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Posted by Lee Elson on August 13, 2004, 6:54 pm
 
I currently have a thermosyphon system in a climate where freezing
seldom occurs. I've learned that the main problem with this system is
overheating.

I'm planning to build a house in a new climate where both freezing and
overheating can be frequent problems and I'd like to come up with a
thermosysphon system (or other passive system that does not require a
pump) that will avoid these 2 problems. While I can easily imagine
avoiding the freezing problem with a thermosyphon antifreeze system
and a heat exchanger, how can I avoid overheating with such a system?
The only system I've seen that avoids both problems is a drainback
system, but that requires pumps.

Suggestions are welcome. References are greatly appreciated!

Lee Elson
lelson@bigfoot.com

Posted by Anthony Matonak on August 13, 2004, 7:25 pm
 
Lee Elson wrote:

First, why avoid the pumps? Sure, they require power to run but
usually only when the sun is shining so a PV panel works well.
Sure, they fail but you can easily design your system to have
redundancy, backup and easy maintenance. For instance, you can
have two pumps. If one fails the other can still operate. You
can add semi-quick disconnect fittings so that the pumps may
be replaced quickly. You can add valves so that one or both
pumps may be removed without draining your system.

Next, you can avoid overheating by using a manual method of
regulating solar energy collected. I.e. throw a cover over
part of the panel to shade it during the months when this
is a problem. You could even make this automatic but that
would require some kind of motor.

I've heard folks have used thermal valves which spill water
when it gets over a certain temperature. This would work but
it seems wasteful to me.

Anthony



Posted by daestrom on August 13, 2004, 9:36 pm
 

If the system 'overheats', that could mean one of two things.  1) Your
storage tank is overheating, or 2) the collector overheats without
overheating the tank.

If the storage tank overheats, then you have more collector than
storage/usage dictate.  You can have a larger amount of storage (up to
limits) and that will absorb the heat from the collector when the sun shines
for use on cloudy days or at night without overheating.

If the collector overheats without getting the storage hot, then you don't
have enough circulating flow.  Use larger piping and fewer fittings.  Elbows
and valves and such can be equivalent to many, many feet of straight pipe.
Thermosyphoning systems don't have a lot of differential pressure to work
with so every tiny bit of flow resistance can be a problem.  Increase the
vertical distance between the collector and storage.  This will increase the
differential pressure available (differential pressure is proportional to
density difference between hot and return water times the vertical rise).
As long as you don't have to add a lot of elbows and fittings to get the
height.

daestrom



Posted by Gary on August 13, 2004, 10:28 pm
 Lee Elson wrote:


Does the overheating occur only in the summer?
If so, you might try tilting the panel more steeply -- this would
increase heat collection in the winter and decrease it in the summer.
For example, in June at 40 deg lat on a sunny day one sqft of panel
sloped at 30 deg picks up 2400 BTU/day, but when sloped at 90 deg it
only picks up 600 BTU/day.  In January, its 1650 for 30 deg slope, and
1730 for 90 deg.
Not sure what effect tilt angle has on the thermosyphon process, but
it seems like it should work as long as the tank is above the panel?

Is the negative effect of overheating simply that the water gets to
hot for safe use (ie scalding), or that the collector or tank are
stressed in some way?
If its only the first, then a mixer valve or anti-scald value that
mixes in cold water with the hot automatically might be an answer?
These are commonly available.

Gary

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