Could energy harvesting make wind more reliable?

Professor Markys Cain

A new European-wide project hopes to standardise energy harvesting devices. These scavengers of energy can power sensors, helping to tackle blade cracking and failure and increasing the lifetime of components….

Energy harvesting technology is on the up, and wind power companies should sit up and take notice. All around Europe, manufacturers are developing tiny devices that fit inside industrial machinery and capture heat and vibration energy lost to the surrounding environment.

Whilst these devices won't solve the looming energy crisis on their own, they do bring forward the potential for autonomous wireless monitoring of industrial machinery, over significant time frames. And this should increase the reliability of components, as well as reduce the risk that they will fail. So could wind turbines also be in line to benefit?

Challenges to implement

Whilst the potential benefits are exciting, there are important factors to consider. Renewable energy producers and engineers looking to incorporate energy harvesting technology within their operations face a major challenge – there is no agreed set of measurement standards that would gauge potential savings.

The lack of such internationally-recognised standards prevents buyers from accurately predicting the amount of wasted energy on offer within different operational environments, together with its capability to feed energy harvesting devices, and power sensor networks. Without this key information, developers are unable to provide meaningful product specifications for commercially-available energy harvesting devices. This means that potentially important markets like the renewable energy sector are forced to buy products and conduct their own trials – often at great expense and time.

Within this context the Metrology for Energy Harvesting project was set up. Made up of 7 European national measurement institutes (NMIs), including the UK's National Physical Laboratory, the project aims to develop traceable (back to national standards) measurement methods that reduce duplication and accelerate innovation and competitiveness in energy harvesting.

It could potentially deliver tangible benefits to the wind power sector, but requires input from manufacturers and developers to bring this potential to fruition.

Back to basics – why energy harvesting for wind?

The economical design of wind turbines – and the ability to monitor them – is increasingly critical as wind power competes with other forms of energy to become a reliable source of power. It is also an important factor in improving public perceptions of the sustainability and practicality of this alternative form of energy.

Tiny fractures within turbine blades, left undetected, can lead to sudden and catastrophic failure. Incidents of dramatic structural collapse in recent years have led to wind-farms being shut down, and provide even more ammunition to those who doubt that wind power can ever play a long term role in energy provision.

Last year for example, Europe's biggest windfarm, in Whitelee, Scotland, was shut down after a giant turbine blade − 46 metres long and weighing 14 tonnes – broke and fell to the ground. The incident led engineers to temporarily halt power generation across all 140 turbines on the site.

This followed a similar incident at the Crystal Rig complex (also in Scotland), when a turbine blade snapped and fell to the ground during strong winds. The incident resulted in £1.25 million in repair costs, and significant downtime for a wind farm that supplies power to 33,000 homes.

And these operational issues will intensify. The pressure on wind farms to generate greater output forces will lead to ever greater rotor diameters, and blades will begin to intersect more complex loading conditions. This results in structural loads that are unaccounted for by blade designers.

In spite of this, manufacturers are looking at blades greater than 50 metres to capture more of the inbound wind energy, in a bid to increase energy production per turbine. With such an increasing blade size, the need to ensure structural integrity becomes even more important. On average, blades experience damage requiring repair (or even replacement) five times per year – a fact that negatively affects the long-term profitability of wind turbines.

It is made more acute as blades are manufactured from various non-metallic materials, which are challenging to inspect by conventional non-destructive methods such as ultrasound and radiography. Thus the acquisition of dynamic output data can be a time-consuming and costly process.

Wireless sensing in wind energy

To address this issue, sensors can be integrated within the blade to monitor structural health at various critical locations. But existing cable-based monitoring systems provide a number of draw backs. They are challenging and costly to install, and leave the turbines vulnerable to lightning.

The most practical method for deploying these sensors is through a network of wireless sensor nodes. The nodes are used to monitor discrete locations of the blade, and transmit data to a central receiver in the turbine's nacelle. This data is analysed and conclusions drawn about the overall health of the system.

As a result, wireless sensor networks are of growing interest to developers and manufacturers that want a system to autonomously detect and locate flaws in the blades. As the networks are wireless, they don't require hard wiring, and don't impact on power generation (or interfere with blade aerodynamics). These tiny, autonomous monitoring systems could extend the life of machines and structures, significantly reducing operating costs and enhancing safety.

Whether temporary or permanent, sensors can provide useful turbine response data. Data showing the effect of turbines exposed to wind and wave loads (as in the case of offshore turbines) can be collected and better used to understand the behaviour of turbines under complex loading scenarios.

And this could lead to improved analytical models, as well as more cost-efficient design procedures. Permanently installed sensors can also serve as the basis for structural health monitoring.

Where sensor networks were once seen as an expensive and troublesome solution, the increasing reliability of energy harvesting devices – over a 15 to 20 year time period – is proving a particularly strong draw for industry budget holders. And as pressure from local authorities forces wind power generators further offshore, monitoring turbines with wireless systems becomes a lot more efficient.

With such a clear need, industry is now beginning to step in and fill the void. The French electrical engineering group CEDRAT has developed a Structural Health Monitoring (SHEM) system for on- and offshore wind turbine blades. Meanwhile Etesian Technologies manufactures the world's first self-powered wireless anemometer systems, which are designed specifically for wind turbines.

The benefits of energy harvesting to wind energy also go beyond powering wireless sensors. Researchers in Germany have developed an active noise-damping system, with piezo-actuators to minimise the hum of the turbines. Terrestrial wind turbines are often forced by regulation to operate under partial load in order to reduce noise caused by the motion of rotor blades or cogwheels in the gearbox.

Passive damping systems have limited effectiveness as they cannot adjust to the changing frequency of the humming as the turbines changes its rotation speed. However, the new German system reacts autonomously to any change in frequency and dampens the noise, regardless of how fast the wind generator is turning. The actuators mounted on the gearbox bearings convert electric current into mechanical motion, generating ‘negative vibrations’ or ‘anti-noise’ that precisely counteracts the wind turbine vibrations.

Analysts are forecasting multi-billion pound growth in energy harvesting over the next decade with wind-farms just one of a range of potential new applications. This all sounds very exciting, but without a set of internationally agreed (and traceable) standards, the benefits of greater energy efficiency and reliable autonomous sensors will remain unrealised.

Metrology for energy harvesting

With this search for agreed and traceable standards in mind, the Metrology for Energy Harvesting project is aiming to provide the standards that will see manufactures in heavy industries like wind energy feel the true benefits of wireless sensors.

Since being set up, it has developed measuring systems for quantifying the electric potential within the piezoelectric harvesting devices being looked at by wind energy producers. These devices convert mechanical strain into electric current.

The project has also developed models to predict the power output of these devices based on the initial force applied.

Over 25 companies from across Europe involved in construction; the automotive industry; transport; mobile communications; and sensors and instrumentation; are part of the programme. These companies are helping to focus the project direction, and inform the development of new standards.

From a commercial point of view it comes down to customers knowing exactly what it is they are getting. Therefore, for energy harvesting to play a role in the renewable energy sector, it is vital that manufacturers and managers of heavy industry parts like wind turbines have a reliable way of comparing the output of different devices in their own particular environment, and are able to predict the power output of harvesters in real world situations.

Achieving this will require expertise and input from all aspects of science and industry including leading research institutions on energy capture and storage; material science; systems engineering; and metrology.

Yet just as important is engagement with businesses. The technology characterised through the Metrology for Energy Harvesting project must reflect what industry wants, and this requires input from companies to find out the issues they currently have, and how the project can address them.

NB: To find out more about how you can follow the progress of the Metrology for Energy Harvesting project please visit here.

The Metrology for Energy Harvesting project is funded by the European Metrology Research Programme, in addition to national metrology research programmes.

About the author: Professor Markys Cain is Knowledge Leader at NPL, one of 7 European Measurement Institutes involved in delivering the energy harvesting project.

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