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.     

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.     

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.     

Comparison of the optics of non-tracking and novel types of tracking solar thermal collectors for process heat applications up to 300 °C

Evacuated CPC (compound parabolic concentrator) collectors with non-tracking reflectors are compared with two novel tracking collectors: a parabolic trough and an evacuated tube collector with integrated tracking reflector. Non-tracking low concentrating CPC collectors are mostly mounted in east–west direction with a latitude dependent slope angle. They are suitable at most for working temperatures up to 200–250 °C. We present a tracking evacuated tube-collector with a trough-like concentrating mirror. Single-axis tracking of the mirror is realized with a magnetic mechanism. The mirror is mounted inside the evacuated tube and hence protected from environmental influences. One axis tracking in combination with a small acceptance angle allows for higher concentration as compared to non-tracking concentrating collectors. Ray-tracing analysis shows a half acceptance angle of about 5.7° at geometrical concentration ratio of 3.2. Losses of well constructed evacuated tube collectors (heat conductivity through the manifolds inside the thermally insulated terminating housing are low) are dominated by radiation losses of the absorber. Hence, reducing the absorber size can lead to higher efficiencies at high operating temperature levels. With the presented collector we aim for operating temperatures up to 350 °C. At temperatures of 300 °C we expect with anti-reflective coating of the glass tube and a selective absorber coating efficiencies of 0.65. This allows for application in industrial process heat generation, high efficient solar cooling and power generation. A first prototype, equipped with a standard glass tube and a black paint absorber coating, was tested at ZAE Bayern. The optical efficiency was measured to be 0.71. This tube-collector is compared by ray-tracing with non-tracking market available tube-collectors with geometrical concentration ratios up to 1.1 and with a low cost parabolic trough collector of Industrial Solar Technology (IST) with an acceptance half angle about 1.5°, a geometrical concentration ratio of 14.4 and a measured optical efficiency of 0.69.     

Theoretical and experimental investigation of the filled-type evacuated tube solar collector with U tube

The filled-type evacuated tube with U-tube, in which the filled layer is used to transfer energy absorbed by the working fluid flowing in the U-tube, is proposed to eliminate the influence of thermal resistance between the absorber tube and the copper fin of the conventional evacuated solar collector. In this paper, the thermal performance of the filled-type evacuated tube with U-tube was researched by means of theoretical analysis and experimental study. The temperature of the working fluid in the flow direction was obtained, and the efficiency of the evacuated tube was also calculated, based on the energy balance equations for the working fluid in the U-tube. The effects of the heat loss coefficient and the thermal conductivity of the filled layer on the thermal performance of the evacuated tube were studied. In addition, the test setup of the thermal performance of the filled-type evacuated tube with U-tube was established. The evacuated tube considered in this study was a two-layered glass evacuated tube, and the absorber film was deposited in the outer surface of the absorber tube. The results show that the filled-type evacuated tube with U-tube has a favourable thermal performance. When the thermal conductivity of the heat transmission component is λ_c = 100, the efficiency of the filled-type evacuated tube with U-tube is 12% higher than that of the U-tube evacuated tube with a copper fin. The modelling predictions were validated using experimental data which show that there is a good concurrence between the measured and predicted results.     

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.     

Experimental studies on solar flat plate collector with internally grooved tubes using aqueous ethylene glycol

In order to cope up with the increase in energy demand and decline in fossil fuels, it has become imperative to use renewable resources efficiently. Among these renewable resources, solar thermal energy is abundant in nature. Solar water heating systems are one of the most important applications of solar thermal energy. Providing internal fins to absorber tube is the technique to improve heat transfer augmentation. Hence in the present study, experiments were performed on solar flat plate collector with different cross section of absorber tubes (plain tube and internally grooved tubes with different helix angles) and by varying the mass flow rates of the working fluids. This study reports the experimental results of flat plate collector, where the working fluid is water and aqueous ethylene glycol (50 : 50). Temperature profile of grooved absorber tube will be compared with plain tube. Since conversion efficiency of solar devices is low, the present study mainly focuses on improving the efficiency of solar flat plate collector.     

Influences of installation and tracking errors on the optical performance of a solar parabolic trough collector

The parabolic trough collector is an important component of parabolic trough solar thermal power generation systems. Coordinate transformations and the Monte Carlo Ray Trace (MCRT) method were combined to simulate the circumferential flux distribution on absorber tubes. The simulation model includes the optics cone with non-parallel rays, geometric concentration ratios (GCs), the glass tube transmissivity, the absorber tube absorptance and the collector surface reflectivity. The mode is used to analyze the effects of absorber tube installation errors and reflector tracking errors. The results are compared with reference data to verify the model accuracy. Influences of installation and tracking errors on the flux distribution are analyzed for different errors, incident angles and GCs. For a GC of 20 and 90° rim angle, X direction installation errors are −0.2%∼0.2%, Y direction installation errors are −1.0%–0.5%, and the tracking error should be less than 4 mrad. As the incident angle increases, the errors become larger, but the errors become smaller as concentration ratios are increased. The results provide foundations for heat transfer analysis of the absorber tube, for parabolic trough plant to ensure the safe intensity, and for economic analysis of the installation process and control system.     

Heat losses from parabolic trough solar collectors

Parabolic trough solar collector usually consists of a parabolic solar energy concentrator, which reflects solar energy into an absorber. The absorber is a tube, painted with solar radiation absorbing material, located at the focal length of the concentrator, usually covered with a totally or partially vacuumed glass tube to minimize the heat losses. Typically, the concentration ratio ranges from 30 to 80, depending on the radius of the parabolic solar energy concentrator. The working fluid can reach a temperature up to 400°C, depending on the concentration ratio, solar intensity, working fluid flow rate and other parameters. Hence, such collectors are an ideal device for power generation and/or water desalination applications. However, as the length of the collector increases and/or the fluid flow rate decreases, the rate of heat losses increases. The length of the collector may reach a point that heat gain becomes equal to the heat losses; therefore, additional length will be passive. The current work introduces an analysis for the mentioned collector for single and double glass tubes. The main objectives of this work are to understand the thermal performance of the collector and identify the heat losses from the collector. The working fluid, tube and glass temperature's variation along the collector is calculated, and variations of the heat losses along the heated tube are estimated. It should be mentioned that the working fluid may experience a phase change as it flows through the tube. Hence, the heat transfer correlation for each phase is different and depends on the void fraction and flow characteristics. However, as a first approximation, the effect of phase change is neglected. Copyright © 2013 John Wiley & Sons, Ltd.     

Investigation of evacuated tube heated by solar trough concentrating system

Two types of solar evacuated tube have been used to measure their heating efficiency and temperature with fluids of water and N₂ respectively with a parabolic trough concentrator. Experiments demonstrate that both evacuated tubes present a good heat transfer with the fluid of water, the heating efficiency is about 70–80%, and the water is easy to boil when liquid rate is less than 0.0046 kg/s. However, the efficiency of solar concentrating system with evacuated tube for heating N₂ gas is less than 40% when the temperature of N₂ gas reaches 320–460 °C. A model for evacuated tube heated by solar trough concentrating system has been built in order to further analyze the characteristics of fluid which flow evacuated tube. It is found that the model agrees with the experiments to within 5.2% accuracy. The characteristics of fluid via evacuated tube heated by solar concentrated system are analyzed under the varying conditions of solar radiation and trough aperture area. This study supports research work on using a solar trough concentrating system to perform ammonia thermo-chemical energy storage for 24 h power generation. The current research work also has application to solar refrigeration.     

Performance of Cylindrical Parabolic Collector with Automated Tracking System

Parabolic solar collector collects the radiant energy emitted from the sun and focuses it at a point. Parabolic trough collectors are the low cost implementation of concentrated solar power technology that focuses incident sun light on to a tube filled with a heat transfer fluid. However, the basic problem with the cylindrical parabolic collector without tracking was the solar collector does not move with the orientation of sun. Development of automatic tracking system for cylindrical parabolic collectors will increase solar collection as well as efficiency of devices. The main aim of this paper is to design, fabricate and analyze the performance of parabolic collector with automated tracking system. The automated tracking mechanism is used to receive the maximum possible energy of solar radiation as it tracks the path of sun. The performance of the parabolic trough collector is experimentally investigated with the water circulated as heat transfer fluid. The collector efficiency will be noted.     

Heat extraction efficiency of a concentric glass tubular evacuated collector

The results of detailed measurements and calculations of the properties of Sydney University/Nitto Kohki evacuated collector tubes have been used to develop a formula for the instantaneous heat extraction efficiency η of a collector panel incorporating the evacuated tubes. The instantaneous efficiency depends on ambient temperature, mean fluid temperature in the collector, solar flux and the design of the manifold used to extract heat from the glass absorber tubes. Manifold design determines the mean temperature difference between absorber tube surface and mean fluid temperature for given operating conditions, and strongly affects the efficiency η of a collector panel. Neither changes in the number of evacuated tubes per unit area of collector, nor variations in solar flux, significantly alter the efficiency decrement Δ η₀ associated with a particular manifold design. Calculated efficiencies agree well with experimental results for collector panels incorporating manifolds of various designs. The formula for efficiency η allows detailed analysis of the relative importance of various energy loss mechanisms in a collector.     

槽式太阳能新型腔式吸热器的热性能研究

提出一种适用于抛物槽集热器的新型太阳能腔式吸热器,该装置具有较高的集热效率,同时连接安装和日常运行维护也相对便利。对其建立一套三维传热模型,并搭建采用新型腔式吸热器的抛物槽集热器实验系统,通过实验测试对比吸热器瞬时效率,验证模型的准确性。此外,定量分析不同环境参数与工作参数对新型腔式吸热器热性能的影响,结果表明:集热效率随着法向直接日射辐照度、环境温度的升高而增加,随着环境风速和吸热器入口传热流体温度的升高而降低,而受传热流体质量流量的影响较小。     

Parabolic trough collector with rhombus tube absorber for higher concentration ratio

In the present study, the concentration ratio of the parabolic trough collector using rhombus tube absorber has been estimated. An analytical technique has been developed to determine the optimum size of the rhombus tube absorber for given trough dimensions. The optimum size of the rhombus tube absorber is 13.8% smaller than the circular tube absorber for the LS3 trough with no change in intercept factor. The maximum improvement in the concentration ratio is found to be 31.5% for the troughs with rim angle 90° in comparison to circular tube absorber. Results also indicate that rhombus tube absorber can be employed for a range of rim angle 75 to 90 degree.

Abbreviations: CR: concentration ratio; CSP: concentrated solar power; MCRT: Monte Carlo ray tracing; PTC: parabolic trough collector     

槽式太阳能直接蒸汽发生系统的热力特性研究

  [目的]  研究典型的槽式直接蒸汽循环系统的热力性能,给出该系统的结构组成和主要性能参数。针对DSG系统的两相区复杂的换热问题,结合两相区的实际流型,考虑分层现象对实际换热过程的影响,更加准确地对两相区的换热过程进行模拟研究。  [方法]  基于槽式集热器内的流动型态,建立槽式集热两相区内的传热模型,利用Matlab编程,针对槽式集热器内的DSG过程进行模拟。  [结果]  揭示了DSG技术沿着管长方向的换热特性变化规律,分层现象大约出现在两相流区域干度为0.7～0.8之间的位置,分层现象的出现降低了管内工质与金属管管壁之间的对流换热系数,减弱了换热效果。  [结论]  该研究为DSG电站的设计和运行提供了参考依据,验证了DSG集热场的最佳布置方式为再循环式。     

Analytic modeling of a solar power plant with parabolic linear collectors

An analytic model for a solar thermal electric generating system with parabolic trough collectors was developed. The energy conversion of solar radiation into thermal power along the absorber tube of the parabolic collector is studied, taking into consideration the non-linearity of heat losses and its dependence on the local temperature. The coupling between the collector and the thermodynamic cycle is made up of three heat exchangers, yielding the characteristic temperatures of the cycle. The conventional Rankine cycle is treated as an endo-reversible Carnot cycle, whereby the mechanical and electric power is calculated. For comparison, we refer to the Solar Electric Generating System VI (SEGS VI), installed in the Mojave desert-CA, whose solar field is composed by LS2 parabolic trough collectors. We simulated the efficiency curves of collectors LS2 with evacuated and non-evacuated absorbers and compared with experimental results. A second simulation was carried out to estimate the optimum quantity of non-evacuated LS2 collectors in series in a collectors’ row, when friction losses along the absorber tubes are considered. Also, the performance of a 30 (MWe) power plant, composed of 50 rows with 16 LS2 collectors in series (total 800 collectors) was simulated. Three fields of different collectors were considered, the first field with evacuated absorbers, the second with non-evacuated absorbers and the third with bare absorbers. Finally, the output power of the plant is analyzed as a function of the evaporation temperature of the water-vapor fluid. A large maximum of the overall cycle efficiency is found for evaporation temperatures around 320 °C. Good agreement is obtained when comparing the results of this model with experimental data belonging to the Solar Electric Generating Systems (SEGS) installed in the Mojave desert. The analytic model developed combines simplicity, precision and flexibility, making it an attractive tool for simulation and design of solar power stations.     

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.     

Performance optimization of evacuated tube collector for solar cooling of a house in hot climate

Evacuating the space connecting cover and absorber significantly improves evacuated tube collector (ETC) performance. So, ETCs are progressively utilised all over the world. The main goal of current study is to explore ETC thermal efficiency in hot and severe climate like Kuwait weather conditions. A collector test facility was installed to record ETC thermal performance for one-year period. An extensively developed model for ETCs is presented, employing complete optical and thermal assessment. This study analyses separately optics and heat transfer in the evacuated tubes, allowing the analysis to be extended to different configurations. The predictions obtained are in agreement with experimental. The optimum collector parameters (collector tube length and diameter, mass flow rate and collector tilt angle) are determined. The present results indicate that the optimum tube length is 1.5 m, as at this length a significant improvement is achieved in efficiency for different tube diameters studied. Finally, the heat generated from ETCs is used for solar cooling of a house. Results of the simulation of cooling system indicate that an ETC of area 54 m², tilt angle of 25° and storage tank volume of 2.1 m³ provides 80% of air-conditioning demand in a house located in Kuwait.     

Study of a new solar adsorption refrigerator powered by a parabolic trough collector

This paper presents the study of solar adsorption cooling machine, where the reactor is heated by a parabolic trough collector (PTC) and is coupled with a heat pipe (HP). This reactor contains a porous medium constituted of activated carbon, reacting by adsorption with ammonia.We have developed a model, based on the equilibrium equations of the refrigerant, adsorption isotherms, heat and mass transfer within the adsorbent bed and energy balance in the hybrid system components. From real climatic data, the model computes the performances of the machine. In comparison with other systems powered by flat plate or evacuated tube collectors, the predicted results, have illustrated the ability of the proposed system to achieve a high performance due to high efficiency of PTC, and high flux density of heat pipe.     

An Experimental Investigation on the Effect of Ferrofluids on the Efficiency of Novel Parabolic Trough Solar Collector Under Laminar Flow Conditions

ABSTRACT

The paper is related to the use of magnetic nanofluids (ferrofluids) in a direct absorption solar parabolic trough collector, which enhances thermal efficiency compared to conventional solar collectors. By applying the right magnetic intensity and magnetic field direction, the thermal conductivity of the fluid increased higher than typical nanofluids. Moreover, the ferrofluids exhibit excellent optical properties. The external magnetic source is installed to alter the thermo-physical properties of the fluid, and the absorber tube does not have selective surface allowing ferrofluids to absorb the incoming solar irradiance directly. In this paper, an experimental investigation of the performance of small scale direct absorption solar collector using ferrofluids as an absorber was conducted. Nanoparticle concentrations of 0.05 vol% at the operational temperatures between 19°C and 40°C were used in the current study. The results show that using ferrofluids as a heat transfer fluid increases the efficiency of solar collectors. In the presence of the external magnetic field, the solar collector efficiency increases to the maximum, 25% higher than the conventional parabolic trough. At higher temperatures, the ferrofluids show much better efficiency than conventional heat transfer fluid. The study indicated that nanofluids, even of low-content, have good absorption of solar radiation, and can improve the outlet temperatures and system efficiencies. The study shows the potential of using ferrofluids in the direct absorption solar collector.