tidal amplitude
(half-range) h
height b
with pumping
without pumping
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.

Getting “always-on” tidal power by using two basins

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/m2 (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/m2 is 50% bigger than the maximum
possible average power density of an ordinary tide-pool in the same lo-

Table G.11. Power density offered by the pumping trick, assuming the boost height is constrained to be the same as the tidal amplitude. This assumption applies, for example, at neap tides, if the pumping pushes the tidal range up to the springs range.