water Article Looking Beyond Energy Efficiency: An Applied Review of Water Desalination Technologies and an Introduction to Capillary-Driven Desalination Seyedsaeid Ahmadvand 1,* , Behrooz Abbasi 1,*, Babak Azarfar 1, Mohammed Elhashimi 2, Xiang Zhang 2 and Bahman Abbasi 2,* 1 Department of Mining and Metallurgical Engineering, University of Nevada, Reno, NV 89557, USA; [email protected] 2 School of Mechanical, Industrial and Manufacturing Engineering, Oregon State University, Bend, OR 97702, USA; [email protected] (M.E.); [email protected] (X.Z.) * Correspondence: [email protected] (S.A.); [email protected] (B.A.); [email protected] (B.A.) Received: 4 March 2019; Accepted: 1 April 2019; Published: 4 April 2019 Abstract: Most notable emerging water desalination technologies and related publications, as examined by the authors, investigate opportunities to increase energy efficiency of the process. In this paper, the authors reason that improving energy efficiency is only one route to produce more cost-effective potable water with fewer emissions. In fact, the grade of energy that is used to desalinate water plays an equally important role in its economic viability and overall emission reduction. This paper provides a critical review of desalination strategies with emphasis on means of using low-grade energy rather than solely focusing on reaching the thermodynamic energy limit. Herein, it is argued that large-scale commercial desalination technologies have by-and-large reached their engineering potential. They are now mostly limited by the fundamental process design rather than process optimization, which has very limited room for improvement without foundational change to the process itself. The conventional approach toward more energy efficient water desalination is to shift from thermal technologies to reverse osmosis (RO). However, RO suffers from three fundamental issues: (1) it is very sensitive to high-salinity water, (2) it is not suitable for zero liquid discharge and is therefore environmentally challenging, and (3) it is not compatible with low-grade energy. From extensive research and review of existing commercial and lab-scale technologies, the authors propose that a fundamental shift is needed to make water desalination more affordable and economical. Future directions may include novel ideas such as taking advantage of energy localization, surficial/interfacial evaporation, and capillary action. Here, some emerging technologies are discussed along with the viability of incorporating low-grade energy and its economic consequences. Finally, a new process is discussed and characterized for water desalination driven by capillary action. The latter has great significance for using low-grade energy and its substantial potential to generate salinity/blue energy. Keywords: capillary-driven desalination; energy grade; viable desalination; emerging technologies 1. Introduction Energy and freshwater production are heavily interconnected, termed the “water-energy nexus” [1–7]. Majority of the water on earth is in the oceans with high salinity and otherwise captured in the icecaps and glaciers [8], while most of human’s energy usage (~90%) originates from fossil fuels [9]. Water desalination is the manifestation of the water-energy nexus with all the strategic considerations regarding to the availability of the two (Figure1)[4,10]. Water 2019, 11, 696; doi:10.3390/w11040696 www.mdpi.com/journal/water Water 2019, 11, 696 2 of 30 Water 2019, 11, x FOR PEER REVIEW 2 of 30 Water Energy B Fresh Water O i Other o H t 3% f h 0.9% u y e N e 1 Surface Water d r l r 0 u s 0.3% 1 c o % l & e % a 2 Ground % r w 5 Water % a s 30.1% t e s n Oil 32% a e c Natural gas 22% % o 7 d e 9 s n n i r a l e % s i a 7 c p S . a a 8 l c 6 e G c I Coal 28% Earth’s Water Fresh Water FigureFigure 1. 1.Current Current availabilityavailability of water and energy energy resources resources [2,4,9] [2,4,9. ]. ThereThere are are two two groups groups of desalination of desalinatio currentlyn currently in use: in physicaluse: physical processes, processes such, assuch reverse as reverse osmosis (RO),osmosis and chemical(RO), and processes, chemical such processes as the newer, such zerovalentas the newer iron zerovalent (ZVI) technology, iron (ZVI discovered) technology in 2010, anddiscovered just starting in to2010 be commercializedand just starting [11 –to17 be]. Throughoutcommercialized this study,[11–17] desalination. Throughout has this been study, mainly revieweddesalination as a purelyhas been physical mainly process: reviewed the as physical a purely separation physical process of salt: andthe physical water [18 separation–28]. In this of salt sense, waterand desalinationwater [18–28] is. In fundamentally this sense, water a thermodynamic desalination is fundamentally process with a a minimum thermodynamic required process work with that is a minimum required work that is intrinsic thereto. This is known as the minimum thermodynamic intrinsic thereto. This is known as the minimum thermodynamic energy of separation (MTES); the energy of separation (MTES); the lowest possible energy that is required to separate the solute from lowest possible energy that is required to separate the solute from water [29]. Attempts to minimize water [29]. Attempts to minimize energy consumption toward MTES are only beneficial if they are energy consumption toward MTES are only beneficial if they are also economically viable [30]. also economically viable [30]. Most researches are mainly focused on the energy and yield efficiency of desalination techniques, Most researches are mainly focused on the energy and yield efficiency of desalination withtechniques, inadequate with emphasis inadequate on emphasis industrial on needs industrial [31–33 nee]. Inds the[31– industry,33]. In the all industry, desalination all desalination systems are designedsystems to are optimize designed the to delivered optimize full the cycle delivered cost to full the cycle consumer cost to as the opposed consumer to energy as opposed consumption to energy [34 ]. Despiteconsumption intensive [34] research. Despite in this intensive area, the research energy consumptionin this area, of the water, energy desalination consumption technologies of water have, notdesalination substantially techn changedologies within have the not past substantially decade [35 changed]. The energy within effi theciency past of decade most current [35]. The desalination energy technologiesefficiency of is most controlled current desalination by the thermodynamics technologies is rathercontrolled than by the the ratethermodynamics of the operation rather [than36,37 ]. Forthe instance, rate of the carbon operation nanotube [36,37] membranes,. For instance, with carbon high nanotube permeability, membranes increase, with the fluxhigh ratepermeability rather than, theincrease energy ethffie ciencyflux rate [38 rather,39]. Also,than the energy energy effi ciencyefficiency often [38,39] serves. Also, in favor energy of reducingefficiency theoften final serves cost, in but in somefavor of cases reducing this synergy the final is cost, violated but in [ 35some]. In cases the latterthis synergy scenario, is violated energy makes[35]. In a the major latter contribution scenario, toenergy the operational makes a majo expenditurer contribution (OpEx) to the but operational not necessarily expenditure to the ( capitalOpEx) but expenditure not necessarily (CapEx) to the [35 ]. Forcapital instance, expenditure some RO (CapEx strategies) [35] o.ff Forer moreinstance, energy some effi ROciency strategies at the offer cost more of adding energy extra efficiency high-pressure at the pumps,cost of which adding leads extra to high a higher-pressure levelized pumps, cost which of water leads (LCOW) to a higher [35 ].levelized cost of water (LCOW) [35]RO. is considered to be the gold standard desalination technique [40–49]. However, recent attemptsRO have is considered not been successful to be the togold reduce standard the gap desalination between the technique current RO[40– technologies49]. However, and recentMTES significantlyattempts have [35]. not Moreover, been successful only high-grade to reduce the energy gap between is applicable the current in RO RO desalination technologies and and additional MTES energysignificantly requirements [35]. Moreover, for pre/post-treatments only high-grade areenergy disregarded is applicable in mostin RO energy desalination analyses and [ 31additional–33,50,51 ]. energy requirements for pre/post-treatments are disregarded in most energy analyses [31–33,50,51]. On the other hand, thermal desalination techniques are more agnostic to the salinity level of the On the other hand, thermal desalination techniques are more agnostic to the salinity level of the intake water, and high-grade energy can be replaced by low-grade energy for the most part [52–55]. intake water, and high-grade energy can be replaced by low-grade energy for the most part [52–55]. However, low-grade energy (i.e., low- to medium-temperature heat, up to 400 C) is harder to control, However, low-grade energy (i.e., low- to medium-temperature heat, up to 400 °C)◦ is harder to control, dissipates faster, and has lower exergy; entropy generation is more significant in thermal desalination dissipates faster, and has lower exergy; entropy generation is more significant in thermal desalination plantsplants [56 –[5658–].58] One. One way way to compensate to compensate for this for energy this energy inefficiency inefficien in thermalcy in desalinationthermal