Science / Medicine : Sun-Powered Car Energizes Goal of Wider Solar Cell Use
With its streamlined profile and gleaming skin, the Sunraycer looks more like a giant, high-tech cockroach than a road vehicle. But in 1987, this experimental car raced 1,950 miles across the middle of Australia to win what was billed as the world’s first international transcontinental road race for solar-powered vehicles.
Fueled only by sunlight, the Sunraycer averaged 43 m.p.h. over 5 1/2 days (including overnight stopovers) and finished more than two days ahead of its nearest rival. The win was not only a triumph for Detroit-based GM Hughes Electronics, which designed and built the car. It was a dramatic success for solar-cell technology, which many scientists believe is approaching the point of widespread practical use.
The Sunraycer is powered by an array of 7,200 solar cells similar to those used in communications satellites. The cells have an efficiency of 16.5% in converting sunlight to electricity--among the highest of all commercially available units. (The cell powering a hand-held calculator typically has an efficiency of less than 8%.) Each is about twice the size of a postage stamp and roughly the thickness of a business card.
Covering 90 square feet of the vehicle’s surface, they provide 150 volts at a peak power of 1,000 watts--about the same as a hair dryer. The car, 19.7 feet long and 6.6 feet wide, weighs a mere 360 pounds. On a sunny day, it can go as fast as 45 m.p.h. on solar energy alone. It also usually uses a battery of rechargeable silver-zinc cells to provide extra power for acceleration and climbing hills, allowing the car to reach 60 m.p.h.
But don’t expect to see it on the road in the near future. Solar cells must become much more efficient before they are cost-effective. Still, their time is coming, owing to recent innovations ranging from cells designed to operate in concentrated sunlight to units that work in the dark to others that imitate the solar-conversion process in green plants.
The most common type of solar cell is made from crystals of the semiconductor silicon and consists of several layers. Atop a glass or plastic base is a thin metallic strip that conducts electricity and acts as an electrical contact. On top of that go two layers of silicon. Traces of different impurities added to the silicon give each layer a different electrical property. The uppermost layer is a metallic grid.
When sunlight shines on the cell’s surface, it frees electrons from silicon atoms in the exposed parts of the upper silicon layer. Because these loose electrons are repelled by the underlying silicon layer, they are forced to make their way through any path--in this case, a wire connecting the top grid to the lower contact--that bypasses the junction between the silicon layers. The current that flows along the wire can drive a motor or other electrical device.
The path from sunlight to electricity, however, is fraught with obstacles. Some sunlight is reflected off the cell’s surface, and even when it is absorbed, materials like silicon take in only a fraction of the light available. Electrons, freed by the sunlight, sometimes bounce around randomly for a while instead of heading directly for the circuit.
Some electrons readily slip back into place among the silicon atoms where they started. All these losses lower the cell’s efficiency. Researchers are looking for new materials and structures that absorb light more effectively and for ways to concentrate sunlight and reduce reflections.
The most efficient device yet produced--the point-contact photovoltaic cell developed at Stanford University--has achieved an unprecedented 28.2% sunlight-to-electricity conversion efficiency in the laboratory. So far, the Electric Power Research Institute in Palo Alto, a utilities-sponsored research center, has invested nearly $8 million to determine whether this kind of cell can be manufactured at a reasonable cost.
“This cell has come the closest in performance to what we feel needs to be achieved for photovoltaic cells used in utility systems,” said Edgar DeMeo, Electric Power Research Institute program manager for solar power systems. The group is encouraged, he said, because “it really looks like the cell can be manufactured using techniques that are well established within the electronics industry.”
The idea is to use a system of tiny lenses to concentrate sunlight onto small photovoltaic cells specially designed to operate efficiently in high-intensity sunlight.
The point-contact cell has several features that make it particularly efficient. First, each single-crystal silicon chip, smaller than a fingernail and only a fraction of an inch thick, has a textured upper surface to spread out incoming light. A mirror-like lower surface helps trap light within the material so that more can be absorbed.
Each of the surfaces has a thin, electrically insulating layer, except at the points where the current is conducted out of the cell. The insulation reduces the chance of light-ejected electrons recombining with the “holes” left behind by departed electrons in silicon atoms. In conventional solar cells, the top and bottom surfaces must be coated with metal films or grids, which decrease a cell’s efficiency.
Alternatively, some labs are examining less-expensive, less-efficient materials spread over a larger area in the form of films much thinner than paper. Such films soak up light more effectively than bulk silicon crystals and offer less resistence to the passage of electrons.
Most thin-film solar cells, including those that drive solar-powered watches and calculators, are made from a noncrystalline, or amorphous, form of silicon. This saves having to grow large single crystals of silicon--a time-consuming, expensive process.
The silicon-based photovoltaic cell, however, is not the only device that converts sunlight directly into electricity. So does the photo-electrochemical cell, which is part solid and part liquid. It can also convert such plentiful materials as water and carbon dioxide into fuels such as hydrogen and methane and can store solar energy for later use.
A typical photo-electrochemical cell consists of slivers of a solid semiconducting material immersed in an electrically conducting chemical soup. A special combination of ingredients is needed to produce the right chemical products and to generate an electric current efficiently and for a long time.
And there are several problems. One is a kind of light-induced rusting that causes the semiconducting material to decompose. In addition, low conversion efficiency results because the junction between the semiconductor and its liquid is much less efficient at separating electrons and “holes” than the junction between layers in a silicon solar cell.
Although the technology of such “liquid-junction” cells still lags substantially behind solid-state systems, scientists have made substantial progress. Recently, Stanford chemist Nathan Lewis and his collaborators reported a photoelectro-chemical device with a solar efficiency of 15% employing gallium arsenide. But stability and efficiency must be improved still further if photoelectro-chemical cells are to become a viable alternative to traditional energy sources.
Stuart Licht and researchers at the Weizmann Institute of Science in Rehovot, Israel, have constructed a novel photoelectro-chemical solar cell that includes the equivalent of a built-in storage battery. Its light-absorbing material is a single crystal of the semiconductor cadmium selenide telluride. The photoelectro-chemical half of the device produces more than a volt of electrical potential at a respectable solar conversion efficiency of 11.8%.
At the same time, part of the generated current is used to convert electrically charged tin into neutral tin metal in the storage half of the device. In darkness or below a certain level of light, the storage half of the cell delivers power by converting metallic tin back into its charged form. The net result is that the cell continues to work regardless of the light level.
“It’s a wonderful system in its simplicity,” said Licht, currently at MIT. “There’s no electronic switching. There’s no computer control. It’s just a chemical system that stores energy and spontaneously releases it when it’s needed.” However, a great deal more research is needed before this experimental cell nears commercial development.
So far, solar cells have found application as power sources for satellites and for communications devices such as microwave receivers and transmitters in remote locations. Their major use is in consumer products, including calculators, portable radios, TV sets and watches.
HOW THE SUN CAN POWER AN AUTOMOBILE
How Artificial Photosynthesis Works
1. Sunlight strikes a pigment “donor,” releasing electrons.
2. A second chemical directs the loose electron away from their source and toward a platinum catalyst.
3. The platinum, suspended in water, reacts to the electron flow by helping the liquid break down into hydrogen gas and a hydroxyl ion (charged particle), thus producing fuel.
The Point-Contact Cell Turns Light Into Power
1. Sunlight strikes silicon atoms, creating loose electrons and ‘holes’ (absence of electrons).
2. A strip across the back of the cell contains 73,000 contact points, half positive and half negative. All contacts of each type are connected.
3. Electrons migrate to negative-type points where the silicon has been doped with phosphorus. Holes find positive-type contacts (boron-doped silicon.)
4. If the two areas are connected with a wire, a current
flow results.
Source: EPRI Journal, General Motors
CAROL PORTER and KARREN LOEB
H2(gas) OH (ion)
ELECTRONS PLATINUM
The Sunraycer, a sunlight-powered car
H 2(gas) OH (ion)
ELECTRONS PLATINUM
THE SUNRAYCER, A SUNLIGHT-POWERED CAR
Source: EPRI Journal, General Motors
