As environmental agencies and governments turn increasingly towards renewable energy technologies in favour of fossil fuel or nuclear based energy, solar-powered desalination has started to become popular. Since it uses a completely renewable source of energy – sunlight – it is seen as a very environmentally friendly method of fresh water generation.
There are two ways to achieve desalination with solar energy: these are known as the direct and indirect methods .
The direct method of solar desalination involves the use of one large unit which is similar to a still. The water is basically distilled using direct heat from the sun, which means that only relatively simple infrastructure is needed.
As the sun heats the sea water, which is fed into the evaporation chamber from an outside source, it begins to evaporate. However, only pure water evaporates, leaving behind any impurities, including salt. Within the chamber there is also an inlet pipe for salt water, an outlet pipe for super-saline water, and some sort of condensation chamber and for the fresh water.
The main downside of direct solar desalination is the relatively low production capacity. Depending on the location and hours of sunlight, the average water production of a direct solar still is only around 3-4 litres per square meter per day. This means that the direct solar still is favored in small desalination plants where the water needs are not very big .
The indirect method of solar desalination involves two steps: the collection of solar energy through photovoltaic cells, or solar panels, and the desalination of water in a conventional desalination plant.
First, energy is collected by an array of solar panels in the same way that any solar panel system works. This energy feeds the desalination plant, which then runs on one of a number of different systems. These systems include Multiple Effect Humidification, Multiple Stage Flash Distillation, Multiple Effect Distillation, Multiple Effect Boiling, Humidification Dehumidification, Reverse Osmosis, and Freeze Effect Distillation.
Indirect solar desalination systems take a lot less space than direct systems, and are able to produce around 200 liters per square meter of photovoltaic panel per day. Since these systems are generally cheaper to produce on a large scale, indirect distillation is the preferred method for larger plants .
Problems with thermal systems
The main problem with thermal desalination systems is the cost of plant construction. Currently, the cost of solar desalination is around four times that of oil-based desalination, which means that without significant government subsidies, solar distillation remains unviable.
A second problem is that the sun only shines, on average, for half of the day. In order to operate with maximum efficiency, desalination plants need to run 24 hours a day. This means that expensive batteries are needed to store the solar energy for the plant to draw from at night, further increasing operating costs.
The third reason that solar desalination plants are not more popular is due to the space that they take. Solar power generation requires a lot of space. In places where land is expensive, it is simply not possible to purchase enough land to construct a profitable plant .
Examples of solar-powered desalination plants
- Although solar desalination can be dated as far back as the ancient Greeks, the first modern plant was built in 1872, when Charles Wilson constructed a small direct plant in Las Salinas, Chile, which produced water for mine workers in the area .
- In the 1960’s and 70’s, the Greek government pushed the construction of a number of solar desalination plants on various islands. These had a production capacity of between 2000 and 8500 cubic meters per day, and provided water to the island communities.
- The first utility scale solar desalination plant is currently under construction in Saudi Arabia, with the aim of creating enough fresh water to supply the city of Al Khafji .