Solar Energy and Photovoltaic Cells

Elizabeth Gruenstein

 

Abstract:  On of the ways of using the sun’s energy is though photovoltaic cells composed of silicon.  These are generally inefficient and costly.  New technologies funded by large businesses will help solar energy become a large part of the U.S. electricity supply.

 

 

There are several ways of capturing sunlight for human use.  One is by constructing houses to maximize sunlight use in heating and cooling.  With proper design, a house can be warm in the winter due to well placed windows and good insulation, as well as being cooled in the summer by evaporation from ‘roof – ponds’.  These are shallow pools of water on the roofs of buildings that are warmed by the sun, causing the warm water to rise to the top of the pool while cooler water sinks to the bottom and cools the space beneath it. (1)

The second way to use the sun’s energy is by solar thermal conversion using flat plate collectors.  Rapp, in his 1981 book on Solar energy, describes flat plate collectors  as “black sheets of metal with tubes attached for heating a fluid” (2)  These can be used to domestic hot water heating, as well as space heating space heating.  These types of collectors only utilize the infarred or heat part of the sun’s energy spectrum.

            The third form way of harnessing solar energy is with photovoltaic cells.  These cells capture the higher energy part of the visible light spectrum.  Photovoltaic or solar cells function on the principle of semiconductors.  Elements on the periodic table that lie between the metals and the nonmetals often can act as conductors of electricity if provided with light or heat, but also act as insulators at low temperatures.  Elements that line this boundary in the periodic table are things such as germanium, silicon and arsenic.  Currently about 90% of solar cells are composed of silicon, with the other ten percent being plastic.(4)  Silicon is cheap and available, but it wastes a lot of energy in the form of heat when it converts light to electricity.  Plastic is also cheap, but currently pretty inefficient.

            Pure silicon is composed of crystals that have no electrons available for movement – they are tied up in the crystalline structure.  To make a solar cell out of silicon one must impregnate the silicon with other elements in a process called “doping”.  These elements do not have a full valence shell of electrons and are therefore less stable than silicon.  One adds impurities that carry either a positive or a negative charge until the silicon itself carries the overall charge.(2, 3)

Oppositely charged layers of silicon are then placed next to each other on the solar cell.  At the point where they meet a potential builds based on the flow of oppositely charged electrons.  When sunlight hits these electrons it excites them and causes them to jump the boundary between layers.  This creates an electric charge that can be harnessed with metal contacts. (2, 3)

            Currently there are several problems associated with solar cells.  Most obviously, the sun does not always shine.  However, even in a very sunny climate, silicon solar cells are only able to work at about 12-15% efficiency, out of a potential 50%.  Part of this is due to the fact that a lot of energy absorbed by the cells is lost in the form of heat while being converted to electricity.  Solar cells do not function well at high temperatures, and so they must be cooled, which takes energy.  However, if they were manufactured to work at high temperatures it might be possible to harness the heat energy as well, as in the case of flat plate collectors. To increase the efficiency of silicon cells a process called waffling has been introduced, where the metal conductors are placed in squares around every few millimeters of silicon in an effort to capture the energy in the form of electricity before it is wasted as heat. (1)

            However, most of the inefficiency is caused by the fact that silicon cells can only absorb and use energy in the higher end of the light spectrum.  This is because electrons have a certain energy threshold they must cross before they can “jump” to the next energy level, therefore being excited and able to flow across the boundary between plates.  This amount of energy needed to excite the electrons is called the band gap of a material.  Silicon has a band gap of 1.07 eV. (5)

            Therefore, the factor that prohibits silicon photovoltaic cells from becoming widely used is that for the amount of energy they produce they are relatively expensive.  Even though silicon itself is cheap and abundant, The technology needed to produce cells is costly.  For the amount of energy produced per dollar spent, silicon photovalic cells are not a very feasible option.   “At a current cost of 25 to 50 cents per kilowatt-hour, solar power is significantly more expensive than conventional electrical power for residences. Average U.S. residential power prices are less than ten cents per kilowatt-hour, according to experts.” (6)

            In the past five years there have been three major breakthroughs in solar cell technology.  The first is in the use of plastics in creating them.  These plastic cells are full of nanorods that act as semiconductors, who then pass the energy along to aluminum electrodes.  While these could be very cheap to mass produce, they do not get a very high efficiency, only about 2-7 %.  (6)  The second advance was thin-film photovoltaics.  These are composed of a building material, like glass, that is coated with a photoreceptor like silicon, and thus could be incorporated into structure more easily than waffle-based silicon cells However, these too are not very efficient, at only a 6-11% efficiency.(7)

            The most exciting breakthrough came in 2002, when scientists at Lawrence Berkeley National Laboratories reported that they had inadvertently discovered that the band gap for indium nitride is 0.7 electron volts, when it was previously thought to have been 2 eV’s.  The significance of this is that by combining indium nitride with the element gallium in an alloy, scientists could create a solar cell that absorbs light from 0.7 eV all the way up to 3.4eV.  This encompasses most of the spectrum of visible light.  By stacking layers of alloys together, each with a slightly different band gap, scientists could create solar cells with a potential of 70% efficiency.  Cells this efficient could be easily implemented, and cheap to mass produce. (8)

            The reason that these breakthroughs have only come very recently is because solar energy research stalled in eighties after a huge boom in the mid seventies due to the oil crisis in the U.S.  In fact, Donald Rapp’s from 1981 on solar power only has a few short pages about photovoltaic cells, since they were so costly and inefficient.  Recently there has been more pressure on countries to develop alternative energy technologies.

            In the U.S., the Bush Administration has concentrated more on hydrogen as an alternative fuel technology, although they have not ignore solar energy.  “The Administration’s budget request would lower overall solar funding from $85.07 million in FY ‘05 to $83.95 million in FY ’06 – a 1.3 percent cut. However, the budget request specifies $4.5 million for a new industry-led Crystalline Silicon Initiative.”(4)  Basically, the administration is interested in the material that has proven to be useful.  However, a lab that depends on government funding and is researching gallium-indium, such as Lawrence Berkeley National Laboratories, will have its funding cut. 

            Luckily, more big businesses have taken an interest in solar technology.  Forbes magazine reports “The world's biggest thin-film maker, indeed the biggest solar-cell manufacturer, is BP Solar, a division of BP Amoco. Its photovoltaic sales this year should run to about $250 million. Other serious contenders are Sharp and Kyocera.” Forbes  also m,entions Akzo Nobel and Royal Dutch/Shell Group unit Shell Renewables.”(9)

            Forbes also reports “U.S. photovoltaic installation by area increased almost 30% in 2000 over 1999.” (9)  It is currently a very fast-growing market, although in 2001 in only accounted for .02% of total U.S. energy supply.(10)   It is likely that solar energy research will progress much faster with large companies funding it, because companies investing in it will have a lot of advertising power, which will make more people interesting in buying the systems.  However, government subsidies wouldn’t hurt while people make the switch to solar power, since it is currently expensive.  As demand increases the comapinies will put more effort into producing solar cells, creating competition, which will drive the price down.  Currently in California  the state offers a toalt 7.5 tax credit for implementation of solar systems for buinessness and residences. (11)

            No one predicts that the whole economy will be run on solar energy, since to meet our world’s current energy needs one would have to cover about 1% of the earth in silicon solar cells.  Additionally, there are problems with storing and transporting electricity produced by solar cells.  The energy can only be used locally, so places that are cloudy or dark for much of the year would have to find alternatives.  However, in conjunction with hydro- and wind power, solar energy could comprise a much larger part of our electricity generation than it does, with little cost.

 

 

 

1.  Daniels, Farrington Direct Use of the Sun’s Energy  Yale Univeristy, 1964 NY

2 . Rapp, Donald Solar Energy Prentice Hall Inc., Englewood Clifs, N.J. 1981 page 11-13

3.  http://www.howstuffworks.com/solar-cell.htm

4.  http://www.solarbuzz.com/News/NewsNAGO223.htm

5.  http://www.nsec.harvard.edu/AP298_2003/Mazur/gapabsorb.pdf

6. http://news.nationalgeographic.com/news/2005/01/0114_050114_solarplastic.html

7.  http://www.forbes.com/2001/05/31/0531solar.html

8.  http://www.lbl.gov/Science-Articles/Archive/MSD-full-spectrum-solar-cell.html

9.  http://www.forbes.com/technology/feeds/general/2004/12/15/generalafx_2004_12_15_AEF_9473-6184-NEW.PRD.EUR.FRA.GBR.NED.ENG.TEL.UTI.CHM..html

10 http://www.blm.gov/energy/solar.htm  .

11.  http://www.akeena.net/solar_energy_benefits/incentive_programsca.html