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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
J. Sol. Energy Eng. 2017;139(5):051006-051006-11. doi:10.1115/1.4037092.

Due to significant reduction in fossil fuel sources, several researches have been conducted recently to explore modern sources of renewable energy. One of the major fields in the category of renewable energy harnessing devices is parabolic trough solar collector (PTC). Several parameters have effect on the overall efficiency of the PTCs. As the effect of these parameters is coupled to each other, a comprehensive investigation is necessary. In the present study, a numerical analysis is performed to examine the efficiency of PTCs via variation of several governing parameters (e.g., wind velocity magnitude, nanoparticles volume fraction, inlet temperature, and reflector's orientation). A detailed set of absorber, reflector, and protection glass in addition to the surrounding environment is modeled to capture sufficiently accurate data. The working fluid is assumed to be nanofluid to inspect the advantage of metallic nanoparticle addition to the base fluid. The Monte Carlo radiation tracing method is utilized to calculate the solar gain on the absorber tube. According to the obtained results, the efficiencies are reduced by 1–3% by rotating the reflector by 30 deg relative to wind direction. Moreover, 14.3% and 12.4% efficiency enhancement is obtained by addition of 5% volume fraction of Al2O3 to the base synthetic oil for horizontal and rotated reflectors, respectively.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(5):051007-051007-8. doi:10.1115/1.4037218.
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This paper proposes new seismic-based methods for use in the wind energy industry with a focus on wind turbine condition monitoring. Fourteen Streckeisen STS-2 Broadband seismometers and two three-dimensional (3D) sonic anemometers are deployed in/near an operating wind farm to collect the data used in these proof-of-principle analyses. The interquartile mean (IQM) value of power spectral density (PSD) of the seismic components in 10 min time series is used to characterize the spectral signatures (i.e., frequencies with enhanced variance) in ground vibrations deriving from vibrations of wind turbine subassemblies. A power spectral envelope approach is taken in which the probability density function (PDF) of seismic PSD is developed using seismic data collected under normal turbine operation. These power spectral envelopes clearly show the energy distribution of wind-turbine-induced ground vibrations over a wide frequency range. Singular PSD lying outside the power spectral envelopes can be easily identified and is used herein along with supervisory control and data acquisition (SCADA) data to diagnose the associated suboptimal turbine operating conditions. Illustrative examples are given herein for periods with yaw misalignment and excess tower acceleration. It is additionally shown that there is a strong association between drivetrain acceleration and seismic spectral power in a frequency band of 2.5–12.5 Hz. The long-term goal of the research is development of seismic-based condition monitoring (SBCM) for wind turbines. The primary advantages of SBCM are that the approach is low-cost, noninvasive, and versatile (i.e., one seismic sensor monitoring multiple turbine components).

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(5):051008-051008-4. doi:10.1115/1.4037484.

The power generated in wind turbine depends on wind speed and parameters of blade geometry like aerofoil shape, blade radius, chord length, pitch angle, solidity, etc. Aerofoil selection is the crucial factor in establishing the efficient wind turbine. More than one aerofoil in a blade can increase the efficiency further. Previous studies of different aerofoils have shown that efficiency of small scale wind turbine increases when NREL S822 aerofoil is used for wind speed on and above 10 m/s. This paper introduces a study on effect of low wind speed (V = 5 m/s) on performance of blade profile. Aerofoils NREL S822/S823 are used for microwind turbine with S823 near root and S822 near tip. Blade of 3 m radius with spherical tubercles over entire span is analyzed considering 5 deg angle of attack. The computational fluid dynamics (CFD) simulation was carried out using ANSYS fluent to study the behavior of blade profile at various contours. The study shows that blade experiences maximum turbulence and minimum pressure near trailing edge of the tip of blade. The region also experiences maximum velocity of the flow. These factors result in pushing the aerofoil in upward direction for starting the wind turbine to rotate at the speed as low as 5 m/s.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(5):051009-051009-10. doi:10.1115/1.4037382.

Reducing levelized electricity costs of concentrated solar power (CSP) plants can be of great potential in accelerating the market penetration of these sustainable technologies. Linear Fresnel reflectors (LFRs) are one of these CSP technologies that may potentially contribute to such cost reduction. However, due to very little previous research, LFRs are considered as a low efficiency technology. In this type of solar collectors, there is a variety of design approaches when it comes to optimizing such systems. The present paper aims to tackle a new research axis based on variability study of heliostat curvature as an approach for optimizing small and large-scale LFRs. Numerical investigations based on a ray tracing model have demonstrated that LFR constructors should adopt a uniform curvature for small-scale LFRs and a variable curvature per row for large-scale LFRs. Better optical performances were obtained for LFRs regarding these adopted curvature types. An optimization approach based on the use of uniform heliostat curvature for small-scale LFRs has led to a system cost reduction by means of reducing its receiver surface and height.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(5):051010-051010-9. doi:10.1115/1.4037383.

Extensive solar field performance testing is often required as part of the plant commissioning process in order to ensure that actual solar field performance satisfies both technical specifications and performance guarantees between the involved parties. In this study, short duration (15 min) steady state performance acceptance test for Kuraymat integrated solar combined cycle (ISCC) solar field was carried out in agreement with the general guidelines of the earlier National Renewable Energy Laboratory (NREL) report on parabolic trough (PT) collector fields (Kearney, 2011, “Utility-Scale Parabolic Trough Solar Systems—Performance Acceptance Test Guidelines,” National Renewable Energy Laboratory, Golden, CO, NREL Report No. SR-5500-48895 and Kearney, 2010, “Development of Performance Acceptance Test Guidelines for Large Commercial Parabolic Trough Solar Fields,” National Renewable Energy Laboratory, Golden, CO, NREL Report No. SR-5500-49367) which is in full agreement with the plant documentations provided by FLAGSOL (2010, “Specification: Performance Test Procedure. Plant Documentations,” Customer Doc-ID: KU1-FLG-000-QP-M-001). This work includes measurement of the thermal power output of PT system under clear sky conditions over a short period during which thermal steady state conditions exist. The methodology of the solar field testing is presented while a special consideration is provided for the model formulation and uncertainty associated with the measured data. The measured results together with the associated uncertainties were compared with model predictions. All tests for both northern and a southern collector subfields that satisfy the test conditions are accepted based on acceptance test evaluation criteria.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(5):051011-051011-11. doi:10.1115/1.4037385.

In this study, a comparative investigation of two types of microheat pipe array (MHPA) flat-plate solar air collectors (FPSAC) based on exergy analysis has been conducted. The thermal performance of MHPA-type solar air collectors (SACs) with two different shaped fins is experimentally evaluated. A detailed parametric study is also conducted to examine the effects of various fins, operation parameters, and inlet air temperature at different mass flow rates on thermal and exergy efficiencies. Results indicated that using V-shaped slotted fins at the specified range of mass flow rates can enhance exergy efficiency. Exergy efficiency can be considered as the main criterion to evaluate the performance of MHPA FPSACs. Attaching V-shaped slotted fins on the condenser section of MHPA is more effective than attaching rectangular fins at high mass flow rates. By contrast, the latter is more effective than the former at low mass flow rates.

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
J. Sol. Energy Eng. 2017;139(5):054502-054502-5. doi:10.1115/1.4037377.

This paper proposes a new method based on a Markov model to calculate the reliability of grid-connected photovoltaic (PV) systems. This system is a grid-connected PV system consisting of PV modules, a multiphase DC–DC converter, an inverter, an inverter controller, and an maximum power point tracking (MPPT) controller at University of Isfahan. This system is considered repairable. Also, different levels of operation are considered for the system equipment. Reliability of the PV modules, the multiphase DC–DC converter, and the inverter has been calculated by the Markov model. Finally, the reliability of the entire PV system is calculated by the Markov model. The proposed algorithm is applied to the PV system positioned at University of Isfahan. Simulation results show the applicability of this method for calculating the reliability of grid-connected PV systems.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2017;139(5):054503-054503-8. doi:10.1115/1.4037379.

Passive cooling by combined radiation–convection from black panels at night is a potential source of significant energy-efficient cooling for both homes and industry. Assessing the technology requires system models that connect cooling load, passive cooling technology performance, and changing weather conditions in annual simulations. In this paper, the performance of an existing analytical model for a passive cooling panel is validated using a full two-dimensional finite differences model. The analytical model is based on a solar hot water collector model but uses the concept of adiabatic surface temperature to create an intuitive, physically meaningful sink temperature for combined convection and radiation cooling. Simulation results are reported for cooling panels of different sizes and operating in both low temperature (comfort cooling) and high temperature (power plant) applications. The analytical model using adiabatic minimum temperature agrees with the high-fidelity finite differences model but is more practical to implement. This model and the validations are useful for the continued study of passive cooling technology, in particular, as it is integrated into system-level models of higher complexity.

Commentary by Dr. Valentin Fuster

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