Assessment of the Pointing Error of Heliostats with a Single Not Polar Rotation Axis for Urban Applications

Significance Statement

In view of increasing energy efficiency with the use of renewable energy, natural daylighting incorporated into buildings has been frequently highlighted due to an achievable reduction in economic cost and greenhouse gas emissions.

Various concentration systems are needed for natural lighting devices for the provision of increased collection of light energy for buildings and urban applications. One of such natural lighting devices is heliostats, which are commonly utilized in small urban applications.

An University of Cordoba researcher team led by Professors Manuel Torres Roldan, Rafael López Luque and Marta Varo Martínez published a research paper in journal, Solar Energy, which analyzes a previously designed heliostat system on its behavior and pointing errors when it’s rotation axis is not oriented along the direction of the Earth’s polar axis.

The authors’ objective was to improve the previous designed polar heliostat system, based on Fahrenheit mechanism, which needs to be oriented towards the Earth’s rotation axis. The improvement deals with the transformation of the polar heliostat mechanism to a generic one with the aid of an automatic control of its rotational speed, thereby eliminating any form of potential architectural restrictions of buildings as a result of the need of redirection in the polar heliostats.

The developed generic heliostat was capable of reflecting the sunbeams with high precision in the desired direction when its rotational speed was controlled automatically.

Considering a fixed focus F located at a height H in the zenithal direction of a terrestrial reference system and a horizontal square plane of side 6H, an average pointing error of 0.01 rad was discovered when the generic heliostat proposed is located at any point of the 36% of the horizontal surface while the average pointing error is below 0.02 rad when the heliostat is located at the 62% of the surface.

When analyzing the behavior of the proposed generic heliostat, it was discovered from graphs of different values of mapping coefficients and angular components that as the mechanism approaches that of the polar heliostat, a better performance was achieved.

The proposed generic heliostat system with Fahrenheit mechanisms and non-polar rotational axis directions can be optimally applied in urban environments due to the achieved admissible pointing errors for smaller urban applications.

Reference

Torres-Roldán, M., López-Luque, R., Varo-Martínez, M.  Assessment of the Pointing Error of Heliostats with a Single Not Polar Rotation Axis for Urban Applications, Solar Energy 137 (2016) 281–289.

Research Group of Physics for Renewable Energies and Resources, University of Córdoba, Albert Einstein Building, Campus de Rabanales, 14071 Córdoba, Spain.

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