Technical state of the art. In its simplest form, the pit store is a buried concrete or steel tank. It is a most reliable storage technique for short term storage of solar heat or heat derived from industrial applications. The high construction cost of the pit store is its major disadvantage. The cost of the pit store is about 30-300 ECU/m3 depending on the volume of the store (Zinko, Hahn, 1994). The rock cavern is a reliable storage technique for combined short term and seasonal heat storage. This type of store would work well for solar heat storage but because of the high construction cost new caverns are not built. Recent research into convective storage has aimed at reducing the construction costs further by using different liners of rubber, and steel. These liners are generally installed in pits with sloping walls, in many cases leakage problems have occurred. There are advanced plans in Sweden to use an old oil storage cavern, in the city of Nynäshamn, for large scale seasonal storage of solar heat. Figure 3.2a) Cavern Storage b) vertical heat exchanger
Technical state of the art. The different types of vertical heat exchanger systems have all proved to be reliable during years of operation. The object of ongoing research is to reduce the cost of the construction and operation. This construction cost can be reduced by: optimizing the design; improving the borehole heat transfer and more efficient construction techniques. System operation can be improved by better system integration. An optimization model, SmartStore, for borehole heat stores in rock and a similar model for clay and soil systems, TecoClay, will be available at the end of 1994. These models minimize the annual storage cost, ie. the annual costs of the investment, heat loss, maintenance and operation. The heat transfer of borehole installations has yet to be fully examined. Natural convection in boreholes could improve the heat transfer considerably, especially important in high-temperature applications. Natural convection increases with the temperature gradient in the borehole and so, borehole pipe installations of materials such as copper should be tested. A more efficient construction technique (less expensive) would be obtained if the drilling equipment was specially designed for heat store drilling. eg. drilling rigs that could drill 4 holes simultaneously. New drilling equipment has been developed, the G-drill, in the Kiruna mine, Sweden, which seems to be very promising for the future. The G-drill, which uses water instead of compressed air, more than doubles the drilling velocity in rock. This technique is however not yet available for deep boreholes. The required system integration studies should include a seasonal heat store combined with a short term store to meet the daily variation of the heat load. This is also important during charging of solar heat, which varies strongly between day and night. With a short term store the seasonal store could be charged during the night also, which means a more efficient operation of the store. With regard to market potential and feasibility, the most important system to develop is a high-temperature store connected to about 50 one-family houses with a low-temperature heating system, eg floor-heating of about 30°C. In such systems heat pumps would not be required.
