Chapter 15 - Template for Sustainability

Hair dryers? In Chapter 24 ‘Exergy Takes Us beyond the Lamppost,’ we’ll use these dryers to help illustrate the difference between energy efficiencies (often meaningless) and exergy efficiencies (always meaningful). My engineering students had no difficulty calculating energy and exergy efficiencies for many technologies – especially for hair dryers, because an example was worked out in their textbook. But when I asked them to calculate how efficiently a hair dryer delivers its service, it caused problems. Is the service dry hair? Not really: If we stand around long enough our hair dries on its own. The service is time saved. It took a while to get this answer. I pushed the issue to remind the engineering students that it isn’t good enough to be able to calculate correctly; it’s even more important to know what you should calculate, and why. Defining the right question is often more important and usually more difficult than calculating the right answer after specifying the right question.

 

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