Abstract
Temporal variability in renewable energy presents a major challenge for electrical grid systems. Tides are considered predictable due to their regular periodicity; however, the persistence and quality of tidal-stream generated electricity is unknown. This paper is the first study that attempts to address this knowledge gap through direct measurements of rotor-shaft power and shore-side voltage from a 1 MW, rated at grid-connection, tidal turbine (Orkney Islands, UK). Tidal asymmetry in turbulence parameters, flow speed and power variability were observed. Variability in the power at 0.5 Hz, associated with the 10-min running mean, was low (standard deviation 10–12% of rated power), with lower variability associated with higher flow speed and reduced turbulence intensity. Variability of shore-side measured voltage was well within acceptable levels (∼0.3% at 0.5 Hz). Variability in turbine power had <1% difference in energy yield calculation, even with a skewed power variability distribution. Finally, using a “t-location” distribution of observed fine-scale power variability, in combination with an idealised power curve, a synthetic power variability model reliably downscaled 30 min tidal velocity simulations to power at 0.5 Hz (R2 = 85% and ∼14% error). Therefore, the predictability and quality of tidal-stream energy was high and may be undervalued in a future, high-penetration renewable energy, electricity grid.
Highlights
It is vital that countries convert to a sustainable low-carbon electricity system, and yet many renewable energy sources exhibit variability in power output over a range of time-scales with low predictability compared with traditional electricity sources (Drew et al 2019)
Using a 10 minute moving average on the 0.5 Hz data, the mean tidal current and associated turbulence intensity (Eq 3), as well as turbine measured power and voltage variability (Eq 4) was calculated; see Figure 6 and Figure 7 for 26-Oct-2014 and 26-Nov2014, respectively
Rapid fluctuations in power generated by renewable energy sources are known to cause problems to power system operation because they result in power unbalance and power quality issues (e.g. Ellis et al 2015)
Summary
It is vital that countries convert to a sustainable low-carbon electricity system, and yet many renewable energy sources exhibit variability in power output over a range of time-scales with low predictability compared with traditional electricity sources (Drew et al 2019). Different solutions are available to mitigate this concern, for example: the use of ‘spinning reserve’ when demand exceeds generation, and curtailment of energy sources when generation exceeds demand – as well as more sophisticated control strategies (see Grotz, 2008; Swain et al 2017; Pinson et al 2017). Both of these solutions result in significant drawbacks: spinning reserves include devices such as diesel generators and, they are potentially polluting and costly. As a result of the variability of renewable energy resources, higher costs are expected to be incurred in a future low-carbon electricity system (Slootweg et al 2003; Albadi and El-Saadany 2010; Joos and Staffell 2018)
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