A geothermal heat pump pulls heat from within the soil, generated by the Earth itself, this temperature stays steady throughout the year. Atmospheric temperatures can overtake the natural warmth of soil in some regions and seasons, but just a few feet below the surface, the Earth maintains a near-constant temperature, in contrast to the summer and winter extremes of the ambient air above ground.
While the sun or summer heat isn't warming your greenhouse, thermal batteries or a heat sink can be used to transfer thermal energy when the ambient temperature cools. This dispersal of thermal energy is known as the 'flywheel effect,' in which excess warmth is released into the cooler air as it comes into contact with the warmed batteries. Sunwarmed barrels of water or dark red brick can absorb and release heat into your greenhouse while the ambient temperature gets colder.
