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How can you compare the data from these Renewable Energy inputs?


The data on these pages is extremely difficult to compare.  You need to understand what you are looking at in order to make any valid comparisons.  Here is a brief attempt at that explaining how you can compare them.  The first difficult thing to do is to level the resource.  There is much, much more energy in stronger winds that lighter winds.  On our 84 foot towers, the winds are about 3 mph stronger than the 42 foot towers.  That is about 50% more energy available to the bigger turbines than the little ones.  So, you can't make a direct comparison unless the winds were identical.  That isn't going to happen in real life.  So what can you do about that?  Not a lot.  

The first myth most people hear about in small wind turbines is the peak output rating of a turbine.  This is basically meaningless.  To demonstrate this, the Whisper H40 has a peak output of about 900 watts, and the Whisper 80 has a peak output of about 1000 watts.  However, in real life, the Whisper 80 produces about twice the energy as the H40.  This is because of the swept area of the turbine.  The Whisper 80 sweeps about 80 square feet, or double the 40 square feet that the Whisper H40 sweeps.  Peak output happens at high winds that don't happen frequently for most people.  Since it doesn't happen very often, it doesn't really matter much in the grand scheme of things.

The best thing to do when comparing these resources is to compare them to themselves.  How does, say, the Whisper H40 perform on a daily, day-in, day-out basis?  How do the strong wind days compare to the low wind days?  For a given month, how well is the energy spread out?  In a renewable energy application, especially for people that are off-grid, you need the energy every day.  You are probably going to dump a fair amount of the energy you get on a windy day, just because your battery cannot hold it all, anyways.  So, you want a turbine that performs better on low wind days with respect to the high wind days.  If you only get energy on high wind days, the turbine won't be very useful.  What is the median, and what is the average output for a month?  That will help demonstrate how reliable the windmill will be at giving you energy on a regular basis.  Since the power in the wind is proportional to velocity cubed, the median output will always be lower than the average.  But it is the median output that is going to keep you from running your generator, NOT the average output.

I took two months of actual data from the Whisper H40 and calculated the AVERAGE daily output.  I then did some theoretical math using that rated daily output.  I assumed that the daily load was equal to the average daily output of the turbine.  I added the input from the windmill every day and subtracted the average load.  It turns out that if the load was 60% of the average daily production, it would meet the daily energy needs very well.  But the wind resource is not constant enough that you can expect the average energy per day based on the average energy per month data.

That said, you probably still want to compare the machines.  The best thing to do in this regard is to compare each machine to itself, then take those comparisons and compare how each machine compares to itself to the way the other turbines compare to themselves.  Since the machines are on different height towers, the resources available to each one are different.

Copyright 2004 Michael A. Klemen                                                Site Table of Contents