by David Hoyle
A Tide Mill Institute email subscriber recently asked TMI why historical tide mill sites are not being used to generate electricity. It’s a fair question: for an 8-foot tidal range, each acre of a tidal basin could power one typical household. The simple answer: the multiple hurdles to be overcome to implement tidal power generation seem to outweigh the benefits. In other words, there does not seem to be a need for tidal power compelling enough to motivate an individual, community, or developer to overcome those hurdles.
First, let’s remember that a historical tide mill required a dam to impound water at high tide and used the difference in head between the tidal basin and the sea to generate mechanical power. Producing electricity in this way is called tidal range generation. (Generating electricity by placing a tidal turbine in tidal currents, without a dam, is called tidal stream generation. While tidal stream generation was not common historically, it has become the de facto standard for new tidal electricity generation projects today.)
Obstacles for Tidal Range Power Generation
In cases where a historical tide mill dam has been transformed into or replaced by a causeway with culverts to permit limited water exchange, the former tidal basin has typically become a lake with little tidal amplitude. Property owners surrounding the lake would be understandably reluctant to support a power generation project that would transform their stable lake back into its original, tidal state, filling and emptying twice a day.
This said, organizations such as the Kennebec Estuary Land Trust (KELT) in Maine are working to remove such tidal restrictions in order to (a) restore the health of tidal marshes and (b) reduce the risk of road washouts caused by storm surge combined with sea level rise. I have spoken to numerous people involved in such efforts about combining tidal power generation with their marsh restoration efforts, but I have never gotten any serious interest. A Maine Department of Transportation hydrologist told me the idea was interesting, but that I shouldn’t expect any support from the DOT!
In cases where a historical tide mill dam has been removed, there will be little appetite to build a new dam, mainly for environmental reasons. The prevailing tendency in Maine is to remove dams rather than build new ones.
One historical site that has been explored is the former tide mill site on Vinalhaven Island. There, the tidal basin still fills and empties through tidal sluices in the extant historical dam. A tidal energy company recently tried to work with the property owner to develop tidal power generation, but the project was ultimately scrapped. On the positive side, the developer told me that the cost of retrofitting tidal energy generation equipment to existing structures is nominal compared to building out new infrastructure.
The U.S. Department of Energy’s Testing and Expertise for Marine Energy (TEAMER) program helps researchers and developers test tidal energy technology and assess tidal energy resources; the TEAMER rules state, in essence, “your project shall not involve a dam!”
Tidal energy researchers and technology developers such as ORPC and Emrgy are generally focusing on tidal stream, riverine, or irrigation channel energy these days. This means that turbines suitable for small scale, low head tidal range generation are not readily available.
There are also significant permitting and capital-cost hurdles to be overcome for tidal power generation installations.
Federal Approvals
Non-grid-connected projects still require a license from the Federal Energy Regulatory Commission (FERC) if they are in navigable waters. Small projects should apply for a Pilot Project License, which is specifically streamlined for small-scale, short-term, removable experimental systems.
The project must secure a US Army Corps of Engineers Section 10/404 permit. A small-scale project may qualify under the USACE Maine General Permit for temporary structures or hydrokinetic testing.
State Approvals (Maine)
Maine offers a “general permit” explicitly for small-scale demonstration projects. This permit is called Maine DEP General Permit for Tidal Energy Demonstration Projects.
If the turbine is anchored to the ocean floor, the project must secure a Submerged Lands Lease or easement from the Maine Bureau of Parks and Lands.
Local and Environmental Consultations
Department of Marine Resources (DMR): The project must consult with the Maine DMR to prove that the deployment will not interfere with local commercial fishing, navigation, or marine mammal migration.
Municipal/LUPC Review: The project must ensure compliance with local Shoreland Zoning rules if the infrastructure touches the shore.
Capital Costs
Finally, there is the capital cost of the project to be financed, and ultimately, the cost of the produced electricity per kWh; these cost figures are not easy to come by. Electricity produced by ORPC’s RivGen turbine in Igiugig Alaska seems to be cost effective when compared to electricity from diesel generators costing approximately $1.00 per kWh.
About the author: David Hoyle is a member of the TMI Leadership Committee and TMI’s representative at the National Hydropower Association and the Marine Energy Council. David also participates in the Marine Energy Council’s Priorities Working Group, which has surveyed marine energy developers and researchers to establish priorities for funding by the U.S. Department of Energy.