Development of low-cost focus control systems for vacuum-membrane solar dish facets

Abstract

Concentrating solar power (CSP) is a growing method of harnessing energy from the sun for generating electricity and process heat, especially in South Africa which boasts one of the most plentiful solar resources globally. A small-scale CSP system, consisting of a multi-faceted concentrator that employs vacuum-membrane technology, is actively being developed at the University of Pretoria. The facets constituting this innovative design are constructed from a reflective polymer-based membrane adhered to the rims of readily available and cost-effective elliptical television antennas. A crucial step involves creating a vacuum within each facet, forming a near-parabolic membrane shape. Previous studies found that the membrane depth shifts slightly due to varying ambient conditions throughout an operational day. These slight depth shifts lead to major focal point shifts, reducing the CSP system’s overall efficiency and performance. The first goal of this research was to examine in more detail how static ambient conditions impact the displacement of membranes used on vacuum-membrane solar-dishes. A controlled-environment enclosure was employed to achieve this, allowing for the independent manipulation of a facet’s ambient pressure and temperature. The second goal was to investigate methods to mitigate membrane displacement. Various manufacturing techniques were investigated within the controlled-environment enclosure, which included alterations in pretension, changes in membrane thickness by removing the removable plastic layer on the EverBright mirror film, and adjustments to overall facet sizes. Results revealed that ambient temperature impacted the membrane displacement significantly more than ambient pressure. It was also determined that opting for a small facet with a thin membrane and high pretension will effectively minimise membrane displacement. This, however, would not suffice to mitigate membrane displacement. The outdoor test results of a facet without a focus control system indicated that solar radiation, specifically global horizontal irradiance (GHI), affected the internal temperature (depending on the wind velocity), and therefore also affected the membrane depth. Furthermore, to further reduce membrane displacement, low-cost focus control systems were investigated. A focus control system for USD 29.34 maintained a constant differential pressure for a vacuum-membrane facet within the required accuracy of ±2 mm membrane displacement. An attempt was made to further mitigate membrane displacement by incorporating the effects of temperature on membrane stiffness, which demonstrated slight improvements. A focus control system consisting of a low-cost Hall effect module actively monitoring membrane depth emerged as the most effective in eliminating membrane displacement, with an increase of about 0.09 mm and a decrease of approximately 0.02 mm from an initial depth of 10 mm. This level of stability will ensure that the facet maintains a consistent optical performance, ultimately advancing the reliability and efficiency of low-cost vacuum-membrane technology.