Dear Ahmad

taller towers allow for larger blades and generally higher outputs

http://www.reddit.com/r/pics/comments/2moxb0/sorry_for_the_potato_quality_but_some_days_my_job/

However, you want ti compare tower height for the same blade diameter I assume. 

Gerro

Dear Ahmad,

there is no general answer to your question as you would have to consider the construction site along with geographic influences as well.

In Tasmania, jet stream is available in less than 100m above ground making any tower >100m obsolete.

In some valleys, you would have to get to as much as 130m to get to the sweet spot. The offshore wind mills in the Alpha-Ventus windpark have a hub-height of about 90-92m above sealevel.

Generally, the tower hight has to be sufficient, to keep the wind speed gradient from the top of the rotor to the bottom of the rotor at a minimum to prevent bending forces to the hub.

Ahmad this is a good question. I say this as here in South Africa recent studies have shown that higher and more reliable wind speeds have been detected using wind speed sensor towers at ~ 100M in height. Recent research conducted countrywide is starting to show that this appears to be applicable countrywide and that earlier estimates were somewhat conservative.

Whether this is applicable elsewhere in unknown but the findings suggest no reason why this might not be possible or applicable elsewhere. I am not an expert in tower height cost versus output but there must be some optimum point.

Martin's response lends some support to this so it might already be well known, but I would guess it would depend upon region and several other factors to find the "sweet spot" as Martin notes/

Dear Ahmad,

Martin nailed quite well the point. The wind and climate characteristics of a given region drive the selection of a wind turbine model: turbulence, temperature, wind profile night vs. day etc just to mention a few. The wind industry tends to manufacture bigger and bigger wind turbines in order to extract more energy per turbine but, a wind turbine manufacturer also recommends a specif wind turbine model depending on the wind characteristics (which is not necessarily the biggest) because of the warranty and to preserve the integrity of a turbine too. 

Once said, to answer your question (Does it depend on geographical location, or wind patterns, or something else?), at ETH with Geographic Information Systems, we developed a model to couple the spatial wind characteristics over a large region (i.e. the variation of the wind direction and speed at different heights, air density, etc) with the technical specifications of commercial wind turbines (i.e. height, rotor diameter, power output, power curve, cost of equipment etc.) and of the land (i.e. the topography and the roughness) in order to identify the optimal layout for different wind turbine models that maximizes the energy generation and thus the estimated return on investment. After having validated the model (which is a critical and fundamental step), if you apply the spatial model to any arbitrary large region you might identify a pattern.

Dear Ahmed:

Your question has been answered by the previous responses. As said above, the local conditions are important as well as what you are trying to do. A wind farm depends on a dispatchable source of electricity so the ultimate customer can have electricity when needed. Thus, the total size of the wind farm depends on the size of the dispatchable source. With this information, the size of turbines and their locations can be estimated.

Hope this helps.

Dear Ahmed;

It is generally better to go higher to catch higher wind energy in the atmospheric boundary layer as you may know; but you have to select the turbine characteristics availabe for the wind pattern at that height with wind data. Then comes the question how to handle the turbine's maintenance and operation. There is a minimum height which is defined by the blade length and the clearence from the ground, but going higher is an engineering desicion making process by considering  cost, maintenance and local restrictions etc. It also depends on your demand of energy and the available turbines. Turbine characteristics must match the wind characteristics at that altitude. Considering the previous responses you might have got answers to your question.

Dear all scientists and colleagues,

Many thanks for all great comments and responses. I have really learnt allot from all your responses. I would guess I had seen an article (should search for it) that suggested that , because of the nature of wind boundary like profile, there is an optimum height that capture the most economical wind energy. Higher than this will increase tower and other costs by increasing a little more of wind energy. However, I have now learnt much more from your very generous and mindful responses.

Thanks allot

Ahmad

Dear Collegue,

The topographic features of a complex area cause a change in wind behavior, which interferes in the acceleration or deceleration of air flow. The effects of the hill slope and the surface characteristics of an area are one of the key elements to a micro siting evaluation, since they lead to a modification of the airflow over it. The correct definition of wind velocity profile and  turbulence intensity is fundamental for the feasibility of wind farms, as small differences result in the infeasibility of the project.

In order to investigate  the topographic and roughness effects in a micrositing,  experimental study in an Atmospheric Boundary Layer Wind Tunnel can better characterize  the airflow in specific areas. The experimental measurements can correlate  the wind velocity profile with  the output power of a specific  wind turbine to investigate and minimize such effects.

Our recent experience for the most optimizated height tower in Brazil is from 80 m to 120 m.

Thanks allot

Jussara Mattuella

Dear Mattuella,

Many thanks for sharing your experience with us. New tower-less wind measuring techniques are offering wind speed measurements upto 1000 meters above the ground. I wonder if any particular research was considered some scale higher than 100 meters for wind energy purpose and what was the outcome.

I would agree with Mattuella . Wind profile and turbulence are the two major parameters which help to select the turbines. Hill top areas (IEC class I and II) have high wind speed along with high turbulence. A taller hub will make the structure mechanically unstable. A shorter hub with shorter blade diameter will be suitable to capture the wind energy. Whereas in plane surfaces with low speed regions (IEC class III and IV), the hub needs to rise high and blades needs to be longer to capture the speed. Turbulence is not of much concern on these sites as speed itself is comparatively slow.

In India most of the class I and II sites are already explored. Agencies are now turning to class III sites where hub height ought to be higher than 100 meter for economic operation.

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