Torque Tube Research Articles

Field measurements of wind load effects in a photovoltaic single-axis tracker mounting rail

The demand for solar photovoltaic power installations has resulted in a highly competitive industry. Equipment suppliers are under pressure to reduce design margins for projects to be feasible. Single-axis trackers are widely used to increase yield and reduce the levelised cost of energy compared to fixed-tilt installations. Load coefficients on tracker arrays are usually quantified using model-scale studies conducted in atmospheric boundary layer wind tunnels. These dynamically sensitive structures have been known to suffer wind damage despite the use of modern design methods. Ongoing research into the support structure design is deemed important to ensure long-term reliability. The current study presents full-scale field data acquired over 109 days on an experimental one-in-portrait single-axis tracker facility located in the Western Cape, South Africa. The purpose of these measurements was to quantify wind load effects in two instrumented PV module mounting rails. Frequency domain data showed modal participation of both the torsional and bending modes of the torque tube. The effect of the bending mode was more pronounced for the rail furthest from a pile. Load coefficients calculated using experimental data were generally lower than those presented in ASCE 7-22. Fluctuating stresses at critical locations led to negligible fatigue damage.

Evaluation of Composite Structural Materials for Heliostat Cost Reduction

Abstract Structures manufactured from steel comprise up to 40% of a concentrating solar thermal power (CSP) heliostat's cost. Composite structures represent a potential opportunity to reduce this cost. A reference heliostat structural model has been created with a reflector area of 25 m2. The design, constructed of low-carbon steel, provides baseline deflection and stiffness under a 21 m/s operating wind speed. Established roster of suitable metal alternative materials is considered including glass, basalt, and carbon fiber-reinforced polymer (GFRP, BFRP, and CFRP, respectively). Three heliostat components are investigated: the pylon, torque tube, and the purlin–strut assembly. Composite material properties are substituted for those of steel, and the beams are re-sized to match the original steel components’ deflection under given wind loads. Weight and cost changes resulting from this resizing are evaluated. It is found that GFRP and BFRP represent a 3 ×–6 × cost premium for the same operating deflection characteristics as steel across all three investigated component classes; with weight reduction only achieved for the purlin–strut assembly. While CFRP components can achieve approximately 25–75% weight savings depending on the application, this comes with a 9 ×–14 × cost increase over the steel baseline for tube-type structures and roughly 5 × cost increase when replacing c-channel structures. This work does not rule out the possibility of cost savings when the heliostat design and kinematics take advantage of composites' specific properties.

Quantifying Structural Shading and Reflection Effects on Single Axis Tracked Bifacial Photovoltaic System Performance

Structural elements of bifacial photovoltaic (PV) systems, such as module frames, module supports, and torque tubes, affect module rear irradiance profiles through both shading and reflection. Standard bifacial energy yield models represent racking effects as a flat rear irradiance de‐rate factor; however, this neglects 1) the impact of racking reflection, and 2) the variation of racking effects over time. In this work, a differential irradiance approach is presented to quantify the effect of racking shading and reflection on bifacial rear irradiance. Using this method, racking effects on system energy yield, irradiance mismatch losses, and bifacial gain are calculated. This is applied to a single‐axis‐tracked 2‐in‐portrait bifacial PV system in Livermore, California with 74.5% reflective racking using bifacial_radiance ray tracing software. Racking reflection counteracts the irradiance reduction and mismatch losses caused by racking shading. Racking reflection reduces average rear irradiance shading by up to 19.2% per timestamp and 9.1% per year compared to absorptive racking. Racking effects vary diurnally, seasonally, and with respect to albedo. In a single day, the impact of racking on rear irradiance varies up to 14.6%. Applying this method will improve energy yield modeling accuracy of bifacial PV systems.

Cryogenic aspects of a 20 MW class low-temperature superconducting generator for the renewables industry

In this paper we give a progress update on the cryogenic design for cooling a 20 MW class, partially superconducting generator with stationary field coils for offshore wind renewables industry [1-2]. This is a continuation of an earlier program on a 10 MW system dating back ten years. Whereas this new power rating increase leads to a radial diameter expansion of the superconducting field coils from 4 to > 9.5 m, it maintains the axial length. We show the design based on this scaled up with enlarged diameter. The field coil size increase asks for higher cooling power that leads to a bigger cold box size to accommodate the cryocoolers. In the design, as many as 8 cryocoolers can be accessed and serviced from the nacelle that houses the main cryogenic components. A typical thermal load balance sheet with all components is given and compared against the available cryocooler cooling power at different operating conditions. Due to the cryocoolers’ local point of contact cooling within the nacelle, the temperature gradient of the thermal shield that fully encloses the cold mass with its embedded field coils needs to be balanced out to minimize the heat burden on the field coils. This requires additional analytical efforts and implementation of further design features. The extended nacelle houses the cold box with its cryogenic infrastructure. The interface for the envisaged cryogenic pushbutton closed-loop circulating system remains invisible, requires no handling of cryogenic liquids and is hermetically closed. The field coil diameter increase also leads to a greater initial helium gas storage volume. In addition, it requires higher initial room temperature fill pressure within the toroidal helium vapor storage tanks and in cooling tubes connecting to the cryocoolers. The toroidal helium gas tanks are thermally coupled with the thermal shield so that helium convection inside the storage tank can improve heat transfer and reduce the temperature gradient of the thermal shield. Besides those heat transfer challenges, additional mechanical strain within this large structure is exerted on the torque tubes during initial cooldown and when energizing field coils. Some of those design challenges are quite unexpected, leading to novel workarounds in order to maintain the chosen cooling strategy. Finally, we assess those design limitations in view of further cryogenic scalability with emphasis on manufacturability and assembly.

Multi-optimization of rod fastened rotor on the distribution of bending stiffness and unbalance

A rod fastened rotor (RFR), which contains serial disks, rods (or a rod), and a torque tube, is the main form of heavy duty gas turbines and light duty aero engines. The roughness of the interface is the main parameter affecting the bending stiffness of RFR, and the jumpiness of the disks is the intuitive representation of the unbalance of the rotor. In this paper, a multi-optimization algorithm, NSGA-Ⅱ, was used to improve the vibration behavior of the RFR-bearing system. First, the quantitative relation between the bending stiffness of the rotor and the roughness of the interface was set up by combining the Persson contact theory and the Greenwood–Williamson model, and the nondimensional bending stiffness (NS) was proposed as the optimization objective of roughness. Then, the optimization objective of the jumpiness of the disk, (A + B), was rendered based on the minimum unbalance force. Finally, the multi-optimization on the roughness and jumpiness of the RFR was carried out, and the vibration behavior was improved greatly. In addition, this multi-optimization method was applied to a practical RFR, and the experimental results showed that the vibration amplitude was reduced by about 82%. The work in this paper can provide the guidance for vibration reduction of the rotor system.

Design Modification of Mechanical Flight Control Circuits of an Intermediate Aerobatic Trainer Aircraft for Improved Efficiency

In a typical Intermediate Aerobatic Trainer Aircraft (IATA) the primary control surfaces viz, Ailerons, Elevator and Rudder are operated mechanically and thus called Mechanical Flight Control System (MFCS) consisting of elements such as push-pull rods, levers, torque tubes etc. Design considerations for such MFCS include strength, stiffness, ergonomics, low circuit friction, kinematic feasibility etc. During the developmental flight trials, in one of the prototype aircraft, it was found that full control surface movement was not achieved under maximum aerodynamic load. This led to the reduction in efficiency of the system. Based on the analytical calculations and Finite Elemental (FE) simulations, it was found that considerable elastic deformation of mechanical elements as well as support structure had lowered the efficiency which was also validated experimentally. Although, aircraft handling qualities were found to be satisfactory during the normal flight testing, it was desirable to improve the efficiency of the MFCS. This paper broadly outlines the methodologies adopted to improve the efficiency of the existing control circuits considering aero-elasticity and structural design aspects

Torsional failure analysis of C/SiC and SiC/SiC combined CMC torque tube

To overcome the inherent limitations of the ceramic matrix composite (CMC) process and increase the torque capacity of CMC torque tubes, this study investigated the failure causes of 2D woven chemical vapor infiltration (CVI) C/SiC and SiC/SiC combined CMC torque tubes. The CT test was used to describe the non-homogeneous density distribution of the CMC torque tubes. Using the Archimedes drainage method to evaluate density and porosity, we simulated the stress distribution and failure strength of CMC torque tubes using an FEM model. Fixtures suitable for universal material testing apparatuses were used for CMC torque tube torsional tests. The stress-strain curves showed that two distinct fiber types of CMC torque tubes displayed different torsional tendencies, and we examined the main causes of failure. The SiC fiber in the CMC torque tube increased maximum shear stress and modulus. Additionally, the strength of the SiC/SiC torque tube, rather than the interface between the C/SiC and SiC/SiC torque tubes, was the primary cause of combined CMC torque tube failure.

Mechanical Stress Prediction of an Aircraft Torque Tube Based on the Neural Network Application

The use of a predictive approach in the aviation industry is an important factor in aircraft maintenance. The main goal of this study was to create a new method for stress prediction during the operation of parts and to apply it on an aircraft torque tube (ATT). The method operates in real time during taxiing, takeoff, and landing using a neural network (NN). The stress calculated by the proposed method can be used in the future to calculate fatigue life and to save maintenance costs related to ATTs. This can play an important role in the evaluation of tests, such as unobserved crack failure. The main contribution of the presented methodology is in the fourth part of this study, where a new method of mechanical-stress prediction using a NN is described. The method essentially replaces finite element methods (FEMs), which require large amounts of time. The new method is much faster than commonly available methods, as the NN predicts the mechanical ATT stress in 0.00046 s, whereas the solution time using FEM is 1716 s for the same load step. In total, 36 regimes were calculated by FEMs in 17 h, 9 min and 36 s, whereas the novel method calculated the ATT stress for 36 regimes in 0.0166 s. The accuracy was also high, with R above 0.99. The main innovation presented in this study is the development of a method that can predict ATT stress in a very short time with a high percentage of accuracy and that can be used for stress and life prediction during the operation of parts. The partial results from the experimental tensile tests are also presented, and they are used for FEM calculations. The FEM results are used as inputs for the stress prediction by the NN.

Impact of torque tube reflection on bifacial photovoltaic single axis tracked system performance.

Among the racking elements of bifacial photovoltaic (PV) single-axis tracked systems, the torque tube (TT) introduces the most shading and reflection, increasing irradiance nonuniformity and electrical mismatch loss. We simulate the impact of TT shading and reflection on the irradiance profiles, electrical mismatch, and energy yield for central bifacial PV modules on one-in-portrait (1P) and two-in-portrait (2P) single-axis trackers. TT reflection increases annual irradiance in 1P and 2P systems by 0.17% and 0.30%, respectively. Overall, TT reflection increases the predicted instantaneous energy yield by up to 0.8% and 0.4%, and the annual energy yield by 0.11% and 0.18% in 1P and 2P systems, respectively.

A novel and predictive process of manufacturing 3DN C/SiC torque tube

In this research, a novel method of CVI + RMI 3DN C/SiC torque tube preparation has been researched. CVI + RMI 3DN C/SiC flat panel was evaluated by Archimedes drainage method for density and open porosity, SEM for morphological characterization, XRD for phase composition characterization, and chemical method for composition mass and volume content. It has an average density of 2.19 g cm−3, and open porosity of 10%. The computed chemical composition method findings are undoubtedly in line with the fiber design volume percentage of 30%. Tensile and shear mechanical tests on 3DN C/SiC standard sample were investigated, with good performance. The average tensile and shear strength was 141.64 MPa and 86.24 MPa respectively. 3DN C/SiC torque tubes were prepared by the same process, and the torsional mechanical tests were carried out. According to the mechanical parameters of the flat panel 3DN C/SiC sample, the mechanical properties of stress distribution and shear stress-strain curves for the torque tube are predicted by using the finite element simulation method, which is in good agreement with the test results. The test and simulation error of the maximum shear strength is only about 2%.

A stress disturbance barrier on a circular torque tube blocking the end effects

AbstractKnowing that the end effects arising from the end torsional loads will disturb the stress distribution in a torque tube, this paper develops a means of blocking the stress disturbances from the end where a torsional load is applied, such that the stress distributions of the Saint-Venant torsion can be maintained. The stress disturbance barrier (SDB) as an end effect shielding barrier is considered installed on an end of a circular toque tube whose material property can be with the cylindrical orthotropy. In general, the SDB is comprised of two layers: the front layer and the blocking layer. The front layer, which is optional and directly contacted with the external load, is for enhancing the durability, while the blocking layer situated between the front layer and the torque tube body is mainly for blocking the stress disturbances. The analysis is based on the state-space formalism, whose uncoupling feature on cylindrically orthotropic materials is beneficial for the further extension of the research. It is found that the parameters crucial to determine an effective SDB with a given thickness are the cylindrical orthotropy parameter and the longitudinal shear modulus. The approach of how to determine the proper material constants for having an effective SDB is clearly provided. As shown by the numerical results presented in the examples, an effective SDB of this research can fully block the stress disturbances from the external torsional loads that can be mathematically expanded by power series.

Torsion and flexural-torsional coupled mechanical properties of different C/SiC torque tubes at room and elevated temperatures

In this paper, the torsion and flexural-torsional coupled mechanical properties of different C/SiC torque tubes were investigated for the testing condition at room and elevated temperatures. Effects of fiber types, fiber preforms, and small hole during fabrication process on torsion mechanical properties were investigated. Flexural-torsional coupled mechanical tests for C/SiC torque tubes with different external diameter and wall thickness were conducted at room and elevated temperatures. The torsion and flexural moments and corresponding shear and flexural strength were obtained. The fracture surface and cracks propagation path were observed and analyzed. The torque and shear strength in T300™-3k torque tube were much higher than those of T300™-1k torque tube. Among 3D needled (3DN), 2D plain-woven [0°/90°] and [±45°] C/SiC torque tubes, the density, torque, and shear strength of 3DN-C/SiC torque tube were the highest. For the C/SiC torque tubes with small hole, the small hole not only increased the densification and uniformity (axial and radial) of the torque tube, but also has the potential to make the damage cracks more zigzag, which improved the fracture toughness of the torque tubes.

A Spoke-Supported Superconducting Rotor With Rotating Cryocooler

This article presents the design of a superconducting (SC) rotor with a spoke supported structure that minimizes the heat loss and enables the motor to be cooled by one cryocooler. The main challenge with the SC machines is the need to maintain the SC coils at cryogenic temperatures. Though usually placed in the vacuum, the cryogenic coils still receive a large amount of heat through the conduction of the torque tube which connects to the ambient temperature. The heat flown into the system exceeds the cooling capacity of a cryocooler and requires the boiling of cryogen, which comes with a more complicated and bulky hardware system. The SC rotor proposed in this article uses a spoke system to support the cryogenic windings. Connecting the windings to the shaft, the flexible spokes with low thermal conductivities transfer the torque from the windings to the shaft. The spoke system has much lower thermal conductivity than a traditional torque tube. Preliminary results show that with the ambient temperature 298 K and the cryogenic temperature at 40 K, the total heat transfer rate from the ambient environment to the cryogenic environment is less than 0.1 W, within the capability of a cryocooler that can be mounted on the rotor. The cryocooler is arranged concentrically with the shaft of the rotor. Thermal straps connecting the cold end of the cryocooler to the windings to conduct the heat. An analytical model is built for the rotor. The rotor’s mechanical property and its thermal behavior are analyzed jointly so that the spoke system is optimized for high stress and lightweight. A physical model is also built to test the cryocooler performance in the rotational environment. A previous study shows that the sterling-cycle cryocooler performance has not deteriorated at 1800 rpm at the temperature of 100 K. The proposed design involves a lower operating temperature at 40 K and a high vacuum environment surrounding the cold head.

A Novel and Predictive Process of Manufacturing 3dn C/Sic Torque Tube

A Novel and Predictive Process of Manufacturing 3dn C/Sic Torque Tube

Fluid-structure interaction analysis of a lightweight sandwich composite structure for solar central receiver heliostats

Central tower concentrating solar power systems are moving to the forefront to become the technology of choice for generating renewable electricity, but their widespread implementation is limited by cost. Heliostats contribute almost 50% to the plant’s cost and are thus the most significant element in central tower systems. For both large and small-area heliostats, the drive elements demonstrate the largest cost element in these systems. While large-area heliostats (>100 m2) have proven offer the best economy compared to other sizes, they require high-torque drives due to the heavy steel-based support structure. Heliostat costs could be reduced by decreasing the support structure’s weight, avoiding large drive units and reducing energy consumption. However, the structure must be able to cope with the aerodynamic loads imposed upon them during operation. Although honeycomb sandwich composites have been widely used where high structural rigidity and low weight are desired, there is an absence of studies that rigorously investigated their suitability as the structure for heliostat mirror. Here, a fluid-structure interaction study investigated, for several loading conditions at various tilt and wind incidence angles, the aero-structural behavior characteristics of honeycomb sandwich composites used as a heliostat support structure. The honeycomb sandwich panel showed markedly different behavior characteristics at various operational conditions. The effect of tilt orientation on the sandwich panel’s maximum deflection and stresses became more pronounced as wind velocity increased above 10 m/s, and increasing wind incidence angle reduced their magnitudes at different rates. The supporting components and torque tube had a noticeable wind-shielding effect, causing pronounced changes in the deflection and stresses experienced by the heliostat. The worst operational condition was at a tilt angle of 30° with wind flow of 20 m/s at 0° to the heliostat surface. However, the heliostat maintained its structural integrity according to relevant optical and material failure standards.

Modeling Delamination in NSTX-U Inner Leg Using “EKILL”

The possibility of thermal strain–induced delamination was anticipated in the original design of the NSTX-Upgrade (NSTX-U). Current “bunching” at the toroidal field (TF) flag causes nonuniform Joule heating of the TF conductor during a shot. This produced through-thickness tensions beyond the measured capacity of the insulation bond. This however occurred where the torsional shear was at a minimum, and the Upgrade design progressed with the understanding that delamination at the core of the TF flag might occur. During the Recovery project design reviews, concern over the extent of delamination was elevated. In various early simulations, the tensile stresses reached 50 MPa. With more accurate through-thickness insulation modulus and thickness, and at end-of-flattop versus end-of-pulse temperature, the tensile stress goes down to 25 MPa, and possibly lower based on higher-fidelity modeling. Insulation delamination has been predicted analytically in coils in projects other than NSTX-U, and indications of delamination have been observed in some machines. Out-of-plane loads on the NSTX TF coil produce local and global twisting of the tokamak. The inner leg supports part of the torsion as a torque shaft or tube. Peak torsion is at the outside radius of the TF central column, away from the regions that don’t carry current, experience less Joule heat, and develop tension. Testing of the shear and tensile fatigue properties of the CTD-425 system was repeated and was a part of this requalification effort. This paper addresses simulations done in ANSYS based on a simple assumption that when the tension and shear stresses exceed an allowable, the elements are “killed” or considered much less capable of carrying tension and shear loads established by fatigue S-N tests. A competing and complementary method, the Virtual Crack Closure Technique (VCCT) was used to augment and validate the EKILL procedure. Determination of Paris constants to support the VCCT analysis is described Fig. 8.

EVALUATE AND RECTIFICATE THE PERFORMANCE OF THE SPIRAL PUMP OF THE HOLLY KARBALA SEWAGE TREATMENT STATION

This paper evaluates the performance of the primary spiral (Archimedes) screw pump in the old sewage treatment station of the holly Karbala. This paper evaluates the pump according to its volumetric flow rate, determines the problems, diagnoses the defects, and finds the solutions. In addition, the paper suggests the optimum design for the pump. The results show a big effect of the side profile, pitch, and radius of the torque tube upon the discharge of the pump.

Parameterizing mismatch loss in bifacial photovoltaic modules with global deployment: A comprehensive study

The effective cost reduction of solar photovoltaic (PV) power generation systems is supported by the widespread use of bifacial modules. To this end, the multiple bifacial modellings are expected to measure the energy yield excluding the mismatch power loss induced by self-shading. But, given the magnitude of non-uniform irradiance may differ between the monofacial and bifacial modules, the conventional full cell-level electrical model is no longer suitable for fast-estimation of bifacial ones. Hence, a fundamentally new equation is reformulated by parameterizing rear-shading mismatch losses (ML) optimally as a function of irradiance variability (standard deviation σ), whereby implicitly taking the effect of rear-shading into account. In Taizhou, the annual ML is found to be as high as 0.8% for close mounts (0.2 m) with a 0.2 albedo and a 22° tilt angle. Panels fixed with or without torque tube have been compared numerically. After the hourly rear-inhomogeneity and configuration dependence on cumulative annual ML of several global-mounted bifacial modules were analyzed. The authors improved the fit with an R2 better than 0.9 for generalization to any worldwide scenarios. Both experimental results and inter comparison with prior empirical equations indicate a better goodness of fits, especially in high-mismatch conditions. In this paper, the presented mismatch model will guide the deployment of revised bifacial gain (BGr)-maximized bifacial systems around the world.

Influence of inertia and aspect ratio on the torsional galloping of single-axis solar trackers

Single-axis solar trackers are currently one of the cheapest systems for electricity generation. However, they may have to face significant maintenance costs depending on environmental and climatic factors. Weather is believed to provoke approximately half of the damages registered in solar tracker systems, and a large part of them are due to dynamic wind load. Torsional galloping – or, more precisely, flutter with one degree of freedom – is a phenomenon that arises when the wind speed exceeds a certain critical value. It causes the tracker to undergo angular oscillations with increasing amplitude until the structure collapses. The phenomenon is intrinsically linked to geometric and structural parameters, some of which exhibit a wide range of variation in the current market, depending on the configuration and design of the trackers. This article presents an analytical and experimental study on how the onset of torsional galloping is influenced by the inertia of the modules and the aspect ratio of the panel; it also includes the effect of the torque tube stiffness. The analytical study starts from the equation of motion involving the aeroelastic derivatives and the torque equation in differential form. Tests have been conducted on aeroelastic models of the structures of interest. It has been found that the critical reduced velocity of galloping changes with tilt angle, but it is essentially independent of the main structural parameters: torsional stiffness, inertia and aspect ratio. The results are finally presented in a Stability Diagram for the correct and optimal dimensioning of these structures against torsional galloping.

Failure analysis of gas turbine torque tube bolts

The cracking of torque tube bolts of a 130 MW gas turbine is investigated in the current work. Visual inspections along with microstructural and finite element analyses were performed for determining the cracking mechanisms. It was observed that the fracture surface was intergranular in appearance indicating crack growth along the grain boundaries. Furthermore, finite element results indicated that crack growth direction was relatively in accordance to the simulated stress distribution in the bolt. Microstructural analysis was performed and a continuous M23C6 carbide layer was observed along the grain boundaries. Although it was not clear what mechanism was responsible for the cracking, one potential mechanism of the cracking could be stress corrosion cracking (SCC). Remedial actions are proposed to minimize future occurrences of cracking.