Subject: Solar closet economics

From: [email-masked] (Nick Pine)
Date: 6 Apr 1995 09:19:04 -0400

This is a rough estimate of the payback period for three houses, counting
only materials costs and annual energy costs, ignoring labor costs and the
time value of money...

House (a) is a conventional 2000 ft^2 two-story frame house, about 32' on
a side, with R30 insulation, 20% R4 window area on each wall, and 0.5 air
changes per hour, in the Philadelphia area. It has a sum of R-values divided
by surface areas of approximately

SAR = (32x16x4+1024 ft^2)/R30 +0.2x32x16x4 ft^2/R4 = 100 +100 = 200,

and an air infiltration rate of about 0.5 x 16,000 ft^3 = 8000 ft^3/hr,

so the average annual space heating requirement is about

5500 degree days x 24 hr/day x (200 + 8000 ft^3/hr/55 ft^3/Btu-F)

    = 5500 x 24 x (200 + 150 ) = 46 x 10^6 Btu/year,

or about the same as the heat contained in 450 gallons of oil, burned in an
old, 70%-efficient oil burner.

In addition, this house needs the heat equivalent of about 400 gallons of oil
per year for domestic water heating, which would cost about $1200/yr, with an
electric water heater, using electricity at 10 cents/kWh. This house uses the
heat equivalent of about 850 gallons of oil per year, altogether. Its oil
burner has an infinite payback period, for comparison with the next two houses.

House (b) is the same house, as redesigned by an architect, as a typical
"passive solar house." It might use 30% less oil than house (a) for space
heating, with 50%, vs. 20% south wall window area, and a masonry floor
in front of the south windows, or a double layer of drywall for additional
thermal mass in the house, at an additional cost of about $10/ft^2 for the
additional windows and thermal mass. The savings here is about 150 gallons
of oil/year, vs. house (a), and the additional cost of the house is about

30% x 512 ft^2 x $10/ft^2  = $1500 for the windows, plus 
50% x 1024 ft^2 x $10/ft^2 = $2500 for the floor.

Ignoring the time value of money, the payback would be about

$4000/(150 gal/yr x $ 0.7/gal) = 40 years,

or perhaps less if the floor is concrete, vs. tile, granite or marble,
which is good, compared to case (a).

House (c) is the same house as redesigned by a residential passive solar
HVAC engineer, with a 4' wide x 16' long x 8' tall solar closet, behind some
clear polycarbonate solar siding, an air heater measuring 24' wide x 16' tall.
Both the closet and air heater would have plastic film dampers to prevent
reverse thermocirculation at night. House (c) might use 90% less energy than
house (a) for space and water heating, at an approximate additional cost of

64 ft^2 x $10/ft^2 = $640 for the solar closet, plus
                     $200 for a small pump and water-water heat exchanger.

The polycarbonate siding might replace vinyl siding _and sheathing_, and the
greenhouse shadecloth and 3 1/2" insulation in the  south wall might replace
5 1/2" of fiberglass insulation. The solar closet would have 3 1/2" of
insulation all around it, and it would contain 32 55 gallon drums full
of hot water, as a sort of stagnant, inefficient solar collector, BUT most
of the lost heat from the closet would go into heating the house. The air
heater wall would provide heat for the house on an average day, in January,
with some sun, and the solar closet would operate in "standby mode," until
its stored heat were needed to heat the house during a string of cloudy days.
Depending on the geometry, the solar closet might have a small, high
temperature fan to help with natural convection, and a horizontal reflective 
surface in front of it, eg a white-painted surface or shallow reflecting pond.

The solar closet would also provide hot water for the house, using a couple
of plastic drums on top of the 4 high x 8 wide drum stack as air-water heat 
exchangers. These bunghole drums would be plumbed together in series, and
low-pressure, high-temperature solar water would be circulated with a pump
through a loop with a water-water heat exchanger located below a conventional
electric water heater. If the electric water heater were located in or above 
the solar closet, the pump might not be necessary. In any case, electric water
heating would seldom be necessary.

The solar closet would be inexpensive floorspace, about 3% of the house area.
It might have a dirt floor, with the drums sitting on a piece of polyethylene
film on the dirt, with each drum supporting the one above it. The rectangular
array of drums might be lightly supported by the closet endwalls, to keep it
from tilting. The inside walls  of the closet might be the foil face of the
fiberglass insulation. The outside walls of the closet, inside the house,
might be drywall. The closet might have a small electrical air damper with a
thermostat to control it, to let warm air heat the house on cloudy days. The
closet could be extended a bit on one end to make a sauna or "warm room,"
for clothes drying or food drying. 

So here we seem to have an additional cost of materials, over case (a)
of about $840, (if the air heater replaces part of the normal south wall,
at the same or lower cost.) The payback period would be approximately

$840/(90% x 450 gal (space) + 400 gal (water)) x $ 0.70/gal)
 
    = $840/$535 = 1.5 years, or less, if water were heated electrically.

Now you might also ask, "Will this work, as described?" I think so, but
I haven't tried it, yet. That's an enginering question that can be answered
by experiments and caclulations involving heatflow. The new Conserval wall is
a thin, unglazed sheet of aluminum, painted black, with about 2% of its area
as 1/32" holes, with air flowing through the holes. It has a measured solar
collection efficiency of 80%, which seems encouraging.

Nick