Survey and evaluation of heating technologies for worldwide agricultural greenhouse applications

16th November 2023

Heating system

• A greenhouse heating system is used to increase the thermal energy storage inside the greenhouse during the day or to transfer excess heat from inside the greenhouse to the heat storage area.

• This heat is recovered at night to satisfy the heating needs of the greenhouse.

• In a composite system, same system is used for heating the greenhouse in winter and cooling in summer.

• It was concluded that to improve the energy efficiency of a greenhouse in winter, the air tightness of the greenhouse must be improved, radiative losses must be reduced by means of using thermal screens or IR-opaque plastic covering materials and the soil efficiency be increased in storing solar energy and releasing it during the night.

• Apart from these factors overall performance of a greenhouse coupled with any heating system is influenced by several interrelated parameters such as size of greenhouse, type of cover material used, heat storage method, quantity of material used, type of cultivation, desired day and night temperature of the inside air, location of the greenhouse and outside ambient conditions.

Water storage systems

• Solar collectors and heat storage media can be put within a greenhouse using water-filled plastic bags and ground tubes.

• During the day, they collect and trap solar radiation, releasing stored heat to the interior via natural convection and radiation at night.

• A heat transfer fluid can also be used to store heat outside the greenhouse.

• A heat exchanger transfers the extra heat from the day to the stored water.

• The storage system might be on the surface of the earth or beneath.

Water storage inside the greenhouse in water tanks/barrels

• Water tanks of various sizes and temperatures have been used to store water in greenhouses.

• Water was held in a 30 m2 polyethylene-covered greenhouse with the north side insulated to prevent heat loss

• Solar radiation flow on tanks can be enhanced by reflecting from the north wall with 0.7-0.8 reflectivity (mylar) reflector sheets.

• Water (18.2 kl) was held in an insulated tank and placed along the north wall of a 40 m2 glass house to grow plants.

• The interior air temperature was kept 4-5 degrees Celsius higher than the lowest outside temperature.

• In a 95 m2 glasshouse, a reflecting north wall was employed to keep the indoor air temperature 3-4°C warmer.

• The device measured 16-22°C higher indoor air temperature than winter ambient air conditions in a filon-covered tiny experimental greenhouse in Arizona.

The north wall

• The north wall of a greenhouse is insulated externally and painted black internally for thermal energy storage, particularly in cold climatic zones.

• This design helps absorb solar radiation and reduce heat losses.

• The north wall can raise the greenhouse air temperature by 1-10°C and cover 35-82% heating needs at various locations and greenhouse sizes.

• However, construction of the north wall increases greenhouse costs but saves on pipes, air pumping, and electricity costs.

• This concept is particularly effective in east-west oriented greenhouses in cold climatic zones.

Summary

1) Water tanks/barrels can reduce annual temperature fluctuations and are more cost-effective due to lesser space utilization.