Turbine Effectiveness

Anything that can spin in the wind, could and probably has been called a turbine.   Turning does indicate some level of effectiveness, but it certainly doesn’t predict market viability.

Some things that differentiate turbines, and help show which may be viable are,

  1. How much of the kinetic energy in the fluid passing through the turbine is converted to useful shaft power?   This is commonly called Efficiency, and expressed as 50% for a turbine capturing one half of the kinetic energy in the wind passing through it.   Engineers often use the term Co-efficient of performance, expressed as  1. for a turbine capturing all the energy passing through it, and Cp 0.5 for a turbine capturing 50% of the energy passing through it.
  2. How much does it cost to build, install and operate a turbine?   With the related question of how long does that turbine last.
  3. What types of sites are accessible to the turbine?  Not all sites are accessible to all types of turbines.    Urban and mountainous sites are generally too turbulent for conventional HAWT systems, which always require tall towers due to their intolerance of turbulence.
  4. Are there other costs or benefits from the turbine.   Some types of turbines have been shown to have substantive effects on local ecology or noise issues that affect site consideration.

 

Background for Comparative Analysis,

To do a comparative analysis of the ART Turbine, it’s important to have the context of both HAWT (Horizontal Axis Wind Turbine) systems, and the alternative VAWTs (Vertical Axis Wind Turbine), both the class of thin bladed “Darius” type of turbine and the thicker bladed Savonius types.

In making comparisons one non intuitive consideration is that the size of a turbine can have a dramatic effect on it’s relative effectiveness.   A small turbine, per/area is not able to capture the same per/area proportion of energy, that a larger turbine of the same design has.    It would therefor be an apples to oranges comparison if a turbine of 2m swept area was compared to a 20m, or 200m turbine.

The ART Turbine, has had about 20 iterations tested in various apparatus, some in water, some in wind.   The tested turbines ranged from about .3m diameter in water, to 2m diameter as a wind turbine.   The testing was mostly done by the inventor, however in some cases 3rd party engineers were used, both in test system design, and data collection.  The most extensively tested ART turbine was 1m diameter and 2.24m tall, with a swept area of .71m.    This testing was done with, and overseen by engineer Jon Scott, an R&D engineer with extensive experience.    Jon Scott Re: ART Turbine  This is the ART Turbine data I will use in my comparison.   For other turbines, where available, I will use the Small Wind Certification Council data.   The ART Turbine test equipment and data were reviewed by engineering masters student Matthew Hall. Support Letter Matt Hall

A further consideration in trying to evaluate turbine performance is that wind tunnel data from previous to 2010, for some turbine types, may not relate to real world performance in the way researchers hoped.   In 2010 turbine researcher Ian Ross published a paper, investigating why the VAWT turbines his company was working on didn’t function as well in the real world, as their wind tunnel based experiments predicted.

Ian Ross Wind Tunnel Blockage Correction Factors

What Ian found was that the size ratio between the size (swept area) of a turbine, and the test area of the turbine (tunnel cross section), made a much more significant difference when testing turbines of high solidity (high proportion of blade area to swept area) than was previously understood.  This increased blockage effect caused turbine performance estimations of 2-6 times higher than expected.    For this reason in comparing turbines, apples to apples, it’s best to use real world performance data, from 3rd party agencies, with turbines of similar sizes.  Even now the research of Ian’s is not widely known and wind tunnel test results therefor can vary widely.

Answering the Questions

Question 1. Efficiency, how much energy is the turbine capturing from the fluid passing around it.

HAWT’s – An industry leader in smaller turbines has a 7m (23′) diameter turbine (38.5m^2 swept area), the Bergy 10kW.   The Small Wind Certification Counsel has tested this turbine and found that it’s peak performance is 30%, or Cp 0.30 .   This is about the smallest effective size for a HAWT.   Commercial HAWT’s smaller than this test substantially lower.

VAWT’s – This is more complex to generalize, as there is no dominant design paradigm, with common performance.   In general there are 2 types, the Savonius and Darius.   Here is the Savonius wikipedia link ,and the Darius wikipedia link.

The Savonius was extensively wind tunnel studied in the 1970’s in the Blackwell report.  This report found that in the best configuration it’s wind tunnel performance was 21% (Cp 0.21).   This figure is commonly still used, however even at the time, real world results were showing a different story. 1964 Savonius real world test results Cp 0.14.    or this MIT student project that needed 20% to work but achieved around 1/4 of that (5%).  If anyone has better data on this, I’d love to see it.  No one who sells a Savonius type turbine has submitted their product for certification, and the few who sell them make claims of around 20%, so 14% -20% is likely reasonable.   Wind tunnel studies have shown that there is a small potential performance advantage to a twisted, or helical Savonius vs a standard model (1-2% increase for helical).

The Darius, has been tested from the 70’s through the 90’s with some test turbines still in existence (Aolius link).   The USA federal government research lab which did a lot of work published in 2012 a meta-analysis of their Darius VAWT work. “A Retrospective of VAWT Technology” found that at the time the best Darius design was the H rotor, with a maximum efficiency of around 40%, the same as the best commercial HAWT’s at the time (this is in a very large turbine, with 160ft long, less than 2ft deep “eggbeater” blades) .   They found that they turbine blades needed to have very long aspect ratio’s to be efficient, and those long thin foils were subject to fatigue failure.   The turbine blades were subject to resonance at a variety of speeds and had to be controlled carefully.   Even with careful control the turbines lifetimes were too short for a commercial product to develop.   Smaller versions of H-rotor Darius’s test at much lower efficiencies ,  11-19%  for the “Windspire”.

The twisted or helical Darius, patented by Dr Gorlov, in 1992, claimed 35% efficiency from a small rotor tested in a water race (similar to a wind tunnel, with the flow constrained around rotor).  In real world testing of the most modern helical variation (produced in China for Urban Green Energy) by the SWCC a peak efficiency of 11% was found, though some evidence was found for a slightly higher efficiency at very high wind speeds.

The ART Turbine has not been

 

 

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