Reuss, M.; Beck, M.; Muller, J.P.
April 1997
Solar Energy, vol 59-4, p. 247-257
Longterm storage of high quantities of thermal energy is one of the key problems for a widespread and successful implementation of solar district heating and for more efficient use of conventional energy sources. Seasonal storage in the ground in the temperature range of up to 90°C seems to be favourable from a technical and economical point of view. Preferably duct systems with vertical heat exchangers can be built in areas without ground water or low flow velocity compared with the geometry of the store and the storage period.
The thermal performance of such systems is influenced by the heat and moisture movement in the area surrounding the heat exchangers. Thermal conductivity and heat capacity are strongly dependent on the water content. This combined heat and moisture transport was simulated on the computer for temperatures up to 90°C. This model calculates the effective heat transfer coefficient and the heat capacity of the soil depending on water content, mineral composition, dry bulk density and shape of soil components. The computer simulation was validated by a number of laboratory and field experiments.
Based on this theoretical work a pilot plant was designed for seasonal storage of industrial waste heat. A heat and power cogeneration unit (174 kWth) delivers waste heat during summer to the ground storage of about 15 000 m3 with 140 vertical heat exchangers of 30 m depth. About 418 MWh/a will be charged into the ground at a temperature level of 80°C, about 266 MWh/a should be extracted at temperatures between 40°C and 70°C and delivered directly to the space heating system. With this design an economic calculation gave energy prices of 39 US$/MWh which is of the same order as conventional energy prices.
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