Geothermal

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source: Kas als Energiebron

Geothermal heat is the heat generated below the earth’s crust. This heat can be mined when it is present in water conducting porous layers of fluid-bearing rocks, known as geothermal aquifers. The hot water from these aquifers can be pumped up and led through a heat exchanger - the heat can be extracted into a separate water circuit and supplied to greenhouses. The resulting cooled water in the geothermal circuit can be pumped back into the soil at the same depth but in another place - these hot and cold pits have to be well-separated so that the cold pit does not cool down the hot pit.

The viability of using geothermal heat is strongly influenced by the location - in many regions, drilling to depths more than 1 km might be required to access geothermal aquifers, which constitutes a significant economic investment. The depth and diameter of geothermal wells can impact the heating/cooling potential. Deeper wells will be less influenced by seasonal variations close to the earth’s subsurface, and thus have more stable temperatures. Despite this, the heat production of a geothermal circuit is quite consistent across the year, unlike solar or wind power. To match this with the fluctuating demands of greenhouses, both short-term daily buffers and long-term seasonal buffers are required.

The geothermal heat capacity for a well is determined by: the well depth (deeper the well, higher the water temperature i.e. greater the available heat capacity), well depth (wider the well, the more water it can contain) and the spacing between the hot and cold well (larger the spacing, more the interference between the wells).

Contribution to energy balance and resource use of greenhouses:

Geothermal energy is a renewable source of heat for the greenhouse - coupled with heat pumps, it can be used for energy management in the greenhouse by heating and cooling.

The consistent supply of heat due to the constant ground temperature can enable more closing of the greenhouse, conserving CO₂.

Possible steps towards sustainable, energy-efficient greenhouses

Unlike intermittent availability of solar and wind power for renewable heat and electricity supply, geothermal energy has a constant availability across the day and seasons. To use geothermal heat supply sustainably, the stored energy from the wells should be depleted relatively slowly, and the energy in the wells should be renewed at approximately the same rate as it is extracted at.