Zhenwu Lu

Design of Novel Compound Fresnel Lens for High-Performance Photovoltaic Concentrator

We present a new design of compound Fresnel-R concentrator which is composed of two lenses: a primary lens (Fresnel lens) that works by total internal reflection at outer sawteeth but refraction at inner sawteeth, and a ringed secondary lens that works by refraction. In contrast to previous Fresnel lens concentrators, this design increases the acceptance angle, improves the irradiance uniformity on the solar cell, and reduces the aspect ratio significantly. Meanwhile several sawteeth of the primary Fresnel lens can correspond to a same ring of secondary lens, which will efficiently lower the complexity of designing and manufacturing. Moreover, in order to reduce the influence of manufacturing tolerances and to increase the optical efficiency further, the central part of the bottom of the secondary lens which directly adhered to the solar cell is designed as a cone-shaped prism to collect the sunlight that does not reach the solar cell. Finally, we provide simulations and analyses of the design method an optical efficiency more than 80% and an aspect ratio smaller than 0.5 can be achieved.

Lithographic fabrication of large diffractive optical elements on a concave lens surface.

We demonstrate experimentally the lithography technique to fabricate a large computer-generated diffractive optical element (DOE) pattern on a concave lens surface with precise alignment by using a laser direct writer. We obtained photoresist film with uniform thickness on the large concave substrate by selecting proper spin-coating parameters, which mainly involve spin rate, spin acceleration, and viscosity of the photoresist. We obtained a square line profile on the concave lens surface. We can write lines that range in width from 0.7 to 10 microm using a single pass of the laser beam. We have designed and fabricated a grating on the concave lens surface using the laser direct writing lithography technique. It is believed that this technique can also transfer large DOE patterns, with a continuous surface relief, onto a convex or concave lens (mirror) surface.

Modeling and simulation of 1 MWe solar tower plant’s solar flux distribution on the central cavity receiver

Abstract The solar flux distribution rule inside a central cavity receiver is of great significance to the safe operation of solar tower power plants. In this paper, a heliostat field model was fully developed to simulate the solar flux distribution on the inner surfaces of a cavity receiver of a solar tower power plant by means of the Monte-Carlo ray-tracing method. In addition, the mathematical modeling process that starts from the incident solar rays to the absorbed energy by the inner surfaces of the cavity receiver was presented in detail. According to the final layout of the heliostat field, a dynamic simulation of the solar flux inside the cavity receiver during the vernal equinox was performed. The results indicated that the incident energy reflected by the heliostat field was mainly distributed on the rear and lateral surfaces throughout the day. Moreover, at different time points, the solar flux distribution rule inside the cavity receiver was also analyzed in detail. In order to verify the validity of this model, the simulation results were taken to compare with the experimental data of a random heliostat. Furthermore, to further testify the accuracy of our model, the simulation results obtained by inputting the coordinates of the CESA-I’s heliostat field into our model were also taken to compare with the published experiment data. Ultimately, both of the comparative results show that they can be good references for the safe design of the whole system.

Fabrication of large diffractive optical elements in thick film on a concave lens surface

We demonstrate experimentally the technique of fabricating large diffractive optical elements (DOEs) in thick film on a concave lens surface (mirrors) with precise alignment by using the strategy of double exposure. We adopt the method of double exposure to overcome the difficulty of processing thick photoresist on a large curved substrate. A uniform thick film with arbitrary thickness on a concave lens can be obtained with this technique. We fabricate a large concentric circular grating with a 10-ìm period on a concave lens surface in film with a thickness of 2.0 ìm after development. It is believed that this technique can also be used to fabricate larger DOEs in thicker film on the concave or convex lens surface with precise alignment. There are other potential applications of this technique, such as fabrication of micro-optoelectromechanical systems (MOEMS) or microelectromechanical systems (MEMS) and fabrication of microlens arrays on a large concave lens surface or convex lens surface with precise alignment.

Dwell time algorithm in ion beam figuring

To get a high-precision optical surface, the deconvolved process of dwell time was transferred to a matrix equation in which the damped factor and the extra removal amount were introduced to expand the freedom of solution. A path weight factor and a surface error weight factor were used to take the scanning path and the initial surface error into account. Combined with the Gerchberg bandlimited extrapolation algorithm for initial surface error map extension, a high-precision final surface could be obtained within a factual aperture. Two surface error maps were calculated to rms = 0.1 nm from rms = 130.23 nm and to rms = 0.08 nm from rms = 82.74 nm. The simulations show that a perfect dwell time solution could be obtained by the revised matrix equation and initial surface error map extension with the help of the least squares QR (LSQR) algorithm.

Optimization procedure for design of heliostat field layout of a 1MWe solar tower thermal power plant

A procedure for designing and optimizing heliostat field layout of solar tower thermal power plant is developed. The ray tracing is used for the calculation of the optical efficiency of field. The mathematical theory of the calculation is derived. The parametric search algorithm, which allows variation of the field parameters within a specified range, is used for the optimization of field. The field layout is made automatically according to the no-blocking loss condition and the heliostats are located at the positions where the annual incident cosine value is higher. In this way, the blocking and cosine losses are lowered. Because the optimization of the distance between fore-and-aft two rows of heliostat is avoided, the computer time is reduced effectively. Using this procedure the heliostat field of a 1MWe solar tower power plant was designed. Four modes of layout including North-South cornfield, North-South stagger, Radial cornfield and Radial stagger were experimented and optimized respectively. The comparison of the field efficiency for the four optimized results was made. It is concluded that the North-South cornfield layout is the optimal decision for the 1MWe solar tower power plant.

Automated optimization of an aspheric light-emitting diode lens for uniform illumination

In this paper, an automated optimization method in the sequential mode of ZEMAX is proposed in the design of an aspheric lens with uniform illuminance for an LED source. A feedback modification is introduced in the design for the LED extended source. The user-defined merit function is written out by using ZEMAX programming language macros language and, as an example, optimum parameters of an aspheric lens are obtained via running an optimization. The optical simulation results show that the illumination efficiency and uniformity can reach 83% and 90%, respectively, on a target surface of 40 mm diameter and at 60 mm away for a 1×1 mm LED source.

Lithographic fabrication of large curved hologram by laser writer.

We fabricated a large curved computer-generated hologram pattern on the concave substrate with diameter of 110mm and radius of curvature of 504mm. The line width of the hologram varied form 39um to 810um. We adopt the single pass and the screw-lines to fabricate this curved hologram precisely and efficiently. 80% of the fabrication time is saved by this method. This work will be useful to the measurement of large convex secondary mirrors that is also hardness at present.

Single step fabrication of microlens arrays with hybrid HfO2-SiO2 sol-gel glass on conventional lens surface.

We demonstrate a novel method for fabricating glass microlens (arrays) with single step on conventional lens surface. In this method, the glass microlens can be achieved by only one step with sol gel glass material. The microlens aperture and focus length can be controlled easily and uniformly. The fabricated sample shows good focusing property. This work will be useful to improve the performance of compound eyes optical system such as camera, telescope, 3D integral imaging and so on.

Third-order aberration fields of pupil decentered optical systems.

By introducing a transformed pupil vector into the aberration expansions of an axially symmetric optical system, the aberration coefficients through third order of a pupil decentered off-axis optical system are obtained. Nodal aberration characteristics are revealed only by means of the pupil decentration vector and the aberration coefficients of the axially symmetric system, which shows great convenience since parameters of individual surface such as radius of curvature, decenter as well as the shifted center of the aberration field are not used in the analysis.