Energy storage for desalination processes powered by renewable energy and waste heat sources

Veera Gnaneswar Gude

There is an acute shortage of potable water in many countries over the world especially in the Middle East region. Currently over one-third of the world's population lives in areas where people are unable to meet their water demand of fresh water [1

In Gulf Cooperation Council (GCC) countries, about 40% of primary energy is consumed for cogeneration based power and desalination plants. In the past, many studies were focused on renewable energies based desalination processes to accommodate 5 fold increase in demand by 2050 but they were not commercialized due to intermittent nature of renewable energy such as solar and wind. We proposed highly efficient energy storage material, Magnesium oxide (MgO), system integrated with innovative hybrid desalination cycle (MEDAD) for future sustainable desalination water supplies. The condensation of Mg(OH)2 dehydration vapor during day operation with concentrated solar energy and exothermic hydration of MgO at night can produce 24 hour thermal energy for desalination cycle without any interruption. It was showed that, Mg(OH)2 dehydration vapor condensation produce 120C and MgO hydration exothermic reaction produce 140C heat during day and night operation respectively correspond to energy st...

Applied Energy xxx (2014) xxx–xxx Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Energy storage for desalination processes powered by renewable energy and waste heat sources Veera Gnaneswar Gude ⇑ Department of Civil and Environmental Engineering, Mississippi State University, Mississippi State, MS 39762, USA h i g h l i g h t s g r a p h i c a l a b s t r a c t Energy storage options for various desalination processes were discussed in detail. Thermal/electrical energy storage options enhance desalination process performance. Energy storage integration provides reliable and continuous desalination operations. Chemical, compressed air, pumped hydrostatic energy storage need further research. Future research should focus on storage media, containers and thermal insulation. a r t i c l e i n f o Article history: Received 12 March 2014 Received in revised form 17 May 2014 Accepted 28 June 2014 Available online xxxx Keywords: Desalination Energy storage Thermal energy storage Renewable energy Cogeneration and batteries a b s t r a c t Desalination has become imperative as a drinking water source for many parts of the world. Due to the large quantities of thermal energy and high quality electricity requirements for water purification, the desalination industry depends on waste heat resources and renewable energy sources such as solar collectors, photovoltaic arrays, geothermal and wind and tidal energy sources. Considering the mismatch between the source supply and demand and intermittent nature of these energy resources, energy storage is a must for reliable and continuous operation of desalination facilities. Thermal energy storage (TES) requires a suitable medium for storage and circulation while the photovoltaic/wind generated electricity needs to be stored in batteries for later use. Desalination technologies that utilize thermal energy and thus require storage for uninterrupted process operation are multi-stage flash distillation (MSF), multieffect evaporation (MED), low temperature desalination (LTD) and humidification–dehumidification (HD) and membrane distillation (MD). Energy accumulation, storage and supply are the key components of energy storage concept which improve process performance along with better resource economics, and minimum environmental impact. Similarly, the battery energy storage (BES) is essential to store electrical energy for electrodialysis (ED), reverse osmosis (RO) and mechanical vapor compression (MVC) technologies. This research-review paper provides a critical review on current energy storage options for different desalination processes powered by various renewable energy and waste heat sources with focus on thermal energy storage and battery energy storage systems. Principles of energy storage (thermal and electrical energy) are discussed with details on the design, sizing, and economics for desalination process applications. Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Tel.: +1 662 325 0345. E-mail addresses: gude@cee.msstate.edu, gudevg@gmail.com http://dx.doi.org/10.1016/j.apenergy.2014.06.061 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved. Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 2 V.G. Gude / Applied Energy xxx (2014) xxx–xxx Nomenclature A AC Ah cp DC I m MDOD Q P PV S v q t T TCM U V area (m2) alternate current ampere hour specific heat (kJ/kg K) direct current solar insolation (kJ/h m2) mass of storage medium (kg) maximum depth of discharge energy or useful energy from solar collectors (kJ/h) pressure (atm), power (kW) photovoltaic entropy (kJ/K) volume of the saline water or storage tank (m3) evaporation energy (kJ/h) time (h, s) temperature (K) thermochemical reaction material internal energy wind speed (m/s) Special characters q density (kg/m3) k latent heat (kJ/kg) g energy (first law) efficiency (%); panel efficiency Dh latent heat for PCM storage 1. Introduction The importance and value of water has been highly pronounced recently due to exploding global population, rapid industrialization and urbanization [1,2]. It was reported that worldwide population estimates tripled while water use estimates increased more than sixfold over the 20th century, suggesting that the world not only has more water users but also a higher water consumption by these users mostly related to improved living standards all over the world [3]. It has been realized in many regions that existing freshwater resources do not have the capacity to meet the escalating demands and in some cases, there are not adequate surface and ground water sources. Therefore, the need for utilizing saline waters from the oceans and the processes to convert the salt water into freshwater have been recognized as logical approaches over past few decades [1]. Desalination technologies initially were both cost and energy prohibitive. The impetus to install desalination plants in many coastal and metropolitan cities for providing freshwater needs comes from: (1) dramatic improvements in energy consumption by many desalination technologies; and (2) reduced investment costs for desalination processes [4]. While desalination of saline waters has now been accepted as a potential alternative for freshwater supplies, the energy demands by the existing desalination technologies for water production continue to pose challenges in their applications (Fig. 1). In 2008, the worldwide installed desalination capacity was 58 million m3/d, and in 2011 it was 65.2 million m3/d which is projected to increase to 97.5 million m3/d by the year 2015 [99–101]. It was estimated by Kalogirou [5] that the production of 1000 tons (m3) per day of freshwater requires 10,000 tons (toe) of oil per year. The worldwide desalination capacity is increasing at a steadfast pace consuming equivalent amounts of fossil fuel sources and associated increase in greenhouse gas emissions. The desalination industry is projected to experience unprecedented growth concurrently with population explosion and increasing standards of living. Since the energy requirements whether thermal or electric, need to be supplied in large quantities, dependence of these technologies over Subscripts 1 TES temperature 2 evaporation chamber temperature a ambient ac alternate current c collector Comb combined CR chemical reaction dc direct current DES desalination EC evaporation chamber h hot stream in inlet, supply e evaporation, electrical l, ls, L latent, losses, load out energy supplied by TES PCM phase change material s solar, collectors, TES sensible sensible heat STC standard conditions t total TES thermal energy storage th thermal finite, conventional fossil fuel sources is not a sustainable approach. Utilization of renewable energy sources such solar, wind, and geothermal sources appears to provide a sustainable alternative. Yet, the major concern with these natural and renewable energy sources is their intermittent nature and the variable intensity which limits their applications in many cases and locations. Costs associated with the renewable energy technologies is another major hurdle for successful implementation of these energy resources. Energy storage can be considered as an option to increase the performance of the renewable energy sources. Energy storage technologies help enhance utilization of these intermittent energy sources and may improve long term sustainability of the investment. Thermal desalination technologies may utilize storage units known as thermal energy storage (TES) to capture, store, and release to match the energy supply and demand trends. TES can be coupled with energy sources whether they are renewable or waste heat in nature. Photovoltaic collectors and wind turbines require batteries to store the energy to be supplied to the process for later use. Fig. 1. Worldwide population and desalination capacity trends and the energy requirements for desalination in the form of oil. Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 V.G. Gude / Applied Energy xxx (2014) xxx–xxx This research-review paper discusses current energy storage options for different desalination technologies using various renewable energy and waste heat sources with focus on thermal energy storage and battery energy storage systems. Principles of energy storage are discussed for the first time with details on design and sizing and desalination process applications. 2. Overview of desalination technologies and renewable energy coupling schemes Currently available desalination technologies can be categorized as phase change (thermal) and non-phase change (membrane) processes. Phase change processes involve heating the feed water (seawater or brackish water) to ‘‘boiling point’’ at the operating pressure to produce steam which is condensed in a condenser unit to produce freshwater. Desalination technologies based on this principle include solar distillation (SD) such as solar stills and active and passive solar desalination systems; multieffect evaporation/distillation (MED); multi-stage flash distillation (MSF); thermal vapor compression (TVC) and mechanical vapor compression (MVC). Non-phase change processes involve separation of dissolved salts from the feed waters by mechanical or chemical/electrical means using a membrane separator between the feed (seawater or brackish water) and product (potable water). Desalination technologies based on this principle include electrodialysis (ED) and reverse osmosis (RO). Other processes that involve a combination of the two principles in a single unit or in sequential steps to produce pure or potable water include membrane distillation (MD) and reverse osmosis combined with MSF or MED processes [2]. A comparison of various desalination technologies, energy requirements and renewable energy source applications are shown in Table 1 [2,5,57,145]. The following paragraphs provide brief description of operational principles of the above mentioned desalination technologies. 2.1. Multi-stage flash distillation (MSF) This process is based on the principle of flash evaporation where seawater is evaporated by reducing the pressure in successive process effects. The energy economy is achieved by successive regenerative heating where the seawater flashing in each flash chamber (effect or stage) rejects heat to the feed water, thereby heating the incoming seawater [102]. Seawater prior to the introduction in the first stage is heated by external heat sources such as low pressure steam form power plants or an extraction steam from a steam turbine plant. This heated seawater then enters the flash evaporation chambers with falling operating pressures in the successive cycles. Typical number of stages for flashing and energy recovery vary between 15 and 30 stages in modern large scale MSF plants [103]. The operating temperatures vary between 90 and 120 °C (also known as top brine temperatures, TBT) which depends on the quality of heat source available in the first stage [104]. Operating the plant at higher temperatures typically allows for higher efficiencies due to higher number of stages. The product water contains about 2–10 ppm which requires post treatment (also known as re-mineralization) prior to water supply in pipelines for human consumption [102]. 2.2. Multi-effect evaporation/distillation (MED) The MED process is the most thermodynamically efficient thermal desalination technology available today [106]. Similar to MSF process, MED process operates in a number of effects to increase the energy efficiency. Contrary to MSF process, the pre-heated seawater is sprayed onto the tubes in the first evaporation chamber 3 which are heated by external heat source derived from a power plant steam [107]. These are operated in a dual purpose power plant scheme. The freshwater vapor evaporated in the first evaporation chamber is allowed to pass through a condenser which serves as evaporating surface in the next effect. The remaining brine from the first evaporation chamber is passed through the next effect where it is sprayed on the evaporating tubes at lower temperatures and lower pressures. This process continues in the successive effects until suitable temperature gradient is available for freshwater evaporation. The energy or steam economy is proportional to the number of effects. The total number of effects is limited by the total temperature range available and the minimum allowable temperature difference between successive effects [108]. Typical number of effects in MED process varies between 4 and 21 which again depend on the heat source temperature and the top brine temperature in the first effect. The top brine temperatures are usually around 90 °C for MED processes, but a lower top brine temperature of 70 °C is also possible which are called low temperature MED (LTMED) [109]. Although this process is considered thermodynamically efficient, a drawback with this process is the requirement for large heat transfer areas. The heat transfer areas for LTMED are considerably higher than MED process often varying between 25% and 40%. 2.3. Vapor compression (M/TVC) Vapor compression is suitable for small to medium scale desalination plants. The principle behind this technology is that energy recovery from the vapor generated in the last effect by compressing it either thermally in a steam ejector or mechanically in a compressor to act as a heat source for the first effect. The compression of the vapor raises the steam pressure and temperature to a level higher than the vapor generated in the first effect [110]. The compressor creates a vacuum in the evaporator and then compresses the vapor taken from the evaporator and condenses it inside of a tube bundle. Seawater is sprayed on the outside of the heated tube bundle where it boils and partially evaporates, producing more vapor. With the steam-jet type of VCD unit, called a thermocompressor, a venturi orifice at the steam jet creates and extracts water vapor from the evaporator, creating a lower ambient pressure [111]. The extracted water vapor is compressed by the steam jet. This mixture is condensed on the tube walls to provide the thermal energy, heat of condensation, to evaporate the seawater being applied on the other side of the tube walls in the evaporator. The difference between two vapor compression techniques is that the mechanical vapor compression requires the installation of the expensive compressor whereas thermal vapor compression requires a simple ejector [112]. MVC compressor is known to have lower efficiency and higher operational and maintenance disadvantages which are not the case with TVC method [112,113]. 2.4. Solar distillation (SD) Solar distillation refers to solar stills and active or passive solar desalination systems that are supported either direct solar energy or indirect solar energy [114]. Indirect solar energy means that the solar energy harvested in the solar collectors (flat panel collectors, parabolic trough collectors, etc.) is supplied to the desalination unit. These applications depend on the type of solar energy harvesting technology and a suitable desalination mechanism. The solar stills are the simplest and cheapest direct solar harvesting desalination units. These units utilize the direct solar energy to evaporate the freshwater from salt water leaving behind the concentrated brines. Solar stills incorporate the evaporating and condensing units into a single chamber. Often, the glass roof of the unit serves as the condensing surface which rejects the latent heat Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 4 Process Process parameters Temperature range Pressure Feed concentration Predominant source Typical freshwater recovery Energy needs (kJ/kg) Energy source Direct solar energy Solar collectors Solar pond Photovoltaic modules PV/Thermal collectors Geothermal source Wind energy Wave energy Costs Small scale (1–100 m3/d) Large scale (m3) Capital costs ($/m3/d) Energy costs ($/m3) Freshwater costs ($/m3) Others Pretreatment Scaling and fouling Maintenance costs Advantages Disadvantages Solar still Multi effect solar still MSF MED LTMED M/TVC MD RO ED 40–80 °C 40–80 °C 80–120 °C 50–90 °C 40–70 °C 40–100 °C 40–80 °C <45 °C <45 °C Atmospheric Atmospheric 1–2 atm 0.1–0.5 atm 0.1–0.4 atm Atmospheric 20–60 atm SW/BW SW/BW SW/BW 35–45% SW/BW 35–45% SW/BW 35–45% SW/BW 25–40% SW/BW 5040 1500 200–350 150–250 150–240 111 BW 35–50% (SW), 50–90% (BW) 120 U U U U U U U U U U U U U U U U U U U U U U U U U U U U <1 m3 NA <1 m3 NA NA NA None None Medium Medium Low Low Small and rural applications possible, low capital and little maintenance costs, no energy costs Not suitable for large scale applications due to lower efficiency Not economical 50–200,000 1600–2300 0.35–1.1 0.77–1.85 100–200,000 550–2100 0.08–1.15 0.87–1.95 Low Low High Low-medium Low-medium Low Large scale applications, reliable process and experience in operations can be combined with power generation (cogeneration) Cost and energy intensive, not suitable for small scale applications Not economical 50–50,000 U U U U 50–50,000 890–1350 0.057–0.4 0.46–5.0 Low Low-medium Low High thermodynamic efficiency, low cost or free waste heat Very low Low-medium High Low specific energy consumption Less experience; large heat transfer areas Electrical and mechanical energy input U <5000 ppm BW 50–90% (BW) 144 0.26–36 Very well applied 100–300,000 900–1700 0.3–0.6 0.55–2.37 0.20–0.35 Low Low-medium Low-medium High product recovery, low temperature operation High High High Reliable and most widely used technology High/medium High Medium Efficient for high quality product from BWRO Limited commercial applications; require membranes High electricity requirements, capital and O&M costs Highly sensitive to raw water quality, pre-treatment 105,000 100,000 V.G. Gude / Applied Energy xxx (2014) xxx–xxx Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 Table 1 Summary of desalination technologies and renewable energy applications. V.G. Gude / Applied Energy xxx (2014) xxx–xxx to the ambient. The energy efficiency of typical solar stills varies between 30% and 40% [115]. The stills with an external condensing surface maintained in shade or at lower temperatures showed higher energy efficiencies up to 70%. The energy efficiency and the distillate product can be improved by adding multiple effects in the same unit. The energy efficiency is calculated as the ratio of the amount of the distillate produced to the amount of solar energy received by the solar still unit. Other indirect solar desalination systems include multi-effect distillation or evaporation system, low temperature multi-effect desalination, humidification– dehumidification, and membrane distillation systems [116]. The efficiency of a thermal desalination plant is described in two forms such as gain output ratio (GOR) or performance ratio (PR). The GOR is defined as the amount freshwater produced for the amount of steam consumed while the PR provides the minimum water cost. Therefore, PR is defined as the units of distillate produced per unit mass (kg or lb) of steam or per 2326 kJ (1000 Btu) heat input which corresponds to the latent heat of vaporization at 73 °C. The PR values for modern large scale MSF plants are in the range of 6.5–10.5 lbs/1000 Btu heat input [105] while the same for the MED plants varies between 10 and 18 with 4–21 effects [108]. When more than one steam pressure will be utilized, or when the desalination system will be integrated with another thermal cycle, it may be more practical to consider how much water is produced per unit of energy consumed. This is often referred to as economy or performance ratio. GOR should be considered at the design stage of a desalination plant when the quantity and economic value of energy and water can be used to compare the capital and operating costs of units with different GORs. Typically higher GOR systems cost more but consume less energy and therefore have lower operating costs (at least the energy component of operating cost is lower). Lower GOR values are typical of applications where there is a high availability of low value thermal energy. Higher GOR values, even up to 18, have been associated with situations where local energy values are very high, when the local value or need for water is high or a combination of both. For MED systems the GOR is directly related to the number of effects expressed as GOR = 0.8 n. More effects directly increases GOR and for systems using thermo-compressors the GOR is also impacted by the pressure of the steam. Higher pressure steam will recycle more process vapor within the MED part of the process thereby improving the GOR and reducing external enthalpy requirements. For MSF systems, the GOR is indirectly related to the number of flash stages. Typically for cross tube MSF the number of stages will be approximately 2.5–3 times the GOR value. For long tube this is typically 3.5–4.0 times the GOR value. This is because the incremental cost of adding a stage to the long tube configuration (during the design phase) is lower than for the cross tube design. For vapor compression systems, very high GOR can be obtained with MED–TVC units. For example, Sidem has designed units with twelve cells and a motive steam pressure of 30 bar, having a GOR of 17 (i.e. 17 kg of distillate water produced per kg of steam fed into the thermo-compressor) [117]. 2.5. Reverse osmosis (RO) Reverse osmosis process is a non-phase change operation where a semi-permeable membrane (allowing water to pass through but not the salts) is used to separate the freshwater from the saline feed water. An external pressure is applied to exceed the osmotic pressure of the feed water to allow the freshwater to pass through the membrane [2]. The amount of energy required for mechanical pumping to create the external pressure depends on the feed water salt concentration. This process does not require heating or phase-change of the feed water. A typical RO plant consists of four major components: feed water pre-treatment, high 5 pressure pumping, membrane separation, and permeate posttreatment [118,119]. The pre-treatment step involves removal of large suspended solids, bacteria and colloidal matter that may cause damage to the membrane operations. A typical pretreatment includes chlorination, coagulation, acid addition, multi-media filtration, micron cartridge filtration, and dechlorination [120,121]. Additionally, the fouling problems should be avoided by using cleaning solutions [122]. The type of pretreatment mainly depends on the feed water characteristics, membrane type and configuration, recovery ratio, and product water quality. Post treatment of the permeate (fresh water) usually is done by re-carbonation and blending with feed water. 2.6. Electrodialysis (ED) The operating principle of this method is based on the migration of ionic salts toward their respective counter charge electrodes. Selective membranes that allow passage of either anions or cations in an alternating fashion result in concentrate and product streams. The anions can pass through the anion-selective membrane, but are not able to pass by the cation-selective membrane, which blocks their path and traps the anions in the brine stream [123]. Similarly, cations move in the opposite direction through the cation-selective membrane under a negative charge and are trapped by the anion-selective membrane. A typical ED system includes a membrane stack with a number of cell pairs, each consisting of a cation transfer membrane, a demineralized flow spacer, an anion transfer membrane, and a concentrate flow spacer. Compartments for the electrodes are at opposite ends of the stack. The electrodes need regular flushing to reduce fouling or scaling [124]. Recycling the concentrate stream and discharging concentrate to waste, or blowdown is common and called feed-and-bleed mode. This is necessary because of the fact that there are sharp differences in flow rates between the product and brine streams. Diluate flow is about 10 times the flow of the brine stream; this difference in flows creates pressure imbalances, requiring concentrate recycle [125]. An ED unit can remove about 50–94% of dissolved solids from a feed water, up to 12,000 mg/L TDS. Voltage input, and process configuration (number of stacks or stages), raw water quality and membrane selection determine the salt removal efficiency of the process. TDS removal is generally limited by economics. The cost of ED increases as the feed water TDS increases. Electrodialysis Reversal (EDR) is similar to ED but the polarity of the electrodes is regularly reversed, thereby freeing accumulated ions on the membrane surface [125]. This process minimizes the effect of inorganic scaling and fouling by converting product streams into waste streams. This process requires increases membrane life and does not require chemical addition and improves membrane and electrode cleaning. 2.7. Membrane distillation (MD) Membrane distillation process can be described as a hybrid process since it combines thermal evaporation and membrane separation principles in a single process unit. The feed water (saline water) is heated by an external heat source often derived from solar energy or process waste heat and is passed through the hot side of the unit to allow for the water vapors to raise and diffuse through the membrane barrier and condense in the permeate flow on the cold side of the unit [126]. Since the hot (saline water feed) and cold (permeate or freshwater) streams are separated by a membrane barrier, a very low temperature differential of 10 °C is sufficient to produce freshwater through this process. The membrane applied in this process should be porous enough allow the water vapor to pass through but not allowing the liquid and preferably non-wetted by the process liquids [127]. There are four Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 6 V.G. Gude / Applied Energy xxx (2014) xxx–xxx Fig. 2. Worldwide desalination capacity by: (a) desalination technology; and (b) source water. main configurations of the MD process namely: (1) direct contact membrane distillation (DCMD); air gap membrane distillation (AGMD); vacuum membrane distillation (VMD); and sweep gas membrane distillation (SGMD) [128]. In DCMD, the membrane is in direct contact with the liquid phases while in AGMD, an air gap is introduced between the membrane and condensation surface to improve the energy efficiency. In VMD, the permeate side maintained at a lower pressure by mechanical pumping to increase the permeate flux, while in the SGMD, a stripping or carrier gas is used to collect the vapor produced. Among these methods, DCMD operates at a reasonable efficiency and a higher permeate rate. The permeate flux is very low for the AGMD. VMD can produce higher permeate but at the expense of higher energy requirements and is suitable for feed waters with volatile contaminants [126–128]. As of June 2011, there were 15,988 desalination plants worldwide which combined produce a total of 65.2 million m3 of freshwater equivalent to 17.5 billion US gallons in over 150 countries supporting 300 million people [129]. Out of these desalination plants, reverse osmosis with about 60% share currently dominates the other desalination technologies [133] (Fig. 2a) and this trend is expected to continue into the future followed by well-established MSF technology (26.8%) and MED technology (8%) with the remaining 5% taken by electrodialysis and other hybrid technologies. Sixty percent of the desalination plants process seawater to produce freshwater (Fig. 2b) followed by brackish water (21.5%), river water (8.3%), wastewater recovery/reuse (5.7%) and other water sources (4.5%) [130]. Energy requirements for desalination technologies vary significantly in quantity and quality. Table 1 presents the energy requirements for various desalination methods [2,5,131–133]. The potential renewable energy-desalination technology combinations are shown in Fig. 3. The renewable energy sources (RES) should be integrated with the relevant desalination technology that has capability to utilize the energy in the most effective manner. Some renewable energy source dependent desalination technologies must be placed on the same site (co-location) and some do not have this requirement. Accordingly, the following thermal desalination-renewable energy combinations require colocation (located on same site): (a) wind–shaft–MVC; (b) solar thermal–TVC; (c) solar thermal–MSF; (d) solar thermal–MED; (e) solar thermal–SD; (f) geothermal–TVC); (g) geothermal–MSF or MED. The other electricity-driven combinations that do not require co-location are: (a) wind–MVC; (b) wind–RO; (c) solar PV–RO; (d) solar PV–MVC; (e) geothermal–MVC; and (f) geothermal–RO. 3. Energy storage options for various desalination processes Desalination technologies that utilize thermal energy and thus require thermal energy storage for uninterrupted process operation are MED, MSF, low temperature MED (LTMED) low temperature desalination (LTD) and humidification–dehumidification (HD) and membrane distillation (MD). Thermal energy storage technology requires a suitable medium for storage and circulation for heat transfer while the photovoltaic/wind generated electricity needs to be stored in batteries for later use as shown in Fig. 3. Similar to TES, the battery energy storage (BES) is essential to store Fig. 3. Desalination technologies coupled with renewable energy and storage systems. Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 V.G. Gude / Applied Energy xxx (2014) xxx–xxx 7 Fig. 4. Thermal desalination system powered by solar collectors or waste heat sources augmented by TES. electrical energy for electrodialysis (ED), reverse osmosis (RO) and mechanical vapor compression (MVC) technologies. 3.1. Thermal energy storage for desalination Energy absorption–accumulation, storage and release are the principal elements of the TES concept which result in improved economics, better resource management and lower environmental emissions of a variable energy source powered desalination system, for instance, solar energy. TES units function in three important stages: (1) charging period; (2) storing period; and (3) discharging period [6]. Thermal energy accumulation and storage can be accomplished by three different principal methods: (1) sensible heat thermal energy storage which includes both solid-state and liquid materials for storage; (2) phase-change thermal energy storage, also known as latent heat storage; and (3) thermochemical thermal energy storage. An example of the sensible heat TES unit for desalination application powered by process waste heat or solar collectors is shown in Fig. 4. The following equation expresses the amount of heat stored in a sensible heat TES in general which depends on the amount of heat transferred from a heat source. Q sensible ¼ m Z T2 cp ðTÞdT ð1Þ T1 where Qsensible is the amount of sensible heat stored, m is the flow rate of the storage medium, cp is the specific heat capacity of the storage medium and T1 and T2 are the outlet and inlet temperatures of the heat transfer fluid. Thermodynamic efficiency of a TES augmented desalination process can be illustrated using the first law and the second law efficiencies [7]. The first law (energy) efficiency of the TES can be expressed as (Fig. 4): gTES ¼ Q out ms cs ðT 1 T 2 Þ ¼ mh ch ðT h T 1 Þ Q in ð2Þ where Qin is the energy supplied to the TES by a hot stream of mass flow rate mh, specific heat ch and inlet and outlet temperatures of Th and T1 respectively. Qout is the energy supplied by the TES to the desalination system with a mass flow rate ms, specific heat cs and inlet and outlet temperatures of T2 (evaporation temperature TEC) and T1 respectively. First law (energy) efficiency of the desalination system (DES) can be found from: gDES mkðT EC Þ ¼ ms cs ½ðT 1 T 2 Þ ð3Þ where m is the mass of the freshwater produced, k is the latent heat of the freshwater at the evaporation temperature TEC which is same as T2. First law (energy) efficiency of the combined system can be found from: gComb ¼ mkðT EC Þ mh ch ½ðT h T 1 Þ ð4Þ Details on the thermodynamic analysis (first law and second law efficiencies) of TES systems augmenting the desalination application at different TES and desalination temperatures can be found elsewhere [7]. Since thermal energy can be stored and transported by fluids in different phases, a generalized expression can be written to represent various fluids and associated operational parameters such as temperature and pressure. Even when a system is exposed to its environment, as long as it is in its equilibrium state so that the temperature T and the pressure P of the system are equal to those of the environment (i.e., T = Tenvironment and P = Penvironment), its internal energy has a well-defined value on the macroscopic level so that the internal energy is a state variable. Being the sum of the kinetic and the potential energy of the constituent atoms or molecules, the internal energy must be proportional to the total number N of these atoms or molecules, which is in turn proportional to their mole number (n): U / N / n, which implies that the internal energy is extensive so that we can express it as U = nu (T, P), where u is the molar internal energy defined by u = U/n. To simplify the illustrations, the previously defined equations will be used in further discussions. The quality of thermal energy that is stored by the storage medium determines its appropriate application which in turn depends on the availability of heat source. Sensible heat TES systems with water as storage medium store thermal energy below 100 °C, therefore their application could possibly be limited to low temperature desalination systems such as solar stills, low temperature multi-effect evaporation systems, membrane distillation, humidification–dehumidification and other novel desalination systems. Solid-state or liquid salt materials can store high grade thermal heat in the range of 200–500 °C which would be ideal for power generation combined with multi stage flash (MSF) distillation, multi-effect evaporation (MEE or MED) and mechanical/thermal vapor compression (M/TVC) desalination processes. Table 2 provides a comparison of the various storage mediums for thermal energy storage and their potential for desalination and power generation applications [8]. Briefly, sensible heat TES technologies can be used in desalination process applications while the phase change and thermochemical TES technologies are ideal for cogeneration purposes. 3.2. Sensible heat TES for desalination The most commonly used sensible heat storage medium is water. In water based TES systems, water serves both as the energy storage and the heat transfer medium. Additionally various solidstate and liquid/molten state sensible heat TES mediums can be used depending on the desalination application. Tables 3 and 4 present the heat storage capacity and costs per unit of thermal energy ($/kW ht) for different solid-state and liquid state sensible Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 8 V.G. Gude / Applied Energy xxx (2014) xxx–xxx Table 2 TES materials and technology applications in desalination and power industries. Technology Molten salt Concrete Phase change material Water/steam Hot water Capacity range (MW h) Annual efficiency Heat transfer fluid Temperature range (°C) Investment cost ($/ kW h) Advantages 500–3000 1–3000 1–3000 1–200 1–3000 98% 98% 98% 90% 98% Synthetic oil Synthetic oil, water/steam water/steam water/steam water 290–390 200–500 Up to 350 Up to 550 50–95 40–60 30–40 40–50 180 2–5 High storage capacity at relatively low cost Experience in industrial applications Well suited for synthetic oil heat transfer fluid Well suited for synthetic oil heat transfer fluid Latent heat storage allows for constant temperature and heat transfer Easily available material Latent heat storage allows for constant temperature at heat transfer Low material requirements Very low cost storage for processes heat below 100 °C Well suited for evaporation/condensation process in direct steam generating collectors Not suitable for pre-heating and superheating in direct steam generating collectors Well suited for evaporation/condensation process in direct steam generating collectors Not suitable for preheating and superheating Disadvantages Desalination and/or power generation application Experience in industrial applications Well suited for preheating and super-heating in direct steam generating collectors Not suited for evaporation/condensation in direct steam generating collectors Recent development Sensible heat storage requires temperature drop at heat transfer Molten salt freezes at 230 °C Cogeneration, power production and desalination Large scale applications for MSF, MED, MVC and RO processes Cogeneration, power production and desalination Very early stage development Cogeneration, power production and desalination Large scale applications for MSF, MED, MVC and RO processes Large scale applications for MSF, MED, MVC and RO processes Experience in industrial applications Sensible heat storage requires temperature drop at heat transfer Not applicable to power generation Cogeneration, power and desalination Solar still, humidification– dehumidification, membrane distillation, other low temperature desalination processes Large scale applications District heating and cooling cogeneration systems with concentrated solar collectors for power generation and water production. In this section, we focus on TES applications in solar stills and solar ponds. Table 3 Solid-state sensible heat storage materials. Storage materials Working temperature (°C) Specific capacity (kW ht/m3 °C) Cost per kg ($/kg) Cost per kW h ($/kW h) Sand–rock minerals Reinforced concrete NaCl Cast steel Silica fire bricks Magnesia fire rocks 200–300 200–400 200–500 200–700 200–700 200–1200 0.61 0.52 0.51 1.30 0.51 0.96 0.15 0.05 0.15 5.00 1.00 2.00 4.2 1.0 1.5 60.0 7.0 6.0 Table 4 Molten salts and high temperature oils. Storage materials Working temperature (°C) Specific capacity (kW ht/m3 °C) Cost per kg ($/kg) Cost per kW h ($/kW h) Mineral oil Synthetic oil Silicone oil Nitrite salts Nitrate salts Carbon salts 200–300 250–350 300–400 250–450 265–565 450–850 0.56 0.58 0.53 0.76 0.83 1.05 0.30 3.00 5.00 1.00 0.5 2.40 4.2 43.0 80.0 12.0 3.7 11.0 heat TES mediums [9,10]. The economics of the energy storage systems vary significantly with different geographical locations, economic packages, and the material costs. Economics of these systems are discussed in the later sections. Sensible heat TES technology has been utilized in numerous solar desalination applications. The applications include solar stills, solar ponds and in 3.2.1. Solar stills In solar stills, the solar radiation is reflected and lost to the ambient. The sinusoidal trend of the solar radiation and the ambient temperatures do not favor freshwater production during nonsunlight hours. During the night time, the solar still productivity could be higher owing to the lower ambient temperatures if the heat source is available during this period. A simple way to increase the heat storage capacity of a solar still is to increase the saline water depth in the still. By increasing the depth (thermal energy storage volume) of the saline water, the heat storage capacity of the device can be increased which would result in continued evaporation during cloudy hours and non-sunlight hours. An optimum water depth should not decrease the water temperature during sunlight hours which may result in lower evaporation rates [97]. Solar stills incorporating thermal energy storage have shown significant improvement on the overall freshwater production rates. To describe this effect a few examples can be discussed. Tabrizi and Sharak [11] investigated a basin solar still integrated with a sandy heat reservoir. The productivity of this configuration was almost twice that of a conventional solar still at the end of a fourteen hour test. Murugavel et al. [12] tested different TES materials such as quartzite rock, red brick, cement concrete, washed stones and iron scraps in a single basin double slope solar still to improve the non-sunlight hour distillate production. Among these materials, 3/4 in. sized quartzite rock was found the most effective, with a productivity of 2.1 kg/m2/d and an enhancement of 6.2% compared to the still with same amount of water but without any Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 V.G. Gude / Applied Energy xxx (2014) xxx–xxx energy storage medium. These studies confirm that TES increases the total freshwater productivity albeit at a slight increase in operational costs in practical applications. A sensible heat TES was employed in a solar (three flat-plate solar collectors) desalination process with three stages of multieffect humidification (MEH)–dehumidification process. The energy storage unit provided the desalination chamber with additional heat for three non-sunlight hours. The experimental results showed that higher seawater mass flow rate through the system (from 0.1 l/s to 0.13 l/s) increase the productivity of the system by 10%. This study concluded that the use of energy storage increased the productivity by 13.5% and the productivity of the experimental system was about three times the productivity of a conventional solar still [13]. 3.2.2. Solar pond A solar pond can be considered as a large solar collector with huge storage volume where the solar energy trapped into the stored brine solution is circulated as a heating medium in the desalination process unit. Solar ponds provide the most convenient and least expensive option for heat storage for daily and seasonal variations [14–16] in solar energy including cloud effects while simultaneously providing for brine management. The solar pond consists of three distinct zones as shown in Fig. 5 [17]. The first zone, located at the top of the pond, contains the low density saltwater mixture. This zone is called the upper convective zone (UCZ) which is the absorption and transmission region. The second zone contains a variation of saltwater densities increasing with depth, is the gradient zone or non-convective zone (NCZ). This zone acts as an insulator to prevent heat from escaping to the UCZ, maintaining higher temperatures at lower zones. The bottom zone is the heat storage zone or lower convective zone (LCZ) with uniform salt density [18]. Sodium chloride (NaCl) is a commonly used storage medium in solar ponds. Magnesium chloride (MgCl2), sodium nitrate (NaNO3), sodium carbonate (Na2CO3), sodium sulfate (Na2SO4), ammonium nitrate (NH4NO3), fertilizer salts like urea (NH2CONH2) satisfy the thermal stability criterion and are considered suitable for solar pond applications [19,20]. Temperature differences between the bottom and top layers of a solar pond are adequate to drive a generator. A heat transfer fluid piped through the bottom layer harvests the heat for direct desalination application. Thermal energy may be circulated through a closed-loop Rankine cycle system to rotate a turbine to generate electricity. The annual collection efficiency of useful thermal energy for desalination is in the order of 10–15% with larger ponds being more efficient than smaller ones. Solar ponds produce relatively low grade thermal energy (less than 100 °C) and are Fig. 5. Solar pond and its convective zones. 9 generally considered suitable for thermal distillation processes. Brine stream generated from the desalination processes can be stored in solar ponds to serve as storage medium. Brine utilization as storage medium in solar ponds provides a convenient method and inexpensive source to maintain solar pond salinity [21]. A study on a single-slope basin solar still integrated with a shallow solar pond (SSP) found that the average productivity and thermal efficiency of this system were higher than those obtained without the SSP by 52.4% and 43.8%, respectively, over a year [22]. Extensive research conducted for more than 20 years at El Paso (Texas, US) solar pond demonstrated the reliability of a salinity-gradient solar pond in desalination application. This research focused on testing various operating conditions for a multi-effect, multistage (MEMS) flash desalination unit. This research provided operation and maintenance procedures of the salinity-gradient solar pond coupled with the desalination [23]. An experimental solar pond with a surface area about 830 m2 and a depth of 2.5 m was studied in Tajoura, Libya [24]. This solar pond was coupled with a 5 m3/d MSF desalination plant. Tahri studied the possibility of combining a MSF desalination plant with a solar pond to recover the waste heat from exhaust gas of a thermal plant [25]. Posnansky [26] presented computer simulations and experimental results on a small sized solar pond for the performance data of the coupled MSF unit [27]. Szacsvay and co-workers developed a desalination system with autoflash MSF unit consisting of a solar pond as the heat source. Performance and layout data were obtained both from computer simulation and experimental results with a small-sized solar pond coupled with desalination subsystem in Switzerland in operation for 9 years. The authors concluded that the cost of distillate could be reduced from $5.48/m3 for small desalination system with a capacity of 15 m3/day to $2.39/m3 for desalination systems with a capacity of 300 m3/day [28]. Solar powered multi effect humidification studied by Müller-Holst et al. used 2 m3 TES increased the production rate to 500 L/day with 38 m2 collector area (about 13 L/m2). Two different humidification units were tested in this study [29]. A multi-objective mixed-integer nonlinear programming model (MINLP) was developed to model an integrated system of reverse osmosis, a Rankine cycle (RC), parabolic trough solar collectors with TES. This study optimized the design and operating conditions with a focus on economic and environmental metrics. A molten-salt thermocline TES was integrated in the system to increase the solar energy utilization. Molten salt was used as the heat transfer fluid (HTF) to transport thermal energy between the TES and the relevant components of the power system (e.g., collector field and RC boiler). The method was applied to a case study of a RO plant coupled with a solar RC and a TES located in Tarragona. This study reported the benefits of TES as improved process performance and efficiency with reduced carbon dioxide emissions up to 55.6% albeit at a slightly higher cost, 14% [30]. 3.2.3. Waste heat utilization and low temperature desalination Thermal energy storage can be used to store the process waste heat to be utilized for desalination purposes as shown in Fig. 4. Low temperature desalination is beneficial from many perspectives which include less resource (thermal) losses, lower operation and maintenance and capital costs. A low temperature desalination process tapping the reject heat from the condenser of a domestic air-conditioning system was studied previously. In this study, evaporation of saline water occurred at near vacuum pressures created by exploiting the principles of local barometric head. The evaporator operates in a temperature range of 40–50 °C with the heat supplied by a TES unit. The energy requirements for the system were less than that required for a MSF distillation process. It was shown that the thermal energy rejected by an absorption refrigeration system (ARS) of cooling capacity of 3.25 kW Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 10 V.G. Gude / Applied Energy xxx (2014) xxx–xxx (0.975 tons of refrigeration) along with an additional energy input of 208 kJ/kg of desalinated water was sufficient to produce desalinated water at an average rate of 4.5 kg/h. This energy consumption was only 60% of a typical MSF distillation process (338 kJ/kg) [31,32]. A TES unit volume of 10 m3 with a solar panel area of 25 m2 was required for this application. In another recent study [33], an integrated process model was developed for a novel application of a sensible TES system for energy conservation and water desalination in power plants. In this configuration, a cold TES was designed to alleviate the negative effects of high ambient temperatures on the performance of on an air-cooled condenser that cools a 500 MW CCPP (combined cycle power plant). The cold TES also provided for the cooling requirements of a final water cooled condenser in a MED unit with a capacity of 950 m3/d. Stack gases from CCPP were used to drive an ARS which maintains the chilled water temperature in a TES tank. A process model integrating CCPP, ARS, TES, and MED was developed to optimize the volume of the TES. Fig. 6 shows the performance profiles of the MED system in that configuration; thermal energy requirements (6a), evaporator (6b) and condenser (6c) areas required for different heat source temperatures, different number of stages, and brine temperatures in the last effect and the condenser temperature differentials (6d). It can be noted that the energy requirements for desalination process increase with thermal energy source temperatures, mainly due to higher heat losses to the ambient. This suggests that low temperature operation and thus low temperature TES system could be more energy-efficient as previously described [7]. In this study, a part of the cooling load from the TES was used to cool the final condenser in the MED system to facilitate additional stages in the MED process. Fig. 6d shows that, the higher the temperature differential available for cooling, the lower will be the condenser and evaporator surface areas required which may result in lower capital costs. Further, as the number of stages increases, thermal energy requirements decrease. Preliminary analysis of this integrated process showed that a cold TES tank volume of 2950 m3 could meet the cooling requirements of ACC and MED in both hot and cold seasons. A potential saving of 2.5% of the power loss in a CCPP was realized on a hot summer day for this TES system along with an estimated desalination capacity of 950–1600 m3/d for top brine temperatures between 100 °C and 70 °C of the MED. 3.2.4. Sizing of TES systems The size of the TES system depends on several factors: (a) the desalination technology (heat load); (b) the heat source whether solar energy or process waste heat (availability and duration); (c) the heat capacity of the storage material; (d) the required storage period; and (e) the expected standby loss (heat losses to ambient). Optimal sizing of the thermal energy storage system is important to maximize the integrated system efficiency (see Eqs. (1)–(4)). Without a supplemental or auxiliary heat source, undersized TES systems are incapable of meeting the energy demands. Oversized TES systems may result in higher capital costs as well as operation and maintenance costs, and can waste energy through standby losses. Sizing the system is even more critical as even optimally sized systems can occupy a large space and require high installation costs. Two important examples on the TES sizing are described below. 3.2.4.1. Thermal energy storage in solar ponds – effect of the thickness of the LCZ. Similar to solar stills (where excess solar energy is stored in the depth of the saline water medium), the lower zone (LCZ) in solar ponds stores the useful heat which it is extracted Fig. 6. Performance profiles for MED desalination system coupled with waste heat from power plant and cold TES. Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 11 V.G. Gude / Applied Energy xxx (2014) xxx–xxx for heating applications. A recent study reported the sizing of salinity gradient solar pond (SGSP) connected to a MED desalination plant of 5 m3/d capacity [27]. It was shown that the thickness of the lower zone affects the amplitude of temperature fluctuations in response to daily solar irradiation (Fig. 7). The temperature variation is in the lower convective zone was between 60 and 80 °C which was found to be suitable for the desalination application. It was reported that a deeper LCZ zone (4 m) results in the smallest surface area required for the solar pond but with increased storage volume and increased capacity. This study also investigated the effects of upper and non-convective zones. The study concluded that the optimum thickness of LCZ zone is of the order of 4 m with 0.3 m thickness for the UCZ and 1.1 m for the NCZ. Solar pond with these specifications will provide working temperatures of the medium suitable for desalination application and to meet the extracted load requirement from the storage zone. Excellent details of heat extraction methods for SGSP and other solar ponds are discussed elsewhere [92,93]. 3.2.4.2. Thermal energy storage in solar desalination. Storage volume is critical to the performance of the TES tank. In desalination application powered by solar collectors, a TES tank is required to mitigate the effects of cloudy hours and non-sunlight hours. In a recent study, the effect of TES tank volume was simulated for a low temperature desalination process (at a capacity of 100 L/d) supported by solar collectors [7]. Temperature profiles over a week of operation for the TES tank at two different volumes (1 m3 and 6 m3) are shown in Fig. 8. As expected, the TES temperature profiles followed the sinusoidal nature of the solar irradiation and the ambient temperature. Smaller TES volume (1 m3) temperatures were more responsive to these variations than higher TES volume (6 m3). Temperatures in smaller TES tank (30–35 °C) were not suitable for desalination application in the non-sunlight hours whereas the temperatures in the higher TES tank (45–48 °C) were adequate for desalination application during non-sunlight hours. Further, simulations on 21 days of continued operation have shown that the storage medium temperature increases with storage time and that higher TES volume increases the quality and quantity of the heat source available for non-sunlight hour operation increases [7]. As a comparison, in desalination systems without energy storage, the desalination unit remains idle during non-sunlight hours requiring a higher desalination production capacity during the sunlight hours. Instead, a TES system can reduce the footprint of the desalination system with reduced downtime. In short, without a TES system, the desalination unit operates in a batch mode but with a TES the desalination can occur in a continuous mode which may result in lower capital and operation and maintenance costs. [7]. In some other novel configurations where adsorption cooling and MED cycles are integrated, the evaporation of freshwater can be achieved at heat source temperatures as low as 35 °C [133]. Finally, to gain optimum storage dynamics and longevity, a few important factors need serious consideration: high energy density in the storage materials; good heat transfer between the heat transfer fluid and the storage medium; mechanical and chemical stability of the storage materials; chemical compatibility between heat transfer fluid, heat exchanger, and storage medium; complete reversibility for a large number of charging/discharging cycles; low cost; and low environmental impact technologies [140]. 3.3. Phase change thermal energy storage Phase change materials (PCMs) store thermal energy as a result of phase change phenomenon, i.e., as heat of fusion (solid–liquid transition), heat of vaporization (liquid–vapor), or heat of solid– solid crystalline phase transformation. Since the heat released during the phase change of a material (solid to liquid; liquid to vapor or gas) is much higher than sensible heat of the materials, PCMs reduce the energy storage unit size significantly. For example, latent heat of water is rather high compared to the sensible heat that can be stored [34]. Some PCMs can absorb heat beyond the phase change. Eq. (5) shows the amount of heat that can be stored in a PCM TES unit. Q PCM ¼ m Z TL T1 Fig. 7. Effect of LCZ depth on the storage medium temperature profiles. Fig. 8. Effect of TES volume on the storage medium temperature profiles. cp ðTÞdT þ mDhjT¼T L þ m Z T2 cp ðTÞdT ð5Þ TL where QPCM is the energy stored in the phase change TES medium with a mass flow rate m, specific heat cp and initial and latent heat temperatures of T1 and TL respectively. Dh is the latent heat, and T2 is the final temperature of the PCM. PCMs can be incorporated into TES units in two ways. PCMs can be suspended in the storage medium in the form of nano- or microscale particles to increase the heat storage capacity. These PCMs flow with the storage medium which remains in liquid state in all stages of TES operation (charging–storing–discharging). The storage system can be a two-tank system with the PCM and storage fluids both contributing to the storage capacity of the system. PCMs can also be used as a stationery medium over which the heat transfer fluid circulates to extract and transfer the heat to the load unit (desalination process). Commonly used PCMs are encapsulated as spheres or other shapes to fill a fixed bed. In this geometry, the heat transfer fluid passes through the void volume of the fixed/ stationery bed. Alternatively, a pipe design such as a heat exchanger within a PCM storage system with charging and discharging cycles can be considered [34]. PCM storage systems have a basic limitation of low power densities caused by the low thermal conductivity of the solid phase of Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 12 V.G. Gude / Applied Energy xxx (2014) xxx–xxx these materials. Metals and metal alloys have conductivities in the range of 100–400 W/m K and do not present this problem when used as PCMs. Salts, however, have conductivities in the range of 0.5 W/m K and limit heat transfer and power density as the storage system discharges. This issue can be addressed by introducing a higher thermal conductivity material while increasing the heat transfer area of the heat exchanger. A large difference in the PCM solid- and liquid-phase densities also presents a design issue and the tank must be sized to accommodate the volume of the lower density phase and also ensure that the heat transfer surfaces are contacted by both solid and liquid phases. Other issues of consideration are the selection of heat transfer design and media since some PCMs exhibit reduced performance after a number of cycles of operation. Additionally, phase change storage materials cost more than the sensible heat storage materials. Sensible heat storage medium usually costs in the range of 0.05–5.00 $/kg, compared to the high cost of latent heat or PCM storage which usually ranges from 4.28 $/kg to 334.00 $/kg. Thermal properties of some PCM storage materials are shown in Table 5 [9,35]. Fig. 9 shows a comparison of the thermal energy storage capacity of different storage materials. It can be noted that most of the phase change and thermochemical storage materials have much higher storage capacities than water which is commonly used as sensible heat TES medium [36]. 3.3.1. PCM energy storage applications in desalination Phase change or latent heat storage mediums are as commonly used as sensible heat storage medium in desalination applications. The latent heat TES systems have many advantages over sensible heat storage systems. The benefits are a larger energy storage Table 5 Phase change thermal energy storage materials. Storage materials Working temperature (°C) Specific density (kg/m3) Latent heat (kJ/kg) Latent heat (MJ/m3) RT100 (paraffin) E117 (inorganic NaNO3 KOH MgCl2 Na2CO3 MgCl2–KCl–NaCl 100 117 307 380 714 854 380 880 1450 2260 2044 2140 2533 2044 124 169 172 149.7 452 275.7 149.7 Unavailable 245 389 306 967 698 306 Fig. 9. Storage capacity vs. temperature for sensible, latent and thermochemical TES. capacity per unit volume, and nearly constant temperature for energy charging and discharging cycles [37]. Table 5 shows the main properties of some PCM energy storage materials. PCM technology is more suited for high temperature desalination technologies such as multi-stage flash (MSF) distillation, multi-effect distillation and thermal vapor compression processes which require very high heat source temperatures (>100 °C) and in the cogeneration plants where the power plants are co-located with the desalination systems. PCMs have been widely used in solar still applications. Additionally, PCMs can be integrated within the still design. Built-in latent heat thermal energy storage (LHTES) was incorporated in a weir-type cascade solar still and the performance was compared with a still without storage. The total productivity of the still without LHTES was slightly higher than the still with LHTES during sunny days. A significant difference in the productivity of stills with and without LHTES was reported. The still without LHTES was preferred for sunny days due to its simplicity and low construction cost and the still with LHTES was suggested for cloudy areas due to its higher productivity [38]. The same group also investigated the effect of water flow rate on internal heat and mass transfer and daily productivity. Higher water flow rates resulted in lower internal heat and mass transfer rates and daily productivities. The maximum and minimum water flow rates resulted in daily productivities of 7.4 and 4.3 kg/m2 d respectively [39]. Since the storage medium has the potential to corrode the process components, Farell and co-workers studied the corrosive effects of salt hydrate PCMs on aluminum and copper heat exchanger materials. The metallographic examination of copper and aluminum samples revealed that aluminum has lower pitting corrosion compared to copper heat exchanger [40]. Thermal cycling tests on a few inorganic and organic PCMs have shown that inorganic PCMs were not found suitable after some cycles while thermal cycling for organic PCMs can be done up to 1000 thermal cycles with gradual change in melting temperature and latent heat of fusion [41]. ElSebaii et al. [42] tested a single basin solar still with PCM as a storage medium in Saudi Arabia. The daily productivity of the still increased significantly with an increase in the mass of PCM due to the increased heat storage. During discharging period of the PCM, the convective heat transfer coefficient from the basin liner (3.3 cm of stearic acid) to water was doubled which increased the evaporative heat transfer coefficient by 27%. This study reported a high daily productivity of 9 (kg/m2 d) with 85.3% energy efficiency compared to a still productivity of 5.0 (kg/m2 d) without the PCM in summer season. The PCM was also found effective for lower masses of basin water in winter season. Another group studied the effect of shape on a PCM heat storage system for rapid heat supply [43]. Four different encapsulated (sphere, cylinder, plate and tube) PCMs were considered in numerical simulations. Among the different shapes considered, the sphere capsules showed the best heat release performance while the tubular capsule with low void fraction was not suitable for the heat release application [43]. Ramasamy and Sivaraman evaluated the performance of a solar still with Paraffin wax as LHTES material. In this study, the still with LHTES material showed higher efficiency (60.11%) compared to a still without LHTES (52.62%). This study also reported a disadvantage of phase change material as corrosion when in direct contact with metal piping or housing [44]. Another study on the weir-type cascade solar still used a heat storage system of paraffin wax (18 kg over 2 cm thickness) beneath the absorber plate. As a result, the daily productivity of the still with PCM was 31% higher than that of the still without PCM [45]. These results are quite different from those reported in [38]. The variations in the still efficiencies suggest that the still efficiencies depend on the water flow rate, solar intensity and the climatic conditions. Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 V.G. Gude / Applied Energy xxx (2014) xxx–xxx 3.4. Thermochemical thermal energy storage Thermochemical storage has the advantage of very high stored energy densities compared to PCM or sensible only thermal storage [33]. Comparing Eqs. (1) and (6), (6) shows an additional enthalpy term associated with the chemical reaction at temperature, TCR. The first term represents the enthalpy associated with heating the reactant(s) from T1 to the chemical reaction temperature, TCR. The third term in the equation accounts for the optional sensible enthalpy for increasing the temperature of the product(s) beyond the reaction temperature, TCR, to temperature, T2. The reaction occurs in two heat exchangers with chemical reaction occurring in forward direction which is endothermic reaction (absorbs solar energy) in one heat exchanger and the backward reaction which is exothermic occurring in the backward direction in another heat exchanger. The reaction occurs at the ambient temperature. the basic principle can be explained as: AB + heat , A + B; using heat a compound AB is broken into components A and B which can be stored separately; bringing A and B together AB is formed and heat is released. The storage capacity is the heat of reaction or free energy of the reaction. A few examples and properties of the thermochemical TES materials that could be utilized in desalination and cogeneration applications are presented in Table 6 [46–50]. Q TCM ¼ m Z T CR T1 cp ðTÞdT þ mDhjT¼T CR þ m Z T2 cp ðTÞdT 13 storage technologies for cooling and high temperature applications are in demonstration stage. While other TES systems for power plants and industrial applications, microencapsulated and slurry PCMs for heating and cooling are in early stage of development and industrialization; thermochemical storage for heating and cooling, waste heat storage and high temperature sensible TES are in very early stage of development. At present, the sensitive thermal energy storage technologies appear to be more practical for the desalination applications. The molten salt and phase change technologies will suit the power generation and desalination cogeneration schemes but further developments are required. In cogeneration schemes, power production with MSF and MED processes may benefit from thermal energy storage whereas low temperature desalination systems may take advantage of the solar and process waste heat sources. For cogeneration schemes using concentrating solar power (CSP) technologies such as parabolic trough, solar tower, Linear Fresnel and Dish-Stirling technologies; the molten salts, high temperature oils, phase change and chemical reaction TES materials will best suit due to the need for high process fluid temperatures (Tables 4–6). Parabolic trough and solar tower require a 2-tank molten salt or PCM storage while the Linear Fresnel and Dish-Stirling methods require short term pressurized storage. For Dish-Stirling technology, the chemical reaction TES is under development [140]. ð6Þ T CR where QTCM is the energy stored in the thermochemical TES medium with a mass flow rate m, specific heat cp and initial and chemical reaction temperatures of T1 and TCR respectively. Dh is the heat released at the chemical reaction, and T2 is the final temperature of the TCM. 3.5. Current status of thermal energy storage technologies Among the available thermal energy storage options, water storage, molten salt for concentrated solar power applications, ice storage for cooling applications, solid sensible storage and PCM for temperature sensitive products have been implemented at commercial scales (Fig. 10). Some underground storage, PCM Table 6 Chemical reaction thermal energy storage materials. Storage materials Working temperature (°C) Enthalpy change during chemical reaction Chemical reaction Iron carbonate Metal hydrides 180 200–300 2.6 GJ/m3 4.0 GJ/m3 Ammonia Hydroxides 400–500 500 67 kJ/mol 3.0 GJ/m3 FeCO3 M FeO + CO2 Metal xH2 M metal yH2 + (x y)H2 NH3 + DH M 1/2 N2 + 3/2 H2 Ca(OH)2 M CaO + H2O 3.6. Economics of thermal energy storage in desalination A comprehensive economic analysis of a desalination system supported by the thermal energy is not yet available. Fig. 11a shows process schematic of a MSF process supported by a solar pond [134]. The MSF process operates in 28 evaporation stages and 3 heat rejection stages with a performance ratio of 10 and a storage zone temperature of 90 °C. The effect of the storage zone temperature and the salt cost on the produced thermal energy cost ($/kW h) is shown in Fig. 11b. It shows that the thermal energy costs are minimum when the salt is available at free of cost which is $0.0138/kW h at 70 °C and increased to $0.0225/kW h when the cost of the salt is $80/ton. Thermal energy cost increased with increasing storage zone temperature due to higher losses to the ambient and lower quantities of useful energy derived. The oversizing the pond, may lead to higher heat losses during summer months which is referred as peak clipping. This affects the heat utilization factor of the desalination plant. The sensitivity analysis of various factors affecting the overall costs showed that the capital costs comprise about 66% of the total freshwater costs. It was concluded in this study that about 1% increase in interest rate for capital costs increases solar pond thermal energy costs by about 13– 15% and desalinated water costs by about 10–13%. The break-even analysis shown in Fig. 11c illustrates the water costs for different fuel costs ($/tonne). Although the cost of product water for the case of 90 °C with a PR of 10 was the lowest, the break even fuel cost was the highest compared to 70 °C with a PR of 6 and 80 °C with a PR of 8. This is because the lowest cost of product water from a Fig. 10. Current status of thermal energy storage technologies. Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 14 V.G. Gude / Applied Energy xxx (2014) xxx–xxx SP/MSF combination system occurs at a higher performance ratio than those of 70 °C and 80 °C, and therefore, the fraction of energy cost for the case of 90 °C represents a smaller portion of the total product water cost when using a dual purpose (power and desalination) plant. More details on the economic analysis can be found elsewhere [134]. In another study where a multi-effect multi-stage flash (MEMS) coupled with a solar pond at the University of Texas, El Paso, the economic analysis of the system has reported the estimated desalination costs of 1.95–2.33 $/kgal (0.52–0.62 $/m3), which depends on the cost of the solar pond liner [135]. Economic analysis on a low temperature desalination process powered by solar collectors augmented by thermal energy storage unit has shown the desalination costs of 1.4 ¢/L or 14 $/m3 and 1.32 ¢/L or 13.2 $/m3 respectively for desalination system with and without thermal energy storage tank system [7]. The following assumptions were made in the economic analysis: (interest rate: 5%, life time: 20 years and plant availability of 90%. When lifetime of the system is considered for 25 years, the costs for the desalinated water are determined as 1.17 ¢/L or 11.7 $/m3 and 1.1 ¢/L or 11.0 $/m3 respectively which are well accepted values for a small scale desalination system. This system was designed to produce 100 L/d of freshwater. This study concluded that the advantages of continuous and uninterrupted water supply provided by TES system regardless of weather conditions (cloudy days and passing clouds) outweighs the small difference in the desalinated water costs. However, the desalination costs for large desalination plants integrated with TES are yet to be reported. It is clear that this will involve high initial capital costs but with continued operation and in the long term operation, the cost factors will become acceptable with payback. 3.7. Electrical energy storage or battery energy storage (BES) for desalination Desalination systems that primarily or partially depend on electrical energy sources are membrane based technologies (Fig. 3) such as reverse osmosis (RO), electrodialysis (ED), capacitive deionization (CD), electrodialysis-reversal (EDR) and membrane distillation (MD). These technologies can be supported by the electrical energy generated by either photovoltaics or wind turbine units. Similar to thermal energy produced by solar thermal collectors, these technologies also suffer from varying source of energy for electricity harvesting and need storage devices to meet the demand–supply logistics and to improve reliability and performance [51]. The solar and wind sources fluctuate very significantly during the course of a day resulting in potential excess generation Fig. 11. Economics of an MSF process integrated with solar pond. Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 V.G. Gude / Applied Energy xxx (2014) xxx–xxx or under supply. Excess electrical energy can be stored in batteries for the desired periods before it is discharged in process applications while the electrical energy that was already stored can be released during the periods of low power generation. Electrical energy storage, in large scale applications, can be divided into three main functional categories such as: (i) power quality: stored energy applied only for seconds or less to ensure continuity of power quality; (ii) bridging power: stored energy applied for seconds to minutes to assure continuity of service when switching from one source of energy to another (uninterrupted power supply, UPS); (iii) energy management: stored energy used to decouple the timing of energy generation and consumption especially in the application of load leveling. Load leveling involves charging of storage in low demand time and use in peak time which enables consumers to minimize the total energy cost [52,53]. The third category is the ideal mechanism for desalination processes. The following sections discuss the energy requirements and energy recovery and energy storage and sizing methods for membrane based desalination processes requiring electricity. 3.7.1. Energy consumption in reverse osmosis (RO) The energy consumed per unit freshwater (kW h/m3) production is defined as specific energy consumption (SEC). This energy is supplied to the pumps in the form of electrical energy which is converted into mechanical energy to provide a pressure impact higher than the osmotic pressure of seawater on the membrane to extract freshwater. The specific energy requirements for RO process can vary between 1 and 10 kW h/m3 which depends on the type of feed water, level of pretreatment, and the membrane module and energy recovery scheme. The highest SEC is observed in the systems without an energy recovery device (ERD). The SEC for seawater desalination by RO process with ERDs such as pelton turbine, pressure or work exchangers varies between 3 and 5 kW h/m3. In a RO process, energy costs may represent up to 50% of the total water production cost [54]. In seawater reverse osmosis (SWRO) plants, the high pressure feed pumps consume 50–75% of the total energy supplied contributing to 35% of the total operating costs [55]. The energy costs ($/m3) are very significant in RO process due to the high pressure requirements (80–100 bar for SWRO and 15–40 bar for BWRO) [56]. Energy consumption in reverse osmosis can be reduced by incorporating energy recovery devices; and by developing high permeability membranes or low energy membranes. More details on energy consumption and recovery schemes can be found elsewhere [57]. 15 Fig. 12. Reverse osmosis system combined with PV or wind or hybrid energy source. Electrical storage sizing involves the following steps: (1) estimation of desalination load requirement; (2) estimation of required PV or wind turbine rating for the desalination load; (3) estimation of the daily power output from the PV array or the wind turbine; (4) estimation of PV array size and wind turbine rotor diameter; and (5) battery storage capacity requirements in A h for the desalination load. 3.7.3. PV–battery system sizing Basis: 100 m3/d of freshwater production from seawater source with specific energy consumption of 5 kW h/m3. The total electrical energy demand for this application will be 500 kW h. The PV array capacity for this application can be calculated using a solar window of 10 h as Pdc ðkWÞ ¼ Load ðkW h=dÞ 500 ¼ ¼ 50 kW Solar window ðh=dÞ 10 For RO process unit, the equivalent AC capacity can be calculated from the efficiency of the PV unit. Considering a typical PV system efficiency (g) of 85% which includes inverter efficiency, dirt and other losses: Pac;STC ¼ Pdc ðkWÞ g The size of the PV array should be more than 1.2 times the desired load to charge the battery while supporting the load [59]. Therefore, the PV array capacity should be adjusted to the equivalent DC load as: Pac;STCðAdjustedÞ ¼ 1:2 Pac;STC ¼ 1:2 60 ¼ 72 kW 3.7.2. PV and wind generated electricity storage The process diagram for PV–wind–battery operated reverse osmosis systems is shown in Fig. 12. A photovoltaic system consists of a PV array (a number of PV modules arranged in series), which converts solar radiation into direct-current (DC) electricity. The system accessories include a charge controller, battery storage, inverter, and other components needed to provide the output electric power suitable to operate the systems coupled with the PV system. PV supported RO and ED processes are the most costcompetitive for small-scale systems where other thermal based technologies are less competitive except those that use direct solar energy which are less common [58]. RO process uses alternating current (AC) for the pumps while the ED process uses DC for electrode stack operation. A DC/AC inverter is required for RO process [58]. But ED can use the energy directly from the PV panels without major modifications. Battery storage systems are used for PV output power to ensure reliable system operation and improved process performance when solar radiation is inadequate. Sizing the battery charging system requires consideration of many physical and operating factors as discussed below. Therefore, for the RO desalination application, 72 kW capacity of PV array with an optimum storage capacity (battery) will be required to support the desalination load for 24 h a day. For a known PV panel efficiency and for an estimated 1 kW/m2 rated PV module, the required surface area of the PV array can be calculated. The efficiency of crystal silicon PV module is around 12.5– 15% [60], which gives PV array area as: Pac;STC ¼ ð1 kW=m2 Þinsolation A g A¼ Pac;STC 72 ¼ 576 m2 ¼ ð1 kW=m2 Þ g 1 0:125 Thus, a PV module area of 576 m2 is adequate to support the desalination load with battery storage. The required battery storage for the desalination application depends on the total energy generated from the PV array. Battery storage capacity can be estimated by multiplying the daily load on battery by number of days it should support the system to provide power for continued operation. Considering a system voltage Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 16 V.G. Gude / Applied Energy xxx (2014) xxx–xxx of 24 V and one day battery storage [61], the ampere hour (A h) rating of the battery bank can be calculated as follows [62]: Daily load in A h @ system voltage ¼ LoadðW h=dayÞ 72 103 ¼ 24 System Voltage ¼ 3000 A h=d The total energy that the battery storage system should store for one day demand at a maximum depth of discharge (MDOD; for lead–acid battery – 80%) would be [61]: Load ðA h=dayÞ No of days 3000 ¼ MDOD 0:80 ¼ 3750 A h Battery storage ¼ The final battery storage capacity for battery (C/20) bank with a discharge rate of 96% at 25 °C would be: Required Battery storage ð25 C; 20 h-rateÞ ¼ ¼ Battery storage Rated capacity 3750 ¼ 3906 A h 0:96 Therefore, 3906 A h of battery storage is required for a 500 kW h/d of load at a system voltage of 24 V with a 72 kW solar PV array system. The above design calculations are based on known PV panel efficiency and electricity generation rate. However, proper design must consider the solar insolation, altitude and the geographical and weather conditions of a particular location. For example, Fig. 13a shows the annual average solar power generation for different locations in the United States. The southern (particularly southwestern states) states have higher solar insolation with higher number of sunny days compared to most of the northern states [136]. These factors need consideration when designing a PV based desalination system and a battery storage unit. The solar power map can be used as a guideline in this design. 3.7.4. Photovoltaic/thermal (PV/T) collectors PVT collectors are hybrid collectors which integrate solar collectors with photovoltaic modules. A thermal energy harvesting system (heat exchanger) is incorporated to harvest the heat absorbed by the photovoltaic module. The circulating fluid cools the surface of the photovoltaic module which increases the electrical efficiency while providing a heat source for desalination application [75]. The energy efficiency of these hybrid systems can be higher than sum of individually operated PV and solar thermal collectors (SC). As a comparison, 1 m2 of the solar thermal collector and 1 m2 of PV would together yield 520 kW h thermal and 72 kW h electrical energy annually; whereas 2 m2 of PVT collector would alone yield 700 kW h thermal and 132 kW h electrical energy [63]. The PVT system also costs about 25% lower than the combined cost of solar collectors and PV panels [64]. The economic and environmental payback times are much shorter for the hybrid collectors compared to PV modules [63,65]. The amount of heat that can be extracted from these collectors depends on the collector type and design, cooling fluid circulation rate and the site specific weather conditions. Two recent studies evaluated the possibility of combining the electricity generation with desalination by using PV/T and concentrated PV/T collectors. One study evaluated modification of photovoltaic modules with a heat exchanger attached to the absorber plate to generate thermal energy that supported a low temperature desalination system operating under natural vacuum for domestic applications. Simulation studies and economic analysis of the integrated process were reported [66]. Another study by Mittelman et al. investigated the performance of multi-effect evaporation desalination system integrated with concentrated PV/T (CPVT) collectors for large scale application and the economics of the process were reported. Details on these studies can be found elsewhere [66,67]. 3.7.5. Wind power battery sizing Similar to PV–battery system sizing, wind power battery storage can be designed for a 100 m3/d of freshwater from seawater source with specific energy consumption of 5 kW h/m3. The total energy demand for this system will be 500 kW h. Wind turbines convert the kinetic energy of the wind into electrical energy and a maximum theoretical conversion of 59.3% was reported which is known as Betz limit (also power coefficient, Cp = 0.593). Available power from wind turbine is expressed [68] as: P¼ 1 C p qAV 3 2 where P is power output from wind turbine (Watts, W), q is the air density (1.225 kg/m3 at 15 °C at sea level), A is area swept by the rotor (m2), and V is the wind speed (m/s) which varies with different geographic conditions. The required wind turbine capacity for the desalination load can be calculated as: Pac ðkWÞ ¼ LoadðkW h=dayÞ 500 ¼ ¼ 25 kW Wind windowðh=dayÞ 20 If an inverter (from AC to DC) is required to support the desalination process, the DC capacity can be calculated using an inverter efficiency which is typically 90%. Pdc;STC ¼ Pac ðkWÞ g ¼ 25 ¼ 27:8 kW 0:9 Similar to PV capacity calculation, 1.2 times the wind turbine capacity will be required for desalination application to charge the batteries while supporting the desalination process. Therefore, the new or adjusted wind turbine capacity for the equivalent DC load will be: Pdc;STCðAdjustedÞ ¼ 1:2 Pdc;STC ¼ 1:2 27:8 ¼ 33:3 kW Considering the DC system voltage as 24 V, load on battery in A h can be calculated for one day as: Daily load in A h @ system voltage ¼ ¼ LoadðW h=dayÞ System Voltage 33:3 103 ¼ 1375 A h=d 24 Considering MDOD of 80% for commonly used lead acid batteries, the battery storage for one day operation is: Load ðA h=dayÞ No of days 1375 1 ¼ MDOD 0:80 ¼ 1719 A h Battery storage ¼ The final required battery storage capacity will be (considering a discharge rate of C/20 batteries of 96% [60]): Final Battery Storage ð25 C; 20 h-rateÞ ¼ ¼ Battery storage Rated capacity 1719 ¼ 1790 A h 0:96 The above calculations illustrate how to size a battery system to match the energy demand in desalination systems powered by photovoltaic or wind energy that require AC or DC power supply. The battery sizing calculation was based on one day of uninterrupted supply but the battery storage will increase proportionately with increase in number of storage days required. The wind energy source Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 V.G. Gude / Applied Energy xxx (2014) xxx–xxx 17 Fig. 13. Solar and wind power maps for United States. will change with seasonal variations, latitude and the location of the desalination unit. A wind power map as shown in Fig. 13b can be used as a guideline to design the desired turbine blade area, height and turbine dimensions. These again depend on the air density which is a function of temperature which in turn affects the wind velocity and finally the energy generated from the swept area by the wind turbine blades. A proper design will consider these factors when sizing the blades and the turbine height and capacity [137]. Fig. 13b shows the annual average wind power for the United States. It can be noted that most of the northern states are rich wind power sources compared to the most southern states [138]. The wind speed depends on the elevation of the location as well as the wind turbine height. For more details on the wind power design and economic analysis, the readers are referred to [139]. 3.7.6. Desalination applications for BES technology Al-Karaghouli et al. reported recent updates on the installed PV–RO and wind–RO desalination plants in 2009 [69]. Zezli et al. reported a few geothermal, wind and PV–RO desalination plants worldwide. These include a geothermal-driven MED plant of 80 m3/d in Kimolos island, a wind energy driven MVC plant (50 m3/d) in Gran Canaria, PV/RO plants of 120, 6 and 1.5 m3/d in Lampedusa island, Brazil and Nevada respectively, and a wind– PV driven RO of (3 m3/d) for seawater desalination in Lavrio, Greece [70]. Table 7 shows the details of PV–RO installations with battery storage details. Some hybrid configurations integrating photovoltaic and wind energies (PV–wind–RO) are also installed. This configuration might have an additional benefit of requiring less battery storage since the wind generation continues round Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 18 V.G. Gude / Applied Energy xxx (2014) xxx–xxx Table 7 PV–wind–RO desalination plants with battery energy storage systems. Capacity (m3/d) Specific energy consumption (kW h/m3) Energy source and storage details Ref PV–RO desalination systems 3.2 16.1–19.7 0.4–0.7 4–5.8 (BWRO) 12 8 53 0.89 (BWRO) 24 1.38–2.77 (BWRO) 5–7.5 2.45 0.8–4.2 18–19 6 3 (BWRO) 10 2.54 36 4.86 0.9 4.3–4.6 50 8 (BWRO) 0.192 2.7 (BWRO) 2 15 8 kWp PV, 46.56 kW h battery 1.2 kWp PV, 4.3 kW h battery 24.5 kWp (pump); 1.22 kWp (control); PV, 132 kW h (pump) and 4.8 kW h (control) battery 19.84 kWp (pump); 0.64 kWp (control equipment) PV, 208 kW h battery 2.59 kWp PV, 60 kW h battery 3.25 kWp PV, 9.6 kW h battery 4.8 kWp PV, 59.52 kW h battery storage 1.1 kWp PV, 9.6 kW h battery 5.6 kWp PV with tracking, 41 kW h battery 125 kWp PV, 160 kVA diesel generator, 1236 kW h battery 0.846 kWp PV, 7.56 kW h battery 10.5 kWp PV, 72 kW h battery 0.136 kWp PV, 0.744 kW h battery 6 kWp PV, 4.8 kW h battery [71,72] [73] [75] [76] [77] [78] [79] [72] [80] [81] [82] [83] [84] [83] PV–wind RO desalination systems 3 – 3.12 16.5 2.2 3.3–5.2 1 3.74 300 4.3 30 – (BWRO) 3.5 kWp PV, 0.6 kWp wind, 36 kW h battery 3.96 kWp PV, 0.9 kWp wind, 44.4 kW h battery 0.846 kWp PV, 1 kWp wind, 7.56 kW h battery 0.6 kWp PV, 0.89 kWp wind, 21 kW h battery 50 kWp PV, 275 kWp wind, grid back up 7.6 kWp PV, 5 kW wind, 5 kVA diesel generator [85] [86,87] [83,87] [80] [88] [89] the clock. Other configurations include integration with the grid power to assure reliable power supply and stable performance [69,71–89]. 3.8. Economics of battery energy storage in desalination Energy storage generally increases the capital costs, thereby affecting the freshwater costs. Evaluation of a few recent comparative studies with and without BES shows varying conclusions. In a study conducted at the Agricultural University of Athens, Greece, unit freshwater costs were reported as 7.8$/m3 without using BES and 8.3$/m3 with a BES system. The specific energy consumption of this system was between 4.3 and 4.6 kW h/m3 by using a Clark pump energy recovery device. However, the freshwater production was higher for the system with BES (0.9 m3) than without BES (0.8 m3) which could be very valuable for remote communities [82]. In another study in Jordan, for a desalination capacity of 0.192 m3/d, the freshwater costs for without BES and with BES were 10 $/m3 and 13 $/m3 respectively [84]. 3.9. Thermoelectric (TE) conversion Similar to PV/thermal collectors, thermoelectric conversion technologies convert the low grade solar or waste heat into high grade thermal energy or prime quality electrical energy both suitable for desalination applications [90,91]. A hybrid desalination scheme comprising of both thermal and membrane based desalination technologies can be considered depending on the desalination needs. Thermoelectric conversion unit can be combined with concentrated solar thermal power plants augmented by thermal energy storage to serve as energy source for desalination systems. Currently, these systems are utilized in very small scale heating and cooling applications. The technology currently suffers from low efficiency and high energy costs indicating scope for research opportunities. 3.10. Geothermal energy sources Geothermal source temperatures vary between wide ranges (50–500 °C) making their application possible for a variety of desalination processes. The advantage with geothermal heat sources is that the heat transfer fluid and the process feed could be derived from the same stream (feed) which is ‘‘water’’. These sources do not require a physical storage unit since they are stored in the aquifers below ground level and can be accessed to meet the process needs. A few demonstrations of geothermal powered desalination systems were reported worldwide and some in power generation [2]. 4. Other energy storage options for desalination The existing energy storage technologies suitable for desalination application can be primarily divided into two categories as thermal and electrical energy storage. Among the thermal energy storage technologies, sensible heat storage technologies are considered mature with many successful demonstrations throughout world in a variety of process application. High temperature sensible TES technologies are still in very early stage of development. Water storage (sensible TES) technology can be considered most simple and economic alternative for low temperature desalination technologies, followed by molten storage and PCMs for high temperature applications including power generation. PCMs involving micro-capsules and nanoparticles still need some demonstration to gain more experience and to overcome the barriers of this technology. For electrical energy storage, the only available option is battery storage with various chemical electrolytes, lithium-ion and lead–acid batteries being the most commonly used. The main foreseeable objection for BES implementation in desalination applications would be their prohibitive costs. With increasing penetration of the renewable energy technologies (PV and wind) into large scale desalination applications, current storage capacity or energy density of the BES technology does not prove to be costcompetitive. Alternate large scale energy storage technologies such as chemical energy storage (CES), compressed air energy storage (CAES), and pumped hydrostatic energy storage (PHES) need further consideration and development. A comparison of the capital costs for different energy storage technologies is shown in Fig. 14 [98]. CES and CAES technologies can be used in power generation schemes to produce electrical energy suitable for desalination applications due their low cost and ability to utilize waste heat from other sources [141–143]. Use of low grade heat for power generation using Rankine cycles is also viable for low cost desalination [144]. Pumped hydrostatic energy, on the other hand, does not Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. Appl Energy (2014), http://dx.doi.org/10.1016/j.apenergy.2014.06.061 V.G. Gude / Applied Energy xxx (2014) xxx–xxx 19 Fig. 14. Capital costs for different energy storage technologies. require energy conversion but provides desalination directly by virtue of the hydrostatic effect. CAES and pumped hydro energy storage technologies have lower capital costs compared to others due to their large scale application feasibility (see Fig. 14). A novel RO desalination process operating under this principle was proposed by Al-Kharabsheh [94]. This system requires a saline water storage tank positioned at a height to create a hydrostatic pressure equivalent to or higher than the osmotic pressure of the saline water from which freshwater is to be produced. The specific energy requirement for this process was 0.85 kW h/m3 which is very close to thermodynamic limit (0.70 kW h/m3). The drawback with this process is the geographical height difference to provide for the required hydrostatic pressure with segregated system components. Ocean communities may utilize the seawater column as a potential hydrostatic head to perform desalination on-site while eliminating the feed collection and brine disposal issues. A submarine RO desalination plant was proposed to take advantage of the hydraulic pressure head of the seawater [95]. In this configuration, the RO unit is submerged and located at the bottom of the seabed and the produced freshwater was supplied to the surface via mechanical pumping. The feed water to the RO process also requires some mechanical pumping which depends on the height of the water column (available hydrostatic pressure) above the RO unit. The energy requirements for this configuration were 1.88 kW h/m3, much less than the conventional RO (5–8 kW h/ m3) process. However, the RO process unit in this configuration has to be located on the seabed which requires a different operational environment and process skill set to operate the process with additional process engineering and safety concerns. Another recent development in this area is a hybrid system that utilizes the wind energy and gravitational potential energy to supply energy required for RO process for sweater desalination. The estimated specific energy consumption was 2.81 kW h/m3 for this process [96]. the combinations of the desalination process and renewable energy technology as discussed earlier. While the benefits of energy storage systems can be realized from energy, environmental and economic perspectives, the energy storage option may not be ideal or economical in all cases since its feasibility depends on the location, type and size of the desalination application and the available renewable energy sources. Whether an energy storage option is feasible for a given application has to be determined by careful evaluation of the aforementioned factors. Apart from the cost issues, other barriers for TES implementation include material properties and stability issues. The design of TES systems needs several considerations and varies considerably for different applications. Future research efforts should focus on the development of storage materials for different temperature ranges, containers and thermal insulation. Phase change and thermochemical storage technologies require additional efforts to improve material reactions along with process parametric optimization for desalination applications. Battery energy storage technologies also suffer from similar issues as thermal energy storage technologies. Significant research efforts need to be devoted to study the electrochemistry of the materials to produce low cost, high density and low environmental impact batteries. Environmental conditions (humidity, dirt, solar insolation and ambient temperature) play an important role in the design and sizing of PV–wind–RO–battery storage systems. With increasing desalination capacity worldwide, these technologies need to be developed in parallel to provide solutions to the energy supply–demand mismatch and to improve the economics of the renewable energy powered desalination systems. Acknowledgements This research was supported by the Office of Research and Economic Development (ORED), Bagley College of Engineering (BCoE), and the Department of Civil and Environmental Engineering (CEE) at Mississippi State University. 5. Summary Energy storage is critical for uninterrupted supply of freshwater sources from desalination technologies that depend on variable energy sources. The type and size of energy storage depends on References [1] Gude VG, Khandan NN, Deng S. Low temperature process to recover impaired waters. Desalination Water Treat 2010;20:281–90. Please cite this article in press as: Gude VG. Energy storage for desalination processes powered by renewable energy and waste heat sources. 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