frost damage to heat pipes in evacuated tubes

[EccentricAnomaly]

I think there are probably quite a lot of different designs around and quoting, for example, one not very well spell checked and obviously wrong¹ Canadian site for a "definitive" answer is not terribly helpful.

¹ A thermal collector which only or mostly used just the UV radiation wouldn't be very effective. UV is a very small proportion of the short-wave energy arriving at the Earth's surface. Most is visible light and near IR.

[KLD]

I always thought these type heat pipes worked on a phase change principle.

Desp

Absolutely. The heat pipe uses the latent heat of vapourisation to transfer energy from one place to another.The medium in the pipe exists normally in two phases, fluid and vapour. With solar input, the fluid vapourises (this step takes up the energy), the vapour can then move quite freely inside the heatpipe, and where the vapour comes into contact with the cooler manifold pipe, it condenses back to the fluid phase (this step releases the same amount of energy).
The higher the heat of vapourisation (which is a material parameter) the less of the medium is required. Water has a higher heat of vapourisation than for instance actone. For any selected medium, you'd have to select the working pressure such that the boiling and condensing happens at the desired temperature. For a solar collector heatpipe, you might want to have some of the medium in the vapour phase at 25°C or so, i.e. if you were using water, you'd have to lower the pressure inside the heatpipe to maybe 100 mbar.

All that "playing" with the boiling point doesn't do much about the freezing point, though. Water will still freeze at 0°C. I guess that the amount of medium in the heatpipe can be relatively little, and should that start to freeze the plug might simply push upwards?

Klaus

[stuartiannaylor]

So are we saying the contents of the heatpipe are distilled water? 

As if so process variations in filling could cause freezing probs if there isn't enough room for expansion. To be honest those pics looked like gas pressure rips. Its a guess but it looked like something escaped and not merely a split from expansion.
lol total guess though

[langstroth3]

Not exactly subjective, but I've not had problems (touch wood) with the (Navitron) tubes for the last three winters - and last winter was particularly cold for Southern England. My 1-wire sensor near to the panels measured down to -7C on a number of nights.  

..given they were then reaching >40C in the often clear bright day that followed; it's quite a regular and large diurnal temp swing panels have to endure.

[zeus]

Not exactly subjective, but I've not had problems (touch wood) with the (Navitron) tubes for the last three winters - and last winter was particularly cold for Southern England. My 1-wire sensor near to the panels measured down to -7C on a number of nights.  

..given they were then reaching >40C in the often clear bright day that followed; it's quite a regular and large diurnal temp swing panels have to endure.

-7C ... that's almost tropical  , midday temperatures didn't get anywhere near that around here on a number of occasions last winter .... -20C at night for some who don't live so close to the equator or the sea ....    

[SteveH]

I suppose it's time to take a straw poll of those on the forum (In the UK) who have a Navitron  evacuated hot water system installed.

Q:- Does it still work OK after the last 2 hard winters...?  Yes or No will do.

[spluger]

yes

and no antifreeze either

[Mike N.]

No antifreeze, no problems but did once split an outside heat dump radiator. 

Mike

[martin]

Yes (of course!)

[langstroth3]

Yes (no antifreeze as it's a direct system)

[Justme]

Wet tube Navitron system (SFD) no antifreeze no glass breakages. For last 3 winters.

Heat pipe non Navitron array no antifreeze no breakages to glass or heat pipe. For last 2 winters

Heat pipe no glass left lying around no damage or leaks. For last 2 winters.

All have been under snow & ice for weeks. Temps dropped to -16c.

[stuartiannaylor]

We still haven't had a definitive reply as to the heat transfer liquid in the heat pipes. Just wondered if anybody has more of an idea? Ivan?

Was the damage caused by pressure from gassing or frozen expansion and is there a minimum temp at which they will withstand.
Do they pass the new TUV tests?

I guess you can write that you have had them for a couple of years and temps have been cold and they are OK. Thing is the TUV is a lifetime stress evaluation technique. Have the germans gone a bit extreme in there testing or have they just stumbled upon many inferior products?

Stuart

[greentangerine]

I've got 65 Sunnpro tubes and they are all fine; been installed about 18 months now.

I've been looking at the specs and also the TUV test report but can't find any reference as to what liquid is in the heat pipe. 

Temperature range since install has been about -10 to +180 at the manifold sensor. 

[djh]

I've been looking at the specs and also the TUV test report but can't find any reference as to what liquid is in the heat pipe.

Materials Safety Data Sheets (MSDS) often provide the most information about what something is made from, if you can find them.

[pj]

http://www.navitron.org.uk/forum/index.php/topic,5571.msg56552.html#msg56552

In Ivan's own handwriting, "the Navitron tubes have got water, with a trace of antifreeze."