Lorenzo Battisti

Modeling Strategy and Numerical Validation for a Darrieus Vertical Axis Micro-Wind Turbine

This paper presents a model for the evaluation of optimal spatial grid node distribution in the CFD analysis of a Darrieus vertical axis micro wind turbine, by analyzing the trends over a 360° rotation of some indicators of near-blade mesh quality. To this purpose, a complete validation campaign has been conducted through a systematic comparison of numerical simulations with wind tunnel experimental data. Both two-dimensional and three-dimensional grids, characterized by average y+ values of 30 and 1, have been tested by applying some statistical techniques as a guidance in selecting the appropriate grid configuration and corresponding turbulence model. Finally, the tip downstream recirculation zone due to the finite blade extension and the influence of spokes have been analyzed, achieving a numerical quantification of the influence of induced drag and spokes drag on overall rotor performance.Copyright

Photonic crystals for monitoring fatigue phenomena in steel structures

This paper introduces the concept and development of a strain sensing system for structural application based on the properties of photonic crystals. Photonic crystals are artificially created periodic structures, usually produced in the thinfilm form, where optical properties are tailored by a periodicity in the refractive index. The idea of using the crystal as a sensor is based on the observation that a distortion in the crystal structure produces a change in the reflected bandwidth. When a photonic crystal is designed to operate in the visible part of the spectrum, a permanent distortion of the film results in a change in its apparent color. This property makes photonic crystals suitable for permanent monitoring of structural elements, as any critical changes in the strain field can be promptly and easily detected by visual inspection. A simple and low-cost example of photonic crystals consists of opals synthesized by vertical deposition. In this contribution we introduce a target application for the fatigue monitoring of wind turbines, and then provide the reader with some basic information concerning modeling of the crystal architecture and fabrication of these structures. Next we discuss their application to strain measurement, specifying how reflection and transmission properties of the opals have to be designed to satisfy the expected strain response of the sensor. Finally, we present the preliminary results of a laboratory validation carried out on thin films applied to a rubber support.

WARM-AIR INTERMITTENT DE-ICING SYSTEM FOR WIND TURBINES

This paper is concerned with the one-dimensional transient heat transfer in a wind turbine blade leading edge that is covered by ice. Intermittent heating of the surfaces, obtained by means of warm air channeled within the blade shell, was modeled to investigate the applicability of cyclical ice removal from wind turbine blades. Finite differencing was used to solve the heat diffusion differential equations. A conservative approach was employed to get the solution in terms of internal energy. A predictor-corrector method was used to tackle the problem of the water phase change. The de-icing capability of the warm air system was investigated for a typical rotor blade tip section, and its operational limits were illustrated. The comparison with usually employed continuous anti-icing systems showed the de-icing system to offer remarkable reduction of the energy consumption required for the ice prevention systems operation.

Potential of electricity generation from renewable energy sources in standard domestic houses

In this paper the potential of several sustainable energy sources is examined referring to a standard domestic house with 4 inhabitants. The selected options were: wind energy, solar energy (photovoltaic), domestic waste and wastewater. The aim was to define how much of the energy demand for the proposed standard domestic house can be covered by wind energy or photovoltaic panels and how much we can obtain by an efficient waste and wastewater management. The feasibility of these solutions was also compared.

In-field testing of a steel wind turbine tower

The strong drive to exploit wind energy has recently led to consideration of new types of location for wind turbines, including mountain regions. A major concern for wind farm installation in these sites is the long-term reliability of the support structures. In a flexible steel tower, a combination of vortex shedding and gusting caused fatigue cracks at the base joint. To identify the conditions critical to the development of this phenomenon, a twoblade down-wind turbine was thoroughly investigated in an in-field experimental campaign. This turbine features a 13m diameter rotor of rated power 11kW, mounted on an 18m tubular steel tower. In operation, the blades rotate at a fixed rate of 2 Hz. The tower, instrumented with 15 accelerometers, was first dynamically characterized in the absence of significant wind. Next, its spectral response to wind excitation was identified both in operation and with the rotor at rest. The outcome of the experiment suggests that the vulnerability to fatigue of this model of turbine is very sensitive to its modal behaviour, this in turn depending on the mechanical admittance of the foundations.

2D Numerical Simulation of a Wind Turbine De-icing System, Using Cycled Heating

At cold climate sites, the icing of the turbine components, mainly the blades, affects the systems performance and safety due to the ice accretion and shedding. Consequently, the economic sustainability of wind turbine utilization in adverse climates needs some special requirements, such as anti-icing or de-icing equipment for the blades. This paper gives a numerical investigation of a new approach, consisting of cyclically de-icing the wind turbine blades by the intermittent warming of the surfaces. De-icing simulations were carried out for a MW-class rotor blade section. A parametric study investigated the sustainability and effectiveness of warm air de-icing systems, in terms of the heating cycle duration and the heating energy consumption.

Analysis of Different Blade Architectures on small VAWT Performance

The present paper aims at describing and comparing different small Vertical Axis Wind Turbine (VAWT) architectures, in terms of performance and loads. These characteristics can be highlighted by resorting to the Blade Element-Momentum (BE-M) model, commonly adopted for rotor pre-design and controller assessment. After validating the model with experimental data, the paper focuses on the analysis of VAWT loads depending on some relevant rotor features: blade number (2 and 3), airfoil camber line (comparing symmetrical and asymmetrical profiles) and blade inclination (straight versus helical blade). The effect of such characteristics on both power and thrusts (in the streamwise direction and in the crosswise one) as a function of both the blades azimuthal position and their Tip Speed Ratio (TSR) are presented and widely discussed.

Reliability of numerical wind tunnels for VAWT simulation

Computational Fluid Dynamics (CFD) based on the Unsteady Reynolds Averaged Navier Stokes (URANS) equations have long been widely used to study vertical axis wind turbines (VAWTs). Following a comprehensive experimental survey on the wakes downwind of a troposkien-shaped rotor, a campaign of bi-dimensional simulations is presented here, with the aim of assessing its reliability in reproducing the main features of the flow, also identifying areas needing additional research. Starting from both a well consolidated turbulence model (k-ω SST) and an unstructured grid typology, the main simulation settings are here manipulated in a convenient form to tackle rotating grids reproducing a VAWT operating in an open jet wind tunnel. The dependence of the numerical predictions from the selected grid spacing is investigated, thus establishing the less refined grid size that is still capable of capturing some relevant flow features such as integral quantities (rotor torque) and local ones (wake velocities).

Normalized performance and load data for the deepwind demonstrator in controlled conditions

Performance and load normalized coefficients, deriving from an experimental campaign of measurements conducted at the large scale wind tunnel of the Politecnico di Milano (Italy), are presented with the aim of providing useful benchmark data for the validation of numerical codes. Rough data, derived from real scale measurements on a three-bladed Troposkien vertical-axis wind turbine, are manipulated in a convenient form to be easily compared with the typical outputs provided by simulation codes. The here proposed data complement and support the measurements already presented in “Wind Tunnel Testing of the DeepWind Demonstrator in Design and Tilted Operating Conditions” (Battisti et al., 2016) [1].

Relevance of Icing for Wind Turbines

The chapter addresses specifically the topic of the effect of icing on wind turbines. General icing characteristics are discussed, the prerequisite for icing occurrence and the ice growth on wind turbine. The problem of ice detection and the main ice detection system, the behaviour of iced sensors is then discussed. An experimental campaign have been undertaken with a dedicated wind measurement station at 2,000 m a.s.l. with heated and not heated anemometers to enlighten the phenomenon of direct icing and icing persistence, and to present a procedure to deduce the number of icing days in the year. The problem of meteorological icing data forecasting is twofold. Not only are historical data and spatial extrapolation tools needed for wind farm design, but continuously up-to-date forecasts of icing events, their intensity and duration are also essential for wind turbine operation programmes. In fact, the latter enable a prediction of the energy available for dispatching to the grid in a given period and a rational use of the IPS. Short-term forecast approaches are therefore reviewed and discussed. A probabilistic-based methodology has been further proposed to evaluate the icing period on a site with few information. The problem of ice throw is analysed as it involves also safety of people and goods. The deriving icing risk has been approached with a dedicated, fully explained model based on a Monte Carlo method. Finally, the economic risks of adopting or not adopting ice prevention system are discussed by the help of a dedicated break-even model. The analysis aims to asses the minimum number of icing days that makes the IPS investment viable.