Heating/cooling: Aquifers

Aquifers are natural formations in the Earth where water accumulates and is stored in permeable rock layers or porous materials underground. These aquifers can be tapped as a water source for various purposes, including drinking water supply, irrigation, and industrial use. Their most promising use for horticulture is thermal energy storage. Hot- and cold-water wells can be drilled into these aquifers at depths of 20 - 100 m underground, from which water is drawn to provide heat or cold, as required, to the greenhouse.

In the summer, when greenhouses have cooling demand - water is withdrawn from the cold well (with a temperature of approx. 7 - 11ºC ) to extract heat from the greenhouse via a heat exchanger - the warmed up water (heated to approx. 18 - 20ºC) is injected into the hot well. In winter, to meet the heating demand, the cycle is reversed - water from the hot well delivers the stored heat to the greenhouse, and on cooling down, it is re-injected into the cold well for summer. For heating, a heat pump is typically used to increase the temperature of the water from the hot-well to a level that is suitable for greenhouse heating. For cooling, the cold-well temperature is sufficient.

The heat source for warming up the water in the aquifers can be either solar thermal heat which is collected in the greenhouse, and transferred via air-to-water heat exchangers, or geothermal heat from the ground surrounding the aquifer. After about one year of storage i.e. creation of the cold and warm wells, the thermal energy stored in the aquifer can be recovered according to the climate needs of the greenhouse.

Contribution to energy balance and resource use of greenhouses:

Aquifers enable storing heat from renewable and sustainable heat sources, and facilitating addition or removal of energy from the greenhouse as desired. Thy also enable more decoupling of the greenhouse from the external environment, which can conserve CO₂ in the greenhouse.

Possible steps towards sustainable, energy-efficient greenhouses

By providing heat/cold on demand, aquifers can balance the intermittent availability of renewable energy sources, and reduce dependence on fossil-fuels for heating. The electricity required for the heat exchanging systems and heat pumps required to exchange heat between the aquifers and the greenhouse can also be sourced with renewable and sustainable electricity sources.