INTRODUCTION
The purpose of this report is to furnish the results of a state-of-the-art investigation of wind turbine systems, including a review of the theoretical premises of its operation.
Problem
Increasing energy crisis and global pollution make any investigation on wind energy very important, because wind energy has the possibility to reduce the above two problems. For example, the demand of electricity has increased very much throughout the world because of increased activity in industrial and digital sectors and that is causing very high fossil fuel emission, which has a direct link with global pollution. In this situation, wind energy brings high hopes of giving pollution-free electricity at low cost, which can help both economy and environment (Gokcek & Genc, 2009; Ilkilic & Turkbay, 2010). The above view is supported by the 2013 half-yearly report presented by the World Wind Energy Association (Gsanger & Pitteloud, 2013), which showed that current production of wind power is only 13% of the target mark of 2000 GW, as set by the International Energy Association (IEA).
Chart 1: World Wind Energy Association
[Adapted from Gsanger & Pitteloud (2013)]
One look at the above diagram suggests that even if that 13% can be stretched to 25%, a huge amount of money could be saved, besides saving the environment from a huge volume of pollutants from fossil fuel emission.
Scope
Although this report works with the database on wind energy to identify the issues that help or block the development of wind energy production and discusses the overall wind energy situation, it limits to the common theoretical approaches that present the architecture and mechanism of wind energy; it will not investigate the actual methods applied in various turbines to implement those theoretical approaches.
BACKGROUND
Theory
There are six dimensions of wind energy, such as mechanical, technical, managerial, social, environmental, and economical dimensions. Out of the above, the first three dimensions fall within the production category, while the remaining dimensions fall within utilization category.
The mechanical dimension of wind energy includes mechanical designs of wind power systems, where the designs vary according to their relationship with production purpose and resultantly, there are structural and functional difference between two different systems. For example, the designs of Horizontal Axis Wind Turbine (HAWT) and Vertical Axis Wind Turbine (VAWT) highly differ from each other (Mathew & Phillip, 2012).
A typical HAWT contains blades, rotor, gearbox, and a generator, where the last two components remain within a nacelle.
Figure 1: Basic components of HAWT (Source: [33, 34])
HAWT uses two types of blades, such as the lifting style wind turbine blade and the drag style wind turbine blade, where the first type is designed to capture the energy of strong, fast winds and second type is designed to capture the energy of heightened winds (HAWT, 2013). It has an increased rotor control that makes it a popular choice, as high rotor efficiency is extremely important to extract high volume of wind energy.
On the other hand, VAWT contains low tip-speed ratio, difficulty in controlling rotor speed and in self-starting, which are its disadvantages. However, it takes less space, emits less noise, takes less tower load, and there is no need to twist the blade, which are considered as its advantages (Alberts, 2006; REN 21, 2013; Shubel & Crossley, 2012).
The basic difference between HAWT and VAWT in terms of design can be depicted like below:
Figure 2: Basic Difference between HAWT and VAWT
[Source: Schubel and Crossley (2012)]
In the mechanical dimension of wind energy, the energy (P) carried by moving air can be expressed as a sum of its kinetic energy (Schubel & Crossley, 2012):
P=12 ρAV3 (1)
Otherwise, the common elements of wind turbines include aerodynamics, electrical transformation system, and control system, and it works with a principle of having lower rotation speed than the speed required from generator. A wind turbine sequentially converts wind power to mechanical energy and then to electrical energy, where the wind turbine mass flows at the entry, between vanes, and at the exit remain constant everywhere of the flow volume (Ilkilic & Turkbay, 2010). This process can be depicted as below:
E = ρAV = ρDADVD (2)
Technical dimension of wind energy include factors such as accurate wind-force data and building, maintenance, and improvement of infrastructure, all of which are crucial to gain benefit from wind turbines. For example, the operators need to correctly estimate the technical and economical wind energy potential of a particular region considering the regional characteristics, besides focusing on wind farm configuration, extrapolation/interpolation of wind speed, and on the shape of Weibull distribution to the hub height to calculate the wind energy production (Schallenberg-Rodriguez, 2013).
Managerial dimension of wind energy deals with the managerial issues that have bearings on the efficacy of a wind firm, such as management of risks, voltage, power reserve, grid and basic safety, are dealt in this dimension (Holttinen et al, 2006).
The social dimension involves all stakeholders’ role in maintaining and developing any wind firm project (Boccard, 2010), while environmental dimension involves issues such as impact of an installation on environment and policies regarding environment protection. It thus deals with factors such as noise and visual pollution, ecological degradation, fossil-fuel emission reduction, or casualties caused by an installation (Kaffine, McBee, & Lieskovsky, 2013).
The last but not the least, economic dimension of wind energy acts as a deciding factor regarding any such installation, since these projects are created to earn favorable return on investment (ROI), besides developing local and regional economy (Schallenberg-Rodriguez, 2013).
History
The use of wind energy has a history as long as 5000 B.C., when the people utilized it to run boats along the Nile River. Later, between 500-900 AD, Persians were found building windmills for grinding grains and to pump water. Documents suggest that around 1000 AD Chinese and Tibetans were found using wind energy, while France and Netherlands were found pioneering windmills operation around 1800 AD, when France had 20,000 windmills in operating condition and Netherlands generated 90% power for industrial use. However, the concept of generating electricity through wind turbine made its mark in 1888 in Cleveland, Ohio, through a large wind machine. A little later in 1891, Poul LaCour of Denmark invented the first wind energy turbine, which was followed by a rapid production of 25 kW wind turbines in Denmark. After that the U.S. held the center stage in improving the performance of a wind turbine through shaping its shafts like aircraft propellers, which encouraged other European countries such as UK, Denmark, France, and Germany to exploit the new design and install powerful wind turbines. The U.S. again surged ahead in wind turbine research in the 1970s, when its government established a research wing for wind turbine to overcome fossil fuel crisis, which resulted in widespread use of wind turbines in the U.S. domestic and agriculture sphere. For example, 16,000 wind turbines were placed in California alone, which generated 20-350 kW, while six million small turbines started pumping water across the length and breadth of the country by 1970. These instances encouraged both European and Latin American countries to exploit the benefit of wind turbine to a larger scale (Hoogwijk, de Vries, &Turkenburg, 2004; Kaldellis & Zafirakis, 2011).
DISCUSSION
It would be pertinent to begin the discussion with a comparative study of HAWT and VAWT, since that would bring a clear understanding regarding their mechanism and utility value.
The above comparison of HAWT and VAWT highlights certain points, such as performance of a wind turbine highly depends on its design, design depends on wind characteristics, and therefore constant research on the relationships between performance, design, and wind characteristics can highly contribute to the improvisation of both HAWT and VAWT, as the past research and development initiatives kept improving the design and performance of wind turbines. For example, although HAWT dominates the commercial electricity sector with its three blades and upwind rotor that help utilizing wind flow and reducing noise (Wiser e. al., 2011), researchers’ speculation is that VAWT has the ability to dominate overall wind-energy sector within a few decades, as it takes less money and little space in comparison to HAWT and the more advancement in power storage technology (e.g. fuel cells, batteries) will definitely make it easier to get more benefit from VAWT (Islam et al., 2013). There are reasons to support improvisation of the design and performance of VAWT, as that would increase its use in offshore wind energy production, where it is difficult to install HAWT. This brings the discussion to the state of onshore and offshore wind energy across the globe.
Figure 3: HAWT and VAWT (Source: [16])
Till 1970s, onshore wind energy system was the only means to exploit wind energy, and the scene was changed when Sweden installed first offshore wind turbine, which encouraged many countries to research on this area, where Denmark made their mark by installing world’s first and second commercial offshore wind farm in 1991 (at Vinderby) and in 1995 (at Tuno Knob). The installation at Vinderby started generating 12 GWh/year, while the second installation at Tuno Knob started generating 16 GWh/year. Apart from that it also constructed world’s largest offshore wind farm that became capable of generating 40 MW in 2002. Offshore installation includes more subsystems, such as system to channelize the power output of all turbines to a central collection point (CCP) and system to transmit the same to the onshore power grid (Madariaga et al., 2012).
Altogether it requires appropriate policies and execution strategies to achieve success in offshore wind energy utilization. For example, the success rate of UK in generating electricity from offshore installations is very high due to its policies and strategies, while U.S. still remains at the early stage of power generation from its offshore installations due to lack of the same (Snyder & M. J. Kaiser, 2009).
Another successful operator in offshore wind power generation is Netherlands, who installed its first offshore wind farm at Lely with a rated power of 2 MW in 1994, followed by another installation at Irene Vorrink region with a capacity of 16.8 MW power generation. Altogether the European countries such as UK, Denmark, Netherlands, and Sweden are currently the major players in this field with UK leading all with 590.8 MW installed capacity (Bilgili, Yasar, & Simsek, 2011), which easily shows that it is impossible ignore the possibility of offshore wind energy. As per the estimate provided by European Union in 2011, the current standings of major players in offshore wind power generation stand as below:
Chart 2: Status of Offshore Wind Power in the EU in 2011
[Adapted from Bilgili et al. (2011)]
However, onshore wind energy contains more possibility than offshore wind energy, as it is indicated by estimates calculated by the experts. Basically, the potential of onshore wind energy surpasses even the current global electricity production regardless of regional variance in its output (Rajgor, 2010). In one such estimate of 2004, Hoogwijk et al. (2004) calculated differences in regional results that emerged from varying wind speed data and other key input parameters such as constraints of land use, density of wind energy development, anticipated performance of the installations, difference among regional categories, etc. and to their surprise they found that in spite of the influence of so many variables, the technical potential of onshore wind energy surpassed the volume of the electricity consumption in 17 regions under lens. In another investigation, Lu, McElroy, and Kiviluoma (2009) discovered that if an onshore HAWT network of 2.5 MW turbines could be freed from forest, ice and urban areas, then that could supply 40 times more than current global electricity consumption, besides generating power that could be five times more than total global energy usage. Some more recent studies (IEA, 2013; REN 21, 2013) also support the above findings and suggest obtaining a symbiotic relationship between the rate of wind energy production capacity and storage capacity to optimize the utilization of wind energy potential. It would be pertinent to present the compilation of four individual research works on global technical potential for onshore wind, which was prepared by Wiser et al. (2011):
Going by the above table, one finds that almost all regions carry the possibility of onshore wind energy. What is important within the context is the information regarding continuation of experiments with design and output factors of wind turbines. Resultantly, some changes can be observed in the outputs. For example, the 1.6 MW average rated capacity of new grid-connected turbines in onshore installations in 2008 was increased to 1.8 MW by 2012, while the 3 MW average rated capacity of turbines in offshore installations in 2008 was increased to 4 MW by 2012. Currently the average capacity of largest commercial wind turbine is 7.5 MW with a rotor diameter of 127 m, while turbines with a rated capacity range between 1.5 MW to 2.5 MW are more popular in the wind energy sector (IEA, 2013).
Although Europe continues to dominate the wind energy sector, regions from other continents also showing marks of significant improvement, which can be observed with a quick look at the compilation of charts presented below:
Chart 3: Status of Wind Energy Generation in Europe, Asia, Middle-east, and North America
[Onshore+Offshore, in GW; Adapted from Gsänger and Pitteloud (2013)]
Apart from the above, Brazil recorded 2.788 GW from Latin America, while Australia recorded .475 GW from Oceania and Morocco recorded .391 GW wind power generation (Gsänger & Pitteloud, 2013). Altogether five points emerge from the above discussion:
Wind energy clearly projects the potential to lead as the major source of renewable electricity;
HAWT turbines are dominating the wind power systems;
VAWT holds enough promise;
Onshore wind power systems carry the potential of covering total global power requirements;
Europe specializes in offshore wind power systems; and
Many countries are coming up in exploiting wind energy.
CONCLUSIONS AND RECOMMENDATIONS
The review and discussion show a very positive future of wind energy, which is also a good message for the environment and global economy. However, it is also important to note that the utilization of this opportunity offered by wind energy totally depends on a united, global effort, due to the variety of associated factors. Thus it is required to create an international organization to promote and develop wind energy in every part of the world. This report finds it is absolutely possible to narrow the gap between estimated power generation of wind energy and its current rate of production, with the help of some common policies. Therefore, this report presents its recommendation through the following diagram.
Figure 4: Recommendation
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