Tidal river energy can be tapped both in the sea environment and in tidal rivers and streams. Its potential is large and a mere 10% of the energy in Great Britain was estimated sufficient to provide more than 5% of that country’s electrical needs a quarter of a century ago. The 8-knot current of the river underneath the Golden Gate Bridge (San Francisco) can provide all the bridge’s needs in electricity. Likewise were the Florida Current to be harnessed 25 GW of electricity could be produced. An “aqua power barge”, capable to “harvest” energy along coasts and on tidal rivers, proposed in 1979, would use a high-impulse low-head turbine; with a 6 knots current 50 kW of installed power could be produced.
Patents have been taken out in the United States since the 19th century for a variety of devices intended to tap directly the energy of waterways; they encompass small units as well as “giant” paddlewheels. AeroVironment Inc., where the Coriolis Project was developed, examined the river energy resource for the Western
United States, the economics of ducted and un-ducted axial flow turbines and even carried out some small-scale rotor model tests.
Davis and Swan sought to develop a ducted Darrieus design. Designs of nonconventional conversion systems have been frequently reviewed (Pratte, Davis, and others). Vertical axis turbines were proposed by Davis and Swan.
A technology assessment conducted by New York University on behalf of the State of New York and dealing principally with the tapping of the tidal current in the East River in New York City, yielded information on a number of devices which could be usable and on the advantages of axial-flow propeller machines. The various types of KHECS included waterwheels, free-ducted and Wells rotor axial flow turbines, Darrieus, Savonius, and cyclo-giro type vertical axis rotors and the Schneider Lift Translator. The conclusion of the studies was that the system would cost less than $1,700/kW installed.
A prototype was installed in the East River’s semi-diurnal Eastern Channel in 1985.
Attached to the side of a bridge the 4.3 m diameter device used a three-blade conformal design. Modest ducts had been attached to the screen hoop to test their potential cost-effectiveness. The unit was dismantled for inspection after a short period of operation.
Though hardly tidal current schemes, many proposals have been ventured to link various seas, streams and canals. Some visionaries, including Theodore Herzl in 1902, have suggested a canal linking the Dead and Mediterranean seas, and
proposed to tap the current to generate electrical power. Some thoughts to that effect had been expressed as early as 1850. It was however the James Hayes Commission which, in 1943, made a first assessment. An Israeli commission recommended moving ahead with a linking project at the end of the 1970s. Apparently the plan has been laid to rest, probably the better so in view of the probable ecological consequences it would have.
The advantages of the Turbodyne Generator were praised in 1982: the amount of turbine material is small and the high speed vertical-axis turbine was shown in theory and actual tests to perform highly with low heads; power generation is possible, in ebb and flow tides, independently of current speed, provided the current has a small head (even <1.5 m); silting risks are low, environmental impact rather benign and no sluices are necessary.
Baker and Wishart conducted a study covering three small estuaries and seventeen sites in Great Britain and, in terms of 1983 dollars, arrived at a cost varying between $6.10 and $6.30, depending on the number of turbines, per kWh. The cost (C) of a barrage is given in (8.8), which includes correction factors for shallow margins and ranges < 1
L08(H + 2)2
wherein L is the length of a barrage, H is maximum depth, A is the basin area and R is the tidal range.
Among recommended sites were the Camel River (Cornwall), the Taw-Torridge estuary (Devonshire), Milford Haven (South Wales), Loughor Estuary and several on the Mersey River.
The Salford Transverse Oscillator could harness energy from tidal currents a. o. in rivers and tidal inlets; it could function in basins as small as 0.5 km2, e. g. Loch Heuran (Scotland). Installation of a prototype was being considered in 1993. If P is power, ю the specific weight of water, Q the water discharge, H the head, then
P = (OQH (8.9)
When the flow reaches as little as 0.49 m/s an immersed Savonius type rotor driving a generator could power a marine beacon, and greater efficiency could be
attained by channeling it through ducts. Grant has discussed the potential use of the tidal flow for navigation buoys.
So-called dynamic dams have also been proposed for tidal streams.