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 exchanger of the solar collector, 
In sunshine time ,the vaccum 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 exchanger, then help the compressor heat the tempeature and increase the pressure of the refigerant 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.