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Fuels and Alternate Heat Sources for Commercial Greenhouses

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Which fuel is best? The right answer depends on price, convenience and availability. Some fuels have a higher heat value than other and some heating units have a greater efficiency. A good way to compare the cost of fuel is on a million Btu basis. What is the cost of one million Btu's of heat?

By definition, one Btu equals the amount of energy required to raise a pound of water 1ºF. It takes 8.3 Btu's to raise 1 gallon of water 1ºF.

Gaseous Fuels

Natural gas 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" which allows them to interrupt the supply in time of extreme need, usually during cold spells when fuel is needed to heat homes. A backup fuel supply and equipment to burn it is essential under these conditions.

For larger users, it is best to buy "direct purchase" form one of the large suppliers near a well head. Adding the transmission cost through the pipelines will give you a total fuel cost.

Propane (Liquefied Petroleum 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. Although more expensive than natural gas it can be readily obtained where natural gas in not available. Maintenance is minimal, but a storage tank and preheater are needed.

Propane is best purchased by the transport load which requires a 30,000 gallon tank, usually buried. Purchase should be made during the time of year when the price is lowest.

Fuel Oil

No. 2 oil 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. It is a relatively clean fuel that demands slightly more burner maintenance than gas. Oil requires 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 temperatures near 0ºF. Insulated tanks or additives to the fuel protect against this hazard.

No. 4 and 6 oils have a higher heating value than No. 2 but because of low sulfur restrictions, they are usually similar in price. These oils require preheating and greater attention to equipment operation. They are subject to the same storage and temperature limitations as No. 2 oil.

Contracting for the winters fuel needs when the price is low, usually August or early September can save considerable money and insure that you have a supply for the winter.

Alternate Heat Sources

A heating system using an alternative fuel can be installed to meet the total heat needs of the greenhouse but is generally too expensive. It is more common to install a system that will supplement the present heating system and provide for part of the heat needs. It is only for a few hours per heating season, the coldest nights, that you need the total installed capacity of the furnaces or boilers.

Most heating systems that use an alternative fuel tend to be more expensive than a conventional furnace or boiler that is mass produced. To be attractive, the fuel or heat source should be readily available at a low cost. Even though there has been little government support for research on alternative fuels, considerable progress has been made in developing more efficient equipment and systems during the past few years.

Systems that burn fuels, such as wood, coal and waste oil require more attention and labor. Maintenance costs may also be greater and should be readily available.

The following is a short review of some of the more common alternative fuels and heat sources. A detailed economic study should be done before purchasing a system. It may be a better choice to invest in upgrading the present system to make it more efficient.

Wood - Where readily available, wood is an important alternative. The fuel is relatively inexpensive as forest residue, mill waste, chips or sawdust. The cost is in the transportation and handling.

For example, the fuel oil requirement for a 20,000 sq ft greenhouse located in Connecticut would be about 20,000 gallons for the heating season. If wood is substituted as a fuel source, it would require about 110 cords of hardwood. This means handling about 220 tons if the furnace was fired by hand. The cost of the wood is only $6000 delivered in log length compared to about $35,000 for the oil. This difference is enough to have a reasonable payback for the equipment and to pay for a night fireman.

Furnaces and boilers are available that burn cordwood, chips and sawdust. Heat value of the fuel depends on the moisture content. For example, green, whole tree chips have a heat value of about 8 million Btu/ton whereas dried chips will give about 14 million Btu/ton.

If the wood is burned at high temperature (more than 1500 ° F) very little smoke or pollution occurs. Most commercial units will pass EPA standards.

Coal - Coal is a low-cost fuel source in some areas. Most coal used in the eastern part of the U.S. is anthracite. Bituminous coal is more common in other parts of the country. The farther you are from the mines, the greater the cost due to the increased transportation. One ton of coal has the equivalent heat value of 150 - 180 gallons of fuel oil or 2,100 - 2,500 therms of natural gas.

A coal heating system requires more labor to operate, produces ashes that have to be disposed of and requires a covered storage area to prevent it from freezing. Operation is usually more difficult as the ignition temperature is 800 ° F to 900 ° F compared to about 550ºF for wood.

Waste oil - Collection centers for waste oil from cars and trucks are common throughout the U.S. Some of the oil is cleaned and reused for lubrication. Much of it is available as a fuel at a few cents/gallon. Waste oil that has not been processed requires a settling tank to trap water and sludge. Considered a hazardous waste, these have to be disposed of to meet EPA regulations.

Waste oil has a heat equivalent of about 135,000 Btu/gallon, similar to #2 fuel oil. Furnaces and boilers designed to burn waste oil are available in many sizes. They have automatic control and operate similar to an oil or gas unit.

Methane - Methane is a combustible gas that is commonly available from landfills or from the decomposition of animal manure. If cleaned of impurities, it could be used to power autos or other engines. Methane straight from a landfill can be burned directly in a furnace or boiler. It has a heat value of about 500 Btu/cu ft about ½ that of pure methane.

The Burlington County Resource Recovery Greenhouse operated in cooperation with the Plant Science Department at Rutgers University, New Brunswick NJ has been operating on methane gas produced at the adjacent landfill for several years. In Connecticut , a fuel cell which generates electricity and heat has been operated off landfill gas for several years.

Waste Heat - There are many sources of heat that are going to waste. Common sources include process manufacturing, incinerators and power plants. Some plants are set up to generate electricity and give heat as a by product. The heat may be available as high temperature water or steam but more commonly can only be obtained as low-grade heat of less then 150 ° F. In these cases, a heat exchanger, such as, the Modine Type GLW unit heater is needed to recover the heat for a greenhouse. In other cases, the water can be put directly into a floor heating system.

Geothermal heat - The temperature of the soil at 8' to 12' below the surface remains fairly uniform at about 50 ° F year-round. Water in wells or ponds also stays about the same temperature. In some areas of the U.S. high temperature water or steam is available from deep well sources. There is a potential to capture this heat either directly as with the high temperature steam or through heat pumps or earth tubes to help heat a greenhouse.

Heat pumps work like a refrigerator in reverse taking the heat from the ground source and transferring it to the greenhouse. Earth tubes capture the heat in ventilation air that is then blown into the greenhouse. The 50 ° F air or water could also be used to cool the greenhouse in the summer. The cost of most geothermal systems is fairly high and many things need to be considered before installing such a system.

Although significant savings are possible with the above systems, a substantial investment in equipment and facilities may be required. A thorough analysis of the economics and fuel or heat reliability should be made. It is also best if you can talk to other growers who have similar installations.

You can make your own comparison of fuel costs using the following formulas:

Comparing Fuel Costs

Fuel Heat Equivalent Burner efficiency Cost-$/Million Btu
Fuel Oil 138,500 Btu/gallon 70% $/MBtu = $/gal x 10.3
Fuel Oil 138,500 Btu/gallon 75% $/MBtu = $/gal x 9.6
Waste oil 125,000 Btu/gallon 70% $/MBtu = $/gal x 11.4
Natural Gas 100,000 Btu/therm 75% $/MBtu = $/therm x 13.3
Natural Gas 100,000 Btu/therm 90% $/MBtu = $/therm x 12.5
Natural Gas 103,000 Btu/ccf 90% $/MBtu = $/ccf x 10.8
Propane 92,500 Btu/gallon 75% $/MBtu = $/gal x 14.4
Propane 92,500 Btu/gallon 90% $/MBtu = $/gal x 12.0
Hard coal 25,000,000 Btu/ton 60% $/MBtu = $/ton divided by 15.0
Hard coal 25,000,000 Btu/ton 75% $/MBtu = $/ton divided by 18.7
Hardwood 20,000,000 Btu/cord 60% $/MBtu = $/ton divided by 12
Softwoods 12,000,000 Btu/cord 60% $/MBtu = $/ton divided by 7.2
Wood Pellets 8,200 Btu/lb 80% $/MBtu = $/ton divided by 13.1
Wood chips      
Green (45% m.c.) 4,500 Btu/lb 75% $/MBtu = $/ton divided by 6.75
Green (45% m.c.) 2,000,000 Btu/cu yd 75% $/MBtu = $/cu yd divided by 1.5
Green (50% m.c.) 4,000 Btu/lb 50% $/MBtu = $/ton divided by 4.0
Dry (10% m.c.) 7,400 Btu/lb 60% $/MBtu = $/ton divided by 8.8
Corn 8,200 Btu/lb 80% $/MBtu = $/ton divided by 13.1
Vegetable Oil 120,000 Btu/gal 70% $/MBtu = $/gal x 11.9
Electricity 3,412 Btu/kilowatt-hour 100% $/MBtu = $/kw-hr x 293


Approximate Heating Value of Common Fuels

Natural Gas 1,030 Btu/cu ft 100,000 Btu/therm
Propane 2,500 Btu/cu ft 92,500 Btu/gal
Methane 1,000 Btu/cu ft  
Landfill gas 500 Btu/cu ft  
Butane 3,200 Btu/cu ft 130,000 Btu/gal
Methanol   57,000 Btu/gal
Ethanol   76,000 Btu/gal
Fuel Oil    
Kerosene 135,000 Btu/gal  
#2 138,500 Btu/gal  
#4 145,000 Btu/gal  
#6 153,000 Btu/gal  
Waste oil 125,000 Btu/gal  
Biodiesel - Waste vegetable oil 120,000 Btu/gal  
Gasoline 125,000 Btu/gal  
Softwood 2-3,000 lb/cord 10-15,000,000 Btu/cord
Hardwood 4-5,000 lb/cord 18-24,000,000 Btu/cord
Sawdust - green 10-13 lb/cu ft 8-10,000,000 Btu/ton
Sawdust - kiln dry 8-10 lb/cu ft 14-18,000,000 Btu/ton
Chips - 45% moisture 10-30 lb/cu ft 7,600,000 Btu/ton
Hogged 10-30 lb/cu ft 16-20,000,000 Btu/ton
Bark 10-20 lb/cu ft 9-10,500,000 Btu/ton
Wood pellets - 10% moisture 40-50 lb/cu ft 16,000,000 Btu/ton
Hard Coal (anthracite) 13,000 Btu/lb 26,000,000 Btu/ton
Soft Coal (bituminous) 12,000 Btu/lb 24,000,000 Btu/ton
Rubber - pelletized 16,000 Btu/lb 32-34,000,000 Btu/ton
Plastic 18-20,000 Btu/lb  
Corn - shelled 7,800-8,500 Btu/lb 15-17,000,000 Btu/ton
cobs 8,000-8,300 Btu/lb 16-17,000,000 Btu/ton
Electricity 3412 Btu/kilowatt hour  
John W. Bartok, Jr.
Extension Professor Emeritus & Agricultural Engineer
Natural Resources Mgt. & Engr. Dept.
University of Connecticut , Storrs CT

Originally written May 2005

Comparing Fuel Costs chart and Approximate Heating Value chart updated 2013

Commercial Horticulture
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Greenhouse Engineering