Chapter 5

Greenhouse Energy Conservation Strategies

Alternative Energy Sources for Greenhouses

While the large majority of energy used in by greenhouses still comes from fossil fuels (coal, oil, and natural gas), there is also tremendous growth in alternative energy technologies. Alternative energy is a term for any nontraditional energy form, source, or technology differing from the current popular forms, sources, or technologies. Today, it is generally used in the context of an alternative to energy deriving from popular fossil fuels and thus includes energy derived from such environmentally preferred sources as solar, water power, biomass, wind, and geothermal.

Biomass Energy

Biomass is a renewable energy source not only because the energy in it comes from the sun, but also because biomass can re-grow over a relatively short period of time compared with the hundreds of millions of years that it took for fossil fuels to form.

Biomass Boilers and Furnaces

A boiler heats a liquid such as water or makes steam, while a furnace heats air directly. Whether to choose a boiler or a furnace in part depends on the heating system location and the number of greenhouses the system will service. A boiler can be centrally located on the site and the hot water piped to the greenhouse, which allows more options for distributing the heat in the greenhouse.

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.

Solar Energy

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. Two of the most commonly used methods for storing solar energy

Active Solar Techniques

Solar Photovoltaics. Photovoltaic (PV) systems convert solar radiation to electricity via a variety of methods. The most common approach is to use silicon panels, which generate an electrical current when light shines upon it.

Solar Thermal Systems. Solar thermal systems seek to store heat from the sun that can be used for a variety of purposes. Solar thermal systems allow for energy collection during the sunnier months of the year, to be used during the darker (and colder) months. Long-term storage requires large insulated holding tanks or other means to contain the energy.

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.

Geothermal Energy

Geothermal energy is thermal energy generated and stored in the Earth, originating from the formation of the planet, radioactive decay of minerals, volcanic activity and solar energy absorbed at the surface. The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface.

Types of Geothermal Resources

Low temperature (50°F +/-). The soil temperature at the surface varies considerably over the year and closely follows the air temperature.

Medium temperature (140 to 300 °F). Thermal wells and springs in some parts of the world can provide hot water that can be used directly for heat.

Geothermal Heating Systems

Geothermal heating systems exchange thermal energy between a greenhouse and the ground. When the greenhouse needs heating, the system extracts energy from the ground and pumps it into the greenhouse where it is concentrated by the heat pump. Conversely, when the greenhouse needs cooling, the heat from the greenhouse is concentrated by the heat pumps and the system removes heat from the greenhouse and pumps it into the ground.

Closed-loop Systems. The term “closed loop” is used to describe a geothermal heat pump system that uses a continuous loop of special buried plastic pipe as a heat exchanger.

Open-loop System. The open-loop system uses well or surface body water as the heat exchange fluid that circulates directly through the geothermal heat pump system.

Wind Energy

Outside the greenhouse, a windbreak can reduce infiltration of cold air and convection of heat away from the greenhouse. Wind speed greatly affects infiltration rates; 15 mile-per-hour winds can double heat loss in a greenhouse. Well-designed windbreaks can reduce wind speeds by 50 percent, reducing heat losses by 5 to 10 percent compared to an open area.

Wind Turbine Designs

Today’s wind turbines come in two basic designs: horizontal-axis wind turbines (HAWTs) and vertical-axis wind turbines (VAWTs). Horizontal-axis wind turbines are the most common and feature either two or three long blades on the nacelle, or the body of the turbine. 

Mechanics of Wind Turbines

There are three essential parts in both horizontal and vertical axis wind turbine designs:

Energy Output of Small Wind Turbines

The amount of electricity a wind turbine generates depends on the wind speed and the turbine's capacity rating.

Capacity Ratings. In the real world a turbine will not be exposed to ideal conditions or the rated wind speed at all times.

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