Optimal operation of novel hybrid district heating system driven by central and distributed variable speed pumps

Highlights: • A hybrid district heating system was proposed to reduce the system pressure level. • A hydraulic model was developed to simulate a hybrid district heating system. • Two cases were investigated under different operation control strategies. • Optimal pressure control strategy has advantages on saving pump energy. -- Abstract: The district heating system with distributed variable speed pumps can provide substantial energy savings, compared with the district heating system with conventional central circulating pump. However, for large-scale district heating systems, using distributed pumps at all substations leads to high pressures of the network terminals, which will increase the risk of pipe bursting and failure of heat supply. In this paper, the dilemma is overcome by combining the central circulating pump with distributed pumps to construct a novel hybrid district heating system, which can improve energy saving and reduce the network pressures of the district heating system. The optimal pressure control strategy for the new type of district heating network is developed to minimize the total pump power. In order to illustrate the feasibility and effectiveness of the proposed configuration, two cases were investigated regarding an existing district heating system with 1 heat source and 112 substations, including (I) varying the flow rate at all substations with the same relative rate, and (II) varying the flow rate at all substations with different relative rates. Analysis results show that compared with the conventional constant pressure difference control strategy, the hybrid system controlled by the optimal pressure control strategy ensures more stable operation conditions for the distributed pumps and can save 57% pump power when flow rate is 0.3.