This entry is the latest in a Worldwatch blog series on innovations in the climate and energy world.
When most people think of solar power, they probably think of photovoltaics (PV)—cells that convert the sun’s rays to electricity. Some people might even conjure up images of concentrated solar power (CSP), a technology that uses mirrors or lenses to concentrate sunlight, convert this energy to heat, and drive an engine to generate electricity. But few people realize that there also exists a hybrid of the two, known as concentrated photovoltaics (CPV).
CPV systems rely on the same energy conversion as traditional PV. Sunlight hits a semiconductor, often silicon, and releases electrons. These electrons, if the semiconductor is connected to an electrical circuit, form an electrical current that can power a load. The difference is that, much like a CSP facility, CPV installations increase the amount of sunlight that hits each cell beyond what would come from the sun alone. These cells therefore promise greater efficiency (in terms of energy generated per unit of area and energy generated per installed capacity), lower resource use, and perhaps eventually lower cost.
CPV is usually divided into three categories based on the degree to which light is concentrated: low concentration (LCPV), medium concentration (MCPV), and high concentration (HCPV), although the ranges for each category are not yet standardized. LCPV represents concentrations up to 10 times greater than standard PV. LCPV systems use silicon semiconductors identical to those in traditional PV and simply add mirrors or optics to increase the irradiance hitting the cells. The idea came into fashion in 2007–08 when silicon shortages were hampering PV growth, because LCPV requires less silicon to generate the same amount of power. Silicon supplies have since grown to the point that the price of solar-grade silicon has dropped 90 percent from its high. The interest in LCPV that remains seems to exist as a hedge against a return to high silicon prices.
HCPV systems concentrate more than 300 times and are a much greater departure from traditional PV installations, using multiple mirrors and optics like dish reflectors or Fresnel lenses to achieve such high concentrations. They are double-axis tracked, which means that they rotate throughout the day to track the sun’s movement and change their orientation throughout the year to adjust to the sun’s changing path through the sky. They also generally use multi-junction (MJ) cells instead of silicon-based cells because of the former’s much greater efficiency.
By combining layers of semiconductors with different band gaps—that is, layers that can absorb light most efficiently at different wavelengths—MJ cells capture energy from a wider range of the light spectrum. The efficiency record is currently 41.6 percent, compared to roughly 25 percent for silicon-based PV systems. MJ cells are almost never seen in standard PV designs, however, because of their high cost.
MCPV systems are often 10–20 times concentrators, but they represent the entire range between LCPV and HCPV. They use silicon-based (or sometimes thin-film) cells instead of MJ cells and rely on either single-axis tracking or simpler low-accuracy trackers, making the systems cheaper and easier to maintain than HCPV systems, but less efficient. The application of stronger concentrators, however, allows MCPV to use slightly higher-quality cells than LCPV systems, and to incorporate at least some degree of tracking to increase power production.
Does it pass the laugh test?
PV works, and CSP works, so it makes plenty of sense that CPV would work as well.
Does it have that WOW factor?
A passerby probably wouldn’t be able to differentiate between CPV and more traditional solar installations, and solar panels haven’t been worth gaping at for a while now.
What does it bring to the table?
The chance for cheaper power generation and lower resource use.
CPV doesn’t open new areas to solar development—in fact it is more restricted than PV generally (see below)—but it does have the potential to lower costs and resource use. By concentrating the power generation of each cell, these systems remove much of the need for scarce semiconductor materials. For example, the largest CPV system in the United States, a 1 megawatt (MW) facility that opened this past May at Victor Valley College in California, uses 1/1000th the cell material of a traditional silicon PV installation of the same size.
In most cases, CPV systems also have a much smaller footprint than traditional PV units, often as much as half. Single-axis tracking increases the footprint because of shading concerns as the cells shift position (double-axis tracking does not have this problem), but this is overcome by the greater energy density of CPV systems. Moreover, because the arrays are on platforms off the ground, very little area suffers permanent shading, and the land can be used for other purposes, such as livestock grazing. In comparison to CSP systems, CPV uses very little water, as none is needed in the course of regular operations.
CPV systems also have a small temperature coefficient, which means that the system output does not decrease as dramatically under high temperature conditions.
How scalable is it?
Very, in places.
One of the chief advantages of CPV is the lower capital investment required due to the technology’s dramatically decreased use of semiconductor material. The need for silicon purification facilities and other capital-intensive equipment would be low relative to other solar technologies, which means that as CPV matures and theoretically reaches a higher volume of production, costs could come down more quickly.
On the other hand, CPV, like CSP, is able to utilize only direct sunlight (as opposed to light reflected off clouds or other surfaces), so it probably can be used only in particularly sunny locations.
How close is it to commercialization?
A couple (big) steps away.
The fact that 1 MW represents the largest CPV installation in the United States is indicative of how far this technology is from commercialization. Germany installed 8 gigawatts of solar capacity in 2010 alone (mostly PV), so competing solar technologies are clearly farther along.
CPV is chugging along, however. Multi-megawatt projects have been announced by many CPV companies, the largest by far being a 59 MW plant in Taiwan. According to CPV Today, an industry news service, installations in the United States will grow from 1.5 MW currently to 75 MW by 2015. Other projections are less sanguine.
What is the biggest obstacle to success?
CPV lacks the track record that PV and CSP technologies have built up over the past few years, as it is behind on the development path. This means that utilities are disinclined to choose CPV projects when the prices proposed by CPV and other solar developers are similar. At this point, CPV systems need to offer a lower price, anecdotally as much as 10–15 percent lower in dollars per kilowatthour, according to PV News, to win contracts. But without a track record, it is also more difficult to attract the financing necessary to offer low prices.
With the solar market expected to grow rapidly in 2011 and beyond both in the United States and globally, CPV manufacturers and developers will certainly have opportunities to win big contracts and start to bring their costs down with economies of scale. Thin-film companies faced a similar challenge at first but have broken through this barrier.
The final word(s):
A closing window.
CPV offers many possible benefits, and it seems reasonable that it would be a major piece of the solar power portfolio as the industry goes mainstream. But at present, it is not clear whether CPV’s potential for lower costs will ever be realized. Costs for traditional PV systems are falling and silicon prices are staying low, and CPV investors may lose patience without marked progress. CPV may have a limited amount of time to establish a foothold in the utility-scale solar world before PV and CSP developers’ costs are low enough that CPV cannot compete without an existing track record.