Integrating solar with a GARN

[benjamin]

I would be tempted to go with a moderate length of 1" pipe. The main reason being that 1" is less than twice the price of 1/2". Also far fewer joints for the amount of surface area. One downside would be the cost to fill it with glycol.

It's not too hard to get a rough estimate of flow and heat transfer. One drawback is the limited availability of DC pumps. You have six panels so it would be nice to get six gallons per minute, but three wouldn't hurt the output too much, especially because the heat exchanger is going to need a hotter fluid.

There's still the possibility of laying a coil of pex out in the bottom of the garn, assuming you can do it without the plastic touching the firetubes. You can get a 300' roll of 1" pex for under $200.

Thanks Benjamin. Pex is an interesting thought but I was wondering about the heat transfer of pex versus using black pipe versus using a commercial heat exchanger? I suspect one could get the pex in the unit without having it touch the firetubes as per your suggestion. The Laing solar pump (DC5) will run about 5-7 gpm (depending upon head). So while being a little low - it should suffice. I really don't understand the concept of a heat exchanger beyond the obvious. It would seem to me that the slower the flow, the more heat would be transferred within whatever type of heat exchanger I end up using. What kind of heat exchange should one be aiming for? i.e. if the temp going in is 140 degrees, what temperature would you want it to come out at? 130? 120? 100?

Yes and no, a slow flow through the heat exchanger will get the most heat out of THAT AMOUNT of water, but more water at the same temperature will always provide more heat exchange, even if the temp drop is much lower, think of the AVERAGE temp through the heat exchanger, if it starts at 180 and exits at 100 the average is 140, if it enters at 150 and exits at 140 the average is 145 and will provide similar (maybe more?) heat than the higher input temperature and bigger temperature drop. If both of those scenarios are on the same solar system and heat exchanger then the first one would be not enough flow and reduced efficiency, the second one would be about right.

In this case with six panels hooked to a garn to provide hot water, I think you should try to get at peak sun-output 140 in and 125 out, that will be with the garn approaching 120, if the garn drops to 90 the solar temp shouldn't reach 140, if it does you're not getting enough flow. You won't get as big of a temp drop with partly sunny skies.

Read everything you can on builditsolar.com, and radiantec.com there are lots of ideas out there, once you understand them you'll feel more comfortable with the design of what you're trying to do, whether it's a flat plate hx or something down and dirty like throwing pex in a garn. There are plenty of comparisons on builditsolar to figure out the heat exchange of pex, which is obviously much less than copper or steel, the advantage of pex is the heat exchange per dollar installed.

If the panels are going to be below the garn, then glycol is the only way to go. I disagree that glycol makes for a simpler and more reliable system, that's like saying an IH is harder to work on than JD, it's a personal opinion to me. Glycol does have to be maintained and deal with stagnation, though stagnation shouldn't be a problem with a garn to dump heat into if you have an adequate heat exchanger. With your design I'd suggest mounting your panels at 45 degrees rather than the steeper angle, as you're not expecting or needing solar in the winter.

 

[Ecky]

Looks like the question of heating 1500 gallons has been pretty thoroughly covered. My personal experience suggests that a smaller and hyperinsulated tank for DHW makes a lot of sense. I use water in my solar panels and drain them during the winter and frost season. Where we are, the amount of heat that you get from solar panels in the winter is dwarfed by what you get from burning a small amount of additional wood in the boiler that you're firing anyway - no point in solar hot water in the winter. I end up burning a small amount of oil to cover shoulder seasons when the sun angle isn't good enough and I haven't fired the wood boiler yet.

My ultimate system would be a DHW tank in the 200 gallon range with two internal hx coils - one connected to my heat storage and one connected to the solar panels. The tank would be mounted above both storage and the panels so that it's heated by thermosiphoning.

Thanks "nofossil". Very good points. However this raises two questions. Why not hook your panels up in series to achieve useful solar gain in the winter? Secondly, even at a return solar temp of 70-80 degrees, if you used that to preheat your incoming well water (or municipal) would that not be a worthwhile venture since you have made the investment anyway?

As to your ultimate system - that would be sweet. However would it work in an area that experiences sub-freezing temps for most of the winter?

 

[Ecky]

ideally you want 1-1.5 gallons of storage per square foot off collector. So for your array size 200- 280 gallons of storage. In the heating season or whenever the Garn is fired the solar will not contribute any. In the summer the un-fired Garn will lose a good percentage of the solar up the flue.

Look for an insulated tank and couple the solar to that. An internal or external HX for DHW. A 3 way zone valve could feed the heating circuit from the solar tank until the solar tank cools, then switch to the Garn.

Your collector array will be more efficient the cooler you return fluid to them.

Also consider the % of glycol and how it will effect that circ. Cold glycol is a tough fluid to circulate with a small pump like that. Here is an example of glycol and pumping requirements.

hr

Thanks for your valuable comments. I was aware that the GARN was too large. However, the intent of the solar was to preheat the return water from the house in the winter (used to heat the house and provide DHW) before re-entering the GARN. I do not expect it to provide any meaningful heat in the winter. I did hope that it would aid in the shoulder seasons and provide all my hot water in the summer. Sounds like this may not be the case. It should be noted that I have insulated the GARN with straw bale walls and a lot of Roxul so the tank should not loose a lot of heat hopefully. The flue is horizontal which should limit heat loss. Your line about the cooler the return line to the collector array is a great point. This leads me back to a question I asked in another post, would not a slower gpm allow for more heat to be transferred in a HX (either externally or internally)? I am aware that glycol is slippier then water but in a closed loop pressurized system I have been lead to believe that this would not be beyond the range of the Laing DC5. You seem to be suggesting otherwise.

The key to solar is a small delta T across the collectors. Some say 3-5 °F . You want to scrub the heat from the collectors as quickly as possible as the major loss is through the glass to the ambient air.

I doubt the Laing D5 solar, running 40-50% glycol would be enough pump. But we need to know the collector spec and the piping and fitting pressure drop to be sure. You could double them up, but they are $$y.

Yes you want a controller even with a PV powered circ. There are times when the sun is shining but you don't want the pump to run. early morning, afternoon, anytime the tank is warmer than the collectors, really.

There are a handful of PV controllers. Caleffi offers 12 and 24VDC. Also Arttecsolar has a new version of their DC controllers.

hr

Thanks hr. Wish I could give you more info about the piping, fittings, panels. (Panels are used with no numbers, and piping and fittings are being determined by this thread. I am now wondering why more people to the north don't plumb there panels in series to overcome the low winter sun and wring more heat out of it? There must be some downside. My thought was to plumb the panels (6 in my case) so that I could switch between a series and a parallel system. Then you would have the best of both worlds with a productive solar array in the winter as well as in the summer. Thoughts?

 

[Ecky]

I would be tempted to go with a moderate length of 1" pipe. The main reason being that 1" is less than twice the price of 1/2". Also far fewer joints for the amount of surface area. One downside would be the cost to fill it with glycol.

It's not too hard to get a rough estimate of flow and heat transfer. One drawback is the limited availability of DC pumps. You have six panels so it would be nice to get six gallons per minute, but three wouldn't hurt the output too much, especially because the heat exchanger is going to need a hotter fluid.

There's still the possibility of laying a coil of pex out in the bottom of the garn, assuming you can do it without the plastic touching the firetubes. You can get a 300' roll of 1" pex for under $200.

Thanks Benjamin. Pex is an interesting thought but I was wondering about the heat transfer of pex versus using black pipe versus using a commercial heat exchanger? I suspect one could get the pex in the unit without having it touch the firetubes as per your suggestion. The Laing solar pump (DC5) will run about 5-7 gpm (depending upon head). So while being a little low - it should suffice. I really don't understand the concept of a heat exchanger beyond the obvious. It would seem to me that the slower the flow, the more heat would be transferred within whatever type of heat exchanger I end up using. What kind of heat exchange should one be aiming for? i.e. if the temp going in is 140 degrees, what temperature would you want it to come out at? 130? 120? 100?

Yes and no, a slow flow through the heat exchanger will get the most heat out of THAT AMOUNT of water, but more water at the same temperature will always provide more heat exchange, even if the temp drop is much lower, think of the AVERAGE temp through the heat exchanger, if it starts at 180 and exits at 100 the average is 140, if it enters at 150 and exits at 140 the average is 145 and will provide similar (maybe more?) heat than the higher input temperature and bigger temperature drop. If both of those scenarios are on the same solar system and heat exchanger then the first one would be not enough flow and reduced efficiency, the second one would be about right.

In this case with six panels hooked to a garn to provide hot water, I think you should try to get at peak sun-output 140 in and 125 out, that will be with the garn approaching 120, if the garn drops to 90 the solar temp shouldn't reach 140, if it does you're not getting enough flow. You won't get as big of a temp drop with partly sunny skies.

Read everything you can on builditsolar.com, and radiantec.com there are lots of ideas out there, once you understand them you'll feel more comfortable with the design of what you're trying to do, whether it's a flat plate hx or something down and dirty like throwing pex in a garn. There are plenty of comparisons on builditsolar to figure out the heat exchange of pex, which is obviously much less than copper or steel, the advantage of pex is the heat exchange per dollar installed.

If the panels are going to be below the garn, then glycol is the only way to go. I disagree that glycol makes for a simpler and more reliable system, that's like saying an IH is harder to work on than JD, it's a personal opinion to me. Glycol does have to be maintained and deal with stagnation, though stagnation shouldn't be a problem with a garn to dump heat into if you have an adequate heat exchanger. With your design I'd suggest mounting your panels at 45 degrees rather than the steeper angle, as you're not expecting or needing solar in the winter.

Thanks for the explanation Benjamin (heat exchanger - rate versus temp drop). It helps to clarify the efficiencies of the heat exchangers. Thanks for the website suggestions. I have now looked at them to aid this process. As far as glycol making a simpler and more reliable system, I guess I didn't explain myself very well. Your points are all correct. I was meaning that building a HX (whether pex, black steel, corrugated stainless steel) would be simpler because I would bypass the 2nd pump needed with an external HX. One pump instead of 2. Drainback would of course be ideal but does not appear to be a viable option in my situation.

ecky

 

[benjamin]

Solar panels are almost never installed in series (correctly). The only exception I know of is that some systems use a flat plate and then evacuated tube collectors in series because the evacuated tube collectors can produce higher temps and maintain their output better in cloudy conditions than flat plate collectors. If the sun is that sparse that a series setup improves output then it's not worth trying to capture that heat. The downside of series is flow and temperature change, you do not want a high temp change with solar because it means your flow is too low to get the most heat.

The only downside of large storage is standby heat loss, unless I'm missing something? If you can seal up the insulation perfectly then I would use the garn for summer dhw heat storage. Strawbale will not work. I don't know anything about the other insulation you mentioned.

Assuming you have a standard flat plate collector of 4x8 or 4x10 you should be able to get the vast majority of your non heating season hot water from the six panels you have.

I'm assuming these panels are aluminum framed, 3/4 or 1" copper headers, and 10- 1/2" copper risers, and they haven't had the plate separate from the risers. The efficiency of these doesn't vary that much.

How's the garn treating you so far?

 

[Ecky]

Solar panels are almost never installed in series (correctly). The only exception I know of is that some systems use a flat plate and then evacuated tube collectors in series because the evacuated tube collectors can produce higher temps and maintain their output better in cloudy conditions than flat plate collectors. If the sun is that sparse that a series setup improves output then it's not worth trying to capture that heat. The downside of series is flow and temperature change, you do not want a high temp change with solar because it means your flow is too low to get the most heat.

The only downside of large storage is standby heat loss, unless I'm missing something? If you can seal up the insulation perfectly then I would use the garn for summer dhw heat storage. Strawbale will not work. I don't know anything about the other insulation you mentioned.

Assuming you have a standard flat plate collector of 4x8 or 4x10 you should be able to get the vast majority of your non heating season hot water from the six panels you have.

I'm assuming these panels are aluminum framed, 3/4 or 1" copper headers, and 10- 1/2" copper risers, and they haven't had the plate separate from the risers. The efficiency of these doesn't vary that much.

How's the garn treating you so far?

Hi Benjamin. Thanks for taking the time to respond. If possible, could you explain the bit about not wanting a high temp change with solar because it means your flow is too low to get the most heat? I am not following. Pardon my ignorance, but if the supply and return are say 1" does that not become the limiting factor? Whether the solar fluid is hooked up in parallel of series, the fluid at the end of the array has to go through the same 1". What is wrong with this line of thought?

I am also wondering why you think strawbale won't work? It certainly works on a house.

The panels are standard 4 x 8's, aluminum framed, 1" copper header, with 10 1/2" copper risers and the plates have not separated from the risers.

As to the Garn, I am afraid I can not answer that. Still not hooked up. That is what I am doing right now. Trying to integrate the solar but that may not happen this year. So many options. The biggest holdup is whether to go with an internal HX (in the Garn) or either a plate or tube-in-shell external. Once I get that figured out I can fill the Garn and start the prep.

 

[benjamin]

An electric electric water heating element puts out roughly 15,000 btus per hour regardless of the temperature, so if there 100 pounds of water per hour flowing through the water heater it will raise the temp 150 degrees and if there are 1,000 lbs/hr it will raise the temp 15 degrees, right? Higher flow makes lower temp, lower flow makes higher temp, but the same heat output.

Not so for a solar collector.

If we use an AE-32 panel as an example that is very close to yours.
http://www.aetsolar.com/literature/SRCC_100-2002-001E.pdf

For a mildly cloudy day and a temperature difference of 36 degrees, say the panel has an average temp of 116 degrees and it's 80 degrees outside the rating is 22,000 btus per panel per day. When the temperature difference goes up to 90 degrees (panels average 170 when it's 80 out) the production drops to 10,000 btu's per panel per day. Now the heat at 170 is much more "useful" than the heat at 116, but if your garn is at 110 and you have the choice of circulating a very little water to get a high output temp, or enough water to get an output temp much closer to the temp you are using, then the higher volume of water in this case will give twice the heat, ie the garn will warm up twice as much with the higher flow. The heat output with evacuated tube collectors drops less with higher temperatures than flat plate collectors because of the better insulation/lower loss.

The 1" pipe is part of the flow calculation, hooking panels in series reduces flow because it increases the length of the pipe and number of turns. For six panels that 1" is nowhere near a limiting factor. The pressure drop with a dc pump and glycol may be as stated above but not the amount of heat that can be moved though a 1" pipe compared to the small amount of heat that six panels will produce.

Strawbale may make a fine boiler shed. I'm a cellulose guy, and what I've read of strawbale performance reinforces that preference. For insulating a solar water storage tank I would want the material that virtually every water heater manufacturer uses for this application, expanding foam, for two reasons, air tightness and high r value. Not sure what the factory has to say about foaming a garn, but they use it for gas water heaters...

If it were me, I'd get the garn hooked up and worry about the solar when you want to stop burning wood in March, the amount of heat you get out of those panels in winter might equal one average sized piece of wood a day.

 

[Ecky]

An electric electric water heating element puts out roughly 15,000 btus per hour regardless of the temperature, so if there 100 pounds of water per hour flowing through the water heater it will raise the temp 150 degrees and if there are 1,000 lbs/hr it will raise the temp 15 degrees, right? Higher flow makes lower temp, lower flow makes higher temp, but the same heat output.

Not so for a solar collector.

If we use an AE-32 panel as an example that is very close to yours.
http://www.aetsolar.com/literature/SRCC_100-2002-001E.pdf

For a mildly cloudy day and a temperature difference of 36 degrees, say the panel has an average temp of 116 degrees and it's 80 degrees outside the rating is 22,000 btus per panel per day. When the temperature difference goes up to 90 degrees (panels average 170 when it's 80 out) the production drops to 10,000 btu's per panel per day. Now the heat at 170 is much more "useful" than the heat at 116, but if your garn is at 110 and you have the choice of circulating a very little water to get a high output temp, or enough water to get an output temp much closer to the temp you are using, then the higher volume of water in this case will give twice the heat, ie the garn will warm up twice as much with the higher flow. The heat output with evacuated tube collectors drops less with higher temperatures than flat plate collectors because of the better insulation/lower loss.

The 1" pipe is part of the flow calculation, hooking panels in series reduces flow because it increases the length of the pipe and number of turns. For six panels that 1" is nowhere near a limiting factor. The pressure drop with a dc pump and glycol may be as stated above but not the amount of heat that can be moved though a 1" pipe compared to the small amount of heat that six panels will produce.

Strawbale may make a fine boiler shed. I'm a cellulose guy, and what I've read of strawbale performance reinforces that preference. For insulating a solar water storage tank I would want the material that virtually every water heater manufacturer uses for this application, expanding foam, for two reasons, air tightness and high r value. Not sure what the factory has to say about foaming a garn, but they use it for gas water heaters...

If it were me, I'd get the garn hooked up and worry about the solar when you want to stop burning wood in March, the amount of heat you get out of those panels in winter might equal one average sized piece of wood a day.

The r-value of a straw bale building is between r30 to r 50 depending upon the tightness of the bales and the quality of the installation. I have Roxul between the Garn and my straw bale walls. As such I am confident that I have a very good r-value. The foam would be a good option but, like you, I am not sure how Garn would feel about this. I have never seen it done. Not sure if it is a cost issue or some other. As to getting the Garn hooked up. You are absolutely right. I just have to decide whether to go with an eternal or external heat exchanger then get at it.

I am confused how you get 22,000 btu's per panel per day with a temp of 116 but only 10,000 at a temp of 170. Any chance you can explain that? After I get that figured out I will move on to the flow part of your explanation. Thanks Benjamin.

 

[benjamin]

The 22,000 btu at 36 degrees and 10,000 btu at 90 degrees are the ratings from the manufacturer's website link. These are typical for any flat plate collector and reflect the increased heat loss from the panel at higher temperatures. There is a rating sticker on most panels with the output at -9degrees (pool heating), +9 degrees (also pool heating?), 36degrees space heating, and 90 degrees for water heating, with sunny, partly cloudy and cloudy sky.

Evacuated tube collectors are in a vacuum so they suffer less from heat loss, but are less efficient than flat plate collectors at low temps.

 

[in hot water]

The efficiency of the solar collectors goes down as the ambient temperature drops, or as you raise the return temperature to the collector (Ti) temperature inlet.

Here is the link to the performance of your collectors.

http://securedb.fsec.ucf.edu/srcc/coll_detail?srcc_id=1999001I

The table on the right shows the performance under several operating conditions Ta- T-i temperature ambient - temperature inlet.

With a piece of graph paper you can define the performance slope of your collector and use the inlet fluid parameter to see the performance under any operating condition. The link below link shows how to do that model. As luck would have it the collector modeled in the examples is your collector.

Notice the difference in performance running 160F compared to 95F return temperatures (Ti) With 95 °F inlet (ti) and 20 °F ambient with 200 BTU/sq.ft. of radiation you will see about a 43 percent efficiency.

I have the very same collectors on my shop, at a 60 degree pitch and they do all my DHW, 80 gallons, even in the winter and contribute a bit to the radiant after that.

I have mine in a drainback system to eliminate glycol antifreeze. Better performance and overheat and freeze protection with just softened water as the transfer fluid.

http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_3_us.pdf

hr

 

[Ecky]

The 22,000 btu at 36 degrees and 10,000 btu at 90 degrees are the ratings from the manufacturer's website link. These are typical for any flat plate collector and reflect the increased heat loss from the panel at higher temperatures. There is a rating sticker on most panels with the output at -9degrees (pool heating), +9 degrees (also pool heating?), 36degrees space heating, and 90 degrees for water heating, with sunny, partly cloudy and cloudy sky.

Evacuated tube collectors are in a vacuum so they suffer less from heat loss, but are less efficient than flat plate collectors at low temps.

Thanks Benjamin. My panels are old enough that the plates are virtually unreadable. I understand that there will be more heat loss with the higher temps, but it still does not make sense to me that the panels would produce 12,000 LESS btu's!

 

[Ecky]

The efficiency of the solar collectors goes down as the ambient temperature drops, or as you raise the return temperature to the collector (Ti) temperature inlet.

Here is the link to the performance of your collectors.

http://securedb.fsec.ucf.edu/srcc/coll_detail?srcc_id=1999001I

The table on the right shows the performance under several operating conditions Ta- T-i temperature ambient - temperature inlet.

With a piece of graph paper you can define the performance slope of your collector and use the inlet fluid parameter to see the performance under any operating condition. The link below link shows how to do that model. As luck would have it the collector modeled in the examples is your collector.

Notice the difference in performance running 160F compared to 95F return temperatures (Ti) With 95 °F inlet (ti) and 20 °F ambient with 200 BTU/sq.ft. of radiation you will see about a 43 percent efficiency.

I have the very same collectors on my shop, at a 60 degree pitch and they do all my DHW, 80 gallons, even in the winter and contribute a bit to the radiant after that.

I have mine in a drainback system to eliminate glycol antifreeze. Better performance and overheat and freeze protection with just softened water as the transfer fluid.

http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_3_us.pdf

hr

Thanks "in hot water". Had a look at the links you included. The formula is actually Ti - Ta. Thanks for the explanation. I was interested to read that you had all your DHW provided in winter. This is contrary to the consensus I was getting. That does provide me with some hope! I may have figured out a way to go drainback and avoid having to use glycol (with its' inherent inefficiency).

 

[benjamin]

I didn't intend to say that you wouldn't get hot water from that setup, but it would have to be very well designed to get ALL of your hot water from it in WINTER (reduced output, increased storage loss). Getting most of your hot water from six panels in summer is easy, the typical hot water system is one or two panels.

Plus, of course in the winter the garn will be in use, so the solar contribution will be reduced if that storage is already hot, but if you have radiant heat that can use low temp water, then you may get a break from burning if you get a week of sunny weather.

I started my first fire last night, getting by with solar and masonry storage, no water tank yet.

 

[in hot water]

The efficiency of the solar collectors goes down as the ambient temperature drops, or as you raise the return temperature to the collector (Ti) temperature inlet.

Here is the link to the performance of your collectors.

http://securedb.fsec.ucf.edu/srcc/coll_detail?srcc_id=1999001I

The table on the right shows the performance under several operating conditions Ta- T-i temperature ambient - temperature inlet.

With a piece of graph paper you can define the performance slope of your collector and use the inlet fluid parameter to see the performance under any operating condition. The link below link shows how to do that model. As luck would have it the collector modeled in the examples is your collector.

Notice the difference in performance running 160F compared to 95F return temperatures (Ti) With 95 °F inlet (ti) and 20 °F ambient with 200 BTU/sq.ft. of radiation you will see about a 43 percent efficiency.

I have the very same collectors on my shop, at a 60 degree pitch and they do all my DHW, 80 gallons, even in the winter and contribute a bit to the radiant after that.

I have mine in a drainback system to eliminate glycol antifreeze. Better performance and overheat and freeze protection with just softened water as the transfer fluid.

http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_3_us.pdf

hr

Thanks "in hot water". Had a look at the links you included. The formula is actually Ti - Ta. Thanks for the explanation. I was interested to read that you had all your DHW provided in winter. This is contrary to the consensus I was getting. That does provide me with some hope! I may have figured out a way to go drainback and avoid having to use glycol (with its' inherent inefficiency).

I don't know exactly where you live I ran a RET Screen for Ottawa, with 4 collectors and 120 gallons, at 140F. Nov shows a 25%, Dec 27%, Jan 43%. A yearly solar fraction of 68%. A fairly good number.

www.retscreen.net has a free calculator to do DHW or combined DHW and heat simulations.

 

[Ecky]

I didn't intend to say that you wouldn't get hot water from that setup, but it would have to be very well designed to get ALL of your hot water from it in WINTER (reduced output, increased storage loss). Getting most of your hot water from six panels in summer is easy, the typical hot water system is one or two panels.

Plus, of course in the winter the garn will be in use, so the solar contribution will be reduced if that storage is already hot, but if you have radiant heat that can use low temp water, then you may get a break from burning if you get a week of sunny weather.

I started my first fire last night, getting by with solar and masonry storage, no water tank yet.

Nor did I take it that way. A friend of mine suggested that I use the hot water from the panels to pre-heat the well water (which is around 40 degrees). This way the panels are pretty well always contributing at least a little bit. I was hoping to use a solar hot water tank for the domestic end of things and the Garn for the radiant floor. The solar would then provide all the domestic hot water from late spring to early fall. I would love to hold off firing as you have. We will see.