Shrinking water supplies will unquestionably constrain the development of future power plants. A hybrid system consisting of concentrated solar thermal power and desalination to produce water for a plant, integrated with a combined cycle or conventional steam plant, may be the simple solution.
Desalination is a water treatment process in which seawater is separated into two process streams: a freshwater stream with a low concentration of dissolved solids and a brine or reject stream with a high concentration of dissolved solids. The desalination device, regardless of type, requires energy to operate and do the separation.
The primary commercial technologies for desalination fall into two main categories: thermal desalination and membrane desalination. Thermal desalination systems utilize energy in the form of heat to effect a phase change of the seawater (from liquid to vapor phase) to separate freshwater from brine. Membrane desalination devices use a pump to force water through a semi-permeable membrane, again separating the freshwater from brine. Reverse osmosis (RO) membranes are most commonly used in this type of desalination system.
Thermal Desalination Technology
Thermal processes utilize energy in the form of steam or hot water, extracted from a power plant steam cycle or produced by an auxiliary boiler, to boil the seawater. Vapor released from the boiling seawater is condensed and collected as product water. Water recovery from thermal desalination processes can range from 10% to 50% of the influent seawater, depending on the technology selected.
Multi-stage flash evaporation (MSF) desalination systems have traditionally been preferred because of their reasonable cost, good efficiency, and long experience. However, in the past 20 years, multiple-effect distillation (MED) technology has become the primary thermal desalination option with the development of new designs that permit lower operating temperatures, minimize corrosion and scaling, and increase efficiency. Figure 1 depicts a typical MED system.
1. Multiple-effect distillation system. Courtesy: Bechtel Power
MED systems are designed with multiple evaporator stages, called "effects," which evaporate seawater to produce desalinated water. While in theory, MED systems may be built with an arbitrarily large number of effects, normally, in large desalination plants the number varies between 8 and 16 due to the low temperature drop per effect. In this type of thermal desalination system, seawater is heated to approximately 70C (158F) by steam from an external source.
Boiling occurs in the sequence of effect vessels, each held at a lower pressure than the last. Because the boiling point of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next. Only the first vessel (at the highest pressure) requires an external source of heat. The seawater and external steam source both enter the MED system in the first effect on opposite sides of the heat exchanger tubes. Steam is fed to the inside of the tubes, and seawater is sprayed over the outside of the tubes in a typical effect. The seawater, upon contacting the heat exchanger tubes, begins to boil. The vapor produced is collected and is transported to the second effect on the inside of the second effect’s heat exchanger tubes. The heating steam from the first effect condenses inside the first effect’s heat exchanger tubes and is returned to the power plant or auxiliary boiler as condensate.
Concentrated seawater brine in the first effect is collected in trays and transported across the outside of the heat exchanger tubes in the second effect, where again it boils by heating and the pressure change. This process continues throughout the series of effects. After the final effect, the vaporized water that has collected on the inside of the effect heat exchanger tubes is cooled and condensed with seawater in a final heat exchanger.
Most MED designs today also include a thermal vapor compressor (TVC) to increase system efficiency. A TVC recycles some of the vapor produced in the desalination process to reduce the total amount of steam required to drive the process. A TVC is essentially a steam ejector that entrains low-pressure vapor from a downstream effect with motive steam and discharges the mixture into the first effect.
Though MED technology was originally developed in the 1960s, it did not begin to gain widespread acceptance until the 1990s. Today, MED plants are generally built in units of about 100 m 3 /day up to 36,400 m 3 /day (0.03 to 8 mgd), allowing this design to be utilized in smaller volume applications. Multiple units may be combined in one plant to further increase capacity. The system produces a very high-quality product water from sea or brackish water with a total dissolved solids (TDS) concentration of 25 mg/l or less. The desalinated water produced is so pure that minerals are normally added back to make it suitable for human consumption and use as potable water.
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