Electricity generator using a natural source of power tm

Introduction

‘The solution of every problem is contained within itself. Its plan, form, and character are determined by the nature of the site, materials used, system using them, life concern, and the purpose of the building itself’ – Frank Lloyd Wright. The United Kingdom is amongst the world’s largest natural resources for wind, wave and tidal energy. Using natural resources to generate electricity is a much-preferred choice over fuel/coal as it offers a relatively clean, safe and above all sustainable source of power. Following the oil crisis in 1974, the ETSU was set up to assess government-funded research and development projects in renewable energy field. Wave, tidal and geothermal powers were initially favoured but from 1984 onwards wind energy and more recently energy crops came to the fore.

Overall system - Wind Energy

Kinetic energy in the wind can be exploited as a promising source of renewable energy. The energy that can be captured by wind turbines is highly dependent on the local average wind speed. Wind turbines sited in windy parts of the countryside could in principle generate perhaps 20% of the UK's electricity. Whereas the potential for wind turbines mounted in shallow water offshore is even larger - perhaps up to 50% of UK electricity requirements, although the cost would be higher than for on-land machines. In spite of these geographical limitations for wind energy project sitting, there is ample terrain in most areas of the world to provide a significant portion of the local electricity needs. Typical modern wind turbines have a rated output (at full power) of around 400kW. They are usually grouped together in `wind farms'.

However, there are some technical constraints. Renewable energy sources are intermittent e.g. winds and waves. It has been suggested that intermittence need not be a major operational problem, if the electricity from these devices is fed into the national grid network. As long as the total contribution from the various intermittent renewable does not exceed 30 to 40% of the total electricity on the grid, it can in effect 'even out ' local variations. So the net overall power available from the grid remains more or less constant, without the need for expensive storage systems.

Table 1-Wind Power Stations Constructed in the UK 1991-98

Description of wind turbine

Wind energy projects are generally more financially viable in "windy" areas. This is due to the fact that the power potential, P, in the wind is related to the cube of the wind speed, U.

, where A = swept areas of blade, s = air density

However, the power production of a practical wind turbine is typically proportional to the square of the average wind speed. The difference is accounted for by the aerodynamic, mechanical and electrical conversion characteristics and efficiencies of the wind turbines. This means that the energy produced by a wind turbine will increase by about 20% for each 10% increase in wind speed.

Modern wind energy systems operate automatically. The wind turbines depend on the same aerodynamic forces created by the wings of an aeroplane to cause rotation. An anemometer that continuously measures wind speed is part of most wind turbine control systems. When the wind speed is high enough to overcome friction in the wind turbine drive train, the controls allow the rotor to rotate, thus producing a very small amount of power. This cut-in wind speed is usually a gentle breeze of about 4 m/s. Power output increases rapidly as the wind speed rises. When output reaches the maximum power the machinery was designed for, the wind turbine controls govern the output to the rated power. The wind speed at which rated power is reached is called the rated wind speed of the turbine, and is usually a strong wind of about 15 m/s. Eventually, if the wind speed increases further, the control system shuts the wind turbine down to prevent damage to the machinery. This cut -out wind speed is usually around 25 m/s.

The major components of modern wind energy systems typically consist of the following:

• rotor, with 2 or 3 blades, which converts the energy in the wind into mechanical energy onto the rotor shaft
  
• tall tower which supports the rotor high above the ground to capture the higher wind speeds
  
• gearbox to match the slowly turning rotor shaft to the electric generator
  
• control system to start and stop the wind turbine and to monitor proper operation of the machinery

solid foundation to prevent the wind turbine from blowing over in high winds and/or icing conditions [CanWEA, 1996].

Figure 1: Wind Energy System Schematic

Grid Connected Applications

In grid-connected applications the wind energy system feeds electrical energy directly into the electric utility grid. Two grid connected application types can be distinguished.

1. Isolated diesel-grid electricity generation, with wind turbine generation capacity typically ranging from approximately 10 kW to 200 kW.
2. Central-grid electricity generation, with wind turbine generation capacity typically ranging from approximately 200 kW to 1.5 MW.

When the blades turn (assuming 3 blades), the metal coil in the generator turn relative to the speed of the blade and produces a change in magnetic flux, which in turn generates electricity (Faraday’s Law).

where e = electromotiveforce and q = total magnetic flux

In most electrical transmission systems, three-phase generation is used although there have been proposals to use six phases to increase the load capacity of existing lines. The main reason for using a three-phase generation is to maintain a smooth varying power load and yet still operate with alternating current. The three-phase generator is then connected to the step-up and step-down transformer for transmission to the respective destination (homes, factories, etc). The reason is being it minimises Joule’s Law losses.

Assuming a 10 by 10 array of 100 relatively small 300kW machines (30MW in all) each with say a 30m diameter blades, and with a 10 diameter separation between each machine would cover an area of 3km by 3km. Since wind is intermittent, wind turbines in the UK can only deliver power on average for about 30% (load factor) of the time. To make a comparison with conventional plants, the DNC conversion figure is adopted by the DTI, UK. For wind turbines it is 43%. So, for 100 30MW-wind farms (assume) would have a generating capacity of 3GW but in practice only equivalent to 1.29GW of conventional 1GW plant.

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