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Open Source project looking for help

post #1 of 5
Thread Starter 
I am working on an open source hardware project and am looking for some help. The project is to develop an open source design for a tracking Concentrated Photo Voltaic system (CPV).

We have a triple junction cell with a surface area of 30.25square mm or put another way, a square with 5.5mm length per side. We will be aiming anywhere from 1000 sun concentrations to 1200 sun concentrations into this tiny area. I understand that 1000 sun concentrations will generate around 1750C in that tiny area.

What I am looking to do is to find a passive cooling system to disperse this heat and ideally keep it at about 10C above ambient. The unit will be outdoors. All this and need to try keep cost and complexity down.

The obvious thing is to mount (weld) the cell onto a big chunk of copper (as we can't afford diamond ). But how big a chunk do we need? What is the ideal shape? Should we have fins? Would drilling holes in the copper be better to let the heat blow away or should we keep it solid? Should we attach a smaller piece of copper to a larger piece of aluminium?

If we could cover the top of the cell using a sealed in non flowing inert clear liquid, would we get greater heat dispersal by removing heat from both sides? What other things can we do to try cool this heat monster? Does cooling to 10C degrees above ambient sound at all feasable?

Please note that active cooling (fans, water cooling, peltier etc) in not an option as the complexity and cost would be too much. I expect to mount 600 of these on my roof and still try to do it economically. Each unit needs to be able to move and should have a life expectancy of 25 to 30 years in hot baking sun, rain, hail, snow etc. I am not asking much am I?

Really appreciate any help you experts can give me.

Regards

Chris
Group home page: http://groups.yahoo.com/group/SunGrid
Group email address: SunGrid@yahoogroups.com
post #2 of 5
Geeeeeeez. There are not many options really then? The basic passive cooling choices are heatsinks or heatsink with heatpipes. For the heatsink, more surface are yields better performance so fins are a necessity. Heatpipe technology can be used in conjunction with the heatsinks. They are designed for very long life. However, I would worry about the temperature range they will be required to operate. How about a schematic of the cells and a diagram of the layout?
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post #3 of 5
Thread Starter 
I have calculated that the power that will be concentrated into that 30.25square mm is around 28.5 watts.

The cells will be mounted individually each in their own small self contained 'box' with the heatsink hanging out the bottom. The cell will most likely be directly welded to the heatsink and the heatsink will act as the anode connection.

How do fins help? The reason I ask is that I thought air is a pertty poor conductor of heat and copper super good. Surely it would be better to use more copper to absorb the heat? Or are the fins for final distribution into the atmosphere? Sorry I am a bit confused about this.

Thanks

Chris
post #4 of 5
Quote:
Originally Posted by chris-is View Post
I have calculated that the power that will be concentrated into that 30.25square mm is around 28.5 watts.

The cells will be mounted individually each in their own small self contained 'box' with the heatsink hanging out the bottom. The cell will most likely be directly welded to the heatsink and the heatsink will act as the anode connection.

How do fins help? The reason I ask is that I thought air is a pertty poor conductor of heat and copper super good. Surely it would be better to use more copper to absorb the heat? Or are the fins for final distribution into the atmosphere? Sorry I am a bit confused about this.

Thanks

Chris
Yes, air is a poor conductor... however, it is a decent for convection! There is plenty of air so it relies on convection. Copper can only absorb so much heat so it must pass the heat into something else to carry the heat away. Generally this is air (but water can be used as well). The heat must go somewhere and that would be the atmosphere. The larger surface area allows the copper to transfer the heat into the air more effectively. This is why all radiators and heatsinks have fins or grooves.
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post #5 of 5
I understand the issue of wanting to keep the complexity down, but that would make a nice water heater. That seems like a lot of heat energy that could be put to some use some where...
Fins are a necessity unless you want to build a coper block to be used as a "heat battery" that is slowly cooled at night. Then your block would need to be about the size of your house. Take a look at the rate air can move heat in a passive environment and compare that with the surface area of your heat sink and the rate your device generates heat to determine the size you need. The balance you must achieve on fins will be where the air flows best yet has the most surface area to flow over. Drilling holes will create pockets of dead air space which is one of the best insulators used in production portable coolers and "Ice chest".
The air MUST flow over the surface to carry off the heat. The air flow will be created by some breeze perhaps but mostly, and reliably, by the action of the heat causing the air to rise so you will want the fins to be able to "vent" the heat vertically with out it reheating your device. The heat sink will likely be much larger than your device as well, but you will have to do the math to find exactly how much larger.
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