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Do we still need met masts?

WORLDWIDE: Remote-sensing devices are cheaper to install and maintain and often gather better data than traditional met masts, but while their use is becoming more widespread offshore, bankability remains an issue, especially in complex terrain.

Blown over… Offshore met masts are expensive to install and vulnerable to high winds, making them a bigger insurance risk (pic: Idermar)
Blown over… Offshore met masts are expensive to install and vulnerable to high winds, making them a bigger insurance risk (pic: Idermar)

Developers and operators "who fail to take advantage of state-of-the-art remote sensing technology and expertise will soon find themselves at a significant disadvantage".

That’s the argument put forward by weather monitoring firm Vaisala in its recent report, Remote Sensing Revolution.

Pascal Storck, Vaisala’s director of renewable energy, puts it more forcefully. "Met masts belong in a museum," he says. "Not yet, but soon."

Both traditional met masts and remote-sensing (RS) devices — Lidar and Sodar (light/sonic detection and ranging), which can be ground-based or mounted on the nacelle — have their strengths and weaknesses.

It should also be noted that RS covers a wide variety of technologies with their own characteristics.

That said, there are a number of general points that can be made comparing remote sensing and met masts.

Crucially, met masts with their cup anemometers are well understood and have broad industry acceptance, offering traceability back to a standard, and thereby confidence for financiers.

"The wind industry was built using data from met masts," notes Michael Fisher, senior product manager at NRG Systems.

On the other hand, met masts generally require siting permits, which can take months to obtain, and are susceptible to mechanical failure and lightning strikes, as well as being highly visible to competitors and opponents.

Among other things, cup anemometers suffer from over-speeding in gusty conditions, may stop working in severe ice and are impacted by tower shadow.


Strengths and weaknesses… Severe ice poses problems for met masts, requiring difficult maintenance work to be carried out at height (left); while Sodar devices (right) can be moved around but cannot measure hub-height temperature, pressure or humidity

For their part, RS devices can get buried in deep snow, and struggle in certain atmospheric conditions.

For example, Lidar suffers in far northern regions where there are few aerosol particles, while Sodar needs air turbulence.

Both technologies require a reliable power supply, and currently neither can measure hub-height temperature, pressure or humidity, which are used to calculate air density and provide indicators of atmospheric stability.

RS devices also have components susceptible to mechanical or electrical failure, of course.

On the plus side, RS devices are portable and relatively easy to deploy — and redeploy. They typically do not need permits and can usually be installed in a single day.

This makes them ideal for prospecting potential sites, especially in remote or difficult-to-access regions, and impacts on overall costs.  

Cost analysis

Direct comparisons across the range of different technologies and options available are difficult to make, but, in general, the purchase price of Sodar and, especially, Lidar are higher than a met mast.

However, RS devices tend to benefit from much lower installation costs, and some also have lower operational costs.

To buy and install a 100-metre met mast in the US, for example, falls roughly in the range of $80,000 to $130,000 (excluding permitting fees).

A shorter tilt-up 60-metre met mast can be deployed for $25,000-40,000. The equivalent for a 200-metre vertical-profiling Lidar would be around $175,000, and for an approximately 120-metre vertical-profiling Sodar $60,000-70,000.

Maintenance costs vary widely, especially if a RS unit has to be returned to the factory for service, although some can be field-maintained.

The cost differential is clearly visible in insurance claims. RS devices are vulnerable to theft and can suffer damage in extreme weather, but on the whole the risks are "more straightforward to mitigate for ... and do not translate into substantial losses", says insurance underwriter GCube.

Met masts, on the other hand, "have traditionally lost insurers money", says Jatin Sharma, the company’s head of business development.

According to GCube, claims typically range from $12,000 to $140,000, starting with icing and snow damage at the lower end, up to tower collapse due to high winds or construction error.

Accidents can occur when working at height, and met masts are easy targets for vandalism. Waiting for a replacement mast, particularly in remote areas, can mean costly delays.

Offshore, the claims can be significantly higher, going up to €1.5 million ($1.9 million), Sharma adds.

But costs are not everything, warns Alistair Marsden, sales and marketing director of renewables developer and consultancy Dulas.

Developers need to look at the long-term benefits and prioritise project returns over short-term cost-cutting, he explains.

They should select the technology according to the site conditions and the outcome they wish to achieve.

There are any number of different variations — of combinations of towers and RS units, of renting or leasing and length of deployment, for example — depending on the type of campaign and the required value of the resulting data.

"The cheapest option is not always the best," Marsden stresses.

Rich data set

RS devices have the significant advantage of being able to capture wind data at and above a height of 200 metres.

As turbines grow taller, building hub-height met masts can be prohibitively expensive, and far harder to get permitted, while using a shorter mast (typically 60-80 metres) means having to extrapolate the data to hub height, increasing uncertainty.

RS devices not only capture wind speed and shear over a large range of heights, but also measure across the full rotor sweep, giving developers a more comprehensive understanding of wind conditions.

This much richer data set is one of the main advantages of RS devices. Because they can be moved around a site easily, they also allow a better spatial characterisation.

"Furthermore, a modern RS device can potentially take advantage of advanced signal processing to identify the uncertainty of every measurement — the holy grail for bankability," Storck explains.

But bankability is a thorny issue for RS devices, with financiers often reluctant to accept wind resource assessment (WRA) data derived from standalone units, even though Vaisala and other manufacturers point to numerous in-house and independent trials proving RS units to be just as accurate as met masts.

For the moment, however, met masts remain the accepted benchmark, with many developers using a combination of masts and RS, building in layers of redundancy in part to reassure the banks.

To improve confidence in the measurement, "the technology must be installed correctly, the measurement campaign must follow industry best practice, including on-site verification of the RS devices, and the location and the campaign must be in line with the capabilities of the technology", emphasises Reesa Dexter, senior energy and performance specialist at DNV GL.

It is also a question of time, as the technology evolves and is better understood. "It boils down to industrial use and demonstrated reliability of remote sensing," NRG’s Fisher says.

"RS devices are very reliable, capable solutions and investors are getting everything they need from them when used properly," he adds.

While the industry is gradually seeing more project finance awarded on the back of well-designed measurement campaigns using properly validated standalone RS devices in simple terrain, complex terrain presents more of a challenge.

"The current generation [of devices] shows a bias in its readings if the wind flow has curvature," Storck explains, although this can be mitigated by using computational fluid dynamics (CFD) modelling to apply a correction.

To address the issue, Vaisala and other manufacturers such as Leosphere, NRG Systems and Zephir, are developing protocols for using RS in complex terrain.

"Ongoing research to demonstrate the potential to use RS as a standalone device will be crucial steps in ensuring ongoing progress," Vaisala says.

The availability of formalised industry guidelines on using RS tools in measurement campaigns should also help make the data more bankable, following in the wake of the IEC standard on power performance testing (see below).

Offshore acceptance

Meanwhile, Lidar units are already replacing met masts for some applications offshore, including bankable WRA, provided the units are properly validated, of course.

This is largely down to cost: installing a met mast offshore costs around €10-12 million ($12-15 million) in Europe.

Lidar devices can be mounted on an existing platform or a buoy, which is much cheaper. The entire measurement campaign for the 588MW Beatrice offshore project, using three Leosphere WindCube Lidar units, cost less than 2% of the price of installing a mast, developer SSE says.

Indeed, floating Lidar devices are becoming "the established alternative method for resource campaigns," says Julian Harland, sales director at Eolos, which later this year will deploy a floating Lidar, incorporating a Zephir 300M device, for a two-year measurement campaign in the Baltic for up to 1GW.

The key drivers behind floating Lidar are "accrued deployment volume and campaigns achieved reaching a critical mass of industry recognition", and the "significantly lower Capex", particularly as projects go deeper and further offshore, Harland says.

"With any technology rollout, it needs a track record to convince multiple stakeholders, and ultimately funders, to be considered bankable. Fortunately, the industry is getting there, and quickly."


Using floating remote-sensing devices, such as the Eolos FLS200 with a Zephir 300 lidar (left) instead of a met mast (right) is becoming more common offshore


Onshore, the industry is rapidly advancing the use of RS devices, taking full advantage of their capabilities in applications ranging from initial site prospecting through monitoring and optimisation.

As RS equipment can easily be moved around, it provides an efficient and cost-effective way to validate a site before deciding whether to proceed to the next stage.

It can also help with micro-siting, wind-farm layout and turbine selection, among other things.

However, some of the most interesting applications are in optimising energy production, and therefore the returns, of existing wind farms.

Again this comes down to portability and ease of deployment. For example, the device can be moved around the site to provide targeted performance data and predict maintenance needs for individual turbines.

"Customers are constantly carrying out power performance tests in their fleet to see if something is amiss," Storck says. While these indicative tests may not meet IEC standards, "it can tell you very quickly if the turbine is performing acceptably", he explains.

If not, then operators can take remedial measures or opt to install a met mast or IEC-compliant remote sensor and undertake a fully compliant IEC test to contest the warranty.

Beyond that, "think about the power of having a few remote-sensing devices at a wind farm and  continuously inspecting the performance of your machines so you can troubleshoot and optimise production", Storck adds.

Ground or nacelle-based units can be used to measure wake effects, yaw misalignment and other loss factors, and to assist with condition monitoring.

"Lidar and Sodar are both seeing great advances, especially in optimising sites," agrees Dulas’s Marsden, citing preventive maintenance and Scada-integration offering live power-curve testing.

There are also possibilities linking the internet of things to wind-data forecasting and turbine-performance forecasting to optimise wind farms, he says.

Scanning Lidar is another "super promising" development, providing information that was not previously available, Fisher notes.

These devices can be flexibly configured to measure wind flow in all directions from the unit — as opposed to a simple vertical profile — to create 3D wind maps delivering measurements over an entire site, reaching more than ten kilometres away.

All of which explains why a growing number of developers and operators are adopting RS and expanding its use across their project portfolios.

As an indication, Vaisala has now sold 1,000 units, and clocked up 20 million hours of data around the globe, Storck reports, while Lidar systems are also being adopted on a comparable scale.

Competitive advantage

It perhaps also explains why some pioneering companies are reluctant to reveal exactly how they use RS devices.

Vaisala says that many of its major clients have deployed Tritons across their portfolios, supplementing met masts in both wind development and operations and maintenance applications.

"Our clients sometimes do not want to publicise their use of Tritons, so that they can maintain their competitive advantage for as long as possible," says Storck.

And Storck sees a future where RS devices no longer need to be validated against a met mast, where the next generation of RS has "vastly improved sensing and processing capability all feeding into more advanced algorithms that will make traceability back to a met tower as a standard a thing of the past", he says.

Which is not to say that met masts will not still have a role, at least in the foreseeable future. "Portability is usually seen as an advantage, but there is also value in permanency," Fisher contends.

"The current prevailing strategy is to use a combination of shorter met masts plus remote sensors."

If owners keep the RS device in the same location for one or two years, they may not be extracting maximum value.

If you want a long-term record, then a short met mast, whose extrapolation uncertainty can be mitigated by the complementary remote sensor, is the logical answer, he argues.

No one doubts, however, that the use of RS will increase. And the market may even be reaching a tipping point, as Storck maintains, where the groundwork for widespread adoption, and broader acceptance by financial institutions, is already in place.

"Developers and operators that consolidate their use o the technology will realise the commercial benefits of cutting assessment times, reducing development costs, improving asset performance and unlocking a greater understanding of the wind resource," the Vaisala report concludes.

International standards

In 2017, the International Electrotechnical Commission (IEC) published its long-awaited guidance covering the use of remote sensing (RS) in wind-turbine power-performance testing, reflecting the challenges associated with increasing hub heights and rotor sizes.

For the first time, turbine owners have formal rules for using a ground-based RS device, but only in conjunction with a short met mast to act as a control and only in flat, non-complex terrain.

The standard also introduces the concept of rotor-equivalent wind speed (REWS), which considers the impact of wind shear and wind veer on turbine power performance.

While broadly welcomed by the industry, the standard is not without its critics. Some argue that by introducing new measurements, it leads to increased costs and uncertainty.

Others, such as Vaisala’s Pascal Storck, consider the guidelines "unnecessarily conservative" because they put all the uncertainty on the RS unit, tied back to a met mast.

"According to the guidelines, the RS device can never be ‘more accurate’ than the met mast," Storck points out.

The introduction of a standard for power-performance testing has also raised concerns about the lack of equivalent guidelines for the use of RS devices in other applications, particularly wind resource assessments (WRA).

This has created confusion about whether remote-sensing data can be used in project financing, Vaisala warns.

However, a new standard for WRA, energy-yield analysis and site-suitability assessment onshore and offshore is currently in the works. All being well, it should be published in 2019.

While the details are not yet clear, "the most important aspect is that the standard has a robust method for determining the true uncertainty of RS devices, not an overly conservative estimate", Storck says.

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