The University of Arizona
Self Focusing heliostats with Closed Loop Tracking
Principal Investigator: J. Roger Angel angelj@email.arizona.edu
DOE ID: 38488-005 Project Title: Self-Focusing Heliostats With Closed-Loop Tracking
PI: Roger Angel, the University of Arizona
NREL TM: Stephanie Meyen
Sandia POC: Randy Brost
POP: 04/04/23–10/31/24
Budget: $399,980
Abstract:
The project goal was to design, manufacture, and test a powerful new type of heliostat and study its application for CSP industrial heat application. The targeted improvement beyond industry baseline heliostat technology focused on concentration and receiver temperature. High temperature requires high concentration, i.e., focusing as much sunlight as possible onto a small receiver. A single heliostat achieves its highest possible concentration (set by the second law of thermodynamics) by forming a disc image of the sun at the optical focusing limit of the sun’s angular diameter. Because the sun’s position changes throughout the day, the reflector shape must change over time as well to maintain a perfect disc image. If the shape does not change, as is the case with state-of-the-art heliostats, defocus and spillage occurs, and the achieved concentration is less. An 8-m2 heliostat was built using a target-oriented dual-axis mount with mechanical coupling to: (1) bend the reflector to maintain a focused disc image of the sun, despite different angles of incidence throughout the day; and (2) use a novel sun-tracking camera, also mechanically linked to the mount drive, for closed-loop automatic control of tracking to high precision. A receiver design to exploit fields of such heliostats for stable receiver temperatures of up to 1500°C (compared to the currently achievable maximum of ~700°C) was also realized.
Actual Outcome: The project was successfully completed within the proposed timeframe. A prototype of this novel heliostat design was built and set up for testing at the University of Arizona grounds. Together with the closed-loop tracking technique, a sharp sun disc image was successfully demonstrated. The heliostat was able to maintain the sun disc image close to the theoretical diameter and reliably centered on the target during a full day (Figure 8 and Figure 10). It worked so well that the team used the heliostat to observe the solar eclipse, which was visible in May 2024 in Arizona (Figure 9).
Figure 8. Demonstration of heliostat while it is focusing on a target at 113 m distance
Figure 9. Close view of the focused sun disc image during a solar eclipse
Figure 10. Top row shows a typical sun disc image of heliostats that don’t change shape. Bottom row shows the sun disc image using this shape-changing heliostat at four different times during a day.
Impact: A design was developed for a solar field comprised of this novel heliostat, producing high concentration and enabling new commercial uses for solar industrial heat at high temperature. This design was proposed to a new funding opportunity for a pilot demonstration project of this design. 431 heliostats of 7 m2 each can reach an annual average thermal power of 1 MW. That power focused on a small spot of less than 1-m diameter enables industrial heating to high temperature over 1,000°C (or 1832°F). The main advantage of this heliostat design lies in commercial risk mitigation of heliostat-based projects. The improvement in the heliostat’s optical performance and reliability reduces uncertainties in overall performance. Additionally, the high temperatures achievable by the concentration improvement enable CSP to explore other markets and industries, such as hydrogen production.
Topic area: Advanced Manufacturing
Topic area: Field Deployment