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[en] This paper proposes a method for representing multiple, three-dimensional images in a single binary digital Fresnel hologram. First, jittered sampling is applied to each image. Second, a Fresnel hologram is generated to represent the sampled images so that each of them will occupy a different area on the focal plane. Finally, the Fresnel hologram is binarized with sign-thresholding. When the binary hologram is illuminated with an on-axis reference plane wave, each source image will be reconstructed at their designated position that is separated from the others. The use of jittered sampling is effective in preserving the shaded region in the images, which otherwise would be lost after the hologram is binarized with sign-thresholding, as well as suppressing the aliasing images. (paper)
[en] The solar thermal performance of a 102 kW rated thermal capacity linear Fresnel reflector system was evaluated experimentally under fixed and variable airflow rate through the oil-air heat exchanger. The solar thermal system consisted of 240 mirrors (linear Fresnel solar reflectors), each of 1485 by 625 mm size to heat the thermal oil, which was used to heat the air using the oil to air heat exchanger (AHE). The results show that the actual maximum efficiency of the solar collector lies in the range of 0.28 to 0.34, whereas the calculated one is about 0.54 for the variable airflow rates. However, for a fixed airflow rate, the actual maximum efficiency is between 0.26 and 0.30. It implies that the actual solar collector efficiency is much lower than the calculated values. One of the major factors, which is not accounted for in the theoretical efficiency calculation, is the effect of dust on the primary and secondary mirrors. The investigated site is observing considerable sand movement, which is affecting the thermal output of the solar collector. The maximum heat exchanger effectiveness, ε, obtained for variable and fixed airflow conditions is 0.9 and 0.8, respectively.
[en] The caustic of a two-dimensional parabola illuminated off-axis is the Tschirnhausen cubic and does not contain any singular points. The caustic of a three-dimensional paraboloid of revolution similarly illuminated consists of two cusped sheets containing a hyperbolic umbilic singularity. We relate these two situations by considering a suitable sequence of illuminated elliptic paraboloids
[en] The energy distribution along the focal axis of a long metallic cylindrical parabolic reflector with a plasma layer on its surface in the presence of an external magnetic field is investigated. The effects of some physical parameters, such as the plasma frequency, the wave frequency and the thickness of plasma layer on the energy distribution and the reflected and transmitted electromagnetic fields, are simulated. These investigations for both S- and P-polarizations have been done separately. It is found that the maximum value of the reflected intensity increases by increasing the incident wave frequency and by decreasing the plasma layer thickness and the plasma frequency for both polarizations. Furthermore, the results show that the increase of the magnetic field strength can cause an increase in the reflected intensity for S-polarization and a slight decrease for P-polarization
[en] A dynamical theory is given for calculating the performances of stratified Fresnel linear zone plates (SFLZP's). The Born expansion extended to the Fresnel diffraction is used to prove a fundamental theorem giving the diffraction efficiency and the diffraction pattern of transmission or reflection SFLZP's. The method is valuable as long as the wavelength is smaller than the characteristic parameter of the zone plate. It allows us to evaluate the performances of reflection multilayer SFLZPs, i.e., the so-called Bragg-Fresnel optics recently developed for x-ray optics. 17 refs., 8 figs
[en] By using a helicon plasma sputtering technique, a one-dimensional Ti/Al multilayer zone plate with an outermost layer width of 76 mm has been successfully fabricated. A Bragg-Fresnel lens has been made by combining this zone plate with a Ge(422) crystal. Comparison of the Ti/Al multilayer zone plate with the Ag/Al zone plate is discussed in terms of focusing efficiency
[en] Highlights: • A scalable linear Fresnel reflector which can supply different temperatures is proposed. • Inclination design of the mechanical structure is used to reduce the end losses. • The maximum thermal efficiency of 64% is achieved in Guangzhou. - Abstract: This paper proposes a scalable linear Fresnel reflector (SLFR) solar system. The optical mirror field which contains an array of linear plat mirrors closed to each other is designed to eliminate the inter-low shading and blocking. Scalable mechanical mirror support which can place different number of mirrors is designed to supply different temperatures. The mechanical structure can be inclined to reduce the end losses. Finally, the thermal efficiency of the SLFR with two stage mirrors is tested. After adjustment, the maximum thermal efficiency of 64% is obtained and the mean thermal efficiency is higher than that before adjustment. The results indicate that the end losses have been reduced effectively by the inclination design and excellent thermal performance can be obtained by the SLFR after adjustment.
[en] Highlights: → A combined domed Fresnel lens - CPC PVT system is designed and characterized. → Electrical and thermal experiments have been performed. → CFD analysis has been used to determine thermal characteristic dimensionless numbers. - Abstract: An advanced solar unit is designed to match the needs of building integration and concentrating photovoltaic/thermal generation. The unit proposed accurately combines three elements: a domed linear Fresnel lens as primary concentrator, a compound parabolic reflector as secondary concentrator and a photovoltaic-thermal module. In this work the photovoltaic-thermal generator is built, analysed and characterized. Models for the electrical and thermal behaviour of the module are developed and validated experimentally. Applying a thermal resistances approach the results from both models are combined. Finally, efficiency electrical and thermal curves are derived from theoretical analysis showing good agreement with experimental measurements.
[en] The aim of this project was the development of a solar cooker for travellers. The cooker shouldn't weight more than 2 kg including the cooking pot, it should be easy to handle and it should cook for two within an hour. The first project phase was an intense theoretical study of the market, patents, competitors and mirror geometries. The result were two innovating ideas: A 'linear Fresnel geometry' and a 'section of a parabola'. Both geometries allow the cooking pot to be near the ground. In the second phase, prototypes were built and tested. A first series allowed to choose which technology to follow and gave us ideas for the advanced models of the second series. These models (of the 'linear Fresnel' type) are foldable due to a scissor structure, they can slightly change their shape according to the sun's height and the cooking pot has its windshield. We reached the goal of an innovative solar cooker weighting less than our limit. It takes a reasonable time to set up, it is well transportable and its stability to wind is sufficient. The cooking power is a bit lower than planned, but rice, vegetables and even a small bread have been successfully cooked. (author)
[en] Highlights: • Linear Fresnel reflectors (LFRs) have great potential for cost reductions. • Concentration in the receiver central strip as high as in trough collectors. • Daily constant flux map in the receiver if the filling factor is adequately designed. • High concentration variation between summer and winter for N–S configurations. - Abstract: The late exponential development of Concentrating Solar Power (CSP) technology has driven to a very high power installed worldwide, but with no time for global optimization of the technology. High feed-in-tariffs have concentrated investments on trough collectors and central towers, previously studied during the 1980s. Linear Fresnel reflectors (LFRs) are regarded as a low efficiency technology, which is mainly due to very little previous research. However, the use of slightly bent mirrors drives to high concentration ratios, with obvious cost advantages over other CSP technologies. This paper studies the radiation flux obtained in a flat receiver using different mirror shapes, and analyzes its variation along the year. Linear Fresnel reflector design variables are reviewed, and a Ray Tracing model of the Fresdemo prototype is carried out. Results show higher performances than expected