A novel system for simultaneous generation of electricity, heating and cooling was proposed and optimized. The core of the system was a heat pump with a flash chamber. To provide the consumed electricity of heat pump and other needs of the building, a gas engine and a Photovoltaic system were used. In addition, a steam cycle was utilized to generate electricity from the exhaust gases of the engine. Considering the thermodynamic analysis, the heat pump had the best efficiency for the heating and cooling modes, when the pressures of flash chamber were 369 and 746 kPa, respectively. Moreover, the steam cycle had the best efficiency at the boiler pressure of 1985 kPa. According to a thermo-economic analysis, the optimum size of the engine was about 90 kW. Finally, a thermo-economic and environmental objective function was defined and the capacity of 80 kW was chosen as the optimum capacity for the gas engine. (C) 2019 Elsevier Ltd. All rights reserved.