(200a) Efficiency and Design of High-Temperature Solar-Thermal Reactors

Efficiency and Design of High-temperature Solar-thermal Reactors

Arto J. Groehn, Allan Lewandowski, Ronggui Yang^* and Alan W. Weimer

Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80303, United States.

^*Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309, United States.

The goal of this work is to facilitate the use of concentrated solar power to drive chemical reactions for such applications as carbothermal reduction and water splitting. A computational model interfacing radiative, convective and conductive heat transfer was employed to investigate optimal reactor design and scale-up.

Typically employed radiation models can be classified into Monte Carlo (MC) and finite volume (FV) based approaches. MC models are often employed to solve radiation in static systems where effects of scattering, absorption, emission as well as anisotropy caused by particles, gases or varying material properties can be neglected. In contrast, these effects are readily accounted for in a FV-based discrete ordinates (DO) model (Chui & Raithby, 1993). Its accuracy, however, is limited by the number of employed discrete solid angles as well as computational cells which may result in ray effect and false scattering errors, respectively (Martinek & Weimer, 2013).

To reduce computational cost without compromising accuracy, previously a hybrid MC-FV radiation modeling scheme was suggested for a solar-thermal process (Martinek & Weimer, 2013). In this approach the incoming specular reflections were solved with a MC code while a FV model was used for the diffusive thermal re-radiation away from the reactor walls. Here, this hybrid concept is extended to allow MC-FV coupling at an arbitrary interface thus enabling simulation of complex physical mechanisms (e.g. temperature dependent emissivity) and their interactions inside the reactor domain.

The accuracy and optimal discretization level of the present approach was evaluated by comparing its predictions with those of a well-established and validated MC ray-tracing code SolTrace (Wendelin, 2003). Simulations with the validated model were conducted to determine flux profiles and thermal efficiencies of different heliostat field, secondary concentrator and receiver designs for large-scale (gigawatt) chemical reactors.

References

Chui, E. and G. Raithby, Computation of radiant heat transfer on a nonorthogonal mesh using the finite-volume method. Numerical Heat Transfer, 1993. 23(3): p. 269-288.

Martinek, J. and A.W. Weimer, Evaluation of finite volume solutions for radiative heat transfer in a closed cavity solar receiver for high temperature solar thermal processes. International Journal of Heat and Mass Transfer, 2013. 58(1): p. 585-596.

Wendelin, T. SolTRACE: a new optical modeling tool for concentrating solar optics. in ASME 2003 International Solar Energy Conference. 2003. American Society of Mechanical Engineers.