Sun-Exposed Cable in Black Conduit

[JohnW2]

In another thread ...

What would be an appropriate de-rating factor for the cable when inside a black conduit that was being heated by solar radiation ?

Just conducted as experiment, 2 feet of 25mm black conduit horizontal in free air and in direct sunlight with a mercury in glass thermometer inside the conduit, .... After 10 minutes the temperature inside the conduit was 110°F = 43.333°C.

I've just had a go with some flexible black conduit, and, even with the haze (virtually no shadows), got figures very similar to yours. Assuming tomorrow is less hazy (and even hotter), I'll try again tomorrow and report back!

Haze now gone, and garden air temp about 32.2°C . After about 10 mins in full sun (hard to photograph because of sun - reads 56.6°C) ....

View attachment 274789View attachment 274790

Kind Regards, John
Edit: Didn't work - I'll try again ...

 

[JohnW2]

In another thread ...

What would be an appropriate de-rating factor for the cable when inside a black conduit that was being heated by solar radiation ?

Just conducted as experiment, 2 feet of 25mm black conduit horizontal in free air and in direct sunlight with a mercury in glass thermometer inside the conduit, .... After 10 minutes the temperature inside the conduit was 110°F = 43.333°C.

I've just had a go with some flexible black conduit, and, even with the haze (virtually no shadows), got figures very similar to yours. Assuming tomorrow is less hazy (and even hotter), I'll try again tomorrow and report back!

Haze now gone, and garden air temp about 32.2°C . After about 10 mins in full sun (hard to photograph because of sun - reads 55.6°C) ....

Kind Regards, John

 

[omph]

Would be interesting to see the same experiment but with a white cable instead.

 

[JohnW2]

Would be interesting to see the same experiment but with a white cable instead.

Yes, I thought of that. I haven't got any white conduit, but I do have some white 21.5mm 'overflow pipe', so I might try that. However, that will probably have to wait until tomorrow, since the 'haze' is now back here!

However, as I said previously, I suspect that the only difference white will make will be to slow down the heating up - so if it remains exposed to intense sunlight for long enough, it may well eventually get to much the same temperature as with the black conduit.

Kind Regards, John

 

[SUNRAY]

Yes, I thought of that. I haven't got any white conduit, but I do have some white 21.5mm 'overflow pipe', so I might try that. However, that will probably have to wait until tomorrow, since the 'haze' is now back here!

However, as I said previously, I suspect that the only difference white will make will be to slow down the heating up - so if it remains exposed to intense sunlight for long enough, it may well eventually get to much the same temperature as with the black conduit.

Kind Regards, John

No it shouldn't as the black absorbs heat and the white reflects it.

Many years ago I purchased a minibus, navy blue with matt black roof. the heat was crazy. In my lunchbreak at work I used 80 grade glasspaper to remove half of the black (drivers side) then primed with woolworths primer/undercoat a sort of yellowy-brown colour, next day I gave a second coat. The temperature difference between the 2 sides was such that I could comfortably put my hand on the primer but not on the black, when all primed/undercoated I added bright yellow gloss and again the temp difference between primer & top coat was significant.

 

[JohnW2]

No it shouldn't as the black absorbs heat and the white reflects it.

Yes, but not completely. My thinking was that, although appreciably less (with white, as compared with black) some energy/min would be continue to be absorbed - in which case, as I said, I would expect the temp to continue rising (so long as it was sun-exposed) and maybe eventually (if the exposure lasted long enough) achieve a temp not much lower than with the black.

In attempting to investigate that this afternoon, after about 5 mins in full sun I got this ...

However, when I looked again about 10 minutes later, the electronics of the thermometer had obviously got rather overcome by heat and it was displaying gobbly-gook

I'm currently cooling it down, and will try again later with some attempt at protecting the works of the thermometer from heat! I might have to try to find a mercury (or something)-in-glass thermometer, like bernard used (but try not to break mine

However, with temps here tomorrow forecast as 22-23 degrees, I might not have long to complete these experiments (at least 'until next time'!)

Kind Regards, John

 

[SimonH2]

FWIW, I have some steel conduit on a west facing wall - painted with the same "sand" coloured paint as the wall. Earlier when it had been in strong sunlight for a while, it was barely warm to the touch. It's gone a bit hazy now, and it feels cool to the touch (which suggests it's no higher than low 30s ˚C)

My thinking was that, although appreciably less (with white, as compared with black) some energy/min would be continue to be absorbed - in which case, as I said, I would expect the temp to continue rising (so long as it was sun-exposed) and maybe eventually (if the exposure lasted long enough) achieve a temp not much lower than with the black.

I think you're forgetting the other half of the energy balance - the conduit will also be losing heat by radiation and convection. I assume white will radiate less heat than black, but it'll still lose heat by convection.
If you think about it, without the loss side of the equation, even in poor sunlight, the conduit would continue to heat up indefinitely until it melted.

 

[SUNRAY]

This was a nice experiment, i've had a visitor here for a bit over an hour, we have similar vehicles (Partner Teepee) the temp sensors are in the drivers side door mirror, parked one in from of other. When he left his read 48°, mine 42° the difference his is black, mine is silver. Not as remarkable as Johns difference but still demonstrates the difference.

 

[SUNRAY]

If you think about it, without the loss side of the equation, even in poor sunlight, the conduit would continue to heat up indefinitely until it melted.

I don't know if it's still in action but a system of mirrors along the top of a ridge tracking the sun and focussing onto a hearth somewhere in mainland Europe was used for melting metals well into 4 figures.

 

[bernardgreen]

I think this one is still operational as a power generator

Solar Tower, Seville

A futuristic-looking 11MW solar tower was completed near Seville, Spain, as part of a 300MW solar power project. Located in…

www.power-technology.com

and I believe this one is the one that melts metals

The Odeillo solar furnace, the world's largest - Sabater Fundimol, Spain

In the French city of Odeillo, the largest solar furnace in the world up-to-date has been built, enabling great advances in materials research, particularly of metals, for industries such as aerospace or electronics.

www.sabater-fundimol.com

 

[RandomGrinch]

I don't know if it's still in action but a system of mirrors along the top of a ridge tracking the sun and focussing onto a hearth somewhere in mainland Europe was used for melting metals well into 4 figures.

...and rather aptly for your username, this reminds me of:

 

[JohnW2]

I think you're forgetting the other half of the energy balance - the conduit will also be losing heat by radiation and convection. I assume white will radiate less heat than black, but it'll still lose heat by convection. If you think about it, without the loss side of the equation, even in poor sunlight, the conduit would continue to heat up indefinitely until it melted.

No, I'm not forgetting the other side of the equation - as you say, the ultimate temp achieved will represent an equilibrium between heat acquisition and heat loss. However, as you say, at any given temp the heat loss by convection from from a white tube will be less than from a black one, whilst heat loss by convection and conduction will presumably be about the same - so the total rate of heat loss will be lower with white.

However, it seems that that is not enough to make my thinking work, since you seem to be correct in your view of the situation. Having cooled down my thermometer and taken steps to protect its electronics from heat, after about 30 mins I'm currently seeing ('only') about 47 degrees. I obviously don't know what is going to happen subsequently, but the sun is starting to 'go down' - so that might well happen to the tube temp as well (and, as previously said, tomorrow will probably be 'useless'!

Kind Regards, John

 

[bernardgreen]

The

whilst heat loss by convection and conduction will presumably be about the same

As I recall it the critical heat losses and gains are by radiation to and from the object.

Incoming radiation increases the temperature of the object.
Outgoing radiation from the object increases as it's temperature increases.

It seems to make sense that the object will heat up to the temperature where outgoing is the same as incoming.

 

[JohnW2]

As I recall it the critical heat losses and gains are by radiation to and from the object.

That sound right, in the context we are discussing, in which I imagine that convection and conduction are much less relevant/important

Incoming radiation increases the temperature of the object. Outgoing radiation from the object increases as it's temperature increases.
It seems to make sense that the object will heat up to the temperature where outgoing is the same as incoming.

Indeed so - that's the 'equilibrium' we're talking about. However, I think that one needs quite a bit more information before one can draw any conclusions.

For example, if, on moving from black to white (and for a given prevailing temp), the reduction in outgoing radiation were equal to the reduction in the amount of incoming radiation that was absorbed, then those things would presumably cancel out'?

Kind Regards, John

 

[SimonH2]

I suspect that it's "complicated".
Taking a leaf from climate science, the issue with greenhouse gasses is that they behave differently to different frequencies.
The incoming solar radiation is at high frequencies (visible light & high infra-red) because of the temperature of the sun. The outgoing radiation is low frequency (low infra-red) because our bit of conduit is at a much lower temperature. I recall from school science that the ability to receive or transmit radiation is reciprocal - so as John says, white will also radiate less than black as well as recieving less for a given frequency. But that doesn't mean it has to have the same difference across the frequency band.
But now we're getting into specialist physics.
P s. I was reading a bit yesterday and it mentioned that National Rail paint the rails white in certain hot spots to reduce solar heating - on the assumption that they'd not spend money without checking first, it woukd seem to work.
Also, similar reading. In hot places they tend to have white roofs. But went on to say that here we get more benefit from some solar heating of the roof when it's colder than we'd get from adapting them to the infrequent heatwaves.