Research on the heat storage characteristic of deep borehole heat exchangers under intermittent operation mode: Simulation analysis and comparative study

Deep borehole heat exchanger (DBHE) extracts heat from mid-deep geothermal energy through heat transfer process. As DBHE commonly applies coaxial borehole heat exchangers with depth of 2–3 km, the volume reaches more than 30 m3, similar to the small heat tank underground. When the heat pump system turns off, the water in DBHE still extracts heat from the high-temperature ground and stores heat. This kind of heat storage characteristic of DBHE was studied in this paper with simulation analysis. Benefiting from the heat storage characteristic, the average heat extraction rate (Qr) of DBHE under intermittent operation is 8.6%–64.7% higher than Qr under continuous operation, with run-stop ratio decreasing from 24 to 0 to 8–16 per day. Then the influence of operation modes, parameters and thermo-physical properties of DBHE on the heat storage characteristic were studied quantitatively, proving the wide-range regulation ability of Qr. Therefore, the regulation ability of DBHE matches the variation of space heating load very well, and also matches the production rules of clean electric power like photovoltaic and wind power. Hence, the DBHE could further participate in the demand response of electrical power system, and absorb clean electric power.