Theoretical Maximum Efficiency

The term theoretical maximum efficiency is tossed around while discussing fuel cells, so what is important to understand is what this term means and how it can be applied to evaluate the practicality of a particular fuel system.

Theoretical maximum efficiency is used primarily because it makes it easier to compare different types of power generation. It is an apples to apples comparison, but does not consider steps in power generation such as production, transportation, and storage. Theoretical maximum efficiency is the best way of comparing the inherent abilities of any particular system. It also provides a standardized method of comparing unrelated systems such as the theoretical maximum efficiency of boat propellers or solar panels. While seldom achieved in real world applications, theoretical maximum efficiency does correlate to the real world efficiency. In other words, the greater the theoretical efficiency, the greater the real world efficiency is likely to be.

While theoretical maximum efficiency relies on a fairly complicated calculation, it can be thought of as a comparison between the amount of energy put into a system and the amount of energy derived from the system. Perfect efficiency would indicate that 100% of the energy put into the system is utilized to do useful work. The deficiencies in most systems imply that energy is lost as a result of friction and heat. For instance, in an internal combustion engine, the gasoline placed into the tank contains a certain amount of energy. Of all of that energy, only 25% is actually used to move the vehicle for work. The other 75% is lost to the environment, usually as heat.

According to the U.S. Department of Energy, the maximum efficiency that an internal combustion engine can achieve is approximately 58%. In other words, an internal combustion engine can, at its maximum, only utilized 58% of the energy stored in gasoline. In comparison, fuel cells of the theoretical maximum efficiency of 85 to 90%.

Theoretical maximum efficiency and actual real life values often differ. For instance, the internal combustion engine has a maximum efficiency of 58% would generally only achieve some efficiency of 25%. Similarly, fuel cells have maximum efficiency ratings of up to 90%, but only achieve efficiencies of 40 to 60% in real world applications.