Greenhouse cooling requirements

Mike Haasl wrote:Could you clarify a couple things.  Worst case, is the outside temp 104F and you want to keep the inside at 81F?  I don't know if they make AC big enough for that.

What style of greenhouse?  Can it have vents at the peak?  

I'm not understanding why you wouldn't want to vent heat in the summer?  

John Steadfast wrote:

Mike Haasl wrote:
I'm not understanding why you wouldn't want to vent heat in the summer?  

I'm just calculating as if I wasn't venting the heat...

Mike Haasl wrote:Ok, that makes a bit more sense.

I don't know how possible it is to keep a greenhouse 20 degrees F below the outside temps if the sun is out.  You're fighting a lot of solar gain.  

I think the way to calculate it is to take your monthly solar gain in your worst month (July or August?) and do some math.  That solar gain multiplied by the glazing area that is perpendicular to the sun is about how much heat you'll pick up.  Think of it as the rectangle that the sun is looking down on from its vantage point.  If the glazing is significantly angled relative to the summer sun, it will help reflect some of that heat but I'm not sure how much.

So that gives you your supply of heat.  Then you need to calculate your heat gain/loss through the glazing due to a difference in air temp.  This is trickier because the temp changes outside day to night.  If the 24 hour outside temp averages 81F I'd consider it neutral and not worry about it.  If it's higher, then you're picking up heat through the glazing as well.  So you'd have to know the R value of your glazing and walls and calculate the loss.  

After that, you have the numbers you need to calculate your cooling needs.  If you put shade cloth on or whitewash the plastic, that would cut down on the solar gain.  Thermal mass in the greenhouse will balance out the highs and lows but not help the average indoor temp over the long run.

I hope that helps a bit.  If you're trying to keep it cooler than the outside then venting won't get you where you need to go.  Maybe move somewhere colder?

Kenneth Elwell wrote:John, you might take a look at what they've done at Tamera in Portugal. Their greenhouse has built-in solar collectors that effectively shade the greenhouse while also putting that solar energy to use. The main take away is that in the summertime, the day length is more than sufficient, so shading prevents overheating, without depriving the plants of necessary light.

S Bengi wrote:Area = 2600sqft = 240sqmeter
Sun/Heat input = Area x solar radiation = 240 x 3.6kw = 870KW/day
Lets say your AC unit had a 3 to 1 ratio you would still need 290kW/day or 12kw/h AC unit.

But not only do you have heat making it's way in from the sunlight, you also have leaks/convection and conduction.