Characteristics of a store. The various storage techniques can only be compared if their characteristics are clearly defined. The principal characteristics of a store are: - the amount of energy that can be stored (storage capacity); - the temperature during charging and discharging; - energy losses in comparison to the capacity; - investment and maintenance costs; - useful life. The amount of energy that can be held in a store is determined by the volume of the store, the storage medium and the temperature range. The amount of energy that can be extracted from the store, depends on the energy losses and the minimal (or maximal) temperature at which the energy can still be usefully applied. The definition of the temperature difference depends on the type of store. Charge and discharge temperatures are not constant, the average temperatures during charging and discharging are used. The actual volume of a store may be greater than the volume defined here. This particularly applies to aquifer storage in which there are separate warm and cold sides. Storage media which can be used are: water (eg. cavern storage), unsaturated soil (e.g. bore hole storage), or saturated soil (eg. aquifer storage). The thermal efficiency of a store is determined by the ratio between the extracted amount of energy and the amount of energy stored. The energy loss is the difference between amounts of stored and extracted energy. The total energy loss may be divided into two thermodynamic categories: energy that could have been retrieved efficiently at a high temperature; and energy at a low temperature that could only have been retrieved with difficulty. Thus thermal efficiency is determined not only by the energy system but also the temperatures used. Energy loss is caused by conduction and convection losses. The magnitude of conduction losses is determined by the temperature difference between the store and its surroundings, the heat resistance between the store and its surroundings, and the surface area of the store volume. Good thermal insulation, reduction of the store's surface are a volume ratio, and raising (cold store) or lowering (heat store) the storage temperature are ways to restrict conduction losses and thus to increase storage efficiency. Convection losses occur under the influence of flow of the storage medium, including flow as a result of density differences caused by temperature differences. This so-called boundary flow can be suppressed by storing warm water above cold water, or by creatin...
