Lahnstr. 16 - 35091 Cölbe - Germanyphone +49-6421-1689400 - fax +49-6421-1689409info@concentratoroptics.com

8.7 Fresnel Lens Optics

Fresnel (pronounced ) lenses have been widely used in solar collectors that concentrate light by refraction. Since it is a flat approximation of a curved lens, a Fresnel lens has less material and weighs less than a corresponding curved lens. This results in lower cost and less optical transmission loss through the lens. An idealized Fresnel lens that focuses parallel rays of light to a point is illustrated in Figure 8.31. The Fresnel lens employs canted facets to approximate the curvature of a lens; the more facets, the better the approximation. Typically, a high-quality linear Fresnel lens will have more than 1000 facets per centimeter.

Several factors limit the effectiveness of Fresnel lenses. The most serious is, perhaps, the sharpness of the facets that can be produced. Any manufacturing process will produce slightly rounded facets as shown in Figure 8.32. Any ray striking the back side of a facet (e.g., ray 1) or the tip or valley of a facet (e.g., rays 2 and 3, respectively), is not properly directed to the receiver. The total fraction of the aperture area represented by these features is ineffective and reduces the ability of a Fresnel lens collector to perform efficiently.

Another feature that limits the performance of a Fresnel lens (or any other lens, for that matter) is dielectric and internal reflection (see Section 8.6). As the angle of incidence of light on a facet increases, the quantity of light reflected due to dielectric reflection increases. Figure 8.29 shows the reflection of light from an air-glass interface. Note that as the angle of incidence approaches 42 degrees, total internal reflection occurs for a glass lens. Thus the edge facets of a Fresnel lens are less effective than facets closer to the center of the lens. The effect is similar for plastic lenses since they have indices of refraction similar to glass (see Appendix D).

A more serious factor limiting the use of refractive optics is the need to track the lens about two axes. This is necessary to keep the concentrated image focused on a receiver that is fixed with respect to the lens. This is true even in the case of a linear lens. Figure 8.33a shows that as the incident rays are inclined (e.g.,

Ray l), in the plane of curvature, the refracted rays move. Thus, as would be expected, a minimum of single-axis tracking is required with a linear Fresnel collector.

Figure 8.33 Two-axis tracking requirements for Fresnel lenses. (a) Effect on refracted beam inclined to lens aperture in the plane of curvature; (b) effect on refracted beam inclined along

linear axis of lens. Solid line is in plane of paper (focus F). Dashed line is out of plane of paper (focus F’); (c) change in focal length of linear Fresnel lens when incident rays inclined at an

angle along the linear axis of the lens. Reprinted with permission from Solar Energy Technology Handbook, Marcel Dekker, New York (1980).

However, a somewhat more subtle effect also due to refraction, occurs if the incident ray is inclined along the linear axis of the Fresnel lens. This is illustrated in Figure 8.33b and 8.33c. The net result, over the entire lens, is the creation of a new focal line for the lens that is above the normal focal line. To maintain the refracted image focused on a receiver that is fixed with respect to the lens, therefore, one must track a Fresnel lens collector (or any lens system) to maintain the incident light normal to the lens aperture. This requires two-axis tracking.

 

8.8 Conclusions

In summary, the impact of the geometric optics of cylindrical and parabolic mirrors is that parabolic optics allow construction of high rim-angle troughs that need track about only one axis, whereas low rim-angle cylindrical troughs may be constructed that need not track at all to maintain a moving linear focus. These discussions of tracking requirements involve only focusing considerations. Complete two-axis tracking of the collector aperture will, of course, increase the amount of insolation incident on the collector aperture by elimination of the cosine effect. The magnitude of this effect for the various tracking modes is discussed in Chapter 4.

Concentrator OpticsConcentrator Optics focuses on nonimaging designs for solar applications. We have developed a multitude of systems, including

Cassegrain optics Fresnel mirror systems

Fresnel lens primaries

Reflective and refractive secondaries

Kaleidoscope homogenizers

We integrate the optical train into module and tracker design in close cooperation with our customers. Our designs are primarily custom-made. The maximum size of a lens parquet in hot embossing technology is defined by the size of the press, i.e. 1250x1300 mm².

Typical designs have concentration ratios of 500-800 X. We design and procure glass secondary optics required to keep the acceptance half-angle above 1.0° for the optical trains used in these systems. Refractive optics limit the concentration ratio due to dispersion. Good designs for the optics and the tools are very important to us. Our design group is well known in the field.

The Book

An indication of the experience of the team of designers shaping Concentrator Optics around co-founder Dr Ralf Leutz is

R. Leutz, A. Suzuki: Nonimaging Fresnel lenses - Design and performance of solar concentrators, Springer 2001

 

Concentrator Optics offers complete solutions encompassing design, prototyping and manufacturing of optical elements for the solar industry. Our speciality are Fresnel lens parquets for concentrating photovoltaics (CPV).

Our customers benefit from our experts' competence gained through academic research on nonimaging optics and commercial development of solar applications. We accompanied several solar projects from design to the market.

Our goal is the technology leadership in the solar lens market. Innovative research is an integral part of our strategy to ensure sustainable growth on a technology basis.

Our PMMA-production line with a full capacity of 30MW/a is installed at our facilities in Cölbe, Germany. In addition to that we are capable of producing Silicone-on-Glass-lenses in prototype volumes and sizes. The industrial process for this product is installed since summer 2010.

We are aware that for larger volumes, we need to license or operate additional lines on location of the power plant. An American subsidiary is being planned, in order to be able to quickly react to customer demand.

Fresnel Lenses are refractive optical elements where the local slope of the surface is decoupled from the global shape of the lens. They offer the degree of freedom needed for a prescribed irradiance distribution on the target. Fresnel lenses are the preferred collector for concentrating photovoltaics.

We offer the design, prototyping, testing and manufacturing of Fresnel lens parquets exceeding an area of 1 m².

Co01 and Co02 Parquets

We offer a standard optical system for CPV modules consisting of a 5x4 lens parquet in two materials and with two types of secondaries, respectively

The optical systems are highly optimized, in turn yielding the best optical efficiency. Tooling for mass manufacturing is on us!

PDF: Co01 & Co02 CPV Optical Systems

Our production line is designed for hot embossing PMMA pre-fab sheets in a vacuum hot/cold process. Our lenses can be made of UV-enhanced and standard PMMA (Polymethylmethacrylate, or acrylic glass).  In the visible range of the solar spectrum, PMMA has a transparency greater than 90% (see graphs below). Standard PMMA gets

transmissive at 380-400nm. This can be enhanced to 360nm in a UV-enhanced PMMA quality which we use as well.

For PMMA lenses, the material is heated above the glass transition temperature Tg (a). The mold is then embossed in the soft material and cooled down below Tg (b). Finally, mold and material are separated (c).

In addition to PMMA lenses we are capable of producing lenses and lens parquets from optical silicones on a glass sheet.

For Silicone-on-glass (SoG) lenses made from two-component optical silicone, a thin layer of the liquid material is applied to the glass surface (a). It is then polymerized in the mold at low pressures and constant temperature and simultaneously laminated on to the glass sheet (b). As soon as the polymerization is complete, the mold can be separated (c).

In the visible range of the solar spectrum our materials have a transparency greater than 90%. In the infrared, all organic materials and silicones have a well developed dip in their transmission curves. The interesting part is the ultraviolet, where standard PMMA opens at 380-400 nm. This can be enhanced to 360 nm in a UV-enhanced PMMA quality which we use as well. Silicones and glass are transparent in the UV.

The refractive indices of our materials are as follows: PMMA 1.49, Silicone 1.41/1.42. All materials appear very clear. Their surfaces can be embossed to a roughness below 10 nm. 

Fresnel losses at the surfaces are depending on the refractive index of the material, the number of surfaces, and the angles of the surfaces the light passes. There are no exact comparative measurements of actual systems made in the used materials. We are working on this, and results are expected  by Spring 2011.

There is no comprehensive study on the longevity and performance of the materials used for lens making for CPV, PMMA, and Silicone. The latter is new, and tests cannot be expected to have produced results. Accelerated aging tests like the one prescribed by the norm IEC 62108 have been passed by PMMA and Silicone under glass. There are numerous singular records that PMMA survives more than 20 years outdoors.

The decrease in efficiency max be given as 0.2% points per year.

PMMA has the strongest track record, we are working with the supplier on guaranteeing the product over 20 years. There is a PMMA study with our participation under way at NREL, and one for Silicone-on-Glass has been started.

Fresnel Lens

Green Fresnel lens

Fresnel lens precision graphics

Product ID: Fresnel Lens

Fresnel LensIt is a type of lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses.The design enables the construction of lenses of large aperture and short focal length without the weight and volume of material that would be required in conventional lens design. Compared to conventional bulky lenses, the Fresnel lens is much thinner, larger, and flatter, capturing more oblique light from a light source.

DescriptionCompared to conventional bulky lenses, this kind of lens reduces the amount of material required compared to a conventional spherical lens by breaking the lens into a set of concentric annular sections known as "Fresnel zones", which are theoretically limitless. In the first variations of the lens, each zone was actually a different prism. Though a Fresnel lens might look like a single piece of glass, closer examination reveals that it is many small pieces. For each of these zones, the overall thickness of the lens is decreased, effectively chopping the continuous surface of a standard lens into a set of surfaces of the same curvature, with discontinuities in steps between them. In fact this product can be regarded as an array of prisms positioned in a circular fashion, with steeper prisms on the edges and a near flat convex center. Fresnel lenses are usually made of glass or plastic; their size varies from small to large. Since plastic lenses can be made larger than glass lenses, as well as being much cheaper and lighter, they are used to concentrate sunlight in multiple uses.

Application

Solar energy is by far the Earth's most available energy source, easily capable of providing many times the total current energy demand.

Solar Lighting System

Hybrid solar lighting is an active solar method of providing interior illumination. HSL systems collect sunlight using fresnel lens that concentrate the sun light and use optical fibers to transmit it inside the building to supplement conventional lighting. In single-story applications these systems are able to transmit 70%~80% of the direct sunlight received.

High Concentrated Photovoltaics

High concentration photovoltaics (HCPV) systems employ concentrating optics consisting of dish reflectors or fresnel lenses that concentrate sunlight to intensities of 300 suns or more. The conversion rate of compound based chip, such as GaAs/GAInP/Ge crystalline (III-V), can achieve around 30% to 41.6% max. HCPV is the compound based solution that provides the highest efficiency. There are two types of HCPV system: one is reflective type using either reflective dish or linear Fresnel technology, the other one is refractive type using circular Fresnel lens or lens array.

Stirling Engine

A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work. The Stirling engine is noted for its high efficiency (up to 40%), quiet operation, and the ease with which it can use almost any heat source like concentrated light source from Fresnel lens.

Solar Cooker

Solar cooker is a device which uses only sunlight as its energy source. Because required no fuel to operate it. There are a variety of types of solar cookers. The fresnel type is used to concentrate sunlight into a hot spot and use some type of reflective metal to reflective the heat to a small cooking area. The temperature can reach up to 400°F or even higher. However, the cooker enable the sun to bake, boil, and steam a wide verity of food while enhancing the taste and benefiting the environment.