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In modern large-scale wind farms, power loss caused by the wake effect is more than 30%, and active yaw control can greatly reduce the influence of the wake effect by deflecting the wind turbine's wake. The yawed wind turbine's wake characteristics are complex, and a deep comprehension of a yawed turbine's wake is necessary. The actuator line method combined with URANS (unsteady Reynold-averaged Navier–Stokes equations) is used to study the yawed wind turbine's wake characteristics in this paper. Compared with an un-yawed wind turbine, a yawed one has two main characteristics, deflection and deformation. With an increasing yaw angle, turbine wake shows an increasing deflection. The results indicated that deflection at different height was different, the wake profile showed the biggest deflection at about the hub height, while the smallest deflection existed at the top and bottom of the yawed turbine's wake. This can be visually demonstrated by the evolution of a kidney-shape velocity distribution at the vertical cross-section. Two-dimensional and three-dimensional presentations of velocity deficit distributions are presented in this paper. The evolution of an irregular kidney-shape distribution is discussed in this paper. It is formed by the momentum exchange caused by the counter-rotating vortex pair. The results indicated that the counter-rotating vortex pair was composed of the streamwise vortex flux brought by the tip vortex. Furthermore, when the wind turbine rotated clockwise and yawed clockwise, the negative vorticity of counter-rotating vortex first appeared in the upper left position. [ABSTRACT FROM AUTHOR]

Exergy analysis are carried out on a novel 100 kW wind-to-heat system to investigate the thermodynamic performances by employing the experimental data. Nine different combinations of the wind speed and water flow rate are selected as the typical operating conditions. Through the conventional exergy analysis, the wind turbine accounts for the most of the system's total exergy destruction at high wind speeds. In terms of the heat pump subsystem, the compressor is the most critical component, followed by the condenser. For further obtaining the optimal design directions of the key components, the advanced exergy analysis is applied to this system. By establishing the mathematical model, the exergy analysis of the ideal conditions, experimental conditions and unavoidable conditions are compared. Averagely 87.32% exergy destruction of the compressor is endogenous and avoidable, which can be decreased by optimizing the internal structure parameters. The average proportions of the avoidable exergy and endogenous exergy destruction of the condenser are 57.48% and 68.41%, respectively. Both the internal parameters of the condenser and interactions with other components must be taken into consideration. The optimizing potential of the wind-to-heat system through exergy analysis methods can be used for further research and improvement of the system. • Exergy analysis is utilized in a novel wind-to-heat system. • A mathematical model is established for the advanced exergy analysis. • The wind turbine is of the most exergy destruction in the system at high wind speed. • The most critical part in the heat pump subsystem is the compressor. • The compressor and the condenser appear great improvement potentials. [ABSTRACT FROM AUTHOR]

Offshore wind energy is booming and floating offshore wind has been proved to be a promising clean energy. Wake effect is one of the biggest challenges in large-scale floating wind farm which results in power loss and increased fatigue load. In this paper, actuator line model (ALM) coupled with unsteady Reynolds-averaged Navier-Stokes (URANS) is adopted to investigate the wake characteristics and evolution of floating wind turbine under motion. Besides, modal analysis is adopted to extract typical features of turbine wake. Velocity fluctuation is the prominent characteristic of floating turbine wake. The increase of surge frequency results in the decrease of velocity fluctuating region and large surge amplitude leads to bigger velocity deficit fluctuations. In addition, the surge motion brings a faster recovery rate than fixed wind turbine. The induced velocity field generated by vortex ring structure is the key to understanding complex evolution of turbine wake and the vortex ring structure is analyzed in this paper. With the deep understanding of FOWT wake, an innovative vortex ring structure model is proposed in this paper. This work is the basic work of developing the engineering wake model of floating wind turbine, to cope with the wake effect challenge in the large-scale floating wind farm. • Wake characteristics and vortex structure is analyzed using actuator line method. • Wake evolution mechanism is clearly revealed through modal analysis. • Instability theory of helical vortices is introduced into the study of turbine wake. • A semi-empirical vortex structure model is proposed based on theoretical analysis. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines, as large equipment for deep-sea wind energy utilization, must be carefully studied for their aerodynamic performances under platform motions. In experiments, the model wind turbine designed by the traditional thrust coefficient similarity is usually insufficient for unsteady aerodynamic reproductions of the prototype wind turbine. In this paper, a dynamic aerodynamic similarity criterion based on the mapping of the optimal tip speed ratio is introduced for wind tunnel model tests, and a model wind turbine is designed using the NREL 5-MW prototype wind turbine. In this case, to avoid the over-height model tower aroused in pitch motions, a method of equivalent surge motions instead of pitch motions is proposed and validated for feasibility. Then dynamic aerodynamic similarities under different pitch amplitudes and frequencies are validated by the Unsteady Blade Element Momentum theory. Results indicate that temporal and spatial characteristics and hysteresis effects of thrust and power coefficients all maintain better similarity with those of the prototype wind turbine than those of the model wind turbine designed by the traditional criterion. This paper provides a criterion with equivalent pitch motions for unsteady aerodynamic studies in wind tunnel model tests, especially the high-fidelity Hardware/Software-In-the-Loop test. • A new similarity criterion is proposed for FOWT wind tunnel model tests. • The feasibility of equivalent surge motions instead of pitch motions is verified. • The problem of over-height model tower is avoided by equivalent pitch motions. • Spatiotemporal and hysteresis characteristics of unsteady aerodynamics are analyzed. • Unsteady aerodynamic performances of the prototype can be reflected more accurately. [ABSTRACT FROM AUTHOR]

Model tests and real-time hybrid simulation tests of floating offshore wind turbines have recently been extensively conducted to explore the aero-hydro-servo-elastic coupling mechanism. The traditional thrust similarity criterion under the Froude scale cannot satisfy the dynamic aerodynamic similarity in the wave basin experiments. Meanwhile, the traditional model wind turbine in model tests shows huge differences in Reynolds number and significant deviations in the optimal tip speed ratio, and the dynamic performance is different from that of the prototype wind turbine. Therefore, a new criterion based on the mapping of the optimal tip speed ratio is proposed, and a new model wind turbine is designed to physically reproduce the dynamic aerodynamics of the prototype wind turbine accurately in wind tunnel experiments. Then the dynamic aerodynamic similarity in this criterion is numerically verified and compared with that in the traditional criterion. The simulation is performed in steady and unsteady inflows by the open-source OpenFAST. The platform motions of the prototype and model wind turbine are inflow-motivated and program-forced, respectively. The calculation results show that the dynamic thrust, power and their coefficients in the new criterion all maintain better similarity compared to those in the traditional criterion. This study is important for wind tunnel model tests and RTHS tests to accurately obtain unsteady aerodynamics, and it can guide the aero-hydro-structure load evaluations and turbine-floater-mooring integrated designs for large-scale floating offshore wind turbines. • A new similarity criterion is proposed for FOWT's wind tunnel model tests. • Dynamic aerodynamics under wind-wave inflows and coupled motions are simulated. • Motions of prototype and model are inflow-motivated and program-forced respectively. • Similarity of dynamic thrust, power and their coefficients are analyzed and verified. • The criterion is important for RTHS tests to accurately obtain unsteady aerodynamics. [ABSTRACT FROM AUTHOR]

The coupling effects between rotor aerodynamics and platform motions are being focused for floating offshore wind turbines (FOWTs). Ignoring these effects during the controller design can deteriorate some essential loads or lead to aero-elastic instabilities. Superimposed proportional model-free adaptive control (SP-MFAC) is proposed to cope with these effects by simultaneously optimizing blade root moments, rotor speed, platform-pitch and -yaw motions. Trailing edge flap (TEF) is used as the active flow control device. Actuators' saturation and energy consumption are considered by introducing penalty factors into control cost function. A typical semi-submersible FOWT is selected for simulation study. A comparison is made against proportional-integral-differential-based (PID-based) TEF control and baseline control in turbulent and extreme gust conditions. Numerical results reveal that SP-MFAC costs less in TEF deflection activity, but realizes better overall performance than PID-based TEF control. Further, the physical mechanisms of load reduction are analyzed with cross-wavelet transformation. The original in-phase aero-hydro-elastic mechanisms in the semi-submersible FOWT are crippled by the TEF deflection activity. • SP-MFAC is presented to reduce loads on floating wind turbines. • The main rotor-platform coupling effects are considered during controller design. • The potential of trailing edge flap for damping platform motions is explored. • SP-MFAC costs less TEF deflection activity but realizes better performances than PID. • The physical mechanisms of load reductions are analyzed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The concept of wind-to-heat system is studied. • A Matlab model is developed to simulate the system. • The simulation model is verified compared with the experimental data. • A suitable hot water flow rate and refrigerants for the system are selected. The growing demand for energy and the pressure of carbon emission increase the proportion of renewable energy in the global energy consumption. In the wind-to-heat system, the compressor is utilized to replace the generator in the wind turbine to allow the wind energy driving heat pump directly, which simplifies the energy conversion process. In this study, a dynamic simulation model of the wind-to-heat system is proposed, which consists of the following six parts: wind turbine, compressor, condenser, thermal expansion valve, evaporator, and controller. Validations of this model was conducted by comparing the simulation result with the experimental data from a 100 kW wind-to-heat system. Effects of different operational conditions, such as hot water flow rate, inlet water temperature and types of refrigerants, on the characteristics of the system are explored and analyzed. The conclusion is drawn that the hot water flow rate is optimized as 3 kg/s, lower inlet water temperature led to lower heating capacity and lower COP, R245fa and R717 have good performances in heating production, which is in favor of the further design and optimize of the system. After debugging the simulation model for different design alternatives, different performances will be acquired to improve the efficiency of the design. [ABSTRACT FROM AUTHOR]