sun. The evaporator will allow while being cooled, a more quantity of condensed water. 2.1. The SSS model Fig. 1 shows the schematic diagram of the SSS installation. It consists of a basin, which is fabricated from plastic materiel that accommodates the brackish water for a maximum depth of water, which is fixed at 30 cm, and is covered by two slopping covers. The height of the lower vertical side of the solar still was kept at 60 cm and the area of the basin is 0.4 m2. The operation of the still is very simple: the incident solar radiation is transmitted through the transparent glass cover to the water. As a result, the water evaporated, and reached the glass cover and then was collected at the distilled water gutter at condensed phase. 2.2. The HSSHP model In order to produce more distiller water we added a heat pump to the SSS model. Fig. 2 shows this model. A condenser is immersed in basin water to increase temperature of water and then evaporated
446
K. Hidouri et al. / Desalination 250 (2010) 444–449
for the case of the laminar or turbulent flow is given by the following equation of Pitts et al. [15] hcw Lv n = Nu = CðGrPrÞ kv
ð1Þ
where: hcw is the convective heat transfer coefficient, Lv characterizes the length; kv is the conductivity thermal of the humid air; and C and n are the coefficient and the exponential of Eq. (1). The coefficient of transfer convective hcw is to determine by
hcw =
kv C n ðGrPrÞ : Lv
ð2Þ
The quantity of distilled water can be obtained by the following equation: Fig. 2. Green house type Simple Solar Still (SSS).
:
:
mev quantity of water will increase. The condenser located near the upper region of the glass cover enhances. The condensation of the water vapor, and the refrigerant (R12), leaving the condenser is then introduced in a heat exchanger filled with fresh water in order to maintain the temperature of the refrigerant. Then the refrigerant enters the evaporator at low pressure inducing the condensation of water vapor. As a consequence, a greater amount of condensed water will be recuperated at the distilled water gutter. The process is done naturally.
hew ðTw −Tevp Þ Q A t Aw t = ev w = L L = 0:01623hcw ðPw Pevp Þ
mev
Aw t L
k A t = 0:01623CðGrPrÞ v ðPw −Pevp Þ w L Lv
ð3Þ ð4Þ
n
Z = 0:01623 Where
kv A t ðP −Pevp Þ w L Lv w
mev n = CðGrPrÞ : Z
ð5Þ
ð6Þ
Taking logarithm on both sides of Eq. (6) gives:
2.3. Experimental parameters
m ln ev = ln C + n lnðGrPrÞ: Z
For the installation, we assigned the value (0) for which the SSS and the HSSHP plants are orientated towards the south and the value (1) periodically directional towards the sun.
Eq. (7) has a linear form, which is written: n
m y = ln ev Z
ð7Þ
ð7aÞ
Position
Vitrage
PAC
x = lnðGrPrÞ
ð7bÞ
0 0 1 1
0 0 1 1
0 1 0 1
fm = n
ð8aÞ
For the glass cover, the value (0) is given when a single glass cover is used and the value (1) is given when we use a double glass cover. Similarly, the value (0) is given in the absence of a heat pump, and the value (1) is given when the heat pump is used. A mercury thermometer measures all temperatures. The still output is measured by a graduated test-tube. The following parameters are measured every hour for the two models as indicated in Figs. 1 and 2:
c
C=e:
ð8bÞ
3.2. Determination of global and interior efficiency 3.2.1. Efficiency of SSS The global efficiency is given by ηg =
qe : G⋅Aw
ð8Þ
The interior efficiency given by Tw Tevp Ta mex
Basin temperature Vapour temperature Ambient temperature Still output
ηi =
qe qw
ð9Þ
where qw = αt GAw and qe = mev Lv . 3. Theoretical consideration 3.1. Convective heat and mass transfer coefficient
3.2.2. Efficiency of HSSHP The global efficiency given by
The Nusselt number (Nu) in the case of natural convection is a function of a number Grashof Gr and Prandlt Pr. A simple correlation
ηg =
qe : ðG⋅Aw + COP ⋅PÞ⋅3600
ð10Þ
K. Hidouri et al. / Desalination 250 (2010) 444–449
Fig. 3. Hybrid system of solar still with heat pump (HSSHP). Hourly variation of convective heat transfer coefficient (W/m2°C).
qe qw
where
ηi =
ð11Þ
qw = αt GAw + qcond qcond = COPPAC ⋅P:
qe ðαt G⋅Aw + COP PAC ⋅PÞ⋅3600
where:
COPPAC =
qcond Tw = : W Tw −Tev
Fig. 4. Hourly variation of evaporative heat transfer coefficient (W/m2°C).
Fig. 5. Hourly variation of experimental yields (l/m2h).
4. Experimental and result
The interior efficiency is given by ηi =
447
ð12Þ
In order to understand the effects of the use of SSS and HSSHP configurations on the yields and the convective and evaporative heat transfer coefficients, let us consider the plots of these parameters as a function of true solar time. The effect of the convective heat transfers for the SSS and HSSHP models are illustrated in Fig. 3. This figure indicates that hcw increases when the system is coupled with the heat pump, hcw reached a maximum value for the (111) model, which is equal to 2.421 W/m2°C. For (001) configuration, maximum value of hcw is 2.114 W/m2°C. It could be noticed, that the convective heat transfer coefficient is greater for the (110) model than the (001) one after approximately 12 TST. This is due to the fact that the double glass cover and the variable inclination have no influence of the augmentation of hcw and hew. Finally, the (000) model exhibits poor values for hcw, in this case, the obtained maximum value is equal to 0.42 W/m2°C. The values of evaporative heat transfer coefficient are shown in Fig. 4. The heat transfer coefficients in the case of HSSHP model are significantly higher than those of the SSS model. Maximum value of
Fig. 6. Hourly variation of theoretical yields (l/ m2h).
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K. Hidouri et al. / Desalination 250 (2010) 444–449
Fig. 7. Variation of the interior and global efficiency according to TSV for the configuration (000). Fig. 9. Variation of interior and global efficiency according to TSV for the (110) configuration.
hew is obtained for the (001) model and is equal to 42.6 W/m2°C. Before 12 TST, the maximum value for hew is equal to 35.5 W/m2°C for the (111) model, at 12 TST, a light reduction is observed for this model, whereas a considerable increase is obtained for the (001) model as well as the (000) one. After 12 TST, hew stripped considerably for (000) and light (111). Figs. 5 and 6 show experimental and theoretical results of the hourly yields for SSS and HSSHP configurations. The HSSHP plant gives an excellent output, reaching the value of 1.65 l/m2h for the HSSHP model. For the SSS configuration, the maximum flow is equal to 0.3 l/m2°C. Theoretical results are comparable with the experimental ones, for statistical analysis, the error is more less for (001) and (111) (14%, 5.27%) than (000) and (110) (21%, 17%). From Figs. 7–10, it's clear that the global efficiency of SSS and HSSHP are increasing functions of true solar time however this growth attenuates for high values of the irradiation. It could be noticed that in
Fig. 8. Variation of the interior and global efficiency according to TSV for the configuration (001).
the configurations (000), the internal efficiency is higher than the global efficiency, whereas for (001) and (111) configurations, the global efficiency becomes higher than the interior one. This was explained by the addition of the heat pump (one must take in account the quantity of (COP·P)). Moreover, we can also notice that the global efficiency of the HSSHP model is definitely higher than that of the SSS because of its greater thermal inertia, which less quickly follows the variations of incidental solar energy. 5. Conclusion The solar still for purpose of traditional greenhouse was taken as reference to show the performances of the solar still combined with a heat pump.
Fig. 10. Variation of the interior and global efficiency according to TSV for the (111) configuration.
K. Hidouri et al. / Desalination 250 (2010) 444–449
Our measurements showed an increase in going flow of 300 ml/m2h with 1700 ml/m2h by combining with a simple still for purpose of greenhouse heat pump with compression where the evaporator and the condenser are jointly used. As for the daily output, it passes from 2 l/m2 to 12 l/m2. The average efficiency passes from 20% to 80%. In the configuration with heat pump, one observed low moisture of the air in the distiller; this is with the presence of the evaporator constituting a surface at temperature of weak dew. Thus, the vapor created in the still is quickly condensed. For (110) and (000) configurations, the efficiency attained a maximum value of 0.8 as compared to the (111) and (001) ones, in the last cases, the maximum value is equal to 1.8. The efficiency is higher for (111) and (001) configurations than for (000) and (110) ones. Symbols HSSHP SSS MSF RO Aw C Gr hcw hew L Lv mex n Nu Pr Pw Pevp Tw Tevp TST Z G qew qev qe qw
Hybrid-Solar Still-Heat Pump System Simple Solar Still—SSS Multi-Stage-Flash Reverse osmosis Area of water (m2) Experimental constant Grashof number Convective heat transfer coefficient (W/m2 °C) Evaporative heat transfer coefficient (W/m2 °C) Latent heat of vaporization (J/kg) Characteristic dimension (m) Experimental distiller output (l/m2 h) Exponent Nusselt number Prandlt number of humid air Saturation vapor pressure of water at evaporation surface (N/m2) Saturation vapor pressure of water at condensation surface (N/m2) Water temperature (°C) Vapor temperature (°C) True solar time (h) Function Solar radiation (W/m2) Rate of heat transfer by convection, (W/m2) Rate of evaporative heat transfer, (W/m2) Heat flux used for the evaporation of water (W/m2) Heat flux actually received by the water mass (W/m2)
αt η ηg COP(PAC) P G
449
Fictitious absorption coefficient of the water mass (0.85). Interior efficiency Global efficiency Coefficient of performance of heat pump Power of compressor equal 200 W Solar intensity (W/m2)
Subscripts cw Convective evp Evaporative w Water
References [1] E. Tzen, R. Morris, Renewable energy sources for desalination, Sol. Energy 75 (2003) 375–379. [2] H. Al-Hinai, M.S. Al-Nassri, B.A. Jubran, Effect of climatic, design and operational parameters on the yield of a simple solar still, Energy Convers. Manag. 43 (2002) 1639–1650. [3] K. Ahmed, K. Schwarzer, F. Christian, M. Christoph, Production décentralisée de l'eau potable à l'énergie solaire, Desalination 168 (2004) 13–20. [4] C. Esteban, J. Franco, A. Fasulo, Construction and performance of an assisted solar distiller, Desalination 173 (2005) 249–255. [5] A.A. Badran, A.A. Ahmed, I.A.E. Salman, M.Z. Odat, Asoalr still augmented with a flat plate collector, Desalination 172 (2005) 227–234. [6] G.N. Tiwari and A.K. Tiwari, Solar Distillation Practice for Water Desalination Systems, Anamaya Publisher, New Delhi, 2007 and Anshan Publisher, UK, 2008. [7] M. Qudais Abu, et al., Experimental study and numerical simulation of a solar still using an external condenser, Energy 21 (1996) 851. [8] J. Siqueiros, F.A. Holland, Water desalination using heat pumps, Energy 25 (2000) 717–729. [9] V.V. Slesarenko, Heat pumps as a source of heat energy for desalination of seawater, Desalination 193 (2001) 405–410. [10] R. Tripathi, G.N. Tiwari, Thermal modelling of passive and active solar stills for different depths of water by using the concept of solar fraction, Sol. Energy 80 (2006) 956–967. [11] M.N.A. Hawlader, K.D. Prasanta, D. Sufyan, C.Y. Chung, Solar assisted heat pump desalination system Elsevier, Desalination 168 (2004) 49–54. [12] A. Kaabi, S. Nafila, Impact of temperature difference (water–solar collector) on solar-still global efficiency, Desalination 209 (2007) 298–305. [13] M.K. Ghosal, G.N Tiwari, N.S.L. Srivastava, Thermal modelling of a controlled environment greenhouse cum solar distillation for composite and warm humid climates of India, Desalination 151 (2002) 293–308. [14] B.S. Romdhane, H. Khaoula, G. Slimanne. Proceeding of the Heat-SET 2007 (2007). Conference 18–20 Avril, Chambéry, France. [15] D.R. Pitts, L.E. Sisson, Theory and Problems of Heat Transfer, McGraw-Hill Book Company, New York, 1977.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Hybrid solar still by heat pump compression☆ Khaoula Hidouri ⁎, Romdhane Ben Slama, Slimanne Gabsi Analysis Laboratory of the Processes, ENIG, Road of Medenine 6029 GABES Tunisia
a r t i c l e
i n f o
Available online 21 October 2009 Keywords: Solar still Hybrid solar still Heat pump Efficiency Convective Evaporative Heat transfer
a b s t r a c t Our study is related to an experimental work and modelling of a simple solar still (SSS), green house type, asymmetrical and a hybrid system of a solar still connected to a heat pump (SSSHP). Simple solar stills have in general very low efficiency and our study aims in improving that incorporation by heat pump. This will increase vapor condensation, improve efficiency and consequently the output per m2 of still surface area. Data obtained from our experimental research are used to determine convective and evaporative heat transfer coefficients such as the experimental and theoretical efficiencies. The nom of the hybrid system is HSSHP. Daily output increased from 2 l/m2 for the SSS up to 12 l/m2 for the HSSHP and average efficiency increased from 20% to 80%. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Potable water demand is increasing due to rapid population increase and also due to uncontrolled pollution of the fresh water resources. For some places, desalination became the only solution to the water quality problem. Conventional thermal desalination methods, as Multi-Stage-Flash (MSF), Multiple-Effect (MED) and minor Thermal and/or Mechanical Vapor Compression found application for large capacity installations. Reverse Osmosis, a membrane operating method which functions by electricity, is used as well in small or large capacity plants. All these methods found application worldwide, but especially in places with a total lack of fresh water and more or less dense population. They are operated by conventional energy sources as fuels. For small or remote communities where there is lack of water but also of electrical grid the only solution is the use of renewable energies, as solar, wind etc., in connection either to small capacity conventional desalination units but better to use solar energy with solar stills. The use of solar energy for desalination and/or for solar distillation has been investigated by many scientists. Tzen and Morris [1] proposed the use of renewable energy sources (RES) coupled with existing technologies as one method, whereby, the need of conventional energy can be reduced, desalination costs lowered in the long run and does not result in environmental degradations. Hilal et al. [2] have studied the solar model for double purpose and triple effect while determining the mathematical equations, which are used for, calculating the production of water. The method for the double effect can reach a value of about 6.1 l. Khedim [3] determined the production of drinking water of using simple solar still. By recuperating and ☆ Presented at the 1st Conference on Environmental Management, Engineering, Planning and Economics (CEMEPE), Skiathos, Greece, 7-10 December 2007. ⁎ Corresponding author. E-mail address: [email protected] (K. Hidouri). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.09.075
reusing the same quantity of thermal energy in several stages, it has been possible to reduce the total energy needed to the extent that solar energy can now be seen as an acceptable energy source for water desalination (COP: 3.0–4.0). Esteban et al. [4] have study the performance of two solar stills: solar still coupled with a collector and solar still without collector. They found that production is 36.5 l for the first whereas it does not exceed 21.5 l for the second. Badran et al. [5] carried out installations assisted with a solar heat collector that is determining the temperatures in the condenser, the evaporator and the solar intensities. They found a relation between the production of water, which reaches 45.1 l, by using a collector at the end of 24 h where the effectiveness is equal to 24.57%. The active of distillation system can be incorporated with electrically operated fan and condensing chamber to increase the distillate output [6]. Fig. 1 shows a hybrid solar distillation system in which single slope basin-type still is attached with active components [7]. The active components (electrical blower and condenser) can be attached with a distillation unit having a collector panel. Hybrid signifies that to run the active components of the system one can use photovoltaic panel, wind tower, hydropower and other sources of electricity. The application of heat pump in water desalination has been receiving attention in terms of its potential for cost reduction. Experimental work on heat pump assisted water purification has been carried out in Mexico since 1981, where much of works were carried out on the use of heat pumps for the desalination of water. Siqueiros and Holland [8] noted that the production cost of drinking water for cities was comparable with that to connect to a thermal factory finely of a heat pump of desalination, which could increase significantly and economically. Selesarenko [9] found that, connecting a heat pump to a thermal desalination plant could increase significantly the economic viability. Tripathi et al. [10] studied the heat and mass transfer coefficients for passive and active solar distillation systems. It is a wellknown fact that the distillate output (the yield) decreases significantly
K. Hidouri et al. / Desalination 250 (2010) 444–449
445
Fig. 1. Hybrid solar distillation system [7].
with the increase of water depth in the basin of the solar still. It is also known that more yields is obtained in the case of active solar distillation system as compared to passive solar still due to higher temperature difference between the water and inner glass cover temperatures in the active mode. The results showed that the heat transfer coefficient per evaporation in the active mode increases with the depth of the basin (0.15 m) during 24 h, and equal to 2.79 W/m2 °C, that of coefficient of transfer convective and equalize 2.6 W/m2 °C in the same mode. Hawlader et al. [11] shows that the performance ratio and the coefficient of performance (COP) have been evaluated. The performance ratio (PR) obtained from the experiments ranges from 0.77 to 1.15 and COP of the system was found to vary between 5.0 and 7.0. Kaabi [12] shows that the resolution of equations, based on method of finite differences, is better for the efficiency that is obtained in maximum temperature difference, as well as we can obtain this latter by a low glass thickness, a gradient (angle of inclination) closer to that of the area latitude, in which our solar still is placed, a low thickness of the solution to be distilled and a high wind velocity. Results issued from this study show clearly the importance of a cooled condensation surface and a hotter evaporation surface. Ghosal et al. [13] show the effect of mass flow rate on the overall thermal efficiency of solar still which indicates that with the increase of mass flow rate the efficiency decreases. It is due to the fact that rate of evaporation decreases with increased mass flow rate resulting in lower output of fresh water. In this paper, analytical expressions for various parameters have been derived for SSS as well as HSSHP [14] Experimental validation has also been carried out by using the following measured climatic parameters: solar intensity on the glass cover and ambient air temperature. The temperature gradient is the driving force of the phenomena that are developed inside the still. On one hand, temperature differences cause natural circulation of the humid air. The proposed model predicts the theoretical values of water temperature, glass temperature, and the yield at hourly intervals. Hence there is a strong need to have a basic knowledge of heat and mass transfer coefficients in detail for SSS and HSSHP. In this communication, an attempt has been made
to study the effect of heat pump in a solar still on the heat and mass transfer coefficients for the SSS as well as HSSHP models and the variation of the efficiency. 2. Experimental set up In our experimental work, two models are used. The first one is called the SSS (simple solar still) model, in which the water output is simply obtained by purely solar energy. This model works only in the day. The second one is named the HSSHP (Hybrid Simple Solar still-Heat Pump system). In this model, a heat pump was used in order to increase the quantity of water output. This works by using both purely solar energy and heat pump, consequently it was used on day and night. It should be noticed that, the condenser will contribute to the heating of water and thus its evaporation especially in the morning and accordingly in midday to compensate for missing the sun. The evaporator will allow while being cooled, a more quantity of condensed water. 2.1. The SSS model Fig. 1 shows the schematic diagram of the SSS installation. It consists of a basin, which is fabricated from plastic materiel that accommodates the brackish water for a maximum depth of water, which is fixed at 30 cm, and is covered by two slopping covers. The height of the lower vertical side of the solar still was kept at 60 cm and the area of the basin is 0.4 m2. The operation of the still is very simple: the incident solar radiation is transmitted through the transparent glass cover to the water. As a result, the water evaporated, and reached the glass cover and then was collected at the distilled water gutter at condensed phase. 2.2. The HSSHP model In order to produce more distiller water we added a heat pump to the SSS model. Fig. 2 shows this model. A condenser is immersed in basin water to increase temperature of water and then evaporated
446
K. Hidouri et al. / Desalination 250 (2010) 444–449
for the case of the laminar or turbulent flow is given by the following equation of Pitts et al. [15] hcw Lv n = Nu = CðGrPrÞ kv
ð1Þ
where: hcw is the convective heat transfer coefficient, Lv characterizes the length; kv is the conductivity thermal of the humid air; and C and n are the coefficient and the exponential of Eq. (1). The coefficient of transfer convective hcw is to determine by
hcw =
kv C n ðGrPrÞ : Lv
ð2Þ
The quantity of distilled water can be obtained by the following equation: Fig. 2. Green house type Simple Solar Still (SSS).
:
:
mev quantity of water will increase. The condenser located near the upper region of the glass cover enhances. The condensation of the water vapor, and the refrigerant (R12), leaving the condenser is then introduced in a heat exchanger filled with fresh water in order to maintain the temperature of the refrigerant. Then the refrigerant enters the evaporator at low pressure inducing the condensation of water vapor. As a consequence, a greater amount of condensed water will be recuperated at the distilled water gutter. The process is done naturally.
hew ðTw −Tevp Þ Q A t Aw t = ev w = L L = 0:01623hcw ðPw Pevp Þ
mev
Aw t L
k A t = 0:01623CðGrPrÞ v ðPw −Pevp Þ w L Lv
ð3Þ ð4Þ
n
Z = 0:01623 Where
kv A t ðP −Pevp Þ w L Lv w
mev n = CðGrPrÞ : Z
ð5Þ
ð6Þ
Taking logarithm on both sides of Eq. (6) gives:
2.3. Experimental parameters
m ln ev = ln C + n lnðGrPrÞ: Z
For the installation, we assigned the value (0) for which the SSS and the HSSHP plants are orientated towards the south and the value (1) periodically directional towards the sun.
Eq. (7) has a linear form, which is written: n
m y = ln ev Z
ð7Þ
ð7aÞ
Position
Vitrage
PAC
x = lnðGrPrÞ
ð7bÞ
0 0 1 1
0 0 1 1
0 1 0 1
fm = n
ð8aÞ
For the glass cover, the value (0) is given when a single glass cover is used and the value (1) is given when we use a double glass cover. Similarly, the value (0) is given in the absence of a heat pump, and the value (1) is given when the heat pump is used. A mercury thermometer measures all temperatures. The still output is measured by a graduated test-tube. The following parameters are measured every hour for the two models as indicated in Figs. 1 and 2:
c
C=e:
ð8bÞ
3.2. Determination of global and interior efficiency 3.2.1. Efficiency of SSS The global efficiency is given by ηg =
qe : G⋅Aw
ð8Þ
The interior efficiency given by Tw Tevp Ta mex
Basin temperature Vapour temperature Ambient temperature Still output
ηi =
qe qw
ð9Þ
where qw = αt GAw and qe = mev Lv . 3. Theoretical consideration 3.1. Convective heat and mass transfer coefficient
3.2.2. Efficiency of HSSHP The global efficiency given by
The Nusselt number (Nu) in the case of natural convection is a function of a number Grashof Gr and Prandlt Pr. A simple correlation
ηg =
qe : ðG⋅Aw + COP ⋅PÞ⋅3600
ð10Þ
K. Hidouri et al. / Desalination 250 (2010) 444–449
Fig. 3. Hybrid system of solar still with heat pump (HSSHP). Hourly variation of convective heat transfer coefficient (W/m2°C).
qe qw
where
ηi =
ð11Þ
qw = αt GAw + qcond qcond = COPPAC ⋅P:
qe ðαt G⋅Aw + COP PAC ⋅PÞ⋅3600
where:
COPPAC =
qcond Tw = : W Tw −Tev
Fig. 4. Hourly variation of evaporative heat transfer coefficient (W/m2°C).
Fig. 5. Hourly variation of experimental yields (l/m2h).
4. Experimental and result
The interior efficiency is given by ηi =
447
ð12Þ
In order to understand the effects of the use of SSS and HSSHP configurations on the yields and the convective and evaporative heat transfer coefficients, let us consider the plots of these parameters as a function of true solar time. The effect of the convective heat transfers for the SSS and HSSHP models are illustrated in Fig. 3. This figure indicates that hcw increases when the system is coupled with the heat pump, hcw reached a maximum value for the (111) model, which is equal to 2.421 W/m2°C. For (001) configuration, maximum value of hcw is 2.114 W/m2°C. It could be noticed, that the convective heat transfer coefficient is greater for the (110) model than the (001) one after approximately 12 TST. This is due to the fact that the double glass cover and the variable inclination have no influence of the augmentation of hcw and hew. Finally, the (000) model exhibits poor values for hcw, in this case, the obtained maximum value is equal to 0.42 W/m2°C. The values of evaporative heat transfer coefficient are shown in Fig. 4. The heat transfer coefficients in the case of HSSHP model are significantly higher than those of the SSS model. Maximum value of
Fig. 6. Hourly variation of theoretical yields (l/ m2h).
448
K. Hidouri et al. / Desalination 250 (2010) 444–449
Fig. 7. Variation of the interior and global efficiency according to TSV for the configuration (000). Fig. 9. Variation of interior and global efficiency according to TSV for the (110) configuration.
hew is obtained for the (001) model and is equal to 42.6 W/m2°C. Before 12 TST, the maximum value for hew is equal to 35.5 W/m2°C for the (111) model, at 12 TST, a light reduction is observed for this model, whereas a considerable increase is obtained for the (001) model as well as the (000) one. After 12 TST, hew stripped considerably for (000) and light (111). Figs. 5 and 6 show experimental and theoretical results of the hourly yields for SSS and HSSHP configurations. The HSSHP plant gives an excellent output, reaching the value of 1.65 l/m2h for the HSSHP model. For the SSS configuration, the maximum flow is equal to 0.3 l/m2°C. Theoretical results are comparable with the experimental ones, for statistical analysis, the error is more less for (001) and (111) (14%, 5.27%) than (000) and (110) (21%, 17%). From Figs. 7–10, it's clear that the global efficiency of SSS and HSSHP are increasing functions of true solar time however this growth attenuates for high values of the irradiation. It could be noticed that in
Fig. 8. Variation of the interior and global efficiency according to TSV for the configuration (001).
the configurations (000), the internal efficiency is higher than the global efficiency, whereas for (001) and (111) configurations, the global efficiency becomes higher than the interior one. This was explained by the addition of the heat pump (one must take in account the quantity of (COP·P)). Moreover, we can also notice that the global efficiency of the HSSHP model is definitely higher than that of the SSS because of its greater thermal inertia, which less quickly follows the variations of incidental solar energy. 5. Conclusion The solar still for purpose of traditional greenhouse was taken as reference to show the performances of the solar still combined with a heat pump.
Fig. 10. Variation of the interior and global efficiency according to TSV for the (111) configuration.
K. Hidouri et al. / Desalination 250 (2010) 444–449
Our measurements showed an increase in going flow of 300 ml/m2h with 1700 ml/m2h by combining with a simple still for purpose of greenhouse heat pump with compression where the evaporator and the condenser are jointly used. As for the daily output, it passes from 2 l/m2 to 12 l/m2. The average efficiency passes from 20% to 80%. In the configuration with heat pump, one observed low moisture of the air in the distiller; this is with the presence of the evaporator constituting a surface at temperature of weak dew. Thus, the vapor created in the still is quickly condensed. For (110) and (000) configurations, the efficiency attained a maximum value of 0.8 as compared to the (111) and (001) ones, in the last cases, the maximum value is equal to 1.8. The efficiency is higher for (111) and (001) configurations than for (000) and (110) ones. Symbols HSSHP SSS MSF RO Aw C Gr hcw hew L Lv mex n Nu Pr Pw Pevp Tw Tevp TST Z G qew qev qe qw
Hybrid-Solar Still-Heat Pump System Simple Solar Still—SSS Multi-Stage-Flash Reverse osmosis Area of water (m2) Experimental constant Grashof number Convective heat transfer coefficient (W/m2 °C) Evaporative heat transfer coefficient (W/m2 °C) Latent heat of vaporization (J/kg) Characteristic dimension (m) Experimental distiller output (l/m2 h) Exponent Nusselt number Prandlt number of humid air Saturation vapor pressure of water at evaporation surface (N/m2) Saturation vapor pressure of water at condensation surface (N/m2) Water temperature (°C) Vapor temperature (°C) True solar time (h) Function Solar radiation (W/m2) Rate of heat transfer by convection, (W/m2) Rate of evaporative heat transfer, (W/m2) Heat flux used for the evaporation of water (W/m2) Heat flux actually received by the water mass (W/m2)
αt η ηg COP(PAC) P G
449
Fictitious absorption coefficient of the water mass (0.85). Interior efficiency Global efficiency Coefficient of performance of heat pump Power of compressor equal 200 W Solar intensity (W/m2)
Subscripts cw Convective evp Evaporative w Water
References [1] E. Tzen, R. Morris, Renewable energy sources for desalination, Sol. Energy 75 (2003) 375–379. [2] H. Al-Hinai, M.S. Al-Nassri, B.A. Jubran, Effect of climatic, design and operational parameters on the yield of a simple solar still, Energy Convers. Manag. 43 (2002) 1639–1650. [3] K. Ahmed, K. Schwarzer, F. Christian, M. Christoph, Production décentralisée de l'eau potable à l'énergie solaire, Desalination 168 (2004) 13–20. [4] C. Esteban, J. Franco, A. Fasulo, Construction and performance of an assisted solar distiller, Desalination 173 (2005) 249–255. [5] A.A. Badran, A.A. Ahmed, I.A.E. Salman, M.Z. Odat, Asoalr still augmented with a flat plate collector, Desalination 172 (2005) 227–234. [6] G.N. Tiwari and A.K. Tiwari, Solar Distillation Practice for Water Desalination Systems, Anamaya Publisher, New Delhi, 2007 and Anshan Publisher, UK, 2008. [7] M. Qudais Abu, et al., Experimental study and numerical simulation of a solar still using an external condenser, Energy 21 (1996) 851. [8] J. Siqueiros, F.A. Holland, Water desalination using heat pumps, Energy 25 (2000) 717–729. [9] V.V. Slesarenko, Heat pumps as a source of heat energy for desalination of seawater, Desalination 193 (2001) 405–410. [10] R. Tripathi, G.N. Tiwari, Thermal modelling of passive and active solar stills for different depths of water by using the concept of solar fraction, Sol. Energy 80 (2006) 956–967. [11] M.N.A. Hawlader, K.D. Prasanta, D. Sufyan, C.Y. Chung, Solar assisted heat pump desalination system Elsevier, Desalination 168 (2004) 49–54. [12] A. Kaabi, S. Nafila, Impact of temperature difference (water–solar collector) on solar-still global efficiency, Desalination 209 (2007) 298–305. [13] M.K. Ghosal, G.N Tiwari, N.S.L. Srivastava, Thermal modelling of a controlled environment greenhouse cum solar distillation for composite and warm humid climates of India, Desalination 151 (2002) 293–308. [14] B.S. Romdhane, H. Khaoula, G. Slimanne. Proceeding of the Heat-SET 2007 (2007). Conference 18–20 Avril, Chambéry, France. [15] D.R. Pitts, L.E. Sisson, Theory and Problems of Heat Transfer, McGraw-Hill Book Company, New York, 1977.