Chapter 5

Greenhouse Energy Conservation Strategies

Greenhouse Heating Systems

Maintenance

Maintaining maximum heating efficiency of the existing heating system is critical to reducing heating costs in the greenhouse. While some loss is inevitable, an efficient system for generating heat and transferring it to the crop will reduce fuel consumption. Poorly maintained furnaces or boilers will burn more fuel to deliver the same amount of heat. Having the heating system inspected and serviced once a year will usually pay for itself in fuel savings and reduce expensive emergency service calls.

Heat Air Distribution

Often, greenhouses are heated with one or two forced-air unit heaters that discharge the heated air above crop level. If two unit heaters are used, they are typically placed in opposite corners on opposite ends of the greenhouse to create a circular airflow pattern. Heaters are often placed high in the greenhouse to allow more room for benches or walkways. Overhead heating is generally provided using forced air systems. Forced air systems are typically less expensive but generally provide less uniform conditions.

In-floor Heating

In-floor heating typically uses a hydroponic, or hot water, heating system. Heat pipes are usually distributed about 12 inches apart in the floor, and the heat warms the floor and transfers to pots sitting on the floor and to the surrounding air. If concrete floors are used, the tubing is installed and the concrete is poured over the tubing.

Under-bench Heating

Under-bench heating can use either a forced-air system or a hydronic system. A forced-air system can consist of a unit heater with a blower-type fan connected to ductwork that distributes the heated air under the bench.

High-efficiency Heaters

Traditional heating systems include atmospheric boilers, which pull air from the room in which the boiler is located and operate at an efficiency of approximately 80 to 82 percent, pressurized combustion boilers, with an efficiency of about 85 to 87 percent, and unit heaters, which can be hydronic (using hot water or steam) or flame-fired (using natural gas, fuel oil, or propane).

Condensing Boilers

Between steam and hot water and water tube and fire tube boilers there exists conventional atmospheric boilers and condensing boilers. The condensing boiler is far more efficient than the conventional atmospheric boiler. Water vapor is a byproduct of the combustion of gas or oil. Normally, this water vapor, which has a temperature of 400 to 500 degrees F, goes up the stack along with the other byproducts and is exhausted into the atmosphere. A condensing boiler incorporates an extra heat exchanger in the flue gas exhaust system so that the water vapor condenses back to a liquid.

Heating Fuels

When choosing heating fuels or systems for a greenhouse, the greenhouse operator may want to compare the cost of different heating fuels or energy sources. Because heating fuels are measured and sold in different units, such as gallons of oil and propane, cubic feet or therms of natural gas, or kilo watt hours (kWh) of electricity, comparing the price of fuels in dissimilar units is not meaningful.

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