Reliable guidelines and accents

All renewable energy (except tidal and geothermal power), and even the energy in fossil fuels, ultimately comes from the sun. The sun radiates 174,423,000,000,000 kilowatt hours of energy to the earth per hour. In other words, the earth receives 1.74 x 10 17 watts of power. About 1 to 2 per cent of the energy coming from the sun is converted into wind energy. That is about 50 to 100 times more than the energy converted into biomass by all plants on earth.The wind rises from the equator and moves north and south in the higher layers of the atmosphere. As the wind rises from the equator there will be a low pressure area close to ground level attracting winds from the North and South. At the Poles, there will be high pressure due to the cooling of the air. Keeping in mind the bending force of the Coriolis force, we thus have the following general results for the prevailing wind direction:

Latitude 90-60oN 60-30oN 30-0oN 0-30oS 30-60oS 60-90oS
Direction NE SW NE SE NW SE

The prevailing wind directions are important when siting wind turbines, since we obviously want to place them in the areas with least obstacles from the prevailing wind directions. Local geography, however, may influence the general results. Local climatic conditions may wield an influence on the most common wind directions. A wind turbine obtains its power input by converting the force of the wind into a torque (turning force) acting on the rotor blades. The amount of energy which the wind transfers to the rotor depends on the density of the air, the rotor area, and the wind speed. The kinetic energy in the wind depends on the density of the air, i.e. its mass per unit of volume. At normal atmospheric pressure and at 15o Celsius air weighs some 1.225 kilogrammes per cubic metre, but the density decreases slightly with increasing humidity. Also, the air is denser when it is cold than when it is warm. At high altitudes, (in mountains) the air pressure is lower, and the air is less dense. The rotor area determines how much energy a wind turbine is able to harvest from the wind. Average wind speeds are often available from meteorological observations measured at a height of 10 metres. Hub heights of modern 600 to 1,500 kW wind turbines are usually 40 to 80 metres, however.The wind speed is always fluctuating, and thus the energy content of the wind is always changing. Exactly how large the variation is depends both on the weather and on local surface conditions and obstacles. Energy output from a wind turbine will vary as the wind varies, although the most rapid variations will to some extent be compensated for by the inertia of the wind turbine rotor.

In most locations around the globe it is more windy during the daytime than at night.From the point of view of wind turbine owners, it is an advantage that most of the wind energy is produced during the daytime, since electricity consumption is higher than at night. Meteorology data, ideally in terms of a wind rose calculated over 30 years is probably your best guide, but these data are rarely collected directly at your site, and here are many reasons to be careful about the use of meteorology data.

The wind power projects

If there are already wind turbines in the area, their production results are an excellent guide to local wind conditions. Both the feasibility of building foundations of the turbines, and road construction to reach the site with heavy trucks must be taken into account with any wind turbine project. With an average wind speed of 4.5 m/s at hub height the 600 kW machine will generate about 0.5 GWh per year, i.e. 500,000 kWh per year. With an average wind speed of 9 metres per second it will generate 2.4 GWh/year = 2,400,000 kWh per year. Thus, doubling the average wind speed has increased energy output 4.8 times. If we had compared 5 and 10 metres per second instead, we would have obtained almost exactly 4 times as much energy output. The reason why we do not obtain exactly the same results in the two cases, is that the efficiency of the wind turbine varies with the wind speeds.Another way of stating the annual energy output from a wind turbine is to look at the capacity factor for the turbine in its particular location. By capacity factor we mean its actual annual energy output divided by the theoretical maximum output, if the machine were running at its rated (maximum) power during all of the 8766 hours of the year. Even if prices are very similar in the range from 600 to 750 kW, you would not necessarily want to pick a machine with as large a generator as possible. A machine with a large 750 kW generator (and a relatively small rotor diameter) may generate less electricity than, say a 600 kW machine, if it is located in a low wind area. Not all wind turbine manufacturers around the world have a good, long reliability record. With any investment, you pay something now to get something else later. The market of second-hand wind turbines is dynamic. There are too many requests for less quantity turbines offered for sale. Here is a historical statistics for the turbine’s cost and total project’s costs:

Total investment cost

Good sites for wind plants are the tops of smooth, rounded hills, open plains or shorelines, and mountain gaps that produce wind funneling.Operating a wind power plant is not as simple as just building a windmill in a windy place. Wind plant owners must carefully plan where to locate their machines. One important thing to consider is how fast and how much the wind blows.Wind power has a unique characteristic: it requires no fuel. Therefore it has zero fuel price risk, zero fuel costs, no external energy dependence and extremely low operation and maintenance costs. Wind is power without fuel. One wind machine can produce 1.5 to 4.0 million kilowatthours (kWh) of electricity a year. That is enough electricity for to power 150-400 homes. New technologies have decreased the cost of producing electricity from wind, and growth in wind power has been encouraged by tax breaks for renewable energy and "green programs". The European wind power industry estimates, that given the right legal and financial support, wind projects could provide energy for 50 million people in Europe in less than 10 years' time.

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