• Post Reply Bookmark Topic Watch Topic
  • New Topic
permaculture forums growies critters building homesteading energy monies kitchen purity ungarbage community wilderness fiber arts art permaculture artisans regional education skip experiences global resources cider press projects digital market permies.com private forums all forums
this forum made possible by our volunteer staff, including ...
master stewards:
  • Anne Miller
  • Pearl Sutton
  • r ranson
  • Nicole Alderman
  • Mike Haasl
stewards:
  • paul wheaton
  • Joseph Lofthouse
  • James Freyr
master gardeners:
  • Carla Burke
  • John F Dean
  • jordan barton
gardeners:
  • Jay Angler
  • Greg Martin
  • Leigh Tate

Greenhouse cooling requirements

 
Posts: 34
1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Hi,
I'm planning ahead for a horizontal geothermal cooling/heating system for a 2600 sqft closed greenhouse. I'm learning everything I can...
To properly size it I would first have to know how much cooling requirements such a greenhouse has. Cooling will be my biggest issue. The greenhouse would be situated in Bodrum, Turkey. USDA Zone 9bish...10a. Wasn't easily recognizable on the map I had. The yearly average solar radiation is 3.6 kWh/m2 per day, and the total yearly radiation period is approximately 2,640 hours. Highest temps would be 40°C/104°F. And I need to keep it best at max 27°C/81°F. Evaproative cooling is not an option. Venting off heat is, but considering global warming and the labour involved in geothermal, I'd like to size it directly as if there was no vents and I had an AC running to cool it.

Can anyone tell me how many kW of AC cooling I would need?

I have searched the internet far and wide and couldn't find any clear data.
I would imagine, that greenhouses will more or less behave in a linear manner according to the USDA zones they're in and the design. Or am I wrong!?
 
master steward
Posts: 9371
Location: Northern WI (zone 4)
2710
hunting trees books food preservation solar woodworking
  • Likes 2
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
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
Posts: 34
1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

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?  




Hi Mike,

Thanks for replying. Sorry, for not having been clear enough. I plan on putting in geothermal cooling tubes. I will not be using a real AC, I would only need to know the kW of a figurative AC to get outside temps from 104F down to 81F inside. With those kW I could size the amount of cooling tubes required and the size of the excavation. The AC doesn't really have to exist. I would just like to know the cooling requirements of the greenhouse.

I still have to pick the exact style of greenhouse, but I think it will be a gable roof, there will be no vents on the top ridge.
It will have fans and they will be used to vent heat, but since temps are globally on the rise and geothermal is a labour intensive project, I'd like to build in a good amount of redundancy.
I hope I was clearer this time around.


 
John Steadfast
Posts: 34
1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

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
master steward
Posts: 9371
Location: Northern WI (zone 4)
2710
hunting trees books food preservation solar woodworking
  • Likes 5
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
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?
 
John Steadfast
Posts: 34
1
  • Likes 2
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

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?



Thanks... It's all in the redundancies... Venting will not get me there, but a little closer. One must also consider the cooling factor of the plants themselves. But yes, thanks you were very helpful.
I even finally found an interesting article on cooling calculation... practically the same you said.

http://www.agroponic.com/wp-content/uploads/2014/02/asktheexperts_SolarGainCooling.pdf
 
pollinator
Posts: 564
Location: Boston, Massachusetts
182
urban books building solar rocket stoves ungarbage
  • Likes 1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
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.

 
John Steadfast
Posts: 34
1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

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.



Hi Kenneth, Thanks, but shading is absolutely not an option.
 
Mike Haasl
master steward
Posts: 9371
Location: Northern WI (zone 4)
2710
hunting trees books food preservation solar woodworking
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
What are you trying to grow in this greenhouse?
 
John Steadfast
Posts: 34
1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
I will be growing cannabis, as soon as we get out license.
 
pollinator
Posts: 3114
Location: Massachusetts, Zone:6/7, AHS:4, Rainfall:48in even Soil:SandyLoam pH6 Flat
324
forest garden solar
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
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.

 
John Steadfast
Posts: 34
1
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

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.



Thanks, but that would still be an empty greenhouse. The plants would take care of a good amount of solar gain, too. And luckily geothermal heat pumps can turn any 1kW into 4kW of cooling power. So let's say I'd be down to 220kW a day taking away a minimum of 33% re plant transpiration we'd be left roughly with 150kW, but it is still a 12.5kW/h, since the 3.6kW are distrubuted over 12h and not 24h, I assume. Now I can at least roughly size the amount of geothermal piping I will need and if I should do it in soil or water.

Thanks again
 
A wop bop a lu bob a womp bam boom. Tutti frutti ad:
100th Issue of Permaculture Magazine - now FREE for a while
https://permies.com/goodies/45/pmag
reply
    Bookmark Topic Watch Topic
  • New Topic