A review on small scale wind turbines
Introduction
Gross Domestic Product (GDP) is the major factor to judge the development of a country [1]. GDP represents the economic growth of a country which is closely related to the amount of the power produced by the nation. To have a sustainable economic growth, a nation should have the resources to continuously produce energy. But due to the precipitous depletion of the conventional energy sources and the increase in the CO2 emissions, the demand for a sustainable and a reliable alternate source of energy has been increasing day by day. Wind energy and solar energy are two sources of energy that hold a chance to serve the purpose. Wind energy, being renewable provides a great opportunity to generate energy due to its abundance.
Wind turbines are those which convert the kinetic energy present in the wind to mechanical energy and eventually into electricity. The history of wind turbines originates around 200 B.C somewhere in Persia but the first practical wind mills were developed in 7th century in Iran and were called the Sistan wind mills. Wind mills were effectively used in farms for producing electricity and pumping water in 1930׳s in USA. The first utility grid-connected wind turbine was built by John Brown & Co in 1951. The total capacity of wind power by the end of 2014 was around 369.6 GW and is expected to touch 666.1 MW by the end of 2019 [2].
Various countries, in order to meet their growing power demand are installing large scale wind farms both on shore and off shore. In 2014, China has installed capacity of around 114,609 MW of wind energy contributing 31.0% of the total wind power followed by USA, Germany, Spain and India which produce 65,879 MW, 39,165 MW, 22,987 MW and 22,465 respectively as shown in Fig. 1 [3]. In the current scenario large scale wind turbines farms (like offshore wind farm “London Array” which is around 100 km2 and onshore wind farm “Alta Wind Energy Centre” which is around 36.5 km2) are required in order to match the increasing power demand. This will lead to expansion of the wind farms and finally would result in large scale continental wind farms.
The effect of large scale wind farms on the climatic conditions has been studied by various authors. Wang et al. [4] have performed simulations using a three dimensional climatic model in order to study the various potential climatic effects of future large scale wind farm installation over land and ocean. The simulations were run on a global scale for a period of 60 years as the temperature changes need a longer duration to show the gradual impact. It was observed that the installation of wind turbines in order to meet 10–15% of global energy demand might cause surface warming by increasing the temperature by 1 °C on land. Similar simulations for the 1 °C increase in temperature over the oceans have also been computed by increasing the ocean surface drag but a further study has to be done on its validation. Fig. 2 shows the temperature changes in one of the models due to the deployment of large scale wind turbines over the land in order to generate 158 EJ/year. All the above conclusions can only be made after considering the special or new parameters that affect the wind turbines. Due to nonlinear variation of climatic changes with surface roughness, defining the optimal arrangement of wind turbines is challenging. Climatic effects increase with the power generated and decrease with conversion efficiency, leaving out the potential environmental effects on birds, weather radar, ambient noise levels etc.
Fiedler et al. [5] have performed simulations for 62 warm seasons on a regional climatic model, and observed that there was 1% increase the precipitation rate. It was also seen that a larger rate of precipitation has occurred for a larger wind farm [6], [7]. From the above results a conclusion can be drawn that installing large scale wind farms might lead to significant weather changes, so there arises a need to effectively use this wind energy without causing any adverse effects to the atmosphere.
The large scale wind farms are not a sustainable viable option for renewable power production. The best option available is by installing the decentralized grid system i.e., by using small scale wind turbines. Small scale wind turbines produce power around 10 kW which is sufficient for our domestic needs. This energy can be effectively utilized so that the energy extracted from the conventional resources could be saved for a larger period of time. Hence there arises the need to understand the characteristics of small scale wind turbines. The majority of work on small scale wind turbines was done over the past few years. The disadvantages of small wind turbines are high initial cost, effective placement, wind fluctuation, change in wind direction and also aero-acoustic noise.
This paper presents a detailed literature review on small scale wind turbines. Initially, scaling of wind turbines was illustrated in order to identify small scale wind turbines. Then, the general classification of the wind turbines was explained. After that, various aspects of horizontal axis wind turbine (HAWT׳s) like performance, blade design, control and manufacturing were reviewed. Next, review of various kinds of Vertical axis wind turbines (VAWT) was presented. Darrieus type VAWT was reviewed in two sections, experimental and numerical analysis. Also, positioning of wind turbines and aeroacoustics of small scale wind turbines were presented.
Section snippets
What is a small scale wind turbine?
Fig. 3 shows the classification of wind turbines based on rotor diameter. A typical large scale wind turbine is one which has a rotor diameter ranging from 50 m to 100 m. It produces power between 1 and 3 MW. When compared to large scale wind turbines, small scale wind turbines are those which have their rotor diameter ranging from 3 m to 10 m and having a power capacity of 1.4–20 kW. Table 1 demonstrates the classification of wind turbines based on power rating. Small scale wind turbines which have
Classification of small scale wind turbines
Small scale wind turbines can be classified based on two categories:
- 1.
Classification based on axis of rotation:
- a.
Vertical Axis Wind Turbines: Vertical axis wind turbines are those whose rotor axis is in vertical direction. These turbines do not have any yawing mechanism or self-starting capability. The generator location for these turbines is on ground and their height of operation is very low hence making them easier for maintenance. The ideal efficiency for these turbines is more than 70%.
The
Horizontal axis wind turbines
As mentioned in Section 3, horizontal axis wind turbines are turbines in which the rotor axis is in the horizontal direction. Since large scale wind farms (generally HAWT) have potential impact on the climatic conditions, work is being done in order to understand the performance characteristics of small scale HAWT by varying various parameters. The work done on small scale HAWT for various parameters has been presented in the upcoming sections.
Vertical axis wind turbine
Vertical axis wind turbine (VAWT) is a turbine in which the rotor axis is in the vertical direction. Since the rotor axis is in the vertical direction, these turbines need not be pointed into the wind to be effective make them advantageous for the usage on sites where the wind direction is highly variable. They are significantly quieter than horizontal axis wind turbines making them particularly useful in residential and urban areas. But the VAWT׳s are less efficient that the HAWTs because of
Position of wind turbines
As mentioned earlier, small scale wind turbines can be used effectively used to fulfill domestic needs but the placement of these turbines also plays an important role in obtaining maximum efficiency. Various experiments have been conducted to determine the best suited position for the deployment of wind turbines.
According to Ledo et al. [67], the important factors which occlude the performance of the turbines are low mean wind speeds, high turbulence and high aerodynamic noise levels generated
Aero acoustics
Aero acoustics is an important factor in designing wind turbine blades as it radiates acoustic noise. The dissonance that is created has to be considered in designing the wind turbine blade before installing it in a population dense region. The rotational speeds of most wind turbines develop constraints in designing of the blades because of acoustic emissions.
Migilore [73] presented the aero acoustic tests that were conducted on wind tunnels. These tests were to investigate the effect of
Findings from actual installation of small scale wind turbines
James et al. (2010) presented important findings of building mounted turbine component of the UK micro-wind trial undertaken by the Energy Saving Trust in 2008–09. Performance of 39 horizontal axis wind turbines in rural, sub-urban and urban areas is monitored. Majority of the micro-wind turbines do not receive sufficient wind resource to make them economical. Among the sites monitored, only two sites met with the NOABL-MCS standard of 5 m/s wind speed. In case of building mounted turbines, none
Conclusions
As large scale wind turbines alter the global climatic conditions and have adverse effects on the atmosphere, small scale wind turbines offer a great scope for producing valuable power which can be sufficient for domestic needs without altering the climatic conditions. From the review, the following conclusions can be made:
- 1.
For Small Scale HAWT, most of the work was focused on effect of various parameters such as TSR, rotor speed, and pitch angle for a specific airfoil. Not much work has been
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