Chapter 5

Greenhouse Heating

Renewable Energy for Greenhouses

Given the upward trend in both price and worldwide demand for a finite supply of fossil fuel, coupled with concerns about global climate change, many greenhouse farmers are switching to renewable energy. Renewable energy is a term for any nontraditional energy form, source, or technology differing from the current popular forms, sources, or technologies. Use of renewable energies instead of fossil fuels has many environmental, social and economic benefits and results in mitigation of the greenhouse effect. Greenhouses require heat and power for the production of various crops. The quantities of electricity and heat needed depend on the local climate, the greenhouse construction and the cultivated crop. In general, it can be said that the most of energy used is consumed for their heating. Among renewable energy sources solar energy, biomass energy, geothermal energy, and wind energy have been used for covering the heating needs of the greenhouses. Depending of the specific area, the local availability of the above-mentioned renewable energy sources is an important factor for their use in greenhouses. However, greenhouses apart from heating require electricity for lighting, cooling and operation of various electric devices (e.g., motors, valves, pumps, fans, etc.).

Heat Pumps

The use of heat pumps is becoming increasingly popular in greenhouse operations. There are myriad benefits that come with heat pumps. In addition to being cost-efficient, they’re an environmentally conscious choice that will reduce the amount of greenhouse gases generated. Heat pumps work by pulling heat from the outside surrounding and pump it into the greenhouse on a cold day and on a hot day they can pull the heat out of the greenhouse and pump it outside. Heat pumps can use heat from the outside air, the ground, a water source, or even a geothermal vent, making them incredibly versatile. In general, air-source heat pumps are more effective in mild climates and ground-source heat pumps are most effective when there is a balanced need for heating and cooling on a daily or annual basis.

Solar Energy for Greenhouses

Greenhouses were used as solar collectors long before scientists began the search for efficient methods of storing and using the sun's energy. As a solar collector, the greenhouse catches and stores solar energy. Unfortunately, the amount of heat retained in the greenhouse is not enough to maintain the desired temperature through the long nights of winter. However, it is very possible to store the solar energy collected during the day to meet part of the heat requirements at night. Solar technologies for storing solar energy are characterized as either active solar or passive solar depending on the way they capture, convert, and distribute solar energy.

Active Solar Techniques

Active solar heating systems use solar energy to heat a fluid—either liquid or air—and then transfer the solar heat directly to the interior space or to a storage system for later use (See Figure 4.20). Air-based systems use fans to distribute the heat that is collected and liquid-based systems use pumps. Active systems also have an energy storage system that is used to provide heat when the sun is not out.

Passive Solar Techniques

Passive solar heating presents the most cost, effective means of providing heat to greenhouses. Generally, the amount of solar energy that falls on the roof of a greenhouse is more than the total energy consumed within the greenhouse. Passive solar applications, when included in initial building design, adds little or nothing to the cost of a building, yet has the effect of realizing a reduction in operational costs and reduced equipment demand. It is reliable, mechanically simple, and is a viable asset to a greenhouse.

Solar Orientation. The ideal orientation for solar glazing is within 5 degrees of true south. This orientation will provide maximum performance. Glazing oriented to within 15 degrees of true south will perform almost as well. and orientations up to 30 degrees off—although less effective—will still provide a substantial level of solar contribution. When glazing is oriented more than 15 degrees off true south, not only is winter solar performance reduced, but summer air conditioning loads also signtflcantly increase, especially as the orientation goes west.

Glazing. Many new greenhouse-glazing materials have emerged in recent decades. Plastics now are the dominant type of glazing used in greenhouses, with the weatherability of these materials being enhanced by ultraviolet radiation degradation inhibitors, infrared radiation (IR) absorbency, anti-condensation drip surfaces, and unique radiation transmission properties.

Solar Heat Storage. For solar greenhouses to remain warm during cool nights or on cloudy days, solar heat that enters on sunny days must be stored within the greenhouse for later use. The most common method for storing solar energy is to place rocks, concrete, or water in direct line with the sunlight to absorb its heat. Brick or concrete-filled cinder block walls at the back (north side) of the greenhouse can also provide heat storage. However, only the outer four inches of thickness of this storage material effectively absorbs heat.

Biomass Energy for Greenhouses

Biomass is fuel that is developed from organic materials, a renewable and sustainable source of energy used to create electricity or other forms of power. Bioenergy is carbon neutral electricity generated from renewable organic waste that would otherwise be dumped in landfills, openly burned, or left as fodder for forest fires. Biomass can be made from agricultural and forestry residues and some industrial wastes and crops grown solely for energy purposes.

Biomass Boilers and Furnaces

Biomass boilers and furnaces can be divided into two main groups: those that require manual stoking, or loading, and those with automatic stoking (See Figure 4.21). Manually stoked boilers and furnaces are generally fueled by cordwood or waste wood, while automatic-stoking boilers and furnaces can handle a wide variety of biomass, including wood chips, wood pellets, biomass pellets (grass, corn fodder, etc.), paper pellets, and grains (corn, rye, and other small grains)

Biomass System Design

Biomass systems are customized to the grower’s needs and conditions in order to fully utilize the advantages of the biomass concept: auger sizing, types of screening used, burn capabilities and most importantly the size of the boiler required. Biomass boiler system manufacturers generally size the units to 60 percent of the total capacity needed to heat the greenhouse in question.

Components of Biomass Systems

Regardless of the end user being served, large-scale biomass systems generally require a similar integrated network of components, the complexity of which will vary according to the heat output and the type of fuel. The following is a brief description of these components. The storage bin needs to be sized in order to ensure an adequate supply of fuel during peak demand and designed to function in sync with delivery vehicles.

Geothermal Energy for Greenhouses

Traditional greenhouses protect crops in temperature extremes by using temperature, humidity, and ventilation controls to maintain an ideal climate. Generating such a climate can result in significant energy costs. Heating and electric combined is typically the third-largest cost for operating a traditional greenhouse. Geothermal systems allow growers to take advantage of these stable temperatures. In general, a system is comprised of two main components: underground tubing or piping to support the flow of a fluid (either air or a glycol solution) and the mechanism to push the air or fluid in and out of the piping.

Geothermal Systems

The flow of heat energy within a geothermal system can be utilized to heat or cool greenhouses, depending on seasonal needs. Air is moved from inside a building and pushed through a network of underground piping. Convection through this piping heats or cools the air, which is then exhausted back into the structure at a more desirable temperature. When the underground temperature is warmer than the ambient temperature above ground, the heat pump or fan pushes the warmer air up to the growing structure that needs to be heated.

Types of Geothermal Resources

Soil and water below ground contains a vast reservoir of thermal energy. Geothermal heating systems recover this energy and convert it to heat that can be utilized in greenhouses and other buildings. Geothermal heat can be classified into three categories.

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