Fish-friendly hydro turbines move center stage

June 24, 2008 at 7:10 am | Posted in Environmental, Power Plant | Leave a comment

Hundreds of U.S. hydro plants come due for license renewal over the next decade, giving river and wildlife advocates an ideal time to push for fish-friendly turbine retrofits. With these new, e-“fish”-ent turbines, early retrofit projects have targeted up to 98% fish-passage efficiency and up to 15% more power output.

The 91,000 MW of hydropower capacity in the U.S. comes from about 180 federal projects and more than 2,000 non-federal projects regulated by the Federal Energy Regulatory Commission (FERC). Although the country has substantial undeveloped hydropower resources, little new construction is expected, and hydropower’s share of the nation’s generation is predicted to decline through 2020, due to a combination of environmental issues, regulatory pressures, and changes in energy economics.

FERC is the lead permitting agency for private hydroelectric plants. The U.S. Fish and Wildlife Service for the eastern part of the country, and the National Marine Fishery Service for the western part, are other agencies that make recommendations on fish mortality goals. In response to the 1986 Electric Consumers Protection Act, these agencies have set their target as “no fish production loss” for all new hydro projects as well as for any projects for which the hydraulic lease is expiring. That last phrase is key, because there are hundreds of plants in the U.S. that must pass through the relicensing minefield over the next decade. About 80% of California’s hydropower is subject to a FERC license, and about half of those facilities—approximately 4,000 MW at 150 projects—are due for license renewal in the next 15 years.

Save the fish

Turbines installed decades ago were designed with one thing in mind: maximizing power output. Although fish ladders were typically built for migratory fish, little thought was given to the side effects of fish injury and mortality caused by passage through the turbines or lower levels of dissolved oxygen in downstream water. Fish mortality rates currently range from 5% for the least-harmful existing turbines to more than 30% for more-damaging turbines.

Fortunately, special fish-friendly designs developed over the past decade have come into operation at several hydro power stations, significantly improving fish-passage efficiency while increasing power output. In fact, the repowering of projects built 50 years ago could eventually improve plant output by as much as 15%, thereby making a fish-friendly retrofit project economically feasible. The fish win, and so do plant owners and electric consumers.

Turbine-passage survival is a complicated function of gap sizes, runner blade angles, wicket gate openings and overhang, and water passageway flow patterns (see box). The consequence to fish populations of unaccommodating turbines is especially serious among migratory species—such as salmon, steelhead, and American shad—and eels that must pass downstream all the way to the sea, perhaps through multiple turbines, to complete their life cycle. That’s why increasing the passage efficiency of juvenile salmonids at Pacific Northwest hydro plants is one of the major challenges facing the industry. According to the Bonneville Power Authority (BPA), juvenile salmonids outmigrating to the Pacific Ocean through the Columbia River Basin during high spring flows experience a 5% to 25% mortality rate at each hydroelectric project they encounter. During this time, BPA—already teetering on the verge of bankruptcy and rising electricity costs—must reduce its power generation potential by $160 million per year through increased spills and other operations designed to increase fish-passage efficiency.

The U.S. Army Corps of Engineers operates eight multipurpose dams on the lower Columbia and Snake Rivers as part of the Federal Columbia River Power System. The Corps’ Columbia River Fish Mitigation program focuses on improving the passage of adult and juvenile salmon either around these dams with fish screens or through them with expensive fish-bypass structures. The Corps estimates that it will spend $1.4 billion implementing its fish-mitigation program, according to a 1998 report. About $908 million of this total will be spent on the construction of fish-passage projects and related studies from fiscal year 1999 through the scheduled completion of the program in fiscal year 2007, although currently shrinking budgets may place this goal in jeopardy.

New turbine designs arrive

The goal of the U.S. Department of Energy’s (DOE’s) Advanced Hydropower Turbine System (AHTS) program is to develop technology that will maximize the country’s hydropower resources while minimizing adverse environmental effects. The AHTS program is closely coordinated with industry and other stakeholders—such as Public Utility District No. 2 of Grant County (Washington), the Electric Power Research Institute, Corps of Engineers, U.S. Bureau of Reclamation, BPA, and National Marine Fishery Service—which all have a part to play in implementing advanced turbine designs.

The survival of turbine-passed fish depends greatly on characteristics of both the hydropower plant—the type and size of the turbine, environmental setting, and mode of operation—and the entrained fish—species, size, and physiological condition. Some small, Pelton-type turbines designed for high-head installations most likely cause complete mortality due to their basic design (Figure 2). On the other hand, the survival of small fish encountering turbine types with larger water passages—such as Kaplan, Francis, and bulb turbines—is commonly 70% or greater for current plants (Figure 3). Among the most fish-friendly conventional turbines, large Kaplan turbines used at the mainstem Columbia and Snake River dams have shown average fish survival (including both direct and indirect effects) of about 88% (Figure 4).

2. Pelton turbine: nearly 100% mortality

In a Pelton or impulse turbine, the water is directed onto clamshell buckets attached to the periphery of the impeller wheel to impart a torque on the turbine impeller. High rotating speeds and tight clearances mean almost 100% fish mortality.

Courtesy: VA Tech Hydro

3. Kaplan turbine

This cross section of a fish-friendly Kaplan turbine shows the details of the stay vanes, wicket gates, and runner with hub and water passages. Water enters through the semi-spiral case, passes through the stay vanes and wicket gates, then through the runner, and finally into the draft tube, exiting to the tail waters.

Courtesy: Voith Siemens Hydro

4. Francis turbine

Francis turbines for power generation typically are used where high flow rates are available at medium hydraulic head—such as the world-famous Niagara Falls. Water enters the turbine through a volute casing and is directed onto the blades by wicket gates. The low-momentum water then exits the turbine through a draft tube.

Courtesy: VA Tech Hydro

That’s why new fish-friendly turbine designs are a vital part of hydro’s future. The U.S. Department of Energy’s Advanced Hydropower Turbine System program has identified specific injury mechanisms, which include:

  • Turbulent flows or cavitation in turbine water passages resulting from low-efficiency designs or plant operating strategies where extremely low water pressures cause the formation of vapor bubbles, which subsequently collapse violently.
  • Turbulent flows and the trapping and cutting of fish in the zone of flow passing near the turbine hub when large gaps between blade and hub exist (characterizing the lower-output operation of Kaplan turbines).
  • Strike of fish by turbine blades or impact of fish on structures including runner blades, stay vanes, wicket gates, and draft tube piers.
  • Shear stress when two bodies of water of different velocities collide across a fish’s body. The highest values of shear stress are found close to the interface between the flow and solid objects it speeds by, such as the blade leading edges, vanes, and gates.
  • Rapid and extreme pressure changes (water pressures within the turbine may increase to several times atmospheric pressure, then drop to sub-atmospheric pressure, all in a matter of seconds).
  • Abrasion and grinding: Abrasion occurs with the rubbing action of a fish against rough turbine surfaces by flow turbulence and is dependent on flow discharge and velocity, number and spacing of turbine blades, and the geometry of flow passages. Grinding injury can occur when a fish is drawn into small clearances (gaps of sizes close to that of the fish) within the turbine system.

Testing of new designs with fish-friendly features has demonstrated even higher survival rates. The mission of the AHTS program has been to develop turbines that achieve 98% survival of turbine-passed fish.

Corkscrew turbine uncorked

The original AHTS design teams selected by the DOE in 1995 are still involved in bringing the fish-friendly designs to market. They are:

  • Alden Research Laboratory (ARL, Holden, Mass.);
  • Concepts NREC (White River Junction, Vt.), which was formed by the merger of Concepts ETI with Northern Research and Engineering Corp; and
  • Voith Siemens Hydro Power Generation Inc (Heidenheim, Germany; U.S. office: York, Pa.) with team members Georgia Tech, Tennessee Valley Authority (TVA), MWH Consultants, and Normandeau Associates.

The latest ARL-designed runner (the “runner” in a hydro turbine is analogous to an impeller in a pump) uses only three long blades, which are wrapped around the central hub in a corkscrew shape to gradually reduce pressure and minimize blade-induced injuries. This is much different than the 6 to 10 blades found on most traditional hydro turbines. The new runner minimizes the number of blade leading edges, reduces the pressure-versus-time and the velocity- versus-distance gradients within the runner, minimizes clearance between the runner and runner housing, and maximizes the size of flow passages.

End result: Fish get more room in the water to swim through the turbine on their way to upstream spawning grounds or downstream to the ocean (Figure 5).

5. New runner: nearly 100% survival

Alden Research Lab designed a fish-friendly runner that minimizes internal clearances and maximizes flow passages with a minimal impact on turbine efficiency. Tests have demonstrated fish survival rates approaching 99%.

Source: U.S. Department of Energy

Despite its dramatically different geometry, the new turbine has a predicted efficiency of only a few percentage points lower than a conventional turbine, although the power density is reduced. This means that, for modernizing a plant, the volume of the power house must increase to produce the same power output. For this reason the ARL design will focus on new installations. “The new fish-friendly turbine is expected to be used for new generation capacity, as replacements for existing turbines, and in fish-diversion systems installed near the main turbines,” says Thomas Cook, project manager for ARL. “We expect the cost of the new fish-friendly turbine to be comparable to existing hydraulic turbines.” Testing of a 2,000-hp full-scale model demonstrated control fish survival up to 99%, depending on fish size and turbine head. ARL has obtained patents for the new designs.

E-“fish”-ent turbines

For its contribution to the AHTS program, Voith Siemens Hydro (VSH) proposed building on the success of ongoing internal R&D programs and investigating the impact of existing turbine designs in relation to fish passage efficiency and developing new turbine designs that improve efficiency and reduce environmental effects for rehab projects. The results include design concepts for improved fish- passage survival in Kaplan and Francis turbines as well as designs for boosting dissolved oxygen levels in discharges from Francis turbines.

Elements of the advanced Kaplan designs were implemented in rehabilitated units recently installed at the Chelan County Public Utility District’s Rocky Reach powerplant, at the Corps of Engineers’ Bonneville Dam, and at the Tennessee Valley Authority’s (TVA’s) Kentucky Dam. An even more advanced design has been developed and model-tested for the Grant County Public Utility District’s Wanapum Dam. These fish-friendly designs generally focused on modifications to a Kaplan turbine that eliminated runner gaps, improved blade shapes, and included an advanced control system to sense the presence of fish and adjust to fish-friendly operation (Figure 6). Other major VSH design features that came out of the AHTS program include:

6. Advanced Kaplan design

Voith Siemens Hydro supplied the first 2 of 10 units being replaced at the Corps of Engineers Bonneville Powerhouse 1. The remaining eight units will be rehabilitated at the rate of one per year through 2008. Test on the first unit showed a fish survival rate of 93.8% to 97.5% for the MGR design.

Courtesy: Voith Siemens Hydro

  • High efficiency over a wide operating range with reduced cavitation potential.
  • A minimum gap runner (MGR) design, characterized as a design with a fully spherical discharge ring, no gap near the hub, and runner blades that tend to be thicker near the leading edge.
  • A non-overhanging design for wicket gates.
  • Environmentally compatible hydraulic fluid and lubricants, and greaseless wicket gate bushing.
  • Smooth surface finishes in conjunction with upgrades for stay vanes, wicket gates, and draft tube cone.

VSH and the Georgia Institute of Technology are employing computational fluid dynamics (CFD) simulations of turbulent jets to better understand the fluid stresses that fish experience as they pass through hydroelectric turbines. VSH is using the same CFD model to relate measured velocity fields in an experimental flume used to expose fish to shear and turbulence to predicted velocity fields in a turbine at the Wanapum Dam on the Columbia River. Additional software (VSH’s “Virtual Fish” model) is being used to estimate flow-induced loads on both flume-passed and turbine-passed fish.

VSH has also developed several advanced computational tools for estimating trajectories of fish-like bodies passing through hydro turbines. The motion of the “virtual fish” is governed by a set of differential equations that account for the fish mass and various flow-induced forces. This model can be used not only to estimate the trajectory of a virtual fish from the forebay to the tailrace, but it also can provide specific information about a variety of flow-induced loads on fish passing through various zones of turbine flow.

Controls too

The fish stand to benefit both from new turbine designs and from new control systems for those turbines. For example, VSH and TVA jointly developed new controls to operate their fish-friendly hydro turbines. And Hydro Resource Solutions (Norris, Tenn.) markets “WaterView,” the state of the art in real-time hydro plant performance optimization when constrained by factors such as fish-passage criteria, civitation limits, avoidance zones, and ramp rates. For example, WaterView can:

  • Limit turbine operation to fish-friendly modes when sensors indicate that fish are present and optimize plant performance whether fish are present or not.
  • Streamline the periodic updates of Kaplan turbine “digital cam surfaces” to most efficient operation at each head and flow, minimizing fish-injuring flow turbulence.
  • Sense active cavitation and limit turbine operation to noncavitating conditions.
  • Optimize plant output when fish are present to achieve targeted fish-passage survival based on fish presence, location, turbine-passage mortality, spillway fish mortality, fish bypass characteristics, and total dissolved gas generated during spilling.

Bonneville Dam

The Corps of Engineers plans to install MGR on all 10 units at the Bonneville Powerhouse 1. The first units to be replaced were Unit 6—put into commercial operation on July 27, 1999—and Unit 4, which returned to service in September 1999. The remaining eight units will be rehabilitated at the rate of one per year through 2008.

The Corps conducted biological tests on Unit 6 after its retrofit to determine the unit’s ability to pass juvenile salmon safely. The survival of turbine-passed fish through Unit 6 was compared with the survival of fish passed through Unit 5, an adjacent, conventional Kaplan turbine. The goal of the biological testing was to determine if the MGR is at least equivalent to the existing machine in terms of fish-passage mortality. The biological tests, co-funded by the Corps, Grant County Public Utility District No. 2, DOE, and BPA, were conducted between November 1999 and January 2000 with a release of 7,200 balloon-tagged fish.

Overall injury rates among turbine-passed fish were impressively low for both units: 1.5% and 2.5% for the MGR and Kaplan unit, respectively. Survival rates of fish passing near the hub also were high (97% or greater) for both units, while survival rates for fish passing through the mid-blade region ranged from 95% to 97% and did not differ between units.

At all four power levels tested, the MGR showed better survival rates than the conventional Kaplan for fish that passed near the blade tip. Survival for blade tip–released fish ranged from 90.8% to 95.6% for the conventional Kaplan and from 93.8% to 97.5% for the MGR.

Chelan County is first

Public Utility District No. 1 of Chelan County in Washington State owns and operates the second-largest non-federal hydroelectric generating system in the U.S. The District’s three hydroelectric projects—Rocky Reach, Rock Island, and Lake Chelan—have a combined generating capacity of over 2,000 MW (Figure 7).

7. Rocky Reach plant

The Chelan County Public Utilities District is nearing completion of a major upgrade of all 11 units at its Rocky Reach plant.

Source: Chelan County Public Utility District

Rocky Reach Units 1 to 7, six-bladed vertical Kaplan turbines rated 140,000 hp, were built in the early 1960s by Allis Chalmers (now VSH). Turbines for the remaining Units 8 to 11, originally vertical propeller turbines rated 177,000 hp, were built in the early 1970s.

In the early 1990s, Units 1 to 7 began experiencing severe runner blade cracking, starting at the interface between the blades and the trunnions and progressing diagonally into the blade leaves. In June 1994, after completing a round of competitive model testing, the District awarded a contract for the rehabilitation of Units 1 through 7 to Voith Riva Hydro (now VSH).

While rehabilitation of Units 1 through 7 was moving forward, the rehabilitation of Units 8 to 11 became more pressing due to increased restrictions on unit operation to aid downstream fish passage. The VSH contract was amended to add turbine rehabilitation for Units 8 to 11.

During the model development phase, new ideas related to the safe passage of juvenile fish through the turbines were identified, building on the Bonneville Powerhouse 1 designs. VSH completed additional testing and model development based on the elimination of wedge-shaped gaps between runner blades and adjacent components and minimizing the runner hub-blade gaps upstream and downstream of the blade trunnion.

Also, a fully spherical discharge ring was developed for Units 8 through 11, closing the gap between the runner blade periphery and the discharge ring along the entire length of the blade. This elimination of the upstream blade-hub gap was later applied to all 11 Rocky Reach turbines, as well as other hydroelectric projects along the Columbia River, such as the Wanapum and Bonneville.

Rehabilitation of all 11 Rocky Reach units is scheduled for completion this spring.

Grant County advances

Last August, the DOE chose Washington State’s Grant County Public Utility District (PUD) to test a single fish-friendly turbine at its 1,038-MW Wanapum Dam on the Columbia River. “The new turbine is designed to allow salmon smolts to pass through the dam without injury,” says Stephen Brown, the PUD’s hydro engineering supervisor. But there’s also a nice side benefit: The proposed fish-friendly turbines are much more efficient than the existing turbines. The eventual uprating of all 10 Wanapum Dam units would provide an additional 300 MW of power to meet peak loads.

New advances in understanding the correlation between the turbine operation point and fish survivability can also contribute to improved operating efficiencies on the mainstem dams. Tests of the existing turbines at Wanapum Dam used balloon-tagged fish to verify many of the fish mortality mechanisms. These tests clearly demonstrated increased fish survival rates at increased flow rates (beyond the turbines’ “best-efficiency” points). The result: increased power generation and improved fish-passage survival.

These results counter the conventional hydro plant wisdom that says fish survival is reduced under conditions of very low operating efficiency. Fish-passage studies of Kaplan turbines at Lower Granite, Rocky Reach, and Wanapum Dams have all failed to detect a direct relationship between the “1% of peak turbine efficiency” target that is employed in the Columbia River basin and probability of highest survival.

“Our goal is to lessen injury on fish,” according to Brown. “We are looking for a 98% survival rate.” Brown said the federal government is picking up half of the $2.5-million engineering and biological testing cost. Grant PUD will cover the other $2.5 million, plus the cost of the turbine and installation, for a total of $14.5 million. The new turbine is scheduled for installation in late 2004, with testing set for spring 2005.

McNary Dam rehab

McNary Dam, at 7,265 ft long and 183 ft high—one of the largest hydroelectric power facilities in the U.S.—is among a succession of dams on the Columbia River between Oregon and Washington that must be navigated by salmon during their annual downstream migration to the ocean. After nearly 50 years of constant use, the turbines at McNary Dam are being studied by the BPA and the Corps of Engineers for total replacement during a 13-year powerhouse overhaul. In June 2002, four contracts were awarded to firms specializing in manufacturing large hydroelectric turbines:

  • VSH;
  • Alstom Power (Littleton, Colo.);
  • G.E. Hydro Power (Pleasanton, Calif.); and
  • VA Tech Voest MCE (Charlotte, N.C.).

Each of these firms will design, build, and test scale models of a new turbine they would propose to install at McNary. The full-size turbine will be subjected to in-stream biological testing and evaluated for both fish survival and hydraulic performance. The test results will be incorporated as part of the National Environmental Policy Act analysis required for the rehab permit.

“The outcome of the testing, analysis, and the public input process could lead to a federal decision to replace all 14 of the existing 80-MW Kaplan turbines,” said Kevin Crum, project manager for the Corps of Engineers.

Fish gotta breathe too

Mechanical injury is not the only fish- damaging aspect of hydro turbines. Dissolved oxygen (DO) levels downstream of a powerhouse tend to drop during warm months, effectively suffocating fish that had successfully passed through the turbines. This is a particular problem in the U.S. Southeast.

In the 1980s, VSH and TVA invested in a joint research partnership to enhance DO concentrations in releases from Francis-type turbines. “Auto-venting” (AVT), or “self-aerating” turbines, which use the low pressures created by flows entering the turbine to induce additional airflows, typically are the most cost-effective technologies for Francis turbines (Figure 8).

8. Adding air to water

Voith Siemens Hydro has revolutionized the design of auto-venting turbines by the use of hollow blades and aerating holes that increase the amount of dissolved oxygen added to the water passing through the turbine.

Courtesy: Voith Siemens Hydro

In the last 10 years, significant progress has been made. TVA has developed reliable line diffuser technologies and surface-water pumps for low-cost aeration of reservoirs upstream of hydro plants and effective labyrinth weirs and infuser weirs for aerating downstream flows.

TVA’s Norris Dam was selected as the first site to demonstrate the AVT technologies. The two Norris AVTs have specially shaped turbine component geometries developed to enhance low pressures at locations for aeration outlets in the turbine water passage, to draw air into an efficiently absorbed bubble cloud, and to minimize aeration auxiliary power. Results show that up to 5.5 mg/l of additional DO uptake can be obtained for single-unit operation with all aeration options operating. In this case, the amount of air aspirated by the turbine is more than twice that obtained in the original turbines with hub baffles. Compared to the original turbines at the plant, these specially designed replacement units increased overall efficiency 3.5% and increased capacity by 10%. The new runners also have shown significant reductions in both cavitation and vibration.

The first uprated and rehabilitated AVT was recently returned to service at the Corps of Engineers’ J. Strom Thurmond plant in Clarks Hill, South Carolina. The 48-yr-old plant—yes, it was named for the ageless senator that long ago—is being upgraded with Francis runners designed to increase the levels of DO in rivers with depleted or diminished gas content. Seasonal stratifications result in the discharge of water with less than 1 mg/l DO during the summer months as far as 16 miles downstream from the dam. Georgia and South Carolina have a standard of 5 mg/l for that portion of the Savannah River.

The modified Francis turbine design includes fewer blades, improved blade shapes, and larger spaces between blades that improve fish-passage survival as well. Initial tests last September of the first installed unit showed DO improvement of slightly more than 4.0 mg/l with minimal impact on turbine efficiency.

The first of seven new aerating turbines arrived at J. Strom Thurmond in March 2002. The second turbine is due to arrive March 2003, and by 2006 all seven should be installed and operating.

By Dr. Robert Peltier, PE, Sr. Editor


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