[varjag]

Sure there's a physical ceiling to energy generation but I feel like there's a great degree of oversimplifcation in his argument.

Take for example solar: right now, pretty much all of that energy (sans the work performed for propelling ocean currents and production of future fossil fuels via photosynthesis of plants) is irradiated back into space anyway. Using more of that energy to perform work on surface of Earth won't change a thing from global thermodynamics perspective. That energy is already on the books.

Or consider hydro (a consequence of solar itself): the moving water already carries energy and performs work. Re-purposing that energy in the form of electricity would have no impact on the overall energy budget.

Granted the energy growth is theoretically limited, but the limit might not be as near if we consider that much of energy generation could be essentially energy conversion.

[hebdo]

Consider this: a) at some point our energy budget will exceed total Sun radiation reaching Earth, and b) indefinite "re-usage" of energy is impossible (entropy).

[varjag]

I actually considered this, making a disclaimer at the beginning and the end of the post :)

My point was that the timeframe would likely be quite different, e.g. millennia instead of centuries. From theoretical perspective doesn't change anything, but perhaps makes the argument a bit less dramatic.

[digi_owl]

What he did was to take the economists golden calf, the 3% growth pr annum, and run with it in physical terms. Demonstrating how, if economists managed to maintain that growth curve, the energy requirements would be staggering.

[dredmorbius]

All "energy generation" is "energy conversion".

To "convert" energy, you need an entropic gradient.

Those are fundamentally limited.

Here's your inventory:

Total human annual energy consumption presently is about 560 million terajoules (TJ), 156,000 TWh, or 530 quad (quadrillion BTU), or 13.3 billion tons of oil equivalent (btoe), or 91.5 billion barrels of oil, or 19.1 billion tons of coal, or 4 million tonnes of processed uranium nuclear fuel.

Oil, gas, and coal combined are 11.0 btoe, or 83%, of all energy consumption. Nuclear and hydro, both possibly close to tapped out, account for another 1.4 btoe.

● Solar. About 1kW/m² at Earth's surface, modulo conversion efficiency -- 37% max for single-layer PV, 85% max for infinite layer, or concentrated solar power at Carnot efficiencies determined by input/output thermal differentials, 40-45% likely upper bound. And spacing factor (dependent on latitude, about 55% if you're in the continental US, lower in Europe, higher in the tropics), capacity factor, 20-30%, the amount of time you can capture sunlight.

● Solar-derived: biomass, wind, hydroelectricity, and wave energy are all based on solar flux. Biomass is largely tapped out and competes with both human food and natural ecosystem uses.

● Biomass is stubbornly persistent as a part of human energy consumption. It's also heavily dependent on fossil-based inputs: fertilisers, pesticides, equiipment, transport, processing, refrigeration and storage. About 6% of present net energy, unlikely to change. Recognise that all wastestream sources (food waste, sileage, sewerage, animal waste, forestry waste, and garbage) either derive directly from biomass or fossil fuels. I've penciled these out to at best 1-2% of present fuel consumption. Britain might be able to run its bus fleet on sewerage-derived biomethane, but that's about it. For the US, most biofuel crop yields would require multiples of total national land area to provide the present level of fuel consumption.

● Hydro's similarly largely tapped out, though some third-world potential remains. My suspicion is that hydro will increasingly fill a storage / dispatchable load role. Mind that it is among the highest return on energy investment options available. It's just not very available.

● Wind actually has good metrics on per plant return, though at roughly 3-4 MW per turbine, you'd need a lot to address global energy requirements (about 4.4 million nameplate, 12-21 million at 21-35% capacity factors). Oh, and the best sites get picked first, so output/capacity will likely fall with build-out.

● Geothermal. Absent a few global hotspots, not likely a large player. Already surprisingly well developed in Iceland, the US, Japan, New Zealand and the Philippines. Possible potential in Kenya, Indonesia and a few other regions. Proven, but limited. Enhanced geothermal, using boreholes and consumptively extracting heat has proven both expensive and disappointing to date. Total global1potential of 35 GW to 2 TW (306 TWh/yr - 18,000 WTh/yr). At the high end, that's 12% of present total energy consumption, which isn't to be sneezed at. But it's also not "unlimited potential underneath our feet".

● Tidal. This is far more limited, difficult to extract, and distributed than people seem to think. It also has massive local environmental impacts. Locally, plants with a few tens of MW of continuous capacity might be created. As storage systems, they're likely more useful than energy. Potential of 0.26% of global use per Tom Murphy.

● Nuclear. Conventional uranium resources are finite, with roughly 60 years' supply at present rates of use. Ramped to full human energy consumption, they'd be good for about six years. Suggestions are that lower-grade ores, or recovery from seawater might be possible, but this is generally unproven at scale. Likewise thorium, despite much cheerleading. Nuclear might offer potential, but it comes with numerous extreme challenges and risks, not all well or fairly discussed by proponents. .

And that's pretty much it. There are some extremely long shots:

● Fusion: still not working after all these years. First successful sustained nuclear fission was achieved within four years of theoretical understanding. Applied plants were operating within a decade, and significant commercialisation within two. Though those plants haven't been without their problems. Now, 70 years after first demonstration, nuclear proponents are assuring us they've got all the kinks worked out. I'm dubious.

● Space-based solar. Factor in several kg per kWh capacity, and launch costs of $200-$10,000 per kg. Your benefits are:

More sunlight per panel. No day/night factor, no latitudinal losses, no weather losses. Greater total area potential. Net gain is about 3 over ground-based desert siting. Half that is lost in transmission to Earth. Launch costs to geosync orbit are $20k/kg. Murphy follows a NASA study which assumes $100-200/kg costs (presently a few thousand to LEO is still quite good). At 1kW/kg, that's 17.7 megatonne of material, and $354 trillion in launch costs -- a transport bill alone, on top of engineering, fabrication, manufacture, etc. Murphy sees this as at least 4x the cost of ground-based installations.

Black holes or antimatter might offer more extreme options, but again those would be exceptionally difficult to engineer and are now entirely theoretical.