  Citation: Montenon, A.C.; Meligy, R. Control Strategies Applied to a Heat Transfer Loop of a Linear Fresnel Collector. Energies 2022, 15, 3338. https://doi.org/10.3390/en15093338 Academic Editors: Abdul-Ghani Olabi, Michele Dassisti and Zhien Zhang Received: 30 March 2022 Accepted: 28 April 2022 Published: 4 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Article Control Strategies Applied to a Heat Transfer Loop of a Linear Fresnel Collector Alaric Christian Montenon 1, * ,† and Rowida Meligy 2,† 1 Energy, Environment and Water Research Center, The Cyprus Institute, 20 Konstantinou Kavafi Street, Aglantzia 2121, Cyprus 2 Mechatronics Department, Faculty of Engineering, Helwan University, Cairo 11795, Egypt; rowida.meligy@h-eng.helwan.edu.eg * Correspondence: a.montenon@cyi.ac.cy; Tel.: +357-222-08-672 † These authors contributed equally to this work. Abstract: The modelling of Linear Fresnel Collectors (LFCs) is crucial in order to predict accurate performance for annual yields and to define proper commands to design the suitable controller. The ISO 9806 modelling, applied to thermal collectors, presents some gaps especially with concentration collectors including LFCs notably due to the factorisation of the incidence angle modifiers and the fact that they are considered symmetric around the south meridian. The present work details the use of two alternative modellings methodologies based on recorded experimental data on the solar system installed at the Cyprus Institute, in the outskirts of Nicosia, Cyprus. The first modelling is the RealTrackEff, which is an improved ISO9806 modelling, and the second is constructed using the CARNOT blockset in MATLAB/Simulink. Both models include all the elements of the heat transfer fluid loop, i.e., mineral oil, with a tank and a heat-exchanger. First, the open loop’s studies demonstrated that the root mean square on temperature is 1 ◦ C with the RealTrackEff; 2.9 ◦ C with the CARNOT and 6.3 ◦ C with the ISO9806 in comparison to the experimental data. Then, a PID control is applied on the experimental values in order to estimate the impact on the outlet temperature on the absorber and on power generation. Results showed that the error on the estimation of the heat absorbed reaches 32%. Keywords: linear fresnel reflector; modelling; control; heat transfer fluid 1. Introduction Linear Fresnel Collectors (LFCs) are one of the four main concentration technologies available [1], which are divided into two categories: the point focusing (central receivers [2] and dish-Stirling [3]) and the linear focusing (LFCs [4] and parabolic trough collectors [5]). The latter ones rely on linear receivers tubes [6–8] assembled in series. They can be evacuated or atmospheric, sometimes mixed [9]. The tubes are absorbers that receive the concentrated solar income reflected by the primary optics or reflector. It transfers the heat to a fluid in motion, a heat transfer fluid (HTF), sometimes for direct steam generation by evaporating water [10]. The reflector is composed of numerous (almost) flat mirrors [11] moving on a single axis to track the sun to reflect the DNI (Direct Normal Irradiance) on the linear receiver. Usually, the LFCs are topped by a secondary mirror above the tubes to harmonise the distribution of the solar flux around the absorber wall [12]. While point-focusing technologies are suitable for electricity generation at high temperatures (>600 ◦ C), the LFCs are perfect candidates for mid- temperature levels (150–450 ◦ C), especially for heat-processing purpose [13]. Although they require more land usage, the LFCs compared to other concentration technologies reportedly offer lower investment costs [14]. However, the reliability of the technology shall be supported by the accuracy of its annual yield prediction for techno-economic considerations [15]. To this end, the ISO9806:2017 is often used to predict the quasi-dynamic behaviour of solar thermal collectors [16]. Energies 2022, 15, 3338. https://doi.org/10.3390/en15093338 https://www.mdpi.com/journal/energies