In message , Franz Heymann
writes

"David Hill" wrote in message
...
Following my posting re reverse thermostat, I have been wondering just
what
ideas the collective brains on this group could come up with for
Greenhouse/poly tunnel heating.
We get a lot of questions re Gas , electric or paraffin heating, but
surely
we can come up with something free/cheep and useable.

Such things are generally mutually exclusive. The closest is wind power
where you can buy reasonable 500W wind turbines for around £600. Or make
them from old alternators and scrap for somewhat less.

Your next best bet is a water wheel - provide you have a stream in the
garden.

I seem to remember talk of having bottles of water with washing soda in
solution in them, the idea being that when the temp dropped to below 50
then
the soda would crystallize giving off heat, and when the water was over
about 60 then the soda would absorb heat as it went back into solution.
Now have this in tanks with some sort of heat exchanger taking heat from
the
top of the house in the day and heating the tanks.........

I see no thermodynamic advantage in that, compared with any massive black
material in the tunnel to absorb heat during the day and release it during
the night. But you are likely to find that the amount of heat you can trap

There is some advantage in that the latent heat of crystallisation is
moderately large so the stuff manages to store more heat than plain
water can. And in theory it provides some weak thermostat action - the
bad news is that it can supercool and then bounce. And sometimes the
containers rupture due to volume change on crystallisation.

in this way is nowhere near enough to keep your greenhouse warm over night.
Let's face it, winter is winter because there is not much heat sculling
around.

At UK latitudes that is true, with short days and very low sun but at
latitude 45 or lower there is still real power available in the winter
sun.

Regards,
--
Martin Brown