What are the best energy upgrades for your greenhouses? How long will it take to pay the investment back?
The following is a summary of 100 audits I have conducted on greenhouse operations in New England for the USDA EQIP and REAP grant programs. The size of these operations varied from 6,000 to over 1 million square feet of greenhouse space. Of these operations, 60 percent had free-standing houses, mostly hoop houses, 13 percent had gutter-connected houses. Seventeen had both.
The audits were conducted with the intention of determining what energy conserving measures a grower had in place and what the payback would be for new or improved measures. The USDA grant programs are designed to help growers reduce energy use and to reduce greenhouse gases and air pollutants in existing greenhouses. Depending on your state, some of the following measures may be eligible for funding.
Infrared poly glazing
Although IR poly has been around since the 1970s, 47 percent of the audited operations could benefit from installing this on some or all of their houses. The heat savings can be up to 30 percent on a cold, cloudless night. Based on an average 12 percent fuel savings, the payback varied from less than one month to about one year. The additional cost of 2-2.5 cents per square foot of material is very small when you consider that the plastic has a four-year life.
Insulating wall areas below benches and on end walls has a good payback, usually less than two years. Only 24 percent of the greenhouse operations had insulation installed. Foil-faced, double bubble insulation is commonly used for this purpose. It works well on hoop houses where the insulations are placed between the hoops and the inflated poly. On greenhouses with permanent polycarbonate or glass walls, foam board type insulation is easy to install and lasts longer.
More than 65 percent of the greenhouses audited could benefit from more accurate controls. The typical Dayton, Penn or Honeywell thermostats have a large differential between on and off. If the thermostat is set to start the heater at 60 degrees Fahrenheit, it will usually not shut off until it reaches the shut off temperature of 66 or 68 degrees Fahrenheit. This overheating increases heat loss and your fuel bill. Having a thermostat such as the Dramm Model T41-SV with a differential of +/- 1 degree Fahrenheit will shut the heater off at 61 or 62. For each degree that the temperature is above the set-point, there is an approximate three percent increase in fuel use. At an approximate $100 cost, payback on accurate thermostats is usually less than 1.5 years. For greenhouses with more heating and cooling equipment, controllers do a better job and offer a good payback of less than three years.
Furnace draft control
About 36 percent of the furnaces and boilers were being operated without proper draft control. Barometric dampers and draft hoods maintain a constant draft on the fire. Excessive draft increases heat loss both when the heater is operating and when it is in standby. The savings in fuel use can be as much as 15 percent on a windy night. Payback is usually less than two years with a cost of about $100 for the unit.
Most new gutter-connected greenhouses are built with an energy screen. Retrofitting existing greenhouses is possible but may be more expensive due to structural design and overhead obstructions. Installing a screen in a hoop house is possible and may have a good payback if the house is heated during the winter. The payback for screens may be as long as eight years.
Roof and side vents: Although reducing fan energy and providing more uniform cooling, the initial expense of retrofitting a greenhouse is high resulting in a long payback.
Root zone heat: About 20 percent of the greenhouse operations audited could benefit from installing root zone heat. Besides saving five to 10 percent in fuel use by having a lower air temperature. It usually shortens production time. Depending on the cropping system, the payback varies from three to 10 years.