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re: thermo-siphon (from re: solar hot water question)
28 jul 2001
david buchner wrote:
>...what's the deal with smaller vs. bigger pipe?
bigger pipe has less resistance to water flow, so more water flows
when the collector is warmer than the tank above, so the collector
temp is closer to the tank temp, ie the collector is cooler when
operating and it and loses less heat to the outdoors, so the solar
collection is more efficient and the rate of solar heat transfer
into the tank is higher.
but, if the pipe is too big or the collector holds too much water,
system efficiency suffers because lots of water cools off overnight
and needs to be reheated to the tank temp the next morning before
thermosyphoning flow starts again.
>i built an expermental thermo-siphoning heater this summer, and
>it's up there on the roof full of hot water, waiting for me to install a
>pressure pump in the house so i can actually use it. i have a ten-gallon
>tank from a little electric water heater, packed in styrofoam, and a
>roughly 4x2 ft panel made of 1/2" rigid copper pipe running back and
>forth under an old window i had laying around.
with 8 ft^2, you might collect about 4k btu/day in wintertime, enough
to heat 4k/(8lb/gx(110f-60 f)) = 10 gallons of water from 60-110 f,
enough for a 3 minute 3 gpm shower, but it sounds like the copper pipe
may not have a good thermal connection to a conductive absorber plate.
if so, it would only absorb heat from the sun that shines on its small
surface, and a little more from the hot air inside the box.
>...would it be more effective with a steeper slant from the panel up
>to the tank?
maybe "yes," because that might mean less overall pipe length, hence
less resistance to flow. the only pressure driving the flow is the temp
difference times the vertical height difference (the horizontal distance
just adds flow resistance.)
>...maybe the question is whether it's more effective to have the water
>circulate quickly -- and get more of the total volume through the whole
>loop more times in a day, gradually heating it. or to have it move more
>slowly, through smaller pipe or up a steeper grade -- and have it get
>much hotter before being dumped back into the tank.
the first way seems better. hotter water loses more heat to the outdoors
through the collector and piping. bigger pipe keeps the collector and pipe
temps lower and maximizes the daily solar energy collection and tank temp.
nick
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