Concentrating Solar Power
By collecting solar energy during daylight hours and storing it in hot molten salt, concentrating solar power technologies like power towers give utilities an alternative method for meeting peak loads. (Warren Gretz)
Concentrating solar power plants produce electric power by converting the sun's energy into high-temperature heat using various mirror configurations. The heat is then channeled through a conventional generator. The plants consist of two parts: one that collects solar energy and converts it to heat, and another that converts heat energy to electricity.
Concentrating solar power systems can be sized for village power (10 kilowatts) or utility-scale grid-connected applications (up to 100 megawatts [MW]). Some systems use thermal storage during cloudy periods or at night. Others can be combined with natural gas, and the resulting hybrid power plants provide high-value, dispatchable power. These attributes, along with world record solar-to-electric conversion efficiencies, make concentrating solar power an attractive renewable energy option in the Southwest and other sunbelt regions worldwide.
The solar resource for generating power from concentrating solar power systems is plentiful. For instance, enough electric power for the entire country could be generated by covering about 9% of Nevada — a plot of land 100 miles on a side — with parabolic trough systems.
The amount of power generated by a concentrating solar power plant depends on the amount of direct sunlight. These technologies use only direct-beam sunlight, rather than diffuse solar radiation.
The southwestern United States potentially offers the best development opportunity for concentrating solar power technologies in the world. There is a strong correlation between electric power demand and the solar resource due largely to air conditioning loads in the region. In fact, the Solar Electric Generating System plants operate for nearly 100% of the on-peak hours of Southern California Edison.
Three kinds of concentrating solar power systems — troughs, dish/engines, and power towers — are classified by how they collect solar energy.
These parabolic troughs are one of five Solar Electric Generating System in California's Mojave Desert.
Trough Systems
The sun's energy is concentrated by parabolically curved, trough-shaped reflectors onto a receiver pipe running along the inside of the curved surface. This energy heats oil flowing through the pipe, and the heat energy is then used to generate electricity in a conventional steam generator.
A collector field comprises many troughs in parallel rows aligned on a north-south axis. This configuration enables the single-axis troughs to track the sun from east to west during the day to ensure that the sun is continuously focused on the receiver pipes. Individual trough systems currently generate up to 80 MW of electricity, although larger systems are possible.
Trough designs can incorporate thermal storage—setting aside the heat transfer fluid in its hot phase—allowing for electricity generation several hours into the evening. Currently, all parabolic trough plants are hybrids, meaning they can use fossil fuel to supplement the solar output during periods of low solar radiation. Typically a natural gas-fired heater or a gas steam boiler is used; troughs also can be integrated with combined cycle or coal-fired plants.
This dish/engine system uses a Stirling engine to produce electricity from heat.
Dish/Engine Systems
A dish/engine system is a stand-alone unit composed primarily of a collector, a receiver, and an engine. The sun's energy is collected and concentrated by a dish-shaped surface onto an receiver that absorbs the energy and transfers it to the engine's working fluid. The engine converts the heat to mechanical power in a manner similar to conventional engines—that is, by compressing the working fluid when it is cold, heating the compressed working fluid, and then expanding it through a turbine or with a piston to produce work. The mechanical power is converted to electrical power by an electric generator or alternator.
Dish/engine systems use dual-axis collectors to track the sun. The ideal concentrator shape is parabolic, created either by a single reflective surface or multiple reflectors, or facets. There are many options for receiver and engine type, including Stirling engine and Brayton receivers.
Dish/engine systems are not commercially available, although ongoing demonstrations indicate good potential. Individual dish/engine systems currently can generate about 25 kilowatts of electricity. More capacity is possible by connecting dishes together. These systems can be combined with natural gas and the resulting hybrid provides continuous power generation.
Solar Two, a demonstration project near Barstow, California.
Power Tower Systems
The sun's energy is concentrated by a field of hundreds or even thousands of mirrors called heliostats onto a receiver on top of a tower. This energy heats molten salt flowing through the receiver, and the salt's heat energy is then used to generate electricity in a conventional steam generator. The molten salt retains heat efficiently, so it can be stored for hours or even days before being used to generate electricity. Solar Two, a demonstration power tower located in the Mojave Desert in California in the 1990s, generated about 10 megawatts of electricity.
The liquid salt at 550°F is pumped from a cold storage tank through the receiver, where it is heated to 1,050°F and then on to a hot tank for storage. When power is needed from the plant, hot salt is pumped to a steam generating system that produces superheated steam to power a turbine and generator. From the steam generator, the salt is returned to the cold tank, where it is stored and eventually reheated in the receiver.
With thermal storage, power towers can operate at an annual capacity factor of 65%, which means they can potentially operate for 65% of the year without a backup fuel source. Without energy storage, solar technologies like this are limited to annual capacity factors near 25%. The power tower and parabolic trough's ability to operate for extended periods on stored solar energy separates them from other intermittent renewable energy technologies.
With one of the best direct normal insolation resources anywhere, the southwestern states are poised to reap large and as yet largely uncaptured economic benefits from this important natural resource. California, Nevada, Arizona, and New Mexico are exploring policies that will nurture the development of their solar-based industries.
In addition to the concentrating solar power projects underway in this country, a number of projects are being developed in Spain, India, Egypt, Morocco, Mexico, Algeria and Australia. Given successful deployment of one or more of these initial markets, additional project opportunities are expected in these and other regions.
Concentrating Photovoltaics
As with other concentrating solar technologies, concentrating photovoltaics (CPV) use optics to concentrate sunlight onto a small area of solar cells. Rather than using the sun's heat to produce steam for electricity generation, these PV cells use semiconductor materials to convert sunlight directly to electricity.
One key competitive advantage of concentrating solar energy systems is their close resemblance to most of the power plants operated by the nation's power industry. Concentrating solar power technologies use many of the same technologies and equipment used by conventional central station power plants, simply substituting the concentrated power of the sun for the combustion of fossil fuels to provide the energy for conversion into electricity. This "evolutionary" aspect—as distinguished from revolutionary or disruptive—results in easy integration into today's central station based electric utility grid. It also makes concentrating solar power technologies the most cost-effective solar option for the production of large-scale electricity generation.
Analysts predict the opening of specialized niche markets in this country for the solar power industry over the next five years. The U.S. Department of Energy estimates that by 2020 there may be as much as 20 gigawatts of concentrating solar power capacity installed worldwide.
Concentrating solar power technologies currently offer the lowest-cost solar electricity for large-scale power generation (10 MW-electric and above). Current technologies cost $3-$3.50 per watt. This results in a cost of solar power of $0.11-0.12 per kilowatt-hour (kWh) in real 2004 dollars. New innovative hybrid systems that combine large concentrating solar power plants with conventional natural gas combined cycle or coal plants can reduce costs to $1.5 per watt and drive the cost of solar power to below $0.08 per kWh.
Advancements in the technology and the use of low-cost thermal storage will allow future concentrating solar power plants to operate for more hours during the day and shift solar power generation to evening hours. Future advances and cost reductions resulting from deployment are expected to allow solar power to be generated for $0.05-$0.06 per kWh in the next few decades.
For more information on current research and development in concentrating solar power, visit the National Renewable Energy Laboratory's Concentrating Solar Power Research.