By Simon Watson, Professor of Wind Energy at Loughborough University
In 1993, nine 300kW turbines were installed on the eastern pier at Blyth Harbour, near Newcastle on England’s east coast. One can question whether this really counted as an offshore wind farm, but it was the UK’s first tentative step towards building wind turbines at sea. Ten years later, the country’s first truly offshore wind farm was built at North Hoyle in Liverpool Bay, 6km from the coast of North Wales. Thirty 2MW turbines were installed which could provide the electricity needs of around 40,000 homes.
Fast forward to 2016 and the UK is the world’s leading developer of offshore wind power with a total installed capacity of more than 5,000MW. The London Array, built in the outer reaches of the Thames Estuary, presently stands as the world’s largest offshore wind farm which, with a capacity of 630MW, can rival a medium-sized gas-fired power station.
The proposed Hornsea Project Two offshore site, which the UK government has just approved, will be larger still.
More than 100km from the east coast (see red dot in photo), the project will, together with neighbouring Hornsea One, provide up to 3,000MW. It will dwarf any of the more than 60 offshore windfarms already built.
These projects are getting bigger – and heading further and further offshore. Where will it end?
Why Windfarms are Going Further out to Sea
The drive to develop offshore wind power, despite its higher cost compared to onshore wind farms, has come from a politically driven perception that onshore wind power is too intrusive in a country which is relatively densely populated and values its landscape, as well as a recognition that winds offshore are stronger and more constant.
A metric known as the “load factor” is used as a measure of how much a power plant produces on average (taking into account it is not at full output all the time) as a fraction of its maximum possible output. For onshore wind power this was 29.4% in 2015 and for offshore it was 33.3%. During a particularly windy December 2015, the London Array’s load factor was 78.9% which is a level closer to that expected from a nuclear power station.
These figures are testament to the higher and less variable winds that are seen offshore. As projects are built further from the coast, average wind speeds increase, along with the load factor.
The UK is also largely surrounded by shallow waters, which also works in its favour. For instance, Dogger Bank, site of another huge proposed windfarm, is more than 100km from the coast, yet the sea is less than 30m deep in much of the area (indeed “Doggerland” was above the waterline during the last ice age, and was home to thousands of humans). This leaves much of Britain’s offshore potential well within the reach of established and relatively cheap foundation designs such as piles driven into the sea bed.
Growing Pains
How much offshore wind power could feasibly be installed in UK waters? The answer will be primarily driven by economics and constraints, which in turn depend on a number of physical and logistical factors.
As we move to deeper water sites, those cheaper foundations are no longer feasible. Indeed, beyond depths of 80m, floating foundations are required. This technology, though common in the oil and gas industry, is still at a prototype stage for offshore wind and will inevitably increase costs.
Greater distances from shore make both construction and maintenance more expensive, as boats have further to travel from the relatively few large ports available to service huge offshore wind farms. In addition, the relatively small number of specialist vessels required for these tasks limits the rate and scale of construction.
The UK has some of the busiest seas in the world both at the surface, criss-crossed by shipping lanes, and on the sea bed in the form of cables, pipelines, shipwrecks and unexploded ordnance. These factors to some extent restrict where offshore wind farms can be built. Beyond 80km or so from the coast, special high-voltage direct current (HVDC) grid connections are required to export the gigawatts of power that will be generated. Though this technology is relatively well established, it is currently more expensive than the more common alternating current (AC) cabling – and connecting multiple offshore wind farms to a common connection point offshore is challenging.
Environmental factors have to be considered too. A site may be off limits if it may have an impact on a particular bird species. For example, one of the reasons that the proposed site at Shell Flats, off the coast of Blackpool, was abandoned was because of its potential impact on the common scoter sea duck.
The number of turbines that can be installed in a given area is also limited by aerodynamics: if turbines are too close, those downstream have to work with reduced wind speeds and increased turbulence. The former reduces the power available and the latter increases fatigue and reduces turbine lifetime.
Taking all of these factors into account, a recent study of the UK’s offshore wind energy potential has suggested that the total amount of economically feasible installed capacity offshore might be up to 675GW. This could provide more than six times the UK’s present national electricity demand. Capacity is currently just 5GW so, in other words, the UK has still exploited less than 1% of its offshore wind potential. In the unlikely event that this amount of offshore wind power was built, it would make the UK a major exporter of renewable energy to continental Europe.
Where next for UK offshore wind power? The study which reached the 675GW figure assumed a levelised cost of energy (LCOE), which factors in construction and maintenance costs, of up to £120/MWh. By 2020, the government wants to get costs down to £100/MWh. A number of the developers in British waters are bullish about such reductions and by 2025, they expect costs of £70/MWh, well below the strike price of £92.50/MWh agreed for the Hinckley Point C nuclear power station which is not expected to come on stream until at least 2023.
However, there are challenges ahead. Even if operational improvements and more efficient designs do manage to keep costs down, there remains the question of how to integrate a variable form of power generation into the UK grid. This will be achieved by more advanced controlled strategies for offshore wind farms, greater network interconnectivity with European neighbours, smart and flexible demand-side management technology and ultimately cost-effective energy storage.
This article was originally posted on The Conversation.
