Project Description
Thermoplastic-based Fresnel Sunlight Concentrators for Photovoltaic Application: in cooperation with Ioffe Institute (Russia)
Supported by the Russian Foundation of Basic Research for the period 2017-2019, the Ioffe Institute uses the provided grants for initiative scientific projects carried out by small groups of scientists. The present research is primarily focused on spreading of 3D printing potentiality for high-efficiency Fresnel-based optics solutions within concentrator photovoltaic technology while overcoming the economic and technological bottlenecks in costly conventional optics processing.
Experimental solar tracker as developed by the Ioffe Institute for Fresnel lens modules. (Picture courtesy of Ioffe Institute)
Project Summary – Ioffe FL Lens Application
Fresnel lenses (FLs) are increasingly used in modern actively developing photovoltaics, becoming the main type of sunlight concentrators in PV modules of different designs. The most justified from the economical point of view is the creation of PV modules, in which a high radiation concentration ratio (higher than 100) is ensured. In using multijunction solar cells (SCs) with the efficiency of greater than 40% (AM1.5 D, 1000 W/m2) in such devices, the overall efficiency of the modules can not significantly exceed 30%.
The key moment for reducing a substantial difference in efficiency values is raising the optical efficiency of Fresnel lenses and ensuring high optical and power parameters in the “FL-SC” pair as a whole. In this case, there exist enough problems associated with the development of models for projecting the refracting profile and methods for its optimization, with searching and synthesizing new optical materials, with peculiarities of fabrication techniques and with estimation of FL optical and power characteristics (OPCs).
General trend for concentrator PV: from large mirrors to small aperture Fresnel lenses at saving high concentration ratio. (Picture courtesy of Ioffe Institute)
Silicon-on-Glass Fresnel Lenses – Advantages and Disadvantages
In the last two decades in the concentrator photovoltaics, the most widely used are combined FLs of “silicon-on-glass” type. Such FLs consist of solar-grade silica glass and of a Fresnel profile formed by polymerization of transparent silicone, what ensures a high optical efficiency of lenses at the low consumption and the cost of materials and has the potential for easy technology transfer to mass production of lens panels. However, matrices being fabricated by the high-precision diamond turning method appear to be rather expensive.
It is obvious that any experiments on the creation of new specimens involve additional expenses and require substantial time. Disadvantages of combined lenses also include significant distinctions in refraction indices of glass and silicone, what does not allow achieving limiting values of the lens optical efficiency due to additional losses for reflection at the “glass-lens” interface.
Thermoplastics based 3D printed Fresnel lenses are versatile in application and easy to rework.
Additive Optics Fabrication of Fresnel Lenses for CPV
Application of 3D printing technology for creating optical elements concentrating radiation allows widening substantially the spectrum of materials being used including those with the increased (compared to glass) refraction index and raising manufacturability of the FL production process. A typical peculiarity of 3D printing is the economical use of basic material, and, in a number of cases, also the possibility for its easy secondary processing, what facilitates carrying out the optics design and rapid prototyping of functional elements.
3D Printing Fresnel Optics: Challenging and Promising
The use of transparent thermoplastics in the production of optical concentrators of a refracting type for solar photovoltaic installations is a comparatively new task. The main challenges related to 3D printing technology are:
– assurance of precise formation of profiles of refracting surfaces on a large area and of sufficient homogeneity of the concentrator material to minimize optical losses;
– selection or synthesis of optical materials with stability of properties under solar radiation.
The integration of the experience of the Photovoltaic Converters Laboratory of Ioffe Institute, in the part of designing and analyzing FL parameters, carrying out optimization procedures and experimental investigations and the experience of the Luximprint additive optics fabrication company in developing the 3D printing method for optical materials allows distributing additive technologies to the field of optical radiation concentrators for the interests of high-efficient solar power engineering.
Creating optical elements by means of additive optics fabrication allows widening substantially the spectrum of materials being used including those with the increased (compared to glass) refractive index.
Luximprint Collaboration with the Ioffe Institute
The impact to the development of new custom optics from the Ioffe Institute including the following points:
– selection of promising solutions for sunlight concentrators of a FL type on the base of thermoplastic materials;
– design of a mathematical model and an algorithm for tracing beams in complex multi-element systems for the sunlight concentration;
– development of a procedure for synthesizing a profile of reflecting (refracting) surfaces of sunlight concentration systems ensuring the required levels of average concentration on a receiver and characteristics of distribution of concentrated radiation on it;
– optimization of constructive parameters of high-efficient FLs including those with a variable pitch and aspherically profiled of working faces allowing obtaining a preset irradiance profile on a SC surface;
– development of a procedure for calculating OPCs of sunlight concentration systems with aspheric refracting surfaces with allowing for the effect of technological and operational factors on this characteristics;
– simulation the effect of technological and operational factors (rounding of vertices and cavities of profile notches, roughness of their surfaces, lens optical material temperature) on FL OPCs;
– determination of sources of geometrical errors of the Fresnel profile of fabricated lenses, registration of profile form geometrical deviations and refracting surface roughness; allowance for obtained parameters in correcting programs of calculation and analysis of lens OPCs;
– investigation of FL optical and physical-mechanical characteristics obtained from thermoplastic materials; determination of achievable values of the optical efficiency and the concentration ratio;
– performance of investigations of spatial and spectral distributions of concentrated radiation in the FL focal plane;
– correction of mathematical models for designing FLs and of calculation of their OPCs to take into account more adequately the effect of technological factors of the 3D printing method factors on them.
A section of a PV module based on two Fresnel lenses and two solar cells. (Picture courtesy of Ioffe Institute)
Unique Project Features
During the project, Ioffe Institute was able to immediately take benefit of some interesting features the Luximprint process has to offer, enabling them to do a fast and effective research job. The main benefits of using additive optics fabrication for this project included, among others:
- No Tooling Involved: with direct CAD-to-Optic manufacture, there was no need to order physical manufacturing tooling. Apart from the cost-savings that were realized, the lead times were limited to days rather than weeks or even months enabling Ioffe to do a fast and effective test job;
- Easy Product Iterations: after testing the initial lens parts, some unexpected aberrations appeared. As there was an option to improve the performance by slightly tweaking the design, a second prototype was prepared and forwarded for testing;
- Multiple Product Variations: along with the second masterpiece, a reference profile was provided to enable the Ioffe Institute to do a fast and effective job in setting up the measurement profile.
Final Conclusions
The cooperation between the Ioffe Institute and Luximprint is very valuable and lead to an even better understanding of improving the CPV cell efficiency and application possibilities at the one hand, while providing a deeper discovery of the manufacturing capabilities and process flexibility at the other hand. In the end, the best compromise between those resulted in a satisfactory outcome for all parties involved.
Being able to keep the cost low and the offering versatile, Ioffe Institute was able to quickly equip its demonstrators, create measurement profiles and test the lenses in its developed in-house application. Not limited by the economic boundaries of conventional optics fabrication technologies, such as long lead times and significant upfront cost, a very efficient and future-proof methodology of testing could be introduced.
Together with the Ioffe Institute, we look forward to contributing to an even greater future for highly-efficient CPV solar cells and greatly encourage its use in the most versatile applications!
Within joint collaboration, the activity of Ioffe Institute is partly supported by Grant 17-08-00247-a from Russian Foundation of Basic Research.
About the Ioffe Institute
The Photovoltaics Lab of the Ioffe Institute is one of Russia’s largest institutions for research in physics and technology with a wide variety of operating projects. It was founded in 1918 and run for several decades by Abram F. Ioffe. So it is quite natural that the Institute bears the name of this outstanding scholar and organizer.
The Photovoltaic converters laboratory deploys extensive studies of heterostructure solar cells for terrestrial applications, which lead to the development of high-efficient solar cells operating under concentrated solar radiation and photovoltaic installations with concentrators and solar trackers.
