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 source 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 are used in greenhouses, listed below.
Daytime storage of heat for nighttime use
Carbon dioxide can increase plant growth. One of the byproducts 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.
A relatively new concept to the greenhouse industry is to use water storage with alternate fuel heating systems with limited cycling. Systems, such as wood, coal and corn burn 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.
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. Westbrook Greenhouse Systems, Ontario Canada has been supplying these tanks to the greenhouse industry for several years.
Design of these systems allows for a smaller boiler as the water storage carries part of the nighttime 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 200°F. The lowest temperature water for distribution in steel pipes or fin radiation is around 150°F. A lower temperature for 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 hoophouses, 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 3,000 to 4,000 gallon 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 fall, winter and spring, there is usually excess heat that needs to be vented. Capturing this heat for nighttime use is a possibility. The amount of usable heat is approximately 200 - 400 Btu/sq ft of floor area depending on where in the U.S. your greenhouse is located. For example, a 30' x 100' greenhouse could have from 600,000 to 1,200,000 Btu of excess heat. It is a low grade heat with maximum temperature of about 90°F. Capturing and storing this heat is not easy. It could be collected with ducting near the ridge and stored below the floor in a rock bed. It could also be collected with a heat exchanger and the temperature increased with a heat pump. It could then be stored in an insulated hot water tank. The cost of the equipment and operation may he prohibitive. An economic study should be done first.
Summer to winter storage
In the 1970's, research at the Ohio Agricultural and Research Development Center at Wooster studied the use of a solar heated salt pond covered by a greenhouse structure.
The advantages included relatively low cost, passive operation and the ability to collect and store summer radiation for use during the winter. The pond was filled with water and sodium chloride or other salt was dissolved in the water to form a uniform concentration in the lower half and decreasing concentration gradient from the pond mid-depth to the surface. The water, which was heated all summer reached a temperature above 150°F was drawn off when heat was needed. Water to air heat exchangers were used to heat an adjacent greenhouse. Due to space limitations and management considerations the concept has not been adopted by the industry.
Current research in Europe and elsewhere has been exploring the installation of heat storage below the greenhouse floor. A water tank or tank filled with wet sand is the storage medium. The soil below the floor could also be used. Collection can be either from the excess heat in the greenhouse or from solar collectors. Recovery is through water pipes or air ducts spaced throughout the storage area. This system can add considerable construction cost to the greenhouse.
When evaluating heat storage, the storage medium needs to be considered. Heat capacity is measured as specific heat. Water has a specific heat of 1.0 Btu/sq ft - °F, whereas concrete, crushed rock and sand are approximately 0.2 Btu/sq ft - °F. On a volume basis, water holds about three times as much heat as the concrete, rock and sand.
Phase change materials, such as, calcium chloride hexahydrate and Glauber's salt have be used. These change phase from a solid to a liquid at about room temperature with a large heat storage capacity similar to the change of ice to water. These materials are expensive and are found mostly in hobby greenhouses installations.
Heat storage can provide a buffer that allows a smaller heating system to be installed. Selection of the system and its size are important to making it economically feasible.
John W. Bartok, Jr.
Extension Professor Emeritus & Agricultural Engineer
NRME Department, University of Connecticut, Storrs CT 06269-4087