Optimising position control of a solar parabolic trough

Puramanathan Naidoo,Theo I Van Niekerk

doi:10.4102/sajs.v107i3/4.452

Puramanathan Naidoo, Theo I Van Niekerk

Open Access

https://doi.org/10.4102/sajs.v107i3/4.452

Copy DOI

Abstract

In today’s climate of growing energy needs and increasing environmental concerns, alternatives to the use of non-renewable and polluting fossil fuels have to be investigated. One such alternative is solar energy. This study is based on the implementation of a mathematical computation – the PSA (Plataforma Solar de Almeria) computation developed at PSA (the European Test Centre for solar energy applications) – embedded in a control algorithm to locate the position of the sun. Tests were conducted on a solar parabolic trough (SPT) constructed at the Solar Thermal Applications Research Laboratory of the Mangosuthu University of Technology (Durban, South Africa) for optimal position control using the PSA value. The designed control algorithm embedded in an industrial Siemens S7-314 C-2PtP programmable logic controller compared the PSA computation to a measured position of the SPT to optimally rotate the SPT to a desired position with the constant movement of the sun. The two main angles of the sun relative to the position of the SPT on earth, the zenith angle and the azimuth angle, both calculated in the PSA from the vertical and horizontal planes, respectively, were applied to the control algorithm to generate an appropriate final tracking angle within a 0.007 radian (0° 24′ 3.6″) tolerance, in accordance to the construction specifications and solar collector testing standards of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE, 1991). These values, together with the longitude and latitude applicable to the geographical location of the SPT, were processed in the control software to rotate the SPT to an optimal position with respect to the position of the sun in its daily path, for solar-to-thermal conversion.

Highlights

A solar parabolic trough (SPT) is a linear solar collector with, as its name implies, a parabolic cross section
Each parabolic trough collector and its associated receiver tube, tracking device and controls form a modular collector assembly that can be connected in various series or parallel-flow circuits to achieve a range of performance characteristics
The calculated and measured values from the Plataforma Solar de Almeria (PSA) computation and incremental encoder, respectively, are expressed as pulse counts because the programmable logic controller (PLC) software processes them in this format to generate a corrective output signal when there is a difference between the two input values at the summing point of the controller (Figure 3)

Summary

Introduction

A solar parabolic trough (SPT) is a linear solar collector with, as its name implies, a parabolic cross section. Its reflective surface concentrates sunlight onto a receiver tube located along the trough’s single horizontal focal line, tracking the sun along only one axis, either north–south or east–west.[1,2,3] A north–south orientation provides slightly more energy per year than an east– west orientation, but its winter output is low in mid-latitudes whereas an east–west orientation provides a more constant output throughout the year.[4] The SPT at STARlab was constructed and installed in a north–south position to track the sun in an east–west direction, referred to as a oneaxis SPT. Each parabolic trough collector and its associated receiver tube, tracking device and controls form a modular collector assembly that can be connected in various series or parallel-flow circuits to achieve a range of performance characteristics. Solar tracking is important in systems that operate under concentrated sunlight.[1,5,6]

Methods

Results

Conclusion