Welcome to “Tuesdays with Scott” segment of the Climate Change program.
When it comes to energy, people talk about efficiency. But what does efficiency mean? For instance, some people may argue that coal has an efficiency of 70 percent, meaning that 70 percent of the coal burned can be converted into electricity. It sounds pretty compelling until you realize how is coal formed in the first place. Coal is a non-renewable energy source that takes millions of years to form. The energy in coal comes from solar energy that was stored in giant plants that lived hundreds of millions of years ago in swamp forests, even before dinosaurs existed. When these giant plants and ferns died, they formed layers at the bottom of the swamps. Water and dirt began to pile up on top of the dead plants. And over thousands of years, pressure and heat would build up on top of the plant remains, undergoing chemical and physical changes and pushing out the oxygen, turning these remains into what we call coal. So simply put, coal was made by plants using solar energy but it took millions of years to become coal. If you compare the energy obtained in the end with the energy coming from the sun, the percentage might be lower than 1 percent in efficiency for coal. Whereas solar panels derive energy directly from the sun without taking millions of years. So when it comes to effiency, we need to consider the original source.
Different technologies are required to harvest different sources of energy. For solar, the sun’s energy comes in several forms, such as visible and invisible light as well as heat. Solar thermal harvests heat where as photovoltaics harvest visible light. So, we need multiple technologies to harvest the radiation from the sun. But, we can’t harvest it all, because of the laws of physics.
One of those laws is called the thermodynamic efficiency limit. It represents the unbreakable upper limit in converting energy from the sun. Thermodynamic efficiency limit is the absolute maximum theoretically possible conversion efficiency of sunlight to electricity. Its value is about 86% (which is the Chambadal-Novikov efficiency, an approximation related to the Carnot limit).
This comes from the fact that every time you transform energy from one source such as photons to electricity, you lose some energy in the process.
Next week, we’ll get into effect of band gap energy and solar spectrum to better understand it in the context of thermodynamic efficiency limit.
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