Where does solar energy stand today, and where does it need to go in order for us to make the transition to renewable energy? Let’s look at solar photovoltaic technology, since that provides most of the solar electric generation in the world.
There are three generations of solar photovoltaic technology. The first generation is wafer-based crystalline silicon, a now mature technology used by 90 percent of installed solar capacity.
First generation solar panels typically comprise solar cells wired together and protected from the elements by glass and other materials. Solar cells are made of semiconducting (light absorbing) materials, such as crystalline silicon, which release electrons when they are hit by sunlight.
Crystalline silicon wafer-based photovoltaic is non-toxic, abundant and reliable, but it does not have good ability to absorb light, so the silicon wafer must be thick, which contributes to its rigidity. These solar panels are complex to manufacture and relatively expensive; but crystalline silicon wafers’ solar-to-electric power conversion efficiency rate has reached 25 percent, the highest for commercial applications.
Second generation solar panels consists of thin-film solar cells made mainly from cadmium telluride (CdTe) and copper indium gallium selenide (CIGS), both of which combinations involve rare and/or toxic metals. Thin-film cells are made by depositing one or more thin layers, or a thin film, of photovoltaic material, onto glass, plastic or metal. They absorb light 10 to 100 times more efficiently than silicon, so they only need to be a few microns thick (a human hair is 90 microns), and are thus flexible and light. Their efficiency is about 20 percent. They are used commercially, mounted on the ground and on roofs like silicon photovoltaic, with cadmium telluride the leading thin-film technology installed in the world today.
Third generation technologies include thin-film solar photovoltaic employing dye-sensitized, organic, quantum dot or perovskite solar cells and novel combinations of semiconductor materials, as well as concentrators.
Dye sensitized solar cells are thin film solar cells composed of titanium dioxide nanoparticles covered with dye that absorbs sunlight. They are simple to make, use inexpensive materials and can work in low-light conditions; they have achieved 12.3 percent efficiency.
Organic photovoltaic cells (also called plastic solar cells) use small carbon-based molecules of abundant materials to absorb light. They can be made into thin film relatively cheaply with inkjet printing, and have achieved 11.1 percent efficiency.
Quantum dot photovoltaic uses nanocrystals made of semiconductor materials that take advantage of the laws of quantum mechanics. Because the size of the nanoparticles can be changed, they can be tuned to absorb energy from different parts of the solar spectrum, including parts of far infrared wavelengths, which constitute half of the sun’s energy. Quantum dot photovoltaic has only reached 9.2 percent efficiency, but it is inexpensive to produce and can be sprayed or painted on.
Using many relatively inexpensive lenses, concentrators focus sunlight onto a small solar cell; because the light is concentrated, it makes the cells much more efficient. Concentrators have reached 46 percent efficiency in lab tests. Since less solar-cell material is needed, concentrators have the potential to lower the cost of solar power. On the other hand, because some concentrators concentrate sunlight by a factor of 300 to 1,000 times onto a small cell, the use of more expensive solar cells combining multiple semiconductor materials to capture a broader range of wavelengths is possible. These types of concentrators that use multi-layer tandem cells are used for space and satellite applications where cost is not a factor, but on land are only used to keep costs down.
With the exception of concentrators, third generation photovoltaic technology is still in the lab stage. Of technologies that are in commercial use today, thin film costs the least in terms of square meters and how much power it can deliver. The leading technology is cadmium telluride, because it’s easier and faster to manufacture, and thus costs less. Ongoing research is focused on three key areas of photovoltaics: higher power conversion efficiency, use of more commonly found and abundant materials, and reduced manufacturing complexity and cost. According to the MIT report, “No single photovoltaic technology today excels in all three key technical characteristics.”
Meanwhile, other exciting applications of solar energy are also being developed.
The Joint Center for Artificial Photosynthesis (USA) , is developing technology that replicates the natural photosynthesis of plants, converting sunlight, water and CO2 into oxygen and fuels made of carbohydrates or sugars. The artificial photosynthesis system, which is 10 times more efficient than natural photosynthesis, is called an artificial leaf or solar fuel generator.
In Sandpoint, a solar roadway is being developed that involves a specially treated glass surface (to provide traction), with solar panels, a heating element and LED lights inside. The solar roadway can produce solar energy, keep roads warm enough so that snow doesn’t collect, generate warnings and instructions to drivers with its LED lights, and potentially provide a charging infrastructure for electric vehicles.
The first solar road has opened in Normandy, France. The kilometer-long stretch of road is expected to generate enough electricity to power a nearby village of over 3000 people.
Solar roads are a controversial and mostly untested new technology. A solar bike path was built two years ago in the Netherlands and only managed to generate enough electricity to power a single home. It was estimated that the cost of that bike path could pay for over 100 times as much electricity from other sources. The solar road in France has even more of an uphill battle. In Normandy, where the road was built, there are typically less than two months of strong sunshine a year. However, if this test produces as much power as it’s expected to, and the company building the roads, Wattway, can bring down the costs of future panels, it’s possible solar roads could become a good investment.
People who want to install PV panels on their roofs soon run into a very basic electrical engineering problem: Solar panels produce DC (direct current) power, but the appliances we plug into the wall require AC (alternating current) power. The solution is a new breed of small devices called “microinverters.” Connected directly to individual solar panels, microinverters enable each panel to output AC instead of DC power. They can also be combined with a wireless monitoring system that allows the performance of each panel to be monitored.
To conclude, “It’s not fair to compare solar with technologies that pollute. We don’t account for the societal costs of pollution from coal. … If we did, we’d see that electricity from coal is a lot more expensive than what we actually pay for it. This would make wind and solar much more appealing.