Solar Thermal energy

A significant portion of sunlight reaching the earth is heat radiation. The greenhouse acts as a solar collector. However, the amount of solar energy entering the greenhouse depends on the latitude, season, time, and weather conditions. Periods of surplus solar energy and insufficient solar energy typically alternate across the days and across seasons. If harnessed efficiently and stored, this energy can be used to heat the greenhouse during darker and colder periods, reducing the dependence on fossil-fuels as a heat source. The surplus solar heat can be stored for the short term (diurnal) in daily buffers, or for the long term (seasonal) storage, such as in underground aquifers.

Long term storage requires large insulated holding tanks or other means (e.g., underground aquifers) to contain the energy. In some cases both warm and cold water storage systems are used allowing for heating as well as cooling of the greenhouse environment. In that case, the use of heat pumps can prove economical. Heat can also be extracted from the hot greenhouse air via heat exchangers, and transferred to water going into the diurnal buffers or seasonal aquifers.

Some other technologies for converting incoming solar radiation into heat are flat plate collectors (in which water or other fluids flow through a panel oriented toward the sun), which collect heat at lower temperatures, and parabolic reflectors and fresnel lenses, which can concentrate the heat to generate higher temperatures, but are significantly more expensive and often depend on using tracking systems to maintain an optimal orientation.

Contribution to energy balance and resource use of greenhouses:

Solar thermal energy provides a renewable, abundantly available, fossil-fuel-free supply of heat to the greenhouse. However, the impact on the energy and resource use depends significantly on how the thermal energy input to the greenhouse is regulated, e.g. through passive means like shading screens or coatings, or through active means like mechanical cooling.

Large unregulated inputs of solar thermal energy can cause undesirable heat build-up, increased water loss through transpiration, and crop damage. In absence of such thermal management strategies, natural ventilation is used to remove excess thermal energy, causing losses of CO₂ and water.

Possible steps towards sustainable, energy-efficient greenhouses

If thermal energy is managed through the use of heat exchangers and heat pumps, the system can be kept closed for longer, avoiding water and CO₂ loss. Adding diurnal and seasonal heat storage to such systems can resolve the challenge of the fluctuating availability of solar thermal heat, and thus significantly reduce dependence on fossil fuels for heat generation.