Solar contribution allocation method in heat and power in a solar-aided CHP system by tracing exergy flows

Solar-aided combined heat and power (CHP) system is a promising technology in North and West China. This study modeled the energy, exergy, and solar exergy balances of key components in a solar-aided CHP system to quantify the solar contribution in heat and power. Besides, thermodynamic performance of integration mode I (i.e., using solar heat to fully replace extraction steam) and integration mode II (i.e., using solar heat for direct heating) were compared under design and off-design conditions. On the design condition, the increased power generation were 7.43 and 4.69 MW for integration modes I and II, respectively. Integration mode I performed better boosting the conversion efficiencies of energy, exergy, and solar exergy by 0.37%, 0.47%, and 2.66%. The off-design conditions were caused by heating load demand and direct normal irradiation (DNI). With increasing heating load demand, the conversion efficiencies of solar energy and solar exergy increased for integration mode I and remained unchanged for integration mode II. Only when the heating load demand was higher than 70%, the conversion efficiency of solar exergy for integration mode I was higher than that of integration mode II. With increasing DNI, the conversion efficiencies of solar energy and solar exergy increased for integration modes I and II. Only when DNI was below 900 W/m2, the conversion efficiency of solar exergy in integration mode I was higher than that of integration mode II. The above-mentioned results provided references to choose the proper integration mode according to the environmental conditions.