The 19th Century American industrialist Andrew Carnegie probably didn’t have solar power in mind when he advised aspiring tycoons to “concentrate your energies.” If he were alive today, however, he might have seen how apt the principle is to turning sunlight into electricity.
Fans of solar power sometimes boast that energy from the sun is “unlimited” when what they really mean is “vast.” Scientists have quantified the average amount of solar radiation (insolation) striking most inhabited land on Earth as ranging from approximately 150 to 300 watts per square meter.
For decades, most photovoltaic (PV) research has been directed at increasing the efficiency of the conversion process – to wring more electricity from that finite amount of light energy. Those efforts have had great success. From an efficiency of just 4 percent for the first cells made from crystalline silicone in the early 1950s, efficiency has increased to around 20 percent today – a five-fold jump.
Scientists Jerry Olson and Sarah Kurtz have taken a radically different approach. They began working in the 1980s on increasing not just efficiency but also on maximizing the total amount of light striking the cell. In other words: concentrating and maximizing the amount of energy that strikes the cells. Their pioneering work was recognized in 2006 when Olson and Kurtz were named Dan David Prize laureates (an international prize that carries a million dollar award). After making giant strides in research at the National Renewable Energy Laboratory (NREL), concentrating photovoltaic power (CPV) has begun to move out of the lab and into production.
Rick Russell, head of engineering for the young CPV manufacturing firm Soliant, showed me how their SE-1000X unit works. The apparatus stood in a giant exhibition hall during the just-ended Solar Power Industry trade show in Los Angeles. Eight gleaming white rectangles (each measuring 14” x 28” and 16” deep) stood out in a room dominated by panels of dark silicon.
“These,” said Russell pointing to the Fresnel lenses that topped each box, “concentrate the sunlight like a magnifying glass does, onto the solar cell below.” The lenses can increase the amount of light focused on the cell by one thousand times. The solar cells themselves aren’t visible, but sit in box so small you could hold it in one hand.
“They may be small,” Russell explained, “but they’re extremely efficient.”
Known in the industry as “triple junction” cells, they’re made from extremely thin layers of Gallium Indium Phosphide and Gallium Indium Arsenide put onto a substrate of Germanium.
Each layer is sensitive to a different part of the spectrum, allowing the triple junction cells to produce even more electricity. These high-efficiency cells were developed for space applications, where small size and peak efficiency are more important than just the price. (Soliant was founded by a group of former NASA and JPL scientists.)
Combining high-efficiency cells with concentrating lenses began bringing the price down to earth (pardon the pun). But Russell pointed to a third engineering feature that is critically important to helping CPV approach grid parity – and achieve it in some locations. Solar tracking, NASA style.
Attaching solar panels to mechanized tracking devices that follow the sun throughout the day isn’t exactly new – although the practice has grown more popular in recent years. What is new, is the precision method adopted from the space agency.
Most tracking devices follow a “virtual sun,” using an algorithm based on known factors like latitude and longitude and time of year.
But that process doesn’t incorporate real world factors.
“Predicting the position of the sun is easy,” Russell explained. “What’s harder to predict -- in fact, what’s impossible to predict with a high degree of accuracy -- is the position of the sun relative to the position of the solar cell at a given moment.”
That’s because the panels are mounted on a roof that can sag and in locations where high winds can shift the panels themselves enough to affect electrical output. Soliant’s solution was to incorporate sensors that allow the array to tilt and turn in three dimensions so that any tracking errors are self-corrected – with a phenomenal accuracy of .1 of a degree.
The result is a commercial CPV rooftop system that can generate double the electrical output of a traditional solar array of the same cost.
There is one caveat, however. Brian Robinson alluded to it earlier in the week, in a panel discussion at SPI.
“CPV is perfectly positioned to grow,” he said, adding, “where the market will be.”
As the CEO of the California-based CPV company, Amonix, Robinson believes in his product 100 percent, but only in the right location.
“CPV is not the best choice for everywhere,” Robinson cautioned.
It only becomes competitive with other energy sources in areas with sufficiently high amounts of solar insolation, places like the Southwest and west Texas. “I wanted a technology strategy that could address the energy markets, not the subsidized solar markets,” Robinson emphasized.
CPV may be the first form of solar power to reach full grid parity on a large scale – without subsidies or incentives, or even a truly level playing field, i.e., one that includes environmental and health costs in its price. It won’t happen everywhere, but, then, why does solar have to be the best energy choice for the entire planet, right now?
As the technologies develop, and as our political will matures to the point that we’re able to deal rationally with climate change, solar power will likely spread.
CPV can flourish now in locations where it’s cost effective – a hint, perhaps, of things to come
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