Optical design of compact linear fresnel reflector systems

https://doi.org/10.1016/j.solmat.2017.12.016Get rights and content

Highlights

  • Geometrical models for CLFR systems with polar orientation are developed.

  • A comparative study is conducted for LFR and CLFR systems through theoretical modelling and experimental investigations.

  • CLFR-hybrid system has the best optical performance compared with the conventional LFR and CLFR-complete systems.

  • Geometrical concentration ratio of 15.14 and ground utilization ratio of 0.95 are achieved with a small CLFR-hybrid system.

Abstract

Compact linear Fresnel reflector (CLFR) system employing multiple receivers is promising with better optical performance and cost effectiveness compared to linear Fresnel reflector (LFR) system, especially for applications with limited ground availabilities. Nevertheless, only few researches have been conducted to evaluate optical design and performance of the CLFR system. In this study, geometrical models for the CLFR system with flat mirrors and receivers are developed on the basis of polar orientation. A comparative study of concentration characteristics among the LFR, CLFR-complete and CLFR-hybrid systems is conducted based on numerical, ray tracing simulation and experimental results. In addition, optical design analyses of the CLFR-hybrid system are carried out from various design aspects. It is noteworthy that the mirror arrangement and focal length should be optimized for the CLFR-hybrid system with considerations of the associated geometrical characteristic and optical performance. For a small-scale CLFR-hybrid system with a solar field width of 2100 mm and a focal length of 1500 mm, the geometrical concentration ratio of 15.14 and ground utilization ratio of 0.95 are achieved respectively. The findings demonstrate the feasibility of the CLFR-hybrid system with flat mirrors and polar orientation, which provide progress to the concentrated solar power technology.

Introduction

The supply shortage of conventional fossil fuels and environmental concern about climate changing lead to a great need to harness renewable energy for a sustainable future. Solar energy is regarded as the largest available carbon-neutral energy on the planet, which offers great potentials to be utilized by various technologies [1]. Besides direct conversion of the solar radiation into electricity with photovoltaic technology, the solar energy also can be transformed into thermal energy by solar collectors. Generally, solar collectors are categorized into non-concentrating and concentrating types. Non-concentrating solar collectors are commonly used in domestic hot water heating systems, whereas concentrating solar collectors are popular in thermal power generations with high efficiencies at medium to high operational temperatures, especially for locations receiving high solar beam radiation [2].

Based on the focus geometry, concentrating solar collectors are classified into parabolic tough concentrator (PTC), solar tower concentrator (STC), parabolic dish and linear Fresnel reflector (LFR) concentrator. Though PTC is a more mature and widely applied technology, its higher initial cost, complex manufacture process and serious challenge of wind resistance are noted [3]. Alternatively, the LFR system in a simple design is capable of harnessing the solar radiation efficiently with less land usage [4]. It has been proved to be a better choice for energy production where land availability is limited [5]. For the LFR system with a single centred receiver as illustrated in Fig. 1, each mirror element is suitably placed at a proposed tilt angle so that the incident solar radiation converges onto the receiver after reflection.

Optical designs of the LFR system have been investigated from different aspects, such as solar field layout and orientation, tracking algorithm and aiming strategy, reflector curvature and receiver configuration [6]. Among them, the solar field orientation influences significantly the other optical parameters. For instance, the complexity of a tracking mechanism depends on the orientation. Three common orientations are namely north-south, east-west and polar axis. Both north-south and east-west orientations are mostly preferred for applications, with which tracking schemes are based on the flat ground. In operation, reflectors follow the sun movement by varying their tilt angles under the control of a tracking system [7], [8]. Consequently, end or lateral losses may occur when the reflected solar rays do not impinge on the receiver due to the longitudinal component of the solar radiation [9], [10]. On the other hand, mirrors involved in the LFR system are commonly predefined with constant width and shift for the sake of simplicity. Such a simple optical design results in shading of the incoming solar radiation and blocking of the reflected solar radiation by adjacent mirror elements [11], which are illustrated in Fig. 2. Numerous researches have been conducted to minimize energy losses owing to the end, inter-row shading and blocking effects. For example, optical variables such as the width and position of each mirror element, gap between consecutive mirrors and receiver height have been considered in literatures [9], [10], [12], [13]. Abbas and Martínez-Val [13] observed an evident increase in the annual energy efficiency for a LFR system with mirrors of variable widths and shifts, but more complicated optical modelling and additional installation cost are required accordingly. On the other hand, the receiver has remarkable impacts on optical and thermal performances of the LFR system as well, which can be assembled horizontally, vertically or in triangular configuration [14]. To address the drawbacks of shading and blocking of the LFR system, Barlev, Vidu and Stroeve [1] suggested increasing receiver height, but the spacing between two adjacent mirror elements causes more ground usage and investment on the receiver tower. Contrarily, the system with polar axis orientation is more efficient with the simplest tracking system and the absorbed solar energy can be maximized while energy losses are minimized [15]. The polar orientated system has great adaptability to terrain with reduced installation requirement and cost. The simple polar mounted system usually adopts one single axis aligned to be roughly parallel to the axis of the earth rotation around the north and south poles. On the other hand, to ensure the solar field perpendicular to the solar rays throughout the year, various designs of the dual-axis solar tracker are proposed to follow the sun movement in both elevation and azimuth directions as reflectors and receiver are fixed at predefined tilt angles and positions [16], [17]. Comparatively, dual-axis tackers are more promising for small-scale applications, whereas more structural considerations should be taken for large scale applications due to variable inclination angles of the entire solar field. Practically, the impacts of high slope and large solar field not only complicate the support structure required to elevate the array, but also increase the wind load which rises exponentially with height above the ground [18].

Alternatively, an innovative concept of compact linear Fresnel reflector (CLFR) system with multiple receivers is developed to minimize energy losses as depicted in Fig. 3. Different from the LFR system, one receiver is installed at each side of the solar field, which allows consecutive mirrors to redirect the sunlight to the two receivers respectively. The CLFR system has better system cost effectiveness [19] and is promising for applications with limited ground availabilities [1].

The CLFR system has been investigated with limited optical designs and available demonstrations. Mills and Morrison [20] evaluated the CLFR concept applied for a large-scale solar thermal electricity generation plant from various aspects, including the receiver orientation and structure, mirror field configuration and packing density. Chaves and Collares-Pereira [21] assessed variable mirror sizes and shapes for the CLFR system. In particular, Montes, Rubbia, Abbas and Martínez-Val [19] presented two CLFR configurations, as shown in Fig. 4. Mirrors are arranged with alternating tilts for pointing to one or the other receiver in the CLFR-complete configuration, while only mirrors located in the central solar field alternate their tilts in the CLFR-hybrid configuration. They observed that both CLFR configurations successfully minimize the blocking and shading losses, whereas lateral losses decrease the optical efficiency due to the great dispersion of reflected rays from mirrors located far from the receivers.

In terms of optical performance evaluations of the LFR and CLFR systems, studies have been conducted through numerical modelling [7], [9], [22], [23], [24], ray tracing simulation [10], [13], [25], [26], [27] and experiment [28], [29], [30], [31]. For instance, Barale, Heimsath, Nitz and Toro [6] adopted a generalized methodology to optimize geometry for a LFR prototype built in Sicily, including optical modelling with ray tracing techniques and effect analyses of mirror and receiver geometries. On the other hand, Pino, Caro, Rosa and Guerra [29] simulated a LFR solar plant using optical and thermodynamic modelling, which was validated with operating data gathered from a supervisor system.

Researches in the optical design and performance evaluation of the CLFR system through simulation and experiment are limited in the literature. The investigations in the existing literature generally have been conducted for the CLFR system with north-south and east-west orientations, while the polar orientated CLFR system has not been concerned much. Furthermore, the optical performance comparison between the LFR and CLFR systems has been rarely mentioned in the literature, which would be desirable for realizing and promoting the CLFR system application. Thus, the optical design and performance evaluation of a small-scale CLFR system with polar orientation are carried out in this study. For the sake of simplicity and cost effectiveness, flat reflection mirrors are employed, which are made of glass. Besides, a comparative study of the concentration characteristics among the LFR, CLFR-complete and CLFR-hybrid configurations is performed through geometrical modelling, ray tracing simulation as well as experiment. To validate the geometrical models, experimental investigations are implemented with a small-scale test rig in Nottingham, UK. Through numerical simulation, optical design of the CLFR-hybrid system is assessed from different design aspects (e.g. mirror array arrangement, solar field area, focal length, receiver tilt angle, etc.) by evaluating associated geometrical parameters and optical performance.

Section snippets

Geometrical modelling of CLFR system

Geometrical modelling of the CLFR system is developed by referring to the methodology in literatures [23], [32]. The modelling is based on polar orientation, flat mirrors are defined with variable widths and spacing to eliminate end, shading and blocking effects. Two flat receivers are designed to absorb the reflected solar rays with a given full width (2B) and an inclination angle (β). In addition, the sun conical angle (represented by 2Ω1=32) is considered for more accurate optical design.

Modelling approach

Matlab is used to solve the developed geometrical models in Section 2, and then the mirror element profile including the width, tilt angle and reference position for a given system can be produced accordingly. To investigate solar radiation distributions on receiver surfaces, ray tracing simulation is implemented with the advanced TracePro program. The ray tracing simulation is capable of verifying the defined tilt angles and shifts of mirrors whether lateral, shading and blocking problems

Experiment description

A small-scale CLFR test rig is designed and constructed in Nottingham (latitude of 52.97° and longitude of 1.18°). Specifications of the test rig are presented in Table 1. The test rig has an overall solar field area of 2.10 m2, which is feasible to track the sun movement freely. Due to the small-scale set up, narrow flat mirrors and horizontally placed flat receivers are employed.

As shown in Fig. 8, the test rig mainly comprises a base framework, a solar field platform with supporting beams,

Mirror array arrangement

Regarding to operability of a small-scale system with polar orientation, the solar field with a width of 2100 mm and a length of 1000 mm is adopted for the LFR, CLFR-hybrid and CLFR-complete systems at a focal length of 1500 mm. In addition, three arrangements are proposed for the CLFR-hybrid configuration with different ratios of the common tilting mirrors to the alternating ones. As listed in Table 2, the total number of mirror elements varies greatly for different configurations. With the same

Conclusions

In this paper, detailed geometrical modelling has been established for the CLFR system with polar orientation, which employs flat mirrors and receivers. Based on the numerical modelling, ray tracing simulation and experimental work, a comparative study of concentration characteristics among the LFR, CLFR-complete and CLFR-hybrid systems has been conducted. Moreover, optical design analyses of the CLFR-hybrid system have been performed from different design aspects (i.e. limited solar field

Acknowledgements

The authors gratefully acknowledge the scholarship support from the Faculty of Engineering of the University of Nottingham.

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