Pit storage. In a pit store, heat is stored as hot water. Pit volumes vary, usually from about 100 m3 to a few 10,000 m3. The relatively small storage volumes indicate that the store is used for short-term storage. The top of the store is usually at the ground surface and consequently the rest of the store is surrounded by soil or sometimes rock. The walls of the store are often sloped so that the bottom area is smaller than the top area of the store (figure 3.1). The store is always thermally insulated at the top and most of the time also at the bottom and walls of the store. This type of store was developed to reduce the cost of cylindrical water tanks. The cost is reduced for two reasons: - The construction could be made much weaker since the pressure of the hot water would partly be carried by the surrounding ground. - The heat insulation could be reduced at the bottom and the sides because of the insulating surrounding ground.
Pit storage. 3.2.2. Rock cavern storage In a rock cavern heat store, heat is stored as hot water in a large uninsulated rock cavern, as shown in Figure 3.2. Heat is supplied, in the form of hot water, to the top of the store, while cold water is pumped out from the bottom. When charging the store with heat, cold water from the bottom of the store is pumped to a heat exchanger, heated and returned to the top of the store. Thermal stratification maintains the temperature difference in the store, with hot water at the top and cold water at the bottom. When extracting heat, the flow is reversed, i.e., hot water is extracted from the top of the store and cooled water returned to the bottom. With careful design, the boundary zone between the hot and cold water can be kept quite thin. This zone moves up or down, depending on whether the store is being discharged or charged. Even if the store is almost empty (of heat), the remaining heat can still be extracted at high temperature. The main advantage of this type of store is that the injection/extraction rate is limited only by the pumping capacity. This means that stores of this type can be used for both short term and seasonal heat storage. The rock cavern heat store has two major limitations: - The rock must be of good quality and - Excavation costs are high. The Lyckebo rock cavern (100,000 m3 of water with a temperature up to 90 oC) for seasonal storage of solar heat has been in full operation since 1984. It became evident after five years of operation that the annual heat losses were up to 50% higher than those predicted assuming three-dimensional heat conduction in the surrounding rock. Xxxxxxxx et al. (1994) concluded that the extra heat loss was due to a convective flow in a closed water loop through cracks from the top of the cavern to a transportation tunnel used during the construction and an expansion tunnel from the bottom of the rock cavern. The transportation tunnel winds down from the ground surface to the bottom of the cavern. The distance between the cavern and the tunnel is about 20 m. The Lyckebo experience shows that volume expansion should be undertaken by an independent system hydraulically isolated from the surrounding tunnels. It is also better to lead the transportation tunnel straight away from the cavern in order to avoid close contact between the tunnel and the cavern. This was done in the Avesta project where convective heat loss was found to be small.
