Average Wind Speeds At Heights Engineering Essay

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The assignment aim is to understand the nature of wind resource and to conduct an analysis of the environment of the site from the measured wind data given. This measured data is given in an excel file, and it consists of:

Average wind speeds at heights of 12m,15m and18m

Standard deviation of wind speeds at heights of 12m, 15m and 18m

Average wind direction at heights of 12m, 15m and 18m

Standard deviation of wind direction at heights of 12m, 15m and 18m

Maximum wind speeds at heights of 12m, 15m and18m

Temperatures in degrees Celsius

All the data is measured at every 10 minutes interval from the date 1/04/10 to 31/01/11 at a site called UCLAN sports arena. Therefore, the objectives of this assignment includes, calculating and analysing:

Wind shear - Comparing average measured data, calculated data of log law and the power law and also comparing the database of NOABL for different height of wind speeds.

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Wind variability - comparing the variation of average wind speeds and temperatures for different time scales (hourly and monthly).

Frequency distribution - comparing the frequency distribution using different types of representation. These types include: frequency distribution, Rayleigh distribution and wind rose.

Turbulence intensity - Calculating turbulence intensity for different wind speed intervals and comparing the results.

Background

Wind energy has been used for thousands of years for milling grain, pumping water and many other mechanical applications. Attempts to generate electricity from wind energy started in early 19th century. It is since 1980s that the wind technology has become sufficiently mature to design and manufacture large wind applications for large electricity productions. The technology is continuously being improved to make it much cheaper and more reliable than any other fossil fuel applications. To conclude wind energy will become an even more economically competitive source of energy for the United Kingdom over coming decades. Boyle Renewable energy for power for sustainable future and

The wind resource is the most obvious factor to concentrate on when choosing a wind turbine location. We have a wide range of options to determine the wind resource of the site. The quality of the tools varies significantly and so does their price. To determine if a sites good enough to install a wind turbine factors such as wind shear, wind variability, frequency distribution and turbulence intensity should be considered. [1]

Wind shear

Wind shear refers to a change in wind speed or direction with height in the atmosphere. The increase of the mean wind speed with height defines the phenomenon called the wind shear.[2]

Wind variability

One of the most critical features of wind generation is the variability of wind. Wind speeds vary with time of day, time of year, height above ground, and location on the earth's surface. [2]

Frequency distribution

Frequency distribution for given is plotted to give a probability of number hours for which the wind speed occurs during the year. Great tool for determining which wind speed dominates at that site accordance to the number of hours.

Turbulence intensity

Turbulence intensity quantifies how much the wind varies typically within 10 minutes. Because the fatigue loads of a number of major components in a wind turbine are mainly caused by turbulence, the knowledge of how turbulent a site is of crucial importance. [3]

Wind shear

Wind shear refers to a change in wind speed or direction with height in the atmosphere. The increase of the mean wind speed with height defines the phenomenon called the wind shear. In wind energy, two mathematical models or laws have generally been used to model the vertical profile of wind speed. The first approach is the log law shown in equation 1 and the second is the power law which is shown in equation 2. [3]

Mean wind speed is the average annual wind speed for that particular height. Power law was calculated using the following formula:

U(z)=(ln z/z_o )/(ln z_ref/z_o ) (Uã€-(zã€-_ref)) (1)

Therefore, a power law can also be determined:

U(z)=ã€- (z/z_ref )ã€-^s (Uã€-(zã€-_ref)) (2)

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Zo is the surface roughness of the ground terrain. Assumption was made this value is 0.03

S is the wind shear exponent which depends the earths roughness can be calculated using equation 3

Z is the tower height

Zref is the anemometer height usually at 12 m in this case

U (zref) is the reference wind speed

Wind shear exponent is calculated by the following equation 3:

S= (1/(ln z/z_o )) (3)

Table 1: Wind shear at heights of 15m and 18m

Height (m) 15 18 12

mean wind (m/s) 2.932866 3.235617 2.608837

Log law (m/s) 2.706 2.785387

Power law (m/s) 2.704213 2.77955

Table 1 shows the mean wind speed, log law and the power law at heights of 15m and 18m. Which the log law and power law was calculated by equations (1-3). It can be observed in Table 1, that the mean wind speed increases with an increase in height. This statement is also true with the log law and the power law. This is known as the wind shear phenomenon. Table 2 was drawn in order to compare with the measured and the calculated data.

Table 2: NOABL database for heights of 10m, 25m and 45m

NOABL database

Height (m) 10 25 45

Wind speed (m/s) 5.1 5.9 6.5

Observing Table 2, the data that was provided by NOABL database was at different heights compared to those that were calculated using the measured data. Therefore some interpolation was needed to compare the both results obtained. Figure 1 shows interpolated data of wind speed for different heights.

Three results were given by the NOABL data base, this included mean wind speeds at 10m, 25m, 45m height only. Therefore, an equation was generated through a function in excel called trend lines by plotting a graph. This equation was then manipulated to calculate different wind speed for different heights. This is presented on Figure 1.

Comparison can be made from the measured data, calculated data and the NOABL data. Observing Table 1 and Figure 1. It can be seen that heights for 15m and 18m are different for measured, calculated and NOABL database. 18m height for measured is approximately 2.9 m/s, for calculated it is 2.7 m/s and for NOABL database it is 5.6 m/s.

To conclude at heights of 45 meters the annual wind speed should be greater than 7 m/s in order to consider installing a wind turbine but as Figure 7 shows, that at 45 meters the wind speed is 6.5 (m/s) which is just below the required amount.

Wind variability

One of the most critical features of wind generation is the variability of wind. Wind speeds vary with time of day, time of year, height above ground, and location on the earth's surface.

Figure 2: Mean hourly wind speed from

Figure 3: Mean hourly temperature from

Wind speed data and temperature data for both monthly and hourly is presented in Figures (2-5) for 18 meters height. Data was given in 10 minutes interval therefore, an average was taken annually for average hours from 0 to 23 and same method was used for the average months.

It can be visually observed at the graphs in Figure 2 and Figure 3, that they both produce similar pattern. Meaning that during the night the temperatures are low as well as the wind speeds. However, during the day as the temperature hits noon it increases and so does the wind speed, therefore they are both correlated. This could be due the reason that near the earth's surface, winds are usually greater during the middle of the day and decrease at night. This is due to solar heating, which causes "bubbles" of warm air to rise. The rising air is replaced by cooler air from above. This thermal mixing causes wind speeds to have only a slight increase with height for the first hundred meters or so above the earth. At night, however, the mixing stops, the air near the earth slows to a stop, and the winds above some height (usually 30 to 100 m) actually increase over the daytime value. []

Figure 4: Mean monthly wind speed form April 210 to March 2011

Observation can be placed on the two Figures (4-5). Where, monthly temperature is linked with the monthly wind speed. During the summer periods the temperature is high and therefore, the wind speed is also high. When it comes to December and reaches temperatures below 0oC the wind speeds also drop to a lower speed. Therefore again this is due to the reason mentioned earlier about solar heating, which causes "bubbles" of warm air to rise during summer. The rising air is replaced by cooler air from above. This thermal mixing causes wind speeds to have only a slight increase with height for the first hundred meters or so above the earth and vice versa happens during winter. However, in the month of February the wind speed is at the highest compared to all other months and the temperature is low for that month. February is considered to be a spring month therefore high wind speed is created due the rise and fall of pressures from cold and warm winds. Where, this phenomenon tries to stabilise itself to creates a conventional wind current.

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Frequency distribution

Frequency distribution in wind resource is the measure of how much the wind speed occurs for a particular wind speed interval or a bin. Figures (6 - 8) shows different types of distribution that can be presented in order to locate velocity of wind occurrence and the direction of the using the wind rose plot shown in Figure 8.

Figure 6: annual wind speed distribution for the annual year of April 2010

Frequency distribution for average wind speed at 18 meters height is shown in figure 6. It can be observed that wind speeds between the intervals of 3-4 m/s occurs more than any of the other wind speed. This shows that most of the yearly hours for the UCLan arena site are fixated between the intervals of 3-4 m/s.

Figure 7: Rayleigh distribution of different wind speed intervals at 18 meter height

Figure 7 shows the Rayleigh distribution of different wind speed intervals and the annual hours for that wind speed occurrence for 18 meters height. Rayleigh distribution was calculated by equation 4. For all the wind data and then using FREQUENCY function excel to allocate different wind speed bins or category of wind speeds.

Equation to calculate Rayleigh distribution:

hours=8760/exp[ã€-Ï€/4 (U/U_mean )ã€-^2 ] (4)

8760 is the number of hours per year

U is the wind speed

Umean is the mean wind speed

Observing Figure 7 it can be concluded that the most common wind speed that the site receives is backed up by the number of hours that the wind speed is received is approximately 3 m/s at a height of 18 meters.

Figure 8: wind rose at 18 meters

Figure 8 shows a wind rose plot of different wind directions and wind speeds at 18 meters height. Wind rose is calculate or drawn up in excel by using the AVERGEIFS function to categorised 16 different directions also with wind speed correlating to that direction.

It can be observed from Figure 8 that wind speeds in the intervals of 0 m/s to 5 m/s is the most dominate wind speed which is roughly about 11% this is also analysed and concluded before in the frequency distribution in Figure 6 that the most dominate wind speeds lie between 3 m/s and 4 m/s. The direction where this wind is coming from is located towards the south west (225o). This is roughly what the expectation for the direction of the wind should be, because the UK's prevailing winds normally blow from the South-West, over the Atlantic Ocean. The prevailing wind direction depends on the time of year. However, on average over the entire year the prevailing direction is from the southwest at an approximate of 245 degrees []. From the measured data this is close to the results expected. http://uk.ask.com/what-is/what_is_the_prevailing_wind_direction_in_the_uk.

Turbulence intensity

Turbulence is the instantaneous, random deviation from the wind speed. Turbulence in the wind is caused by the dissipation of the winds kinetic energy into thermal energy via the creation of and destruction of progressively smaller gusts []. The basic measure of turbulence is the turbulence intensity. The turbulence intensity changes with mean wind speed, with surface roughness and with the atmospheric stability. []

The turbulence intensity is frequently in the range of 0.1 and 0.4

Therefore the turbulence intensity is defined by:

I=σU/U ̅ (4)

Where:

I is the turbulence intensity

σU is the standard deviation of the wind speed

U Ì… Is the mean wind speed

Table 3: Turbulence intensity at wind speed intervals of: a) 2-4 m/s, b) 6 to 8 m/s and c) 11 to 13 m/s. Also showing the average wind speed between the in the intervals of a, b and c.

wind speed intervals (m/s) Turbulence intensity Average wind speed (m/s)

2 to 4 0.277306 2.960365

6 to 8 0.238645 6.815984

11 to 13 0.252491 11.84847

Table 3 shows the turbulence intensity at different wind speed ranges. It can be observed that the turbulence intensity is high at low wind speeds and low at high wind speed, however, observing at intervals between 6 m/s to 8 m/s the turbulence decreases.

Conclusions

To conclude the assignment objectives were met where an analysis of wind speed was carried out from the given data. It can be said that this site relates to having a mean wind speed of 3.2 m/s at a height of 18 meters. However a good commercial site for installing a wind turbine at 45 meters should have annual wind speeds above 7 m/s. NOABL data base have given a figure of 6.5 m/s which is not far off therefore it could be considered at bigger height rather than 18 meters. Also looking at Figure 5 it can be alleged that during the winter time where the winds are low, the turbine could be stopped for maintenance.

When installing the turbine at this site some points must be considered such as:

Is causing any obstruction to any building on the site.

Environmentally-sensitive areas such as bird habitats, wetlands, and historical preservation areas.

People may be concerned about the visual impact of the tower and turbine so you should discuss this with them before proceeding.

People are often concerned about the potential noise of the system but a wind turbine generally creates no more noise than the average home refrigerator.

Looking at the points above it can be stated that the turbine at that site is mainly planned to be installed on an open site and therefore, it would be viable to install this turbine as some of the points would not affect the installation.