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Development and Test of an Imaging Instrument for Measurement of Water Droplets in Icing Conditions
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.ORCID iD: 0000-0002-5324-002X
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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 \gls{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 \gls{ai} and empirical data.

The new instrument, called \gls{dii}, is based on shadowgraph imaging using \gls{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, for example, real-time in-situ icing condition measurements and assimilation and verification of data in \gls{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 \gls{nwp} models and other instruments. The \gls{cv} for a given value of the concentration is lower than \SI{1.6}{\percent} for droplets \SI{25}{\micro\meter} 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 a limited 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.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University , 2019. , p. 68
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 306
Keywords [en]
Icing measurements, field study, shadowgraph imaging, edge detection, LWC, MVD.
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:miun:diva-37639ISBN: 978-91-88947-22-2 (print)OAI: oai:DiVA.org:miun-37639DiVA, id: diva2:1368439
Public defence
2019-12-05, C326, Holmgatan 10, Sundsvall, 10:00 (English)
Opponent
Supervisors
Note

Vid tidpunkten för disputationen var följande delarbete opublicerat: delarbete 5 (accepterat).

At the time of the doctoral defence the following paper was unpublished: paper 5 (accepted). 

Available from: 2019-11-07 Created: 2019-11-07 Last updated: 2019-11-07Bibliographically approved
List of papers
1. Liquid Water Content and Droplet Sizing Shadowgraph Measuring System for Wind Turbine Icing Detection
Open this publication in new window or tab >>Liquid Water Content and Droplet Sizing Shadowgraph Measuring System for Wind Turbine Icing Detection
2016 (English)In: IEEE Sensors Journal, ISSN 1530-437X, E-ISSN 1558-1748, Vol. 16, no 8, p. 2714-2725, article id 7384444Article in journal (Refereed) Published
Abstract [en]

This study shows that the liquid water content (LWC) and the median volume diameter (MVD) can be derived from images of water droplets using a shadowgraph imaging system with incoherent LED illumination.

Icing on structures such as a wind turbine is the result of a combination of LWC and MVD and other parameters like temperature, humidity and wind speed. Today, LWC and MVD are not commonly measured for wind turbines. Systems for measuring these properties are often expensive or impractical in terms of location or remote reading. The aim of this study is to gain knowledge about how to design a single instrument based on imaging that has the ability to measure these properties with enough precision and accuracy to detect icing conditions for wind turbines.

A method to calculate both the LWC and the MVD from the same images is described in this paper. The size of one droplet is determined by measuring the shadow created by the droplet in background illumination. The concentration is calculated by counting the measured droplets and estimating the volumes in which these droplets can be observed.

In the described study, the observation volume is shown to be dependent on the particle size and the signal to noise ratio (SNR) for each measured particle. An expected coefficient of variation of the LWC depending on the droplet size is shown to be 2.4 percent for droplets 10 µm in diameter and 1.6 percent for 25 µm droplets. This is based on an error estimation of the laboratory measurements calibrated using a micrometer dot scale.

Place, publisher, year, edition, pages
IEEE Sensors Council, 2016
Keywords
LWC, MVD, Icing, Clouds, Image processing, Machine vision, Meteorology, Optical microscopy, Wind power generation
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-27321 (URN)10.1109/JSEN.2016.2518653 (DOI)000372419100061 ()2-s2.0-84962128668 (Scopus ID)STC (Local ID)STC (Archive number)STC (OAI)
Funder
Swedish Energy Agency
Available from: 2016-03-22 Created: 2016-03-22 Last updated: 2019-11-07Bibliographically approved
2. Droplet Imaging Instrument Metrology Instrument for Icing Condition Detection
Open this publication in new window or tab >>Droplet Imaging Instrument Metrology Instrument for Icing Condition Detection
2016 (English)In: 2016 IEEE INTERNATIONAL CONFERENCE ON IMAGING SYSTEMS AND TECHNIQUES (IST), IEEE, 2016, p. 66-71, article id 7738200Conference paper, Published paper (Refereed)
Abstract [en]

An instrument for measuring water droplets is described and constructed. It is designed to measure the volume concentration and the size distribution of droplets in order to detect icing conditions in a natural fog. The instrument works by shadowgraph imaging, with a collimated blue LED as background illumination. We show how to use a reference object to obtain a calibration of the droplet size and the measurement volume. These properties are derived from a measurement of the object's shadow intensity and its edge second derivative. From the size of every measured droplet and its expected detection volume, a measure of the liquid water content (LWC) and the median volume diameter (MVD) can be estimated. The instrument can be used for continuous measurement in a remote weather-exposed location and is tested in a small environment chamber. We also describe this chamber and how we can change the LWC using an ultrasonic fog generator and a fan.

Place, publisher, year, edition, pages
IEEE, 2016
Series
IEEE International Conference on Imaging Systems and Techniques, ISSN 2471-6162
Keywords
atmospheric measurements, fog chamber, image analysis, liquid water content, machine vision
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-29765 (URN)10.1109/IST.2016.7738200 (DOI)000388735200012 ()2-s2.0-85004010273 (Scopus ID)STC (Local ID)978-1-5090-1817-8 (ISBN)STC (Archive number)STC (OAI)
Conference
IEEE International Conference on Imaging Systems and Techniques (IST) / IEEE International School on Imaging, OCT 04-06, 2016, Chania, GREECE
Available from: 2016-12-22 Created: 2016-12-22 Last updated: 2019-11-07Bibliographically approved
3. Field Study of LWC and MVD Using the Droplet Imaging Instrument
Open this publication in new window or tab >>Field Study of LWC and MVD Using the Droplet Imaging Instrument
2019 (English)In: IEEE Transactions on Instrumentation and Measurement, ISSN 0018-9456, E-ISSN 1557-9662, Vol. 68, no 2, p. 614-622Article in journal (Refereed) Published
Abstract [en]

The droplet imaging instrument (DII) is a new instrument for cost-effective in situ measurements of the size and concentration of water droplets. The droplet size distribution and the concentration of atmospheric liquid water are important for the prediction of icing on structures, such as wind turbines. To improve the predictions of icing, there is a need to explore cost-effective working solutions. Through imaging, a wide range of droplet sizes can be measured. This paper describes a study of the atmospheric liquid water content and the median volume diameter using the DII and a commercial reference instrument--the cloud droplet probe 2 from Droplet Measurement Technologies Inc. The measurement is done at a weather measurement station in mid-Sweden. For a second validation, the result is compared with predictions using a numerical weather prediction model. The size measurement of the DII is verified using polymer microspheres of four known size distributions. The study shows that the DII measurement is precise, but there is a systematic difference between the two compared instruments. It also shows that droplets larger than 50 μm in diameter are occasionally measured, which we believe is important for the prediction of icing.

Place, publisher, year, edition, pages
IEEE, 2019
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-34304 (URN)10.1109/TIM.2018.2843599 (DOI)000454332000030 ()
Projects
SMART (Smarta system och tjänster för ett effektivt och innovativt samhälle)
Note

Date of Publication: 20 June 2018

Available from: 2018-08-28 Created: 2018-08-28 Last updated: 2019-11-07Bibliographically approved
4. Particle Measurement Volume and Light Intensity in a Shadowgraph Imaging System
Open this publication in new window or tab >>Particle Measurement Volume and Light Intensity in a Shadowgraph Imaging System
2018 (English)In: IST 2018 - IEEE International Conference on Imaging Systems and Techniques, Proceedings, IEEE, 2018, article id 8577170Conference paper, Published paper (Refereed)
Abstract [en]

A method is sought to find the measurement volume of an optical instrument for particle measurement is sought. The study shows that the measurement volume depends on a combination of the optical homogeneity of the illumination light and the camera system. The result from a mix of illumination cases and positions shows that, if the true size and the background brightness are known, the measurement volume can be determined with an average precision of four percent using adual term exponential fit. Using a lens with lower magnification increases the measurement volume since both the field of viewand the depth of field increase. However, a larger field of view increases the required amount of light as well as the sensitivity to other optical errors.

Place, publisher, year, edition, pages
IEEE, 2018
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-34825 (URN)10.1109/IST.2018.8577170 (DOI)000455657200028 ()2-s2.0-85060703908 (Scopus ID)978-1-5386-6628-9 (ISBN)
Conference
2018 IEEE International Conference on Imaging Systems and Techniques, IST 2018; Krakow; Poland; 16 October 2018 through 18 October 2018
Projects
SMART (Smarta system och tjänster för ett effektivt och innovativt samhälle)
Available from: 2018-10-25 Created: 2018-10-25 Last updated: 2019-11-07Bibliographically approved
5. Measurement of Atmospheric Icing and Droplets
Open this publication in new window or tab >>Measurement of Atmospheric Icing and Droplets
(English)Manuscript (preprint) (Other academic)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-37641 (URN)
Available from: 2019-11-07 Created: 2019-11-07 Last updated: 2019-11-07Bibliographically approved

Open Access in DiVA

The full text will be freely available from 2020-07-28 12:05
Available from 2020-07-28 12:05

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Rydblom, Stefani Alita Leona

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