IMPROVED SUPPLY OF DISTRICT HEAT TO HYDRONIC SPACE HEATING SYSTEMS

Abstract: The studies presented in this thesis concern hydronic space heating systems connected via heat exchangers to a district heating network: referred to as an indirect connection. The objective has been to improve the reliability of district heat supplies and to lower the return temperature from consumer substations. Two of the disadvantages of an indirect connection have been addressed: the thermodynamic loss and the local dependency on electricity involved during the use of a heat exchanger. The results show that the influence of these disadvantages can be reduced by employing several variants of so-called low-flow procedures in hydronic heating systems. Concerning the reliability of district heat supplies, the present work has led to new knowledge regarding the dependence of the district heating technology on electricity. A substantial heat supply can be maintained in numerous buildings in case of an electric power failure since natural circulation can be expected to take place in the heating systems. A turbine-driven circulation pump has the potential to further reduce the dependence on electricity. Concerning a low return temperature from consumer substations, which is a key performance measure for district heating substations, a new method for the control of the heating system has demonstrated a potential of reducing the district heating return temperature. The method involves the control of both the supply temperature and the flow rate in the heating system. The possibility of achieving a low return temperature from different connection schemes of the substation has also been studied.