Solar Hydronic Heating.
Calculations which might be done to size system.
I'm trying to put in a hydronic loop on the
north side of my house, also.
The power from the sun maxes out at about
1Kw/sq.meter. If we were to start with one
4'x 8' collector, we would have about 3 sq.meters, so at
50% efficiency we would have 1.5Kw of energy in full
sun.
One would have to find the average hours of
sun for a specific location to see how much average energy
one can expect to collect.
For my area with 2.5 hours of sun, I might expect
to capture 3.75Kw of energy a day. In S.Ca with 6
hours of sun a day, one might expect to get 9Kw
of energy per day. Arizona might be more.
Looking it up,I get 3412BTU/Kw-hr, so I'd be
collecting 3412*3sq m=around 5,000BTU/hr.
at 50% efficiency with the panel. For
2.5 hours, I'd get a total of an average of
12,500 BTU per day with this system, in my region.
I could, for example, use a drainback system
from a hot water heater in which
the panels are located on the roof and a pump
starts pumping water from the tank to the
panel whenever the temperature in the panel is
greater than that in the tank. When the temperature
of the panel falls lower than that of the tank or
the temperature in the tank gets near boiling, the
pump will stop and the water will drain back into
the tank. In this way no antifreeze would be
required in freezing climates. I would capture all
useable energy.
Whenever the thermostat calls for heat, the
hydronic system will pump hot water from the
solar storage tank whenever its temp is greater
than a certain temperature. If its temperature
drops below this setpoint, alternative heating
sources could be employed.
I could use one tank as a solar tank and a
second tank as a alternate fuel source heated
water tank, for example. I would draw off the
solar tank as long as the temperature was great
enough. Then the solar tank pump or valve would
be switched off and the water would come from
the alternate tank.
To figure the amount of water required is
not too hard. The greater the amount of water,
the more efficiency from the solar panels.
This is because the greater the
delta t (difference in temperature between the
panels and the water) the faster the heat
transfer will occur and the less energy will be
lost from the panels into the outside air.
If the water is stored in the area to be heated,
then all heat lost will actually not be lost.
The more thermal mass in the area to be heated,
the less the temperature will fluctuate with
large inputs and withdrawals of energy.
If you want to calculate the amount of thermal
mass, you can do this also, based on the amount
required to absorb the maximum daily heat input
with only a couple degree change in temperature.
For example, the heat capacity of pure water,
by definition states that it takes 1 calorie
to raise 1 gram of water 1 degree celsius.
1 calorie is 0.00397BTU, so we can make up a
chart or graph and plot the number of
grams of water against the temperatures at a
given BTU input. We can plot this at the
maximum and minimum expected energy inputs
to get the possible temperature ranges for a
given amount of water. 1 gram of water takes
up a volume of 1 cubic centimeter (more or
less, depending on the temperature).
We can figure from this the number of gallons,
knowing there are 0.0002642 gallons in one cc.
Hope this was not too complicated. With only
3 sq meters of collector area it shouldn't
require a whole lot
of water to keep it from boiling.
People I know tell me the simplest
systems are the best. They say passive solar
requires zero maintenance. It can also be quite
efficient if it is designed properly.
For example, I could use a hot air collector
of the same size and a simple fan and baffle
to circulate the air when the temperature of
the panel is greater than that of the house.
I could employ enough thermal mass in the house
to absorb the heat from the panel with
minimal temperature fluctuation.
One friend of mine built a thermosiphon
system which collected hot water on the roof
thermosiphoning to a blower/water to air
heat exchanger in the dwelling unit below.
When the temperature got above a certain
point the thermostat on the blower came on,
releasing the heat into the house. If anyone
is interested in patenting his design, and
making a production version, let me know.
To simplify this further, I could employ
glass on the south facing side
of the habitation which is superinsulated and
contains a substantial amount of stone within.
An overhang could shade the windows in the
summer and possibly insulating blinds could be
employed at night in the winter.
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