What is worse?

[sandovalch]

My first homemade solar panels were almost totally sealed up, so no dirt and dust would go into the panel. The problem I found with these panels, was that they would retain very much heat and on very hot days the output would drop quite considerably.

On the other hand I started to do some modifications on my newest panels and left some holes in the back to allow air to recirculate in them. But the problem was, that we have quite much dust here and it was getting into the panels and was accumulating on the solar cells.

So my question is: Where would you go, seal the panels up, not allowing anything to go into the panels, and having losses due to heat or leave them open for dust to come into them and opening them up after some time and "cleaning" them up, but allow the heat to go out?

I would very much appreciate your comments...

[pyrocasto]

Why dont you just put some filters on the back to keep out dust. If water doesnt accumulate inside it wouldnt be a bad idea.

[stm]

Dust can cause a permanent damage to the panels since you cannot disassemble the panels to clean them up.

Since you are creating your own panels, then a larger space etween each cell and a white background could decrease the problem with the heat?

/Steffen

[ghurd]

I would leave them sealed. Just my opinion.

Always allow a very good distance between and beneath them for circulation, at least 30cm.

A little more angle tilt will help circulation.  15 degrees steeper than the latitude I have read.  It will double your angle but not reduce the exposure to the sun much.

Guatemala is listed at 15 30N, that would be about 30 degrees tilt, for those interested.

G-

[DanG]

With roof mounted panels, especially your low slung ones trying very hard not to attract attention you have only a few options. One fix common to the US southwest desert region is add extra capacity, more panels and overvolting; say 60V or 72V system to be able to utilize 48V on hottest days. Expensive but effective.

I admire your work: http://www.otherpower.com/images/scimages/2333/P3160308.JPG

use radiative, conductive and convective cooling schemes. Also use every trick to keep non-generating-area sunshine from being absorbed.

More heat could be radiated from panel backs, ie: white paint + adding more surface area via rough sanded surface or fine grooves. Radiated heat is mostly infrared and acts like light so remember the IR shine shouldn't shine on adjacent cooling fins or be reflected right back by roofing. Having panels in close rows puts heat streaming off back of 1st panel onto 2nd panels front, that then dumps its heat onto 3rd panel, etc. I've read where an objects heat load can be reduced by 20% using passive heat shedding design. Painting the front aluminum frame bright white would be something easy too.

After the concrete is fully cured, painting your roof bright white would reduce the areas overall temperature, panels might absorb more reflected light as heat but have cooler air around them to help convection cooling. Probably slight overall gain.

Consider how to make "smokestack" convection work in your favor, a false roof platform  that helps reflect light back towards sun (missing the panels) while promoting a better draft both above and beneath and channeled to reduce panel temperatures....

Conductive cooling could be making sure individual cells are not suspended in air, but have the backs in contact with a larger mass to keep the absorbed heat flowing instead of just super-heating the air around it...

[DanG]

I missed a something - I would not vent the panels. Flush with nitrogen or argon maybe  if I see moisture inside, but not leave open to atmosphere for more than a week or two.

The .4% output loss per degree Centigrade now is nothing compared to 100% a few years down the line, and all the years after it, when corrosion randomnly snips cells out of the arrays. Evergreen Solars 10-year 90% / 25-year 80% warranty is a strong hint some of these cells will be around thirty or fourty years.

[BeenzMeenzWind]

I mentioned this elsewhere, but..

I've helped a guy I know build a couple of 'Ebay-panels'. His method is to stick sachets of silica gel around the outside of the frame to keep down moisture and to partially evacuate the panels once they're sealed up by dragging the air out through a little glass tube, then heating the tube and twisting it off to seal it.

He reckons that the lower air pressure makes the panel less prone to heating, as more of the heat is radiated away instead of warming up a pocket of trapped air, which obviously serves to keep the cells warm.

His panels are finished in laquered, polished aluminium on the outside, the back (facing the roof) is matt black to improve black body radiation and everything inside (except the cells, obviously) is either polished or painted white.

His reasoning seems to make sense. What do you guys think?

[DanG]

A plate heatsink painted gloss white will lose 3% of the heat transmisivity of the same one painted flat black - but I bet in Sandovalch's case black paint would absorb more from the surrounding hot roof then using even glossy white paint - the roof has the panels beat for surface area Use flat white if you can, and keep the paint as thin as you can while still getting unmottled coverage.

As far as pulling a partial vaccum on the cell arrays - it would be the same as putting hot coffee in a vaccum 'thermos' bottle to keep it hotter longer. Only about 30% of an flat objects heat can get radiated away in IR, continuous solar input without the conduction-convection 70% would push cell surface temps higher. I've read in older solar research papers where each 10C above 50C reduces useable output lifetimes by 50%, but I don't know if that still applies at all to Evergreen-type PV cells.

[BeenzMeenzWind]

Sounds as if he was going about it the wrong way then. I wasn't sure what the ratios of heat transfer due to radiation, convection and conduction were. I just looked it up.

By the way, I have a wierd question! I have a little solar powered squirrel-cage fan. I was playing with it the other day. It will spin fine outdoors or under 5 of the six halogens in the kitchen, but not under the other one. Why?

The six lights are all the same make, in the same fittings and the same wattage. All appear to give the same light output and are the same colour. What gives?

[BeenzMeenzWind]

Oh, yeah! If partially evacuating the panels would have a negative effect heat wise, what would happen if you pressurised them? Just wondering.