The Tide is Turning For Renewable Energy In The United Kingdom

Tidal lagoons are a simple concept involving the adaptation of standard, proven components used in global engineering projects.  The Swansea Bay lagoon will comprise a U.K. standard sand-core breakwater or rock bund, similar to many seen in coastal defense schemes and harbor walls. The generating equipment of bulb hydro turbines have been used for many years on run-of-river hydro power schemes as well as some landmark tidal barrages.  The hydro turbines are mounted inside concrete turbine housings and are permanently submerged so the resulting view is of a ring-shaped harbor wall with one section of concrete casing. 

As the sea outside the breakwater rises and is held back a difference in water levels is created, known as ‘head’, and once a sufficient head height is reached sluice gates are opened and water flows into the lagoon through turbines that generate electricity.  This process then occurs in reverse, on the ebb tide, as sea levels start to fall and a tidal head is created by holding water back within the lagoon. This way the tides can flow through these turbines four times daily to generate power. Fig. 2 illustrates the tide movements relative to the turbines. Turbines work regardless of the direction of water flow direction, which changes from ebb tide to flood tide.

System proposals involve the use of existing technology in the form of low-head bulb turbines that generate electricity when water flows the past turbine blades.  The flow is created by gravity through the difference in head (or tidal height) between the inside and outside of the lagoon walls. This type of turbine has been used in many hydro power applications, a good example of which is La Rance barrage in Brittany, France, which has been successfully operational for half a century.

The primary difference between previous applications and the tidal lagoon is the efficiency of electricity generation in both directions, on ebb and flood tides.  Designers are working with the world’s leading turbine manufacturers to optimize turbine technology and energy outputs.  Accurate tidal data has been used in energy output studies undertaken from 2011 onwards, to assess the potential for energy extraction through a tidal lagoon.  Results have been extremely encouraging and the predictable power output offers the U. K.’s National Grid an entirely-predictable source of electricity capacity.

The proposed turbines will have a runner diameter (span of the turbine blades) in the region of 7 m and will be permanently submerged below the low water level (Fig. 3).  Turbine optimization and modelling is ongoing as designers assess the most efficient configuration.  

Tidal Lagoon Power Ltd. (TLP) plans to develop six lagoon power stations in the U.K. - the first of which is a 320 MW power station in Swansea Bay where construction is due to commence during the first half of 2015. The entire lagoon construction program will be spread over a 12-year period (from 2015 to 2027). Once fully operational in 2027, the six lagoons would generate 30 TWh of electricity per annum, equivalent to 8% of forecasted U.K. electricity production in 2027 and enough to power 7.9 million homes.

Developing a full-scale tidal lagoon industry in this manner (as opposed to building a single power plant) can benefit from significant economies of scale in both construction and operation. Developing six lagoons with overlapping construction schedules allows bulk orders of components, shared resources and operational efficiencies which can be applied across multiple lagoons.

A key advantage of lagoon construction is the domestic oriented nature of the supply chain. A majority of the value of components and construction contracts required to complete a tidal lagoon can be sourced in the U.K. This means that a higher share of the benefits associated with these large scale investments can be retained within the domestic economy.

Developing a tidal power industry can also benefit the consumer in the form of cheaper renewable electricity. Tidal lagoon power could be cheaper than offshore wind and comparable with nuclear generation for larger lagoons. The long lifespan of each lagoon means that consumers will be able to benefit from low-cost electricity from a domestic renewable energy resource long after the initial capital investment has been repaid.

The combination of these benefits means that tidal lagoon power presents an excellent opportunity for the U.K. to strengthen its electricity generation portfolio, helping to guarantee security of supply, while also achieving long term climate change objectives at a lower cost to the consumer and providing a significant final demand stimulus to the economy through the investment program.

The U.K. is legally committed to reducing greenhouse gas emissions by 80% by 2050 and to meeting 15% of energy demands from renewable sources by 2020. To achieve these renewable energy and emissions goals, the UK must increase the amount of electricity generated from renewables almost 5-fold on 2009 levels by 2020. The Department for Energy and Climate Change (DECC) wants electricity from renewable sources to play a key role in helping to decarbonize our energy sectors.

New capacity is needed over the next number of years due to the planned closure of fossil fuel plants. This presents an opportunity for the UK to reshape the electricity generation industry in a way that meets climate change targets and insures the country against exposure to price volatility in fossil fuels sourced from abroad.

Wind power remains the dominant renewable energy technology in the U.K. with 8.9 GW of installed capacity in 2012 and its share of generating capacity is projected to continue to increase as new schemes are commissioned. However, wind power suffers from several significant downsides that limit its ability to fill the gap in UK electricity generating capacity left by the closure of fossil fuel plants. Wind power has a highly variable output which means that it cannot produce the predictable generation that is needed by the National Grid to meet base load electricity demand. In addition, wind turbines are generally assumed to have a short lifespan of between 20 and 25 years which adds to their long term investment cost relative to other technologies.

Power from tidal lagoons represents an important component of this future electricity generation portfolio. Due to the cyclical nature of tides, power generation from tidal lagoons can be predicted years in advance. This gives tidal lagoon power a distinct advantage over some other renewable technologies as it does not suffer from the same forecast error. Therefore, it often requires a lower stand-by capacity (normally provided by a gas-fired plant which can be activated on demand) to balance supply and demand at peak times. At times when stand-by capacity is required, it can be provided by tidal lagoons operating to different tide timetables in other locations, or booked long in advance, thereby reducing unit cost.