Thermodynamic Evaluation and Comparison of Efficiency Performance for a Thermally Driven Combined HP-ORC System using Conventional Expansion Valve or Two-Phase Ejector in the CO2 VCC HP Cycle

Abstract

conventional domestic heating systems consume significant amount of energy generated mostly from fossil fuel. Most of these heating systems reject considerable amount of heat energy to the atmosphere to meet the second law of thermodynamics concept. Combined heat and power cycles designed to convert the wasted heat into useful energy modes. Recently, thermodynamics coupling of heat pump (HP) and ORC cycles for heating application are proposed and investigated. However, many synthetic working fluids used in these thermodynamic cycles have been band or predicted to be phased out due to environment concerns. Natural working fluid such as CO2 used in the vapor compression heat pump cycle is proposed to replace synthetic refrigerant due to its unique thermophysical characteristics and eco-friendly behavior. In this study, thermodynamic evaluation and comparison are conducted between conventional expansion valve and two-phase ejector used in the CO2 VCC HP cycle in the combined system. The steady state results show that combined system using CO2 HP ejector cycle has achieved HP COP and ORC thermal efficiency values higher than the expansion valve cycle by 27.2 and 3.7% respectively. In addition, in terms of fuel to heat efficiency, the combined system with CO2 HP using two phase ejector has achieved a value of around 140% which is higher than the combined system using expansion valve (118.8%). Overall, both combined systems have achieved final water temperature range between 65-79 ^oC which is suitable for various domestic heating application.