Friday November 27th 2009- It was a perfect day for a visit to the big turbines- sunny, mild and windy with a building nor’ wester.
I had flown down to Wellington on the 7:00 am flight and was picked up at the airport by wind engineers Simon and Nina from Aurecon (a large consulting firm involved in wind farm development).
We then drove over the Rimutaka ranges into the south Wairarapa, fuelling up on coffee and food en route. Travelled along the wide plains through Featherston and Martinborough towards Genesis Energy’s Hau Nui wind farm.
Driving down towards the plain we stopped to get an overview of the hills in the area. Directly across the valley three hills known as the Canoes of Kupe (Nga Waka O Kupe) stood prominently on the skyline and to the right of them a gentle curved hill where the Hau Nui Wind Farm is located. Even though the weather was fine the Hau Nui turbines were barely visible through the atmospheric haze. The next time I saw the turbines they were looming impressively through the cars windscreen.
The Hau Nui Wind farm consist of a total of 15 wind turbines, scattered in four groups of two to seven turbines along the ridgeline. The first group we encountered consisted of seven German-built Enercon E40 turbines. These mid-size turbines stand 46m high with 40m diameter blades. The E40s are a slightly older design and you can clearly see the generator (alternator) directly behind the hub with its large stator.
A few hundred metres away rise the next cluster of turbines which from the road look like they are in a single group due to the perspective. These are the more recent E44s which have a 44 m diameter span and produce up to 600 KW of electrical power, slightly more than the 550MW nameplate capacity of the E40s. The E44s have a more streamlined appearance with an egg-shaped housing for the nacelle.
The blades were spinning around at a fair clip in the wind which was probably about 10 m/s at the hub height. Contrary to reports often reported in the media of unbearable noise created by the turbines they were barely audible above the ambient noise of the environment. A gentle swishing noise was all that could be heard above the stiff breeze.
The generators (alternators) on the turbines output variable frequency AC with a raw voltage of around 800 volts. This is immediately rectified to DC and then inverted to 50 Hz AC. After inversion to AC it is at about 800 volts and is then stepped up to 33 KV by a transformer at the base of the tower and fed into the grid.
From here Simon and Nina went into reconnaissance mode, the main purpose of the trip being to evaluate other potential turbine sites identified in the office through computer modeling.
For a wind farm to be successful it must be located in an area of high average wind speed and not be subject to excessive turbulence caused by topography and nearby objects such as trees and buildings.
One of the early stage evaluations of wind potential is through meso-scale modeling. Initially there is unlikely to be actual wind measurements at the desired height (50m above ground level) in the area of interest. Therefore the likely wind flows are modelled over a broad area using powerful computer simulations. The meterological data set (6 hourly, three dimensional weather data for NZ over the last few decades) is used to model average wind flows at grid scales of several kms to a reasonable level of accuracy. The resulting mesoscale map is usually normalised to a level of 50m above the land surface, roughly the height of the turbines hub and above.
To narrow down suitable locations further analysis follows, combining different data sets in a GIS (geographic information system). Once this has occurred sites are visited in the field and if they still look favourable (this will depend on factors such as accessibility) a wind mast may be erected to measure real wind speeds, wind directions and durations at multiple levels.
The wind mast we looked at was 80 m high and has 4 anemometers (spinning cups to measure wind speed) spaced at regular intervals up the mast. Data is collected at 10 minute as well as 1 second intervals. The average wind speed had picked up by then and was howling at around 20 m/s with unusually little wind shear (meaning that the wind speed was fairly constant from ground level to 80m).
Another way to measure wind speed at multiple levels is to use a remarkable piece of equipment called a LiDAR (Light Detection And Ranging) which avoids excessive construction of further wind masts with all of the attendant engineering of concrete pads and guy ropes.
This type of LiDAR uses a laser beam to measure the wind speed using the Doppler effect. Small particles suspended in the wind reflect and back-scatter the laser light. The reflected light is Doppler-shifted in frequency due to the particles motion. By measuring the amount of frequency shift between incident and reflected photons the velocity can be easily calculated, The Doppler LiDAR data (which also contains information on turbulence) is then correlated with the wind mast data and calibrated. The LiDAR can then be easily moved from site to site collecting accurate data on wind speed. Typically the LiDAR, about the size of a large dishwasher, would stay on site for 4 to 5 weeks.
See source website here.
Airborne LiDAR is also used for very accurate 3D mapping of the land surface and vegetation height which can be an important data set used in the modelling of wind farms.
Statistical analysis of the wind data can then be performed and used to inform a more detailed level of micro-scale modelling for the design and planning of wind farms. Software such as Windographer or WindPRO is used for this purpose and can predict the expected energy yield for different turbine locations and arrangements. This allows the wind farm to be optimised to the specific location and evaluated for its feasibility.
For a wind farm to go ahead it has to be granted a Resource Consent under New Zealand’s Resource Management Act.
Also see the Hau Nui wind farm animation I made with Adobe Flash.
Filed under: Renewable Energy, Uncategorized | Tagged: carbon, Doppler Effect, electricity, emissions, generator, Genesis Energy, GIS, Hau Nui, laser, LIDAR, mesoscale, modeling, noise, power, sustainability, turbine, wind, wind farm | Leave a Comment »