Abstract
Combined district heating (DH) systems have at least one base load heat plant and several peak-shaving heat sources, which can be installed in the primary network or in the heating substations. This article studies the combined DH system with peak-shaving gas-fired boilers in heating substations, where the peak boilers can be connected to the heat exchangers in series or parallel mode. We analyze the thermal balances for these two connection modes and obtain the analytical operational regulation formulas for them. Then we study the operational methods for each connection mode, considering the heat distribution control strategies in both primary and secondary networks. Adopted control strategies are (1) variable temperature (VT) control, (2) variable flowrate (VF) control, and (3) variable temperature and flowrate (VT-VF) control. The combinations of these three strategies in primary and secondary networks are compared in terms of pumping energy consumption. The results indicate that operational strategies should be phased into different periods according to whether the peak heating is needed or not. In addition, the pumping energy consumption for the parallel connection mode is less than that that for the series connection mode with the same operational strategy, and the VF control strategy shows better energy saving potential.
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