Accepted Manuscripts

Hamidreza Abedi, Lars Davidson and Spyros Voutsinas
J. Sol. Energy Eng   doi: 10.1115/1.4035887
The aerodynamics of a wind turbine is governed by the flow around the rotor, where the prediction of air loads on rotor blades in different operational conditions and its relation to rotor structural dynamics is one of the most important challenges in wind turbine rotor blade design. Because of the unsteady flow field around wind turbine blades, prediction of aerodynamic loads with high level of accuracy is difficult and increases the uncertainty of load calculations. An in-house Vortex Lattice Free Wake (VLFW) code, based on the potential, inviscid and irrotational flow, was developed to study the aerodynamic loads. Since it is based on the potential, inviscid and irrotational flow, it cannot be used to predict viscous phenomena such as drag and boundary layer separation. Therefore it must be coupled to tabulated airfoil data to take the viscosity effects into account. Two different turbines, NREL and MEXICO, are used in the simulations. Predicted normal and tangential forces using the VLFW method are compared with the Blade Element Momentum (BEM) method, the GENUVP code and the MEXICO wind tunnel measurements. The results show that coupling to the 2D static airfoil data improves the load and power predictions while employing the dynamic stall model to take the time-varying operating conditions into consideration is crucial.
TOPICS: Stress, Vortices, Blades, Rotors, Wind turbines, Flow (Dynamics), Airfoils, Momentum, Aerodynamics, Separation (Technology), Viscosity, Drag (Fluid dynamics), Simulation, Wakes, Structural dynamics, Boundary element methods, Boundary layers, Design, Engineering simulation, Uncertainty, Turbines, Unsteady flow, Wind tunnels
Technical Brief  
Matthew T. Boyd
J. Sol. Energy Eng   doi: 10.1115/1.4035830
Three grid-connected monocrystalline silicon photovoltaic arrays have been instrumented with research-grade sensors on the Gaithersburg, Maryland campus of the National Institute of Standards and Technology (NIST). These arrays range from 73 kW to 271 kW and have different tilts, orientations, and configurations. Irradiance, temperature, wind, and electrical measurements at the arrays are recorded, and images are taken of the arrays to monitor shading and capture any anomalies. A weather station has also been constructed that includes research-grade instrumentation to measure all standard meteorological quantities plus additional solar irradiance spectral bands, full spectrum curves, and directional components using multiple irradiance sensor technologies. Reference photovoltaic (PV) modules are also monitored to provide comprehensive baseline measurements for the PV arrays. Images of the whole sky are captured, along with images of the instrumentation and reference modules to document any obstructions or anomalies. Nearly all measurements at the arrays and weather station are sampled and saved every 1 second, with monitoring having started August 1, 2014. This report describes the instrumentation approach used to monitor the performance of these photovoltaic systems, measure the meteorological quantities, and acquire the images for use in PV performance and weather monitoring and computer model validation.
TOPICS: Solar cell arrays, Instrumentation, Sensors, National Institute of Standards and Technology, Meteorology, Wind, Electrical measurement, Shades and shadows, Temperature, Solar radiation, Computers, Model validation, Photovoltaic power systems, Silicon
Afroza Nahar, Md Hasanuzzaman and N.A. Rahim
J. Sol. Energy Eng   doi: 10.1115/1.4035818
Photovoltaic module performance decreases significantly with increasing its cell temperature due to overheating. The excess heat can be effectively removed and utilized in various thermal applications. Photovoltaic thermal collector (PVT) is an appropriate technology to harvest both electricity and heat simultaneously that also increase performance of the system to cool down the PV module. This paper investigates a 3D numerical model and simulation to analyze the performance of a PVT collector with pancake shaped flow channel. The flow channel is attached directly to the backside of the PV module by using thermal paste. The governing equations are solved numerically by using finite element method (FEM) with Galerkin's weighted residual technique in the commercial software COMSOL Multiphysics®. The numerical results show that the cell temperature reduces about an average 42oC for both of copper and aluminum with increasing inlet velocity from 0.0009 to 0.05 m/s. It is also found that output power and electrical efficiency increases by 20W and 2% respectively with increasing inlet velocity. On the other hand, overall efficiency of PVT system drops about 13% with increasing the inlet temperature from 20 to 40oC. It was also found that cell temperature increases about 5.4 and 9.2oC for every 100 W/m2 increase in irradiation level of the PV module with and without cooling system respectively. Regarding flow channel material, it has been observed that use of either copper or aluminum produces almost similar performance of the PVT module.
TOPICS: Flow (Dynamics), Heat, Temperature, Copper, Aluminum, Cooling systems, Computer simulation, Irradiation (Radiation exposure), Simulation, Finite element methods, Computer software, Appropriate technology, Electrical efficiency
Technical Brief  
Wongyu Choi, Michael B. Pate, Ryan D. Warren and Ron M. Nelson
J. Sol. Energy Eng   doi: 10.1115/1.4035755
A grid-connected dual-axis tracking photovoltaic (PV) system was installed in the Upper Midwest of the U.S., defined as a cold region, and then evaluated and monitored for a one-year period. This system serves as a real-world application of PV for building energy generation in a region long overlooked for PV research studies. Additionally, the system provides an opportunity for research, demonstration, and education of dual-axis tracking PVs, again not commonly studied in cold regions. In this regard, experimental data for the system were collected and analyzed over a one-year period. During the year of operation, the PV system collected a total of 2,173 kWh/m2, which equates to 5.95 kWh/m2 on average per day, of solar insolation and generated a total of 1,815 kWh, which equates to an energy to rated power ratio of 1,779 kWh/kWp of usable AC electrical energy. The system operated at an annual average conversion efficiency and performance ratio of 11 percent and 0.82, respectively while the annual-average conversion efficiency of the inverter was 92 percent. The performances of the tracking system are also compared to a stationary PV system, which is located in close proximity to the tracking PV system. The tracking system's conversion efficiency was 0.3% higher than the stationary system while the energy generation per capacity was 40% higher although the PV module conversion efficiencies were not significantly different for the two systems.
TOPICS: Cold climates, Performance evaluation, Photovoltaic power systems, Energy generation, Solar energy, Education

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