A compact linear Fresnel reflector (CLFR) – also referred to as a concentrating linear Fresnel reflector - is a specific type of linear Fresnel reflector (LFR) technology. Linear Fresnel reflectors use long, thin segments of mirrors to focus sunlight onto a fixed absorber located at a common focal point of the reflectors. These mirrors are capable of concentrating the sun’s energy to approximately 30 times its normal intensity

Intensity (physics)

In physics, intensity is a measure of the energy flux, averaged over the period of the wave. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech...

. This concentrated energy is transferred through the absorber into some thermal fluid (this is typically oil capable of maintaining liquid state at very high temperatures). The fluid then goes through a heat exchanger

Heat exchanger

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 to power a steam generator

Boiler (steam generator)

A boiler or steam generator is a device used to create steam by applying heat energy to water. Although the definitions are somewhat flexible, it can be said that older steam generators were commonly termed boilers and worked at low to medium pressure but, at pressures above this, it is more...

. As opposed to traditional LFR’s, the CLFR utilizes multiple absorbers within the vicinity of the mirrors.

History

The first linear Fresnel reflector was developed in Italy in 1961 by Giovanni Francia of the University of Genoa

University of Genoa

The University of Genoa is one of the largest universities in Italy.Located in Liguria on the Italian Riviera, the university was founded in 1471. It currently has about 40,000 students, 1,800 teaching and research staff and about 1,580 administrative staff.- Campus :The University of Genoa is...

. Francia demonstrated that such a system could create elevated temperatures capable of making a fluid do work. The technology was further investigated by companies such as the FMC Corporation during the 1973 oil crisis

1973 oil crisis

The 1973 oil crisis started in October 1973, when the members of Organization of Arab Petroleum Exporting Countries or the OAPEC proclaimed an oil embargo. This was "in response to the U.S. decision to re-supply the Israeli military" during the Yom Kippur war. It lasted until March 1974. With the...

, but remained relatively untouched until the early 1990s. In 1993, the first CLFR was developed at the University of Sydney

University of Sydney

The University of Sydney is a public university located in Sydney, New South Wales. The main campus spreads across the suburbs of Camperdown and Darlington on the southwestern outskirts of the Sydney CBD. Founded in 1850, it is the oldest university in Australia and Oceania...

 in 1993 and patented in 1995. In 1999, the CLFR design was enhanced by the introduction of the advanced absorber. In 2003 the concept was extended to 3D

Three-dimensional space

Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...

 geometry. Research published in 2010 showed that higher concentrations and / or higher acceptance angles

Acceptance angle (solar concentrator)

Acceptance angle is the maximum angle at which incoming sunlight can be captured by a solar concentrator. Its value depends on the concentration of the optic and the refractive index in which the receiver is immersed...

 could be obtained by using nonimaging optics

Nonimaging optics

Nonimaging optics is the branch of optics concerned with the optimal transfer of light radiation between a source and a target...

 to explore different degrees of freedom in the system such as varying the size and curvature of the heliostat

Heliostat

A heliostat is a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun's apparent motions in the sky. The target may be a physical object, distant from the heliostat, or a direction in space...

s, placing them at a varying height (on a wave-shape curve) and combining the resulting primary with nonimaging secondaries.

Reflectors

The reflectors are located at the base of the system and converge the sun’s rays into the absorber. A key component that makes all LFR’s more advantageous than traditional parabolic trough mirror systems is the use of "Fresnel reflectors". These reflectors make use of the Fresnel lens

Fresnel lens

A Fresnel lens is a type of lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses.The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design...

 effect, which allows for a concentrating mirror with a large aperture

Aperture

In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

 and short focal length

Focal length

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 while simultaneously reducing the volume of material required for the reflector. This greatly reduces the system’s cost since sagged-glass parabolic reflector

Parabolic reflector

A parabolic reflector is a reflective device used to collect or project energy such as light, sound, or radio waves. Its shape is that of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis...

s are typically very expensive. It should be noted, however, that in recent years thin-film nanotechnology

Nanotechnology

Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...

 has significantly reduced the cost of parabolic mirrors.

A major challenge that must be addressed in any solar concentrating technology is the changing intensity of the incident rays (the rays of sunlight striking the mirrors) as the sun progresses throughout the day. The reflectors of a CLFR are typically aligned in a north-south orientation and turn about a single axis using a computer controlled solar tracker

Solar tracker

A solar tracker is a generic term used to describe devices that orient various payloads toward the sun. Payloads can be photovoltaic panels, reflectors, lenses or other optical devices....

 system. This allows the system to maintain the proper angle of incidence

Angle of incidence

Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...

 between the sun’s rays and the mirrors, thereby optimizing energy transfer.

Absorbers

The absorber is located at the focal point

Focus (optics)

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

 of the mirrors. It runs parallel to and above the reflector segments to transport radiation into some working thermal fluid. The basic design of the absorber for the CLFR system is an inverted air cavity with a glass cover enclosing insulated steam tubes, shown in Fig.2. This design has been demonstrated to be simple and cost effective with good optical and thermal performance.

For optimum performance of the CLFR, several design factors of the absorber must be optimized.

• First, heat transfer between the absorber and the thermal fluid must be maximized. This relies on the surface of the steam tubes being selective. A selective surface
  Selective surface
  In solar thermal collectors, a selective surface or selective absorber is a means of increasing its operation temperature and/or efficiency...
  
   optimizes the ratio of energy absorbed to energy emitted. Acceptable surfaces generally absorb 96% of incident radiation while emitting only 7% through infra-red radiation. Electro-chemically deposited black chrome is generally used for its ample performance and ability to withstand high temperatures.
• Second, the absorber must be designed so that the temperature distribution across the selective surface is uniform. Non-uniform temperature distribution leads to accelerated degradation of the surface. Typically, a uniform temperature of 300 °C (573.2 K; 572 °F) is desired. Uniform distributions are obtained by changing absorber parameters such as the thickness of insulation above the plate, the size of the aperture of the absorber and the shape and depth of the air cavity.

As opposed to the traditional LFR, the CLFR makes use of multiple absorbers within the vicinity of its mirrors. These additional absorbers allow the mirrors to alternate their inclination, as illustrated in Fig. 3. This arrangement is advantageous for several reasons.

• First, alternating inclinations minimize the effect of reflectors blocking adjacent reflectors’ access to sunlight, thereby improving the system's efficiency.
• Second, multiple absorbers minimize the amount of ground space required for installation. This in turn reduces cost to procure and prepare the land.
• Finally, having the panels in close proximity reduces the length of absorber lines, which reduces both thermal losses through the absorber lines and overall cost for the system.

Applications

In March 2009, the German company Novatec Biosol constructed the Fresnel solar power plant known as PE 1. The solar thermal power plant is based on CLFR technology and has an electrical capacity of 1.4 MW. PE 1 comprises a solar boiler with mirror surface of approximately 18000 m² (1.8 ha; 4.4 acre). The steam is generated by concentrating sunlight directly onto a linear receiver, which is 7.4 metres (24.28 ft) above the ground. An absorber tube is positioned in the focal line of the mirror field where water is heated into 270 °C (543.2 K; 518 °F) saturated steam. This steam in turn powers a generator.

The commercial success of the PE 1 has led Novatec Biosol to design a 30 MW solar power plant known as PE 2. PE 2 will be constructed in Murcia, Spain in 2010. Novatec Biosol has also obtained permits for another 60 MW of related projects.

In April 2008, the solar thermal company AREVA Solar (Ausra) opened a large factory in Las Vegas, Nevada that will produce linear Fresnel reflectors. The factory will be capable of producing enough solar collectors to provide 200 MW of power per month.

AREVA Solar (Ausra) has finished construction of the 5 MW Kimberlina Solar Thermal Energy plant in Bakersfield, California. This is the first commercial linear Fresnel reflector plant in the United States. The solar collectors were produced at the Ausra factory in Las Vegas.

AREVA Solar (Ausra) also built and operates a linear fresnel reflector plant in New South Wales, Australia. This reflector plant supplements the 2,000 MW coal-fired Liddell Power Station. The power generated by the solar thermal steam system is used to provide electricity for the plant's operation, offsetting the plant’s internal power usage.

Solar Fire, an appropriate technology

Appropriate technology

Appropriate technology is an ideological movement originally articulated as "intermediate technology" by the economist Dr...

 NGO in India, has developed an open source

Open source

The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...

 design for a small, manually operated, 12 kW peak Fresnel concentrator that generates temperatures up to 750 °C (1,023.2 K; 1,382 °F) and can be used for various thermal applications including steam powered electricity generation.

See also

• Concentrating solar power
• List of solar thermal power stations
• Solar energy
• Solar thermal energy
  Solar thermal energy
  Solar thermal energy is a technology for harnessing solar energy for thermal energy . Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are flat plates generally used to heat...