Tribological challenges of scaling up tidal turbine blades

Generating electricity from renewable resources (wind, wave and tidal) is of increasing interest. Of all marine renewables, tidal energy, by comparison, possesses the higher persistency and predictability over long time scales and the higher density of water than air results in greater power output from a tidal turbine than a wind turbine with similar dimensions. However, due to the nature of the tides, developing a reliable device for such environments, especially with an increased rotor diameter, raises more challenges to be addressed including the tribological challenges such as sediment erosion, cavitation erosion and their possible synergistic effects on the tidal turbine blades. This research focuses on testing and developing materials for improved tribological performance in tidal environments. This includes producing a variety of composite materials with different fibres and layouts reinforcement to evaluate two main tribological issues of composite materials in tidal environments: matrix cutting and reinforcement fracture using a loped test rig, which measures the effects of impact angle, particle size and concentrations at different tip speeds. The test samples are analysed using scanning electron microscopy (SEM) to conduct a surface topography and characterisation

Tribology of tidal turbine blades : impact angle effects on erosion of polymeric coatings in sea water conditions

Tidal energy, of all marine renewables energies, possesses higher persistency and predictability over long time scales. Due to the aggressive marine environment, there are barriers in the development of tidal power generation technology. In particular, with regard to increased rotor diameter, the selection of material presents significant challenges to be addressed including the tribological environment, such as solid particle erosion, cavitation erosion, the effect of high thrust loading on the turbine blade tips, and the synergy between sea water conditions and such tribological phenomena. This research focuses on producing and testing a variety of advanced materials and surface coatings to investigate two main tribological issues in tidal environments: matrix cutting and reinforcement fracture. In our previous work, a G10 epoxy glass laminate was tested in this environment and the results revealed tribological issues. In this present work, G10 epoxy glass laminate base erosion resistant polymeric coatings have been tested for the range of sand particles size in our our previous work and in NaCl solution. The test results reveal that the coating has enhanced the quality of performance of the composite with respect to tribological behaviour, and has diminished the synergy between sea water and tribological phenomena. This indicates progress toward the selection of advanced materials to manufacture tidal turbine blades

Impact angle effects on erosion maps of GFRP : applications to tidal turbines

Tribology in marine renewable technologies has become of increasing interest due to the implications for developing improved materials for tidal and wave energy conversion devices. This on-going research mainly focuses on tidal devices; the materials of interest are primarily polymer based composite materials that are used to provide structural integrity while reducing weight. These are specifically applied to turbine blades to withstand the high impact loadings in sea water conditions. At present, current materials in test trials have demonstrated some limitations in service. In this paper, some advanced experimental research has been carried out to investigate the tribological mechanisms of potential candidate composite materials to be used in tidal turbines by firstly considering the effects of various erosion parameters on the degradation modes, with and without particles in still and sea water conditions. The erosion mechanisms of composite materials used in tidal turbine blades have been evaluated using Scanning Electron Microscopy techniques to analyse the surface morphologies following testing in water representative of the constituents of coastal sea water. Generic erosion maps and the mechanistic maps have been constructed as a key to identify regions of minimum erosion for the operating conditions and to identify the significant effect of the sea water environment on the degradation of the composite. This research outcome will further help us to deeply understand and identify the erosion rates at different impact velocities and angles

Mapping synergy of erosion mechanisms of tidal turbine composite materials in sea water conditions

Tidal energy, of all marine renewables energy, possesses higher persistency and predictability over long time scales. Moreover, the higher density of water than air also results in greater power output from a tidal turbine than a wind turbine with similar dimensions. Due to the aggressive marine environment, there are barriers in the development of tidal power generation technology. In particular, with regard to increased rotor diameter, the selection of material presents significant challenges to be addressed including the tribological environment, such as solid particle erosion, cavitation erosion, the effect of high thrust loading on the turbine blade tips, and the synergy between sea water conditions and such tribological phenomena. This research focuses on producing and testing a variety of composite materials with different fibres and reinforcement layouts to evaluate two main tribological issues in tidal environments: matrix cutting and reinforcement fracture. A slurry pot test rig was used to measures the effects of different impact angles and particles sizes at constant tip speeds

The impact of limiting tidal turbine capacity on the output characteristics from three complimentary tidal sites in Scotland over the lunar cycle

ABSTRACT Recent Government policy has set specific targets for renewable energy technologies to contribute to the national electricity supply make up. Currently, wind power is the preferred technology. However, it is widely accepted that due to the stochastic nature of wind, planning restrictions and the finite availability of suitable sites there is an upper limit to the capacity that can be accommodated. This paper demonstrates the potential of tidal energy to provide firm power and shows that by limiting the capacity of the power generated, provides base load supply without compromising power quality. This increases the capacity factor of the installed system, thus improving the economic viability and commercial competitiveness of tidal farms. KEYWORDS Renewable energy, tidal current, base load generation, electricity supply. INTRODUCTION Recent energy policy developments in the UK have favoured the development of renewable technologie

Development and initial application of a blade design methodology for overspeed power-regulated tidal turbines

The range and variability of flow velocities in which horizontal axis tidal stream turbines operate introduces the requirement for a power regulation method in the system. Overspeed power regulation (OSPR) has the potential to improve the structural robustness and decrease the complexity associated with active pitch power regulation methods, while removing the difficulties of operating in stalled flow. This paper presents the development of a methodology for the design of blades to be used in such systems. The method requires a site depth, maximum flow velocity and rated power or flow speed as input parameters. The pitch setting, twist and chord distribution were set as input parameters, variable through the use of alteration functions. Rotor performance has been broken down into OSPR performance metrics which consider coefficients of power and thrust, and cavitation inception. Three visual-numerical tools have been developed: the OSPR performance metrics were used in conjunction with a one-at-a-time sensitivity analysis approach to develop a design space; cavitation inception analyses gave plots of converging cavitation and pressure terms for each blade section; the local angle of attack and torque distribution across the blade designs were plotted at key turbine operation states. Alterations to pitch setting and twist distribution are shown to have most impact upon the design requirement of increased gradient in the rotor speed-efficiency relationship in the overspeed region; coupled with such alterations, targeted changes to the chord distribution have been shown to increase the maximum efficiency. The prevention of cavitation has been highlighted as a driver for speed-limiting design alterations. While facilitating blade design, the methodology also produces experiential knowledge which can be stored, and shared in graphical format

The effect of bathymetry interaction with waves and sea currents on the loading and thrust of a tidal turbine

This paper reports work undertaken to advance analytical methods used to evaluate the influence of bathymetry on wave- current interactions with tidal turbines. The model takes in to account the wave transformation due to a sudden depth change in the sea level. The functions developed provide solutions for wave transformation by changes in bathymetry to find how this change effects the torque and thrust exerted over a tidal turbine. Costal site data for the west coast of the US, from the US DoE, has been used to access the robustness of these analytical methods. The high resolution data sets used have monitored wave, sea and climatic conditions over a period of 8 years

The Soft X-ray Lightcurves of Partially Eclipsed Stellar Flares

Most stellar flares' soft X-ray lightcurves possess a `typical' morphology, which consists of a rapid rise followed by a slow exponential decay. However, a study of 216 of the brightest flares on 161 pre-main sequence stars, observed during the Chandra Orion-Ultradeep Project (COUP), showed that many flare lightcurves depart from this typical morphology. While this can be attributed to the superposition of multiple typical flares, we explore the possibility that the time-variable eclipsing of flares by their host stars may also be an important factor. We assume each flare is contained within a single, uniform plasma density magnetic loop and specify the intrinsic variation of the flare's emission measure with time. We consider rotational eclipse by the star itself, but also by circumstellar discs and flare-associated prominences. Based on this simple model, we generate a set of flares similar to those observed in the COUP database. Many eclipses simply reduce the flare's maximum emission measure or decay time. We conclude therefore that eclipses often pass undetected, but usually have only a modest influence on the flare emission measure profile and hence the derived loop lengths. We show that eclipsing can easily reproduce the observed atypical flare morphologies. The number of atypical modelled flare morphologies is however much less than that found in the COUP sample. The large number of observed atypical flare morphologies, therefore, must be attributed to other processes such as multiple flaring loops.Comment: 11 pages, 9 figure

Improving effective contraception uptake through provision of bridging contraception within community pharmacies:findings from The Bridge-it Study process evaluation

OBJECTIVE: To present process evaluation results from the Bridge-it Study, a pragmatic cluster randomised cross-over trial to improve effective contraception uptake through provision of the progestogen only pill (POP) plus sexual and reproductive health (SRH) clinic rapid-access to women presenting to community pharmacies for emergency contraception (EC). RESEARCH DESIGN AND METHODS: A multimethod process evaluation was conducted to assess intervention implementation, mechanisms of change and contextual factors. Data were gathered from screening logs (n=599), observations of pharmacist training, analysis of data from 4-month follow-up questionnaires (n=406), monitoring of contemporaneous events and qualitative interviews with 22 pharmacists, 5 SRH clinical staff and 36 study participants in three participating UK sites in Lothian, Tayside and London. RESULTS: The intervention was largely delivered as intended and was acceptable. Pharmacists’, SRH clinical staff and participants’ accounts highlighted that providing a supply of POP with EC from the pharmacy as routine practice may have positive impacts on contraceptive practices in the short term, and potentially longer term. Key mechanisms of change included ease of access, increased awareness of contraception and services, and greater motivation and perceptions of self-efficacy. Few participants took up the offer to attend an SRH service (rapid-access component), and existing barriers within the SRH context were apparent (eg, lack of staff). Participant accounts highlight persistent barriers to accessing and using routine effective contraception remain. CONCLUSIONS: Implementation appeared to be acceptable and feasible, highlighting the potential for provision of POP within EC consultations as routine practice in community pharmacies. However, lack of engagement with the rapid access component of the intervention and existing barriers within the SRH context suggest that signposting to SRH services may be sufficient. Wider implementation should consider ways to address key implementation challenges to increase effectiveness and sustainability, and to overcome persistent barriers to accessing and using effective contraception. TRIAL REGISTRATION NUMBER: ISRCTN70616901

Tow-tank testing of a 1/20th scale horizontal axis tidal turbine with uncertainty analysis

Tidal turbine developers and researchers use small scale testing (i.e. tow tank and flume testing) as a cost effective and low risk way to conduct proof-of-concept studies and evaluate early stage device performance. This paper presents experimental performance data for a three-bladed 1/20th scale NREL S814 tidal turbine rotor, produced at the 4.6 × 2.5 m and 76 m long Kelvin Hydrodynamics Laboratory tow tank at Strathclyde University. The rotor performance was characterised from very low tip speed ratios to runaway for four carriage speeds. A maximum CP of 0.285 and a maximum CT of 0.452 were recorded at tip speed ratios of 3.53 and 4.45 for a carriage speed of 1 m/s. The uncertainty in the instrument calibration and experimental measurements was quantified, allowing accurate representation of the experiments in numerical models. The methodology behind the uncertainty calculations is described in this paper. The uncertainty in the experimental measurements was found to be less than 5% for over 87% of the tests. Reynolds number scaling effects were found to be influential on the rotor performance in the range of velocities tested