Further Design Pitfalls in Glycol Solar Hot Water Systems
I recently received a post on my blog from a subscriber in the U.K. by the name of Cleland who has installed numerous glycol solar hot water systems. He said he was starting to encounter many of the problems I’ve been describing lately. In particular, he mentioned the following headaches…
- Stagnation and boiling of glycol
- Corrosion of pop-off valves
- Degeneration of the glycol
- Failure of expansion tanks
- Need to change out the glycol every five years (minimum).
Feel free to read his entire comment and post your own thoughts or questions about glycol systems. Luckily, there’s a much better way to design a solar hot water system as I’ll be illustrating next time.
Getting you in hot water,
Dr. Ben
3 Responses to “Further Design Pitfalls in Glycol Solar Hot Water Systems”
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Ben,
As I mentioned somewhere else at this site, glycol can be used successfully with the proper expansion tank design. The method is now being called “steamback”. Personally, I still much prefer drainback because of cost and efficiency.
I just don’t think that it’s fair to eliminate glycol on technical limitations because those limitations don’t exist anymore.
Yes, I saw your comment and went to the web site to read about steamback systems. I hadn’t heard of them before.
Frankly, I don’t like the idea. While it may work technically, glycol solutions age badly at only 200F, so letting it go to 300F or more seems like an invitation to acid breakdown in short order. Also, all expansion tanks have a lifetime. When the bladder goes, what then? Seems like a short term solution with an unhappy ending. While they may “work”, what’s the point when there is a more durable, cheaper, and more efficient design available?
One reason I am conservative with this technology is because I have seen so many systems break down, or cost a fortune in maintenance and then get scrapped five years down the road. That gives us all a bad name. I have drainback systems that have gone 25 years with little to no maintenance. The military has already told us they don’t do maintenance, they only fix things when they break. This is why they are now requiring drainback systems. I just can’t endorse a system with built-in weaknesses that don’t have to be there.
See the comments from Cleland in the UK about his experience with glycol systems.
Regards,
Dr. Ben
I designed and built my PV pumped, closed loop glycol solar hot water system over 22 years ago. PV pumping has several advantages- one is variable speed as less sunlight heating the glycol translates also to slower pump speeds, keeping loop temps above what a simple on off Drainback system would. PV pumping also eliminates the need for sensors and controls, as the Sun is your control. This also eliminates the parasitic losses inherent in Drainback systems, as it takes a fair amount of energy to push the water back up to the collector at least once a day if not many times on partly cloudy days. Because there are no parasitic losses with PV pumped systems, they are the most efficient solar water heating system. I disagree with a few points put forward on your blog. One is that you claim the pressure relief valve in the loop must be at the top of the loop. I’ve seen many of them in other places in the loop, they can be wherever you consider it most convenient. Also, a simple vacation bypass valve that bypasses the check valve allowing a thermosiphon at night is a much better solution to overheating conditions then a heat dump. This is opened when no one is using any water for a period of time, but if overheating is common with normal usage the system was designed poorly.
There are some online groups devoted to pushing the envelope in solar water heating. In these groups people have discovered ways to totally eliminate the expansion tank in the closed loop as well as the air eliminator. This brings the cost of the DIY antifreeze closed loop to less than the Drainback, which requires a separate Drainback tank. Even with the normal hardware in a closed loop, the same size system available in either Drainback or closed loop antifreeze is less expensive to buy as a PV pumped closed loop.
While the closed loop antifreeze versus Drainback debate rages on,I admit there are pros and cons to each system, neither is the best in every situation. For example, many installations have the collectors out in the yard, where drainback is difficult if not impossible. Some collectors are not plumbed for drainback. Drainback requires more pumping power to repeatedly lift water to the collectors, which almost all the time means grid powered pumps. Since such pumps are performing more work, they wear faster than circulation pumps, as do the contactors controlling them. If a cloud bank passes over a drainback, it drains the fluid down, and then needs to add the energy back again when the sky clears, this can occur many times during the day. PV pumped systems will just reduce the pump speed to closely match heat gained in the panel, never requiring the considerable energy to lift a column of fluid. Being grid powered, drainback has parasitic losses requiring large differential settings compared to PV pumped. Most drainback systems are not set up for variable flow, PV pumping naturally provides variable flow. Because of these last two points, PV pumped is the most efficient. Don’t take it from me, read what the experts have to say-
From Homepower Issue #126, Aug, Sept 2008 Article titled “Under control-solar water heating- SHW controllers” Page 60
“Parasitic loss occurs when energy is consumed or lost in order for a system to make more energy. In grid powered SHW systems, the pumps and controllers take a certain amount of electrical energy to operate. If the parasitic losses are greater than the solar energy being put into the SHW system, a net loss of energy results. In PV-direct DC systems, the pump energy losses are inconsequential because the energy from the PV module is there regardless, and at no cost. But in AC systems, a wider temperature differential is used to ensure that the parasitic losses are not greater than the solar energy gained. Since there is no utility generated parasitic power consumed with a PV powered DC differential controller, the turn off differential is lower-sometimes zero is appropriate if the pipe losses are negligible.”
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This makes it clear that PV pumped SHW systems are more efficient because they have a zero degree differential, gaining solar heat whenever collector temps are above tank temps, while AC powered system differentials range as high as 18*F, to overcome losses caused by carbon based grid powered pumps. PV pumped systems also have the ability to moderate flow according to the amount of sun received, not just on or off like AC systems.Doug
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Subject: Most efficient solar water heater
Homepower Issue #121, Oct. and Nov. 2007 “Pick the right pump” article by Chuck Marken states on Page 86-
“Using a utility powered AC pump for your solar water heating system will give you a COP between 12 and 25, this is an excellent value compared to electric water heaters, which have a COP of 1. But the COP will never be as good as a DC PV powered SHW system. DC hot water circulation pumps have a higher COP than AC pumps because there is no traditional energy input if a PV module powers the system. If you use a solar-electric module to power the pump, your COP is infinite- you`re not adding any energy input. The sun provides it all, and you get something for nothing after the initial investment. PV powered systems are immune to utility outages.”
“PV powered DC pumps are normally the optimal choice for a solar heating system except in high-head drainback and very large antifreeze systems.”
http://www.builditsolar.com/Projects/SolarHomes/Doug/DougsProjects.htm
http://www.youtube.com/user/sundug69