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How hot will this greenhouse get?

Posted by shimana (My Page) on
Thu, Aug 12, 10 at 12:47

Hi,

I'd like to place 2 transparent pieces of plastic (30 centimeters apart), over a well insulated box. The box would be about 2x2x2 meters.

In Western Europe or Britain:

On a sunny day in the summer (outside temperature 25 degrees Celsius) how hot would it get inside?

On a sunny day in the winter (outside temperature 0 degrees Celsius) how hot would it get inside?

The box would be placed so that as much sun as possible would fall on the plastic.

Thank you for your help!


Follow-Up Postings:

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RE: How hot will this greenhouse get?

30 centimeters is one foot.

25 degrees celsius is 77 degrees fahrenheit.
0 degrees celsius is 32 degrees fahrenheit.

2x2x2 meters is 7x7x7 feet.

Again, thank you for any replies!


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RE: How hot will this greenhouse get?

  • Posted by kudzu9 Zone 8b, WA (My Page) on
    Thu, Aug 12, 10 at 22:11

Sorry, but there are too many variables to give you an accurate answer. In general, though, I can say this:

1) In summer, it can get quite hot and you could fry your plants unless you vented it. I have a 10' X 15' greenhouse and the roof is covered with shade cloth. On a 25C day, it can get as hot as 40-45C inside the greenhouse even with my 4 roof vents open and a fan going.

2) In winter it can also get surprisingly warm compared to the outside temperature, but it will probably get down to the ambient outside temperature a couple of hours after sunset, so you could have plants that were fine in the day and die from the cold at night.

I know this is general, but you really can't get accurate answers without engineering analysis...and this would be dependent on details like seasonal variation in solar gain, cloud cover, materials of construction, etc. You'll just have to build it and see...and then make some modifications, perhaps.


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RE: How hot will this greenhouse get?

Thanks for your reply!

Actually, I'd like to see how hot I could make it ;)

So you have a greenhouse that gets to 45C on a 25C day in the summer, even with roof vents and a fan going, and a shade cloth.

If you removed the shade cloth, and closed the vents and the fan, how hot do you think it could get?

How is your greenhouse constructed? Double-paned glazing? Single sheet of plastic? Or?


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RE: How hot will this greenhouse get?

  • Posted by kudzu9 Zone 8b, WA (My Page) on
    Fri, Aug 13, 10 at 15:22

shimana-
I've never tried to see how much higher I could get it, because it would only be good for giving myself heat stroke, and I don't know any plants that would be happy at those elevated temps for long. The greenhouse is an aluminum frame with single pane, tempered glass. The shade cloth is white, covers the roof, and comes down about 1/3 on the sides.


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RE: How hot will this greenhouse get?

Thanks for your reply :)

I've found this: http://www.kyoto-energy.com/kyoto-box.html in the meantime.

Not really a greenhouse, but that looks like it can get up to 165C.


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RE: How hot will this greenhouse get?

Are you building a greenhouse, or a walk-in oven? I don't understand why you would want that much heat. Plants will die without a way to exhaust excess heat on a warm, sunny day.


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RE: How hot will this greenhouse get?

I'm trying to understand the principles of greenhouses, so I can apply this to building a solar water heater.

I'm still struggling with understanding where to find the insolation (strength of sunlight) on a particular time of day, region, season, and how this varies with cloud cover.

I'm also struggling with insulation values, and how the two combine to change the temperature.

What effect does the size of the greenhouse have?

And I'm thinking about what material to put inside the solar heater. Metal gives off a lot of heat under sunlight, so maybe that is a good choice? But why? Water stores heat energy, but what other materials do as well?

Etc.

(You could use the same principles to heat your greenhouse in winter and cool it in summer, and average the temperature over night and day.)


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RE: How hot will this greenhouse get?

Now this starts to make sense. This idea has been around for a long time. I remember looking at a solar pool heat collector design back in 1974 or 5 at a farm science review when I was majoring in agriculture. There should be some very explicit tables as to latitude and degree/cloud days for your area. I'm sorry I can't take the time right now to pull up your research for you but I can tell you it is out there.


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RE: How hot will this greenhouse get?

  • Posted by kudzu9 Zone 8b, WA (My Page) on
    Mon, Aug 16, 10 at 12:49

shimana-
If you want to get clear answers, you should clearly state what you're doing. So, despite what you titled this post, you're not actually trying to build a greenhouse, but a solar collector...for some purpose? Take a run at this again and explain precisely what you want to accomplish.


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RE: How hot will this greenhouse get?

Thanks for your replies :)

I tried to keep it simple ;)

I'm trying to understand the principles behind this, so I can use those principles for future projects, which might involve a solar heater or a greenhouse (in winter).

I've thought a bit more about it. I think the problem can be divided into several questions:

>>> How much of the sunlight is converted into heat, and how does that work?
>>> How much of the created heat escapes?

I always find it easier to try to understand things if I try to work out an example, so for the first question (I'll concentrate on this one first) that might be:

(Insolation)Sunlight: 1000 W / m^2
Angle of sunlight: 90 degrees
Surface area: 1m^2
Color of surface: blue ;-)

So, how does sunlight get converted to heat?

From wikipedia: >>> Infrared (IR) light is electromagnetic radiation with a wavelength between 0.7 and 300 micrometres (...) IR wavelengths are longer than that of visible light (...) Bright sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation. <<<

That information divides my original question into two questions:

>>> How does visible light get converted to infrared radiation?
>>> Where to put the original infrared radiation?

Again from Wikipedia:

>>> Heat
Main article: Thermal radiation

Infrared radiation is popularly known as "heat" or sometimes known as "heat radiation", since many people attribute all radiant heating to infrared light and/or all infrared radiation to heating. This is a widespread misconception, since light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun only accounts for 49% of the heating of the Earth, with the rest being caused by visible light that is absorbed then re-radiated at longer wavelengths. (...) It is true that objects at room temperature will emit radiation mostly concentrated in the 8 to 25 micrometer band (see black body and Wien's displacement law).

Heat is energy in transient form that flows due to temperature difference. Unlike heat transmitted by thermal conduction or thermal convection, radiation can propagate through a vacuum. <<<

In particular:

>>> The concept of emissivity is important in understanding the infrared emissions of objects. This is a property of a surface which describes how its thermal emissions deviate from the ideal of a black body. To further explain, two objects at the same physical temperature will not 'appear' the same temperature in an infrared image if they have differing emissivities. <<<


Useful Wikipedia links:

http://en.wikipedia.org/wiki/Insolation
http://en.wikipedia.org/wiki/Infrared

http://en.wikipedia.org/wiki/Black_body
http://en.wikipedia.org/wiki/Emissivity
http://en.wikipedia.org/wiki/Wien%27s_displacement_law

I suspect the last three links provide the answer, but I don't understand them yet ...


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RE: How hot will this greenhouse get?

Another useful link:

http://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation


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RE: How hot will this greenhouse get?

  • Posted by kudzu9 Zone 8b, WA (My Page) on
    Tue, Aug 17, 10 at 14:13

shimana-
Sorry to keep asking questions, but: 1) do you want to acquire a theoretical, scientific education about how solar energy works, or 2) do you want to acquire enough practical knowledge to build something? It looks to me from the references you cite that you're digging into a lot of theoretical areas that are interesting, but won't help you easily convert the knowledge into results. If you want to acquire knowledge at a less theoretical level and build something, then put the words "Solar greenhouse" into the Amazon Books search engine: there are many good ones out there that will get you to a goal faster than the approach you seem to be using. I'm not trying to insult you, but I studied all this stuff in engineering school and it's fascinating...but there is a big gap between theoretical calculations and building something that works. Also, for a general understanding of how heat loss is quantified, play with the greenhouse calculator shown in the link below...

Here is a link that might be useful: GH calculator


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RE: How hot will this greenhouse get?

I'd like to get a more theoretical understanding about how solar energy works. I think it's just a few pages of information. The challenge is to integrate information from multiple places into one understandable format.

If I have that, I can model different solutions and materials before building the thing.

I could of course just start building using the basic knowledge I have, and I'm sure I'd get an acceptable solution, as at that level it's not such a difficult problem. But it'd really like to understand the problem and model it beforehand, because of the trouble and cost of making the thing ...

I think it's just a few equations I need to understand -- not that difficult at all.

Here is a link that might be useful: Lecture - 10 Thermal Radiation - 1


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RE: How hot will this greenhouse get?

Thanks for the tips, btw :)


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