SOLAR DISTRICT HEATING WITH A COMBINED PIT AND DUCT STORAGE IN THE UNDERGROUND

Solar District Heating with a Combined Pit and Duct Storage in the Underground

Reuß, Manfred; Jens Peter Müller

Bayer. Landesanstalt für Landtechnik, Technical University Munich,

Voettinger Str. 36, D-85354 Freising, Germany,

Fax: ++49-8161-714048

Seasonal thermal energy storage is one of the key problems for a successful implementation of solar district heating and efficient use of conventional energy sources. Heat storage in the underground is favourable from a technical and economical point of view. Within a research project (SOLEG) sponsored by the Bayerische Forschungsstiftung, a new concept for a combined pit and duct storage has been developed. The system considered consists of a cylindrical water tank made of concrete with neither thermal insulation nor liner, but surrounded by a ring of ducts.

Pits or big underground water tanks have a high volumetric heat capacity. Solar heat can be delivered directly to the heating system and they can be charged and discharged easily. These operational advantages have to be paid for by a relatively high investment. Duct storage systems use the heat capacity of the underground, the heat is transferred indirectly via vertical heat exchangers in boreholes. Especially the combination of underground storage with solar collectors requires a big buffer storage capacity to manage the high variations in thermal power from the solar system and to adjust it to the duct storage and the district heating system. This combination of the two systems promises to combine the operational advantages of the water tank with the economical ones of the duct storage. If the temperature of the heat is high it will be charged into the water tank and if it is low into the duct storage. The heat from the tank is directly transferred to the building load whereas the one from the duct store is discharged via an absorption heat pump.

A rural community nearby Freising is developing a new housing area of about 25 low energy buildings. Within a feasibility study different concepts of solar district heating with different underground storage techniques are investigated from a technical and economical point of view. The main objective of the energy concept is 50 % savings of primary energy at least for the whole area. For two major concepts system simulations with TRNSYS were carried out. In the first one, a low temperature concept, the solar heat is delivered to the storage and/or to the low temperature district heating system. In each building a heat pump will raise the temperature level to the required one. In the other concept high temperature storage and district heating is considered with a central backup (heating fuel). For both concepts the calculations were carried out for duct storage and the new hybrid storage technique. Detailed results of the performance of the different concepts regarding solar fraction, storage efficiencies, energy savings as well as a cost estimation will be given in the paper.

References: Reuß, M.; Müller, J.; Röhle, B.; Weckler, M.; Schölkopf, W.: A New Concept of a Hybrid Storage System for Seasonal Thermal Energy Storage in Solar District Heating. – In: Proceedings of 2^nd Geothermal Conference at Richard Stockton College, New Jersey, USA. 16.-17.3.98; Ed.: L. Stiles, Richard Stockton College, Pomona, New Jersey, USA, 1998

Keywords: Solar district heating, seasonal storage, underground thermal energy storage, borehole storage, pit storage.