Mittuniversitetet

Structural icing is a persistent challenge for the production of renewable energy from wind. It is mainly caused by supercooled atmospheric droplets of water, which are very common in cold climates. In the most exposed wind parks in Sweden, more than 10 per cent of annual energy production can be lost. Some properties of liquid water are included in current Numerical Weather Prediction (NWP) models and are used as input parameters for the estimation of icing, but they are rarely measured in-situ for verification or validation.

To address this problem, a new instrument was developed.This compilation thesis is a collection of five articles describing the development, testing and verification of this instrument. Finally, icing and ice loads are measured and compared with a standard model and a model using Artificial Intelligence (AI) and empirical data.

The new instrument, called Droplet Imaging Instrument(DII), is based on shadowgraph imaging using Light Emitting Diode (LED) light as background illumination and digital image processing. The components were selected with the possibility of low-cost volume production in mind. The applications of a commercial instrument based on this technique include, forexample, real-time in-situ icing condition measurements and assimilation and verification of data in NWP models. The instrument, alongside a reference instrument, was tested in two locations with different icing conditions. Shadowgraph imaging and its limitations as a measurement method for droplet size and concentration were investigated.

The work presented shows that measurements of the size and concentration of water droplets using shadowgraph images can be used for the comparison and validation of NWP models and other instruments. The Coefficient of Variation (CV) for a given value of the concentration is lower than 1.6 % for droplets 25 µm in diameter, based on uncertainty in the size measurement only. The accuracy of the sampling volume can be improved by measuring the background light intensity in the position of the measured droplet.

A fog chamber was used for initial tests. However, to evaluate models of ice accumulation, in-situ measurements are necessary. These measurements should use a temporal resolution of at least one sample per minute, preferably higher. With alimited amount of data, multivariate data analysis can be used to estimate the level of ice accretion. Together with a heuristic model of erosion/ablation, the resulting figures can be used to simulate the ice load.

All of the instruments, as well as many other components used during the described field measurements, did at some point break due to the difficult weather conditions. An instrument for measurement of icing conditions needs to be designed with high environmental protection and endurance. The results in the attached papers may help and motivate further technical development of instruments that can measure atmospheric liquid water in icing conditions.