Heat transfer performance enhancement and thermal strain restrain of tube receiver for parabolic trough solar collector by using asymmetric outward convex corrugated tube
Introduction
Global environmental degradation is one of the serious threats facing humankind as a result of its consuming of fossil fuel energy around the world [1], [2]. The compatible development of both the efficient energy utilization and environment protection is the hottest issue in the 21stcentury [3]. Solar resource is the world's most abundant source of energy with the potential to meet a significant portion of the world's energy requirements [4], [5]. Solar energy is widely available and one of the renewable forms of energy with little impact on the environment [6]. Concentrated solar power technology is a technology which can be capable of producing clean, renewable and grid-scale energy [7], [8]. Due to the present situation, the concentrated solar thermal power generation technologies have attracted increasing interests in the renewable energy fields [9]. These technologies are classified by their focus geometry as either point-focus concentrators (central receiver systems and parabolic dishes) or line-focus concentrators (parabolic-trough collectors, and linear Fresnel collectors) [10], [11], [12], [13], [14]. Among these technologies, parabolic trough solar thermal power technology has been widely adopted among these solar thermal power utilizations [15].
Parabolic trough type solar thermal power plant consists of parabolic trough collectors (PTC), parabolic trough receivers (PTR) with heat transfer fluid (HTF)/steam generation system, Rankin steam turbines/generator cycles, and optional thermal storage and/or fossil-fired backup systems [16]. The heat transfer fluid in the metal tube of PTR is heated by concentrated solar radiation [17], [18], [19]. The Chinese government had set up a research project in China 863 Program of “the research and demonstration of parabolic trough solar thermal power generation technology” during the National 12th Five-Year Plan, which aimed to develop the mass production technology of parabolic trough solar collector system [20].
The metal tube with evacuated glass envelop used as PTR is the key component where concentrated solar energy is converted to thermal energy [21]. The thermal strain caused by large temperature gradient during operation may cause bending of metal tube and rupture of glass envelope. The thermal deformations of PTR are related to the thermal conductivity of metal tube and glass envelop, flow pattern of HTF and asymmetry of the temperature distribution on the metal tube [22]. Wu et al. [23] had investigated the bending of stainless steel PTR by indoor experiments and numerical simulations and field measurements, and their researches indicated that the thermal deformation in the stainless steel tube changed inversely with HTF velocity and temperature. Lei et al. [24] indicated that the residual stresses were generated not only by the differences of the thermal expansion coefficients between the glass and the metal but also by the sealing geometry which had a significantly negative influence to the seal strength. It should be noted that an overlapped thermal strain of PTR for parabolic trough type solar thermal power plant would appear and vary with time during the operation.
It is a consensus that enhancing heat transfer performance in the PTR can reduce the temperature gradient which can results in less thermal deflections and improve the reliability of PTR. Aggrey et al. [25] had presented a new type of PTR with perforated plate inserting for parabolic trough solar collector to reduce the temperature gradient and peak temperature, their results indicated that the maximum reduction of temperature gradients in tube receiver was up to 33%. A new type of PTR with helical screw-tape inserting was proposed by Song et al. [26] to homogenize the temperature distribution on the metal tube of PTR and improve the thermal performance. Wang et al. [27] had investigated the effects of inserting metal foams in metal tube of PTR on heat transfer performance enhancement, and the results presented that the maximum circumferential temperature difference on the out surface of receiver tube decreases about 45% which can greatly reduce the thermal deformation of PTR.
Besides, optimizing the structure of PTR is also an effective way of reliability enhancement. Montes et al. [28] designed a new module that all the components are made of structural aluminum with the aim to decrease the thermal deformation and weight. With the aim to minimize heat flux gradient which in turn can reduce thermal deformation of tube receiver, glass cover with elliptic-circular cross section is put forward by Wang et al. [29]. Khanna et al. [30], [31] had introduced the explicit expressions to obtain the temperature distribution of PTR and the corresponding deflections in the central axis of PTR, they also investigated the effects of desired rise in fluid temperature, optical errors and rim angle of PTC on temperature distribution and deflection of PTR with two types of structural constraints. He et al. [32], [33], [34] proposed to use milt-longitudinal vortexes and metal foam inserting in PTR to reduce temperature gradient and enhance the convective heat transfer with less pressure drop.
In the previous studies, the authors had introduced a symmetric outward convex corrugated tube for PTR to enhance the heat transfer performance, and an optical-thermal-structural sequential coupled method is developed to analyze the heat transfer performance and thermal deformation of glass envelope and metal tube of PTR [35]. The authors had also put forward an asymmetric corrugated tube for nuclear engineering devices for enhancing heat transfer [36]. Fig. 1 presents the asymmetric outward convex transverse corrugated tubes fabricated through a hydraulic pressure method. The authors had numerically investigated the flow and heat transfer characteristics of two tube types named as symmetric corrugated tube (SCT) and asymmetric corrugated tube (ACT), their investigation indicated that ACT can improve the overall heat transfer performance up to 18% over that of SCT [36].
Although the symmetric outward convex corrugated tube had been introduced as the metal tube of PTR and showed excellent heat transfer performance and reliability, none researchers had proposed to use asymmetric outward convex corrugated metal tube as the metal tube of PTR in solar thermal power utilizations. In this paper, the asymmetric outward convex corrugated tube receiver (ACPTR) is proposed with the aim to enhance the heat transfer performance and reliability of PTR. The Monte Carlo Ray Tracing (MCRT) method coupled with Finite Volume Method (FVM) and Finite Element Method (FEM) are adopted to solve the optical-thermal-structural coupled problem. The effects of geometry parameters variation on heat transfer performance enhancement and thermal strain reduction were analyzed to give theoretical instructions for application.
Section snippets
Physical model
The schematic diagram of parabolic trough solar collector with PTR is shown in Fig. 2. As shown in this figure, the bottom periphery of the PTR suffers the incoming solar radiation concentrated by PTC, while the top periphery of the PTR subjected to the non-concentrated solar radiation. The tube receiver of PTR is made of stainless steel coated with selective coatings which have the function of increasing the energy absorption in the solar spectrum range and minimizing the energy emission in
Model of sunlight transmission and concentration
Monte Carlo Ray Tracing (MCRT) method is a powerful tool in the field of solar energy application for performing irradiative equilibrium calculations and also suitable to solve complex geometries [38]. Therefore, the transmission and concentration processes of solar radiation in this study are calculated by codes compiled based on MCRT method.
The MCRT method involves the stochastic trajectories of a large amount of solar rays, as they meet with solar components [39], [40]. A series of
Grid independent verification
In order to obtain higher precision and better convergence for simulation, all of the structured hexahedral meshes used for this study are generated through O-grid generation technique. The three dimensional views of corrugated tube and axial cross section view of structured hexahedral meshes generated with O-grid generation technique for ACPTR are shown in Fig. 6. For the computational fluid dynamics analysis, the PTR is divided into four domains distinguished by different colors: glass
Model validation
In this study, the sequential validation steps are adopted for the optical-thermal-structural coupled problem. First, the heat flux distribution calculated by the authors is compared with that obtained by Hachicha et al. [58]. The concentrated heat flux distribution obtained from the MCRT method is imported into the heat transfer performance analyses of PTR by UDFs by fitting curve method with tiny interpolating error [50], [51]. Then, the temperature distributions on the parabolic trough solar
Effects of optical errors on heat flux distribution
The mirror roughness error () is the error caused by roughness of parabolic trough collector, which has significant influences on the direction of reflected sunlight. Fig. 11 illustrates the heat flux distribution on the bottom periphery of tube receiver variation with the change of circumferential angle at different mirror roughness errors. Seven cases, = 0, 2, 4, 6, 8, 10 and 12 mrad, are investigated. As seen in this figure, the peak value of heat flux distribution on the
Conclusions
In this study, the asymmetric outward convex corrugated tube is introduced as the metal tube of PTR (ACPTR) to increase the overall heat transfer performance and reliability. An optical-thermal-structural sequential coupled method was developed to study the heat transfer performance and thermal deformation of tube receiver for parabolic trough solar collector system. The following conclusions have been drawn:
- 1)
Mirror roughness errors and pointing errors decrease the peak value of concentrated
Acknowledgements
This work was supported by National Natural Science Foundation of China (Grant No. 51406239, 51476042, 51506034), and the China Postdoctoral Science Foundation (No. 2015M580261). Besides, a special acknowledgement is made to the editors and referees whose constructive criticism has improved this paper.
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