Renewable Tech – solar PV

by Richard Perkins
This entry is part 3 of 6 in the series Renewable Tech

In my previous posts in this series, I described hydropower and wind power. Both of these renewable technologies could be classified as electromechanical systems. They convert the mechanical energy of falling water or moving air into electrical energy through the principle of electromagnetic induction.

Today, I’d like to introduce a technology of an entirely different class: one with no moving parts at all. Photovoltaics, or PV panels, are solid state electrical generation systems. With the growing popularity of PV, you’ve probably seen a PV installation like the one shown to the right from Solar Depot. Unlike spinning wind or hydro turbines, PV panels reveal nothing to the naked eye about their mysterious energy transformation. Everything is hidden behind that innocuous looking veil of blue-black crystal an shimmering glass. Now that you understand how hydropower and wind power work, you may have been wondering what makes PV panels tick?

Solar PV power simplified

In order to understand the photovoltaic effect, let’s first talk about a class of material called the semiconductor. Materials of this type allow electricity (as in the movement of electrons) to flow through them sometimes. And sometimes they block the flow of electricity. It all depends on what state they’re in at the time. (I know. This analogy makes semiconductors sound moody and fickle, doesn’t it!)

To visualize this process, imagine a firebucket brigade. When every firefighter has a bucket and no one has a free hand the buckets of water can’t move from one end of the brigade to the other. That’s like a semiconductor in its insulating state. But when someone puts one of the buckets down, they suddenly have a free hand. We’ve created a hole in the line. The next firefighter can hand over a full bucket to the one with the free hand and suddenly we have movement! A full water bucket has moved in one direction while the gap, or hole, in the bucket brigade has moved in the opposite direction. This concept is called an electron-hole pair in semiconductor terminology.

So what makes the firefighter put down that first bucket? What makes an electron jump from the valence band to the conduction band in a semiconductor? Sometimes it happens spontaneously (think of a firefight getting tired and setting the bucket down for a moment). But in such cases the firefighter is more likely to pick up their own bucket again than to grab the bucket of the next firefighter in the line. This is what’s known as recombination in semiconductor terminology, and its one of the things you try to prevent in PV cell design.

Now imagine the scenario where a foreman comes along and says, “You there! Put that bucket down!” The firefighter closest to the foreman will not only drop the bucket, but he’ll probably reach for the next bucket in the line instead of the old one. In the case of PV cells, you can think of sunlight as the line foreman, who comes along and pulls one random bucket out of the line by knocking one of the electrons in the valence band up into the conduction band.

Now let’s go back to our firebucket brigade. How will the firefighter know whether to take the bucket on the left or the right? The firefighter won’t, unless the fire is visible in one direction and the pond is visible in the other. And if the buckets aren’t whisked along quickly enough, that bucket that was pulled out of the line could fall back into it’s old position and bring the line to a halt again. In our PV cells, one part of the semiconductor material has been doped to make it either electron rich or electron poor. The electron rich side is the pond, and the electron poor side is the fire. Once sunlight (our foreman) liberates some electrons and creates mobile electron-hole pairs, electrons will flow in one direction and holes will flow in the other. Now if you hook up some conductive electrodes on either side of the cell, the result is something that behaves an awful lot like a DC battery when sunlight hits it.

Small arrays of PV cells can be used to power electronics that run on DC electricity like calculators, watches, parking meters, and landscape lights, just to name a few. But to be useful for household power applications, the output of a PV cell has to be converted to AC. This is where inverters come into play. Now modern solid state inverters merit their own seperate entry in this renewable tech series, so won’t go into very much detail describing how they work here. Suffice it to say that they take the DC power from the solar panels and chop it by switching it on and off at a frequency synchronized with the utility grid. They then filter the resulting output so that it meets utility quality requirements and pump the power out so people can use it in their homes and offices.

Crystalline silicon PV cells

Crystalline silicon panels like the Evergreen unit shown to the right are still the most commonly manufactured and installed PV systems. These devices are manufactured from the same high grade silicon that goes into today’s high tech computer chips and microprocessors. Unfortunately that means that demand is high and supplies are sometimes limited. Crystalline cells are fairly efficient at converting sunlight to electricity compared to other PV cell types. But they also have high manufacturing energy and labor requirements, which makes their raises their cost of fabrication and installation. Still, they have a longer field record of proven reliability than any other type of PV on the market, which explains their dominant market share.

Thin film PV cells

Thin film PV modules, like the one shown here from First Solar, are manufactured very differently than their crystalline counterparts. Sometimes called second generation devices, thin film development has focused on ways to reduce the manufacturing costs of PV. Rather than slicing, doping and sandwiching bulk silicon wafers between electrodes and transparent layers of glass, thin film PV modules are made by depositing a thin film of photovoltaic material onto a substrate. The film can be deposited through electroplating, vapor deposition, or even through ultrasonic nozzles ( think of a very high precision ink jet printer). The substrate can be unprocessed silicon, glass, or in some cases a flexible film. The result is a PV cell which requires a lot less active material, reducing material and labor cost over traditional cell manufacturing techniques. However, thin films cells generally have a lower conversion efficiency than crystalline cells. So you need a bigger patch of thin film cells to generate the same amount of power as a crystalline panel. But for applications like large solar farms where land space is not a obstacle, the efficiency hit is an acceptable trade-off for lower production costs.

Multi-junction and concentrator PV systems

Multi-junction PV cells are not new per se. These high efficiency cells were originally developed for space applications well before we earth bound mortals started using them. But traditionally, their manufacturing costs have made them prohibitively expensive for marktet adoption, except in places where price is not a factor (like space and some military applications).

What makes multi-junction cells more efficient? They are made by sandwiching several different types of PV cells, each specifically tuned to a certain color or portion of the electromagnetic spectrum. The combination of cells are chosen to cover the entire range of  energy available in the incoming solar radiation, a feat which is not possible for a single type of cell like crystalline or thin film PV. This allows multi-junction cells to reclaim some of the energy that would otherwise be wasted by a traditional PV cell. But putting multiple types of PV cell together in the same device is a challenging and time consuming manufacturing process, so the cost of such cells is high.

But multi-junction and other high efficiency PV cells have come back into the spotlight recently with a concept called concentrator PV. As the picture from Solar Systems at the left shows, the idea here is to combine an expensive yet very high efficiency PV cell with a bunch of inexpensive focusing mirrors and mechanical supports. This one tracks the sun and focuses at 500x intensity onto a single high efficiency PV module. Of course, there is added cost for cooling (ever fry an ant with a magnifying glass?) and the motors to drive the tracking system. But the added efficiency of energy conversion has the potential to make systems like this very cost competitive in a dollars per sense. Like thin film, this technology is still relatively new on the scene and doesn’t have a very long track record. But a handful of innovative technology companies acattered around the globe are working to commercialize their own versions of cost competitive concentrator PV.

Other systems

There is an incredibly fertile field of ongoing research aimed at pushing the boundaries of PV performance and cost of production. You’ll read about light absorbing dies, organic or polymer PV cells, quantum dots and others just emerging in experimental labs all over the world. Most of them are more than ten years away from any possible market introduction. But who knows which one of them might turn into a breakthrough in affordable renewable energy generation? Ultimately, the demand for energy in our world is only going to grow. Our future will require all of the methods we can discover to provide for that demand in an environmentally sound and sustainable manner. So for those of you researching ground breaking techniques in PV and other renewable energy sources, keep the light burning. I’m cheering for you. :D

One Response to “Renewable Tech – solar PV”

  1. [...] other things to occupy my time. Today it was a new entry in my Renewable Tech series, this time on Solar PV. Wander over to the Professional page to have a look and drop me a comment while you’re [...]