Integration of geothermal water into secondary network by absorption-heat-pump-assisted district heating substations

Abstract

Integrating renewable energy into District Heating (DH) systems can significantly save the consumption of fossil energy and receive environmental benefits. The 4th Generation District Heating (4GDH) is the most promising technique for future DH systems by decreased supply temperature and use of low-temperature renewable thermal sources. As current district heating networks are mostly designed with supply temperature around 100°C, high hydraulic resistance and large pressure loss may often occur if decreasing the supply temperature to 30–70°C. This paper has proposed a Composite District Heating Substation (CDHS) with integrated geothermal water into the secondary network by Absorption Heat Pump (AHP). This has been demonstrated to be a feasible and effective solution for using low-temperature geothermal water in DH systems, without decreasing supply temperature of the primary network. The AHP is driven by hot water from the primary network, and the heat of the geothermal water could be used more intense by reducing its return temperature by the AHP and the plate heat exchanger. In this study, both the AHP and the whole substation were simulated by MATLAB, and the results indicated that the heating capacity of AHP increased when increasing the temperature or the mass flow rate of geothermal water, the mass flow rate of the primary network, or when decreasing the return temperature of the secondary network. Additionally, it was found that using temperature regulation with flow changing in stages in the secondary network could receive a higher COP of the AHP and a higher ratio of geothermal energy used in the substation. A case study was carried out as well and its results showed that comparing to conventional DH substations, the CDHS could save 8.0% of the annual cost and reduce 25.6% of the pollutant emissions for a heating season.