Light and Thermal Energy

Solar energy will rapidly expand in 2014, with analysts predicting that 44.5 gigawatts will be added globally (Bloomberg New Energy Finance has revealed) on the next few years.

The molten salt typically used in renewable energy applications is a mixture of 60% sodium nitrate and 40% potassium nitrate (aka saltpeter). This mixture melts at 230 degrees Celcius into a liquid which can be pumped around a concentrated solar system, and passed through heat exchangers (to collect heat from sunlight, and turn water into steam) etc.

Molten salt is non-toxic and also non-flammable so it is very safe to use, and it flows much as water does. Common table salt (sodium chloride NaCl) is not used as its melting point is over 800 degrees Celcius. The molten salt acts effectively as a solar heat battery. By keeping the hot molten salt in a well insulated tank it is possible to generate solar electricity at any time of night and day, and to store solar energy for up to a week after it was collected.

When water freezes (i.e. goes from liquid to solid state) it expands. That is why water pipes can burst during the winter. Fortunately molten salt contracts when it returns to the solid state and so will not damage any pipework or equipment if it cools down.

Pictured above is a schematic showing how molten salt is used with a solar power tower as a heat store .In this example, the molten salt is kept at least 550F (290C) in the cold storage tank, pumped up through the receiver at the top of the tower where it is heated to over 1000F (537C), and pumped to the hot storage tank. The hot molten salt is then passed through a steam generator to create steam which turns a turbine to generate electricity. Long after the sun has set, the stored hot molten salt can be used to generate electricity.