Thermal energy storage involves the storage of heat in one of three forms; Sensible heat, Latent heat and thermo-chemical heat storage.
Sensible heat storage is the most common method and has been employed for hundreds of years as hot water tanks. Sensible heat storage simply means changing the temperature of storage medium. The storage medium is most commonly water but rock, sand, clay and earth can also all be used.
Latent heat energy storage involves the storage of energy in Phase-Change Materials (PCM’s). Thermal energy is stored and released with changes in the materials phase. The most common phase change to exploit is the solid-liquid transition, as the liquid –gas transition is impractical and solid-solid (crystalline structure) transitions usually have too low an energy density to be useful. When a PCM is heated initially it behaves like sensible heat energy storage and the materials temperature is increased. However, once the transition temperature is reached the material will continue to absorb heat at a constant temperature while it changes state. This heat absorbed at constant temperature is known as the latent heat of the transition. To retrieve the energy the PCM can be changed back from the liquid to the solid phase and the energy stored as latent heat is released.
Thermochemical heat energy storage involves storing heat energy in chemical bonds. A reversible chemical reaction which absorbs heat is used to absorb the heat energy that is to be stored. This reaction can then be reversed to release the stored heat. The most common reactions used for this process is the hydration of salts. The energy storage is based on the release of the heat of hydration. Hence, a salt hydrate storage system is charged by the endothermic thermal dehydration of the respective higher hydrated salt.
Characteristics and applications
Hot water tanks are probably one of the best known thermal energy storage (TES) technologies and are a fully commercial technology. They are already widely used at a building scale in combination with electrical or solar thermal water heating systems to store water over a number of hours from when it is heated (e.g. at night when electricity is cheaper or during the day, when the sun is shining) until it is needed. In the future it is anticipated that larger versions will be combined with heat pumps to increase coefficients of performance. On a larger scale hot water tanks have been used in district heating systems. One of the largest of these schemes is in Pimlico in London, consisting of a 3.4 MWth combined heat and power (CHP) plant and an accumulator that can store 2,500m3 of water at just less than 100oC. The other major form of heat storage is electrical storage heaters. These use off-peak electricity to store heat in high density bricks, which is released throughout the course of the day.
Larger storage volumes and longer storage periods (up to months) can be achieved by storing hot (or cold) water underground. Naturally occurring aquifers (e.g. a sand, sandstone, or chalk layer) are most frequently used. Groundwater is extracted from the layer and then re-injected at a different temperature level at a separate location nearby. Boreholes can also be used to provide the heat storage and exchange. In Drake Landing Solar Community, Alberta Canada, the homes get 97% of their year-round heat from a district heat system (in operation since 2007) that is supplied by solar heat from solar-thermal panels on the garage roofs. This is enabled by inter-seasonal heat storage in a large mass of underground rock. The thermal exchange occurs via 144 boreholes, drilled 37 metres into the earth.
Sensible heat energy storage is advantageous due to its low cost but the losses are often increased significantly at high energy density (high temperature) and materials that can tolerate repeated cycling up to very high temperatures are rare. Latent heat energy storage in PCM’s offer an increase in energy density and a decrease in losses. Due to this higher energy density PCM’s are the subject of much research, especially concerned with the integration of PCM’s into building materials for both heat and coolness storage (see Baetensa, et al., 2010 ). PCM’s can be split into organic and inorganic. Most organic PCMs are non-corrosive and chemically stable, exhibit little or no sub-cooling, are compatible with most building materials and have a high latent heat per unit weight and low vapour pressure. Their disadvantages are low thermal conductivity, high changes in volume on phase change and flammability. Inorganic compounds have a high latent heat per unit volume and high thermal conductivity and are non-flammable and low in cost in comparison to organic compounds. However, they are corrosive to most metals and suffer from decomposition and subcooling, which can affect their phase change properties.
Thermochemical energy storage is a newer approach that offers even higher energy density than PCM’s, but is less advanced. In principle these systems can achieve very high efficiency with negligible loss over time.