Many forms of renewable power generation, such as wind and solar, suffer from differing power output depending on how intense the natural energy source is at any particular point in time. One neat exception to this is a tidal barrage with multiple lagoons. By managing the water level in each, it is possible to smooth out power generation between tides, as well as make output constant between days with bigger tides and those with smaller tides. It is also possible to use such systems to store excess energy from other renewable generation sites (such as winds farms running at full power during times of low demand) and to release energy at times of maximum demand, or when output from other renewable options is flagging.
With two lagoons and pumps for both, there are a huge number of options. You can maintain one pool at a ‘high’ level, and the other at a ‘low’ level, topping up the former using natural high tides or pumping and drawing down the latter in the same ways. When the tide is high, you can generate power by letting water flow into the low pool from the sea, or by letting water flow into the low pool from the high pool. When the tide is low, you can generate power by letting water flow from the high pool out to sea, or from the high pool into the low pool. Whenever you are producing power, you can use it for any mixture of supplying the grid, pumping up the high pool, and pumping down the low pool.
The combination of pumping with tidal lagoons is even better than conventional pumped storage. This is because you can actually produce more energy letting the previously pumped water flow than it took to do the pumping. Wikipedia explains:
If water is raised 2 ft (61 cm) by pumping on a high tide of 10 ft (3 m), this will have been raised by 12 ft (3.7 m) at low tide. The cost of a 2 ft rise is returned by the benefits of a 12 ft rise. This is since the correlation between the potential energy is not a linear relationship, rather, is related by the square of the tidal height variation.
David MacKay’s book also has a detailed section on tidal pumping and two-lagoon arrangements.
Of course, tidal power is not without environmental consequences. It will certainly alter the marine ecosystems that exist wherever facilities are built, and may create consequences in river systems located behind the barrage. That being said, the many advantages of tidal power as an energy generation and energy storage option mean that it probably has an important role to play in building a sustainable global society.