The first photovoltaic cells were created in 1954 at Bell Labs, and the solar energy companies that sprang up in the 1970s, primarily in California, used this technology, employing silicon wafers. The percentage of the sun's energy that could actually be captured for use as energy was relatively small, only 4% at first and later 11% (Lund et al.), a number that has continued to grow with technological advances.
Bulk photovoltaic cells are composed of silicon, "like computer chips, and for the same reason. They rely on that element's properties as a semiconductor, in which negatively charged electrons and positively charged 'holes' move around and carry a current as they do so" (Economist, Another silicon valley).
How Stuff Work explains the basic technology behind these cells: "when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely" (Aldous). The energy comes out in the form of Direct Current (DC), which must then be converted to Alternating Current (AC) for practical uses.
Silicon wafer solar cells come in two main types: polycrystalline and monocrystalline. Monocrystalline solar cells are made from a single crystal and are slightly more efficient than polycrystalline, which are made from many crystals. Less expensive, polycrystalline cells are currently the most common (Youngester).
In isolated locations, solar cells offer an excellent source of power. "Solar power has so far always [been] a better option in rural areas where grid power is not possible," says Jayantha Nagendran of the World Bank. (Samath) Stand-alone solar is useful in a variety of situations. In Iraq, for instance, solar powered street lamps are helping to make up for a dysfunctional grid. (Zavis) However isolated solar cells provide energy only for immediate use or for battery storage, which currently is impractical on a large scale.
Buildings connected to the electrical grid may use conventional energy when needed, while employing solar cells when the sun is out and the energy is available. In many locations, solar energy may actually be fed back into the electrical grid, causing the meter to run backwards, a phenomenon known as net metering. Yet, "For many years the utilities did not allow the feeding in of solar electricity into their grid and . . . in most countries the utilities fight this idea forcefully once it comes up" (Greenpeace 48). In many locations, then, returning energy to the grid is not yet possible.
On top of net metering, solar advocates would like policies tied to the changing value of energy depending on time of day. This is known as demand pricing, and "requires a meter that can track hourly as well as total usage, differentiating between peak and off-peak consumption" (Derbyshire). Because solar is strongest during day-time, peak hours, it is more valuable than a flat rate would indicate; yet, in many countries, those who invest in solar panels are not rewarded for this value. Travis Bradford advocates variable pricing, which "would charge consumers for electricity based on a variety of factors, including the amount of power the customer uses, the time of day they use it, and customer density" (149).
Two major factors have hamstrung the use of photovoltaic cells for generating energy: cost and intermittency. Except in special situations, such as off-grid uses, solar is simply not competitive with fossil fuels. Explains Time magazine, "stiff up-front cost has always been the biggest barrier to residential use of solar power. An average set of rooftop panels costs $20,000 to $30,000 and takes 10 to 15 years to produce enough electricity to pay for itself" (Walsh). On a larger scale, this amounts to solar being "at least two to three times as expensive as the typical electricity generated in America for retail customers" (Economist, Bright).
Still this is changing due to experimentation, in combination with practical application. According to the Economist, "decades of research have improved the efficiency of silicon-based solar cells from 6% to an average of 15% today, whereas improvements in manufacturing have reduced the price of modules from about $200 per watt in the 1950s to $2.70 in 2004. Within three to eight years, many in the industry expect the price of solar power to be cost-competitive with electricity from the grid" (Economist, Bright).
Although growth has been steady, a worldwide silicon shortage has frustrated expectations for an even faster rate by raising cost: "after decades of steady decline, prices increased from a low of $2.70 per watt in 2004 to about $4 per watt in the spring of 2006" (Economist, Bright). Nevertheless, other factors have kept the situation in check: "Thanks to economies of scale, rising conversion efficiencies, and more-efficient use of polysilicon in conventional cells, average PV module prices declined in 2007, even as polysilicon prices rose" (Sawin).
Still the shortage of refined polysilicon, due to a lack of facilities, has kept prices high. This seems puzzling, as silicon is one of the most abundant materials, found, for instance, in sand. As Greenpeace explains, "23% of the earth's crust consists of silicon. However, the process of producing the pure silicon needed for crystalline solar cells is complex. The period from planning a new silicon factory to its first output is approximately two years. The dynamic development of the PV market led to a shortage of silicon" (13). Competition from computer chip makers, which also use silicon, has added to the excess of demand over supply. Still, various projections claim that the worldwide shortage should end in 2009, as production ramps up. For instance, "The European Photovoltaic Industry Association projects 80,000 tons of annual production by 2010, up from just over 37,000 tons in 2007" (Sawin).
Improved technology, increased silicon production, and the increasing cost of energy are acting to make photovoltaic cells more competitive. At some point, solar should start to out-compete fossil fuels.
As solar power becomes more efficient, it is becoming more competitive. The company SunPower is notable here, "converting 22 percent of the sun's rays to electricity, compared with industry norms of about 10 to 15 percent" (Wolgemuth). Still the cost for SunPower cells is high; just what mix of cost and efficiency will dominate the industry remains to be seen.
An alternative to polysilicon solar cells is thin film, which
uses metallic compounds to capture the sun's energy. Cheaper and
easier to install than wafer based PV cells, "thin- film cells
use as little as 1 per cent of the volume of materials that ordinary
PV cells demand" (Daviss).
Installation is also flexible, since thin film "can be integrated
into roof shingles, siding, and the windows of buildings" (Sawin).
Many in the industry, then, see thin film technology as the future of solar: "Today, thin-film PV modules that use materials such as amorphous silicon (a-Si); cadmium telluride (CdTe); or copper indium gallium selenide (CIGS) are attracting much attention and are growing at an impressive rate" (Energy Information Administration). However, thin film as of yet captures a smaller percentage of the sun's energy than do wafer based cells. Overall, "thin-film solar cells are inefficient but cheap. Where there's room to put up a lot of them, they're cost effective, but to compete elsewhere, they'll have to get more efficient" (Takahashi). Still thin film efficiency is rapidly increasing. According to one report, "thin-film technology has now reached a critical mass and is poised to start taking 'significant market share' from incumbent technology. Thin-film silicon technologies from turn-key vendors will be ramping up in large scale during the second half of 2008" (Solid State). Numerous experiments with the best combination of thin film materials make it likely that efficiency will increase significantly in coming years. In one test thin film technology reached 19.9% (NREL) although how quickly and easily this material can be manufactured for mass consumption remains to be seen.
Disadvantages of Solar Cells
A major disadvantage of both wafer-based and thin film solar energy is intermittency. The sun does not shine at night, and is diminished by overcast skies and storms. Energy from solar cells therefore cannot be counted on at all times.
This means that decentralized energy from solar cells cannot supply what the energy industry calls baseline power, which supplies a constant energy need. Currently coal plants are the major supplier of base-load electricity, while nuclear is also excellent at this task (although expensive to bring on-line).
For peak demand times, as well as sudden surges (such as during a heat wave when air conditioners work overtime) power must be added. The energy needed for this part-time demand is called "intermediate-load electricity, as opposed to the base-load electricity that is needed twenty-four hours a day" (Bradford 13). Natural gas is currently the favorite method for supplying intermediate-load electricity, although renewable sources, such as solar and wind, are well suited to the task.
Intermittency is actually less of a problem for solar cells than for wind power. This is because solar tends to be most available during times of peak demand, particularly working hours: Intermediate load power, "which represents some 30 percent of all the electricity supply" must be provided primarily during daylight hours (Bradford 130). This is fortuitous for solar energy, since, "in the middle of the afternoon when the sun is at its peak and solar panels are producing at their optimum, demand and pricing for electricity also peaks" (Canberra Times). In addition, solar and wind can complement each other, since times of low sunshine are often excellent for wind power, notably in winter.
Still, the use of solar cells and wind power will be limited until more efficient storage methods can be developed to conserve energy when it's generated and use it when it's most needed. Bradford explains that "there are potential technical limits to widespread adoption of intermittent sources of electricity beyond 15 percent of total grid capacity without the added inclusion of energy storage solutions" (132) (other recent studies put the number at 20% for wind power). Currently, batteries are the method of choice for storing solar energy, although these need to be replaced regularly. Solid oxide fuel cells employing hydrogen technology show great promise, but need to be further developed (see http://www.csa.com/discoveryguides/fuecel/overview.php). For hydrogen storage to fulfill its promise it needs to derive its power from nonpolluting sources, such as electrolysis powered by solar energy. (Bradford 87-88) (see http://www.csa.com/discoveryguides/hydrogen/overview.php).
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List of Visuals
- Photovoltaic array.
2007 Solar Decathlon, Washington, DC.
Photo by author.
- Monocrystalline solar panel
Skycom Elesmt Trading Ltd.
- Net metering lets your meter run backwards
CalFinder Residential Solar Power Contractors
- This PV array has one onlooker gazing upward in awe.
2007 Solar Decathlon, Washington, DC.
Photo by author.
- Thin film solar panels