Modelling of parabolic trough direct steam generation solar collectors

Solar electric generation systems (SEGS) currently in operation are based on parabolic trough solar collectors using synthetic oil heat transfer fluid in the collector loop to transfer thermal energy to a Rankine cycle turbine via a heat exchanger. To improve performance and reduce costs direct steam generation in the collector has been proposed. In this paper the efficiency of parabolic trough collectors is determined for operation with synthetic oil (current SEGS plants) and water (future proposal) as the working fluids. The thermal performance of a trough collector using Syltherm 800 oil as the working fluid has been measured at Sandia National Laboratory and is used in this study to develop a model of the thermal losses from the collector. The model is based on absorber wall temperature rather than fluid bulk temperature so it can be used to predict the performance of the collector with any working fluid. The effects of absorber emissivity and internal working fluid convection effects are evaluated. An efficiency equation for trough collectors is developed and used in a simulation model to evaluate the performance of direct steam generation collectors for different radiation conditions and different absorber tube sizes. Phase change in the direct steam generation collector is accounted for by separate analysis of the liquid, boiling and dry steam zones.     

On the sensitivity of soot and thermal radiation simulation results to the adopted PDF for temperature underhighly sooting combustion conditions

The impact of the turbulent temperature fluctuations on soot processes and consequently on radiation heat transfer in a small furnace is investigated. A simplified approach based on presumed PDFs for temperature is adopted. Of interest here is the sensitivity of the simulation results to the shape of the used PDF. To cover a wide spectrum of turbulence possibilities two widely varying PDFs are adopted. One is a simple two-deltas function and the other is a beta function. Results show that (as compared with the case when the beta PDF is used) more soot is generated in the furnace, hence more heat is radiated to the furnace walls, when the two-deltas PDF is employed. However, regarding thermal radiation results, under the relatively high sooting conditions of the present furnace, the difference between the two cases is found to be insignificant.     

Solar reforming of methane in a direct absorption catalytic reactor on a parabolic dish: II—Modeling and analysis

The CAtalytically Enhanced Solar Absorption Receiver (CAESAR) test was conducted to determine the thermal, chemical, and mechanical performance of a commercial-scale, dish-mounted, direct catalytic absorption receiver (DCAR) reactor over a range of steady state and transient (cloud) operating conditions. The focus of the test was to demonstrate “proof-of-concept” and determine global performance such as reactor efficiencies and overall methane conversion. A numerical model was previously developed to provide guidance in the design of the absorber. The one-dimensional, planar, and steady-state model incorporates the following energy transfer mechanisms: solar and infrared radiation, heterogeneous chemical reaction, conduction in the solid phase, and convection between the fluid and solid phases. Improvements to the model and improved property values are presented here. In particular, the solar radiative transfer model is improved by using a three-flux technique to more accurately represent the typically conical incident flux. A spatially varying catalyst loading is incorporated, convective and radiative properties for each layer in the multilayer absorber are determined, and more realistic boundary conditions are applied. Considering that this test was not intended to provide data for code validation, model predictions are shown to generally bound the test axial thermocouple data when test uncertainties are included. Global predictions are made using a technique in which the incident solar flux distribution is subdivided into flux contour bands. Reactor predictions for anticipated operating conditions suggest that a further decrease in optical density (i.e., extinction coefficient) at the front of the absorber inner disk may improve absorber conditions. Code-validation experiments are needed to improve the confidence in the simulation of large-scale reactor operation.     

带有透明蜂窝漂浮式选择性吸热器的小型净水太阳池数值模拟分析

提出了一种低热损小型净水太阳池，采用带光谱选择性吸收面的漂浮式吸热器及透明蜂窝结构，可大幅度抑制热损。工作时，从池底抽出的水喷射在吸热器的背面，能使后者恒处于较低的温度，基于一维准稳态假定提出一个计算模型，可用于计算一天中池水温度随深度的变化及一年中各月份的的热性能。     

FEM Simulation of the Bending Angle During the Laser Forming of FGM Plates

Functionally graded material (FGM) plate forming by using a modern laser technique is taken into consideration in this article. By using finite element simulation, the effects of movement speed, laser diameter, laser power, plate thickness, and the number of passes on the value of the FGM plate transverse bending angle are investigated. Finite element (FE) simulation is performed with the ABAQUS/CAE standard software package. By comparing the simulation with analytical formulas, it is observed that the FE results are in good agreement with the analytical results.     

Thermal enhancement of solar parabolic trough collectors by using nanofluids and converging-diverging absorber tube

Parabolic trough collectors are the most mature technology for utilizing the solar energy in high temperature applications. The objective of this study is the thermal efficiency enhancement of the commercial parabolic collector IST-PTC by increasing the convective heat transfer coefficient between the working fluid and the absorber. There are two main factors which influence on this parameter, the working fluid type and the absorber geometry. For this reason three working fluids are investigated, thermal oil, thermal oil with nanoparticles and pressurized water. Moreover, a dimpled absorber tube with sine geometry is tested because this shape increases the heat transfer surface and increases the turbulence in the flow. The final results show that these two techniques improve the heat transfer coefficient and the thermal efficiency of the collector. More specifically, the use of nanofluids increases the collector efficiency by 4.25% while the geometry improvement increases the efficiency by 4.55%. Furthermore, collector parameters such as the heat loss coefficient, the exergetic efficiency, the pressure losses and the absorber temperature are presented for all the examined cases. The model is designed with Solidworks and is simulated by its flow simulation studio.     

Numerical investigation of an enhanced PTC absorber tube using cylindrical inserts

The main purpose of this study is to investigate numerically the thermal performance of a parabolic trough solar collector's absorber tube that contains a novel kind of inserts with the objective to improve the heat transfer between the heat transfer fluid and the absorber tube. In the first part of this paper, the diameter and the length of the cylindrical inserts are investigated based on finite volume method and Monte Carlo ray tracing method for Reynolds number ranges from 2.36 × 10⁴ to 7.09 × 10⁴. In the second part, the eccentricity of the cylindrical inserts is investigated under the same operating conditions. The Therminol®VP1 is the HTF that is used in this investigation's intermediate fluid. The numerical simulation indicates that the perturbators enhance the thermal behavior of the receiver and reduces the absorber tube's temperature difference.     

多孔介质太阳能吸热器性能分析

采用4种多孔骨架中辐射传输模型,包括:忽略多孔骨架内部辐射模型(模型A)、Rosseland模型(模型B)、均匀内热源模型(模型C)与吸热器中辐射传输满足Beer定律的模型(模型D),推导得到了局部非热平衡条件下4种模型所对应的吸热器中多孔骨架温度、空气温度和吸热器热效率的解析解,分析了多孔骨架孔隙率、导热系数和孔隙直径对吸热器性能的影响。结果表明,对模型A和模型B,吸热器中最高温度位于吸热器进口处;对模型C,吸热器中最高温度位于吸热器出口处;而在模型D中,吸热器中吸热器内部或吸热器的出口处温度最高。吸热器效率取决于多孔骨架导热系数、孔隙率和孔隙直径等参数,当吸热器中内热源均匀分布时,吸热器效率是最高的。     

Optimization of the tubular absorber using a compound parabolic concentrator

The paper deals with the optimization of the tubular absorber of a compound parabolic concentrator (CPC) solar collector. In order to minimize the radiation thermal losses from the absorber, a modified absorber with multi-cavities is proposed. The cavities are introduced at the circumferential area with relatively high solar intensities. These areas were determined by the use of a ray-tracing technique. This has been adopted using the AutoCAD^® package. The analysis was carried out and applied to a CPC with an acceptance angle of 10 and a concentration ratio of × 4.0.     

The thermal and economic characteristics of solar air collectors with different delta turbulators arrangement

The benefits of using a solar air collector with different array delta turbulators absorber plate are experimentally investigated to assess this type of collector under western and middle of Iraq climate conditions. The solar collector was investigated with four different absorber plate in which flat plate, in‐line delta turbulators, staggered delta turbulators and inclined staggered delta turbulators with different mass airflow rate. The economic characteristics and overall thermal performance of the collector are compared with other heating systems. The major findings show that the delta turbulators enhance the economic characteristics and the overall thermal performance of collectors due to vortex generation and damping the development of the thermal boundary layer in the direction of airflow. A substantial enhancement is observed in lowering both life‐cycle cost and increasing energy saving with delta turbulators. This study will likewise provide a new direction to the work trend in western and middle of Iraq climate conditions during winter days.     

Cover systems for high temperature flat-plate solar collectors

A simulation study has been conducted of the influence of cover design on the thermal performance of flat-plate solar collectors for use at temperatures of 150°C. Detailed results are presented of the effects of changes in cover materials, cover surface treatments, cover system configuration and absorber plate surface treatments on both the instantaneous efficiency and the long term solar contribution of flat-plate collectors. For the simulation conditions, it is shown that flat-plate collectors consisting of a single high transmittance cover, a convection suppressing device such as a honeycomb and a selective absorber surface yield long term solar contributions comparable to those of evacuated tubular collectors, whereas other configurations simulated (single and multiple cover systems) are significantly inferior.     

Investigation on the forced response of a radial turbine under aerodynamic excitations

Rotor blades in a radial turbine with nozzle guide vanes typically experience harmonic aerodynamic excitations due to the rotor stator interaction. Dynamic stresses induced by the harmonic excitations can result in high cycle fatigue (HCF) of the blades. A reliable prediction method for forced response issue is essential to avoid the HCF problem. In this work, the forced response mechanisms were investigated based on a fluid structure interaction (FSI) method. Aerodynamic excitations were obtained by three-dimensional unsteady computational fluid dynamics (CFD) simulation with phase shifted periodic boundary conditions. The first two harmonic pressures were determined as the primary components of the excitation and applied to finite element (FE) model to conduct the computational structural dynamics (CSD) simulation. The computed results from the harmonic forced response analysis show good agreement with the predictions of Singh's advanced frequency evaluation (SAFE) diagram. Moreover, the mode superposition method used in FE simulation offers an efficient way to provide quantitative assessments of mode response levels and resonant strength.     

V型多通道平板太阳能集热器的热性能研究

提出一种V型多通道平板太阳能集热器,对其建立稳态传热模型,利用Matlab软件编程进行求解,并进行实验测试验证模型的准确性.利用已验证的传热模型,模拟分析V型多通道平板太阳能集热器的结构和物性参数对其热性能的影响,结果表明当透明盖板和吸热体发射率变大时,集热器的集热效率会呈下降趋势;当V型槽顶角、吸热体长度及空气夹层厚...     

Thermal Post-Buckling Analysis of Nanoscale Films Based on a Non-Classical Finite Element Approach

A size-dependent finite element (FE) formulation including surface free energy effect is developed in this article to study the post-buckling behavior of nanofilms under the action of thermal loads. The Gurtin-Murdoch surface elasticity theory is utilized to consider the surface effects. Moreover, the principle of virtual work is used so as to derive the equilibrium equations. The proposed FE formulation is based on the first-order shear deformation theory (FSDT). The von Kármán nonlinear relations are also employed to take the geometric nonlinearity into account. After deriving the FE equations, the resulting set of parameterized non-linear equations is solved using the pseudo arc-length continuation algorithm, and bifurcation diagrams of nanofilms are obtained. Selected numerical results are presented for the influences of surface stress on the thermal post-buckling characteristics of nanofilms subject to different types of boundary conditions.     

Detailed numerical simulation of thermal radiation influence in Sandia flame D

In order to investigate the influence of thermal radiation in turbulent combustion processes, Sandia flame D is numerically simulated, with multiple-time scale (MTS) k–ε turbulence model for turbulence, the combination of probability density function (PDF) transportation method, Lagrangian flamelet model (LFM) and the detailed chemical reaction mechanism GRI 3.0 (consisting of 53 species and 325 elemental reactions) for combustion and finite volume/correlated-k (FV/CK) method for radiation heat transfer. To account for turbulence’s influence on radiation, the effects of turbulence–radiation interactions (TRI) are investigated in radiation calculations and it is recommended that for detailed numerical simulation TRI should be considered. Numerical results with and without radiation influence being taken into accounted are compared with experimental data. Different from reports by other researchers, our simulation results show that although the magnitude of thermal radiation is relatively small, its influence on combustion process is significant. It is suggested that turbulence and chemical reactions may magnify the influence of thermal radiation.     

Thermodynamic performance analysis of gas-fired air-cooled adiabatic absorption refrigeration systems

In China, the application of small size gas-fired air-cooled absorption refrigeration systems as an alternative for electric compression air conditioning systems has shown broad prospects due to occurrence of electricity peak demand in Chinese big cities and lack of water resources. However, for conventional air-cooled absorption refrigeration systems, it is difficult to enhance the heat and mass transfer process in the falling film absorber, and may cause problems, for example, remarkable increase of pressure, temperature and concentration in the generators, risk of crystallization, acceleration of corrosion, degradation of performance, and so on. This paper presents a gas-fired air-cooled adiabatic absorption refrigeration system using lithium bromide–water solutions as its working fluid, which is designed with a cooling capacity of 16 kW under standard conditions. The system has two new features of waste heat recovery of condensed water from generator and an adiabatic absorber with an air cooler. Performance simulation and characteristic analysis are crucial for the optimal control and reliability of operation in extremely hot climates. A methodology is presented to simulate thermodynamic performance of the system. The influences of outdoor air temperature on operation performances of the system are investigated.     

A novel advanced box-type solar cooker

An advanced version of the box-type solar cooker is presented: a fixed cooking vessel in good thermal contact with a conductive absorber plate is set into the glazing; the results are improved thermal performance, easier access to the cooking vessel and less frequent maintenance due to protection of all absorbing and reflecting surfaces. Outdoor tests show that 5 L of water per sq m of opening surface can be brought to full boiling in less than one hour. A finite element simulation model of the advanced box cooker is presented. It is shown that the most decisive parameters are absorber-to-pot heat transfer and absorber conductivity. Field tests in Ethiopia and India are under way, local production in India has started.     

Simulation and optimisation studies on a solar parabolic collector: an experimental investigation

The present paper has focused on parametric optimisation of a solar parabolic collector. This work has been performed in two phases. In the first phase, simulation studies have been done to identify the influential parameters. Based on simulation results, an experimental set-up of the parabolic collector has been fabricated using the materials selected by priority-score method. In the second phase, experiments are conducted according to Taguchi's L₉ orthogonal array. Parabolic collector parameters, namely reflector materials, absorber materials, positions of the absorber tube and angle of the absorber tube, are optimised with the consideration of multi-responses such as temperature, enthalpy, optical efficiency and thermal efficiency. The obtained experimental data are analysed using the desirability functional analysis approach and optimal levels of input parameters have been identified. Analysis of variance also has been performed to know the contribution of influential parameters on the responses.     

Multi-linear performance model for hybrid (C)PVT solar collectors

A performance model is presented that enables yield predictions of hybrid photovoltaic and thermal (PVT) collectors. It applies for both non-concentrating (PVT) and concentrating (CPVT) systems. The model is based on considerations on energy balance, heat transfer and the dependence of the photovoltaic efficiency on absorber temperature. Similar to the quasi dynamic model for thermal collectors, linear parameterizations of both electrical and thermal power outputs are derived. The respective linear coefficients are obtained from multi-linear regression on solely standard measurement data.The model is applied to measurement data of a CPVT collector to exemplify the procedure and to validate the model. Good agreement of empirically determined collector coefficients with expectations from known parameters is achieved.