Integrating solar with a GARN

[Ecky]

So here is what I am trying to do - integrate solar hot water with a Garn WHS1500 (holds 1500 US gallons - 5000 L). The Garn is going to provide space heating (2 floors of radiant heat in the house) and DHW. By integrating solar, I would presumably shorten the wood heating season (spring / fall). In the summer the Garn would become a heat dump and provide the DHW (hopefully) without having to fire the Garn. The 6 panels are used flat plate 4' x 8' collectors. I was hoping to use a single solar circulator (Laing Solar DC-5) which is supposed to produce a max of 7 gpm. By running 1/2" black pipe back and forth inside the bottom third of the Garn I would avoid a heat exchanger and a 2nd solar pump (Shurflo 2088). I am wondering if this is a good idea (the downside being if a leak developed inside the Garn I would have to drain it). Not sure how much black pipe I would need to simulate a heat exchanger. I do know that I have to use black pipe in order to avoid the problem of dissimilar metals. I choose a 1/2" thinking it gives me the best surface area to volume ratio. Any suggestions?

Hopefully that gives you an idea of what I am hoping to accomplish. As a bit of an aside, I am also looking for a DC pump that will push about 5 gpm through a 1" ID pex from the Garn (non-pressurized) to the house.

 

[benjamin]

Have you considered doing a drain back solar system utilizing the Garn water? Skip the heat exchangers altogether if the Garn is heating the radiant directly to maximise the efficiency.

My system is almost 300sf of panels heating the same water that goes through the floors and my boiler, but my boiler is a $50 cast iron boiler core, not a $$$Garn.

Of course with any drainback system, you want to be certain that it will drain all the way, every time.

 

[Ecky]

Thanks for the suggestion. Drainback was my first choice. Not going to happen (unless you know something I don't) since a few of the collectors (ground mounted) are lower than the Garn in elevation. Roof mounting would not work for a couple of reasons. Any suggestions?

 

[benjamin]

I don't know what kind of access you have to the inside of a Garn, but that and the efficiency loss from a heat exchanger on the solar side would make me try to do drainback. If you want to try a heat exchanger inside the garn, then it would be just like any coil inside an unpressurized storage tank. I don't know the difference between the conductance of copper and black iron, or whether that would make a big difference in the output in this case. Some people have used plastic pipe as well for heat exchangers so you could compare the plastic and copper figures and interpolate a figure for steel. Conventional wisdom says to avoid pex on solar systems but mine works fine, but since it's open it would never reach it's failure point.

If all of your collectors were below the level of the garn heat exchanger you might consider an alternative heat transfer liquid/vapor, maybe ethanol? evacuate the system and use it like a refridgerant. You might be on your own with that one though, I don't remember if I've ever seen that used before.

 

[Ecky]

I talked to someone from Garn. They were strongly opposed to using copper inside the Garn. Access inside the Garn is excellent. Drainback would just seem not to be an option. Ethanol is an interesting thought but since I am having enough problems trying to do something that would not seem to be that unusual - I hesitate to guess the types of difficulty I would have with ethanol. Someone else suggested that the pressure loss using piping inside the Garn would be substantial. He is suggesting that an external HX would be the better option. I like the supposed simplicity of an internal HX but the pressure issue could be a problem.

 

[benjamin]

Agreed, don't use copper inside of an open and pricey tank.

Pressure loss is what you make it. If the access is good, then you can use however many circuits of whatever size pipe you need to get the flow you want.

The external heat exchanger may be the simplest way to go. Even if it reduces your heat exchange and output from the panels, the internal heat exchanger would also do this to some extent.

 

[Ecky]

Kind of a catch 22. I would think you would want a smaller diameter pipe (better surface area to volume ratio) but this reduces flow and increases pressure loss. Larger pipes would give you better flow and less pressure loss but then you don't get the heat exchange efficiency. Hmmm....I wonder if a few circuits as you suggested would allow the use of smaller pipes but limit pressure loss? Wish I understood this better!

 

[btuser]

Smaller pipes in paralell should be cool, although you're talking about a lot of heat coming off 6 full sized panels.

 

[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.

 

[ewdudley]

Thanks for the suggestion. Drainback was my first choice. Not going to happen (unless you know something I don't) since a few of the collectors (ground mounted) are lower than the Garn in elevation. Roof mounting would not work for a couple of reasons. Any suggestions?

Might be more elaborate than you had in mind, but worth considering would be a drain-back tank and sub-system below the collectors in a little mini pump house or such.

 

[brad068]

Ecky,

I have solar tied into my home-built garn style unit. I"m using a two pump system controlled by a Caleffi solar controller. Can't remember the number but it is the one that can control two pumps w/ variable speed. I exchange through a 30 plate hx. I have 4, 4'x10' panels heating @1300 gals. and in my opinion that is about 500 gals to much. The hottest my tank ever got was @145*F in the middle of Aug.

Solar system is filled with propylene antifreeze @ -15*F right now but I have another pail to dilute with. I like the Caleffi controller but it is quite a complex controller that can handle about 100 tasks more than what I need it to do.

 

[tom in maine]

As a manufacturer of unpressurized tanks, we also were contractors at one time and installed hundreds of drainback systems in unpressurized tanks of various sizes--100g to 1500g.

It seems that a simple drainback to a Garn tank would be a natural system. You will need to use a pump that can lift water to the top of the collectors.
You will need some excess pump head capacity to make sure that you can overcome the friction in the piping as well.
Since Garn does not want copper in their tanks, I would suggest a separate tank with a hx for dhw like an Amtrol or Superstor or perhaps a plate hx to tie to a conventional electric tank.

The use of a heat exchanger in any loop, solar or heat output is not going to lose much efficiency in the system, it is going to require a circulator pump to run a little bit longer. This extra amount of electric energy is minimal.

Drainback is going to require all solar lines can drain back to the tank. The return line from the collectors has to be vented to allow a positive air inlet into
the system when it shuts off and has to drainback. Drainback should be done in a couple minutes after shutdown.

 

[in hot water]

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

 

[Ecky]

Smaller pipes in paralell should be cool, although you're talking about a lot of heat coming off 6 full sized panels.

The fact that the panels are ground mounted and subject to some shading will lessen the heat generated. Something that you would not be aware of, of course. Not sure how I would run the pipes in parallel unless you are talking having each section go out and back coming off a manifold. Is that what you are suggesting?

 

[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?

 

[Ecky]

Thanks for the suggestion. Drainback was my first choice. Not going to happen (unless you know something I don't) since a few of the collectors (ground mounted) are lower than the Garn in elevation. Roof mounting would not work for a couple of reasons. Any suggestions?

Might be more elaborate than you had in mind, but worth considering would be a drain-back tank and sub-system below the collectors in a little mini pump house or such.

Thanks Eliot. That is an interesting thought but I would be concerned about keeping this tank from freezing. (I am at the 45th parallel.) My reading suggested that these tanks needed to be located within heated spaces. Is this not the case or am I missing something?

 

[Ecky]

Thanks Tom.

 

[Ecky]

Ecky,

I have solar tied into my home-built garn style unit. I"m using a two pump system controlled by a Caleffi solar controller. Can't remember the number but it is the one that can control two pumps w/ variable speed. I exchange through a 30 plate hx. I have 4, 4'x10' panels heating @1300 gals. and in my opinion that is about 500 gals to much. The hottest my tank ever got was @145*F in the middle of Aug.

Solar system is filled with propylene antifreeze @ -15*F right now but I have another pail to dilute with. I like the Caleffi controller but it is quite a complex controller that can handle about 100 tasks more than what I need it to do.

Thanks Tom. 145 would work fine for me although I would be surprised if I got those kind of numbers. Are you happy going with the external HX? I take it you did not go with DC pumps? What was your reasoning for this? Dumb question, but do you need a controller if you go DC?

 

[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.

 

[ewdudley]

Might be more elaborate than you had in mind, but worth considering would be a drain-back tank and sub-system below the collectors in a little mini pump house or such.

I would be concerned about keeping this tank from freezing. (I am at the 45th parallel.) My reading suggested that these tanks needed to be located within heated spaces. Is this not the case or am I missing something?

I guess there's heated spaces and then there's freeze-proofed facilities. It takes some attention to detail but freeze-proofing isolated pumps, tanks, and pipes can be done robustly.

Here at the 43th I doubt we get the quantity of cold weather, but we can get 10 day stretches with the temperature below 0/-18 day or night and all the far-flung livestock have plenty of liquid water available all day every day.

The main trick is to avoid any movement of air near pipes. Then with a some heat tape and a little insulation it's surprising what you can get away with. Or maybe a little underground dog house with two inches of styrofoam all around and a 60-watt bulb. There's ways to do it, but it's hard to say if it would be too much bother.

--ewd

 

[Ecky]

Might be more elaborate than you had in mind, but worth considering would be a drain-back tank and sub-system below the collectors in a little mini pump house or such.

I would be concerned about keeping this tank from freezing. (I am at the 45th parallel.) My reading suggested that these tanks needed to be located within heated spaces. Is this not the case or am I missing something?

I guess there's heated spaces and then there's freeze-proofed facilities. It takes some attention to detail but freeze-proofing isolated pumps, tanks, and pipes can be done robustly.

Here at the 43th I doubt we get the quantity of cold weather, but we can get 10 day stretches with the temperature below 0/-18 day or night and all the far-flung livestock have plenty of liquid water available all day every day.

The main trick is to avoid any movement of air near pipes. Then with a some heat tape and a little insulation it's surprising what you can get away with. Or maybe a little underground dog house with two inches of styrofoam all around and a 60-watt bulb. There's ways to do it, but it's hard to say if it would be too much bother.

--ewd

Good ideas / points. It does bring up the issue of finding a DC pump that will overcome the head. I just thought the supposed simplicity of running a closed loop system with anti-freeze and a heat exchanger located within the Garn and a single pump would be the best option. According to Ramlow (Solar Water Heating - 1996), closed loop antifreeze systems are the only option that should be considered for areas which experience a lot of freezing. Comments?

 

[ewdudley]

I just thought the supposed simplicity of running a closed loop system with anti-freeze and a heat exchanger located within the Garn and a single pump would be the best option. According to Ramlow (Solar Water Heating - 1996), closed loop antifreeze systems are the only option that should be considered for areas which experience a lot of freezing. Comments?

Considering all the valves, pumps, and relays it would take to implement a drain-back tank at the bottom of the system, I'd have to agree that a closed-loop antifreeze system would be a lot simpler, easier, and more reliable.

But note that the Ramlow book is from the pre-internet era when we could be told what ideas to consider or not to. Nowadays we can kick around bad ideas and we can even use bad spelling doing so if we want, which may be waste of time, but where's the harm?

Cheers --ewd

 

[Ecky]

I just thought the supposed simplicity of running a closed loop system with anti-freeze and a heat exchanger located within the Garn and a single pump would be the best option. According to Ramlow (Solar Water Heating - 1996), closed loop antifreeze systems are the only option that should be considered for areas which experience a lot of freezing. Comments?

Considering all the valves, pumps, and relays it would take to implement a drain-back tank at the bottom of the system, I'd have to agree that a closed-loop antifreeze system would be a lot simpler, easier, and more reliable.

But note that the Ramlow book is from the pre-internet era when we could be told what ideas to consider or not to. Nowadays we can kick around bad ideas and we can even use bad spelling doing so if we want, which may be waste of time, but where's the harm?

Cheers --ewd

OK - glad that we seem to agree that I have made the right decision (for my particular situation) in regards to closed loop versus drainback. As to the concept of kicking ideas (good OR bad) via the internet, I could not agree with you more Eliot. It gives people who think "outside the box" a chance for equal billing. And there can be no harm in that! Well put. Thanks for providing me some potential options! Your time and efforts are appreciated.

 

[in hot water]

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

 

[Nofossil]

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.