Improved solar power reflector

Dr. Neil Canter, Contributing Editor | TLT Tech Beat December 2009

Researchers develop a new lightweight and durable reflective surface to replace glass in parabolic mirrors.

KEY CONCEPTS
• Parabolic mirrors arranged in a trough shape are used to concentrate incoming solar radiation over 70 times to capture sunlight at a rate of 1,000 watts per square meter.
• A new silver-metalized reflective film has been developed that is lower in weight, more durable and more cost-effective than glass.
• An upgrade in the thermal stability of the heat transfer fluids used to transfer solar energy captured by the parabolic mirrors is desirable.

Development of alternative energy sources has been a daunting task from both the technology and economic standpoints. A case in point is capturing solar energy for use in generating electricity.

In a previous TLT article, a heat transfer fluid is found to be an integral component in a parabolic mirror used to capture solar energy for the generation of electricity. The parabolic mirror efficiently captures and focuses solar rays onto a receiver pipe containing a heat transfer fluid (1).

Keith Gawlik, senior engineer at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), says, “The use of parabolic mirrors to capture solar energy dates back to the first installations in the 1980s. In a period between 1984 and 1991, the Solar Electric Generating Systems (SEGS) was established in the U.S. in the Mojave Desert in California. SEGS includes nine plants with a total capacity of 354 megawatts.”

Parabolic mirrors are arranged in a trough shape that concentrates the incoming solar radiation over 70 times to capture sunlight at a rate of 1,000 watts per square meter. Focusing the solar rays on the receiver pipe raises the temperature of the heat transfer fluid close to 400 C, according to Gawlik. The typical heat transfer fluid is a synthetic based on aromatic technology.

The curved shape of a parabola is advantageous in utilizing a higher concentration of sunlight. Flat collectors will not concentrate incoming sunlight to any extent. Gawlik says, “A high concentration of solar energy is not collected through the use of a flat surface. Incoming solar radiation is captured in a linear fashion and leads to raising the temperature of a heat transfer fluid only to between 65 C and 82 C.”

Flat collectors are appropriate for domestic use such as placement on the top of houses. In contrast, a series of parabolic mirror collectors are placed in frames and situated on the ground to form solar power plants. One-axis tracking is utilized to enable them to follow the sun’s movement across the sky during the day.

Gawlik says, “The early parabolic troughs were radically different than today’s designs. The frame in particular is lighter and stiffer to improve efficiency and reduce cost.”

But the reflective surface of the mirror has not changed. Gawlik says, “Glass has been used because of its optical quality and ease of cleaning. It is a mature technology but has disadvantages, which are weight, ease of breakage and cost.”

The high weight of glass increases the cost of the rigid frame used to support it. Glass can break fairly easily, which means that replacement costs are fairly high and time-consuming as the mirrors are situated in remote locations.

Utilization of a lighter-weight and more durable reflective surface may improve the efficiency and reduce the cost of parabolic mirror-based solar plants. Such a surface has not been available until now.

SILVER-METALIZED FILM
Engineers at NREL, in collaboration with SkyFuel, Inc., in Albuquerque, N.M., have developed a new material that can replace glass in the parabolic mirrors. Gawlik says, “We prepared a highly reflective surface that contains multiple layers of polymer film in combination with an inner layer of pure silver. This surface is laminated to aluminum sheets to create the mirror and incorporated into an aluminum space frame to produce the actual mirror unit.”

The silver-metalized film is known as Reflec Tech® Mirror Film and exhibits superior durability compared to glass. The fully assembled solar collector is known as SkyTrough.™

Gawlik says, “The film is much less susceptible to breakage than glass. Dropping objects onto the silver-metalized film just leads to the formation of small dents that hardly affect its overall performance.”

The weight savings achieved with the silver-metalized film compared to glass are also significant. Gawlik explains, “A conventional parabolic trough with glass mirrors, with dimensions of 8 meters long-by-5 meters wide and an aperture of 40 square meters, weighs approximately 1,200 kilograms. In comparison, a 14-meter long-by-6-meter wide trough using the silver-metalized film reflectors containing an aperture of 82 square meters weighs 50% less or 600 kilograms.”

Of significance is that the silver-metalized film-based trough is much bigger in size and has twice the aperture of the trough using glass mirrors. Yet, the former weighs much less. The aperture represents the area that solar radiation actually impinges on the collector. A larger area is favored to increase the amount of energy captured from the sun.

NREL has been working to optimize the performance of the silver-metalized film trough. Gawlik says, “We have set up an advanced prototype trough atop South Table Mountain in Colorado to evaluate its performance. One of the key parameters measured is the optical efficiency, which is the ratio of the thermal energy generated to the solar energy collected by the mirror.”

The prototype trough uses a lower temperature heat transfer fluid (water glycol-based) in order to reduce heat losses to a minimum. The trough is also on a two-axis tracker with the second axis moving in the vertical direction. Figure 1 shows snow being cleaned off the prototype trough.


Figure 1. A prototype trough containing a silver-metalized film reflector has been developed as a more cost-effective alternative to glass. The film weighs less and is more durable. (Courtesy of the Department of Energy’s National Renewable Energy Laboratory)

Gawlik indicates that work is concluding on the optimization of the trough using silver-metalized film. Among the modifications are improvements to the frame and in the tube supports. At the same time, lab work has been conducted to improve the reflectivity and durability of the silver-metalized film.

Gawlik believes that an upgrade in the thermal stability of the heat transfer fluid also will enhance performance. He says, “We are limited by the physical properties of the current aromatic-based synthetic fluids, which are stable up to 400 C. Systems can be run at 600 C without any difficulty. A higher temperature would improve the efficiency of the power cycle and reduce cost.”

One concern is that focusing solar energy on the heat transfer fluid can lead to the generation of hydrogen. The receiving pipe containing the heat transfer fluid is maintained under vacuum in a similar fashion to a thermos. A high level of hydrogen can lead to the loss of thermal insulation. Getters are commonly used, according to Gawlik, to absorb the hydrogen. More stable heat transfer fluids are needed to minimize hydrogen formation.

This technology has the potential to improve the cost-performance of solar collectors. It is a recipient of the 2009 R&D Top 100 Award given out by R&D Magazine. Further information on the silver-metalized film can be found in a recent patent (2) or by contacting Gawlik at keith.gawlik@nrel.gov. 

REFERENCES
1. Canter, N. (2009), “Heat Transfer Fluids: Selection, Maintenance & New Applications,” TLT, 65 (5), pp. 28–35.
2. Jorgensen, G. and Gee, R. (2006), “Durable Corrosion and Ultraviolet-Resistant Silver Mirror,” U.S. Patent 6,989,924 B1.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.