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Passive Solar Water Heating
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Most everyone has experience with passive solar water heating.
How many times have you turned on the hose in the yard and nearly
burnt yourself with hot water? While you weren't looking, old
Sol was quietly working to give you hot water, even if you didn't
want it. Well if it's that easy, imagine what you can do if you're
actually trying to make hot water. Passive solar hot water systems
are probably the oldest commercially available solar systems.
At the turn of the century there were large numbers of solar
water heating systems on roof tops, especially in Los Angeles
and Florida. Very little has changed from the original concept.
Put a water holding tank in a box, with glass on the side facing
south and fill it with water. No moving parts, nothing to break
down, free fuel and no pollution. The passive solar water heater is known today by many names;
PSWH, Batch Heater and Bread Box are the most common and then
there is the very technical; Integrated Collector and Storage
System (ICS). The PSWH of today usually starts with a 40 gallon, glass lined
tank. These tanks come disguised as ordinary electric water heaters,
which are stripped of their appliance shell and insulation. Painted
flat black, with high temperature engine or barbecue paint and
they're ready for solar.
The box should be well insulated to prevent energy loss and the amount of insulation should reflect your local climate. The typical box is constructed with 2X4s or 2X6s, using fiberglass batt insulation. The exterior siding may match that of your home, or some other material suitable for your area. The interior sheathing is often ridged insulation, preferably with a foil face facing in which works to reflect more energy onto the tank. Ridged insulation comes in various thicknesses which can help increase your insulation R-value.
The size of the box must be big enough for the tank, but also
large enough to allow adequate solar gain. Typical glass sizing
is 1 sq. ft. of glass for every 2 to 2 1/2 gallons of water.
A standard size, double glazed, patio door replacement glass
(34"X76") is ideal for a 40 gal. water heater. Of course
if you already have a piece of glass....... A water heater has an inlet and outlet and how you attach
your plumbing does make a difference. The cold water inlet has
a dip tube which extends down nearly to the bottom of the tank,
to deliver the cold water to the right place. The hot water outlet
takes the hottest water from the top of the tank. If your design
calls for the tank to lie on its side be sure that the cold inlet
is at the bottom. Be aware that the 40 gallon tank when filled with water will
weigh over 350 lbs. Add to that the weight of the box/glass and
it's time to reconsider putting this monster on your mobile home.
Ground mounted PSWH are very common. As always be sure the system
will receive full sunshine from 9 am to 4 pm. Remember, if your
installing a solar system and you're working in the shade, there's
something wrong. If the collector will be installed on a frame roof it's best
to attach in such a way as to spread the weight over a few rafters,
and if possible, provide additional support with braces extending
up to the rafters from interior walls. The ideal location is
as close to the existing water heater as possible. Shorter plumbing runs are not only more efficient, they decrease
the winter freezing potential. The chances of freezing 40 gallons
of water are minimal but frozen pipes are a reality. With the
tank installed close to the water heater the freezing potential
is minimized but not eliminated. All plumbing between the existing
water heater and the PSWH is insulated, with more insulation
on any pipes exposed to the outside. Also be sure to carefully
insulate all plumbing in attic areas. Plumb the system by first supplying cold water to the solar
tank. From there, the hot water outlet is plumbed to the cold
water inlet on your existing water heater. As long as the solar
water entering your water heater is above the thermostat setting,
your water heater does nothing. When the temperature of the solar
water entering the water heater is less than the thermostat setting,
your water heater makes up the difference. The temperature of the water from a PSWH depends on many variables.
The amount of sunshine, ambient air temperature, the amount of
insulation used, the temperature of the supply water as well
as the hot water demand all effect outlet temperature. Under
ideal weather conditions, and no hot water used since morning,
the water temperature at 5 pm can exceed 180 degrees F. You may
consider installing a tempering valve which allows you to set
the temperature for the water before it reaches the faucet. How much of your hot water demand will be met by your PSWH
varies depending on a number of factors. Have you installed low-flow
shower heads and aerators? Have you installed a water heater
blanket and set the thermostat to 120 degrees F? When do you
use the most hot water? If you normally wash clothes/shower etc.
in the evening, there probably won't be any solar water in the
morning. If someone is normally home during the day and clothes
washing is scheduled for around solar noon, you can stretch your
solar water. After normal water use in the am, the sun heats
the water all morning and then that water is used for the laundry
(if necessary). This schedule allows time for the water to heat
up again during the afternoon. Be sure your installation meets all local plumbing codes etc. |
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