re: central heat SOLAR.KnowledgePublications.com

re: central heat
30 dec 1999
adowning@webtv.net wrote: 

>   nick says solar is best; not if you live in the pacific northwest!
>we don't even see the sun for 6 months at a time. 

maybe you aren't looking in the same direction as the national renewable
energy laboratory, eg south from seattle, where nrel has measured an
average 0.8 kwh/m^2 (303 btu/ft^2--about 1.5 hours) of direct beam sun
per day in december (24-hour average outdoor temp 40.5 f, with an average
daily max of 45.1) for the last 30 years. seattle gets 303/(70-40.5)
= 10.3 btu/ft^2 of beam sun per degree day.

garys@wwnet.net wrote: 

>not in se michigan. we get some long stretches of bitter, gloomy weather.
>solar might be great for some people but not up here.

nrel says a south wall in detroit gets an average daily dose of
1.1 kwh/m^2 (417 btu/ft^2) of direct beam sun in december, when
the average outdoor temp is 28.3 f. detroit gets 417/(70-28.3)
= 10 btu/ft^2-dd (vs albuquerque with 1668/(70-35.3) = 48 :-)

solar heating in cloudier climates is more difficult, but not impossible.
detroit has more sun than seattle, but it's colder, so detroit houses
might have less solar collection area and more heat storage. both can be
frugal if combined with other functions like attics or sunspaces that add
floorspace to houses, or tanks for rainwater storage or more efficient
higher temp sewage treatment, which can eliminate the need for expensive
wells and septic systems. 

solar attics have advantages over sunspaces: their geometry suits reflective
fixed solar concentrators like engineer howard reichmuth's ecotope solar
greenhouse in seattle, which has a 20-year successful track record. these
concentrators can heat water to a higher temperature with greater efficiency
and lower cost than sunspaces, and supply hot water for showers and hydronic
floorslabs for heat distribution. attics are unlikely to be shaded by trees
or other houses. they don't require extra real estate or add much taxable
value to houses. they can provide powerful daylighting with inexpensive
efficient skylights in the attic floors, with movable insulating reflective
foamboard covers that are hinged on the north edges. 

houses need attics anyway. a single layer of $1/ft^2 20-year clear corrugated
polycarbonate plastic over purlins on 4' centers is cheaper and more mold-
resistant than asphalt shingles over tarpaper and sheathing and rafters on
16" centers. with more solar heat and slipperieness and thermal conductance,
it can shed snow more easily.

a 32'x32'x16' tall house with real r24 6" sip walls and ceiling, 176 ft^2
of r4 windows with 50% solar transmission, and 0.2 house air volumes per
hour of air leaks has a thermal conductance of 176ft^2/r4 = 44 btu/h-f for
the windows plus 78 for the walls plus 85 for the ceiling. the air leakage
rate is 0.2achx32x32x16/60 = 109 cfm, which adds about 109 btu/h-f to the
conductance, making the total about 316 btu/h-f. the house needs about
24h(70-40.5)316 = 224k btu to stay warm on an average december day in
seattle, when 250, 100, 170, 170 and 420 btu/ft^2 of sun fall on a level
surface and north, east, west, and south windows. if 10, 15, 25, and 50%
of the windows face north, east, west, and south, they will collect a
total of 25k btu/day of sun (solar heating the house about 10%.) a frugal
300 kwh/month of internal electrical usage adds 34k btu/day of heat, so
the house needs an additional 224k-25k-34k = 165k btu/day for 100% solar 
heating in december.

with a 4' stemwall above the attic floor and a 3:4 pitch roof with 90%
solar transmission, we can collect about 0.9x250x32'x16'= 115k btu of
overhead sun and about 0.9x420x32'x16' = 194k of sun from the south,
309k btu/day altogether. line the north wall with masonite covered with
nielsen's (www.snomo.com/mylar.html) 90% reflective 9 cent/ft^2 film and
collect about 0.9x0.9x0.9x303x16'x32'= 113k btu/day of south sun in a
32' long x 4' wide 30" diameter $20 polyethylene tube filled with 2" of 
water along the base of the north wall. the reflector would focus at
y^2/(4x) = 16'^2/(4x16') = 4' from the north wall at dawn, and closer 
during the day.

if we withdraw 165k btu/day of heat from the attic, some warm air as well
as heat from skylights and sun concentrated in water, and most of the attic
walls are well-insulated, with an average attic air temp t over a 6 hour
solar collection day in december, the r1 south attic roof will lose about
6h(t-43f)16'x20'/r1 btu/day = 309k - 165k, making t = 43 + 144k/(6x16x20)
= 118 f (which will also lower the daytime heat loss from the house ceiling.) 

putting 113k btu/6h = 18.8k btu/h of sun into the 4'x32'r0.67 = 192 btu/h-f
tube makes the water collection temp 118+18.8kbtu/h/192btu/h-f = 216 f,
which seems good enough :-) if the water turns out to be cooler, we might
raise that temp with a controller that only fills or pumps the trough
during times of direct beam sun. 

the house needs 5d(224k-34k) = 950k btu for 5 cloudy 40.5 f days in a row.
(are cloudy days warmer in seattle?) if this comes from g gallons of 150f
rainwater cooling to 80 f, (150f-80f)gx8btu/gal = 950k, so g = 1696. we
might use a couple of $419.95 1500 gallon 84"dx60" tall polyethylene tanks 
in/under the house, surrounded by insulation... 

nick

nicholson l. pine                      system design and consulting
pine associates, ltd.                           (610) 489-1475/0545 
821 collegeville road                           fax: (610) 489-7057
collegeville, pa 19426                     email: nick@ece.vill.edu

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