Wind Resource Assessment and Micro-siting

Science and Engineering

Matthew Huaiquan Zhang

• Publisher: Wiley
• Year: 2015
• ISBN:
  9781118900130
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Power Resources
• Index: Technology & Engineering

Covers all the key areas of wind resource assessment technologies from an engineer's perspective

• Focuses on wind analysis for wind plant siting, design and analysis
• Addresses all aspects from atmospheric boundary layer characteristics, to wind resource measurement systems, uncertainties in measurements, computations and analyses, to plant performance
• Covers the basics of atmospheric science through to turbine siting, turbine responses, and to environmental impacts
• Contents can be used for research purposes as well as a go-to reference guide, written from the perspective of a hands-on engineer
• Topic is of ongoing major international interest for its economic and environmental benefits

Chapter 1
Introduction

Energy supply is undoubtedly one of the most challenging issues facing human beings in the 21st century. Limited traditional fossil fuels are being used up gradually, let alone air pollutants and global warming caused by the combustion of those dirty fuels. Renewable energy has therefore attracted increasingly more attention in recent years. Wind energy, being one of the most commercially viable forms of renewable energy at the moment, has already played an important role in quenching our society's energy thirst. Yet there is still a long way to go before we can fully exploit the wind potential of our planet.

Wind is the ‘fuel’ for wind power generation. Its characteristics, that is wind conditions, are therefore of the upmost importance when it comes to determine the economics of a wind farm project. Wind conditions are set by nature, but how well we can understand or estimate them is another question. Wind resource assessment in essence is the estimation of wind conditions based on wind data available and topographical (roughness, obstacles and terrain) and meteorological (e.g. atmospheric stability, boundary layer structure, weather system) features of a given site.

Being invisible already makes it hard for us to picture the wind, and to make matters worse it varies constantly and dramatically in time and space, influenced by a great number of factors, some of which we may not even know of. However, on the other hand, building wind farms is very capital intensive and those wind farms have to generate profit for their owners. Profitability has to be predicted before wind farms are built with a reasonable risk premium. Such stringent requirements from the industry have raised sometimes almost impossible challenges for wind resource assessment professionals. After all, the results of wind resource assessment and micro-siting will determine the success of the investment of a wind power project.

This book endeavours to bring together pieces of core knowledge used in wind resource assessment and to put them into a logical order and to explain them, adding in the author's own experience obtained in day-to-day work scenarios. This kind of effort has rarely been made before, at least to the author's knowledge, even though a few publications covering a few sections of the domain can be found in the market.

1.1 Wind Resource Assessment as a Discipline

From a meteorological point of view, the study of a wind resource for the purpose of energy production can be described as wind energy meteorology, which has developed into an independent division of meteorology. In fact, a monograph named Wind Energy Meteorology by Emeis [1] has recently been published in early 2013, a milestone of the discipline. Petersen et al. [2] describe wind energy meteorology as applied geophysical and fluid dynamics, a combination of meteorology and applied climatology.

Despite its importance, wind energy meteorology has not been a major area of expertise required by the industry to produce satisfactory wind resource analysis results until the last decade or so. In the last decade especially, wind turbines have substantially grown in size and height, which means that they are exposed to much more complicated atmospheric boundary layer structures. Simplified engineering models, which worked well before, have to be re-examined based on the study of wind energy meteorology. The fact that wind turbines are usually erected in more complex terrain conditions, and even offshore nowadays, has also promoted the development of the discipline. Therefore a significant portion of time will be spent on this subject in order to form a physical profile of wind resource analysis for readers.

Wind resource assessment takes us one step closer to the wind energy industry, setting off from the ivory tower of physics. The domain of wind resource assessment should at least consist of wind data analysis, site analysis, wind turbine selection, wind turbine siting (micro-siting), wind flow modelling, power production estimates, wind park optimization and uncertainly analysis. Statistical tools are predominantly used in the process owing to the stochastic nature of the wind. Therefore, statistics becomes another pillar of wind resource analysis, the first one being the physical models explained by wind energy meteorology, such as the boundary layer profile and atmospheric stability. In order to ensure quality calculations, we need to understand how the wind should be measured as well as interaction mechanisms between wind and wind turbines and amongst turbines (wake effects).

The development of wind resource assessment has been accompanied and motivated by the commercial evolution of wind turbines and the construction of large-scale wind power projects. It will continue to do so in the foreseeable future. As a matter of fact, the expertise in wind resource assessment has become a core competence for many organizations in the industry and therefore well sought after.

1.2 Micro-siting Briefing

Micro-siting is really a meteorological definition, because in the eyes of a meteorologist, a few hundred metres is really on a micro scale. Micro-siting can be defined as the process of strategically positioning wind turbines within a given project area, in order to maximise power production with minimised turbine loads, that is optimising the wind park. Petersen et al. give an alternative definition of micro-siting, that is an estimation of the mean power produced by a specific wind turbine at one or more specific locations [2].

A full siting procedure includes considerations such as the availability and capacity of the power grid, the present and future land use, and so on, but these aspects are not considered in this book. However, one important issue concerning the siting of wind turbines is their environmental and health impact, such as noise and flickering, which can turn into a dominant factor in some cases and is explicated in Chapter 11.

1.3 Cascade of Wind Regime

The wind in nature almost never travels along a straight line; rather its track resembles circles. Those ‘circles’ are of all sizes, driven or dominated by different forces and induced by various mechanisms. Bigger ‘circles’ break into smaller ones and then even smaller ones until dissipated into heat, that is vibration of air molecules.

The wind we feel is a superposition of all the ‘circles’ of air movement at one spot. The scale of wind regime (or the size of the ‘circles’) can be described by two dimensions: temporal and spatial. The temporal scale and spatial scale of a wind regime are closely related. We can imagine that the bigger it is in space, the longer it takes to finish a circle. This cascade of wind regime should be the first physical model of the wind one should formulate before getting into the world of wind resource assessment. Wind regime is also referred to as wind climate or wind system. Chapter 9 will present wind systems of various scales in detail.

1.3.1 Global Scale Wind Regime

The atmosphere is a very complex heat engine whose energy is supplied by the heating of the earth's surface by the sun. Because the earth is tilted and also because of its uneven surface, different parts of the earth receive substantially different amounts of energy from the sun, which in turn induces air circulations with a spatial scale of the entire globe and a temporal scale of one or many years. This partly explains why wind resources are distributed so unevenly around the globe, as shown in Figure 1.1 [2].

Figure 1.1 Energy flux of the wind at 850 hPa (about 1500 m.a.s.l.) in W/m² from 8 years of the NCEP/NCAR reanalysis [2]

Source: Risø National Laboratory

Long-term wind data measured around the globe are required to analyse wind climate on this scale, but such efforts are commonly hindered by poor data quality (usually measured at 10 m height and contaminated by local features and inconsistent through time) and insufficient measurement points.

In recent years, however, the advances in computational power, the availability of nontraditional meteorological datasets with global coverage (such as satellite data), in addition to the traditional ones used in the global meteorological network (e.g. the Global Observing System [3]), and the advances in weather prediction models have together made it possible to reconstruct the global scale weather situation at every instant over recent decades. Global meteorological models are able to provide dynamic, consistent wind data and statistics, while avoiding some of the setbacks associated with the direct use of wind data (in fact most reanalyses do not consider low-level wind data in the analysis because of their ‘contamination’ with local influences) [4]. Figure 1.1 [2] is a good example of such applications and indicates the global wind resource variation, though the figure is rather dated. Figure 1.2 demonstrates the distribution of wind power density in China.

Figure 1.2 Wind power density distribution of China

Source: China Meteorological Administration (CMA)

Global meteorological models are usually made with spatial resolutions too coarse to be used in project-based wind resource analysis. For a higher level of spatial resolution, which includes smaller-scale phenomena of significant influence on the wind ...