self heating greenhouse

[cortina1498]

Hi, I mentioned a long, long time ago that I was trying to build a self heating greenhouse, well, I have partially succeeded.
It consists of an old computer cooling fan (12v) fitted into the end of some fall pipe, this is positioned up in the ridge of the green house, approx in the centre. the fall pipe is then led down the eaves and wall to the top of a metal box, approx 2' x 18" x 18", the box is painted matt black and filled with small black pebbles (about golf ball size) and there is an pipe outlet from the bottom of the box.
How it works, the fan runs 24/7 and during the day it takes the hot air from the top of the green house and puts it into the box, warming the pebbles, so that the air leaving the box is cooler. During the night the same fan blows the air over the warm pebbles and the air from the outlet is warmer! The whole system is run from a leisure battery and a small solar panel so it costs nothing to run.
It is not really a self heating green house, but it has considerably lowered my day time temperature and, more important, has increase my night time temperature, making for a better greenhouse environment
Hope this is of interest to somebody.
Cheers
barry

[Archie]

Simpler than the buried insulation that I have heard about. If the pebbles are in a box does it matter they are black.
Also an improvement on the black painted plastic water barrel that I have.

[cortina1498]

Hi Archie, I don't know if the pebbles being black helps, I just worked on the theory that matt black absorbes heat better, but being in an enclosed box does it really matter?
The truth is I had a load of small black pebbles.
Cheers
barry

[Archie]

I have some storage heater bricks. not quite the same thing...would take too long to heat perhaps.
Are you growing low light winter stuff....like lettuce?

[Muddypause]

Here's some theory, if you are interested:

Painting the pebbles black isn't really going to make much difference - this is something that only applies to radiant heat (remember that heat can be moved about by either conduction, convection or radiation). I would contend that in this situation, the heat is being transferred from air to pebble (and back again) mainly by conduction.

In the case of water barrels, painting them black will make a big difference if they are in direct sunlight. In that situation a good deal of the heat they absorb will be radiant heat. Conduction and convection will play a part here, too, but the colour of the barrels will not affect that. When the barrels are re-emitting their heat (when their surroundings have become cooler than the water in them), the black colour will also help them radiate some of their heat - though most of it will probably be lost by convection.

Generally, I think pebbles would work better than large storage heater bricks, because in this situation the temperature difference is relatively small, and fairly small amounts of heat energy are being dealt with, so the larger surface area of lots of small things will maximise the amount of energy transferred. Presumably, also, the air can fairly easily flow between and around the individual pebbles when they are piled together. ISTR someone on telly recently did something similar, but used crushed glass as the heat store.

Another factor to consider it the heat capacity of the material used - that is to say the amount of heat which it can store. Weight for weight, water has a very high heat capacity, but glass and stone are probably pretty good, too (depending upon the type of stone).