The mass and energy balance provides the quantitative foundation for a Life Cycle Analysis (LCA)—often called a cradle-to-grave assessment—which evaluates the total environmental impact of a process or project from initial construction through operation to final decommissioning. I’ll cover the details of conducting an LCA later in this book, but first I want to share a story from Bill Banholzer, based on a talk he gave at Princeton in 2022 titled “Perspectives on Separating Possible vs. Practical Sustainable Solutions for Energy and Materials.”

In his talk, Bill reminds us that as engineers, we all want to build a world powered by sustainable energy—but good intentions aren’t enough. Achieving meaningful progress requires hard numbers, rigorous thermodynamics, and a willingness to question overly optimistic ideas. Too often, enthusiasm for “green” solutions overlooks real constraints like energy density, conversion efficiency, and process economics. By applying mass and energy balances to examples such as carbon capture, biofuels, and green hydrogen, we can cut through the hype and focus on solutions that actually work.

One case Bill discussed involved a carbon-capture proposal that used sodium hydroxide (NaOH) to absorb CO₂ (see video 23:30–30:35). When he first reviewed the project, his immediate question was, “Did you ever close the energy balance on this?” The company had not—and had overlooked the enormous electrical costs required to produce NaOH in the first place. It’s a classic example of assuming certain molecules, like NaOH or hydrogen, exist freely in nature, without accounting for the energy needed to create them. Bill’s story underscores a critical lesson: without a complete and accurate mass and energy balance, even the most promising sustainable technologies can fail the test of practicality.