Collecting and storing solar energy for greenhouse heating

Greenhouses consume heating energy but are also powerful solar energy collectors. Based on these characteristics, the “Agrichauff Project” was developed to validate and then commercialize a solar heating system for greenhouses. Researchers at the French Technical Institute of Horticulture ASTREDHOR in collaboration with the Agrithermic and Clauger societies have developed a method to collect warm air in the greenhouse and store its heat into water tanks placed under the growing benches. At night, heat contained in the water is released to the air in the greenhouse by convection through a regulated system that controls fans in order to optimise electricity consumption. This system has been tested for three years and monitored in an experimental greenhouse with climate, heating and electricity consumption sensors. Successive experiments with measurement analysis, evaluations and improvements were realized: warm air collection was improved by mapping air movement with smoke to optimize heat storage in water tanks. Greenhouse air temperature dynamics were adjusted accordingly. In 2016, the optimised system was compared with a control greenhouse at the Paris Horticultural Production Center, with a 14°C night set point temperature. Heating consumption was 49% lower and electricity consumption only 6% higher, for similar air temperatures. The COP exceeded 10 during sunny days. It was shown that the Agrichauff system saved up to 40% energy. From the economic point of view, with an installation cost of less than € 30 m^‑2, the return on investment was calculated between six to ten years, which makes the system feasible for commercial operations.