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re: desperate for solar heating
26 aug 2005
ed earl ross considers the following:
>build a 4' by 12' (2' deep) box in front of a south facing window,
>waterproof it, and fill it with water.
the box would be 4' wide (ns) x 12' long (ew) x 2' deep (ie a 2' water
depth), outside the house, to the south of a 12' wide (ew) south window?
>put glazing over the box and window--the glazing will slope from
>the top-front of the box to the top of the window--sort of making
>half an a-frame.
would the box have its own horizontal transparent cover? where do you
live? winter sun arrives at a max elevation of 90-lat-23.5 degrees at
noon on 12/21, eg 26.5 degrees above the horizon at 40 n latitude, so
vertical collectors work better than horizontal collectors.
>during the day, sunlight will provide heating for the house and the
>pool of water. during the evening, heat in the pool can be for
>later use at night.
sounds like the box will lose lots of heat through the glazing at night.
>after insulating the box, there will be about 35 cu ft or 260
>gallons of water, which will weigh about 16,000 pounds.
...35ft^3x62.33lb/ft^3 = 2182 pounds.
>according to my source, water holds 1 btu per pound per degree f.
yes. getting the heat in and out of the box at a reasonable rate may
be difficult. that requires lots of surface, with a 1.5 btu/h-f-ft^2
slow-moving airfilm conductance. for instance, moving 10k btu/h from
80 f water to 70 f air requires a thermal conductance of 1000 btu/h-f,
eg 1000/1.5 = 667 ft^2 of uninsulated surface with water on one side
and air on the other.
>i found a rule of thumb that says one needs about 30 btu per sq-ft
>to heat a home. thus, a 2000 sq-ft home would need 60,000 btu
>heating. would that be per hour?
yes. if a 2400 ft^2 house needs 72k btu/h to stay 70 f inside on a 30 f
night, it has a thermal conductance of about 72k/(70-30) = 1800 btu/h-f
from indoor to outdoors. that's high by today's standards.
a 40'x60'x8' 2400 ft^2 house with an r40 ceiling and r30 walls and 192
ft^2 of r4 windows and 0.5 air changes per hour would have a thermal
conductance of 2400ft^2/r40 = 60 btu/h-f for the ceiling + 192/4 = 48
for windows + 1408/30 = 47 for walls (excluding windows) + 0.5x2400x8/60
= 160 for 160 cfm of air leakage (about 10x more than ashrae's fresh air
standard), for a total thermal conductance of 315 btu/h-f. air leaks are
more than half of this, so they are an obvious target for improvement.
an older house might have 1 or 2 ach.
you might forget the box and add lots of low-mass sunspace glazing or
transparent polycarbonate "solar siding" over the south wall of the
house and let warm air circulate between the sunspace and the living
space during the day and stop the airflow at night. that way, there's
no heat loss through the glazing at night.
if you want a high solar heating fraction (more than 30% or so), you
might put more thermal mass in the house, eg concrete furniture or
mass in the ceiling above the sunspace air outlet. with a low-e surface,
ceiling mass can stay a lot warmer than house air without overheating
the space below and store lots of heat for nighttime and cloudy days.
if cloudy days are like coin flips, a house that can store heat for
1 day can be at most 50% solar-heated... 2 days makes 75%; 3, 88%;
4, 94%; and 5, 97% :-) you might end up with some plywood platforms
below the ceiling with foil glued underneath and flat poly film ducts
with 2" of water above, or fin tube pipe around the ceiling perimeter
or near the sunspace air outlet with a large tank on the ground and
cool water pumped from the bottom up through the tubes during the day
and warm tank water thermosyphoning up from the tank top through the
tubes at night and on cloudy days, with a slow ceiling fan in series
with a room temp thermostat and an occupancy sensor.
any questions? join pe drew gillett and phd rich komp and me for
a workshop on solar house heating and natural cooling strategies
at the first pennsylvania renewable energy festival on saturday
september 24, 2005 near allentown. see
http://www.paenergyfest.com/workshop-info.shtml
nick
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