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Research Papers

J. Sol. Energy Eng. 2017;139(5):051001-051001-8. doi:10.1115/1.4037090.

Numerical simulation enables the optimization of a solar collector without the expense of building prototypes. This study details an approach using computational fluid dynamics (CFD) to simulate the performance of a solar thermal collector. Inputs to the simulation include; heat loss coefficient, irradiance, and ambient temperature. A simulated thermal efficiency was validated using experimental results by comparing the calculated heat removal factor. The validated methodology was then applied to five different inlet configurations of a header–riser collector. The most efficient designs had uniform flow through the risers. The worst performing configurations had low flow rates in the risers that led to high surface temperatures and poor thermal efficiency. The calculated heat removal factor differed by between 4.2% for the serpentine model and 12.1% for the header–riser. The discrepancies were attributed to differences in thermal contact between plate and tubes in the simulated and actual design.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(5):051002-051002-9. doi:10.1115/1.4037091.

The stability of platform is the most fundamental guarantee for the safe operation of floating wind turbine in complex marine environment. The helical strakes used on spar platform in the traditional oil industry are useful and effective. This paper is to investigative the validity of helical strakes when used for offshore wind energy harvesting. The National Renewable Energy Laboratory (NREL) 5 MW wind turbine based on OC3-Hywind spar-buoy platform with the attachment of helical strakes is modeled for the purpose to analysis the impact of helical strakes and its design parameters (number, height, and pitch ratio) on the dynamic response of the floating wind turbine spar platform. The dynamic response of spar platform under wind, wave, and current loads is calculated and analyzed based on the radiation and diffraction theory, the finite element method, and the orthogonal design method. The research result shows that the helical strakes can effectively suppress the dynamic response of the platform but enlarge the wave exciting force, and helical strakes cannot change peak frequency of response amplitude operator (RAO) and wave exciting force of spar in frequency-domain. The best parameter combination is two pieces of helical strakes with height of 15%D and the pitch ratio of 5. Height and pitch ratio of the helical strakes have significant influence on pitch response, while the number and interaction of height and pitch ratio have slight effect.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(5):051003-051003-11. doi:10.1115/1.4036855.

In this paper, analytical expression for characteristic equation of double slope solar still (DS) included with series connected N identical evacuated tubular collectors (N-ETC-DS) has been developed. The derivation is based on fundamental energy balance equations for various components of the proposed system. The analytical result of the proposed N-ETC-DS has been compared with results reported by earlier researchers for the same basin area under similar climatic condition. It has been concluded that daily energy efficiency is higher by 23.90%, 26.45%, and 42.65% for N-ETC-DS than N identical partially covered photovoltaic thermal (PVT) compound parabolic concentrator collectors (CPC) integrated double slope solar still, N identical partially covered PVT flat plate collectors (FPC) integrated double slope solar still, and conventional double slope solar still (CDS), respectively, at 0.14 m water depth under optimized condition. Moreover, daily yield, exergy, energy and exergy efficiency have been computed.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(5):051004-051004-11. doi:10.1115/1.4037191.

The design and characterization of an upward flow reactor (UFR) coupled to a high flux solar simulator (HFSS) under vacuum is presented. The UFR was designed to rapidly heat solid samples with concentrated irradiation to temperatures greater than 1000 °C at heating rates in excess of 50 K/s. Such conditions are ideal for examining high-temperature thermal reduction kinetics of reduction/oxidation-active materials by temporally monitoring O2 evolution. A steady-state, computational fluid dynamics (CFD) model was employed in the design to minimize the formation of eddies and recirculation, and lag and dispersion were characterized through a suite of O2 tracer experiments using deconvolution and the continuously stirred tank reactors (CSTR) in series models. A transient, CFD and heat transfer model of the UFR was combined with Monte Carlo ray tracing (MCRT) to determine radiative heat fluxes on the sample from the HFSS to model spatial and temporal sample temperatures. The modeled temperatures were compared with those measured within the sample during an experiment in which Co3O4 was thermally reduced to CoO and O2. The measured temperatures within the bed were bounded by the average top and bottom modeled bed temperatures for the duration of the experiment. Small variances in the shape of the modeled versus experimental temperatures were due to contact resistance between the thermocouple and particles in the bed and changes in the spectral absorptivity and emissivity as the Co3O4 was reduced to CoO and O2.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(5):051005-051005-7. doi:10.1115/1.4037161.

A theoretical mathematical model that considers the continuous linear porosity or pore diameter distribution is established to develop a novel porous absorber with variable pore structure, which will result in a thermopressure drop improvement. Efficient performance can be achieved based on reconstruction of the velocity, temperature, and radiation fields. Collimated and diffusive radiative heat fluxes and the heat loss mechanism from the irradiated surface are analyzed in the presence of the volumetric effect. This study analyzes three typical linear pore structure distributions: increasing (I), decreasing (D), and constant (C) types, respectively. In general, the D type porosity (φ) layout combined with the I type pore diameter (dp) distribution would be an excellent pore structure layout for a porous absorber.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Sol. Energy Eng. 2017;139(5):054501-054501-4. doi:10.1115/1.4037089.

An engineering design for a novel 1-kW solar-driven reactor to capture carbon dioxide via the calcium oxide-based two-step carbonation–calcination cycle has been completed. The reactor consists of a downward-facing cylindrical dual cavity. The inner cavity serves as the radiation receiver, while the outer cavity is the reaction chamber that contains a packed- or fluidized-bed of reacting particles. Several aspects have been incorporated in this reactor design, including high flexibility, mechanical rigidity and simplicity, high-temperature and thermal shock resistance, accommodation of thermal expansion, low convective heat losses, uniform gas distribution inside the reaction chamber, and simple reactor assembly. The final reactor design is presented, and the reactor assembly is illustrated.

Commentary by Dr. Valentin Fuster

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