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Concentrating Solar Power

SunCatcher: The next step in solar

The SunCatcher is the latest addition to the ever diversifying market of solar power technologies. Photo by the Salt River Project

The sun is practically one of the most often used natural resources in human history. From the heat it provides, man has reaped a multitude of benefits. Early civilizations have used the sun to even tell the time. But in today’s modern and industrial world, people have utilized the sun in a completely revolutionary way: to generate electricity.

Presently, the solar power market is on the rise with numerous projects having already been established and in full operation in different international locations. In Europe, Germany and Spain lead the way while Japan and China are the Asian leaders. In the US, the state of California holds a bevy of companies that provide solar power services as well as various laboratories and testing facilities for the most cutting-edge in solar technology.

Just this August, a recent innovation in an increasingly lengthening list of solar power devices made history. The SunCatcher solar power system, a product of American company Stirling Energy Systems and its sister company Tessera Solar, set a new record as it hit a solar energy conversion efficiency rate of 31.25% under test conditions. The previous record for converting solar energy into grid-quality electricity was set in way back in 1984 at 29.4%.

Tessera Solar has already partnered with major American utility Salt River Project (SRP) to develop the Maricopa Solar Plant, a 1.5-megawatt (MW) solar project, in Peoria, Arizona. The project, which reportedly will consist 60 of these SunCatcher dishes, is expected to be completed in January 2010 and will be operated by Tessera Solar under a 10-year agreement with SRP.

The SunCatcher deconstructed

The SunCatcher is the latest in a growing list of concentrating solar-thermal power (CSP) technologies being tested and used today. Unlike typical photovoltaic (PV) technology which uses gathered sunlight to cause chemical reactions in PV cells to generate electricity, CSP devices use the sun’s heat instead to provide thermal energy to power a connected turbine or engine to produce electricity.

The way the device operates offers a slight modification on the way other CSP technologies work. It has a 25-kilowatt-electrical (kWe) solar energy system which consists of a concentrator dish structure that supports an array of curved glass mirror facets which concentrate its gathered solar energy onto a patented power conversion unit (PCU). The PCU is coupled with, and powered by, a completely re-engineered Stirling engine.

This engine involves a closed-cycle, high-efficiency, four-cylinder, reciprocating device utilizing an internal working fluid that is recycled through it. It consequently operates with heat input coming from the dish’s mirrors onto the PCU's solar receiver tubes which contain hydrogen gas. The incoming solar thermal energy heats and pressurizes the gas in the heat exchanger tubing and this gas, in turn, powers the Solar Stirling Engine to generate electricity.

Waste heat from the engine is transferred to the ambient air via a radiator system similar to those used in automobiles. The gas is cooled by a radiator system and is continually recycled within the engine during the power cycle. The conversion process does not consume water, as is required by most thermal-powered generating systems. The only water necessary for the entire operation is the minimal amount needed to wash the glass mirrors.

However, the real integral factor and modification in how the SunCatcher got such an impressive efficiency rate was the device’s unique parabolic radial dish design. It is programmed in such a way that it automatically tracks the sun at various times of the day. This feature in itself already places the device a step above other solar arrays that are traditionally flat planes that only face a set location or placement of the sun.

Generational efficiency

In the greater scheme of things, the SunCatcher’s record-breaking conversion efficiency highlights how solar technologies and research have been steadily improving over the years and across the so-called “generations” of solar cells.

At present, it is the single-junction, silicon-wafer, PV cells of the first generation that are garnering the most amount of solar market attention. In fact, roughly 86 percent of the entire market inventory is comprised of first generation cells. However, their peak efficiency is pegged at 15 to 20 percent at the most. Though rates could potentially go higher, the main challenge is particularly rooted in the high production costs involved in acquiring pure silicon for the cells.

Then there are the second generation or thin-film cells. These devices are cheaper than the silicon-wafer PV cells since they use amorphous or micromorphous silicon instead. Some have even forgone the use of silicon altogether. An added feature is that thin-film cells are more flexible and lightweight than their predecessors, marking an improvement in usability. But despite resolving the issue of production costs, it is their conversion efficiency that becomes the tradeoff. Though rates go up as much as 15 percent, it plateaus at around 10 percent or less in actuality.

Third generation solar cells have banked on the efficiency of the first generation and the reduced costs of the second generation to produce more innovative technologies. They include polymer solar cells that use plastic instead of silicon; nanocrystalline cells that are made up of zinc or titanium dioxide nanoparticles which absorb ultraviolet energy to generate electrons; as well as organic and dye-sensitized cells.

The major drawback in all of these, though, is that they are still in their research and testing stages and are not yet available for large-scale commercial consumption. Add to this how their conversion efficiency rates are currently below 10 percent given their initial stages of development.

But the success of the SunCatcher shows that the scientific and engineering communities are still hard at work discovering, re-discovering, and ultimately improving their findings in order to come up with new devices that could potentially be the next big thing not just in solar energy market, but in the renewable energy industry as a whole.

Though great care must be taken in order to ensure that further developments do not inadvertently cause more carbon emissions and pollutants to be dispersed in the environment, it will only be a matter of time until a cheaper, more efficient way of providing clean, solar-based energy will give the SunCatcher a run for its money.

-  Joseph Dayrit

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