碟式聚光太阳热发电技术

太阳热发电技术是利用太阳集热器把太阳能聚集起来,将工质加热到一定的温度,驱动热机带动发电机发电。由于整个系统的热源来自太阳能,所以称为太阳热发电技术。太阳能热发电系统一般由聚光系统、集热系统、热传输系统、蓄热贮能系统、热机系统等组成。当前太阳热发电技术主要有以下5种:(1)中央塔式(2)槽式(3)单碟独立电站和碟群体系(4)太阳烟囱(5)太阳池。本文通过介绍国外碟式太阳热发电技术的现状及发展趋势,提出发展我国碟式热发电技术的建议与思考。1碟式太阳热发电技术的历史与现状a.碟式热发电系统现代碟式太阳能热发电技术在20世纪70…     

Optical performance of spherical reflecting elements for use with paraboloidal dish concentrators

While paraboloidal dishes have traditionally been used for high flux/high power solar concentration devices, the manufacture of multi-facet collectors has been complicated somewhat by the need to produce reflecting elements having different curvatures for different regions of the paraboloidal surface. This complication could be minimised by using identical spherical reflector sub-components mounted with a paraboloidal orientation on a space frame dish structure. This paper compares the optical performance and manufacturing feasibility of collectors having such a combination of surfaces.     

Paraboloid simple and compound cone concentrators for spherical absorbers

Various circular concentrators with spherical absorbers have been examined with the objective of obtaining a high performance low cost configuration with minimum tracking requirements. The paraboloid concentration is considered and the optimum design established. Three conical arrangements are compared to the paraboloid—simple truncated cone, Fresnel layout and dished compound cone. The simple cone is found to have unsatisfactory performance, the Fresnel to have moderate performance and the lowest reflective surface area but an unduly large overal diameter. The compound cone may approach closely the performance of the paraboloid, and experimental results are shown to confirm the data given.     

Parabolic reflectors formed by inflation

Paraboloids of revolution over 1 meter in diameter have been formed from flat sheets by inflation without the use of a mold or template. Quality is adequate for microwave use and for most high-concentration applications for focusing solar energy. The process is suitable for making reflectors in ones or twos to special focal lengths and diameters, but could also be automated for production runs.     

Mathematical modeling of thermoelectric generator driven by solar parabolic dish collector

This paper presents mathematical modeling of thermoelectric power generator driven by solar parabolic dish collector. The system is modeled by set of mathamatical equations from the first law of thermonadynamics for the subcompoents of solar parabolic dish collector and thermoelectric power generator. The model is solved analytically for the a set of operating and design parameters. Modeling results can be useful for further development of the system to study it economic viability.     

Parametric analysis and yearly performance of a trigeneration system driven by solar‐dish collectors

Solar‐driven polygeneration systems are promising technologies for covering many energy demands with a renewable and sustainable way. The objective of the present work is the investigation of a trigeneration system, which is driven by solar‐dish collectors. The examined trigeneration system includes an organic Rankine cycle (ORC), which operates with toluene, and an absorption heat pump, which operates with LiBr/H₂O. The absorption heat pump is fed with heat by the condenser of the ORC, which operates at medium temperature levels (120°C to 150°C). The absorption heat pump produces both useful heat at 55°C and cooling at 12°C. The ORC produces electricity, and it is fed by the solar dishes. The examined ORC is a regenerative cycle with superheating. The total analysis is performed with a developed model in Engineering Equation Solver (EES). The system is investigated parametrically for different ORC heat‐rejection temperatures, different superheating levels in the turbine inlet, and various solar‐beam irradiation levels. Furthermore, the system is investigated on a yearly basis for the climate conditions of Athens (Greece) and for Belgrade (Serbia). It is found that the yearly system energy and exergy efficiencies are 108.39% and 20.92%, respectively, for Athens, while 111.38% and 21.50%, respectively, for Belgrade. The values over 100% for the energy efficiency are explained by the existence of a heat pump in the examined configuration. For both locations, the payback period is found close to 10 years and the internal rate of return close to 10%. The final results indicate that the examined configuration is a highly efficient and viable system, which operates only with a renewable energy source.     

A simple clear skies model for the luminous efficacy of diffuse solar radiation on inclined surfaces

In the present work we study the luminous efficacy of diffuse solar radiation incident on vertical surfaces for a clear sky and mean hourly values of diffuse irradiance and diffuse illuminance. We develop a model easy to use, similar to a model previously obtained for horizontal surfaces. To develop the present model for vertical surfaces we assume that the slope of the surface influences diffuse illuminance and diffuse irradiance in the same way. As a consequence of this hypothesis, the luminous efficacy of diffuse solar radiation is assumed to be the same for both horizontal and inclined surfaces.     

消除复合抛物面聚光器二次反射的设计研究

建立未截短的原复合抛物面聚光器(CPC)、1/2截短CPC以及消除二次反射设计CPC的数学模型,根据光线追踪法和蒙特卡罗法,在几何聚光比为2~10内,计算CPC高度与出射光孔宽度的比值,并依据该比值分析CPC的经济性,计算CPC最佳均匀面上的非均匀度和最大辐照度,进而分析CPC聚光均匀性.结果表明:在相同几何聚光比下,消除二次反射设计CPC的经济性和聚光均匀性最好.     

Experimental investigations of frost release by hydrophilic surfaces

Frost formation occurs when water vapor in the surrounding air comes into contact with cold surfaces through heat and mass transfer. It is usually an undesirable phenomenon in most refrigeration and cryogenic systems. A few studies have shown that changing the surface energy, such as increasing the surface hydrophilicity or hydrophobicity, has significant effects on frost growth. In this paper, a kind of hydrophilic polymer paint is formulated to counteract frost deposition on cold surfaces. The coated surface can retard frost formation up to three hours under low plate temperatures (− 15.3°C) and high air humidity (72%). To test the antifrosting performance of the hydrophilic paint under more practical conditions, it is applied to a fin-and-tube heat exchanger and a domestic refrigerator at a coating thickness of 30 μm. Comparisons of frost deposition, pressure drops, and outlet temperatures are made between uncoated and coated heat exchangers. Under conditions of high air temperature (2.2°C) and relative high air humidity (90%), the paint prolongs the defrosting interval from 80 to 137 min. Experimental observations also show that the coated hydrophilic fins are free of frost deposition during the entire course of the test and that the coating has no significant additional thermal resistance.     

Droplet pinning by PEM fuel cell GDL surfaces

In this work, side view images of liquid–gas–solid interfaces are observed during the evaporation of liquid water droplets on various commercially available untreated gas diffusion layers (GDLs). The change in contact diameter as a function of evaporative volume loss is measured to quantify the unpinning rates of micro-sized droplets. This contact diameter pinning behaviour during evaporation is correlated to the material topography, which is quantified through profilometry measurements. The carbon fibre paper with the smallest average roughness (15 μm) exhibits the strongest degree of pinning (unpinning at a rate of 0.13 mm/μL). Higher average surface roughnesses for felt (30 μm) and cloth yarn (32 μm) result in higher unpinning rates, 0.21 mm/μL and 0.19 mm/μL, respectively. These results indicate that common GDL materials exhibit Cassie–Baxter wetting behaviour, and reduced GDL roughness promotes droplet pinning. The material-specific droplet contact diameter progression should be considered during GDL selection for polymer electrolyte membrane (PEM) fuel cells. This work provides insight into the effect of GDL material properties on gas channel water management, as water droplets are expected to experience similar pinning to that observed in this work within the cathode gas channels of a PEM fuel cell.     

The ignition of gases by rapidly heated surfaces

A surface heated by a frictional impact was simulated by the discharge of a bank of capacitors through a strip of tungsten foil. The resultant rise and fall of temperature were investigated by means of a two-color pyrometer. The heated foil was exposed to various concentrations of flammable gases, and the probability of ignition was investigated as a function of several variables, including gas concentration, temperature profile, and foil size. It was found that the peak temperature necessary for ignition decreased as the width of the foil was increased, and that 7% was the most easily ignitable concentration of methane in air. The size of the heated surface was of the order of 200 mm² and this necessitated a working range of peak temperatures of approximately 1500°C to 2200°C in the study of the ignition of methane-air. Some preliminary experiments indicated that propane-air could be ignited at much lower temperatures.     

槽式太阳能辅助燃煤发电系统热性能研究

针对槽式太阳能集热系统,建立了集热场得热及热损失等数学模型,并以典型槽式集热器EUROTROUGH-150(ET-150)组成的集热系统为例,进行了以集热场热效率最优为目标的优化;在此基础上以槽式太阳能辅助燃煤发电系统(即互补发电系统)为研究对象,采用一定的评价准则和集成方式,对直射辐射强度(DNI)设计值的选取进行分析研究.结果表明:不同回路数、不同纬度分布的集热场存在不同的最优列间距,且存在最佳取值范围的直射辐射强度设计值,使系统年性能最优.     

Low temperature passivation of silicon surfaces by polymer films

A novel surface passivation method for silicon carrier lifetime measurements and solar cells using a polymer film is introduced. It is easy to apply, no special pre-treatment, e.g. no hydrofluoric acid (HF)-treatment, is necessary. The surfaces to be passivated are covered with the polymer solution, dried at 90°C and encapsulated. Surface recombination velocities (S) as low as S=30 cm/s for various doping concentrations have been observed, nearly independent of the bulk injection level. The passivation is stable for at least 6 h. For a polymer-passivated rear contact solar cell the same open circuit voltage is achieved as for a cell with thermally grown oxide.     

Effective conductivity bounds by inserting adiabatic or isothermal surfaces

Structure-dependent bounds of the effective thermal conductivity ETC in polyphasic materials, with a regular inclusion array, are proposed. The bounds account for the contact resistance and are based on the enhancement of the ETC by insertion of any isothermal surface (for the upper bounds) and on the decrease of the ETC by insertion of any adiabatic surface (for the lower bounds). The method is above all able to introduce bounds for the high contrast filled composite polymers, whose inclusions are much more conductive than the polymer matrix. Computations for the ETC of a simple cubic array of spheres, which simulate well the heat conduction features in filled polymers, show that two such lower bounds and an upper one are easily computable. For all ETC higher than 3 or 4 times the matrix thermal conductivity, the bounds by insertion of isothermal or adiabatic surfaces are tighter than the other known structure-dependent bounds which account for the imperfection of the perfect contact – the 3rd order variational bounds of Torquato and Rintoul [S. Torquato, M. Rintoul, Effect of the interface on the properties of composite media, Phys. Rev. Lett. 75 (1995) 4067–4070].     

Reduction of reflection losses of PV-modules by structured surfaces

Structuring the transparent cover of solar cell modules reduces reflection losses, particularly at large angles of incidence. Relevant aspects are good transmission efficiency independent of wavelength and a low sensitivity to pollution. The macroscopic, linearly grooved structure proposed in this article shows good performance only in combination with a textured cell because large angles are likely to occur inside the structured cover. A classification is made with the concept of annual averaged transmission efficiency for the climatic zone of Freiburg. Calculations and measurements for different combinations of smooth and structured covers and solar cells are presented. From the calculated 97.8% entering the structured glass cover, a measured 93.2% can be coupled into a pyramidal textured monocrystalline solar cell. This is an absolute improvement of 17% compared to a smooth, incoated solar cell with a smooth glass cover. Outdoor measurements showed that a textured solar cell with a structured cover has between 5 and 10% higher values of short-circuit current than a textured cell with a smooth cover.     

Techno-economic appraisal of a dish/stirling solar power plant in Greece based on an innovative solar concentrator formed by elastic film

As the environmental problems caused by the use of conventional fuels have risen significantly and due to the increase in fossil fuel prices, the importance of reducing society dependence on non-renewable energy sources becomes more and more urgent. It is clear that this could only happen by switching to extensive use of clean energy sources such as renewable energy sources. In this paper a technical feasibility and economic viability study of a dish/Stirling solar power plant in Greece is presented. The proposed power plant uses Dish/Stirling technology and has nominal power 10 MW. The proposed solar concentrator system consists of a primary mirror, formed by elastic film, used to focus the sun's rays onto a secondary mirror which then deviates this radiation into the receiver (Stirling engine). TRNSYS program was developed to simulate Dish/Stirling power plant operation in order to investigate its performance in Greece. The simulation predicted that the proposed power plant could produce 11.19 GWh annually. Project investment cost is approximately 27,000,000 €, while payback is achieved after 16 years of operation. The total profit at the end of the analysis period of 25 years is estimated to be 25,500,000 €.     

Transport of thermal energy by a simple two-phase loop

A loop filled with a phase changing fluid is proposed as a device capable of transferring heat power from a generation place to a user. Through the solution of a set of non-linear equations, operating conditions of the system are determined. It is seen that even large amounts of energy can be transported under relatively small mass flow rates and differences of pressure and temperature. Then only a small degradation of the energy conveyed takes place. The operating conditions of the system are sensitive to the filling degree and the inner diameter of the loop pipe.     

Progress in particle resuspension from rough surfaces by turbulent flows

This article deals with the resuspension phenomenon whereby particles adhering on a wall surface can be re-entrained by a flowing fluid. This is an area where significant progress has been achieved over the last years from an experimental, theoretical and numerical point of view. A first purpose of the present work is to report on the advances that have clarified our understanding of the physics of particle resuspension. It will be seen that new pictures have emerged about the physical processes involved in particle resuspension and, correspondingly, that new models have been proposed. A second purpose of the review is to put forward a general framework that allows both experimental analysis and new modelling ideas to be developed in terms of the fundamental interactions at play. These interactions are made up by the particle–fluid, particle–surface and particle–particle forces which are, in turn, related to the three specific fields of fluid dynamics, interface chemistry and surface roughness. Such a separation is helpful to highlight the actual physical processes while emphasising their relative importance in different situations and to provide useful guidelines for the necessary modelling efforts. In particular, it is stressed that new models which capture particle motion along a wall and simulate the complete particle dynamics represent an improvement over more classical static approaches. It is proposed that these new approaches be pursued and brought to higher levels of maturity.In this paper, attention is first focussed on the case where only a single layer of particles is sticking on the surface and, thus, can be re-entrained. A detailed review of the experimental works brings out the essential mechanisms and particle resuspension is shown to result from a balance between particle–fluid interactions and particle–surface interactions influenced by surface heterogeneities (roughness). The numerical models which have been proposed are then thoroughly discussed with respect to a new hierarchy of modelling approaches which is introduced. The present paper also outlines the mechanisms of multilayer particle resuspension which is still an open subject and where our present understanding remains preliminary. In this situation, resuspension is shown to be also governed by particle–fluid and particle–surface interactions but with the addition of particle–particle interactions (through cohesion forces or impaction). Finally, suggestions about the areas that still need to be addressed as well as about the issues that remain to be improved are addressed.