Two novel high-porosity materials as volumetric receivers for concentrated solar radiation

doi:10.1016/j.solmat.2004.01.039

Solar Energy Materials and Solar Cells

Volume 84, Issues 1–4, October 2004, Pages 291-304

https://doi.org/10.1016/j.solmat.2004.01.039 Get rights and content

Abstract

This article reports on two novel porous materials, which have been foreseen as volumetric receivers for concentrated solar radiation: a double-layer silicon carbide foam and a screen-printed porous silicon carbide material. Volumetric receivers are used in the solar tower technology. In this technology ambient air flows through the porous solid, which is heated by concentrated solar radiation. A heat exchanger then transfers the energy to a conventional steam turbine process. The general thermophysical and permeability properties of materials required for this application are reviewed. Experimental set-up and results of pressure loss and laboratory scale tests in concentrated solar radiation are reported. For the foam, efficiency data could be determined from the test results. Finally, a comparison is presented between the efficiency properties of the foam and those of materials used for the same application until now. This comparison shows, that the efficiency of the double-layer foam material is significantly higher. Up to now, porous materials consisting of a parallel channel geometry with thin walls showed disadvantageous permeability properties. By applying a new manufacturing process and modifying the channel geometry, the permeability properties of the printed material could be significantly changed, so that it now meets the requirements for an application as a volumetric receiver.

Introduction

Volumetric receivers consist of high-porosity materials and are used in the solar tower technology for the conversion of concentrated solar radiation into heat. This heat is then used to feed a conventional steam turbine process. As a heat transfer medium ambient air is used, which flows through the open pores of the material. This technology has been developed during the past 20 years, a number of large scale tests have been carried out [1], [2]. A first 10 MW_el commercial plant is currently planned for Southern Spain [3], [4]. The principle of the receiver is illustrated in Fig. 1. along with a typical flow chart of a solar power plant in Fig 2. The general requirements and the resulting material properties of a material to be applied as a volumetric receiver are summarized in Table 1.

A lot of theoretical research work has been conducted on properties and physical restrictions of the open volumetric receiver principle [5], [6]. The results significantly depend on how detailed the numerical calculation is performed and how reliable the material properties assumed correspond to realistic material data. In general, due to the increasing viscosity of air with increasing temperature, the permeability of the hot channels of the receiver is lower. This leads to a mass-flow distribution shown in the right sketch of Fig. 1. In this case a Gaussian distribution of the incoming concentrated radiation is assumed.

It has been shown by Pitz-Paal and Hoffschmidt [7] and Buck [8], that the flow through the porous material is significantly influenced by thermal conductivity and permeability properties of the material. Obviously, using a long receiver made out of a material with a high thermal conductivity leads to a heat flow perpendicular to the flow direction of the air and thus to a more homogenous temperature distribution of the receiver. Air flow conditions in each channel are then dominated by the homogenized temperature of the receiver.

It could also be shown, that permeability properties of the porous material influence the homogeneity of the air flow significantly. This is mainly caused by the temperature-dependent viscosity of air, which increases with increasing temperature following a T^0.7 law. Consequently, having a uniform pressure difference over the length of a multi-channel array, the permeability in hot channels is lower. In some cases this may lead to a hot blocking of the channels and the formation of by-pass flows in cold channels. If the pressure loss (Δp/L) as a function of the fluid velocity (U_L) shows a linear shape according to the Darcy-law (L: length of the receiver, μ_DYN: dynamic viscosity of air, K: permeability) $Δ p L = μ_{DYN} K U_{L}$ this tendency is significantly stronger compared to materials with a pressure loss following the Forchheimer-law describing a quadratic dependence of the pressure loss as a function of air flow velocity (K₁: viscosity coefficient, K₂: inertial coefficient, ρ_f: density of air): $Δ p L = μ_{DYN} K_{1} U_{L} + ρ_{f} K_{2} U_{L}^{2} .$

Generally, porous structures with straight channels show linear behavior, structures with changes in channel diameter show quadratic behavior.

Homogeneity of the air flow is a prerequisite for high efficiencies and the avoidance of local overheating, which may cause a destruction of the absorber. Past receiver concepts have been based on materials like metallic wire meshes, catalyst carriers with parallel channels for exhaust systems or ceramic foam materials, which have been optimized for other applications. In this paper two new approaches are reported, which are aimed at developping materials with the properties described above, giving special attention to a high ratio of K₁/K₂: A porous silicon carbide material manufactured via the Direct-Typing-Process (called typed material in the further course of the text) and a fine celled silicon carbide foam (called foam). A detailed description of the materials themselves as well as the manufacturing technologies are described in the 3rd paragraph. The foam was developed and manufactured by the Fraunhofer IKTS¹ within a common project, the typed material was developed and manufactured by the company Bauer R&D within a subcontract according to the design of the authors. It was then delivered to DLR, where it was tested.

Section snippets

Experimental tests

This article presents results of permeability measurements and tests in concentrated solar radiation (solar furnace tests). Permeability measurements have been carried out with the simple set-up shown in Fig. 3. A fan generates an air flow through the porous sample. The pressure difference between the outlet and the inlet is measured. A mass flow and a temperature sensor allows the determination of the air flow velocity. Pressure loss as a function of air flow velocity can be measured in the

Ceramic foam receiver samples

Ceramic foams have been employed as volumetric receivers several times. The new approach reported here was to combine two foam materials of different cell density. The reason for this design is, that a high cell density is only required in the extinction volume, the volume in which radiation is absorbed and the heat transfer material to air takes place. The other part of the receiver may also consist of a low cell density material with a higher permeability and a higher effective heat

Results and discussion

The results of the pressure loss measurements are presented in Fig. 7. In this diagram the pressure loss is plotted as a function of the air velocity. The results obtained with the material manufactured via the Direct-Typing-Process (here denoted as “screenprinted material”) are compared with the results of a typical catalyst carrier with simple parallel channels without any artificial obstacles. The effect of the additional perforated plane in the Direct-Typing-Process material is significant.

Conclusions

Two novel porous materials for an application as a volumetric receiver have been presented and tested. First, a double layer silicon carbide foam showed improved performance in an experiment with concentrated solar radiation compared to a single layer material, which has allready been applied before. Secondly, the direct typing process has been employed to show the feasibility of manufacturing a porous ceramic structure with predefined permeability properties. Material samples, meeting the

Acknowledgements

The authors acknowledge the support of this work by the German Federal Ministry of Education and Research within the program “Vernetzungsfond Erneuerbare Energien” and the “Deutsche Forschungsgemeinschaft (DFG)”.

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