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How To Cool A Greenhouse

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Ways to Cool a Greenhouses


Too much heat is caused by either too much light or not enough ventilation or both. Professional greenhouse temperature control is a dynamic system that responds efficiently and quickly to temperature changes. The temperature inside a greenhouse can get out of control very fast. The ways that people cool greenhouses depends significantly on climate and the value of crops. Here are some effective ways to cool a greenhouse.



Reflectivity in the greenhouse


Usually you want as much of the greenhouse as possible to be white. This reflects light around the greenhouse (good for plant growth) and means that the greenhouse absorbs less heat.


--white ground cover


--white heating and irrigation pipes


--white walls against the header house (non-transparent areas)



Exclude light (and heat) from coming in


  • Use whitewash (paint-like material) that is sprayed onto the outside of the greenhouse for summertime in many areas. There is also chemical used to remove the white wash when one is done with it.


  • Install greenhouse curtains. Traditionally these are placed inside the greenhouse above the trusses (and can be open and closed by your climate computer).


  • Find a way to place curtains outside the greenhouse. These are unusual in North America, but I have seen them in China.


  • If you have excess light/heat year-round, you can install plastic/glass with some light exclusion built in.


  • There are some bubble systems which blow soap-like bubbles between the layers of glazing. These can have different characteristics including light exclusion. I know it sounds a bit crazy, but apparently it works. Note, though, that as far as I understand there is some dispute about who invented this (and owns the patent for it). Also I talked to one company about this for our greenhouse, but they were not very interested in quoting for us since we are out in the middle-of-nowhere and our greenhouse is measured in partial hectares not tens of hectares. I don’t know of anyone who works with small greenhouses on this technology.


  • Grow in a warehouse or underground (like in an old mine) instead of in a greenhouse and use lights rather than heat/cool with other systems.



Increase the humidity. 


  • Install misters (good ones are quite expensive, poor ones can create problems).


  • Find a way to wet the floor of the greenhouse during the day.


  • Swamp coolers (running air through a cardboard-like woven material that wicks water). This needs some extra design elements as they can only cool a certain amount of distance away from them.


  • Cooling foggers (there is probably a different name for these, but they basically create a mist throughout the greenhouse).



  • This only works in dry areas; you should try very hard to keep your greenhouse below 85% humidity—in most cases this is much more important than manipulating heat



Active Cooling Systems


  • Air conditioning. Ridiculously expensive (but effective). If you have specialized orchids or some other very challenging but valuable crop this might be worth exploring


  • Heat pumps. Basically a compressor system that runs water from underground or from a lake/river/ocean/pond into the greenhouse through radiator-like mechanisms to remove heat.



Venting 


  • Let as much air from outside run through the greenhouse as possible


  • Roof vents are most common. Some greenhouses allow the roof to open nearly 100%, but most just have vents at the roof that open as the greenhouse is too cold


  • Side vents. Usually the side wall (or much of it opens and allows a breeze through).


  • Fans. Fans push hot air out (negative pressure) or push outside air in (positive pressure — better for insect control if you screen on the outside of the fan). You can also get foggers/misters/swamp coolers that attach to many fans and thus combine these with humidity increase strategies.



Air movement

Use horizontal airflow fans (HAF) to move air in a circular pattern around the greenhouse. Traditionally these are installed above the crop, but I have also seen them installed under benches.


This helps to reduce stratification (layers of different temperature). This way you don’t have a situation where the head of, say, your tomato plant which is 11 feet above the ground (or whatever you have) is much hotter than the bottom of the plant. The plants will respond to the temperature of their head more than other parts, so often you want the whole plant to be similar in temperature.


This also keeps pockets of increased humidity from building around your plant leaves (you want to prevent this).



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