Chapter 5

Greenhouse Energy Conservation Strategies

Greenhouse Structure

The first line of defense in efficient heating and cooling 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.

Glazing Materials

Glazing selection affects energy use and crop production. Growers will often need to choose between glazing that is very transparent but doesnít retain heat or a covering that has reduced light transmission and better ability to retain heat. Glass and rigid plastic glazing have the potential to keep thermal transmission to less than 3 percent, while regular films lose about 50 percent of the radiant heat.

Double-layer of Polyethylene

Many greenhouses use a double-layer of polyethylene to reduce heat loss. This solution helps to retain heat but itís far from perfect. Standard polyethylene film allows infrared or long wave radiation to be transmitted to the outside, especially on clear nights and can cause condensation to form on the inside surface.

Greenhouse Insulation

Insulating Side Walls

Greenhouses are designed for maximum light transmission, and as a result, often have limited insulating properties. Some portions of the side walls and end walls can be modified to improve their insulating capacity with very little impact on light transmission.

North-facing Side Wall

In the past, some growers opted to add temporary insulating boards to the entire north-facing side wall in an attempt to reduce greenhouse heat loss during the winter when the sun entered the greenhouse at low angles (northern hemisphere).

Building Perimeter

Wet soil is a good conductor of heat. When the soil immediately outside the greenhouse perimeter is wet and in direct contact with the soil or concrete floor inside the greenhouse, a conduit is created for heat to flow from the inside to the outside of the greenhouse.

Glass Greenhouses

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.

Greenhouse Orientation

Generally, it’s best to orient single-span greenhouses so the length runs east–west but the rows have to be oriented north-south to minimize shadings. This orientation maximizes winter sunlight and heat gain in the greenhouse.

Single Bay vs. Gutter-connected Greenhouses

A single bay greenhouse will lose more heat than a gutter connected greenhouse because the single bay greenhouse will have more surface area exposed to the outdoors.

Greenhouse Air Leaks

In many ways, a greenhouse is no different from a home or any other structure with energy conservation. Air leaks can be a major cause of heat loss. Caulking and weather stripping around glass laps and other locations can reduce the amount of outside air entering the greenhouse.

Windbreaks

Outside the greenhouse, a windbreak can reduce infiltration of cold air and convection of heat away from the greenhouse. Wind speed greatly affects infiltration rates; 15 mile-per-hour winds can double heat loss in a greenhouse. Well-designed windbreaks can reduce wind speeds by 50 percent, reducing heat losses by 5 to 10 percent compared to an open area.

Roof and Sidewall Greenhouse Vents

Traditional designs incorporate vents at the peak of the greenhouse, often on both sides of the ridge, but sometimes only on the leeward side (based on the prevailing winds). Single span naturally-vented greenhouses typically have inlet vents along the side walls.

Open-roof Greenhouse Designs

Many greenhouse manufacturers make structures in which the entire roof can open to provide ventilation (“open-roof” designs). These structures can often maintain relatively uniform greenhouse conditions at or slightly below ambient outdoor temperatures. Natural ventilation works primarily on the principle that wind blowing across a roof vent will create a low-pressure zone that will suck the air out of the greenhouse while letting air in the side vents.

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