Exergy Analysis of Photo-Thermal Interaction Process between Solar Radiation Energy and Solar Receiver

A unified theory of non-equilibrium radiation thermodynamics is always in search as it is meaningful for solar energy utilization. An exergy analysis of photo-thermal interaction process between the solar radiation energy and solar receiver is conducted in this paper. The non-equilibrium radiation thermodynamic system is described. The thermodynamic process of photo-thermal interaction between the solar radiation and solar receiver is introduced. Energy, exergy and entropy equations for the photo-thermal process are provided. Formulas for calculating the optimum receiving temperatures of the solar receiver under both non-concentration and solar concentration conditions are presented. A simple solar receiver is chosen as the calculation example to launch the exergy analysis under non-concentration condition. Furthermore, the effect analysis of solar concentration on the thermodynamic performance of the solar receiver for solar thermal utilization is carried out. The analysis results demonstrate that both the output exergy flux and efficiency of the solar receiver can be improved by increasing the solar concentration ratio during the solar thermal utilization process. The formulas and results provided in this paper may be used as a theoretical reference for the further studies of non-equilibrium radiation thermodynamic theory and solar thermal utilization.     

Impact of heliostat curvature on optical performance of Linear Fresnel solar concentrators

The present paper gives a numerical investigation of the effect of mirror curvature on optical performance of a Linear Fresnel Reflector solar field installed recently in Morocco. The objective is to highlight and discuss the effect of mirror curvature on the flux density distribution over the receiver and the system optical efficiency. For this purpose, a Monte Carlo-ray tracing simulation tool is developed and used to optimize the optical design taking into account the curvature degree of the heliostat field. In order to assess the accuracy of the numerical code developed and the validity of simulation results, a set of verification tests were developed and detailed within this article. Then, the optical performance of the system is evaluated as a function of mirror curvature and receiver height. The major challenge of this study is to find a trade-off between heliostat curvature and receiver height since lower and smaller receivers may reduce the system cost. It has been found that the flux distribution over the receiver and the optical efficiency of the system are relatively sensitive to the mirror curvature. We have demonstrated quantitatively how the use of curved mirrors can enhance the optical performance and reduce the required receiver size.     

Effect of receiver geometry on the optical and thermal performance of a parabolic trough collector

In this study, the optical and thermal performance of a Parabolic Trough Collector PTC system is investigated theoretically. A series of numerical simulations and theoretical analysis has been conducted to investigate the effect of the receiver geometry and location relative to the focal line on its optical performance. The examined receiver geometries are circular, square, triangular, elliptical and a new design of circular- square named as channel receiver. The thermal performance of PTC is studied for different flow rates from (0.27 to 0.6 lpm) theoretically. Results showed that the best optical design is the channel receiver with an optical efficiency of 84% while the worst is the elliptical receiver with an optical efficiency of 70%. Thermally the best design is the elliptical receiver with a thermal efficiency of 85% while the worst is the circular receiver with a thermal efficiency of 82%.     

A method of estimating monthly average solar radiation on shaded receivers

The geometry of vertical solar radiation receivers shaded by overhangs is described. The instantaneous mean solar radiation on a shaded receiver is defined leading to a method of estimating , the monthly average daily radiation on a shaded receiver. Values of the monthly average beam irradiated fraction of the receiver area, , are presented in a series of figures. The effects of overhang extension and receiver azimuth on are considered. Example calculations are presented and the overall effect of an overhang as a shading device is discussed.     

Optical efficiency analysis of cylindrical cavity receiver with bottom surface convex

The bottom surface of conventional cavity receiver cannot be fully covered by coiled metal tube during fabrication, which would induce a dead space of solar energy absorption. The dead space of solar energy absorption can severely decrease the optical efficiency of cavity receiver. Two new types of cavity receiver with bottom surface convex are put forward with the objective to solve the problem of dead space of solar energy absorption and improve the optical efficiency of cavity receiver. The optical efficiency and heat flux distribution of the two new types of cavity receiver are analyzed by Monte Carlo ray tracing method. Besides, the optical efficiency comparisons between conventional cavity receiver and the two new types of cavity receiver are conducted at different characteristic parameter conditions.     

Modeling and performance of microscale thermophotovoltaic energyconversion devices

We analyze the feasibility of energy conversion devices that exploit microscale radiative transfer of thermal energy in thermophotovoltaic devices. By bringing a hot source of thermal energy very close to a receiver fashioned as a pn-junction, the near-field effect of radiation tunneling can enhance the net power flux. We use the fluctuational electrodynamic approach to microscale radiative transfer to account for the spacing effect, which provides the net transfer of photons to the receiver as a function of the separation between the emitter and receiver. We calculate the power output from the microscale device using standard thermophotovoltaic device relations. The results for the performance of a device based on indium gallium arsenide indicate that a ten-fold increase in power throughput may be realized with little loss in efficiency. Furthermore, we develop a model of the microscale device itself that indicates the influence of semiconductor band-gap, energy, carrier lifetime and doping     

High temperature solar thermal central-receiver billboard design

The design of central receivers in solar thermal power plants is critical for efficient plant operation and sufficient operational lifetimes. The high, non-uniform concentration ratios used in central receivers lead to high, non-uniform receiver temperatures. For the same operational conditions, small changes to the receiver design can make a big impact on the expected lifetime of the receiver. This is due to limitations of the receiver materials to high temperatures and thermal cycling. In this study, we investigate the effect of several engineering concepts on the resultant surface temperatures of tubular billboard receivers. Four tubular billboard designs are investigated along with the sensitivity these designs have to high temperatures resulting from changes in the aiming point of the heliostat array. We examined a receiver with single diameter tubes, an ideal flow receiver, a receiver using various diameter tubes and a receiver made of tube panels in series. The single-diameter and multi-diameter receivers were found to have high temperatures and high sensitivity under non-standard irradiation. The multi-pass receiver was found to out-perform the other designs by reducing both the maximum surface temperatures under standard irradiance and the risk of high temperatures from irradiance changes. The results provide insights into tubular billboard receiver design, material selection and design for extended life.     

Investigations on heat losses from a solar cavity receiver

Thermal as well as optical losses affect the performance of a solar parabolic dish-cavity receiver system. Convective and radiative heat losses form the major constituents of the thermal losses. In this paper, an experimental and numerical study of the steady state convective losses occurring from a downward facing cylindrical cavity receiver of length 0.5 m, internal diameter of 0.3 m and a wind skirt diameter of 0.5 m is carried out. The experiments are conducted for fluid inlet temperatures between 50 °C and 75 °C and for receiver inclination angles of 0° (side ways facing cavity), 30°, 45°, 60° and 90° (vertically downward facing receiver). The numerical study is performed for fluid inlet temperatures between 50 °C and 300 °C and receiver inclinations of 0°, 45° and 90° using the Fluent CFD software. The experimental and the numerical convective loss estimations agree reasonably well with a maximum deviation of about 14%. It is found that the convective loss increases with mean receiver temperature and decreases with increase in receiver inclination. Nusselt number correlations are proposed for two receiver fluid inlet temperature ranges, 50-75 °C and 100-300 °C, based on the experimental and predicted data respectively. Besides no-wind tests, investigations are also carried out to study the effects of external wind at two different velocities in two directions (head-on and side-on). The wind induced convective losses are generally higher than the no-wind convective loss (varying between 22% and 75% for 1 m/s wind speed and between 30% and 140% for the 3 m/s wind speed) at all receiver inclination angles, the only exception being the loss due to side-on wind at 0° receiver inclination angle. This is because the wind acts as a barrier at the aperture preventing the hot air to flow out of the receiver. The head-on wind causes higher convective loss than the side-on wind. Nusselt number correlations proposed in this work are compared with the existing correlations in the literature. It is found that the correlations available in literature under-predict the convective losses at mean receiver temperatures between 100 °C and 300 °C. This is due to the fact that the correlations are developed for certain receiver geometries having the ratio of aperture diameter to receiver diameter equal to or lesser than one.     

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.     

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.     

Performance of a concentrating photovoltaic/thermal solar collector

The performance of a parabolic trough photovoltaic/thermal collector with a geometric concentration ratio of 37× is described. Measured results under typical operating conditions show thermal efficiency around 58% and electrical efficiency around 11%, therefore a combined efficiency of 69%. The impact of non-uniform illumination on the solar cells is investigated using purpose built equipment that moves a calibrated solar cell along the line of the receiver and measures short circuit current. The measured illumination flux profile along the length shows significant variation, despite the mirror shape error being less than 1 mm for most of the mirror area. The impact of the illumination non-uniformities due to the shape error, receiver support post shading and gaps between the mirrors is shown to have a significant effect on the overall electrical performance. The flux profile transverse to the receiver length is also investigated. Peak flux intensities are shown to be around 100 suns. The impact on efficiency due to open circuit voltage reduction is discussed.     

Performance of optimized solar central receiver systems as a function of receiver thermal loss per unit area

Recent efforts in solar central receiver research have been directed toward high-temperature applications. Associated with high-termperature processes are greater receiver thermal losses due to thermal radiation and convection. This article examines the performance of central receiver systems having optimum heliostat fields and receiver aperture areas as a function of receiver thermal loss per unit area. The results address the problem of application optimization, where the receiver design, temperature and consequently thermal loss per unit area may vary. A reasonable range of values for the primary independent variable L (the average thermal loss per unit area of receiver aperture) and a reasonable set of design assumptions were first established. Heliostat field analysis and optimization required a detailed computational analysis. Results are discussed for tower focal heights of 150 and 180 m. Values of L ranging from 0.04 to 0.50 MW per square meter were considered, roughly corresponding to working fluid temperatures in the range of 650–1650°C. As L increases over this range, the receiver thermal efficiency and the receiver interception factor decrease. The optimal power level drops by almost half, and the cost per unit of energy produced increases by about 25% for the base case set of design assumptions. The resulting decrease in solar subsystem efficiency (relative to the defined annual input energy) from 0.57 to 0.35 is about 40% and is a significant effect. Unoptimized systems would experience an even greater degradation in cost-effectiveness.     

Performance of a cylindrical parabolic trough using a fin receiver: Limb darkening effects

The distribution of local concentration ratio on a fin receiver used with cylindrical parabolic solar concentrators has been calculated analytically considering the variation of intensity over the solar disc. The effect of the pointing error of the concentrator and that of the lateral shift and tilt of the fin receiver have also been studied. The results are plotted graphically and discussed.     

Experimental and theoretical analysis of a dynamic test method for molten salt cavity receiver

Test methods for estimating the thermal performance of the molten salt receiver are a matter of ongoing concern. To date, test methods in the literature require receiver to be operated in steady state or quasi-steady state. However, the receiver is always operating in the unsteady state with ongoing changes in power absorption and flow rate. Therefore, research into dynamic test method for the molten salt cavity receiver is required. The Transfer Function Method (TFM) is a successful dynamic test method for solar collectors. In this paper, a theoretical analysis of the TFM was applied to the molten salt cavity receiver and then verified by indoor transient experiments. The TFM predicted outlet temperature of the receiver was compared with experimental data. The results showed that the TFM accurately predicted the outlet temperature trends despite some errors between predicted and measured outlet temperature. The errors may have originated from the changing flow rate. The TFM is a good candidate as a dynamic test method for the concentrated solar receiver.     

Effect of non-uniform θ-temperature distribution on a receiver of an evacuated tubular collector on its performance parameters

A performance of an evacuated tubular collector (G.E. design) fixed at the focus of a compound parabolic concentrator is investigated. In the G.E. design, heat is transmitted to the circulating fluid inside a U-tube. The U-tube is in contact with the receiver only on a line along the length of the receiver. This results in a non-uniform temperature distribution on the receiver in the θ-direction. The effect of the non-uniform temperature distribution on the performance parameters of the collector, viz. overall heat loss coefficient, plate efficiency factor and heat removal factor, has been studied. The results are presented in the form of a graph.     

An experimental study: Thermal performance of molten salt cavity receivers

In solar power plants, a molten salt receiver always works in unsteady state conditions. Therefore, it is necessary to research the thermal performance of a receiver in an unsteady state condition. For this purpose, an indoor testing system with a molten salt cavity receiver was developed. Experimental research was conducted to determine the thermal performance of a 100 kWt molten salt receiver. The effect of the input power and flow rate on the thermal performance of the receiver was investigated. In addition, a simple unsteady model was established to research the characteristics of the variation of the internal energy of the receiver and the characteristics of the heat loss. The results indicated that the efficiency of the receiver was in direct proportion to the flow rate. However, the influence was small. In the initial stage of the transient process, the increments of the internal energy of the receiver and the fluid were large (approximately 20% in the energy which is not removed by the mass flow of the fluid). Over time, the thermal inertia of the receiver decreased with the transient process. As a result, any energy not taken away by the fluid was transformed into heat loss.     

Thermo-Fluid-Dynamics of a Ceramic Foam Solar Receiver: A Parametric Analysis

Abstract

The high efficiency of concentrated solar power (CSP) in energy conversion makes it a very attractive device for using solar energy as a substitute of nonrenewable energy sources. The open volumetric receiver plays an important role in the performance of a CSP. The optimized design of solar receiver implies the thorough knowledge of the heat transfer between the air and the foam and of the temperature distribution in the receiver. Heat transfer in the cylindrical SiC porous volumetric receiver of a solar tower, undergoing the impact of concentrated solar radiation, is investigated numerically in this paper. Governing equations are written with the volume averaging technique. A two-equation model for the energy equation, under the local thermal nonequilibrium assumption, is used. Numerical simulations are carried out through the commercial code COMSOL Multiphysics. The solid and fluid temperatures, the fluid velocity and the pressure drop, for various boundary and morphological conditions, are predicted and discussed. The receiver efficiency is finally maximized carrying out an extended parametric analysis of process parameters.     

Performance Characteristics of a Volumetric Solar Receiver: Presence of an Absorber Plate with a Selective Surface

Performance characteristics of a concentrated solar volumetric absorber are examined numerically. The thermal system considered consists of parabolic trough, glass tube, absorbing plate, and slurry containing 7% lauric acid as a phase change material and water as a carrier fluid. To assess the effect of the absorbing plate on performance characteristics, two locations of the absorbing plate on the glass tube surface are incorporated in the analysis. A selective surface is considered at the absorber plate surface for improved absorption of solar radiation and reduced thermal emission due to temperature increase at the surface. Temperature ratio, gain parameter, and pump power loss parameters are introduced to quantify the performance characteristics of the volumetric receiver. The study is extended to include the effect of Reynolds number on the receiver performance characteristics. A heating model incorporating radiation, convection, and conduction is adopted to simulate the thermal process. It is found that the gain parameter of the concentrated solar volumetric receiver improves by 15% when the absorber plate is located at the left face of the glass tube opposing the trough surface. The effect of Reynolds number on gain parameter is found to be inconsiderable.     

LiMoNAED: a limited motion, non-shading, asymmetric, ecliptic-tracking dish

The conventional design of a parabolic dish for a small solar conversion system places the receiver along the line between the center of the dish and the sun. This forces the receiver to move in a large arc during tracking, and produces some shading of the dish. In some applications, such large movement of the receiver is not acceptable. A new concentrator design is proposed for small systems with a constraint of limited mobility of the receiver. This is accomplished by using a first polar axis and a second axis that is aligned with the normal to the ecliptic plane. The new design features limited motion of the receiver, with inclination changing only within ±23.45°; off-axis reflector to eliminate shading; constant rotation speed in both axes; and constant flux distribution on the receiver.     

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.