Why tidal power?

We need tidal power to complement solar and wind

Why tidal range energy?

Tidal energy can come from either tidal stream or tidal range generation.

Tidal stream generation is like to having the equivalent of the more familiar wind turbines in the water. These consist of one or more stand-alone units which rely on the velocity of the water passing their blades to generate electricity. The flow velocities need to be greater than 2.5m/s for this to be an attractive commercial proposition. Each individual stand-alone turbine would be expected to produce energy in the range 0.25MW to 5MW.

There are no suitable locations for significant tidal stream generation in the Severn Estuary.

Tidal range generation uses a difference in water levels across a barrier (a barrage or a lagoon) to generate electricity on rising and falling tides. A barrage will capture the greater proportion of the available energy as the tide enters and leaves an estuary. Lagoons operate in a similar way although they will capture a much smaller amount of energy. In both cases, the amount generated will be orders of magnitude greater than for tidal stream: for individual lagoons this will be up to 2.5GW and for a barrage it will be up to 15GW.

The Climate Change Committee’s targets for renewable electricity in the balanced pathway suggest that the UK will soon face a generating ‘emergency’. The only way to begin to address this situation is by harnessing as much as possible of the large tidal movement in the Severn Estuary. This is the case for a large barrage.

How electricity is generated

Electricity is generated from turbines in the water coupled to generators. This is well established and has been used in thousands of applications around the world.

The Severn Estuary requires low head, high flow turbines. This means that the difference in the water levels from upstream to downstream of the turbines should be 2m or less and they will need large apertures.

The turbine that has been used extensively for this purpose to is the Kaplan bulb turbine. It was first used commercially in 1922 for a run-of-the-river power plant. More recently, it has been adapted for tidal applications where the head to drive the turbine varies throughout the tidal cycle as well as needing to generate in both directions (on both the ebb and flow tides).

Although the basic turbine design remains similar to those used on run-of-the-river power plants, there have been improvements to accompany its use in tidal applications. These include:

• body has become more streamlined

• variable speed induction generators with frequency converters

• turbines ‘pump’ water at either end of the tidal cycles to seek, as far as it is practically required, to match the upstream water levels to those existing before construction of the barrage

• reversible and variable runner pitch angled blades and guide vane angles for optimising performance

• 3-bladed operation and lower speeds of rotation to reduce significantly the disorientating effects on fish and other estuarine fauna passing through the turbines

The combination of the last two points leads to the concept of triple regulation to promote efficient generation from varying head and flow rates. This is the most significant recent development to maximise the energy capture across the whole of the tidal cycle.

These improvements are supplemented with advances in optimisation software for fine control to maximise the length of generation and power output.

Whilst the Kaplan bulb turbine has a long and established track record, two other potential turbine types will be reviewed during the design development:

• An axial flow turbine consisting of two contra-rotating variable pitch blade rows. Whilst this was proposed by Atkins and Rolls-Royce as part of DECC’s Severn Embryonic Technologies Scheme (SETS) in 2010, it remains at concept design stage and would need development and full-scale testing before it could be considered for use. The period for this activity has been estimated as up to four years. However, the present proposal is for consideration of its viability including whether it is likely to have the necessary robustness as well as a suitably long interval between major overhaul for use in the Severn Estuary.

• A vertical axis turbine based on the Darrieus concept. This turbine comprises up to four fixed hydrofoil blades that are connected to a shaft that drives a generator. Such a turbine has been used in the Kislogubskaya Tidal Power Plant in the Murmansk region of Russia. It has operated successfully and almost continuously since it was built in 1968.

  The main advantage of this turbine is its simplicity. It would be easier to build but there is less information on its robustness for the more challenging conditions in the Severn Estuary. There are also questions about whether it would be able to generate over as much of the tidal cycle as the latest Kaplan turbines and thereby achieve comparable power outputs.

  The details of this turbine are confidential but we have the contacts necessary to explore this option further.

At this time, the default turbine will be the one that is tried and tested - the Kaplan bulb turbine for which the internal schematic is shown.

The selected turbine would operate with the tip speed of the blades below the ‘limit of negligible fish mortality’ suggested by Oak Ridge National Laboratory Research, as published by Idaho National Laboratory. It is accepted that this information will need to be reviewed for the diversity of fish in the Severn Estuary and whether any concerns over indirect as well as direct mortality are justified. There is further discussion on this in the Section on ‘Environmental benefits’.

Complementing wind and solar

Tidal movements are predictable for centuries ahead. These would allow a pattern of barrages and lagoons around the coast of Britain with different tidal times to be used to plan a near continuous supply. An alternative approach would be to complement the barrages with electricity generated from a hydrogen grid. Alternatively, more pumped storage facilities could be built and filled at times of high electricity generation to supply the Grid at times when barrages are unable to meet demand which would be around high and low tides.

With such provisions, the electricity output from tidal range generation would be made a relatively steady part of the base load for planning purposes.

Whilst wind generating capacity is increasing and will continue to do so for the foreseeable future, it has major drawbacks in terms of providing reliable base load. One is the effect of wind droughts leading to a variability of output which can be as low as 10% of the design output. This is illustrated in the image below captured from the Drax Electric Insights website for low carbon and renewables generation on 18 March 2019.

A second drawback is that care is needed with wind turbines during severe storms when wind speeds are very high. There are two commonly cited examples: the first is the widely circulated video footage of a wind turbine catching fire and breaking up in Ardrossan, Ayrshire. Another incident occurred in September 2018 when there was a storm in South Australia with gusts up to 140km/hour, representing a one-in-50-year storm. This resulted in a catastrophic chain of events which caused the wind turbines to be shut down and the State’s interconnector to suddenly become severed. Not enough gas generators were switched on to maintain the frequency and so the entire state blacked out since the Australian Energy Market Operator (AEMO) had not foreseen that this might happen. Notwithstanding and even with better preparation, this incident highlights the need to maintain sufficient standby backup to wind turbines.

Solar power is by its very nature only available during the daytime and fluctuates depending on climatic factors such as the time of year and cloud cover.