Thought Leaders

Tidal Stream Turbines: The Future of Clean Energy Generation

Thought LeadersDr Danny ColesResearch Fellow - Tidal Stream Industry Energiser ProjectUniversity of Plymouth

The importance of accelerating net-zero technology use in society is growing. Whilst in the UK wind turbines will provide the primary source of renewable power in the future, they present significant challenges that prevent us from fully relying on this type of power source. Dr. Danny Coles speaks to AZoCleantech about tidal stream power and its potential to complement wind power by delivering predictable, reliable renewable energy.

1. How did you begin your research into tidal turbines?

During my degree at the University of Manchester, I enrolled in a renewable energy module that covered tidal stream power, along with other technologies such as wind and wave power. It was interesting to compare the benefits and challenges of each technology, and how they can complement each other to achieve the common goal of net-zero. The predictable, periodic nature of tidal power seemed very important at the time, and even more so now. This led me to take up a PhD in tidal stream energy resource modeling at the University of Southampton, where I focused on quantifying the tidal energy resource in the Alderney Race, and other sites around the Channel Islands.

2. What is tidal stream power and how does it work?

Tidal stream turbines convert tidal power to electrical power, using the same components as a wind turbine. Because of this, tidal stream turbines look very similar to wind turbines but are located underwater.

Tidal stream turbine blades act like wings on an airplane, where seawater flowing over the turbine blades creates a lift force. The blades are orientated in such a way that the lift force on each of the blades causes them to rotate. This rotation is transmitted to a generator, through a shaft. The generator converts the shaft power to electrical power by using the rotation of the shaft to rotate copper windings through a magnetic field. The electrical power is transmitted to the onshore grid using an underwater cable that lies on the seabed.

3. What are the main differences between tidal stream turbines and wind turbines?

Firstly, tidal stream turbines are smaller. The reason for this is two-fold. First, the density of seawater is approximately 800 times greater than air. The power generated by a turbine is related to the density of the medium being used to rotate the blades, and the length of the blades (amongst other variables). So if the density is greater, blade length can be lower.  The second reason is that the length of tidal stream turbine blades is limited by the depth of the water. The maximum blade length of a tidal stream turbine currently is around 10 m, whereas the length of wind turbine blades, which are exceeding 100 m in some cases, are less constrained.

Another difference is their prevalence. Wind turbines have been widely adopted in the UK, with approximately 10 gigawatts of offshore wind and 12 gigawatts of onshore wind currently installed. In contrast, the current installed capacity of tidal stream turbines is just 0.01 gigawatts. This is equivalent to just  0.05% of the total wind installed capacity. It is important to highlight that wind power was adopted far earlier than tidal stream, helped by early and continued financial support from Government to grow to this extent, whereas the tidal stream industry has had limited patchy support. However, with the Government’s announcement that tidal stream energy projects will get ring-fenced access to financial support in 2022, this is slowly changing.

4. How is it possible to predict how much power tidal stream turbines will generate?

The amount of power generated by a tidal stream turbine is dependent on the speed of the tidal flow incident on the turbine. The movement of the tides is driven by the astronomical motions of the sun, earth and moon. For example, as the moon rotates around the earth, the moon's gravitational force on the earth’s oceans creates a tidal bulge, where water is effectively pulled towards the moon. Humans have understood this phenomenon for centuries. With this knowledge, we can predict the tidal flow speed, and therefore turbine power, at any location, this time tomorrow, next week, next month, even this time 100 years from now.

5. What are the benefits of tidal stream power?

The main benefit of tidal stream power is that it is predictable. This means that it is possible to forecast how much power can be generated at any time in the future. Predictability is important because it makes it possible to plan which power generators and energy storage are needed to meet electricity supply with demand at any given time in the future. For example, if a natural gas power station needs to be shut down for maintenance, it becomes possible to plan the shutdown to occur when the power generation from other sources can cover the deficit. Long-term forecasting of wind and solar power is not possible because power generation is dependent on weather conditions that can only be forecast over relatively short periods into the future.

tidal stream

Image Credit: Andrey Armyagov/Shutterstock.com

Another benefit of tidal stream power is that it is not visible (in the case of tidal stream turbines standing on the sea bed), as they are completely submerged underwater. In the case of floating turbines, the floating structure is visible, but they are far less noticeable than an offshore wind turbine, for example.

The energy density of tidal stream power is relatively high. This means that a tidal stream turbine farm covering 1 acre will produce more energy than a solar PV farm or wind farm of the same size. As an island nation, space is limited, so using space efficiently to generate clean power will be important, particularly as the demand for electricity increases in the future.

6. Is there a potential for this to be adopted throughout the UK and across the world? What challenges may be faced and how could these be overcome?

Tidal stream turbines have already been adopted in the UK, as well as Canada, USA, China, and Japan, but at a relatively small scale. The UK is currently leading the way, with around 10 megawatts of current power capacity.

My recent research broadly supports findings that the UK could generate approximately 11% of its current electricity demand using tidal stream power. This is particularly important given the changing energy landscape of 2021 and early 2022, where imported fossil fuel price volatility is causing strategists to re-think how to ensure energy security as we transition to net-zero. Tidal stream cannot provide the answer on its own (nor can any technology), but it can certainly provide an answer.

There are four key challenges faced by the tidal stream sector; cost of energy, availability, environmental impact and power transmission.

Starting with the cost of energy, tidal stream energy is currently around five times more expensive than wind energy. Given tidal stream energy’s later adoption, it is on a far steeper cost reduction trajectory, which is expected to make it cost-competitive with technologies such as nuclear, biomass and natural gas in the near future. I expand on how costs will be brought down in my answer to question 8. 

On availability, it is necessary for tidal turbine operators to demonstrate that turbines can operate continuously over long periods without component failure. This is starting to happen, with high levels of availability reported from turbines operating in the USA and Japan, but it needs to be more widespread across other worldwide projects for confidence in the technology to build.

Another key challenge is demonstrating that tidal stream turbines do not cause detrimental environmental impacts. Evidence is building from field measurements taken at existing operational tidal stream turbines that support this. However, it is currently unclear if low environmental impacts will be maintained as the size of turbines and farms (i.e. number of turbines) scales up. This will require ongoing environmental monitoring to detect any changes.

Finally, the transmission of tidal stream power is a key challenge because typically, tidal stream resources are remotely located away from demand centers. For tidal stream power to be useful, significant upgrades to grid infrastructure will be needed.

7. What further research/developments are needed before tidal stream turbine usage is ramped up?

While it is well known that tidal stream power is predictable, the benefits of this characteristic to the energy system remain poorly defined. For example, going back to my previous example, the predictability of tidal stream power means that if a natural gas power station needs to be turned off for maintenance, it is possible to schedule the shutdown for a spring tide period (i.e. when tidal stream turbines are generating maximum power). This has potential cost benefits because it can reduce reliance on reserve power from elsewhere (such as expensive imported fuels). Quantiying these benefits is the focus of my research.

8. What is needed to reduce the cost of tidal stream turbine installation?

Cost reduction will come from four main areas. First, economies of scale. This means building larger turbines to increase energy yield per turbine. In doing so, the costs relating to things such as turbine installation can be reduced on a pounds per unit of energy produced basis.

The second is economies of volume. This means building larger farms of tidal stream turbines. By manufacturing higher numbers of turbines, manufacturing processes become more efficient and the supply chain scales up and becomes more competitive. Similarly, by installing more turbines per project, shared costs (e.g. grid connection costs) are reduced on a cost per unit energy produced basis.

Thirdly, technological innovation will bring down costs. We are seeing lots of innovation currently, particularly the adoption of floating turbines, as opposed to turbines that stand on the seabed. Floating turbines are able to reduce operational costs significantly, because maintenance tasks can be carried out onboard the device, or once the turbine has been brought to the quayside using a small vessel. This differs from seabed-mounted turbines that typically require very large, expensive vessels with onboard cranes to lift the turbine from the seabed to gain access to the device for maintenance.

Finally, changes in finance mechanisms will bring down costs. For example, currently, insurance is expensive, because the tidal stream industry is still relatively young, and as a result only just about achieving ‘acceptable’ levels of power output availability. It is vitally important for projects to demonstrate that turbines can produce power reliably over long periods without component failures to build confidence that operational expenditure and downtime will not remain high.

In summary, the cost of tidal stream energy is on a steep downward trajectory. Currently, the cost of tidal stream energy is around 5 times more expensive than offshore wind, but this is changing. Whilst tidal stream energy is unlikely to ever become as cheap as offshore wind at a technology level, it is likely to become cost-competitive with electricity generated from natural gas and biomass in the near future, whilst also providing additional whole-system cost savings.

9. How will this technology help us reach climate goals?

The Climate Change Committee estimates that the UK’s electricity demand will more than double by 2050, mainly due to the electrification of transport and heating. To meet net-zero targets, wind turbines will be the primary source of renewable power. However, we cannot rely solely on wind power. For example, in Autumn 2021, a 3-week lull in wind meant that wind power contributed about 60% less to the overall UK electricity supply than expected. This coincided with low nuclear power availability, and increasing imported natural gas prices, highlighting the risk of relying on intermittent renewable power and volatile fossil fuels.

Whilst the UK’s tidal stream energy resource remains largely untapped, it has the potential to contribute clean electrical power and significant security of supply, given its ability to provide predictable, reliable power.

10. What are the next steps for the project?

I am currently working on the Tidal Stream Industry Energiser project, known as TIGER. Next on the to-do list is to simulate tidal stream turbines within the Ramsey Sound, located in Pembrokeshire, Wales.

When turbines are added to a tidal flow, they create a blockage. If too many turbines are installed, the flow will take a path of less resistance around the farm, resulting in low power generation per turbine. I want to understand how many are too many turbines?

I am also working on an energy systems model that considers how much wind, solar PV, tidal stream, and short-long duration energy storage the Isle of Wight should install to reduce its future cost of energy.

Tide as part of the energy mix | Danny Coles | TEDxAberystwyth

Video Credit: TEDx Talks/YouTube.com

Where can readers find more information?

The Conversation article: https://theconversation.com/tidal-turbines-could-generate-11-of-the-uks-power-new-research-168890

UK tidal stream energy resource review paper: https://royalsocietypublishing.org/doi/10.1098/rspa.2021.0469

Tidal resource in the Alderney Race paper: https://www.sciencedirect.com/science/article/pii/S0360544217301974

About Dr. Danny Coles

Dr. Danny Coles is a Research Fellow at the University of Plymouth, a leading institute in offshore renewable energy research. He works on the Tidal Stream Industry Energiser project, known as TIGER. In this role, his primary focus is on better understanding the tidal stream energy resource in the Ramsey Sound, Pembrokeshire, Wales, and the cost of tidal stream energy reduction through turbine and farm optimization.

His research has recently been cited in the House of Commons debate: Tidal Energy Generation: Ringfenced Funding.

Danny is a member of the Marine Energy Council and the British Standards Institute (BSI) Committee, where he contributes to the International Electrotechnical Commission standards on tidal stream energy resource modeling and tidal stream turbine power curve testing.

During his time working in a previous role at the tidal stream energy developer SIMEC Atlantis Energy, his work on farm optimization for the MeyGen Phase 1A project in Scotland was recognized when he was awarded the Analyst Award at the Scottish Renewables Young Professionals ceremony. Danny’s research has also been featured in a TEDx talk and in national and regional newspapers. 

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.

Laura Thomson

Written by

Laura Thomson

Laura Thomson graduated from Manchester Metropolitan University with an English and Sociology degree. During her studies, Laura worked as a Proofreader and went on to do this full time until moving on to work as a Website Editor for a leading analytics and media company. In her spare time, Laura enjoys reading a range of books and writing historical fiction. She also loves to see new places in the world and spends many weekends looking after dogs.

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