Hydrodynamic and control optimization for a heaving point absorber wave energy converter

The thesis aims at studying the non-linear performance of a designed 1/50 scale point absorber wave energy converter (PAWEC) in heave motion. In particular, designs of the PAWEC geometry and control strategy are considered to optimize the power capture. Experimental and computational fluid dynamics (CFD) data are provided to evaluate the studies. Specifically, this thesis can be summarized into four parts.Firstly, a numerical wave tank (NWT) is constructed in a commercial CFD package ANSYS/LS-DYNA. The main objective associated with the NWT is to closely reproduce the physical wave-PAWEC interactions. To achieve this, physical experimental data from two specified WECs are provided to verify the capability of the NWT. One of the devices is the PAWEC designed at University of Hull. Free decay, excitation force and water splashing tests, etc., are conducted. As a result, the developed NWT is validated to be capable of representing the non-linear behaviors of the PAWEC compared with the costly physical experiments. The second part focuses on investigating the extent to which the non-linear hydrodynamic characteristics of the PAWEC need to be considered. By comparing with the CFD data from a series of tests, a non-linear mathematical modeling involving a quadratic viscous term is verified. The results show that the non-linear PAWEC behavior for the conditions of large oscillations (e.g., near resonance or at a large wave heights) can only be predicted realistically by considering a correct viscous effect. This study highlights that the linear counterpart derived from potential flow code ANSYS/AQWA fails to describe the PAWEC behavior and would mislead the control strategy and power take-off (PTO) designs. Additionally, the results show that the viscous damping is significantly larger than the inviscid radiation damping for the flat-bottom cylindrical heaving PAWEC.It is found that the viscous effect can induce clear energy losses during device oscillations which is unwanted for a PAWEC system. Therefore, in the third part, besides the originalflat-bottom PAWEC, two streamline-bottom counterparts are proposed to improve the capability of power capture. Free motion tests are conducted in the NWT regarding the three different geometric devices. The results indicate that for the streamlined devices, the added mass and hydrodynamic damping decrease by up to 60% compared with the flat-bottom device. More importantly by simulating PTO system in the NWT, it is found that there exists a clear mutual interaction among the designs of the device geometry and PTO damping. Applying a proper PTO damping to the streamlined PAWEC can prominently improve the optimal power absorption efficiency by up to 70% underboth regular and irregular waves, compared with the flat-bottom PAWEC. Finally, a fuzzy logic control strategy with particle swarm optimization algorithm (PSO-FLC) is implemented on the developed non-linear modeling to adaptively tune the PTO damping for power absorption maximization. The fuzzy rule base is initialized according to the power capture characteristics achieved through the NWT tests. PSO algorithm is then used to search for more efficient rules. It is found that applying a well-designed fuzzy inference system can adaptively tune the PTO damping for power capture optimization in contrast to the passive control with constant PTO damping

Nonlinear Modeling and Verification of a Heaving Point Absorber for Wave Energy Conversion

Although the heaving Point Absorber (PA) concept is well known in wave energy conversion research, few studies focus on appropriate modelling of non-linear fluid viscous and mechanical friction dynamics. Even though these concepts are known to have non-linear effects on the hydrodynamic system, most research studies consider linearity as a starting point and in so doing have a weak approach to modelling the true dynamic behaviour, particularly close to resonance. The sole use of linear modelling leads to limited ability to develop control strategies capable of true power capture optimisation and suitable device operation. Based on a 1/50 scale cylindrical heaving PA, this research focuses on a strategy for hydrodynamic model development and experimental verification. In this study, nonlinear dynamics are considered, including the lumped effect of the fluid viscous and mechanical friction forces. The excellent correspondence between the derived non-linear model and wave tank tested PA behaviours provides a strong background for wave energy tuning and control system design

Viscosity effect on a point absorber wave energy converter hydrodynamics validated by simulation and experiment

To achieve optimal power in a wave energy conversion (WEC) system it is necessary to understand the device hydrodynamics. To maximize conversion efficiency the goal is to tune the WEC performance into resonance. The main challenge then to be overcome is the degree to which non-linearity in WEC hydrodynamics should be represented. Although many studies use linear models to describe WEC hydrodynamics, this paper aims to show that the non-linear viscosity should be carefully involved. To achieve this an investigation into the hydrodynamics of a designed 1/50 scale point absorber wave energy converter (PAWEC) in heave motion only is implemented to indicate the non-linear viscosity effect. A non-linear state-space model (NSSM) considering a quadratic viscous term is used to simulate PAWEC behaviors. The non-linear model is compared with the linear counterpart, and validated by computational fluid dynamics (CFD) and experimental data. A conclusion is drawn that the non-linear PAWEC hydrodynamics (including amplitude and phase responses, conversion efficiency) close to resonance or at high wave heights can only be described realistically when the non-linear viscosity is correctly taken into account. Inaccuracies in its representation lead to significant errors in the tuning procedure which over-predict the dynamic responses and weaken the control system performance

Energy expenditure of type-specific sedentary behaviors estimated using sensewear mini armband: a metabolic chamber validation study among adolescents

SenseWear Mini Armband, an accelerometer with multiple physiological sensors, could be a practical means to estimate energy expenditure (EE) of children and adolescents, but its validity reported for these age groups has not been consistent within the literature. EE of twenty-six healthy Chinese 12-year-old adolescents was measured simultaneously using both SenseWear Mini Armband (SWMA) and metabolic chamber (MC) during a 16-hour stay in a MC. SWMA systematically underestimated the adolescents’ EE during sedentary behaviors, resting metabolic rate (RMR), basal metabolic rate (BMR), and total EE, with the absolute error rate ranging from 14.85% to 28.65%. The SWMA significantly underestimated EE compared with MC in Chinese adolescents. However, the amount of error can be reduced by applying correction equation proposed in this study

UK perspective research landscape for offshore renewable energy and its role in delivering net zero

Acknowledgements This work was conducted within the Supergen Offshore Renewable Energy (ORE) Hub, a £9 Million programme 2018–2023 funded by Engineering and Physical Sciences Research Council (EPSRC) under grant no. EP/S000747/1.Peer reviewedPublisher PD

On the use of constrained focused waves for characteristic load prediction

Physical experiments investigating the extreme responses of a semi-submersible floating offshore wind turbine were conducted to allow a comparison of design wave methods. A 1:70 scale model of the IEA 15MW reference turbine and VolturnUS-S platform was studied focusing on the hydrodynamics under parked turbine conditions. A comparison of characteristic load predictions was made between design standard recommendations by the IEC and DNV covering different design wave types and post processing methods. Constrained waves are permitted for predicting characteristic loads for fixed offshore turbines but the extent to which they are suitable for floating devices is questionable. A constrained wave method for characteristic load prediction is applied and it is concluded that in general characteristic responses related to pitch may be estimated well with single response conditioned focused waves but for response types where the low frequency surge is important, e.g. mooring loads, constrained focused waves need to be applied

Extreme responses of a hinged raft type wave energy convertor

Much attention has been paid in recent years to the determination of design loads for moored floating structures and the application of established methodologies for fixed structures have been found to be ineffective. This paper experimentally investigates extreme responses of a lazy S moored 1:20 scale model of Mocean Energy’s Blue Star wave energy convertor (WEC) along the 1 year return contour. The device is a hinged raft type WEC and the extreme responses studied include mooring loads and snatch load events. Long irregular wave time series are used in the estimation of extreme value distributions of the mooring load for particular sea states. Conditional random response wave and constrained new wave profiles are used to study and support the predictions. Wave calibration and the impact of wave breaking on the physical realisation of response conditioned focus waves and extreme value distribution (EVD) predictions are discussed

Analysis of COVID-19 Guideline Quality and Change of Recommendations: A Systematic Review.

Background Hundreds of coronavirus disease 2019 (COVID-19) clinical practice guidelines (CPGs) and expert consensus statements have been developed and published since the outbreak of the epidemic. However, these CPGs are of widely variable quality. So, this review is aimed at systematically evaluating the methodological and reporting qualities of COVID-19 CPGs, exploring factors that may influence their quality, and analyzing the change of recommendations in CPGs with evidence published. Methods We searched five electronic databases and five websites from 1 January to 31 December 2020 to retrieve all COVID-19 CPGs. The assessment of the methodological and reporting qualities of CPGs was performed using the AGREE II instrument and RIGHT checklist. Recommendations and evidence used to make recommendations in the CPGs regarding some treatments for COVID-19 (remdesivir, glucocorticoids, hydroxychloroquine/chloroquine, interferon, and lopinavir-ritonavir) were also systematically assessed. And the statistical inference was performed to identify factors associated with the quality of CPGs. Results We included a total of 92 COVID-19 CPGs developed by 19 countries. Overall, the RIGHT checklist reporting rate of COVID-19 CPGs was 33.0%, and the AGREE II domain score was 30.4%. The overall methodological and reporting qualities of COVID-19 CPGs gradually improved during the year 2020. Factors associated with high methodological and reporting qualities included the evidence-based development process, management of conflicts of interest, and use of established rating systems to assess the quality of evidence and strength of recommendations. The recommendations of only seven (7.6%) CPGs were informed by a systematic review of evidence, and these seven CPGs have relatively high methodological and reporting qualities, in which six of them fully meet the Institute of Medicine (IOM) criteria of guidelines. Besides, a rapid advice CPG developed by the World Health Organization (WHO) of the seven CPGs got the highest overall scores in methodological (72.8%) and reporting qualities (83.8%). Many CPGs covered the same clinical questions (it refers to the clinical questions on the effectiveness of treatments of remdesivir, glucocorticoids, hydroxychloroquine/chloroquine, interferon, and lopinavir-ritonavir in COVID-19 patients) and were published by different countries or organizations. Although randomized controlled trials and systematic reviews on the effectiveness of treatments of remdesivir, glucocorticoids, hydroxychloroquine/chloroquine, interferon, and lopinavir-ritonavir for patients with COVID-19 have been published, the recommendations on those treatments still varied greatly across COVID-19 CPGs published in different countries or regions, which may suggest that the CPGs do not make sufficient use of the latest evidence. Conclusions Both the methodological and reporting qualities of COVID-19 CPGs increased over time, but there is still room for further improvement. The lack of effective use of available evidence and management of conflicts of interest were the main reasons for the low quality of the CPGs. The use of formal rating systems for the quality of evidence and strength of recommendations may help to improve the quality of CPGs in the context of the COVID-19 pandemic. During the pandemic, we suggest developing a living guideline of which recommendations are supported by a systematic review for it can facilitate the timely translation of the latest research findings to clinical practice. We also suggest that CPG developers should register the guidelines in a registration platform at the beginning for it can reduce duplication development of guidelines on the same clinical question, increase the transparency of the development process, and promote cooperation among guideline developers all over the world. Since the International Practice Guideline Registry Platform has been created, developers could register guidelines prospectively and internationally on this platform

Satellite On-Orbit Anomaly Detection Method Based on a Dynamic Threshold and Causality Pruning

It is difficult for existing deep learning-based satellite on-orbit anomaly detection methods to define the residual-based detection threshold and identify false anomalies. To solve the above problems, this paper proposes both a detection threshold determination and dynamic correction method and a causality-based false anomaly identification and pruning method. We use the GRU (Gated Recurrent Unit) to model and predict the telemetry parameters to obtain the residual vector; determine and dynamically correct the threshold according to the prescribed false positive rate; propose an improved multivariate transfer entropy method to identify the causal relationships between the telemetry parameters; and, based on the causality, determine whether the detected parameter anomalies are false. Experiments show that the precision, recall, and F1-score of the method proposed in this paper are superior to the current typical method, and the false positive rate is significantly reduced, demonstrating the effectiveness of the proposed method

Enhancement of wave energy absorption efficiency via geometry and power take-off damping tuning

In this work a three dimensional computational fluid dynamic (CFD) model has been constructed based on a 1/50 scale heaving point absorber wave energy converter (PAWEC). The CFD model is validated first via wave tank tests and then is applied in this study to investigate the joint effects of device geometry and power take-off (PTO) damping on wave energy absorption. Three PAWEC devices are studied with the following geometrical designs: a cylindrical flat-bottom device (CL); a hemispherical streamlined bottom design (CH) and a 90°-conical streamlined bottom structure (CC). A PTO force via varying damping coefficient is applied to compare the power conversion performances of the aforementioned devices. Free decay, wave-PAWEC interaction and power absorption tests are conducted via the CFD model. The results show that for CH and CC designs the added mass and hydrodynamic damping decrease by up to 60% compared with the CL device. Moreover, the CC design is the best of the three structures since its amplitude response increases by up to 100% compared with the CL. Applying an appropriate PTO damping to the CC device prominently increases the achievable optimal power by up to 70% under both regular and irregular waves (compared with the CL device)