Integration of Space Heating and Hot Water Supply in Low Temperature District Heating

摘要

District heating makes it possible to provide heat for many consumers in an efficient manner. In particular, district heating based on combined heat and power production is highly efficient. One disadvantage of district heating is that there is a significant heat loss from the pipes to the surrounding ground. In larger networks involving both transmission and distribution systems, the heat loss is most significant from the distribution network. An estimate is that about 80-90 % of the heat loss occurs in the distribution system. In addition, the heat loss is naturally highest from the forward pipes, where the water is at the highest temperature. The heat loss may be lowered by decreasing the temperatures in the network for which reason low temperature networks are proposed as a low loss solution for future district heating. However, the heating demand of the consumers involve both domestic hot water and space heating. Space heating may be provided at low temperature in modern low energy buildings. Domestic hot water, however, needs to reach sufficient temperatures to avoid growth of legionella bacteria. If the network temperature is below the temperature demand, supplementary heating is required by the consumer. In the present paper we study conventional district heating at different temperature levels and compare the energy efficiency, the exergetic efficiency and annual heating cost to solutions that utilize electricity for supplementary heating of domestic hot water in low temperature district heating. Four different supplementary heating solutions are studied: direct electric heating and three heat pump solutions. Heat pumps with R134a and R744 are studied. The results show that conventional solutions at lowest possible temperature have the highest exergetic efficiency of 28 % and lowest annual cost of C 690 (5200 DKK) for a 159 m2 house. The best low temperature system is an R134a heat pump with hot water storage on the district heating side. This system reaches an exergetic efficiency of 25 % with a heat pump COP of 4.0.