Greenhouse heat storage can save on energy costs

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Storage of heat for future use is an old idea used in industry and in solar homes.

August 6, 2009

John W. Bartok Jr.

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 heat source and the storage medium. Selection of a system and its size are important to making it economic.
 Heat can be stored for short periods of time, as from day to night, or for longer periods such as from summer to winter. Several heat storage concepts are used in greenhouses.

Heat storage for nighttime use
Carbon dioxide (CO2) can increase plant growth. One of the byproducts of the combustion of fossil fuels is carbon dioxide. Capturing carbon dioxide from the flue gases and distributing it in the greenhouse costs very little. As carbon dioxide 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 day to be used at night when the heat demand is the greatest.

Tanks with capacities of 1,000 gallons to more than 500,000 gallons are available. They are usually steel with an interior liner or anti-rust coating and 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 in Beamsville, Ontario, Canada, has been supplying these tanks to the greenhouse industry for several years.

The design of these systems allows for a smaller boiler as the water heat storage supports part of the nighttime load. A 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 water temperature for distribution in steel pipes or fin radiation is around 150ºF. A lower water temperature can be used if a 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 per hour of boiler heat capacity.

For small growers with a good wood supply and a few hoop houses, an outdoor wood boiler may be a good alternate fuel source that will lower heating costs. These are available with capacities up to 1 million Btu per hour 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 sunny days in the fall, winter and spring, there is usually excess heat that needs to be vented from the greenhouse. Capturing this heat for nighttime use is a possibility. The amount of usable heat is approximately 200–400 Btu per square foot of floor area depending on where a greenhouse is located. For example, a 30- by 100-foot greenhouse could have from 600,000 to 1.2 million Btu of excess heat. It is a low grade heat with maximum temperature of about 90ºF.

Capturing and storing this heat is not easy. It could be collected with ducting near the ridge of the greenhouse 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 equipment and operation may be prohibitive. An economic study should be done first. Several research projects are exploring this concept.

Summer to winter storage
In the 1970s, research at the Ohio State University-Ohio Agricultural and Research Development Center in Wooster studied the use of a solar heated salt pond covered by a greenhouse structure. The advantages of this system 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 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 150ºF, and 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.

Heat storage medium
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 per square feet - ºF, whereas concrete, crushed rock and sand are approximately 0.2 Btu per square feet - ºF. On a volume basis, water holds about three times as much heat as concrete, rock and sand.

Phase change materials, such as calcium chloride hexahydrate and Glauber’s salt, have been 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 greenhouse installations. 

John Bartok Jr. is faculty emeritus, University of Connecticut, Department of Natural Resources Management and Engineering,