The motion of tides, currents, and ocean waves can be used to produce energy. A few places, like the Bay of Fundy, have high tides, and the water rushes through a narrow channel four times a day. If the speed is greater than about 5 knots (2.6 m/s), the current can drive an electric generator, but there are very few such sites. A new method called Vivace87 is claimed to work at speeds as low as 2 knots (1 m/s). Flexible cylinders are anchored to plates on the sea bottom. Currents flowing back and forth make the cylinders flex and wobble, and this motion is used to generate electricity. How they do that and what the cylinders are made of are not revealed. Tides and waves also make the ocean level go up and down. Several systems using this effect are based on the same principle. A rigid tube is anchored to the sea bot­tom. A diaphragm inside the tube is driven up and down by a buoy floating on the top. As the buoy moves up and down, the diaphragm drives air in and out of an aperture at the top end of the tube. This flow of air turns a turbine to generate elec­tricity. An underwater cable carries the electricity to shore. This method requires a floating object that can be seen and collided into.

The most publicized system is the Polamis (“sea snake” in Greek),88 designed to capture wave energy. It looks like a series of giant sea snakes floating in the ocean. Each snake points in the direction of wave motion, perpendicular to the wave crests, and consists of metal cylinders with the size of railway cars hinged to each other so that the snake flexes with the waves. In between the cylinders are air pistons push­ing air back and forth with the wave motion. This air drives onboard electric gen­erators. These Polames have been built in several countries, Portugal for one.

Cost and power have been calculated, but none of these ideas has been worked out for impact on the environment, wildlife, and ship traffic. Engineering for 30-year lifetime in the sea may be difficult. The resistance of materials against salt water dam­age, so important in offshore wind turbines, has not been mentioned, for instance. The power is also not constant, so that some storage mechanism is needed to level it out. It is clear that these entrepreneurial ventures cannot yet be taken seriously.