I am sure this has come up before, but I could not find it. Is there an efficiency differencial here? I know with cars we use the gycol for it's heat transfer and freezing level capabilites, and the same in home systems. But if you move toward a drain back system where you now do not need to be concerned with possible gycol leaks into your water systems nor frozen pipes, then what are the benefits?

Is one benefit you do not need the heat exchangers as you would with a glycol closed loop system?

Just curious of the pros and cons.

Bill

Hi Bill,
there are infact several advantages and disadvantages  with Glycol (no toxic propolyne glycol not etholyne glycol)

The main advantage is one does not have to worry about freezing.  For a simple hot water installation with few panels it is not a problem, just use a simple drainback system.  When the location gets extreems in temperatures and the system has more complex collector configurations, one may not always  be able to completely drain the system and freeze rupture in piping can be costly.  Hence use glycol.

In a closed loop glycol system one can use a low cost IRON pump with only about one sixth of the power required since in a sealed glycol system you have a continuous loop.  The pump will not oxydize nor does it have to have a high head preasure needed in a drain back system to lift the water up to the collectors. 
Drain back systems need  distilled or treaded water and sometimes more expensive brass pumps if there is any chance of adding new water or oxygen to the system.   Yes you can eliminate a heat exchanger in a drain back system but a heat exchanger is still required to separate potable drinking water from the recirculated drainback water.


The efficiency of heat transfer in a glycol system is less than plain water.  it depends on the concentration.  In a 40% glycol to water mixture you get protection down to about -9 degs F, but heat tranfer is reduced about 7%

In a drainback system care must be taken to have all piping and manifolds slope about .25 in per foot.  In a vacuum tube collector system a row of six collectors will change hight as much as 10 inches.  It might not look very pleasing, To some people they may think the roofers started the after work happy hour a little early.  For systems that are ground mounted there is no choice but to use glycol in a high freeze area.

If designed correctly a drainback system does work to eliminate freezing and summer over heating (stagnation) Care must be taken not to restart the system in the middle of a hot summer day after an early  drain down occured hours earlier.  Steam may result.  An example would be when the system shuts down when the high temp tank limit is reached.   The tank is subsequently depleted of its heat when a Jucuzzi is filled. As the collectors are now hotter than the tank water the system may attempt to restart.

Hope this helps
Dennis

Understand though, that any glycol system must have double-jacketed heat exchangers, under Code, which are frightfully expensive. This is true no matter the glycol chosen, and you would want this, since they all are toxic to some degree. Just use a little care in installing the exterior horizontal lines. The evac-tube reservoir does not have to completely drain.

You are correct, but the requirement for a double wall hx depends on the local building department. Here where I live they require it but in a neighboring county they do not,  Many departments  however do no permit open systems which mix potable water with the slab heating water'
Dennis

I'm betting that 95% of building inspection departments require double-jacketing, as most use the IBC. Those who don't are really out on a limb. Are we going to have to argue about this?

BTW, drainback systems do not need distilled or treated water, but it would be best. I would recommend it. (and in fact gave you that information)

Dennis & Quantum, Thanks. My previous system as stated was glycol and did have the double wall heat exchanger. I have to admit it was very economical and since it came with the house when I bought it, no installation cost. When I sold, I advertised the system as a way to keep electric bills low. So now I get to install from stratch and it looks to me one could go either way.

I am not sure what the code is in my area, but I feel that one would want to isolate the heating system from the potable system and therefore the choice as I see it is either drainback with a distilled water system or glycol. I am leaning toward the glycol for a couple of reasons. One, is you do not need to worry about designing the system to completely remove all pockets of water in case of a freeze, and two is it is active all the time and as stated above by Dennis, you do not have to be concerned with creating a steam situation. It was a very simple system in my old home and I do feel comfortable with it.

My next question would be lets say you have several collectors for winter use but too many for the summer, so you need to shut one or more down. With a water system this seems easy and a no brainer, its just drained and isolated. But I think you cannot do that with glycol can you? Can you leave it full, but isloated (ie no heating) or will it also have to drained for the summer season?

Thing about a glycol system is it must always be under some measure of pressure. Usually this is done with an expansion tank. Seems alot more fussy and expensive to me, but to each their own. Remember, there's no need to completely drain a water system in order to prevent freeze damage; even as much as 50% is enough. With drainback, no thinking allowed.

And I don't see anything wrong with running all collectors in Summer. They will never get the tank over 212 unless you're grossly malsized, and high tank temps mean no domestic hot water costs; in fact set up an absorption air conditioner, and you've got solar cool!

I can wait for solar residential chillers... Yazaki has one that just got UL approved but their local distributor says it is $30k for a 5 ton unit...rediculously expensive
In systems with small storage tanks and a large number of collectors overheating can occur in the summer. The standard solution is an active or passive heat dump.
A cooling loop is added before the hot glycol goes to the storage tank. In the passive approach a high temperature relief valve vents the circulating glycol to a radiator system of buried pex or copper tubing. You can also use baseboard radiator cores. In the active system a controller will turn on a small pump at a preset temperature and send fluid through the cooling loop.
As previously stated a well designed system will not overheat but.....this is usually true for solar fractions of less than 50%. It is when the customer wants a larger solar fraction ie. wants most of radiant heat to come from solar that provisions must be made for dumping heat in the summer, having pool or jacuzzi makes things easier. As also stated a closed glycol loop needs to be maintain pressure. the use of an expansion tank (before the pump) will prevent premature pump failure. When the system is first charged with glycol we pressurize it to 25 psi. Yes you can drain a glycol loop or sub loop but it is complicating things requiring recharging every fall. Keep it simple.
I have seen but never designed a system using glycol in a drainback loop using a glycol reservoir and a heat exchanger, anyone want to comment ?

Dennis

Man Dennis, absorption systems have been around for >50 years.

BTW, I do not give up info under duress.

Quantum,

Interesting. I looked up absorption chillers. Though I did not see anything for residential use, but I am sure there is something out there. In NM we usually go for the evaporative coolers. Though cheap to run electrically their drawback is water usage. That said the extra humidity it adds to the living quarters is also helpful.

Bill, for the most advanced swamp-cooler check out a unit called 'Coolerado'.

There is always something new. Very interesting about the Coolerado. But it did say is uses the same amount of water an evaporative (swamp) cooler. Also since we will be building out of adobe there was an article about the affects of humidty that makes the adobe asorb and transfer heat more efficently, making the use of a swamp cooler a plus. But I cannot put my finger on the article. I will have to print it out this time.

Quantum,
True absorption chillers have been around a while. Back in the 40s they use to have gas refridgerators. The general principle is the same.. a phase change in this case from liquid to a gas and then reversed. But significant cooling often requires several stages of adsorption. These systems are large and require a cooling tower as well. Factories often use this technology because it is cost effective and they have a lot of room. Residential chillers have smaller power and space requirements and no one wants a cooling tower on their roof. Only recently have new technologies appeared that use silica gel and lithium bromide and no cooling tower. If you know of any residential units that are available now (and affordable) let me know , we could sell them as part of a complete solar system.

Dennis

Information is so hard to come by. It takes days and days of research just to come up with a small bit of information. Some people only take and take, and I am sick of giving to such selfish ones. I need to know where you are located.

Be advised that LiBr has been in use since the '40's. And absorption refrigerators since the 1910's.

ABsorption (LiBr, ammonia) units are radically different from ADsorption units. Only one company has developed and is now making ADsorption units (Nishiyodo/HIJC), but theirs are only on industrial scale. Enough good cannot be said about aDsorptive-cycle though, and they are highly compatible with solar hot water.

Hi Quantum,
It does take a while to collect info doesn't it. We have people who come to our store all the time looking to solve problems with a system they purchased on the internet. They shop us for price and then go buy the cheapest they can find on the internet and when it doesn't work they want us to tell them how to fix it. I feel likr telling them to go ask the internet.

Thanks for the input. i have in fact been at that web site. I am getting my technologies mixed up. We are in Carson City, NV at The Solar Store
Dennis

Yep, those exact same folks came to my computer store I had in Orlando FL. Boy they get around...lol

Thanks guys. I will surely be looking for someone nearby to help design my system.

Now, Now, lets not get political here. Some of us can be both a conservationist and conservative...or is that the same thing...lol It's amazing what we can do with our language.

Interesting topic Quantum. You never know what little light will come on about something you read or heard in the past and can be applicable to solving a problem one has.

Fact is, certain political bents have done massive damage to our environment, not to mention ideas, advancement, our service(wo)men, and our (now unpayable) national debt... individuals aside.

Friendliness to a toxic culture is maladaptive.

please i need to know the cooling rate of glycol as used in indirect solar water heating. thank you