Michael Graham - Wind Energy Handbook

Figure 1.1shows the remarkable growth in the installed capacity of wind power worldwide over 15 years to 2019. The typical annual rate of increase of capacity was more than 10%. Figure 1.2shows the growth of wind energy capacity by country, dominated by China and the USA. Figure 1.3summaries current capacity (2019) by country and region of the world.

Figure 1.1 Wind power capacity worldwide (World Wind Energy Association 2020).

Figure 1.2 Wind power capacity by country (US Energy Information Administration 2019; REN21 2020).

Figure 1.3 Installed onshore wind power capacity in countries with more than 10 GW, regions, and total offshore (Global Wind Energy Council 2020).

The development of wind energy in some places has been more rapid than in others, and this cannot be explained simply by differences in the wind speeds. Important factors include the financial support mechanisms for wind generated electricity, access to the electrical network, the permitting process by which the local civil authorities give permission for the construction of wind farms, and the perception of the general population, particularly with respect to visual impact. The development of offshore sites, although at considerably increased cost, is in response to these concerns over the environmental impact of onshore wind farms.

Figure 1.4 Onshore wind turbines in flat terrain.

Source : Stockr/Shutterstock.com.

Figure 1.4shows modern wind turbines in flat open terrain, and Figure 1.5shows an offshore wind farm.

When it was a new electricity generation technology, wind energy required financial support for some years to encourage its development and stimulate investment from private companies. Such support was provided in many countries in recognition of the contribution that wind generation makes to mitigating climate change and the security of national energy supplies. Feed‐in Tariffs continue to be offered in a number of countries. These are fixed prices paid for each kWh generated from renewable sources with different rates for wind energy, photovoltaic solar energy, and other renewable energy technologies. This support mechanism has the benefit of giving certainty of the revenue stream from a successful project and is credited by its supporters for the very rapid development of wind energy, and other renewables, in these countries.

Large wind farms are now often supported through competitive auctions that establish a price that a project developer can expect for the electrical energy. This acts to reduce uncertainty and hence project financing costs. The cost of generating electricity from wind power continues to fall and is now below the retail price of electricity in most countries and lower than the cost of generation from alternative sources of energy under favourable conductions of high site wind speed and low wind farm constriction costs. These cost reductions mean the need for national subsidies is rapidly reducing.

Figure 1.5 Offshore wind farm.

Source: fokke baarssen/Shutterstock.com.

The power output from a wind turbine is given by the well‐known expression:

ρ is the density of air (1.25 kg/m3)

Cp is the power coefficient

A is the rotor swept area

U is the free wind speed

The density of air is rather low, 800 times less than water, which powers hydro turbines, and this leads inevitably to the large size of a wind turbine. Depending on the design wind speed that is chosen, a 3.5 MW wind turbine may have a rotor that is 100 m in diameter. The power coefficient describes that fraction of the power in the wind that may be converted by the turbine into mechanical work. It has a maximum value of 16/27 or 0.593, and rather lower peak values are achieved in practice (see Chapter 3). Incremental improvements in the power coefficient are continually being sought by detailed design changes of the rotor, and by operating at variable speed it is possible to maintain the maximum power coefficient over a range of wind speeds. However, these measures will give only a modest increase in the power output. Major increases in the output power can only be achieved by increasing the swept area of the rotor or by locating the wind turbines in higher wind speeds.

Figure 1.6 Largest commercially available wind turbines.

Hence, over the last 25 years there has been a continuous increase in the rotor diameter of commercially available wind turbines from around 40 m to several manufacturers offering turbines of more than 170 m ( Figure 1.6). A tripling of the rotor diameter leads to a nine times increase in power output. The influence of the wind speed is even more pronounced, with a doubling of wind speed leading to an eightfold increase in power. Thus, there have been considerable efforts to ensure that wind farms are developed in areas of the highest wind speeds and the turbines optimally located within wind farms. In certain countries, with modest wind speeds, very high towers are being used to take advantage of the increase of wind speed with height.

In the past a number of studies were undertaken to determine the optimum size of a wind turbine by balancing the complete costs of manufacture, installation, and operation of various sizes of wind turbines against the revenue generated (Molly et al. 1993). However, these early estimates would now appear to be too low, and more recent studies indicate that the lowest cost of energy is obtained with rotors approaching 150 m diameter, although smaller turbines may be preferred on some sites for reasons of environmental impact and difficulty of transporting very large components to the site. Even larger turbines give the lowest cost of energy offshore, where the foundation and cabling costs of individual turbine are high and the very large blades can be transported by ship directly from the factory to the site.

All modern electricity generating wind turbines use the lift force derived from the blades to drive the rotor. A high rotational speed of the rotor is desirable to reduce the gearbox ratio required, and this leads to a low solidity rotor (the ratio of blade area to rotor swept area). The low solidity rotor acts as an effective energy concentrator, and as a result the energy generated over a wind turbine's life is much greater than that used for its manufacture and installation. An energy balance analysis of a 3 MW wind turbine showed that the expected average time to generate a similar quantity of energy to that used for its manufacture, operation, transport, dismantling, and disposal was six to seven months (European Wind Energy Association 2009). A similar time was calculated for offshore wind turbines. Offshore the higher mean wind speeds, and hence greater energy output, compensate for the higher wind farm costs and energy expended in construction and operation.