frost damage to heat pipes in evacuated tubes

[djh]

There's a posting at http://www.boards.ie/vbulletin/showthread.php?t=2056381258 that says:

"The German ST test facility TUV has changed the test parameters for heat pipe collectors.
The reason for this was a massive shortcoming of ST energy harvested in a very large collector field, the actual thermal harvest being much lower than what was predicted and guaranteed.

The official test regime for heat pipe collectors demand a half an hour frost resistance. For the collector. But not for the individual heat pipe. So the half an hour test is easily passed. Most manufacturers ad the term " to be used in combination with glycol ".

But the heat pipes themself are bursting, the inner copper tube holding the fluid, the thermal carrier."

There are links to more details, including the TUV site and some photos, in that posting.

This seems like something to be aware of. 

[desperate]

Well that's pretty odd really, water is almost unique in expanding as it nears its freezing temp, I think, and I am pretty sure that the heat pipe doesn't contain water, at least in the Navitron ones. Some types contain the circulating fluid, which should be an antifreeze mix, which shouldn't freeze either.
Maybe someone can confirm what the working fluid is in the heat pipe type.

Desp

[pj]

Despite having antifreeze in the solar loop, I also have the anti-freeze option set on my Resol controller set at 4degC. In light of the above, this seems a good thing to do?
Has anyone had burst heatpipes?

[pj]

Desp,
I'm sure in the past the contents of the heat pipes was discussed on this forum, one thread I can find is
http://www.navitron.org.uk/forum/index.php/topic,427.msg2587.html#msg2587
I think there were other threads, but they all said that the heat pipes just contain (pure) water. I don't think Ivan ever indicated to the contrary?

[desperate]

Hey well spotted pj, I always thought these type heat pipes worked on a phase change principle. I've learnt something new today, thanks.

Desp

PS not sure about this though, but I have no idea if these are the type Navitron have used??

http://en.wikipedia.org/wiki/Heat_pipe

[martin]

My swift perusal of the linked documents seems to suggest that the problematical heat pipes were of a different design to the Navitron pipes - I've certainly never heard of any problems with the "house brand" over several years (including loads of cold weather), suggesting that the affected systems are not comparable.......

[daftlad]

I thought the contents of the navitron heat pipes was a bit of a closely guarded secret, something to do with it not deteriorating with age or something. all speculation really until Ivan turns up.

peas

[desperate]

Single malt maybe?? that gets better with age

Can't really work out which type of tube those docs are refering to, they talk about heat pipes which doesn't quite mean the circulation type, maybe??

Desp

[EccentricAnomaly]

Despite having antifreeze in the solar loop, I also have the anti-freeze option set on my Resol controller set at 4degC. In light of the above, this seems a good thing to do?

Would keeping the liquid in the manifold warm help much with the heat pipes?

Well that's pretty odd really, water is almost unique in expanding as it nears its freezing temp, I think, and I am pretty sure that the heat pipe doesn't contain water, at least in the Navitron ones.

But maybe the damage would happen as the heat pipe contents melted, e.g., if it froze from the outside in forming a plug in the pipe then melted from the bottom up. Compare with concerns about thawing hydrazine in the Ulysses spacecraft:

http://science.nasa.gov/science-news/science-at-nasa/2004/17mar_ulysses/

[EccentricAnomaly]

I'm sure in the past the contents of the heat pipes was discussed on this forum, one thread I can find is
http://www.navitron.org.uk/forum/index.php/topic,427.msg2587.html#msg2587
I think there were other threads, but they all said that the heat pipes just contain (pure) water. I don't think Ivan ever indicated to the contrary?

Are you sure that parenthetical "water" there isn't just a reference to the analogy being made with a gravity HW system?

[Philip R]

I was told they had propanol (propyl alcohol) in them. At least they would have some degree of freeze protection.

If one of mine gives up the ghost, I will cut the end off with a pipe slice. ( wearing goggles and nomex/ kevlar gloves). Then I will find out if water was used.

PhilipR

[SteveH]

 I suspect this has more to do with the wall thickness of the heat pipes & the internal pressure in relation to atmospheric.

  I had a chat with Ivan about Navitron heat pipes construction on the installers course, I think there are no concerns in connection with the anomalous expansion of water at freezing point....

 I'm also going to wait for Ivan's opinion.

[rogeriko]

Acetone

http://www.solarheatingcanada.com/evacuated-tube-solar-collector.html

An evacuated tube is made up of 2 layers of strong borosilicate glass that are joined together. There is a vacuum in between the 2 layers of glass. This layer of vacuum allows the tube to retain its heat once it is gathered. The UV rays are absorbed by an absorbing film which lies in between the 2 layers of glass on the inside layer. This special absorbing film absorbs more than 92% of the UV rays that incident the tube and rejects less than 8% back to the atmosphere.

The heat is transferred by conduction to a copper fin inside the inner layer of glass. A heat pipe that is inserted into the fin absorbs this heat and transfers it to the copper header at the top of the collector. The heat pipe is a thin copper tube which contains a small amount of acetone. Once the heat pipe is subject to a small amount of heat the acetone vaporizes and turns into steam. It then rises to the top of the heat pipe where it reaches a condenser. Once the heat is removed from the condenser by a heating fluid such as water or a mixture of water and glycol, the acetone reverts to a liquid and returns to the bottom of the heat pipe where it repeats its cycle.

The condenser is inserted into a copper manifold where the heat is transferred to the heating fluid passing through the manifold. A manifold is a copper pipe with a set of sleeves distributed along its length. Each sleeve receives the condensing end of a heat pipe. At this intersection the fluid flowing through the manifold absorbs all the heat from the condenser and continues through the manifold.

[pdf27]

Acetone boils at + 60 deg C, and freezes at -100 deg C - see http://webbook.nist.gov/cgi/cbook.cgi?ID=C67641&Mask=4
Isopropanol is virtually the same - see http://webbook.nist.gov/cgi/cbook.cgi?ID=C67630&Mask=4#Thermo-Phase

Wiki states that distilled water is commonly used as the heat transfer medium - see http://en.wikipedia.org/wiki/Heat_pipe#Solar_Thermal

[stuartiannaylor]

I am not a fan of heat-pipe versions, big fan of directflow u-tube. Better all round, but you get what you pay for.

Saying that I can't see any reason a heat pipe will freeze. Also I am pretty sure it don't expand. I thought Methanol someone said acetone. Either way you ain't going to freeze that stuff. The ruptures are more likely evaporation expansion and prob defective or cheap heat-pipes on the copper-wall side.

The article mentioned is pants though as it talks about total failures and a burst heat pipe will not do that. Total failures are glycol / water (transfer side) and unless you have strong mixtures then well below 20C (whatever that means) does start to get tricky.

Bad winter plus wind chill and someone skimping on the glycol then yeah prob could happen to (transfer side) otherwise I can only think they are popping whilst hot.