solar heater for polytunnel

[tracie]

Can anybody remember!!.

A while ago I remember reading an article about making a solar heater for your polytunnel/greenhouse useing a old radiator, double glazing and copper pipeing.

Can anybody else remember this and if so can you point me in the right direction to find it again.

Thanks


Tracie

[Nature's Babe]

I guess simply painting a radiator black would retain heat
A few more ideas here -


http://www.ehow.com/way_5865565_homemade-solar-greenhouse.html

http://www.builditsolar.com/Projects/Sunspace/DougSolarGreenhouse.htm

http://hacknmod.com/hack/free-heat-using-pop-cans-and-plexiglass/

http://www.youtube.com/watch?v=173owKST-w4&feature=related

[Johnboy]

Hi Nature's Babe,
Actually if you paint a radiator black it will emit more heat than if painted white. Scientific fact!
JB.

[Nature's Babe]

Well that's weird Johnboy, because I had a black plastic solar shower, it certainly absorbed heat very quickly then we used the emitted heat, hot water shower.

[alan refail]

Black absorbs more heat - white reflects heat rays.

"Scientific fact"

So a black radiator filled with water would absorb more heat in the sun and have more heat to give out later.

[alan refail]

A question nobody has asked is: Can you or should you try to heat a polytunnel?

Given the greater heat loss through polythene, compared to through glass, wouldn't heating be a waste of time and energy?

[tracie]

Hi All,

thanks for all you wonderfull replys,

I was just looking to put a bit of heat in the the winter and early spring to get things on the move a bit. There is no electricity on site which is why I wanted a solar alternative.

Please keep your suggestions comming in.

Gratefull Tracie

[Johnboy]

Hi Alan,
What you say is certainly true but also it is true in reverse. Black emits more heat than white. In a solar heating system the whole idea would be to take heat in using black and assuming that you had some way of storing heat, emitting heat using white.
Apart from the point that it is totally unsuitable for a greenhouse or tunnel.
JB.

[alan refail]

Hi Johnboy

I totally agree with you on both points

[JohnN]

I'm puzzled! Why should a black object emit "more" heat (heat that's already inside it) than a white object? I can see why it absorbs more heat, because it does not reflect as much, but what is the "mechanism" that makes it emit more? Or are we getting our terms mixed, confusing "emitting faster" with "emitting more"? Only arsking!
John N

[Johnboy]

Hi John,
You are going to love this answer!
It is to do with the difference between the electromagnetic frequency of the colours.
JB.

[Geoff]

Does anybody know if storing some daytime heat like this makes much difference? How about building a central path from recycled storage heater bricks?

[JohnN]

Johnboy: Oh yes, of course.

Geoff: You've given me a wonderful idea I have lots of old storage heater bricks, so I think I'll lay them about 3 wide in a straight line, and then try and devise a "hinge" mechanism that will run along one side of the length. Sections of polytunnel/cloche could be hinged on this and, when the sun is out, lifted up to allow the bricks to heat up ready for the night time when the tunnel is closed up. The veg. would have to be in a narrow trough parallel to the bricks, or maybe in "sections" between lengths of brick. Perhaps the hinges could be long bamboo canes? Ideas on construction, anyone?
John N

[richard p]

thermal mass. ie a good solid thick brick wall is one of the musts in a solar heated eco house,.... thick brick internal walls will absorb solar energy during the day and release it into the house during the dark hours.... sometimes tanks of water are used in a similar manner.
brick paths in a tunnel will absorb energy during daylight and emit heat at night... how much and what effect it will have on the plants is anybodys guess...... but i suspect to get much of a bonus we are looking at making thick paths.... maybe a couple of feet thick rather than a single layer of bricks....
but its going to work well in a polytunnel in summer....least it'll stop the tomatoes getting chilly on a cold august night

[Colin_M]

sometimes tanks of water are used in a similar manner

Agreed - one of the classic ways of keeping an even temperature in a greenhouse is to have some large containers of water in there. Water has a very good ability to absorb thermal energy so is ideal.

Obviously there's the consideration of space and in a polytunnel that needs some thought. In a greenhouse, there's sometimes room under the staging. Some people use oil drums or plastic drums.

In the early spring, this approach will pick up a limited amount of thermal energy. When you get further into the year, it can help to absorb heat during the sunny parts of the day then protect against cold nights (eg. in May & June when there's still the occasional frost).