The borehole heat store in Luleå, Sweden, was built in 1982/83. The store consists of 115,000 m of chrystalline rock. The rock volume is perforated by 120 boreholes to a depth of 65 m of which 3.5 m penetrate the soil. The total active borehole length is 7380 m. The rock temperature varies between 30 to 60 °c during the year. During six months of the summer season 2.0 GWh is charged of which 50% is recovered during the winter season. The extracted heat is utilized for space heating of one of the university buildings. The research project includes evaluation of the construction work and five years of operation. The measurement programme was completed in May 1988. A final report will be published in the autumn of 1988. The storage system is now in its 6th charging period. This paper gives a brief description of the plant. Experiences from the operation period and operation data of the plant are given.
As part of the Swedish energy policy the Swedish council for Building Research (BFR) gives financial support to seasonal heat storage research. The research was inititally concentrated on waste heat storage but the long-term aim Of the research programme is to store solar heat during the summer for space heating in the winter. There are other advantages with seasonal heat storage. A largescale heat storage system reduces the construction cost of the heat production plant since its peak capacity need is reduced by the storage system. There are also environmental advantages since the emissions from the the heat plant to the air is reduced.
The borehole heat store in Lulea was built in 1982/83 for experiments and demonstrations and to get experiences for future heat stores. The store is located close to Lulea University of Technology. The store is owned and operated by the Public Works of Luleå, distributers of heat and electricity to the main part of the city, with a population of 70,000.
The research is conducted by WREL at Lulea University of Technology.
A borehole heat store is a rock volume, perforated by some hundreds of vertical boreholes. The store is usually covered with a soil layer through which the drillings are carried out. The heat is stored in the heated rock volume. The boreholes work as heat exchangers. The rock volume is heated by hot water circulation in the boreholes. When heat is extracted cold water is circulated. The low heat conductivities of soils and rocks make it possible to store the heat with relatively small heat losses in large storage volumes. The heat capacity of granite and gneiss is about 0.6 kWh/m3, °C. Thus a 400e temperature rise of 1000 m3 of rock means that 24,000 kWh is charged. This is the annual heat demand of a one-family house in northern Europe. Figure 1 shows the storage capacity as a function of storage volume. Figure 1. storage capacity as a function of storage volume.