Effects of geometrical parameters of a dish concentrator on the optical performance of a cavity receiver in a solar dish‐Stirling system

Dish‐Stirling concentrated solar power (DS‐CSP) system is a complex system for solar energy‐thermal‐electric conversion. The dish concentrator and cavity receiver are optical devices for collecting the solar energy in DS‐CSP system; to determine the geometric parameters of dish concentrator is one of the important steps for design and development of DS‐CSP system, because it directly affects the optical performance of the cavity receiver. In this paper, the effects of the geometric parameters of a dish concentrator including aperture radius, focal length, unfilled radius, and fan‐shaped unfilled angle on optical performance (ie, optical efficiency and flux distribution) of a cavity receiver were studied. Furthermore, the influence of the receiver‐window radius of the cavity receiver and solar direct normal irradiance is also investigated. The cavity receiver is a novel structure that is equipped with a reflecting cone at bottom of the cavity to increases the optical efficiency of the cavity receiver. Moreover, a 2‐dimensional ray‐tracking program is developed to simulate the sunlight transmission path in DS‐CSP system, for helping understanding the effects mechanism of above parameters on optical performance of the cavity receiver. The analysis indicates that the optical efficiency of the cavity receiver with and without the reflecting cone is 89.88% and 85.70%, respectively, and former significantly increased 4.18% for 38 kW XEM‐Dish system. The uniformity factor of the flux distribution on the absorber surface decreases with the decreases of the rim angle of the dish concentrator, but the optical efficiency of the cavity receiver increases with the decreases of the rim angle and the increase amplitude becomes smaller and smaller when the rim angle range from 30° to 75°, So the optical efficiency and uniformity factor are conflicting performance index. Moreover, the unfilled radius has small effect on the optical efficiency, while the fan‐shaped unfilled angle and direct normal irradiance both not affect the optical efficiency. In addition, reducing the receiver‐window radius can improve the optical efficiency, but the effect is limited. This work could provide reference for design and optimization of the dish concentrator and establishing the foundation for further research on optical‐to‐thermal energy conversion.     

基于三角腔体吸收器的透射式菲涅尔定焦线系统聚光特性分析

为解决线性菲涅尔太阳能集热系统单轴跟踪过程中出现的聚光焦线偏移以及降低系统跟踪能耗等问题,提出一种透射式菲涅尔定焦线太阳能聚光器.该聚光器采用极轴跟踪方式与线性菲涅尔透镜定期滑移调节方式相结合,可实现固定焦线聚光.将该聚光器与三角腔体吸收器所组成的太阳能集热系统,利用基于蒙特卡罗光线追迹法的TracePro光学软件分析...     

Thermal performance of linear Fresnel reflecting solar concentrator with trapezoidal cavity absorbers

Thermal performance of the four identical trapezoidal cavity absorbers for linear Fresnel reflecting solar device were studied and compared. The absorbers were designed for operating in conjunction with a prototype Fresnel solar reflector. Rectangular and round pipe sections were used as absorber by placing in the trapezoidal cavity. The absorber pipes were coated with ordinary dull black board paint and black nickel selective surface. The bottom of the cavity was provided with plane glass to allow the solar radiation to be reflected from the Fresnel reflector. The other three sides of the cavity absorber were insulated to reduce heat loss. Thermal performance of the Fresnel reflecting concentrator with each trapezoidal cavity absorber was studied experimentally at different concentration ratio of the reflector. The study revealed that the thermal efficiency was influenced by the concentration ratio and selective surface coating on the absorber. The thermal efficiency decreased with the increase in the concentration ratio of the Fresnel reflecting collector. The selective surface coated absorber had a significant advantage in terms of superior thermal performance as compared to ordinary black painted absorber. The round pipe (multi-tube) receiver had higher surface area to absorb solar energy as compared to rectangular pipe receiver. Thermal efficiency of the solar device with round pipe absorber was found higher (up to 8%) as compared to rectangular pipe absorber.     

Heat Transfer Characteristics of a Volumetric Absorption Solar Collector using Nano-Encapsulated Phase Change Slurry

ABSTRACT

The conventional solar collectors which absorb solar energy through surface of the receiver have much energy waste during energy conversion process due to heat loss from the pipe surface. Volumetric absorption solar collectors (VASC) can overcome this problem through directly absorbing solar energy by nanofliud with excellent optical absorption property. Nano-encapsulated phase change material (NPCM) is a kind of novel composite PCMs widely adopted in thermal energy storage system. The NPCM slurry (NPCS) has great potential to be used in VASC since it can be used as both the heat transfer fluid and energy storage medium. In the present study, a numerical model based on the Eulerian-Eulerian approach is built to investigate the heat transfer characteristics of NPCS in a parallel plate channel for volumetric absorption of solar energy. Influences of different parameters such as the extinction coefficient, flow velocity, radiative intensity on the performance of collector are studied through the numerical simulation. The results indicate that the NPCS shows better performance in the VASC compared with the conventional nanofluids without phase change. The information provided is helpful in the further study of solar energy volumetric absorption.     

Performance simulation of a parabolic trough solar collector

A new analytical model for optical performance and a modified integration algorithm are proposed and applied to simulate the performance of a parabolic trough solar collector with vacuum tube receiver. The analytical equation for optical efficiency of each point at reflector is derived first, then the optical efficiency of the system is simulated by numerical integration algorithm. The cosine factor, receiver efficiency, heat loss and efficiency of conversion of solar energy into net heat energy at any time can be calculated with the program. The annual average efficiency is also simulated considering discard loss. The effects of optical error, tracking error, position error from installation of receiver, optical properties of reflector, transmittance and absorptivity of vacuum tube receiver on efficiencies of the trough system are simulated and analyzed as well as optical parameter.     

Exergetic optimization for solar heat receiver with heat loss and viscous dissipation

The exergetic efficiency of heat receiver in solar thermal power system is optimized by considering the heat loss outside the receiver and fluid viscous dissipation inside the receiver. The physical models of heat loss and pumping power consumption for solar heat receiver are first proposed, and associated exergetic efficiency is further induced. As the flow velocity rises, the pumping power consumption and heat absorption efficiency significantly rises, and the maximum absorption efficiency and optimal incident energy flux also increase. Along the flow direction of solar receiver, the exergy flux increment and the flow exergy loss almost linearly increase, while the exergetic efficiency varies very slowly at high flow velocity. According to the exergetic efficiency loss from flow viscou’s dissipation, the exergetic efficiency of solar heat receiver will first increase and then decrease with the flow velocity. Because of the coupling effects of heat absorption efficiency and exergetic efficiency from fluid internal energy, the exergetic efficiency of solar heat receiver will approach to the maximum at proper inlet temperature. As a result, the exergetic efficiency of solar heat receiver will reach the maximum at optimal inlet temperature, incident energy flux and flow velocity.     

Radiative properties of a solar cavity receiver/reactor with quartz window

An energy transfer and conversion model for high-temperature solar cavity receivers has been developed using the transport behaviour of solar radiation as described by the spectral radiative exchange factors. A Monte-Carlo ray-tracing method coupled with optical properties was adopted, to predict radiation characteristics of the solar collector system by calculating radiative exchange factors. A cavity receiver with a plano-convexo quartz window was proposed, based upon the directional characteristics of the focal flux and the redistribution effect of the quartz window. Parametric studies on the windowed receiver provided a more uniform flux distribution, higher efficiency and lower loss than the windowless receivers. The predicted results serve as a design reference for the solar receivers or reactors in high-temperature applications.     

Review of study on solid particle solar receivers

The solid particle solar receiver (SPSR) is a direct absorption central receiver that uses solid particles enclosed in a cavity to absorb concentrated solar radiation. The SPSR is a candidate for applications of solar energy in a thermo-chemical water-splitting process to produce hydrogen. This paper presents a review of the study on SPSRs, including the idea originality, design concepts, advantages and disadvantages, the solid particle identification, a conceptual design in Sandia National Laboratories and detailed studies performed on this design. The geometry, particle size, calculating domain selection, the wind effect, the aerowindow and other factors which influence the cavity efficiency have been studied and the results are presented.     

塔式太阳能热管接收器表面热流密度分布及光学性能研究

该文设计一种用于小型塔式太阳能电站的热管接收器。基于蒙特卡洛光线追迹（MCRT）算法和混合编程方法,开发光学仿真程序。详细研究单根热管表面热流密度时空分布规律和热管接收器表面能量动态分布规律,并分析接收器瞬时光学性能。研究结果表明单根热管表面热流密度具有强烈非均匀性。夏至日正午,吸热面中心单根热管吸收能量约为6.9 kW。春分日和夏至日时,接收器最大光学效率约为75%;冬至日最大光学效率约为61%。研究结果有助于进一步研究热管接收器的光热耦合机理。     

A novel integrated simulation approach couples MCRT and Gebhart methods to simulate solar radiation transfer in a solar power tower system with a cavity receiver

An integrated simulation approach, which couples Monte Carlo ray tracing (MCRT) and Gebhart methods, is proposed to simulate solar radiation transfer in a solar power tower system with a cavity receiver. The MCRT method is used to simulate the solar radiation transfer process from the heliostat field to interior surfaces of the cavity receiver, and the Gebhart method is used to simulate the multiple reflections process of solar radiation within the cavity. This integrated simulation method not only reveals the cavity effect on receiver performance but also provides real-time simulation results. Based on this method, the reflection loss of the cavity receiver and solar flux distributions are discussed in detail. The results indicate that the cavity effect can significantly reduce the reflection loss and homogenize the concentrated solar energy distributed on interior surfaces to some extent. Moreover, the surface absorptivity has less effect on the reflection loss when cavity effect is considered. The cavity effect on homogenizing solar flux distributions is greater with lower surface absorptivity. In addition, although the concentrated solar energy is distributed on the cavity aperture with similar shapes at different times, the shape of the solar flux distribution on interior surfaces varies greatly with time.     

Solar flux distribution study in heat pipe cavity receiver integrated with biomass gasifier

Technological advances have taken place in the field of solar receivers, gasifiers, and heat pipes, however, the integration of these technologies is not significantly available. In this paper, the conceptual design of a novel biomass gasifier is presented. The system facilitates the solar capture and gasification process separately. It is fitted with a heat pipe arrangement to transfer heat from the solar receiver zone to the gasifier zone. Collection of heat pipes comprised of few straight tubes and few innovative semi-‘S’ shaped tubes. Solar receiver geometry is modified to semi-cylindrical shape and the evaporator section of heat pipes is arranged circumferentially inside the solar receiver. The conventional gasifier is modified with an arrangement to distribute uniform solar heat throughout the fixed bed of biomass feedstock. This paper aims to present the optical analysis of the proposed heat pipe embedded solar receiver. Heat pipe disposition inside the cavity, receiver positioning on focal planes and slope error are varied to perform optical performance of the proposed solar reactor. Average solar flux is found to increase up to 1.1-fold to 1.7-fold placing cavity receiver below focal height by 16 and 32 mm respectively. Also, the magnitude and flux profiles incident on surfaces are affected with concentrator slope error. Average flux reduces up to 21.7% with 4 mrad as compared to 2 mrad error.     

Performance evaluation of v-trough solar concentrator for water desalination applications

The working principle and thermal performance of a new v-trough solar concentrator are presented in this paper. Compared with the common parabolic trough solar concentrators, the new concentrator has two parabolic troughs which form a V-shape with the focal line at the bottom of the troughs. This is beneficial for the installation and insulation of the receiver, and the shadow on the reflective surface is avoided. The new v-trough collector does not require high precision tracking devices and reflective material. And therefore the cost of the system could be significantly reduced. Various experimental tests were carried out both outdoor and indoor using different types of receiver tubes. The results show that the collector system can have thermal efficiency up to 38% at 100 °C operating temperature. System modelling was used to predict the rate of fresh water produced by four different solar collector systems which include both static and one-axis solar tracking technologies. Comparison of the solar collectors at different temperature ranges for humidification/dehumidification desalination process using specific air flow rate were considered. At each temperature range, suitable solar collectors were compared in the aspect of fresh water production and area of solar collector required. Results showed that the new v-trough solar collector is the most promising technology for small to medium scale solar powered water desalination.     

Heat transfer performance and exergetic optimization for solar receiver pipe

The basic physical model of solar receiver pipe with solar selective coating is established, and associated heat transfer and exergetic performances are analyzed and optimized. Because of the heat losses of natural convection and infrared radiation, the energy absorption efficiency has a maximum at optimal incident energy flux. As the pipe radius decreases or flow velocity rises, the wall temperature drops for higher heat transfer coefficient, while the heat absorption efficiency increases. Along the flow direction, the heat absorption efficiency almost linearly decreases, while the exergetic efficiency will first increase and then decrease. As the inlet temperature rises, the heat absorption efficiency of the solar receiver pipe decreases, while the exergetic efficiency of absorbed energy obviously increases, so the exergetic efficiency of incident energy will reach maximum at the optimal inlet temperature. Additionally, the maximum exergetic efficiency of incident energy and optimal inlet temperature both increase with flow velocity.     

具有二次聚光器的新型大开口槽式太阳集热器设计与光学性能分析

针对大开口和更高运行温度的槽式太阳能热发电系统,提出一种可实现高聚光比、低辐射热损及能流密度均匀的新型槽式太阳集热器,即在集热管内放置外壁具有太阳选择吸收膜层和内壁具有反射膜层二次聚光器的大开口槽式太阳集热器。建立圆弧为微元段的自适应设计新方法,提出3种典型的二次聚光器面型,利用蒙特卡洛光线追迹方法仿真新型集热器的能流密度分布特性,验证该光学仿真方法,分析影响集热器光学性能的各种因素。结果表明,该集热器可显著提升集热效率。     

The role of the optical properties of solids in solar direct absorption process

Solid particles are considered to be good candidates for absorbing solar energy in an endothermic reaction, due to both their ability to absorb solar radiation directly (as opposed to heat transfer mechanism) and their high ratio of absorbed radiation to mass. Only a limited number of studies have been devoted so far to convert solar energy to chemical energy by means of direct absorption. The major advantages of this concept are the inherent high efficiency of the light absorption, and the short time required to reach the desired reaction temperature.In the work described, an attempt to evaluate the effect of the optical properties of the particles and their temperature dependence, upon the solar driven chemical reaction was carried through. The particles investigated were oil shale particles. Their optical properties were measured in the optical lab at the DLR, Stuttgart, whereas the solar gasification experiments of the oil shale, were conducted in the central receiver of the Weizmann Institute.The results show that temperature profiles for the gasification experiments of oil shales can be predicted with good accuracy by assuming isotropic scatter and a particle albedo of 0.5. This is due to the relatively limited role of direct absorption in low expanding fluidized beds. Calculations of a set-up however, where direct absorption is dominant, show resulting temperature profiles to be sensitive to optical properties. Temperature differences arising from calculations based on assumed optical properties compared to those based on measured optical properties are as high as 300°C.     

Efficiency improvement of a solar direct volumetric receiver utilizing aqueous suspensions of CuO

In this paper, an experimental study was performed to investigate the photothermal conversion properties of CuO‐H₂O nanofluid‐based volumetric receiver mainly considering the effects of nanoparticle (NP) concentration, irradiation time, and receiver depth. First, stable aqueous suspensions of CuO with NPs having average diameter close to 10 nm were produced by the precursor transformation method. The spectral transmittances of CuO‐H₂O nanofluids decrease with increasing the NP concentration (0.01‐0.25 wt%) at wavelengths of 200 to 1350 nm. The photothermal conversion performance of CuO‐H₂O nanofluids is sensitive to the receiver depth, irradiation time, and NP concentration. The higher NP concentration causes stronger optical absorption in the upper part and reduces the temperature at the bottom accordingly. The temperature difference between CuO‐H₂O nanofluid and distilled water increased initially and then decreased with the increase of penetration depth, and there existed an optimal depth of 1 cm with respect to the best photothermal conversion performance. The receiver efficiency decreased with increasing the light irradiation time, and an efficiency improvement up to 30.4% was achieved for the 0.25 wt% nanofluid at the optimal depth of 1 cm as compared with water. This work shows that volumetric receivers provide a potential alternative for solar thermal energy utilization versus surface‐based absorber especially under concentrated solar radiation.     

An alternative method for calculation of semi-gray radiation heat transfer in solar central cavity receivers

This paper describes a general method to calculate the semi-gray radiation heat transfer that occurs within an enclosure comprised of diffuse surfaces. The method is implemented in an existing solar power tower cavity receiver model in the System Advisor Model (SAM, NREL, 2011). The semi-gray radiation model is used to find an optimal distribution of emissivities for the thermal- and solar radiation wavelength bands for surfaces that comprise the solar central cavity receiver. The optimal distribution of emissivities maximizes the overall thermal efficiency of a cavity receiver. The model shows an effective way to reduce heat loss from the cavity is to minimize the temperatures of the passive surfaces through manipulation of their radiative surface properties.For the cavity receiver design considered, an optimal emissivity distribution for the active absorber surfaces of the cavity is a selective surface with high absorptivity in the solar wavelength band and low emissivity in the thermal wavelength band. Passive surfaces within the cavity should be highly reflective for radiation over the full spectrum. For absorber surfaces with solar absorptivity of 0.95, thermal emissivity of 0.1 and reflective passive surfaces with emissivities of 0.1, for the full spectrum, the thermal efficiency of the receiver can be increased by about 0.7% in comparison to gray surfaces having an emissivity of 0.95 for all wavelengths.     

Optimization of nanofluid volumetric receivers for solar thermal energy conversion

Improvements in solar-to-thermal energy conversion will accelerate the development of efficient concentrated solar power systems. Nanofluid volumetric receivers, where nanoparticles in a liquid medium directly absorb solar radiation, promise increased performance over surface receivers by minimizing temperature differences between the absorber and the fluid, which consequently reduces emissive losses. We present a combined modeling and experimental study to optimize the efficiency of liquid-based solar receivers seeded with carbon-coated absorbing nanoparticles. A one-dimensional transient heat transfer model was developed to investigate the effect of solar concentration, nanofluid height, and optical thickness on receiver performance. Simultaneously, we experimentally investigated a cylindrical nanofluid volumetric receiver, and showed good agreement with the model for varying optical thicknesses of the nanofluid. Based on the model, the efficiency of nanofluid volumetric receivers increases with increasing solar concentration and nanofluid height. Receiver-side efficiencies are predicted to exceed 35% when nanofluid volumetric receivers are coupled to a power cycle and optimized with respect to the optical thickness and solar exposure time. This work provides insights as to how nanofluids can be best utilized as volumetric receivers in solar applications, such as receivers with integrated storage for beam-down CSP and future high concentration solar thermal energy conversion systems.     

Prediction and optimization of the performance of parabolic solar dish concentrator with sphere receiver using analytical function

Parabolic solar dish concentrator with sphere receiver is less studied. We present an analytic function to calculate the intercept factor of the system with real sun brightness distribution and Gaussian distribution, the results indicate that the intercept factor is related to the rim angle of reflector and the ratio of receiver angle to the optical error when the optical error is larger than or equal to 5 mrad, but is related to the rim angle, receiver angle and optical error in less than 5 mrad optical error. Furthermore we propose a quick process to optimize the system to provide the maximum solar energy to net heat efficiency for different optical error under typical condition. The results indicate that the parabolic solar dish concentrator with sphere receiver has rather high solar energy to net heat efficiency which is 20% more than solar trough and tower system including higher cosine factor and lower heat loss of the receiver.     

Thermal performance of solar concentrator/cavity receiver systems

To utilize solar energy at a high temperature, a parabolic dish/cavity receiver configuration is often used. The energy loss mechanisms of such a system are analyzed. System efficiency is defined as the power absorbed by the working fluid circulating in the cavity divided by the solar power falling on the concentrator aperture. Power profiles produced in cavities of varying geometry with concentrators of varying rim angle are also discussed. It is found that varying concentrator rim angle and cavity geometry can greatly affect the cavity power profile without a large effect on system efficiency. Cavity isothermality often requires a nonlinear power profile , particularly in a thermochemical system. The methodology described can be used to optimize concentrator/cavity design variables.