Thread: Solar Inverter Sun Screen

[QUOTE=BobL;1845263]

Originally Posted by Neil317

BTW I would consider painting the ally sheet white to reduce reradiating to the other side..

My thinking was Inside surface. Wasn't thinking reflector, thinking low emitter. The invertor doesn't care how hot the aluminium gets if its not emitting IR. Now without looking it up I'm pretty sure paint is a pretty good emitter. So maybe paint the outside white and leave/polish the inside? Granted I could be over thinking things again.

Stuart

Whoa! I was 100% wrong!
Thanks for putting me on track Bob.
Neil

[QUOTE=Stustoys;1845276]

Originally Posted by BobL

My thinking was Inside surface. Wasn't thinking reflector, thinking low emitter.

From what I understood of Bobl's post, the easiest solution is to simply plate both sides of the sheet with gold. Good IR reflection and low emissivity.

I wouldn't paint it. (although gold plating would look good ) The function it has to perform is to block UV, if it shades the inverter and keeps it cooler, that's just a bonus side effect.

In any case the emmissivity of Aluminium, ( whether oxidised or not is very low) , much lower than white paint... typically less than 0.2, whereas white paint is 0.8 or thereabouts.

Ray

I wonder if painting it any colour would make much difference. If polished or painted white say, the aluminium plate absorbs less heat from the suns radiation, and conversely if painted matt black say it will absorb more of the suns radiant energy, but a hot plate will begin to heat the surrounding air, which will form a convection current just as a chimney does, and that rising air will also draw in extra air ( the Bernouli principal) which will tend to cool both the inverter and the sunshade.
One could improve the cooling effect by increasing the length and making it in the form of a hollow tube perhaps, and even just painting black the upper section above the inverter would improve the draft, without any re radiated heat going the inverters way. I'm not sure just when the Local Government Authority would expect you to submit a D.A. though, and I don't know the best way to keep the swallows out either.
Great work as usual Bob by the way, and plenty of interesting thoughts from all contributors too.

I received this response today -

Hi Robert

We can sell a new top lid which has the clear Perspex for $140.75+GST+shipping. I suppose
this is purely aesthetic. Alternatively, you can download Sunny Explorer and monitor your
system with a laptop or PC which has Bluetooth connectivity. The software is very easy to
use, and provides more information and beyond of what you see through the screen.

The link to Sunny Explorer is here:
www.sma-australia.com.au/products/monitoring-control/sunny-explorer.html

As much as we are able to sell you a new lid, Sunny Explorer offers the monitoring you
need for your inverter system if that's what you ultimately need. That said, if you wish
to purchase a new lid, call us on 02 9491 4200 to process an order and pay by credit card.

Kind Regards,

Cyril Laugeon
Technical Service Engineer
SMA Solar Technology


time for me to embrace Bluetooth

The Bluetooth solution looks good Bob, but could you mill out the degraded perspex and Sikaflex in a small piece of plate glass in your present inverter lid, because a replacement seems pretty expensive for what you get. The free bluetooth app is nice, but it seems strange to me that it runs on a P3 windows machine running XP with sp2, but not any Linux or MacOS machines. One good thing I guess is that a P3 with XP sp2 would be pretty cheap at a salvage centre, if you were unable to source one from the roadside on a hard rubbish day

Hi Bob,

Nice job on the sunscreen.

LCD displays can also degrade from heat to the point where they go black and can't be read. I have observed this in the first generation of electronic electricity meters which are now about 20 years old. These are not exposed to UV being in switchboards. They also don't generate much heat, rather, it is the environment (welcome to the tropics) and manufacturing tolerances. Some go black faster than others even though the date of manufacture is similar.

Good to see SMA have spares available. About what you would expect from a premium brand. Schneider (spelling?) are also pretty good. The noisiest inverters around at the moment seem to be Solax which have a fan that runs up as the inverter powers up. Probably this is part of the power on diagnostics as it then switches off the fan before the inverter goes online. The Solax are setup for battery useage (I think, not too sure), so may have extra circuits to cool. The majority of inverters I have been around (likely a couple thousand by now) seem pretty quiet in use.

With regards to painting your sunshade? I don't see the point for a number of reasons. A roof exposed to the suns heat will reach an equilibrium temperature with it's environment. That temperature will be same no matter what the material the roof is made from nor the colour the roof is painted. Lighter colours will probably take longer to get to the same temperature as a dark colour, but it will be the same temperature none the less. A more emmisive roof will radiate heat back into the air making the air adjacent to the roof hot which then radiates/conducts the heat back into the roof. Your roof and mine is exposed to a certian level of insolation in terms of watts per square meter. Place any two objects in a furnace set to 100 degrees Centigrade and sooner or later they will both be at 100 degrees. It makes no difference what colour you paint the objects nor what they are made from.

To go even further out on a limb, to make a real difference to the emmisivity of an item the surface colour has to be better than painted on. I say that because if you look at the heatsinks used on electronic components, they are anodized aluminium and the black pigment has been absorbed into the anodized layer. Painting would be cheaper than anodizing and if it worked anywhere near as well would be the coating of choice. Aluminium is used for heatsinks because it's about the third best conductor of heat after silver and copper and is the cheapest of the three.

So, having played devil's advocate here, I'll just slip out to somewhere cool and shady hehe.

Cheers

The Beryl Bloke

Originally Posted by Theberylbloke

With regards to painting your sunshade? I don't see the point for a number of reasons. A roof exposed to the suns heat will reach an equilibrium temperature with it's environment. That temperature will be same no matter what the material the roof is made from nor the colour the roof is painted. Lighter colours will probably take longer to get to the same temperature as a dark colour, but it will be the same temperature none the less. A more emmisive roof will radiate heat back into the air making the air adjacent to the roof hot which then radiates/conducts the heat back into the roof. Your roof and mine is exposed to a certian level of insolation in terms of watts per square meter. Place any two objects in a furnace set to 100 degrees Centigrade and sooner or later they will both be at 100 degrees. It makes no difference what colour you paint the objects nor what they are made from.

Without going into the science, clearly you have never set foot on a galvanised roof on even a warm day.
Even right now with the air temperature at a pleasant 28ºC with a decent breeze blowing, the galv roof on my back veranda is 42º.
When the air is still the temperature difference will be a lot more than 14ºC.

Originally Posted by BobL

Without going into the science, clearly you have never set foot on a galvanised roof on even a warm day.
Even right now with the air temperature at a pleasant 28ºC with a decent breeze blowing, the galv roof on my back veranda is 42º.
When the air is still the temperature difference will be a lot more than 14ºC.

Hi Bob,

Just for fun, I'd like to hear about the science. My understanding is that it's all about the absorptivity to emissivity ratio,

So if we want an optimal solution we want the front to have low absorptivity, polished aluminium 0.09, white titanium oxide paint 0.17, so we leave the front unpainted.
and the back to have high emissivity, black paint = 1.0, white titanium oxide paint = 0.92, not a whole lot in it, but black is better.

So paint the back black, and leave the front alone. will lead to the coolest panel? does that sound right?

Ray

I'd like to hear the science as well.

Why do you want a high emissivity back Ray? Yes you get the coolest panel.....but thats not what we are after. The inverter doesn't care how hot the panel is, just how much IR it is receiving from it.(ok it probably doesn't even care about that )

My thoughts......

Originally Posted by Theberylbloke

With regards to painting your sunshade? I don't see the point for a number of reasons. -snip- Centigrade and sooner or later they will both be at 100 degrees. It makes no difference what colour you paint the objects nor what they are made from.

The furnace analogy while correct doesn't apply to this case.
Leave the furnace door open, place your objects(say one black and one white) outside the furnace. Now measure the the temp of the back surface(in IR, not the actual temp of the surface). In this case you don't care if the white object is the same temp as the black object(which it wont be), you only care how my IR it is radiating.

Originally Posted by Theberylbloke

To go even further out on a limb, to make a real difference to the emmisivity of an item the surface colour has to be better than painted on. I say that because if you look at the heatsinks used on electronic components, they are anodized aluminium and the black pigment has been absorbed into the anodized layer. Painting would be cheaper than anodizing and if it worked anywhere near as well would be the coating of choice.

Guess what...... they paint Aluminium heatsinks as well. Now painting some heatsinks would be a PITA so maybe thats when they anodize?, can't really say, but I can say that there are painted heatsinks. Further I think, with most heatsinks you'd be more interested in convection than radiation.

So question to BobL, are emissivity and convection related? or it emissivity strictly an IR thing? (I think the later but I'm not sure)

Stuart

Originally Posted by Stustoys

I'd like to hear the science as well.

Why do you want a high emissivity back Ray? Yes you get the coolest panel.....but thats not what we are after. The inverter doesn't care how hot the panel is, just how much IR it is receiving from it.(ok it probably doesn't even care about that )

Good question, I was thinking cooler shield would be better... leaving aside convection cooling..

Which solution results in the coolest inverter,

A.. Where the back is high emissivity, and therefore the shield is cooler, so emitting less infrared (?)
or
B..Where the shield is hotter and has lower emissivity...

I guess it's a black and white question, not sure the answer will be quite so black and white...

Ray

Here's a little predictive test for you guys

I just put 4 pieces of metal in direct sunlight.
A) White paired steel both sides
B) White Painted steel front, black back side
C) Unpainted slightly oxidised Al both sides
D) Unpainted slightly oxidised Al front with a black back

Air temp is 28 degrees.
The Ally is slightly oxidised and the white paint has some scratches etc to simulate real world situation.
Black paint is pot belly black

What do you think the temperature differences will be between the air temp and the temp of the front of each plate.
Hint, see example involving my roof in a previous post

Originally Posted by BobL

Here's a little predictive test for you guys

I just put 4 pieces of metal in direct sunlight.
A) White paired steel both sides
B) White Painted steel front, black back side
C) Unpainted slightly oxidised Al both sides
D) Unpainted slightly oxidised Al front with a black back

Air temp is 28 degrees.
The Ally is slightly oxidised and the white paint has some scratches etc to simulate real world situation.
Black paint is pot belly black

What do you think the temperature differences will be between the air temp and the temp of the front of each plate.
Hint, see example involving my roof in a previous post

In order of hottest to coolest C D A B, probably not a big difference between A and D of course if you used an IR thermometer, then you would have to make sure you correct for emissitivity on the Al..

Originally Posted by RayG

In order of hottest to coolest C D A B, probably not a big difference between A and D of course if you used an IR thermometer, then you would have to make sure you correct for emissitivity on the Al..

My prediction was similar, C A,D B, and while there was bugger all in it, the measurements came out at A=D, C, B

All measurements were done with a thermocouple in the middle of the plate, the reading took about 5 seconds to reach a steady value but I left it there for 10 seconds just to make sure. Plates were left for 10 minutes to equilibrate in direct sunshine on top of a bare wood outdoor table.

Ray is right about not a lot of difference between A and D, in fact they were both the same at 13º above air temp
But they were also the coolest

Then came C at 14º above air temp, and then B at 15º

In other words not much difference between them.

Can I explain this, not really, the numbers are too close and there are too many variables.
We tend to focus on colours, but reflection and emission are also very sensitive to surface roughness and dirt, which are difficult to quantify and even harder to maintain in the real world.

Then there is "what else is around" that will reflect and radiate heat. e.g. painting the back black may absorb significant heat radiating away from a nearby brick wall.

Just moving D) [ally front/black back] from the top of the bare timber outdoor table ~600 mm above the (warm) patio brick paving rapidly raised the temperature difference from 13 to 16º.

I think it was Stu that said the temp of the plate was not that relevant and it should be the temp at some distance behind the plate that we should be looking at. If there was more than a couple of degrees difference in what we've seen so far I might pursue it but give that its so close I'm done.