Consumer Homes.

Question:

>I hope to add a Solar panel to heat water for my house, because of the house >design the top of the panel would be 2.1 meters below the hot water inlet of >my tank and 1.2 meters below my cold water outlet, the panel would be about >9 meters (horizontal distance) from the tank. >Meaning that the hot water "return distance" would be 9 meters slightly >inclined then 2.1 meters vertically.

So the total loop length is about 22 meters (72 feet)? >If the hot water pipe was well insulated would thermosiphoning work over >this sort of distance and set up?

Maybe. Say the tank is 150 F and you want to keep the panel 160 F max and it collects 68K Btu/h (2 kW) of sun power. Then you need a water flow Q, where 3600s/hx64Btu/F-ft^3Q(10F) = 68K, so Q = 0.0294 ft^3/s, or 14 gpm.   Water weighs about 63.74 – 0.0158T lb/ft^3, with T in degrees F. The density difference between up and down pipes causes a pressure difference proportional to the height of the water column. With 2.1 m (7 feet) of height and a 10 F temperature difference, dP = 1.09 lb/ft^2. Here’s a formula for laminar flow in a pipe with radius r and length L in feet and pressure difference dP: Q = Pir^4dP/8MuL ft^3/s. Mu is the viscosity, about 8.17×10^-6 lb-s/ft^2 for 150 F water. If L = 72′, Q = 0.0294 = Pir^4(1.09)/(8×8.17×10^-6×72′) makes r = 0.08′ or 0.96", eg a 50 mm pipe (or larger, with some fitting head losses.) The pipe would hold about 100 pounds of water. With 1" foam insulation (US R5, with k = 0.0167 Btu/h-F-ft), R = ln(4"/2")/(2xPix0.0167×72) = 0.092 F-h/Btu and RC = 9.2 hours. Over 18 hours in 70 F air, the pipe would cool to 70+(160-70)exp(-18/9.2) = 82.7 F. Warming it to 160 F every day would take 7.7K Btu (2.3 kWh), a significant part of the collector’s daily output… Nick

Response:

>> The pipe would hold about 100 pounds of water. With 1" foam insulation > (US R5, with k = 0.0167 Btu/h-F-ft), R = ln(4"/2")/(2xPix0.0167×72) = > 0.092 F-h/Btu and RC = 9.2 hours. Over 18 hours in 70 F air, the pipe > would cool to 70+(160-70)exp(-18/9.2) = 82.7 F. Warming it to 160 F > every day would take 7.7K Btu (2.3 kWh), a significant part of the > collector’s daily output… >So unless I misunderstand it would be far more effective to circulate the >water by pump rather than rely on thermosiphon?

Well, it looks like heating the water in that big long pipe would waste lots of solar energy every morning. A 2 kW peak collector might make 10 kWh on a good day, and 2.3 would go for pipe water heating. Reducing the pipe diameter would lower the storage loss but also raise the collection temp more than 10 F above the tank temp and lower collection efficiency. But I just made up these numbers. You’d want to refigure things using the actual loop length (is it all in 70 F air?) and collector size and height. Using a smaller pipe and a DC pump makes sense to me. Nick

Response:

> Yes, and very simply. Some use a DC pump, and connect the output of > the PV panel directly to the pump. That way it only pumps when the > sun is shining

I was thinking about this in relation to a garden fountain pump. What happens when the PV output voltage drops below what is required to run the pump, yet it still puts out current?  Would the current to a stalled pump damage anything?  How do you size the PV panel to a pump to minimize problems? And, does anyone offer a pond kit that is solar powered? Rex Burkheimer

Response:

http://www.voltscommissar.net/minimax/minimax.htm

– Hide quoted text — Show quoted text -> I hope to add a Solar panel to heat water for my house, because of the house > design the top of the panel would be 2.1 meters below the hot water inlet of > my tank and 1.2 meters below my cold water outlet, the panel would be about > 9 meters (horizontal distance) from the tank. > Meaning that the hot water "return distance" would be 9 meters slightly > inclined then 2.1 meters vertically. > If the hot water pipe was well insulated would thermosiphoning work over > this sort of distance and set up? > if not would a small solar powered pump work? > TIA.

Response:

http://www.voltscommissar.net/K4/kernkraft.htm http://www.voltscommissar.net/minimax/minimax.htm

– Hide quoted text — Show quoted text -> I hope to add a Solar panel to heat water for my house, because of the house > design the top of the panel would be 2.1 meters below the hot water inlet of > my tank and 1.2 meters below my cold water outlet, the panel would be about > 9 meters (horizontal distance) from the tank. > Meaning that the hot water "return distance" would be 9 meters slightly > inclined then 2.1 meters vertically. > If the hot water pipe was well insulated would thermosiphoning work over > this sort of distance and set up? > if not would a small solar powered pump work? > TIA.

Response:

I sell this solar pump and controller. Automatically changes speed depending on the sun intensity and has anti stall characteristics. They aint cheap though….400USD including controller. 70GPH. 12V PV panel input. use A 20w PANEL. I have these too – 337USD. THis is the coolest and most reliable solution. I have bigger models and all the specifications somewhere. Let me know–

– Hide quoted text — Show quoted text -> There is a German company, the name of which escapes me but I can tell you > tomorrow if you are interested, that makes a pump controller.  Takes the > output from the pv panel and controls the pump.  This eliminates any > problems that might occur with a stalled pump. The one we have on it is a > Grundfoss 12v dc central heating pump.  Been running for years without > problems.  Its also a differential controller but normally if there is > enough power to run the pump the water in the panels is hotter than the > storage tank. > Regards, > Stuart Hudson > > Yes, and very simply. Some use a DC pump, and connect the output of > > the PV panel directly to the pump. That way it only pumps when the > > sun is shining > I was thinking about this in relation to a garden fountain pump. > What happens when the PV output voltage drops below what is required to > run > the pump, yet it still puts out current?  Would the current to a stalled > pump damage anything?  How do you size the PV panel to a pump to minimize > problems? > And, does anyone offer a pond kit that is solar powered? > Rex Burkheimer

Response:

20w for $337 US? ROFLMAO — Steve Spence Subscribe to the Renewable Energy Newsletter: http://www.webconx.com/subscribe.htm Renewable Energy Pages – http://www.webconx.com Palm Pilot Pages – http://www.webconx.com/palm X10 Home Automation – http://www.webconx.com/x10 (212) 894-3704 x3154 – voicemail/fax We do not inherit the earth from our ancestors, we borrow it from our children. —

– Hide quoted text — Show quoted text -> I sell this solar pump and controller. Automatically changes speed depending > on the sun intensity and has anti stall characteristics. They aint cheap > though….400USD including controller. 70GPH. 12V PV panel input. use A 20w > PANEL. I have these too – 337USD. THis is the coolest and most reliable > solution. I have bigger models and all the specifications somewhere. > Let me know– > There is a German company, the name of which escapes me but I can tell you > tomorrow if you are interested, that makes a pump controller.  Takes the > output from the pv panel and controls the pump.  This eliminates any > problems that might occur with a stalled pump. The one we have on it is a > Grundfoss 12v dc central heating pump.  Been running for years without > problems.  Its also a differential controller but normally if there is > enough power to run the pump the water in the panels is hotter than the > storage tank. > Regards, > Stuart Hudson > > > Yes, and very simply. Some use a DC pump, and connect the output of > > > the PV panel directly to the pump. That way it only pumps when the > > > sun is shining > > I was thinking about this in relation to a garden fountain pump. > > What happens when the PV output voltage drops below what is required to > run > > the pump, yet it still puts out current?  Would the current to a stalled > > pump damage anything?  How do you size the PV panel to a pump to > minimize > > problems? > > And, does anyone offer a pond kit that is solar powered? > > Rex Burkheimer

Response:

> > Using a smaller pipe and a DC pump makes sense to me.

So can anybody enlighten me — what’s the deal with smaller vs. bigger pipe? 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. One solid tidbit of useful information I’d picked up before (probably in this newsgroup), was about the placement of the top pipe, the one that returns hot water to the tank, being down a ways from the top. This tank conveniently had threaded fittings in the side in just about the right spots, where the TPR valve and drain plug used to be. ** I thought about the pipe size for a bit, didn’t come up with a solid rationalization, and just went ahead with 1/2" as a nice in-between size. I had considerend, hmm, smaller would heat up faster but more water would run through bigger and the water from the tank would get cycled through the sun more times. But I couldn’t make up my mind and I felt more like soldering than thinking. It seems to work as it is. I also wondered if there might be something useful about making the top pipe bigger or smaller than the bottom, because the water on the return trip has expanded or something…? ** For the pipes going to and from the panel, I have short lengths of 1/2" flexible copper, bent into big "U"s, because the panel is leaning against the front of the tank. After I monkeyed around with relative heights a bit, getting the panel low enough that there was enough of an up-slope to the tank, it started circulating splendidly (I started with clear hose for the connections, so I could watch for movement). The water gets pretty danged hot in there — much hotter than my black plastic tank batch-heaters of previous summers — making me wish I’d hurry up and get a way to actually *use* that water! Here’s another thing I’m wondering, though: would it be more effective with a steeper slant from the panel up to the tank? It seems like this would require a greater temperature difference between the high and low pipes, so maybe the water in the panel would "wait" longer and get hotter before climbing up? (times like these I realize how messed up and spotty my whole basic science education is) So 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. Which way would be quicker, which way would end up with a higher temperature? I dunno. — David

Response:

>…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

Response:

> 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.

Thank you! > [....] 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.

I made a bit of an effort in this direction: the backing in the box is a piece of pretty heavy-gauge sheet steel, and I used a bunch of those little copper strap doohickeys and rivets to hold the pipe down to it fairly tight. There’s still space, though, so I don’t know how effective it is. I wonder if there’s some kind of goo I could glurp into those gaps… (Whole works is painted black together, of course.) > [....] The only pressure driving the flow is the temp difference times the > vertical height difference (the horizontal distance just adds flow > resistance.) > [....] 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.

Again, thanks for the useful explanations. — David

Response:

>…the backing in the box is a piece of pretty heavy-gauge sheet steel,

Steel isn’t a great heat conductor. For good solar collection efficiency, you might (arbitrarily) try making the back plate thick enough that the thermal resistance from the plate to the air in the box is ten times the resistance between the plate and a tube, measured from the line halfway between the tubes. If the resistance from plate to still air is US R0.7, with a perfect thermal connection between the tubes and the plate and a 1′ tube spacing, the plate needs R0.07 max across a 6" width with a 1′ length and a d inch thickness, ie a d/12 ft^2 cross sectional area. Steel has a 26 Btu/h-F-ft thermal conductance, ie R0.0032 per inch, ie R0.019 for a square foot 6" thick, so we need 0.07 = 0.019/A or A = 0.27 ft^2 = d/12, ie d = 3.25", ie a steel plate 3.25" thick… Remember that thermal mass problem? Steel has about the same specific heat by volume as water. Reheating a square foot of steel 3" thick from 40 to 120 F every morning takes about 1×3/12ft^3×64Btu/F-ft^3(120-40) = 1280 Btu, ie about 5 hours of full sun before the collector starts to supply useful heat. Copper has a 226 Btu/h-F-ft conductance, so a copper plate would only have to be 26/226×3.25 = 0.37" thick. Of course you could decrease the tube spacing. >I used a bunch of those little copper strap doohickeys and rivets >to hold the pipe down to it fairly tight. There’s still space, though, >so I don’t know how effective it is. I wonder if there’s some kind >of goo I could glurp into those gaps…

Solder it down? Or maybe use some of that hard black high temp furnace cement, vs the kind with fibers. Or maybe some mortar, or sand and "water glass" (sodium silicate.) Anything’s better than air. Or make your collector a flat shallow water trough under a Mylar film concentrating parabolic reflector Nick

Response:

>me: >I used a bunch of those little copper strap doohickeys and rivets >to hold the pipe down to it fairly tight. There’s still space, though, >so I don’t know how effective it is. I wonder if there’s some kind >of goo I could glurp into those gaps… > Solder it down? Or maybe use some of that hard black high temp furnace > cement, vs the kind with fibers.

Good idea. That stuff that comes in a caulking tube and hardens into a sort of brittle ceramic crust – at least on the woodstove it does, maybe hitting it with a torch would set it up nicely? – I’ll try that next time I take the thing all apart. (When I said my backing was pretty heavy steel, I meant sheet metal of a pretty stiff gauge – not some kind of heavy plate. It was just the thing closest to the right size, that offered a bit of structural rigidity, that I had laying in my boneyard.) — David

Response:

> [....] 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. > I made a bit of an effort in this direction: the backing in the box is a > piece of pretty heavy-gauge sheet steel, and I used a bunch of those > little copper strap doohickeys and rivets to hold the pipe down to it > fairly tight. There’s still space, though, so I don’t know how effective > it is. I wonder if there’s some kind of goo I could glurp into those > gaps… (Whole works is painted black together, of course.)

        There is a "Goo" specifically made for this! It’s called "heat transfer compound" and is used extensively in the plastics industry for improving the heat transfer between heater bands and extrusion machine flanges.           http://www.nphheaters.com/products/products.htm These guys should have what you are looking for or be able to point you in the right direction. David W. Beard

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