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Better Turbines for Small Hydro

Natural Resources Canada is devoting significant time and money to small hydro research and development. Work focuses on developing new types of turbines for low-head situations, improving turbine efficiency, and making turbine performance testing less expensive.

By Tony T.P. Tung, Jinxing Huang, Cynthia Handler, and Ghanashyam Ranjitkar

Canada has a great deal of untapped small hydro potential. Much of this potential is at low-head, run-of-the-river sites. For example, there are about 10,000 low-head dams and hydraulic structures in Canada that are used for flood control and water supply/irrigation. Significant opportunity (more than 10,000 MW) exists by adding small hydro plants at these low dams and structures.

The Hydraulic Energy Group of CANMET Energy Technology Centre, Natural Resources Canada (CETC-NRCan) is supporting many emerging technologies designed to increase the commercial viability of this existing hydropower potential. The group is supporting development of several innovative small hydro technologies, such as low- and very-low-head turbines with variable speed operation. By lowering the costs of equipment and civil works and improving efficiency, these new technologies can make feasible sites that would not be economical to develop using traditional technology.

Small hydro potential in Canada

Installed capacity in Canada from small (less than 50 MW) hydro facilities is 3,300 MW. This capacity is expected to reach 6,000 MW in ten years, mostly due to development by independent power producers (IPP), provincial/municipal utilities, and remote communities. Development of small hydro facilities in the country represents about C$500 million in annual business activities.

Much of the small hydro potential in Canada is at low-head, run-of-river sites that do not require large reservoirs or dams. Most small hydro facilities provide decentralized power and connect to local grids. These sites can respond quickly to fluctuations in demand and are a reliable source of electricity for rural and remote communities.

However, further development of small to medium hydro largely will rely on emerging technology that allows the construction of more reliable, efficient, and environmentally friendly stations with lower upfront costs. CETC-NRCan is investing in several research and development projects to address the technical gaps.

CETC-NRCan technology research

The trend toward privatization and opening of the electricity market, as well as the need to prevent major grid failure, have led to operational changes at many small and medium hydro plants. With the recent focus on the reliability and security of the distributed generation system, aging small hydro turbine-generating equipment needs to be upgraded and strengthened. In addition, for very low-head (less than 3 meters) sites with large flow, developing an innovative concept for the turbine-generator system is necessary to reduce civil costs.

CETC-NRCan has put high priority on the following four projects:

– Fish-friendly, variable speed low-head turbine-generator;

– Very-low-head turbines;

– Exit stay apparatus for Francis turbines; and

– Economical turbine performance field testing.

Fish-friendly, variable speed low-head turbine-generator

Affordable, efficient, and fish friendly turbine-generator systems are needed for low-head applications. Successfully implementing this project would allow development of many small, low-head hydro sites with high energy efficiency and without using costly mitigation structures such as fish ladders, forebay guidance devices, and spillway weirs. The main barriers hampering the development of low-head hydro are its low economic viability and the environmental effects.

The primary aspect affecting economic viability of small hydro systems is varying water input. During low water periods, available electricity drops in proportion to the head and flow. Conventional single-regulated turbines cannot operate with significant flow changes and must be shut down. This downtime affects the financial viability of the installation. Double-regulated Kaplan turbines may operate over a significant turndown ratio,1 but their capital costs are much higher. A better approach is to develop a system that combines low capital cost with the operational efficiency and turndown ratio of a double-regulated unit.

Regarding environmental effects of low-head systems, fish survival is a significant concern. Many fish pass through the turbine, making a properly designed fish-friendly turbine highly desirable to reduce injury and mortality.

To meet these challenges, CETC-NRCan and its partners – Rapid-Eau Technologies Inc., Swiderski Engineering Inc., and Laval University – are developing a non-regulated turbine system coupled with a variable-speed generator to operate at the optimum rotational speed with varying flow rate. This type of turbine will improve fish survival, increase productivity, and reduce the overall cost for low-head run-of-river applications. Figure 1 shows a rendering of the turbine casing.

The two aspects to be considered in this development are the turbine and generator. Unlike conventional turbines that have guide vanes, wicket gates, and a relatively large number of short runner blades, this will be a non-regulated (no guide vanes or wicket gates) vortex propeller turbine having fewer but longer and thicker runner blades. By meeting three important criteria – low probability of the leading edge of the runner blades striking fish, shear rate, and static pressure change ratio in the turbine – this arrangement will prevent fish mortality and injury. The unit includes a special turbine casing designed to create the required tangential momentum.2


Figure 1: The fish-friendly turbine being developed for low-head applications features a special turbine casing designed to create the tangential momentum needed to prevent fish mortality and injury.
Click here to enlarge image

Based on permanent magnet technology with a high number of poles, the generator will be able to generate electric power with improved efficiency at low and variable-speed operations.3 The power generated at variable frequency will be converted to utility quality power using commercially available frequency converters modified for this application.

Benefits of this system are twofold:

1) Increased power generation. The turbine connects to the variable-speed generator through direct-drive. Operating at variable speed, the propeller turbine can run over a wide operating range (low or high flows) with high efficiencies and high turndown ratio, thus generating more overall power; and

2) Environmentally friendly. The turbine could reduce fish mortality to less than 5 percent (typically mortality is 5 to 10 percent for the best existing turbines and 30 percent or greater from other turbines). Furthermore, the run-of-river application has only limited effects on the environment because the water level in the river is basically unchanged.

To date, the theoretical design has been optimized through computational fluid dynamics (CFD) studies. A review of the design for fish mortality has been carried out and incorporated in the turbine design. The final design of the model turbine for testing has been achieved. Manufacturing of the model turbine is in progress, and the model performance test was to be carried out in the Hydraulic Machinery Laboratory at Laval University by the end of 2006.

Very-low-head turbines

Very-low-head hydro has the potential to generate green electricity with minimal environmental effects. In addition, it is one of the best options for decentralized power generation. Development of Canada ’s low-head hydro potential is very low because of its high costs, particularly for the related civil works, which represent 40 to 50 percent of total development costs. As a result, an innovative very-low-head turbine design involving less civil cost is highly attractive. This new turbine could target sites that have high natural flow in order to achieve a high annual running time.

Traditionally, turbine manufacturers seek to reduce runner diameters in order to reduce the equipment costs while maintaining high performance. Usually, the decrease in diameter requires more complex civil structures to convey water from the intake to the runner and to recover the kinetic energy at the runner exit. Typically, long intakes and draft tubes are needed for this purpose.

The partners of this project – MJ2 Technologies S.A.R.L. and Atelier ONMEC Inc. – have developed a groundbreaking concept. The Very Low Head (VLH) turbine4 takes a completely different approach to the traditional design: using larger runners to practically eliminate the expensive civil structures of traditional concepts. Larger but simpler runners rotating very slowly, like windmills, will be installed in sluice passages. These passages could be the existing ones adjacent to dams (or weirs) or could be built within a weir.

The best sites for installation of the VLH turbine are those with highly regulated flows at existing weirs with drops of 1 to 3 meters. Such sites often are encountered along navigation canals, irrigation canals, and municipal intakes.

The VLH turbine has the following features:

– Very low rotation speed and low flow velocity through the turbine;

– Advanced low-speed generator directly coupled to the turbine runner;

– Draft tube significantly shortened or eliminated because the runner diameter is such that the kinetic energy of the flow (low flow velocity) at the exit of the runner is less than 20 percent of the total available potential energy of the site;

– Runner has many blades that will be able to close the flow passage. This allows control of the discharge and shutdown of the turbine unit, eliminating the requirement of a gate or movable distributor;

– Turbine is easily accessible for maintenance by translating it to a top position and can be easily raised from its guide using a mobile crane;

– Fish-friendly because of the low velocity and low pressure flow through the runner; and

– Expected turbine hydraulic efficiency of about 80 percent and global efficiency of about 70 percent.


Model testing is under way to verify the design of the Very Low Head (VLH) turbine. Results of this testing are being incorporated into the prototype, which is expected to be fully operational in France in October 2007.
Click here to enlarge image

The hydraulic and mechanical designs have been completed, and designs have been optimized through CFD analysis. The model turbine has been manufactured and is being tested in the Hydraulic Machinery Laboratory at Laval University. Preliminary results show that the turbine performance is very close to the design expectation. The prototype is being improved based on preliminary results and is expected to be fully operational in France in October 2007.

Exit stay apparatus for Francis turbines

A very advantageous alternative, both environmentally and economically, to developing more hydro sites is to make existing equipment more efficient. Therefore, NRCan decided to support the development of a Francis turbine with an exit stay apparatus (ESA). Dr. Alexander Gokhman invented a reaction hydraulic unit with an ESA in 2002. This invention promises to improve efficiency and decrease flow pulsation amplitude at partial loads in Francis and axial propeller turbines.

Many hydropower plants use Francis turbines. At small hydro sites, because of the hydrological conditions, there are wide variations in the water flow, and Francis turbines frequently operate under off-peak load conditions. At large hydro sites, in a deregulated market, utilities have a monetary incentive to operate their units between off-peak load and full load, depending on market demand and price. The head at large hydro sites also may vary significantly from optimal, especially at newly erected power plants with a large reservoir.

Francis turbines, working off the optimal operating regime, experience a significant loss of efficiency. Furthermore, the demand of operation under off-peak conditions has significant detrimental effects on the turbine unit because it often is exposed to pressure pulsation and dynamic loadings. These can cause material fatigue and significantly reduce the life expectancy of the equipment.

This project targets these two issues by developing and implementing an ESA to eliminate the central helix vortex (and thus increase the energy efficiency for off-peak regimes) and achieve reliable operation and a longer life expectancy for existing and new plants. The ESA comprises a stay crown and several exit stay vanes to be put immediately behind the conventional Francis runner exit. (See Figure 2 on page 38.) The ESA can be incorporated into newly manufactured Francis turbines. It also can be retrofitted into any Francis turbine if its draft tube cone is not embedded.

The ESA project is expected to meet the following objectives:

– Substantially increase efficiency at loads less than optimal power. Efficiency should increase by 5 percent at the optimal head and a discharge equal to 43 percent of the optimal power; and by 7 percent at the head equal to 80 percent of optimal head and the load equal to 33 percent of optimal power;

– Increase efficiency by at least 2 percent at the optimal head for the flow rate corresponding to the maximal power for the turbine without ESA; and

– Drastically decrease the amplitude of pulsations at operating regimes off the optimum, resulting in stable operating conditions and longer life expectancy for the equipment and power plant.

Work on this project is planned in two phases. Phase I mainly focuses on a model study to prove the concept, and Phase II focuses on a demonstration. The main project partners are independent consultant Dr. Alexander Gokhman5 and the Hydraulic Machinery Laboratory of Laval University.

Dr. Gokhman has completed hydraulic and mechanical designs of the ESA for an existing Francis turbine supplied by GE. This unit was put in place at the Hydraulic Machinery Laboratory at Laval University in August 2006. Manufacturing and installation of the ESA on a model Francis turbine was to be carried out in the fall of 2006, and model testing was to be completed by the end of the year.

Economical turbine performance field testing

Among Canada ’s 500 hydro stations, more than half are considered small (less than 50 MW installed capacity). These small hydro plants typically operate with only the turbine manufacturer ’s information on performance, and the units usually are developed for specific design criteria.


Figure 2: The exit stay apparatus (ESA) being developed for a Francis turbine promises to improve efficiency and decrease flow pulsation amplitude at partial loads in both Francis and axial propeller turbines.
Click here to enlarge image

There is no economical and affordable methodology to verify the as-built and installed unit performance characteristics for small power stations, or to ascertain wear and make adjustment-related performance determinations. Most turbine prototypes vary from the design by having a shift of the best operating point and the maximum output due to empirical step-up factors used to convert model test results to prototype performance. As a result of the high cost of performance testing ($20,000 to $100,000 for index tests), or an absolute performance test, operators of small hydro stations typically lack the data to analyze how much power they lose when deviating from the best operating point.

This project will develop an economical turbine testing procedure, including equipment for hydroelectric generating units, that will help small and medium hydro stations improve operating efficiency. The purpose of the turbine testing procedure will be to determine the actual turbine and unit performance characteristics.These data will provide operators with a basis for daily operating decisions, allowing them to choose the best operating mode.

The test equipment will be designed to have a universal setup to fit most hydroelectric stations. It will be a comprehensive system that includes a variety of flow measurement tools, as well as power measurement and head measurement tools and other test parameters.

To the extent practical, the test methodology will be developed to be in accordance with the American Society of Mechanical Engineers (ASME) and International Electrotechnical Commission (IEC) test codes. The test methodology will be designed to be accurate, economical, and provide immediate results. Test procedures will be developed to suit the comprehensive system of new, universal test equipment and instrumentation. A standard test report will be designed and formatted to satisfy the needs of operators, designers, and consultants. The report is to be produced on site before test equipment is dismantled. The main project partner is HydroPower Performance Engineering Inc.6,7

An economical turbine testing procedure (including equipment) for small and medium hydroelectric power stations will provide several benefits:

– Operating hydro stations at their best operating point increases power production and optimizes water usage because this point has the highest power production per cubic meter of flow; and

– Because hydro generation does not produce any greenhouse gases, improved energy production means more displaced greenhouse gas emissions.

Developing the new test equipment and procedures will reduce the setup time and the data analysis and reporting. The target of this project is to make testing economical ($10,000 to $20,000 per test) and demonstrate to hydroelectric power producers the benefits of investing in knowledge acquisition to operate efficiently, to utilize water resources efficiently, and to improve power production for the same available water resources.

The concept design of measuring modules has been completed. The modules fit most low-head hydro plants with minimal alterations, are easy to assemble and dismantle, and are interchangeable without affecting the structure strength or rigidity of the modules. The current meter and side supporting frames have been designed and approved by professional engineers. Next steps involve system integration and calibration, as well as demonstration of the effectiveness of the universal performance measurement system in low-head small hydro plants.

The authors may be reached at CANMET Energy Technology Centre, Natural Resources Canada, 580 Booth Street, 13th Floor, Ottawa, Ontario K1A 0E4 Canada; (1) 613-996-6119 (Tung), (1) 613-992-4379 (Huang), (1) 613-947-4122 (Handler) or (1) 613-944-4407 (Ranjitkar); E-mail: tung@nrcan.gc.ca, jhuang@nrcan.gc.ca, cyhandle@nrcan. gc.ca, or granjitk@nrcan.gc.ca.

Notes

  1. Turndown ratio is the ratio of the design flow to the minimum flow at which a turbine will run efficiently.
  2. de Montmorency, D., “The SBR Turbine, A Simplified Design System for Axial Flow Turbines, ” Proceedings of the 13th International Seminar on Hydropower Plants, Institute for Waterpower and Pumps and Institute for Testing and Research in Materials Technology at the Vienna University of Technology, Vienna, Austria, 2004.
  3. KWI Architects Engineers Consultants, “Status Report on Variable Speed Operation in Small Hydropower, ” Energy 2002, European Commission, www. europa.eu.int/comm/energy/res/sectors/doc/small_hydro/statusreport _vspinshp_colour2.pdf.
  4. Fonkenell, J., “Hydro Turbine Generating Set for Very Low Head, ” Proceedings of the 13th International Seminar on Hydropower Plants, Institute for Waterpower and Pumps and Institute for Testing and Research in Materials Technology at the Vienna University of Technology, Vienna, Austria, 2004.
  5. Gokhman, A., Exit Stay Apparatus, U.S. Patent Office Application Number 10/224,442.
  6. Mikhail A.F., et al, “Performance Testing of the Robert Moses Niagara Power Plant Using the Gibson and Index Methods to Verify Unit Similarity, ” Waterpower XIV Technical Papers CD-Rom, HCI Publications, Kansas City, Mo., 2005.
  7. Mikhail, A.F., and R.J. Knowlton, “Performance Testing of the Robert Moses Niagara Power Plant and Sir Adam Beck Generating Station, ” Proceedings of the IGHEM Conference, International Group for Hydraulic Efficiency Measurement, Toronto, Ontario, Canada, 2002.

Tony Tung is senior advisor to the hydraulic energy group, Jinxing Huang is senior advisor for the hydraulic energy group and computational fluid dynamics, Cynthia Handler is senior renewable energy engineer for renewable energy technologies, and Ghanashyam Ranjitkar is hydraulic energy engineer for the hydraulic energy group of renewable energy technologies in the CANMET Energy Technology Centre of Natural Resources Canada.


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