Optical performance analysis of an innovative linear focus secondary trough solar concentrating system

Abstract

The parabolic trough solar concentrating system has been well developed and widely used in commercial solar thermal power plants. However, the conventional system has its drawbacks when connecting receiver tube parts and enhancing the concentration ratio. To overcome those inherent disadvantages, in this paper, an innovative concept of linear focus secondary trough concentrating system was proposed, which consists of a fixed parabolic trough concentrator, one or more heliostats, and a fixed tube receiver. The proposed system not only avoids the end loss and connection problem on the receiver during the tracking process but also opens up the possibility to increase the concentration ratio by enlarging aperture. The design scheme of the proposed system was elaborated in detail in this paper. Besides, the optical performance of the semi and the whole secondary solar trough concentrator was evaluated by using the ray tracing method. This innovative solar concentrating system shows a high application value as a solar energy experimental device.

Keywords

secondary parabolic trough solar concentrator / ray tracing method / linear focus / concentration ratio / optical performance

Introduction

As energy demands increasing and environmental issues becoming severe, there is growing attention to the utilization of renewable energy. The concentrating solar power (CSP) system, as a promising source of high temperature using the clean and sustainable solar energy, has experienced a wide variety of studies in recent decades. In the field of CSP applications, the parabolic trough solar power plant is the first commercial technology with an operating history of more than 20 years [¹,²]. Until the end of 2017, there had been nearly 3.7 GW capacity of more than 50 plants running in Spain, USA, UAE, Thailand, Morocco, South Africa, Egypt, Algeria, etc. The Israeli-American company called Luz International Ltd., founded in 1979, designed three generations of parabolic trough concentrating systems, referred to as LS-1, LS-2 and LS-3, and installed in Solar Electric Generating System (SEGS) plants [³–⁶]. In 1998, a consortium of European companies and research laboratories developed a new generation of parabolic trough concentrating systems for cost-efficient CSP plants, called Euro Trough, since LS-3 was no longer competitive [⁷,⁸]. Recently, various designs of electricity production have appeared according to the Euro Trough philosophy.

The parabolic trough concentrating system focuses on direct solar radiation along the focal line. The key component of the system, parabolic trough collector, works by one-axis tracking to ensure solar rays falling parallel to its axis [⁹–¹¹]. The receiver tube supported at the collector focal line by the support bracket rotates along the movement of the collector. The receiver carries heat transfer fluid inside and raises its enthalpy by absorbing the concentrated solar energy from outside walls [¹²]. Although it has been developed as a relatively mature technology after decades study, there are still some technical problems unsettled. Both continuous rotating movements during working process and inevitable thermal stress on support brackets pose challenges to connecting receiver tube sections [¹³]. Moreover, the concentration ratio of parabolic trough collector has limitations since it is impossible to drive a collector with infinite aperture size. Based on the typical problems in current commercial parabolic trough concentrating method, this paper aims to overcome the difficulties at least costs for maximum benefit by maintaining certain geometric parameters of key components and make it available on the market [¹⁴–¹⁶].

The advantage of the secondary concentrating system is widely noticeable since the receiver can keep stationary during operation. The secondary concentrating was first proposed as solar furnace. The largest solar furnace at Odeillo in France has been working for more than 40 years, which contains 63 heliostats each having a surface of 45 m², and a paraboloid reflector of 2000 m² intercepted area [¹⁷,¹⁸]. A high flux solar furnace is operated at Cologne. The optical design is a two-stage off-axis configuration which uses a flat 52 m² heliostat and a concentrator composed of 147 spherical mirror facets. The heliostat redirects the solar light onto the concentrator which focuses the beam out of the optical axis of the system into the laboratory building [¹⁹]. Moreover, several studies have been conducted for optimized design of the key components in CSP. For instance, the Renewable Energy Institute of the National University of Mexico (IER-UNAM) has built a high radiative flux solar furnace for the solution of the drift and backlash problems in the heliostat [²⁰]. Lately, more and more solar thermal systems equipped with secondary concentrating technology have served experiments that require the stability of high temperature and high heat flux [²¹–²⁴], which shows extensive potential in more areas.

Based on the advantage of the secondary concentrating method, in this paper, an innovative linear focus secondary concentrating design is proposed to provide a solution for setbacks of the parabolic trough system. The trough concentrator remains still on the ground, which offers an approach to obtain a higher concentration ratio by enlarging the aperture of the parabolic concentrator. The receiver is also stationary, so there is no concern about connection problem. Besides, the semi-trough concentrator structure makes near-ground experiment possible. The optical performance of the semi and the whole secondary solar trough concentrator has been evaluated by using the ray tracing method [²⁵–²⁷]. The analysis indicates that this innovative linear solar concentrating system has the feasibility and the potential application value on solar energy collecting experimental devices.

Linear focus secondary parabolic trough concentrating system and ray tracing method

Design principles of linear focus secondary parabolic trough concentrating system

In the working process of the parabolic trough concentrating system, the primary reflector, also called heliostat, always tracks the sun and continuously reflects the sunlight onto the secondary reflective parabolic trough concentrator. The sunlight reflected is then concentrated onto the tube located in the focal plane. The heat transfer fluid such as thermal oil inside the tube is kept flowing and heated up. Eventually, the solar energy is converted into thermal energy to be used. The schematic diagram of the linear focus secondary trough concentrating system is shown in Fig. 1.

Structural design of linear focus secondary parabolic trough concentrating system

The system consists of a primary heliostat, a secondary parabolic trough concentrator, and a tube receiver. The components are arranged from the south to the north, with the order of parabolic trough concentrator, tube receiver, and heliostat, all of which are independent.

The primary reflector can be either composed of multiple biaxial-tracking heliostat arrays or a single-axis tracking reflector with a large length-width ratio, accorded with the secondary concentrator. The heliostat is composed of several flat silver mirrors with high reflectivity, a mirror bracket, and a tracking control system. The silver mirror facets are fixed onto the supporting structure through the connection pieces, and the tracking system controls the whole structure to track the sun position. Regarding multiple biaxial tracking heliostats arrays, plat reflectors are displayed along the direction of east–west following the sun all day along. They always keep the sunlight reflected onto the secondary parabolic trough concentrator and maintain the reflected light in parallel with the symmetry center of the secondary parabolic trough concentrator, which is also named as the theoretical parabolic symmetry center plane. On the other hand, for a single-axis tracking reflector with a relative large length-width ratio, its length direction can be placed along the east–west director, making sure that the sunlight illuminates the secondary parabolic trough concentrator aperture in parallel with its symmetry center plane.

The secondary parabolic trough concentrator consists of a highly reflective parabolic silver mirror and a supportive bracket. The former is attached to the latter by connection pieces. The concentrator can be a form of the traditional parabolic trough concentrator, a complete parabolic shape, i.e., bilaterally symmetric along the center of the symmetry plane. Also, the concentrator can be non-symmetric to the center of the symmetry plane for lowering the height of the tube receiver. As illustrated in Fig. 2, the portion of a traditional parabolic trough concentrator along the width direction can be utilized as the secondary parabolic trough concentrator. The secondary parabolic trough concentrator is settled along an east–west direction, and its ideal parabolic symmetry center plane is in horizontal direction. The secondary parabolic trough concentrator is located on the south side of the primary reflector. The front side with the parabolic mirrors is facing the north, and the other directing the south, both of which extend into the ground and are connected with the foundation by anchor bolts and locking nuts. To achieve a straight line of concentrated sunlight, adjustment of each parabolic reflector is needed. The tube receiver is placed along the focus line of the secondary parabolic trough concentrator and fixed on the ground by bracket structure.

Optical calculation model

As discussed above, there are two kinds of tracking strategies for heliostat, consistent with the length-width ratio of the trough concentrator. Table 1 gives two cases for the evaluation of optical performance of the secondary concentrating system while Fig. 3 presents the schematic of trough concentrator structures.

Based on the parameters in Table 1, the analysis of the optical model is performed by using the Monte Carlo ray tracing method. Since this method relies on numerously repeated samplings to obtain the results, a sufficient number of random rays are traced from the primary reflector. Each incident sunlight is treated as a light beam cone. After figuring out whether a ray has the intersection point with the reflective surface, the heat flux is obtained from the total number of rays reaching on the receiver tube. All reflection procedure is processed with unit direction vectors, and the optical calculation is done in an appropriate set of right-handed coordinates. The calculation is done by self-programmed C++ codes, and detail procedure is illustrated in Fig. 4.

Calculation results and discussion

Because the tube is cylinder-shaped, it can be expanded along a specific horizontal line on the tube surface for showing a more visual heat flux result. Figure 5 demonstrates the heat flux distribution on fixed tube receiver in the secondary semi-parabolic trough concentrating system at 12AM in spring equinoxes. The value of DNI is obtained from the Eq. ²⁸. suitable for Chinese weather condition. From the simulation result, it is known that the peak average heat flux is 43.2 kW/m²as DNI reaches 957.9 W/m². It is obvious that the heat flux distribution along the circumferential direction is fluctuating and asymmetrical with respect to the centerline. The peak heat flux does not appear on the central line, which is natural because of the half of the saddle-shaped concentrated solar irradiation distribution resulted from the typical structure of the trough parabolic concentrator. There is a small drop at the center due to the shield of the receiver tube, which is in accordance with the actual situation. Moreover, since the incident solar ray is treated as a light beam cone with the 9.3 mrad radial angle, diffused light appears on the other half of the receiver tube that should not be directly illuminated. Figure 6 depicts the variation of the average heat flux along the circumferential direction of the receiver tube with time. The shape of concentrated heat flux can stay in good form even though the solar position changes from time to time. Figure 7 gives the variation of peak concentration ratio and DNI with time, and the total efficiency of the system. The solar evaluation angle is large around the noon time. It leads to the fact that the parabolic trough concentrator is not fully illuminated even though the heliostat size is larger than the concentrator. Therefore, the concentration ratio declines before noon and increases due to the increasing illuminating area on the concentrator. However, the concentration ratio can stay around 47 for this structure.

Figures 8–10 show the calculation results of the second case in Table 1. Unlike the semi-structure, the concentrated heat flux is of Gaussian distribution. The reason for this is that the receiver diameter is relatively small compared to the aperture size of the concentrator and the ratio of the focal distance to the aperture width of the trough concentrator is a bit large than the former case [²⁹]. The peak heat flux is 147.5 kW/m² in Fig. 8. As is the case in the previous simulation result, the heat flux distribution can maintain in good shape and there is no or little change of solar position influent on the gathering thermal energy. Since the tracking strategy is one-axis tracking, the peak concentration ratio shows the same trend as the variation of DNI on the premise of full illumination on the concentrator. The peak concentration ratio remains around 157. The efficiency of this system is much higher than that of the previous one. The reason for this is that the heliostat in this case is matched with the concentrator size. In other words, more solar rays are reflected by the heliostat incident throw onto the concentrator. The simulation results are reasonable according to the heat flux on the receiver tube of the parabolic trough system [²⁹].

Conclusions

This paper introduces a linear focus secondary trough solar concentrating system. The main difference between the linear focus secondary trough solar concentrating system and the existing conventional system is that a primary reflective heliostat is adopted. Meanwhile, the secondary reflective parabolic trough concentrator and the tube receiver are fixed, which will bring great convenience either to the installation or replacement of the heat absorption tube. It also benefits the flow and heat transfer process of this system since the receiver is stationary. The fixed concentrator will also have a larger aperture and width which can ensure its stiffness and strength of wind resistance.

The calculated results show that the linear focus secondary concentrating system can get a higher concentration ratio compared to the traditional parabolic trough collector, which is good for the thermal performance of tube receivers. The innovative linear focus secondary concentrating system can be designed to fit different mounting angles and positions of the receiver tube, which shows the potential high application value of solar energy collecting experimental devices.

References

[1] Wang F, Cheng Z, Tan J, Yuan Y, Shuai Y, Liu L H. Progress in concentrated solar power technology with parabolic trough collector system: a comprehensive review. Renewable & Sustainable Energy Reviews, 2017, 79: 1314–1328

[2] Mills D R, Morrison G L. Compact linear Fresnel reflector solar thermal power plants. Solar Energy, 2000, 68(3): 263–283

[3] Pharabod F, Philibert C. LUZ solar power plants: success in California and worldwide prospects. DLR (German Aerospace Center) and IEA-SSPS (Solar PACES), Cologne, Germany, 1991

[4] Dudley V E, Kolb G J, Mahoney A R, Mancini T R, Matthews C W,Sloan M, Kearney D. Test results: SEGS LS-2 solar collector. Technical Report, Sandia National Labs, 1994

[5] Nava P, Aringhoff R, Svoboda P, Kearney D. Status rep on solar trough power plants. Technical Report. Pilkington Solar International, Germany, 1996

[6] Mills D. Advances in solar thermal electricity technology. Solar Energy, 2004, 76(1–3): 19–31

[7] Geyer M, Lüpfert E, Osuna R, Esteban A, Schiel W, Schweitzer A, Zarza E, Nava P, Langenkamp J, Mandelberg E. EUROTROUGH-Parabolic trough collector developed for cost efficient solar power generation. In: 11th International Symposium on Concentrating Solar Power and Chemical Energy Technologies, Zurich, Switzerland, 2002

[8] Herrmann U, Nava P. Performance of the SKAL-ET collector of the Andasol power plants. In: Proceedings of 14th International Solar PACES Symposium on Solar Thermal Concentrating Technologies, Las Vegas, USA, 2008

[9] Price H, Lupfert E, Kearney D, Zarza E, Cohen G, Gee R, Mahoney R. Advances in parabolic trough solar power technology. Journal of Solar Energy Engineering, 2002, 124(2): 109–125

[10] Lamba D K. A review on parabolic trough type solar collectors: innovation, applications and thermal energy storage. In: Proceedings of the National Conference on Trends and Advances in Mechanical Engineering, YMCA University of Science & Technology, Faridabad, Haryana, 2012

[11] Ren L, Wei X, Lu Z, Yu W, Xu W, Shen Z. A review of available methods for the alignment of mirror facets of solar concentrator in solar thermal power system. Renewable & Sustainable Energy Reviews, 2014, 32: 76–83

[12] Cheng Z, He Y, Cui F, Xu R J, Tao Y B. Numerical simulation of a parabolic trough solar collector with nonuniform solar flux conditions by coupling FVM and MCRT method. Solar Energy, 2012, 86(6): 1770–1784

[13] Forristall R. Heat transfer analysis and modeling of a parabolic trough solar receiver implemented in engineering equation solver. National Renewable Energy Lab., Golden, CO, USA, 2003

[14] Gee RC, Hale MJ. Solargenix energy advanced parabolic trough development. National Renewable Energy Laboratory (NREL), Golden, CO., USA, 2005

[15] Castañeda N, Vázquez J, Domingo M, Fernández A, León J. Sener parabolic trough collector design and testing. In: SolarPACES Congress, Spain, 2006

[16] Relloso S, Castaneda N, Domingo M. New Senertough collector development in collaboration with key components suppliers. In: 14th International SolarPACES Symposium, Las Vegas, USA, 2008

[17] Trombe F, Vinh A L. Thousand kW solar furnace, built by the National Center of Scientific Research, in Odeillo (France). Solar Energy, 1973, 15(1): 57–61

[18] Perez Lopez I, Benoit H, Gauthier D, Sans J L, Guillot E, Mazza G, Flamant G. On-sun operation of a 150 kWth pilot solar receiver using dense particle suspension as heat transfer fluid. Solar Energy, 2016, 137: 463–476

[19] Neumann A, Groer U. Experimenting with concentrated sunlight using the DLR solar furnace. Solar Energy, 1996, 58(4–6): 181–190

[20] Perez-Enciso R, Riveros-Rosas D, Sanchez M, Pérez-Rabago C A, Arancibia-Bulnes C A, Romero-Paredes H, Estrada C A. Three-dimensional analysis of solar radiation distribution at the focal zone of the solar furnace of IER_UNAM. Energy Procedia, 2014, 57: 3031–3040

[21] Villafán-Vidales H I, Arancibia-Bulnes C A, Riveros-Rosas D, Romero-Paredes H, Estrada C A. An overview of the solar thermochemical processes for hydrogen and syngas production: reactors, and facilities. Renewable & Sustainable Energy Reviews, 2017, 75: 894–908

[22] Almeida Costa Oliveira F, Cruz Fernandes J, Badie J M, Granier B, Guerra Rosa L, Shohoji N. High meta-stability of tungsten sub-carbide W2C formed from tungsten/carbon powder mixture during eruptive heating in a solar furnace. International Journal of Refractory Metals & Hard Materials, 2007, 25(1): 101–106

[23] Li Q, Bai F, Yang B, Wang Z, El Hefni B, Liu S, Kubo S, Kiriki H, Han M. Dynamic simulation and experimental validation of an open air receiver and a thermal energy storage system for solar thermal power plant. Applied Energy, 2016, 178: 281–293

[24] Wang F, Bai F, Wang T, Li Q, Wang Z. Experimental study of a single quartz tube solid particle air receiver. Solar Energy, 2016, 123: 185–205

[25] Jeter S M. Calculation of the concentrated flux density distribution in parabolic trough collectors by a semifinite formulation. Solar Energy, 1986, 37(5): 335–345

[26] Huang W, Han Z. Theoretical analysis of error transfer from the surface slope to the reflected ray and their application in the solar concentrated collector. Solar Energy, 2012, 86(9): 2592–2599

[27] Mwesigye A, Bello-Ochende T, Meyer J P. Minimum entropy generation due to heat transfer and fluid friction in a parabolic trough receiver with non-uniform heat flux at different rim angles and concentration ratios. Energy, 2014, 73: 606–617

[28] Xu Y, Cui K, Liu D. The development of a software for solar radiation and its verification by the measurement results on the spot. Energy Technology (Weinheim), 2002, 23(6): 237–239

[29] Zhao D, Xu E, Wang Z, Yu Q, Xu L, Zhu L. Influences of installation and tracking errors on the optical performance of a solar parabolic trough collector. Renewable Energy, 2016, 94: 197–212

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