Greenhouse - Conserving Energy

From an energy standpoint, greenhouse growers have been blessed with a warm fall. However, the heating season is now upon us. With the high cost of fuel, growers should consider all economical methods to reduce energy costs. The following are some suggestions.

Conserving Energy in Greenhouses

The Greenhouse Structure

• The first line of defense in efficient heating of a greenhouse is the structure itself. Losses vary depending on the greenhouse covering and the age of the structure. In general, newer structures will have better seals around the coverings and openings than older houses.
• Double poly - Double polyethylene (poly) coverings reduce heating costs about 50% compared to single poly coverings. Most greenhouses in Virginia that are used for winter production are inflated double poly houses. Different polyethylene films vary from 35% to 60% heat loss. Ask your supplier about the film's thermal value. Selecting films that reduce water condensation will enhance light transmission and improve heat retention. Maintaining proper inflation between double poly layers is critical to maximizing the insulation value of the covering.
• Retrofitting - A glass greenhouse can be covered with one or, preferably, a double, inflated layer of poly for extra insulation during the winter. A single layer of film over glass can reduce annual heating costs by 5% to 40% whereas a double (inflated) layer can reduce costs 40% to 60%. Remember that there is a tradeoff between increased energy efficiency and reduced light transmission with additional layers of poly.
• Winterize openings - A tight greenhouse with few air leaks around vents, fans or doors will cost less to heat. Repair any holes in the plastic, glass or doors. Keep doors closed and caulk or weatherstrip door frames and other openings.

Maximize the Insulation

• Endwalls - Insulate the endwalls of the greenhouse, especially the north endwall. In most parts of Virginia, the north endwall provides very little light for crop production. This wall can actually be constructed of a solid material like wood. Plywood (1/2-inch thick) will lose about the same amount of heat as a double poly wall. At least, insulate this wall for winter production. Reflective (foil backed) insulation boards provide better insulation than other rigid foam boards. Place them with the reflective side facing into the greenhouse. If possible, add windbreaks outside the greenhouse along the north wall. These may be conifers planted for screening or a temporary fence material to divert the wind over the greenhouse. The south endwall can be insulated with an extra layer of plastic.
• Foundations on new construction - On new construction, foundation heat loss can be reduced by half through the installation of 1 to 2 inches of polyurethane or polystyrene insulation. This insulation should be installed 1.5 to 2 feet deep around the foundation wall with care given not to leave gaps or openings. This is especially important when installing any type of floor heating system.
• Existing foundation and side walls - If the foundation of the greenhouse was not insulated during construction, make sure that all gaps or holes below the foundation board are filled or repaired. If the greenhouse has a concrete kneewall, insulating the inside of it with insulation board can significantly reduce heat loss. Reflective insulation boards can be added to the inside of any flat greenhouse wall but should not extend above the crop or bench height. Leave a small airspace between the insulation and the sidewall to prevent freezing of the greenhouse wall. Be sure that the reflective surfaces are not in contact with perimeter heating pipes. Sidewall insulation can reduce annual heating costs 5% to 10%.
  Fans and vents - To reduce other air leaks, insulate secondary fans and vents to reduce heat loss through unused areas during the winter. Do not cover all of the vents; remember that winter ventilation is required for humidity control and to restore the oxygen/carbon dioxide balance in the greenhouse. Keep these vents in good working condition so that they close tightly when not in use.

Add a Thermal Blanket

• Up to 85% of the heat loss from a greenhouse occurs at night. Using a thermal blanket to retain heat at night can be a cost efficient investment. These blankets are easier to install and create less shading in gutter-connected houses than in a quonset house. Remember to use a porous curtain material so that condensation from the underside of the roof of the greenhouse will not pool above the plants. For greenhouse structures where an internal curtain cannot be installed, external curtains are available that can reduce radiation loss from the greenhouse at night.
• Reductions in heat loss - Blankets offered primarily for heat retention can reduce energy use by up to 50%, whereas blankets offered as combination thermal blanket and summer shade protection can reduce winter energy use 25% or more.
  Recouping installation costs - With purchase and installation costs running $1.00 to $1.50 per square foot, these systems pay for themselves in one to two years­ or less under high fuel prices.
• Installation details - Make sure that the blanket fits the greenhouse walls tightly to reduce heat loss above the blanket. Heating or water lines should be located below the blanket or be well insulated to reduce heat loss.
• Open slowly - Take care not to open the blankets too quickly over a chill-sensitive crop. On 10°F to 20°F nights, the temperature above the thermal blanket could be 30°F in a 60°F greenhouse. Open the blanket 6 to 12 inches for about 30 minutes to allow mixing of the air before opening it completely. Some growers wait until the sun has risen and warmed the air above the blanket before opening it and allowing that air to mix with the rest of the greenhouse air. That may be dictated by the light requirements of the crop.
  Keep it open during snowstorms - In the case of snow storms, the blanket should be left open to allow the heat to reach the roof to prevent snow accumulation on the roof of the greenhouse.

Heating System Efficiencey

• Maintaining maximum heating efficiency of the existing heating system is critical to reducing heating costs in the greenhouse.
• Annual maintenance - Examine the equipment for physical damage to any parts of the system. Check the vent pipe and air inlet or discharge pipes for obstructions (i.e., bird nests). Furnaces should be cleaned and adjusted at least once per year. Check that the boiler, burner and backup systems are operating in peak efficiency. Clean the soot from inside the furnace. A 1/8-inch layer of soot can increase fuel consumption by as much as 10%.
• Fuel choice - Use the proper fuel for the system for maximum efficiency.
• Insulation - Insulate boiler or distribution pipes in areas where heat is not needed.
  External air for combustion - Install an air inlet pipe for direct fired heaters to provide fresh air for combustion from outside the greenhouse.
• Clean radiation surfaces - Clean heating pipes or other heat radiation surfaces frequently.
  Motors and pumps - Keep all motors and pumps properly maintained for maximum efficiency.
• WARNING: Do not inhibit the fresh air supply to the greenhouse heater. If you are using a heater that requires greenhouse air for combustion, be sure to leave about 1 square inch of opening for each 2,000 Btu/hr of heater output. If possible add an inlet pipe from outside air to serve the burner.

Add Horizontal Air Flow (HAF) Fans

• Reducing air leaks and heat loss in the greenhouse will make the house "tighter" which will also tend to increase the relative humidity. Regardless of the type of heating system used, install a sufficient number of horizontal air flow (HAF) fans to adequately circulate the air inside the greenhouse. Good air circulation will improve temperature and humidity uniformity in the greenhouse, which reduces the incidence of cold pockets in the greenhouse and improves plant quality and uniformity. Monitor the humidity level in the house, generally keeping it below 80% to minimize disease incidence, and vent when necessary.
• Air speed - Air circulation by the HAF fans should be maintained at 2 to 3 cubic feet per minute over the floor surface of the greenhouse. For example, a 28-foot x 96-foot greenhouse requires an airflow of 5,376 cubic feet per minute (28 x 96 x 2 cubic feet per min per square foot = 5,376 cubic feet per minute). This greenhouse would require four HAF fans capable of moving air at 1,440 cubic feet per minute. This could be provided by four 16-inch fans with 1/15-horsepower motors at 1,600 revolutions per minute. (See reference D. Ross, UMd Bulletin 351 for more details.) Horizontal air flow fans are generally available in two air flow capacities, but check the fan specifications to determine that they meet the calculated needs.
• Fan location - The HAF fans should be located 2 to 3 feet above the plants and aligned parallel to the sidewalls of the greenhouse so that the air is circulated around the house in a rotational pattern.
• Winter operation - The HAF fans should be run continuously during the winter to improve temperature and humidity uniformity in the greenhouse.

Environmental Control

• Use aspirated thermostats - Thermostats should be aspirated with greenhouse air and be placed near the plant canopy in locations representative of the rest of the greenhouse (not near sidewalls, fans, or doors). Aspirated thermostats save 2% to 3% of the total fuel bill by improving fan and heater operation.
• Electronic thermostats - Switching to solid-state electronic thermostats can also improve efficiency by reducing the differential between the on and off modes, usually down to 1°F instead of the 3°F to 4°F of mechanical thermostats.
• Calibrate sensors - Calibrate the sensors regularly to maintain proper environmental temperatures. If you are lowering greenhouse growing temperatures, sensors must be accurate to avoid chilling damage to the crop.

Information extracted from "Dealing with the High Cost of Energy for Greenhouse Operations" by Joyce G. Latimer, Extension Specialist, Greenhouse Crops; Virginia Tech