Growing in a year-round greenhouse often means contending with heat. For most crops, photosynthesis slows when temperatures exceed 95 F. As the temperatures start to climb, the most energy-efficient ways to cool a greenhouse need to be explored.
The first is to reduce light and heat gain to only what is necessary for growth. While light requirements vary by crop, most commercial greenhouse growers shoot for a Daily Light Integral (a measure light useful for growth) of 12 – 20.
The Daily Light Integral maps from Purdue University show outdoor light levels across the US. Keep in mind that siting and shading factors will significantly reduce light inside the greenhouse. If you already have a greenhouse, you can more accurately measure indoor light levels with a light meter.
Once you know target light levels, you can select a shade fabric. Fabrics range from 10 to 100% shading factor. 100% shade cloth is also known as a black-out or light deprivation fabric. Ensure that your shade system is automated or easily removable so it can be taken down in the winter. It is also a good idea to use UV-stabilized fabrics, which increases longevity.
For the best results, install the shade cloth on the exterior of the greenhouse roof, to keep the heat out of the building. Shade cloth under the roof still helps, but is less effective because the heat has already entered the greenhouse. In places with high winds, expect to replace the shade cloth every few years.
Ventilating the greenhouse is the easiest and least expensive way to cool a greenhouse, and should be used whenever outdoor air temperatures allow. Methods can be active (using fans) or passive.
Passive ventilation requires operable vents (both intake and outtake vents). Passive vents don’t require electricity and are an excellent tactic for efficient cooling, but one that must be designed into the greenhouse when you choose the structure.
Active ventilation uses exhaust fans and intake vents. A qualified greenhouse designer can help customize fans for your cooling load. A common rule of thumb is to size fans they can exchange the entire volume of greenhouse air at least once per minute, though this varies by climate.
Ventilation should be used when possible. In very hot climates this may only be at night, necessitating additional cooling methods.
The cooling effect of evaporation has been used for millennia. As water changes from a liquid to vapor, it absorbs heat from the air. Evaporative coolers, misting systems and wet walls take advantage of this process to efficiently cool greenhouses.
Wet walls are the most common systems in commercial greenhouses. Large fans blow air over a wet pad on one wall of the greenhouse. Residential greenhouses can employ the same methods using portable evaporative coolers often called swamp coolers. Misting systems cool the air by spraying a light mist into the greenhouse which quickly evaporates.
In all cases, these systems can cool a greenhouse by many degrees while only using a small amount of energy. Their primary limitation is that they work best in dry climates. Their performance decreases when there is more water vapor in the outdoor air (something you can see in performance charts of evaporative coolers). However, technology is expanding to enable wet wall systems for humid climates as well. Florida growers will rejoice.
Overheating mainly occurs during the day. Thermal mass and heat exchangers store excess heat gain in a medium. This both cools the greenhouse during hot periods and heats the greenhouse when it gets cold. Thermal storage tactics can be very simple as well. Water walls are a longtime method to help even out daily temperature swings. Their primary drawback is they take up space in the greenhouse.
All heat storage strategies work best in climates with some temperature fluctuation. Water walls and PCM work best when nighttime temps cool down sufficiently. GAHTTM systems / climate batteries work best when the soil deep underground has a moderate temperature, typically in seasonal climates.
In very hot climates like Arizona, year-round growing requires completely enclosing and air conditioning the greenhouse. This is an expensive recourse but often necessary when outdoor temperatures are too hot for growth, and / or if you require a highly controlled environment.
Though chillers and air conditioning units are expensive to run, the industry has made gains in efficiency. Chillers cool water via a refrigeration cycle. Cool water is then circulated in the greenhouse or combined with air handlers to directly cool indoor air.
The last cooling strategy is a preventive one. Designing an energy-efficient greenhouse greatly reduces the need for cooling in the first place. Talk to an experienced greenhouse designer about selecting the right materials – particularly glazing – for your climate. Greenhouses in Arizona, for example, should have lower light transmittance glazing than those in Alaska. If conditioning the space with chillers or wet wall systems, insulated walls help keep the cool air inside. An insulated wall on the north side makes the greenhouse much more energy-efficient, while still achieving sufficient light levels (as a major principle of passive solar greenhouse design).
Lindsey Schiller is with Ceres Greenhouses Systems CeresGS.com.