Roman Bader

Design of a Solar Reactor to Split CO2 Via Isothermal Redox Cycling of Ceria

The design procedure for a 3 kWth prototype solar thermochemical reactor to implement isothermal redox cycling of ceria for CO2 splitting is presented. The reactor uses beds of mm-sized porous ceria particles contained in the annulus of concentric alumina tube assemblies that line the cylindrical wall of a solar cavity receiver. The porous particle beds provide high surface area for the heterogeneous reactions, rapid heat and mass transfer, and low pressure drop. Redox cycling is accomplished by alternating flows of inert sweep gas and CO2 through the bed. The gas flow rates and cycle step durations are selected by scaling the results from small-scale experiments. Thermal and thermo-mechanical models of the reactor and reactive element tubes are developed to predict the steady-state temperature and stress distributions for nominal operating conditions. The simulation results indicate that the target temperature of 1773K will be reached in the prototype reactor and that the Mohr-Coulomb static factor of safety is above two everywhere in the tubes, indicating that thermo-mechanical stresses in the tubes remain acceptably low.

A 9-m-Aperture Solar Parabolic Trough Concentrator Based on a Multilayer Polymer Mirror Membrane Mounted on a Concrete Structure

A large-span solar parabolic trough concentrator is designed based on a multilayer polymer mirror membrane mounted on a rotatable concrete structure. The multilayer membrane is contained in a transparent protective air tube and generates a multicircular profile that approaches the trough parabolic shape. An analytical model of the mechanical behavior of the membrane mirror construction coupled to a Monte Carlo ray-tracing simulation is formulated and applied for design and optimization and for elucidating the influence of manufacturing and operational parameter variations on the radiative flux distribution. It is found that the parabolic shape can be well approximated with four stacked membranes that generate an arc-spline of four tangentially adjacent circular arcs. A 45-m-long 9-m-aperture full-scale prototype concentrator was fabricated and experimentation was carried out to validate the simulation model. Highest measured peak solar radiative flux concentration was 18.9, corresponding to 39% of the theoretical maximum value for an ideal parabolic trough concentrator.

An Air-Based Cavity-Receiver for Solar Trough Concentrators

A cylindrical cavity-receiver containing a tubular absorber that uses air as the heat transfer fluid is proposed for a novel solar trough concentrator design. A numerical heat transfer model is developed to determine the receiver’s absorption efficiency and pumping power requirement. The 2D steady-state energy conservation equation coupling radiation, convection, and conduction heat transfer is formulated and solved numerically by finite volume techniques. The Monte Carlo ray-tracing and radiosity methods are applied to establish the solar radiation distribution and radiative exchange within the receiver. Simulations were conducted for a 50 m-long and 9.5 m-wide collector section with 120°C air inlet temperature, and air mass flows in the range 0.1‐1.2 kg/s. Outlet air temperatures ranged from 260°C to 601°C, and corresponding absorption efficiencies varied between 60% and 18%. Main heat losses integrated over the receiver length were due to reflection and spillage at the receiver’s windowed aperture, amounting to 13% and 9% of the solar power input, respectively. The pressure drop along the 50 m module was in the range 0.23‐11.84 mbars, resulting in isentropic pumping power requirements of 6.4510 4 0.395% of the solar power input. DOI: 10.1115/1.4001675 Cavity-receivers are typically used in point-focusing solar concentrating systems e.g., dishes and towers to efficiently capture incoming radiation through multiple internal reflections, while providing sufficient heat transfer area for heat removal by a heat transfer medium or by chemical reactions. In contrast, tubular receivers are typically used in line-focusing solar concentrator systems e.g., parabolic troughs to efficiently absorb incident solar radiation through the application of selective coatings and vacuum insulations. However, when the heat transfer fluid HTF has low volumetric heat capacity and thermal conductivity, as is usually the case for gases, cavity-receivers are an interesting alternative to conventional tube receivers, as they offer the potential for larger heat transfer area and flow cross section without significantly affecting the reradiation losses from the absorber. Cylindrical cavity-receivers have been previously analyzed for an annular flow cross section 1 and for a cavity containing a single absorber tube or an array of absorber tubes 2‐4. Air is used as the HTF in the present case. The advantages are fourfold: 1 Performance loss and operating temperature constraints due to chemical instability of the HTF are avoided; 2 operating pressure can be close to ambient, eliminating the need for sophisticated sealing; 3 a packed-bed thermal storage can be incorporated to the system and heated directly by air, eliminating the need for a heat exchanger between HTF and thermal storage medium; and 4 costs for the heat transfer fluid are removed. Further, by employing conventional materials of construction and avoiding selective absorber coatings, vacuum insulation, or getters, significantly lower fabrication costs per unit receiver length are expected than those for existing receivers. On the other hand, the disadvantages of air-receivers are associated with the larger mass flow rates and surface area needed due to the lower volumetric heat capacity and thermal conductivity of air as compared with those of thermo-oils, molten salts, sodium, or other heat transfer fluids proposed. These drawbacks translate into higher pressure drops and concomitant energy penalties. In this paper, a numerical heat transfer model of an air-based cylindrical cavityreceiver is developed and applied to investigate the influence of air mass flow rate on outlet air temperature, receiver’s absorption efficiency, pumping power requirements, and thermal losses.

Optical Design of Multisource High-Flux Solar Simulators

We present a systematic approach to the design of a set of high-flux solar simulators (HFSSs) for solar thermal, thermochemical, and materials research. The generic simulator concept consists of an array of identical radiation modules arranged in concentric rows. Each module consists of a short-arc lamp coupled to a truncated ellipsoidal specular reflector. The positions of the radiation modules are obtained based on the rim angle, the number of concentric rows, the number of radiation modules in each row, the reflector radius, and a reflector spacing parameter. For a fixed array of radiation modules, the reflector shape is optimized with respect to the source-to-target radiation transfer efficiency. The resulting radiative flux distribution is analyzed on flat and hemispherical target surfaces using the Monte Carlo ray-tracing technique. An example design consists of 18 radiation modules arranged in two concentric rows. On a 60-mm dia. flat target area at the focal plane, the predicted radiative power and flux are 10.6 kW and 3.8 MW m(-2), respectively, and the predicted peak flux is 9.5MW m(-2).

Optical Design of a Novel Two-Stage Solar Trough Concentrator Based on Pneumatic Polymeric Structures

An innovative concept for fabricating solar trough concentrators based on pneumatic polymer mirrors supported on precast concrete frames is presented. Optical aberration is corrected by means of a secondary specular reflector in tandem with a primary cylindrical concentrator. The optimal design is formulated for maximum solar flux concentration. The Monte Carlo ray-tracing technique is applied to determine the effect of reflective surface errors and structural beam deformations on the performance of the combined primary and secondary concentrating system. The numerical results are validated with field measurements on a 49.4 m length, 7.9 m width sun-tracking prototype system. Theoretical maximum solar concentration ratio is 151 suns; the measured one with a flat secondary reflector was 55 suns.

Experimental and Numerical Heat Transfer Analysis of an Air-Based Cavity-Receiver for Solar Trough Concentrators

A cylindrical cavity-receiver containing a tubular absorber that uses air as the heat transfer fluid is proposed for a novel solar trough concentrator design. A numerical heat transfer model is developed to determine the receiver’s absorption efficiency and pumping power requirement. The 2D steady-state energy conservation equation coupling radiation, convection and conduction heat transfer is formulated and solved numerically by finite volume techniques. The Monte Carlo ray-tracing and radiosity methods are applied to establish the solar radiation distribution and radiative exchange within the receiver. Simulations were conducted for a 50 m-long and 9.5 m-wide collector section with 120°C air inlet temperature, and air mass flows in the range 0.1 – 1.2 kg/s. Outlet air temperatures ranged from 260 to 601 °C, and corresponding absorption efficiencies varied between 60 and 18 %. Main heat losses integrated over the receiver length were due to reflection and spillage at the receiver’s windowed aperture, amounting to 13% and 9% of the solar power input, respectively. The pressure drop along the 50 m module was in the range 0.23 to 11.84 mbar, resulting in isentropic pumping power requirements of 4 6.45 10 ⋅ % 0.395 % of the solar power input.

Efficient ceria nanostructures for enhanced solar fuel production via high-temperature thermochemical redox cycles

Syngas synthesis by solar energy-driven two-step thermochemical redox cycles is a promising approach for large-scale industrial production of renewable fuels. A key challenge is developing durable materials capable of providing and sustaining high redox kinetics under harsh environmental conditions required for efficient operation. Here, we demonstrate that nanostructured ceria with a high surface area and porosity can significantly enhance the initial and long-term syngas production performance. Three types of ceria morphologies were synthesised and comparatively investigated against commercial powders in two-step thermochemical redox cycles, namely nanostructured flame-made and flower-like agglomerates and sol–gel sub-micro particles. Their syngas production performance was assessed in terms of redox kinetics, conversion stoichiometry and structural stability. The flame-made ceria nano-powders had up to 191%, 167% and 99% higher initial average production rates than the flower-like, commercial and sol–gel ceria powders, respectively. This resulted in the highest H2 (480 μmol min−1 g−1) and CO (230 and 340 μmol min−1 g−1) production rates and redox capacity (Δδ = 0.25) so far reported for ceria. Notably, the grain morphology played a key role in the long-term performance and while the redox kinetics of the flower-like ceria rapidly decreased below that of the commercial powders, the flame-made agglomerates maintained up to 57% higher average production rate until the last cycle. These findings show that the thermochemical stabilisation of nano-scale structural features, observed in the flame-made agglomerates, is key to engineering efficient materials for enhanced thermochemical solar fuel production.

Experimental and numerical characterization of a new 45 kW el multisource high-flux solar simulator.

The performance of a new high-flux solar simulator consisting of 18 × 2.5 kWel radiation modules has been evaluated. Grayscale images of the radiative flux distribution at the focus are acquired for each module individually using a water-cooled Lambertian target plate and a CCD camera. Raw images are corrected for dark current, normalized by the exposure time and calibrated with local absolute heat flux measurements to produce radiative flux maps with 180 µm resolution. The resulting measured peak flux is 1.0-1.5 ± 0.2 MW m-2 per radiation module and 21.7 ± 2 MW m-2 for the sum of all 18 radiation modules. Integrating the flux distribution for all 18 radiation modules over a circular area of 5 cm diameter yields a mean radiative flux of 3.8 MW m-2 and an incident radiative power of 7.5 kW. A Monte Carlo ray-tracing simulation of the simulator is calibrated with the experimental results. The agreement between experimental and numerical results is characterized in terms of a 4.2% difference in peak flux and correlation coefficients of 0.9990 and 0.9995 for the local and mean radial flux profiles, respectively. The best-fit simulation parameters include the lamp efficiency of 39.4% and the mirror surface error of 0.85 mrad.

Optics of solar central receiver systems: a review

This article reviews the state of the art in optical design, modeling and characterization of solar central receiver systems.

A Solar Trough Concentrator for Pill-Box Flux Distribution Over a CPV Panel

An integral methodology is formulated to analytically derive the exact profile of a solar trough concentrator that delivers a uniform radiative flux distribution over a flat rectangular target area at the focal plane. The Monte Carlo ray-tracing technique is applied to verify the analytical solution and investigate the effect of sun shape and mirror surface imperfections on the radiation uniformity and spillage. This design is pertinent to concentrating photovoltaics at moderate mean solar flux concentration ratios of up to 50 suns.