Desalination

Desalination refers to any of several processes that remove excess salt and other minerals from water. This water then can be used for either human consumption or irrigation. Most of the modern interest in desalination is focused on developing cost-effective ways of providing fresh water for human use in regions where the availability of fresh water is limited such as the world’s hot deserts.

Removing salt from water is a process that has been used for a long time, in the form of distillation. The natural process of evaporation from the surface of the sea forms clouds, which result in rain, is the most widespread distillation process. Boiling salty water and condensing the steam, or even putting a dish of water in the sun and collecting the vapour on a clear cover are both very simple methods of distillation.

Commercial desalination plants have been operating now for decades, using the distillation process. When distilling large quantities of water there are practical problems to be dealt with: firstly, the energy needed to evaporate water is considerable, so the process can be very expensive, unless a cheap source of electricity or heat is available. For instance, running a power station and a desalination plant together (commonly called cogeneration) can be cost effective, since the waste heat from a generator can be used, as well as cheap electricity.

A more recent development, and now more widely used, relies on what is called a semi-permeable membrane to separate salt from water. A synthetic membrane is made, with pores so tiny that water molecules can pass through it, but other molecules, especially salts, cannot. This separation does not happen easily, though, and it requires very high pressures to force the water through the membrane. A natural process, called osmosis, operates in all living cells, to equalize the salt concentration on either side of the membrane. Because the process for desalination is the exact opposite, it is called reverse osmosis, or just RO. A pre-treatment step is required before RO to provide high quality water and reduce membrane fouling. The most common pre-treatment steps include coagulation and filtration or microfiltration.

The seawater desalination concept depicted to the right, uses reverse osmosis where ocean water is brought into the desalination plant, desalinated water is produced and concentrated brine remaining from the process is returned to the sea. In the desalination plant the ocean water is pressured before entering the RO apparatus where the desalinated water is forced out one side of the impermeable membrane and the remaining brine flows back to the ocean.

Large-scale desalination plants typically use large amounts of energy as well as specialized, expensive infrastructure, making the water they produce costly compared to fresh water from rivers or groundwater, which are sources not available in the desert.

As shown above there are sustainable ways of producing the energy required to desalinate ocean water that in turn mitigate the expected effects of global warming. It is an objective of the current invention to use OTEC, offshore wind energy and/or STE to provide the power required to desalinize sufficient ocean water that significant portions of the world’s hot deserts may be irrigated.

The world's largest desalination plant is the Jebel Ali Desalination Plant in the United Arab Emirates. It is a dual-purpose facility that uses multi-stage flash distillation and is capable of producing 300 million cubic meters of water per year.
The largest desalination plant in the United States is the one at Tampa Bay, Florida, which began desalinizing 25 million gallons (95000 m³) of water per day in December 2007. The Tampa Bay plant runs at around 12% the output of the Jebel Ali Desalination Plants.

The International Desalination Association has estimated that worldwide, 13,080 desalination plants produce more than 12 billion gallons of water a day.

Waste heat from the turbines used in CSP plants can be used for the desalination of seawater. The spent steam from the turbines is used to raise the temperature of seawater (via a heat exchanger) causing it to evaporate. The water vapour that comes off is then condensed as fresh water. This is normally done in a succession of stages (multi-stage flash distillation) to improve overall efficiency. A vacuum is applied at all stages to promote evaporation.

The several processes for desalinating ocean water 100 are well known and do not form a part of this inventive concept. It is an objective of the current invention however to use desalinated ocean water 102 both as a means of cooling portions of the world’s hot deserts by evaporations and as a means of irrigating said deserts. It is a further objective of the current invention to provide sufficient electricity from non-carbon and sustainable sources that sufficient ocean water can be desalinated to irrigate a substantial portion of the world’s hot deserts.