This is the sixth article in a continuing series on renewable energy technology. Today we’ll continue our discussion of ocean power. As previously mentioned, the term covers virtually any device that extracts power from the sea. That’s a broad range of technologies, including wave and tidal energy conversion systems or the more exotic concepts of ocean thermal energy conversion (OTEC) and salinity gradient devices.
The previous article covered wave energy. This article explores tidal energy and ocean current energy schemes. Most of the power used on this planet comes (directly or indirectly) from the sun. That includes fossil fuels, wind, traditional hydropower, biofuels, wind, solar PV and wave energy. Tidal power comes from the moon, or more accurately, the gravitational interaction between the earth, moon, and the oceans. That makes tidal and ocean current energy unique. But how do tidal and ocean current power systems work?
Tidal and ocean current power simplified
Tidal and current power systems share a lot of characteristics with wind turbines. In mechanical terms, they both extract energy from a fluid (sea water and air, respectively). In simple terms, tidal and current power systems use lunar induced changes in sea level to push water through a turbine, hydrofoil or other device, which turns an electric generator very much like a wind turbine or hydroelectric plant.
The classic tidal energy system features a turbine placed at the mouth of a tidal estuary, bay, or lagoon. When the tide rises, water flows into the estuary through the turbine. The turbine spins a generator and produces electricity. As the tide drops, water flows back out through the turbine, again generating electricity. The greater the difference between high and low tides, the greater the potential for energy generation.
Tidal and current power is generally classified into three main types: barrage systems, tidal lagoons, and tidal stream systems.
Classic barrage systems are essentially dams placed across the width of a tidal estuary. They convert potential energy into electricity by capitalizing on the difference in water height across the barrage, in much the same way as a hydroelectric dam. Most traditional systems, like the world’s first tidal power system on the Rance River, use ebb generation methods, which means they only generate power at ebb or low tide.
Sluice gates open when the tide comes in, filling the basin behind the barrage. Once the tide starts to fall, sluice gates are shut, trapping the water in the estuary. Once the sea level drops far enough outside the barrage, water is allowed through the turbines to produce power. Once the water level in the estuary drops back to the same level outside the barrage, the turbines are shut down and the sluice gates reopened to begin the cycle again.
The alternative to ebb generation is flood generation, where power is generated when the tide is rising, as it is with the Sihwa Lake Tidal Power Station under construction in south Korea. However, it’s easier to maintain a large pressure differential across the turbine using ebb generation, so it’s rare to find a barrage system that uses the flood technique. It’s also possible to use the turbines as pumps, turning a barrage system into a large-scale pumped storage scheme.
There aren’t many barrage tidal power systems in the word today, in large part due to their high construction costs and environmental impact. Barrages alter the flow rate of salt-water into and out of the estuary. This can result in changes to salinity, oxygenation, suspended solids, silting, and solar penetration, all of which shift the estuary’s natural ecosystem.
Tidal lagoons are similar to barrage systems, but are often built as self-contained structures, reducing the environmental impacts from damming an entire marine estuary.
Power lagoons, like the one proposed in 2004 near China’s Yalu River, can be formed in shallow water off-shore by using impoundment walls and low-head turbine generators, like those used in run-of-river or micro-hydropower.
Like most renewable technologies though, tidal lagoons are not without controversy. Arguments over which approach to use for a proposed tidal energy project in the Severn Estuary have gone on for years. The debate is fiercest over questions about environmental impact, construction costs, and generation capacity.
Tidal stream systems are a different beast. Instead of converting potential energy (from the difference in water height across a barrage or impoundment wall), tidal stream systems convert the inherent kinetic energy of moving water currents to power turbines or hydrofoils. Tidal stream technology is still in its infancy compared to barrages. As a result, the technical approaches are still widely diverse.
Axial turbines, like those made by Marine Current Turbines and Verdant Power are the most common. They’re quite similar to the horizontal axis wind turbine that dominate the wind energy markets today.
Meanwhile, companies like Neptune Renewable Energy and Ocean Renewable Power Company are experimenting with cross-flow turbine designs, counterparts to the wind energy market’s vertical axis devices. These devices feature Darreius or Gorlov type turbines with helical blades that look a little bit like the old-style water wheels that used to power saw mills. However, instead of the water flowing over or under the wheel, it flows directly through the pitched blades, turning the turbine around an axis perpendicular to the flow.
Then there are some real innovators in the market, like BioPower Systems and others, who are developing oscillating devices powered by hydrofoils instead of turbines. BioPower’s approach is to use a bio-mimetic design (the tail of a swimming species like shark or mackerel, in this case) to generate power from the flow in ocean currents.
Tidal stream devices are an area of growing interest right now. Developers think such devices can be built more cost-effectively than barrage or lagoon systems which require heavy engineering for dams and impoundment walls. Tidal stream systems are also expected to have lower ecological impact, which is another big advantage. Theoretically, any of these designs could be used to tap the energy in thermal ocean currents like the Gulf Stream as easily as tidal currents, which opens up even more opportunity for growth in this sector.
It’s pretty clear from the variety of devices in play that the tidal stream market hasn’t converged on an optimal solution the way wind power has over the last decade. The technical challenges of tidal power are different than those wind power technologists have faced, so the solution this market converges toward may look very different than the large horizontal axis turbines that wind power developers have embraced.
State of the art
A sustainable energy source as predictable as the tides would be a real boon for utilities faced with rising demand for clean, renewable power. Unfortunately, tidal power isn’t a perfect fit for every market. As with wind power, tidal power tends to be concentrated in certain belts where geographic conditions favor strong tidal currents and large tidal swells.
As with wave energy, tidal power is an immature technology, and it faces challenges in survivability, energy capture, and regulatory restrictions. The survivability issues aren’t quite as severe as they are in wave energy systems, especially for tidal current schemes that can be submerged below the worst effects of severe ocean storms. The ocean is still a harsh mistress though, and designing for for such an environment adds an unavoidable cost component to any ocean power generation system. Regulatory concerns are the main barrier to further construction of barrage systems, and the impetus behind tidal lagoon developments in geographies where barrage systems might have been considered forty years ago.
There is a lot of activity in tidal power today. You can find a lot of companies dabbling in the space: joint ventures, prototype projects and big investments from firms putting their weight behind one technology or another. What will we have when the water clears? Hopefully, a focused suite of technologies to add to our growing arsenal of clean, sustainable power generation methods. At the rate world energy demand is growing, we’re going to need everything that we can get to work.
The text of this article was previously published on Associated Content.