Efficient utilization of solar energy and industrial waste heat requires heat storage facilities. Such facilities can be provided in groundwater aquifers. Two aquifer heat storage projects currently being carried out in Sweden are presented.
Efficient utilization of solar energy and industrial waste heat presupposes the availability of short-term and seasonal heat storage facilities. Storage in natural groundwater aquifers is at present being studied in Sweden as one possibility. Various kinds of geological formation can be used, primarily in sedimentary rocks or Quaternary deposits. An ideal storage facility provides natural boundaries against water and heat flow from the facility. Methods are being developed to control this flow. Heat storage in aquifers could be used for internal heat storage in factories or as part of a district heating system. Low temperature storage can be used in combination with heat pumps. In this paper, two projects conserning heat storage in connection with district heating are presented.
A pilot low energy housing project is being planned in Landskrona, in the south of Sweden. The basic concept of the project offers a range of possible energy-saving and energy-collecting installations. One example is a glass-covered street or arcade which functions as a giant solar collector for nine one-family houses, see Fig. 1.
The low temperature heating system (40oC) will be built around heat-pumps which use groundwater (+5 to 20oC) for heat storage and extraction. The groundwater will be pumped up and injected by means of a two-well system, consisting of one "warm" well and one "cold" well. The system is primarily planned for short-term storage, for example, from sunny days to cloudy days. Experiments will be performed to find out if it is also possible to use the system for long-term storage, i.e. from summer to winter. The system is schematically illustrated in Fig. 2. On sunny days, the heat-pumps use warm air from the arcade to heat the cold groundwater being pumped up from the cold well. The temperature of the groundwater is raised from min +50C to max +20°C. It is then injected through the warm well into the aquifer. As a result, the temperature in the arcade will be kept at a comfortable level. On cloudy days, heated water will be taken from the warm well and used for heating. The whole pumping-system is closed and the groundwater is never in contact with the air. The maximum water flow has been calculated to be 200 m3/day. There is a suitable aquifer in the sedimentary rock about 30 m below the ground surface. The rock is more than 500 m thick and consists of Cretaceous limestone with banks of flintstone. The Quaternary layers covering the aquifer are mainly clay and clayey till. The aquifer is fracturous as well as porous and it is very extensive in area. The experimental siteis located near the sea and the natural hydraulic grandient is very small.