Passive Solar Heaters

Designed and tested by Tarland Young Scientists with help from CFG

These little heaters can be positioned in your greenhouse near your most vulnerable plants, to help keep the plants frost free. You can easily make them yourself.

  • find a clear (not coloured) transparent plastic container. Keep the top but take off any labels. If possible use a container with a wider, more stable base, than a fizzy drinks bottle.
  • the larger the container, the better it will work, but the small ones as shown (this is around 1.5 litres) are invaluable for placing alonside vulnerable plants and are easy to move.
  • find some black plastic either bin bags or go for a walk and pick up bits of black plastic silage wrap
  • stuff the container with crumpled up black plastic reserving enough to wrap the whole container in black plastic (critical to do this).
  • fill the container, preferably with old spare olive oil. Otherwise use ordinary cooking oil. Keep going until you have got as many air bubbles out as possible.
  • do not use water, as that will burst your container if it freezes.
  • put the top in and seal it with something like electrical insulating tape to stop it leaking if it falls over, and place it near the plants you want to protect in either a frame or a greenhouse
  • these heaters do not need to be in direct sunlight, but if they are, that’s even better.
  • they work because we tend to have sunshine on really cold nights so the heat in the greenhouse is good during the day. They hang onto the heat and let it out during the night.
  • they will not heat a greenhouse in the depths of a really cold winter, or in a period of long grey days, but they are invaluable when you have short sharp frosts in the spring

The science :

Test results show that the heater (blue) radiates heat gently long after the air temperature (green) has dropped.

They use the properties of black bodies – black things absorb and give out heat more effectively than white or shiny things – hence why log stoves are usually black

They also use the properties of thermal mass because dense things cling onto the heat within them and let it out more slowly, than air.

If you can fill it with olive oil, this also “freezes” (i.e. turns solid) in a refrigerator at about 4 to 5 degrees Celsius. That is really useful as it means you also get heat given out by the act of freezing, which is known as a “phase change” – i.e. it turns from one state of matter to another.

Phase changes take in a lot of heat when going from solid to liquid or liquid to gas, and release a lot of heat when it goes the other way. Hence why you heat your water to make steam, and cool it to release the heat to make ice cubes. (This property is used by apple growers to protect blossoms in the spring from frosts by spraying them with fine droplets of water).

The results above were using candle wax from old candles not oil, which is nice and dense but very difficult to use to fill the containers and the process is highly flammable, so we do not recommend that. Candle wax also only changes to a liquid at 49 degrees, so the phase change effect never comes into play.

Cooking oil is much simpler and much safer and in really cold conditions should release more heat exactly when you want it, around the time that air frost is a real possibility.

We tested it in a larger container last winter and it was also effective as the temperatures dipped but we were unable to monitor the phase change effect as it never got cold enough to do so.