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Small wind turbines (SWTs) are used in two main areas:
- ‘Autonomous’ electrical systems (also called ‘stand-alone’, ‘grid-isolated’ or ‘off-grid’), in other words those that are not connected to any larger electrical system and are therefore solely responsible for the control of voltage and frequency; and
- ‘Distributed generation’, in other words systems with small generators connected to a larger public distribution network, where there is a network operator responsible for overall control (this is also often called ‘grid-connected’ or ‘on-grid’ generation).
Despite the attention given to multi-megawatt wind farms, the markets for autonomous electrical systems and distributed generation using small wind turbines can be attractive if prices of conventional electricity and fossil fuels are sufficiently high or, as in many developing countries, where hundreds of millions of people live without access to electricity.
However, in spite of the maturity reached on the development of the large- and medium-sized wind technology for wind farms, the state of the art for small wind turbines is far from technological maturity and economical competitiveness. Average costs for current stand-alone wind turbines vary from EUR 2,500 to 6,000 per installed kW, while in distributed generation, a small wind turbine can vary from EUR 2,700 to 8,000 per installed kW, the additional cost mainly due to the power converter required for grid connection. Both these figures contrast with the specific costs of large wind turbines, which are in the region of EUR 1,500/kW.
Concerning the performance analysis for small wind turbines, the average power density is around 0.15 to 0.25 kW/m2 because of the limited wind potential in sites where the energy is required, compared to typical sites for large wind turbines in wind farms.
The technology of small wind turbines is clearly different from that used in large wind turbines. These differences affect all of the subsystems: mainly the control and electrical systems, but also the design of the rotor. Most of the SWTs existing on the market are machines that have developed in an almost ‘hand crafted’ way, with lower maturity compared to that achieved by large wind turbines.
SWTs have great potential but some challenges have to be addressed to produce reliable machines. IEC standards do exist for SWTs (IEC61400-2 for design requirements for SWT) and there are applicable standards from large wind, such as power performance or noise emissions measurements; however, something more has to be done in order to develop more appropriate standards and simpler ways to display the results obtained to end users.
In spite of these barriers, the market in developed countries is promising for grid-connected and off-grid applications due to promotion policies (such as capital cost buy-down, feed-in tariffs and net metering), and even more so for developing countries because of the continuing decrease in specific costs and the increasing need for energy.
Table 6.1 gives a useful categorisation of commercial SWT ranges by power rated, from a few watts to 100 kW.
Table 6.1: Classification of SWT
Rated power (kW) Rotor swept area (m2) Sub-category Prated < 1 kW A < 4.9 m2 Pico wind 1 kW < Prated< 7 kW A < 40 m2 Micro wind 7 kW < Prated< 50 kW A < 200 m2 Mini wind 50 kW < Prated< 100 kW A < 300 m2 (No clear definition adopted yet)
The values that define the ranges for this classification have been chosen from the norms and legislation affecting SWTs. The value of 40 m2 was the limit established in the first edition of the IEC-61400-2 standard, and is the range intended at the present time for integration of SWT into the built environment; the 200 m2 limit was established in the second edition of the above mentioned IEC-61400-2 standard in 2006, and includes most SWT applications. Finally, the limit of 100 kW is defined in many countries as the maximum power that can be connected directly to the low voltage grid. The pico-wind range is commonly accepted as those SWTs smaller than 1 kW.
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