I recently came across a statement to the effect that once we transition away from fossil fuels to renewable energy like solar, wind, and hydro, we would essentially be home free for the long run—tapping into inexhaustible flows. It is a very pleasant notion, to be sure, and one that I believe is relatively common among enthusiasts for renewable energy.
Naturally, I am concerned by the question of: what magnificent things would we do with everlasting copious energy? As an excellent guide, we can ask what amazing things have we done with the recent bolus of energy from fossil fuels? Well, in the course of pursuing material affluence, we have eliminated 85% of primeval forest, made new deserts, created numerous oceanic dead zones, drained swamps, lost whole ecosystems, almost squashed the remaining wild land mammals, and initiated a sixth mass extinction with extinction rates perhaps thousands of times higher than their background levels—all without the help of CO2 and climate change (which indeed adds to the list of ills). These trends are still accelerating. Yay for humans, who can now (temporarily) live in greater comfort and numbers than at any time in history!
But the direction I want to take in this post is on the narrower (and ultimately less important) technical side. All the renewable energy technologies rely on non-renewable materials. Therefore, inexhaustible flows are beside the point. It’s like saying that fossil fuel energy is not practically limited by available oxygen for combustion, so we can enjoy fossil fuels indefinitely. Or that D–T fusion has billions of years of deuterium available, when there’s no naturally-occurring tritium (thus reliant on limited lithium supply). In a multi-part system, the limiting factor is, well, the limiting factor. Sure, into the far future the sun will shine, the wind will blow, and rain will fall. But capturing those flows to make electricity will require physical stuff: all the more material for such diffuse flows. If that stuff is not itself of renewable origin, then oops. The best guarantee of renewability is being part of natural regeneration (i.e., of biological origin). If solar panels, wires, inverters, and batteries were made of wood and the like: alright, then.
Recognizing that biological organisms—plants and the animals that directly or indirectly draw energy from them—have already figured out how to tap into (essentially) inexhaustible flows—solar, primarily—I became interested in comparing the performance of the human animal to that of a solar panel or wind turbine, in terms of mineral requirements. After all, the biosphere gets by without mining the depths. So let’s dig into the material requirements of life.
A Pinch of Dirt
Human construction requires very few mineral elements that do not come to us from water and air. I think that’s really cool. Our caloric intake consists of carbohydrates, fat, and protein—the whole set requiring only four elements that are obtained from air and water.
This Wikipedia page provides a compositional breakdown of the human body—presented both by mass and by atoms. Note, however, that the two forms listed on the Wikipedia page are not wholly self-consistent, so I arbitrarily adopt the by-atom numbers (more significant digits, meaningful or not) and produce by-mass fractions from them—though the result is not qualitatively different if starting from the by-mass numbers instead. Below are two tables that capture approximate numbers, broken up according to elements that derive from air and water, and those that we get from the ground. The first table also notes the elemental origins within our environment.
| ELEMENT |
% MASS |
% ATOMS |
SOURCE |
| Oxygen |
61.2 |
24.0 |
direct from air and water |
| Carbon |
23.0 |
12.0 |
air via plants/photosynthesis |
| Hydrogen |
10.0 |
62.0 |
water: direct and via plants/sugars |
| Nitrogen |
2.5 |
1.1 |
air via plants and microbes |
| Totals |
96.7 |
99.1 |
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So, about 99% of the atoms in our bodies come from air and water, often processed by other lifeforms before entering our mouths. That’s a very neat trick! By mass, it’s 97%. The difference is due to the most abundant elements in our bodies being on the lighter side (especially hydrogen), while the rarer minerals tend to be heavier atoms. Now for the dirt elements.
| ELEMENT |
% MASS |
% ATOMS |
| Calcium |
1.4 |
0.22 |
| Phosphorus |
1.1 |
0.22 |
| Potassium |
0.19 |
0.03 |
| Sulfur |
0.19 |
0.038 |
| Sodium |
0.14 |
0.037 |
| Chlorine |
0.14 |
0.024 |
| Magnesium |
0.06 |
0.015 |
| Totals |
3.22 |
0.574 |
Other minerals appear in trace amounts, totaling a small fraction of one percent. The elements in the table above are typically found in soils and rocks, accessed by fungi and roots. The comparatively small amount of ash left over from burning a log completely tends to be composed of elements on this list. The sources of these elements in our diets can be found on this website (and similar variants on the URL for other elements).
Power Performance
Now let’s look at the material efficiency of the human body and compare to that of a solar panel. According to the United Nations’ Food and Agriculture Organization, the global average caloric intake is 2,800 kcal per day, translating to an average continuous power of about 135 W. The mineral requirements to accomplish this constitute just over 3% of body mass, or 2 kg for the global average body mass of 62 kg. Thus, a human achieves roughly 70 W per kilogram of minerals. Note that even though the human body is only 20–25% efficient at converting metabolic energy into external mechanical work, the rest is not waste to us: it provides crucial thermal energy to keep body temperature up, and thus counts as a critical contribution.
Let’s look at solar panels. Typical 60-cell panels produce 300 W in full sun, and have a mass around 20 kg. Straight away we compute 15 W/kg—a factor of five lower than human performance. But to be fair, we must account for the fact that the sun is not always directly in front of the panel, producing a typical capacity factor of 20%, or an average power delivery of 60 W. Now the deployed panel delivers 3 W/kg: less than 5% as “efficient” as a human, in mineral terms.
Massive wind turbines at 20% capacity factor (typical global average) score even worse, at 0.4–0.6 W/kg. Without the mass-dominant concrete pad, a wind turbine would pump out 1.6–2.4 W/kg, for the short time it remained standing.
Just as a wind turbine needs a mounting base, a realistic utility-scale solar deployment has a material mass far in excess of the bare panels: support structures, interconnect wiring, inverters, storage (if truly replacing fossil fuels). I would not be surprised if a whole-system figure dropped to 1 or 2 W/kg, while humans stay smugly perched at 70. The score for wind would erode as well once other necessary components are considered—especially storage. Moreover, the minerals needed by humans are in wide circulation within the community of life at the surface: no mining (and associated tailings, energy, processing, pollution) necessary.
Thus, biology has far exceeded technology in capturing the inexhaustible flow from the sun using a minimum of minerals—and those being extracted from and re-deposited to the soil in a continuous, self-sustaining cycle, importantly. Biology and evolution really figured things out! Modernity looks like a bumbling idiot by comparison—like R2D2 in a stair-climbing competition against an athlete.
Replacement Considerations
What about the fact that the human body does not store its minerals indefinitely, but requires dietary replenishment? By contrast, solar and wind infrastructure lasts a few decades (it is not indefinite, either). To get a lower limit for replenishment times, I look at the recommended daily allowance (RDA) of minerals, provided at this site, represented as the RDA column in the table below.
| ELEMENT |
RDA (G) |
IN BODY (G) |
DURATION (DAYS) |
| Calcium |
1.0 |
870 |
870 |
| Phosphorus |
0.7 |
670 |
960 |
| Potassium |
3.0 |
120 |
40 |
| Sodium |
1.5 |
85 |
60 |
| Chlorine |
2.0 |
85 |
40 |
| Magnesium |
0.35 |
40 |
110 |
| Total |
8.55 |
1870 |
220 |
Dividing the amount of elemental mass in the body (obtained via mass percentage in the second table) by the RDA produces a timescale for complete replacement, as indicated in the last column of the table above. It makes some sense to me that calcium and phosphorus—locked up in bones—would persist for a long time, while mediators of biochemistry might flush more routinely. Still, I would imagine the RDA numbers to be conservative (a bit overkill; while staying short of harmful), translating into a more mineral-hungry portrait than is actually necessary. For instance, it seems unlikely that the average dwell time of calcium in your bones is just a few years. I’d be willing to bet that an RDA-consuming person passes unabsorbed calcium (and other minerals) in their poop. But I’m not likely to wade in there, experimentally.
In any case, in a 24-hour day, our 135 W standard human cranks out 3.2 kWh of energy, requiring daily intake of 8.55 grams of minerals according to the RDA standard. In order to compare to renewable energy figures, I’ll translate into tons per TWh to get about 2,600 tons of mineral input needed to produce one TWh of human metabolic energy (probably a lot less under actual body requirements).
According to Table 10.4 of the Department of Energy Quadrennial Technology Review, the production of electricity entails the following material requirements (in the form of aluminum, concrete/cement, copper, glass, steel, etc.):
| TECHNOLOGY |
TON/TWH |
FACTOR |
| Coal |
1185 |
0.45 |
| Gas |
572 |
0.22 |
| Solar PV |
16447 |
6.2 |
| Wind |
10260 |
3.9 |
| Hydro |
14068 |
5.3 |
We see that on this measure as well renewable energy technologies are more mineral-hungry than biological systems (at 2,600 ton/TWh) by substantial factors—and more if RDA is conservatively overstated. Moreover, the required elements are different from those needed for life—more “exotic” so-as to require mining, vs. readily at hand on the surface in biological circulation.
To help appreciate this difference, imagine placing an end-of-life solar panel and all its accompanying stuff out in the forest. What components are eagerly eaten by the resident biology? At the same time, put a dead plant or animal next to the solar junk and come back in ten years. One will be much the same, while the microbes and fungi have consumed the other, leaving no discernible trace.
Note that the fossil fuel entries in the table above are “cheating” by not including the mass of the fuel itself. The intent is to capture the infrastructural “machinery” needed to convert the flow to electricity. At energy densities of 6 and 13 kcal/g for coal and gas, respectively, the corresponding ton/TWh numbers translate to 143,000 and 66,000—numbers typical of chemical energy. I could make the case that the numbers in the table are still fair in the materials sense, counting the elements that are not provided by derivatives of air and water via photosynthetic processes—much as we ignored the bulk of the human mass (and food intake) for the same reasons. Missing in the other direction is ore purity and thus mine tailings, which can exceed the end-product material mass by factors of hundreds, so that the total extracted mass is far larger than indicated in the table above. Still, this post is not intended as an argument for or against fossil fuels.
One point to note is that for every ton of fossil fuel removed from the land, another six tons are removed in the form of sand, metal, rock, and wood. Inferring from the table and figures above (and common sense), these materials are not primarily devoted to the machinery needed to burn fossil fuels (i.e., engines and power plants). They are going to the human enterprise called modernity: buildings, roads, consumer goods, etc. Replacing modernity’s engine with another source, like renewable energy, aims to keep the bulk of material extraction in full swing—in fact enhancing it to supply the extra materials necessary for diffuse renewable energy to function.
The Inexhaustible Point
For all intents and purposes, biology has figured out a way to tap into the continuous and (seasonally) reliable flow of solar energy using a bare minimum of mineral requirements from the land’s surface. It took billions of years to solve this very hard problem. One could consider the result to be a “circular economy,” in that minerals are recycled into the environment and taken in by microbes, fungi, plants, and on up the food chain. By working within the strictures of multi-level selection (evolution) subject to long-term ecological viability in relation to other life, the result has the word “sustainable” effectively built in: sustain-a-built. No? Okay, yeah, that’s pretty lame.
Our technologies are clumsy and materially insatiable, by comparison—no surprise, given the short development time and our complete disregard for the unforgiving constraint of sustainable practices. Make no mistake: “renewable” energy is not the same as sustainable technology. The only demonstrated sustainable technologies to date are those found outside modernity, in the biodiverse ecological realm (including things made from wood and plant materials, for instance). Until a technology achieves closed-loop sustainability in concert with the rest of the community of life—which may not be possible—it’s not truly “renewable.” Systems that require mining, produce mine tailings/pollution, destroy habitats, and result in collateral damage in the form of permanent species extinctions can’t be considered to be long-term viable, in my view—just part of the jaw-dropping fireworks show that will soon shock itself by self-terminating. Nobody could have seen it coming!
A typical unsubstantiated knee-jerk reaction is that aggressive/complete recycling could address the concerns. But recycling yield is always going to disappoint, so that a moratorium on new mining (or simple exhaustion of economically recoverable material as the low-hanging fruit is depleted) would result in a slow dwindling of available materials until the weakest link falters below some minimum threshold required to keep the industry alive—likely on a timescale that is lightning-fast compared to that of ecological evolution. Recycling also consumes copious energy: more and more as higher and higher yields are sought. It becomes self-defeating: from what source does such energy come, and at what additional material cost? Plus, I always return to the question of what we use the energy to do. Thus far, it’s been 99.9% unsustainable activity (my crude guess: vanishingly little goes into restoration of ecological damage). Sixth mass extinction, anyone?
So, is technology on the verge of inexhaustibly tapping into inexhaustible flows?
I don’t think so.
It should not be surprising that we have not yet been—and may never be—able to engineer long-term-sustainable modernity (i.e., high-tech). I strongly suspect that’s not even a thing. Why on Earth would we just assume that it’s possible? Where does that hubris come from? It’s not from a thorough analysis in full ecological context, and certainly not from any demonstration. It’s just a lazy and wishful assumption based on the brief and highly anomalous window on the world to which we’ve been exposed. Comparing modernity-relevant timescales to those relevant to evolution, and looking at the profligate rate of one-time inheritance spending (i.e., of non-renewable resources) that has been required to produce modernity tells us a lot. Unlike biology, this ain’t built to last. I know which team is a better long-term investment—the ultimate victors unless everyone loses first.
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ABOUT THE AUTHOR
Tom Murphy is a professor of physics at the University of California, San Diego. An amateur astronomer in high school, physics major at Georgia Tech, and PhD student in physics at Caltech, Murphy has spent decades reveling in the study of astrophysics. He currently leads a project to test General Relativity by bouncing laser pulses off of the reflectors left on the Moon by the Apollo astronauts, achieving one-millimeter range precision. Murphy’s keen interest in energy topics began with his teaching a course on energy and the environment for non-science majors at UCSD. Motivated by the unprecedented challenges we face, he has applied his instrumentation skills to exploring alternative energy and associated measurement schemes. Following his natural instincts to educate, Murphy is eager to get people thinking about the quantitatively convincing case that our pursuit of an ever-bigger scale of life faces gigantic challenges and carries significant risks. To learn more about this author and whether you should dismiss some of his views as alarmist, read his Chicken Little page.
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