[XCSSA] Solar and Green events in SA
xcssa@xcssa.org
xcssa@xcssa.org
16 May 2007 23:24:42 -0500
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I saw a design a few years ago that eliminated a lot of those problems.
When the house was built, they dug a couple of BIG holes in the ground
next to it. They lined the first hole with some high-efficiency
insulation, filled both with crushed rock, and covered them with
insulation & dirt. In among the rock, they ran copper heat transfer
coils. In effect -- they turned the holes into big heat storage bins!
The solar heated water would run through the rock in the first hole
during the day heating it up with the glycol solution. A separate coil
would pick up heat for transfer to the house (this was in a much colder
climate than San Antonio). I don't remember how they handled hot water
supply -- but the heat all came from the same place. I think the hot
water supply ran directly through the rock with a circulation pump to
keep it hot at the tap.
The reason for two holes was one was a "dump" or "overflow" in case the
first one reached saturation (calculated automatically by internal heat
measurements and input/output glycol temperatures). It then switched
over to the second hole which was basically just a way to dump the
excess heat to keep the collectors from overheating (overheating damaged
the collector's absorption material). And I seem to remember something
about turning half to three quarters of the collectors upside down
during the summer months since they didn't need heat for the house --
only hot water. It was a way to avoid excess heat capture.
Well, that was one way someone dreamed up. Seems it would solve a lot
of the problems you've been mentioning.
Chuck
The typical "indirect" solar water heater is microprocessor
controlled, and does indeed do something like that. My beef (or
misinformed guess) is that that they don't take the "convection
radiator" part all that seriously. In several hours of web
searching, I found only one example of a "heat dumping" device for a
solar water heater, it it was a simple home-brew coil of tubing. It
didn't look adequate to me to deal with 1-2Kw of excess power on a
continuous basis. They seemed to be relying on (1) system "sizing",
(2) vacation covering, and (3) overall random heat leakage to deal
with the problem. They also assume that the main water tank is OK
with having its temperature vary over a wide range, from hot (say 140
degrees F) to near boiling. That appears to be a big part of the
implied heat dumping strategy. And then occasional pressure
release. Seemed like poor design to me. With a good heat dumping or
rejection strategy, you could keep the temperature within a
relatively narrow range, increasing lifespan of all components, and
not worry about system sizing or vacations.
BTW, I like the "hot tub" heat dumping strategy! Assuming it's not
in air conditioned space.
Then elsewhere I found some reliability studies. One of the big cost
reliability problems is main tank leakage from a history of large
thermal cycling. Connect the dots. And typically, in order to
withstand the temperature cycling, much higher quality tanks are used
than in conventional water heaters. Such as solid stainless steel.
But it can break at the welds. Other problems could be traced to
heat abuse of the heat transfer glycol. In an indirect system, the
glycol flow shuts down thermostatically when heat isn't needed or
available. Then it cooks in the hot sun, getting partly solidified.
Then that destroys the pump. Pump failure is probably the single
most common problem in an indirect system.
You can imagine that the "heat dumping" isn't typically a big selling
point. Typical consumer might wonder why heat needs to be dumped at
all...it seems like a waste. Also the question is where to put the
heat dumper. Probably outside, in the shade, and out of human/pet
reach if you are seriously dissipating 2Kw at about 200 degrees F.
But I didn't find any designs that did that. In one diagram it was
right next to the tank.
I'm spending thousands of dollars, I want a tank that's going to last
for decades, and a pump that's going to last for at least one decade.
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<FONT SIZE="3">I saw a design a few years ago that eliminated a lot of those problems. </FONT>
<BR>
<FONT SIZE="3"></FONT>
<BR>
<FONT SIZE="3">When the house was built, they dug a couple of BIG holes in the ground next to it. They lined the first hole with some high-efficiency insulation, filled both with crushed rock, and covered them with insulation & dirt. In among the rock, they ran copper heat transfer coils. In effect -- they turned the holes into big heat storage bins!</FONT>
<BR>
<FONT SIZE="3"></FONT>
<BR>
<FONT SIZE="3">The solar heated water would run through the rock in the first hole during the day heating it up with the glycol solution. A separate coil would pick up heat for transfer to the house (this was in a much colder climate than San Antonio). I don't remember how they handled hot water supply -- but the heat all came from the same place. I think the hot water supply ran directly through the rock with a circulation pump to keep it hot at the tap.</FONT>
<BR>
<FONT SIZE="3"></FONT>
<BR>
<FONT SIZE="3">The reason for two holes was one was a "dump" or "overflow" in case the first one reached saturation (calculated automatically by internal heat measurements and input/output glycol temperatures). It then switched over to the second hole which was basically just a way to dump the excess heat to keep the collectors from overheating (overheating damaged the collector's absorption material). And I seem to remember something about turning half to three quarters of the collectors upside down during the summer months since they didn't need heat for the house -- only hot water. It was a way to avoid excess heat capture.</FONT>
<BR>
<FONT SIZE="3"></FONT>
<BR>
<FONT SIZE="3">Well, that was one way someone dreamed up. Seems it would solve a lot of the problems you've been mentioning.</FONT>
<BR>
<FONT SIZE="3"></FONT>
<BR>
<FONT SIZE="3"></FONT>
<BR>
<FONT SIZE="3">Chuck</FONT>
<BR>
<FONT SIZE="3"></FONT>
<BLOCKQUOTE>
<PRE><FONT COLOR="#737373"><FONT SIZE="3"><I>The typical "indirect" solar water heater is microprocessor </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>controlled, and does indeed do something like that. My beef (or </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>misinformed guess) is that that they don't take the "convection </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>radiator" part all that seriously. In several hours of web </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>searching, I found only one example of a "heat dumping" device for a </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>solar water heater, it it was a simple home-brew coil of tubing. It </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>didn't look adequate to me to deal with 1-2Kw of excess power on a </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>continuous basis. They seemed to be relying on (1) system "sizing", </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>(2) vacation covering, and (3) overall random heat leakage to deal </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>with the problem. They also assume that the main water tank is OK </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>with having its temperature vary over a wide range, from hot (say 140 </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>degrees F) to near boiling. That appears to be a big part of the </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>implied heat dumping strategy. And then occasional pressure </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>release. Seemed like poor design to me. With a good heat dumping or </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>rejection strategy, you could keep the temperature within a </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>relatively narrow range, increasing lifespan of all components, and </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>not worry about system sizing or vacations.</FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I></FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>BTW, I like the "hot tub" heat dumping strategy! Assuming it's not </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>in air conditioned space.</FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I></FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>Then elsewhere I found some reliability studies. One of the big cost </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>reliability problems is main tank leakage from a history of large </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>thermal cycling. Connect the dots. And typically, in order to </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>withstand the temperature cycling, much higher quality tanks are used </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>than in conventional water heaters. Such as solid stainless steel. </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>But it can break at the welds. Other problems could be traced to </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>heat abuse of the heat transfer glycol. In an indirect system, the </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>glycol flow shuts down thermostatically when heat isn't needed or </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>available. Then it cooks in the hot sun, getting partly solidified. </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>Then that destroys the pump. Pump failure is probably the single </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>most common problem in an indirect system.</FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I></FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>You can imagine that the "heat dumping" isn't typically a big selling </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>point. Typical consumer might wonder why heat needs to be dumped at </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>all...it seems like a waste. Also the question is where to put the </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>heat dumper. Probably outside, in the shade, and out of human/pet </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>reach if you are seriously dissipating 2Kw at about 200 degrees F. </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>But I didn't find any designs that did that. In one diagram it was </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>right next to the tank.</FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I></FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>I'm spending thousands of dollars, I want a tank that's going to last </FONT></FONT></I>
<FONT COLOR="#737373"><FONT SIZE="3"><I>for decades, and a pump that's going to last for at least one decade.</FONT></FONT></I></PRE>
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