<<Back to stories

Greenhouse Energy Conservation

Reducing Natural Gas/Propane Use for Greenhouse Space Heating

The Tomato Magazine
April 2005

By Scott Sanford
Sr. Outreach Specialist
Wisconsin Focus on Energy / Rural Energy Issues
University of Wisconsin, Biological Systems Engineering

Natural gas and propane prices have been increasing over the past few years due to increasing demand and the continued instability in oil producing areas of the world.
In October 2004, natural gas prices climbed to an all time high of $9 per mmBtu on the NYMEX commodity exchange for natural gas delivered to the Henry Hub in Louisiana before retreating to the $7 range in December 2004. Prices for 2004 were 149 percent higher than five years ago and 80 percent higher than two years ago. The Future prices for 2005 are trending down from current levels to $6.40 to $6.50 per mmBtu until November 2005, when prices are forecasted to rise to about $7.30 for the 2005-2006 heating season.

Propane prices have also taken a dramatic jump with wholesale prices reaching record levels at $0.97 per gallon in October 2004, 64 percent above October 2003 prices and 115 percent higher than the average price for October 1999, five years earlier.

Energy costs are the second largest cost for greenhouse owner behind labor costs, with greenhouse heating consuming 70 – 80 percent of the total energy budget. The long-term natural gas price forecast predicts prices to decline only about 5 percent as new wells come on line. Current propane prices for 2005 to 2007 are forecasted to be 15 percent above the 2004 price levels. Political instability and high demand will continue to affect future prices for both natural gas and propane. More information on what drives propane prices can be found at http://tonto.eia.doe.gov/FTPROOT/other/Propane_Prices_Pub.pdf.

Heat Loss
The following are some things that can be done to reduce the impact of higher energy costs before next winter:

IR and anti-condensation-treated films: A double polyethylene covered greenhouse reduces infiltration losses. However, it will allow infrared radiation to transmit out of the greenhouse unless it’s using an IR-treated film on the inside to reduce infrared radiation loss. Condensation on the inside of a poly covered greenhouse can reduce thermal radiation loss by up to 50 percent, however, condensation also reduces light levels at the plant and the amount of solar radiation entering the greenhouse. Dripping condensation can also lead to plant quality issues, so it is important to keep greenhouse covers free of condensation.

The IR/anti-condensation treated films cost about $0.015 per square foot more than untreated films but reduce energy use by 15 - 20 percent. In Wisconsin, the payback on the incremental cost for purchasing IR/anti-condensation-rated films should be less than one heating season.

Insulated side walls: Greenhouses that use bench systems can insulate the side walls, end walls and perimeter with 1- or 2-inch foam insulation board. Insulation should be dug in 12 - 24 inches (preferred) deep and can be extend up to the plant height. The foam should have a protective cover, such as aluminum foil, to protect the foam from UV deterioration and to reduce fire hazards. Spray-on foams on framed walls also provides excellent insulation but also need to be protected. If foam is placed on the inside of the greenhouse, a reflective coating towards the inside will reflect direct solar radiation back to the crop canopy aiding in plant growth. Two inches of foam insulation around the knee wall of a 28- by 100-foot greenhouse will save about 400 gallons of fuel oil, 610 gallons of propane or 558 therms of natural gas per year.

Night curtains: Research indicates that 80 percent of the energy to heat a single-glazed greenhouse is required at night, so reducing heat loss at night can pay dividends. A movable insulted curtain can reduce the heat loss by up to 70 percent when the curtain is closed. There are several types of blanket materials available with different advantages and disadvantages. Porous blankets save about 20 percent when closed but can be used for shading in the summer; they allow water to drain through it. Non-porous aluminized materials provide the most savings—up to 70 percent when closed. Installation costs (2001) can range from about $1.10 to $3.35, depending on the size of greenhouse, blanket material and type of track/drive system used.

Infiltration losses: Infiltration is air movement into and out of a greenhouse through cracks and small openings in the shell of the building. New construction greenhouses can range from 0.5 - 1.5 air exchanges per hour while old construction glass-glazed greenhouses can range from 1 - 4 air exchanges per hour. Wind velocity has a direct effect on the infiltration rate.

Weather stripping: Weather strip, replace gaskets and caulk joints around doors and other opening in the greenhouse shell. Pay particular attention where the greenhouse cover or glazing attaches to the foundation, side walls and end walls and seals around vents. A 1/8-inch wide crack around a 36-inch-wide door will allow 500 cubic feet per minute of air to infiltrate and require 25,000 Btu’s per hour of additional heating.

Glass houses: Glass greenhouses inherently have more infiltration because of the larger number of joints. Covering glass greenhouses with a single or double layer of poly film reduces infiltration and heat loss. The cover can be installed permanently or just during the winter months. Reducing infiltration can lead to increased humidity levels and a rapid depletion of carbon dioxide. Mechanical ventilation may be needed to control humidity and can be used to replace the carbon dioxide. If additional carbon dioxide is needed, it can supplied by purchasing compressed carbon dioxide or using a special natural gas or propane burner to enrich the air. The light levels will be reduced by 18 percent because of the poly films which need to be taken into account in an economic analysis. A double poly cover can reduce heat losses by up to 50 percent.

Wind breaks: In open, windy areas, wind breaks in the path of the prevailing winter wind will aid in reducing infiltration losses. A temporary wind break can be made from a 10- to 12-foot high snow fence placed 40 - 60 feet away from the greenhouse to protect the typical 11- to 14-foot high greenhouse. A more permanent wind break would be four or five rows of deciduous and evergreen trees planted four to six mature tree heights up wind of the greenhouse. A mix of tree species is best to guard against losing the entire windbreak from disease or insects. Fertilization and irrigation can accelerate tree growth to provide benefits in about five years.

Heating Systems
Thermostats: Clean thermostats regularly. A dirty thermostat will not accurately sense temperature. Calibrate thermostats annually. If purchasing a new thermostat or controller use electronic models with 1ºF differentials.

Furnace checkup: Have furnaces and unit heaters serviced and tested yearly. Replace older inefficient furnaces. A two percent increase in efficiency will save about 125 gallons of fuel oil, 190 gallons of propane or 174 Therms of natural gas per year for a 28- by 100-foot greenhouse. This savings would more than pay for the inspection and tune up for your heating systems for the coming winter.

Gas Burners — Flame should burn as blue as possible; yellow flame indicates insufficient air. Check gas supply line pressures, check all fittings for leaks.

Oil Burners — Replace the nozzle with one that meets the furnace manufacturer’s specifications. Change oil filters twice per year. Check pump output pressure, typically between 100 -120 psi; low pressure causes incomplete combustion. Check spark jump between igniter contacts; new 14,000-volt electronic igniters are recommended.

Clean igniter contacts and ignition sensors. At temperature below 20ºF oil viscosity increases, water droplets freeze and paraffin precipitates out. Moving the oil storage tank inside, adding fuel treatments and raising pump pressures can reduce problems.

Chimneys — Should be air tight, same diameter as the furnace connection, at least eight feet high and at least two feet above the greenhouse peak. A chimney cap will reduce back-drafts and keep rain out.

Central heating systems: Insulate pipes and air ducts in head houses and boilers rooms. Many head houses are overheated because of poor insulation of heating pipes running through to the greenhouse. Insulation is simple to install and usually has paybacks of less than two years. Each uninsulated linear foot of a two-inch heating supply pipe will loose about $4 worth of heat during the winter months. Have the heating system serviced regularly. This includes soot removal inside the firebox, changing fuel filters, cleaning nozzles, checking valves and controls, checking and aligning belts, lubricating bearings and testing combustion efficiency. Soot can build up in fire tubes due to incomplete combustion of fuel caused by improper air-to-fuel ratios or plugged nozzles. A 1/8-inch soot deposit can increase fuel consumption by 10 percent or more.

Bottom heating
: Moving heating pipes and air distributions systems from overhead to under bench, on-floor or in-floor can save 20 - 25 percent in heating costs and results in faster plant growth. One study reported a 7 percent average yield increase from greenhouse tomato production, largely believed to be due to a 7ºF higher root medium temperature. A heating pipe under the gutters will still be needed on gutter-connected greenhouses to aid in melting snow.

Alternate fuels: If a central heating system is used, it may be easier to take advantage of alternate fuels such as wood or other bio-mass. Before investing in alternate fuel, make sure you are considering all costs (labor, maintenance, ash removal and ash disposal), and check with you state environmental regulatory agency about permits and ash disposal requirements.

Waste heat/cogeneration: Cogeneration opportunities using waste heat are limited to sites adjacent to power plants or industrial sites. The waste heat source must be able to dependably supply 90 - 100ºF water. A radiant floor system is ideal for this system. Depending on the purchase agreement, energy costs can be reduced substantially.

Cogeneration may also be accomplished by powering an electrical generator with an internal combustion engine. The electricity not used for greenhouse operations is sold to the utility while recovering the waste heat from the engine cooling system and exhaust system for heating the greenhouse. The method can result in an overall efficiency of about 75 percent — 25 percent for the conversion of electricity and 50 percent for the use of waste heat. The disadvantage of such a system is the initial cost and maintenance and the need to have a backup heating system.

Considerations for New Greenhouses
Roof slopes: Gothic or peaked roofs with slopes of six in12 pitch (six inches of rise for every 12 inches of horizontal) will allow condensation to run off, reducing reduction in light levels caused by condensation on the glazing.

Side wall height: Adding a foot or two to the sidewall heights to a greenhouse increases heat loss by only about 5 percent but gives room for hanging baskets and may allow room for night curtains to be used.

Gutter connected greenhouses: Six 30- by 100-foot individual greenhouses with 10-foot sidewalls have 37 percent more surface area than a gutter-connected greenhouse covering the same growing area. If individual growing rooms are needed, poly wall dividers can be installed between bays so there are different heating zones. It is also easier to take advantage of a centralized heating system with a gutter connected greenhouses.

Site location: Chose a sheltered site to reduce wind induced infiltration heat losses as long as it doesn’t reduce lighting levels.

Natural ventilation
: Greenhouses with roof vents or opening roofs and side wall vents can take advantage of thermal buoyancy for cooling. The air temperature at crop level should be no more than 5ºF above the ambient air temperature in a well designed system. Each vent should be 15 - 20 percent of the floor area, and the sidewall vents should be equal in area to the ridge vent. Some ventilation fans may be need even in a greenhouse that can be naturally ventilated when only a little cooling is needed or the cold outside air could cause plant damage.

Space utilization: Increasing the amount of plants that can be grown in your greenhouse will reduce your production costs per plant. This usually means reducing aisles or using tiers or racks to increase plant density.

Peninsular bench layout: A traditional straight row bench system utilizes 60 to 70 percent of the floor area for plant production while a peninsular bench layout provides over 75 percent of the floor area for growing.

Movable bench system: Movable growing systems can increase plant production area to over 90 percent. There are two types of movable systems: moving benches and transport trays. Both provide high space efficiency and can increase labor efficiency and reduce energy use per plant. The disadvantage associated with movable benches is the limited space for maneuvering. This can to overcome by using a narrow portable belt conveyor or an overhead trolley to move plants into or out of the growing area.

Rack growing systems: The use of growing racks can double the growing space and create conditions similar to a forest canopy. This is useful when many different plants are being grown that have different light requirements.

Central control system: A computerized control system coupled with a weather station can control the different operating parameters required for normal greenhouse operation as well as anticipate changes such as increased wind speeds or rain may dictate the need to close vents or low solar levels during cold weather may dictate closing thermal blankets and turning on supplemental lighting. These systems can also alert personnel to equipment failures or operating parameter that is outside control limits by an audible alarm, light or telephone.
These control systems save energy due to smaller differential ranges on sensors, having safe guards that the fans and the heating system aren’t running at the same time and anticipating changes that reduce energy input and increase plant growth.

© 2005 Columbia Publishing

>> Return to top

Columbia Publishing & Design  |   1-800-900-2452