Load Management in District Heating Systems

1. I. District heating as a heat source District heating is in most cases based on the availability of low-cost heat, for instance, residual heat o f industrial processes such as the generation of electric power. To utilize this heat, facilities for transport, distribution and delivery must be provided. The greater part of the expenditure on district heating consists of these capital-intensive provisions. For the economic base of district heating systems it is of primary importance that the load factor of the parts o f the system, related to the design power, is as large as possible. Beside large-scale systems, generally based on cheap residual heat from industrial origins, small-scale systems can also be considered. In these systems, heat is generated by a heat pump or a small total energy installation [1]. With these systems the transport over relatively long distances can be abandoned. They can also be completed within a short t ime, consequently making the initial deficit smaller. Notwithstanding the higher product ion costs of heat, these systems can be the most economic solution in a number of cases. In the majori ty of cases, district heating is a public utility delivering heat to more than one, and of ten to a large number of different customers. The transport medium is, as a rule, water which has almost wi thout exception a temperature of < 1 0 0 °C at the delivery point. The characteristics of the means of production (e.g., heat pumps) can induce the applicat ion of considerably lower temperatures. Usually the supply temperature is controlled according to the outside temperature. The heat supply mainly serves the space heating of the connected buildings. According to the current trend of bet ter insulation of buildings, the supply of heat for the heating of tap-water is becoming increasingly important. Since the demand for warm tapwater is almost independent of the season, this heat delivery can contr ibute considerably to the economy of the system. However, the need of a heat supply for tap-water heating means that the water supply temperature cannot decrease below 70°C outside the heating season. This causes relatively large distribution losses, therefore a central heatexchanger, placed in a substation and serving a separately circulated warm tap-water network is a viable alternative.