Tidal and Wave Energy

March 24, 2014

When we think about renewable energy sources, solar energy is the first thing that springs to mind. The average insolation arriving at the top of Earth's atmosphere from the Sun is about 1360 watts per square meter. At ground level, however, we have a daily average insolation of just 250 watts per square meter.

The deficit is a consequence of atmospheric absorption, the absence of sunlight at night, and the fact that the Sun is usually not overhead, so its rays project on Earth's surface at an angle. When we factor in the less than 25% efficiency of today's photovoltaics, we have a solar energy collection ability of just slightly more than fifty watts per meter, averaged over a year. Then there's the problem of storing the energy during the day for use at night.

There's another form of renewable energy, and that's lunar energy, better known as tidal power. You can extract tidal power at all times of day, since the tides are always ebbing or flooding, although there is variability from high vs low tides.

Time and tide wait for no man. Cnut the Great of the North Sea Empire is famous for setting his throne at the water's edge and commanding the tide to stop. (From the Genealogical roll of the kings of England (c. 1300 - c. 1340), Royal MS 14 B VI of the British Library, via Wikimedia Commons.)

One method of extracting energy from the tides is a variation of hydroelectric power known as a tidal barrage. As in a conventional hydroelectric power plant, a dam is involved, which isolates a bay or other estuary from the sea tides. Water will move through turbine generators into the bay or estuary at high tide, and it will move in the opposite direction at low tide.

A tidal barrage can generate power equivalent to a small nuclear power plant, with no high technology required. As shown in the figure, a massive tidal barrage was proposed as early as 1921 at the Severn Estuary at the border between England and Wales.

Diagram of a 1921 plan for a tidal barrage at the Severn Estuary in the United Kingdom at the border between England and Wales. (Illustration from a 1921 issue of Popular Mechanics, via Wikimedia Commons.)

Tidal stream generators are essentially wind turbines placed underwater, although there are architectural variations on this theme that are difficult to do on land. Under the sea, you can duct the tidal flow to produce a higher velocity stream at your water turbine. These water versions of wind turbines can be much smaller, since the working fluid is the much denser water, which packs a lot of momentum into its flow.

Why a duct? If we consider water to be an incompressible fluid, conservation of momentum in a circular duct requires that V2 = V1(D1/D2)2 (Illustration by the author using Inkscape.)

Tidal power is not the only way that energy can be extracted from the oceans. You can extract solar energy as well as lunar energy from the oceans through ocean waves. Ocean waves are driven by the wind, which, in turn, is driven by the Sun. Wind excites waves over large areas of the ocean, and these can travel great distances before reaching a shore. This means that waves will still appear in your locale even when it's not locally windy.

Even waves of a modest height are still raising large volumes of water against the force of gravity, so they contain considerable energy. A novel method of energy extraction from ocean waves is being explored by a team of mechanical engineers led by professor M. Reza Alam of the Mechanical Engineering Department of the University of California, Berkeley. The device is an elastomer carpet coupled to hydraulic cylinders.[1-2] Says Alam,

"There is a vast amount of untapped energy in the oceans, and with increasing worldwide demand for power, the need to find cleaner alternatives to fossil fuels is critical... We are also seeing greater population growth along coastal cities, so the ocean-based system we are developing would produce electricity in a carbon-neutral way right where it is needed."[1]

The device, as shown in the photograph, is a sheet of rubber mounted on a grid of hydraulic cylinders. The rubber sheet moves up and down with the waves, pumping the cylinders, which transmit hydraulic pressure back to shore for conversion into electricity. The rubber elastomer, of course, is the weak point of the system, since it needs to repeatedly flex, and it's located in a corrosive environment. Best placement of the carpet would be at a sixty foot depth in shallow coastal waters.[1]

Wave energy converter in a wave tank. An online video shows how the motion of the carpet tracks the wave motion.[2] (Still image from a YouTube video.[2])

Studies have shown that wave power worldwide has the potential to provide more than 2,000 terawatt hours of electricity per year. This is about 10 percent of global electrical needs.[1] The Berkeley experiments have shown that their system could extract more than 90% of incoming wave energy, and that just a square meter of the carpet could generate the electrical requirements for two US households. A hundred square meters of carpet placed at the California coastline can generate as much power as 6,400 square meters of photovoltaics.[1]

The Berkeley team most recently presented results of their experiments at the 10th 10th European Wave and Tidal Energy Conference, Aalborg University, Denmark, September, 2013.

References:

1. Sarah Yang, "Seafloor carpet catches waves to generate energy," University of California, Berkeley, Press Release, January 28, 2014.
2. Berkeley Team Producing Energy from Ocean Waves, YouTube video produced by Roxanne Makasdjian and Phil Ebiner, January 28, 2014.