In the August 2023 issue of Physics Today (page 22), Rachel Berkowitz nicely examines how turbines installed in strong tidal currents could provide power for small communities that currently rely on diesel generators. The article emphasizes the reliability and predictability of tidal energy compared with solar and wind power. Unfortunately, tidal power is limited at the global scale and cannot contribute significantly to humanity’s overall needs.
To illustrate this point, compare current human power consumption of about 20 terawatts (TW) with the availability of various renewable energy sources. Insolation at Earth’s surface is approximately 100 000 TW, showing the potential of solar power. Wind power, with dissipation of close to 1000 TW, also has potential. Tidal dissipation is a mere 3.7 TW. Simple models indicate that this is a reasonable upper bound to what could be extracted in principle, though extracting more than a small amount is not technically feasible and would cause significant changes in global tides as well as major changes locally.
Places such as Cook Inlet in Alaska, the Bay of Fundy in Canada, Pentland Firth north of Scotland, and Cook Strait in New Zealand offer a potential of hundreds of megawatts or even more than a gigawatt (GW), but a strong current of, say, 3 m/s corresponds to a head of only 0.5 m. Providing significant amounts of power in such a situation requires huge fluxes of water through turbines—several tens of thousands of cubic meters per second per GW, depending on the details of the installation. A large turbine array would also slow the flow, which would limit the available power and potentially have a significant environmental impact. In places where the strong current is associated with a tidal range of several meters, exploitation using a barrage or lagoon seems preferable, although this would have other disadvantages.
Instead, the best energy-related use of strong, cold tidal currents might be to provide cooling water for nuclear reactors. For example, in the Bay of Fundy, which has the world’s highest tides and where the use of tidal turbines is proposed, a CANDU-6 pressurized heavy-water reactor at Point Lepreau uses 26 cubic meters per second of cooling water for a power output of up to an average 600 MW of electricity. That’s just 43 cubic meters per second per GW, less than the tens of thousands of cubic meters per second required for the same output using in-stream turbines. Many factors need to be taken into account in comparing potential sources of power but, even where a large-scale in-stream tidal-power project is feasible, it seems appropriate to quote the late David MacKay’s freely available book Sustainable Energy—Without the Hot Air: “Please don’t get me wrong: I’m not trying to be pro-nuclear. I’m just pro-arithmetic.”
