JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 3, 043108 (2011)
Sustainable low temperature desalination: A case for renewable energy Veera Gnaneswar Gude,1,a) Nagamany Nirmalakhandan,2 and Shuguang Deng1 1
Chemical Engineering Department, New Mexico State University, Las Cruces, New Mexico 88001, USA 2 Civil Engineering Department, New Mexico State University, Las Cruces, New Mexico 88001, USA (Received 9 August 2010; accepted 10 June 2011; published online 27 July 2011)
In this paper, different configurations for running a low temperature desalination process at a production capacity of 100 liters=day are presented. Renewable energy sources such as solar and geothermal energy sources are evaluated as renewable, reliable, and suitable energy sources for driving the low temperature desalination process round the clock. A case study is presented to evaluate the feasibility of sustainable recovery of potable water from the effluent streams of wastewater treatment plant. Results obtained from theoretical and experimental studies demonstrate that the low temperature desalination unit has the potential for large scale applications using renewable energy sources to produce freshwater in a sustainable manner. The following renewable energy=waste heat recovery configurations may produce around 100 liters=day of desalinated water: (1) solar collector area of 18 m2 with a thermal energy storage (TES) volume of 3 m3; (2) photovoltaic thermal collector area of 30 m2 to provide 14–18 kW electricity and 120 liters=day freshwater with an optimum mass flow rate of the circulating fluid around 40–50 kg=h m2; (3) A geothermal source at 60 C with a flow rate of 320 kg=h; and (4) waste heat rejected from the condenser of an absorption refrigeration system rated at 3.25 kW (0.95 tons refrigeration), supported by 25 m2 solar collector area and 10 m3 TES volume. Additionally, the secondary effluent of local wastewater treatment plant was processed to recover potable quality water. Experimental results showed that >95% of all the water contaminants such as biological oxygen demand (BOD), total dissolved solids (TDS), total suspended solids (TSS), ammonia, chlorides, nitrates, and coliform bacteria can be removed C 2011 American Institute of to provide clean water for many beneficial uses. V Physics. [doi:10.1063/1.3608910]
I. INTRODUCTION
In many parts of the world, desalination has become an imperative and inevitable solution to overcome the shortage of potable water. Current desalination technologies are based on thermal evaporation or membrane separation principles. Thermal desalination technologies require large quantities of energy and fossil fuels have traditionally been used to provide the energy requirements for desalination of seawater or brackish waters. The idea of utilizing the fossil fuels to produce freshwater through desalination processes is not a sustainable approach any more due to the rapid decline in these resources and resultant high fuel costs and negative environmental impacts. In an effort to conserve the depleting natural fossil fuel resources, desalination industry has been adopting several energy-saving measures in recent years. Examples a)
Author to whom correspondence should be addressed. Present address: Civil Engineering Department, Oregon Institute of Technology, 3201 Campus Drive, Klamath Falls, Oregon 97601, USA. Electronic mail:
[email protected]. Tel.: 1(530) 751 6061. FAX: 1(575) 646 7706.
1941-7012/2011/3(4)/043108/25/$30.00
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C 2011 American Institute of Physics V
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include recovery and recycling of energy as in the case of staging, low temperature desalination, and utilization of waste heat or renewable energy. Various renewable energy resources are available that suit the energy needs of different desalination processes. Currently, the resources which are well explored and exploited for desalination applications include solar energy (harvested by solar collectors or photovoltaic modules), wind energy, geothermal energy, and wave energy.1 Sustainable use of these resources depends on the conversion technology employed and the end-user process configuration. Sustainability, in this context, can be interpreted as how the available energy resource is being utilized. Conserving, recycling and increasing the efficiency of the conversion technologies are some approaches which result in a sustainable use of an energy resource. For instance, daily solar energy available on the surface of the earth is roughly 15 000 times greater than the daily energy consumption of the world population,2 which means that only much less than 1% of the daily solar energy captured in energy devices could solve the energy problems of the world. However, the maximum energy conversion rate of the current photovoltaic modules in the market has not exceeded 15% to date. This indicates that the energy harvested through these resources, though freely available, still have a very high value and need to be utilized efficiently or utilized in a “sustainable manner”. One way to utilize the renewable energy sources more efficiently is by coupling with an energy-efficient desalination process. Thermal desalination processes operating at higher temperatures such as multi-stage flash distillation (MSF), multi-effect distillation (MED) technology require high quality heat sources at higher temperatures and result in higher fugitive losses and consumption of prime non-renewable energy sources. On the other hand, low temperature desalination processes have lower specific energy requirements and a higher thermodynamic efficiency. Apart from the above, other advantages include lower corrosion rates, low-cost materials of construction with a longer plant life, lower scaling, lower heat losses, and shorter start-up periods. The motive energy for driving the low temperature processes can be provided by low grade heat sources (renewable energy) or process waste heat rejections, so that better economies of the overall processes can be achieved.3–5 In this research, a new low temperature desalination process has been developed which can utilize low grade heat sources such as waste heat releases, solar, photovoltaic=thermal (PV=Thermal), and geothermal energy sources. Since the process operates at lower temperatures, energy losses and, hence, the energy requirements for desalination are reduced. As this process utilizes renewable energy and waste heat releases, it does not directly contribute to any greenhouse gas emissions and can be considered a sustainable process. Results obtained from theoretical modelling studies and experimental studies are presented in this paper to demonstrate the viability of the proposed desalination process. Different configurations in which the proposed process can be driven using different energy sources at a desalination production capacity of 100 liters=day and the energy requirements are discussed. This paper focuses on theoretical development of the low temperature desalination system using different renewable energy sources with a limited analysis of experimental results. A. Description of the desalination system
The premise of the proposed system can be explained by considering two barometric columns at ambient temperature, one filled with freshwater and the other with saline water as shown in Fig. 1. The barometric columns contain the head equivalent to local atmospheric pressure and when closed, a vacuum will be created in the headspace by the amount of the fluid volume displaced by gravity. Due to the natural vacuum generated by this process, the head space of these two columns will be occupied by the vapors of the respective fluids at their respective vapor pressures. If the two head spaces are connected to one another, water vapor will distill spontaneously from the freshwater column into the saline water column, because the vapor pressure of freshwater is slightly higher than that of saline water at ambient temperature. However, if the temperature of the saline water column is maintained slightly higher than that of the fresh water column to raise the vapor pressure of the feed water side above that of the
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FIG. 1. Physical principle of the low temperature desalination system.
fresh water side, water vapor from the saline water column will distill into the fresh water column. A temperature differential of about 10–15 C is adequate to overcome the vapor pressure differential to drive this desalination process. Such low temperature differentials can be achieved using low grade heat sources such as solar energy, process waste heat, thermal energy storage (TES) systems, etc. A schematic arrangement of the low temperature desalination system based on the above principles is shown in Fig. 2. Components of this unit include an evaporation chamber (EC), a natural draft condenser, two heat exchangers, and three barometric 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 (seawater tank), BT (brine tank), and DWT (desalinated water tank), respectively. The brine tank holds the concentrate removed from the evaporation chamber to maintain the salt concentration in the evaporation chamber. The
FIG. 2. Low temperature desalination system powered by renewable and waste heat sources.
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evaporation chamber can be designed to use direct solar energy (with glass top exposed to solar radiation) and waste heat sources (as shown in Fig. 2). The EC is installed atop the three columns at a height of about 10 m above ground level to create vacuum naturally in the headspaces of the feed, withdrawal, and desalinated water columns. This configuration drives the desalination process without any mechanical pumping.6 The saline water enters the evaporation unit through a tube-in-tube heat exchanger.1,2 The temperature of the head space of the saline water column is maintained slightly higher than that of the desalinated water column. Since the head spaces are at near-vacuum level pressures, a temperature differential of 10 C is adequate to evaporate water from the saline water side and condense in the distilled water side.3–5 In this manner, saline water can be desalinated at about 40–50 C, which is in contrast to the 60–100 C range in traditional solar stills (SSs) and other distillation processes. This configuration enables the brine to be withdrawn continuously from the EC through heat exchanger 1 (HE1), preheating the saline water feed entering the EC.6,7 Further, by maintaining constant levels of inflow and outflow rates in SWT, BT, and DWT, the system can function without any energy input for fluid transfer in the desalination system. The heat input to EC is provided by TES tank through a heat exchanger 2 (HE2) which in turn is fed by a low grade waste heat or renewable energy source. Different heat sources evaluated in this study are solar collectors, photovoltaic thermal collectors, geothermal energy sources, and process waste heat. Experimental results obtained for a configuration using glass top evaporation chamber to utilize direct solar energy were compared with theoretical results obtained in Sec. III. A closed top evaporation chamber was also tested for recovering potable quality water from the secondary effluent of the local wastewater treatment plant. B. Theoretical analysis of the desalination system
Mass and energy balances around the EC yield the following coupled differential equations, where the subscripts refer to the state points shown in Fig. 2. The variables are defined in the Appendix. Mass balance on volume of water in EC, d ðqVÞ ¼ m2 m6 m3 : dt
(1)
d ðqVCÞEC ¼ m2 C2 m6 C6 : dt
(2)
Mass balance on solute in EC,
Energy balance for volume of water in EC, d ðqVcp TÞEC ¼ QEC þ ðmcp TÞ2 ðmcp TÞ6 m3 hLðTÞ Ql ; dt
(3)
where QEC is the rate of energy input (load on the TES) to the EC and Ql is the rate of energy loss from the EC. The energy input, QEC, to the evaporation chamber can be supplied by the solar collectors, photovoltaic thermal collectors, geothermal water sources, and process waste heat sources to the TES as discussed in Sec. I A and is written as QEC ¼ ms cs ðTs TEC Þ;
(4)
where, ms, cs, and Ts, are the mass flow rate, specific heat, and temperature, respectively, of the water from the TES and TEC is the temperature of the saline water in the evaporation chamber. Theoretical expressions for the different heat sources are discussed next. Desalination efficiency is defined as
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g¼
J. Renewable Sustainable Energy 3, 043108 (2011)
MhLðTÞ ; RðQEC DtÞ
(5)
where hLðTÞ ¼ 3; 146 2:36ðT þ 273Þ:
(6)
Evaporation rate as a function of pressure and temperature,8 " m3 ¼ AEC k fðCEC Þ
pðTEC Þ ðTEC þ 273Þ0:5
pðT5 Þ
#
ðT5 þ 273Þ0:5
;
(7)
where pðTÞ ¼ ½expð63:02 7139:6=ðT þ 273Þ 6:2558 lnðT þ 273Þ 102 Pa:
(8)
The above coupled equations are solved using Extend (Imagine That Inc.) and Engineering Equation Solver (EES) simulation software. Details of heat transfer relations for evaporation chamber and condensation surface and heat losses by convection and radiation are presented elsewhere.7,8 Model parameters for the low temperature desalination process are presented in Table I. II. THEORETICAL STUDIES
In this section, theoretical analyses for different energy sources and the results from the modeling studies are presented. The expressions for different energy sources can be substituted in the overall energy balance (Eq. (3)) to generate the simulations. Schematics for different configurations are shown in Fig. 3. A. Solar collectors
Flat plate solar collectors supplying low grade heat in the range of 50–70 C can be used to drive the proposed desalination system during sunlight hours (Fig. 3(a)). The sensible heat stored in the TES will provide the heat source to the evaporation chamber during non sunlight hours. Energy balance across the solar panel can be written as dðmcTÞsc ¼ FR AC ½ðsaÞIs UL ðTSC Ta Þ QS ; dt
(9)
where Qs is the solar energy harvested by the solar collectors and stored in the TES tank and is given as Qs ¼ mR cR ðTSC Ts Þ;
(10)
TABLE I. Model parameters for the low temperature desalination system. Parameter
Value 2
Parameter
Value 2
EC area
m
1–5
Solar insolation
W=m
200-1000
Condenser area Water depth in the EC
m2 m
1–5 0.05-0.1
Seawater concentration Seawater density
% kg=m3
3.5 1020
Height of EC
m
0.5
Seawater, TES reference Temperature
C
25
1
Ambient temperature
C
3 to 35
TES volume
3
m
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Gude, Nirmalakhandan, and Deng
J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 3. Energy balance on different heat sources: (a) solar collectors, (b) photovoltaic thermal collectors, (c) geothermal source, and (d) process waste heat.
where mR and cR are the mass flow rate of the collector fluid, Tsc is the temperature of the water exiting the solar collector, and Ts is the temperature of TES tank. The energy balance on the TES can be written as8 d ðMcTÞs ¼ Qs QEC Qls ; dt
(11)
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where Ms (q) is the total mass of water in the TES, cs, and Ts are as defined earlier, and Qs is the thermal energy supplied by the solar collectors. Actual energy supplied from the TES tank to the evaporation chamber, QEC can be calculated using Eq. (4). Qls are the energy losses from the TES. 1. Performance of the low temperature desalination system
Temperature profiles of the desalination system driven by the solar collectors are shown in Fig. 4(a) for both evaporation and solar collector areas of 1 m2. The TES temperatures reach a maximum value of 57 C during sunlight hours and evaporation temperatures reach a maximum value of 45 C. The maximum ambient temperature is 34 C. The energy supplied from the TES and the energy utilized for evaporation are shown in Fig. 4(b). Hourly freshwater production rates and cumulative product are shown in Fig. 4(c). A daily product of 6.5-7 liters can be produced for 1 m2 evaporator and 1 m2 solar collector areas. These results are compared to those reported previously.6 The evaporation efficiency of the process ranged between 60% and 90%, most of the time in the range 75%-90% as shown in Fig. 4(d). The TES volume used in this simulation was 0.1 m3. Hourly and daily freshwater production rates are shown for a solar collector area of 18 m2 and a TES volume of 3 m3 in Fig. 5. As it can be seen from Fig. 5, the freshwater production rates for this case start to stabilize after 72 h of operation. The freshwater production rate was lower during initial hours as some of the energy supplied from the solar collectors is utilized to increase the sensible heat of the total volume of the water in TES. The stabilized freshwater production rate for this configuration is 104 liters=day. 2. Use of Thermal Energy Storage System
In this configuration, the need for thermal energy storage tank was evaluated through simulations. Fig. 6 shows the TES performance for two different volumes (1 m3 and 6 m3) over 168 h (7 days). The TES temperatures fluctuate in relation with the daily solar insolation and ambient temperatures in both cases. TES temperatures for a TES volume of 1 m3 respond quickly to the changes in the solar insolation and ambient temperatures with very little sensible heat available for desalination during nonsunlight hours. Temperatures during sunlight hours reach as high as 65–68 C and fall down to as low as 30–35 C during nonsunlight hours. TES
FIG. 4. Low temperature desalination system driven by solar energy; (a) temperature profiles, (b) energy utilization, (c) daily production rates, and (d) desalination efficiency.
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Gude, Nirmalakhandan, and Deng
J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 5. Low temperature desalination system driven by solar collectors round the clock (TES volume: 3 m3, solar collector area: 18 m2).
temperatures for a TES volume of 6 m3 respond slowly to the daily solar insolation, and ambient temperatures increasing sensible heat of the bulk of the water in the tank. For this tank volume, the TES water temperature during nonsunlight hours was around 45–48 C, which is still a good heat source for evaporation during nonsunlight hours enabling 24 h operation. The maximum and minimum TES temperatures are shown in Fig. 7 for different TES volumes in the range 1–6 m3. From Fig. 7, it can be observed that as the TES volume increases the heat source available for nonsunlight hour operation increases. Relation between the solar collector areas in
FIG. 6. TES performance over 7 days (168 h).
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FIG. 7. Effect of TES volume on TES and EC temperature profiles.
connection with the TES volumes is presented in Fig. 8. For higher TES volumes, it is obvious that the solar collector area requirement is high for the freshwater production rate of around 100 liters=day. It is because sensible heat losses to the ambient from the TES tank are to be provided by the same collector area. Therefore, the collector area required for 1 m3 TES volume is 15 m2, which is increased by 25% and 40% for TES volumes of 3 m3 and 6 m3, respectively. Hourly and daily freshwater production rates for a fixed solar collector area of 15 m2 over 21 days of operation are shown in Fig. 9. At the end of 7 days of operation, the average daily freshwater production for TES volume of 1 m3 remained at 100 litres=day and for TES volume of 6 m3 at 68 liters=day. It should be noted that the average freshwater production rates continue to increase for the TES volume of 6 m3 and reach 86 liters=day at the end of 21 days of operation. From these simulations, the maximum TES temperatures for TES volume of 1 m3 remained at 68 C, whereas for TES volume of 6 m3, the temperatures increased from 53.8 C to 54.5 C at the end of 21 days of operation. This indicates that the energy stored in the TES is available for longer periods of time enabling continuous and stable freshwater production
FIG. 8. TES volume effect on solar collector area and freshwater production rate.
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Gude, Nirmalakhandan, and Deng
J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 9. TES volume effect on daily production rates and cumulative product over 21 days.
rates. Therefore, to determine the optimum solar collector areas, long term performance of the TES needs to be considered. The configuration with small TES volumes will suffer from the changes in daily solar insolation and ambient temperatures. High TES volume may initially result in lower freshwater production rates but with continued operation the productivity will be increased. For small TES volumes, the freshwater production rates cease during nonsunlight hours leaving the unit idle for 33%–50% of the day. If the high storage volume of TES is a constraint, freshwater has to be stored in water tank for rest of the day, cloudy, and rainy day needs as well. Continuous operation allows for downsizing the desalination unit and reduces the equipment cost. Batch operation requires a large evaporation area and equipment with higher costs. Continuous process mode is easily adaptable to other low grade waste heat sources and can be scaled to large applications to provide freshwater for small rural communities.
3. Cloudy Day Effect on Thermal Energy Storage System
The effect of cloudy days on different TES volumes was investigated. One cloudy day per week was considered over 3 weeks (21 days). As can be seen from Fig. 10, the average daily production over 21 days of operation decreases from 100 liters=day to 92 liters=day (by 8 liters=day) for TES volume of 1 m3 with a solar collector area of 15 m2. For the same collector area, the average daily production over 21 days decreased from 93 to 87 (by 6 liters=day) and 86 to 81 liters=day (by 5 liters=day), respectively, for TES of 3 m3 and 6 m3 volumes. Considering the performance on the cloudy days alone, the reduction in the daily product amount was 100 to 63, 93 to 67, and 86 to 75 liters=day, which are 37%, 28%, and 13% reductions as shown in Fig. 10. The hourly freshwater production rates decreased from 7.5 to 4.6, 5.1 to 3.5, and 4.5 to 3.5 liters=h for TES volumes of 1, 3, and 6 m3, respectively, the smallest variation being observed for 6 m3. Similar comparison for different TES volumes with a solar collector area of 18 m2 was done over 21 days of operation. Based on this analysis, the daily production increased for TES volume of 6 m3 from 75 liters=day (15 m2 solar collector area) to 88 liters=day (18 m2 solar collector area). The hourly production rates also stabilized with this configuration. As shown in Fig. 11, the solar collector area requirements based on 21 days operation are 6% and 13% only compared to 7 days operational performance analysis, 25% and 40% for TES volumes of 3 and 6 m3, respectively. Therefore, based on the above analysis, it is clear that the TES performance has to be evaluated on a long term performance basis and the
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J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 10. TES volume effect on daily production rates and cumulative product over 21 days with cloudy day effects.
advantage of the TES is recognized when long term operations are considered. The process conditions are more stable with variations in energy supply and demand trends. The need for the TES, however, depends on the type, scale, and economics of a particular application. B. PV=Thermal collectors
Photovoltaic thermal collectors have the ability to produce both electrical and thermal energy from solar energy. The overall efficiency of the PV=Thermal collectors is higher than the sum of the efficiencies of separate photovoltaic modules and solar thermal collectors.9 Thermal energy produced from PV=Thermal collectors is suitable for low temperature desalination by the proposed process while electricity produced can be used for domestic uses (Fig. 3(b)). Theoretical analysis for the desalination system utilizing photovoltaic thermal energy is as follows. The energy balance on the PV=Thermal collector and the absorber plate can be written as follows10:
FIG. 11. TES volume effect on solar collector area and freshwater production rate based on 21 days performance.
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Mp Cp
J. Renewable Sustainable Energy 3, 043108 (2011)
dTPVT ¼ Qsp Qlp Pe Qu ; dt
(12)
where Mp and Cp are the mass and specific heat capacity of the absorber plate, respectively, Qsp is the incident solar energy, Qlp are the losses from the PV=Thermal collector, Pe is the electrical energy derived from the module, and Qu is the useful energy (thermal) extracted by the collector fluid. Solar energy absorbed by the PV=Thermal panel material is given as Qsp ¼ Is sg ap :
(13)
Heat losses through radiation and convection from the PV=Thermal absorber plate to the glass cover can be written as10 Qlp ¼ ea eg rfTp4 Tg4 g þ hpg ðTp Tg Þ:
(14)
Electrical energy generated by PV=Thermal collector system is given as11 Pe ¼ Is sg Fc gstd f1 0:005ðTp 298:15Þg:
(15)
Useful thermal energy derived from PV=Thermal collectors can be expressed as Qu ¼ mCpf ðTf Ti Þ;
(16)
where Tf is the collector fluid exit temperature and Ti is the collector fluid inlet temperature. m and Cpf are the mass flow rate and specific heat capacity, respectively, of the collector fluid which is water in this case. The circulating fluid exit temperature, Tf, can be calculated as follows:10 ðx=dÞNu þ Ti ; Tf ¼ ðTPVT Ti Þ 1 exp 4 Re: Pr
(17)
where TPVT is the absorber plate temperature, x and d are the length and diameter of the circulating fluid tube, respectively. The heat input to the EC is the useful heat extracted from the PV=Thermal collectors and stored in thermal energy storage tank and is given by dðvqcTÞs ¼ Qu QEC Qlo ; dt
(18)
where v is the volume of the storage tank, Cps is the specific heat of the water in the storage tank, and Ts is the storage tank temperature. QEC is the heat supplied to the EC and Qlo is the energy losses from the storage tank. Actual amount of heat supplied to the EC from TES can be obtained by using Eq. (4). Thermal and electrical efficiencies of the PV=Thermal collector at a given time are as follows:12 mCpf ðTf Ti Þ Thermal efficiency of the PV=Thermal collector ¼gPVT;th ¼ ; (19) IAc Total PV=Thermal collector efficiency ¼gPVT ¼
mCpf ðTf Ti Þ þ Pe ; IAc
(20)
where Ac is the collector area.
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1. Integrated PV=Thermal-desalination system
Numerical simulations have been performed for a site in southern New Mexico. Parameters used in the numerical simulations are presented in Table II for the PV=Thermal collector system. The solar insolation and ambient temperatures on a summer day in June varied between 225–1000 W=m2 and 16–33 C, respectively. The temperature profiles for the photovoltaic thermal system are shown in Fig. 12. The absorber plate temperatures reached up to 68.7 C, while the collector fluid temperature reached a maximum value of 64.5 C in the middle of the day. The glass cover was in contact with the ambient air and reached a maximum temperature of 43.1 C, while the maximum ambient temperature was 33 C. In this simulation, the mass flow rate of the circulating fluid was 40 kg=h m2. The volume of the TES was 1 m3 with a volume to collector area ratio of around 40 litres=m2. Thermal and electrical energy production rates and their efficiencies during sunlight hours are shown in Fig. 13. Thermal energy is the useful energy extracted by the circulating fluid in the PV=Thermal collector. Total PV=Thermal system efficiency of 59.4% and thermal efficiency of nearly 50.5% are predicted. These results are comparable with the previous studies.13,14 The primary energy saving efficiency of PV=Thermal system is predicted as 72.7%, which considers the total energy that will otherwise be required to generate the electricity by a conventional power plant. A photovoltaic collector (Sharp NT-S5E1U, cell efficiency 17.5%, and module efficiency 14.2%) was used in these simulations. It is well known that the photovoltaic cell efficiency would decrease with increasing absorber plate temperature. The electrical efficiency of the PV=Thermal system varied between 12.5% and 8.5% during the sunlight hours. 2. Low Temperature Desalination System Driven by PV=Thermal System
Useful energy extracted from the PV=Thermal system was used as the heat source to drive the desalination system. The mass flow rate of the circulating fluid between the TES and the evaporation chamber was fixed at 60 kg=h m2 in these simulations. The resulting evaporation temperatures in the desalination system and freshwater and ambient temperatures are shown in Fig. 14. The maximum saline water temperature in the evaporation chamber is predicted as 51.2 C at the maximum TES and ambient temperatures 57.2 C and 33 C, respectively. The saline water temperature decreased with the collector fluid temperature and eventually reached ambient temperatures during non-sunlight hours. The useful energy supplied to the evaporation chamber is utilized for evaporation, with energy losses ranging from 10% to 20% of the total useful energy supplied, resulting in 80%–90% evaporation efficiencies. The hourly freshwater production rate is shown in Fig. 15. As expected, the evaporation rate increased with increase in the heat source temperature and a maximum evaporation rate of 15 liters=h is obtained. The cumulative amount of freshwater produced from the desalination system under the specific conditions was 120 liters. The optimum PV=Thermal collector area for this application was found to be 30 m2 which can also provide 14–18 kW h of electricity needs for a household.15 The mechanical energy required to circulate the collector fluid is calculated as 4 kJ=kg of freshwater produced.
TABLE II. Model parameters for photovoltaic thermal collector system. Parameter Cell material PV=T module area (m2) Total PV=T module area (m2) Cell efficiency (%)a) Coefficient of temperature inversion (%= C) a)
Value
Parameter
Value
Mono-Si variable
Material of absorber Absorption factor, PV cell
aluminum 0.9
20–30
Radiation factors, cover glass-eg, and absorber ea
0.05, 0.5
17.5 0.5
Absorption factor, cover glass-eg, and absorber ea Collector fluid (kg=h=m2)
0.05, 0.16 Water, 1–80
At Ta ¼ 25 C, Is ¼ 1000 W=m2.
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Gude, Nirmalakhandan, and Deng
J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 12. Temperature profiles of the PV=Thermal collector system.
FIG. 13. Energy and efficiency profiles of the PV=Thermal collector system.
FIG. 14. Temperature profiles of the integrated PV=Thermal-low temperature desalination system.
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J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 15. Evaporation rate and cumulative product of the low temperature desalination system.
C. Geothermal energy
Geothermal energy sources deliver an energy quantity of 160–200 kW h=m2 annually which is much higher than photovoltaic (50–100 kW h=m2), biomass (15–45 kW h=m2), and wind energy resources (11–18 kW h=m2).16 Low grade geothermal source with temperature of about 60 C can be used directly to heat the saline water or to maintain a thermal energy storage tank which can then provide the energy needs to the evaporation chamber (Fig. 3(c)). The amount of thermal energy supplied by the geothermal source, QG, can be quantified as QG ¼ mG cpg ðTgi Tgo Þ;
(21)
where mG and cpg are the mass flow rate of the geothermal water and Tgi and Tgo are the inlet and outlet temperatures of the geothermal water, respectively. Saline geothermal energy sources can be used both as feed (saline water) and heat source. 1. Geothermal Energy Requirements
Geothermal source flow rate for a known freshwater production rate can be estimated using the following energy balance: QG ¼ mG cpg ðTgi Tgo Þ ¼ me fcpe ðTw Ti Þ þ hLðTw Þ g;
(22)
mG ¼ R; ðme Þg
(23)
where, me is the desired evaporation rate (freshwater production rate, litres=day), cpe is the specific heat of the water, Tw is the evaporation temperature of the brackish water, Ti is the inlet temperature of the brackish water, hL is the latent heat of the brackish water at evaporation temperature, and g is thermal efficiency of the desalination system. R is the ratio of geothermal source to the mass of freshwater to be evaporated. Based on Eq. (23), for a fixed evaporation rate of 100 liters=day, the energy requirements and geothermal water flow rates were calculated (Fig. 16). As expected, the energy and flow requirements increase with the geothermal water temperatures. This also indicates that when the heat resource is limited, low temperature operation can provide the benefits of higher thermodynamic efficiency and higher freshwater production rates.4
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043108-16
Gude, Nirmalakhandan, and Deng
J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 16. Energy and flow rate requirements for the low temperature desalination by geothermal energy
2. Performance of the Low Temperature Desalination System
Temperature profiles for the low temperature desalination system driven by a geothermal source at 60 C are shown in Fig. 17. The saline water temperatures in the evaporation chamber are slightly higher during the sunlight hours due to higher ambient temperatures, and the temperature gradient available between the evaporator and condenser is lower during the daytime resulting in lower evaporation rates. Temperature gradients between the evaporator and condenser are higher during the nonsunlight hours which favor the convection and condensation of water vapors from the evaporator side to the condenser. From the simulations, it was observed that the freshwater production rate was around 3.9 liters=h during sunlight hours whereas it was around 4.8 liters=h during nonsunlight hours. Fig. 17 also shows the temperature profiles for fluids flowing in and out of the HE1. Saline water enters (cold in) the tube-in-tube heat exchanger at 25 C and exits at 36 C before entering the EC. The brine in (saline water from EC) temperature is same as the saline water temperature in the EC and is about 51 C and exits HE1 at 30 C. Thus, the heat exchanger operates in the range 75%–85% of thermal efficiency preheating the saline water entering the EC. A geothermal water flow rate of 320 kg=h has been considered for numerical simulations in Fig. 18. Theoretical simulations show that this system can produce up to 100 liters=day of desalinated water with an average evaporation rate of 4.5 liters=h at a geothermal water temperature of 60 C. The evaporation rates for geothermal temperatures at 50 C, 70 C, and 80 C are 3 liters=h, 6.5 liters=h, and 8.6 liters=h, respectively. Geothermal sources have potential for large scale application of the low temperature desalination system. High temperature geothermal waters (80–100 C) are suitable for multi-effect low temperature desalination process to provide freshwater for small rural communities. They provide continuous source of water and qualify as a new source of water as the feed itself can be the geothermal water or depending on the availability of brackish water=seawater sources. D. Waste heat sources
A general scheme for low temperature desalination system utilizing the process waste heat is shown in Fig. 3(d). Examples of process waste heat include reject heat from the condenser of domestic air-conditioning system, exhaust gases from diesel generators, and circulating cooling water jacket type reactors. The feasibility of the proposed system using a TES system storing the waste heat rejected by a Li-Br absorption refrigeration system (ARS) has been simulated.8,17 A schematic of this configuration is shown in Fig. 19. In this configuration, the EC area was 5 m2 and the TES was sized to maintain its temperature at 50 C. An ARS system rated at 3.25 kW (0.975 tons of refrigeration) along with an additional energy input of 208 kJ=kg of desalinated water was adequate to produce desalinated water at an average rate of 4.5 kg=h. On a
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043108-17
Desalination using renewable energy
J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 17. Low temperature desalination system driven by geothermal energy; (a) EC temperature profiles and (b) HE1 temperature profiles.
typical day, the heat demand by the EC (including heat loss) on the TES varied from 8700 to 14 200 kJ=h over a 24-h period; a TES tank volume of 10 m3 was found to be adequate to maintain its temperature at 50 C to provide the energy needs of the EC. The operating characteristics of the ARS system were as follows: absorber temperature ¼ 28 C, condenser temperature ¼ 55 C, evaporator temperature ¼ 12 C, generator temperature ¼ 100 C,
FIG. 18. Low temperature desalination system driven by geothermal energy; (a) daily and (b) hourly production rates.
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043108-18
Gude, Nirmalakhandan, and Deng
J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 19. Low temperature desalination system driven by waste heat rejected by ARS system.
condenser=generator pressure ¼ 15.75 kPa, absorber=evaporator pressure ¼ 1.403 kPa, and coefficient of performance ¼ 0.72. In this configuration, the ARS was powered by the solar collectors and augmented by an auxiliary electric heater. The solar collector system was sized to maintain the generator of the ARS at 100 C by maintaining the storage tank of the solar panel at 110 C. The energy provided by the auxiliary heater is equal to the difference between the energy required by the generator and that can be collected from the solar insolation. The energy contributed by solar collectors from the solar energy is called the solar fraction. For this design, the solar fraction was 0.4 (40%). The optimum area of the solar collectors for this application was found to be 25 m2. Thermal energy supplied or transferred between the condenser of ARS system and the TES tank and the heat transfer between TES tank and desalination system depend on the available temperature gradients and ambient temperature conditions. The TES volume was determined to be 10 m3 to maintain temperature of the TES water medium within 60.01 C during winter conditions. This unit can be designed to drive the absorption refrigeration unit round the clock with the solar energy harvested by solar collectors (by increasing the solar fraction). Results from the modeling studies discussed above show that the proposed process has the potential to be driven solely by renewable energy sources or waste heat releases and can be operated on a continuous basis with moderate yields. More details of the theoretical modeling results are presented elsewhere.7 E. Specific energy requirements
Specific energy required to produce 1 kg of freshwater for the four energy sources are shown in Table III. Specific energy requirements include the heat energy used for evaporation and mechanical energy for pumping the heat source. This table suggests that the above configurations have the potential to produce freshwater either on batch or continuous modes of operation. The specific energy requirement is also dependent on the mode of operation. The ARS configuration is the most suitable for domestic applications, since free energy is available from the ARS condenser and the specific energy requirements are much lower than the remaining configurations.7
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043108-19
Desalination using renewable energy
J. Renewable Sustainable Energy 3, 043108 (2011)
TABLE III. Specific energy requirements for the low temperature desalination process using different sources of energy.
Energy source
Mode of operation
Thermal energy required (kJ=kg)
Mechanical energy required (kJ=kg)
Total Energy required (kJ=kg)
Solar collectors
Batch=continuous
3118
4.1
3122.1
PV=Thermal collectors Geothermal source
Batch=continuous Continuous
3118 2934
4 144
3122 3078
Continuous
194
14
208
ARS configuration
III. EXPERIMENTAL A. Direct solar energy
In this section, the theoretical modeling results are compared with experimental results for the system using direct solar energy. The top of the evaporation chamber was exposed to the incident solar energy to cause evaporation in the evaporation chamber, thus, the system can be called solar still under vacuum (SSV) as shown in Fig. 20. Experimental results obtained for other two configurations (solar still under vacuum with a reflector (SSR) and external photovoltaic energy (SSPV)) are also presented. All of the results are compared with the traditional solar still configuration to illustrate the benefits of low temperature desalination unit. The details of theoretical modeling and experimental results are presented elsewhere.7
1. Temperature and Freshwater Production Profiles
The experimental studies were conducted in summer at engineering research facility in Las Cruces, USA. The solar insolation varied between 400–1100 W=m2 while the ambient temperatures ranged 15–35 C during summer. The maximum ambient temperature recorded was 35 C, and the maximum temperature of the brackish water in the EC was 52.75 C. The predicted
FIG. 20. Photo of experimental desalination system driven by direct solar=photovoltaic energy.
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043108-20
Gude, Nirmalakhandan, and Deng
J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 21. Saline water temperature and distillate production profiles for different configurations.
maximum temperature was 52 C as shown in Fig. 21(a). The correlation between the predicted and measured EC temperature was satisfactory with r2 ¼ 0.943, F ¼ 2358.2, and p < 0.001. As a comparison, the maximum saline water temperatures measured for different configurations were as follows: low temperature desalination process using direct solar energy (solar still configuration, SSV)—50 C, low temperature desalination process using direct solar energy fitted with an external reflector (solar still configuration, SSVR)—53 C, and low temperature desalination process using solar energy as well as photovoltaic energy (SSPV)—55 C, respectively, as shown in Fig. 21(b). These temperatures are lower than those commonly reported for solar stills which are in the range of 60–75 C.18,19 The predicted distillate volume during the above test is compared against the measured distillate volume in Fig. 21(c). Cumulative volume predicted by the model for a 24-h period was 5.25 liters=day m2 while the measured value was 4.95 liters=day m2. The difference (of 5.5%) in the cumulative distillate volume is mainly due to the assumption that the entire volume of the vapor distilled on the freshwater side, whereas during the test it was observed that some of the vapor condensed on the roof of the evaporator and trickled back to the saline water. Correlation between the predicted and measured distillate volume as a function of time was strong with r2 ¼ 0.988, F ¼ 11,839.4, and p < 0.001. Daily freshwater production rates for different configurations are shown in Fig. 21(d). The low temperature desalination process as a SSV produces freshwater of about 5 liters=d m2, nearly 1.5 to 2 times that of typical solar stills.18,19 This improvement can be attributed to the reduction in energy losses by the low temperature desalination process. The near-vacuum pressures created by natural means of gravity and barometric head allow for the evaporation of freshwater to occur at low temperatures resulting in higher energy efficiency. This configuration, when fitted with a reflector (SSVR), produced about 7.5–8 liters=day of distillate which is three times that of typical solar still. As the solar insolation incident on the solar still was intensified by the reflector, the saline water temperatures rose quickly resulting in evaporation of freshwater as shown in Fig. 21(d). Low temperature desalination process powered by photovoltaic energy (SSPV) produced over 12 l=day when fitted with a reflector. Photovoltaic area required for this configuration was 6 m2. Photovoltaic energy generated during the day is sufficient to produce freshwater of 4–5 liters=day during the night time. The efficiency of the PV modules is 14.2%. The process can
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043108-21
Desalination using renewable energy
J. Renewable Sustainable Energy 3, 043108 (2011)
be designed to operate round the clock with addition of an external heat source (during nonsunlight hours) as in the case of solar collectors. Although, this configuration (SSPV) may not prove economical, it could be beneficial in arid areas where the need for freshwater and energy are highly pronounced. 2. Thermal Efficiency and Specific Energy Consumption
Traditional solar stills have a thermal efficiency around 30% and rarely exceed 45%.18,19 Normal solar still (SS) operating with an efficiency of 45%, requires 5040 kJ of thermal energy per kg of freshwater produced. The proposed process (SSV) operates at higher thermal efficiencies with a specific energy consumption of 3900 kJ=kg of freshwater. While the measured thermal efficiency of SSV configuration was 61%, that of the configuration with a reflector (SSVR) was 75% with a specific energy consumption of 3200 kJ=kg. The specific energy required for the configuration with photovoltaic energy (SSPV) is only 2800–2900 kJ=kg of freshwater with thermal efficiencies ranging between 80% and 90%. In the case of traditional solar stills and SSV, major energy losses occur through the glass cover during sunlight hours. However, for SSPV, the glass cover can be covered with insulation during non-sunlight hours to reduce the energy losses through the glass cover. Additionally, lower ambient temperatures during nonsunlight hours favor the convection and condensation of freshwater vapors from the evaporation chamber to the condenser side.4 Specific energy consumption for different operational modes are summarized in Table IV.5 B. Recovery of potable water from secondary effluent using low grade waste heat 1. Using Low Grade Heat Source
Feasibility of running the low temperature desalination process using a low grade thermal source was demonstrated. A hot water tank was used as heat source in this configuration.20 The design of the unit was slightly modified in this configuration to integrate the evaporation and condensation sides of the desalination unit. The condenser plate was arranged at the top of the evaporation chamber to dissipate the heat of condensation to the ambient, thus reducing the footprint of the unit. During these tests, the circulation rate of the hot water was maintained at 9 kg=h, while the temperature of the source was varied between 50 C70 C. Typical temperature profiles recorded over 24 h operation with continuous operation mode are presented in Fig. 22. Results from these tests showed that the low temperature desalination system operated with higher efficiency at lower evaporation temperatures since the losses to the ambient are reduced. At higher evaporation temperatures, the heat dissipation rate depends on the condenser surface area available and, thus mass of water evaporated. Hourly freshwater production rates varied between 77–91 ml=h. Thermal energy supplied through the hot water source for temperature range 50–70 C varied between 355 and 395 W, while thermal efficiency declined from 53% to 47%. The effect of cooling the condenser surface was tested with a small flow rate of cooling water of 500 ml=h which was collected at the bottom of the condenser. An increase in
TABLE IV. Specific energy requirements for the low temperature desalination process by different configurations using solar energy.
Description
Mode of operation
Specific energy requirement (kJ=kg)
Case 1
Direct solar energy
Batch
3900
Case 2 Case 3
Direct solar energy with a reflector Solar energy during sunlight hours, photovoltaic energy during non-sunlight hours Solar and photovoltaic energy together
Batch Continuous
3118 2926
Batch
3325
Experiment
Case 4
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043108-22
Gude, Nirmalakhandan, and Deng
J. Renewable Sustainable Energy 3, 043108 (2011)
FIG. 22. Temperature profiles of the low temperature desalination system driven by low grade heat source.
thermal efficiency of 10–15% was observed with cooling. Thermal efficiency increased from a maximum value of 53%–67% for heat source temperature at 50 C. The energy efficiency can be improved further with adequate insulation between the evaporator basin and condenser top, addition of fins on the condenser plate for more efficient heat dissipation and external cooling by recycling the product water. This configuration can be scaled to utilize low grade heat sources for large scale applications. As an illustration, a case study for the City of Las Cruces wastewater treatment plant is presented in the next sub-section. The source water contained biochemical oxygen demand (BOD), total dissolved solids (TDS), total suspended solids (TSS), nitrates, nitrites, chlorides, and coliform bacteria. However, the process was able to achieve more than 90% reductions for each of the above contaminants as shown in Table V. The process produces high quality distillate with TDS < 50 ppm which is suitable for many non-potable uses. 2. Case Study
The City of Las Cruces treats an average of 10 million gallons per day (MGD) of wastewater. The wastewater treatment plant has anaerobic sludge digester in place to process the TABLE V. Characteristics of secondary effluent and product water. Low temperature desalination using low grade heat
Parameter
Secondary effluent
Recovered water
% Reduction
USEPA drinking water standard
BOD (mg=l)
12.7
—
—
—
TDS (mg=l)
783
16
98
500
TSS (mg=l) Nitrate (as N mg=l)
8 2.6
0.3
