Advancements, challenges, and opportunities in the measurement of high heat flux for concentrated solar thermal systems

Concentrated solar thermal (CST) systems, capable of driving high-temperature processes up to 2000 K, hold significant potential for chemical and industrial applications. This paper reviews methods for measuring concentrated solar flux, focusing on their role in calculating key optical performance metrics such as concentration ratio and optical efficiency. It also explores the challenges associated with these measurements. Flux measurement methods are categorized into direct and indirect approaches. Direct methods use heat flux gauges positioned on a stationary or moving target to measure solar flux, while indirect methods rely on a CCD or CMOS camera, a Lambertian target, and a heat flux gauge. The camera captures images of solar radiation, which are calibrated to heat flux values using readings from the heat flux gauge. Heat flux gauges are a critical component of both direct and indirect measurement methods. This study also reviews and compares various types of heat flux gauges, including Gardon radiometers, Schmidt-Boelter gauges, Kendall radiometers, heat flux microsensors, and calorimeters. The comparison considers factors such as maximum flux rating, response time, durability, water-cooling requirements, cost, and measurement uncertainty. Calibration techniques for these gauges are also discussed. It is observed that indirect methods are commonly used for measuring concentrated solar flux due to their higher resolution, which enables them to identify hot spots and measure concentrated solar flux more accurately than direct methods. The key challenges in measuring concentrated solar flux include calibrating sensors under solar conditions, ensuring sensor durability, accounting for environmental factors, and managing cost considerations.