Solar Process Heat Pyrolysis

Solar process heat has long been used as a green energy source for small-scale and remote heating applications. The use of concentrated solar power (CSP) allows for the increase in scale and temperature parameters of heating applications. However, re-emittance of energy from the absorbing media reduces the efficiency of photothermal conversion. Selective solar absorber (SSA) surfaces ‘select’ which wavelengths of incoming solar radiation are reflected, transmitted, and absorbed to minimize the emittance of thermal radiation, greatly increasing the efficiency of photothermal conversion and the temperature of the absorbing surface. SSAs allow for the potential to remotely produce process heat for high temperature processes such as pyrolysis, annealing, smelting, and calcination, introducing operating flexibility in process engineering with the elimination of infrastructural power.
SSAs operate on three parameters: solar concentration (input), cutoff wavelength (material property), and operating temperature (output). Most SSA applications surround water heating and use metal to provide reflectance. However, at temperatures needed to provide sufficient process heat for materials processing applications above 400 °C, metallic surfaces lose thermal stability due to diffusion (intralayer or oxidation). All-ceramic designs with high thermal stability have shown promise but remain at a low technical readiness level (TRL) due to the lack of application exploration. To explore an increase in the TRL of SSAs, a lab scale pyrolysis reactor has been designed and constructed to test the SSA in a production relevant environment. A lack of SSA design methodology accompanies the low TRL, so a model to determine all SSA operating parameters was developed in tandem with the reactor design. By defining convective heat transfer losses from the reactor model and inefficiencies in the CSP and SSA components, the necessary solar concentration of the system was able to be defined, something that has not been shown in previous SSA studies.
Using a simple absorber-reflector tandem design allows material selection to determine the cutoff wavelength. The absorption begins at the absorber layer’s band edge, increasing with frequency. With a defined relationship between all SSA parameters, the necessary solar concentration can be determined once the absorber layer material and operating temperature for any design. Complications arise from the bandgap shift at operating temperatures and undefined absorbing layers, such as non-stoichiometric oxynitrides. Current research aims to map the optical properties of titanium oxynitride across varying compositions to tune the absorber layer for performance at temperatures necessary for pyrolysis.