By using mirrors and lenses to concentrate the rays of the sun, solar thermal systems can produce very high temperatures—as high as 3,000 degrees Celsius. This intense heat can be used in industrial applications or to produce electricity.
Solar concentrators come in three main designs: parabolic troughs, parabolic dishes, and central receivers. The most common is parabolic troughs—long, curved mirrors that concentrate sunlight on a liquid inside a tube that runs parallel to the mirror. The liquid, at about 300 degrees Celsius, runs to a central collector, where it produces steam that drives an electric turbine.
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| Parabolic trough concentrators. Source: NREL |
Parabolic dish concentrators are similar to trough concentrators, but focus the sunlight on a single point. Dishes can produce much higher temperatures, and so, in principle, should produce electricity more efficiently. But because they are more complicated, they have not succeeded outside of demonstration projects.
A more promising variation uses a stirling engine to produce power. Unlike a car’s internal combustion engine, in which gasoline exploding inside the engine produces heat that causes the air inside the engine to expand and push out on the pistons, a stirling engine produces heat by way of mirrors that reflect sunlight on the outside of the engine. These dish-stirling generators produce about 30 kilowatts of power, and can be used to replace diesel generators in remote locations.
The third type of concentrator system is a central receiver. One such plant in California features a "power tower" design in which a 17-acre field of mirrors concentrates sunlight on the top of an 80-meter tower. The intense heat boils water, producing steam that drives a 10-megawatt generator at the base of the tower. The first version of this facility, Solar One, operated from 1982 to 1988 but had a number of problems. Reconfigured as Solar Two during the early to mid-1990s, the facility is successfully demonstrating the ability to collect and store solar energy efficiently. Solar Two’s success has opened the door for further development of this technology.
To date, the parabolic trough has had the greatest commercial success of the three solar concentrator designs, in large part due to the nine Solar Electric Generating Stations (SEGS) built in California’s Mojave Desert from 1985 to 1991. Ranging from 14 to 80 megawatts and with a total capacity of 354 megawatts, each of these plants is still operating effectively.
As a result of state and federal policies and incentives, more commercial-scale solar concentrator projects are under development. Modified versions of the SEGS plants are being constructed in Arizona (one megawatt) and Nevada (65 megawatts). In addition, Stirling Energy Systems received approval from the California Public Utility Commission in October 2005 to build a 500-megawatt facility (with the option to add 350 megawatts) in the Mojave Desert using the parabolic dish design. Beginning in January 2009, the plant will supply power to Southern California Edison under a 20-year contract that will help the utility meet its requirements under the state’s renewable electricity standard.
In 1839, French scientist Edmund Becquerel discovered that certain materials would give off a spark of electricity when struck with sunlight. This photoelectric effect was used in primitive solar cells made of selenium in the late 1800s. In the 1950s, scientists at Bell Labs revisited the technology and, using silicon, produced solar cells that could convert four percent of the energy in sunlight directly to electricity. Within a few years, these photovoltaic (PV) cells were powering spaceships and satellites.
The most important components of a PV cell are two layers of semiconductor material generally composed of silicon crystals. On its own, crystallized silicon is not a very good conductor of electricity, but when impurities are intentionally added—a process called doping—the stage is set for creating an electric current. The bottom layer of the PV cell is usually doped with boron, which bonds with the silicon to facilitate a positive charge (P). The top layer is doped with phosphorus, which bonds with the silicon to facilitate a negative charge (N).
The surface between the resulting “p-type” and “n-type” semiconductors is called the P-N junction (see the diagram below). Electron movement at this surface produces an electric field that only allows electrons to flow from the p-type layer to the n-type layer.
When sunlight enters the cell, its energy knocks electrons loose in both layers. Because of the opposite charges of the layers, the electrons want to flow from the n-type layer to the p-type layer, but the electric field at the P-N junction prevents this from happening. The presence of an external circuit, however, provides the necessary path for electrons in the n-type layer to travel to the p-type layer. Extremely thin wires running along the top of the n-type layer provide this external circuit, and the electrons flowing through this circuit provide the cell’s owner with a supply of electricity.
Most PV systems consist of individual square cells averaging about four inches on a side. Alone, each cell generates very little power (less than two watts), so they are often grouped together as modules. Modules can then be grouped into larger panels encased in glass or plastic to provide protection from the weather, and these panels, in turn, are either used as separate units or grouped into even larger arrays.
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Click on the graphic above to
see a full-size diagram of a
PV cell. Illustration: Amanda
Wait/DG Communications |
The three basic types of solar cells made from silicon are single-crystal, polycrystalline, and amorphous.
- Single-crystal cells are made in long cylinders and sliced into round or hexagonal wafers. While this process is energy-intensive and wasteful of materials, it produces the highest-efficiency cells—as high as 25 percent in some laboratory tests. Because these high-efficiency cells are more expensive, they are sometimes used in combination with concentrators such as mirrors or lenses. Concentrating systems can boost efficiency to almost 30 percent. Single-crystal accounts for 29 percent of the global market for PV.
- Polycrystalline cells are made of molten silicon cast into ingots or drawn into sheets, then sliced into squares. While production costs are lower, the efficiency of the cells is lower too—around 15 percent. Because the cells are square, they can be packed more closely together. Polycrystalline cells make up 62 percent of the global PV market.
- Amorphous silicon (a-Si) is a radically different approach. Silicon is essentially sprayed onto a glass or metal surface in thin films, making the whole module in one step. This approach is by far the least expensive, but it results in very low efficiencies—only about five percent.
A number of exotic materials other than silicon are under development, such as gallium arsenide (Ga-As), copper-indium-diselenide (CuInSe2), and cadmium-telluride (CdTe). These materials offer higher efficiencies and other interesting properties, including the ability to manufacture amorphous cells that are sensitive to different parts of the light spectrum. By stacking cells into multiple layers, they can capture more of the available light. Although a-Si accounts for only five percent of the global market, it appears to be the most promising for future cost reductions and growth potential.
In the 1970s, a serious effort began to produce PV panels that could provide cheaper solar power. Experimenting with new materials and production techniques, solar manufacturers cut costs for solar cells rapidly, as the following graph shows.
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| Source: NREL |
One approach to lowering the cost of solar electric power is to increase the efficiency of cells, producing more power per dollar. The opposite approach is to decrease production costs, using fewer dollars to produce the same amount of power. A third approach is lowering the costs of the rest of the system. For example, building-integrated PV (BIPV) integrates solar panels into a building’s structure and earns the developer a credit for reduced construction costs.
Innovative processes and designs are continually reaching the market and helping drive down costs, including string ribbon cell production, photovoltaic roof tiles, and windows with a translucent film of a-Si. Economies of scale from a booming global PV market are also helping to reduce costs.
Historically, most PV panels have been used for off-grid purposes, powering homes in remote locations, cellular phone transmitters, road signs, water pumps, and millions of solar watches and calculators. Developing nations see PV as a way to avoid building long and expensive power lines to remote areas. And every year, experimental solar-powered cars race across Australia and North America in heated competitions.
More recently, thanks to lower costs, strong incentives, and net metering policies, the PV industry has placed more focus on home, business, and utility-scale systems that are attached to the power grid. In some locations, it is less expensive for utilities to install solar panels than to upgrade the transmission and distribution system to meet new electricity demand. In 2005, for the first time ever, the installation of PV systems connected to the electric grid outpaced off-grid PV systems in the United States. As the PV market continues to expand, the trend toward grid-connected applications will continue.
This distributed-generation approach provides a new model for the utilities of the future. Small generators, spread throughout a city and controlled by computers, could replace the large coal and nuclear plants that dominate the landscape now.
The Future of Solar Energy
Solar energy technologies are poised for significant growth in the 21st century. More and more architects and contractors are recognizing the value of passive solar and learning how to effectively incorporate it into building designs. Solar hot water systems can compete economically with conventional systems in some areas. And as the cost of solar PV continues to decline, these systems will penetrate increasingly larger markets. In fact, the solar PV industry aims to provide half of all new U.S. electricity generation by 2025.
Aggressive financial incentives in Germany and Japan have made these countries global leaders in solar deployment for years. But the United States is catching up thanks particularly to strong state-level policy support. The rolling blackouts and soaring energy prices experienced by California in 2000 and 2001 have motivated its leaders to create new incentives for solar and other renewable energy technologies. In January 2006, the California Public Utility Commission approved the California Solar Initiative, which dedicates $3.2 billion over 11 years to develop 3,000 megawatts of new solar electricity, equal to placing PV systems on a million rooftops.
Other states are following suit. Arizona, Colorado, New Jersey, and Pennsylvania have specific requirements for solar energy as part of their renewable electricity standards. Many more states offer rebates, production incentives, and tax incentives, as well as loan and grant programs. Even the federal government is offering a 30 percent tax credit (up to $2,000) for the purchase and installation of residential PV systems and solar water heaters.
As the solar industry continues to expand, there will be occasional bumps in the road. For example, demand for manufacturing-quality silicon from the solar energy and semiconductor industries has led to shortages that have temporarily driven up PV costs. In addition, some utilities continue to put up roadblocks for grid-connected PV systems. But these problems will be overcome, and solar energy will play an increasingly integral role in ending our national dependence on fossil fuels, combating the threat of global warming, and securing a future based on clean and sustainable energy.
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