Decentralised heat supply in district heating systems : Implications of varying differential pressure

There is a rising interest for the integration of decentralised heat supply in district heating (DH) systems in the form of so-called prosumers, i.e., customers that both can withdraw and supply heat to the grid. The interest comes from a growing interest in local energy supply among owners of property as well as a growing awareness among DH companies about the need to view their customers more like partners rather than just consumers of heat.In a previous study, decentralised solar heat plants in Sweden were mapped out and their performance were evaluated. In general, the performance in terms of delivered heat was at least 20% lower than expected. The main reason for this is deficiencies regarding the feed-in of theheat to the grid, caused by an inability of the control system to handle the variation of the differential pressure between the supply and the return pipe in the DH network. These variations, caused mainly by the rapid load fluctuations caused by consumption of domestic hot water, has so far been overlooked when designing the control system. This paper describes and pins down this problem with support from measurements and simulations of differential pressure.There are different ways to connect decentralised heat supply, where a primary return/supply connection is the most common, implying the heat being added to the DH water before the customer's substation or directly to the DH supply pipe. Although the field study-objects utilise solar energy, it must be emphasised that the results from the project will be of general interest for any type of decentralised heat supply, e.g. surplus heat from local cooling machines or industrial processes. Suggestions for improved control strategies is given in the paper and future work will aim to support them.Keywords: Prosumers, decentralised heat supply, differential pressure, return/supply conne