- Email: [email protected]
Combined desalination and solar-assisted air-conditioning system
Energy Conversion and Management 49 (2008) 3326–3330
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Combined desalination and solar-assisted air-conditioning system Veera Gnaneswar Gude, Nagamany Nirmalakhandan * Civil Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA
a r t i c l e
i n f o
Article history: Available online 24 June 2008 Keywords: Waste heat utilization Desalination Absorption refrigeration Thermal energy storage Process modeling Solar collectors
a b s t r a c t Analysis of a new desalination process utilizing low grade thermal energy is presented. In this process, fresh water is distilled from saline water under near-vacuum pressures created by passive means, enabling low-temperature distillation with lower energy requirements. The energy for low-temperature distillation is provided by a thermal energy storage (TES) system maintained at 55 °C utilizing any low grade waste heat source. In this study, heat rejected by the condenser of a modified absorption refrigeration system (ARS) is evaluated as a possible source to drive this desalination process. The energy for the generator of the ARS is provided by a combination of solar collector system and grid power. Results of this study show that the thermal energy rejected by an ARS of cooling capacity of 3.25 kW (0.975 tons of refrigeration) along with an additional energy input of 208 kJ/kg of desalinated water is adequate to produce desalinated water at an average rate of 4.5 kg/h. This energy consumption is competitive with that of the multi-stage flash distillation process of similar capacity (338 kJ/kg). An integrated process model and performance curves of the proposed approach are presented in this paper. Effects of process parameters on the performance of the system are also presented. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
2. Description of the proposed system
Due to increasing cost of energy, more attention is being paid to improve efficiency and economics of thermal processes. One option to improve the thermodynamic and economic performance of thermal processes is to utilize low grade thermal energy and waste heat releases [1]. This study presents the feasibility of lowtemperature desalination using low grade waste heat. Low-temperature technologies for desalination can be beneficial due to the following advantages: low corrosion and scaling rates, higher thermodynamic efficiency, better flexibility and reliability, and high-purity distillate [2]. Often times, low grade thermal sources (50–100 °C) are readily available free of cost, as in the case of heat pumps and condensers that reject waste heat to the environment. Cogeneration units producing electricity as well as desalinated water have become successful with use of low grade waste thermal energy sources. Many multi-effect desalination and multi-stage flash desalination units are in operation around the world that utilize low grade heat sources [3]. Several modifications have been made on the equipment and process units to improve the performance of low-temperature desalination processes [1]. In this study, an innovative low-temperature desalination process that utilizes waste heat rejected by an absorption refrigeration system is evaluated.
A schematic of the proposed system is shown in Fig. 1. Major components of the system are a desalination unit, a sensible heat thermal energy storage (TES) unit, and an absorption refrigeration unit. The desalination unit includes an evaporation chamber (EC), a condenser (CON), two heat exchangers (HE1 and HE2), and three 10-m tall columns. These three columns serve as the saline water column; the brine withdrawal column; and the desalinated water column, each with its own holding tank, SWT, BT, and DWT, respectively. The heat input to EC is provided by the TES, which, in turn, is maintained at 50 °C by the absorption refrigeration system (ARS). The EC is installed atop the three columns at a height of about 10 m above the free surfaces in the three holding tanks, creating a Torricelli’s vacuum in the head space of the EC. The temperature of the head space of the feed water column is maintained slightly higher than that of the desalinated water column. Since the head spaces are at near-vacuum level pressures, temperature differential as small as 10 °C is adequate to evaporate water from the saline water side and condense in the fresh water side. In this manner, saline water can be desalinated at about 40–50 °C, which is in contrast to the 60–100 °C range employed in traditional solar stills and other distillation processes. A continuous stream of brine is withdrawn from the EC through HE1 preheating the saline water feed entering the EC and maintaining the desired salt level in the EC. This configuration drives the desalination process without any mechanical pumping [4].
* Corresponding author. Tel.: +1 5056465378. E-mail address: [email protected] (N. Nirmalakhandan). 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.03.030
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Condenser, CON
Evaporation chamber, EC
Flat panel solar collector Hot water storage
Solar collector system
HE2 HE1
Thermal energy storage, TES
~ 10 m
Auxiliary power Generator
Evaporator
SWT
BT
Cooling load
DWT Condenser
Saline water
Brine
Freshwater
Absorption refrigeration system, ARS
Desalination system
Fig. 1. Schematic of the proposed desalination system.
Thermal energy to maintain the EC at the desired temperature is provided by the thermal energy storage (TES) system, whose temperature is set at 50 °C. The thermal energy required to maintain the TES at this temperature is provided by the heat rejected by an absorption refrigeration system (ARS). The ARS evaluated in this study operates with LiBr–H2O as refrigerant under a pressure range of 1–16 kPa. Energy required to heat the generator of ARS is supplied by a solar collector during sunlight hours and by an auxiliary electric heater during non-sunlight hours. In this manner, the thermal energy to drive the desalination process is available round the clock. The generator of the ARS is maintained at 100 °C. Since the evaporator of the ARS feeds the cooling load, the proposed system performs two functions of continuous desalination and cooling with reduced amount of external non-renewable energy input.
Heat balance for the EC
d _ _ ðqV cTÞEC ¼ Q TES Q e Q l;EC þ ðqVcTÞ i ðqVcTÞw dt
ð3Þ
where QTES, Qe, and Ql,EC are the rates of heat input from the TES, heat for evaporation, and heat loss. The energy for evaporation, Qe, is determined from
Q e ¼ ðqqe ÞhLðTsÞ
ð4Þ
where qe is the volumetric evaporation rate. In this study, qe is estimated following the model proposed by Bemporad [5] where the evaporation rate is controlled by the pressure and temperature gradients between the heating surface and the condensing surface
AEC am
"
pðT d Þ ðT s þ 273Þ1=2 ðT d þ 273Þ1=2
#
3. Modeling of the system A process model for the integrated system has been developed based on mass and energy balances and solved using ExtendÒ and EESÒ simulation software packages.
The process variables are defined in Appendix A. The desalination efficiency, g, is defined as
3.1. Modeling the desalination unit
g¼P
me hL ðQ TES DtÞ
An evaporator area of 5 m2 and a height of 0.5 m are considered. In all calculations, the reference temperature used is 25 °C. All heat exchangers are assumed to have 80% efficiency. The following mass and heat balance equations apply to the evaporation chamber, EC: Mass balance on water in the EC
d ðqVÞwater ¼ qi V_ i qw V_ w qe V_ e dt
ð1Þ
Mass balance on solute in the EC
d ðqVCÞsolute ¼ qi V_ i C i qw V_ w C w dt
q
fðCÞ
pðT s Þ
qe ¼
ð2Þ
ð5Þ
ð6Þ
3.1.1. Solar collector system Absorption refrigeration system is driven primarily by solar energy during sunlight hours and by auxiliary power source during non-sunlight hours. The efficiency of solar collectors is expressed in terms of solar fraction, which is the contribution of the solar energy to the total load in terms of the fractional reduction in the amount of external energy that must be supplied. A storage tank volume of 0.125 m3/m2 has been considered and the required optimum area of solar collectors is found from the solar fraction graph [6]. The optimum number of collectors is the lowest number of collectors for which a 100% solar fraction is achieved at the hour of maximum solar radiation. The pumping requirements are calculated using EESÒ software.
V.G. Gude, N. Nirmalakhandan / Energy Conversion and Management 49 (2008) 3326–3330
Heat balance across solar collector system, SC
d ðmcTÞSC ¼ FASC fsaIsolar U SC ðT SC T a Þg AU L ðT g T a Þ dt ðmcÞr ðT SC T g Þ
ð7Þ
3.1.2. Low-temperature thermal energy storage system The sensible heat TES system [7] stores heat rejected by the condenser of the ARS, Q. The optimal volume of TES to maintain and supply constant heat source is estimated by solving the heat balance for the TES by trial and error [8]. Heat balance for TES:
d ðqVcTÞTES ¼ Q ARS Q TES Q l;TES dt
ð8Þ
where QARS is the heat rejection rate of the ARS. The model Eqs. (1)– (8) were solved using the ambient temperatures for a site in Southern New Mexico. 4. Results One of the objectives of the modeling exercise was to verify that a properly sized TES would be able to provide the required thermal energy to the evaporator to maintain the desalination rate over a 24-h period. Fig. 2 shows the variation in rates of heat supplied by the TES, the heat consumed for evaporation, and the heat lost over a 24-h period for a summer day, when the ambient temperature ranged from 25 to 35 °C. The desalination efficiency defined by Eq. (6) is also plotted in Fig. 2. As expected, the energy lost by the EC is higher during non-sunlight hours than that during sunlight hours due to lower ambient temperatures during non-sunlight hours. Under the base case conditions, the energy available for desalination is about 12,500 kJ/h (=3.45 kW) which is the waste heat rejected by the condenser in ARS. However, the net heat transfer is dependent on the temperature gradient between the transfer medium and the heat source. The actual mass of water that can be evaporated in the EC and hence, the desalination efficiency, will depend on the heat input rate from the TES, the ambient temperature at which the condensation takes place, and the brine withdrawal rate, as discussed later. Since the driving force for evaporation is the temperature difference between the EC and the condenser, the heat input to EC during the day is lower than that input during the night. During the night, the ambient temperature is low and the freshwater temperature is also low which favors higher desalination rate, thus resulting in higher heat input and vice versa.
The variations in the saline water temperature in the EC and the fresh water temperatures with respect to ambient temperature are shown in Fig. 3. The temperature of saline water varied from 43.5 to 46 °C and the ambient temperature ranged from 25 to 37 °C while the fresh water temperatures ranged from 35 to 40 °C. From Figs. 2 and 3, it is concluded that the TES is able to maintain the desalination efficiency and the temperature of the evaporation chamber at the desired operating conditions. As can be seen from these plots, ambient temperature is an important variable because condensation occurs at the ambient temperature, which indirectly determines the desalination rate in this process. 4.1. Analysis of ARS The ARS configuration employed in the proposed system is designed for two functions- for maintaining the TES at the desired temperature and for providing the cooling load. As such, the proposed ARS operates under slightly different conditions compared to the traditional systems used for cooling alone. Operating conditions for typical ARS used in cooling and those for the ARS proposed in this study are compared in Table 1, for the same cooling load of 3.25 kW. The notable difference is the pressure rangesabout 1–6 kPa versus 1.5–15.75 kPa, respectively. This is necessary to run the condenser at 55 °C to maintain the TES at 50 °C. 4.2. Volume of TES tank Winter conditions were assumed to determine the size of the TES necessary to provide the heat energy to the EC. This volume was found by solving Eq. (8) by trial and error so that the temperatures at the beginning and the end of a 24-h period would be within ±0.1 °C.
50 Evaporation chamber
Temperature [ºC]
3328
40
Freshwater Ambient
30
20
10 8:00 AM
2 8:00 AM
8:00 PM
Time of day Fig. 3. Evaporation chamber, freshwater, and ambient temperature variations over 24 h.
100
Efficiency 12,000
80
Q
TES
60 8,000
Qe 40
4,000 20 Qlosses 0 8:00 AM
8:00 PM
Time of day Fig. 2. Rates of heat exchanges and efficiency over 24 h.
0 2 AM 8:00
Efficiency [%]
Heat transfer rate [kJ/hr]
16,000
Table 1 ARS system parameters: typical values versus and values in this study Parameter
Typical value
This study
Absorber temperature (°C) Condenser temperature (°C) Evaporator temperature (°C) Generator temperature (°C) Condenser/Generator pressure (kPa) Absorber/Evaporator pressure (kPa) Energy transfer rate at absorber (kW) Energy transfer rate at condenser (kW) Energy transfer rate at evaporator (kW) Energy transfer rate at generator (kW) Coefficient of performance, COP (-)
30 35 8 100 6.27 1.073 4.32 3.49 3.25 4.43 0.73
28 55 12 100 15.75 1.403 4.43 3.49 3.25 4.67 0.72
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V.G. Gude, N. Nirmalakhandan / Energy Conversion and Management 49 (2008) 3326–3330
4.3.1. Energy requirements Apart from the solar energy, the proposed system requires additional mechanical energy to drive the pumps and additional heat energy for the generator to drive the ARS during non-sunlight hours. Simulation results show that the additional mechanical energy requirement is 16 kJ/kg of product plus auxiliary heat energy of 192 kJ/kg of product, totaling to a specific energy requirement of 208 kJ/kg. In comparison, a typical multi-stage flash distillation
Desalination rate [kg/hr]
200 180 160
30
140 120
20
100 80 60
10
40 20
0
0 0
5
10
15
20
25
Cooling load [kJ/hr] Fig. 6. Desalination rates at different cooling rates and solar panel areas.
100
45.8
90
80
45.7
70
60
Temperature of EC [ºC]
The solar collector, augmented by an auxiliary heater, is to be sized to provide for the TES and the cooling load. The desired temperature of the storage tank of the solar collector is set to 110 °C in order to maintain the generator temperature at 100 °C. The energy to be provided by the auxiliary heater is equal to the difference between the energy required by the generator and that can be collected from solar insolation. Fig. 5 illustrates this difference and the solar fraction over a 24-h period. The optimal area of the collectors was found from Eq. (7). For the base case considered here, solar collector area of 25 m2 can satisfy a cooling load of 3.25 kW at an average desalination rate of 4.5 kg/h. The relationships between desalination rate, solar panel area, and cooling load are presented in Fig. 6.
Efficiency [%]
4.3. Solar collector for ARS
40
Solar panel area [sq m]
A tank volume of 10 m3 was found to be adequate to maintain a temperature of 50 °C throughout a 24-h period and to provide the energy needs of the EC. Fig. 4 shows that the TES temperature remained constant at the set value of 50 °C while the ambient winter temperature ranged from 2 to 15 °C.
45.6 0
5
10
15
20
25
30
Withdrawal rate [kg/hr] Fig. 7. Relationship between withdrawal rate, efficiency and temperature of EC.
60 50
Temperature [ºC]
TES
process requires mechanical energy of 44 kJ/kg of product plus heat energy of 294 kJ/kg of product, totaling to a specific energy requirement of 338 kJ/kg [9]. Based on simulation results, the proposed process can be an energy-efficient and sustainable alternative for desalination.
40 30 20
60
4.3.2. Brine withdrawal versus system performance Brine withdrawal rate is the primary control variable in this system, which has positive as well as negative impacts on the performance of the system. At low withdrawal rates, salts build up in the EC, and evaporation rates decrease as shown by Eq. (3). High salt levels also reduce the enthalpy of saline water that can further reduce evaporation [10]. For example, when salinity increases by 1%, evaporation is also reduced by about the same percentage. Even though better salt removal can be achieved with higher withdrawal rates, large amounts of sensible heat are also simultaneously removed from the EC, resulting in decline of EC temperature. Simulation results presented in Fig. 7 shows that both EC temperature and the desalination efficiency decline with increasing withdrawal rate. For example, the desalination efficiency dropped from 90.5% to 80% when the withdrawal rate increased from 2.5 kg/h to 25 kg/h.
40
5. Conclusions
Ambient
10 0 8:00 AM
8:00 PM
2 8:00 AM
Time of day Fig. 4. Ambient and TES temperature variations over 24 h.
Temperature [ºC]; Solar fraction [%
120
Target temperature for SC 100
Actual temperature of SC
80
20
Solar fraction 0 8:00 AM
8:00 PM
Time of day Fig. 5. Solar fraction and optimum solar fraction area.
8:00 AM
Based on simulation results, the proposed low-temperature desalination process is capable of producing fresh water at 4.5 kg/h and providing a cooling load of 3.25 kW, consuming 208 kJ/kg of freshwater. The system requires thermal energy storage volume of 10 m3 and solar panel area of 25 m2. Model simulations show that the proposed system can achieve a desalination
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efficiency of 80–90%. Based on the results from this study 100% withdrawal rate is suggested to prevent scale formation which affects the evaporation rate. The energy requirements for the proposed system are less than that are required for a multi-stage flash distillation process. The proposed system has the potential to be operated entirely on renewable or low grade waste energy sources and offers a sustainable approach for desalination. The performance of the system may be further improved by incorporating double or triple-effect configurations. Acknowledgement This study was funded in part by a grant from the New Mexico Water Resources Research Institute. Appendix A A c C F(C) F hL m q Q T U V V_
a g
surface area (m2) specific heat capacity (kJ/kg-°C) solute concentration (%) correlation factor for effect of solute concentration (%) fraction solar insolation latent heat (kJ/kg) mass of fluid (kg) volumetric rate (m3/s) heat transfer rate (kJ/h) temperature (°C) heat loss coefficient (kJ/h-m2-°C) volume of fluid (m3) volumetric flow rate of fluid (m3/h) absorptivity, experimental coefficient (107–106 kg/m2Pa-s-K0.5) desalination efficiency
q s
mass density (kg/m3) transmittivity of glass (–)
Subscripts a ambient conditions d distilled water conditions e evaporation stream from EC g generator conditions i input stream to EC l loss from EC or TES r generator-solar collector recirculation stream s solute w withdrawal stream from EC References [1] Wang SC. The exergetic efficiency of MSF process and the contributions of desalination by waste heat. Desal 1983;44:39–49. [2] Gustavo K, Fredi L. Low-temperature distillation processes in single- and dual purpose plants. Desal 2001;136:189–97. [3] Jacques G, Dominique L. Cogeneration applied to very high efficiency thermal seawater desalination plants. Desal 1999;125:203–8. [4] Al-Kharabsheh S, Goswami DY. Theoretical analysis of a water desalination system using low grade heat. J Solar Energy Eng 2004;126:774–80. [5] Bemporad GA. Basic hydrodynamic aspects of a solar energy based desalination process. Solar Energy 1995;54(2):125–34. [6] Luis HA, Jorge EG. Simulation of an air-cooled solar-assisted absorption air onditioning system. J Solar Energy Eng 2002;124:276–82. [7] Dincer I. Thermal energy storage systems as a key technology in energy conservation. Int J Energy Res 2002;26:567–88. [8] Gadhamshetty V, Nirmalakhandan N, Myint M, Ricketts C. Improving aircooled condenser performance. ASCE J Energy Eng 2006;132(2):81–8. [9] Kaligirou SA. Seawater desalination using renewable energy sources. Prog In Energy Comb Sci 2005;31:242–81. [10] Keren Y, Rubin H, Atkinson J, Priven M, Bemporad GA. Theoretical and experimental comparison of conventional and advanced solar pond performance. Solar Energy 1993;51:255–70.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Combined desalination and solar-assisted air-conditioning system Veera Gnaneswar Gude, Nagamany Nirmalakhandan * Civil Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA
a r t i c l e
i n f o
Article history: Available online 24 June 2008 Keywords: Waste heat utilization Desalination Absorption refrigeration Thermal energy storage Process modeling Solar collectors
a b s t r a c t Analysis of a new desalination process utilizing low grade thermal energy is presented. In this process, fresh water is distilled from saline water under near-vacuum pressures created by passive means, enabling low-temperature distillation with lower energy requirements. The energy for low-temperature distillation is provided by a thermal energy storage (TES) system maintained at 55 °C utilizing any low grade waste heat source. In this study, heat rejected by the condenser of a modified absorption refrigeration system (ARS) is evaluated as a possible source to drive this desalination process. The energy for the generator of the ARS is provided by a combination of solar collector system and grid power. Results of this study show that the thermal energy rejected by an ARS of cooling capacity of 3.25 kW (0.975 tons of refrigeration) along with an additional energy input of 208 kJ/kg of desalinated water is adequate to produce desalinated water at an average rate of 4.5 kg/h. This energy consumption is competitive with that of the multi-stage flash distillation process of similar capacity (338 kJ/kg). An integrated process model and performance curves of the proposed approach are presented in this paper. Effects of process parameters on the performance of the system are also presented. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
2. Description of the proposed system
Due to increasing cost of energy, more attention is being paid to improve efficiency and economics of thermal processes. One option to improve the thermodynamic and economic performance of thermal processes is to utilize low grade thermal energy and waste heat releases [1]. This study presents the feasibility of lowtemperature desalination using low grade waste heat. Low-temperature technologies for desalination can be beneficial due to the following advantages: low corrosion and scaling rates, higher thermodynamic efficiency, better flexibility and reliability, and high-purity distillate [2]. Often times, low grade thermal sources (50–100 °C) are readily available free of cost, as in the case of heat pumps and condensers that reject waste heat to the environment. Cogeneration units producing electricity as well as desalinated water have become successful with use of low grade waste thermal energy sources. Many multi-effect desalination and multi-stage flash desalination units are in operation around the world that utilize low grade heat sources [3]. Several modifications have been made on the equipment and process units to improve the performance of low-temperature desalination processes [1]. In this study, an innovative low-temperature desalination process that utilizes waste heat rejected by an absorption refrigeration system is evaluated.
A schematic of the proposed system is shown in Fig. 1. Major components of the system are a desalination unit, a sensible heat thermal energy storage (TES) unit, and an absorption refrigeration unit. The desalination unit includes an evaporation chamber (EC), a condenser (CON), two heat exchangers (HE1 and HE2), and three 10-m tall columns. These three columns serve as the saline water column; the brine withdrawal column; and the desalinated water column, each with its own holding tank, SWT, BT, and DWT, respectively. The heat input to EC is provided by the TES, which, in turn, is maintained at 50 °C by the absorption refrigeration system (ARS). The EC is installed atop the three columns at a height of about 10 m above the free surfaces in the three holding tanks, creating a Torricelli’s vacuum in the head space of the EC. The temperature of the head space of the feed water column is maintained slightly higher than that of the desalinated water column. Since the head spaces are at near-vacuum level pressures, temperature differential as small as 10 °C is adequate to evaporate water from the saline water side and condense in the fresh water side. In this manner, saline water can be desalinated at about 40–50 °C, which is in contrast to the 60–100 °C range employed in traditional solar stills and other distillation processes. A continuous stream of brine is withdrawn from the EC through HE1 preheating the saline water feed entering the EC and maintaining the desired salt level in the EC. This configuration drives the desalination process without any mechanical pumping [4].
* Corresponding author. Tel.: +1 5056465378. E-mail address: [email protected] (N. Nirmalakhandan). 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.03.030
3327
V.G. Gude, N. Nirmalakhandan / Energy Conversion and Management 49 (2008) 3326–3330
Condenser, CON
Evaporation chamber, EC
Flat panel solar collector Hot water storage
Solar collector system
HE2 HE1
Thermal energy storage, TES
~ 10 m
Auxiliary power Generator
Evaporator
SWT
BT
Cooling load
DWT Condenser
Saline water
Brine
Freshwater
Absorption refrigeration system, ARS
Desalination system
Fig. 1. Schematic of the proposed desalination system.
Thermal energy to maintain the EC at the desired temperature is provided by the thermal energy storage (TES) system, whose temperature is set at 50 °C. The thermal energy required to maintain the TES at this temperature is provided by the heat rejected by an absorption refrigeration system (ARS). The ARS evaluated in this study operates with LiBr–H2O as refrigerant under a pressure range of 1–16 kPa. Energy required to heat the generator of ARS is supplied by a solar collector during sunlight hours and by an auxiliary electric heater during non-sunlight hours. In this manner, the thermal energy to drive the desalination process is available round the clock. The generator of the ARS is maintained at 100 °C. Since the evaporator of the ARS feeds the cooling load, the proposed system performs two functions of continuous desalination and cooling with reduced amount of external non-renewable energy input.
Heat balance for the EC
d _ _ ðqV cTÞEC ¼ Q TES Q e Q l;EC þ ðqVcTÞ i ðqVcTÞw dt
ð3Þ
where QTES, Qe, and Ql,EC are the rates of heat input from the TES, heat for evaporation, and heat loss. The energy for evaporation, Qe, is determined from
Q e ¼ ðqqe ÞhLðTsÞ
ð4Þ
where qe is the volumetric evaporation rate. In this study, qe is estimated following the model proposed by Bemporad [5] where the evaporation rate is controlled by the pressure and temperature gradients between the heating surface and the condensing surface
AEC am
"
pðT d Þ ðT s þ 273Þ1=2 ðT d þ 273Þ1=2
#
3. Modeling of the system A process model for the integrated system has been developed based on mass and energy balances and solved using ExtendÒ and EESÒ simulation software packages.
The process variables are defined in Appendix A. The desalination efficiency, g, is defined as
3.1. Modeling the desalination unit
g¼P
me hL ðQ TES DtÞ
An evaporator area of 5 m2 and a height of 0.5 m are considered. In all calculations, the reference temperature used is 25 °C. All heat exchangers are assumed to have 80% efficiency. The following mass and heat balance equations apply to the evaporation chamber, EC: Mass balance on water in the EC
d ðqVÞwater ¼ qi V_ i qw V_ w qe V_ e dt
ð1Þ
Mass balance on solute in the EC
d ðqVCÞsolute ¼ qi V_ i C i qw V_ w C w dt
q
fðCÞ
pðT s Þ
qe ¼
ð2Þ
ð5Þ
ð6Þ
3.1.1. Solar collector system Absorption refrigeration system is driven primarily by solar energy during sunlight hours and by auxiliary power source during non-sunlight hours. The efficiency of solar collectors is expressed in terms of solar fraction, which is the contribution of the solar energy to the total load in terms of the fractional reduction in the amount of external energy that must be supplied. A storage tank volume of 0.125 m3/m2 has been considered and the required optimum area of solar collectors is found from the solar fraction graph [6]. The optimum number of collectors is the lowest number of collectors for which a 100% solar fraction is achieved at the hour of maximum solar radiation. The pumping requirements are calculated using EESÒ software.
V.G. Gude, N. Nirmalakhandan / Energy Conversion and Management 49 (2008) 3326–3330
Heat balance across solar collector system, SC
d ðmcTÞSC ¼ FASC fsaIsolar U SC ðT SC T a Þg AU L ðT g T a Þ dt ðmcÞr ðT SC T g Þ
ð7Þ
3.1.2. Low-temperature thermal energy storage system The sensible heat TES system [7] stores heat rejected by the condenser of the ARS, Q. The optimal volume of TES to maintain and supply constant heat source is estimated by solving the heat balance for the TES by trial and error [8]. Heat balance for TES:
d ðqVcTÞTES ¼ Q ARS Q TES Q l;TES dt
ð8Þ
where QARS is the heat rejection rate of the ARS. The model Eqs. (1)– (8) were solved using the ambient temperatures for a site in Southern New Mexico. 4. Results One of the objectives of the modeling exercise was to verify that a properly sized TES would be able to provide the required thermal energy to the evaporator to maintain the desalination rate over a 24-h period. Fig. 2 shows the variation in rates of heat supplied by the TES, the heat consumed for evaporation, and the heat lost over a 24-h period for a summer day, when the ambient temperature ranged from 25 to 35 °C. The desalination efficiency defined by Eq. (6) is also plotted in Fig. 2. As expected, the energy lost by the EC is higher during non-sunlight hours than that during sunlight hours due to lower ambient temperatures during non-sunlight hours. Under the base case conditions, the energy available for desalination is about 12,500 kJ/h (=3.45 kW) which is the waste heat rejected by the condenser in ARS. However, the net heat transfer is dependent on the temperature gradient between the transfer medium and the heat source. The actual mass of water that can be evaporated in the EC and hence, the desalination efficiency, will depend on the heat input rate from the TES, the ambient temperature at which the condensation takes place, and the brine withdrawal rate, as discussed later. Since the driving force for evaporation is the temperature difference between the EC and the condenser, the heat input to EC during the day is lower than that input during the night. During the night, the ambient temperature is low and the freshwater temperature is also low which favors higher desalination rate, thus resulting in higher heat input and vice versa.
The variations in the saline water temperature in the EC and the fresh water temperatures with respect to ambient temperature are shown in Fig. 3. The temperature of saline water varied from 43.5 to 46 °C and the ambient temperature ranged from 25 to 37 °C while the fresh water temperatures ranged from 35 to 40 °C. From Figs. 2 and 3, it is concluded that the TES is able to maintain the desalination efficiency and the temperature of the evaporation chamber at the desired operating conditions. As can be seen from these plots, ambient temperature is an important variable because condensation occurs at the ambient temperature, which indirectly determines the desalination rate in this process. 4.1. Analysis of ARS The ARS configuration employed in the proposed system is designed for two functions- for maintaining the TES at the desired temperature and for providing the cooling load. As such, the proposed ARS operates under slightly different conditions compared to the traditional systems used for cooling alone. Operating conditions for typical ARS used in cooling and those for the ARS proposed in this study are compared in Table 1, for the same cooling load of 3.25 kW. The notable difference is the pressure rangesabout 1–6 kPa versus 1.5–15.75 kPa, respectively. This is necessary to run the condenser at 55 °C to maintain the TES at 50 °C. 4.2. Volume of TES tank Winter conditions were assumed to determine the size of the TES necessary to provide the heat energy to the EC. This volume was found by solving Eq. (8) by trial and error so that the temperatures at the beginning and the end of a 24-h period would be within ±0.1 °C.
50 Evaporation chamber
Temperature [ºC]
3328
40
Freshwater Ambient
30
20
10 8:00 AM
2 8:00 AM
8:00 PM
Time of day Fig. 3. Evaporation chamber, freshwater, and ambient temperature variations over 24 h.
100
Efficiency 12,000
80
Q
TES
60 8,000
Qe 40
4,000 20 Qlosses 0 8:00 AM
8:00 PM
Time of day Fig. 2. Rates of heat exchanges and efficiency over 24 h.
0 2 AM 8:00
Efficiency [%]
Heat transfer rate [kJ/hr]
16,000
Table 1 ARS system parameters: typical values versus and values in this study Parameter
Typical value
This study
Absorber temperature (°C) Condenser temperature (°C) Evaporator temperature (°C) Generator temperature (°C) Condenser/Generator pressure (kPa) Absorber/Evaporator pressure (kPa) Energy transfer rate at absorber (kW) Energy transfer rate at condenser (kW) Energy transfer rate at evaporator (kW) Energy transfer rate at generator (kW) Coefficient of performance, COP (-)
30 35 8 100 6.27 1.073 4.32 3.49 3.25 4.43 0.73
28 55 12 100 15.75 1.403 4.43 3.49 3.25 4.67 0.72
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4.3.1. Energy requirements Apart from the solar energy, the proposed system requires additional mechanical energy to drive the pumps and additional heat energy for the generator to drive the ARS during non-sunlight hours. Simulation results show that the additional mechanical energy requirement is 16 kJ/kg of product plus auxiliary heat energy of 192 kJ/kg of product, totaling to a specific energy requirement of 208 kJ/kg. In comparison, a typical multi-stage flash distillation
Desalination rate [kg/hr]
200 180 160
30
140 120
20
100 80 60
10
40 20
0
0 0
5
10
15
20
25
Cooling load [kJ/hr] Fig. 6. Desalination rates at different cooling rates and solar panel areas.
100
45.8
90
80
45.7
70
60
Temperature of EC [ºC]
The solar collector, augmented by an auxiliary heater, is to be sized to provide for the TES and the cooling load. The desired temperature of the storage tank of the solar collector is set to 110 °C in order to maintain the generator temperature at 100 °C. The energy to be provided by the auxiliary heater is equal to the difference between the energy required by the generator and that can be collected from solar insolation. Fig. 5 illustrates this difference and the solar fraction over a 24-h period. The optimal area of the collectors was found from Eq. (7). For the base case considered here, solar collector area of 25 m2 can satisfy a cooling load of 3.25 kW at an average desalination rate of 4.5 kg/h. The relationships between desalination rate, solar panel area, and cooling load are presented in Fig. 6.
Efficiency [%]
4.3. Solar collector for ARS
40
Solar panel area [sq m]
A tank volume of 10 m3 was found to be adequate to maintain a temperature of 50 °C throughout a 24-h period and to provide the energy needs of the EC. Fig. 4 shows that the TES temperature remained constant at the set value of 50 °C while the ambient winter temperature ranged from 2 to 15 °C.
45.6 0
5
10
15
20
25
30
Withdrawal rate [kg/hr] Fig. 7. Relationship between withdrawal rate, efficiency and temperature of EC.
60 50
Temperature [ºC]
TES
process requires mechanical energy of 44 kJ/kg of product plus heat energy of 294 kJ/kg of product, totaling to a specific energy requirement of 338 kJ/kg [9]. Based on simulation results, the proposed process can be an energy-efficient and sustainable alternative for desalination.
40 30 20
60
4.3.2. Brine withdrawal versus system performance Brine withdrawal rate is the primary control variable in this system, which has positive as well as negative impacts on the performance of the system. At low withdrawal rates, salts build up in the EC, and evaporation rates decrease as shown by Eq. (3). High salt levels also reduce the enthalpy of saline water that can further reduce evaporation [10]. For example, when salinity increases by 1%, evaporation is also reduced by about the same percentage. Even though better salt removal can be achieved with higher withdrawal rates, large amounts of sensible heat are also simultaneously removed from the EC, resulting in decline of EC temperature. Simulation results presented in Fig. 7 shows that both EC temperature and the desalination efficiency decline with increasing withdrawal rate. For example, the desalination efficiency dropped from 90.5% to 80% when the withdrawal rate increased from 2.5 kg/h to 25 kg/h.
40
5. Conclusions
Ambient
10 0 8:00 AM
8:00 PM
2 8:00 AM
Time of day Fig. 4. Ambient and TES temperature variations over 24 h.
Temperature [ºC]; Solar fraction [%
120
Target temperature for SC 100
Actual temperature of SC
80
20
Solar fraction 0 8:00 AM
8:00 PM
Time of day Fig. 5. Solar fraction and optimum solar fraction area.
8:00 AM
Based on simulation results, the proposed low-temperature desalination process is capable of producing fresh water at 4.5 kg/h and providing a cooling load of 3.25 kW, consuming 208 kJ/kg of freshwater. The system requires thermal energy storage volume of 10 m3 and solar panel area of 25 m2. Model simulations show that the proposed system can achieve a desalination
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efficiency of 80–90%. Based on the results from this study 100% withdrawal rate is suggested to prevent scale formation which affects the evaporation rate. The energy requirements for the proposed system are less than that are required for a multi-stage flash distillation process. The proposed system has the potential to be operated entirely on renewable or low grade waste energy sources and offers a sustainable approach for desalination. The performance of the system may be further improved by incorporating double or triple-effect configurations. Acknowledgement This study was funded in part by a grant from the New Mexico Water Resources Research Institute. Appendix A A c C F(C) F hL m q Q T U V V_
a g
surface area (m2) specific heat capacity (kJ/kg-°C) solute concentration (%) correlation factor for effect of solute concentration (%) fraction solar insolation latent heat (kJ/kg) mass of fluid (kg) volumetric rate (m3/s) heat transfer rate (kJ/h) temperature (°C) heat loss coefficient (kJ/h-m2-°C) volume of fluid (m3) volumetric flow rate of fluid (m3/h) absorptivity, experimental coefficient (107–106 kg/m2Pa-s-K0.5) desalination efficiency
q s
mass density (kg/m3) transmittivity of glass (–)
Subscripts a ambient conditions d distilled water conditions e evaporation stream from EC g generator conditions i input stream to EC l loss from EC or TES r generator-solar collector recirculation stream s solute w withdrawal stream from EC References [1] Wang SC. The exergetic efficiency of MSF process and the contributions of desalination by waste heat. Desal 1983;44:39–49. [2] Gustavo K, Fredi L. Low-temperature distillation processes in single- and dual purpose plants. Desal 2001;136:189–97. [3] Jacques G, Dominique L. Cogeneration applied to very high efficiency thermal seawater desalination plants. Desal 1999;125:203–8. [4] Al-Kharabsheh S, Goswami DY. Theoretical analysis of a water desalination system using low grade heat. J Solar Energy Eng 2004;126:774–80. [5] Bemporad GA. Basic hydrodynamic aspects of a solar energy based desalination process. Solar Energy 1995;54(2):125–34. [6] Luis HA, Jorge EG. Simulation of an air-cooled solar-assisted absorption air onditioning system. J Solar Energy Eng 2002;124:276–82. [7] Dincer I. Thermal energy storage systems as a key technology in energy conservation. Int J Energy Res 2002;26:567–88. [8] Gadhamshetty V, Nirmalakhandan N, Myint M, Ricketts C. Improving aircooled condenser performance. ASCE J Energy Eng 2006;132(2):81–8. [9] Kaligirou SA. Seawater desalination using renewable energy sources. Prog In Energy Comb Sci 2005;31:242–81. [10] Keren Y, Rubin H, Atkinson J, Priven M, Bemporad GA. Theoretical and experimental comparison of conventional and advanced solar pond performance. Solar Energy 1993;51:255–70.