Cellular Ceramics / 5

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5.7

Solar Radiation Conversion

Thomas Fend, Robert Pitz-Paal, Bernhard Hoffschmidt, and Oliver Reutter

5.7.1

Introduction

This chapter covers the use of cellular ceramic materials as absorbers in volumetric solar receivers. A receiver is a central element of solar tower technology, which converts concentrated solar radiation into high-temperature heat. In a volumetric absorber cellular material is employed to absorb concentrated solar radiation and to transfer the energy to a fluid flowing through its open cells. This is explained more in detail in Section 5.7.2. The concentrated radiation is generated by a large number of controlled mirrors (heliostats), each of which redirects the solar radiation onto the receiver as a common target on the top of a tower (Fig. 1). There are different concepts to exploit the heat generated by a volumetric receiver.

In one concept ambient air is forced through the open pores of the material and is heated to about 700 C. It is then used to generate steam for a conventional steamturbine process (Fig. 2). This idea of an open volumetric air receiver was first presented in a study in 1985 [1]. Since then, the technology has been successfully proven in a number of projects [2–4]. Key components of a 2.5 MWth facility were tested by the TSA (Technology Program Solar Air Receiver) consortium under the leadership of the German company Steinm4ller on top of the CESA 1 tower at the

Fig. 1 Views of the Californian 10 MW test plant Solar Two (left) and the Spanish 1.2 MW test plant Cesa 1 (right).

Cellular Ceramics: Structure, Manufacturing, Properties and Applications.

Michael Scheffler, Paolo Colombo (Eds.)

Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-31320-6

5.7 Solar Radiation Conversion 525

In the same type of closed receiver, cellular ceramic materials have been used as an absorber/reactor for chemical reactions. Part of the solar energy transferred by the absorber to the fluid is stored in chemical energy. For example, solar reforming of methane was investigated by the Sandia National Laboratories (SNLA) and the German Aerospace Centre (DLR) in the CAESAR Receiver [9, 10], as well as by the Weizmann Instute of Science (WIS) and the DLR in the SCR (Solar Chemical Reac- tor-Receiver) [11–13]. The absorber is catalytically coated, and the inlet fluid is a mixture of methane and carbon dioxide.

5.7.2

The Volumetric Absorber Principle

The principle of the volumetric absorber is illustrated in Fig. 4. To demonstrate the advantages of the volumetric absorber the principle of a simple tubular absorber is shown for comparison. Because cold ambient air enters the material at the front of the volumetric absorber, where it is facing the radiation, the material temperature can be kept low. In ideal operation, the temperature distribution shown on the lower right-hand side of Fig. 4 should be realized. The low temperature at the front minimizes thermal radiation losses, which occur in accordance with the well-known Ste- fan–Boltzmann law q ¼ erT4. In the inner absorber volume the temperature increases and the temperature difference between fluid and solid vanishes. Usually, this is already the case after a couple of cell diameters, for example, in the case of an 80 ppi ceramic foam after 1–2 mm. In contrast to this increasing temperature distribution from the inlet to the outlet of the absorber module in case of an ideal volumetric absorber, the temperature distribution of a simple tubular absorber is disadvantageous. It is shown in the graph on the lower left-hand side of Fig. 4. Here the fluid which is to be heated flows inside a tube. The solar radiation heats the tube, which in turn heats the fluid. The significantly higher temperature at the outer tube surface leads to higher radiation losses. The temperature at the outer tube surface is limited by the temperature resistance of the material employed. To avoid destruction of the tube material, the intensity of the concentrated radiation must be kept low compared to volumetric absorbers. This makes it necessary to install larger absorber apertures to achieve similar amounts of total power.

Volumetric absorbers usually consist of materials with a high open porosity, which must withstand temperatures of 1000 C and more. A high porosity is needed to allow the concentrated solar radiation to penetrate into the volume of the cellular material. This volume is called extinction volume. The structures of the porous material have to be small to achieve the large surface areas necessary to transfer heat from the material to the gaseous fluid flowing through the open pores of the material. Even though the extinction volume decreases with decreasing structure size, the increased surface area and the increased heat transfer due to smaller hydraulic diameters leads to the desire for structures that are as small as possible as long as the porosity can be kept high.

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Fig. 4 Open volumetric absorber principle compared to a tubular absorber.

When volumetric absorbers are used as chemical reactors, solar energy is converted into heat and chemical energy. For example in the SCR Receiver for methane reforming [11–13] an aluminum oxide ceramic foam is used as absorber. A wash coat made of highly porous c-aluminum oxide was applied to enlarge the active surface area, and rhodium was used as catalyst.

5.7.3

Optical, Thermodynamic, and Fluid-Mechanical Requirements of Cellular Ceramics for Solar Energy Conversion

The general requirements and the resulting material properties of a material to be used as a volumetric absorber are summarized in Table 1. To achieve high values of optical absorption in the 250–2500 nm wavelength range, that is, the solar spectrum, dark ceramics must be employed as solar absorbers or black coatings must be applied to the front surface of the cellular ceramic. Silicon carbide has been used for this purpose and reaches weighted solar absorption values of 0.9. Oxide ceramics usually have white, highly reflecting surfaces. Black coatings can be used to increase the solar absorption. For example Pyromark high-temperature paint was used for coating a cordierite cellular ceramic material, which then reached absorption values of 0.95 and more. In porous materials the radiation enters the extinction volume of the material, and the pores or channels then act as small cavities which improve absorption due to multiple reflections at the cell walls. The use of selective coatings or selective absorbers has been investigated, and theoretical predictions state that

5.7 Solar Radiation Conversion 527

Fig. 5 Spectral absorptance of a silicon carbide short-fiber mesh material, spectral solar radiation, and spectral black-body radiation at 1300 K.

thermal radiation losses can be minimized [14]. However, due to the overlap of the spectral emission curve of a black-body radiator at 1300 K and the solar spectrum, the effect of selectivity is much smaller than for less concentrating technologies such as flat-plate collectors or parabolic trough systems [2]. Figure 5 shows the spectral solar irradiation (not concentrated), the spectral absorptance of a silicon carbide short-fiber mesh material, and the spectral black-body radiation at 1300 K.

Table 1 Optical, thermodynamic, and resulting material requirements of candidate absorber materials

Experimentally the absorptance e in the solar spectrum can be determined indirectly by measuring the solar weighted hemispherical reflectance 2pS and using the simple equation e = 1– 2pS. This equation holds under the assumption that the transmittance is negligible, which it is for all feasible absorbers. It is calculated from the spectral hemispherical reflectance 2p(k) according to a standard [15]. The quantity 2p(k) is measured in the wavelength range 250 < k < 2500 nm with a UV-Vis- NIR spectrometer equipped with an integrating sphere. Then a standard solar spectrum is used [16] to obtain solar weighted hemispherical reflectance values from the following formula:

i¼0

where E(ki) denotes direct normal spectral irradiance at wavelength ki in 5 nm steps (k0 = 250, k1 = 255,..., k450 = 2500 nm).

High porosity is needed to let the solar radiation penetrate into the volume of the material. As shown schematically in Fig. 6, the intensity of the radiation I is attenuated in the cellular material due to a multiple reflection/absorption process approximated by an exponential law

Thus, a measure for this property is the optical extinction coefficient k.

Fig. 6 Attenuation of the intensity of solar radiation in a cellular ceramic material used as a volumetric absorber.

Experimentally the extinction coefficient k can be measured indirectly by performing transmittance measurements on samples of cellular materials of different thicknesses, as illustrated in Fig. 7. The measured intensities I(xi) are then fitted with the above-mentioned exponential function to determine the extinction coefficient k. Alternatively, pictures of the cross section of the cellular material can be evaluated to find the penetration depth and thus the extinction coefficient [13].

In the case of regular geometries such as extruded monoliths, this coefficient can also be easily calculated from the channel diameters, wall thicknesses, and the aperture angle of the incoming radiation. In the case of fiber materials or foams, models must be used to describe the geometry of the material. For example, considering a fiber mesh as layers of parallel cylinders, as in Fig. 7, leads to the following equation

5.7 Solar Radiation Conversion 529

Fig. 7 Principle of measurement of extinction coefficients.

In the case of a fiber material the hydraulic diameter dH is simply the fiber diameter. If the diameter increases at constant porosity this means that the distance between neighboring fibers increases and thereby k decreases.

The third important requirement of a cellular ceramic used as a volumetric solar absorber is the heat-transfer surface area, which is a more general requirement that is also important for high-temperature heat exchangers. The quantity describing this property is called specific surface area AV [m2 m–3]. This surface area is responsible for efficient solid-to-fluid heat transfer. High values of AV are achieved by using small cells or small fiber diameters. Similar to the extinction coefficient, models can be used to estimate this quantity.

A comparison of the different structures can be found in Section 5.7.4, and values of thermophysical properties are listed in Table 2. In the case of ceramic foams, the following empirical equation yields the specific surface area as a function of the cell density nppi. It was derived from a numerical investigation of various micrographs [17]:

Table 2 Geometric and thermophysical properties of selected cellular ceramics used as volumetric solar absorbers [19].

* Results from a 20 ppi/80 ppi sandwich foam sample.

In the case of fibrous materials AV can be easily calculated from the porosity P0 and the fiber diameter dH. The porosity of the fibrous material can be calculated from the total volume V and the volume of the fibers Vfiber by

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Assuming l >> dH for the length of the (cylindrical) fibers, the surface area Ofiber and volume Vfiber of n fibers can be calculated by

Combining these equations and using the definition of the specific area AV = Ofiber/V yields the following simple equation:

Comparing Eqs. (3) and (8) shows that a higher value of AV leads to a higher extinction coefficient and hence a lower penetration depth, because of the geometric similarity of a smaller pore structure of the material.

Up to now we showed how the material properties porosity P0 and cell dimensions dH influence the specific surface area and the extinction coefficient, but how do they affect the efficiency of a volumetric absorber? This can be shown by considering that P0 and dH also influence the heat-transfer properties of the cellular ceramic. The solid- to-fluid heat transfer of a cellular ceramic can be described by the equation

where a denotes the convective heat-transfer coefficient, TS and TF are solid and fluid temperature, respectively, and V is the total volume of the cellular body. An important result from the theory of volumetric absorbers is the following conclusion for the efficiency of a volumetric absorber g, which is defined as the power of the fluid mass flow at the outlet of the absorber divided by the power of the concentrated solar radiation penetrating through the aperture area of the absorber [18]. It states that the deeper the radiation penetrates into the absorber volume (small k) and the better the heat transfer (large aAV) the better the volumetric effect of the absorber. This means that the front temperature of the absorber and hence the thermal radiation losses decrease. This leads to higher efficiencies if the other parameters like irradiance and mass flow are kept constant.

Equation (3) can be approximated numerically in the relevant range of porosity from 0.3 to 0.95 and an incident angle of radiation of 20 as the following function of the porosity

5.7 Solar Radiation Conversion 531

Since the convective heat-transfer coefficient is independent of the porosity it is easy to conclude

These calculations conclusively show that both a high porosity and a high cell density (corresponding to a low characteristic length dH) are beneficial for the efficiency of a volumetric absorber.

Furthermore, materials that retain their strength and corrosion resistance at high temperatures are needed if high solar radiation fluxes on the order of 1000 kW m–2 are to be absorbed. Silicon carbide materials, which offer excellent high-temperature properties and thermal shock resilience, can be used. Material properties are discussed in more detail in Chapter 4.3.

Thermal conductivity is another property essential for a safe operation of a volumetric absorber. This and the permeability of cellular ceramics to air flow have a significant influence on the flow through the open pores of the material [13, 19]. It could be shown that permeability properties of the porous material especially influence the homogeneity of the air flow. This is discussed in detail in the Section 5.7.6. Permeability of a solid with open cells can be characterized by the Forchheimer equation describing the dependence of the pressure loss Dp divided by the length L of the absorber as a function of fluid flow velocity U0:

where K1 is the viscosity coefficient, K2 the inertial coefficient, f the density of the fluid, and ldyn the dynamic viscosity of the fluid.

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In this equation the linear term has a major contribution when frictional forces dominate the flow, for example, as in catalyst supports with straight channels. The quadratic term has a major contribution when inertial forces dominate the flow, for example, as in foams or sponges, in which the flow has to follow the tortuous paths of the linked cell walls.

Experimentally, permeability measurements can be carried out with a simple setup shown in Fig. 8. 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.

Fig. 8 Test setup used for pressure-loss measurements

5.7.4

Examples of Cellular Ceramics Used as Volumetric Absorbers

An overview of some of the physical properties of materials for volumetric absorbers is given in Table 2. Although the materials are at least partly some form of silicon carbide because of its favorable heat and heat shock resistance properties there is still a wide variety of construction parameters and hence physical properties.

5.7.4.1

Extruded Silicon Carbide Catalyst Supports

Silicon carbide extrusion technology has been widely employed for manufacturing diesel particle filters. Thus, this technology also offers the chance for cost-effective manufacturing of solar absorbers, because only a slight modification of the manufacturing process is necessary. After various laboratory-scale tests a 3 MW modular receiver system was manufactured and tested at the Plataforma Solar de Almer%a (PSA) in Southern Spain within the project SOLAIR, co-funded by the European Commission and a consortium of European companies and research institutions [5]. This receiver system contains absorber elements made of extruded silicon carbide ceramic and is known as Hitrec (high temperature receiver). The absorbers, manufactured by the Danish company Heliotech, have a rectangular honeycomb structure with parallel channels (Fig. 9). The channel width of 2 mm and wall thickness of 0.8 mm represent a compromise between material stability and optimal