Wind Energy Systems

REPORT FORMAT

Use the following template to complete the individual report and submit a PDF file. Use of Latex to create a similar layout is also permitted. Ensure that all of the introductory information and the instructions under each section heading are deleted prior to submission. The font in the template is 12pt Times New Roman and this should be maintained in the final document. You should decide on appropriate pages for each section. The total page count is limited to 12 pages. References do not count towards the page limit. Assume any appendices are not marked.

INTRODUCTION & SCENARIO

Early plans for development of an offshore wind farm at the “Atlantic Array” site in the Bristol Channel were abandoned in 2013. However, another developer has now initiated new studies to investigate the feasibility of installing a 450MW wind farm in a region of the original Atlantic Array development zone situated closest to the onshore grid connection point, where soil conditions and water depth are also favourable (see Figure 1). The developer is working with a turbine manufacturer that is progressing the concept design of a new, very large turbine, capable of generating 18MW with a three-bladed rotor of 208m diameter and operating with variable speed, pitch regulated control.

Figure 1 shows the original Atlantic Array Development Zone boundary (area enclosed by red line) and the area allocated for the 450MW wind farm being considered by the developer (area enclosed by blue line). Although more detailed layout optimisation studies are still to be conducted, the developer can comfortably fit the twenty five 18MW turbines required using 5 rows of 5 turbines with a 5D x 10D spacing (where D is the turbine diameter, the larger separation of 10D is in the direction of the prevailing wind direction and the smaller separation of 5D is lateral to the prevailing wind direction). Electricity produced by the wind farm is to be stepped up from 66 kV to 120 kV by an offshore substation located at the point where the export cable corridor meets the site boundary, before being transmitted to the onshore substation.

In order to conduct the analysis tasks required within the project, an Ashes model of the baseline 18MW, variable speed, pitch regulated (VSPR) wind turbine has been supplied on Blackboard. The model should be downloaded and saved within your personal user directory.

For each section, the percentage of marks associated with that section are shown. This should give you a guide to the approximate amount of space to allocate to each section in the final report.

All explanatory text prior to this point should be deleted before final report submission, along with any explanatory text for the individual sections.

1. DESIGN A PITCH CONTROL SCHEDULE (20%)

The baseline wind turbine (termed the WES 18-MW) is a 208m diameter turbine designed to produce rated power at 12.5m/s windspeed and 8.84RPM Currently, there lacks a suitable pitch control strategy, which you will have to design to produce a power curve for the turbine.

For this section, you should use an idealized set-up; use the RNA only, and uniform. oncoming flow conditions (no wind sheer). In Ashes, run dynamic simulations (as usual) for this exercise. If there is currently a pitch-controller turned on, you will need to disable this so that you can tune the pitch values manually (click on “RNA in the parts menu, then set “demanded pitch scheme” to “live” which will make the pitch angle in the control window editable).

Complete the following tasks:

· Design an appropriate pitch-control schedule for a range of wind speeds between 0m/s and the cut-out speed of the turbine. You can either manually change the pitch angle to achieve this, or design a PID controller in Ashes. This should achieve the conventional ramp-up region then rated region and you will need to decide appropriate cut-in and cut-out speeds. Graphically, plot the variation of the following quantities with wind speed (you can decide how many graphs and appropriate scales):

o Power, torque and thrust;

o TSR and RPM;

o Pitch angle;

o In-plane and out-of-plane root moments;

· For a number of wind speeds (you decide which points and how many), plot the spanwise variation of in-plane (driving force) and out-of-plane (thrust) forces from the blade root to the blade tip;

· Explain the features of the power curve and loading plots at points between cut-in and cut-out;

· Describe and explain any key modelling assumptions that are utilized in the calculation of the power curve. How might the real performance of the turbine be different to the simulated power curve and why?

· Why is this rotor diameter for an 18MW generator only likely to be suitable for high wind speed sites? What design changes could be made to the rotor to make it more suitable for lower wind speed-sites and what are the likely design implications on the tower and foundations?

2. SYSTEM DYNAMICS AND STRUCTURAL CHECKS (30%)

You have also been tasked with an investigation of the key dynamic, structural and foundation properties of the WES 18-MW wind turbine system. For this section, you will  need to use the “Offshore bottom fixed” set-up in Ashes. A model turbine has been provided in Blackboard. The initial dimension of the turbine has been set as:

· Tower height: 125m

· Tower diameter: 3m (top); 7m (bottom)

· Tower thickness: 30 mm (top); 100 mm (bottom)

A uniform.  water depth of 40m is assumed at this location. For an initial assessment, the turbine is supposed to be fixed at seabed level, although this condition will be relaxed in the following of this section. The rated wind speed is 12.5 m/s and wave height is set at 6m with a period of 10s. An automatic pitch control is set, although this may differ from what you have determined in section 1.

Structural checks:

· Using Ashes, undertake a loading “simulation” for about 1 minute. Extract the load at three different locations of the tower: at seabed level, mid height and top of the tower and carry out the relevant structural checks, determining the section utilization ratio. For this exercise, assume that extracted load conditions are already factorized and that the design (factorized) yield strength of the steel is 355MPa.

· Analyze and comment on how the load on the structural element changes with time for the simulation performed in the point above.

Modal analysis :

· Using Ashes, undertake a “Modal analysis” (Eigenmodes) and calculate the natural frequencies of the first two tower bending modes (set azimuth angle to zero) and the first five blade bending modes using fixed conditions at seabed level. Present the results in tabular form. identifying each bending mode and its corresponding natural frequency.

· Assuming that the wind turbine will have a cut-in rotor speed of 6 RPM to work then at the fixed 8.84RPM for the rated wind speed of 12.5m/s, produce a chart showing normalized power spectral density versus frequency which includes wave loading, “1P” rotational frequency and “3P” rotational frequency. In your graph add a typical Jonswap spectra with a peak at the frequency f=0.1Hz. Plot the first two tower natural frequencies.

· Produce a Campbell chart and plot the first two tower natural frequencies and the first blade natural frequency, each assumed for simplicity to be constant across the range of wind speeds.

· Consider the graphical results and provide a full response to the following.

o Identify any coincidences of tower or blade natural frequencies with 1P, 3P or 6P harmonics. Explain the significance of these 1P, 3P and 6P harmonics in the context of the applied loading (excitation) of the turbine and, therefore, the consequence of their coincidence with system natural frequencies in terms of dynamic response, loading and fatigue of the wind turbine. When the turbine is operating above the rated wind speed, explain whether the tower can be described as a “soft-soft”, “soft-stiff” or “stiff-stiff” structure.

o List the key characteristics of the wind turbine, tower and foundation which will influence the values of the tower natural frequencies. Identify any further issues which must be taken into account when considering how the natural frequencies may change when the turbine is installed offshore on a suitable support structure.

o Modify the provided Ashes model by introducing a monopile foundation having the same diameter as the base of the tower. Consider an embedment depth of monopile (L_emd=30m) and model the monopile-soil interaction as a series of linear springs (uniform. with depth). Assuming appropriate values of soil stiffness (derived from published p-y curve for clay and selecting an appropriate displacement level), investigate how the natural frequency of the tower is affected by considering the foundation flexibility. Comment on your results suggesting changes for the potential cases in which the tower dynamics would not be appropriate. Include a discussion on how the soil spring stiffness may change during the lifetime of a wind turbine.

3. ANNUAL ENERGY YIELD ANALYSIS (30%)

The specific development site within the Atlantic Array is shown in Figure 1. Extracting this and assuming a straight boundary on the south-west side, Figure 2 gives the outer boundary of the development site with vertices (labelled A-G) as defined in Table 1. A wind survey of the site has shown a substantial variation in hub-height average wind speed with direction. Table 2 gives the average hub-height wind speed across a year by direction (the cardinal direction given is where the wind is blowing from).

As noted on page 1, the proposed farm layout is a grid of 5 rows each of 5 turbines. The rows are spaced 5D apart and the columns 10D. The grid is rotated to align with the prevailing wind direction (south-west) such that the turbines are spaced 10D apart in the flow direction, and 5D normal to it.

For this section, you will need to calculate probability data for the directions and use this to determine the expected annual energy production. For the analysis, assume no ABL and work at hub-height for all calculations.

Complete the following tasks:

· Assuming the wind obeys a Weibull distribution with a shape factor of 2, plot the wind probability distributions for the direction with the highest average wind speed (south-west) and the lowest average wind speed (north);

· Calculate and produce a wind rose for the site. You should decide the directions to plot, the intervals and the scales used;

· Plot the proposed wind farm layout showing the site boundary;

· Using FLORIS as the wind farm simulation environment, determine the expected power for each wind direction. You should use your power curve derived in section 1. Using these results, calculate the annual expected power (AEP) of the farm. Compare your answer to the UKs typical annual energy requirement of 350TWh;

· Modern control systems usually align turbines to directly face the oncoming wind direction. Discuss any positive or negative benefits that purposely mis-aligning the turbine yaw values with the oncoming wind direction may have. You may choose to perform. further simulations in FLORIS to underpin your discussion.

4. MULTI-ROTOR SYSTEM (20%)

There is clearly an enormous challenge involved in the development and engineering of a very large cost-effective offshore wind turbine of 18MW with a 208m diameter. Before proceeding with such a development into prototype production and testing, the wind turbine manufacturer wishes to consider an alternative, more innovative and radical approach which may have the possibility of reducing development risk by making use of smaller components. The approach proposed is to develop the 18MW wind turbine based on the use of a multiplicity (at least 2 and perhaps many more) smaller rotors all mounted on a single support structure. A recent example of such a multi rotor turbine has been developed as a prototype by Vestas at 900kW scale (see figure 3).

Your task is to undertake a simple scaling analysis to investigate for the multi rotor configuration how the rotor diameters and system mass (and cost) compare with those for the single rotor turbine. In this analysis it can be assumed that the small rotors of the multi rotor configuration are simply scaled down versions of the large, three bladed, single rotor.

Complete the following tasks and in doing so, provide the workings of your analysis in addition to the numerical result:

· Recalling that the power output and hence energy yield of a turbine is proportional to the swept area of its rotor, and assuming that the 18MW turbine is made up of 4 rotors on a single support structure, calculate the diameter of each of the 4 rotors to ensure the same total power and energy yield as that from the single 208m diameter rotor analysed in earlier sections of this project;

· A common and reasonable scaling rule approximation is that the mass and hence cost of a rotor system is proportional to its volume and hence to the cube of rotor diameter. Based on this assumption, calculate the mass (and hence cost) of the 4-rotor system as a percentage of that of the single 208m diameter rotor.

· For a multi rotor system with “n” rotors, derive expressions purely in terms of n for the ratios of:

1. Diameter of each of the n rotors to the diameter of the single large rotor;

2. Total mass (and hence cost) of the n rotor system to the mass of the single large rotor.

· Identify and explain advantages and disadvantages of a multi rotor configuration relative to a single rotor solution, highlighting the key challenges and risks involved in such a development. Consider issues such as design complexity, reliability, safety and structural integrity, performance, fabrication, installation, operation and maintenance and their potential impact on LCOE.

· Provide a critical recommendation on whether further investment into more detailed modelling/analysis and design of a multi-rotor configuration can be justified before making a final decision on whether to proceed with a single-rotor or multi-rotor turbine for the site  (i.e. do the potential advantages of this relatively immature technology warrant further development investment and if so, why?).

REFERENCES

[1] Tony Burton et al, Wind Energy Handbook Second Edition (page 17), John Wiley & Sons Ltd, 2011.

Make sure that you reference all data gained from external sources.