re: solar closet diagnostics
28 dec 1996
larry arsenault wrote:
>...i think you are leaving out an important part of the solar equation.
hmmm. which equation?
>keeping the thermal units in after the sun's angle decreases to
>the point of being ineficient.
i'm not sure what you mean by that. are you talking about collecting heat in
the dim sunlight of dawn and dusk, or keeping heat in a thermal store with
night insulation? sounds like the latter, altho the former can be a problem
in cold climates with little sun and single glazing: solar space heat needs
to be collected at some temperature that is at least as warm as a house, if
it is to be useful without using a heat pump. that can mean waiting until
a collector warms up in the morning. having a low-thermal-mass air heater
collector that warms up quickly and leaves no heat "on base" at night helps,
as do reflectors and double glazing. my simulations using hourly weather data
indicate that low-thermal mass sunspaces do not throw away much sun because
it is below some critical intensity, where i live.
harvard physicist bill shurcliff, who has studied this for years, says:
one could shorten the warm-up time of the enclosure and increase
the amount of heat delivered to the rooms by making the enclosure
virtually massless--by greatly reducing its dynamic thermal capacity.
this can be done by spreading a 2-inch-thick layer of lightweight
insulation on the floor and north wall of the enclosure and then
installing a thin black sheet over the insulation. then, practically
no heat is delivered to the massive components of floor or wall;
practically all of the heat is promptly transferred to the air.
and since the thermal capacity of the 100 or 200 lb. of air in
the room is equal to that of one fourth as great a mass of water
(about 25 to 50 lb. of water), the air will heat up very rapidly.
i estimate that its temperature will rise about 40 f. degrees in about
two minutes, after the sun comes out from behind a heavy cloud cover.
at the end of the day, little heat will be "left on base" in the
collector floor or north wall and, accordingly, the enclosure will
cool off very rapidly.
new inventions in low cost solar heating--
100 daring schemes tried and untried
by william a. shurcliff, phd
brick house publishing, 1979, 293 pages, $12
a non-masonry sunspace would do almost as well, eg something with indoor-
outdoor carpet on the ground, or dark lightweight poorly conducting lava
or expanded shale, or some sort of dark wooden deck, with a vapor barrier
on the ground, and perhaps some dark green shadecloth hung a few inches
away from the attached house wall, to keep sun-warmed air away from the
cool sunspace glazing.
>sunshades, i believe is the term.
sunshades sound like sun-blocking awnings or shadecloth or overhangs to me.
>insulation that can be easily applied over the glassed areas at the
>proper time, usually after 3p.m. depending on the geography involved.
over, as in over the outside, like steve baer's drumwall house in new mexico,
which has large manually-operated exterior insulating shutters, hinged near
the ground, that fold down to become daytime reflectors, or something like
the saskatchewan conservation house, which had motorized thermal shutters
over the windows, hinged at the top, which might serve as winter reflectors
if folded up to 45 degrees? it seems to me that cost, air seals, wind, snow,
ice, leaves and children can be serious problems here. insulated garage doors
over glazing might have automatic photocontrols, but they seem expensive and
thermally inefficient too. and none of these systems add any living space to
a house. shutters that fold onto the ground subtract lawn space...
i like automatic systems, vs movable insulation that has to be moved by
people twice a day. most people get tired of that after a season or two.
a $10 per square foot thermal curtain using r5 material that leaks a little
air around the edges seems very expensive and thermally inefficient to me,
compared to an r20 house wall that has to be there anyway. why not put
most of the windows of a house on the south side of a thermally-isolated
low-thermal-mass sunspace? that space might be inexpensive and "dramatic,"
and it would be warm on many winter days. it would get cold at night, but
it could be heated up quickly for occasional use, eg for a new year's eve
party. perhaps such a space will be seen as more useful when more people
actually live at home during the day, vs driving to work. it seems to me
that most people do not have a great urge to look out of windows at night,
so why put lots of windows in a 24 hour living space? why not use those
inside walls for bookshelves and art, with just a few windows?
>it is not enough to just get, and store heat,
>the transmitting areas need be insulated or they will transmit cold,
>nullifying a significant portion of the heat units gained.
agreed. altho many say neither mass nor heat units can be nullified :-)
air can stop flowing through automatically-controlled openings in an
insulated house wall to the north of a low-thermal-mass thermally-isolated
sunspace at night, or at dawn and dusk, or on a cloudy afternoon or even a
cloudy 5 minutes, when the sunspace is cooler than the house or the house
warm enough. a 2 watt motorized damper or two in series with two thermostats
(a heating thermostat for the house and a cooling thermostat for the sunspace)
can control sunspace-house wall openings.
air can also cease to circulate between a massy solar closet and its low-
thermal-mass air heater, through controlled openings in an insulated south
wall of the closet, leaving the warm thermal mass in a box completely
surrounded by insulation. a small fan controlled by a differential thermostat
might make this happen when the air heater is cooler than the closet.
nicholson l. pine system design and consulting
pine associates, ltd. (610) 489-0545
821 collegeville road fax: (610) 489-7057
collegeville, pa 19426 email: firstname.lastname@example.org
computer simulation and modeling. high performance, low cost, solar heating
and cogeneration system design. bsee, msee. senior member, ieee. registered
us patent agent. web site: http://www.ece.vill.edu/~nick