Sustainable desalination using solar energy

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    Solar energySustainable desalinationProcess modelPrototype systemPhotovoltaics

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    and barometric pressure head to maintain near vacuum conditions in an evaporation chamber. Because

    ater duncernAdmi

    the total water withdrawal in the US; consumptive use of water forelectricity production could more than double from 3.3 billion gal-lons/day in 1995 to 7.3 billion gallons/day in 2030 [2]. Although thisconsumptive use is not high compared to the total US consumptionof 100 billion gallons/day, large volumes of water are to be dedi-cated to thermoelectric power plant operation.

    Future demand for potable water will be much higher in theglobal context. According to the World Health Organization, nearly

    as 123 10 MW h/year [3]. It has been estimated that productionof 1 m3 of potable water requires the equivalent of about 0.03 tonsof oil [4]. Extraction and rening of fossil fuels and production ofenergy not only places additional demands on water, but also re-sult in pollution of water sources and air. Thus, the projected globaldemand for clean water supply for the future will signicantlyaccelerate not only depletion of fossil fuel reserves but also theconcomitant environmental damage and emission of greenhousegases.

    One of the solutions to this dilemma is to develop sustainableapproaches that can utilize renewable water and energy sources

    * Corresponding author. Tel.: +1 530 751 6061.

    Energy Conversion and Management 51 (2010) 22452251

    Contents lists availab

    n

    lseE-mail address: john_us@nmsu.edu (V.G. Gude).tions, the US population is expected to grow by about 70 millionby 2030 [1]. The direct domestic water demand and the indirectindustrial, agricultural, and environmental water needs to sustainthis growth is expected to place serious strains on the currentlyavailable water resources. At the same time, this growth in popula-tion is expected to increase the electricity demandby approximately50% [1], whichwill place additional demands on availablewater. Forexample, in 2000, thermoelectric power plants accounted for 48% of

    WHO, UNDP, UNICEF, etc.) expect that 24 of the least developedcountries, many of them along coastal areas without access tofreshwater and electricity, need to more than double their effortsto reach the Millennium Development Goals (MDGs) for basichealth, sanitation, and welfare.

    Provision of clean water inevitably requires energy, which iscurrently being provided essentially by nonrenewable fossil fuels.Total energy demand for providing the US water needs is reported

    61. Introduction

    Increasing demand for potable wand rapid development is a major coAccording to the Energy Information0196-8904/$ - see front matter 2010 Elsevier Ltd. Adoi:10.1016/j.enconman.2010.03.019of the vacuum conditions, evaporation occurs at near ambient temperature, with minimal thermal energyinput for phase change. This conguration enables the process to be driven by low-grade heat sourcessuch as solar energy or waste heat streams. Results of theoretical analysis and prototype scale experi-mental studies conducted to evaluate and demonstrate the feasibility of operating the process using solarenergy are presented. Predictions made by the theoretical model correlated well with measured perfor-mance data with r2 > 0.94. Test results showed that, using direct solar energy alone, the system could pro-duce up to 7.5 L/day of freshwater per m2 of evaporator area. With the addition of a photovoltaic panelarea of 6 m2, the system could produce up to 12 L/day of freshwater per m2 of evaporator area, at efcien-cies ranging from 65% to 90%. Average specic energy need of this process is 2930 kJ/kg of freshwater, allof which can be derived from solar energy, making it a sustainable and clean process.

    2010 Elsevier Ltd. All rights reserved.

    e to population growthnationally and globally.nistration (EIA) projec-

    2.8 billion people (40% of the world population) currently haveno access to safe drinking water and, water-borne diseases accountfor 90% of all infectious diseases in the developing world. TheWorld Resources Institute predicts that by 2025, at least 3.5 billionpeople will experience water shortages. Global agencies (includingKeywords:Desalination

    sustainable approach. In this paper, a sustainable phase-change desalination process is presented thatis driven solely by solar energy without any reliance on grid power. This process exploits natural gravitySustainable desalination using solar ener

    Veera Gnaneswar Gude *, Nagamany NirmalakhandaNew Mexico State University, Las Cruces, NM 88003, USA

    a r t i c l e i n f o

    Article history:Received 4 August 2009Accepted 21 March 2010Available online 14 April 2010

    a b s t r a c t

    Global potable water demlimited. Production and surently being derived fromrequires water, current pr

    Energy Conversio

    journal homepage: www.ell rights reserved.is expected to grow, particularly in areas where freshwater supplies arey of potable water requires signicant amounts of energy, which is cur-nrenewable fossil fuels. Since energy production from fossil fuels alsoe of potable water supply powered by fossil fuel derived energy is not a

    le at ScienceDirect

    and Management

    vier .com/ locate /enconman

  • Nomenclature

    A surface area, m2

    cp specic heat, kJ/kg KC concentration of solute, kg/kghL(T) latent heat at temperature T, kJ/kgI(t) solar insolation as a function of time, kJ/h m2

    q mass density, kg/m3

    s transmissivity ()g efciency ()

    Subscripts

    2246 V.G. Gude, N. Nirmalakhandan / Energy Conversion and Management 51 (2010) 22452251without consuming any nonrenewable resources (fossil fuels andwater) and without causing any environmental harm. Even thoughwater is one of the most abundant resources covering three-fourths of the planets surface, about 97% of this volume is saline,and only 3% is fresh water suitable for humans, plants, and ani-mals. The amount of water in the oceans, however, can serve asan inexhaustible and equitable source for the planets freshwaterneeds, if sustainable and cleaner technologies can be developedfor desalination.

    While a range of mature technologies are available for desalina-tion, most of them are cost-prohibitive, energy-intensive, and fossilfuel-dependent. Table 1 summarizes the energy requirements andgreenhouse gas emissions associated with currently available tech-nologies. With increasing costs and uncertainties of fossil fuel sup-plies and the environmental impacts associated with energyproduction, currently available desalination technologies are notreliable, affordable, and sustainable solutions for meeting futurewater needs, particularly for low-income rural and remotecommunities.

    Use of renewable energy sources (such as wind, solar, geother-mal) to drive desalination processes can be a sustainable andaffordable approach to reclaim potable water from seawater andbrackish waters. Solar energy, in particular, has been identied asa convenient renewable energy source for this application, because

    m mass ow rate, kg/hM total daily mass of distillate, kgQ heat ow rate, kJ/ht time, hT temperature, CV volume, m3

    Greek symbolsa absorptivity ()j experimental constant (107 106 kg/m2 Pa s K0.5)it is more widely available and can be stored in batteries via pho-tovoltaic (PV) arrays, and converted to heat or mechanical energywith reasonable efciency. Although solar energy is free thehardware necessary for capturing it, converting it to useful forms,and storing it can add signicantly to the cost. Additional costs willincur depending on the type of desalination technology that isused. Economic factors are the main barriers to the use of solar en-ergy for desalination. However, for rural and remote applications,where grid power or fossil fuels to generate energy may not beavailable at affordable costs, solar energy-driven desalinationmay be economically attractive. Thermal desalination technologies

    Table 1Comparison of proposed process with traditional desalination processes.

    MSF MED

    Specic energy (kJ/kg) Thermal 294 123Mechanical 44 26Total 338 149

    CO2 emissions (kg CO2/kg H2O) 0.09 0.04

    MSF multi-stage ash distillation; MED multi-effect distillation; MVC mechanical vphotovoltaic, this study [4].In this study, a new low temperature desalination process hasbeen developed which can utilize low-grade heat sources such aswaste heat releases or solar energy. Since the process operates atlower temperatures than traditional thermal desalination pro-cesses, energy losses and hence the net energy requirements forthis process are lower and its thermodynamic efciency is higher[5]. As this process utilizes waste heat releases and renewable en-ergy, it does not contribute directly to any greenhouse gas emis-sions, and can be considered a sustainable process.

    2. Proposed system

    The premise of the proposed process can be illustrated by con-sidering two barometric columns at ambient temperature, onewith freshwater and one with saline water. The headspace of thesetwo columns would be lled by the vapors of the respective uidsat their respective saturated vapor pressures. Suppose these head-require large quantities of energy. Traditionally, fossil fuels havebeen used to provide the energy requirements for desalination ofseawater or brackish waters. In an effort to conserve fossil fuel re-sources, desalination industry has been adopting several energy-saving measures in recent years. Examples include recovery andrecycling of energy as in the case of staging, low temperature desa-lination, and utilization of waste heat or renewable energy.

    BB battery bankEC evaporation chamberg glassl lossesPV photovoltaic panelspaces are connected to one another. Since the vapor press