Thermal storage is the concept of storing energy in form of heat. There are different approaches to storing large amounts of heat, one of the most promising is the concept of phase changing materials (PCM). These materials are capable of holding large energy amounts when changing from one phase into another. We know this effect from a simple ice cube. If you heat the cube up it stays at 0° C (273° K) until completely molten. The amount of energy used to melt an ice cube is equivalent to the amount you need to heat water to 80° C (353°K). The same effect, but with a higher energy density, holds the mol-ten salt storage concept, containing a combination of sodium and potassium, which is cur-rently the first choice in several solar projects. However, research is being done to find other thermal storage concepts using materials that are cheaper and easier to handle.18
Because heat is the lowest form of energy, it hasn’t been really considered for storing energy because the most common way to produce electricity is to power steam turbines; it would be just too inefficient to transform already produced electricity through steam once again into heat just to power a steam turbine again. However, the concept is relevant for balancing solar thermal power plants, as they use the heat of the sun during the day to sim-ultaneously produce electricity and “fill up” the thermal storage tanks, which allows them to generate electricity at night.