Wind turbines, are they feasible?

In order to use wind turbines efficiently, good site selection and positioning of the turbines is necessary. A good site would have strong, steady wind and low levels of turbulence. According to the UAE Climate, prepared by the ministry of communication, the UAE has an average wind speed of 4.3 knots yearly.

Although wind turbines are usually installed in rural areas -where low population density combines with fewer turbulence inducing obstructions than in urban areas – the installation of wind turbines in urban areas was found to be financially and environmentally recommended.

In urban areas it is unusual to have an open space for wind turbines similar to rural areas, so mounting the wind turbines on the top of high rise buildings would be an optimal solution.

 

The average wind speed for the mean maximum stations in the table below, which is 4.2 m/s, will be used in this article, since the average mean for all the 20 stations in UAE, which is 2.2 m/s, is below the cut in speed required by the conventional wind turbine.

Wind turbines are usually classified by rated power, but wind turbine energy output also depends on the time the turbine spends producing a certain power.

A rough capacity factor can be obtained from the following graph. This factor depends on the cut-in speed of the wind turbine and the average wind speed on site.

For example, using the graph below, where the cut-in speed is 10 MPH, if the average wind speed in a certain site is 15 MPH, the rough capacity factor will be 20%. This means that the total number of hours in a year is: 8,760 hours x 0.2 = 1,752 hour.

So, having a wind turbine with a rated power of 1.5 MW will give output energy of: 1500 KW x 8760 hr x 0.2 = 2,628,000 KWh (American Wind Energy Association, 1998).

The shape of the idealised power curve graph came from the cubic power, where P = ½ π r ² v³, and this is why the slightest increment in wind speed would yield a substantial increase in power output, since the power is directly proportional to the cube of the wind velocity.

Considering the general electric 1.5 MW wind turbine with cut-in speed = 3.5 m/s and a Rated Speed = 12 m/s, the Rough capacity factor will be 2.2% for the average of the mean maximum stations wind speed in U.A.E (4.2 m/s), as shown in the following graph. (GE Energy, 2008)

From that graph, a table is obtained showing the monthly working hours.

According to the Danish Wind industry association, the average price for wind turbines is US $1,000 per KW. So, the average capital cost for a 1.5 MW wind turbine is US $1,500,000.

Taking the installation cost as 30% of the capital cost, which is US $450,000, gives a total initial payment on this wind turbine investment of US $ 1,950 000. Considering an expected life time for the building to be 25 years, and estimating the maintenance cost to be 1.5% of the capital cost per year (US $22,500), in accordance with the Danish Wind industry association, the total maintenance cost over the 25 years period will be US $562 500. As per the electric utility in Dubai and Northern Emirates, the price of 1KWh = 0.09 USD. So, the total cost of energy produced per year is:
1500 KW * 8,760 hr/Yr * 2.2% * 0.09USD/KWh = $ 26,017 USD/year.

This shows that the value of energy produced is not sufficient to cover the own cost.

The Wal-Mart Experience provides rare data regarding wind turbines and their actual energy production. Two experiments were implemented in McKinney, Texas and Aurora, Colorado.

The expected annual production for the 50 KW, total rated peak power, wind turbine in Aurora was 94.6 MWh, while the measured production was 7.18 MWh.

 

The measured production is less than 8% of the expected production; this means that the number of actual full load hours is 143.6 hours instead of 1,892 hours.

In Texas, the expected annual production for the 50 KW total rated peak power McKinney wind turbine was 80 MWh, while the measured production was 24 MWh for the first 12 months and 21 MWh for the last 12 months.

Again, the measured production is much less than the expected production, the measured is about 28% of the expected, with almost 450 full load hours instead of the expected 1600 hours. (ASHRAE, 2007)

Back to the UAE, the following graph shows a relationship between elevation and wind speed, as an actual balloon experiment done in Abu Dhabi International Airport.

The red line represents the actual wind speed with respect to elevation, while the other lines show a comparison between the actual and computer modeled results (TECHNOTES).

The graph actual data shows that the wind speed at the ground level is about 2 m/s, but it reaches 5 m/s at 200 m elevation (Applied Meteorology and Climatology, 2008).

Calculations indicate a wind speed of 3.86 m/s at 200 m elevation. The same calculations give an average wind speed in the UAE, at an elevation of 200 m, of 8.1 m/s (Exell, R. H. B, 2004).

Air will be thinner at 200 m elevation and the relative density of air compared to ground is approximately =0.98 (The Engineering ToolBox, 2005).

The power generated at 200 m elevation is substantially higher (13.8 times) and will be:

1,500 KW x 8,760 hr/Yr x 31% x 0.98*0.09USD/KWh = US $359,274 per annum.

At US $22 500 for annual maintenance costs, the net annual saving is US $359 274 – US $22,500 = US $336,774 per annum and the payback in this case is US $ 1,950,000 / US $ 336,774 per annum = 5.8 Years.

This is an impressive turn around when you compare ground level data with just a 200 m elevation and shows that in this application wind turbines are a feasible option.

Although the world is excited about using wind energy to generate clean electricity, the Wal-Mart Experience came with data that shows the sustainability and complexity of wind turbines cannot be easily overcome with simple traditional wind turbines.

The main reason for the poor field data is probably that Wal-Mart wind turbines were placed at a low elevation.

 

New products with new technologies are needed in the market to replace the conventional wind turbines. Examples include axial wind turbines that can improve the cut in speed to 2.5 m/s instead of 3.5 m/s, and may have usable hours of 59%, an improvement on conventional wind turbines.

Proper application and field data at elevated levels are required. In our region we are looking forward to the Bahrain World Trade Centre starting normal operation and gaining valuable field data from the application.