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Solar Research and Development

Solar research can be divided into a number of categories beginning with research into active and passive technologies. While there is some research into passive solar design, that is generally limited to the architectural and landscape planning fields. It is more about the application of known principles than about advancing technology.

In the active solar research category, there are a number of sub-categories. The most relevant are photovoltaic and concentrated solar. Photovoltaic research occurs at governmental, NGO, university, and private levels. Concentrated solar is primarily, though not exclusively, a government venture when implemented on the largest scales. On the small scale, there are a limited number of private companies investing in concentrated solar research and development.

This article will explore some of the overarching themes in solar research and take a look at goals for the sector as a whole. Subsequent articles will look at selected research centers throughout the world to explore the specifics of their endeavors.

Photovoltaic Research and Development

Cost Reduction through Increased Efficiency in Silicon Processing

Though more complex materials are always under investigation, silicon has proved itself to be indispensible and wafers are still the dominant technology. Major research focus in PV cells is making them less expensive. This would drive down the cost of solar technology considerably and bring the prices in line with standard forms of energy generation. The average home would require an installation of PV panels costing roughly $25,000 USD in order to replace grid electricity completely. At this rate, the system only pays for itself after 20 to 25 years, the point at which components must be replaced and another large investment made. If system cost could be reduced to the point that solar pays for itself after five years, more people would invest. There are several efforts in this area as follows.

Silicon is the primary product in most solar cells and the reason they are so expensive. Though this element is exceptionally abundant, it comes bound to oxygen in the form of silica (SiO2). Separating the silicon from silica is an energy-intensive process that requires reaction with carbon at a temperature of 1700 oC. At this rate, it takes two years for a solar panel to produce as much energy as was invested into its construction.  Current efforts to reduce cost include lower temperature reactions of 900 oC and reduced amounts of silicon in solar panels.

At lower temperatures, Silicon can be separated from silica in a molten salt bath with the help of electrolysis. This process produces porous silicon rather than the monocrystalline and polycrystalline forms used in most PV cells. There is research to understand how to use silicon in this form.

Reducing the amount of silicon in PV cells is very promising. Much like creating faster and smaller microprocessors, reducing the amount of silicon in a PV cell is iterative. Current research has allowed researchers to use a single silicon wafer to create one 140 watt PV panel. With conventional technology, the same solar panel would require 60 wafers. Though not exactly a one to one process, it is easy to see how such advances could reduce the cost of a solar panel significantly.

Silicon Processing and Thin Panel Technology

One of the advantages of thinner panels is the ability to create multijunction connections. This allows the panels to extract more energy from the sun by utilizing light at multiple wavelengths. The National Renewable Energy Laboratory in the United States is at the forefront of multijunction technology.

Thin-film PV cells can be multijunction of unijunction. In either case, they use less than 1% of the expensive materials that traditional panels use. This technology is farther from market than is the multijunction technology mentioned above.

Conductive Polymers and Dyes

Silicon is not the only material that can be used to convert solar energy into electricity. While research into other semi-conducting elements is ongoing, there is also interest in organic polymers and pigments that can perform the same job as semi-conducting elements, but are much less expensive to produce.

The major hurdles to organic polymers and dyes are their fragility and cold tolerance. Both chemicals can be damaged by UV light. Additionally, they lose a great deal of the functionality in colder weather. Freezing is detrimental to performance of these solar cells as they generally rely on liquid electrolyte to operate. Research into solid electrolytes as well as different organic molecules is ongoing.

Carbon

Carbon nanotubes embedded in conductive polymers are showing some promise as films for increasing optical efficiency in solar panels. This research is very new, the efficiencies as high as 42% have been achieved. The price is currently out of the range of commercial viability.

Other Light Sources

Most solar panels use visible light to produce electricity, but the sun also emits infrared and UV light, both of which reach the surface of the Earth in large quantities. Infrared light is basically heat energy and there is interest in using nanotubes (carbon tubes on the nanometer scale) to absorb heat. This technology has been placed on thin polymer films and is flexible enough to be used on almost any shape surface. There has been some difficulty in extracting electrical energy from the heat generated.

One of the amazing things about ultraviolet light (UV) is that Japanese researchers have produced a cell that absorbs UV, but allows visible light to pass through. This allows traditional solar cells to be paired with UV solar cells to extract even more energy from what is available.

Concentrated Solar

Major investments in concentrated solar are in large scale power generation facilities using power tower designs and in heat storage systems. Molten salt storage was demonstrated to be feasible by the Solar Two power tower produced by the United States government. Such systems can generate power for up to eight hours without sunlight before they need to rely on back-up sources of energy.

Bridging the Gap

There is no rule that says PV and concentrated solar must remain separate. In fact, there are good reasons to combine the two as demonstrated by luminescent solar concentrators. Rather than pairing concentrated solar with a heat engine, these concentrate sunlight on a PV panel. This raises the efficiency of the entire system, allowing the PV cell to be used at maximum efficiency. This means fewer PV panels are needed to produce the same amount of electricity, which results in substantial cost savings.

Interestingly, research at the Massachusetts Institute of Technology has resulted in windows be converted into solar concentrators for the production of electricity. These cells combine light absorbing dyes with internally reflective glass panes. The panels can concentrate sunlight by a factor of 40 and are expected to be commercially available within a few years.

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