Energy Bites: Solar Panel Efficiency
The other day I heard somebody say that solar capacity factors are going up. She misspoke, and I chose not to correct her.
(I wish I could show the same restraint with my wife. Happy wife, happy life, and all that.)
What my friend meant was that solar efficiencies are going up.
A solar installation’s capacity factor is the actual output of electricity divided by the maximum potential output.
While capacity factor is often generalized (e.g., fixed solar is 12% and tracking solar is 18%, etc.), it is really a characteristic of a particular installation. For example, if I put my solar panels in the basement, the capacity factor will be zero. If I put my panels on a south-facing hill in Vermont, I might reach 14%.
Efficiency relates to potential: it measures a solar panel’s ability to convert sunlight into electricity. Efficiency is an attribute of the panel and not its installation. No matter where you put the panel, it has the same efficiency. Even if it’s in the basement.
A square meter of direct sunlight delivers about a kilowatt worth of energy. I installed an array of solar panels in 2008. They were about 12% efficient. A few years later, I added panels. The new panels had an efficiency of about 16%. Panel efficiency is improving. Today, you can buy panels at the hardware store that have efficiencies in the low 20s. And the cost per watt has dropped.
Dr. William Shockley (left) & Carl Betz as Dr. Alex Stone and his television family (right)
The maximum theoretical efficiency of a solar cell is 30%, and that’s called the Shockley-Queisser limit.* The efficiency of a panel can be boosted by stacking cells atop one another. A stack of infinite height would have a limit of about 68%, but you couldn’t fit such an arrangement in your pickup truck.
There is a similar limit to the efficiency of wind turbines. Betz’s Law** states that a wind turbine cannot extract more than 59% of wind’s kinetic energy. Turning the extracted energy into electricity introduces other inefficiencies. Currently, wind turbines operate at a maximum efficiency of about 40%.
Hydro-electric turbines? There’s no theoretical maximum efficiency.
By the way, that 1kW per square meter of sunlight? That’s on Earth’s surface. If you could move your panels into orbit, the sunlight–unaffected by the atmosphere–would contain more energy. The efficiency of the panels would remain the same. And if you could avoid Earth’s shadow, you could achieve a capacity factor of 100%.
*If you are of a certain age, the name Shockley might be familiar to you. William Shockley won the 1956 Nobel Prize in physics for his work in semiconductors. He promoted extreme views on eugenics and the relationship between race and intelligence. He sued a journalist for comparing his ideas to those of Nazis. He won the suit and was awarded damages of one dollar.
**A lot of people think that Betz’s Law was developed by Carl Betz, who played Dr. Alex Stone–Donna Reed’s pediatrician husband on the Donna Reed Show. This is just not true. Betz’s Law was derived by the German physicist Albert Betz. They never even met one another. Honest.
Carl’s television daughter, Mary, was played by Shelley Fabares, who sang “Johnny Angel,” a number-one hit in 1962. She went on to star in movies with both Elvis Presley and Peter Noone (of Herman’s Hermits). Neither Shelley, Peter, nor Elvis ever met Albert Betz. Honest.
Mark Whitworth, President of Energize Vermont