The New York Power Authority is determining whether the use of the Alden turbine during rehab of its 12-MW Crescent plant can help alleviate some of the environmental issues associated with fish passage at NYPA hydropower projects.
By Peter Ludewig, George Hecker, Norman Perkins and Paul Jacobson
When the New York Power Authority (NYPA) in the U.S. decided to learn how it could improve fish passage at its hydroelectric plants, the company looked into the Alden turbine. This new turbine design has features, such as only three blades, relatively thick rounded leading edges and no gaps between moving and stationary parts, which dramatically increase fish passage survival. The Alden turbine has been tested at reduced scale for performance and fish passage but its use at Crescent would be the first full scale application. Power generation modeling shows that the Alden turbine would provide a small (about 5%) increase in annual power.
The desire to be proactive was due, in part, to fish passage concerns expressed in the Federal Energy Regulatory Commission (FERC) relicensing process of a hydropower plant located just downstream from Crescent. However, NYPA did own the downstream hydro project.
NYPA is one of the largest state public power organization in the U.S. and has a total combined generating capacity of about 28.7 billion kWh per year from 16 plants.
NYPA owns seven hydropower projects: 800-MW St. Lawrence-Franklin D. Roosevelt on the St. Lawrence River in Massena, the 1,160-MW Blenheim-Gilboa pumped storage in the Catskill Mountains north of Blenheim and Gilboa, 2,441-MW Niagara on the Niagara River in Lawrence, 9-MW Gregory B. Jarvis at the Hinckley Reservoir in Oneida County, 5-MW Ashokan at the Ashokan Reservoir in Ulster County, 12-MW Crescent in Albany and Saratoga counties, and 12-MW Vischer Ferry on the Mohawk River in Schenectady and Saratoga counties.
NYPA also operates nine natural gas-fueled power plants and produces some of the cheapest electricity in North America.
In essence, a fish-friendly turbine is one that provides at least 95% survival during fish passage through a turbine. At the same time, this turbine needs to offer similar efficiency and capacity to other modern turbine designs.
The technology discussed here was conceived by Alden Research Laboratory Inc. (Alden). The aptly named Alden turbine has been designed and model tested, and the next steps included building a full-scale prototype for field testing.
This technology development received resources from the Electric Power Research Institute (EPRI), U.S. Department of Energy (DOE), Electricite de France, NYPA and New York State Energy Research and Development Authority. Other supporters include Brookfield Renewable Power, Dairyland Power Cooperative, Voith Hydro, Puget Sound Energy, SCANA Corp. and Southern Co. All of the contributors seek the shared goal of creating an efficient, advanced fish-friendly turbine design.1
The new turbine, under development since 1995, is designed to minimize fish injury and mortality during passage through the turbine, which may help offset the need for other measures (i.e., intake screens, fish passage spills and alternative downstream fish bypasses) that can be cost-prohibitive and decrease power generation.
In theory, the new design reduces injury to fish because the Alden runner only has three blades and there is no clearance between the blades and their housing (crown and discharge ring). A shroud attached to the blade tips rotates with the runner. With the exception of small areas around blade leading edges, pressure and velocity (shear) gradients meet established bio-criteria for safe fish passage.
Alden turbine on Mohawk River
The Alden turbine was first designed for deployment in New York for the 38.8-MW School Street Project, owned and operated by Erie Boulevard Hydro L.P. The scheme includes a dam, located on the Mohawk River, downstream of Crescent, that was constructed in 1831. The Alden turbine was intended as a totally new unit housed in an addition to the existing power house. Its possible application at Crescent would be as a retrofit, replacing the existing runner but using all other components of the existing turbine
Power generation at School Street commenced in 1916, and additional generating units were added in 1922 and 1925. The project’s 16-foot-high dam, located about 4,000 ft above Cohoes Falls, impounds a 100-acre reservoir. Water is diverted to a power canal, conveyed to a powerhouse just below Cohoes Falls and then returned to Mohawk River.
FERC issued an original license for the project to Niagara Mohawk on June 11, 1969, with a term expiring on Dec. 31, 1993. FERC, in the relicensing effort for School Street that began in 1993, included as a condition that the project must employ methods and quantifiable measures that address fish survival. New license conditions included use of an Alden turbine to enhance fish passage survival if the latter was equal to or greater than survival using a new conventional fish bypass. During an energy generation study with and without an Alden turbine added to the project, it became clear that any additional unit would not be economical. The existing units effectively used the available river flow. Therefore, application of the Alden turbine, or any other type of turbine, at School Street was not pursued.
As designed for School Street, the Alden turbine was about one third larger than a conventional unit, which generally makes the Alden turbine a more costly alternative. To minimize this factor, EPRI and Alden initiated a study to re-design the Alden runner for use in a retro-fit application, for which only the runner would be replaced in an existing turbine. NYPA was receptive to using Crescent as a prototype for this study, which was initiated in 2012.
Site conditions differ significantly. School Street has a 92 ft (28 m) net head compared to the 26.5 ft (8.1 m) head at Crescent provided by two curved (crescent shaped) dams in the Mohawk River. The Alden unit proposed for School Street, when scaled down to fit in the available space at Crescent Unit 1, indicated the Crescent unit’s power output and efficiency were not optimized using this design.
As a result, NYPA had to pursue developing a unique Alden turbine design that included greater capacity at a reduced runner diameter for installation at Crescent. This involved re-design of the runner blade geometry to be compatible with the existing Crescent wicket gate height.
The current Unit 1 at the Crescent powerhouse is a 2.8.-MW Francis turbine. Voith Hydro worked on the re-design of an Alden turbine to accommodate the conditions at Crescent.
Maintaining the same circle intersecting the center of all wicket gate pivots currently being used in Crescent Unit 1 would allow reusing the existing turbine’s flow distributor. Additionally, by using the existing generator, spiral case and draft tube, the main performance goal for the redesigned Alden runner would be equal to or higher capacity and energy production, compared to the existing Francis unit. In addition to performance goals, a significant increase in fish survival during turbine passage was to be achieved.
Using the existing generator (or rewinding the existing generator at some later time) fixes the rpm of the turbine runner. Thus, the re-designed Alden runner must operate at the same speed as the existing runner rather than at an optimum speed for the Alden turbine.
During the re-design phase by Voith, a number of features were included:
• Increase the Alden runner wicket gate height to current Unit 1 dimensions to help boost unit capacity to 3.1 MW from 2.8 MW;
• Select a semi-circular 8-in (203-mm) leading blade edge thickness to increase survival from blade strikes during fish passage; and
• Use a higher-flow runner rather than an optional lower-flow runner to increase energy production by an average of about 5%.
The diameter of the runner discharge is smaller than the diameter of the existing draft tube entrance. This requires a transition to be inserted below the runner. CFD simulations indicate the area expansion and use of the existing hydraulically inefficient draft tube result in an Alden turbine efficiency peak value of about 88%. This is less than the peak efficiency of 94% for the Alden turbine designed for School Street which used optimized flow conveyance components.
No cavitation is indicated at this design point and although cavitation is minimal at other design points, cavitation can be addressed in the final hydraulic design.
The energy analysis assumed an allocation of river flows distributed between 250 cfs low (habitat) flow releases over the crescent shaped dams (not available for generation), possible future turbine bypass flows of 300 cfs to guide fish from the power house intakes back to the river, and turbine flows.
Engineers considered two base cases:
• Maintaining the existing 250 cfs minimum (bypass) flow over the dams; and
• A possible future requirement of an additional 300 cfs (8.5 cms) turbine bypass flow at the powerhouse intakes.
For each case, energy production estimates were made with and without a redesigned Alden runner replacing the existing Unit 1.
Results indicate that replacing the present Unit 1 with the redesigned Alden runner would increase the turbine capacity and energy production at a reasonable cost compared to replacing the unit with a conventional Francis runner.
The capacity of the redesigned Alden turbine for installation at Crescent Unit 1 is 3.1 MW at a flow of 1,625 cfs (46.0 cms), which is slightly higher than that of the existing Unit 1. At this design point, the overall efficiency of the redesigned Alden turbine is estimated to be about 88%, which is somewhat lower than the original Alden turbine design, but, as previously stated, this is due in part to poor flow conditions in the exiting draft tube.
The performance hill chart for the proposed Alden runner was used to develop a flow versus power relationship for the constant site net head of 26.5 ft (8.1 m). This relationship was used in a generation model with 50 years of recorded river flows for two base cases: the present habitat bypass flow of 250 cfs (7.1 cms) and an additional future turbine bypass flow of 300 cfs (8.5 cms). The energy production for each case was predicted with and without the redesigned Alden turbine installed.
Energy production modeling indicates installation of the redesigned Alden unit would increase the average annual generation at Crescent by 4.3% for the existing bypass conditions and 6.6% for postulated future bypass flow requirements.
Fish survival estimates were made using a predictive blade strike probability and mortality model that incorporates extensive strike survival data from laboratory tests with moving blades that were funded by EPRI and conducted at Alden,2,3 as well as data from the literature describing the effects of short-term low-pressure spikes on fish mortality.4
Survival was predicted to increase dramatically with the use of the Alden turbine for 300-mm-long adult fish, from the present 83% to 99.8%, and the survival of 100-mm-long juvenile fish was predicted to increase from the present 96% to 99.8% (see Figure 1). These estimates are considered accurate to about 1% because there is no method available to predict the effects of turbulence in the existing draft tube.
An optional physical model could be considered to improve performance predictions and additional CFD analysis is anticipated in the future during the final design phase if NYPA decides to proceed with installation of the Alden turbine.
The objective of the study was to produce an Alden turbine design optimized to replace Unit 1 within the current site conditions at NYPA’s 12-MW Crescent Hydropower Plant. The study met this objective and turbine design is now ready for final engineering if NYPA decides to proceed.
The estimated cost for final engineering and fabrication of the redesigned Alden unit, including fabrication and installation of a transition to the existing draft tube, is about $3 million.
Costs for removing Unit 1 and installing the Alden runner will be assumed by NYPA and is not included in the $3 million estimate.
NYPA is currently reviewing the results of this work and has neither made a commitment to move forward with the retrofit nor has it decided not to use the turbine.
Similar to this study, an Alden turbine retrofit design may be developed for other hydro project owners. The consideration would be for runner replacement to help achieve comparable performance at a competitive cost while also achieving the goal of high fish survival.
1Perkins, N., et al., “Development Status of the Alden ‘Fish-Friendly’ Turbine,” Hydro Review, Volume 32, No. 2, March 2013, pages 46-55.
2Evaluation of the Effects of Turbine Blade Leading Edge Design on Fish Survival, EPRI Report No. 1014937, Alden Research Laboratory Inc., Holden, Mass., 2008.
3Additional Tests Examining Survival of Fish Struck by Turbine Blades, EPRI, Palo Alto, Calif., U.S., 2011.
4Brown, Richard S., et al., “Quantifying Mortal Injury of Juvenile Chinook Salmon Exposed to Simulated Hydro-Turbine Passage,” Transactions of the American Fisheries Society, Volume 141, No. 1, 2012, pages 147-157.
Hecker, G.E., Perkins, N.F., Amaral, S., Jacobson, P.T., Fay, C., Ludewig, P. & Walsh, J., “Retrofit of the Alden Turbine at New York Power Authority’s Crescent Hydroelectric Project,” presented at HydroVision International 2017, Denver, Colo., U.S., 2017.
The authors thank Steve Amaral with Alden Research Laboratory, Celeste Fay with Hydro Consulting Specialists and Jim Walsh with Rennasonic for their help in developing
Peter Ludewig, PE is director of hydropower at the New York Power Authority. George Hecker, PE, is senior technical fellow and Norman Perkins, PE is chief operating officer with Alden Research Laboratory. Paul T. Jacobson, PhD, is EPRI waterpower program senior technical leader.