Wednesday, 28 September 2016
09:00 - 10:30 Cold climate issues in resource assessment
Resource assessment  
          

Sites subject to cold climate not only have temperatures outside the normal limits of standard wind turbines but also atmospheric icing conditions which are frequent and may account for a significant loss in annual production. This session addresses the most recent advancements in the field of atmospheric icing effects on wind resource yield assessment. Assessing, measuring and estimating icing losses in the resource assessment phase of a project is of crucial importance for the successful business case of a wind farm in cold climates. Icing of the rotor blades can significantly reduce the energy yield of a wind farm up to 10% or more of the annual production and it also influences wind measurements by reducing availability.

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Learning objectives

• Learn state-of-the-art measurement techniques and data analysis approaches for cold climate sites and therefore decrease the uncertainty in yield assessments;
• Discover the latest findings from over 20 sites and 100 met mast years of data in cold climates from Scandinavia and Germany;
• Learn to execute more reliable pre-construction energy yield assessments in cold climate sites.

This session will be chaired by:

Øyvind Byrkjedal Kjeller Vindteknikk, Norway
Co-authors:
Øyvind Byrkjedal (1) ^F Bjorn Nygaard (1) Finn Nyhammer (1) Oyvind Welgaard (2)
(1) Kjeller Vindteknikk, Kjeller, Norway (2) Statnett, Oslo, Norway

New advances in icing measurements and icing predictions

Introduction

Experiences from wind power in cold climate have shown that large production losses can be expected due to atmospheric icing of wind turbine blades. The losses are related to aerodynamical degradation of the turbine blades or complete stops of the energy production caused by the ice coating on the blades. For wind farms operated in exposed areas in Sweden monthly production losses of 60% and more caused by icing have been reported on several occasions. Icing on wind turbine blades also represents a potential safety hazard when it falls or is thrown from wind turbine blades. Ice buildup on power lines can lead to a complete collapse of towers and lines, galloping, sagging and increase in the fault rate of the power line.

The ongoing research project FRonTLINES is focused on innovation within icing monitoring and icing measurements, as well as further development of numerical icing models. The underlying idea is to develop a toolbox that combines state-of-the art methods and models to calculate the impacts from frost and rime ice on overhead transmission lines. The tools will be applied both for design purposes and for monitoring lines under operation, including forecasting of potentially critical events. The innovation gained in the project is however, also applicable for wind energy site assessment as well as production forecasting at sites exposed to severe icing climate.

Approach

A measurement program comprises three test sites, all located in exposed mountain areas in western and southern Norway. The Instrumentation used at the test sites are:

• A newly developed ice load sensor (KVT Ice-Troll)
• An improved heated web camera system for reliable ice monitoring
• Wind and temperature sensors
• Real-time ice load measurements in overhead line test spans and in operational high voltage transmission lines


Main body of abstract

The first experiences and results from the new ice load sensor developed at Kjeller Vindteknikk (preliminarily named “KVT Ice-troll”) will be presented. The sensor is constructed according to the ISO description of a standard reference object [1], and the new design has allowed for forced rotation of the cylinder in combination with automatic load monitoring, and no requirement of added heat. The forced rotation ensures that the icing on the cylinder grows radially on the cylinder and results in a relatively smooth buildup of ice on the instrument. From these measurements it is possible to derive the icing intensity in accordance with the standard.

Meso scale models are often used for the icing assessments in wind farms; for the estimation of potential icing losses and for risk analysis related to ice throw. One of the missing links in the validation of icing calculations from meso scale modelling (e.g [2]) is observations of cloud liquid water and droplet size distributions. The measurements of icing intensity from the forced rotation measurement it is possible to better quantify these variables.

In this presentation we give an overview of the setup and instrumentation of the test sites, and provide an analysis of the measurements collected through the winter season 2015-2016.


Conclusion

The results show that the icing calculated from the meso scale model correspond very well to the measurements in terms of ice accumulation on test spans and operational transmission lines, but the occurrence of ice shedding from the lines seems somewhat less predictable.

The calculations carried out from the icing intensity measurements make it possible to quantify the liquid water content and droplet distributions within certain ranges. This is important knowledge for the development of icing calculations from meso scale models.


Learning objectives
The measurements of icing from a forced rotation cylinder that is constructed according to the ISO standard makes it possible to better describe the icing climate at a wind power site. Better knowledge about the icing conditions at the site will make it possible to make better judgments regarding potential ice prevention systems and the impact that the icing will have on the wind farm.