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Tom Murphy

Tom Murphy

Tom is an associate professor of physics at the University of California, San Diego. This post originally appeared on Tom's blog Do the Math.

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Energy from Waste: A Waste of Time or a Genuine Energy Solution?

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.

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.”


“…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.

If you’ll forgive the personal question, how much of your 100 W metabolic intake (chemical energy) would you guess is undigested and remains available in your poop? Are starving people tempted to avail themselves of the energy within? Obviously there is some energy left, or flies would not be attracted to lay eggs in the stuff. And bacteria can make do breaking down the remaining material. But no large animal (reptile, bird, mammal), to my limited knowledge has evolved to fill a niche in the ingenious animal kingdom out of harvesting the waste streams of other animals (cow pies are not treats to any animal I might have as a pet, despite the suggestive name).

I’m going to pick 10% as a waste energy fraction I would imagine to be at the high end of the spectrum—meaning 10 W of continuous power might be extracted from an individual’s human waste. Before you quibble that the number isn’t right, let’s see that it won’t matter. At 10 W per person, we could potentially replace one four-hundredth of the 4,000 W personal oil stream. That’s 0.25%, and I think this is on the high side.

Now re-read the quotes above and see if they get your ire up too. Okay, if you want to capture energy from a waste stream at a point of concentration (sewage plant), I’m all for it. But can we please refrain from hyperbole that gives voting citizens the impression that we’ve got a lock on the energy problem?

Happy Meals for Wheels

Biodiesel can be made from cooking oil, even after the cooking oil has been used. Some clever folks have taken to using waste streams of cooking oil (e.g., from fast-food restaurants) as the feedstock for biodiesel. I have no doubt that if I were rambling down the road powered by waste cooking oil, I would have a smile on my face—and not just because I love the smell of french fries: I would relish being free from oil. And with little effort, a search engine can point you to many claims of generating biodiesel from cooking oil as a path to energy independence. As with the others, we ask: can this represent a significant fraction of our reliance on oil? Can many of us expect to do this?

First question. Have you ever seen a tanker truck loaded with gasoline? Lots of times, right?. You’ve seen them on the freeway, unloading in gas station parking lots, and spectacularly blowing up in any number of action films. Now, how about cooking oil trucks delivering fresh supplies to restaurants, or picking up waste oil? I’ve seen them, but nowhere close to the frequency I’ve seen fuel trucks.

Another way to frame this is that you seldom pass a gas station when someone is not fueling up. During a day, hundreds of tanks are filled; thousands of gallons (tens of thousands of liters). But you could park behind a restaurant for a week without seeing a cooking oil transfer. And when that does happen, it may be a single 55 gallon drum. I used to work at a fast-food chicken place, and I can vouch for the fact that the oil is not changed as often as you might wish it to be. True, restaurants outnumber gas stations, but you surely get the sense that the flow of cooking oil is miniscule compared to the flow we demand in our vehicles.

Let’s put some numbers on it before leaving this one alone. The U.S. uses about 20 million barrels of oil per day, or about 3 gallons per person per day (300 million people; 42 gallons per barrel). Even if there were one fryer-equipped restaurant per 1,000 people in the U.S., and each had 4 deep fryers at 5 gallons each, and even if the oil is replaced once per day (we should be so lucky), we’re dealing with 0.02 gallons per person per day—more than 100 times less than the petroleum flow. We could replace less than 1% of our oil consumption with recycled cooking oil, even though I intentionally stacked the numbers in favor of cooking oil (by making larger estimates than I think are realistic).

PetroPlastic to the Rescue

Recently, a Japanese company began selling a home-use machine to turn waste plastic into fuel. Sounds fantastic: eliminate recycling and trips to the gas station! The device got significant attention, as described in an article on the United Nations University site. Here, we find the inventor’s quote:

“The home is the oil field of the future.”

The device takes one kilogram of plastic, and produces about 70–80% of that mass in oil (about one liter) over the course of three hours. It consumes somewhere between 1–3 kWh of electrical energy (specifications unclear, claiming 1 kW/hr—which is a nonsense unit, either meaning that it takes 1 kWh or runs at 1 kW for its three hour run-time). The result is something like 9 kWh of oil energy per batch. So it seems to be energy-positive, unless 3 kWh of delivered electrical energy comes from 10 kWh of fossil fuel (thermal) energy at the power plant. In any case, the oil would need further refining to work in our vehicles.

But how much oil demand could we displace with such a device? For starters, 7% of oil extraction goes to making plastics, so this is an upper bound if all plastic were recycled (and if all types were appropriate to the machine, which is not true). At best, turning plastic into oil cannot be a substitute for oil extraction (since oil is the feedstock for plastic), but is rather a small adjustment to the efficiency with which we turn extracted petroleum into combustible fuel. In any case, the claim I want to examine is at the personal scale: that the home becomes your own oil field.

My wife and I are not prolific consumers, but we do buy items with an eye toward being able to recycle the packaging. As a result, our recycle bin receives much more volume than our trash bin. A typical plastic container for 2 liters of fluid has a mass of 75 g. Even ten such containers (or equivalent plastic) per week is less than one kilogram of material—and this is far more than we generate in our household. But if you did generate a kilogram of plastic waste in your household each week, we’re dealing with one liter of oil per week. Compare this to the U.S. average of about 12 liters of oil per day per person—therefore approximately 30 liters per day per household, or 200 liters per week. The resulting 0.5% puts us back into the familiar territory of a household contribution less than 1%.


Is the Theme Clear?

So we have seen a number of clever ideas with proof of concept (working models) that prove to be inconsequential at a relevant scale. There are a lot more cute, but tiny ideas where this came from. Perhaps we could implement 100 different 1% energy solutions and call it good. But this is unlike any economic model we have seen in the past, where a select few energy sources dominate the infrastructure.

The Danger of Hype

Ever since I became aware of our looming energy predicament, I have shared my concerns with friends and acquaintances. I routinely hear references like: “The other day I heard about a way that we could use [some hitherto unappreciated energy 'source'] just by [exercising some cute trick].” We are bombarded with cute “solutions” to energy issues. We have enough awareness to know that we want solutions, but not enough honesty in presentation or comparative analysis to judge whether an idea has real potential.

Demonstration, or proof of concept, is often taken as enough evidence to satisfy our skeptical nature. And even if half of the things we hear about are over-hyped, we hear enough of them to placate our worries. The result is that we do not have an all-hands-on-deck effort to plot our energy future. Reliance on market forces, human ingenuity, and a track record of successful substitution short-circuits our ability to get serious.

I am one of many highly-educated researchers with a full toolbox of instrumentation and analysis skills, representing a substantial investment on the part of our taxpayers (much of my education and research has been funded by federal scholarships, fellowships, and grants). One reason people like me are supported by our society is because we have collectively learned the value of keeping a talent pool at hand to solve our big problems. But there has been no trumpet call for this mother of all problems. No Manhattan Project. No Apollo. Fractionally, very little research funding is directed toward a meaningful transition away from fossil fuels. I visit physics departments around the country and very seldom find anyone working substantively on the energy problem (including me personally, as motivated as I am). The public assumes that scientists will solve the problem in time. But in my circles, I hear crickets chirping.

I largely place blame on the fact that the public is not sufficiently educated about the challenges ahead to demand action from their political representatives. And when we are constantly exposed to cute “solutions,” why should we be surprised that folks aren’t fired up—even if the solutions are a load of [what word comes to mind]?

By. Tom Murphy

This is a guest post by Tom Murphy. Tom is an associate professor of physics at the University of California, San Diego. This post originally appeared on Tom's blog Do the Math.

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