Frontiers 2023 Green Energy

Offshore wind turbines: how big can they go?

Written by Johan Daelman

Johan Daelman leads floating turbine design at Thistle Wind Partners (TWP), which is developing two ScotWind offshore wind projects – the Ayre and Bowdun offshore wind farms. He will be speaking in this year’s Orkney International Science Festival about the engineering challenges ahead.

The TWP team is very excited to be partnering with this year’s Festival and taking part in this smorgasbord of science, technology and art.

We are sponsoring the Sun, a vast and glowing artwork in St Magnus Cathedral, which starts our conversation in the logical place: the Sun itself. By heating our atmosphere so unevenly, making it rise and fall, the Sun moves the air, which creates the wind, which will drive our turbines!

We’ll then dive into some astounding technologies coming up in offshore wind. We’ll bring in some theatre (where engineers of the past challenge us!) and I will give a lecture on floating wind technologies on the opening day, leading some floating wind experiments for visitors at the Family Day. You can see our schedule below.

Offshore wind turbines: how big can they go?

One of the hot topics for us, as we survey future markets and begin our design process, is: How big should our wind turbines be? And how big can an offshore wind turbine physically go?

Let’s go back to the beginning. The first offshore wind turbine was installed in 1991 at Vindeby in Denmark by Ørsted: each of the wind farm’s 11 turbines had a capacity of just 0.45MW (and some said that would be impossible!) Since then things have certainly billowed: today’s most powerful turbine (Vestas V236) has a 15MW capacity – that is 33 times more! Its blades are 115.5m (or eight UK buses) long, dwarfing Vindeby’s 17m blades

And so, we’ve seen how offshore wind engineers crash through barrier after barrier to create ever-bigger turbine sizes. So, can it go on forever? Godzilla-sized turbines

Firstly, I would say that there are a few constraints on turbine size from a technical point of view – the main issue here is that making the rotors bigger can place excessive loads on the foundation (the increased thrust can generate a gigantic bending movement at the bottom of the tower). An intriguing possibility exists to find alternatives to the three-blade model (spiral blades or even bladeless oscillating towers) – which would make turbines well beyond 25MW possible. But it remains an academic question for now.

The other big three restraints on size would be:

Manufacturing: A turbine manufacturer needs to make an existing model profitable before investing in a new (and bigger) next model, so this calms things a bit. Indeed, increasing the size of wind turbines also implies increasing the size of factories and tools, in particular the moulds for the blades, and the furnaces for the flanges. These are huge investments, and an ethical question arises: Would supersizing turbines to this extent effectively exclude local ports, supply chains and small businesses from the industry?

Less problematic than on land, transporting ever larger and heavier blades and nacelles by sea nevertheless requires ever larger boats. As with factories, any ship owner will first seek to make their existing boat profitable before investing in the design of a larger boat

More difficult than on land, the installation of an 18MW wind turbine at sea presupposes having a boat equipped with a crane capable of lifting blades that are 130m long and nacelles weighing 800 tons. In shallow water, a wind turbine is typically assembled using a jack-up vessel on a jacket foundation, i.e., placing a fixed structure on a fixed structure (as on land). But in deep water, this is no longer possible, it is therefore necessary to move the turbine from a floating platform (boat) to a fixed structure (jacket). If the wind turbine is itself floating (semi-submersible), it is then necessary to move from floating to floating (unless you attach the turbine to the floating foundation on land and then tow it out).

Having said this, I am confident that most of these logistical challenges can be overcome. If the increase in the unit power/size of offshore wind turbines were to stop one day, it would most likely be for legislative, economic or environmental reasons. For example, ethical questions are arising: Do we want to see such big structures in the seascape? Would they affect marine flora and fauna adversely? And would they cause too much obstruction to other marine users?

Are bigger and bigger turbines inevitable?

Of course, offshore wind developers like larger turbines as they capture far more wind and help achieve a lower cost of energy. The cost of development and operation is much lower across fewer turbines.

That’s the pro. The big con, however, is that the bigger you make a wind turbine, the riskier it becomes. If you have fewer and bigger turbines, and one of your turbines fails, you lose a lot more energy production and money than if one of many small turbines fails.

And so, this is where we are at TWP: we need to make some very fine judgements about the turbine models we choose for our Ayre and Bowdun projects. While we are only part-way through our investigations, we certainly think that 18MW turbines would be feasible for our projects (if we choose to go for bigger turbines). We think there is an outside chance that 25MW turbines could be around by the end of the decade, but may not be realistic for our projects.

Final thought: small turbines?

For utility-scale projects like ours, smaller turbines (under 15MW) would simply not produce enough power in a cost-effective way. However, that is not to say they do not have their place in the energy mix. A recent research project led by entrepreneur Trevor Hardcastle of Frontier Technical in Tyneside has shown how small, modular turbines could be built on floating platforms by remote and island communities themselves (in a ‘flat-pack’ fashion) and then maintained locally. The project worked with researchers in Bangladesh to model how small fishing communities could maintain such turbines themselves at very low cost.

Small arrays of turbines for off-grid communities (called ‘non-interconnected zones’ in jargon) have also played a historic role in providing test laboratories for innovative approaches and technologies. In many island communities, imported diesel is still often the only source of fuel, which is an expensive choice for them and a high-carbon option. It is exciting to think about how small-scale wind, wave and tidal energy could play an important, complementary role to the Titans of the sea.

When and Where – TWP’s Offshore Wind Shows

Thursday 7 September: Power from the Sea Wind, A Talk by Johan Daelman, 11.30 – 12.30, Phoenix Cinema, Pickaquoy, Kirkwall.

Saturday 9 September: TWP Offshore Wind Stand and Live Experiments, Family Day, Kirkwall Grammar School (further school engagements TBA).

Saturday 9 September: Impossible Engineering?, an Engineering Play with Mike Stevenson – meet the Goddess of the Wind Zephyra, a grumpy Archimedes, Scottish pioneer James Watt and see who wins the debate! Performances at 10 am, 12.30 pm and 3 pm in the St Magnus Centre, Kirkwall.

About the author

Johan Daelman

Johan Daelman leads floating turbine design at Thistle Wind Partners (TWP), which is developing two ScotWind offshore wind projects – the Ayre and Bowdun offshore wind farms.