About Heat Storage for Greenhouses: The how and why

About Heat Storage for Greenhouses:

The How and Why

Storage of heat for future use is an old idea used in industry and in solar homes. It is becoming popular now that alternate energy systems are being installed for greenhouse heating. Many systems have been developed depending on the source of the heat and the storage medium.

Heat can be stored for short periods of time as from day to night or for longer periods such as from summer to winter. Trees store energy for a century or more. Coal and oil store the sun’s energy for thousands of years. Several heat storage concepts can be used in greenhouses. Let’s look at a few of them.

Daytime storage of heat for night time use

Carbon dioxide can increase plant growth. One of the by-products of the combustion of fossil fuels is CO2. Capturing this from the flue gases and distributing it in the greenhouse costs very little. As CO2 is effective only during the day and heat is not normally needed at this time, storage of the heat is required to make the system efficient. Large insulated water storage tanks are used to store the heat for use at night.

In the greenhouse industry the system is used for water storage with alternate fuel heating systems with limited cycling. Matter such as wood and coal are most efficiently if operated at a constant fire rate. Adding a large, insulated water buffer tank can store excess heat during the daytime operation to be used at night when the heat demand is the greatest. This can reduce the size of the heating system needed.

Tanks with capacities of 1,000 gallons to over 500,000 gallons are available. They are usually steel with an interior liner or anti-rust coating and a heavy insulation on the outside. An exterior metal jacket protects the insulation. Smaller tanks are delivered by truck. Larger tanks are assembled on site.

Design of these systems allows for a smaller boiler as the water storage carries part of the night-time load. Typical design looks at the maximum heat needs for the coldest day. It also considers the maximum tank water temperature that can be achieved, the lowest water temperature that can be used and the storage period. Maximum water temperature is around 200ºF. The lowest temperature water for distribution in steel pipes or fin radiation is around 150ºF. A lower temperature water can be used if root zone heating system is installed. Storage period may be from one to two days. Typically storage capacity is one gallon per 200 – 300 Btu/hr of boiler heat capacity.

For small growers with a good wood supply and a few plastic tunnels, an outdoor wood boiler may be a good alternate fuel source that will lower heating cost. These are available with capacities up to one million Btu/hr output. Installing a 11 356 to 15 142 litres insulated water tank can provide the buffer capacity needed to store excess heat for the night.

Capturing excess greenhouse heat

On bright, sunny days in the autumn, winter and spring, there is usually excess heat that needs to be vented. Capturing this heat for night-time use is a possibility. The amount of usable heat is approximately 61 – 400 Btu/sq meter