solar panel

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Hi Freaks,

Among all the different compounds used for making solar panels which one has the highest solar power to electrial power conversion ratio for a given size?

I am looking for some panels to make a simple battery charger for NiMH AA cells and would like to look at smallest size and highest power.

Thanks.

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Sunpower claims they are getting 20% with their panels.

Imagecraft compiler user

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Small size and high power doesnt come cheap with solar cells...
Triple junction GaAs (gallium arsenide) is the most efficient cells, but they do cost botha arms and legs. I think we paid about 2-2,500$ for solar cells to our cubesat, total area about 15x15cm.

Monocrystalline silicon is alot cheaper, but also less efficient. Multicrystalline is cheapest and have the lowest efficency, but gives the highest energy pr dollar i think.

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Wikipedia has a good summary of efficiencies to date
http://en.wikipedia.org/wiki/Solar_cell#High_efficiency_cells
Single crystal cells are approaching the theoretical max of 33%, the record for multiple junction cells is ~41%.

But back up, cell voltages are small and temperature dependent and you need to string them together to charge a battery. Any excess voltage will be wasted unless you use a DC converter which will have its own losses (It would be nifty to have a cell that was divided into a few hundred subcells that could be switched into optimum serial/parallel configuration using onboard FETs). As the battery nears full charge the efficiency will fall to zero unless you make use of the energy for other purposes.

So what voltage and currents do you need, over what time period and temperature range?

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I have bought this product:
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http://www.walmart.com/catalog/p...
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The battery is rechargeable lithium polymer. Don't know the specs (mAh) yet. I will be able to tell when I get the battery.

How do you calculate how much solar energy will you need to charge a given battery say 200 mAh @ 4.5V?

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I'd guess more like 1000mAh but we'll go with your number. It seems to charge through USB, so let's say 200 mAh at 5 volts, which is 1 watt-hour or 3600 joules. Typical no-clouds insolation is a kilowatt per square meter = 100 milliwatts/cm2 or an electrical output of 10 millijoules/cm2/sec assuming 10% cell efficiency. If you want to recharge in an hour you'd need 100 cm2 of cells, but to get 5 volts those 100cm2 would have to be divided into 10 cells at 25C, probably more like 12 cells under usual non-winter conditions. Off the shelf "6 volt panels" which are 9 volts open circuit would have 18 cells or around 200 cm2 for your needed current of 200 mA, the excess voltage would have to be dissipated by a zener diode, or better converted into more current through inductance. Possibly the player already has a buck converter that could handle 9 volts input. You could ask wal-mart technical support :)

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Here's a solar USB charger just out:
http://www.powerfilmsolar.com/downloads/pdf/USB+AA%20Flysheet_LOWRES.pdf

1400 mW, around 400cm2 of thin film cells, street price ~$75. The USB port seems powered by a DC-DC converter and may require one or both batteries installed since it puts out "up to 1 amp" at 5 volts.

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Don't you hate it when you have an idea and then you see it on those tiny shelves near the registers when you are checking out at Walmart? :(

Anyways, it seems pricey. Is it mostly the solar panels? If not, maybe I can make something cheaper.

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Assuredly you could make something cheaper. Here's a battery-powered USB charging kit for $19.50, with a lot of educational links:
http://www.adafruit.com/index.php?main_page=product_info&products_id=14&sessid=a85c723a56445362fa93f35dd078fe45
And a range of small solar panels for ~$10 a watt:
http://www.siliconsolar.com/1-5-watt-solar-modules.html
I'd suggest a fixed solar installation that charges a larger battery all day, and use that to quick charge other batteries through USB. A lead-acid battery could be directly connected if charged at less than about 0.01C (e.g. 100 ma for a 10 amp-hour battery), but lithium-ion with a simple charge controller would be cheaper in the long run.

I use a 75 watt panel (~$4/watt) to charge salvaged 1200 mAh laptop battery cells (free, can catch fire), soon to be replaced with 10 amp-hour Headway LiFePo4 cells (~$20, safe chemistry).

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The ladyada kit looks amazing. I wish they sold it assembled (sounds like a job opportunity :)