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Modelling Space Heating Systems Connected to District Heating in Case of Electric Power Failure

Since district heating (DH) is the dominating heating system in Scandinavia, and fairly common in large parts of Europe, it is of vital interest to study the possibility for buildings connected to DH to receive heat during an electric power failure. Our studies have proved it possible to achieve natural circulation in space heating systems indirectly connected to DH if the DH network operation can be maintained. Natural circulation in modern heating systems has been simulated. The model shows good resemblance with field studies. By using a model the influence of different parameters can be studied, e.g., a changed DH supply temperature and different outdoor conditions. INTRODUCTION A large-scale power failure affects many vital functions of society, e.g. space heating. Obviously, heating systems using electricity (either directly or via a heat pump) cannot operate at all, but systems using a boiler (oil, natural gas, pellets, etc.) will generally not work either since electricity is required for the operation of burners and control equipment. District heating (DH) is common in many countries and it is the dominating heating system in e.g. Denmark, Finland and Sweden. Therefore, it is of vital interest to study what can be expected to happen in buildings connected to DH in case of a power failure, assuming that the production of DH can be maintained. Hydronic radiator systems in modern buildings are fitted with circulation pumps that will stop when electricity supply fails. Consequently, the heating of buildings connected to a DH network are generally assumed to stop functioning in such an event. Nevertheless, our recent studies have proved that in many cases a substantial natural circulation effect caused by an increased water density differential takes place in many cases. The major power failures that have occurred in recent years have led to an increased focus on the possibility of local production and distribution of electric power during a breakdown of the national power grid, socalled ‘island operation’. In order for the production of electricity in combined heat and power (CHP) stations to be maintained, it is necessary that excess heat continues to be removed from the power generating plant. If it is possible to use the DH network as a heat sink, i.e., if buildings connected to the DH network carry on consuming heat energy, provisions for additional cooling of the plant may not be necessary. One must keep in mind that, even if an island power grid were established, the access to power would generally be strictly limited and possibly directed to prioritized users (e.g. hospitals, authorities). For our society, it is essential that building heat supplies be maintained, especially in the event of a power failure of long duration during harsh weather conditions. If evacuations of people from buildings can be avoided, or at least delayed, this will help protect especially elderly people and persons in need of health care from risky exposure to low air temperatures. Because of the concern for these matters from DH utilities as well as from authorities and owners of property, the present study was carried out, with the aim at finding out what could be expected to happen in buildings connected to the DH network, in the event of a severe power failure. The objective of the overall study was to investigate what can be expected to happen in space heating systems connected to DH when a power failure occurs. The study is limited to so-called indirect connection, i.e. that primary (DH) and secondary (house-internal) systems are hydraulically separated by heat exchangers (HEX), which in this context must be considered as a worst-case scenario compared to direct connection. We have found that natural circulation can arise in modern space heating systems; most buildings can achieve a space heat supply corresponding to 40-80 per cent of the amount prior to the interruption, at low outdoor temperature [4]. Natural circulation was used many years ago, before pumps were introduced in space heating systems, and is based on the difference in density between hot and cold water. I.e., hot water from the heat source rises in the vertical supply pipes. In reverse, when the water is cooled off in the radiators, the cold water descends back to the heat source. With natural circulation in a modern system, the flow becomes significantly smaller and, therefore, very hot in the HEX. With a high supply temperature to the radiators and a high temperature drop Eleventh International IBPSA Conference Glasgow, Scotland July 27-30, 2009