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Posts: 283
Reply with quote  #1 
For anyone interested here is a useful site i came across for performing the  calculations for finding the BTU requirements of a greenhouse.
Regarding the area calculator,if you have a quonset or hoop type, the formula used for calculating the circumference of a semi circle is (2*pi*r), r here is your height and pi is 3.142

I have used the calculator and have ended up with a figure of 125040 BTU(56kW) in know that my geographic location is of importance.What data do i need to use to figure out how many square meters(or ft) of panel i will need to keep the greenhouse at its required internal temperature?

For my calculations ive used 14 f(-10 celcius) as my lowest outdoor temperature it is quite rare that the temperatures here get that low.It was used as a gauge for extreme conditions its quite likely i dont need all the BTU.
Here are the links to the calculators ive used.



I  have attached my calculations in the images below .
Monthly_heating.jpg btu_to_kw.jpg 
Regards Mattie


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Posts: 2,240
Reply with quote  #2 
Hello, Mattie,

The first thing is to find your local irradiation value ("insolation").

I found you THIS: http://solarelectricityhandbook.com/solar-irradiance.html

(Maynooth is a nice place, but I prefer Castlerea...)

Next you need to do a solar survey - this is HERE...  http://www.builditsolar.com/SiteSurvey/site_survey.htm

Then we'll get on to working out your panel size, using your heating requirement.
(BTW, that was nice tools U found, up there...)

(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...

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Posts: 283
Reply with quote  #3 
Thanks again,
Yeah i found that site useful,hopefully it can help a few others too.
Ill be back when i have the details i need.
Regards Mattie

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Posts: 283
Reply with quote  #4 
Hello Again
Here is the insolation data for the location nearest me,

Optimal 37 degree

optimal 37.jpg



A site survey is not possible at the minute, as i don't have one yet.The solar elevation and azimuth gauge are very useful when i reach that stage.For now its preliminary work .The Google sketch-up technique also looks highly useful.
I know if i perform whats needed here regarding the site it will paint a more accurate picture but its not possible at the minute.
Is there a way i can get a generalized result from the insolation values and from that work out how much panel i would need? Im trying to get a figure on what the whole build will cost, so i can make better judgment of what type of system might be the best route to take.
Regards Mattie


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Posts: 283
Reply with quote  #5 
Heres another way to get azimuth for anyone interested http://tools.solmetric.com/Tools/roofazimuthtool

Posts: 243
Reply with quote  #6 
Hi Mattie,

You might find this link interesting 
http://www.builditsolar.com/Projects/Sunspace/sunspaces.htm#Greenhouse Plans
Its the first one on the U of Missouri greenhouse.
There approach is to build a GH that has much less heat loss so not so much solar heating is required to keep it up to growing temperatures.


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Posts: 283
Reply with quote  #7 
Thanks Gary,
This seems a lot more logical from a cost point of view(comparing to L90ft * W 18ft hoop house i seen for 1500 uk or 2500 roughly in dollars). I will be renting land and need to have the option of moving if needs be,this greenhouse is a lot more bare-bones which i like a lot.When building it could simply be a case of designing it to be taken apart again.

Another option here would be to use a drain back type solar panel system with some of the barrels used for the thermal mass also functioning as the thermal storage.The greenhouse has the insulation, would have to look at the insulation specs on the original tank design to see how well they would compare,perhaps some insulated barrels could provide buffer energy if its required(thermostats between thermal mass and thermal storage).

I also looked into trying to size the required panel area for the 56 kw power requirement i had earlier.Going on what i found so far, i needed to include the solar fraction for my area, the efficiency of panels and work out from there what area of panels i would need. I read somewhere 800 btu/ft^2 of panel as rule of thumb not sure how accurate this is.I know that a panel output is entirely dependent on the input so it varies a lot so not entirely sure here.
Regards Mattie

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Posts: 2,240
Reply with quote  #8 
Hey, Mattie,

First off, hats off to Gary, for posting and hosting that University of Missouri GH project (cf. Item #6).

I read the whole thing with interest, and noted a few salient points:

  • The MO GH was built for a worst-case scenario (on top of a windy hill), on the basis that it "could only do better" in another locality…
  • Even during one of their coldest winters on record, the GH never dropped below freezing point.
  • On the coldest night ever recorded with GH greenhouse in operation ("bitterly cold –13°F in 1996") the plants did not suffer, as the interior temperature held steady at 33°F.
  • Their barrel water minima were 43 to 79°F, and the maxima were 48 to 91°F

I see your project requires to cover for minima of 14°F, and a required GH temp of 65°F.

A "steady 33°F" is obviously not enough for your purposes.
Therefore the MO GH needs beefing up in some way, to produce the extra heat.

For reference, here's your input data:
  • minimum outside temp               = 14°F (or -10°C)
  • constant GH interior temp req't   = 65°F (or 18.3°C).
Therefore, a priori, you will need additional heat in the coldest of winter, to meet the 65°F requirement (because the MO water minima dropped to 43°F… which is too cold for your spec.).

(I guess this minimum is for FISH reasons, rather than plants ?).

(in which case, perhaps for the coldest period of the year, you could just warm the WATER (with a heater…) rather than designing & heating the entire greenhouse, year-round on the basis of 65°F)  ?

(Just a thought...

In any case, IMO, it is not a panel (or panels…) that will be affording you *that* degree of protection, because the panel(s) would be as cold as the GH itself… and presumably if it is *that* cold anyhow, then even a panel is not off to work becos no sun…

I found this site, concerning your local weather…


Your area is one of the coldest in Ireland !

The coldest areas are found inland: Clones (County Monaghan) and Mullingar (County Westmeath) both have the lowest annual mean temperature, at 8.8 °C (47.8 °F)[UNQUOTE]

However, it looks like your mean December temp is not below 4°C.

Also, your overall Winter mean, although one of the very lowest, is not THAT cold,  compared with the Missouri Study…  (Still, that fact will be of little comfort to your FISH...).

Severe freezes occur only occasionally in winter, with temperatures below −10 °C (14°F) being very uncommon, and temperatures below freezing uncommon in many coastal areas.[UNQUOTE]

I note the last line – 14°F) is « very uncommon »  as this corresponds to your own input minimum…
So your minima is (at least) "uncommon"..
(even if your maxima is "demanding" !)
============= =================

So although your above post addresses a panel question, excuse me for ignoring the panel business, since my first reaction would be to compare your heating requirement in more detail, against the reference MO GH, to put things into some sort of perspective.

Once that is done, you could then examine improvements (as they suggest...) to adapt the MO GH design to meet your requirement of 65°F.

Again, not wanting to be a bore, IMO the required top-up heat would NOT come from a solar panel in any case, but from another heat source than solar (such as gas or electric heating, generator, lighting etc.).

So (finally...) turning to your heating requirement, of 56 kW of heat.
(apologies for using metric again…) (it’s become a bad habit…)

your temperature difference is 65°F – 14°F
that is 18.3 (say 19°C) minus –10°C
giving a TD of 29 kelvin

56 kW ÷1.163 = 48,150 kcal
÷ 29 = 1660 liters of water
(= 438 USG, or 365 imperial gallons)
crosschecking : from your spreadsheet:
power = 193,244 BTU ÷ 51°F = 3789 lbs of water
divided by 8, = 473 USG.
(The 35 gallon difference in the USG is because I calculated my kelvins for 19°C, not for 18.3 °C, which is your 65°F requirement).

Therefore, to recap,

  1. the Missouri Greenhouse could almost meet your requirements, as-is, using the same approach but perhaps with the additional insulation that they forgot to install on the backwall...
  2. You will most likely need additional heating to hit the 65°F requirement.
  3. This would come from another heating source than solar (gas, electric, generator etc.)
  4. paid for by the money you would save by not constructing panels.

In addition, without getting involved in your business plan, if as you say U are likely to have to change site at some time, and your GH is presumably knock-down, not having to transport and re-erect panels would no doubt be a considerable saving.

Good luck with the rest of your thinking !


(1)  "Heat goes from hot to cold, there is no directional bias"
(2) It's wrote, "voilà" unless talking musical instruments...

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Posts: 283
Reply with quote  #9 
Thanks to both Garage hermit and Gary,on point and insightful; as allways.[thumb]
Regards Mattie
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