5. Comparison with field data for smaller wind turbines.
The Bergey Excel S-60, Proven 6 and the Honeywell WT600 wind turbines.
0n this webpage, we will look firstly at two popular turbines to show the uncertainties that can arise in establishing the power curves and the mean power results calculated from them. Each turbine uses a different technique of overload protection at high wind speeds. The fact that these turbines are among the more popular suggests that their users are satisfied with their overall performance and reliability. In addition, a new turbine - the Honeywell WT6000 - is also considered. At the time of updating this webpage (September 2009), this turbine is not yet commercially available but it is of an unusual design and, in view of the reputable nature of the company that designed it, it could make a big impact on the small turbine market.
Bergey Excel S-60 with Gridtek 10 inverter.The Bergey Excel is a popular US turbine design. It has a blade diameter of 7 metres and comes with electrical regulators for connecting it either to a mains electricity system (the Gridtek 10 inverter) or for charging batteries (the Opticharge regulator). The manufacturer's literature shows that the different electrical regulators have a significant effect on the peak power delivered by the turbine. In the case of the a.c. output from the Gridtek 10, the peak power is shown to be around 12 kilowatts at a steady wind speed of 19 metres/second. By contrast, with the battery charging regulator, peak power is reduced to 8 kilowatts at 15 metres/second. The Bergey unit uses a tail fin furling arrangement to turn the turbine away from the wind at high wind speeds.
The Bergey Excel with the Gridtek inverter has been the subject of a number of independent tests by the American National Renewable Energy Laboratory (NREL) in Colorado. The reports are all available from the NREL website. The tests have involved turbines with different aerofoil sections and different rotor diameters. Only one of the reports (NREL/EL-500-33546) seems to have been on a standard Excel S-60 test - primarily to check the durability of the turbine over a period that altogether took about two years from 1999 through to 2001. Generally, the Bergey Excel seemed to perform well and it is probable that some reliability issues that arose over the performance of the electrical inverter have now been remedied. The mechanical reliability of the turbine itself seemed of a high standard.
The figure below shows a scatter plot of NREL results obtained over 10 second averaging periods of output power against wind speed. It nicely illustrates the problems of extracting a 'steady' power curve from this type of data as discussed in the opening paragraph of this web page. The tail fin furling mechanism comes into play at around 19 metres/second and it is clear that the inertia of the whole turbine system is creating significant time delays between the power output and the wind speed when this mechanism starts to operate. At lower speeds, the scatter is greatly reduced. Also shown by the green dots are the average power over several months obtained from power output readings within a number of wind speed bins. The bin widths are 1 metre/second so, for example, the 10 metre/second results are obtained from power results at wind speeds between 9.5 metres/second and 10.5 metres/second. Up to about 15 metres/second, the agreement between these binned results and the 10 second averaged results is good but, beyond this point, they diverge significantly. At first sight, this is hard to explain but it is almost certainly a consequence of the erratic behaviour of the furling mechanism in an unsteady wind. Even if a power curve had been obtained in the steady flow of a wind tunnel, this dynamical effect would not be taken into account in the WindPower program. Thus, estimating the mean power output at the higher wind speeds when the furling mechanism is coming into play will be a process of uncertain accuracy - see later discussion. The red line in the figure is the 'steady' power curve given in the manufacturer's literature. In common with most manufacturers, it is not stated how this power curve was obtained but it is close to the mean of the NREL 10-second averaged results.
Using the Bergey power curve, the figure on the right shows the turbine efficiency. These are quite low which may be a reflection of the fact that the turbine blades are untapered and without twist. Of course, it is important to stress that low efficiencies in themselves are not particularly important because it maybe that the cost of manufacturing the Bergey blades leads an overall cost for the turbine that is much lower than a more sophisticated design. From a users' point of view, it is the cost per kilowatt-hour that is the more important parameter.
The WindPower program has been used to calculate the mean power curves using on the one hand the manufacturer's power curve (the red line) and, on the other, the NREL binned data (the green dots). In both cases, it has been assumed that the power curve is flat from the last data point to the WindPower program limit of 30 metres/second. The results of these calculations are shown below with default standard deviation for the wind speed of 62%.
The table below shows the annual energy production based on the two mean power curves above.
|Annual energy production in kilowatt-hours (Bergey Excel S-60)|
|Mean wind speed (m/s) =||5||6||7||8||9||10|
|Binned NREL data||7,558||11,615||15,423||18,639||21,167||23,043|
Up to a mean speed of 8 metres/second, there is not much difference between the two sets of predictions because the influence of the differences in the assumed steady power curves at higher speeds does not contribute overmuch to the mean power estimates at the lower speeds. In consequence, the annual energy estimates up to 8 metres/second are almost certainly reasonably accurate. Beyond that, the actual energy output will probably lie somewhere between the two curves.
Proven 6.The Proven 6 is a small turbine manufactured in Scotland. It has a 5.5 metre diameter rotor and a ingenious hinged blade mechanism that allows the blades to deflect in high winds and, in this way, both damage avoidance and power regulation at high speeds are achieved. There is no cut-out speed.
Once again, the Proven 6 has been the subject of independent tests. The first figure shows another scatter plot of power against wind speed obtained from a Proven 6 on top of a residential block in London - the Ashenden report (see references).
The red line shows the mean power obtained from averaging the binned results at wind speed intervals of 0.5 metres/second. The Ashenden site had a mean wind speed of only 3.8 metres/second so that there are not many data points at the higher speeds.
In addition to the Ashenden results, the National Engineering Laboratory (NEL)in East Kilbride also carried out some tests on the Proven 6 and their results are shown on the left below along the Ashenden results and the manufacturer's curve obtained from a Proven brochure. The figure on the right shows the turbine efficiencies calculated from the power curves. It will be noted straightaway that the efficiency figures from the manufacturer's data at the lower speeds are too high to be realistic. For good aerodynamic reasons, small turbines cannot achieve the same efficiency values as their larger counterparts and a good design would be doing well to achieve peak efficiencies in the 30-35% range. The NEL and Ashenden results are in reasonable agreement and give peak efficiencies close to 30%. This is far more plausible - but still at the higher end of small turbine performance. The Proven blades seem to have taper and twist which suggests that they may have been designed with some care.
Using the steady power curves above, the mean power output has been calculated using the WindPower program with the default wind speed standard deviation of 62% of the mean wind speed. The graph below shows the results of these calculations for the Proven power curve data and the NEL test data.
Most manufacturers seem to prefer presenting potential performance figures for their turbines in terms of the energy produced by a turbine over some period of time - usually a year. From the WindPower program results shown above, the energy output in a year for the Proven 6 is easily calculated by multiplying the turbine mean power by the number of hours in a year i.e. 8760. The table below shows the kilowatt-hours for a range of mean wind speeds. The figures based on the NEL test data are almost certainly a better guide to the energy output but, even here, it should be stressed that these figures assume there is no down time for the turbine and also that there are no other losses in the system.
|Annual energy production in kilowatt-hours (Proven 6)|
|Mean wind speed (m/s) =||5||6||7||8||9||10|
The Honeywell Windgate WT6000 wind turbineThis is a new wind turbine in the small wind turbine market. Honeywell is a large and reputable company and so one would expect this turbine to be well manufactured and backed up by reliable performance data. Unfortunately, the initial advertising material (including a launch video that can be seen on YouTube) does not seem to provide this. This is a shame because there are aspects of this machine which seem attractive but uncertainties about its power data casts doubt on its likely performance.
The turbine is unusual in that it is a horizontal axis turbine of 1.7 metres diameter with apparently twenty blades. Counter to intuition, it should be noted that having a large number of blades does not confer any significant improvement in aerodynamic efficiency compared with a three-bladed or even a two-bladed turbine. The generator is housed in an annulus around the blades. It is claimed that this results in a low frictional resistance to rotation so that the cut-in speed is close to 1 metre/second. It is argued that this is a good thing in that the turbine will start to generate power at much lower wind speeds than a more conventional wind turbine. However, this is actually not very significant in itself because the power generated at these low wind speeds is miniscular - literally just a few watts. A more significant point is that the turbine seems able to operate over wide speed range up to to about 20 metres/second without any mechanical feathering of the blades. What happens above this speed is not altogether clear. The figure below shows an image of the turbine with what is apparently a steady speed power curve obtained from an advertising pamphlet that is available from www.earthtronics.com/pdf/Energy-Generation-Information-4.pdf.
The noteworthy feature of the power curve is that it shows an ever increasing power output with speed and apparently the turbine will produce a power of 2 kilowatts or more at higher speeds approaching 20 metres/second. However, the figure below shows the turbine efficiency based on this power curve - given in a tabulated form as well as a graph. Not only is the Betz limit exceeded but the efficiency rises above 100% at speeds below about 3.5 metres/second! Clearly, this casts doubt on the whole performance of the turbine. It is a puzzle to know how such data was produced. In the advertising material, Honeywell mention that they have a wind tunnel available in which they tested the turbine. As mentioned at the beginning of this webpage, it would need to be a very large tunnel in order to avoid problems over blockage effects. To take an extreme example, if a turbine were tested in a tube of the same diameter as the turbine, the results would be completely erroneous and much higher powers could be extracted from a turbine in these circumstances than would occur in a unconfined stream.
The price of this turbine is quoted at around $4,500. This seems a very competitive price but until power data is available in which confidence can be placed, it is difficult to make a judgement about this turbine. It has been included on this webpage as an example of how careful a potential buyer of a small turbine must be when faced with a manufacturer's claims about their wind turbine performance.
J. T.G. Pierik, R.W. Dunlop, W.K. Lee, J. Gabriel (2001). Performance evaluation methods for autonomous, applications orientated wind turbine systems. Energy Research Centre of the Netherlands Report ECN-RX--01-062. Presented at European Wind Energy Conference and Exhibition, Copenhagen, Denmark, 2-6 July, 2001.
J van Dam,M Meadors (2003). Duration test report for the Bergey Excel-S60.US National Renewables Energy Laboratory report NREL/EL-500-33546.
Southwark Council (2008). Ashenden Wind Turbine Trial; Phase 1 results. Southwark Council, London South Bank University, Brian Dunlop Associates report.
Next is a discussion on obtaining estimates of mean wind speed.