Improving the district heating operation by innovative layout and control strategy of the hot water storage tank

Abstract

In the district heating (DH) systems, the thermal storage tank is widely used for domestic hot water (DHW) preparation to shave the peak load. Compared to the space heating, the domestic hot water demand exists for all seasons, and the variation of which is more dramatic within short time span. Therefore, the well-controlled storage tank is of great help to stabilize the district heating operation and to improve the overall performance. This study develops an innovative control strategy of the local DHW tank with the heat delivered by DH. According to the dynamic DHW load profile, the controller can switch to the corresponding charging mode to obtain the optimal performance without violating any requirements. A smart-metered multi-storey building in Denmark is selected as the reference case for DHW loop renovation. The proposed control strategy is applied in two new system layouts with the external heat exchanger and the micro heat pump by numerical models. The stratifications of the storage tank, the stability of the DH operation, and the DH return temperatures of the three scenarios are investigated as the key performances. The results show that, to cover the same DHW demand, the total energy consumption of the three scenarios show little difference. However, the proposed charging method of the DHW tank can limit the peak load for DH. Moreover, the resulting DH return temperature can be lowered by 4.9–16.4 °C and the DH water flow can be saved by 8%–28% compared to the reference case.