Wind Power Could Supply Global Electricity Needs 40 Times Over

The enormous potential of wind power

Wind turbines on land could provide more than 40 times the world’s current electricity consumption or over five times its total energy needs. That’s the latest assessment using wind data from meteorological sources [1]. A network of 2.5-megawatt (MW) turbines on land restricted to non-forested, ice-free, nonurban areas operating at as little as 20 percent of their rated capacity would do the trick; allowing for the fact that the wind does not blow constantly. To put this into perspective, wind turbines installed in the US in 2004 and 2005 operate on average 36 percent of rated capacity.

For the United States, the central plain states could accommodate enough wind turbines to provide as much as16 times its total current demand for electricity.

Wind power is on a steep ascent. It accounted for 42 percent of all new electrical capacity added to the US in 2008; but it is still only a tiny fraction of the total capacity, 25.4 GW out of 1 075GW. The Global Wind Energy Council projected a 17-fold increase in wind-powered generation of electricity globally by 2030.

Simulating global wind fields based on state-of-the-art data

Xi Lu and Michael McElroy at Harvard University, Cambridge Massachusetts in the United States and Juha Kiviluoma at the Technical Research Centre of Finland based their study on a simulation of global wind fields from version 5 of the Goddard Earth Observing System Data

Assimilation System (GEOS-5 DAS) that includes global meteorological data from a wide variety of sources including surface and sounding measurements, measurements and observations from aircraft, balloons, ships, buoys, dropsondes (radio probe dropped by parachute) and satellites; the gamut of data that can provide the world with the best possible meteorological forecasts enhanced by application of these data in a retrospective analysis.

The land-based turbines are assumed to have a rated capacity of 2.5 MW with somewhat larger turbines, 3.6 MW, deployed offshore, to take account of the greater cost of construction and the economic incentive to build larger turbines to capture the higher wind speeds available there. In siting turbines on land, the study excluded densely populated regions and areas occupied by forests and environments distinguished by permanent snow and ice cover (notably Greenland and Antarctica). Turbines located offshore were restricted to water depths less than 200 m and to distances within 92.6 km off shore.

Optimal spacing of the turbines in an individual wind farm involves a trade off between various costs: turbines, site development, laying power cables, routine operations and maintenance. Turbines must be spaced to minimize interference in airflow and requires a compromise between maximizing power generation per turbine and maximizing the number of turbines sited per unit area. For example, restricting overall power loss to < 20 percent requires a downstream spacing >7 rotor diameters, and a cross-wind spacing of > 4 diameters.

The power yield is assumed to be only a fraction (20 percent) of the maximum potential to account for the variability of the wind over the course of a year.

In this way, a world map of the annual wind power potential (W/m2) is obtained; and  the country by country potential for both on land and off shore wind power also represented.

Wind power potential worldwide

The total global potential power source for wind is estimated at 2 470 EJ (ExaJoule = 1018J).

Table 1 gives the wind power potential of 10 countries identified as the largest emitters of CO2 in 2005, though China has surpassed the US to be the biggest emitter in 2006.

Table 1. Wind power potential for the 10 biggest CO2 emitting countries

As can be seen, wind power could supply close to 18 times the electricity consumption for China, the bulk of which, 89 percent, could be derived from land wind turbines. The potential in the US is 23 times the current electricity consumption, the bulk 84 percent supplied on land. The UK’s wind potential is 30 times its electricity consumption, with 41.5 percent derived from land. In terms of wind power potential, Russia ranks number one, followed by Canada, with US in third position. Much of the wind power potential in Russia and Canada is located at large distances from population centres, however.

Wind power for the US

In the US, demand for electricity peaks twice a year in summer and winter respectively separated by minima in spring and fall. Demand is greatest in summer due to air-conditioning, when it exceeds the minimum in spring/fall typically by some 25 to 35 percent. There is a negative correlation between the monthly averages of wind power production and electricity consumption. Very large wind power can produce excess electricity during large parts of the year. This allows the option of converting electricity into other energy forms. For example, plug-in hybrid electric vehicles could take advantage of short-term excesses in the electricity system, while energy-rich chemicals such as H2 – from electrolysis of water - could provide for longer term-storage [2] (Harvesting Energy from Sunlight with Artificial Photosynthesis, SiS 43)..

The annual onshore wind potential on a state-by-state basis shows a high concentration in the central plains extending northward from Texas to the Dakotas, westward to Montana and Wyoming, and eastward to Minnesota and Iowa. The resource in this region could provide 16 times the total current demand in the US. As resource is significantly larger than the local demand, it will require extending the existing power transmission grid to exploit this resource. The Electric Reliability Council of Texas, the operator responsible for the bulk of electricity transmission in Texas, estimates that the extra cost of transmitting up to 4.6 GW of wind generated electricity is ~$180/kW, or about 10 percent of the capital cost for installation of the wind-power generating equipment.

Micro-generation with small wind turbines

The study convincingly shows that wind power can supply the world’s energy use many times over; though it implies that big turbines and wind farms are necessary, which is not the case. Like solar heating and photovoltaic, local micro-generation of wind power is eminently feasible, and has been encouraged by the Ministry of Agriculture, Food & Rural Affairs in Ontario, Canada, for several years [3]. There it costs $2 000 to $8 000 per kilowatt to purchase a small wind turbine; but that represents only 12 to 48 percent of the total costs of the wind energy system, which includes inverters and batteries, sales tax, installation charges and labour. The cost of energy produced by small (<10 kW) wind turbine over its life time has been estimated to vary from $0.07/kWh, for a low cost turbine in a high wind area to $0.96/kWh for a high cost turbine in a low wind area.

In the UK, micro wind electricity generation is increasingly popular for households [4]. The average UK household uses around 4 000 kWh a year, which can be produced with a 1.5 kW wind turbine. If a house is already linked to the national grid, a wind turbine can supplement the mains supply. When the wind turbine is not generating enough energy, mains electricity is used. When the turbine generates more than is needed, the excess can be exported to the national grid. A 1.5 kW wind turbine costs around £3 000 to £ 5 000 (2007 prices). UK’s Department for Business Enterprise and Regulatory Reform (BERR) runs a Low Carbon Buildings Programme that provides grants for micro-generation technologies for householders as well as public building [5]. The micro-generation technologies supported include solar electricity, wind turbines, water turbines (small scale hydro), solar hot water, ground source heat pumps, air source heat pumps, wood-fuelled boilers (biomass), automatic pellet-feed wood burning stoves (biomass), renewable combined heat and power, and fuel cells. 

The current cost of micro wind generation is still rather high, but it could come down considerably. William Kamkwamba from a remote village in Malawi built his first wind turbine from scrap when he was 14 years old, and Max Robson in the UK has been inspired to produce an Envirocycle Scrap Wind Turbine prototype at £20 budget, that he claims cost £2 000 on the market [6] (Harnessing the Wind with Scrap, SiS 44). Such low cost micro generation options are particularly appropriate for developing countries.

A cheap micro wind turbine at last?

In another development, John Gregg, an international expert in spin electronics and magnetic instrumentation at the University of Oxford has designed and built a wind turbine prototype in his mother’s garden that uses a standard induction motor as a generator [7].

In an ordinary wind turbine, the rotor blades rotate in the wind and in doing so, spin a shaft leading from the hub of the rotor to a generator. The generator transforms the rotational energy into electricity. The simplest generator works by electromagnetic induction to produce an electrical voltage – a difference in electrical potential – that can drive an electric current through an external circuit. Whenever an electrical conductor moves relative to a magnetic field, voltage is induced in the conductor. If a coil is spinning in a magnetic field, then the two sides of the coil moves in opposite directions, and the voltages induced in each side add up to produce a direct current (DC) through the external circuit. In order to fit in with the 60 cycles alternating current (AC) of the domestic electricity supply, an inverter is needed to convert the DC into 60 Hertz AC, and this is complicated as the voltage produced depends on the speed of the rotor, which in turn depends on the wind speed. The high costs of wind turbines are due to custom-built generators, invertors, storage batteries and complex circuitry.

Gregg struck on the idea of using an electric (induction) motor as a generator as the result of a question asked by a student: How can an induction motor work as a generator?

An electric motor uses electromagnetic induction to create motion, which is the opposite of a generator. It consists of an electromagnet rotating in the field of a permanent magnetic (or another electromagnet) on the simple principle that like poles repel and opposite poles attract.

In trying to answer the student’s question, Gregg spotted a novel and very cheap way of using an induction motor as a generator, basically by running it backwards. Induction motors can be found in everything from domestic appliances such as washing machines to industrial machines.

The electricity generated by using an AC inductor motor is not at constant voltage or frequency. But, Gregg realises that hot-water tank heater elements don’t mind variable voltages or frequencies. “That’s why we can make it cheaply and why it performs well because we are not handcuffed by the necessity to deliver 249V 50 Hz,” Gregg said.

Instead, Gregg designed a patented electronic control method, drawing inspiration from Swiss locomotives. Instead of a mechanical gearbox, the train changes gear electrically as the field windings on the magnet on the motor are switched to give maximum acceleration at all speeds. “Our generator works in a similar fashion,” said Gregg. “Because the generator is configured as a constant power source and acts effectively as a generator and a continuous variable electronic gearbox, the turbine aerofoils operate on the peak of their performance curves at all times, and all the power they deliver is harvested and channelled to the load.”

The wind turbine has a six-metre diameter blade and a standard 7.5 kW induction motor used as a generator. Because of planning permission, it cannot be sited high enough to catch the optimum amount of wind. Nevertheless, early results show the equivalent of 1 kW continuous power. The turbine provides electricity for a heat exchanger tank, which in turn feeds the domestic hot-water tank and also feed surplus heat into the domestic central heating, so saving on oil as well as electricity bill.

Five years ago, when it all started, it would have cost £33 000 to install an equivalent commercially available turbine.

With co-inventor Mazhar Bari, Gregg is now proposing a spinout company, Renewox, though Isis Innovation, the technology transfer company of Oxford University.