Clipping- What would my number have been?

[russ]

I get the point of what Ian was saying. He didn't say "wind chill", he said "wind".

In reply to a different question - we forgive him though - he is a liberal type

[Ian S]

New science from Ian? Wind chill does not affect a solar panel. Cool temperatures OK but not wind chill.

Surely you're not disagreeing with my contention that wind can cool a solar panel!

[Ian S]

he is a liberal type

You bet I am and proud of it!

[inetdog]

Surely you're not disagreeing with my contention that wind can cool a solar panel!

Any effects which are calculated based on evaporation (wet bulb versus dry bulb temperature) will have no effect on cooling a panel because it does not sweat.
But the effect of heat removal by mass transport rather than convection alone will certainly make a difference. I think Russ is simply having a bad day.

On the other hand, conventional wind chill is based on heat loss from a body at or near 98.6F and so the weatherman's number will not apply directly.
A temperature of 0F with wind chill of -20F will cause an equilibrium panel temperature lower than 0F with no wind, but it will generally NOT be as cool as a panel in a -20F zero wind environment.
So, both sides win. Or, as I prefer to look at it, both sides lose because of oversimplification and underexplanation.

[Ian S]

I get the point of what Ian was saying. He didn't say "wind chill", he said "wind".

Yes, because I knew Russ would obsess over it. Although if you look into the general concept behind wind chill factors and such, they very much depend on wind removing heat from the surface of a body but it's more directed toward what humans perceive. However, there is no doubt that in your case with panels in full sun and ambient temperature below freezing, wind will cool them below what they would be in still air.

[Ian S]

Any effects which are calculated based on evaporation (wet bulb versus dry bulb temperature) will have no effect on cooling a panel because it does not sweat.

Apparently, in the US and Canada, humidity is not taken into consideration for wind chill index because the index is only applied at low temperatures where the humidity is also expected to be low.

[inetdog]

Apparently, in the US and Canada, humidity is not taken into consideration for wind chill index because the index is only applied at low temperatures where the humidity is also expected to be low.

True. Humidity plays a role in the high temperature Comfort Index, which also will not be particularly applicable to solar panels.
In the case of a survival index for low temperatures, the effects of evaporation resulting from air being forced through your clothing and your body sweating because you are working hard to survive might be an issue. But in those cases you want to regulate your clothing layers to prevent sweating anyway.

[russ]

From the wiki as I don't have interest in more search

"The speed of cooling has different effects on inanimate objects and biological organisms. For inanimate objects, the effect of wind chill is to reduce any warmer objects to the ambient temperature more quickly. It cannot, however, reduce the temperature of these objects below the ambient temperature, no matter how great the wind velocity. For most biological organisms, the physiological response is to maintain surface temperature in an acceptable range so as to avoid adverse effects"

[inetdog]

From the wiki as I don't have interest in more search

"The speed of cooling has different effects on inanimate objects and biological organisms. For inanimate objects, the effect of wind chill is to reduce any warmer objects to the ambient temperature more quickly. It cannot, however, reduce the temperature of these objects below the ambient temperature, no matter how great the wind velocity. For most biological organisms, the physiological response is to maintain surface temperature in an acceptable range so as to avoid adverse effects"

And since a solar panel does have a source of heat input, its temperature will always be higher than the ambient air temperature.
During the summer, it could easily be 20C or more above the ambient air temperature if the air flow under the panels is restricted.

So during the winter, with an ambient of 0C and a wind chill of -20C, the temperature of the panel may be lowered to, say, 10C with no wind and 5C with wind.
It will not be cooled below 0C, but it will get significantly closer to it than with no wind.
So you are perfectly correct that the ambient temperature with no heat input will be exactly the same regardless of wind speed. But that is not the situation that we have.

Oh, and an odd paradox to consider:
If the sky is clear the temperature of the panel just before sunrise may actually be cooler than the ambient air temperature. In that case the higher wind speed and lower wind chill index could actually cause the panel to be warmer.

We are not dealing with either adiabatic or isothermal equilibrium conditions in the system.

Russ, you are perfectly free to argue whether the difference is significant or not, you just cannot argue that there is no difference.
I do not think (I hope) that anybody is advocating calculating the temperature effect on V by plugging in the wind chill temperature itself.

[russ]

This started off as a question about wind chill affecting a panel which the wiki refers to.

[J.P.M.]

And since a solar panel does have a source of heat input, its temperature will always be higher than the ambient air temperature.
During the summer, it could easily be 20C or more above the ambient air temperature if the air flow under the panels is restricted.

So during the winter, with an ambient of 0C and a wind chill of -20C, the temperature of the panel may be lowered to, say, 10C with no wind and 5C with wind.
It will not be cooled below 0C, but it will get significantly closer to it than with no wind.
So you are perfectly correct that the ambient temperature with no heat input will be exactly the same regardless of wind speed. But that is not the situation that we have.

Oh, and an odd paradox to consider:
If the sky is clear the temperature of the panel just before sunrise may actually be cooler than the ambient air temperature. In that case the higher wind speed and lower wind chill index could actually cause the panel to be warmer.

We are not dealing with either adiabatic or isothermal equilibrium conditions in the system.

Russ, you are perfectly free to argue whether the difference is significant or not, you just cannot argue that there is no difference.
I do not think (I hope) that anybody is advocating calculating the temperature effect on V by plugging in the wind chill temperature itself.

My $.02, FWIW:

1.) The term "Wind Chill" or "Wind Chill Index" as they are commonly and usually defined have no meaning in a discussion of how wind velocity affects a solar panel temperature. Those terms apply when dealing w/ human perception of wind's effects on the human body.

2.) What all this comes down to in an engineering sense is doing a heat balance on a PV panel. Most of the time this is an iterative process, depending on the confidence level required and the instrumentation available. It also requires some knowledge of heat and mass transfer, among other things. Usually, for solar devices, the inputs are not well known. Because of this uncertainty, some effects, particularly those due to wind velocity and direction are the hardest to estimate. Also and unfortunately, those can often have the largest effect on panel temp. after the irradiance input.

3.) Depending on something often called the "effective radiant sky temperature", any object exposed to ("seeing") the sky may well be radiatively cooled to below ambient temp. That's most of the reason, for example, why a frost is possible on a clear still night even though the ambient temp. is above freezing. This often happens just before sunrise, but in theory can happen any time, depending on the heat balance on the object.The literature is full of empirical and other correlations to estimate the radiant sky temp.

4.) When I was working, doing a heat balance on power generating and handling equipment was an everyday thing. Some situations were vexing. Each was unique. Two sort of common characteristics about doing heat balances on solar devices is that the inputs are much more variable than for, say, a power boiler, and there are usually more of them to consider, particularly the radiative and wind convection heat transfer contributions which are often an order of magnitude or so more important than for industrial equipment.

5.) Some of my current retirement efforts are in trying to estimate just how the wind around my array affects the panel temp. Lots of fun. More questions generated than answers so far. I need more temp. sensors on the array which, given my luck will probably screw up the temp. profile I'm trying to measure.

[pleppik]

So..

Just out of curiosity, does anyone instrument their PV panels with temperature sensors? It would be interesting to have some real data.

[Ian S]

So..

Just out of curiosity, does anyone instrument their PV panels with temperature sensors? It would be interesting to have some real data.

I will do some checking next time I'm on the roof. I have a flat foam roof so it's easy to do. I thought I had done some temperature measurements some time back but can't seem to find the results. I've seen it claimed elsewhere that in full sun, panels run about 20C above ambient. That would allow for significant cooling by the wind.

[J.P.M.]

So..

Just out of curiosity, does anyone instrument their PV panels with temperature sensors? It would be interesting to have some real data.

I'm experimenting w/ the idea. Tried a couple of 10K sensors on the backside of the panels and compared #'s to IR thermometer readings. Mixed bag on results. The idea is to get reasonable looking #'s that appear to make some sense and correlate w/ irradiance, temp. and wind data from the Davis weather instrument adjacent to the array. I'm considering more sensors and a multichannel data logger for the sensor output. Still in design phase.

[pleppik]

I'm experimenting w/ the idea. Tried a couple of 10K sensors on the backside of the panels and compared #'s to IR thermometer readings. Mixed bag on results. The idea is to get reasonable looking #'s that appear to make some sense and correlate w/ irradiance, temp. and wind data from the Davis weather instrument adjacent to the array. I'm considering more sensors and a multichannel data logger for the sensor output. Still in design phase.

You can also just get a couple of Davis temperature sensors and feed it into your weather station. I did that for my geothermal system. The Davis add-on sensors are a little spendy, but because I already have the weather station there's no need for a separate data logger.