Garbage In, Garbage Out: Talking Waste To Energy

Editor’s Note: EarthTechling, always looking to bring you interesting cleantech reading, is proud to repost this article via partner Do The Math. Author credit goes to Tom Murphy, an associate physics professor at University of California, San Diego.

How many times have you heard it: if we could tap into the energy embedded in our copious waste streams, we could usher in a new era of energy independence—freeing ourselves of the need to support oppressive regimes who happen to sit atop the bulk of the oil reserves in the world. In fact, these sorts of claims are abundant enough to give the impression that we have a cornucopia of fresh (and sometimes not so fresh) energy solutions to pursue if we got really serious. This is a hasty and dangerous conclusion, so in this case, waste makes haste.

I consider this perceived abundance of technological solutions to be one of our worst enemies in developing sensible solutions to the coming fossil fuel energy crunch. If ideas abound, each claiming some ability to free us of foreign oil, then surely we’ve got the situation under control and don’t need to invest substantial time and energy today to solve what looks like a non-problem of tomorrow. But what if the claims are overblown, hyped, or just plain wrong? At best, this is irresponsible behavior. At worst, the resulting sense of complacency could delay substantive action to our ruin.

image via Shutterstock

Example waste streams include human waste (sewage), household trash, agricultural byproducts, and commercial/manufacturing waste. I have even heard ambitions of capturing acoustic energy in noisy environments to generate useful electricity. I almost drove off the road when I heard this one, considering that a loud 100 dB corresponds to 0.01 W/m² of power density. At great expense of installation (hundreds of square meters of collection), we could maybe power a nightlight or two!

The problem is that ideas like this sound good when the public lacks the background to evaluate the quantitative potential, and when the press release is too lazy to put what sparse numbers are provided into context. Addressing this quantitative/analytical shortcoming is the whole point of Do the Math.

In this post, we’ll look at human waste, waste cooking oil, and household garbage as examples. We bypass for now what is perhaps the biggest potential waste stream listed above: agricultural byproduct. The point of this post is not to say that waste streams are, well, a waste of time. Valuable energy can be recouped by such methods—sometimes economically so. The point, rather, is to put numbers on ideas that are sometimes portrayed as big solutions, when they are not.

I Wouldn’t Eat That if I Were You

A recent story describes a new venture to use sewer gas for fueling hydrogen cars in the Los Angeles area. Two quotes from the article got my goat:

“This is a paradigm shift. We’ll be truly fuel-independent and no longer held hostage by other countries. This is the epitome of sustainability, where we’re taking an endless stream of human waste and transforming it to transportation fuel and electricity. This is the first time this has ever been done.”

and

“…a third of all cars on the road in the U.S. could eventually be powered by ‘biogas,’ made from human waste, plant products and other renewable elements.”

I’m so flummoxed I don’t know where to start. Deep breaths. Start small. When we import 60% of our oil, even reaching the goal of powering one-third of cars on the road with biogas can’t claim to make us “truly fuel-independent.” I could maybe be convinced that all-out (labor intensive) efforts to derive biogas from plant products could replace substantial fractions of our oil. But human waste (which gets top billing in the quote)?! Let’s do some numbers.

We saw in the post on personal energy cubes that the average American is responsible for 10 kW of continuous (thermal) power production. Petroleum represents about 40% of that, so 4,000 W from oil. Meanwhile, typical diets are in the neighborhood of 2000 kcal/day, which converts to 100 W.

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