Efficiency of Solar PV and Wind Power

A young lady from a faraway land writes: My name is Alexandra, and I am a pupil at Herschel Girls School in Cape Town. I am doing a Science investigation and I am researching wind and solar energy, and which one is more efficient.

What an excellent question. And how lucky you are to go to such a wonderful school (pictured here) in such a beautiful part of the world (see below).

To get to the point:  You probably mean: How much electricity does the device produce as compared to the total energy incident upon it? In that case, the prize goes to wind.  Today’s large wind turbines convert about 45% of the total kinetic energy in the wind itself into electricity.  The theoretical limit, btw, is only 59%, so 45% is really pretty good.  Here’s a bit more on that: http://2greenenergy.com/2015/01/27/clean-energy-efficiency-wind-turbines/.  This figure, 45%, is far higher than solar, which is about half that, in the mid-20s.

Then there is a related term called “capacity factor” (CF) which means: In the course of an average day, what is the energy production as compared to what the device can do when it’s running at maximum performance?  Obviously, PV doesn’t work when the sun isn’t shining, and wind doesn’t work when the wind stops blowing.  Here too wind has an advantage; if a wind turbine is sited in excellent conditions, its CF can approach 50% and will average 30% – 40%.  For PV, this averages about 25%, and can be as low as 10% in cloudy places like Germany.

There are other complicating factors as well.  For example, PV happens to correlate well with our demand for energy, so storing extra energy for later use isn’t a big deal.  That’s not the case with wind, where, in many places with the best conditions, wind tends to blow hardest in the middle of the night when the demand is least.  Thus, we have lots of people figuring out how to store inexpensively all this “off-peak” wind energy.

Here’s another issue.  Where in a science class we’re concerned about efficiency as described above, outside in the real world we’re more concerned about cost-effectiveness.  That is, what is the average cost of a certain amount of electrical energy when averaged across the lifetime of the equipment?  The good news is that, for both wind and PV, the cost of the fuel is zero.  But that means that someone has to find all the cash at the beginning when the devices are made, bought, and installed.  Essentially the opposite is true of fossil fuel plants, when the majority of the cost is, for instance, in mining and transporting coal.  But then, they need to deal with the fact that generating their energy has released a tremendous amount of poison into our skies and oceans.  We act as though this isn’t true, but someone eventually is going to have to pay to clean that up.

OK, there I’ve presented a few thoughts.  I hope you do a good, thorough job in your investigation, and that you get a big fat “A” in the class.

In any case, keep in mind that you are enjoying a very privileged life.  Accordingly, I know you will do what you can to make the world a better place.