Glycol and Solar efficiency

Unknown

simply because your solar system collection exceeds your usage does not mean it is equally efficient.

look at it another way: you could have matched your usage with less collector, if your system were more efficient.

or you could service greater usage.

more efficiency means higher flow rates (lower average temperature across the collector) and no glycol.

this does not mean evac tube systems are not adequate; it does mean they are less efficient than they could be.

scott markle_2

glycol and solar efficiency

I had a discussion about solar today that left me questioning what I was being told.

I'm fairly clear on the drainback vs. closed loop issues, not trying to revive that debate per-say. I think it's fair to say that both methods have merits and problems. I'm getting close to doing a small supplemental DHW system on my own house and for various reasons I'm leaning towards closed loop flat plate.

Anyway the supply house were I do business is expanding it's line to include prepackaged systems which are drain-back. Seems like a good value and a quality american made product, but for various reasons, I'm leaning the other way.

Anyway we are having a quick exchange about the pros. and cons, he seems to favor drain-back, but one of the reasons seemed flawed and I wanted to run it by the community here for some feedback.

In the con list for closed loop systems was the argument that glycol (because of it's lower heat transfer capacity) made closed loop systems less efficient. No argument that a glycol solution has %15 or so less heat caring capacity that water. But for this to lower the efficiency of a solar collector or a heating system wouldn't I have to be approaching the ceiling of what I can transfer at a given flow rate and delta? In other words I could see this being an issue if the amount of heat being generated is more than what can be "extracted" at the flow rates available, but for my application I don't think I would be anywhere near this ceiling. Can anyone help me debunk this argument or if I'm wrong explain why? Thanks

CC.Rob_7

(educated) HO opinion

or at least somewhat educated about this stuff. I have a closed loop system. The guts are 2 flat plates (80 sf), HTP SSU-119 tank, Steca controller.

IMHO and experience, you're probably right. There are other inefficiencies that are likely to impact performance before you get to the reduced heating capacity of glycol. A good example is simple differential control. For example, this particular Steca (0301) starts circulating when it sees a 16F dT between the panels and the bottom of the tank, and shuts off when the dT drops to 8. This is user-adjustable, but is in the ballpark of the range that is doing useful work. A higher-end controller like the Caleffi isolar3 can do VS based on dT, which might help increase the harvestable BTUs. You could also do a fancier circulator like an ECO and run it on dT with a simpler controller set wider. At that point the minimal power draw probably makes it well worth it to keep circulating. With a standard 007 this is probably true, too, given what BTUs are harvestable.

In the Northeast, this is probably a non-issue for 6 months of the year. From mid-April to about October, this system has no trouble getting 120 gal of water up to 160F on a sunny day. More than we need for even a couple days. In the winter, sure, there is probably a performance hit. Maybe the tank would get to 100 instead of 95.

After pondering drainback and closed loop at length, and deciding on the latter, I like the simplicity of it. Installation was easy. Maintenance is simple. No need for an extra circ or other strategy to fill the system. etc.

And, of course, one big pine tree in the wrong place will do a lot more than glycol to reduce system performance....

scott markle_2

Thanks for your input, still this is not what I wanted to here. My thinking is that in the winter when there is less energy to collect any performance hit from the transfer fluid would be even less of an issue.

The effectiveness of the transfer medium certainly impacts the pumping efficiency but as long as we are not bumping up against the flow limits I don't see how the medium would have a significant effect on the collector efficiency, I believe it's incorrect to equate the 15% loss in transfer capacity with a 15% loss in transfer. All this means is we need to pump 15% more to get the same transfer. While this has a "cost" it's hardly like loosing 15% of our available solar energy. Am I missing something?

scott markle_2

eco has no dT mode

The eco has only two modes and no interface. The first mode is constant pressure, the second is some kind of "fuzzy logic" setback.

This web vendor has some very good pricing on tekmar products. The 157 is my pick. Variable speed can get more from a collector by maintaining a constant delta and it also saves electricity.

http://www.plumbinggoods.com/product_dtl.asp?pID=7955&brand=Tekmar

CC.Rob_7

put that way

I think that sounds reasonable.

It also could (should?) be more about the collector than the fluid. What type of absorber plate, that sort of thing. Take a look at the SRCC-100 test methods to get an idea of the variability of performance they are trying to measure, and how it is measured.

Sweating this stuff out into the margins may be interesting from a theoretical standpoint, but once you get out of theory -- and out of the lab -- the practical differences in total system installation and lifecycle cost, maintenance, etc. should probably play into the thinking. Much like the choice of a boiler and control strategy.

CC.Rob_7

oops

My mistake. I thought it did dT. Heard there was a thermistor in it and assumed it could. Some other one then.

Pretty sure constant dT is a good thing. Under what conditions, however, would it not be? Any?

The 157 would be good. tekmar fan myself....

Larry Weingarten

Possibly...

...a way to make up for heat transfer fluid that is 15% less efficient at carrying heat is to oversize the heat exchanger to compensate. Or use a non-toxic fluid and a single wall exchanger ;~)

Yours, Larry

Drew_2

Propylene Glycol Heat Transfer

Scott
I've cut and pasted some information from a document that is on the Noble Website. www.noblecompany.com If you would like to read it complete, go to the tech section.
As you'll see PG looses 10% in heat transfer at a 50% solution. Most system call for a 40% solution.


The following discussion is based on information collected from chemical companies, equipment manufacturers, hydronic system design manuals and research performed by the Hydronics Institute, Inc. It represents a synthesis of the best information available at the time of publication. The intent is to provide guidelines to help licensed contractors and engineers in designing, servicing and maintaining hydronic systems that use glycol based anti-freeze.

This discussion applies to 100% Glycol with inhibitors (Super Noburst)& Non-Aluminum Systems

A comparison of propylene glycol and ethylene glycol is shown in Table 2 below:
Table 2. Glycol Properties

Inhibited 100% Ethylene Glycol Inhibited 100% Propylene Glycol
Heat transfer @180F with no increase in flow rate
EG PG
20% solution .96 .97
50% solution .87 .90
Flow Rate Correction Required (with no change in pump curve)
EG PG
100F 116%

140F 115%

180F 114% 110%

scott markle_2

Rob, ultimately If I can move the heat at an equivalent rate by flow compensation, I don't see how transfer fluid is going to have an effect on the COLLECTOR efficiency, as in it's ability to capture thermal energy.

Yes it will effect pump efficiency (but since most systems use fixed speed slightly oversized pumps even this largely is moot). Higher pumping cost for glycol is nothing compared to drain-back requirements.

Yes glycol diminishes the energy holding capacity of water. But as long as this is compensated for by a higher flow rate, so what. I hate to be nit picky but I think it's really important to have clear information to make these comparisons.

It seems totally false to equate a 10% loss in carrying capacity of glycol to a 10% loss in solar collection capacity (assuming corrective flow rates).

P.S I am overlooking the Tank coil, Thanks Larry. As deltas narrow I can see how the glycol could effect performance, if this is the bottle neck correcting it with increased flow will have no effect.

Unknown

whatever you are pushing could be greater with the same energy if you didn't have glycol. whatever faster that is would lower your average collector temperature, and increase collection efficiency.

so you can't equivalently just "increase flow rate". the presence of glycol reduces your flow rate from what it could be otherwise.

it's the same double whammy as always: it's harder to pump faster, and it carries less heat for what flow you do get.

the drainback pump power requirement is for a few minutes on system startup... there is no need to run big pumping power the rest of the day once the siphon is established.

I'm not saying it's a one for one situation, but there is no doubt and no argument about whether or not glycol is less efficient than straight water... it is, noticeably so. it's only a question of whether the efficiency drop is significant to the system you are proposing or not.

I'm also not sure about the reduced maintaence and such you talk about; glycol is a maintenance item, especially in a solar system. stagnation can really wipe out inhibitors, can't it?

scott markle_2

Rob, I can see situations where the penalty from glycol would be an issue. Like heating 5000sq.ft with a single 15-58.

In this case you are up against the performance limit of your system and a 10% reduction in transfer could be an issue.

A small residential DHW solar system is not likely to need the full capacity of it's circulator. Thats why variable speed pumping based on maintaining a optimal delta t is such a nice feature. Under low load this feature will keep a small continuous flow of heat moving from the collector instead of cycling the pump on and off.

No argument that glycol has a lower transfer capacity than water but unless the system is very close to it's performance limit with water I don't see how glycol is a penalty except in it's effect on pumping efficiency.

A collectors efficiency is determined by it's glazing and the delta between ambient temperature and return water. Pure water can move more energy (for a given volume) than a glycol solution, but if glycol can do the job at a higher flow the only penalty I see is in pump performance. As far your point about glycol "reducing your flow rate from what it could have been otherwise", yes but does that mater?... we don't want to send water screaming through our collectors, generally we have some headroom as far as flow is concerned.

Unknown

again: higher flow is lower delta T is lower average temperature across the collector. I am told by very knowledgeable fellows that this is not negligible in effect. small, perhaps, but then, maximizing solar collection is a worthy goal IMHO. I also cannot say that I am personally an expert, but I certainly trust the opinions I get on this matter.

I also occasionally get involved with glycol, evac tube solar systems. so don't think I'm a zealot here!

But If you're going to spend thousands getting panels on your roof, shouldn't you try to maximize its benefit?

scott markle_2

Rob, I'd hardly call you a zealot, lots of smart people see advantages in drain back. You know a thing or two about moving thermal energy and I appreciate you knocking this around with me.

I get your point about delta and flow. Obviously the transfer medium is important- imagine blowing air through the pipes instead of water.

Still I question this argument against glycol, Seems to me that biggest issue for efficiency is providing an effective sink for the available energy. If we have a place to put it then moving it doesn't seem like the big issue, glycol or not.

As far as "rated systems" are concerned do drain-back systems have higher efficiencies than closed loop glycol? The term efficiency is at some risk of losing it's meaning these days, it's just not that simple to track energy paths.