Non-uniform Solar Flux Distribution Research Articles

Design, Simulation and Optimization of a Novel Transpired Tubular Solar Air Heater

In this paper, a novel tubular solar air heater is introduced. In this air heater, the hot boundary layer is drawn into the absorber tube and can provide thermal energy at moderate temperatures. Several different cases were simulated and a correlation was proposed to predict the collector’s effectiveness as a function Rayleigh number and Reynolds number. An equation was derived to find the effectiveness of this collector. Finally, a real case was studied with non-uniform solar flux distribution, as well as radiation heat loss. Good agreement was found between the results and those derived by the proposed analytical method. For different suction values, the first-law and the second-law efficiencies were calculated. Based on the exergy analysis, exergy destruction in absorption is the dominant factor that is unavoidable in low-temperature collectors. It was shown that there is an optimum suction value at which the second-law efficiency is maximized. At the optimum point, temperature rise can reach 54 K, which is hardly possible with a flat plate collector. Based on the exergy analysis, the relation between tube wall temperature and air outlet temperature in their dimensionless forms at the optimum working condition was derived, and it was shown that effectiveness at the optimum working condition is around 0.5. This means that the air temperature rise shall be half of the temperature difference between collector wall and the ambient temperatures. A high outlet temperature besides the low cost of construction and maintenance are the main advantages of this air heater. With such a high temperature rise, this type of collector can increase the use of solar energy in domestic applications.

Thermal-fluid-mechanical analysis of tubular solar receiver panels using supercritical CO2 as heat transfer fluid under non-uniform solar flux distribution

The tubular receiver is one of the most attractive options for the directly heated supercritical CO2 (S-CO2) solar receiver, of which tubular panels are the basic modules. Due to the high working pressure and the non-uniform solar flux distribution, the thermal-fluid-mechanical analysis becomes an important research topic for the S-CO2 solar receiver. In this paper, the thermal-fluid-mechanical characteristics of the S-CO2 tubular receiver panel under non-uniform solar flux distribution using S-CO2 as the heat transfer fluid is numerically studied. The effects of solar flux distribution and flow arrangements on the thermal and mechanical performances are discussed with emphasis. The results indicate that: (1) the non-uniform solar flux of the receiver panel will result in the increase in the thermal loss and the thermal stress; (2) adjusting the flow arrangement to match the solar flux distribution can decrease thermal loss and thermal stress; (3) the proposed matching factor can be used as an indicator for the thermal-mechanical performances, and a larger matching factor means a good performance; (4) although the I-type flow arrangement lead to uneven flow allocation with more fluid in middle tubes, it cannot always yield good performances under Gaussian-shaped solar flux distribution, which is greatly depended on the standard deviation of the Gaussian distribution; (5) an ideal flow allocation can be easily determined by the matching factor being set to 1, which provides guidelines for the design and optimization of the tubular solar receiver panel.

Buoyancy effects on convective heat transfer of supercritical CO2 and thermal stress in parabolic trough receivers under non-uniform solar flux distribution

The buoyancy influenced heat transfer characteristics of S-CO2 have been investigated widely in previous studies, but limited to the conditions with constant wall temperature or constant heat flux. The present study numerically investigates the buoyancy effects on the convective heat transfer of S-CO2 and the thermal stress in parabolic trough receivers (PTRs) under the non-uniform solar flux distribution based on a coupled optical-thermal-fluid-mechanical numerical model. The results indicate that the drastic density variation associated with the large temperature gradient of the fluid occurs on the cross-section under the non-uniform solar flux distribution, which further induces the secondary flow. As the secondary flow improves the synergy between the velocity vector and the temperature gradient, the bulk average heat transfer is prominently enhanced due to the buoyancy effect. However, the buoyancy effects on the local heat transfer characteristics are more complex. The buoyancy effect enhances the heat transfer at the bottom but impairs the heat transfer at the top, and the heat transfer enhancement area is expanded along the flow direction. The temperature distribution in the absorber tube wall is greatly flattened due to the buoyancy effect, and the thermal stress is consequently reduced. The buoyancy effects on the thermal and mechanical performances become less significant with the increase in the operating pressure, the inlet velocity and the inlet temperature but with the decrease in DNI. The buoyancy effect in the PTR under non-uniform solar flux distribution can be ignored when Se/Re<0.3, Ri<0.04, Bup<0.002, Buj<80, or Grq/Grth<70.

A design method for optimizing the secondary reflector of a parabolic trough solar concentrator to achieve uniform heat flux distribution

Non-uniform solar flux distribution in a parabolic trough concentrator (PTC) causes large temperature gradients on absorber tube surface, making the PTC inefficient and damaged. To solve the problem, a novel method is proposed for designing the additional secondary reflector (SR) in a PTC to improve uniform heat flux distribution on absorber tube surface. In the design, the heat flux between upper and lower surfaces of absorber tube is well balanced by optimizing the location of absorber. Then, the SR with segmented broken-line type composed of multiple plane mirrors is delicately designed with local heat flux compensation strategy. Two case studies are conducted to compare the performances of the newly designed SRs with other existing SR designs. It is shown that the uniformity of heat flux distribution can be improved to over 90% for present SR design, much higher than those for other existing SR designs. Also, the optical efficiency of the PTC with present SR design is also increased as compared with other designs. The results indicate that this proposed method is competent for designing SR in a PTC with uniform heat flux distribution.

A coupled optical-thermal-fluid-mechanical analysis of parabolic trough solar receivers using supercritical CO2 as heat transfer fluid

Supercritical CO2 (S-CO2) is an attractive heat transfer fluid (HTF) candidate for application in parabolic trough receivers (PTRs). The thermal and mechanical performances are of serious concern for the PTR using S-CO2 as HTF which suffers from the rigorous conditions of high operating pressure and non-uniform solar flux distribution. The present study develops an integrated numerical model for the optical-thermal-fluid-mechanical analysis of PTRs by coupling Monte Carlo ray-tracing method, finite volume method, and finite element method. It is found that the non-uniform solar flux distribution can induce significant secondary flow of S-CO2 which improves the synergy of the velocity vector and the temperature gradient in the fluid, benefiting the convective heat transfer. However, both of the high operating pressure and the non-uniform solar flux distribution can induce remarkable mechanical stress in PTR, and the superposition of the operating pressure and the solar flux distribution aggravates the mechanical stress. It should be noticed that the operating pressure has no contribution to the axial stress component, while the radial stress component induced by the non-uniform solar flux distribution can be neglected. Flattening the solar flux distribution by adding a secondary reflector is recommended as an effective approach to reducing the thermal stress, and the maximum thermal stress can be reduced from 50 MPa to 10 MPa under the conditions studied in this paper.

Optical analysis and heat transfer modeling of a helically baffled cavity receiver under solar flux non-uniformity and windy conditions

Optical and thermal analyses of a parabolic dish collector (PDC) with spiral baffles embedded in the annular space of a cylindrical cavity receiver was studied in windy conditions under realistic non-uniform solar flux distribution. The asymmetric distribution of solar flux on the cavity walls was obtained by a nonsequential ray tracing and finite element (FEM) coupled simulation technique. The performance of the system was analyzed in windy conditions, for two different wind directions of inward and outward of the receiver aperture. The slope error of the dish surface has a significant effect on the distribution of reflected rays on the receiver walls. For higher slope errors, the dishes that are more concave and for smaller slope errors more flat dishes give higher thermal efficiency and have better performance. For a dish collector with a focal point of 1.5 m, when the slope error of the dish surface is only 10 mrad, about 13% of the total solar flux received by the dish is lost. When the slope error of the dish surface increases, the system thermal efficiency decreases, and this reduction is more severe for larger focal points and more flat dishes. The effect of wind direction on the system performance for inward aperture mode is more than that of outward mode, and by increasing the wind speed from 1 to 10 m/s, the system thermal efficiency is reduced up to 50%.

Experiment and optimization study on the radial graded porous volumetric solar receiver matching non-uniform solar flux distribution

Radial graded porous volumetric solar receiver is designed to match the non-uniform solar flux distribution. Based on the computed tomography and image-processing techniques, uniform and radial graded porous volumetric solar receivers are reconstructed. The 3D printing technique and suitable post processing are implemented to fabricate complex porous samples using super-alloy Inconel 718 as material. Both experimental and numerical studies are conducted to investigate the fluid flow and heat transfer processes in porous volumetric solar receivers. The results present that the 3D printed porous samples are suitable for solar thermal energy absorption and high temperature utilization. As for uniform porous receivers, porous media with small pore diameter has larger thermal efficiency because of enhanced convective heat transfer. Compared with the uniform porous receiver with highest thermal efficiency, the radial graded porous volumetric solar receiver with large pore diameter inside could further relatively increase the thermal efficiency by 4.1% while relatively decreases the flow resistance by 8.6%. The reasonable distribution of pore diameter of porous media could regulate the mass flow distribution and direct more air to the high heat flux region. Moreover, local overheating phenomenon is observed in the uniform porous receiver using air as heat transfer fluid. By applying the coupled optimization method, an optimum pore diameter distribution is determined for the radial graded porous volumetric solar receiver.

Multi-scale modular analysis of open volumetric receivers for central tower CSP systems

Open volumetric air receivers (OVARs) are an interesting Concentrating Solar Power (CSP) technology, due to the high temperature that can be reached by the heat transfer fluid (air at ambient pressure), leading to high efficiency of the power cycle.In this paper, a novel multiscale and modular computational approach is proposed and applied to the analysis of a conventional OVAR, consisting of several honeycomb cups, each including a large number of parallel prismatic channels. The methodology proposed in this work consists in scaling-up the receiver numerical model, from the single channel (micro-scale), through the cup (meso-scale), to the whole receiver (macro-scale), exploiting the results obtained at the micro-scale to characterize the model applicable at the meso-scale, which is then used to modularly generate the model at the macro-scale. The rationale of the multi-scale approach is to take into account at each scale the most relevant phenomena that affect the receiver performance at that scale, which cannot be considered in detail at larger scales. Starting from the thermal-hydraulic characterization at the micro-scale, obtained by means of a detailed 3D CFD model of the generic (single) channel, the meso-scale problem is addressed introducing an equivalent 1D porous medium model for the generic cup, implemented in the Modelica environment. The latter model is finally exploited to assemble a macro-scale model able to describe the entire receiver behavior, considering also the return air flow that laps the lateral walls of the cups.The receiver macro-scale model has been preliminarily compared with the data collected during the SolAir 200 experimental campaign, conducted at the PSA in the early 2000s, demonstrating that the model developed here provides credible results. A first application of the developed multi-scale model is also presented, in order to demonstrate the potential of this approach. The results from this analysis are discussed, underlining the influence of the phenomena that affect the receiver performance at the different scales. Particularly, the effect of the return air flow is investigated, as well as the effect of a non-uniform solar heat flux distribution. In the latter case, the model has been used to calibrate the opening of the orifices installed at the cup rear side to regulate the mass flow rate among the cups forcing an almost uniform air outlet temperature. Moreover, the full receiver model is applied to a simple transient scenario consisting of a fast passing cloud, proving the ability of the model to simulate dynamic conditions in a reasonable way.

Design and characterization of the non-uniform solar flux distribution measurement system

It is important to accurately measure the solar flux distribution on the heated surface for the utilization of solar energy. To this end, in this paper a new measurement system was proposed to measure the solar flux distribution of the absorber tube surface by taking advantage of the end loss effect. The new measurement system does not disturb the collected lights path and influence the operation of the solar collector. The solar flux distribution at different angles can be measured with a few number of heat flux gage by rotating the proposed solar flux measurement system. Compared with existing measurement techniques, the proposed solar flux measurement system has distinct advantages such as simple structure, flexible and reliable operation and easy application. An experimental platform was designed and constructed, and the solar flux distributions on the outer surface of the absorber under differentconditions are measured. The feasibility and effectiveness of the proposed measurement method are validated by comparing the experiment results and the numerical simulation results.

Multi-objective optimization of the aiming strategy for the solar power tower with a cavity receiver by using the non-dominated sorting genetic algorithm

The extremely non-uniform solar flux distribution in the solar power tower plant can badly cause some crucial problems for the solar receiver such as the local hot spot, the thermal stress, and the thermal deformation. Homogenization of the solar flux distribution is an effective method to avoid these problems, and has become an important research topic. The objective of the present study is to homogenize the solar flux distribution on the inner surfaces within the cavity receiver while keeping the optics loss as low as possible by replacing the conventional single-point aiming strategy with optimal multi-point aiming strategies. Multi-objective optimizations of the aiming strategy for the solar power tower with cavity receivers are performed by using the non-dominated sorting genetic algorithm. The distribution of the aiming points on the cavity aperture and the allocation of the aiming points for each heliostat are optimized simultaneously. The following conclusions can be made: (1) The uniformity of the solar flux distribution on the aperture does not always signify the uniformity of the solar flux distribution on the inner surfaces, where the later one is what we are truly concerned about. Therefore, the optimization of the aiming strategy should take charge of the solar flux distribution on the inner surfaces rather than on the aperture. (2) The multi-objective optimization can provide the trade-off between the non-uniformity of the solar flux distribution and the optics loss in the form of Pareto optimal fronts. (3) The optimal aiming strategies provided by the multi-objective optimization can significantly homogenize the solar flux distribution on the inner surfaces within the cavity at a minimum cost of optics loss. (4) For the optimal aiming strategies at all time except the noon, there exists a west-east asymmetry of the aiming point distribution on the aperture. Moreover, the asymmetry gets less obvious as the time gets closer to the noon.

Non-uniform characteristics of solar flux distribution in the concentrating solar power systems and its corresponding solutions: A review

In this paper, the non-uniform characteristics of solar flux distribution and the consequent challenges in concentrating solar power (CSP) systems including parabolic trough system, linear Fresnel reflector system, solar power tower, and solar dish system are summarized. The corresponding solutions proposed to tackle the challenges caused by non-uniform solar flux distributions are particularly reviewed. Meanwhile, the authors’ previous work is also introduced. It is can be seen that the solar flux distributions in both tubular receiver and volumetric receiver are extremely non-uniform for the 4 types of CSP systems. The non-uniform solar flux leads to the non-uniform temperature distributions in the absorber. There exist high local temperature and large temperature gradient, which can causes great challenges for the safety and high-efficiency operation of CSP systems. The selective coating and the heat transfer fluid tend to degrade in high temperature environment; and the absorber also could be burned out in the hot-spot area. Thermal stress and thermal deformation resulted from large temperature gradient can cause damage of the key components of solar receivers. Then the authors pointed out the reason that the non-uniform solar flux distribution could cause the above challenges is attributed to the mismatching between the non-uniform solar flux distribution in the absorber and the uniform heat absorption capacity of the heat transfer fluid, which leads to the non-uniform temperature distribution. Accordingly, the existing solutions for tackling the challenges caused by non-uniform solar flux distribution can be classified into 2 groups: (1) Optimizing the heat absorption capacity to match the non-uniform solar flux distribution; (2) Homogenizing the solar flux distribution in absorber/receiver to match the uniform heat absorption capacity. Finally, the guiding principle of matching between the collecting process and the absorbing process is proposed for solving the problems caused by the non-uniform characteristics of solar flux distribution. The solar collectors and solar receivers in CSP systems should be optimized under the guidance of this principle to improve the safety and efficiency of CSP systems.

Integrated numerical study on the coupled photon-thermal conversion process in the central solar molten salt cavity receiver

In a solar power tower system, the concentrating and collecting subsystem, including the heliostat field and the receiver, contains some of the most costly and technically challenging components. Many types of solar receivers make up the solar power tower system. Among them, the cavity molten salt receiver has been widely applied due to its lower heat loss. The accurate simulation of the coupled heat transfer process in the cavity receiver and the prediction of receiver performance are of great importance for the receiver design and safe operation. The solar flux distribution on cavity interior surfaces is extremely non-uniform, which has great effect on the receiver thermal performance. Taking the non-uniform solar flux distribution into consideration, the photon-thermal conversion process is simulated integrally in this paper. A hybrid simulation approach, which couples Monte Carlo ray tracing and Gebhart methods, is applied to investigate the complete solar radiation transfer process in the solar power tower system with a cavity receiver and obtain the non-uniform solar flux distribution. On this basis, the coupled photon-thermal conversion process in the molten salt cavity receiver is simulated. The effects of the fluid flow layout on the receiver performance under the non-uniform solar flux distribution are particularly revealed. Also, the variation of the receiver performance with time is analyzed in detail. The results show that the solar flux distribution on the interior surfaces of the cavity receiver is extremely non-uniform, under which the molten salt flow layout has great effects on the receiver performance. In the case of flow layout that the cold molten salt flows into the receiver in the high-flux area, the temperatures of both the absorber wall and the molten salt increase rapidly along the flow path. The temperature of the whole absorber wall is relatively high, which leads to greater heat loss and less amount of hot molten salt, and even seriously, “heat transfer deterioration” near the outlet of the receiver. When the cold molten salt flows into the receiver in the low-flux area, the temperature of both the absorber wall and the molten salt increase slowly along the flow path. The temperatures of the whole absorber wall is relatively low, which results in less heat loss and larger amount of hot molten salt. Although the shape of the solar flux distribution on the interior surfaces varied greatly with time, the evolutions of the molten salt (absorber wall) temperature at different time display similar variation tendency along the flow path. Moreover, the reflective loss varies with time greatly, while the heat loss varies with time slightly.

Design and characteristics of a novel tapered tube bundle receiver for high-temperature solar dish system

Most of current receivers for high-temperature (>1000 K) solar thermal utilizations were confronted with non-uniform solar flux distribution on absorbers, which would result in cracking and melting as well as flow instability. This paper proposes a new design of tapered tube bundle receiver (TTBR) by investigating the geometrical principle of dish concentrators, which makes each passage of the absorber parallel with the concentrated sun rays to absorb uniform solar thermal. The performance of the new design was confirmed using the theory of energy balance, Ray-tracing simulation and CFX simulation. Calculations based on the theory of energy balance indicated that the thermal efficiency of TTBR achieves 80%. Ray-tracing simulation results showed that solar rays are able to irradiate the entire tubes and uniformly distribute on each tube. And according to the flow field simulation by CFX, attributed to the circumferential inflow structure, almost equivalent air mass rate was obtained in each tube, indicating the designed TTBR receiver can achieve uniform flow in the absorber passages, which laid a good foundation for the further stable and efficient heat transfer procedure.

Influence of circumferential solar heat flux distribution on the heat transfer coefficients of linear Fresnel collector absorber tubes

The absorber tubes of solar thermal collectors have enormous influence on the performance of the solar collector systems. In this numerical study, the influence of circumferential uniform and non-uniform solar heat flux distributions on the internal and overall heat transfer coefficients of the absorber tubes of a linear Fresnel solar collector was investigated. A 3D steady-state numerical simulation was implemented based on ANSYS Fluent code version 14. The non-uniform solar heat flux distribution was modelled as a sinusoidal function of the concentrated solar heat flux incident on the circumference of the absorber tube. The k–ε model was employed to simulate the turbulent flow of the heat transfer fluid through the absorber tube. The tube-wall heat conduction and the convective and irradiative heat losses to the surroundings were also considered in the model. The average internal and overall heat transfer coefficients were determined for the sinusoidal circumferential non-uniform heat flux distribution span of 160°, 180°, 200° and 240°, and the 360° span of circumferential uniform heat flux for 10m long absorber tubes of different inner diameters and wall thicknesses with thermal conductivity of 16.27W/mK between the Reynolds number range of 4000 and 210,000 based on the inlet temperature. The results showed that the average internal heat transfer coefficients for the 360° span of circumferential uniform heat flux with different concentration ratios on absorber tubes of the same inner diameters, wall thicknesses and thermal conductivity were approximately the same, but the average overall heat transfer coefficient increased with the increase in the concentration ratios of the uniform heat flux incident on the tubes. Also, the average internal heat transfer coefficient for the absorber tube with a 360° span of uniform heat flux was approximately the same as that of the absorber tubes with the sinusoidal circumferential non-uniform heat flux span of 160°, 180°, 200° and 240° for the heat flux of the same concentration ratio, but the average overall heat transfer coefficient for the uniform heat flux case was higher than that of the non-uniform flux distributions. The average axial local internal heat transfer coefficient for the 360° span of uniform heat flux distribution on a 10m long absorber tube was slightly higher than that of the 160°, 200° and 240° span of non-uniform flux distributions at the Reynolds number of 4000. The average internal and overall heat transfer coefficients for four absorber tubes of different inner diameters and wall thicknesses and thermal conductivity of 16.27W/mK with 200° span of circumferential non-uniform flux were found to increase with the decrease in the inner-wall diameter of the absorber tubes. The numerical results showed good agreement with the Nusselt number experimental correlations for fully developed turbulent flow available in the literature.

Heat transfer analysis and numerical simulation of a parabolic trough solar collector

Parabolic trough solar collector is the most proven industry-scale solar generation technology today available. The thermal performance of such devices is of major interest for optimising the solar field output and increase the efficiency of power plants. In this paper, a detailed numerical heat transfer model based on the finite volume method for these equipment is presented. In the model, the different elements of the receiver are discretised into several segments in both axial and azimuthal directions and energy balances are applied for each control volume. An optical model is also developed for calculating the non-uniform solar flux distribution around the receiver. This model is based on finite volume method and ray trace techniques and takes into account the finite size of the Sun. The solar heat flux is determined as a pre-processing task and coupled to the energy balance model as a boundary condition for the outer surface of the receiver. The set of algebraic equations are solved simultaneously using direct solvers. The model is thoroughly validated with results from the literature. First, the optical model is compared with known analytical solutions. After that, the performance of the overall model is tested against experimental measurements from Sandia National Laboratories and other un-irradiated receivers experiments. In all cases, results obtained shown a good agreement with experimental and analytical results.