Heat transfer and thermodynamic performance of a parabolic trough receiver with centrally placed perforated plate inserts
Introduction
Parabolic trough solar collectors are one of the most technically and commercially developed technologies of the available concentrated solar power technologies [1], [2]. The parabolic trough’s linear receiver is a central component to the performance of the entire collector system. Its state and design greatly affects the performance of the entire collector system. The performance of the receiver is significantly affected by the thermal loss and heat transfer from the absorber tube to the working (heat transfer) fluid [3]. The conventional receiver consists of an evacuated glass envelope to minimize the convection heat loss and a selectively coated absorber tube to minimize the radiation heat loss [2]. Numerous studies have been carried out to characterize the thermal performance of the receiver and to determine the thermal loss at different receiver conditions [4], [5], [6], [7], [8], [9]. From these studies, it has been shown that: the thermal loss is majorly dependent on the state of the annulus space between the glass cover and the absorber tube, the absorber tube selective coating, the temperature of the absorber tube, the wind speed and the heat transfer from the absorber tube to the heat transfer fluid.
With the availability of lightweight materials, the use of higher concentration ratios has become feasible [10]. Higher concentration ratios ensure shorter and less expensive collectors given the reduction in the number of drives and connections required. However, larger concentration ratios mean increased entropy generation rates [11], increased absorber tube circumferential temperature gradients as well higher peak temperatures.
The presence of circumferential temperature gradients in the receiver’s absorber tube is a major concern. At low flow rates, higher temperature gradients existing in the tube’s circumference can cause bending of the tube and eventual breakage of the glass cover [12], [13]. And the peak temperature in the absorber tube facilitate degradation of the heat transfer fluid especially as these temperatures increase above 673.15 K [14], [15]. The degradation of the heat transfer fluid results in hydrogen permeation in the receiver’s annulus. With formation of hydrogen in the receiver’s annulus, the receiver’s thermal loss increases significantly thereby affecting the collector thermal performance [16].
Temperature gradients and temperature peaks in the receiver’s absorber tube exist due to the non-uniform heat flux profile received on the absorber tube, with concentrated heat flux on the lower half of the absorber tube and nearly direct solar radiation on the upper half [17], [18], [19], [20]. Most failures of parabolic trough receivers, especially the breakage of the glass cover have been attributed to the circumferential temperature gradients in the absorber tube [2], [13]. Therefore, reducing these temperature gradients and temperature peaks can go a long way in increasing the life span of the receiver and avoiding the thermal loss due to vacuum loss and hydrogen permeation in receiver’s annulus space. The maximum temperature gradient for safe operation of receiver tubes is about 50 K [21].
Enhancement of convective heat transfer in the receiver’s absorber tube is one of the relevant solutions to the above concerns. With improved convective heat transfer in the absorber tube, circumferential temperature gradients and peak temperatures in the absorber tube can be reduced and risks of breakage and hydrogen formation can be minimized. As such, heat transfer enhancement in the receiver’s absorber tube has received considerable attention in the recent past. Ravi Kumar and Reddy [22] numerically analyzed a receiver with various porous fin geometries and compared its performance with a receiver having longitudinal fins. Ravi Kumar and Reddy [3] investigated the performance of the receiver with a porous disc at different angles of orientation, different heights and different distances between the consecutive discs. Muñoz and Abánades [13] analyzed an internally helically finned absorber tube with a view of improving thermal performance and minimizing the temperature gradients in the absorber tube. Absorber tube temperature difference was reduced by between 15.3% and 40.9%. All these studies used an approximate heat flux boundary condition on the receiver’s absorber tube. The use of realistic non-uniform heat flux boundary condition is crucial in determining the temperature gradients, peak temperatures as well as entropy generation rates in the receiver.
Recently Cheng et al. [23] analyzed the heat transfer enhancement of a parabolic trough receiver using unilateral longitudinal vortex generators with a realistic non-uniform heat flux boundary condition. The wall temperatures and thermal loss were found to decrease with each geometrical parameter considered. Wang et al. [21] investigated heat transfer enhancement using metal foams in a parabolic trough receiver for direct steam generation using realistic non-uniform heat flux boundary condition. They showed a maximum circumferential temperature difference was shown to reduce by 45%.
Several other studies have been carried out on heat transfer enhancement for various applications using different techniques as reviewed by Manglik [24], [25]. Studies on heat transfer enhancement in parabolic trough receivers with realistic non-uniform heat flux boundary conditions are not wide spread. Moreover, most studies on heat transfer enhancement have only focused on heat transfer and fluid friction performance. Investigations of the effect of heat transfer enhancement on thermodynamic performance of enhanced devices are still few. Therefore, in this paper, a numerical investigation of heat transfer, fluid friction and thermodynamic performance of a receiver with a centrally placed perforated plate is carried out. The plate is centrally placed to provide heat transfer enhancement in the core flow thereby avoiding any possible hot spots that can facilitate degradation of the heat transfer fluid [14] which are characteristic of heat transfer enhancement methods with recirculation, separation and re-attachment. In addition to heat transfer performance, using the entropy generation minimization method [26], the effect of heat transfer enhancement on the thermodynamic performance of the receiver is also investigated and presented. To the author’s best knowledge, the use of centrally placed perforated plate inserts for heat transfer enhancement in a parabolic trough receiver has not been studied previously.
Section snippets
Physical model
The perforated plate assembly is considered to be supported on a thin axially placed rod as shown in Fig. 1(a). The placement of the perforated plate defined by spacing between the two consecutive plates (p), the diameter of the plate (d) and the angle of orientation measured from the positive y-axis (β). β is negative in the clockwise direction and positive in the anti-clockwise direction. In our analysis, we have considered a simplified model of the parabolic trough receiver in which the
Governing equations
For the range of Reynolds numbers considered, the flow is in the fully developed turbulent regime. As such, the governing equations used in our analysis for steady-state and three-dimensional turbulent flow are the continuity, momentum and energy equations given by;
Continuity
Momentum equation
Energy equation
The additional
Solution procedure
The numerical solution was implemented using a commercial software package ANSYS® 14. The governing equations together with the boundary conditions were solved using a finite-volume approach implemented in a computational fluid dynamics code ANSYS FLUENT [30]. The computational domain was discretized using tetrahedral elements with a structured mesh in the absorber tube wall normal direction and as structured mesh in the receiver’s annulus space. The coupling of pressure and velocity and was
Validation of numerical results
Our numerical analysis was validated in several steps. For a receiver with a plain absorber tube, we have compared our results with experimental data from Dudley et al. [7] for receiver’s temperature gain and collector efficiency to ensure that our receiver model is accurate. Table 4 shows the comparison of the present study receiver’s temperature gain and collector efficiency with Dudley et al. [7] experimental data for a receiver 7.8 m long, 66 mm absorber tube internal diameter, 70 mm absorber
Conclusion
In the present study, a numerical investigation was carried to investigate the thermal, fluid friction and thermodynamic performance of a parabolic trough receiver with centrally placed perforated plate inserts.
From the study, the Nusselt number and friction factor are strongly dependent on the spacing and size of the insert as well as flow Reynolds number. For the range of Reynolds numbers, temperatures and geometrical parameters considered, the Nusselt number increases about 8–133.5% with
Acknowledgement
The funding received from NRF, TESP, and Stellenbosch University/University of Pretoria, SANERI/SANEDI, CSIR, EEDSM Hub and NAC is duly acknowledged and appreciated.
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