Solar Thermal

For heat storage at a grid scale and for electricity production from that heat, a much higher temperature is needed. These plants typically cover a large area of land with solar concentrators – parabolic mirrors focusing sunlight onto a small area or volume so that the heat is concentrated on that region. The power plants are called Concentrating Solar Thermal Power plants (CSP).

Commercial power plants exist and work. Wikipedia lists about 68 of them – typically in desert areas. The USA has several, including two of the world’s largest ones at 394MW and 310MW in the Mohave Desert, though the USA has far more land taken up by oil and gas than by solar thermal plants. Morocco now has the biggest one (510MW). Spain has several 150MW and 200MW installations as well as a large number of 50MW – 100MW plants. Israel, India, South Africa and China all have a few. For those countries with large desert areas, this is an excellent way of producing the power they need.

In order for the systems to work efficiently in gathering as much heat as possible during sunlight hours, the mirrors usually need to track the sun to keep the heat focused on the correct point.

Parabolic trough mirror designs concentrate the heat onto pipes carrying a receiving or collecting fluid which is pumped round to carry the heat to a heat engine and storage tank. Some of the heat can be used to produce electricity immediately; some can be stored for later use. The typical heat-carrying fluids are either some form of synthetic oil or molten salt or pressurised steam. The advantage of molten salt is that it is perfectly OK at normal atmospheric pressure and it does not require any pressurised vessels to keep it contained, so it is much safer. The usual molten salt for the solar concentrators is a mixture of sodium nitrate, potassium nitrate and calcium nitrate which will melt at about 130^oC but can be stable up to a little over 500^oC.

Power tower designs concentrate the heat from a huge area, typically about 2 square miles, of mirrors onto a central point at the top of a tower where it heats molten salt to over 500^oC; this is then circulated into an insulated storage tank. The molten salt in the storage tank is, in turn, circulated to produce steam to run generators whenever electricity is needed.

Dish designs have a parabolic mirror, shaped like a satellite dish, concentrating heat onto something at its focal point, which might be a steam engine/turbine or some other engine which can convert heat to spinning motion to run a generator. These are relatively small-scale, and overall, more efficient. However, there is no concept of storage – it will simply produce electricity when the sun is shining and not otherwise.

Fresnel reflectors have long narrow, flat mirrors all directing light and heat at long linear collecting/receiving pipes suspended above them – otherwise they work in the same way as the parabolic trough collectors.

Concentrating Solar Thermal Plants can extract only about 2.75% of the energy of the sunlight falling upon them. The total solar energy reaching the ground on the earth is usually quoted as 1kW per square metre, so 1000m² of land would receive 1MW of solar energy when the sun shines. However, even the most efficient CSP would take nearly 40,000 m² of land to extract 1MW of energy from the incoming sunlight.