Augmenting Offshore Wind-Farm Yield with Tethered Kites

A Parametric Investigation Into the Effect of Low Induction Rotor (LIR) Wind Turbines on the Levelised Cost of Electricity for a 1 GW Offshore Wind Farm in a North Sea Wind Climate

This paper posits that a low-speed wind turbine design is suitable for harnessing wind energy in Africa. Conventional wind turbines consisting of propeller designs are commonly used across the world. A major hurdle to utilizing wind energy in Africa is that conventional commercial wind turbines are designed to operate at wind speeds greater than those prevalent in most of the continent, especially in sub-Sahara Africa (SSA). They are heavy and expensive to purchase, install, and maintain. As a result, only a few countries in Africa have been able to include wind energy in their energy mix. In this paper, the feasibility of a novel low-speed wind turbine based on a Ferris wheel is demonstrated for low wind speed applications in Africa. The performance of Ferris wheel wind turbines (FWT) with 61 m (200 ft), 73 m (240 ft) and 104 m (341 ft) diameter rims and an 800 kW generator are evaluated for selected African cities. The research also compares the Weibull wind distribution of the African cities of interest. A comparison between the FWT and the conventional commercial wind turbines in terms of efficiency, rated wind speed, cost, performance, and power to weight is included. Results show that the FWT has the potential for economic power generation at rated wind speeds of 6.74 m/s, which are lower than the average of 12 m/s for conventional wind turbines and have lower power to weight ratios of 5.2 kW/tonne as compared to 6.0–9.2 kW/tonne for conventional wind turbines.

Current research is exploring a new design concept for offshore wind turbines whereby the electrical generator in a conventional wind turbine is replaced by a large positive displacement pump that supplies pressurized sea water to a centralized hydro-electric plant. This paper investigates the potential of applying this concept to concurrently exploit thermocline thermal energy through deep sea water extraction in conjunction with offshore wind energy. A performance analysis is presented for a single wind turbine-driven pump supplying combined power and thermal energy by delivering pressurised deep sea water to a land-based plant consisting of a hydro-electric generator coupled to a heat exchanger. The steady-state power-wind speed characteristics are derived from a numerical thermo-fluid model. The latter integrates the hydraulic characteristics of the wind turbine-pump combination and a numerical code to simulate the heat gained/lost by deep sea water as it flows through a pipeline to shore. The model was applied to a hypothetical megawatt-scale wind turbine installed in a deep offshore site in the vicinity of the Central Mediterranean island of Malta. One year of wind speed and ambient measurements were used in conjunction with marine thermocline data to estimate the time series electricity and thermal energy yields. The total energy yield from the system was found to be significantly higher than that from a conventional offshore wind turbine generator that only produces electricity. It could also be shown that in regions where the offshore wind resource is not as rich, but where the ambient temperature is high as a result of a hotter climate, the cooling energy component that can be delivered is relatively high even at periods of low wind speeds.

Issues related to environmental concern and fossil fuel exhaustion has made wind energy the most widely accepted renewable energy resource. However, there are still several challenges to be solved such as the integrated design of wind turbines, aeroelastic response and stability prediction, grid integration, offshore resource assessment and scaling related problems. While analyzing the market of wind turbines to find the direction of the future developments, one can see a continuous upscaling of wind turbines. Upscaling is performed to harness a larger resource and benefit from economy of scale. This will pose several fundamental implications that have to be identified and tackled in advance. This research focuses on investigating the technical and economical feasibility and limits of large scale offshore wind turbines using the current dominant concept, i.e. a three-bladed, upwind, variable speed, pitch regulated wind turbine installed on a monopile in an offshore wind farm. Thus, the objective of this research is to investigate how upscaling influences the offshore wind turbines. Specifically, following questions are of interest: 1. How do the technical characteristics of the larger scales change with size and can these technical characteristics appear as a barrier? 2. How does the economy of the future offshore wind turbines change with size? 3. What are the considerations and required changes for future offshore wind turbines? To address these questions, a more sophisticated method than the classical upscaling method should be employed. This method should provide the detailed technical and economical data at larger scales and address all the design drivers of such big machines to identify the associated problems. However, interdisciplinary interactions among structure, aerodynamics and control subject to constraints on fatigue, stresses, deflections and frequencies as well as considerations on aeroelastic instability make the development of such a method a cumbersome and complex task. Among many different methods, integrated aeroservoelastic design optimization is found to be the best approach. Therefore, the scaling study of this research is formulated as an multidisciplinary design optimization problem. This method enables the design of the future offshore wind turbines at the required level of details that is needed to investigate the effect of size on technical and economical characteristics at larger scales. Using this method, 5, 10 and 20 MW wind turbines are designed and optimized, including the most relevant design constraints and levelized cost of energy as the objective function. In addition to the design of these wind turbines, the method itself shows a clear way forward for the future offshore wind turbine design methodology development. Based on these optimized wind turbines, scaling trends are constructed to investigate the behavior of a wind turbine as it scales with size. These trends are formulated as a function of rotor diameter to properly reflect the scale. Loading, mass, cost and some other useful trends are extracted to investigate the scaling phenomenon. Blades and tower as the most flexible load carrying components are examined with more attention. Using these results, the challenges of very large scale offshore wind turbines up to 20 MW range are explored and identified. These results demonstrate that a 20 MW design is technically feasible though economically not attractive. Therefore, upscaling of the current wind turbine configurations seems to be an inappropriate approach for larger offshore wind turbines.

Wind turbines can be considered as structures that are in between civil engineering structures and machines since they consist of structural components and many electrical and machine components together with a control system. Further, a wind turbine is not a one-of-a-kind structure but manufactured in series production based on many component tests, some prototype tests and zeroseries wind turbines. These characteristics influence the reliability assessment where focus in this paper is on the structural components. Levelized Cost Of Energy is very important for wind energy, especially when comparing to other energy sources. Therefore much focus is on cost reductions and improved reliability both for offshore and onshore wind turbines. The wind turbine components should be designed to have sufficient reliability level with respect to both extreme and fatigue loads but also not be too costly (and safe). In probabilistic design the single components are designed to a level of reliability, which accounts for an optimal balance between failure consequences, cost of operation & maintenance, material costs and the probability of failure. Furthermore, using a probabilistic design basis for reliability assessment it is possible to design wind turbines such that site-specific information on climate parameters can be included. Illustrative examples are presented considering uncertainty modeling, reliability assessment and reliability-based calibration of partial safety factors for structural wind turbine components exposed to extreme loads and fatigue loads.

Abstract. The Hybrid-Lambda Rotor is an aerodynamic rotor concept that enables low-specific-rating offshore wind turbines in order to increase the power output in light winds and to limit the loads on the very long and slender rotor blades in strong winds. In this paper, the rotor concept is scaled to wind tunnel size and validated under reproducible inflow conditions. The objectives are to derive a scaling methodology, to investigate the influence of the steep gradients of axial induction along the blade span, and to characterize the wake of the Hybrid-Lambda Rotor in wind tunnel experiments. The scaling objectives are to match the axial induction distribution and to incorporate the change in the angle of attack distribution when switching between the light-wind and strong-wind operating mode. The derived model rotor with a diameter of 1.8 m is experimentally investigated and compared to a conventional model wind turbine in the large turbulent wind tunnel in Oldenburg under tailored inflow conditions produced with an active grid. A two-dimensional laser Doppler anemometer is used to measure the axial induction in the rotor plane, and the wake is characterized by means of a hot-wire rig. The measurement data are supplemented with free-vortex wake simulations of both scaled rotors. The results demonstrate that switching the operating modes with the characteristic change in the angle of attack distribution works similarly for the model and the full-scale turbine. The strong gradients of axial induction along the blade span lead to complex three-dimensional flow structures, such as an increased radial flow component in the rotor plane. The low-induction design of the outer part of the rotor reduces the load overshoots in gust events compared to the conventional model turbine. The wake characterization reveals an outer annulus with reduced wake deficits, an additional shear layer and vortex system, and overall reduced wake deficits over a wide range of wind speeds below rated power. The derived results can improve the understanding of the unique flow patterns that are introduced by the Hybrid-Lambda Rotor and provide a valuable complementary data set to the simulations on the full-scale rotor.

Background: During the whole life cycle of an offshore wind farm, from cradle to grave, logistics is needed. Particularly during wind turbine construction, vessels are intensively used. However, no study has been found to evaluate logistics costs during this particular turbine construction phase. Objective: In order to close this gap, this paper aims to propose an Offshore Logistics Cost (OLC) framework, evaluate OLC impact on total cost and identify recommendations to reduce this impact. Methodology: This study considers two quantitative approaches to evaluate OLC share. First quantitative approach is based on cost information found in twenty related studies from literature. Second quantitative approach was developed in two steps. A first step is a bottom up calculation model. This model performs OLC calculations for each scenario identified. In a second step, OLC results are used as input to two existing calculation models: National Renewable Energy Laboratory (NREL) simplified model and commonly accepted model developed by Megavind. Variance of results are then analyzed to determine a representative range for OLC impact on total cost. Results: Using some related values from literature review, this study assesses wind turbine construction offshore logistics to represent around 1.2% of Levelized Cost of Energy (LCOE). Using bottom up approach, this study evaluates offshore logistics during wind turbine construction with a broader contribution, conservatively from 0.6 to 7.6% of LCOE for offshore wind turbines over 4 MW power rating. It is also demonstrated that the higher the wind turbine size, the lower offshore logistics impact on LCOE. Conclusion: Offshore wind logistics costs during construction phase can represent between 0.6 and 7.6% of LCOE and it appears necessary to optimize these costs to contribute to a competitive LCOE. As main offshore logistics cost drivers have been identified, areas to reduce LCOE impact are suggested: increasing weather limits to reduce waiting on weather costs, minimizing work offshore, improving processes, using economy of scale and optimizing vessel use.

Innovative design method and experimental investigation of a small-scale and very low tip-speed ratio wind turbine

In recent years, floating offshore wind turbines have attracted more attention as a new renewable energy resource while bottom-fixed offshore wind turbines reach their limit of water depth. Various projects have been proposed with the rapid increase in installed floating wind power capacity, but the economic aspect remains as a biggest issue. To figure out sensible approaches for saving costs, a comparison analysis of the levelized cost of electricity (LCOE) between floating and bottom-fixed offshore wind turbines was carried out. The LCOE was reviewed from a social perspective and a cost breakdown and a literature review analysis were used to itemize the costs into its various components in each level of power plant and system integration. The results show that the highest proportion in capital expenditure of a floating offshore wind turbine results in the substructure part, which is the main difference from a bottom-fixed wind turbine. A floating offshore wind turbine was found to have several advantages over a bottom-fixed wind turbine. Although a similarity in operation and maintenance cost structure is revealed, a floating wind turbine still has the benefit of being able to be maintained at a seaport. After emphasizing the cost-reduction advantages of a floating wind turbine, its LCOE outlook is provided to give a brief overview in the following years. Finally, some estimated cost drivers, such as economics of scale, wind turbine rating, a floater with mooring system, and grid connection cost, are outlined as proposals for floating wind LCOE reduction.

Wind turbine blade is considered as one of the most vital component of the wind turbine system meant to produce energy from the power of the wind. A finite element model of a three bladed small-scale Kestrel e230i horizontal axis wind turbine will be used in this study. Firstly, the three-dimensional solid geometry of the conventional wind turbine was generated using a customized aerofoil measurement technique in the laboratory. The generated geometry was then cleaned to remove all abrupt changes in surface profile. Then the actual conventional wind turbine blade was statically tested and resulting strains measured in different positions along its blade length. The blade was rigidly fixed at its root by bolting it into a laboratory test bench. The three-dimensional model was subsequently loaded and supported at its root in a similar manner. Finite element analysis and model updating was performed in ANSYS v 19.0 by iteratively correlating stress values between the finite element model and the test. A bio-inspired version of the updated finite element model was generated by adding undulations/corrugations to the bottom surface of the conventional blade in a longitudinal direction. Modal analyses were then performed for the two models to compare their natural frequencies in first two bending and first twist modes. These two modes are reported to be the most important modes in wind turbine dynamic response and may also be very influential in energy production of wind turbines. The numeral results show that the inclusion of undulations/corrugations increases the natural frequencies, which typically means that for low-speed inland applications, the risk of wind turbine blade structural failure due to resonant or near-resonant excitations is heavily averted due to increased potential operating frequency ranges.

Future offshore wind farms around the world will be built with wind turbines of size and capacity never seen before (with diameter and hub height exceeding 150 and 100 m, respectively, and rated power exceeding 10 MW). Their potential impacts at the surface have not yet been studied. Here we conduct high-resolution numerical simulations using a mesoscale model with a wind farm parameterization and compare scenarios with and without offshore wind farms equipped with these ‘extreme-scale’ wind turbines. Wind speed, turbulence, friction velocity, and sensible heat fluxes are slightly reduced at the surface, like with conventional wind turbines. But, while the warming found below the rotor in stable atmospheric conditions extends to the surface with conventional wind turbines, with extreme-scale ones it does not reach the surface, where instead minimal cooling is found. Overall, the surface meteorological impacts of large offshore wind farms equipped with extreme-scale turbines are statistically significant but negligible in magnitude.

The main aim of this work is to design, fabricate, and test a wind turbine for power generation applications in rural areas. Vertical Axis Wind Turbines were selected to harness the energy from wind through the drag forces induced due to vehicular movements. Various parameters were analyzed for the design of a low‐cost wind turbine. A Savonius blade was selected for the design, which could be accommodated on the median of the highways. By using recycled materials, a low‐cost wind turbine was fabricated at a cost of $117.5 approximately. The wind turbine was placed on the houses and on the highway medians to test the power output at various operating conditions. Average electricity consumption at selected rural houses were calculated. The calculated average electricity demand during power cuts in the selected rural houses was around 0.2–0.6 kWh/day. Average generated electricity from the turbine at highways was observed to be around 0.67 kWh/day. The Levelized cost of electricity (LCOE) of the generated electricity from the proposed SWT on highways is around $0.04/kWh. The LCOE of the proposed design is relatively cheaper when compared with the conventional horizontal axis wind turbines. The energy demand during power cuts was met completely when the SWT was placed on the highways. © 2018 American Institute of Chemical Engineers Environ Prog, 38: 278–285, 2019

Offshore wind power is one of the most popular renewable sources of energy. However, there are many challenges during the design, construction and operation of offshore wind farms. One of these challenges is the stability of offshore wind turbines. The main loads on the foundations of wind turbines are from the environment (wind and wave) and there are other loads arising due to their operations (known as rotor frequency loads-1P and blade passing loads-2P/3P). All these 4 loads are unique in terms of magnitude, number of cycles and the strain they apply to the supporting soil. Furthermore, due to innovation in turbine technology, the sizes of turbines also increased few folds (3MW to 12MW) in a span of about 5 years and these large turbines need customised foundations. Due to the attractiveness of this new technology and the reduction of LCOE (Levelized Cost of Energy), offshore wind turbines are also sited not only in deeper waters but also in seismic areas and other disaster-prone areas (typhoon and hurricane). Any new foundation must be validated using scaled model tests (i.e. study of Technology Readiness Level) to satisfy the industry requirements. This thesis developed techniques for scaled model testing to study different aspects of long-term performance of foundations. The novel testing methodology and apparatus is based on understanding of the loads on the foundations. The apparatus consists of two eccentrically loaded gears which can be customised to apply cycloid loads. The apparatus can be easily upscaled to study bigger models and is very simple to assemble and operate. The apparatus can also apply millions of cycles of loading of different amplitude and frequency which is representative of a real wind turbine. Results from scaled model tests on few types of foundations are presented and they revealed interesting Soil-Structure Interaction. In a wind turbine system, long term performance is mainly governed by the SSI and this thesis summarised the limited field observations reported in the literature and compared with the laboratory observations. One of the scientific challenges is the prediction of long-term performance of these relatively new and novel technologies. While scaled model tests can identify the physics, this is not a practical tool for routine design as it is difficult to create model tests for each of the sites. As a result, this thesis aimed to link the understanding of SSI to element testing of soil. This will allow to use the recovered sample from offshore wind farm location to carry laboratory tests to obtain design parameter. This thesis proposed a simple method to obtain the strain level in the soil which is beneficial for planning offshore Site Investigation. Offshore wind turbines are currently designed for 25 to 30 tars and the number of cycles of loading are in the range of 100 million. This thesis presented data from element testing of soil where up to 50,000 cycles of loading were applied. The general trends of behaviour were noted, and it was observed that the soil behaviour was attaining a steady state. All the above helped to understand some SSI aspects of offshore wind turbines. Future work is also suggested

Far offshore wind conditions in scope of wind energy

Capital expenditure and levelized cost of electricity of photovoltaic plants and wind turbines – Development by 2050