Kannagawa Hydropower Plant, Japan

Dr. Robert Peltier, PE

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Blade runner

Worldwide hydropower generation technology is a century old but is just now reaching technical maturity. In the past, technological innovation focused on improving turbine efficiency by just 0.1%. By contrast, TEPCO's development of the pump-turbine runners for Kazunogawa and Kannagawa increased turbine efficiency by more than 1.5%. The new runners also expanded turbine output adjustment range and operational water level range by more than 10% while reducing by half the stresses from water pressure acting on runners (Figure 4). To sum up, the new runners featured the best of both worlds: better hydraulic performance as well as improved turbine quality and reliability.

4. Split the difference. The split-runner design increases pumping efficiency and system operating range. Source: Tokyo Electric Power Company Inc.

In addition, due to limits on water pressure inside pump-turbines, worldwide pump-turbine technology had stagnated at an effective head of about 1,800 ft and a unit output of 300 MW since 1980. On the Kazunogawa and Kannagawa projects, TEPCO introduced the world's first high-efficiency high-head pump-turbines—with an effective head of over 2,100 ft and a unit output greater than 400 MW—by developing and using splitter runners.

It was accepted practice in pump-turbine design that increasing the number of blades (or runners) would increase frictional losses and reduce unit efficiency. However, recent computational fluid dynamics (CFD) analyses have revealed that in runners, turbulence losses are far greater than frictional losses. The analyses also found that increasing the number of blades reduces turbulent loss and dramatically increases turbine efficiency (Figure 5).

5. Traditional vs. innovative. A conventional runner design found on typical vertical pumps (L), compared with a splitter runner design, in which multi-blade runners consisting of long and short blades are arranged in turn. Source: Tokyo Electric Power Company Inc.

However, in the past, increasing the number of blades narrowed flow paths and prevented runners from discharging enough water to allow turbines to produce large outputs (even at high flow rates). TEPCO overcame this problem by fitting runners with both long and short blades. As a result, the company was the first in the world to develop and deploy pump-turbines with splitter runners (Figure 6).

Hydraulic performance tests on a 1/9-scale mockup revealed the following advantages of the new, splitter runners (compared with traditional runners):

An increase in turbine efficiency of more than 1.5% across the entire operating range (an increase of 4%, compared with turbines built pre-Kazunogawa).
Prevention of increases in pressure pulsation, even in the part-load (lower output) operating range.
A widening of turbine operating range by 10%, enabling operation at lower output (reducing the lower output limit).
An increase in turbine stability across the whole operating range, expanding the unit's output range and load adjustment range.

It's worth noting that TEPCO's advanced pump/turbine runner design has been chosen for use on the Xilongchi Pumped Storage Power Plant Project in Shanxi Province, China. The first of four units at the plant—which will have a maximum head of 2,306 ft and a total capacity of 1,200 MW—is scheduled for startup in August 2008.

Boring story

TEPCO matched its innovation in runner design by applying some novel techniques to the construction of Kannagawa's underground water infrastructure. In the pumped-storage world, it is common practice to connect two reservoirs by first drilling a small pilot tunnel from the bottom reservoir up, and then drilling and enlarging the tunnel from the top down. To avoid this long and laborious process, TEPCO used a "full-face tunnel boring machine" (TBM) to drill the penstock tunnel (which is inclined 48 degrees, approximately 20 ft in diameter, and more than 3,000 feet long) in one fell swoop, from the bottom to the top.

The TBM (Figure 7), weighing in at 600 tons, is self-supporting, so it doesn't slide down the steep incline. It also is equipped with a boring machine to survey soil up to 100 ft ahead. Ultimately, the TBM shortened the time needed to drill the penstock tunnel by about six months. Drilling through sold rock, it worked at an average speed of 225 ft/month.

Another of TEPCO's technical innovations was its use of high-strength steel in the lower portion of the penstock to cope with extremely high system pressures. Kannagawa is the first hydro plant to use HT-100 for penstock piping. Because HT-100 is lighter than standard steel pipe, the switch reduced penstock construction time by about three months.

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