A detailed nonuniform thermal model of a parabolic trough solar receiver with two halves and two inactive ends

In this paper a detailed one dimensional nonuniform thermal model of a parabolic trough solar collector/receiver is presented. The entire receiver is divided into two linear halves and two inactive ends for the nonuniform solar radiation, heat transfers and fluid dynamics. Different solar radiation and heat transfer modes can be taken into consideration for these four different regions respectively. This enables the study of different design parameters, material properties, operating conditions, fluid flow and heat transfer performance for the corresponding regions or the whole receiver. Then the nonuniform model and the corresponding uniform thermal model are validated with known performance of an existing parabolic trough solar collector/receiver. For applications, the uniform thermal model can be used to quickly compute the integral heat transfer performance of the whole PTC system while the nonuniform thermal model can be used to analyze the local nonuniform solar radiation and heat transfer performance characteristics and nonuniform heat transfer enhancements or optimizations. Later, it could also be effectively used with an intelligent optimization, such as the genetic algorithm or the particle swarm optimization, to quickly evaluate and optimize the characteristics and performance of PTCs under series of nonuniform conditions in detail.     

Experimental and numerical investigation on solar parabolic trough collector integrated with thermal energy storage unit

Solar parabolic trough collector (PTC) is the best recognized and commercial‐industrial‐scale, high temperature generation technology available today, and studies to assess its performance will add further impetus in improving these systems. The present work deals with numerical and experimental investigations to study the performance of a small‐scale solar PTC integrated with thermal energy storage system. Aperture area of PTC is 7.5 m², and capacity of thermal energy storage is 60 L. Paraffin has been used as phase change material and water as heat transfer fluid, which also acts as sensible heat storage medium. Experiments have been carried out to investigate the effect of mass flow rate on useful heat gain, thermal efficiency and energy collected/stored. A numerical model has been developed for the receiver/heat collecting element (HCE) based on one dimensional heat transfer equations to study temperature distribution, heat fluxes and thermal losses. Partial differential equations (PDE) obtained from mass and energy balance across HCE are discretized for transient conditions and solved for real time solar flux density values and other physical conditions of the present system. Convective and radiative heat transfers occurring in the HCE are also accounted in this study. Performance parameters obtained from this model are compared with experimental results, and it is found that agreement is good within 10% deviations. These deviations could be due to variations in incident solar radiation fed as input to the numerical model. System thermal efficiency is mainly influenced by heat gain and solar flux density whereas thermal loss is significantly influenced by concentrated solar radiation, receiver tube temperature and heat gained by heat transfer fluid. Copyright © 2016 John Wiley & Sons, Ltd.     

Heat transfer analysis of parabolic trough solar receiver

Solar Parabolic Trough Collectors (PTCs) are currently used for the production of electricity and applications with relatively higher temperatures. A heat transfer fluid circulates through a metal tube (receiver) with an external selective surface that absorbs solar radiation reflected from the mirror surfaces of the PTC. In order to reduce the heat losses, the receiver is covered by an envelope and the enclosure is usually kept under vacuum pressure. The heat transfer and optical analysis of the PTC is essential to optimize and understand its performance under different operating conditions. In this paper a detailed one dimensional numerical heat transfer analysis of a PTC is performed. The receiver and envelope were divided into several segments and mass and energy balance were applied in each segment. Improvements either in the heat transfer correlations or radiative heat transfer analysis are presented as well. The partial differential equations were discretized and the nonlinear algebraic equations were solved simultaneously. Finally, to validate the numerical results, the model was compared with experimental data obtained from Sandia National Laboratory (SNL) and other one dimensional heat transfer models. Our results showed a better agreement with experimental data compared to other models.     

Thermal performance analysis of small-sized solar parabolic trough collector using secondary reflectors

ABSTRACT

In this paper, theoretical analysis of receiver tube misalignment, the design of secondary reflector and experimental analysis of a small-sized solar parabolic trough collector (PTC) with and without secondary reflectors are represented. Experimental analysis of PTC has been done using a parabolic secondary reflector (PSR) and triangular secondary reflector (TSR) and compared with PTC without secondary reflector (WSR). The maximum outlet temperature of heat transfer fluid is observed as 49.2°C, 47.3°C and 44.2°C in the case of PSR, TSR and WSR conditions, respectively. The maximum thermal efficiency of 24.3%, 22.5% and 17.8% is observed in the case of PSR, TSR and WSR conditions, respectively. The circumferential temperature difference on the outer surface of the receiver tube is obtained more uniform in the case of PSR and TSR than WSR condition. This indicates that the use of a secondary reflector can improve the performance of a solar PTC system.     

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.     

Heat transfer analysis of receiver for large aperture parabolic trough solar collector

Parabolic trough solar collector (PTSC) is one of the most proven technologies for large‐scale solar thermal power generation. Currently, the cost of power generation from PTSC is expensive as compared with conventional power generation. The capital/power generation cost can be reduced by increasing aperture sizes of the collector. However, increase in aperture of the collector leads to higher heat flux on the absorber surface and results in higher thermal gradient. Hence, the analysis of heat distribution from the absorber to heat transfer fluid (HTF) and within the absorber is essential to identify the possibilities of failure of the receiver. In this article, extensive heat transfer analysis (HTA) of the receiver is performed for various aperture diameter of a PTSC using commercially available computational fluid dynamics (CFD) software ANSYS Fluent 19.0. The numerical simulations of the receiver are performed to analyze the temperature distribution around the circumference of the absorber tube as well as along the length of tube, the rate of heat transfer from the absorber tube to the HTF, and heat losses from the receiver for various geometric and operating conditions such as collector aperture diameter, mass flow rate, heat loss coefficient (HLC), HTF, and its inlet temperature. It is observed that temperature gradient around the circumference of the absorber and heat losses from the receiver increases with collector aperture. The temperature gradient around the circumference of the absorber tube wall at 2 m length from the inlet are observed as 11, 37, 48, 74, and 129 K, respectively, for 2.5‐, 5‐, 5.77‐, 7.5‐, and 10‐m aperture diameter of PTSC at mass flow rate of 1.25 kg/s and inlet temperature of 300 K for therminol oil as HTF. To minimize the thermal gradient around the absorber circumference, HTFs with better heat transfer characteristics are explored such as molten salt, liquid sodium, and NaK78. Liquid sodium offers a significant reduction in temperature gradient as compared of other HTFs for all the aperture sizes of the collector. It is found that the temperature gradient around the circumference of the absorber tube wall at a length of 2 m is reduced to 4, 8, 10, 13, and 18 K, respectively, for the above‐mentioned mass flow rate with liquid sodium as HTF. The analyses are also performed for different HTF inlet temperature in order to study the behavior of the receiver. Based on the HTA, it is desired to have larger aperture parabolic trough collector to generate higher temperature from the solar field and reduce the capital cost. To achieve higher temperature and better performance of the receiver, HTF with good thermophysical properties may be preferable to minimize the heat losses and thermal gradient around the circumference of the absorber tube.     

Effect of using nanofluids on efficiency of parabolic trough collectors in solar thermal electric power plants

In this paper, forced convection heat transfer nanofluid flow inside the receiver tube of solar parabolic trough collector is numerically simulated. Computational Fluid Dynamics (CFD) simulations are carried out to study the influence of using nanofluid as heat transfer fluid on thermal efficiency of the solar system. The three-dimensional steady, turbulent flow and heat transfer governing equations are solved using Finite Volume Method (FVM) with the SIMPLEC algorithm. The results show that the numerical simulation are in good agreement with the experimental data. Also, the effect of various nanoparticle volume fraction on thermal and hydrodynamic characteristics of the solar parabolic collector is discussed in details. The results indicate that, using of nanofluid instead of base fluid as a working fluid leads to enhanced heat transfer performance. Furthermore, the results reveal that by increasing of the nanoparticle volume fraction, the average Nusselt number increases.     

Three-dimensional CFD analysis for simulating the greenhouse effect in solar chimney power plants using a two-band radiation model

The greenhouse effect in the solar collector has a fundamental role to produce the upward buoyancy force in solar chimney power plant systems. This study underlines the importance of the greenhouse effect on the buoyancy-driven flow and heat transfer characteristics through the system. For this purpose, a three-dimensional unsteady model with the RNG k–ε turbulence closure was developed, using computational fluid dynamics techniques. In this model, to solve the radiative transfer equation the discrete ordinates (DO) radiation model was implemented, using a two-band radiation model. To simulate radiation effects from the sun's rays, the solar ray tracing algorithm was coupled to the calculation via a source term in the energy equation. Simulations were carried out for a system with the geometry parameters of the Manzanares power plant. The effects of the solar insolation and pressure drop across the turbine on the flow and heat transfer of the system were considered. Based on the numerical results, temperature profile of the ground surface, thermal collector efficiency and power output were calculated and the results were validated by comparing with experimental data of this prototype power plant. Furthermore, enthalpy rise through the collector and energy loss from the chimney outlet between 1-band and two-band radiation model were compared. The analysis showed that simulating the greenhouse effect has an important role to accurately predict the characteristics of the flow and heat transfer in solar chimney power plant systems.     

Investigation of temperature distribution on a new linear Fresnel receiver assembly under high solar flux

A linear Fresnel collector design with an operation temperature of 300°C or above typically requires a solar flux concentration ratio of at least 20 on the surfaces of the receiver assembly. For the commercial linear Fresnel collector design in this work, the receiver assembly includes a secondary reflector and an evacuated receiver tube. The high‐concentration solar flux may impose additional operating‐temperature requirements on the secondary reflector and receiver tube. Thus, a careful heat‐transfer analysis is necessary to understand the operating temperature of the receiver assembly component surfaces under design and off‐design conditions to guide appropriate material selections. In this work, a numerical heat‐transfer analysis is performed to calculate the temperature distribution of the surfaces of the secondary reflector and receiver glass envelope for a commercial collector design. Operating conditions examined in the heat‐transfer analysis include various wind speeds and solar concentration ratios. The results indicate a surface temperature higher than 100°C on the secondary reflector surface, which suggests that a more advanced secondary reflector material is needed. The established heat‐transfer model can be used for optimization of the other types of linear Fresnel collectors.     

An experimental investigation of the heat losses of a U-type solar heat pipe receiver of a parabolic trough collector-based natural circulation steam generation system

The performance of a parabolic trough collector (PTC)-based steam generation system depends significantly on the heat losses of the solar receiver. This paper presents an experimental study of the heat losses of a double glazing vacuum U-type solar receiver mounted in a PTC natural circulation system for generating medium-temperature steam. Field experiments were performed to determine the overall heat losses of the receiver. Effects of wind, vacuum glass tube, radiation, and structural characteristics on the heat losses were analyzed. The thermal efficiency of the receiver was found to be 0.791 and 0.472 in calm and windy days, respectively, at a test temperature of about 100 °C, whereas the thermal efficiencies became 0.792 and 0.663, respectively, while taking the receiver element into consideration. The heat losses were increased from 0.183 to 0.255 kW per receiver for the two cases tested. It was shown that neither convection nor radiation heat losses may be negligible in the analysis of such U-type solar receivers.     

Numerical study of heat transfer enhancement by unilateral longitudinal vortex generators inside parabolic trough solar receivers

This study presents numerical computation results on turbulent flow and coupled heat transfer enhancement in a novel parabolic trough solar absorber tube, the unilateral milt-longitudinal vortexes enhanced parabolic trough solar receiver (UMLVE-PTR), where longitudinal vortex generators (LVGs) are only located on the side of the absorber tube with concentrated solar radiation (CSR). The novel absorber tube and the corresponding parabolic trough receiver with smooth absorber tube (SAT-PTR) are numerical studied by combining the finite volume method (FVM) and the Monte Carlo ray-trace (MCRT) method for comparison and verification from the viewpoint of field synergy principle (FSP). Then the effects of Reynolds number, heat transfer fluid (HTF) inlet temperature, incident solar radiation and LVG geometric parameters were further examined. It was found that the mechanism of heat transfer enhancement of this novel absorber tube can be explained very well by the field synergy principle, and that the proposed novel UMLVE-PTR has good comprehensive heat transfer performance than that of the SAT-PTR within a wide range of major influence factors of diverse working conditions and geometric parameters.     

Validation of CFD simulation for flat plate solar energy collector

The problem of flat plate solar energy collector with water flow is simulated and analyzed using computational fluid dynamics (CFD) software. The considered case includes the CFD modeling of solar irradiation and the modes of mixed convection and radiation heat transfer between tube surface, glass cover, side walls, and insulating base of the collector as well as the mixed convective heat transfer in the circulating water inside the tube and conduction between the base and tube material. The collector performance, after obtaining 3-D temperature distribution over the volume of the body of the collector, was studied with and without circulating water flow. An experimental model was built and experiments were performed to validate the CFD model. The outlet temperature of water is compared with experimental results and there is a good agreement.     

CFD modeling of a polymer solar collector

A polymer solar collector is developed and its behavior is investigated both experimentally and with computational fluid dynamics (CFD). Solar irradiation as well as convection and heat transfer in the circulating fluid and between the parts of the collector is considered in the model. The temperature and velocity distribution over its area as well as the collector efficiency at nominal flow rate were used in order to validate the CFD model. Temperature distribution during operation and average collector efficiency were found to be in good agreement between the experimental data and the results of the CFD modeling.     

Flow distribution in a solar collector panel with horizontally inclined absorber strips

The objective of this work is to theoretically and experimentally investigate the flow and temperature distribution in a solar collector panel with an absorber consisting of horizontally inclined strips. Fluid flow and heat transfer in the collector panel are studied by means of computational fluid dynamics (CFD) calculations. Further, experimental investigations of a 12.5 m² solar collector panel with 16 parallel connected horizontal fins are carried out. The flow distribution through the absorber is evaluated by means of temperature measurements on the backside of the absorber tubes. The measured temperatures are compared to the temperatures determined by the CFD model and there is a good similarity between the measured and calculated results.

Calculations with the CFD model elucidate the flow and temperature distribution in the collector. The influences of different operating conditions such as flow rate, properties of solar collector fluid, solar collector fluid inlet temperature and collector tilt angle are shown. The flow distribution through the absorber fins is uniform if high flow rates are used. By decreased flow rate and decreased content of glycol in the glycol/water mixture used as solar collector fluid, and by increased collector tilt and inlet temperature, the flow distribution gets worse resulting in an increased risk of boiling in the upper part of the collector panel.     

A novel PVT/PTC/ORC solar power system with PV totally immersed in transparent organic fluid

A novel hybrid PVT/parabolic trough concentrator (PTC)/organic Rankine cycle (ORC) solar power system integrated with underground heat exchanger has been proposed. The evaporator unit consists of a transparent flat PVT solar collector and a PTC connected in series. The first transparent solar collector has transparent covers and consists of solar cells totally immersed within a pressurized transparent organic fluid that allows the solar radiation to reach the solar cells, cools them effectively, and captures all thermal losses from the solar cells. The second concentrator is a conventional one with opaque black receiver used to reheat the transparent organic fluid to higher temperatures. Both solar collectors (the PVT and PTC) perform as the boiler and superheater for the ORC. The performance of the proposed system is investigated by a steady‐state mathematical model. The results show that, at design conditions, the efficiency of the PV modules stabilizes around 12%, absorber efficiency varies within 64% to 75%, and the ORC efficiency varies within 7% to 17%.     

Heat transfer dynamics in an inflatable-tunnel solar air heater

A mathematical model that describes the dynamics of the heat transfer in an inflatable-tunnel solar collector for air heating is proposed and validated. The model is distributed-parameters, one-dimensional and unsteady-state. It considers the thermal inertia of a pebble bed acting as the absorber surface and is constituted by three equations that describe the temperature distributions of the three system components: polyethylene cover, transfer fluid (air) and absorber surface.To solve the governing equations, a novel numerical scheme that differs from the standard method of finite differences in the form of generating the discretization equations is proposed. In this scheme, the dimensionless versions of the equations are reduced to linear canonical forms of first order and then are solved analytically in small spatial domains to produce discretization equations in an explicit form.To validate the quality of the present model, some experimental tests in a 50 m long inflatable-tunnel solar collector were carried out. Results of the model compare favorably with experimental results.     

开放式热管中温太阳能空气集热装置的试验研究

设计一种使用简化CPC(非追踪式复合抛物线聚光板)集热板和新型开放式热管组合的全真空玻璃集热管中温太阳能空气集热装置。每个集热单元包括一个简化CPC集热板,一根全真空玻璃集热管,在玻璃集热管内安装一个铜管和外部的一个蒸汽包连接构成一个开放式热管结构。蒸汽包内安装螺旋换热管加热通过换热管的流动空气工质。分别使用水和CuO纳米流体作为热管工质,以空气作为集热工质,对热管式中温空气集热器的传热特性进行了实验研究。分析了不同工作压力、不同工质及纳米流体质量分数对热管集热传热特性的影响,详细比较了热管水工质和纳米流体工质在集热传热性能上的优劣。试验结果表明:本系统只使用2根玻璃集热管构成集热器,空气最大出口温度在夏天可达到200℃,在冬天可接近160℃,系统平均集热效率达到0.4以上,整个系统表现了良好的中温集热特性。以纳米流体为工质的热管热阻比以水为工质时平均降低了20%左右     

Thermal analysis of solar parabolic trough with porous disc receiver

In this paper, 3-D numerical analysis of the porous disc line receiver for solar parabolic trough collector is presented. The influence of thermic fluid properties, receiver design and solar radiation concentration on overall heat collection is investigated. The analysis is carried out based on renormalization-group (RNG) k–ε turbulent model by using Therminol-VP1 as working fluid. The thermal analysis of the receiver is carried out for various geometrical parameters such as angle (θ), orientation, height of the disc (H) and distance between the discs (w) and for different heat flux conditions. The receiver showed better heat transfer characteristics; the top porous disc configuration having w = d_i, H = 0.5d_i and θ = 30°. The heat transfer characteristic enhances about 64.3% in terms of Nusselt number with a pressure drop of 457 Pa against the tubular receiver. The use of porous medium in tubular solar receiver enhances the system performance significantly.     

Assessment of three types of heat pipe solar collectors

New comparative tests on three different types of heat pipe solar collectors are presented in this paper. These three collectors are installed in parallel and tested at the same working conditions.Results are also presented in terms of efficiency plotted against temperature of the heat transfer fluid entering the collector minus the ambient air divided by the global solar irradiance upon the aperture plane of the collector. This allows representing the comparative characteristics of the three collectors when operating under variable conditions, especially with wide range of incidence solar radiation.     

Optimal energy use of the collector tube in solar power tower plant

As one of the most important parts of the solar power tower plant, the receiver plays an important role in the high-efficiency operation of the solar power tower system. Obtaining the maximum available energy in the receiver is highly desired in real-world operations. In this paper heat transfer and exergy transfer methods are used to model the energy transfer process in a collector tube. Different from common analysis methods, in order to ensure the molten salt to obtain the maximum available energy, an objective function is proposed to convert the task into a constrained optimization problem. The gravitational search (GS) algorithm is employed to search for the optimal solution of the proposed objective function. Numerical results indicate that the proposed optimization method can find the optimal operation parameters under different conditions. The heat transfer and exergy transfer characteristics along the collector tube under the optimal working condition are revealed, which quantifies the available energy along the collector tube, as well as reveals the location of energy degradation in the tube. The research findings will provide a beneficial reference for the effective use of the solar energy.