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This sixth in our series on the physical limits to energy resources on Earth covers the remaining potential major sources that we know about: tides and geothermal energy. Previous articles covered chemical (including bio-), nuclear fission, fusion, solar, and wind, water, and wave energy. Energy from Earth's internal heat and from Earth's graviational interaction with the Sun and Moon are the only remaining energy flows large enough to induce noticeable effects on the planet, and potentially large enough to supply human needs. Let's look at the numbers in more detail.
Tides tap into the orbital energy and motion of Earth and Moon, and of the Earth around the Sun. For lunar tides, the Moon's mass and orbital speed give the scale of total energy available. Total lunar mass is 7.3x10^22 kg, and the Moon orbits at about 1 km/sec, so with kinetic energy of 1/2 m v^2 that amounts to 3.65x10^28 J, or about 10^8 years of current world energy use.
Thanks to Earth's shape the tides raised by lunar and solar effects result in energy dissipation, as the water crashes into shorelines around the globe. Models of this process show a total tidal dissipation rate of about 2.42 TW; about 32% of this coming from solar tides, and the remaining 68% lunar. The reaction effect on the Moon results in its orbit slowly expanding and its motion slowing down on a time-scale of hundreds of millions of years. At less than 20% of today's human energy use, Earth's tidal energy flow is unfortunately significantly less than the scale needed to power human society.
Geothermal energy originates from the heat of Earth's interior. A number of Earth's elements release energy through natural radioactive decay – their present and past energy releases are responsible for Earth’s high core temperature of 5000-7000 K. With iron's heat capacity of about 500 J/kg/K and a core mass of at least 10^24 kg, the instantaneous total energy content of Earth's inner heat amounts to over 2x10^30 J, or 5 billion years of present world energy use.
Total energy flow to the surface from this source is about 45 TW, cooling the interior on a billion-year time-scale (but the elements continue to decay, so it will actually cool more slowly than that). Technology to tap into the deeper interior is conceivable, so the natural 45 TW scale may not be the limit. At the least geothermal can provide a few times current world energy demand.
In addition, the steady but not very warm temperatures of the first few hundred feet below the surface can be used to efficiently provide moderate heating and cooling to buildings with geothermal heat pumps, but this doesn't actually tap into the interior heat, it just provides a convenient storage mechanism to more efficiently use the energy flows available to us from elsewhere. Only registered users can write comments.
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