CHAPTER 9 SOLAR DESALINATION John H. Lienhard,1,¤ Mohamed A. Antar,2 Amy Bilton,1 Julian Blanco,3 & Guillermo Zaragoza4 1 Center for Clean Water and Clean Energy, Room 3-162, Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, USA 2 Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia 3 Plataforma Solar de Almeria, Carretera de Senes s/n, 04200 Tabernas (Almeria), Spain 4 Visiting Professor of Electrical Engineering, King Saud University, Riyadh, Saudi Arabia ¤Address all correspondence to John H. Lienhard E-mail: [email protected] In many settings where freshwater resources or water supply infrastructure are inadequate, fossil energy costs may be high whereas solar energy is abundant. Further, in the industri- alized world, government policies increasingly emphasize the replacement of fossil energy by renewable, low-carbon energy, and so water scarce regions are considering solar-driven desalination systems as a supplement to existing freshwater supplies. Even in regions where petroleum resources are copious, solar-driven desalination is attractive as a means of con- serving fossil fuel resources and limiting the carbon footprint of desalination. Finally, in set- tings that are remote and ‘off-the-grid,” a solar driven desalination system may be more eco- nomical than alternatives such as trucked-in water or desalination driven by diesel-generated electricity. This article reviews various technologies that couple thermal or electrical solar energy to thermal or membrane based desalination systems. Basic principles of desalination are reviewed. Solar stills and humidification-dehumidification desalination systems are dis- cussed. Membrane distillation technology is reviewed. Current designs for solar coproduction of water and electricity are considered. Finally, photovoltaic driven reverse osmosis and elec- trodialysis are reviewed. The article concludes by summarizing the prospects for cost efficient solar desalination. 1. INTRODUCTION Water scarcity is a growing problem for large regions of the world. Scarcity results when the local fresh water demand is similar in size to the local fresh water supply. Figure 1 shows regions of the world in which water withdrawal approaches the difference between evaporation and precipitation, resulting in scarcity.1¡3 The primary drivers of increasing water scarcity are population growth and the higher consumption associated with rising standards of living. A lack of infrastructure for water storage and distribution is also a fac- tor in the developing world. Over time, global climate change is expected to affect existing water resources as well, potentially altering the distribution of wet and arid regions and ISSN: 1049–0787; ISBN: 1–978–56700–311–6/12/$35.00 + $00.00 277 °c 2012 by Begell House, Inc. 278 ANNUAL REVIEW OF HEAT TRANSFER NOMENCLATURE 2 Acol area of solar collector, m LEP liquid entry pressure, bar Amem reverse osmosis membrane surface L distance between water surface and area, m2 glass cover, m 2 ¡1 Apanel PV panel area, m M molecular weight, g mol ¡1 C0 PV panel performance constant, V m_ p mass flow rate of purified water, kg s ¡2 C1 PV panel performance constant, Md hourly distillate collected, kg m VK¡1 N_ molar flow rate, mol s¡1 Cfc average concentration of water in n PV model diode ideality factor (Sec. 7) the membrane feed channel, mg L¡1 n depreciation period in years (Sec. 6) Cp concentration of reverse osmosis Nu Nusselt number permeate water, mg L¡1 P pressure, Pa cp specific heat at constant pressure, pf polarization factor J kg¡1K¡1 ppm parts per million, mg kg¡1 Enet annual net electricity delivered to Pw water partial pressure (at Tw), mm Hg the grid, kWh Pwg water partial pressure (at Tg), mm Hg FF membrane fouling factor Q_ rate of heat transfer into system, J s¡1 Fs radiation shape factor Q_ least minimum (reversible) rate of heat G Gibbs energy of per mole, J mol¡1 transfer to separate, J s¡1 ¡2 Grad solar irradiation, W m q charge of an electron, C GOR gained output ratio qb heat loss through still material to H enthalpy per mole, J mol¡1 surroundings (ground), W m¡2 Hsol daily solar incidence on solar qc convection heat transfer from water to collector, J m¡2day glass cover, W m¡2 ¡1 hfg latent heat of vaporization, J kg qga heat transfer from the glass cover to h heat transfer coefficient, W m¡2K¡1 ambient air, W m¡2 hfg latent heat of evaporation qe evaporation heat loss from water to (difference between the enthalpy of glass cover, W m¡2 saturated vapor and that of saturated Rs PV panel series resistance, ­ liquid at specified temperature), Ra Rayleigh number ¡1 J kg Rsh PV panel shunt resistance, ­ I PV panel current, A Re Reynolds number ¡1 ¡1 I0 reverse saturation current, A S entropy per mole, J mol K Iph PV panel light generated current, A S_gen rate of entropy generation in system, k thermal conductivity, W m¡1K¡1 J s¡1 K¡1 KA membrane permeability for water, SW specific work (per unit mass of m bar¡1s¡1 purified water), J kg¡1 KB membrane permeability for salt, Tcell PV cell temperature, K m s¡1 T temperature, K Kfuel annual fuel cost, e T0 system temperature, K Kinvest total investment of the plant, e TH high temperature from which heat is KO&M annual operation and maintenance supplied, K costs, e TCF water permeability temperature k Boltzmann constant, J K¡1 correction factor kd real debt interest rate Ub heat transfer coefficient between the ¡2 ¡1 kinsurance annual insurance rate basin and surrounding soil, W m K SOLAR DESALINATION 279 NOMENCLATURE (Continued) V PV panel operating voltage, V w water Vp volume of purified water produced per day, m3day Acronyms _ Vp volume flow rate of purified water, AGMD air gap membrane distillation m3s¡1 CSP concentrating solar power W_ rate of work transfer into system, CSP+D concentrating solar power J s¡1 and desalination W_ least minimum (reversible) rate of work DCMD direct contact membrane to separate, J s¡1 distillation DNI direct normal irradiance Greek Symbols ED electrodialysis ® absorptivity LEC levelized electricity cost ¯ angle of inclination of glass cover LT-MED low-temperature ¹ dynamic viscosity of air (for Re multieffect distillation calculation), kg m¡1 s¡1 LT-MED-TVC low-temperature multieffect º kinematic viscosity, m2 s¡1 distillation powered by ¢p pressure difference, Pa thermal vapor compression ¢P¹ average pressure applied across LWC levelized water cost the membrane, bar MD membrane distillation ´pump isentropic efficiency of pump MED multieffect distillation ´pv energy conversion efficiency MENA Middle East and North Africa of photovoltaic device MSF multistage flash distillation ´th efficiency of solar thermal PT parabolic trough collector PT-CSP parabolic trough ¢¼¹ average osmotic pressure applied concentrating solar power across the membrane, bar PV photovoltaic ½ PVED photovoltaic electrodialysis σ Stefan Boltzmann constant PVRO photovoltaic reverse osmosis ¿ transmissivity RO reverse osmosis SEGS solar energy generating Subscripts systems (California, a air (ambient) 1984–1991) b basin SGMD sweeping gas membrane brine property of concentrated brine distillation stream TVC thermal vapor compression g glass TVC-MED multieffect distillation least value in the reversible limit powered by thermal vapor pure, p property of purified water stream compression saline, sw property of saline feed stream VMD vacuum membrane distillation raising the salinity of some coastal aquifers. Among these factors, consumption in the de- veloped world can be moderated relatively quickly by government policies aimed at reduc- ing per capita water use, and new supplies can be established through technology; however, 280 ANNUAL REVIEW OF HEAT TRANSFER FIG. 1: Regions of water stress, in which total water withdrawals approach the difference between precipitation and evaporation, are show in orange and red.1 population growth can be moderated only over very long time scales and infrastructure may not be developed quickly. All of these pressures are moving water-scarce regions toward purification of water supplies that are otherwise too saline for human consumption. Purification of saline water involves chemical separation processes for removing dis- solved ions from water. These processes are more energy intensive than the standard treat- ment processes for freshwater supplies. In many settings where fresh water resources or water supply infrastructure are inadequate, fossil energy costs may be high whereas so- lar energy is abundant. Such locations include sub-Saharan Africa and southern India. In the industrialized world, particularly the European Union, government policies increas- ingly emphasize the replacement of fossil energy by renewable, low-carbon energy, and so water-scarce regions such as Spain or the southwestern United States are considering solar-driven desalination systems as a supplement to existing fresh water supplies. Even in regions where petroleum resources are copious, such as the Arabian or Persian Gulf, solar-driven desalination is attractive as a means of conserving fossil fuel resources and limiting the carbon footprint of desalination. Finally, in settings that are remote and “off- the-grid,” a solar-driven desalination system may be more economical than alternatives such as trucked-in water or desalination driven by diesel-generated electricity. Desalination systems are of two broad types, based upon either thermal distillation or membrane separation.4;5 In a solar context, the thermal systems will heat saline water and separate the relatively pure vapor for subsequent condensation and use; the engineer’s pri- mary challenge is to recover and reuse the heat released in condensation, with minimal temperature difference, so as to make an energy efficient distillation system. Membrane separation systems usually rely on solar-generated electricity either to drive high-pressure pumps that overcome osmotic pressure differentials or to create electric fields that drive electromigration of ions in solution.