Determining best site locations for small scale wind energy
When trying to determine which locations may be the most appropriate for wind, there are several things you should take into consideration.
Which direction does the wind usually come from? Public information sources, such as state agencies may provide maps that show which is the most common direction for wind in your particular area. The US Department of Energy offers leads on this information and directs you to maps and website specific to your area on their website: http://www.windpoweringamerica.gov/wind_maps.asp
Here in New York State, one source that offers a general idea is through a subsection of NYSERDA's website: http://www.powernaturally.org
A wind turbine should optimally be at minimum 300 feet away from any obstruction, particularly in the upwind direction. Of equal consideration, the wind turbine would optimally be placed higher than the closest line of an (upwind) obstruction. Examples of obstructions are buildings, homes, barns, silos, trees, towers, hills or mountains. Again, this distance is optimal, but your location may not give you these ideal conditions. Therefore, you'll have to use you best judgment when trying to pre-determine which locations may be the best to locate your equipment and conduct any measurements.
There are costs associated with the distance from the renewable energy equipment to the building. To transmit the electricity being generated, usually a trench is excavated from the renewable equipment to the building, and then the wires and conduits are installed all the all the way to the building. There is also cost associated with the installation of the wires from the entry point in the building to the battery bank location inside the building. So optimally, if you have the choice, the less expensive installation location would be to locate the renewable energy equipment as close to the building as feasible, and if you are lucky yet, as close to the location of the battery bank inside the building. You'll save on installation costs.
After you've discovered which location could meet the basic requirements, narrow those down by taking into consideration the impact of each location on your neighbors, your community and the environment. You want to be a good steward by considering locations that meet your energy generation needs and will try not to be disruptive to the environment and your community. Rural businesses and farms fortunately are often located a good visual distance from other residences so there may be many options that meet all of these considerations and will not burden your community.
horizontal axis wind turbine
Here in New York, we have a state funded agency called NYSERDA, which suggests a minimum 10 mph average wind speed at hub height for wind turbine viability (hub height would be the height above the ground that the wind turbine is located). If you do not measure the wind speeds at hub height, there are formulas that will allow you to approximate the wind speed at hub height versus the actual height from where you measured. They say it is ideal to conduct your wind speed measurements at actual hub height. Having said that, let's take what is ideal versus what is practical and realistic.
For all practical purposes, most people would not want to invest in a tall radio tower just to measure wind speeds at the estimated optimal height. If you are one of those who would rather test possible locations more affordably, there is a solution. Identify the locations you think will most likely garner the best wind (tips on this step are located in another area of this section). Then create or purchase an inexpensive mounting pole and build it to the highest height you feel practical (that would not require special permitting from your local municipality) and then attach the weather station. A weather station can be purchased online or at many stores that sell outdoor recreational equipment (more on weather stations later in this section).
The measurements taken, for instance, at a height of 10 feet, could then be applied into the mathematical formula that helps you estimate what kind of wind speeds you could garner at that location at varying higher heights. Generally speaking, the rule of thumb given by engineers is the lower the hub height, the lower the wind speed; the higher the hub height, the higher the wind speed. I have been told by a variety of engineers that, for example, when measuring the wind speed average at a hub height of 10 feet, you can expect the wind speed to increase by 40% at a hub height of 80 feet.
If you want to avoid the investment of building large towers just to test wind speeds at different locations, and you build your own testing pole at a lower height, here is a mathematical formula that can help you determine the estimated wind speed at different hub heights based on the averages you collect from your actual test height:
V2 = V1 x (H2/H1)n
V2 = wind speed at a given hub height (for instance, if you need a minimum of 10 mph to power your system, this number indicates at what height you can meet or exceed that mph)
V1 = wind speed measured at the current height (for instance, if your weather station measured average wind speeds of 8 mph, this is the number you would plug in)
H2 = turbine height that would achieve the required/needed wind speed (the hub height that would produce the average minimum wind speeds that you need)
H1 = actual height at which you have measured the wind speed (the number of feet off of the ground that your weather station's wind measuring tool was affixed) n = this is called the shear exponent; in other words, the numerical value assigned to particular ground conditions. Here are the values assigned given the different topography variables (I found these values on http://www.aerostarwind.com/adjusting_wind_speeds.html):
smooth surface = 0.10
low grass or fallow ground = 0.15
high grass or row crops = 0.18
tall row crops or low woodlands = 0.20
high woods or small towns = 0.3
if it doesn't match any of these descriptions, use = 0.14
I have come across this formula at many renewable energy educational sources, enough for me to conclude it is a formula available in the public domain and therefore I am sharing it here as the generally accepted method of calculation. A few online locations where this formula can be referenced are:
Here is an example of how this calculation was used for our project in order to consider possible turbines and heights:
V2 = this is the number we are trying to determine: the wind speed. Generally, we need to exceed 10 mph on average, so we're using this formula to determine if we can exceed this number, by plugging in the following variables:
V1 = our weather station measured an average wind speed of (10.27) mph at the current height
H2 = selected hub height; we'll sample this formula to see what we can harness at (20) feet
H1 = our weather station's wind measuring tool was approximately (7) feet off of the ground n = the particular ground conditions at our site would best be defined as open field, therefore, the numerical value assigned was (0.18)
Since we are already achieving the minimum average wind speed of 10 mph at only 7 feet off of the ground, we use this formula to try to estimate what wind speeds we could achieve at varying heights. Here is an example of how we could estimate our wind speed at a hub height of 20 feet when using the above information to plug into this formula
Using the formula [ V1 ] x [ (H2/H1)n ] = V2
the result was: [10.27] x [1.208] = 12.406
This formula would seem to indicate that at a wind turbine hub height of 20 feet at our location, we could expect an average wind speed of 12.406 mph.
That would seem to indicate all is well at that height, right? Well, we have to be careful, even with this calculation. Why? Average speed is just that: an average. Therefore, it would be important for me to also try to find out how often and for what length of time wind may not meet that minimum wind speed at this location. This analysis is often referred to as the location's wind distribution curve.
Let me explain this next step and why it is important. Some wind turbines do not generate electricity below certain levels of wind; the minimum wind speed that a turbine may need is referred to as the cut in speed. [Note: In another part of this website, under Our Planned Solution: Renewable Energy Equipment, we've complied a database that contains a detailed list of a variety of wind turbines and the specific requirements for each, including cut-in speeds. The information you gather during this step will help you better utilize that database.]
Therefore, what I would like to know is, the actual distribution curve of the wind at the selected location. Welcome to probability and statistics class! To sum it up, the distribution curve analysis would give me a statistical determination of the frequency with which I could expect different wind speeds. Have no fear: I can gain this information by using a data-logging weather station that sends and downloads this information automatically onto my computer.
Let's put this consideration into application. Wind turbines, generally speaking, do not start producing electricity until 8 mph. If the distribution curve at my site indicates that I have numerous readings below 8 mph for extended periods of time, for instance, I would need to do one of the following. Either:
(1) seek wind turbines that can start producing electricity at wind speeds lower than 8 mph
- or -
(2) increase my tower height to possibly decrease the frequency at which wind speeds ever get below 8 mph at that location
Knowing the wind distribution curve, as well as the cut-in speeds of different wind turbines, becomes important when determining which kind of wind turbine could work efficiently at the location and height under consideration, or, what kind of wind turbine is needed given the limitations of the location and height under consideration.