All over the planet, wind turbines are continuously being put up to produce cheap and sustainable electricity. During recent decades, the power produced by new turbines has grown steadily as they have reached a high level of development. They are now very reliable, and can harvest energy at a rate that is close to their theoretical limit. In order to obtain further economic savings it is extremely important to install the turbines in favourable locations where there is strong and steady wind.
The strength of the wind is such an important factor for the profitability of a wind turbine. This is because the energy of the wind depends on its speed to the power of three. That means that a site with as little as 15% faster winds offers around 50% more energy that can be potentially harvested. Wind speed data is, therefore, of great importance, but unfortunately it is not easy data to acquire.
Measuring wind at the right height
The available wind speed maps give a good first indication of general locations for wind farms, but are not detailed enough. Measurements on the ground cannot be used for greater heights with enough accuracy. The wind speed must, therefore, be measured at the height of a wind turbine’s tower before this tower is actually there. Traditionally, expensive and immobile meteorological masts have been built and operated for several months or years at potential wind sites. This is an unfavourable method for data collection.
Using lidar – measuring the wind with laser beams
A sophisticated technology called lidar which stands for light detection and ranging, offers an alternative method to measure the wind remotely. Commercially available devices based on this technology are placed on the ground and measure the vertical wind speed profile by sensing the wind field above in a fascinating way. Unlike radars that emit sound waves, lidars emit laser light into the sky. This laser light hits particles that are naturally found in the air, which then scatters the light in all directions. A tiny fraction of this scattered light is directed back towards the measurement device that detects it.
Estimating all the wind components
The received radiation is then compared with the emitted radiation and a certain frequency shift is found. That is not a surprise because whenever there is wind the particles don’t stand still but are transported with the air flow. That means they move relative to the direction of the laser beam, causing what is known as the Doppler effect. After the signals are analysed, the radial wind speed in the direction of the specific laser beam is estimated. Several of these radial measurements from beams pointing in different directions are then combined to estimate the horizontal and vertical wind components.
More methods of measuring wind from afar
In addition to assessing the wind conditions for future wind parks, there are other possible applications for remote wind measurements. When a remote sensing device is placed on the tower of an existing turbine it can determine whether the turbine’s rotor is aligned perfectly towards the wind. This can improve the efficiency of harvested energy. Another field of development is to detect wind gusts and use the signal to either prepare the turbine to make perfect use of it, or to protect it against damage if the wind gets too strong. Both these cases, can be seen as economically favourable for electricity generation from wind turbines.
Reducing green costs further
The use of lidars for wind energy applications is still a very active and ongoing field of research that has potential to further reduce the costs of renewable energy. The ENERSENSE research area at NTNU is grabbing this challenge with both hands, and making large efforts to improve the use of sensor technology for a greener energy future.
This blog post was written by PhD Candidate Felix Kelberlau, Department of Energy and Process Engineering, NTNU
Research area: ENERSENSE – energy and sensor systems