Cellular Ceramics / 5

5.7 Solar Radiation Conversion 543

For foam ceramics the values of K2 are even lower and can be less than 10–4 m. This means in practice that when a ceramic foam is used as a volumetric air absorber at a solar flux of 2000 suns instabilities are not expected. However, if other fluids are used, instabilities can occur at significantly lower flux densities. This is valid, for example, for solar methane-reforming receivers.

Fig. 20 Quadratic pressure drop difference as a function of the air outlet temperature for several values of the inertial coefficient.

The mathematical description of the more general case is shown in the following. Setting the derivative of the pressure drop difference to zero results in the equation:

If the ratio of K1 and K2 is greater than f(I0,Tout) there is no ambiguity in the pressure drop for different temperatures. The maximum was also found in the usual

way by solving the system of equations by setting the partial equations to zero. The maximum of f(I0,Tout) is found for b = 1 at Tout = 1342 K and I0 = 4.167 MW m–2. This gives a critical ratio of K1 and K2 at:

Considering the given boundary conditions of a certain setup it is possible to find suitable material parameters which give stable operation of the solar absorber.

In the above-discussed simple model used to study the systematic dependency on the parameters under which instabilities occur, not all parameters are included. A further important parameter of flow stability is thermal conduction of the absorber. In the case of an inhomogeneous temperature distribution, heat flow can level out

544 Part 5 Applications

local hot regions. The better the thermal conduction in the absorber is, the less possible instabilities become. Nevertheless, even without instabilities a positive feedback between solar radiation and mass flow distribution is observed. This means that in locations with increased solar irradiation the local mass flow density is reduced.

5.7.6.1

Experimental Determination of Nonstable Flow

Flow instability can be investigated in a transient experiment, sketched in Fig. 21. Samples of cylindrical shape are heated to 800 C in a tube heater at zero fluid velocity U0. At a defined time t1, U0 at the entrance of the unblocked section is set to U0 » 1 m s–1. At a time t2 » t1 + 0.5 s fluid velocity in the blocked section is also set to U0 » 1 m s–1. Experimentally, this time shift is realized by a blocking mechanism which covers an area in the lower half for 0.5 s. By monitoring the front surface of the sample with an infrared camera, the temperature distribution during the cooling phase can be recorded.

Fig. 21 Experimental setup for investigating the front-tempera- ture distribution of a heated porous monolith cooled with ambient air.

As an example, the homogeneity of the temperature distribution during the cooling phase of a porous material through which cold air is flowing is compared for two materials: a cordierite ceramic foam (20 ppi) and a cordierite catalyst support (400 cpsi). Though having similar bulk thermophysical properties, the geometric structure of the material leads to different pressure-drop characteristics and thus to completely different flow properties.

Figure 22 shows the results of such an experiment. Slight temperature inhomogeneities observed at the front of the foam are rapidly compensated, whereas the catalyst support shows a permanent inhomogeneous temperature distribution. The evolving temperature distribution not only depends on the initial blocking but also on inhomogeneities in the initial temperature before the onset of fluid flow.

In conclusion it can be said that instabilities are a problem for volumetric absorbers, but they can be overcome by choosing an appropriate design. Instabilities are more likely to occur at higher concentrations, but this is no relief as volumetric

5.7 Solar Radiation Conversion 545

Fig. 22 Cooling behavior of air-cooled preheated porous materials: a cordierite catalyst supported (top) and a cordierite foam (bottom).

absorbers are designed for high solar concentrations. Care should be taken to choose a design of an absorber in which the pressure drop characteristic has a strongly quadratic behavior and radial heat exchange is high. In designing an volumetric receiver the feedback between temperature and mass flow must be taken into account.

5.7.7

Summary

Volumetric absorbers are the absorbers of choice for high-temperature applications utilizing highly concentrated solar radiation. As ceramics offer high temperature resistance they play a major role in the design of volumetric absorbers. Many different designs of porous structures can be manufactured from ceramics. When the ceramic is coated with a catalyst it can be used as a chemical absorber/reactor and thus allows thermochemical storage of solar energy. Many successful tests have shown the feasibility of ceramic absorbers in high-temperature solar applications. By considering the different physical properties needed for high efficiency together with construction parameters to avoid unstable flow, it is possible to design highly efficient absorbers with efficiencies above 80 %.

References

4Chavez, J.M., Kolb, G.J., Meinecke, W., Second Generation Central Receiver Technologies –