Tidal Stream vs Wind Energy

The value of predictable energy when combined with battery storage

When writing a blog about the Channel Islands the other day (link here) we had an idea regarding the fact that there are good renewable sources available to these islands, but some are still reliant upon diesel generators. Taking an example of the island of Alderney, could we integrate existing renewable devices to significantly reduce reliance upon diesel generation? What would be the best resource to provide this base load?

As a means to make this comparison tidal current, wind speed and electricity demand data for the island were required. The demand came from a published value of 7.3GWh/year and we scaled the UK national grid 5min demand data to match the overall annual demand for Alderney. We acknowledge this was not totally accurate as the islands experience a tourist season which could mean the distribution differs from that of the entire UK, but for the purposes of a quick study, this is what we used.

The tidal current data for the Alderney Race is available and has been used in research papers in the past (see Goss et al, 2020) and the wind speed data for a site just off the western shore was taken from NOAA-CIRES-DOE hindcast reanalysis V3 dataset. We now had all the inputs for a MATLAB model of an energy system.

The two systems used for comparison were that of a 2MW Orbital tidal turbine and a 2MW offshore wind turbine (recognising that a 2MW turbine would not be a new unit, but one that had been repurposed for this task).  The 2MW capacity was based on the fact that a daily peak demand was close to 1.5MW and the fact that the renewable sources are intermittent we would still likely want to charge the battery to cover the non-generating time. In combination with either renewable asset was a 4MWh battery with a peak charge/discharge rate equivalent to the rated power of the renewable source (2MW).

At any given instance in time the system could either:

  • provide renewable energy to the grid (demand) as needed
  • charge the battery using the renewable source
  • discharge the battery to meet the demand
  • dump excess renewable power - we recognise that this would not be likely, rather units would be run at lower power or turned off to prolong their lifetime
  • request alternative power source to fill in any gaps
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Tidal Stream Power, Wind Turbine Power and Demanded Power. 1yr, January and August

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Wind turbine power, demanded power, battery charge level and the required power from another source for the wind case (1yr, January and August)

Running this model for 2013 produced some interesting results. In all of these plots the wind energy is green, the tidal energy blue, the demand orange, the battery energy level yellow and alternative power grey.

The top level results of this study are quite interesting with the tidal stream case able to meet almost all of Alderney's demand, requiring only ~480MWh of energy from alternative sources. Whereas the wind turbine case required ~1825MWh of power from alternative sources.

Some more details from the study:

  • Due to the energetic tidal current in the Alderney Race the capacity factor of the tidal stream turbine was ~52%
  • The offshore wind turbine capacity factor was slightly higher than the tidal case at 56%
  • In order to make the wind energy case as effective at reducing the need for diesel generators the battery would need to be ~20x the size at (80MWh). This is due to the seasonality of wind speeds compared to the repetitive, consistent tidal cycles.
  • Looking at the January example the tidal case needed 106h of generation from another source (diesel generators), whilst the wind case required 179h.
  • For August the tidal case required only 42h of generation from an alternative source, whilst this was 364h for the wind case.

The accompanying plots show the renewable power, demanded power, battery status and alternative power for a winter month (January) and a summer month (August). It is easy to see how the wind resource varies with the season and thus relies more significantly on alternative sources than the consistent, cyclic tidal power. The predictable tidal stream power in combination with storage allows the energy system to provide a much more stable base load than could be achieved with wind energy. This study shows the value of tidal energy in combination with storage as it has a fantastic ability to provide base load power.

We are fans of all types of renewables and to-date offshore wind has been the standard setter for all other renewables to follow. However, all our energy cannot come from offshore wind, we will need other energy sources that are more predictable and can provide base load power. Tidal stream energy has the potential to do exactly that, as shown by this small study for Alderney and could be applied (at a much larger scale) to the UK as a whole.

Examples of tidal with energy storage can be found at EMEC (link) where tidal energy is used to produce hydrogen (which is essentially a means of long term storage) and within Nova's Shetland Array (link)

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Tidal turbine power, demanded power, battery charge level and the required power from another source for the tidal case (1yr, January and August)

Post blog note: we have noted that our conclusions are supported by a very recent research paper on the topic (link here).

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