Chapter 5

Greenhouse Heating

Types of Fuels for Greenhouses

The most common sources of greenhouse heat energy are liquid propane gas (LPG), natural gas, and No. 2 fuel oil. Generally, natural gas is the cheapest of these three on a per energy unit basis. Determining the lowest cost fuel can be difficult because of the various units of measure and independently fluctuating cost for each fuel. The British Thermal Unit (BTU) is the American standard measure of energy content of a fuel and can be used to compare fuel costs. The efficiency of heating equipment depends, to some extent, on the type of fuel used. Natural gas and propane boilers are typically more efficient than other types of heaters. The efficiency of the heater also has an impact on the final cost per useable unit of energy. Larger heating systems can often use more than one type of fuel and can switch depending on which has the lowest current cost. In some cases, it may be worthwhile to retrofit existing equipment for dual fuel use to improve efficiency and reduce costs.

Gaseous Fuels

Natural Gas

This is one of the most economical fuels, although it is not available to growers in all areas. It needs no on-site storage as it is piped from transmission lines. Natural gas burns cleanly, requires little equipment maintenance and may be used in central boilers or remote unit heaters. Some suppliers include an "interruptible clause" that allows them to turn off your supply in time of extreme need, usually during cold spells when fuel is needed to heat homes.

Propane

Liquid propane (LP) gas is a clean, gaseous fuel much like natural gas. It is obtained as a byproduct of oil refinery operations or by stripping natural gas. It is liquefied by moderate pressures at normal temperatures.

Fuel Oils

Fuel oils require on-site, above-ground storage tanks that must have a containment in event of a leak or spill. Oil stored in outdoor, above-ground tanks may become difficult to pump in very cold weather. Insulated tanks or additives to the fuel protect against this hazard. Fuel oils are available in five grades, designated Nos. 1, 2, 4, 5, and 6. No. 1 is slightly heavier than kerosene and is generally used to heat private homes. The oil becomes heavier (more viscous) as the number increases. No. 2 is usually comparably priced with natural gas but may be more expensive in some locations, especially where it has to be transported a long distance from the supply point.

Biomass Fuels

The use of alternative fuels for heat is on the rise, an increase that can be attributed to the price surge and volatility of traditional fossil fuels, as well as a general desire to use more cost efficient and environmentally friendly materials to reduce greenhouse gas emissions. Biomass fuels have been successfully permitted and are being effectively used on a consistent basis as fossil fuel substitutes by many greenhouse operations.

Wood Pellets

Wood pellets are compressed by-products from the forest products industry made primarily of sawdust, wood shavings and fines left over after processing trees for lumber and other wood products. They are a locally available and a cost-effective heating fuel with several advantages over other types of biomass. Wood pellets are a condensed uniformly sized form of biomass energy, making them easier to store and use than many other biomass fuels.

Wood Chips

Wood chips require more storage capacity because the volume is about four times that of wood pellets and require more operation and maintenance efforts. On the other hand, wood chips have a significant cost advantage over wood pellets.

Comparing the Cost of Heating Fuels

When choosing heating fuels, you 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 kilowatthours (kWh) of electricity, comparing the price of fuels in dissimilar units is not meaningful. A more useful comparison is the price or cost of fuels based on the heat content of the fuels, such as dollars per million British Thermal Unit (Btu) of heat content. The British Thermal Unit, or BTU, is the American standard measure of energy content of a fuel and can be used to compare fuel costs. The efficiency of heating equipment depends to some extent on the type of fuel used.

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