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Concentrating solar power (CSP) systems capture the sun's heat and transform it into electricity. CSP plants have been on line in California since 1984 and are the least expensive technology for making solar electricity.
The Southwest United States is ideally suited for CSP. The amount of power generated depends on the amount of direct sunlight, which is abundant in Southern California, Nevada, Arizona, New Mexico, and Texas. In these states, solar power plants can produce the greatest amount of electricity at the same time the greatest demand for power occurs—when people turn on their air conditioners. The hottest part of the day is also the time that generating plants running on coal or natural gas become less efficient.
How It Works
CSP plants use mirrors (sometimes called reflectors or heliostats) to focus sunlight onto a receiver that can withstand being heated to very high temperatures. A gas or liquid inside the receiver then transfers the heat to a power generation system such as a piston-driven external combustion engine and generator (with dish-engine systems) or a steam turbine and generator (with parabolic troughs and power towers).
Some CSP systems can store heat (using molten salt or high-temperature oil) to generate electricity during cloudy periods and at night. Other systems operate in tandem with gas turbines to ensure that high-value power is always available on demand.
In most CSP systems, mechanical drives slowly turn the reflectors during the day to keep sunlight focused on the receiver.
The three types of CSP systems are parabolic troughs, power towers, and dish-engines.
Parabolic troughs collect and concentrate the sun's energy with large, U-shaped (parabolic) reflectors that have oil-filled pipes running along their center, or focal point, as shown in Diagram 1. The mirrored reflectors focus sunlight on the pipes and heat the oil inside to as much as 750°F. The hot oil is then used to boil water, which makes steam to run conventional steam turbines and generators.
 Diagram 1: A parabolic trough system |
Power towers, also called central receivers, use many large, flat heliostats to track the sun and focus its rays onto a receiver. As shown in Diagram 2, the receiver sits on top of a tall tower in which concentrated sunlight heats a fluid, such as molten salt, as hot as 1,050°F. The hot fluid can be used immediately to make steam for electricity generation or stored for later use.
 Diagram 2: A power tower |
Dish-engine systems use mirrored dishes (about 10 times larger than a backyard satellite dish) to focus and concentrate sunlight onto a receiver. As shown in Diagram 3, the receiver is mounted at the focal point of the dish. To capture the maximum amount of solar energy, the dish assembly tracks the sun across the sky. A single dish can produce 2 kW to 25 kW of electricity. Illustration
 Diagram 3: A solar dish-engine system |
In modern systems, the receiver is integrated into a high-efficiency Stirling engine, which is an "external" combustion engine. The Stirling engine has thin tubes containing hydrogen or helium gas that run along the outside of the engine's four piston cylinders and open into the cylinders. As concentrated sunlight falls on the receiver, it heats the gas in the tubes to very high temperatures, which causes hot gas to expand inside the cylinders. The expanding gas drives the pistons. The pistons turn a crankshaft, which drives an electric generator. The receiver, engine, and generator comprise a single, integrated assembly mounted at the focus of the mirrored dish.
Advantages
CSP is a good choice in today's unpredictable power markets. CSP uses proven technology. CSP systems can store thermal energy or be combined with conventional fossil fuels to enable them to operate in cloudy weather or to shift power output to peak demand periods. CSP can deliver power on demand, and its power generation methods resemble those already used by the nation's power industry. CSP is affordable, and it is versatile. Whether for small-scale distributed power generation or for a large-scale power plant, a new CSP system can be brought on line relatively quickly.
CSP is a reliable technology. Parabolic trough power plants in the Southern California desert have sold nearly 7 billion kWh of solar electricity since 1984. Over time, their output has increased by 35%, and costs have dropped by 40%. Because these plants operate in conjunction with gas turbines, they deliver power when it is needed, not just when the sun is shining.
Power towers, designed for use in large solar power plants, don't just provide electricity when the sun is shining. They can also store energy for power generation on cloudy days or at night. (Heat storage is also under development for parabolic trough systems.)
Solar dish systems are the most efficient solar electric systems in the world and have demonstrated a solar-to-electric conversion efficiency of 29.4%. They are well-suited for utility line support and distributed power generation. Dish systems can be installed individually for rural industries or grouped to form a small power plant.
CSP systems are easy for utilities to work with. They use many of the same technologies and equipment as conventional power plants. CSP systems simply substitute concentrated sunlight for fossil fuels.
CSP is good for the environment. CSP plants produce no emissions of pollutants or greenhouse gases during electricity generation.
CSP is the least expensive solar electricity for large-scale power generation. Today's solar power plants produce electricity for 12¢/kWh to 14¢/kWh—competitive costs for high-demand peak periods. With improved technology and more widespread use, costs are projected to drop to 4¢/kWh to 6¢/kWh within a decade.
Disadvantages
A disadvantage of CSP is the relatively high cost of building a new solar energy power plant. To recover these costs, plant operators need to be able to sign long-term power purchase agreements. Until recently, such agreements were virtually nonexistent in today's deregulated power markets.
As California residents discovered, however, deregulated power markets don't do a very good job of ensuring adequate electricity supplies or controlling costs to consumers when fuel prices rise. Many people now recognize that using a fuel-free, renewable energy system such as CSP is like having a fixed-rate mortgage rather than an adjustable-rate mortgage (ARM). While an ARM fluctuates with the market, a fixed-rate mortgage stays the same. Like a fixed-rate mortgage, a CSP system may cost more in the beginning, but you're protected from huge price increases if natural gas costs skyrocket or supplies of electricity run low.
Since the 1970s, CSP technology has come a long way. However, more research and development is necessary to make CSP cost-competitive with fossil fuels for everyday power generation. Improved component design, advanced systems, and stronger markets for the technology at home and abroad will lower costs.
Applications
Small CSP systems can power remote villages and provide distributed power generation in hot and sunny rural areas. Large CSP plants can provide electricity on nearly the same scale as coal- or natural gas-fired power plants. For instance, nine CSP plants in California's Mojave Desert produce 600 million kWh of electricity each year—enough to meet the needs of more than 350,000 people. The parabolic trough systems used in these facilities have a proven track record with utilities.
Parabolic trough and power tower systems are under consideration for grid-connected power plants in the United States, Crete, Spain, and Jordan. In the Southwest United States, for example, utilities are exploring the use of CSP to augment conventional power generation by gas turbines on hot, sunny days.
Small dish-engine systems are being developed for distributed and residential uses in the Southwest and for export to developing countries. Homeowners and businesses wanting a "green" energy option will soon be able to choose CSP. In areas without access to power lines, CSP systems can be integrated with other renewable energy systems to provide an attractive alternative to diesel generator sets.
The fastest growing market for standalone solar power technologies is in developing countries in Asia, Africa, and Latin America. Dish-engine systems, ranging in size from 2 kW to 25 kW, are under development for remote applications such as water pumping and village power. Solar power projects are currently planned for Egypt, India, Iran, Mexico, and Morocco.
For more information, visit the following Web sites:
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