PV Gap

Francisco Said:

What is the average efficiency of commercially available Solar Photovoltaic cells / panels today ?

We Answered:

large no of cells have been manufactured with areas 2*2cm, efficiencies approaching 10%, and operating at 28 degrees celsius..

for terrestrial applications, silicon solar cells have shown operating efficiencies of about 12-15%

these are actual efficiencies but the theoretical efficiencies are of the order of 25%

Rafael Said:

The use of photovoltaic cells is limited by which of the following?

We Answered:

I'd go with A: low cost, or lack thereof.

B is reasonable, too, if you're talking about dollars per watt efficiency

C no.

D There are some scales at which solar is a poor choice, such as getting something cost effective in the 10-watt range. But photovoltaic scales better than most other technologies.

E While this is true, I don't think it's a limiting factor. If all the other economics lined up, there are vast areas of land that people would be willing to use for solar. Solar farms in Germany are an example. Also, the large areas of land do not need to be all in one block. It could be 10,000 individual rooftops throughout a city.

Annie Said:

Would a concave lens help the efficiency of a photovoltaic cell?

We Answered:

Only if the surface area is greater than that of the solar cell. Given that solar panels are a series of solar cells edge to edge, it would be pointless to try to put a lens over each of them. No gain would result.

However, if a really HUGE lens was created, much larger than the surface area of the solar panel, then it might concentrate more light on the panels.

This notion would fail, however, if the lens was not directly pointed at the light source. Solar cells can catch light, at diminished strength, coming from a fairly wide angle. However, the lens would block that light, only being able to focus a light source that is almost directly perpendicular to it.

That's about how I see it anyway.

Kirk Said:

How does the geographic position of a photovoltaic cell affect its efficiency?

We Answered:

It doesn't affect it at all.
It may affect the total output of energy over a period of time, but the efficiency (watts/(lumen?m²)) is a function only of the design of the cell.

Roberta Said:

How does the operating temperature of the photovoltaic cell affect its efficiency?

We Answered:

For crystalline silicon cells, the carrier mobility goes up as the temperature rises, the same as a bipolar semiconductor device. And like a diode or bipolar transistor, the voltage goes down when it gets hot. If you have an electrical engineering friend, or are an EE student yourself, you can look in a Semiconductor 101 book and see why this is so.

In a solar cell, the current goes up with heat, but not enough to balance the loss of voltage, so ultimately, the cell is less efficient when it gets hot. This is why residential panels are mounted several inches above the roof - so they can stay as cool as possible.

In the spec sheet below, look at page 2. You'll see that peak power goes down about half a percent for every degree C rise in temperature. This is about the same for any silicon cell. That formula is an approximation, of course.

David Said:

A typical photovoltaic cell has an efficiency of approximately;?

We Answered:

Common commercial cells today are in the range of 15% to 24% for crystalline silicon. When that goes under glass for a solar panel, a couple percent is lost. I think the answer they're looking for is B.

Answer A is for the 1970's.
Answer D is for exotic cells today.
Answer C is for the future.

Jeffery Said:

are sollar cell's (photovoltaic cells) efficiency going to increase a lot in the future?

We Answered:

Solar cells are already incredibly efficient at gathering energy, but are expensive to produce. Current solar cells that are sold commercially only collect light at a limited range of wavelengths, and are therefore not very efficient. NASA has developed solar cells that collect every wavelength of light and are amazingly efficient, in fact they are used on the Mars rover that has now been operating for years after it was supposed to run out of energy. This is because the solar cells are so efficient. The only catch is they cost millions of dollars. Research is currently being conducted to find a cheaper way of making these cells, and is making progress.