The UK government is committed to a net zero emission target by 2050 and as such around two thirds of existing power stations are expected to close by 2030.
The UK government is committed to a net zero emission target by 2050 and as such around two thirds of existing power stations are expected to close by 2030. In addition, the further electricity demand has led to the National Grid System Operator predicting that generation capacity will have to double by 2050 [3]
The government is currently proposing that the main component of this low-carbon generation will consist of offshore wind, solar, nuclear and gas-power used in combination with carbon capture and storage. However, there are doubts whether these technologies will be able to provide the generation capacity required at the current rate of growth. Furthermore, it is well documented that increasing integration of volatile, unpredictable sources of renewable energy such as wind and solar power jeopardises the stability of the power grid.
In contrast the tides are very predictable and the UK has potential to generate large amounts of low-carbon energy from them since it is blessed with some of the highest tidal ranges in the world particularly along the western coastline from the Severn to the Solway. Tidal range is a mature technology, with tidal range power plants having a history that extends back over 50 years and in the last 10 years there has been increased commercial interest and an R&D resurgence.
Figure 1: The UK Tidal Resource [4]
Tidal range power is generated from a head difference between two bodies of water. To create this difference an impoundment dam is used to separate the two areas and as the tide flows in or out, the dam blocks the flow of the tide and creates a head difference. When the head difference has reached an optimum level, the water passes through the turbines placed within the dam. With two tidal cycles per day, this head difference is created four times each day (as the tide comes in and out). Thus, energy can be generated in either direction, known as flood and ebb modes and in both directions, known as dual mode. Generating electricity using tidal barrages is mature and reliable. The most notable example is the 240 MW tidal power station located at La Rance river opened in 1966.
Figure 2: La Rance Tidal Power Plant
Figure 3: How tidal range power works
There are several tidal range projects already in development in the UK that offer an achievable 10 GW installed capacity and delivering over 20 TWh/yr (or roughly 5% of UK energy use). The most newsworthy recent proposal for the Mersey could alone have 5 GW potential capacity and in February 2020 the Liverpool City Region Authority awarded £2.5 million to develop preferred options for the project.
Despite independent government reports such as the ‘Hendry Review’ being in favour of tidal range in principle, there are several factors that preclude development most notable are the environmental concerns and cost. Unknown or difficult to quantify environmental impacts have prevented numerous developments from being approved and the government takes a risk averse approach. Nevertheless, it can be argued that concerns about impact to existing wildlife needs to be balanced against future environmental challenges such as sea level rise due to climate change. In addition, no developer has yet reached an agreed Contracts for Difference (CfD) price with the UK government with the next cheapest option, offshore wind, being nearly half that of tidal range [7]. Nevertheless, one conclusion of the Hendry Review was that a tendering process taking into consideration the approximate 120-year project lifetimes could see tidal range play a competitive role as part of the UK’s energy mix in spite of the high capital costs.
In a joint paper from Dr Sean Petley and Professor George Aggidis and a team from Liverpool University, it was argued that tidal range energy projects should be developed holistically and recognise that they are unique among power generation installations, in that they inherently provide opportunities beyond energy generation, including environmental, societal and economic opportunities.
Figure 4: Vision for Mersey Tidal Barrage from Lancaster University and Liverpool University
By considering the needs of people by offering possible road and rail transport links, leisure and tourism options, and the need to create compensatory habitats for wildlife, organic architectural designs show how developers can enhance, rather than detract, from estuaries like the Mersey.
Figure 5: Mersey Tidal Barrage concept from Lancaster University and Liverpool University
It is suggested that, to date, this position has been inadequately addressed in the formulation of energy strategy, especially in respect of barrages' potential strategic roles in flood defence and transportation planning. In addition, the economic benefit of the range of employment opportunities will be far-reaching; not just in design, build and manufacturing, but also in the related services, such as tourism, recreation, recruitment, and legal and financial services. It follows, therefore, that apart from the direct appraisal of energy capture, other complementary investigations must be sufficiently advanced to enable proper input in decision-making in respect of these "secondary" functions, as well as the various potentially adverse issues.
In summary, the experiences at La Rance and elsewhere have demonstrated both the function and longevity of tidal barrages, however a UK based project has never progressed beyond the planning stages. The UK is blessed with some of the highest tidal ranges in the world and this potential energy can offer a significant contribution towards regional, as well as national, net zero targets. Nevertheless, it has long been noted that the success of a tidal barrage scheme depends upon striking the right balance between the mode of operation and biological harmony. It has been argued that by considering the design holistically rather than solely focussing on energy generation tidal barrages can be utilised to achieve a sustainable future.