Rather than generating heat on their own, heat pumps use a refrigerant liquid or gas to pull heat out of the source air. In a refrigerator, the cold air is directed into the cold compartment and the warmth is exhausted outside the unit. Conversely, in a heat pump, the air is just distributed in reverse - the colder air is sent outside while the warm air is distributed into the greenhouse space.
Although geothermal heat pumps typically work with higher temperatures and therefore can provide more heat, they require electricity to operate and are significantly more expensive to operate and difficult to repair.
