Development of a Planar Focusing Collector for CSP
In the field of the concentrating solar power (CSP) technology, the parabolic trough is a common choice for a solar collector. Parabolic troughs are highly reflective curved mirrors that take incident sunlight and focus it onto a line-shaped receiver that runs down the center of troughs. The receivers are generally heat-absorbing pipes containing a heat transfer fluid that delivers the collected thermal energy to steam turbines or engines for generating electricity. However, parabolic troughs are constrained by wind loads and cost of materials and support structure. The goal of this project was to develop a planar focusing collector (PFC) that would serve as an alternative to the parabolic trough. An ideal PFC is ultrathin (less than a few microns) and lightweight, and could be manufactured in a manner that is scalable and low cost. Furthermore, these features would be attractive to CSP technology because of the potential to reduce cost and complexity of installation and maintenance. The work pursued in this project attempted to actualize the PFC concept by utilizing advances in the emerging area of metasurfaces, which rely on the nanopatterning of surfaces to control the amplitude and phase of light at length scales smaller than an optical wavelength. Academic pursuits of metasurface technology predominately employ electron-beam lithography (EBL) for fabrication because the technique offers highly reliable nanopatterning of surfaces with a spatial resolution of ~10 nm. However, due to the techno-economic constraints of the DOE’s SunShot program, particularly cost-effective scalability, the use of EBL was avoided in this project. Rather, fabrication approaches that lend themselves to scalable manufacturing by way of stamping or imprinting were favored. Ultimately, the tradeoffs with using such methods was the larger feature size—on the order of ~150 um critical dimension—resulting in poorer control of light compared to structures fabricated by EBL. A summary of our 2-year investigation on this topic is presented herein. We begin by presenting highlights of our 1 st -year efforts using plasmonics, i.e., metallic nanostructures, as the building block for our PFC. The limitations in this approach are subsequently summarized. We then review the strategy we employed during the final year of our project, in particular utilizing insulating or dielectric materials as the light controlling medium. We round out the report discussing the results of our findings from using this approach. The report is relatively self-contained in that and no prior knowledge of optics or nanotechnology is assumed and technical jargon is kept at a minimum.