Introduction to Small-scale Wind Energy Systems (including RETScreen case study)

Q&A Session

Q: How accurate is the use of data coming from weather maps?

Data on weather maps can be used for a first estimation to find a good location. However it does not take into account the specific wind speed at the location where the wind turbine will be located. An error of 10% in the measurement will have a deviation of 33% on the energy output. See (http://www.retscreen.net/ang/accuracy_of_project_cost_estimates_image.php).

Q: Are vertical axis small wind turbines commercially available and usable?

On the website www.allsmallwindturbines.com you can find information about horizontal and vertical wind turbines.

Q: How do the power curves from vertical compare with the horizontal ones?

On the website www.allsmallwindturbines.com the power curves are given for both types. In general the horizontal ones generate more energy compared to the vertical ones.

Q: Are vertical turbines more appropriate for urban settings where there is more turbulence?

In general vertical turbines are easier to install in urban areas. In general they are less noisy and less sensitive to turbulent flows compared to the horizontal ones.

Q: What is the absolute minimum average yearly wind speed in urban areas to consider a small windmill and what is the best way to measure it?

It is not the average wind speed that is important, but the wind distribution. In general the wind speed should be higher then the cut in wind speed of the wind turbine, below this value the wind turbine will not produce any energy at tall. If possible you should measure at least during a couple of month the wind speed, preferable one year.

Q: In Poland, most small-scale wind systems are home-made, and directly coupled with water boilers, as hot tap water is very commonly required standard. Are there other turbine designs, usable at lower wind speeds?

At low wind speed the wind turbine blades can not rotate. It needs at least a certain wind speed for the wind turbine can operate by itself. The wind speed where the wind turbine starts delivering energy is called the cut in wind speed. Below this wind speed no energy will be produced by the wind turbine.

Q: Can you suggest an affordable wind speed measurement tool for small installations?

You can buy one yourself or hire it. As mentioned before an error of 10% means a deviation of 33% in the energy. No particular device to recommend.

Q: I have a small turbine on a site on a very large lake.  Would locating the turbine fan at a low altitude just over the lake surface give comparable power without need for a high tower?

Close to a large lake there will be no obstacle so it could be that your turbine can be located without the need of a high tower. According to the formula here below, in your case the wind shear factor is 0,1, which means that doubling the height in your case would lead to 7% more speed, so 23% more power. To be considered. With heavy trees or buildings doubling the height doubles the power.

The wind shear can be expressed as

v / v_o = (h / h_o)^α        

where

v = the velocity at height h (m/s)

v_o = the velocity at height h_o (m/s)

α = the wind shear exponent

The wind shear exponent varies with the terrain.

Q: Are the manufacturer published power curves reliable?

Curves are measured within laboratories conditions. In the field it is hard to measure similar power curves since there will be some turbulences etc. that could influence the wind speed. RETScreen database is regularly updated, so power curves are reliable. Check you are using the most recent RETScreen version.

Q: Do you know how much is the investment cost by kw installed?

In general the average value for small wind turbines is about 4 to 5 Euro/W.

Q: How does fluctuation of wind speed affect power output?

This can be seen in the power curves of the wind turbine. Below the cut in wind speed and beyond the cut out wind speed no energy will be produced. Between the rated wind speed and cut out wind speed, wind turbines will deliver the rated power. Between the cut in wind speed and rated wind speed, there is a cubicle relation between wind speed and energy.

Q: How can we control overcome friction in the wind turbine if wind speed is higher than 10 m/s?

Turbines are designed for higher wind speeds than 10 m/s. At higher wind speed the wind turbine will be in safe mode, meaning that turbine will be shut off. This is called the cut off wind speed. Also, aerodynamic design of blades provides self-protection to the engine at high wind speed.

Q: I have an average wind speed of 6.1 m/s and I am thinking of installing a 6 kW proven wind turbine. Would it make more sense to install a small turbine that will work more efficiently at that speed?

This depends on the power curve and also on the wind distribution at your location. Based on the wind distribution at your location you can calculate with the given power curve of your wind turbine and other ones which would give you the highest energy output.

Q: How does the density of the air affect the efficiency of the wind turbine, for example in a mountain?

There is a linear relation between the power and the density of the air: P=Area*1/2*density*velocity^3. You can test this effect in RETScreen model by setting the air temperature and atmospheric pressure.

Q: I am supposing that wind speed is not constant over time.

Correct. This is the reason why you should need the wind distribution at your location when considering a wind turbine. Variability in the wind resource results in the turbine operating at continually changing power levels.

Q: When buying wind turbine, which one to consider: variable pitch blade or fixed pitch blade in terms of high wind speed?

If you now you have high wind speed it could make sense to have variable pitch blades. However, note that most of the small wind turbines will use fixed pitch blades.

Q: What would be the maximum and minimum length of wind turbine wings?

The larger the area the blades of a wind turbine cover, the larger the output will be. For this reason wind turbines have higher heights compared to a couple of years ago. At higher altitude the wind speed is higher, the blades can be larger and so the generator can be larger, meaning more energy output. Nowadays rotor diameter can be larger than 100 m.

Q: Why would you choose a vertical axis over a horizontal one?

In general vertical axis are less noisy, the wind direction is immaterial, less sensitive for turbulences, create fewer vibration, however it will also produce less energy.

Q: I have a 400 watt wind turbine and it noisy and vibrating all time, from your experience what will be the best solution?

Older smaller wind turbines do make more noise then the newer ones. It is hard to give some general information about reducing the noise for your specific situation. 

Q: Where are the wind turbines over the motorway located?  Do you know what their rated output is?  Are they commercially available and what do they cost?

The picture shown in the presentation is an idea from the University of Arizona State. The idea was that the turbulence of passing cars would drive the turbines. We are not sure if this commercially available and what the costs would be.

Q: In RETScreen in the ''annual savings and income'' section I only have Fuel cost - base case. How do I put in other factors to get the cash flow graph to look like yours?

Check that you are using the most recent version of RETScreen software. In financial section, start filling the blanks, then new cells will appear to complete your case study.

Q: There is no any information about battery bank if we are using the system at home (off-grid system)? How we can calculate the required battery capacity?

Since there will not always be wind, it is possible that the wind turbine will not produce energy every time. During high wind speed you can use battery for storage of energy. Calculation of the batteries depends on the wind distribution, the power curve of the wind turbine and the energy you consume. The difference between the energy produced by the wind turbine and the energy you consume should be covered by the batteries. RETScreen software provides also case analysis for off-grid installations, check this option in the first sheet of analysis.

Q: What does p(u) represent in a Weibull graph?

The p(u) are the number of hours each year. A p(u) of 0.1 means 10% of the total hours a year , so 8760*10% = 876 hours.

Q: What is the practical way of handling (or protecting against) very high wind speeds as in hurricanes?

At very high wind speeds, typically 25 m/s, most wind turbines cease power generation and shut down. The wind speed at which shut down occurs is called the cut-out speed. Having a cut-out speed is a safety feature which protects the wind turbine from damage. Shut down may occur in one of several ways. In some machines an automatic brake is activated by a wind speed sensor. Some machines twist or "pitch" the blades to spill the wind. Others use "spoilers": drag flaps mounted on the blades or the hub which are automatically activated by high rotor RPMs, or are mechanically activated by a spring loaded device which turns the machine sideways to the wind stream. Normal wind turbine operation usually resumes when the wind drops back to safe levels

Q: What is the best location for turbines in mountainous topography for a wind farm?

In general close to the top of the mountain in case there are no obstacles. Wind speed is accelerated in the top of the mountain, as the air flow has less section to go across.

Q: Can you provide references to CDM methodologies for carbon calculations please? How could you classify this as source of energy which is zero carbon on the domestic side?

A dedicated webinar on CDM methodologies will be scheduled in Leonardo ENERGY. Please, check or subscribe at http://www.leonardo-energy.org/drupal/discussion-webinars to be informed.

Q: Does RETScreen take into consideration wind direction?

RETScreen does not take wind direction into account since RETScreen is a monthly model, not hourly or sub-hourly, where wind direction would have an effect. Wind direction will have an impact on turbine placement and spacing, which are design issues. RETScreen is not really concerned with wind farm design, and is instead concerned with wind farm feasibility. Looking at the figure "Accuracy of project cost estimates" from the RETScreen training course (http://www.retscreen.net/ang/accuracy_of_project_cost_estimates_image.php), the wind direction will have an impact in the engineering and design phase, not necessarily at the pre-feasibility phase. You can learn more about this chart and RETScreen's use in project analysis here: http://www.retscreen.net/ang/voice_slides_overview_intro.php

Q: Could I have more information on tower building and mounting?

This is a design and engineering issue. RETScreen is more concerned with looking at the feasibility of the project as a whole or of its parts. However, you may find some information at the RETScreen Marketplace (http://www.retscreen.net/ang/13.php). Industry journals and magazines are also an excellent resource for this type of information.

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