| For calculations Density of fresh water ρw = 1000kgm-3Density of sea water ρsw = 1025kgm-3 Density of air ρa = 1.29 kgm-3For a deep water wave, For an irregular wave spectrum of sea states, the Power per unit of crest length is given by: P’ (Wm-1) = or approximately P’ (kW/m) = 0.5 Gravitational constant g = 9.81ms-2The specific heat capacity of water, c = 4.2kJ/kg°CThe energy, Q (J) required to heat a substance of mass, m (kg) and specific heat capacity, c (J/kg°C) by a temperature Δt is given by ‘Q = mcΔt’ |
Question 1 Wind Energy Technology
i) Explain the parameters ‘load factor’, ‘array efficiency’ and ‘availability factor’ for a wind farm development and their importance to site economics.
ii) A 500kWe wind turbine has a blade diameter of 40 metres and reaches a peak efficiency Cp of 0.39 at a nominal wind velocity vn of 9ms-1 and TSR of 8.
Determine:
• the blade tip rotational speed in rpm and in radians/second in this operating condition; and,
• the value of Cp at the rated wind velocity vr of 15ms-1.
iii) Define the term ‘Tip Speed Ratio’ (λ) and discuss its importance to the efficient extraction of energy from the wind by a horizontal axis wind turbine.
iv) If a wind turbine radius is doubled (whilst all other factors such as tip speed ratio and blade geometry remain constant) determine the change in:
- power output;
- rotor rotational speed;
- torque; and,
- rotor weight.
Comment briefly on the implications of these characteristics for the ultimate physical size of wind turbines and their design.
v) Describe the mechanism of active pitch control as a means of controlling the power output from a wind turbine (use diagrams as appropriate).
Question 2 Wave Energy
i) Describe the processes involved in the formation of waves from calm conditions to a fully developed sea and discuss the nature of available power within a random sea regime.
ii) Determine the characteristics of angular frequency, wavelength, phase velocity and power per unit of crest for a deep-water wave of period 11.5 seconds and amplitude 1.7 metres.
iii) Explain what is represented by a wave energy spectrum. Describe the difference in spectral characteristics between a swell sea and a wind sea.
iv) Explain why hydraulic power take-off (PTO) systems are often the preferred option for offshore wave devices, and discuss the importance or otherwise, for wave energy devices to be ‘tuneable’.
v) An offshore wave energy device with a frontal width of 5 metres, an overall efficiency of 38% and a rated capacity of 350kWe has been operating for 6 hours in seas which are described by an Hs of 4 metres and Te of 9.5 seconds. Determine the electrical energy generated over the period and the load factor.
Question 3 Tidal Stream Energy
i) By considering a narrow tidal channel with favourable tidal current speeds, describe how a natural tidal regime may vary spatially and temporally within the water-column.
ii) Describe the effect of installing one or more tidal turbines in a narrow tidal channel. How might the natural tidal regime be shifted? How might resulting environmental and economic consequences limit the practical number of turbines?
iii) A tidal turbine has a diameter of 16 metres and develops a peak efficiency of 41% at a Tip Speed Ratio of 3.8 and current speed of 2.9ms-1. Determine the power output of the device at these conditions and the rotational speed of the rotor in rpm.
Question 4 Resource and Site Selection
Describe and explain how you would select a site for one (i) a small commercial tidal stream farm, (ii) a small commercial wave farm or (iii) an offshore wind farm. Answers should cover from the first theoretical prospecting for a suitable region, through to a description of survey and measurement methods and analysis. (Answers should be less than 1000 words excluding illustrations and references. Copied and pasted illustrations may be used, but must be clearly identified and referenced).
Question 5 Engineering, Structures and Moorings
Describe and explain the calculation, analysis and design necessary in preparation to deploy a prototype wave energy conversion (WEC) device in water of approximately 30 metres depth at a site strongly affected by currents and wind. (You may choose a particular device and mooring design as illustration, but you must give some justification for choices. Answers should be less than 1000 words excluding illustrations and references. Copied and pasted illustrations may be used, but must be clearly identified and referenced).
Question 6
A coastal housing development (500 houses) is to be established. As a renewable energy consultant, you have been commissioned to advise the developer on the potential for maximising the self-sufficiency of the scheme in terms of its energy use. The decision on location is yours, however in selecting the most obvious criteria is to choose a country and location area which you are familiar with. The selection should be based on a realistic need for housing and established infrastructure
Several key technologies have already been identified as potential energy providers: –
- Small or medium scale wind turbines;
- A small offshore wind, tidal or wave development (depending on the location that you chose).
- Domestic solar water heaters;
- Photovoltaic modules; and
- A community biomass CHP plant.
There may be other technologies that you can identify.
Please note that though “solar” and “biomass” should not be considered. It will be appreciated if you can chose a location and design that makes offshore wind, wave or tidal generation practical, but avoid unrealistic proposals.
Answer the following in short sentences or 2 paragraphs at most:
- Advise the developer on the anticipated energy needs of the scheme, including seasonal variations;
- Explain the technical operation of the potential technologies involved, their efficiencies, likely load factors, an indication of likely payback periods and any technical limitations;
- Assess the energy resources available at the site / required by the site including annual variations;
- Select the 3 technologies which you judge to be most appropriate and explain your choice;
- Define the recommended deployment and scale of each technology and its integration within the scheme;
- Discuss any materials or energy storage implications and opportunity for selling excess energy; and,
- Highlight any other recommendations which you would have on the implementation and operation of the scheme.
