Thomas Fend

Porous materials as open volumetric solar receivers: Experimental determination of thermophysical and heat transfer properties

Porous solids like extruded monoliths with parallel channels and thin walls made from various oxide and non-oxide ceramics, ceramic foams and metal structures have been tested in the past with the objective of applying them as open volumetric receivers in concentrated solar radiation. In this application, ambient air flows through the solid, which is heated by concentrated solar radiation. A heat exchanger then transfers the energy to a conventional steam turbine process. In all cases, to obtain high efficiencies, high absorptivity in the visible and near infrared range has to be combined with a high porosity to create large surfaces for convective heat transfer from the solid absorber to the fluid. However, it can be shown that especially high performance absorbers tend to be sensitive to inhomogeneous flux distributions, which may cause local overheating of the material. In various tests with specific kinds of materials, flow instabilities occurred, which partly leads to hot spots and a sudden destruction of the receiver. To achieve both high efficiencies and reliable operation, an optimised combination of geometrical as well as thermal conductivity and heat transfer parameters has to be selected. A precise knowledge of these quantities for a number of various materials is necessary to estimate the limits for stable flow conditions on the basis of complex numerical simulation programs. Finally, efficiency and performance tests with candidate materials have been carried out. In this paper, the experimental work on a variety of porous materials is reported. The paper will report on methodology and results of thermal conductivity, convective heat transfer coefficient and efficiency measurements of these monolithic materials. It will also present an experimental set-up designed to investigate how the properties of the porous materials affect flow stability. Based on these results, a recommendation for the design of volumetric absorbers will be given.

Comparative assessment of solar concentrator materials

Abstract This paper reports results from long-term durability tests of reflector materials to be used for solar concentrating systems. The studies have been conducted under the auspices of an IEA–SolarPACES collaboration between the National Renewable Energy Laboratory (NREL, USA), the Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT, Spain) and Deutsches Zentrum fur Luft- und Raumfahrt (DLR, Germany). In this co-operative effort, accelerated ageing tests as well as outdoor exposures at a number of test sites having various climatic conditions have been carried out since 1995. In addition to materials already in use at solar power stations, newer materials offering the chance of a significant cost reduction in solar electricity and process heat generation are being investigated. Comparative optical tests are carried out to assess the efficiency as a function of exposure/service time in a solar concentrator. Among the materials showing promise for long-term outdoor applications are various silvered glass mirrors, a silvered polymer film, and an anodized sheet aluminium having an additional protective polymer coating. In addition to durability tests of reflector material samples, practical results are also reported for experiences with field applications of silvered thin glass and anodized sheet aluminium mirrors.

Applicability of highly reflective aluminium coil for solar concentrators

Abstract Because of their manufacturing flexibility and their low costs, mirrors based on anodized or coated sheet aluminium are a promising alternative as primary or secondary concentrators in a number of solar energy applications. They offer solar weighted reflectances of 88–91%, good mechanical properties and are easy to recycle. However, problems occur due to their limited corrosion resistance. Therefore, prior to application, lifetime tests including outdoor and accelerated ageing tests are necessary to prove their optical durability in terms of achieving a 10-year service lifetime. In this study the optical properties of a number of different aluminized reflector materials after accelerated and outdoor exposure tests have been investigated. Optical testing has been performed by measuring the spectral hemispherical reflectance of exposed samples and calculating the solar weighted value. Additionally, specular reflectance has been measured with a simple mobile reflectometer. Materials involved are standard commercial anodized sheet aluminium with layers of different thicknesses and standard high specular aluminium with a metaloxide layer system plus an anti-oxidation polymer coating. Results show that optical degradation is strongly dependent on climatic conditions. Non-organic coatings involved are primarily attacked by humid climates with higher amounts of atmospheric pollution. Standard anodized materials withstand outdoor and accelerated weathering. However reflectance tends to become less specular, which limits their application in concentrating technologies. Finally, small scale application tests have been performed to demonstrate the applicability concerning handling and mechanical connection with support structures. By measuring power density in the focus of a test collector, minimum specular reflectance requirements for trough systems can be defined.

Characterization of Air Flow Through Sintered Metal Foams

This study investigates air flow in metallic foams, which are produced by the SlipReactionFoamSintering (SRFS)-process. It was conducted as part of the collaborative research center (SFB) 561 “Thermally Highly Loaded, Porous and Cooled Multi-Layer Systems for Combined Cycle Power Plants”. The flow through a porous medium is analysed by Darcy’s equation with the Dupuit/Forchheimer extension. All measurements can be described very well by this equation and permeability and inertial coefficients are obtained for a large quantity of samples with different base materials and different porosities. A threshold porosity of 70 % is observed, above which the pressure loss starts sinking significantly with porosity. Additionally, it was found, that the permeability was anisotropic. Permeability is lower in the direction of gravity during foaming. Scattering in the data of the permeability and inertial coefficients versus the porosity is observed and discussed.

TEMPERATURE DEPENDENCY OF THE EFFECTIVE THERMAL CONDUCTIVITY OF NICKEL BASED METAL FOAMS

This article presents experimental results of the thermal conductivity of sintered metal foams. The foams are intended to be used in advanced combined power plants, investigated by the cooperate research center ‘Thermally Highly Loaded, Porous and Cooled Multi-Layer Systems for Combined Cycle Power Plants’ SFB-561. Porous materials are required for the active cooling of the gas turbine combustion chamber wall by effusion cooling. For design purpose, knowledge of the thermophysical properties of this new material is required within the temperature range up to 1000°C. The investigated metal foams were manufactured by the Slip Reaction Foam Sintering (SRFS) process with porosity ranges from 0.55 to 0.85. The overall porosity may be divided in two parts. The primary porosity with pore size levels about 1-3 mm and a range form zero to 0.68 results from a metal-acid-reaction. These primary pores are embedded in a packed bed of sintered metal grains (<150µm), which is also porous. This secondary porosity with pore size levels around 20µm results in porosities of about 0.5. The thermal conductivity of cellular solids differs from that of their corresponding dense material. Therefore, the various pore size level effects contributing to the thermal conductivity are accounted for by introducing an effective thermal conductivity eff . For the determination of the effective thermal conductivity the Transient Plane Source Technique, also known as Hot Disk was employed. The thermal conductivity of the sintered packed bed without primary pores was determined up to 700°C and compared to similar materials. For the foams, eff was determined for a primary porosity of 0.68 up to 700°C. In this article, a dependency between the primary porosity and eff can be shown. The linear rise of eff up to 400°C can be due to the increase of the thermal conductivity of the solid phase. The measurements are validated by comparison of the received specific heat with values received by thermogravimetry measurements. The general applicability of the measurement method to heterogeneous materials such as metal foams is discussed and an outlook about further investigations is given.

Innovative Volumetric Solar Receiver Micro-Design Based on Numerical Predictions

Due to the continuous global increase in energy demand, Concentrated Solar Power (CSP) represents an excellent alternative, or add-on to existing systems for the production of energy on a large scale.In some of these systems, the Solar Power Tower plants (SPT), the conversion of solar radiation into heat occurs in certain components defined as solar receivers, placed in correspondence of the focus of the reflected sunlight.In a particular type of solar receivers, defined as volumetric, the use of porous materials is foreseen. These receivers are characterized by a porous structure called absorber. The latter, hit by the reflected solar radiation, transfers the heat to the evolving fluid, generally air subject to natural convection.The proper design of these elements is essential in order to achieve high efficiencies, making such structures extremely beneficial for the overall performances of the energy production process.In the following study, a parametric analysis and an optimized characterization of the structure have been performed with the use of self-developed numerical models.The knowledge and results gained through this study have been used to define an optimization path in order to improve the absorber microstructure, starting from the current in-house state-of-the-art technology until obtaining a new advanced geometry.Copyright

GAS FLOW IN HOT POROUS MATERIALS: THE SOLAR AIR RECEIVER AND SPIN-OFF APPLICATIONS

This article presents an overview on research results from various projects, which deal with one common problem: gas flow in hot porous materials. First, the solar air receiver, which converts concentrated solar radiation into heat in an air circuit, is described as far as the basic principle and the materials employed are concerned. Then, results from experiments in concentrated solar radiation are presented. Materials employed in these applications are extruded ceramic materials as well as metal and ceramic foams with pore sizes on the milli- and micrometer scale. As it turned out, the material properties significantly influence the efficiency of the solar air receiver. It is shown, that under specific conditions flow instability occurs, which may lead to a thermal overload of the material. Measures to avoid these overloads are proposed. Two approaches how to predict gas flow theoretically are reported. Additionally, it is shown, how material quantities such as pressure drop characteristics influence the flow behaviour and the temperature distribution inside the material. Finally, before a conclusion is given, two further applications, which have been dealt with because similar phenomena occur, are reported: an advanced cross flow particle filter and a gas turbine cooling system.

Experimental Investigation of Heat Transfer and Pressure Drop in Porous Metal Foams

Metal foams made by the SlipReactionFoamSintering (SRFS)process are investigated. In these foams the pores are produced by a reaction between iron and a weak acid. The generated hydrogen forms pores in a metal powder slurry. These pores remain in the foam after sintering. Also secondary pores are found in these foams because of the sintering of the metal powder slurry. The metal powder base of the foams is Inconel 625 and Hastelloy B. Foam samples with a variety of different porosities of the two metals in the range of about 62 % to 87 % are used as well as samples made out of sintered metal powder which were not foamed with porosities of around 50 %. The motivation for this study is to use these foams as combustion chamber walls in gas fired power plants. By using porous walls effusion cooling can be applied to keep the wall temperatures low. Air is used as a fluid to study the flow characteristics of these samples. Their pressure drop with air at room temperature is measured in the range of velocities of up to around 1 m/s. The parameters characterizing the foams are obtained using the Darcy-Forchheimer equations resulting in the permeability and the inertial coefficients. The dependency on the porosity is discussed. The volumetric heat transfer is measured for the foams by a transient method based on an air flow with a sinusoidal temperature wave, which is attenuated by the sample. The obtained volumetric heat transfer coefficients are discussed and transferred to Nu-Re correlations. Correlations between the heat transfer coefficients and the pressure drop coefficients are made.

Effect of Sintering Atmospheres on the Processing of SiC/AlN Ceramic Composites

In an attempt to provide a new advanced carbide/nitride ceramic material with high sinterability and density for high-temperature and solar energy applications, this work inspected the effect of different sintering atmospheres on the processing of near-fully dense SiC/AlN ceramic composites. Several SiC/AlN (0 – 40 wt.%) composites were produced by pressureless sintering at a temperature of 2080°C for 2 hrs using a sintering additive of 2.5% yttria + alumina. Influences of argon/vacuum and nitrogen/vacuum atmospheres on the reaction response and the densification behavior of SiC/AlN composites were examined and analyzed. Results show that sintering of SiC/AlN ceramics in a nitrogen atmosphere increases mass loss of the different composites during sintering and leads to a decrease in their densification parameters. However, sintering in an argon atmosphere promotes both the sintering and densification processes, making argon atmosphere more convenient for sintering SiC/AlN ceramics. The use of SiC/AlN composites prepared by pressureless sintering is suitable for high-temperature applications.

Characterisation of Flow and Heat Transfer in Sintered Metal Foams

Since sintered metal foam is an innovative material with promising properties such as high porosity, good thermal conductivity, beneficial mechanical properties like strength and weldability, it has been considered to be applied as an open porous wall element in combustion chambers of gas turbines. In this application, the foam serves as a functional material capable to lead cooling air through micro- and minichannels into the inside of the combustion chamber. This cooling technique also known as effusion cooling keeps the combustion chamber walls below critical temperatures and therefore enables the burning process to be more effectively operated at higher temperatures. For a proper design of the wall element, the temperature distribution along the path of the fluid inside the foam must be known. For an exact calculation of the temperature flow and heat transfer processes inside the foam must be known. Therefore in this study the permeability and heat transfer properties of the foam have been characterized experimentally. The methods are described and the results in terms of permeability coefficients, convective heat transfer coefficients and effective thermal conductivity are presented as functions of the foam’s porosity. The method of the calculation is described and finally, the results of the calculation are presented, showing that due to the fine grained structure of the foam, the heat transfer from the solid to the cooling fluid takes place in a thin layer close to the inner surface of the camber wall.