SOLAR TECHNOLOGY

Low-priced plastic photovoltaics

Solar cells and panels, which tap the power of the sun and convert it to electricity, offer a green - and potentially unlimited - alternative to fossil fuel use. So why haven't solar technologies been more widely adopted?

Quite simply, they're too expensive, Researchers have come up with a technology that might help bring the prices down.

To collect a lot of sunlight you need to cover a large area in solar panels, which is very expensive for traditional inorganic - usually silicon - photovoltaics. The high costs arise because traditional panels must be made from high purity crystals that require high temperatures and vacuum conditions to manufacture energy potential.

A cheaper solution is to construct the photovoltaic devices out of organic compounds - building what are essentially plastic solar cells. Organic semiconducting materials, and especially polymers, can be dissolved to make an ink and then simply "printed" in a very thin layer, some 100 billionths of a meter thick, over a large area.

Covering a large area in plastic is much cheaper than covering it in silicon, and as a result the cost per Watt of electricity-generating capacity has the potential to be much lower.

One major difficulty with doing this, however, is controlling the arrangement of polymer molecules within the thin layer. Scientists have developed an advanced structural probe technique to determine the molecular packing of two different polymers when they are mixed together. By manipulating how the molecules of the two different polymers pack together, they have created ordered pathways - or "nanowires" - along which electrical charges can more easily travel. This enables the solar cell to produce more electrical current.

This work highlights the importance of the precise arrangement of polymer molecules in a polymer solar cell for it to work efficiently. Researchers and scientists expect polymer solar cells to reach the commercial market within 5 to 10 years.

TO CLEAN OR NOT TO CLEAN...THAT IS THE QUESTION

Cleaning Solar Panels Often Not Worth the Cost

Don't hire someone to wash your dirty solar panels. That's the conclusion of a study recently conducted.

Researchers found panels that hadn't been cleaned, or rained on, for 145 days during a summer drought in California, lost only 7.4 percent of their efficiency. 

Overall, for a typical residential solar system of 5 kilowatts, washing panels halfway through the summer would translate into a mere $20 gain in electricity production. For larger commercial rooftop systems, the financial losses are bigger but still rarely enough to warrant the cost of washing the panels. On average, panels lost a little less than 0.05 percent of their overall efficiency per day. The study is focused on smaller systems, for very large installations, economies of scale may mean that washing panels is worth it.

Dust on PV panels does make a difference but it's not a big enough factor in Wisconsin to warrant cleaning. Researchers believe that this is the largest study quantifying losses of electricity output due to dirty solar panels conducted so far. Typically, particulate matter from air pollution, agriculture, construction and traffic accumulates on the panels, as well as pollen and sea salt.

Researchers also found that solar panels mounted at an angle of less than five degrees caused bigger losses in efficiency. That's because dirt slips off panels that are installed at a steeper angle. Here in SE Wisconsin we angle solar arrays at slightly less than 45 degrees.

But solar panels heavily soiled with bird droppings should be cleaned. That's because the droppings essentially block all sunlight and will not be washed away when it rains. Engineers also found that photovoltaic panels were dirty enough to warrant cleaning due to very specific and localized circumstances. For example, being directly next to and downwind of a highway, factory or agricultural field may generate enough dirt to warrant cleaning.

Next steps in the study would be looking more closely at the sites that did warrant cleaning and determine what caused the panels to get so dirty. Finally, researchers could add collectors at specific sites to determine what kind of dirt accumulates on the solar panels; whether special materials could keep dirt from accumulating; and whether special, less costly washing systems would do a better job at removing dirt from the panels.

Update on SOLAR IMPULSE

Solar plane on cross-country trek

The Solar Impulse solar plane landed in St.Louis early Tuesday, completing the third leg of a planned five-flight trek from San Francisco to New York.

After taking off Monday morning from Dallas/Fort Worth International Airport, the aircraft landed in St. Louis after a flight of 21 hours and 21 minutes, its longest flight to date.

After the landing the Swiss-built aircraft was moved to an inflatable hangar originally designed for a planned round-the-world flight, but brought into action after a storm caused heavy damage to the airport hangar originally reserved for Solar Impulse.

The inflatable hangar was brought in to the USA for testing purposes and in fact it allowed the mission to stay on schedule. This exercise is now a proof of concept: rather than taking the airplane to a hangar, the hangar has been taken to the airplane.

Two men, both pioneers and innovators, both pilots, are the driving force behind Solar Impulse.

• Bertrand Piccard, doctor, psychiatrist and aeronaut, who made the first non-stop round-the-world balloon flight, is the initiator and chairman.
•  André Borschberg, an engineer and graduate in management science, a fighter pilot and a professional airplane and helicopter pilot, is the co-founder and CEO.

The first leg, beginning May 3 and piloted by Piccard, started from NASA's Moffett Field in the San Francisco Bay Area and ended in Phoenix; a Phoenix-to-Dallas flight with Borschberg at the controls began May 22.

The long flight times of each leg meant Solar Impulse had to fly through the night; 12,000 solar cells built into the wing use sunlight to charge the batteries for night flight.

A fourth flight will see the plane land in Washington, D.C., this month, while the final leg will end at JFK Airport in New York in early July.

You can follow the excitement of this fantastic voyage on the web at:

www.solarimpulse.com