What is an example of high quality energy and low quality energy, and how is either energy efficient?

Question

What is an example of high quality energy and low quality energy, and how is either energy efficient?

Answer 1

Thermally and thermodynamically speaking, high quality energy is either highly concentrated form of energy (any kind of fuel or nuclear) or high temperature (concentrated solar). Low temperature (exhaust fumes or moderately hot water from cooling systems) or highly diffused energy (landfill gas, if not concentrated) will be low quality.

Using energy to do work always involves transferring heat from a high temperature source to a low temperature destination and extracting a bit of it as work in the process. So you want the source to be able to create high temperatures to achieve maximum energy efficiency (low temperature destinations are often considered to be ambient air or water).

In case you didn't already know, carnot efficiency (maximium theoretically efficiency from a thermodynamic engine) is given by an equation (T'high - T'low)/T'high.

There are some applications however where Carnot efficiency isn't applicable (water heating for example) and there what you would consider low quality might be acceptable quality, if I need to confuse a bit more about the "qualifying" something as high or low quality.