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re: thermosyphoning freeze protection
16 jul 2003
daestrom  wrote:

>"andy"  wrote:

>> the system may well thermosyphon through each tube alone - ...

seems unlikely, without a return path through another pipe. 

>> ...i know in my place the 3/4" vertical hot water pipe outlet is
>> always hot hot hot on the second floor above due to thermosyphoning
>> within the pipe itself.

then again... 
 
>while internal circulation within the riser and return may well happen, that
>wouldn't create any flow through the header box.

i could imagine a little flow. if thermosyphoning can occur within
a single pipe, why not within the header pipe as well? but that seems
like a matter of hope, vs a more pro-active and predictable design.

>to get circulation through the header you need to somehow keep the riser
>warmer than the return.

or vice-versa, assuming no check valve (as in the aug 2003 hp system.)

>the storage tank will do this for a while unless some cold supply water is
>admitted to replace warm water drawn off by the homeowner

not exactly something to count on either.

>stripping insulation from the riser and leaving insulation on the return
>could also work if enough of the vertical run is in the warm house (and
>the house stays warm ;-)

this might work better with both pipes bare.

>but warming the water halfway up the riser is not as effective because
>it has less elevation to 'work' through.

agreed...

>like i said before, warn the homeowner *not* to use any water when the power
>is out.  cold supply will cool the bottom of the tank quickly and stall the
>thermosyphon.

i agree that it doesn't seem like a good idea to depend on such warnings,
but a cold tank might be ok, if the supply and return pipes are warmer... 

another freezing scenario: it's cold outdoors, and the kids have exhausted
the solar hot water, leaving 38 f water in the tank bottom, and then the
power fails. if the house heating fails at the same time, we drain the pipes
and go somewhere else, eg florida. but suppose the house is still warm, and
we are all sitting around the woodstove with candles... we might avoid
collector freezing, if the pipes inside the house have enough conductance
to warm house air, to a) supply the header heat loss to the outdoors and b) 
warm the water that flows out of the tank to a thermosyphoning temperature. 

roughly speaking, suppose the pipes have a conductance of g btu/h-f to
70 f house air and the pipe water is about 50 f and the header has 1 btu/h-f
of conductance to -10 f outdoor air. then the house air has to supply
(50-(-10))1 = 60 btu/h to the header. suppose the flow in the pipe loop
q = 1000dt lb/h, which supplies 1000dt^2 btu/h of heat to the header. 
then dt = sqrt(60/1000) = 0.245 f, which makes q = 245 lb/h, which means
we have to heat 245 lb/h from 38 to 50 f. (70-50)g = 60+(50-38)245 = 3000
makes g = 150 btu/h-f, eg 40' of fin tube pipe :-) not too practical,
altho this might work with slightly cooler pipe water. then again, we'd
also like it to work with a slightly cooler house. 

most of that heat (98%) is needed to warm the cold water that comes out
of the tank. without that requirement, g = 3 btu/h-f, eg 10' of bare pipe
and 0.5 gpm of flow. is there a way to add some sort of bypass pipe between
the supply and return pipes to complete the loop at the bottom and avoid
having to heat the cold tank water? it might tend to short out the pumped
solar loop in normal times. perhaps this could work with a normally-open
check valve that closes when the pump runs but allows thermosyphoning
through the bypass pipe with the pump off.

nick




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