DESIGN OF A SEASONAL THERMAL-ENERGY STORAGE IN THE GROUND

Longterm storage of high quantities of thermal energy is one of the ke y problems for a widespread and successful implementation of solar dis trict heating and for more efficient use of conventional energy source s. Seasonal storage in the ground in the temperature range of up to 90 degrees C seems to be favourable from a technical and economical poin t of view. Preferably duct systems with vertical heat exchangers can b e built in areas without ground water or low dow velocity compared wit h the geometry of the store and the storage period. The thermal perfor mance 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 combin ed heat and moisture transport was simulated on the computer for tempe ratures up to 90 degrees C. This model calculates the effective heat t ransfer coefficient and the heat capacity of the soil depending on wat er content, mineral composition, dry bulk density and shape of soil co mponents. The computer simulation was validated by a number of laborat ory and field experiments. Based on this theoretical work a pilot plan t was designed for seasonal storage of industrial waste heat. A heat a nd power cogeneration unit (174 kW(th)) delivers waste heat during sum mer to the ground storage of about 15 000 m(3) with 140 vertical heat exchangers of 30 m depth. About 418 MWh/a will be charged into the gro und at a temperature level of 80 degrees C, about 266 MWh/a should be extracted at temperatures between 40 degrees C and 70 degrees C and de livered directly to the space heating system. With this design an econ omic calculation gave energy prices of 39 US$/MWh which is of the same order as conventional energy prices. (C) 1997 Elsevier Science Ltd.