A Brighter Outlook for the Other Tidal Power
by Ted W. Verrill, with Halcyon Tidal Power in Southport, Connecticut
[Ed. A discussion in the June 2014 Project Finance NewsWire suggested that tidal power is still some years away from being able to compete with other forms of electricity generation except in remote locations. The CEO of a tidal power company argues that his company has a form of tidal power that has the potential to reach scale more quickly.]
The “other” tidal power is tidal range power - power created from the head pressure of tidal waters against an enclosure.
Tidal range power is an existing technology to which the addition of key advancements could lead to worldwide deployment of a zero-emission renewable resource capable of satisfying 10% to 20% of global electricity demand, a scale that would have a meaningful impact on climate change in short order.
At scale, tidal range facilities can produce power at market rates for the first 20 to 30 years and below-market rates for the remainder of their 120-year useful lives, without subsidy. Tidal range power can provide all of this while also offering compelling financial returns.
Jigar Shah, founder of SunEdison - one of the world’s leading solar services companies - succinctly states the case for existing technologies in his new book Creating Climate Wealth: Unlocking the Impact Economy: “We must invest money in deployment of existing technologies that drive economic growth and jobs, demonstrate the scale needed to impact climate change and offer compelling returns.”
Why Existing Technologies?
Despite a few naysayers, climate change “waits for no man.” Experimental renewable energy development is useful, but is it likely to have a significant impact on climate change in the next 10, 20 or 30 years when, at its current pace, climate change is likely to become irreversible? According to the International Panel on Climate Change, irreversibility will likely occur when the Earth’s surface temperature passes the 4° C threshold. In fact, many climatologists suggest that if we do not leave 80% of known fossil fuels in the ground over the next 20 odd years, we will easily pass the point of no return. But this is a bit of a digression.
Even though wind power on terra firma is now technologically well-established - producing reasonable returns and offering market rate power - can we wait another 20 odd years of government subsidized experimentation and development for the next new renewable resource to leave the drawing board? Is it worth the effort when such experimental renewable resources may have other shortcomings - such as the inability to satisfy more than a small fraction of worldwide electrical demand, or have a relatively insignificant impact on climate change, or have an indeterminate intermittency, or have a low capacity factor?
Is it also worth the effort when there are existing technologies that with advancement or refinement can create climate wealth while having a significant impact on climate change now?
Tidal range power is an existing technology. Successful modern tidal range facilities have been in continuous operation for decades, including a 240-megawatt tidal range facility in La Rance, France (in operation for about 60 years) and a 20-megawatt tidal range facility in Annapolis-Royal, Nova Scotia (in operation for more than 20 years). Tidal range facilities have also been seriously considered in the Bay of Fundy, Canada for a century and in Bristol Bay, United Kingdom for at least as long. Indeed, “tide mills” (grain milling facilities using the release of tidal waters captured in a reservoir behind an enclosure to turn a water wheel) have been in operation since the Middle Ages with more than 750 operating in the United States and Europe during the 18th and 19th centuries.
Notwithstanding the successful operation of several modern tidal range facilities, they have not proliferated.
This has been due principally to the environmental harm created by existing facilities, the perceived or anticipated environmental harm expected from some of the newly-conceived facilities, and the high cost of construction. Rather than focus on remedies or advancements in technology to resolve these issues, developers, government agencies and other promoters have shifted their focus to hydrokinetic forms of tidal power, or “underwater windmills,” if you will. However, the development of hydrokinetic tidal power is at a pre-commercial stage and, therefore, cannot provide any certainty that deployment at scale is possible, and it is unlikely to have any meaningful effect on climate change in the next 20 to 30 years. It would be prudent to consider existing technologies that can be exploited now.
What are the key advancements to tidal range power that could bring the resource to the forefront of clean renewable power?
The key to bringing any technology to market, beyond its efficacy, is the cost to end users. If the cost of electricity generated by a particular technology is far beyond current market rates, then the technology may require years of subsidy until costs are contained or the technology is improved. As mentioned in the June 2014 issue of the Project Finance Newswire, the cost of electricity from existing in-stream tidal pilot-scale projects is at least $320 a megawatt hour. Nova Scotia is subsidizing the deployment of developmental and pre-commercial hydrokinetic tidal power devices at between $375 and $575 a megawatt hour. This is essentially the same evolutionary scenario that has played out with land-based wind power, with hydrokinetic sources of power potentially requiring several decades of improvements before commercial-scale deployment and power production at or about market rates.
Possible Cost Reductions
In order to make tidal range power economic currently, something had to be done to reduce the cost of construction.
Historical embankment or barrage construction has consisted of substantial quantities of stone and earth placed at the narrowest part of an estuary. Even then, the cost of construction has proven to be prohibitive with the width of the barrage increasing exponentially with the depth of the surrounding waters. This is essentially the same as levee construction on the US Gulf coast or dike construction in The Netherlands. Not only is this type of construction expensive, it has been shown to be short-lived: witness the effects of Hurricane Katrina in New Orleans.
The construction of the La Rance tidal range project included the cost of two cofferdams and required four years to complete. After 60 years of operation, it now produces power for less than $30 a megawatt hour; however, it took several decades to amortize the construction costs and bring electricity prices in line with or below market prices.
Tidal Lagoon (Swansea Bay) plc is currently considering a 320-megawatt tidal range facility using existing technologies in Swansea Bay, on the Welsh side of Bristol Bay. (See tidallagoon-swanseabay.com.) As the name of the company suggests, a tidal lagoon structure is being employed. Although similar to traditional embankment construction (a pyramid structure with a large base and narrow top), the project will use geotextile casings, known as “geotubes,” that will be filled with dredged sandy material from within the lagoon. Additional sand, small rocks and then larger rocks will protect the outer layer against degradation.
Unfortunately, this construction methodology may retain some of the shortcomings of embankment construction: it is meant for shallow locations as it becomes prohibitively expensive to construct at depth, construction time may be rather lengthy and removal is difficult at best. The low-head bulb bi-directional turbine technology to be used is similar to what Halcyon will deploy, although Swansea Bay will use a Kaplan-style runner blade. While Halcyon believes that its tidal range patented construction and operational methodologies are superior to those of Swansea Bay, the cost, scale and near-term deployment using existing technologies are probably sufficient to move the Swansea Bay tidal lagoon concept forward.
Rather than inventing a completely new technology to replace typical barrage or embankment construction, Halcyon has borrowed two existing technologies – pile-supported technology perfected over several decades in the offshore oil and gas and bridge construction industries, and reinforced concrete resistant to seawater from the bridge and dam construction industries. (An article in The Washington Post on July 13, 2014 about the 50th anniversary of the construction of the Chesapeake Bay Bridge and Tunnel illustrated the use of concrete pilings driven in sand - not bedrock - to support the bridge.) By setting pilings in bedrock, prefabricating concrete panels on shore, floating or barging the panels to the facility site and locking them between the pilings, Halcyon has reduced the cost of constructing a tidal range enclosure by 50%, reduced construction time by 50% and permitted construction of a “Halcyon enclosure” at depth and over long distances, creating tidal lagoons that avoid sensitive estuarine environments.
This prefabricated modular construction method permits step-wise construction, carried out almost exclusively from the water side, without the need for a cofferdam or other construction enclosure. It also permits ready decommissioning of the facility at the end of its useful life, an unlikely event for traditional embankment or barrage construction. Illustrations of the Halcyon enclosure can be found on the Halcyon website at hal-cyontidalpower.com.
What does a 50% reduction in cost mean to the price of electricity provided by a Halcyon tidal range facility? In the first instance, it means that special government subsidies are unnecessary to support the development, construction and operation of the facility. Depending on the jurisdiction, it may also mean that such a tidal range facility will produce power at current market prices out of the gate. In almost all cases, it means that the price of electricity produced will be below market prices after amortizing the cost to construct over 20 years.
The ability of a Halcyon tidal range facility to avoid sensitive estuarine environments is a perfect segue to the further reduction or elimination of environmental harm caused by traditional tidal range facilities, the remaining key to the proliferation of tidal range power. Halcyon can now locate its tidal range facilities along shorelines, creating tidal lagoons that do not enclose sensitive estuaries. Along with a construction methodology that limits or avoids environmental harm, Halcyon has also engineered an advanced operating cycle from existing technologies. In collaboration with the hydro turbine division of Alstom Power, Halcyon has taken bulb turbines off the shelf and will be deploying them horizontally at the base of its enclosure, generating power at both the ebb and flood tides as well as using them as high-volume pumps during slack tides to realign sea water (within the basin created by the Halcyon enclosure) with the natural intertidal zone.
This effort is critical to maintaining natural hydrology, preventing sedimentation and otherwise maintaining the marine environment. As feeding or migrating fish and invertebrates move in and out of the basin, care must be taken to assure that most, if not all, are able to do so unscathed. Most of the fish and invertebrates will have little difficulty moving through the three-meter diameter turbines themselves or through the sluicegates, which will be open at various times throughout a tidal cycle. However, in order to accommodate the normal activity of marine life, Halcyon and Alstom have further modified the bulb turbines by reducing the number of impellers from four to three, by thickening the leading edges of the impellers and by reducing the speed of the impellers through gearing modifications. Furthermore, safe-guards will be put in place to prevent cetaceans or other large marine mammals from entering the basin or at least guiding them away from the turbines. Finally, Halcyon will make special accommodation for unique species that use the basin on a case-by-case basis.
Another complaint levied against tidal range power is the use of an enclosure. Many detractors have likened the enclosure to a dam. The definition of a dam is a barrier that impounds water or diverts water from its natural course. Hoover Dam is a dam because it impounds a large portion of the water volume from the Colorado River in Lake Mead. Existing tidal range facilities share most of the attributes of a dam: they impound water, produce power only on the ebb tide, change the characteristics of the intertidal zone and ultimately fill the basin with sediment.
This is not the case with a Halcyon tidal range facility, which produces power on both the ebb and flood tides without impounding or diverting water from its natural course, maintains the natural intertidal zone, with pumping if necessary, and sustains the natural hydrology of the water, preventing sedimentation. While obviously of lesser importance, a Halcyon enclosure does not look like a typical dam either, as over 75% of the enclosure lies permanently below the surface of the sea. The portion of the enclosure sitting above the water line can be modified to accommodate aesthetic considerations.
Where are tidal range facilities likely to be deployed?
The website GreenRhinoEnergy.com suggests that there are only five regions in the world where tidal range power could be generated. It is assumed that this site determination was based on the economics of those facilities where the tidal range is more than eight meters (the distinguishing criteria apparently used). This conclusion appears to be drawn from facilities using typical embankment or barrage construction rather than the far less expensive Halcyon enclosure construction methodology. With Halcyon advancements, a tidal range facility can be constructed efficiently and economically, on five continents, not just five regions, as well as many more locations on these continents. Please see the seminal work on tidal power by L.B. Bernshtein, Tidal Energy for Electric Power Plants, published in 1961 and translated from the Russian in 1965, for a discussion of these locations.
Halcyon is currently developing a 25-megawatt facility in Cobscook Bay, Maine and considering an 1,100-megawatt facility in Scott’s Bay, Nova Scotia. It is also proposing to develop a facility on the English side of Bristol Bay. The Cobscook Bay power plant, sized to be the “showcase” Halcyon tidal range facility, is economically viable at five meters of tidal range. In sum, Halcyon intends to construct its facilities as tidal lagoons along shorelines in places where the tidal range is approximately five meters or more, such as the Bay of Fundy and Bristol Bay, where, in actuality, several of these facilities could be considered without altering the natural hydrology.
Tidal range power deserves a serious second look by the renewable energy and clean tech sectors and, in particular, the ocean energy subsector.