Lettuce and the Laws of Thermodynamics

by Bill Duesing

First broadcast on WSHU/WSUF-FM, October 31, 1997

The laws of thermodynamics govern the flow of energy in the universe, and in our everyday lives. These laws aren't like campaign-finance laws or speed limits; the laws of thermodynamics can't be broken. Government and business leaders who talk about energy use and the global climate treaty seem especially ignorant of the second law of thermodynamics, and of the guidance it provides in choosing ways to satisfy our needs.

The first law states that energy is neither created nor destroyed; it can only be transformed - changed, for example, from fuel into motion. Efficiency according to the first law is computed by comparing the amount of energy that goes into a system with the amount of work that results. A car which gets 40 miles per gallon of gas is more efficient at converting gasoline's energy into distance traveled than a car that gets 15 miles per gallon.

The second law states essentially, that energy transformation goes in one direction only. A car's waste heat and motion can't be converted back to gasoline's original, concentrated, high-quality energy. And, this one-way energy conversion creates disorder or entropy which tends to accumulate. If more energy is transformed, more entropy is created. Air pollution and climate change are just two of the forms that the entropy (from the conversion of gasoline to motion) takes.

To measure efficiency according to the second law of thermodynamics, the energy that we do use is compared to the minimum energy required to accomplish the same work. In effect, the question is, "How can we produce less disorder and still satisfy our needs?"

A head of iceberg lettuce provides a good lesson. With the help of the educators in a workshop I presented at the "Connecticut Energy Council for Teachers Conference," I compared the energy conversions required and the resulting entropy for several ways of satisfying a single need. In this case, the need for crunchy green plant material for salads and sandwiches.

But first, a little background information. Lettuce is over 95% water. This head of iceberg lettuce, like more than 90% of the lettuce eaten in this country, was grown in the far west, on the California coastal plain or in the California or Arizona desert. I bought this lettuce at a neighborhood market in Derby. It was the first lettuce I've bought in many years. Before I could get the money out my pocket, the owner put the plastic wrapped head into another plastic bag.

The teachers and I brainstormed the numerous ways that energy was transformed to produce and deliver that lettuce. The resulting list, just for that one head of lettuce, filled two full blackboards.

Of course, each time energy is transformed, entropy is created. Truck exhaust, hazardous wastes from pesticide and plastics factories, the plastic wrapper and empty bag as well as the ecological changes caused by large-scale irrigation are just a few examples of the entropy created by this one head of lettuce.

Each of these transformations can be made more efficient according to the first law of thermodynamics. Making pesticides could require fewer kilowatt hours of electricity, trucks may get more miles per gallon, store coolers may use less energy, and on and on through all the steps.

However, it is second-law efficiency that we need to understand. For that, we compare the energy used in all of those steps to the minimum energy required to provide us with crunchy greens. One teacher quickly suggested that growing lettuce in a home garden which is hand-tended and fertilized with compost uses very little energy. This is possible here for six to eight months of the year. I suggested sprouting seeds on the kitchen counter for the winter months. Sprouts provide crunchy greens with additional nutritional benefits.

Compared to garden lettuce or sprouts, the second-law efficiency of California iceberg is nearly zero. Stated another way, despite first-law efficiency improvements, the California lettuce is almost absolutely inefficient according to the important second law.

If we really want to slow down climate change and still satisfy our needs, the second law of thermodynamics provides clear guidance. While globalization increases the energy transformations and entropy produced in satisfying our needs, the second law points to the importance of simple, local solutions.

This is Bill Duesing, Living on the Earth

This page and its contents are copyright © 1997 by WSHU-FM, Fairfield, CT, and by Bill Duesing.