Lecture 14 - Fluid Mechanics for Offshore Renewable Energy
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- Speaker: Dr. Christopher Vogel
Offshore renewable energy technologies currently in use include:
- Wind
- Wave
- Tidal stream
- Tidal barrage
Additional technologies under exploration include salinity and temperature gradient energy systems.
Wind Energy
Fundamentals of Wind Turbines
- The power generated by a turbine depends on the flux of kinetic energy through the rotor.
- The turbine must exert sufficient resistance to the flow to maximise power extraction.
- The Betz limit, which represents the maximum theoretical efficiency of a wind turbine, is \(\frac{16}{27}\) (approximately 60%).
- \(P = \frac{1}{2} \rho U^{3}_{\infty}A C_{P}\)
A low streamwise force results in high mass flow but low power extraction. A high streamwise force leads to low mass flow but high power extraction.
Current Status of Wind Energy in the UK
Wind energy's contribution to total UK electricity generation is as follows:
- 2019: 20%
- 2020: 25%
- 2021: 21%
- 2022: 25%
- 2023: 29%
The majority of installed capacity is onshore, comprising 9,145 turbines with a total capacity of 15,587 MW.
Current development efforts are focused offshore, with 2,765 turbines generating 14,733 MW.
Global Distribution of Wind Resources
Wind resources vary significantly across different regions.
Variability Scales
Long-timescale variability influences the total available wind resource, while short-timescale variability affects the mixing rate of individual turbine wakes.
A wind rose is a graphical representation of wind speed and direction, typically based on 10-minute averages.
Correlations in Wind Drought Events
Wind droughts occur when multiple wind farms simultaneously experience low wind conditions.
However, low wind conditions are not necessarily synchronised across all regions.
It is advisable to distribute wind farms across multiple regions to mitigate the impact of wind droughts.
Predicting Wind Farm Yield
Predicting the yield of a wind farm differs from that of a single turbine and is subject to temporal variations.
Tidal Stream Energy
Tidal stream energy is generated by gravitational interactions among the Sun, Moon, and Earth.
Amplification of tidal flows often occurs near coastlines, leading to accelerated currents.
Tidal energy is uncorrelated with wind energy, as they originate from distinct sources.
Design Considerations for Tidal Turbines
Underwater turbines are subjected to higher mechanical loads compared to wind turbines.
Hydrodynamic rotor design can leverage constructive interference effects to enhance performance.
This approach can result in increased power output.
However, designing relatively short, heavily loaded blades presents a significant challenge in composite materials engineering.
Demonstration of Constructive Interference Effects
Turbines designed to exploit constructive interference achieve a 20% increase in power output with only a 10% increase in thrust.
Benchmarking Studies for Tidal Turbines
Reducing uncertainty improves modelling accuracy and lowers costs.
Developing experimental setups and models is both challenging and costly.
Benchmarking exercises provide critical data for validation and improvement.
Unsteady Loading Tidal Turbine Benchmarking Study - Supergen ORE
Key challenges in the offshore renewable energy (ORE) sector and research include:
- Scale: Strategies for scaling up offshore renewable energy to achieve net-zero targets, and the associated challenges.
- Uncertainty: Enhancing the reliability of farm yield models and identifying missing physical processes.
- Resilience: The role of offshore renewable energy in building a resilient and reliable zero-emission energy system.