My colleagues are going to hate me for posting this, but ... well, I'm not Elon Musk*, okay? So here's a bit of a beginnings for constraints on the future of renewables. And when I say 'beginnings' I mean 'beginnings'! If one wants to do this properly one needs to start with so-called 'boundary conditions' and work inwards from there.
In 2010, we humans used fossil fuels at an annual rate of 400 EJ (1 exojoule = 10^18 joules), which is equivalent to about 13 TW (1 terawatt = 10^12 watts). In raw thermodynamic terms (i.e. ignoring the capital costs, and the fact that it takes energy to build and maintain the generating capacity), would renewables be able to replace this? A quick summary of the potential of wind, solar, hydro, wave, tidal, geothermal and biofuel (photosynthesis), based on a recommended book by Vaclav Smil’s:
Wind
About 870 TW of solar radiation is transferred to global wind’s kinetic energy (Peixoto & Oort, 1992). A recent study (Marvel, Kravitz & Caldeira, 2013) finds that, in terms of sheer geophysical potential, “wind turbines placed on Earth’s surface could extract kinetic energy at a rate of at least 400 TW, whereas high-altitude wind power could extract more than 1,800 TW.” Although that all sounds like a lot, the accessible energy 80m above ground is estimated to be in the range of 72 TW (Archer & Jacobson, 2005), but we need to bear in mind that turbines need to stand about 5 rotor diameters apart. Computer simulations of a world where accessible areas are covered by 100m tall 2.5 MW turbines with capacity factors of 20% conclude that it’s possible to harnass a maximum of 78 TW (Lu McElroy & Kiviluoma, 2008). So that’s about six times our current fossil fuel use. Hurrah for wind!
Solar
About 120 PW (1 pentawatt = 10^15 watts) of solar radiation reaches the biosphere, of which about 25 PW is absorbed by land. If we knock out excluded areas like polar- and steep mountain regions we’re left with a usable flux of about 15 PW. So that’s about a thousand times our current fossil fuel use. Hurrah for sun!
Hydro
We at SCENE have mixed feelings about hydro. We love the small stuff, though! The total potential of Earth’s runoff is about 10.5 TW, but only about 15% (WEC, 2007) of this is technically exploitable (before even taking the economics into account). Still, hurrah for small-scale hydro!
Wave
Wind-driven ocean waves have a kinetic energy of some 60 TW, only 3 TW of which is dissipated along coasts. Well, it’s worth a try!
Tidal
Tidal energy amounts to about 3 TW, of which only 60 GW is dissipated in coastal zones. Better than nothing, especially if you’re miles away from the nearest electricity grid. Beats running a generator.
Geothermal
Earth’s geothermal flux is on the order of 42 TW (Sclater, Jaupart & Galson, 1980), but mostly (like 80%) in the form of low-temperature diffuse heat on ocean floor which isn’t going to help us out much. Some (Bertani, 2009) reckon that, by using steam, we can tap into 140 GW by 2050. Worth a try if you’re lucky enough to live near a resource – just look at Iceland (cheapest electricity in the world!)
Biofuel (photosynthesis)
Terrestrial (= on land) photosynthesis proceeds at a rate of about 60 TW, about 3 TW of which currently gets used for energy. Hurrah for plants!
Now time to start thinking about how much of this renewable resource is realistically available to power human lives.
*(i.e. he has more spare time than I have)