re: underground heat storage
21 mar 2006
>... basically you dig a deep 'foundation' and in the middle of the
>greenhouse build a plenum from the bottom of the pit to the top of
>the greenhouse. at the bottom of the plenum you would cut several
>4" holes and attach 4" black sewer pipe with holes in it to go all
>directions. then fill the foundation with some sort of thermal mass
>material... prolly rocks or gravel. then a blower is attached to
>the top of the plenum to blow air down in to the rocks. i would
>presume that if a slab foundation were to be poured there would be
>some sort of vents around the edge to allow the warm air to escape
>back in to the greenhouse. the blower would blow the hot air from
>the day down to heat the rocks and then at night the rocks would
>warm the cooler air.
i think this could work well for a house in a dry climate, but it might
become a mold factory in a greenhouse. green leafy plants in the sun
can evaporate lots of water, about 1 pound per day per square foot of
floorspace. this (1000 btu/ft^2) is about the same as the amount of sun
that falls on a south wall in phila in january, so a greenhouse might
decide to use all of the sun that falls on it for evaporation, with no
sensible heat gain...
the usual way people prevent greenhouses from becoming swamps is to turn on
ventilation fans to blow lots of water vapor (and solar heat) out of the
greenhouse during the day. then they turn on propane heaters at night.
an energy-conserving greenhouse without lots of daytime venting still
has to deal with humidity buildup. from heat transfer and energy storage
points of view, humidity might be good, with condensation indoors.
we might even reuse the distilled water. pa has an approved "sundrive"
alternative wastewater treatment system using an large greenhouse with
plants (eg reeds) on top of a waterproof liner, but the greenhouse has
to be almost as big as the house it's treating wastewater for, since
the water leaves the greenhouse in vapor form. a condensing or multiple
effect distillation greenhouse might be good for water reuse in the
southwest, where water is scarce and sun is plentiful...