Performance assessment of a novel combined heating and power system based on transcritical CO_2 power and heat pump cycles using geothermal energy

摘要

The combined heating and power technology is an encouraging measure to ameliorate energy utilization efficiency and meet the ever-growing heating and power demands in buildings. Meanwhile, transcritical CO2 cycles have been demonstrated to be powerful and ambitious competitors for exploitation of low-grade heat sources. In this paper, a novel geothermal driven cogeneration system is conceptually designed by coupling an ejector expansion compression heat pump cycle into a transcritical CO2 Rankine cycle. The system can simultaneously supply power, hot water and hot air. The mathematical model is built from the thermodynamic and economic perspectives under reasonable assumptions. Parametric study is first performed to assess relationships between key physical parameters and system performance. Results indicate that turbine inlet pressure is optimized in research scope for the greatest exergy efficiency. Decreasing turbine outlet pressure and increasing evaporation temperature can raise exergy efficiency but induce negligible influence on thermal efficiency. The optimal turbine outlet pressure and evaporation temperature exist to cause the lowest total product unit cost. Finally, the Non-dominated Sorting Genetic Algorithms-II is employed to execute multi-objective optimization with targeting to maximize system efficiencies and minimize total product unit cost. In the final optimal operating condition, the optimization results suggest 122.88%, 37.17% and 23.53 $/GJ for thermal efficiency, exergy efficiency and total product unit cost, respectively.