Even an older home can be retrofitted with a thermal mass or batch heater system.
MOTHER EARTH NEWS STAFF
Maybe in your mind the term "solar power" conjures up images of blue photovoltaic panels lining a broad expanse of south-facing roof on a residential home, or perhaps vast arrays of mirrors aimed to concentrate the sun's light in one spot. And maybe you also think about the cost of these big ticket implementations, and know they are well beyond your means. Well don't despair. Cheap solar power isn't an oxymoron. Tapping the sun's energy for heating purposes is affordable and relatively simple. Here are two ways to do it.
When I was around ten, my mother decided to build herself a
modest 3' x 7' step-in greenhouse as a small addition to
the family room. She and my father sat around the kitchen
table for weeks, poring over plans and deciding how they
could piece it together without going broke. And each day I
came from school to find something different; a large hole
in the south-facing wall, sheets of plastic as temporary
windows, piles of bricks, concrete. For a while it was
better than television. In a month, construction was
finished, and we had a handsome little topiary bubble
sticking out of the house like a turret. The first hot
weekend in May, however, brought a shock for my Mom. That
bubble, while we were away for just one warm, sunny
weekend, had managed to kill all but the hardiest of Mom's
plants and turn the family room into a convection oven. It
was my first lesson in just how much heat a few windows can
muster. Mom and Dad restocked the plants, invested in some
reflective shades and monitored the weather a little more
closely after that.
Nearly all homes have some southern facing glass, but most
homes, especially older ones, were not built to take
advantage of the heat energy those windows can transmit.
One low-cost and simple way of taking advantage of southern
exposure is through the addition of thermal mass material,
which absorbs the heat energy and stores it, radiating the
stored heat during the cool fall and winter evenings. You
can do this by placing a substantial amount of heat-holding
material such as masonry, concrete, or containers of water
where the sun can shine on them. This thermal mass will
help to prevent the sunny room from being overheated and
will reduce the need for opening windows or shading out the
sun's free energy just to maintain comfort. At night, this
mass will radiate its stored solar heat and reduce the need
for conventional heat.
There are many ways to add thermal mass to a room. The
challenge is to add enough. Try to provide 2 to 4 gallons
(1/4 to 1/2 cubic feet) of water per square foot of south
window, or 1/2 to 1 cubic foot. (75 to 150 lbs.) of
concrete or masonry per square foot. If the mass is not in
direct sunlight, double or triple these amounts. If you
have a concrete slab, you already have plenty of thermal
mass — just allow the sun to reach it. The mass
should be dark-colored to absorb radiation and should be
placed as close to the window as possible so that the
sunshine hits it all day. It's important, though, that the
windows should normally overheat the space where the
thermal mass material is placed. If the windows don't
overheat the space, the mass won't attain the higher
temperatures needed to help with heating, except by
moderating uncomfortable temperature change.
If you have a strong floor, you can pour a thin 2" - 4"
concrete slab over it, or lay down brick or heavy
Steel drums or racks of water-filled containers could act
as partitions or furniture.
An interior wall that receives sunlight could be lined with
stone or brick.
The addition of thermal mass does not increase the amount
of solar energy available to a space, but it lengthens the
time you can use the heat.
Thermal mass moderates temperature swings between day and
night, keeping the space more comfortable.
Excess heat that might otherwise be thrown away is saved
and used later.
Thermal mass has no moving or complex processes. It simply
sits there and does its job without your time or
Thermal mass materials are common and inexpensive. Concrete
runs $40.00 to $60.00 per cubic yard (27 cubic feet). Water
is almost free. Containers, such as steel drums or plastic
jugs, can be scrounged or bought secondhand. Figure $1 to
$2 per square foot of window area.
Thermal mass requires bulky materials and often takes up a
lot of space. It's often difficult to add more in a
graceful way that's acceptable to your existing
The structure of your house may need strengthening before
it can support the considerable weight of thermal mass. To
be sure, check out the foundation and floor before you load
in the mass.
Another way to take advantage of the sun's considerable
heat energy is through a batch heater (sometimes called a
breadbox). A batch heater, nothing more than a greenhouse
for heating water, is a water tank painted black, mounted
in a well-insulated box, and covered by a sheet of glazing.
The batch heater preheats the cold water from your well or
municipal water supply, reducing the amount of energy your
water heater uses. The best location for a batch heater is
an unshaded area in the yard on the south side of your home
or as close as possible to your existing tank.
The sun's energy passes through the glazing and is absorbed
by the black surface of the tank. This heat is immediately
transferred to the colder water in the tank, supplied from
your well or municipal water service. On a warm sunny
summer day, the water in the tank may reach 140°F. This
preheated water then flows to a well insulated outlet pipe
to your home's water heater. As the warm water is delivered
to your home, it is replaced in the tank by cold water from
the inlet pipe and the cycle continues.
To get the most hot water from this solar system, use only
a single layer of glass. Including a reflector in the
design will increase the amount of solar energy the tank
receives. The water lines going to and from the solar tank
should be well insulated and kept as short as possible.
The batch heater needs to be drained at the first sign of a
severe frost and refilled in the spring when all danger of
frost has passed. This heater needs little maintenance —
every two years add a fresh coat of paint and check the
sacrificial rod that is placed in the tank to reduce
A batch heater is an excellent choice for a water-heating
system. It's inexpensive and takes a moderate amount of
experience with carpentry and plumbing to build. It will
pay for itself very quickly, especially if it is combined
with hot water conservation measures. The batch heater
is flexible — if you have a sunny wall, you can build
a freestanding version.
Some commercial solar water heaters are based on this
design. By using advanced materials, they do not freeze in
the winter and can be used year round.
Construction of a batch heater, including $50 for energy
conservation measures on your existing hot water system,
will cost between $500 and $ 1,000. A batch heater can cut
your hot water bill by half when it operates, or up to 1200
kWh per year.
The batch heater can be oriented in a north-south direction
or it can also stretch in an east-west direction.
versions of this design use several layers of glazing and
operate year round
This hot-water system is the cheapest and simplest to
build. For the cost it puts out a good amount of hot water.
- The total hot water
production is lower than some other systems because the
batch heater is not used all year.
Adapted from The Fuel Savers (Morning Sun
Press, 1992) by Bruce N. Anderson. Copyright © 1992 by
Bruce N. Anderson
What It Costs to Run Electrical Appliances
Unless indicated, costs are measured per hour. Estimated electricity rate: 14.7¢ per kwh. Figures courtesy of Consolidated Edison
Coffee maker 1.2¢
Television, color 2.1
Television, B&W 0.7
Computer monitor 0.8
Computer and disk drive 1.2
(100 W) 1.5
(60 W) 0.9
(25 W) 0.4
Electric blanket 1.1
Blow dryer (per use) 2.0
Clock (per month) 29.4
Vacuum cleaner 9.7
Washing machine (load) 3.8
Clothes dryer (load) 44.1
Room air conditioner (10,000 BTUs/500 hrs for one summer)
New unit (10.0 EER): $77
Old unit (7.0 EER): $110
Refrigerator (dollars per year)
12 cu ft, manual defrost: $72
14 cu ft, manual defrost: $113
14 cu ft, frost free: $108
18 cu ft, frost free: $115
Freezer (stand alone)
15 cu ft chest, manual defrost: $70
16 cu ft upright, manual defrost: $97
16 cu ft upright, frost-free: $129
Sizing a Room Air Conditioner
Floor area Type of Outside Wall
100 4,500 5,300
125 5,150 6,100
150 5,700 6,800
175 6,200 7,500
200 6,650 8,100
250 7,550 9,300
300 8,300 10,400
400 9,700 12,400
500 11,000 14,250