Small Wind Turbine Technology

Abstract

Small wind turbines are an attractive alternative for off-grid electrification and water pumping, both as stand-alone applications and in combination with other energy technologies such as photovoltaic, small hydro or Diesel engines. Under these conditions, the cost of energy alone is often not the only criterion to consider, and aspects like system performance, suitability for a given wind regime, reliability under normal and extreme wind conditions, and overall system life are often equally important. Where no grid connection is available or the grid is unreliable, it is the energy-providing service that matters, not its precise cost. In grid-connected situations, the actual vs. the rated performance may be of more interest in order to achieve the cost saving benefits proposed in the design of the project. In either case, an uninterrupted service with a performance close to the one specified by the provider is a key requirement for a successful small wind project. While all wind turbines, both MW-class utility turbines and small wind generators, are subject to the fluctuating nature of the wind, there are several reasons why it is more difficult to guarantee the performance of a small wind turbine. First, the smaller inertia of rotor/generator leads to significant transient effects in response to changing wind speeds. Moreover, often small wind turbines rely on passive mechanisms for aligning the rotor with the wind direction, such as lifting forces acting on a tail vane in the case of an upwind rotor or axial (drag and lift) forces acting on the rotor in the case of a downwind turbine. Therefore, under conditions of varying wind directions, an incomplete alignment of wind turbine and wind direction may occur, and the alignment error can be expected to be a function of the turbulent time scales present at the site. In the case of furling systems, a technology used by many manufacturers to passively protect the turbine from overspeeding and generator overheating, the situation is still far more complex since the mechanism can be triggered both by sustained high winds and gusts, also exhibiting a hysteresis depending on the specific design parameters chosen. Another important aspect relates to maintenance. While large wind turbines are routinely serviced as part of an ongoing service agreement during the lifetime of a wind farm and are constantly monitored by means of a SCADA (supervisory control and data acquisition) system, small wind generators are expected to function largely unsupervised and with only occasional maintenance. Therefore, the only indication for a typical owner of a small wind turbine is the charge level of the battery, which is only very indirectly related to the (integrated) wind turbine system performance in the period prior to the observation.