The CO2 heat Pumps are Air Source Heat Pump systems, so they take energy from the ambient air and transfer it to a water source.
However the CO2 units work under much higher pressure than standard heat pumps, which is why they can achieve extremely high water temperatures of up to 90 degrees.
The CO2 units are extremely efficient with a very small electrical consumption. Due to the extremely high water temperatures the water flow from the units is much slower, so these systems are installed using buffer cylinders to allow the stored temperatures to raise steadily.