Heat storage

The requirement for heat storage arises due to daily and seasonal imbalances between heat demand and heat requirement across the year: the greenhouse acts as a solar collector during those hours of the day and those months of the year which receive significant sunlight; whereas, the heat demand is highest during night-time hours and during winters. Thus, storage systems are required which can collect heat during the surplus hours and supply it to the greenhouse during the deficit hours. The main considerations in such a system are:

• the assembly for collection and release of heat, such as heat pumps, heat exchangers, and circulation pipes, and

• the storage medium used for the heat

Daily storage is typically implemented by using large, well-insulated buffers which can store the hot water generated by the boiler which is used to enrich CO₂ during the day. Other possibilities to heat up water for the buffer can be - capturing heat from warm greenhouse air using air-to-water heat exchangers, or using solar collectors.

For seasonal storage, the heat has to be retained for a much longer period, hence storage structures such as underground aquifers are required. There are possibilities for storage of heat in rocks or in phase-change materials, but these need to be optimized a lot before widespread commercial implementation.

Heat can be stored either as sensible heat, as latent heat, or via thermo-chemical means. In sensible heat storage systems, heat ‘charges’ i.e. heats up the medium (water, rock, sand, clay). This heat can be extracted from/fed into the medium using heat transfer fluid and heat exchangers. Latent heat based systems have storage media which can change phase e.g from solid to liquid, and in this process, absorb/release the latent heat associated with the phase change to the surroundings or to heat exchangers. Thermochemical heat storage involves using chemical reactions which absorb/release heat in order to store and release the associated heat energy.

The parameters to be considered for such systems applied to greenhouses are: the heat capacity of the medium (which will affect the required storage volume), the temperature at which heat is stored and released (especially for latent and thermo-chemical heat storage), the efficiency and stability of the storage-discharge cycle, and safety and toxicity considerations.

Contribution to energy balance and resource use of greenhouses:

By addressing the imbalances between availability and demand of heat, seasonal storage makes it possible to supply heat to greenhouses through sustainable sources like solar or geothermal power, in place of fossil fuels.

Use of seasonal storage for climate conditioning in the greenhouse can enable the closing of the greenhouse, preventing CO₂ loss to the outside air.

Implementing daily and seasonal storage will involve the use of heat exchangers and heat pumps in order to transfer the heat to the storage medium (e.g. water), which would increase the electricity consumption.

Possible steps towards sustainable, energy-efficient greenhouses

By providing a consistent supply of heat as required, heat storage systems reduce dependence on fossil energy for heat supply. To further reduce the carbon footprint, the energy to power the heat exchangers and pumps can be sourced through renewable sources.

Implementing seasonal storage systems, such as water in underground aquifers can be limited by geographical constraints. Also a large volume of the storage medium can be required, especially in sensible heat storage systems. Exploring alternate high-storage density media which are independent of geographical constraints can make the adaptation of seasonal storage easier and more widespread.