April 1, 2009

Fossil Fuels + Solar Energy = The Future of Electricity Generation

Dave Ugolini and Dr. Justin Zachary, Bechtel Power Corp., and Hyung Joon Park, Bechtel Enterprises

CSP: Steaming with Solar

Concentrated sunlight has performed useful work for humans for many years. There is one notable example of Auguste Mouchout, a French inventor, who in 1866 successfully powered a steam engine with sunlight, making him the first known person to construct a concentrating solar-powered mechanical device.

Today, a typical CSP system requires several unique components to produce electricity: a concentrator, a receiver, a heat transfer system, and a power conversion device. There are several ways to combine these components to produce a useful solar energy – powered, electricity-producing plant, although the key enabling technology is the collector. Descriptions of the currently available options follow. All of the available CSP technologies require direct sunlight to function and are of limited use in locations with significant cloud cover.

Parabolic Trough Collector. The parabolic trough is considered the most proven technology of all the CSP options. More than 350 MW of electric power have been installed using this technology since the 1980s.

The parabolic trough design begins with a very large curved mirror. The parabolic shape is designed to concentrate solar energy and reflect it onto a single point. The mirror position follows the sun’s movement in the sky, using a motorized device. The cylindrical parabolic reflector is traditionally made of thick glass silver mirrors (4 to 5 mm, or 0.15 to 0.2 inches), but thin glass, plastic films, and other polished metals are also used.

A receiver tube located at the focal point of the parabolic mirror collects the concentrated solar heat energy. This metal tube uses special coatings to maximize energy absorption and minimize heat losses. Flowing inside the tube is a conventional heat transfer fluid (HTF), which absorbs the thermal energy from the concentrated sunlight. Another glass tube, kept under a vacuum to further reduce the heat losses, envelops the receiver tube.

Several receivers connect to a circulating HTF loop, which enables the enlargement of the system. Many loops combine to form a plant that can now leverage its scale to produce economical power. About 4 to 5 acres will generate 1 MW of energy.

The hot HTF produced by the combined loops, serving as the "fuel" source, next enters a steam generator to produce superheated steam. The cooled HTF returns to the CSP modules to be heated again in a closed loop. The generated steam is used to produce power in a conventional steam-bottoming plant. The maximum HTF temperature is ~395C (743F), mainly due to the operational limitation of the synthetic heat transfer fluid (Figure 1).

1. Solar trough technology. Parabolic reflectors focus the sun’s energy on a glass tube filled with a heat transfer fluid in this demonstration plant at the Plataforma Solar de Almería in southern Spain. The heat transfer fluid is used as “fuel” to produce steam that can then produce electricity in a standard steam-bottoming cycle. Courtesy: DLR

The two main disadvantages of trough technology are the relatively low maximum HTF operating temperature, which limits the thermal efficiency of the steam turbine system, and the added complexity of the binary fluid steam generator. However, trough technology is well understood and has a good operations record on a relatively large scale. This experience base may give trough designs an advantage over other, more interesting CSP technologies that are still in their infancy.

Fresnel Collector. The Fresnel solar collector is a line-focus system similar to the parabolic trough, although it uses an array of Fresnel reflectors to concentrate the sun’s energy on a series of receivers. Normally, single-axis tracking flat mirrors fixed to a steel structure are used. Several frames are connected together to form a module, and the modules form a long row up to 450 meters (1,470 feet) long (Figure 2).

2. Fresnel lens technology. Fresnel reflectors concentrate the sun’s energy on a receiver at the Plataforma Solar de Almería, Spain.Courtesy: DLR

The receiver consists of one or more tubes located above the mirrors at a predetermined height. The metal tubes have an absorbent coating very similar to that used in trough technology to increase heat absorption. Inside the tubes flows water or a mixture of water and about 70% quality steam. At the exit of the tubes, water and steam are separated, and saturated steam is produced either for process use or to generate electricity using a conventional Rankine cycle power block.

The Fresnel collector approach does have several technical advantages over the more familiar parabolic lens CSP technology: It can generate steam directly, without the need for an intermediate HTF or binary steam cycle; the optical precision required of the Fresnel lens is less than for a parabolic lens; and the system more easily lends itself to factory production, which means less field construction is required.

However, Fresnel technology is much less mature, and the lower steam temperatures keep the steam turbine cycle efficiency low. In addition, the lower optical efficiency of the Fresnel receiver increases heat losses due to the absence of insulation around the receiver tubes.

Solar Power Tower. In this concept a boiler or gas turbine on top of a tall tower receives concentrated solar radiation from a field of heliostats, which are two-axis tracking mirrors (Figure 3). The heat transfer media could be water or steam, molten salts, or compressed air, although water is usually selected. The heated water temperature — close to 545C (1,013F) — is higher than in any other line-focus system.


3. Power tower technology. German Aerospace Center (DLR) scientists were ableto demonstrate a hybrid solar-powered gas turbine system (230 kW) at Plataforma Solar de Almería. The sun’s energy is focused on a point at the top of the tower to produce heat energythat may be used to generate steam or to heat air. This plant uses three receivers, connected in series, to gradually heat the compressor air of a 250-kW gas turbine to 800C (1,472F). Courtesy: DLR/Steur

The power tower can be connected to a molten salt storage system, thus allowing the system to operate for periods of low or no incident solar energy. The main advantage of this technology is its ability to provide high-temperature superheated steam. The design does require very accurate alignment with solar rays, plus heliostat controls, to avoid potential damage to the receiver on top of the tower.

Note that each of these three CSP technologies operates at different working fluid temperatures (Table 1). This is a key point: Which CSP technology is selected for a solar hybrid plant, be it a gas turbine combined cycle or a conventional steam power plant, will likely be determined based on the temperature of the generated steam and how that temperature matches the fossil fuel – generated steam uses that can be replaced.


Table 1. Summary of concentrated solar technologies. Source: Bechtel Power Corp.