Untapped resource: An analysis of tidal stream energy extraction in Ireland and the UK


Events in Ukraine have highlighted the dependency in the west on oil and gas imports, so a focus on local energy production – preferably renewables – is required, and in the UK and Ireland generation of renewable electricity has grown over the past 20 years or so, writes Mike Barrett.

Wind-generated electricity accounts for some 34% of our electricity demand in Ireland, according to the Sustainable Energy Authority of Ireland (SEAI) ‘Energy in Ireland, Report 2021’ and 24% of the UKs requirements according to the UK Office for National Statistics.

While the contribution of wind energy in both Ireland and the UK is substantial the output from tidal energy (tidal stream or wave) is zero in Ireland and statistically insignificant in the UK.

Tidal stream energy generation has advantages over wind in that it is predictable and, due to the difference between air and seawater densities, it can extract more energy per volume.

However, the commercialisation of wind energy systems has far exceeded that of tidal stream turbines and is a mature technology with a range of support services. This article will explore the reasons for this difference in the technologically similar wind and tidal stream contributions to the national grids of both states.         

Tidal energy extraction

Tides occur due the gravitational pull of the Moon and (to a lesser extent) the sun on the world’s oceans. Springs (or high tides) occur at the places on the Earth’s surface nearest to the moon. Neap (or low tides) occur at places at an angular position of 90° from the location of the spring tides.

The dates, times, velocity and range (height) of tides are predictable and are published in tide tables worldwide. Therefore, the energy that may be produced by tides can be calculated for any particular location taking into consideration local geographical influences that can impact on energy levels.

There are two methods are in use for tidal energy extraction:

  • The barrage method, which is based on tidal range;
  • Tidal stream, which is based on the velocity of the tidal stream.

Tidal barrage systems

The barrage system is a dam-like structure erected across a fast-flowing tidal inlet to which turbines are attached which capture the rising and falling tides. There are two major implementations of tidal barrages worldwide, La Rance River installation in France (1966) and the Sihwa Lake barrage in South Korea, completed in 2011. 

Figure 1:  La Rance River Barrage

The La Rance River system, in Saint Malo, Britany, occupies an area of 22km2 with a wall length of 332.5 metres. The tidal range is 8.2 metres average and 13.5 metres at spring tide.

The installed capacity is 240MW.The Sihwa, South Korea, barrage system, placed between Sihwa Lake and the West Sea, occupies an area of 30km2with a 12.7km long seawall. The total installed capacity is 254MW. The seawall has a series of sluice gates.

The method of operation is: the gates are closed against the flood tide from the West Sea until it (the tide) reaches an optimum height at which point the gates are opened allowing tidal flow through the turbines, thus generating electricity. Electricity is generated in one tidal direction only. The tidal range at the site is 5.6 metres average value (across neaps and springs).

A barrage power plant was commissioned in Annapolis Royal, Bay of Fundy, in Nova Scotia in 1984 with 20MW of installed power. The tides in the Bay of Fundy are among the greatest in the world with ranges of 15-16 metres at different locations. This station was shut down in 2019 principally due to technical and environmental concerns.

The barrage method is controversial because of the environmental impact and high construction costs. On the positive side however, there has been a constant power output over the years from the La Rance River and the Sihwa lake sites. Furthermore, as the turbines are land based, maintenance is less problematic than those deployed in tidal streams.

Tidal lagoon in Swansea

The barrage type of installation involves a structure that spans an entire river estuary. This method, as previously stated, is controversial in an environmentally conscious era. An alternative structure is a tidal Lagoon. This encloses an area of coastline. One such implementation is at the development stage in Swansea, Wales.

Figure 2:  Proposed Swansea Tidal Lagoon

The Swansea area, being in the Severn estuary, has the advantage of a high tidal range. The lagoon, as proposed, will be enclosed by a 9.5km breakwater wall with gates to control the flood and ebb tides.

Bidirectional turbines will be installed in housings in the wall sufficient to provide power, according to the promoters, Tidal Lagoon Power, to 155,000 homes. The Swansea project has wide acceptance across stakeholders in Industry and the public generally.

It is proposed that the lagoon project will incorporate aquamarine projects, water-sports facilities and a visitor and education centre. The project, apart from its obvious purpose of generating clean and renewable electricity, should lead to industrial development in the south Wales region.

Turbines

Turbines, whether tidal-stream or wind are basically similar.

The blade or propeller is driven, typically at slow revolutions, by the tidal current or wind. The function of the gearbox is to convert the slow revolutions to a higher speed to allow the generator to generate an EMF, e, according to:

                               e=Ndt  ,                                                                        

where, e is the generated EMF in volts, N the number of stator winding turns, ϕ the magnetic flux density in Webers, traversed in t seconds.  As the frequency of the EMF generated is a function of the blade speed it must be converted to the grid frequency.

The converter rectifies the three-phase voltage from the generator and generates a grid-synchronized three-phase voltage. The (theoretical) extractable power (P, Watts) by the turbine is determined by the sweep area of the blade (A), the velocity of the medium (air or seawater) (V) and the medium density (𝝆) and expressed as:

                                          P= 12 ρAV3                                                                   

When comparing the output from wind or tidal-stream turbines the density of the medium (𝝆) is an important factor, given that, for air   𝝆 = 1.225Kg /m³, while for seawater the density is 1026Kg /m³. 

So theoretically, if a wind turbine generating 1MW is uprooted and placed in a tidal-stream of similar velocity as that of the wind the output power will be 837MW.

In reality however, a 1MW wind generator will have a rated wind speed of around 16m/s, a value not found in tidal streams, while the rotor diameter would be some 50 metres which would not survive the impact of the tidal-stream!  What this does mean, however, is that a much smaller tidal turbine will produce the same power output as the larger wind system.

Tidal resource of Ireland and the UK

The Irish tidal-current resource figures come from a study commissioned by SEAI in 2004. In summary, the theoretical tidal energy was calculated as 230TW/h per year.

The technical tidal current energy was a calculation based on areas where the tidal flow peak velocity was greater than 1.5m/s. and the efficiency of the turbine is taken as being 0.39 and was calculated as being 10.46TW/h per year.

The practical tidal current energy-resource calculations depended on water depth, excluded shipping channels, areas where submarine cables and pipes were laid and military zones. The practical tidal-current energy was calculated as 2.633TW/h per year.

The accessible tidal current energy resource is limited by manmade rules and regulations such as environmental and planning constraints. The areas on which the assessments of practical tidal current energy resource were calculated were free of those constraints, so the resource didn’t change (2.633TW/h per year).

The viable tidal current energy resource calculation is based on commercial limitations and is 0.915 TW/h. (915 GW/h per year), which was predicted as being 2.18% of the electricity requirement for the country in 2010.

The SEI 2004 study, makes the point that this figure could rise as turbine technology develops. This potential contribution to Ireland’s electrical energy capacity is especially important as the wind sector contribution is not constant. SEAI reports 2021 as being a ‘low wind’ year, down 18% on 2020.

The most advantageous sites for tidal current energy exploitation identified in the 2004 report are the north/east coast with a viable energy resource of 273GW/h per year, Strangford Lough with 130GW/h, Tuskar Rock and Carnsore Point with 177GW/h and Shannon Estuary with 111GW/h.

The first and only, grid-tied tidal stream turbine, in Ireland, was installed in Strangford Lough. Regarding other areas where the tidal currents velocities are less than optimal, a paper by a researcher who has studied this (2004) report, makes the point that tidal turbines could be designed to suit those sites.

The UK resource

A report titled ‘UK Tidal-Resource Review’, commissioned by the Sustainable Development Commission, reviewed the potential for tidal stream and tidal range energy extraction around UK coastal waters.

From the data produced by this review the extractable tidal stream resource in the UK was calculated as 22TWh/y. Other sources, publishing later, estimate the UK technically extractable tidal resource figure as 34TW/h per year, which would be equivalent to 11% of the UK electricity demand, and 20.6–29TW/h per year. The actual installed capacity is 24MW (2022).

The most significant tidal-stream resource is focused in a limited number of sites including the Pentland Firth and the Channel Islands.

Tidal range extraction was also considered in the report. The areas with the greatest potential were identified as the Severn which includes the Welsh and Devon coasts. The west coast, which includes the areas of Conway, the Mersey and Liverpool, Duddon, Morecambe Bay and Wyre up to the Solway Firth and the Wash on the east coast are areas of tidal range potential.     

A short history of the tidal stream sector in the UK

Peter Fraenkel could justly claim to be the founder of the tidal stream electricity generation sector in the UK.

Frankel, who founded Marine Current Turbines (MCT) deployed a tidal stream turbine project, called Seaflow, some 1.1km off the Devon coast. This was a demonstration project which developed up to 310KW (its rated power being 300KW).

The generator was not grid tied but yielded useful design information and informed future turbine designs. Data collected from the project focused on turbulence, velocity shear, vortex shedding and the effects of passing waves.

Frankel and MCT implemented the SeaGen project in Strangford Lough in in the north in 2008. This was licensed by the Northern Ireland administration. The site was in a fast flowing tidal stream, peaking at 4m/s, between the Lough and the Irish Sea, known locally as ‘The Narrows’, near the village of Portaferry.

Figure 3: SeaGen in Strangford Lough

Prior to the installation a detailed environmental study was carried out. The installation consisted of a fixed base with two 600KW turbines mounted on crossbeams which could be raised and lowered to facilitate physical inspection of the turbines.

SeaGen was grid-tied and was the subject of a study by the Irish Electricity Supply Board (ESB), MCT and Chalmers University, Sweden to study the power quality (PQ). The focus of the study was to assess that the power exported to the grid was compliant to the EN50160 standard. The study, which ran for four months, consisted of a Power Quality (PQ) analysis of:

  • Power production;
  • Power frequency;
  • Voltage variation;
  • Voltage imbalance;
  • Total Harmonic Distortion (THD);
  • Flicker.

Item 1 indicated that 186.1 MW/h was delivered in a 34-day period, for five days of which only one turbine was operational due to a technical issue. Items 2-6 were found to be compliant to EN50160.

The estimated cost of the installation was £12m. The company (MCT) received a £5.2m UK government grant and £500,000 from the Northern Ireland Assembly. The power produced was purchased by ESB International Energy for sale to customers in the republic and the north.

SeaGen delivered in excess of 8GW/h between 2008 and 2014 and was a case study for further projects by MCT. SeaGen was decommissioned and the installation recovered from the sea bed in 2016.

The European Marine Energy Centre

The European Marine Energy Centre (EMEC) was established in 2003 with funding from the Scottish, the UK government and the EU to kick-start a marine industry in the UK by providing ‘pathways to commercialisation’.

It is the world’s first such facility. The EMEC, based in Orkney, provides wave and tidal test facilities with access to the grid and other facilities, for client industries in the marine energy extraction sector.

Orkney was selected because of the fast-flowing tidal currents in the area. Clients include Atlantis Resources, OpenHydro, Voith Hydro, Andriz Hydro Hammerfest, Alstrum, Megallanes, Orbital and Verdant Power. A tidal test site was opened at the Fall of Warness, off the Isle of Eday in 2006. The centre become self-sufficient financially in 2011. Many marine energy devices now deployed worldwide were developed at the EMEC. 

MeyGen project by Simec Atlantis Energy 

MeyGen is located in a 3.5km2 site between the northeast of Scotland and the island of Stoma, south of Orkney, in the fast-flowing currents between the North Sea and the Atlantic. This is a three-phase project. Phase 1A is currently operational and consists of the deployment of 4 x 1.5MW turbines in a multi-turbine array.

Figure 4: A turbine for the MeyGen Project (SIMEC Atlantis Energy)

The turbines, a combination of Atlantis Resources AR1500 and Hammerfest AH1000 turbines, each mounted on 250-350 tonne foundation, delivered 17 GW/h to the grid in 2019. Phase 1B, now in construction, will add an additional two Atlantis AR2000 turbines to the array.

Conclusion

The slow deployment of tidal-stream turbines worldwide has been attributed by many researchers to high Operation and Maintenance (O&M) costs impacting on commercialisation.

This is due to the challenging sub-sea conditions of fast-flowing tides, turbulence, corrosion and access difficulties. There is however, on-=going international research to address those issues and thus increase investor confidence.

The relatively fast turbine deployment in the UK (24MW installed capacity) compared to the zero installed capacity in Ireland may be attributed to two factors, the pioneering work of Peter Fraenkel and the establishment of the EMEC.

In addition to the tidal stream resource, an SEAI report ‘Renewable Energy in Ireland 2013’ estimated that 21TW/h per year could be provided from wave energy.

Clearly, to start a marine-energy extraction industry in Ireland more state intervention is required to make the sector commercially attractive to investors.

To follow the UK model, which is producing positive results to date, would require more investment in the marine research establishments around the Irish coast with a view to fast-tracking turbine deployment.

A suggested site for large commercial turbine deployment would be Strangford Lough. It has many of the attributes required for a suitable site, namely the tide velocity, grid access, a completed environmental study and the fact that it has already hosted the SeaGen project. While the recent government offshore wind announcement is to be welcomed, a review of our marine resource is urgently required. 

Author: Mike Barrett is a former lecturer, now research active with an interest in condition monitoring of tidal-stream turbines


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