Hi Guys,
I'm finishing construction of my big (128SF) drainback solar panel for hot water heating (space and domestic). I have built the 700 gallon insulated storage tank, ready to put in the liner. My water pressure is just 12 psi, from a hill mounted tank.
I'm interested in the "batch" tank within my tank heat exchanger for my domestic hot water, but am having a hard timing finding an affordable tank. I can't use a water heat tank since the outside of those tanks would corrode when submerged. New galvanized steel tanks are out of sight, cost wise, and stainless steel pressure tanks seem insane as well.
My last resort is a copper tube exchanger, but in winter my incoming water is near freezing, so according to the figures I've seen, I would need at least 180 feet of 1/2" copper ( as 6-8 coils in parallel) at 2 GPM. (38F to 105F). This is $360 just for the soft copper, perhaps $420 all together for copper cost. I think that even 180 feet is marginal.
Any ideas would be appreciated.
PS- I can't use a big coil of 1" pex as the exchanger/batch heater as I don't tolerate hot water from pex for showers. Too much plasticizers smell, especially the first year.
Quote from: BruceM on March 14, 2014, 02:23:30 PM
......PS- I can't use a big coil of 1" pex as the exchanger/batch heater as I don't tolerate hot water from pex for showers. Too much plasticizers smell, especially the first year.
Crank the water temp up, over 150F if you can, and cook it out for a couple weeks. Then use it ?
But don't expose the PEX to sunlight / UV kills it quick.
Sadly, having to deal with domestic water pressure, I don't see an alternative to the spendy SS pressure tank or the copper coils. You might be able to save a little building a heatex bundle out of straight copper and elbows... 1/2" X 10' copper is $9.95 and 1/2 elbows are about .38 each in bulk(HD prices). Depending on how long a bundle your tank could accomidate, you could probably do 200' for under $250. Of course you would be a little more complex with all the unions.
Another option might be a smaller heat exchanger loop in the top of the storage tank feeding a standard domestic water tank alongside the storage tank. This would require a small circ pump and some temp sensing/control to circulate water thru the loop to "charge" the water tank. the good thing about this method would be if for some reason there is not enough heat in storage to charge the domestic tank, eventually the heat level drops to a point where the normal electric elements kick in as a backup to maintain temp. I am working on something similar for my radiant floor to maintain boiler tank temp, with the electric as a backup to guarantee warm floors when solar or my woodchip/pellet/wasteoil boiler is not able to provide:)
Are you doing anything to deal with possible growth in the tank/solar storage water? Copper dosn't react well with any form of chlorine added to the water...
Thanks for your suggestion, Mike. Alas, I've already tried that experiment before abandoning Pex for shower hot water. I love Pex for most other uses.
Ronmar, thanks for your thoughts. I've thought about going to rigid copper at half the price of soft, but with so many fittings to fold it into my 5x5x6 foot tank it doesn't end up being such a bargain.
I'll have to think more about the possibility of adding another pump to circulate the hot water through my propane hot water tank. This would allow a more modest copper coil in the storage tank. Alas, with my basic pilot type propane water heater, I'd have to turn the propane off, since there is no practical way to suspend it's turn on. (I wonder if there is some way to trick it.) If I end up needed more propane assist than I hope to achieve, it would be a real pain, and very inefficient.
My initial plan was to keep it simple and have no additional pump, use the storage heat exchanger/tank as a pre-heater in the periods of cloudy winter weather (very rare), but to allow for manual bypass of the propane heater so it can be turned off most of the year. So even when the propane water was turned on and "in the loop", the incoming water would be pre-heated and little propane would be consumed. The propane heater would have to maintain tank temperature with this approach, but not do any serious heating.
Used stainless steel, closed top barrels (30 and 55 gallon) are available on ebay, but I think 12 psi is too high for their flat top/bottom design. Stainless pressure tanks seem to command very high prices.
On the biological front, I will monitor the water and will resort to silver and/or copper ion generators or non-chlorine biocides if need be. Gary Reysa has had no biological problems with his copper collector /EPDM tank drainback system, and I hope my experience will be the same.
instead of PEX, there is also flexible poly pipe, in garden (40psi) and NSF (100psi) grades. We us it with a large (200') loop laid out in the sun, and outdoors, we smell nothing, and when I prep the pipe by dipping in boiling water to force it over barb fittings (backed with a hose clamp) I smell nothing. But it will kink easily, and I'm not sure of it's pressure/heat ratio.
You don't have to trick the propane unit. You just set the propane thermostat lower than the op temp of the circ pump system. The circ pump draws heat from the storage tank and maintains tank temp untill it no longer has the heat in storage to do so. When it can no longer maintain temp, the temp then falls to a point where the propane kicks in normally...
Ronmar- Let me see if I understand this arrangement with the 2nd circ pump cycling hot water from the propane tank through a copper coil in the storage tank... It would require the pump and heat exchanger coil in the tank(140F max) to transfer enough heat to keep the tank above a minimum useable shower temperature. I'd add another thermistor to the propane tank, with differential control between it and the storage tank temperature to turn on the pump. I'd draw the water off of the hose bib (bottom of the tank), where the ccccold winter water comes in, put it back in a tee at the hot outlet.
It seems like I'd might still need about the same amount of the copper needed for the pumpless preheat coil in order to keep the tank temperature from dropping below the propane turn on point, since the storage tank temp may not be too far about a useable shower temp. I suppose if I set the stock propane thermostat low enough, it might not kick on.
Any ideas where I could find a calculator or equations for copper tube in tank heatex ???
The calcs are fairly straightforward. Copper has a heat transfer co-efficient of 231. That is 231 BTU per hour, per sq/ft, per degreeF of temperature difference. 1/2" tube has a circumference of 1.57". Multiply that by 10'(120") = 188 sq/in or 1.31 SQ/FT of surface area per 10'. Inlet water from the ground is probably going to be in the 50F range and lets assume a safe target HW tank temp of 120F. 50 gallons of water weighs 400 pounds. SO 90 degrees(50 to 140) X 231 BTU = 20790 BTU x 1.31 sq/ft = 27235 BTU. 1 BTU will raise 1 pound of water 1DegreeF. 27235 BTU divided by 400# of water means it would raise 50 gallons of 50F water 68F in 1 hour. This assumes a lot of course, that the water temps around the copper maintains 140F and that you can turbulate the water inside the tube enough to maintain the biggest temp difference. A piece of solid copper wire run inside the 1/2" pipe will help to turbulate the flow thru the pipe. That 140F target temp is of course going to drop as you extract heat from the tank. Also the transfer rate slows as the temp differences gets closer to one another. 110 to 140 is only 30F. So 30X231X1.31 is 9078 BTU/HR. 9078 BTU/hr will raise 50 gallons of 110F water 22.7F in an hour.
You can take these basic numbers and apply them to various sizes of heat exchangers and varying tank temps to come up with something that meets your needs...
Uh, now that I am completely dazzled by science and math, and as pertaining to the closed pressure tank question, um Beer Keg? They do come in stainless and aluminum, 12lbs of pressure would be no problem.
And if this option is off the table, never mind. ::)
I'll sit down and be quiet now.
Ron.
Hey Ron, I think his heatex needs to sustain domestic water pressure, and I am not sure a keg would do that. Emptying one and then pressure testing in a tank full of water might be entertaining though:) "Hey Bubba hold my beer while I open this air valve"...
Thanks for the black pipe idea, Mike. I'm not normal since a sick building/aircraft composites injury, so I'd rather avoid plastics mixed with hot water.
Thanks very much Ronmar. I'll play with some numbers tonight to see how much copper I'd need. Copper is king for long life, I think. Some vendors claim 1/2" coils at 20" radius have good turbulence. I'll have to check their performance figures against the equations to see how close they come.
I did find a bargain galvanized tank at my local Tractor Supply, which makes that approach tempting. $269 for a 43 gallon tank. Very little plumbing, and no extra pump is appealing. Limited lifespan is the only downside.
I also thought about SS beer kegs, Ron. If I can find a cheap source it might be a good way to go. So far, no luck, and mail order is about $140 each for tanks bad enough to be decommissioned. A bit of a plumbing frig with only the top hole but doable via the small pipe in a bushed down big tee method. I'm going to check on availability tomorrow . My water pressure is only 12psi but the tanks are rated around 130 psi.
The good news of the day- we got the 32 foot by 4 foot high panel mounted on my shop south wall this morning before the wind got horrible. Jacks and blocks and straps- it went pretty fast. Tomorrow we'll add the polycarbonate glazing and steel braces for tilting the bottom out about 33 degrees after the glazing is on. (It's hung via 9 leftover 4" door hinges from the top edge.) We got the 1" Pex (sloped to drain back) running to the storage tank installed this afternoon.
Dear Bruce,
This is a very interesting project and I would really like to see some pictures of it.... Solar water heating is the next big to do around here too.
I am not clear on the details. Is 12psi the pressure at your tap? So the heatexchanger tank within the storage tank would be exposed to 12psi on the inside and a few psi on the outside? waddya mean the water will be close to freezing? Do you mean the incoming tap water going to the storage tank?
Two possibilities come to mind...a brand new radiator old school copper....OR....why not use a stripped down domestic water heater? You mentioned corrosion...so make the storage water non-corrosive. Everybody here has to do that with their boilers...its not that much fuss.
I'm sitting here thinking about how I would have to do it...I would need the storage water to have anti-freeze and the heat exchanger to be absolutely leak proof or maintain constant pressure on the inside.
OK, I think I would have a surflo pump wired to start pumping when the panel warmed up, then I'd have the heat exchanger at the drainback, then the fluid leaving the heat exchanger would go to the storage tank. I think I would look at either those thermostatic zone valves for hydronic systems or hydraulic solenoids (12v), to make a manifold to heat the exchanger when the panel is "off".
Please take all this as mere armchair speculation...my main interest would be to build something like this with the parts I have on hand, and one thing around here that runs really hot and uses twelve volts are the hydraulic units. I'd even consider using oil as the panel/storage fluid, as here, freezing is more an issue then efficiency.
DF
DF- If you research hot water systems starting at builditsolar.com you can get a good education. My system is a copy of the diy copper tube/aluminum fin design developed by Gary at builditsolar. Closed systems to the solar panels are to be avoided if at all possible, drainback systems have the longest, collector service life by far, require no antifreeze. Antifreeze in the storage tank is to be avoided at all costs- it's insanely expensive and an ongoing cost as the antifreeze must be replaced. Closed systems have a fatal flaw- if there is a system failure, the antifreeze gets overheated, and if at any time of the year you have excess capacity, you MUST have a system to either cover the panels or another active system to dump that heat. Drainback systems drain in case of failure, have no antifreeze and have the only downside of requiring a lot more pump to get the flow started (though only slightly more once the return flow is established). A drainback system can be safely turned off when your storage tank gets to it's maximum temperature. Because of the expense of copper and aluminum for the collector, I decided that I would do drainback, single tank and single pump system, even though for me that meant having to build a mostly below ground tank, cutting through the slab in my shop floor. It eliminates the need for a heat exchanger for the solar loop.
I'll post some pictures of my Hiser (horizonal riser) solar panel and tank soon. Yes, my domestic water pressure is 12psi from gravity pressure. No boost pumps, no pressure tanks, just one 2000 gallon storage tank up on the hill. I love the simplicity of it and oversized my pipes for the lower pressure. It works fine. I use a gas fired pressure washer with 75 feet of pressure line for cleaning patios and cars- the garden hose is the only place you notice the low pressure, especially on more than 30 feet of hose.
What methods could be used to prevent corrosion of a 43 gallon galvanized tank within my storage tank?
+1 for Dualfuels' suggestion of a school radiator.
Or .. consider connecting 3 or 4 sections of 3" pipe in series to form a submersible radiator.
The corrosion allowance on sched.40 pipe means the thing will take 473.347 years to corrode through ;) .
Not sure of the resources you have, but a bit of welding is necessary to join each end with 3/4" pipes.
The "pipe exchanger" can be made from any diameter you choose to get the desired retention time.
Then lay it flat on it's side in the tank. Perhaps on legs which to raise it off the floor and place it in the center of the reservoir.
veggie
Just thinking out loud here, why not buy a good a good 1/2" tubing bender and get some stainless tube. You can then bend up a shape to best fit your new tank. Or once a time I used to make heat exchangers by winding copper tube around a drum. Eliminating the fittings will save a lot of cost and failure points.
Hi Bruce,
here is a try.
Buy 20 foot lengths of type L hard copper, then solder on a cap at one end. fill the pipe a couple of inches shy full, of fine silica sand, then solder on another cap.
Build yourself a white oak jig to the radius you need for the bends to fit the tank. Then with map gas heat the radius and bend it around the jig.
Cut the caps off each end; blow/wash the sand out. HD sells a copper swagging tool for about 9.99, that you use with a hammer, then map gas the end, and then swagg the end up, then solder up. No fittings needed.
You can do the whole Heat/X in copper, with no fittings.
Lloyd
Quote from: BruceM on March 14, 2014, 02:23:30 PM
Hi Guys,
I'm finishing construction of my big (128SF) drainback solar panel for hot water heating (space and domestic). I have built the 700 gallon insulated storage tank, ready to put in the liner. My water pressure is just 12 psi, from a hill mounted tank.
I'm interested in the "batch" tank within my tank heat exchanger for my domestic hot water, but am having a hard timing finding an affordable tank. I can't use a water heat tank since the outside of those tanks would corrode when submerged. New galvanized steel tanks are out of sight, cost wise, and stainless steel pressure tanks seem insane as well.
My last resort is a copper tube exchanger, but in winter my incoming water is near freezing, so according to the figures I've seen, I would need at least 180 feet of 1/2" copper ( as 6-8 coils in parallel) at 2 GPM. (38F to 105F). This is $360 just for the soft copper, perhaps $420 all together for copper cost. I think that even 180 feet is marginal.
Any ideas would be appreciated.
PS- I can't use a big coil of 1" pex as the exchanger/batch heater as I don't tolerate hot water from pex for showers. Too much plasticizers smell, especially the first year.
Thanks for all the ideas, guys. My head is spinning with alternatives, now.
Best Wishes,
Bruce
QuotePS- I can't use a big coil of 1" pex as the exchanger/batch heater as I don't tolerate hot water from pex for showers. Too much plasticizers smell, especially the first year.
I read that charcoal filters don't like hot water but your system might really profit from a temping valve and a whole house sized filter on the hot water side.
Casey
I'd rather skip the plastic for hot water and avoid the whole filtering issue (yes, hot is a problem for carbon filters, and no, I don't have the extra psi to spare for one anyway). My water is fabulous- no ag or chemical contaminates, no iron or manganese sulfate like some other areas west of here.
Watts 3/4 and Honeywell tempering valves are both pretty low head at (about half a psi) my modest flow rates. More of a problem for the in floor heat as the circ pump is presently only 10 watts and gutless. I'd have to use a much stronger pump or I may do something custom via a 3 way motorized ball valve and a Picaxe chip. I like the EL-Sid 10 watt pump I'm using now- it has two h-bridges and with twice the usual number of poles of the "electronic-magnetic" motor, it has less pulsing of the water and doesn't make the 1" Pex pipe hum where I strapped it inside a wall in the kitchen.
Not sure I fully understand the requirements and setup here but instead of a tank in tank set-up, what about a Flat plate heat exchanger coupled to the heat source being the main tank and then the load? Would work a lot better than copper coils etc and be easier to install.... If it's practical in the application.
If a really large one was acquired, it may just be possible to submerge the thing and still get enough heat out of it.
Dear Bruce,
I am having trouble pinpointing exactly what you are building on the builditsolar website...Gary has many articals. Although, like you wrote, its sure is a wonderful educational resource.
Could you link me to a specific desiign you are working with?
Thx.
My collector is copper pipe with aluminum fins, per Gary's prototype:
http://builditsolar.com/Experimental/CopperAlumCollector/CopperAlumCol.htm
The difference is that my panel is turned sideways, so the risers are horizontal. This is called a Hiser configuration (for horizontal risers).
Here's an example-
http://www.builditsolar.com/Projects/SpaceHeating/Hiser/Hiser.htm
My panel is 7 runs of 1/2" copper, 32 feet long in parallel, with a return of 3/4" copper across the top. It is mounted on the wall with 1/4" per foot of run drain back slope, or 8" higher at the far end. The Hiser project above was not a drain back system.
Here's a good article on some tricks for doing drain back system:
http://www.builditsolar.com/Projects/WaterHeating/DBAirLine/DBAirLine.htm
Because I have little vertical between the panel and tank, I added an optional air bleed tap on the highest point of the panel, in case I need to assist the drainback with an air bleed solenoid.
My tank is unusual in that it is all below my shop floor, except for the last foot of height. The extra foot lets me use a non-submersible pump, without a hassle, since the pump can be mounted just below the water level, outside the tank.
Here's an example of an EPDM lined homebuilt tank:
http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm
We just put in the liner today, and 31 of 55 inches of water in the 800 gallon tank.
Best Wishes,
Bruce
Nice job Bruce. I can dream that good but somehow when their feet hit the ground they don't often come this good. Excellent.
Casey
Here's the state of my solar project as of today. Panel up but not plumbed all the way yet, insulated storage tank built and liner in but not trimmed.
Thanks, Casey. It's a struggle for me now, MS related memory problems make me feel lost in the details most of the time, and the physical work is getting so painful I'm going to be avoiding big projects for some time, I think.
I decided to stick with copper for the domestic heat exchanger/preheater, no pump. Spendy but low risk and very long lived. The house in-floor heat will be direct from the insulated storage tank, no exchanger needed, same for the shop heat. Shop heat plumbing deferred until I see if I do have excess capacity next winter. If I have excess capacity, I'll run the shop heat whenever the storage tank hits 140F.
Best Wishes,
Bruce
Here's the completed an installed copper heat exchanger.
The system is working now, and though the sun angle is getting increasingly off axis this time of year (intentionally) I still get 10 degrees of heat gain at 3.1 gallons per minute. I did some calculations and it would take 50 feet of Slantfin Fineline 30 radiant baseboard to radiate that many BTUs at 140F and that's at 65F room temperature! Good grief, I need a new plan for shop heat/heat dump.
I have run into an issue with the twinwall polycarbonate glazing. After stagnating for about 5 warm sunny days straight while I was fiddling with my new custom pump controller electronics and a new EMI filter install, the material cracked where bulging from expansion along the top of the panel. Not sure show I'm going to solve that issue just yet. Tempered glass would be nice but my quote for 4 pieces of 4'x8' tempered glass was $1800 plus tax. The twinwall Lexan was only $324. The twinwall gets pretty wild and wavy with one side heated to 200+F. The outer layer does not get over 150F. So it's a structural issue- it does not expand in a flat and uniform manner, and while held loosely to allow expansion, the the edges still must be constrained in a straight line. The joys of DIY design...
I also ran into an issue with running my in floor heating system from the hot water storage tank (no heat exchanger, atmospheric pressure only). When using the propane hot water heater as a secondary booster, so much air was released on raising the water temperature that the system would air lock due to lack of circ pump head sufficient to push water higher than the air trap above the water heater. Bleeding the air by temporarily pressurizing and venting it would only solve the problem for 20 minutes, if the propane water heater was on.
My solution was to modify my Watts brand air vent valve by JB Welding a brass tube port on the former air vent opening, and connecting to a 7 gallon portable compressed air tank converted to be a vacuum tank with gauge. The Watts air vent now "sees" the system as being pressurized and continues to function normally despite the water being at atmospheric pressure only. This solves the problem and lets me continue to use my very low head 12V EL-SID pump to heat my house. Most of the heating season I will be leaving the propane water heater off and bypassed, so air will not be released into the system from heating in the water heater. The heating of water in the big storage tank does not cause the problem since it is vented to the room. Hopefully I won't have to recharge the vacuum tank more than a couple times a heating season.
I got quite a surprise a few days ago. I checked out the system in preparation for this year's heating season, opened the valves, purged air, turned on the pump for a test run. Everything A-OK, house manifolds hot, system running absolutely quiet. Turned off the thermostat.
I noticed later that day that the floor was pretty warm near the manifold- but thought it was just a delayed heating from the short test run.
The next day, I noticed that the floor was STILL warm, and in fact warmer even further from the manifold. This despite having windows slightly open day and night.
On the third day my house was getting uncomfortable- up to 78F. I went and checked my 800 gallon storage tank- sure enough, 25 degree drop in temperature. The manifolds were hot in the house - hot water was flowing even though the circ pump was off!
The in-floor heat system presently is a non-pressurized almost closed loop which has a buried hot water tank, then the hot water rises to the circ pump, then up above the propane tank which stands on an 18" pedestal. This 8 foot rise, plus the cold return from the house (a couple feet above the shop) and back into the buried tank is apparently enough to thermosiphon pump my whole house in floor heat. Slowly, but it does pump a lot of BTUs.
I was able to stop it manually by closing a valve, but now my design dilemma is how to make it stop automatically. I'm thinking perhaps a vertical loop in the cold return before it's returned to the storage tank.
It also makes me wonder if instead I should raise the hot water line up through the ceiling into the insulated attic and dispense with the circ pump altogether?!! The thermostat could control a motorized ball valve instead.
Thermosiphon flow sure is contrary- it won't when you want it to and will when you don't!
I tried putting anti-thermosipon loops on both the cold return and the hot supply to my in floor heat; it still would not stop circulating once started by the pump.
I finally got it fixed by adding a swing type check valve, mounted so that the swing flap has to be lifted vertically by the pump to make it flow. That was enough restriction to stop the thermosiphon.
I also added a water catching bottle to the air bleed-vacuum system; same as brake bleeding bottle. Didn't want my steel vacuum tank to rust out.
A lot of plumbing and purging but now I'm ready for winter and will find out how my tank and hot water panel size work out for meeting my space heating needs. I'm still using the patched twinwall polycarbonate glazing, with some 2.5" galvanized steel strips held down over the former bulge/cracks, which are silicone caulked and foil taped. Not pretty but it works and will let me continue testing the system through the winter. Single glazing with 3/32 polycarbonate is probably my best bet.
Yep, a simple swing type check valve will stop the thermosiphon in it's tracks:)
For your tank within a tank system, Thought about using an old Electric Hot Water Cylinder? I have used a couple now for indoor pot plant holders (when cut in half). The ones I have used are low pressure cylinders for use with a header tank in roof, about 180 litres, are copper inner tank with insulated material glued onto outside and clad with galv sheet steel. It only takes me about 1/2 hour total to cut off cladding with a skinny disk on grinder and then attack insulation with a sharp spade. This would give you rust resistance along with all the threaded pipe fittings you would need.
No copper inner tanks available here, though I did think of using aluminum beer kegs before deciding to just spend a bit more and go with copper tubing for it's low risk and longevity.
If you'll check the thread above, you can see my copper tubing exchanger, 180 feet of 1/2" soft copper (3 rolls), configured as 4 parallel loops, fed from bottom to top. It has worked well for my domestic hot water. It's been a nice luxury to use "endless" hot water for laundry and such since this spring, when the system became operational. My washing machine now annoys me as it does not allow for a hot or warm rinse unless I fill the tank manually. I have not turned on the propane backup, since I have excess capacity for just domestic HW.
The in-floor heat is run direct from the main storage tank, with no heat exchanger. The big question to be answered this winter is how often will I have to use the backup propane hot water heater. At 5600 feet elevation in the White Mountains of AZ (central eastern) we do have sunny but real winters with sub zero temps and snow.
The copper pipe in the heat storage tank above the water line has a dark black oxide layer- seems 150F temp and the steam from when the system first starts is a fairly oxidizing situation. No change to the copper in the water. In the 6 months of operation, no smell of bacteria in the tank water- so far. Perhaps just the copper will be sufficient.