Energy consumption and water production cost of conventional and renewable-energy-powered desalination processes

Desalination technologies improve water quality, greatly reduce water shortage problems, and improve quality of life and economic status. Two main technologies are currently used in water desalination: thermal (phase-change) processes and membrane processes. The primary thermal distillation processes include multistage flash distillation (MSF), multi-effect distillation (MED), and vapor compression (VC). The VC process encompasses two types: mechanical (MVC) and thermal (TVC). The common membrane desalination processes include reverse osmosis (RO) and electrodialysis (ED and EDR). Energy cost, operational and maintenance cost, and capital investment are the main contributors to the water production cost of any of these processes. The energy cost is responsible for about 50% of the produced water cost. For thermal distillation processes (MSF, MED, and TVC), two energy forms are required for the operation: (1) low-temperature heat, which represents the main portion of the energy input and is usually supplied to the system by a number of external sources (e.g., fossil fuel, waste energy, nuclear, solar) and (2) electricity, which is used to drive the system’s pumps and other electrical components. For the MVC thermal distillation process, only electricity is needed. For membrane processes (RO and ED), only electricity is required as an energy input. Renewable energy systems such as solar thermal, solar photovoltaic, wind, and geothermal technologies are currently used as energy suppliers for desalination systems. These renewable resources are now a proven technology and remain economically promising for remote regions, where connection to the public electric grid is either not cost effective or feasible, and where water scarcity is severe. As the technologies continue to improve, and as fresh water becomes scarce and fossil fuel energy prices rise, renewable energy desalination becomes more viable economically. The technical features, energy consumption, environmental considerations, and potential of renewable energy use in driving the main desalination processes are reviewed and analyzed in this paper. The current and projected costs of water produced from conventional and renewable-energydriven processes are discussed and compared. & 2013 Elsevier Ltd. All rights reserved. 1. Conventional desalination processes Desalination technologies are categorized as thermal (phasechange) and membrane desalination, and these are further divided into subgroups. The main thermal distillation technologies are multi-stage flash (MSF), multi-effect distillation (MED), vapor compression (VC), whereas the main membrane technologies are reverse osmosis (RO) and electro-dialysis (ED and EDR) . 1.1. Thermal distillation technologies 1.1.1. Multi-stage flash MSF distillation is an energy-intensive process that requires both thermal and electrical energy. The thermal energy is in the form of low-pressure bleed steam (1 to 3 bars) for the feed-brine heating, and medium-pressure steam for the ejectors to generate the required vacuum in different sections of the unit. The electrical energy is required for driving the unit’s various pumps such as recycle, cooling water, distillate product, brine blow down, condensate, and chemical dosing pumps. MSF units typically range from 10,000 to 35,000 m/day and consist of a series of stages, ranging from 4 to 40 each, with successively lower temperature and pressure that cause flash evaporation of the hot ll rights reserved. brine followed by condensation as fresh water. In this process, the feed seawater moves in heat exchangers through the stages and gains some heat that helps to reduce the external thermal energy needed for hot brine and also to condense the water vapor for collection as fresh water in each stage. External heat from fossilfuel boilers, power-plant waste heat, nuclear reactor, renewable energy, or any other heating source is supplied to the intake preheated seawater to raise its temperature to the required top brine temperature of 901 to 110 1C. The heated brine water is then moved through stages that are held at successively lower pressure in which a small amount of water flashes to vapor in each stage and the remaining brine flows to the next stage for further flashing until it is finally discharged. The vapor from each stage is condensed and collected as fresh water [2,3]. Fig. 1 shows a schematic diagram of the MSF unit. Flashing of the steam forms scales and deposits on the tubes, so periodic cleaning and removal is required. MSF is currently the second-most desalination process installed worldwide after the RO process. 1.1.2. Multi-effect distillation The MED process consists of a series of stages (usually from 2 to 16) that are maintained at decreasing levels of pressure.  (2013) 343–356 344 External heat from a fossil-fuel boiler, power-plant waste heat, solar, or other sources is supplied to increase the brine temperature of the first stage to around 70 1C, to be used to evaporate some of the brine inside the stage that is kept at low pressure. The water vapor produced from the stage is transferred inside a tube to the next heating stage for boiling additional seawater, which produces water vapor in a series fashion. MED units are generally built at capacities of 600 to 30,000 m/day and the design is based on two arrangements: the vertical tube in which the seawater boils in a thin film flowing inside the tube and vapor condenses on the heat-transfer tubes, and horizontal tube where the seawater feed is sprayed on the outer surface of the tubes and vapor flows inside the horizontal tubes, where it condenses to produce water. Fig. 2 shows a schematic diagram of the MED unit. The earliest distillation plants used MED, but MSF displaced it due to its lower cost and less tendency to scale . In the past few years, the interest in the MED process has been renewed and appears to be gaining market share. 1.1.3. Mechanical vapor compression Distillation plants using vapor compression rely on the heat generated by the compression of water vapor to evaporate salt water, and two methods are employed—mechanical vapor compression (MVC) and thermo vapor compression (TVC). The feed water enters the VC process through a heat exchanger, and vapor is generated in the evaporator and compresses by mechanical (MVC) or thermal (TVC) means. Compression the vapor raises its temperature by a sufficient amount to serve as the heat source. The concentrated brine is removed from the evaporator vessel by the concentrate reticulating pump. This flow is then split, and a portion is mixed with the incoming feed and the remainder is pumped to the waste. Fig. 3 show both types. MVC use electricity to drive the compressor, whereas in TVC a steam jet creates the lower pressure. These units are usually used in smalland medium-sized applications. MVC capacity ranges between 100 and 3000 m/day, and TVC capacity ranges between 10,000 and 30,000 m/day . 1.2. Membrane desalination technologies 1.2.1.  Reverse osmosis (RO) is a form of pressurized filtration in which the filter is a semi-permeable membrane that allows water, but not salt, to pass through. This yields permeated fresh water and leaves a concentrated solution on the high-pressure side of the membrane. It has four subsystems: (1) pre-treatment, (2) high-pressure pump, (3) membrane, and (4) post-treatment. Feed-water pre-treatment involves filtration, sterilization, and addition of chemicals to prevent scaling and biofouling. The high-pressure pump generates the pressure needed to force the water to pass through the membrane; therefore, the energy needed is electricity to drive the pumps. The pressure needed for desalination ranges from 17 to 27 bars for brackish water and from 55 to 82 bars for seawater. The membranes are designed to yield a permeate water of about 500 ppm and made in a variety of configurations. Several types of membrane are available Fig. 3. Schematic diagram of VC (MVC and TVC) units. Fig. 4. Schematic diagram of RO system. Letter to the Editor / Renewable and Sustainable Energy Reviews 24 (2013) 343–356 345 in the market, with the two most commonly used ones being spiralwound and hollow fine fiber. The post-treatment removes gases such as hydrogen sulfide and adjusts pH. Fig. 4 is a schematic diagram of an RO system. RO is a mature technology that is the most commonly used desalination technique. Its installed capacity ranges between 0.1 m/day (used in marine and household applications) to 395,000 m/day (for commercial applications) [6–12]. Fig. 5. Schematic diagram of ED unit. 1.2.2. Electro-dialysis and electro-dialysis reversal Electrodialysis (ED) is an electrochemical separation process that operates at atmospheric pressure and uses direct electrical current to move salt ions selectively through a membrane, leaving fresh water behind. The ED unit consists of the following components: pretreatment system, membrane stack, low-pressure circulation pump, direct-current power supply (rectifier or photovoltaic system), and post-treatment system. The operational principle of ED is as follows: electrodes (generally constructed from niobium or titanium with a platinum coating) are connected to an outside source of direct current in a container of salt water containing an ionselective membrane connected in parallel to form channels. When brackish water flows between these channels and electricity is charging the electrodes, positive salt ions travel through the cationpermeable membrane toward negative electrodes, and negative salt ions travel through the anion-permeable membrane to the positive electrode, which results in the removal of salinity from the water. This creates alternating channels—a concentrated channel for the brine and a diluted channel for the product f