Subject: An inexpensive, 100% solar house?

From: [email-masked] (Nick Pine)
Date: 20 Oct 1995 09:18:44 -0400

Here's a house that looks like it could be 100% solar-heated, while 
providing close to 100% solar hot water as well, inexpensively...

One might start with Jim Cahill's house, designed by engineer Lyle Rawlings
at (609) 466-4495. This was manufactured by AvisAmerica at (800) 284-7263,
and built by Jim Cahill at (508) 677-3533. It is described in the
September/October 1995 issue of Solar Today, pp 24-27.
 
This very nice house in Falmouth, MA (5800 F DD), has 2530 ft^2 of floorspace.
The selling price is listed as $185K, including dealer/builder markup and
a 2 kW PV system, so the basic price of the house would be about $170K,
without the PV system, and about $160K without the backup heating system
or other solar features. Building it without a basement should also lower
the price.

The estimated annual non-solar fuel requirement is the heat equivalent of 
140 gallons of oil (although the house has a natural gas-fired forced hot
water backup heating system.) Minimal hot water usage would add another
60 gallons of oil a year to that requirement, making 200 gallons of oil
per year. How can we lower that backup heating requirement to zero? (One
might also ask, why bother to do anything at all to this superinsulated
house, with a basic yearly heating bill of $660, but that's another story.)

As designed, the house has 419 ft^2 of south-facing glass, with minimal
glazing on the other walls. The ceiling has R38 insulation, and the walls
are R27. The house has a remarkably low air infiltration rate of 0.0125 ACH,
based on a 50 Pascal air infiltration rate of 0.25 ACH. The house is 44'
long and 28' wide and two stories tall.

The south windows seem to be the biggest heat losers here:

Sum (Ai/Ri) = 420 ft^2/R2 + 28'x44'/R38 + (16'(28'+44')x2-420)/R27
              windows       ceiling       walls
            = 210         + 32          + 70      =   312.

It looks like these south windows account for about 70% of the heat loss of
the house, ignoring the air infiltration, which is 1/8 of the ceiling loss.
The south windows also contribute solar gain, when the sun is shining.

Suppose we somehow change this house so most of the south wall is an
insulated frame wall, like the rest of the house walls... ("Oh, it will be
less dramatic!" :-) And add some curved galvanized steel pipes and plastic
glazing to make a low-thermal mass sunspace, with a solar closet behind it.
How big will the sunspace and solar closet have to be, in order to provide
100% of the space heating and close to 100% of the hot water for the house?

With an insulated south wall, the new sum above becomes about 120 Btu/hr-F.
If the average temperature in December is 32 F, the house would need about
100K Btu of heat on an average December day. If the the sunspace provides
a net solar gain of say, 750 Btu/ft^2/day, it will have to have

      24hr x 120 (68-32) = 138 ft^2 of glazing.

Let's make it 200 ft^2, so the solar closet can provide hot water as well.
If the sunspace were 16' tall, it would be about 12' wide. The house
would look something like this:

                      28'
        . . . . . . . . . . . . . . . . 
        .                             .
        .                             .
        .                             .
        .                             .
        .                             .
        .                             .
        .                             .
        .                             .
        .                             .
        .                             .
        .                             . 44'
        .                             .
        .                             .
        .                             .
        . 4'                          .
. . . . . . .                         .
.       .   .                         .
.       .   .                         .
.       .   . 12'  sauna?             .
.       .   . . ./ clothes drying?    .
.       .   .   .                     . 
. . . . . . . . . . . . . . . . . . . .
    8'
                      . 
                    . f .
                  .   \   .
polycarb roof?  .     s \   .         I added a solar attic here too, for fun,
              .       s   \   .       using corrugated clear polycarbonate
            .         s     \   .     plastic, and a fan with a backdraft 
          .           s       \   .   damper at the top, to blow down warm
        . md. . . . . . . . . . . g . air from the peak of the attic into
       ..             .             . the house, where it wends its way back
      . .             .             . up to the attic through a motorized
     .  .dhw          .             . return damper, md. g are airflow grates.
    .   . . . g . . . . . . . . . g . The fan is controlled by an attic fan
   .    . f .         .             . thermostat in series with a house 
  .     .   .         .             . thermostat. This is another way to make
 .      .   .     <== g             . a low-thermal-mass sunspace.
. . . . . . . . . . . . . . . . . . . . . . . . . 

s is some greenhouse shadecloth (optional), f is 10' of fin-tube radiator
pipe, and dhw is a conventional or indirect-fired water heater ("geyser"
in the UK), which is heated by natural water convection using the fin-tube
as an air-water heat exchanger.

The low-thermal mass sunspace ($1000?) would work best with a fan controlled
by an attic thermostat and a house thermostat in series, as well as plastic
film backdraft dampers, to prevent reverse airflow at night. The solar closet
($500?) would store enough heat for 5 days without any sun, using

        5 days x 100K Btu/day /((130F-80F)x 55 gal x 8lb/gal) = 24

55-gallon sealed drums full of water, assuming an initial water temperature
of 130 F, and a minimal usable space heating closet water temperature of 80 F.

Who will be the first to order one of these houses?

Nick