Sustainable low temperature desalination: A case for renewable energy

Download Sustainable low temperature desalination: A case for renewable energy

Post on 23-Dec-2016




4 download

Embed Size (px)


<ul><li><p>Sustainable low temperature desalination: A case for renewable energyVeera Gnaneswar Gude, Nagamany Nirmalakhandan, and Shuguang Deng </p><p>Citation: Journal of Renewable and Sustainable Energy 3, 043108 (2011); doi: 10.1063/1.3608910 View online: View Table of Contents: Published by the AIP Publishing </p><p>Articles you may be interested in A sustainable renewable energy mix option for the secluded society J. Renewable Sustainable Energy 6, 023124 (2014); 10.1063/1.4873129 </p><p>Exceptional ion rejection ability of directional solvent for non-membrane desalination Appl. Phys. Lett. 104, 024102 (2014); 10.1063/1.4861835 </p><p>Potential role of renewable energy in water desalination in Australia J. Renewable Sustainable Energy 4, 013108 (2012); 10.1063/1.3682060 </p><p>Industrial effluent treatment: Theoretical and experimental analysis J. Renewable Sustainable Energy 3, 013107 (2011); 10.1063/1.3558862 </p><p>SolarPowered Desalination: A Modelling and Experimental Study AIP Conf. Proc. 941, 249 (2007); 10.1063/1.2806091 </p><p> This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: Downloaded to IP: On: Tue, 29 Apr 2014 10:38:21</p></li><li><p>Sustainable low temperature desalination: A case forrenewable energy</p><p>Veera Gnaneswar Gude,1,a) Nagamany Nirmalakhandan,2 andShuguang Deng11Chemical Engineering Department, New Mexico State University,Las Cruces, New Mexico 88001, USA2Civil Engineering Department, New Mexico State University, Las Cruces,New Mexico 88001, USA</p><p>(Received 9 August 2010; accepted 10 June 2011; published online 27 July 2011)</p><p>In this paper, different configurations for running a low temperature desalination</p><p>process at a production capacity of 100 liters=day are presented. Renewable energysources such as solar and geothermal energy sources are evaluated as renewable,</p><p>reliable, and suitable energy sources for driving the low temperature desalination</p><p>process round the clock. A case study is presented to evaluate the feasibility of</p><p>sustainable recovery of potable water from the effluent streams of wastewater</p><p>treatment plant. Results obtained from theoretical and experimental studies</p><p>demonstrate that the low temperature desalination unit has the potential for large</p><p>scale applications using renewable energy sources to produce freshwater in a</p><p>sustainable manner. The following renewable energy=waste heat recoveryconfigurations may produce around 100 liters=day of desalinated water: (1) solarcollector area of 18 m2 with a thermal energy storage (TES) volume of 3 m3; (2)</p><p>photovoltaic thermal collector area of 30 m2 to provide 1418 kW electricity and</p><p>120 liters=day freshwater with an optimum mass flow rate of the circulating fluidaround 4050 kg=h m2; (3) A geothermal source at 60 C with a flow rate of 320kg=h; and (4) waste heat rejected from the condenser of an absorption refrigerationsystem rated at 3.25 kW (0.95 tons refrigeration), supported by 25 m2 solar</p><p>collector area and 10 m3 TES volume. Additionally, the secondary effluent of local</p><p>wastewater treatment plant was processed to recover potable quality water.</p><p>Experimental results showed that &gt;95% of all the water contaminants such asbiological oxygen demand (BOD), total dissolved solids (TDS), total suspended</p><p>solids (TSS), ammonia, chlorides, nitrates, and coliform bacteria can be removed</p><p>to provide clean water for many beneficial uses. VC 2011 American Institute ofPhysics. [doi:10.1063/1.3608910]</p><p>I. INTRODUCTION</p><p>In many parts of the world, desalination has become an imperative and inevitable solution</p><p>to overcome the shortage of potable water. Current desalination technologies are based on ther-</p><p>mal evaporation or membrane separation principles. Thermal desalination technologies require</p><p>large quantities of energy and fossil fuels have traditionally been used to provide the energy</p><p>requirements for desalination of seawater or brackish waters. The idea of utilizing the fossil</p><p>fuels to produce freshwater through desalination processes is not a sustainable approach any</p><p>more due to the rapid decline in these resources and resultant high fuel costs and negative envi-</p><p>ronmental impacts. In an effort to conserve the depleting natural fossil fuel resources, desalina-</p><p>tion industry has been adopting several energy-saving measures in recent years. Examples</p><p>a)Author to whom correspondence should be addressed. Present address: Civil Engineering Department, Oregon Institute</p><p>of Technology, 3201 Campus Drive, Klamath Falls, Oregon 97601, USA. Electronic mail: Tel.:</p><p>1(530) 751 6061. FAX: 1(575) 646 7706.</p><p>1941-7012/2011/3(4)/043108/25/$30.00 VC 2011 American Institute of Physics3, 043108-1</p><p>JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 3, 043108 (2011)</p><p> This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: Downloaded to IP: On: Tue, 29 Apr 2014 10:38:21</p></li><li><p>include recovery and recycling of energy as in the case of staging, low temperature desalina-</p><p>tion, and utilization of waste heat or renewable energy.</p><p>Various renewable energy resources are available that suit the energy needs of different</p><p>desalination processes. Currently, the resources which are well explored and exploited for</p><p>desalination applications include solar energy (harvested by solar collectors or photovoltaic</p><p>modules), wind energy, geothermal energy, and wave energy.1 Sustainable use of these resour-</p><p>ces depends on the conversion technology employed and the end-user process configuration.</p><p>Sustainability, in this context, can be interpreted as how the available energy resource is being</p><p>utilized. Conserving, recycling and increasing the efficiency of the conversion technologies are</p><p>some approaches which result in a sustainable use of an energy resource. For instance, daily so-</p><p>lar energy available on the surface of the earth is roughly 15 000 times greater than the daily</p><p>energy consumption of the world population,2 which means that only much less than 1% of the</p><p>daily solar energy captured in energy devices could solve the energy problems of the world.</p><p>However, the maximum energy conversion rate of the current photovoltaic modules in the mar-</p><p>ket has not exceeded 15% to date. This indicates that the energy harvested through these</p><p>resources, though freely available, still have a very high value and need to be utilized effi-</p><p>ciently or utilized in a sustainable manner.</p><p>One way to utilize the renewable energy sources more efficiently is by coupling with an</p><p>energy-efficient desalination process. Thermal desalination processes operating at higher tem-</p><p>peratures such as multi-stage flash distillation (MSF), multi-effect distillation (MED) technology</p><p>require high quality heat sources at higher temperatures and result in higher fugitive losses and</p><p>consumption of prime non-renewable energy sources. On the other hand, low temperature</p><p>desalination processes have lower specific energy requirements and a higher thermodynamic ef-</p><p>ficiency. Apart from the above, other advantages include lower corrosion rates, low-cost materi-</p><p>als of construction with a longer plant life, lower scaling, lower heat losses, and shorter start-up</p><p>periods. The motive energy for driving the low temperature processes can be provided by low</p><p>grade heat sources (renewable energy) or process waste heat rejections, so that better economies</p><p>of the overall processes can be achieved.35</p><p>In this research, a new low temperature desalination process has been developed which can</p><p>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 tempera-tures, energy losses and, hence, the energy requirements for desalination are reduced. As this</p><p>process utilizes renewable energy and waste heat releases, it does not directly contribute to any</p><p>greenhouse gas emissions and can be considered a sustainable process. Results obtained from</p><p>theoretical modelling studies and experimental studies are presented in this paper to demon-</p><p>strate the viability of the proposed desalination process. Different configurations in which the</p><p>proposed process can be driven using different energy sources at a desalination production</p><p>capacity of 100 liters=day and the energy requirements are discussed. This paper focuses ontheoretical development of the low temperature desalination system using different renewable</p><p>energy sources with a limited analysis of experimental results.</p><p>A. Description of the desalination system</p><p>The premise of the proposed system can be explained by considering two barometric col-</p><p>umns at ambient temperature, one filled with freshwater and the other with saline water as</p><p>shown in Fig. 1. The barometric columns contain the head equivalent to local atmospheric pres-</p><p>sure and when closed, a vacuum will be created in the headspace by the amount of the fluid</p><p>volume displaced by gravity. Due to the natural vacuum generated by this process, the head</p><p>space of these two columns will be occupied by the vapors of the respective fluids at their re-</p><p>spective vapor pressures. If the two head spaces are connected to one another, water vapor will</p><p>distill spontaneously from the freshwater column into the saline water column, because the</p><p>vapor pressure of freshwater is slightly higher than that of saline water at ambient temperature.</p><p>However, if the temperature of the saline water column is maintained slightly higher than that</p><p>of the fresh water column to raise the vapor pressure of the feed water side above that of the</p><p>043108-2 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)</p><p> This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: Downloaded to IP: On: Tue, 29 Apr 2014 10:38:21</p></li><li><p>fresh water side, water vapor from the saline water column will distill into the fresh water col-</p><p>umn. A temperature differential of about 1015 C is adequate to overcome the vapor pressuredifferential to drive this desalination process. Such low temperature differentials can be</p><p>achieved using low grade heat sources such as solar energy, process waste heat, thermal energy</p><p>storage (TES) systems, etc.</p><p>A schematic arrangement of the low temperature desalination system based on the above</p><p>principles is shown in Fig. 2. Components of this unit include an evaporation chamber (EC), a</p><p>natural draft condenser, two heat exchangers, and three barometric columns. These three col-</p><p>umns serve as the saline water column, the brine withdrawal column, and the desalinated water</p><p>column, each with its own holding tank, SWT (seawater tank), BT (brine tank), and DWT</p><p>(desalinated water tank), respectively. The brine tank holds the concentrate removed from the</p><p>evaporation chamber to maintain the salt concentration in the evaporation chamber. The</p><p>FIG. 1. Physical principle of the low temperature desalination system.</p><p>FIG. 2. Low temperature desalination system powered by renewable and waste heat sources.</p><p>043108-3 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011)</p><p> This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: Downloaded to IP: On: Tue, 29 Apr 2014 10:38:21</p></li><li><p>evaporation chamber can be designed to use direct solar energy (with glass top exposed to solar</p><p>radiation) and waste heat sources (as shown in Fig. 2).</p><p>The EC is installed atop the three columns at a height of about 10 m above ground level</p><p>to create vacuum naturally in the headspaces of the feed, withdrawal, and desalinated water</p><p>columns. This configuration drives the desalination process without any mechanical pumping.6</p><p>The saline water enters the evaporation unit through a tube-in-tube heat exchanger.1,2 The</p><p>temperature of the head space of the saline water column is maintained slightly higher than</p><p>that of the desalinated water column. Since the head spaces are at near-vacuum level pres-</p><p>sures, a temperature differential of 10 C is adequate to evaporate water from the saline waterside and condense in the distilled water side.35 In this manner, saline water can be desali-</p><p>nated at about 4050 C, which is in contrast to the 60100 C range in traditional solar stills(SSs) and other distillation processes. This configuration enables the brine to be withdrawn</p><p>continuously from the EC through heat exchanger 1 (HE1), preheating the saline water feed</p><p>entering the EC.6,7 Further, by maintaining constant levels of inflow and outflow rates in</p><p>SWT, BT, and DWT, the system can function without any energy input for fluid transfer in</p><p>the desalination system. The heat input to EC is provided by TES tank through a heat</p><p>exchanger 2 (HE2) which in turn is fed by a low grade waste heat or renewable energy</p><p>source. Different heat sources evaluated in this study are solar collectors, photovoltaic thermal</p><p>collectors, geothermal energy sources, and process waste heat. Experimental results obtained</p><p>for a configuration using glass top evaporation chamber to utilize direct solar energy were</p><p>compared with theoretical results obtained in Sec. III. A closed top evaporation chamber was</p><p>also tested for recovering potable quality water from the secondary effluent of the local waste-</p><p>water treatment plant.</p><p>B. Theoretical analysis of the desalination system</p><p>Mass and energy balances around the EC yield the following coupled differential equations,</p><p>where the subscripts refer to the state points shown in Fig. 2. The variables are defined in the</p><p>Appendix.</p><p>Mass balance on volume of water in EC,</p><p>d</p><p>dtqV m2 m6 m3: (1)</p><p>Mass balance on solute in EC,</p><p>d</p><p>dtqVCEC m2C2 m6C6: (2)</p><p>Energy balance for volume of water in EC,</p><p>d</p><p>dtqVcpTEC QEC mcpT2 mcpT6 m3hLT Ql; (3)</p><p>where QEC is the rate of energy input (load on the TES) to the EC and Ql is the rate of energyloss from the EC. The energy input, QEC, to the evaporation chamber can be supplied by thesolar collectors, photovoltaic thermal collectors, geothermal water sources, and process waste</p><p>heat sources to the TES as discussed in Sec. I A and is written as</p><p>QEC mscsTs TEC; (4)</p><p>where, ms, cs, and Ts, are the mass flow rate, specific heat, and temperature, respectively, of thewater 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.</p><p>Desalination efficiency is defined as</p><p>043108-4 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011)</p><p> This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: Downloaded to IP: On: Tue, 29 Apr 2014 10:38:21</p></li><li><p>g MhLTRQECDt ; (5)</p><p>where</p><p>hLT 3; 146 2:36T 27...</p></li></ul>