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re: thermosyphoning freeze protection
14 jul 2003
daestrom wrote:
>a cloudy day or two and the lower part of the tank would be quite cool
>just from supply water. but the homeowner would get hot water from
>the upper, electric element...
with some downward heat mixing/diffusion, i ween.
>...the bottom of the tank could be close to 40 due to cold supply
>temperature. around here (ny) our city water temp. is about 38
>in the wintertime for a couple of months straight.
which part of ny you live in? nrel's average yearly temps:
albany 47.4 f
binghamton 45.9
buffalo 47.7
massena 49.1
nyc 54.7
rochester 47.6
syracuse 47.4
>so a cloudy day, take a short shower, then lose power to the circ water pump
>that night. doesn't sound very implausible at all. typical weather,
>typical usage, one failure...
the most probable freezing scenario may be very cold weather and hot water
use (filling the lower part of the tank with 38 f water) and power failure,
simultaneously. the cure for this might be a microprocessor that energizes
the lower electric heating element to keep the lower tank water just warm
enough to keep the collector from freezing, given the time of year, assuming
the solar input is insufficient to do this.
ashrae lists massena with the coldest ny -13 f 99% winter design temp, ie
the expected outdoor temp is only colder than -13 f for 22 hours per year,
which are probably not consecutive hours. this might determine the min lower
tank temp in very cold weather. we might maintain this low temp under such
unlikely conditions with a very small energy penalty.
>> with inside insulation, the header needs (40-(-10))0.4 = 20 btu/h to avoid
>> freezing at -10 f. that might come from 32' of bare 3/4" indoor pipe with
>> 6.3 ft^2 of surface and 9.4 btu/h-f of conductance to t (f) house air,
>with
>> (t-40)9.4 = 20, so t = 42 f.
>
>frankly, i doubt that insulation inside the header vs. outside would make
>that much difference (ua of 0.4 vs 4).
feel free to doubt. you have my numbers. where are your numbers?
>such a small size, one can hardly use flat heat transfer equations anymore,
>may have to shift to radial calculations since the inside surface
>area is so much different than the outside area.
feel free to recalculate this using a radial calc or log mean area.
>and what about conduction of the metal in the heat pipes? i understand the
>fluid/vapor inside the heat pipes won't be circulating, but you would still
>have a number of metal pipes sticking out the bottom of the header box. do
>they have some sort of 'thermal break' in the metal construction?
the pipes are surrounded by evacuated tubes which provide insulation, and
they are below the header, which provides insulation for downward heatflow.
evacuation can provide very good insulation at low temperatures, given the
nature of radiant heatflow. consider a dewar flask vs a high-temp thermos.
>...natural circulation systems typically have the two legs of piping offset
>vertically to start the flow. putting the inlet and outlet on the same
>elevation may leave the system stalled for some time...
steve baer says he has often seen "stuck" thermosyphoning systems with
unequally-insulated legs. he thinks we get the most reliable performance
with equal legs, thermally-speaking.
>given typical mid-west or new england weather, imho i think the system would
>freeze up the first circ-water pump stops out for 8 hours or more at night.
>i believe freeze protection is more complicated than this because this
>design is flawed (as far as freeze protection is concerned).
tant pis. happy bastille day.
nick
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