tidal amplitude (half-range) h(m) |
boost height b(m) |
power with pumping (W/m ^{2}) |
power without pumping (W/m ^{2}) |
---|---|---|---|

1.0 | 1.0 | 1.6 | 0.8 |

2.0 | 2.0 | 6.3 | 3.3 |

3.0 | 3.0 | 14 | 7.4 |

4.0 | 4.0 | 25 | 13 |

level at high tide can be pumped up to the maximum. Table G.11 gives

the power delivered if the boost height is set to *h*, that is, the range in the

pool is just double the external range. A doubling of vertical range is easy

at neap tides, since neap tides are typically about half as high as spring

tides. Pumping the pool at neaps so that the full springs range is used

thus allows neap tides to deliver roughly twice as much power as they

would offer without pumping. So a system with pumping would show

two-weekly variations in power of just a factor of 2 instead of 4.

Here’s a neat idea: have two basins, one of which is the “full” basin and

one the “empty” basin; every high tide, the full basin is topped up; every

low tide, the empty basin is emptied. These toppings-up and emptyings

could be done either passively through sluices, or actively by pumps (using

the trick mentioned above). Whenever power is required, water is allowed

to flow from the full basin to the empty basin, or (better in power terms)

between one of the basins and the sea. The capital cost of a two-basin

scheme may be bigger because of the need for extra walls; the big win is

that power is available all the time, so the facility can follow demand.

We can use power generated from the empty basin to pump extra water

into the full basin at high tide, and similarly use power from the full basin

to pump down the empty basin at low tide. This self-pumping would

boost the total power delivered by the facility without ever needing to buy

energy from the grid. It’s a delightful feature of a two-pool solution that

the optimal time to *pump* water into the high pool is high tide, which is

also the optimal time to *generate* power from the low pool. Similarly, low

tide is the perfect time to pump down the low pool, and it’s the perfect

time to generate power from the high pool. In a simple simulation, I’ve

found that a two-lagoon system in a location with a natural tidal range of

4 m can, with an appropriate pumping schedule, deliver a *steady* power of

4.5 W/m^{2} (MacKay, 2007a). One lagoon’s water level is always kept above

mean sea-level; the other lagoon’s level is always kept below mean sealevel.

This power density of 4.5 W/m^{2} is 50% bigger than the maximum

possible average power density of an ordinary tide-pool in the same lo-