Experimental Investigation of the Influence of Longitudinal Tilt Angles on the Thermal Performance of a Small-Scale Linear Fresnel Reflector  
[PERSON]  
1 Polytechnic School of Engineering of Cijon, University of Oviedo, 33203 Asturias, Spain; [EMAIL_ADDRESS]  
2 Department of Electrical Engineering, University of Oviedo, 33003 Oviedo, Spain; [EMAIL_ADDRESS]  
[PERSON]  
2 Department of Electrical Engineering, University of Oviedo, 33003 Oviedo, Spain; [EMAIL_ADDRESS]  
[PERSON]  
3 Department of Mathematics, University of Oviedo, 33003 Oviedo, Spain  
33 Department of Mathematics, University of Oviedo, 3  
energy sources that will be able to replace fossil fuels given that this energy is clean, free, and unlimited. Photovoltaic systems produce electricity directly and can be divided into two technologies: nonconcentrating photovoltaic systems (\(NCPV\)s) and concentrating photovoltaic systems (\(CPV\)s). In an \(NCPV\) system, the \(PV\) cells receive nonconcentrating solar irradiance. These systems can use a solar tracking system or remain at a fixed tilt angle throughout the year. Solar tracking systems are classified according to their motion: (i) two rotation axes (dual-axis trackers) or (ii) a single rotation axis (single-axis trackers). Although dual-axis trackers generate the most power, single-axis trackers are the most widely used. In contrast, in a \(CPV\) system, optical devices (mirrors, lenses) concentrate the solar irradiance incident on the collector area into a much smaller area, achieving a significant increase in the flow of solar energy incident on the photovoltaic cells. These systems require a solar tracking system. Depending on the geometric concentration ratio (defined as the number of times solar irradiance is concentrated), expressed in number of sums (1 sun equals 1000 (W/m\({}^{2}\)), \(CPV\) technology can be classified into [3] high-concentration photovoltaics (\(HCPV\)s) (between 100 sums and 1000 sums), medium-concentration photovoltaics (\(MCPV\)s) (between 10 sums and 100 sums), and low-concentration photovoltaics (\(LCPV\)s) (from 2 suns to 10 sums). Some of the differences between concentrating and nonconcentrating systems are shown below:  
1. \(PV\) cell efficiency. \(NCPV\) systems use commercial silicon cells with efficiencies between 10% and 20% [4]. In contrast, \(CPV\) systems use multijunction cells with an efficiency as high as 39% [4]. 2. The solar tracking system. A solar tracking system is mandatory in a \(CPV\) system. However, solar tracking is optional in an \(NCPV\) system even though the use thereof increases electricity production. The use of solar tracking systems in \(CPV\) and \(NCPV\) systems complicates their implementation and increases their cost. 3. Incident solar irradiance on \(PV\) cells. The beam, diffuse, and ground-reflected components of solar irradiance are incident on the \(PV\) cells in \(NCPV\) systems. In contrast, only the beam component is incident on the \(PV\) cells in \(CPV\) systems. The available diffuse solar irradiance often represents a significant fraction of the solar irradiance on a tilted surface. There are even some locations where this component is decisive. 4. The operating temperature of the \(PV\) cells. The energy production of a \(PV\) system depends on the operating temperature of the \(PV\) cells [5]. For example, manufacturers of crystalline photovoltaic cells estimate a decrease of between 0.4% and 0.65% in electrical efficiency per \({}^{\circ}\)C increase over the reference operating temperature [6]. In \(CPV\) systems, the concentration of solar irradiance causes an increase in the temperature of the \(PV\) cells, but it is easier to implement a cooling system due to the size of the \(PV\) cell system. In contrast, it is not possible to implement a cooling system in \(NCPV\) system due to the large size of the \(PV\) cell system.