Geothermal heating is considered to be one of the low-carbon-energy technologies for building heating. Aiming at the problem that the operating cost and investment cost of geothermal heating systems are still high, the conventional geothermal heating system coupled with energy storage for office building heating is studied in this paper. Four operational strategy models of the coupled system are established based on time-of-use electricity prices. A genetic algorithm is used to find the optimal value of each decision variable using minimization of levelized cost of heat (LCOH) as the objective function. The influences of electricity and equipment prices on the optimal values of the decision variables are discussed. Four operation strategies are investigated. If only operating cost is considered in the optimization, comparison shows that the best operation strategy is the one giving high priority to use the energy storage tank for heating during the peak electricity period. However, if the investment cost is further considered in the optimization, the best operation strategy is the one using the energy storage tank for heat load peak-regulating. In addition, based on the minimization of LCOH, an optimal energy storage ratio is found for each scenario, and suitable conditions of using energy storage tank are discussed. The geothermal heating system coupled with energy storage can have a good performance when the peak-valley electricity price difference is higher than CNY 0.566/kW·h (USD 0.0847/kW·h)⁺ or the energy storage tank price is lower than CNY 900/m³ (USD 134.64/m³). The results obtained from this study prove that the cost of geothermal heating systems can be effectively reduced by choosing an optimal operation strategy and using an energy storage device with an optimal energy storage ratio.

As obtained geofluids from enhanced geothermal systems usually have lower temperatures and contain chemicals and impurities, a novel power cycle (NPC) with a unit capacity of several hundred kilowatts has been configured and developed in this study, with particular reference to the geofluid temperature (heat source) ranging from 110 °C to 170 °C. Using a suitable CO₂-based mixture working fluid, a transcritical power cycle was developed. The novelty of the developed power cycle lies in the fact that an increasing-pressure endothermic process was realized in a few-hundred-meters-long downhole heat exchanger (DHE) by making use of gravitational potential energy, which increases the working fluid’s pressure and temperature at the turbine inlet and, hence, increases the cycle’s power output. The increasing-pressure endothermic process in the DHE has a better match with the temperature change of the heat source (geofluid), as does the exothermic process in the condenser with the temperature change of the sink (cooling water), which reduces the heat transfer irreversibility and improves the cycle efficiency. Power cycle performance has been analyzed in terms of the effects of mass fraction of the mixture working fluids, the working fluid’s flowrate and its DHE inlet pressure, geofluid flowrate, and the length of the DHE. Results show that, for a given geofluid’s temperature and mass flowrate, the cycle’s net power output is a strong function of the working-fluid’s flowrate, as well as of its DHE inlet pressure. Too high or too low of a DHE inlet pressure results in a lower power output. When geofluid temperature is 130 °C, the optimum DHE inlet pressure is found to be 11 MPa, corresponding to an optimum working-fluid flowrate of 6.5 kg/s. The longer the DHE, the greater the corresponding working-fluid flowrate and the higher the net power output. For geofluid temperature ranging from 110 °C to 170 °C, the developed NPC has a better thermodynamic performance than the conventional ORC. The advantage of using the developed NPC becomes obvious when geofluid temperature is low. The maximum net power output difference between the NPC and the ORC happens when the geofluid temperature is 130 °C and NPC’s working fluid mass fraction (R32/CO₂) is 0.5/0.5.

In this study, a power cycle (IPEC), with an increasing pressure endothermic process in a downhole heat exchanger (DHE) and a CO₂-based working fluid mixture, was developed for geothermal power generation. The increasing pressure endothermic process, which cannot be achieved in a conventional evaporator on the ground, was realized using the gravitational potential energy in the DHE. The parameters of the power cycle and the structural size of the DHE were optimized simultaneously. Using CO₂-R32 as the working fluid of the IPEC provides the highest net power output. The net power generated with the IPEC was compared with a single-flash (SF) system, a trans-critical CO₂ (t-CO₂) system, and an organic Rankine cycle (ORC) under the same heat source and sink conditions. Six selection maps were generated for choosing the optimum power cycle for electricity production, in which four power generation systems (ORC, t-CO₂, IPEC, and SF) were included, and two DHE diameters (0.155 m and 0.22 m) were investigated. It was found that the IPEC system had more net power output than the other three systems (ORC, t-CO₂, and SF) under the conditions that the geofluid’s mass flow rate was less than 10 kg/s and its temperature was lower than 180 °C.

The thermodynamic analysis of an increasing-pressure endothermic power cycle (IPEPC) integrated with closed-loop geothermal energy extraction (CLGEE) in a geothermal well at a depth from 2 km to 5 km has been carried out in this study. Using CLGEE can avoid some typical problems associated with traditional EGS technology, such as water contamination and seismic-induced risk. Simultaneous optimization has been conducted for the structural parameters of the downhole heat exchanger (DHE), the CO₂ mixture working fluid type, and the IPEPC operating parameters. The CO₂-R32 mixture has been selected as the optimal working fluid for the IPEPC based on the highest net power output obtained. It has been found that, when the DHE length is 4 km, the thermosiphon effect is capable of compensating for 53.8% of the pump power consumption. As long as the DHE inlet pressure is higher than the critical pressure, a lower DHE inlet pressure results in more power production. The power generation performance of the IPEPC has been compared with that of the organic Rankine cycle (ORC), trans-critical carbon dioxide cycle (t-CO₂), and single-flash (SF) systems. The comparison shows that the IPEPC has more net power output than other systems in the case that the DHE length is less than 3 km, along with a DHE outer diameter of 0.155 m. When the DHE outer diameter is increased to 0.22 m, the IPEPC has the highest net power output for the DHE length ranging from 2 km to 5 km. The application scopes obtained in this study for different power generation systems are of engineering-guiding significance for geothermal industries.