Lower electricity use via natural ventilation

Photos courtesy of John Bartok

Fan ventilation has been a primary method of cooling greenhouses.

At 8-10 cubic feet of air per square foot of floor area, this system gives about one volume air change per minute, adequate to keep the growing area within a few degrees of the outside air temperature. Its main advantage is that it provides a positive flow of air through the crop with the fans exhausting heated air from the greenhouse and replacing it with cooler air from outside.

One disadvantage is that it creates a 7º-12º F differential between the intake shutter end and the fan end of the greenhouse, which can affect plant growth. Another disadvantage is that the fan ventilation is often the largest part of the monthly electric bill.

Greenhouse fans usually operate about 2,000 hours per year. Depending on the installed capacity, the efficiency of the fans and desired daytime temperature of the greenhouse, this can amount to 0.5 to 1 kilowatt hour per square foot of greenhouse floor area per year.

According to the U.S. Energy Information Administration, current commercial electricity rates are between 7.6¢/kwhr in Idaho to 17.9¢/kwhr in Massachusetts, with a U.S. average of 11.2¢/kwhr. If we use the average cost and assume that the fans will use three-fourths of a kilowatt hour/sq ft, the cost will be 8.4¢/sq ft, or $840 for cooling a 10,000 sq ft greenhouse for the year.

Natural ventilation.

Greenhouses with roof and sidewall vents operate on the principle that heat is removed by a pressure difference created by wind and temperature gradients.

Wind plays the major role in cooling greenhouse temperatures. In a well-designed greenhouse, a wind speed of 2-3 mph provides 80% or more of the ventilation, so orientation of the greenhouse is important. Wind passing over the roof creates a vacuum and sucks the heated air out of the vent. Cool outside air enters through sidewall vents, open doors or through the bottom of the roof vent.

Buoyancy, the effect of warm air, also aids ventilation. Heavy cool air near the floor becomes lighter as it is heated and rises towards the roof. On cool days, the large temperature difference creates excellent air exchange. On hot days, when the temperature difference is only 5º-10º F, the buoyancy effect is minimal. HAF fans should be shut off to avoid destratifying the warm air and also save on electricity.

If the greenhouse is designed properly with large enough vents or roll-up sides, the temperature will be much more uniform throughout the greenhouse than a greenhouse with fan ventilation.

Determine the cost

Most greenhouses can be retrofitted with a sidewall natural ventilation system. For poly covered hoophouses, this is fairly simple and involves adding a splice rail 4’ to 5’ above the ground and a drop down or roll-up curtain system the length of the sidewalls. Cost of the materials is about $10 per linear foot. The system can be motorized for another $500.

Adding roof vents to a hoophouse or gutter-connected greenhouse is more involved. A separate frame to support the glazing needs to be installed. A rack and pinion drive, gearmotor and controls are also needed to adjust the amount of opening. There are several good systems available from greenhouse manufacturers and equipment suppliers.

The cost of retrofitting to natural ventilation will depend on the size of the roof vents and the mechanism and controls used. A typical range is from $50 - $75 per linear foot. In some states, the USDA Natural Resource Conservation Service EQIP incentive payment program will pay up to $50 per linear foot towards the installation.

The operating cost of a natural ventilation system is much less than with fans. The vent motors should operate no more than ½ hour/day to make adjustments to the vent opening.

The gearmotors are very small, usually one-twentieth to one-fifth horsepower. This makes the operating cost only a few cents per day. The payback can be fairly short for growers that use the greenhouses during hot weather.

I usually recommend that growers leave one operating, thermostatically controlled fan in each hoophouse or one per bay in a gutter-connected greenhouse to provide a small amount of ventilation air to remove humidity or when the vents may be frozen shut.

Significant savings are possible by converting to a greenhouse to natural ventilation. The system needs to be designed and installed properly and the payback calculated.