Optimized district heating system for combined operation with seasonal heat storage

Abstract

This study optimizes the district heating network side of a high temperature community heating system powered by decentralized solar collectors and seasonal thermal energy storage (STES). Six network configurations are considered which have the potential to improve system performance compared to a base scenario. The base scenario consists of a 2-line network with a fixed supply temperature where the decentralised solar collectors feed in over the heating network. All alternative configurations aim to improve system performance by lowering the temperature of consumed and/or produced heat. Lowering the temperature in the heating network reduces heat losses and decreases heat pump utilization. Lowering the operational temperature of the solar collectors increases their efficiency. The strategies explored by the different configurations include variable supply temperatures, a 4-line network (where the solar collectors do not feed into the heating network), and ways to mitigate temperature constraints imposed by domestic hot water production regulations. In the neighbourhood “”Karwijhof” of Nagele, 24 consumers will make the switch to a solar+storage district heating system. In order to assess their performance, all configurations and the base scenario are modelled in Matlab/Simulink. The system performance is measured in terms of levelised cost of heat (LCOH) and seasonal coefficient of performance (SCOP). They are compared to a scenario where the dwellings are fitted with individual high temperature air to water heat pumps. Making the supply temperature variable (dependent on the ambient temperature) reduces pipeline thermal losses and reduces heat pump utilization. The transition from a 2-line network to a 4-line network where the solar collectors are separately connected to the buffer was found to significantly increase solar collector efficiency. The combination of these two measures reduces the LCOH by 4.5 %. Slightly oversizing the buffer volume and solar area significantly increases the SCOP with small impact on LCOH. When comparing the improved community solar heating system with a scenario where every house is heated with an individual heat pump instead, it is found that the community solar system achieve a 15.7 % lower LCOH while having a SCOP of 4.4 compared to just 2.75 for the heat pump scenario.