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Before getting into the challenges of solar
space heating let's review the basics of
hydronic (water) heating systems.
Hydronic heating systems come in many forms but
forced hot water baseboard systems are by
far, the most popular.
In a
baseboard system, water is heated and
circulated through
your home's radiators. As the radiators heat
up that heat is carried to the room by natural
convection.
In order to keep the radiators relatively
small (compared to steam radiators),
baseboard systems run at a relatively high
temperature of about 180-200F. At this
temperature (and flow rate of about 1 gallon
per minute), enough energy can be transferred
to baseboard radiators so they can heat
relatively large spaces.
After dumping its energy, the hot water
returns to the furnace at about 150-170F
ready to be heated again.
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So why can't we heat baseboard water with
solar hot water systems? The problem is one
of volume. To heat your home, you need a
large volume of very hot water. Fossil fuels
are well suited to this task because they
provide a large amount of energy in a small
package, and until recently, at a low price
(not counting the environmental impact).
For example, burning one pint of fuel oil
will generate about 16,250 BTUs (British
Thermal Units) of energy. In a typical
furnace, about 80-85% of that energy is
useful (the rest goes up the chimney with the
exhaust gas). The remaining 13,000 BTUs is
enough energy to raise 10 gallons of water
(~84 lbs) about 150 degrees F!
By comparison, on a sunny day, a solar hot
water panel will capture about 20,000 BTUs.
On a cloudy day, that panel might capture
3,000 BTUs (depending on ambient
temperature). So a typical 4'x8' solar
collector-day captures roughly the same
amount of energy as does burning a pint of
heating oil.
With this relatively low rate of energy
capture compared to oil, you need a rather
large collector array and storage system to
make a real dent in your oil usage.
Furthermore, in the winter, solar hot water
systems have a difficult time making water
hot enough to put into your baseboard system.
In fact, the solar hot water would likely
cool your furnace. (we design our
standard solar hot water piping for summer
temperatures when solar hot water might get
to 160F. Winter temperatures usually hover
around 90F).
So can you heat space with solar hot water?
Yes. But not with traditional baseboard
radiators. As it turns out, radiant floor
systems are nicely compatible with solar
heated water. However, you still must plan on
a large number of solar collectors (about 8
will cover half the heating load of a 2000
square foot area) and a large hot water
storage tank (several hundred gallons). In my
next issue I'll go into more detail on
radiant floor systems and solar hot water.
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In today's Boston Globe, I read about a home
restoration in the Back Bay. The wealthy
homeowner is combining two town homes into a
single 24,000 square foot living space. I
can't imagine the energy required to run the
home,but at more than 8 times the square
footage of my home, I'm sure it is large.
I am truly puzzled. How is this homeowner
being a responsible citizen of the planet? I
hope he includes some solar power in the $23
million project.
I appreciate that he can do whatever he wants
with his money but what about the common good?
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Try to reduce waste, use a fan instead of an air conditioner, turn off the lights, switch to compact fluorescent bulbs, put computer gear on a power strip and turn it off at night, unplug unnecessary appliances, ventilate your attic, take shorter showers, walk instead of drive, ride a bike, group your errands, inflate your tires, grow your own food, eat locally-produced food, recycle, use your most efficient vehicle, get a hybrid vehicle, wash your clothes in cold water, buy less packaging, use a clothesline instead of the dryer, drink tap water, print with narrower margins, use both sides of the page, buy less, compost, install a renewable energy system...
Thanks for Reading the Energy Miser,
 Mark Durrenberger
New England Breeze, LLC
phone:
978-567-WIND (9463)
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