Evacuated Tube Solar -- Freezing Heat Pipes ...

[jebatty]

I'm working on a proposal for a 10 - 30 tube solar hot water evacuated tube system to operate year-round, including very cold MN winters when temps into the -30 to -40F range can be expected. Preliminary research indicates that tube collectors may suffer from failure due to freezing of the heat pipes as temps get near 0F and below. Any experience by anyone on this issue or others? The attractiveness of evacuated tubes relates to cold weather performance, as sunny days with below 0F temps are common.

 

[woodgeek]

I'm working on a proposal for a 10 - 30 tube solar hot water evacuated tube system to operate year-round, including very cold MN winters when temps into the -30 to -40F range can be expected. Preliminary research indicates that tube collectors may suffer from failure due to freezing of the heat pipes as temps get near 0F and below. Any experience by anyone on this issue or others? The attractiveness of evacuated tubes relates to cold weather performance, as sunny days with below 0F temps are common.

I know you are looking for first hand experience, but I personally don't see how this could be a problem. The heat pipes are filled with the vapor of some volatile solvent (like toluene), along with a slight excess of liquid. You could suppose that that solvent could freeze, but that would never be a problem (since, unlike water, it would contract on freezing). When the sun comes out, it melts and you are in business. On the water loop side, you have glycol, and you have formulated its strength for you min design temp.

Or maybe I am not understanding the question?

Edit: Ok, more research suggests that some (many?) tubes are using a small amount of distilled water as the transfer fluid...maybe a quarter teaspoon. Add too much (as a manufacturer defect) and then you have a problem. Then you make a warranty claim.

 

[jebatty]

My research showed the same, but perhaps this problem has been solved by using a different solution, so looking for updates from anyone in a very cold climate where the tubes would be subject to freezing (< 0F).

 

[woodgeek]

There's nothing magic about 0F or -30F here. The water in the heat pipe would freeze at 32°F. I think the idea is there is only a couple drops present, so it can (and does normally) expand freely without breaking the pipe. A defective pipe (with too much water) would fail in my climate as well as yours. I can only guess that water gives better performance than other solvents due to the higher heat of vaporization, and is not a problem in a properly made tube.

 

[semipro]

Is it possible the freezing problem associated with the evacuated tubes is actually at the manifold rather than in the tubes themselves?
Don't some applications use water for the heat transfer rather than a glycol or other low freezing point liquid?

 

[GaryGary]

Hi,
I've not heard of evac tubes failing in low temperatures, but they do have the reputation for collecting ice/hard snow on the outside of the tubes which cannot easily be cleaned off without the risk of breaking the tubes. They take a much longer time to clear the ice themselves as the temperature of the glass envelope is low with the vacuum insulation.

I know this evacs vs flats gets debated and lot and people feel very strongly about one or the other, but I've not seen any hard data that says the evacs outperform the flats.
Some side by side comparisons here: http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Evacuated Tube

You can also use this calculator to compare a few sample flats and evacs at various temperatures: http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm
For example, with an ambient temp of 0F, average collector temp of 110F, and full sun:
A Heliodyne Gobi gives: 45.8% efficiency
A Seido evac tube gives: 42.5%

This depends on the particular flat plate or evact tube that you choose -- while the flat plates don't vary much from brand to brand, there is a lot of variation in the efficiency curve between brands of evacs.

But, I think that unless you live in Fairbanks, 0F is awfully low to use for average winter performance. WeatherSpark.com is your friend in determining realistic average daily high temps for your area. I think its best to use the daily high temperature as this is fairly close to the temperature you will do the most collecting at -- or knock the daily high down a few degrees if you like.


The SRCC site has performance reports on every collector they have certified -- (broken link removed to http://www.solar-rating.org/ratings/index.html)
Just bare in mind that the collector output is given for the whole collector, so if the area of the two collectors you are comparing are different, than you have to correct for that.-- just divide the total panel output by the area of the panel to get the output per sqft.

Gary

 

[Wolftech]

Hello, I have first hand experience with this issue. Years back evacuated tubes were sold in the southern regions and northern regions of the US. Some evacuated tube company's(non-SRCC certified) sold a where not able to handle the cold climates and split the copper tubing in the evacuated tube. The thing to look for is that SRCC label as Gary said.

As for if you want glycol system or a water system that depends on alot of other conditions not just the cold. I have closed loop glycol system and it works for what I wanted, but I could of put in a open loop drainback in just as easy. I guess its just what fits in your or your customer application. Hope that helps.