2 tank systems Vs. One tank system SDHW

michael_34

So what do think is more efficient. a two tank system ( a solar storage tank tied to existing water heater/side arm tank or a one tank system with a back up built into the solar tank?

There is also the use of a tankless, but that's another discussion.

Thanks,

Michael

Kevin_in_Denver_2

One Tank

For electric backup systems:



I've read lots of case studies, and they all conclude that one-tank is more efficient overall.

Retrofits are usually easier and cheaper to do with two tanks.

In new construction, one tank systems are easier and cheaper.

hot_rod

collector efficiency

goes up as the supply to the collector drops. So the piping or tank layout that provides the coldest and temperature to the collector will drive up that efficiency number. I suppose it depends on how much hot water they use. If they use 80 gallons per day then a 40 gallon solar tank and a 40 gallon back up tank would seem to be more efficient. With an 80 gallon one tank system, the solar only has the ability to heat 1/2 of the tank, as the back up source will keep the rest of the tank warmed.

Fortunat

one tank wins...

> If they use 80 gallons per day then a 40 gallon solar

tank and a 40 gallon back up tank would seem to be more efficient. With

an 80 gallon one tank system, the solar only has the ability to heat

1/2 of the tank, as the back up source will keep the rest of the tank

warmed.



I've heard a number of people say this, or something like this, but it doesn't make any sense.



In a single two coil tank, the solar can heat the bottom half from cold AND the top half from 120 (or the maintained temp) to 180 (or the max tank temp). In a two tank system, the solar can't heat the boiler tank at all (unless you add a transfer pump or some other way to flip the two tanks).



For that reason and because the single tank has lower heat loss, we almost always prefer the single tank solution (unless limited overhead space or some other constraint forces our hand).



Cheers,

~Fortunat

www.revisionenergy.com

Royboy

hmmm

working with the standard rule of thumb (at least standard to me, in my northern climate) of 1-1.25 gal of storage/sf of collector - I'm usually looking for 60-80 gallons of storage for the solar heat. my impression is that its hard to get that in a 1 tank system without going to a 120 gallon tank, so I usually opt for 2 tanks.



how do you size tanks relative to collector area, Fortunat, for your single tank systems? and are you using single tanks with gas backup or just electric?

michael_34

I have always heard and read...

that a two tank system is more efficient on the facts that HR pointed out. Let's say there is a cloudy morning. The back up on the single tank system kicks on to heat the water. Sun comes out by 11am and the solar kicks on. If the water is already hot what is there for the solar to do? The reason I posted this question is that I went to a "training" this past weekend and the guy was a proponent of a single tank system. His thoughts went contradictory of everything I have heard or read. I would like to here more input on the single tank systems (or both types). Thanks for your thoughts Michael

Kevin_in_Denver_2

Collector efficiency vs. system efficiency

Yes, collector efficiency is higher with a two- tank system.



But there's never been a report  that showed a two tank system having a higher SYSTEM efficiency, and of course, that's more important.



Most of the reports I speak of are so old that I haven't found them online anywhere.

Royboy

and in addition

to system efficiency there is cost-efficiency, which can be yet a higher priority for many folks ...



in terms of operating costs, single tank systems are pretty much limited to electric backup, are they not? (unless they are indirects with an upper tank coil). and with electricity and the upper element only, its hard to get by with interruptible or off-peak electricity - so you end up paying the higher standard rates, which around here are more expensive than LP, oil, or for sure natural gas.



so collector efficiency favors 2 tank

system efficiency single tank

cost efficiency - ???

Fortunat

One tank IS two tanks, just tightly packaged

>Let's say there is a cloudy morning. The back up on the single tank

system kicks on to heat the water. Sun comes out by 11am and the solar

kicks on. If the water is already hot what is there for the solar to do?



In a well designed dual coil tank (or a solar tank with integral upper electric element), the water heated by the backup source (boiler or element) is ONLY in the top portion of the tank. because of the buoyancy of hot water, the heat from the upper coil does not make it's way to the 'solar' portion of the tank, the lower 1/2.



In this way the single tank is identical to the dual tank as far as solar efficiency goes. If the sun hasn't been out in a while, the bottom of the tank just sits at well water temperature (even as the top is heated to 120 or 130 by the upper coil or element). People tend to find this hard to believe, but it is true and can easily be verified by starting with a cold tank, turning on the backup and then running a temp probe down through the tank. You get a very distint thermocline, just below the bottom of the top coil where in the matter of a few inches, the water temp will drop from 130 to 50 degrees.



Keeping the boiler heat (or electric heat) away from the bottom of the tank is critical and is why the Euro solar tanks have a recirc return port between the two heat exchangers as well as a cold water inlet baffle to prevent swirling. That keeps the solar portion cold, unless it is heated by the sun.



The relatively new Vaughn solar tank, on the other hand, has solar and boiler coil side by side (or right near one another) and why anyone would want to use that tank for anything but mooring a sailboat is a mystery to me.



hope that helps,



~Fortunat

www.revisionenergy.com

Fortunat

storage tank sizing

Roy,



We use a similar rule of thumb, roughly 1-1.5 G per sf for flat plates. So for a typical residential system which consists of a pair of 4x8 collectors, we'd use an 80 Gallon tank.



In this case, remember that while the top of the tank is maintained at DHW temp, it is still available for solar energy storage (something that isn't true of the two tank system). So with half the tank starting at 50 degrees, and the other half starting at 120 degrees, I still have over 70,000 BTU's of solar storage available to me before I bump against the tank high limit.



Yes, we do single tank systems with gas (and oil) as well as electric.



~Fortunat

hot_rod

I think you hit it

Kevin, it depends on which efficiency you are talking about.



Same with the question "is radiant heat more efficient'. You have to consider boiler efficiency, distribution efficiency, building efficiency, etc.



I can agree with a single tank if that bottom cool portion can absorb a days worth of solar input.



A single tank solar needs to be sized to supply the household enough hw with just the top half, when you have days, or weeks on no sun. Seems like 80- 120 gallons would be needed to assure most 4 person households enough hw. So then size the solar to the 120 gallons for 100% solar in sunny months? 80- 120 square feet of collector.



hr

Kevin_in_Denver_2

Baffle Design

These baffles seem to be pretty important, but they can be easily homemade.  Fortunat, have you seen a good sketch of one?

hot_rod

tank baffles

I've seen a number of methods to lessen that mixing at the bottom. The Buderus tank uses a stratification wand, I have seen some fabric devices, also the Lochinvar Lock-Temp, which looks like a box built inside the tank bottom.



hr

Tehiru

One Tank Wins

In that case you are looking at a Piggyback system from TrendSetter. It's a small 40-gallon storage tank that you place a conventional electric/gas water heater on top of. The heat exchanger in the piggyback tank thermally couples both tanks together via thermosiphon. The fact that it doesn't take up any more floor space is a bonus.

Fortunat

Baffle design

I can't remember seeing a sketch of the cold water baffle, but we have a cut-away tank in our showroom that shows it pretty well. Maybe I'll try to take a photo...



that tank is a Stiebel Eltron SBB plus and in their tank, the bottom of the tank is a rounded hemispherical shape and the cold inlet comes in right at the center and strikes a baffle plate which is sort of umbrella shaped and has maybe a 1-2 inch seperation from the tank bottom. This seems to work rather well (though having the cold water connection on the bottom of the tank means it has to have 'legs'.



~Fortunat

www.revisionenergy.com

Kevin_in_Denver_2

Stratification Wand or Lance

I found this old sketch - easy to make.  Stratification improves overall performance by at least 8%.



See attached

Kevin_in_Denver_2

Trendsetter redefines pricey

Trendsetter products fill an important niche for the solar industry.  They seem to be high quality and well-designed, with important options.



The best prices I've found for them, however mean they're not going to sell very well.



http://shop.solardirect.com/product_info.php?products_id=490



Would you believe over thirty dollars per gallon?

michael_34

Good info

I'm stilled not sold on a one tank system, but the info presented will not make ever shy away from using it. It has to be the right tank and situation. Thanks I really appreciate it.



What I have been doing if a customer has a gas/propane hot water heater already in use (and in good shape) I plumb a bypass to avoid the backup when there is enough solar gain. obviously this can be done with a motorized three way valve and a good controller or 2 ball valves.



I also prefer using a tankless tied to the solar for back up.



Michael

Karl_Northwind

one tank

For systems that are a light combi system, I like to go with a one tank.  there is a bigger tank to absorb heat in the non heating season, and therefore close to 0 backup energy used for the water heating. 

In new construction where the hot water loads are conservative, I'd be OK with a single tank system.  if someone wants to fill a 100 gal tub anytime, use a real backup water heater.  my wife and I get by with an 80 gallon single tank system with electric backup (no choice due to utility room constraints)  if it's been cloudy for a few days, we have to be careful to give the element time to catch up between showers.  ususally not a problem, except when I have to take a shower right after her, getting ready to go somewhere.



we turn the element off from april to october usually, only turn it on if it's cloudy for a week. 



in short it all depends.   in wisconsin we're "strongly encouraged" to do a double tank system.

Bob Gagnon plumbing and heating

I Vote For Two Tanks

I have two tanks on my system and it allows me to collect a lot more heat. During the heating season I run my radiant heat from the second solar tank whenever that tank is above 72 degrees. That means I can collect solar heated water when the collector gets above 75 degrees, this tank- collector setup collects energy all day long, while my domestic tank and collector with higher, gradually increasing temperatures, waits around to make 120 to 140 degree water . My low temperature space heating tanks collects WAY more energy, than my higher temperature domestic tank.



Thanks, Bob Gagnon