Lunar Energy
"Lunar energy" sounds like esotericism, but it is solid physics. Every day the moon moves masses of water against which every power plant looks tiny.
Do not laugh straight away: there really is an energy source that goes back to the moon. All tides, that is, the daily rising and falling of the sea, and a large part of the ocean currents arise through the gravitational pull of the moon (and, to a lesser extent, the sun) on the water masses of the Earth. That makes tidal energy, alongside geothermal heat, the second major source that does not come from sunlight but from gravity in space.
How the moon moves the sea
The moon pulls on the Earth, and because water is more mobile than rock, it bulges toward it. Thus on the side facing the moon a tidal bulge forms, on the far side a second. As the Earth turns, these bulges travel around the globe: twice a day the water rises and falls on the coasts. The difference in height is called the tidal range. In some bays it piles up extremely; in the Canadian Bay of Fundy it is over ten metres. Precisely this regular rise and fall can be captured as energy.
A bit of history
The idea is ancient. Already in the Middle Ages, tide mills ground grain on European coasts: the flood was let into a basin, closed off, and at ebb the outflowing water was used. The first great modern tidal power plant arose in 1966 at the Rance estuary in France and runs reliably to this day.1 It shows that the technology works, yet it found surprisingly few imitators.
How it is used today
One distinguishes two ways. Tidal barrage power plants work like a hydropower plant: a barrage dams the flood, and the in- and outflowing water drives turbines. Tidal stream turbines, by contrast, stand freely in the sea and turn in the tidal current, similar to wind turbines under water, without damming a river.
Despite decades of attempts, tidal energy so far plays only a tiny role worldwide. It remains a footnote in the great energy mix, and it is worth asking why such a reliable source is used so little.
Advantages
The great advantage: the tides are absolutely predictable. Unlike with sun and wind, you can calculate to the minute when the flood will come in a hundred years, for the moon obeys no moods of the weather. That makes tidal electricity plannable like hardly any other renewable energy. Moreover the amount of energy in the sea is vast and in operation CO₂-free.
Disadvantages
So why does hardly anyone use it? First, there are only few suitable places: you need coasts with a very high tidal range, and only a handful of those exist worldwide. Second, the technology is expensive and the construction of a coastal barrage a heavy intervention in the ecosystem, threatening tidal flats, fish and birds. Third, salt water is a destructive opponent: it eats at turbines and anchorings, and repairs under water are costly. So tidal energy often remains simply too costly, measured against what it delivers.
The critical look
Lunar or tidal energy is a fine object lesson about the difference between possible and sensible. Physically the matter is crystal clear and the amount of energy huge. And yet it mostly fails on costs, sites and nature conservation. Not every impressive force of nature makes a clever energy source.
Precisely in this lies the emancipatory value of this chapter. Whoever understands that a source can be "inexhaustible" and yet impractical more easily sees through the grand promises bandied about in energy debates. The decisive question is never only "Is there enough energy?" but always also "What does it cost to fetch it, and who pays the price for it?" The moon sends its power anyway, whether we use it or not. Whether it is worthwhile is revealed to us by no force of nature, but only by a sober look at the account.