Solar Cell

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The BEAM Glossary says:


  • A device designed and used to convert (at least a portion of) available light into electrical energy without the use of either chemical reactions or moving parts.
Solar cells can be purchased from long time BEAM supporter Solarbotics.com, and other suppiers of hobby and robotics related parts and materials


Contents

[edit] History

The development of the solar cell stems from the work of the French physicist Antoine-César Becquerel in 1839. Becquerel discovered the photovoltaic effect while experimenting with a solid electrode in an electrolyte solution; he observed that voltage developed when light fell upon the electrode. About 50 years later, Charles Fritts constructed the first true solar cells using junctions formed by coating the semiconductor selenium with an ultrathin, nearly transparent layer of gold. Fritts's devices were very inefficient, transforming less than 1 percent of the absorbed light into electrical energy.

By 1927 another metalÐsemiconductor-junction solar cell, in this case made of copper and the semiconductor copper oxide, had been demonstrated. By the 1930s both the selenium cells and the copper oxide cell were being employed in light-sensitive devices, such as photometers, for use in photography. These early solar cells, however, still had energy-conversion efficiencies of less than 1 percent. This impasse was finally overcome with the development of the silicon solar cell by Russell Ohl>/span> in 1941. In 1954, three other American researchers, G.L. Pearson, Daryl Chapin, and Calvin Fuller, demonstrated a silicon solar cell capable of a 6-percent energy-conversion efficiency when used in direct sunlight. By the late 1980s silicon cells, as well as those made of gallium arsenide, with efficiencies of more than 20 percent had been fabricated. In 1989 a concentrator solar cell, a type of device in which sunlight is concentrated onto the cell surface by means of lenses, achieved an efficiency of 37 percent due to the increased intensity of the collected energy. In general, solar cells of widely varying efficiencies and cost are now available.

[edit] Structure

Modern solar cells are based on semiconductor physics -- they are basically just P-N junction photodiodes with a very large light-sensitive area. The photovoltaic effect, which causes the cell to convert light directly into electrical energy, occurs in the three energy-conversion layers.

Diagram courtesy U.S. Department of Energy

The first of these three layers necessary for energy conversion in a solar cell is the top junction layer (made of N-type semiconductor ). The next layer in the structure is the core of the device; this is the absorber layer (the P-N junction). The last of the energy-conversion layers is the back junction layer (made of P-type semiconductor).

As may be seen in the above diagram, there are two additional layers that must be present in a solar cell. These are the electrical contact layers. There must obviously be two such layers to allow electric current to flow out of and into the cell. The electrical contact layer on the face of the cell where light enters is generally present in some grid pattern and is composed of a good conductor such as a metal. The grid pattern does not cover the entire face of the cell since grid materials, though good electrical conductors, are generally not transparent to light. Hence, the grid pattern must be widely spaced to allow light to enter the solar cell but not to the extent that the electrical contact layer will have difficulty collecting the current produced by the cell. The back electrical contact layer has no such diametrically opposed restrictions. It need simply function as an electrical contact and thus covers the entire back surface of the cell structure. Because the back layer must be a very good electrical conductor, it is always made of metal.

[edit] Operation

Solar cells are characterized by a maximum Open Circuit Voltage (Voc) at zero output current and a Short Circuit Current (Isc) at zero output voltage. Since power can be computed via this equation: P = I * V

Then with one term at zero these conditions (V = Voc / I = 0, V = 0 / I = Isc ) also represent zero power. As you might then expect, a combination of less than maximum current and voltage can be found that maximizes the power produced (called, not surprisingly, the "maximum power point"). Many BEAM designs (and, in particular, solar engines) attempt to stay at (or near) this point. The tricky part is building a design that can find the maximum power point regardless of lighting conditions.

For solar cell selection and comparison information, see the solar cell section of the BEAM Reference Library's BEAM Pieces collection. Also see the Starting Block article on solar cells.

[edit] Efficiency

When it comes down to buying solar cells, most of your decisions will come down to trading off a number of factors -- primarily:

  • Price

Oftentimes, you get what you pay for ; sometimes, you get more; other times you get less. You really need to do your math before you decide that a higher-priced cell is too expensive (given its performance), or that a cheap cell is a good buy. I've pulled together comparison data in large part to help the BEAM community decide what is and isn't a good deal.

  • Availability

You can often find really good deals on solar cells from surplus houses -- but when their supply is gone, it's gone. If you want to take advantage of such situations, you should consider keeping a reserve fund on hand for "opportunity buys." If / when you later find a high-performance cell at a good price, you'll be prepared to buy more than you need in the near term. Essentially you'll have to take a stance of "buy now, design (the 'bot) later" -- since what kind of cells will be (temporarily) cheap in the future isn't predictable.

  • Size

The solar cell area you require will be a function of two things -- the amount of power you need (which in turn is a function of how much power your 'bot draws, and how often you want it to be active), and the performance of the solar cells you pick. Lower performing cells have larger cell area -- this may or may not "fit" with your desired BEAMbot design aesthetically, and can also cause mass problems.

  • Mass

Heavier cells put more of a load on your BEAMbot, which usually means more of a load on your motor(s), which means more power consumption (which primarily has to be addressed by adding more cells...). Bear this in mind when you're looking at solar cell performance vs. size, particularly if you're considering buying encapsulated solar cells.

  • Cell Voltage

Most BEAMbot designs require at least 3 volts from their solar cell(s). This means, of course, that if you buy 0.5 V cells, you need to wire together at least six of them to do the job. This may or may not be something you want to mess with; it may or may not fit with the aesthetic design you're shooting for.

To start with, I tabulated advertised data on solar cells sold by all the small-cell vendors I could find (including some surplus houses). The vendors I found were (alphabetically) All Electronics, Electronics Goldmine, Plastecs, Radio Shack, Scientifics, and Solarbotics. Note that I collected data on 3 cell types -- regular solar cells ("C**" ID numbers), flexible cells ("F**" ID numbers), and encapsulated cells ("E**" ID numbers).

Image:solar_cell_table.gif

Note that solar cells are available in a number of output voltages -- the most common being 0.5 V (this being a function of solar cell physics), but higher voltages also being available. Since most BEAMbots require solar cell output voltages of 3 - 5 volts, if you buy 0.5 V cells you must connect a number of them in series in order to provide a useful voltage.

In order to compare cells of various voltages, sizes, and costs, I had to construct some performance metrics. While Voc * Isc is not the maximum power a cell can produce (it's easily 20% higher than that), it is at least roughly proportional to a cell's peak power output. So, in order to find solar cells that produce a high power level for a given cost, and a high power level for a given size (and roughly, mass), these are the two metrics I used:

  • Voc * Isc / Width * Length -- measured in mW / mm*2, metric of performance vs. cell area (thus also a rough metric of performance vs. mass)
  • Voc * Isc / Cell price -- measured in mW / $(US), metric of performance vs. cost

When I plotted these cells on a chart using the selected metrics, here's what I came up with (note that here, the best performing cells for a given size and cost will be located toward the top right of the chart):

Image:solar_cell_comparison.gif

What can we deduce from this?

  • Cells tend to group pretty neatly into two clusters -- a higher-efficiency (i.e., higher performance per unit area) one, and a lower-efficiency one. The higher performance cluster also has slightly better performance against cost, so in general the smaller required cell area doesn't translate into higher price tags.
  • Encapsulated cells lie solidly in the lower performance cluster. Their advantages (water resistance, slight improvement in performance in light collected from off-normal angles) must be weighed against their increased size and mass (due both to size and their plastic encapsulation).
  • Flexible cells also lie solidly in the lower performance cluster. The design flexibility (no pun intended) you gain from them must be offset against the larger size required for a given power requirement.
  • Occasionally you can find a really good deal in surplus cells. C01 and C02 on this chart were surplus calculator solar cells -- good performance vs. size, and a really exceptional deal (mW / $). Only annoying thing about them was their low (0.5 V) output voltage -- should have bought more when I had the chance.


[edit] External References


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