How to measure wind resource

Met mast, weather vane and anemometer

I have to  start with a disclaimer – I’m not a specialist in wind resource analysis.

However, through the year I’ve seen several time this process so I think I can summarize it with a reasonable level of accuracy (and obviously if you spot a mistake, please let me know).

The wind resource assessment is done for several reason: to define the most suitable wind turbine given the local meteorology, to define the layout of the wind farm and, above all, to calculate the expected energy production of the wind farm – which is obviously a key input in the calculation of the profitability of the project.

To calculate the wind resource you will need to measure several variables:

  • Wind speed
  • Wind direction
  • Wind shear
  • Wind turbulence
  • Air density
  • Roughness of the area

This variables are usually measured installing one or more meteorological mast (“met mast”) in the area where the wind farm is planned  - obviously with the exception of the roughness, which is assessed by a specialist keeping into account the topography and the vegetation of the area.

This activity is called “site measurement campaign”.

The met mast is a tower made of steel (or, more unusually, in concrete) where the measuring equipment is installed. Ideally the met mast should have the same high of the wind turbines that are going to be installed in the area – however, to save money sometimes shorter masts are used.

The equipment installed on the met mast include usually the following:

  • Anemometers (usually there are several anemometers at different heights)
  • Weather vane (to record the direction of the wind)
  • Barometer
  • Thermometer

All the information collected is safely stored in an element called “data logger”. Auxiliary elements in a met mast are solar panels, a protective lightning rod on top, anti-vandalism fence and obviously the foundation.

Ideally, at least 1 year of data should be recorded. However longer measurements (2 to 5 years) have less uncertainties and capture better the seasonal and intraday variability of the wind in the area.

After the site measurement campaign the wind resource assessment start.

The first step is to “clean” all records before processing, removing errors that can occur due to malfunctioning of the instruments.

After, several key parameters are defined:

  • Mean speed
  • Wind rose
  • Wind speed distribution
  • Wind shear
  • Wind turbulence
  • Air density, pressure, temperature

The last step is to use this parameters to estimate electrical power production. There are quite a lot of commercial software in the industry, being some of the most widely used WAsP, WindPRO and OpenWind .

This software will try to optimize the wind farm layout to maximize energy production considering certain limitations – for instance, they will leave a distance of at least 6 rotors from one wind turbine to the other in the direction parallel to the wind.

Finally, when the layout is defined, the software will combine the power curveof the WTG with the wind speed distribution of the site to have the power output.

 

Wind sector management – how to put more wind turbines in the same area

Wind sector management - image curtesy of wasp.dk

Wind sector management - image curtesy of wasp.dk

In many project my colleagues from the wind and site department (the people who calculate the best wind turbine model and the optimal layout in a wind farm) are forced to put quite a lot of wind turbines in a reduced space.

Each of these wind turbines generate a “wake effect” – basically, they create turbulence in the wind.

These turbulences can affect other turbines nearby, increasing loads. This is not good, because higher loads usually means more problems due to component failures.

Wind sector management it’s a solution to this problem – basically, when the wind is blowing from a certain direction some turbines are automatically shut down.

There are basically 2 alternatives: you can shut down the turbine upstream (the one creating the turbulence) or the one downstream (the one suffering the increased loads).

Stopping one or more wind turbines will obviously result in a loss of production. However, the guys in wind and site often found that, even considering these losses, the global output of the wind farm is higher in a densely packed wind farm with wind sector management then in a configuration without it.

In the market there are also more advanced solutions that, instead of stopping completely the wind turbines, change only some parameters of the WTGs. For instance the optimization algorithm could decide to change the speed of the rotor or the pitch of the blade.

Wind sector management is one of the curtailment that a wind farm can have. Other typical restrictions are linked to environmental issues (noise, shadow flickering, birds or bats) or to requirements coming from the grid.

Wind site assessment

The wind site assessment (or "wind and site" assessment) is one of the most important steps in the development of a wind farm.

Basically is a in depth analysis of the site conditions of the area where a wind farm could be built.

Purpose of this assessment is calculate energy production and suitability of a specific WTG model to the local conditions.

Such study is usually performed by different stakeholders – external consultancies (possibly on behalf of financial institutions), wind turbines manufacturers and even developers (if they are big enough to have a wind & site department in house).

The inputs for the site assessment are:

  • Wind data
  • Topography & Roughness conditions
  • Other environmental conditions

Wind data includes usually raw data from one or more met masts. Measurement period should be sufficiently long, ideally several years. Key data are the wind rose (from where the wind is blowing), the distribution of the wind speeds (it follows a Weibull distribution) and the normal and extreme wind speeds.

Topography & Roughness conditions have an impact on turbulence, flow inclination and local speed up effects that can be key in the selection of the correct wind turbine.

Other environmental conditions include parameters such as temperature (both very high and very low temperature will need a special “package” and usually leads to decreased energy production), air density (will change the loads, and if very high or low could lead to a derating) and seismic actions (will the tower withstand earthquakes?).

All this data is cross checked against standard wind classes. These classes have been defined by the IEC, an international committee of experts, and are often used to categorize a wind turbine model. For instance, wind turbine A could be certified for wind class I (strong wind) while wind turbine II could be certified for wind class III (weak wind).

It’s important to highlight that usually there is uncertainty on one or more parameters. Therefore different assumptions are made by the wind & site engineers. The interesting part of the story is that, depending on where you are working, you will be interested in “twisting a bit” the numbers in a different direction.

For instance, an external consultant will usually be more conservative when analysing energy production (as he doesn’t want to be blamed if the actual production is lower).

Conversely, a WTG manufacturer could possibly give you higher number when calculating energy production. This is good for 2 reasons: because more production means more money for the prospective customer, and because considering higher loads put the engineer on the safe side when assessing expected life of the key components of the wind turbine.

Guy-wire supported mast for a permanent met mast

Here you have some interesting pictures that we’ve received trying to define the meteorological tower (also known as “met mast”) foundation for one of our wind farms.

Strangely, our customer is going to use a wired met mast for both towers (the permanent mast of the wind farm and the temporary mast used to calibrate the power curve).

As you can see from the pictures the mast has an interesting hinged base joined to the foundation of the WTG with 4 screws - looks like an effective technique.

The mast is tower model is KT470 from Kintech engineering.

Meteorological towers in a wind farm

In a standard wind farm, 3 different Meteorological Tower (known as Met Mast) are installed.

First of all, a tower is needed to provide several years (normally at least 2) of wind data. This tower is used to collect not only wind data, but also other information such as temperature, humidity, rain and so on.

Normally these towers are lower that the WTGs that will be installed on site, with an average height of 40 meters: the wind speed at the hub height will be interpolated at the correct elevation using one of the available formulas.

If after a deep study of the energy that can be provided by the wind a promising location is found and the wind farm construction start, one or more temporary met mast are erected.

This towers measures the actual wind speed in the positions where a WTG will be erected, at the hub height (80 meters or more) and are used to calibrate the power curve, that is if the produced energy is in line with the expected values.

After a couple of weeks (normally 2 months) this tower is removed and a turbine is installed nearby.

The third tower is the wind farm tower: this tower will be used to transmit to the control centre the meteorological situation in the location and it has a paramount importance for the management of the site. It normally has the same height of the turbines of the wind farm.