Chapter 29 - Latent heat and chemical transformation storage

Abstract

The energy associated with a change of phase for a given material can be used to store energy. The phase change may be one of melting or evaporating, or it may be associated with a structural change such as in lattice form or content of crystal-bound water. The storage in a chemical system with two separated components, of which one draws low-temperature heat from the environment and the other absorbs or delivers heat at fairly low or medium temperature, is referred to as a chemical heat pump. The use of high-temperature chemical heat reactions in thermal storage systems is fairly new and to some extent related to attempts to utilize high-temperature waste heat and to improve the performance of steam power plants. The chemical reactions that are used to store the heat allow, in addition, upgrading of heat from a lower temperature level to a higher temperature level, a property that is not associated with phase transition or heat capacity methods. Conventional combustion of fuel is a chemical reaction in which the fuel is combined with an oxidant to form reaction products and surplus heat. This type of chemical reaction is normally irreversible, and there is no easy way that the reverse reaction can be used to store thermal energy. The process of burning fuel is a chemical reaction whereby energy in one form (chemical energy) is transformed into another form (heat) accompanied by an increase in entropy. The change in bond energy for a reversible chemical reaction may be used to store heat.