piclist 2002\01\22\003009a >
Thread: Enclosed outdoor circuitry, moisture prevention
www.piclist.com/techref/index.htm?key=enclosed+outdoor
picon face BY : Russell McMahon email (remove spam text)



> >Been trying to remember something I meant to try in a
> >greenhouse once (but never did). The idea was that you
> >filled black tubes with a compound that has a high latent
> >heat (higher than water).

Water has a rarther high heat capacity compared to most other common
materials.
Where you can win is if you can get a phase change somewhere in your
temperature range.
You can store 10 or 20 times more energy in a solid-liquid or liquid-gas
phase change than in the whole temperature swing between  0 & 100 degrees C.

>>During the day they heated up
> >and then released the heat during the night to stop frosts.

A reasonable alternative is SAID to be softdrink bottles filled with water
which is blackened just enough that you can't see sunlight through it when
in the bottle.

> I seem to remember something like this called a "nightstor heater" 'cept
it
> was 12 hours out of phase ;)

Nightstore uses the fact that from SOME power boards you can get a rate for
power offpeak that is MUCH cheaper than on peak. Not all power boards offer
this. (Mine offer a $NZ0.065 / kWh night rate (11pm - 7am on) compared with
typically $0.096 to $0.115 / kWh daytime rates depending on other factors)

The original nightstores used wax as an energy storage medium. This did have
a phase change (the wax melted during heating) so had very good capacity.
However, there were a number of fires with these for reasons I'm not aware
of (ie I would have thought  that it SHOULD be possible to achieve proper
control - maybe they got massive temperature buildups near the elements and
inadequate circulation) and AFAIK they recalled them and changed to bricks
as the storage medium. Bricks are not known to exhibit phase changes across
the temperature range of interest :-) so the thermal capacity is much worse.



       RM

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