Numerical study of conduction and convection heat losses from a half-insulated air-filled annulus of the receiver of a parabolic trough collector

Grid-quality parabolic trough collectors utilize expensive receivers that maintain vacuum in their annuli to reduce convection losses. On the other hand, receivers with air-filled annuli, currently used mainly for process heat applications, are significantly less expensive, but their thermal performance is inferior to evacuated receivers. A promising technique that can bridge the cost and performance gap between the two types of receivers is introduced in this work. A heat-resistant thermal insulation material is fitted into the portion of the receiver annulus that does not receive concentrated sunlight. The presence of this insulation material is expected to reduce not only convection heat losses, but also radiation losses. This study focuses on the calculation of conduction and convection heat losses from the proposed receiver using numerical modeling. The performance of the proposed concept is compared to that of a conventional receiver with an air-filled annulus. The results have shown that the combined conduction and convection heat loss from the proposed receiver can be smaller than that from a receiver with an air-filled annulus by as much as 25% when fiberglass insulation is used. However, the fact that the thermal conductivity of the insulating material increases with temperature reduces the benefit of the proposed concept at high temperatures. As a result, the proposed receiver is expected to be suitable as a replacement for receivers with air-filled annuli or as an economical alternative to evacuated receivers that are used at the lower temperature end of utility-scale solar power plants.     

Numerical investigation of parabolic trough receiver performance with outer vacuum shell

The useful heat gain of a parabolic collector system is directly dependent on the heat loss from the absorber at its operating temperature. Selective coatings with evacuated/non evacuated glass tubes are employed to control radiative and convective heat losses. A concentric glass shell under vacuum is investigated for its thermal performance as this method circumvents the need for direct sealing between the glass envelope and the metal receiver to maintain vacuum and its related technical challenges. The performance is compared against a non evacuated receiver and its influence under different wind velocities; emissivities are calculated by a one dimensional theoretical model and solved by an iterative method.     

抛物槽式集热器热效率研究

介绍槽式集热器的结构及其工作过程,对集热器进行热性能分析,研究已有集热器热力学模型,并对其进行优化,利用该模型计算各个部位的热损失大小以及集热器热效率,分析得出影响集热器热效率的主要因素,定量分析这些因素对集热器效率的影响趋势,并解释其原因。     

Eulerian multiphase study of direct steam generation in parabolic trough with OpenFOAM

Direct steam generation (DSG) in parabolic trough solar collectors is a feasible option for economic improvement in solar thermal power generation. Three-dimensional Eulerian two-fluid simulations are performed under OpenFOAM to study the turbulent flow in the evaporation section of the parabolic trough receiver and investigate the phase change, and pressure drop of water as a heat transfer fluid. First, the model's validity has been tested by comparing the numerical results of a laboratory scale boiler with the available correlations and semi-correlations of boiling flows from the literature. Simulations agreed well with Rouhani–Axelsson correlation for horizontal tubes, with a mean relative error of less than 7.1% for all studied cases. However, despite a mean relative error of less than 13.19% compared to the experimental data in the literature, the reported pressure drop factor remains valid; overprediction remains tolerable for most engineering applications. Second, the scaling effect on the mathematical model, from laboratory to commercial-scale configuration, was tested with experimental data of the DISS test loop in Platforma Solar de Almeria, Spain. The Monte Carlo Ray Tracing method under the Tonatiuh package allowed for obtaining the nonuniform heat flux distribution. Due to the large size of the evaporation section in the DISS loop (eight collectors), each collector is considered independently in the simulations. Thus, simulations follow each other, taking the numerical results of each collector output as input data in the next collector and so on until the last. The numerical results showed an excellent agreement for the void fraction with 3.53% against the Rouhani–Axelsson correlation. Frictional pressure losses are within a 17.06% error of the Friedel correlation, in the range of previous work in the literature, and the heat loss is less than 4.69% error versus experimental correlation.     

Potential of performance improvement of concentrated solar power plants by optimizing the parabolic trough receiver

This paper proposes a comprehensive thermodynamic and economic model to predict and compare the performance of concentrated solar power plants with traditional and novel receivers with different configurations involving operating temperatures and locations. The simulation results reveal that power plants with novel receivers exhibit a superior thermodynamic and economic performance compared with traditional receivers. The annual electricity productions of power plants with novel receivers in Phoenix, Sevilla, and Tuotuohe are 8.5%, 10.5%, and 14.4% higher than those with traditional receivers at the outlet temperature of 550°C. The levelized cost of electricity of power plants with double-selective-coated receivers can be decreased by 6.9%, 8.5%, and 11.6%. In Phoenix, the optimal operating temperature of the power plants is improved from 500°C to 560°C by employing a novel receiver. Furthermore, the sensitivity analysis of the receiver heat loss, solar absorption, and freeze protection temperature is also conducted to analyze the general rule of influence of the receiver performance on power plants performance. Solar absorption has a positive contribution to annual electricity productions, whereas heat loss and freeze protection temperature have a negative effect on electricity outputs. The results indicate that the novel receiver coupled with low melting temperature molten salt is the best configuration for improving the overall performance of the power plants.     

Three-dimensional numerical study of heat transfer characteristics in the receiver tube of parabolic trough solar collector

The solar energy flux distribution on the outer wall of the inner absorber tube of a parabolic solar collector receiver is calculated successfully by adopting the Monte Carlo Ray-Trace Method (MCRT Method). It is revealed that the non-uniformity of the solar energy flux distribution is very large. Three-dimensional numerical simulation of coupled heat transfer characteristics in the receiver tube is calculated and analyzed by combining the MCRT Method and the FLUENT software, in which the heat transfer fluid and physical model are Syltherm 800 liquid oil and LS2 parabolic solar collector from the testing experiment of Dudley et al., respectively. Temperature-dependent properties of the oil and thermal radiation between the inner absorber tube and the outer glass cover tube are also taken into account. Comparing with test results from three typical testing conditions, the average difference is within 2%. And then the mechanism of the coupled heat transfer in the receiver tube is further studied.     

球形腔式太阳能吸热器性能的数值研究

利用蒙特卡洛光线追踪法分析了6种不同开口比(D/d)的球形腔式吸热器的光学性能,并以光学模拟所得壁面能流作为热分析的边界条件导入CFD软件中,运用CFD软件对6种不同开口比的球形腔式吸热器进行流固耦合传热计算,获得了球形腔式吸热器和内部流体的温度场分布。通过计算球形腔式吸热器的反射光损失、对流热损失和热辐射损失,得到聚光器/球形腔式吸热器系统的光热转化效率为81.9%～84.4%,球形腔式吸热器的最佳开口比1相似文献   

Numerical and experimental investigation of parabolic trough collector with focal evacuated tube and different working fluids integrated with single slope solar still

This study deals with the design and fabrication of parabolic trough solar collectors (PTCs) used to increase the yield of a single slope solar still. The designed parabolic trough solar collector is investigated numerically using Ansys Fluent 18.2. The proposed solar still is coupled with a parabolic trough solar collector with an evacuated tube receiver in its focal axis using different working fluids. The working fluids are water (case 1), oil (case 2), and nano-oil (CuO/mineral oil 3% vol; case 3). In the case when the working fluid is not water, then a heat exchanger serpentine should be used in the solar still basin. The PTC has a rim angle of 82° and an aperture width of 0.9 m and length of 2.8 m. An assessment of the performance for the studied systems was accomplished under the weather conditions of Ismailia, Egypt, during summer months, June, July, and August 2019. The outcomes of closed-loop working fluids different flow rates are investigated. The experimental results of the accumulated freshwater productivities record 2.955, 3.475, 4.29, and 5.04 L m⁻² d⁻¹ for the traditional solar still and the modified cases 1 to 3 solar stills, respectively. The modified solar still in case 3 has the highest daily accumulated freshwater productivity with a percentage increase of 71.2% than the traditional solar still. The maximum daily efficiency is 46% and 26.9% for the traditional and modified (case 3) solar stills, respectively. The cost of 1 L of fresh water is 0.057 and 0.062 $/L for the traditional and the modified (case 3) solar stills, respectively.     

Numerical simulation of the heat flux distribution in a solar cavity receiver

In the solar tower power plant, the receiver is one of the main components of efficient concentrating solar collector systems. In the design of the receiver, the heat flux distribution in the cavity should be considered first. In this study, a numerical simulation using the Monte Carlo Method has been conducted on the heat flux distribution in the cavity receiver, which consists of six lateral faces and floor and roof planes, with an aperture of 2.0 m×2.0 m on the front face. The mathematics and physical models of a single solar ray’s launching, reflection, and absorption were proposed. By tracing every solar ray, the distribution of heat flux density in the cavity receiver was obtained. The numerical results show that the solar flux distribution on the absorbing panels is similar to that of CESA-I’s. When the reradiation from walls was considered, the detailed heat flux distributions were issued, in which 49.10% of the total incident energy was absorbed by the central panels, 47.02% by the side panels, and 3.88% was overflowed from the aperture. Regarding the peak heat flux, the value of up to 1196.406 kW/m² was obtained in the center of absorbing panels. These results provide necessary data for the structure design of cavity receiver and the local thermal stress analysis for boiling and superheated panels.     

Thermal and hydraulic characteristics of a large-scaled parabolic trough solar field (PTSF) under cloud passages

To better understand the characteristics of a large-scaled parabolic trough solar field (PTSF) under cloud passages, a novel method which combines a closed-loop thermal hydraulic model (CLTHM) and cloud vector (CV) is developed. Besides, the CLTHM is established and validated based on a pilot plant. Moreover, some key parameters which are used to characterize a typical PTSF and CV are presented for further simulation. Furthermore, two sets of results simulated by the CLTHM are compared and discussed. One set deals with cloud passages by the CV, while the other by the traditionally distributed weather stations (DWSs). Because of considering the solar irradiance distribution in a more detailed and realistically way, compared with the distributed weather station (DWS) simulation, all essential parameters, such as the total flowrate, flow distribution, outlet temperature, thermal and exergetic efficiency, and exergetic destruction tend to be more precise and smoother in the CV simulation. For example, for the runner outlet temperature, which is the most crucial parameter for a running PTSF, the maximum relative error reaches −15% in the comparison. In addition, the mechanism of thermal and hydraulic unbalance caused by cloud passages are explained based on the simulation.     

Numerical study on coupled fluid flow and heat transfer process in parabolic trough solar collector tube

A unified two-dimensional numerical model was developed for the coupled heat transfer process in parabolic solar collector tube, which includes nature convection, forced convection, heat conduction and fluid-solid conjugate problem. The effects of Rayleigh number (Ra), tube diameter ratio and thermal conductivity of the tube wall on the heat transfer and fluid flow performance were numerically analyzed. The distributions of flow field, temperature field, local Nu and local temperature gradient were examined. The results show that when Ra is larger than 10⁵, the effects of nature convection must be taken into account. With the increase of tube diameter ratio, the Nusselt number in inner tube (Nu₁) increases and the Nusselt number in annuli space (Nu₂) decreases. With the increase of tube wall thermal conductivity, Nu₁ decreases and Nu₂ increases. When thermal conductivity is larger than 200 W/(m K), it would have little effects on Nu and average temperatures. Due to the effect of the nature convection, along the circumferential direction (from top to down), the temperature in the cross-section decreases and the temperature gradient on inner tube surface increases at first. Then, the temperature and temperature gradients would present a converse variation at θ near π. The local Nu on inner tube outer surface increases along circumferential direction until it reaches a maximum value then it decreases again.     

Effect of gas radiation on radiative heat transfer in urban street canyon model

In this paper, we describe the results of numerical simulation of radiative heat transfer between the human body and an urban street canyon (building walls, pavement, and the sky) in the presence of participating non‐gray gas mixtures consisting of H₂O and CO₂. The ambient temperature in typical summer conditions and the concentration of gas mixtures during summer in Tokyo were assumed. Further, the parallel infinite plane model and simple urban street canyon model were used. The results show that the participating gas significantly affects the infrared radiation field in an urban street canyon. The radiation flux emitted by the participating gas is approximately 35% of the total radiation flux incident on the human body surface. This causes a homogenization of the infrared radiation field surrounding the human body. Gas radiation plays an important role in the heat transfer between the human body and the environment under hot and humid summer conditions. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20258     

Double diffusive Buoyancy-driven flow in a fluid-saturated elliptical annulus with a neural network-based prediction of heat and mass transfer

This paper presents a numerical study of buoyancy-driven double-diffusive convection within an elliptical annulus enclosure filled with a saturated porous medium. An in-house built FORTRAN code has been developed, and computations are carried out in a range of values of Darcy–Rayleigh number Ra_m (10 ≤ Ra_m ≤ 500), Lewis number Le (0.1 ≤ Le ≤ 10), and the ratio of buoyancy forces N (−5 ≤ N ≤ 5). In addition, three methods are used, namely the multi-variable polynomial regression, the group method of data handling (GMDH), and the artificial neural network (ANN) for the predictions of heat and mass transfer rates. First, results are successfully validated with existing numerical and experimental data. Then, the results indicated that temperature and concentration distributions are sensitive to the Lewis number and thermal and mass plumes are developing in proportion to the Lewis number. Two particular values of Lewis number Le = 2.735 and Le = 2.75 captured the flow's transition toward an asymmetric structure with a bifurcation of convective cells. The average Nusselt number tends to have an almost asymptotic value for Le » 5. For the case of aiding buoyancies N > 1, the average Nusselt Number $\overline{\mathit{Nu}}$ decreased by 33% when the Lewis number increased to its maximum value. Then, it increased by 10% when the Lewis number increased to Le = 1 for the case of opposing buoyancies N < 1 and then decreased by 33% when the Lewis number increased to its maximum value., contrary to the behavior of the average Sherwood number $\overline{\mathit{Sh}}$ that increased by 700% for both cases N > 1 and N < 1. New correlations of $\overline{\mathit{Nu}}$, and $\overline{\mathit{Sh}}$ as a function of Ra_m, Le, and N are derived and compared with GMDH and ANN methods, and the ANN method showed higher performance for the prediction of $\overline{\mathit{Nu}}$ and $\overline{\mathit{Sh}}$ with R² exceeding 0.99.     

Characteristics of combined heat transfer of superheated steam drying: Numerical study on coupled convection and gas radiation heat transfer

A numerical study on a combined radiation and forced convection heat transfer of superheated steam, which is a radiation participating real gas, in thermally developing laminar flow through a parallel‐plate channel has been conducted to investigate characteristics of superheated steam drying. The integrodifferential energy equation was solved using an implicit finite‐difference technique with a marching solution procedure and an exponential wide‐band model for the treatment of the radiative transfer part. Comparison of results with and without gas radiation in various conditions shows that fluid radiation decreases the temperature of the main stream, but increases the total heat flux at a heat transfer surface. Furthermore, the results show that the fluid radiation decreases the inversion point temperature approximately to 150 to 240 °C with the increase of optical thickness. This numerical result agrees in an order of magnitude with the previous experimental studies, but is about 100 K lower than that of former theoretical predictions without considering fluid radiation. © 2000 Scripta Technica, Heat Trans Asian Res, 29(5): 385–399, 2000     

Efficient utilization of waste heat from molten carbonate fuel cell in parabolic trough power plant for electricity and hydrogen coproduction

Direct steam generating parabolic trough power plant is an important technology to match future electric energy demand. One of the problems related to its emergence is energy storage. Solar-to-hydrogen is a promising technology for solar energy storage. Electrolysis is among the most processes of hydrogen production recently investigated. High temperature steam electrolysis is a clean process to efficiently produce hydrogen. In this paper, steam electrolysis process using solar energy is used to produce hydrogen. A heat recovery steam generator generates high temperature steam thanks to the molten carbonate fuel cell's waste heat. The analytical study investigates the energy efficiency of solar power plant, molten carbonate fuel cell and electrolyser. The impact of waste heat utilization on electricity and hydrogen generation is analysed. The results of calculations done with MATLAB software show that fuel cell produces 7.73 MWth of thermal energy at design conditions. 73.37 tonnes of hydrogen and 14.26 GWh of electricity are yearly produced. The annual energy efficiency of electrolyser is 70% while the annual mean electric efficiency of solar power plant is 18.30%.The proposed configuration based on the yearly electricity production and hydrogen generation has presented a good performance.     

Simulation-based life cycle assessment of energy efficiency of biomass-based ethanol fuel from different feedstocks in China

Interests in biomass-based fuel ethanol (BFE) have been re-boosted due to oil shortage and environmental deterioration. Biomass-based fuel ethanol is renewable and, apparently, environmentally friendly. Biomass-based E10 (a blend of 10% ethanol and 90% gasoline by volume) is a promising conventional gasoline substitute, because vehicle engines require no modifications to run on E10 and vehicle warranties are unaffected. This paper presented life cycle assessments (LCAs) of energy efficiency of wheat-based E10 from central China, corn-based E10 from northeast China, and cassava-based E10 from southwest China. The respective energy flow-based evaluation model of wheat-, corn-, and cassava-based E10 was built based on data from pilot BFE plants. Monte Carlo method is applied to deal with the uncertain parameters and input and output variables of the evaluation model because of its wide application and easy development of statistical dispersion of calculated quantities. According to the assessment results, the average energy input/output ratio of wheat-based fuel ethanol (WFE), corn-based fuel ethanol (CFE), and cassava-based fuel ethanol (KFE) is 0.70, 0.75, and 0.54, respectively, and biomass-based E10 vehicle can have less fossil energy demand than gasoline-fueled ones.     

凝汽式汽轮发电机组冬季低真空运行余热供暖的节能分析

凝汽式汽轮发电机组热效率低,改造成抽汽供热和冬季低真空运行排汽余热供暖后,获得了巨大的节能效果,具有极好的应用效果。本文利用汽轮机变工况计算程序,对改造后不同运行工况进行了详细计算分析,并针对机组运行情况给出一些重要结论。     

Numerical study on heat and mass transfer in falling film crystallization by an integrated model

In this paper, a numerical study of the falling film crystallization process for purifying material uses an integrated model to solve unsteady coupled heat and mass transfer problems with phase change. The model divides the melt continuously coming down from the top of the tube into numbers of flow bundles according to their entering time sequences (steps). The heat and mass transfer equations in the coordinate system are integrated from the top of the tube to the bottom for each fluid bundle, so that the transient crystallizing amount and crystal concentration can be calculated along the tube, using conventional formulas for fully developed heat and mass transfer of falling film liquid between the liquid and tube wall. The concentrations on both liquid-solid sides of the interface are kept in equilibrium according to their temperature-concentration phase diagram so that the solving procedure must be iterated at each time step. The calculated results of some important parameters, such as crystallization rate and naphthalene concentration, coincide qualitatively with our experiments. Finally, several factors that influence the process are studied as a good reference for designing and operating the separator. © 1999 Scripta Technica, Heat Trans Asian Res, 28(4): 307–321, 1999     

Kinetic Monte Carlo simulation of hydrogen production from photocatalytic water splitting in the presence of methanol by 1 wt% Au/TiO2

In this research, using the kinetic Monte Carlo simulation (KMC), the hydrogen production from a water-methanol mixture using Au/TiO₂ photocatalyst is investigated. A mechanism is proposed, and the rate constants of the reaction steps are specified. The reaction rate constants of different steps and the concentration of the active sites on the photocatalyst surface were determined. An excellent match between simulated and experimental data confirms the results. The electron-hole pair production, methanol adsorption on the photocatalyst surface, and electron-hole recombination steps are considered the most critical steps. To study the effects of independent variables (initial concentration of methanol, photocatalyst dosage, pH, and time of reaction) on the produced hydrogen, a combination of KMC simulation and design of experiment was employed. The concentration of photocatalysis has the highest and pH has the lowest effect on the hydrogen production. The optimal conditions for photocatalytic hydrogen production are presented.     

物性参数对环形空间流动和传热影响的模拟研究

利用有限容积法,建立了环形空间内单相流体竖直向上流动过程中流动和传热的稳态模型。模型将环形空间内管设置为具有固定生热速率的发热体;流体与内管壁之间设置流动和传热边界层,以更精确的描述壁面位置流体与固体之间动量和热量的耦合传递过程。通过与常物性模型的对比,流体密度、导热系数和黏度随温度变化的变物性模型,在传热能力上具有一定的减少,流体与固体传热面之间的界面剪切力稍有下降。通过比较常物性模型和变物性模型的Re和Ri,结果表明,随着流体强制循环速度的加大,流体物性变化对流动和传热过程的影响逐渐减小。