• Energy Research

This paper aims to investigate the development of a floating artificial sustainable energy island at a conceptual design level that would enhance the energy independence of islands focusing on a case study on the island of Crete. This paper provides a baseline assessment showing the immense potential of wind and solar energy in and around Crete integrating the third significant renewable energy source (RES) of ocean waves into the energy island. The selection of the best location for the floating offshore platforms that compose the energy island is addressed through exploiting the great potential of the above-mentioned RES, taking into consideration criteria with regard to several significant human activities. To this end, the concept of an innovative floating modular energy island (FMEI) that integrates different renewable energy resources is proposed; in addition, a case study that focuses on the energy independency of a big island illustrates the concept referring to the substitution of the local thermal power plants that are currently in operation in Crete with sustainable energy power. Although focused on the renewable energy resources around Crete, the work of this paper provides a basis for a systematic offshore renewable energy assessment as it proposes a new methodology that could be used anywhere around the globe.

The failure of power converters is currently the main reason of stopping for wind turbines. The unpredictable nature of wind power flow causes temperature swings to the semiconductor devices, which leads to additional thermal stresses and, eventually, unexpected failures. A suitable way to avoid this is using condition monitoring systems, which detect the degradation of the devices and reduce the stopping time by a planned maintenance. The choice of the most appropriate early failure detector is therefore essential to make sure that the condition monitoring system is accurate and indicates a fault only when this is actually occurring. For this reason, this paper focusses on the sensitivity analysis of switching parameters in response to a variation of the load and the dc-bus voltage, which may introduce uncertainty in the detection of early fault as they have a direct influence on the device temperature. The results of the experimental investigation show a substantial sensitivity of switching parameters to dc-bus voltage variation and a moderate sensitivity to load variation.

An increasing number of engineering applications require accurate predictions of the flow around buildings to guarantee performance and safety. This paper investigates the effects of variations in the turbulent inflow, as predicted in different numerical simulations, on the flow pattern prediction around buildings, compared to wind tunnel tests. Turbulence characteristics were assessed at several locations around a model square high-rise building, namely, above the roof region, at the pedestrian level, and in the wake. Both Reynolds-averaged Navier–Stokes (RANS, where turbulence is fully modelled) equations and large-eddy simulation (LES, where turbulence is partially resolved) were used to model an experimental setup providing validation for the roof region. The performances of both techniques were compared in ability to predict the flow features. It was found that RANS provides reliable results in regions of the flow heavily influenced by the building model, and it is unreliable where the flow is influenced by ambient conditions. In contrast, LES is generally reliable, provided that a suitable turbulent inflow is included in the simulation. RANS also benefits when a turbulent inflow is provided in simulations. In general, LES should be the methodology of choice if engineering applications are involved with the highly separated and turbulent flow features around the building, and RANS provides reliable information when regions of high wind speed and low turbulence are investigated.

Predicting flow patterns that develop on the roof of high-rise buildings is critical for the development of urban wind energy. In particular, the performance and reliability of devices largely depends on the positioning strategy, a major unresolved challenge. This work aims at investigating the effect of variations in the turbulent inflow and the geometric model on the flow patterns that develop on the roof of tall buildings in the realistic configuration of the University of Birmingham’s campus in the United Kingdom (UK). Results confirm that the accuracy of Large Eddy Simulation (LES) predictions is only marginally affected by differences in the inflow mean wind speed and turbulence intensity, provided that turbulence is not absent. The effect of the presence of surrounding buildings is also investigated and found to be marginal to the results if the inflow is turbulent. The integral length scale is the parameter most affected by the turbulence characteristics of the inflow, while gustiness is only marginally influenced. This work will contribute to LES applications on the urban wind resource and their computational setup simplification.

Abstract The effect of free stream turbulence on a DU96w180 wind turbine aerofoil is investigated through wind tunnel experiments. Wind turbine blades experience large scale, high intensity turbulent inflow during their service life. However, the effect of turbulence is normally neglected in the assessment of their aerodynamic performance. This is normally justified based on common assumptions on the effect of the integral length scale of turbulence, which supposedly only acts in the low-frequency range of the energy spectrum, hence affecting the angle of attack instead of the aerodynamic behaviour. In this study, an experimental setup implementing passive grids is developed to vary independently turbulence intensity and integral length scale in wind tunnel testing, with a range spanning I~5–15% and L ~ 8 − 33 c m respectively. Results show that turbulence effects are not negligible even at the largest integral length scales, provided that a critical value for the turbulence intensity is achieved. Turbulence is found to increase mean and fluctuating Lift and delay separation in stalled conditions.

Abstract Grouted connections for offshore wind turbines are formed by attaching overlapping steel piles with an ultra-high strength cementitious grout. The structural performance of grouted connections is critical for the substructures in order to exhibit sufficient resistance to environmental loads. The long-term integrity of the grout core can be compromised due to the complex stress states present, leading to unexpected slippage and gaps in the steel-grout interface, grout cracking and water ingress. This paper presents the results of an experimental investigation on damage evolution and failure mechanisms occurring within grouted connections in laboratory-based bending tests using acoustic emission. A parametric analysis of the detected acoustic emission signals has been conducted. The acoustic emission activity has been correlated with load-displacement measurements and the observed specimen failure modes. For the tested grouted connections, the number of acoustic emission hits and the signal duration were employed to identify damage evolution during load application. Root mean square and the ratio of rise time to amplitude were found to be useful Key Performance Indicators (KPIs) for damage prognosis. Finally, an improved b-value analysis has been performed, and the computed drops were well-associated with grout cracking within the connection.

The human migration from rural to urban areas has triggered a chain reaction causing the spiking energy demand of cities worldwide. High-rise buildings filling the urban skyline could potentially provide a means to improve the penetration of renewable wind energy by installing wind turbines at their rooftop. However, the above roof flow region has not received much attention and most results deal with low-rise buildings. This study investigates the flow pattern above the roof of a high-rise building by analysing velocity and pressure measurements performed in an atmospheric boundary layer wind tunnel, including four wind directions and two different roof shapes. Comparison of the surface pressure patterns on the flat roof with available low-rise building studies shows that the surface pressure contours are consistent for a given wind direction. At 0° wind direction, a separation bubble is detected, while cone vortices dominate at 30° and 45°. The determining factor for the installation of small wind turbines is the vicinity to the roof. Thus, 45° wind direction shows to be the most desirable angle by bringing the substantial amplification of wind and keeping the turbulence intensity low. Decking the roof creates favourable characteristics by overcoming the sensitivity to the wind direction while preserving the speed-up effect.