Quantifying the reduction in panel solar load due to sun shields

[123MB]

Hi All

Part of my job is to calculate the size and number of fans which are required to keep the inside of outdoor electrical enclosures at a suitable temperatures.

It's a fairly standard approach of quantifying the heat load produced by the equipment in the panel, the solar heat load, and the amount of heat which is dissipated from inside to outside by the panel surface. The balance of the heat load represents the amount which must be dissipated by the fans.

A fairly common approach for outdoor panels is to fit sunshields to the exterior faces. These shields are made of the same material as the panel and are placed so that there is an inch or two gap between the panel and the shield. The idea is that the sun hits the shield and not the panel and this is supposed to reduce the overall temperature inside.

My problem is how do I represent this in the calculations? It doesn't seem correct to assume that the sun shield results in zero solar load on the panel, as the air between the panel and the sunshield would be heated by the sun.

One idea that I have is to calculate the resulting air temperature between the sunshield and the panel, and use this to recalculate the amount of heat that the panel surface itself can dissipate. I could then only include a small portion of the solar heat load (i.e. area of the panel not covered by heat shields).This would be conservative as it would assume that the space between the panel and the sun shield would be enclosed while natural air movement would occur.

Does anyone have some feedback on this? how do you quantify the affect of sunshields yourself? What do you think of my proposed method above?

Thanks

Regards, Michael.

 

[IRstuff]

"My problem is how do I represent this in the calculations? It doesn't seem correct to assume that the sun shield results in zero solar load on the panel, as the air between the panel and the sunshield would be heated by the sun."

This is partially true; shields would typically be open at the ends so that ambient airflow would remove some of the heat from the outer plate. Nevertheless, you can treat the air as an insulation layer in series with the plates on either side, as a worst case, since that might represent a zero wind, natural convection case where the airflow might be stopped because someone put their clipboard on the top of the panel.

As for the calculation, I would typically set up something like this as a set of simultaneous equations, driven by conservation of energy.

heat_in(outer layer) = heat_out(outer layer)
heat_in(air gap) = heat_out(air gap)
heat_in(inner layer) = heat_out(inner layer)
etc.

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[VEBill]

The sun shield itself would reach a certain temperature. That heat that would then transfer to the other side. That inner surface temperature would then radiate energy onto the surface of the panel.

As noted, airflow may not be fully effective, so above ambient hot air is another energy path.

Selection of material would make a huge difference. Colour, reflectivity, insulation (thermal conductivity), emissivity (ability to radiate).

Outer surface colour and reflectivity may be limited by neighbourly considerations.

 

[itsmoked]

Sad you have to use lousy sun shields. It's always better to use sun roofs as then the radiation is down onto the smaller tops of the enclosures where they naturally are hotter due to what's in them and hence the smaller delta allows less heating by the top-down radiation. There is also enough vertical space to entirely eliminate all convection heating since the hot air will pile up under the top-shade never reaching down to the enclosures below. Top shades also block hard rain and hail.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[123MB]

Thanks everyone for taking the time to respond.

Keith, can you post a link to a 'sun roof'? I am not sure that I know what you're referring to. Cheers.