re: seek passive solar design faq/guide
18 may 1996
william r stewart wrote:
>> william r stewart wrote:
>> >nick pine wrote:
>> >> william r stewart wrote:
>> [etc - snip!]
>> >> 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.
>> (sounds like a solar panel to me :-)
but they are also very expensive, and normally live on the roof, which makes
them fundamentally different from these inexpensive sunspaces. these panels
are often heavy boxes added on to houses, mounted on brackets, shipped from
afar at great expense and manufactured in relatively small quantities, with
ducts and plumbing that go through the house roof, and pumps and blowers
that eat lots of electrical energy every year. (yes, that electrical energy
can come from pvs, altho they are very expensive now and usually waste 90%
of the sun that falls on them, the very same solar energy we are trying to
collect with the add-on boxes :-)
some solar air heating panels have low thermal mass. water heating panels
have more thermal mass. i saw a new rooftop-type water heating panel for
sale for about $40/ft^2 with lots of intentional thermal mass. it was about
4x8x1' thick, and contained 30 gallons of water, a batch heater with no
insulation between the water and the absorber plate and the glazing as
far as i could tell. they also had a 50 gallon model...
brick sunspaces have lots of thermal mass behind glass with no night
insulation, as do many trombe walls and direct gain houses. this is a
very inefficient way to heat a house in a partly cloudy climate, because
the thermal mass stores lots of solar heat during the day, and most of that
solar heat disappears through the glazing at night. during a week without
sun, an isolated sunspace can just get cold, even if it is full of bricks,
which is ok. but a trombe wall or a direct gain house with south glazing in
the living space will lose a lot of backup house heat to the outside world
through the relatively low thermal resistance of the glazing.
>> there is a philosophical difference here that boils down to whether
>> you want the sunspace to be part of the living space.
>> if you like sunspaces, fine!
>thank you! i've been trying to get the point across that some people
>have their own preferences.
of course people have preferences and that's fine. but let's recognize
that that's a lifestyle choice involving a compromise with aesthetics and
money and fossil or wood fuel consumption. let's not drag in false physics.
a low-thermal-mass sunspace could be a commercial plastic film greenhouse
adjacent to a house, costing 50 cents/ft^2, put up by 1 person in one day,
more like a tent than a building. the glazing might be poly film costing
5 cents/ft^2, with a 3 year guarantee, changed every 3 years in an hour,
and recycled. or it might be clear mylar glazing, slightly more expensive
and longer lasting, if this inexpensive sunspace leans against a house wall.
or it might be flat very clear polycarbonate glazing, costing $1/ft^2,
with a 10+ year solar lifetime, that comes in rolls 49" wide, so it might
be simply attached to 2x6s on 4' centers in a simple lean-to sunspace that
forms the weather south wall of a house.
and to me, a "low-thermal-mass sunspace" can also be $1/ft^2 clear plastic
"solar siding," ie a sunspace 2" thick, that takes the place of, say, vinyl
siding on the south wall of a house, with no sheathing underneath and only
3 1/2" of insulation in a 2x6 wall. this costs _less_ than normal house
construction (no sheathing, less labor), and it collects solar energy.
another way to make a low-thermal-mass sunspace is to make the steep south
roof of a house with the same single-layer corrugated polycarbonate plastic,
with an insulated attic floor. again lower first cost than normal
construction (no shingles, no tarpaper, no sheathing, and 4' x 12' panels
that attach with a few hex head screws) and it can collect solar energy,
with a low power fan that blows warm air from the peak of the attic down
a large cheap uninsulated duct (eg a poly film tube duct that costs 50
cents per linear foot), into the house when the sun is shining. house air
might return to the attic through a $200 2' x 2' motorized damper that lets
daylight into the house when it is open. the value of such daylight might
be $200/year of electrical power savings, vs the fluorescent equivalent,
if it is well distributed inside the house, as well as the aesthetic value
>> i am going to suggest that a dedicated sunspace need not be part of
>> the design for space heating.
>> if you design your house to be superinsulated, it has been shown that,
>> even for houses orientated to minimise passive solar gain, addittional
>> energy costs for space heating can be made insignificant.
absolutely true. but at what price?
>> with windows available now (r-10+) and orientation to make best use of
>> winter sun i think it is possible to reduce space heating costs to
>> zero in an otherwise normal house.
i agree, if you are willing to live with very few windows.
>> these houses cost little more than a normal house
>> then the main effort needed is to reduce lighting and water heating costs.
true. and how do you reduce water heating costs? superinsulation doesn't
heat water. can we try to look at more of the whole picture here, and make
new houses that have these functions as beautiful integral parts, rather than
adding on kludgey afterthoughts, box after box after box, more expensively?
>> on the latter a recent design here in the uk (scotland in particular)
>> has aimed at reducing cost rather than increasing efficiency of a
>> solar panel.
what a strange and excellent idea :-)
>some of the low-e windows i have seen lately have been close to what
>nick had mentioned; one was sc=.59 and another 'heat mirror' sc=.41
they are also more expensive...