Explainer: PV and Solar Thermal
PV at the California Academy of Sciences | Photo: Mitzi Young/Flickr/Creative Commons License

With the exception of geothermal and tidal energy, most renewable energy is solar at its root. Wind is caused by the sun heating the air. Biomass stems from sun-fed plants. Hydro works because the sun evaporates water from oceans and lakes, which then falls on the land as rain or snow.

But when we talk about solar energy, we mainly mean energy derived from the sun without all those intermediary steps. We use two basic methods to harness the energy of the sun that directly: photovoltaics (PV) and solar thermal. In PV, sunlight strikes semiconductors that turn light into electrical power. Solar thermal harnesses solar radiation as heat.

We'll take a look at PV first. The "photovoltaic effect" -- that certain substances give off electrical energy when illuminated -- has been known since the 1830s, and people started building the first, very inefficient PV cells fifty years later. The first practical PV unit was built in the 1950s at Bell Laboratories. and found application in the space program as a power source for the first satellites.

The first PV units that were cheap enough to be commercially viable appeared in the early 1970s, and their cost per watt has pretty much dropped since then.

The photovoltaic effect isn't a particularly easy thing to explain. (Albert Einstein did so in 1905, work for which he won the 1921 Nobel Prize in Physics.) But the basics are reasonably straightforward: each PV cell is essentially two stacked electrodes composed of semiconductors. Light, made up of photons, hits the electrode in front. Some of those photons is absorbed by the atoms that make up the electrode, increasing the atoms' energy. If the photon striking an atom has a certain amount of energy, it knocks an electron off the atom, and the freed electrons migrate to the back electrode. This sets up a difference in electrical potential -- a.k.a. voltage -- between the electrodes, and if the electrodes are connected by wiring, electrical power begins to flow.

That's a vastly oversimplified explanation, but it'll do for our purposes. If you want to dig more deeply into the physics of the process, Wikipedia isn't a bad place to start.

The first commercially viable solar cells used silicon crystals as their semiconductors. Many of them are still in use today, and various types of crystalline silicon still make up a large percentage of PV cells sold today. Cells cut from a single crystal of silicon -- called "monocrystalline silicon" -- are the most efficient PV cells, converting as much as 17 percent of the light that hits them into electricity. But monocrystalline silicon is quite expensive to build, and so PV cells based on other forms of silicon -- "polycrystalline" and "amorphous" -- are far more economical to use, despite producing a little more than half as much power per square inch.

Monocrystalline and polycrystalline silicon PV cells have another disadvantage: they're bulky, heavy, and somewhat fragile. Amorphous silicon, however, can be fabricated in thin enough layers that PV cells using it can be made far lighter. This is the "thin-film" PV you may well have heard of. Thin film PV trades weight and cost for efficiency: it puts out less power per square inch than mono- or polycrystalline silicon cells. However, the cost is generally so much lower that it's still cheaper to install enough additional cells to make up for the lack of efficiency.

Thin film PV can be made with semiconductors other than silicon. Cadmium telluride (CdTe) is one very widely used PV semiconductor: First Solar, the second-largest PV company in the world, uses cadmium telluride exclusively in its PV cells. Cadmium telluride does have a couple of problems. Cadmium is a toxic heavy metal, and can be released into the environment as a cell degrades with exposure to the elements. (This seems to be happening more quickly than anyone expected in the California desert.) The other element making up CdTe, tellurium, poses its own toxicity issues, but the real problem with tellurium is that it's rarer than platinum. If CdTe production continues to climb, a tellurium shortage may well be in the offing.

Other thin film PV semiconductors now seeing some use include copper indium gallium selenide (CIGS), which may offer the possibility of "spray-on" PV cells on buildings and other structures, and which has been pushed to about a 15 percent efficiency in the lab, and a number of organic compounds, which may allow extremely economical power cells but which have yet to overcome durability and efficiency drawbacks.

Solar thermal technology is far easier to understand, in essence, than PV. You don't need quantum physics to explain solar thermal to the lay person: if you set something in the sun it warms up. Focus enough sunlight on that thing and it warms up a lot. If you use sunlight to boil water or some other fluid, the resulting hot liquid (a.k.a. heat transfer fluid) can be used to run a turbine to generate electricity.

Two main methods are used these days to generate solar heat to run a turbine. The most common is focusing the sun, usually using parabolic mirrored troughs but occasionally relying on fresnel reflectors, on a series of transparent pipes running throughout the power generating installation. The pipes channel the heat transfer fluid through the focal point of the reflected sunlight, and by the time the fluid runs all the way through the installation it can be very hot indeed. It's then used to run a turbine that generates electricity, cooled once it leaves the turbine and then piped back into the field of mirrors.

The other main design for solar thermal power generation is the power tower, in which a field of mirrors all aim their reflected sunlight at a single point, namely a boiler atop the eponymous tower. The fluid in the boiler heats and, as in the parabolic trough and other designs, powers a turbine.

According to the National Renewable Energy Laboratory, power towers are somewhat more efficient at generating electricity than parabolic trough systems, and do so at a lower cost per kilowatt. However, as the towers tend to be eyesores -- occasionally blinding ones -- and a parabolic trough or fresnel setup can be hidden behind a fence, it's a lot easier to site a trough setup than a power tower facility.

The largest use of solar thermal technology in the US has little to do with electrical power other than that it reduces consumption of other energy. Buildings built with the sun in mind can use its heat to warm the interior, and using heavy walls as thermal mass means that heat can be retained until quite late at night. And many buildings in California sport solar thermal water heaters on their roofs or next to their heated swimming pools: for decades, such water heaters were the most cost-effective way for homeowners to use the sun to reduce their energy bills. Though they increasingly have to share their place in the sun with PV, there are more than half a million solar thermal water heaters in use in the United States and that number keeps growing.