How useful is tidal-stream energy for electricity supply?

Renewable energy could increase access to electricity, bringing communities out of fuel poverty, and provide grid-connected communities with a low carbon secure energy supply. However, temporal variability exists in non-thermal renewable energy sources, such as clouds for solar-PV and gusts for wind energy. The change to temporally variable renewable energy sources, from controllable and predictable thermal power stations, is a therefore a challenge for electricity grid system engineers. Due to the regular periodicity of the tide, tidal-stream energy is often stated as predictable. Here, we use various metrics to understand the temporal variability and asses the quality of tidal-stream electricity. Direct high-resolution measurements of power and electricity from a 1 MW tidal turbine (Orkney Islands, UK), and incoming hub-height tidal current velocity, were interpolated to a common timeseries at 0.5 Hz. Variability of shore-side measured voltage was found to be well within acceptable levels (∼0.3% at 0.5 Hz) and fine-scale temporal variability did not significantly affect resource yield estimates (<1%). Tidal power temporal variability, at sub 10-minute scales, was low (standard deviation 10–12% of rated power), with decreasing power variability for higher flow speeds. Fine-scale tidal current variability (i.e. turbulence) followed a normal distribution, and turbulence intensity levels reduced for higher mean flow speeds. Measured power variability was driven by this flow variability, allowing a statistical model to successfully predict power at 0.5Hz using broad-scale tidal resource model data. The synthetic power variability model reliably downscaled 30-minute tidal velocity simulations to power at 0.5 Hz (85% skill and 14% error), using a “t-location” distribution of observed fine-scale power variability in combination with an idealised power curve. The predictability and quality of tidal-stream energy was therefore considered high, which may be undervalued in future electricity system design. Moreover, tidal energy appears suitable for off-grid renewable energy systems as control measures and storage solutions can be designed to provide a firm supply, perhaps cheaper than some estimates of Levelized Cost of Energy would suggest.