ENVIRONMENTALLY FRIENDLY UTILITY-SCALE MARINE POWER
Although tidal range has a proven operational track record, adverse effects on the marine environment have been a major obstacle to global deployment. Tidal power presents complex and interconnected challenges which can only be addressed using an integrated approach. Over the last decade, Halcyon Tidal Power has pioneered multiple innovations that directly address and solve the problems associated with tidal power. Collectively, these form the Halcyon Solution, an integrated system which generates affordable utility-scale power from the tides while preserving the ecological integrity of the marine environment.
THE PROBLEM: Conventional Tidal Power Damages the Marine Environment
Historically, the process of extracting power from the tides (the operating cycle) has led to a major disruption of the natural tidal cycle. All conventional operating cycles reduce the net flow of seawater in and out of the basin (see “What is tidal power” for the principles of operation). This flow reduction has led to a succession of adverse effects.
Reduction in flow leads to a reduction in tidal amplitude and therefore a reduction in the intertidal zone, the area of the shoreline which is alternately submerged and exposed by the rise and fall of the tides. The intertidal zone is a critically important marine environment. It is an essential habitat for a diverse assemblage of organisms including fish, which use them as nurseries, hundreds of different invertebrates that burrow in the mud, an array of shellfish, and migrating and resident shorebirds that feed on the mud-flats. Many of these intertidal zones are protected under the RAMSAR convention. The ecology of the macro-tidal environment is dependent on maintaining the natural tidal flow. The Severn Barrage, a massive 8800 MW ebb generation project proposed in 1983, 1989, and 2008 in the UK, would result in the loss of about 15,000 hectares of intertidal zone, carrying a compensatory cost for habitat loss ranging between £690 million to £2 billion!
Conventional tidal range facilities also increase the residence time that water is held in the basin, which results in siltation, just like the effect of hydroelectric dams. Progressive siltation has been observed in the basin of the 240 MW La Rance tidal power plant in France. Eventually, an enclosed basin becomes filled with silt, rendering the tidal power plant inoperable. Additionally, increased residence time of organic nitrates results in (sometimes toxic) algal blooms.
THE HALCYON SOLUTION: The Free Flow Power Cycle
Halcyon’s Free Flow Cycle was developed to fully preserve the volume of tidal water flowing in and out of the basin, which retains the natural intertidal zone and avoids sedimentation. The cycle employs horizontal bulb turbines which can generate power in both directions, as well as serve as high volume pumps. These capabilities make them ideal for use in generating tidal power. The Free Flow Power Cycle is illustrated in the figure below. Sea water passes back and forth through the tidal power plant just as it does during the flood and ebb of the tides. Additional pumping phases during slack tides keep the tidal amplitude at its naturally occurring levels, and residence time in the basin unchanged. The result is that the tide rises and falls in the basin just as it would in the absence of the tidal power plant, maintaining the integrity of the intertidal zone and preserving water quality.
THE FREE FLOW POWER CYCLE
It is important to point out that pumping generates more power than it consumes. Since pumping occurs during slack tides when there is a smaller head, the amount of power required for pumping is minimized, while the increase in head created by pumping provides an incremental increase in output. The interested reader can consult numerous technical accounts for more detail (for example, LB Bernshtein, Tidal Energy for Electric Power Plants, translated by Israel Program for Scientific Translation, Published by US Dept. of the Interior & the National Science Foundation). The result is a generating cycle that preserves the natural ebb and flow of the tides, floods the intertidal zones to their natural levels, eliminates sedimentation, and maintains natural residence times.
VALIDATION OF THE FREE FLOW CYCLE USING ADVANCED MODELING
The Free Flow Power Cycle was modeled in detailed computer simulations carried out by Alstom Power, the world’s largest hydroelectric power equipment provider. A graphical representation generated by Alstom is shown to the right. The blue line represents the natural rise and fall of the tides. The violet line represents the rise and fall of the water in the basin once the tidal power plant is in operation. The graph shows how the rise and fall of the tides in the basin mimics the natural rise and fall of the tides, except for a brief delay. Technical studies carried out by Halcyon Tidal Power in collaboration with Alstom show that the cycle has a capacity factor of 39%, higher than all other existing tidal cycles.
To further preserve the natural flow, Halcyon will use smaller diameter (3.2 m) turbines, which can be more widely distributed across the marine enclosure. This wider distribution of tidal flow into and out of the basin more closely reflects the natural tidal flows. While less widely distributed larger turbines in the enclosure may generate the same quantity of power as a larger array of smaller turbines, the ability to replicate the natural tidal cycle and the concomitant maintenance of the intertidal zones and prevention of sedimentation is significantly diminished.
FISH FRIENDLY TURBINE INNOVATIONS
Halcyon has worked closely with Alstom Power to modify their bulb turbines to protect fishes, which can, in most instances, pass directly through the turbines! These innovations include a reduction in the number of impeller blades from 4 or 5 to 3, and the use of a gearbox to maintain high generator speed while reducing the impeller speed to a low 92 rpm. These modifications reduce the risk of physical fish strikes by the turbine blades, as well as damage to fish bladders due to significant changes in water pressure gradients caused by high impeller speeds.
The relatively small diameter 3.2 meter turbines further reduce fish mortality because the blade tip spins more slowly than a larger turbine. Finally, the leading edges of the blades have been thickened significantly permitting fish to slide off the impeller blades, all without reducing the output or efficiency of the turbine. Halcyon continues to innovate towards zero fish mortality.>
SMALL FOOTPRINT – LOW IMPACT CONSTRUCTION
Halcyon’s Modular Pile Supported Construction provides an environmentally sound alternative to conventional construction because it requires minimal dredging and takes up a fraction of the sea-floor space. Conventional construction methods require massive structures to support tidal loads.
The two conventional methods of constructing marine enclosures are embankments and caissons. Embankments (pictured right) are essentially long sloping mounds of rock, concrete and sand, but they are economical only in shallow water, growing exponentially in volume and cost with increasing water depth. They destroy large areas of the sea floor, and once built, these massive structures are essentially permanent.
Caissons (pronounced “kay-sonns”; not pictured) are large reinforced concrete boxes placed side by side to form the marine enclosure. Caissons can be powerhouse caissons containing the turbine generators, sluice caissons to control the flow of water, and blank caissons tying the structure together. Caissons can be 70 meters in length and weigh 60,000 tons, and must rest on a flat surface. Therefore the seabed must be dredged using suction cutter dredgers, removing millions of cubic meters of surficial material and rock, destroying the sea floor.
Halcyon’s novel modular design employs pile supported construction using large diameter methods initially developed for supporting offshore oil & gas platforms. Providing piling support removes the need for massive embankments or caissons, with the marine enclosure only about 3 to 4 meters in width. Its footprint is a tiny fraction of an embankment or caisson. Seafloor modifications required for the Halcyon Marine Enclosure are negligible compared to those required for caisson construction. By providing piling support the Halcyon Powerhouse is lighter and smaller than a conventional powerhouse. Both the footprint and construction impact are greatly reduced, thereby reducing the environmental impact.
FLEXIBLE SITING TO AVOID ECOLOGICALLY SENSITIVE AREAS
The modularity of Halcyon Tidal Power Plants allows them to be configured as barrages or shore connected lagoons. Lagoons bring to bear all of Halcyon environmental innovation. The Free Flow Cycle can be implemented for many different sizes and types of lagoons. Lagoons can be built along any coastline with a tidal range greater than 5 meters, which greatly expands the potential of tidal power around the world. The amount of power potential from lagoons worldwide is many times that of barrages, contributing at a significant scale to GHG reduction. Lagoons can also be sited away from sensitive estuaries and spawning rivers, further reducing environmental impact. Fish friendly turbines can be incorporated into each lagoon further reducing fish mortality.
Barrages are an important but secondary application of the Halcyon Solution, because the Free Flow Operating Cycle is not universally applicable to large barrage basins. Furthermore, our construction methods eliminate the cost advantages that barrages once held over lagoons. Because barrages typically span the seaward mouths of rivers, extra care must be taken to prevent negative environmental impacts. Our technology prevents negative environmental impacts when properly sited, and therefore we do intend to build our first facility in a barrage configuration, especially because it represents one of the few modest sized tidal projects available for an initial project. Offshore lagoons have also been considered by some companies but we do not see them as being cost competitive.
At the end of its 120+ year economic life, a Halcyon Tidal Power Plant can be fully and economically decomissioned. All the major elements can be refloated and towed away. Piles are cut to the seafloor level using methods developed by offshore oil and gas. Once decommissioned, a Halcyon Tidal Power Plant leaves no visible trace. This is in sharp contrast to embankments, which are permanent structures. Decommissioning is a major environmental issue for all power plants and one that has been poorly addressed by the tidal power industry to date.