Paint It Black - Your GH Wall!

No the wall radiates out the heat so it is radiation. I think the wavelength might be different, can't remember whether it is longer or shorter but it makes it more likely to be bounced back by the glass.
I think the weakness is the back outside wall. The brick wall will absorb the heat so preventing some being bounced back into the greenhouse but it will release most stored heat out to the outside. Now if there was some way of insulating the other side??
The water or any heat store inside the greenhouse will be a winner as it will absorb radiation in daytime and then there is only one place for it to reradiate out to.

 

Hi Pete. It is a bit complicated as heat is both gained and lost by three mechanisms - conduction, convection and radiation. It's difficult to know the actual mix, without a precise experiment. We tend to think that radiation is only associated with very hot things and this is largely but not totally true.

Every body radiates heat to every other body. The heat given off is related to the fourth power of the absolute temperature in degrees Kelvin, which is degrees C + 273. So on a sunny day we actually receive 5,800^4 from the sun, but give back 300^4. Given that the temperature of the sun is 5,800K and our temperature is roughly 300K (or 27C). Its a no contest we are the winner; but you must remember it also depends on the area, we radiate heat 360 degrees, whilst what we receive from the sun is only a small angle. But at night we are still giving out 300^4 but we don't get much back. On a cloudy night we get about the same back, ie 273^4 if the clouds are at freezing point (0C), but if it is a clear night what we get back comes from outer space, which is only at about 2K. So we get back 2^4. Although these look like small figures compared to the sun, they are big enough to explain why we get a sharp frost on a clear night and but not on a cloudy night.

Another illustration is that when people are found on mountain tops suffering from the cold, they are first wrapped in a very thin reflective melinar sheet. This reflects their own radiation back to them, and hence cuts out any radiation heat loss.

Sorry it it is a bit complicated. The real solution from this is to have reflective melinar curtains, which are drawn at night to reflect the radiation back into the greenhouse. :D

Sorry Geoff, I crossed with you.

 

Thanks for that PeterS. Now all I need is curtains in the GH - problem with that is I'd have to remember to open them every morning :D

Here's a link with some quite interesting stuff about thermodynamics for anyone interested in expanding on Peter's explanation-
http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html

 

Thanks Dave - thats a nice site. There is a lot of info there.

I have been tempted to do a little calculation. On one hand the sun is very hot and the figure becomes huge when raised to the fourth power, but on the other hand it is an exceedlingly small percentage of the sky, and we get virtually no radiation from the rest of the sky. By contrast the greenhouse is at a much lower temperature than the sun, but it radiates heat to the entire sky.

My calculation shows that the greenhouse (or the garden as a whole) radiates to outer space heat at the rate of 17% of what it receives from the sun. During the day it receives so much more that it heats up. But on a clear night the greenhouse is still radiating heat out at the same rate, and getting nothing back. Though, as I said above, on a cloudy night it radiates out at the same rate but gets almost the same back from the clouds.

I am amazed that the figure is so high. But it comes about because the sun is such a small percentage of the sky. The figure is logical. It does explain why clear nights in winter are so frosty compared with cloudy nights.

 

I have been thinking about the above figure that the ground/greenhouse radiates out at 17% of the rate of the energy that it receives from the sun. I did this calculation purely based on the sun's temperature and its size and distance from the earth. There was no allowance for the effect of the atmosphere or the angle of the sun's rays - so this figure would have applied at the equator. I thought it was a lot - but I now think it is too small.

If we take the planet earth. It receives 100% of its heat from the sun by radiation. But it also re-radiates exactly the same amount of heat into outer space. There is no convection or conduction for the planet as a whole. If there were any difference between these two figures the earth would be either heat up or cool down. But we know that the temperature has on average been constant for the last 500 million years.

What this says is that radiation from a cool body is very important. 100% of the heat that we receive we re-radiate. 50% during the day and 50% during the night, averaged over the year, and over the globe. Not the 17% I previously calculated. Its obviously a lot more complicated than that. We don't know if the ground radiates it all, or if convection carries the heat up to the clouds and it is then radiated from there.

 

As you say Peter very compicated, I just remember someone saying once that most of the heat that you get from a Radiator is convected and not radiated.
Surely thats why we blow air through a car radiator.
I know thats pretty basic stuff and may be wrong, but thats me,.....basic.

 

Convection is concerned with movement. The radiator radiates out heat to its outer surface and passes it to air it comes into contact with. The convection is when that air rises up - You have convection currents. Unless you have molten metal moving the heat , as in the earth's core , then you don't get convection.
I don't think I go along with the calcalations. There are so many variables that affect a garden's absorption of energy. the amount of vegetation is one, vegetation soaks up more radiation than bare soil which is inclined to reflect it, the colour of the vegetation affects how much is reflected, the colour of the soil, whether the soil is wet or dry, whether it is clay, sandy etc. The amount absorbed will also depend on the angle of the sun. also a lot of soalr radiation is bounced off the outer atmosphere, absorbed by the outer atmosphere, bounced off by clouds, dust particles etc.

 

I have been pondering this - its a subject I have never really thought about before. But I do believe that the figure of 50% is in the right ball park.

Did you see the calculation on the site that Dave found. It said that sunlight provides about 1000 watts per square meter, of which 30% is reflected by human skin. ie the skin receives 700 watts per square meter. It also said that the human body radiates about 550 watts from 1.5 square meters of surface area. ie about 370 watts per square meter - or 50% of what it receives. Whilst the sun shines 12 hours of the day, the human body is radiating 24 hours a day.

Bare earth, at the same temperature, will do the same. Earth is darker than skin so it will absorb more that 70% of the suns rays, but it will also radiate more. Absorption and radiation coefficients are always the same.

And so will a greenhouse. The heat radiated from a greenhouse is essentially from the floor area, rather than the glass area. Even so, a 6 foot by 6 foot greenhouse is radiating from 4 square meters and at 370 watts per sq m - thats 1500 watts of heat being radiated. Thats at body heat 36.6C, its only about 60% of that at 0C.

 

I find this a really interesting subject, the problem is there are so many variables involved that it's nigh on impossible to come up with anything other than approximations. The important thing is that with a basic understanding of the principles of thermodynamics we can perhaps make better use of the "free" heat our green houses receive.

I bet that someone, somewhere has a nice little computer model that will do the calculations. Years ago when we first put in a central heating system I built a spreadsheet to calculate our room by room and overall heat losses and heating requirement. However that was a fairly crude and simple calculation compared with the one under discussion.

 

I did exactly the same Dave for house heating. I wrote a little program in basic. But the key thing was that it had the exact dimensions of every wall, window and door of my house not the 'average house'. I could also adjust the u-value of every part, so I could double glaze just a single window, or all of them, and I could turn individual radiators on and off. I played with the program and found that I learnt a lot.

The two main things that I remember, is that even after double glazing the main heat loss is still through the double glazing. And the other is that most information about loft insulation is untrue for me. If you heat the upstairs of your house to 70F then you lose a lot of heat through the loft. But I only heat the downstairs, consequently the upstairs is much cooler and very little heat is lost through the loft. So extra loft insulation doesn't help.

I have found thinking about radiation most interesting. As you say it is very complicated. But I have realised that it is much more important than I had realised. The fact is that all the heat the world receives from the sun during a 24 hour period, is re-radiated to outer space. Wind and convection will move it from one place to another, but eventually it all has to be radiated.

On a more practical note - I think there is a good arguement for having an automatic blind system with reflective melinar (like the survival blankets). It really could keep your greenhouse warmer. http://www.outdoorscotland.co.uk/shop/cs02_reflective_survival_blanket.htm

The other thing is to not have glass at the bottom or at the back of the greenhouse. It loses a lot of heat, but doesn't contribute much.

 

Well I think I've learnt something, but I must admit I'm not quite sure what. :D
I think it all goes back to the fact that even the victorians and before knew the benefits of growing on walls.
And if the wall be housed inside a greenhouse, so much the better.

 

On Radio 4 'Gardener's Questions' it was suggested that a good steaming compost heap be built inside the greenhouse/poly tunnel during winter to keep the temps up. They didn't say anything about the pong though

 

Or the not so friendly mini-beasts.

 

Reminds me of the pineapple forcing pits I saw at Heligan a few years back, Froggy.

 

A great idea in theory, but sadly beyond the means of the majority of amateur gardeners. IF!! I had the space and resources I'd look at building a big heap and extracting the heat to the GH using water filled tubing. If I could get a heap big enough I could run a methane extraction plant too :D

One can but dream