When the power on, the compressor start working, it compress the refrigerant from low pressure to high pressure, low temperature to high temperature, then discharge to heat exchange of the solar collector, 

In sunshine time ,the vacuum tube can absorb the sunshine to heat the water of the water tank, then the hot water will translate the heating to refrigerant by the exchange, then help the compressor heat the temperature and increase the pressure of the refrigerant again,so it  just need use the small compressor driver the system running, just use the less power to cool or heat the room,to save more energy.