Desalination by any process involves energy. The exact energy amount depends on the temperature and the amount of salt in the water. Higher salt concentrations require more energy. Thus, energy costs go up with each recovery reiteration. As a bit of water is removed and purified, the rest of the water has a higher salt concentration and thus, more energy is needed to separate the next unit of water.The figure shows the energy needed to desalinate water with higher and higher percentages of salt. This is from the DWPR Report #78 "Improving the Thermodynamic and Economic Efficiencies of Desalination Plants: Minimum Work Required for Desalination and Case Studies of Four Working Plants."
Figure 1. Energy require to convert saline water to fresh water for a range of salinity and recovery levels. 1 kJ/kg (1.05 kWh/kgal). Seawater is 3.5% dissolved salts by weight.
The method used to extract fresh water from saline does not matter. The "thermodynamic minimum energy" for desalination (the energy required for separating an infinitely small amount of fresh water from salt water) is the same for all processes. Energy differences between processes are due to the difference in efficiency of the process. For instance in distillation, all the water must be heated to the boiling point before any distillate can be produced. Efficiency in improved by using the bottoms water to help heat more water through heat exchangers. Still, at the end of the system a volume of hot water must be cooled before discharge back to the source.
In reverse osmosis, all the water must be pressurized to a level greater than the osmotic potential before any water will permeate through the membrane. The concentrate is still under pressure. Efficiency of the process is increased by putting the pressurized concentrate to work to pressurize the feed water or to generate power to run the high pressure pump.
The most efficient seawater reverse osmosis system tested so far is the Affordable Desalination Collaboration's pilot plant tested in Port Hueneme, Clifornia at the Naval Facilities Engineering Service Center Seawater Desalination Test Facility. For the process alone ADC managed to desalt seawater using only 6 kWh/kgal. Fresh water recovery was 42%, temperature 15°C (59°F), and operating pressure of 675 lb/in2. From Figure 1, the theoretical minimum energy under these conditions is about 2.1 kWh/kgal.
Thermal processes are not as efficient as reverse osmosis because they involve a phase change in the state of water – converting water to steam requires a certain amount of energy depending on the concentration of salt, the pressure, and temperature. The energy to vaporize water that is already at 100 °C is 40.65 kJ/mol or 2376 kWh/kgal. Some of this energy can be recovered with heat exchangers when the vapor condenses and releases the same amount of heat. The efficiency of heat exchange depends on the type and condition of the contacting surfaces.
Energy Efficiency for Desalination
Most of the energy for desalination is for pressurizing water for membrane desalination, and heating or freezing water for thermal methods. As discussed above, once the maximum economic portion of water is recovery, energy remains in the concentrate left behind in the form of pressure, heat, or cold. A pressure exchanger, such as produced by Energy Recovery, Inc., transfers pressure to another stream of water. In a similar manner, heat exchangers transfer temperature.
Figure 2. State of energy efficiency. Projects listed in Figure 2:
|9081||Optimization of Energy Recovery Seawater Reverse Osmosis Desalination Systems||2005||2009||Steve Dundorf|
|Report Title||Performing Agency||Date||Report #||See also|
|Reduced Energy Consumption Evaporator for Use in Desalting Impaired Waters||Water Reuse Technology; Alamo CA||Dec-94||11||Thermal|
|Desalination Research and Development Workshop Report||National Water Research Institute; Fountain Valley, CA; USBR; Denver, CO||Jan-01||64||Decision Support|
|Improving the Thermodynamic and Economic Efficiencies of Desalination Plants: Minimum Work Required for Desalination and Case Studies of Four Working Plants||University of Nevada; Reno, NV||Nov-03||78||Ancillary Improvements,
|Photovoltaic Reverse Osmosis Desalination System||ITN Energy Systems, Inc.; Littleton, CO||May-04||104|