The first live test is underway off Maine for a new technology that relies on strong tides and currents to power underwater generators. It took innovative engineering and precision execution to make the first test happen, but it could open the door to a new renewable energy source. In Europe, South American and Japan, which have expensive grid power costs, tidal energy is expected to become competitive. This article delves deeper into the engineering behind underwater turbines.
On A Cloudy Mid-August Afternoon
Chris Sauer, president of Ocean Renewable Power, fixated on a barge-mounted crane slowly lowering a 100-ft.-wide by 18-ft.-tall set of tidal turbines onto a support frame anchored to the bottom of Cobscook Bay.
The 150 kW TidGen power system was Ocean Renewable’s first full-scale attempt to prove it could generate electrical power economically from ocean tides. The 40-ton system was so large, the team was unable to match it with the frame on land.
“They had to be within an inch tolerance,” Sauer said. “We took every measurement possible, made final adjustments and completed computer simulations to predict how the units would behave, but until the actual ocean installation of the structure, it was impossible to predict the outcome.”
Cobscook Bay was not making it easier. Located on Maine’s northern coast, it lies just off Bay of Fundy, home of the world’s most powerful tides. Twice daily, 160 billion tons of water flow in and out of Fundy, lifting the water level in some places in Nova Scotia by 50 ft. or more.
The crew had only a 40-minute window to install the huge turbines when the currents slowed before the tide began to reverse.
Soon after the barge and divers began lowering the 40-ton system, the wind picked up and lightning flashed in the distance. Sauer worried that the approaching storm would churn the water and scuttle the operation.
“When the radio would go silent, I thought something was wrong,” Sauer recalled. “But in the end, we nailed it. The components matched up perfectly. One of the divers said the unit didn’t even scratch the paint.”
A month later, he received a call from his chief technology officer, Jarlathe McEntee, saying the device had just delivered the first electricity to the Emera Maine grid.
“That was a big moment,” Sauer said.
It was also an important step for the roughly 14 U.S. and 60 international companies developing technologies to harness the kinetic energy of rivers, oceans, tides, and currents, according to the U.S. Department of Energy.
More than half the U.S. population lives within 50 miles of coastlines. DOE believes tidal power could provide clean, renewable electricity for many of them.
As a power source, it is as predictable as the tides. There are two tidal cycles daily and a transition period in each cycle called slack tide when currents are slow. On an average day, Sauer estimates a tidal energy device would generate power for 20 out of 24 hours, with the output varying with current flow along a sinusoidal curve.
“The output is variable but totally predictable,” he said. “At any given minute, you can predict exactly how much energy you’re going to be generating.”
Despite its perilous tides, the Bay of Fundy region is an attractive resource. According to Fundy Ocean Research Center for Energy, models suggest 50,000 MW of energy exist in the Bay of Fundy.
Ocean Renewable wants to extract some of it. It began testing turbines in Cobscook Bay in 2009. When the 150 kW Cobscook Bay TidGen project went live in September 2012, it became the first revenue-generating tidal project to deliver power to a grid in the United States.
Yet it was only the first step in a long journey. Ocean Renewable still has to prove that ocean tides—and river currents—can be a competitive source of renewable power.
Commercial tidal power is not new. France’s Rance Tidal Power Station opened 50 years ago. It uses a dam-like tidal barrage to trap water during high tide and release it during low tide. Despite a peak capacity of 240 MW, it averages 57 MW overall. Projects like these are very expensive and have very long payback periods.
Tidal stream generators, another technology, work like underwater wind turbines. Because water is denser than air, flowing water carries more power than wind. A turbine in the Bay of Fundy’s 10 mph water current can generate as much electricity as a similarly sized wind turbine in a 90 mph wind. Of course, it takes expensive, heavy-duty construction to withstand such powerful currents.
Ocean Renewable’s TidGen is also a stream generator, but its cross-flow design uses twisted composite blades that look like the business end of a manual lawn mower.
TidGen’s 150 kW unit mounts four sets of these blades on a single shaft, supported by a composite and steel frame. As water rushes through, the blades turn at 30-60 rpm. This produces low-speed/high-torque energy that a Rolls-Royce permanent-magnet generator turns into electricity.
After retrieving the TidGen unit and analyzing with data collected during 2013, the team realized the system was still not fully reliable and too expensive to deploy.
“We were way above estimates,” Sauer said.
“We had to determine what caused the biggest headaches in terms of unplanned outages, identify which components were prohibitively expensive, and find opportunities to improve the design to increase extraction efficiency,” Sauer said.
First, Sauer focused on “nagging issues,” such as bolts that came loose in strong tidal flows and caused unplanned outages.
“We had to redesign how we connected the blades to the struts that hold it to the shaft,” he said. “All of the bolted connections had to be redesigned to assure we wouldn’t have any loosening bolts.”
Bearing friction from the drive line was robbing up to 15 percent of power output and wearing out the bearings.
“We probably would have had to replace them every year or two,” Sauer said.
Working with European bearing suppliers, Ocean Renewable developed a new bearing arrangement that virtually eliminated friction and had a five-year design life.
Then they turned to massive deployment costs.
“Installation of the bottom support frame cost three times more than we thought,” Sauer said. “Pile driving and deploying a big crane barge costs a half a million dollars per month and when you have fog, wind, and bad weather, you’re still paying that charge.”
First, the engineers replaced the frame with a gravity-based structure, a type of heavy concrete or steel system commonly used to anchor buoyant offshore oil platforms or wind turbines.
Then they added an elliptical buoyancy pod to TidGen. This made it buoyant enough to tow into place. By letting air out of the pod, they could lower it to the level where currents ran strongest. They will then moor it into place with tension cable attached to the weighted structure below.
The new TidGen 2.0 will produce up to 600 kW of power when deployed in Cobscook Bay in 2020. An array of ten generators could one day produce 6 MW of power.
While tidal power cannot yet compete with cheap natural gas in Maine, Ocean Power believes it makes perfect sense in towns like Igiugig, Alaska.
The isolated settlement of 80 is situated hundreds of miles southwest of Anchorage, where the Kvichak River spills out of Lake Iliamna, Alaska’s largest lake. Without direct road access, the cost of delivering diesel fuel for the village’s generator drives electricity costs to $0.80/kWh.
Ocean Renewable believes a smaller version of its TidGen cross-flow generator—RivGen—could tap the Kvichak’s current to slash electricity costs and limit diesel use to emergencies only.
“We have determined the best early-adopter market for this technology is these remote communities on diesel micro-grids,” Sauer said. “Throughout North America, there are hundreds of communities that rely 100 percent on diesel. Fuel delivery to these remote areas jacks up the cost, but most of these communities are sitting on very good river or tidal resources.”
Armed with a $2.3 million Department of Energy grant, Ocean Renewable plans to install a full-scale, 35 kW river system at Igiugig this June (or as soon as the ice melts), said Monty Worthington, the company’s director of project development. A second system will follow within two years.
RivGen is essentially a smaller TidGen designed for rivers as shallow as 18 ft. Two turbine blades and a generator are mounted between two floating pontoons, so it looked like a catamaran without a mast. Filling the pontoons with water would submerge the unit into position.
Ocean Renewable built a 25-kW RivGen prototype in 2014 and tested it in Maine. Then it had to ship it to Igiugig.
First, the team disassembled the unit and sent the components to Anchorage in trailers. Since there are no roads through the mountainous north side of Cook Inlet, which connects Anchorage with the Gulf of Alaska, they drove the trailers 200 miles down the south side to Homer.
From there, they boated the trailers 80 miles across Cook Inlet, drove them 20 miles down a dirt road (open only in summer) to Lake Iliamna, and floated them 80 miles across the lake to Igiugig.
“One of the project’s goals,” Worthington said, “was to prove we could handle all the remote logistics.”
The entire operation took about a month. Then the engineers discovered the bolts did not line up with the pontoon, which had been fabricated in Alaska. Fortunately, Jocko Niemi, a talented village welder, was able to make critical modifications.
Next, the team tested whether it could tow RivGen on Lake Iliamna using a local landing craft and a converted fishing vessel.
“We wanted to make sure we could push the device fast enough with the local vessels so we could move it against the river’s current, but it had too much drag,” said Ryan Tyler, a project manager at Ocean Renewable.
Plan B involved a 30-ft.-by-80-ft. barge from a nearby village, which they had used to deploy Riv-Gen’s 10,000 lb. concrete mooring anchors.
“We knew we could push the barge in the river,” Worthington added. “So we considered, ‘What if we put the RivGen on it, push it into position, attach RivGen to the mooring system, and then pull the barge out from underneath it?’ It’s like pulling a tablecloth out from underneath your dishes on your table.”
The idea was dodgy, especially since they had to find a way to tip the barge’s nose into the rapidly moving river to slide RivGen into the water.
The solution they devised was to drive two excavators onto the barge and have them slowly crawl forward until the nose tipped downward. Too far, and the river would flood the barge, which would be held down by the tension cables running from RivGen to the anchors.
After a thorough analysis, the team decided it wasn’t too risky to rule out.
“Everything went super smooth like we planned, but the crazy operation using excavators never became standard operating procedure,” Worthington said.
Over the winter, the team redesigned RivGen. They eliminated a cross-member between the two pontoons to reduce drag, so that a single vessel could push the RivGen turbine and generator to the mooring location.
They also slapped a hydrodynamic fairing onto the front of RivGen to funnel flowing water into the turbines, improving efficiency by 30 percent.
The second deployment in 2015 went much smoother. When connected to Igiugig’s microgrid, it generated up to 33 percent of the town’s power, a huge milestone.
Still, there were snags. The unit used an “umbilical cord,” a sheathed cable containing air and water hoses, to sink and refloat the device. Its position was marked with a buoy with a cable going down to the cord. When the team tried to pull it up, the cable snapped.
The river had buried the umbilical under several feet of gravel and strong currents made diving impossible.
Again, the team had to improvise. They rigged a high-pressure water hose to an excavator bucket to blow away the gravel. Then, they attached a camera to a long hook so they could pull the umbilical to the surface.
“After retrieving the umbilical hoses, the crew was able to pump air into the RivGen pontoons to deballast and float the system,” Worthington said.
“Pulling up the RivGen was the only other option, but that required an enormous amount of lifting capability we didn’t have in the village,” added Worthington.
So far, 10 customers in remote communities of Alaska and Canada have shown an interest in Riv-Gen’s performance. “They want projects to replace diesel,” Sauer said.
Like many successful startups, Ocean Renewable has found a way to build production volume and move its technology forward even though its original TidGen is not yet competitive with low-cost natural gas. Sauer believes RivGen 2.0, which will be installed this coming June, can compete in communities paying more than $0.50/kWh electricity.
Yet the company remains committed to ocean power. As engineering barriers fall, the emphasis is on overseas installations.
A half-size version of TidGen is scheduled for installation in Quebec in November 2019. Sauer puts the market potential of Canada, which seeks to move to green and renewable power, at more than $1 billion. It is clearly Ocean Renewable’s essential first market, Sauer said.
The company is also looking at Cook Inlet southwest of Anchorage. The tides there are the highest in the United States and have a potential 18 GW of extractable tidal power, the equivalent to 119 TWh per year, Worthington added.
In Europe, South America, and Japan, which have expensive grid power costs, tidal energy is expected to become competitive as TidGen technology improves, Sauer said. For domestic markets, he predicts costs below $0.15/kWh within 15 years.
“The biggest markets for us in the next decade or two are going to be outside the U.S.,” Sauer said.