Desalination by cogeneration and renewable energy

Desalination by either evaporation (EV) or reverse osmosis (RO) is very energy intensive. However certain methods have been developed for reducing energy usage and greenhouse gas (GHG) emissions, as well as for powering installations where it is not feasible or is impractical to connect to the electricity grid.

Cogeneration

In the Middle East and North Africa, it is now fairly common to build dual-purpose facilities. These plants produce both electricity and potable water. Exhaust heat from the cogeneration power plant is used for the EV desalination unit. Such a combined facility consumes less fuel than would otherwise be needed for two separate facilities.

RO desalination plants consume less energy than EV plants, but can have less potential advantage from cogeneration.

Hybrid plants utilize both EV and RO units. They use heat from a cogeneration plant for the EV unit and for pre-heating water for the RO unit. The RO unit is supplied by electrical power from the cogeneration plant or from the electricity grid. Hybrid plants have the advantage that they can continue to produce freshwater when the cogeneration unit is out of operation.

Wind turbines

Since desalination units are usually situated at the seaside, there is often enough wind for powering a RO unit with wind turbines. That is a good option for remote or isolated areas like small islands that have abundant wind but no connection to the electricity grid. The Canary Institute of Technology (ITC) has developed RO desalination plants that are mechanically powered by wind energy, without electricity as an intermediate energy carrier.

Photovoltaic (PV) systems

PV systems are still more expensive than wind turbines. Nevertheless it can be a good solution for rural areas with an abundance of sun and land but not much wind. In remote areas where there is no electricity grid and little wind, it may be the only viable option. A PV surface of approximately 500m2 is needed to supply 265 individuals each with 150 litres of freshwater per day. The large area of land required makes this approach an impractical method for developed urban areas.

Grid connected or not

If a RO desalination plant is powered by an intermittent renewable energy system, some type of back-up system is needed. Since most desalination plants are designed to operate with continuous energy input, the most obvious back-up is the electricity grid. If no grid is readily available or it is too costly to build a connection, a back up can be provided by a diesel generator or by batteries. A more intelligent system is to adapt the desalination plant to a variable energy input and provide back-up in the form of freshwater storage tanks.