Limitations of Glycol Solar Water Heating Systems (Part III)
In this third part of my mini-series on pressurized glycol solar water heating systems, I go into some of the details regarding potential overheating, which is of particular concern for commercial systems. Remember, glycol systems do not scale up easily so the larger the system, the more heat you have to get rid of during periods of low demand. Engineers typically introduce heat dump systems into the design to help disperse the excess energy. But think about it: the owner of the system ends up paying extra money for equipment designed to throw away the energy for which he spent money to capture. Not only is that inefficient, it’s nonsensical! Also keep in mind of one the cardinal rules of engineering: the more complicated the design, the more points of failure you will deal with over its lifespan. Fortunately, there is a better way…
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Dr. Ben
10 Responses to “Limitations of Glycol Solar Water Heating Systems (Part III)”
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Hello Dr. Ben,
Thank you for explaining the glycol system, it’s very clear this way.
I’m looking forward to the explanation of the drainback system.
Kind regards,
Stefan Martens
Hi Dr. Ben,
In the last few years, some solar practitioners and engineers have developed “steamback” systems. In short, it eliminates the high temperature and stagnation problems with closed systems by letting the collector fluid vaporize. The increased pressure and volume of the system are accommodated by proper expansion tank sizing and relief valve pressure setting.
I still think drainback is better, but it’s nice to know these problems have been solved elegantly. Another arrow in the quiver if you will.
Here’s John Siegenthaler’s explanation of the strategy:
http://www.pmengineer.com/Articles/Column/BNP_GUID_9-5-2006_A_10000000000000570238
And a lengthy discussion of the pros and cons:
http://www.heatinghelp.com/forum-thread/133064/Steamback-System-Design
Hi Dr Ben
Your blog and videos are very informative.
We install sealed pressurised glycol systems in the UK. For a while all appeared to be fine but it is becoming clear from all the repairs we carry out on our own and others’ systems that there are some recurring issues which ultimately can lead to system failure and expensive servicing unless caught early.
So we are looking at drain back as an option and hope to start installing them one day. Thanks again.
All the best
Cleland
Cleland, thanks for your feedback. What type of maintenance do you typically have to perform in order to keep the glycol systems running smoothly? How often do you have to recharge the glycol?
Dr. Ben
Hi Ben,
Yes, typical problems revolve around stagnation and boiling of the glycol due to over-sized collectors, under-sized cylinders or customer absence in sunny weather. The steam gets into all the components. Externally-mounted pressure relief valves can rust and so cause pressure loss due to not re-seating correctly. Pumps run dry and burn out if not flooded. Flow gauge glass windows have cracked due to steam. Some glycol turns to a black tarry goo and literally blocks narrow bore pipes. Expansion vessels lose pressure over time; perhaps accelerated due to steam pressure. Extremely high cylinder temps can occur as controller attempts to cool cylinder and/or collector leading to customer concern. Some customers appear not to be aware of how to change their boiler timings for hot water so keep heating the cylinder in the morning so exacerbating the boiling issues.
The glycol itself lasts about 5 years in well designed systems.
Most of the problems occur with direct flow evacuated tubes which stagnate poorly and don’t empty well. So we have switched to Thermomax heat pipes with their useful cut-off valve of 95C. Flat plates don’t seem to cause so many issues.
The UK solar industry is reasonably new so a lot of systems have gone in which are poorly designed with components mounted in the wrong order or wrong way (e.g.expansion vessels too small or up the wrong way so trapping steam). We run around fixing them and so pick up a lot of interesting information. Hence the desire to find a design which is more bullet proof and more maintenance-free. There are not many drainback collectors available in UK…or at least not according to their warranty information. There are some Dutch collectors and designs.
Your 6 x points in favour of drainback are getting more attractive as the summer gets closer!
Cleland
Cleland,
Your comments parallel the experience here in the US. A glycol system is only about 85% as efficient as a drainback design and has about 1/3 less lifetime due to glycol chemistry problems. They can also be a maintenance nightmare as you describe. All these problems are magnified when you try to scale a glycol system up to commercial sizes.
Please note that the drainback system is not based on a specific collector design, other than it must be able to drain when the pump stops. All the various heat pipe, focussing, and evacuated tube collectors are simply not necessary for domestic hot water DHW applications. Their efficiency is typically lower in the DHW range, they cost a lot more, and are prone to breakage/leakage with time.
A drainback system is a design more than it is equipment. Well designed equipment will enhance the performance and provide longer life, as any well designed product will.
I am about to go pick up a 250 gallon Fluid Handling System that is over 25 years old and still in perfect working condition (according to the dealer who installed it). Why am I picking it up? Well, the facility that owns it got a grant to replace all their HVAC equipment, including the solar. They wanted new solar equipment and they will get it.
I am going to use the FHS as a demo unit to show how long a drainback design can last.
As you may have noticed in this blog, we just started up a factory again last September and have been focussing on large commercial systems. However, residential and small commercial systems are now our most asked for products.
Maybe someday, we can get the product into Europe and the UK, but we have to get up to speed in the US first.
Dr. Ben
Dr. Ben,
I am trying to assess the best method for heat to the water in our new swimming pool in the summer and diverting the energy to our DHW in the winter.
When will your drainback YouTube be visible to us?
Thank you for your explanations of the gylcol system.
Greg
Greg,
The very best applications for solar thermal are those that go year round. So, for a residence, you have DHW year round, space heating in the winter, and pool heating in the spring and fall when there no space heating. I don’t know your locale, but in North Carolina, we can generally add one month to the beginning and ending of the swimming season (for an in-ground pool). This assumes the solar system is sized for space heating and DHW to start with.
The concept is simple. To a space heating and DHW system you add an exchanger and controls for the pool circuit.
Hoewever, if you are thinking of a small DHW only system, then you won’t get much energy into the pool out of a couple of collectors (unless the pool is very small), and the expense to add a pool exchanger circuit and controls may not be worth it. This is always a judgment call. Technically, doing a pool application is straight forward, the economics usually drive the decision.
You asked about the YouTube videos. They are all at http://www.youtube.com/user/DrBenGravely.
Thanks for your inquiry!
Dr. Ben