A simulation study of hybrid solar-geothermal heat pump system for residential applications using carbon dioxide was carried out under different operating conditions. The system consists of a solar unit (concentric evacuated tube solar collector and heat storage tank) and a CO2 heat pump unit (three double-pipe heat exchangers, electric expansion valve, and compressor). As a result, the differential of pressure ratio between the inlet and the outlet of the compressor increases by 19.9%. and the compressor work increases from 4.5 to 5.3 kW when the operating temperature of the heat pump rises from 40 degrees C to 48 degrees C. Besides, the pressure ratio of the compressor decreases from 3 to 2.5 when the ground temperature increases from 11 degrees C to 19 degrees C. The operating time of the heat pump is reduced by 5 h as the daily solar radiation increases. As the solar radiation increases from 1 to 20 MJ/m(2), the collector heat rises by 48% and the maximum collector heat becomes 47.8 kWh. The heating load increases by 70% as the indoor design temperature increases from 18 degrees C to 26 degrees C. However, the solar fraction is reduced from 11.4% to 5.8% because of the increases of the heating load. (C) 2012 Elsevier Ltd. All rights reserved.