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Small Hydropower Plant Perlen - Switzerland; A siphoned rebuild - high efficiency at low cost

H. Leutwiler, P.-J. Frank, ITECO Engineering Ltd., Switzerland In Switzerland, a remarkable amount of energy potential exists at low head hydro plants which are currently out of service or obsolete. The example of the reconstructed ultra low head plant at Perlen shows that, with innovative technology, renovation can be economic, even in the context of industrial self-consumption and low energy tariffs. At Perlen, a small hydro plant with single-regulated siphon-turbines was installed for the first time in Switzerland.

Historical background

The power plants of the Perlen paper mill on the river Reuss in the Canton of Lucerne, Switzerland, were built in the middle of the 19th century by Theodor Bell, a Swiss industrialist. They are situated on a factory- owned diversion canal 3.2 km long with two small hydro plants built in series as run-of-river plants, each with a head of 2.7 m and each providing approximately 1 MW of power. The electrification took place at the beginning of the 20th century with the installation of four new generating units. In parallel with them, three new turbines were connected mechanically with a main shaft to the belt-drive milling machines. In the 1940s, the shaft was coupled to a generator, and since then the small hydro plant has successfully been in service without any significant investment.

In the 1980s, mechanical problems became frequent and, one by one, the turbines were shut down. Repair or renovation was out of the question because of the low efficiencies, high costs and personnel-intensive operations involved. At the beginning of the 1990s, the plant was decommissioned.

Studies and concept for renovation

A pre-feasibility study reviewed optimal solutions taking into account all the possibilities, including demolition, upgrading, replacement, and reconstruction. Since all the energy generated was consumed by the paper mill, any refurbishment had to compete economically with the low energy purchase prices available to a large customer.

The results were remarkable: A concept with a single machine based on the law of economies of scale proved not to be economic. A solution with two turbines resulted in high turbine and civil construction costs. A concept with two horizontal shaft turbines was less attractive than three turbines with vertical shafts, for which reduced construction costs would be possible. A potential solution based on minimal structural changes and the installation of seven speed-regulated compact submerged turbines without mechanical regulation of the vanes and blades proved more expensive as well.

The management of the paper mill therefore decided in favour of a total reconstruction of the plant with three vertical Kaplan turbines.

Siphon construction type

Evaluation of the construction risks and associated tender prices for civil construction showed that renovation of the old brick powerhouse would not be a good solution compared with the construction of a new powerhouse.

In addition, tenders for the turbines pointed unequivocally towards single-regulated Kaplan turbines, since the water flow is quite constant. With the possibility to fix the guide vanes, a siphon arrangement was selected. With the siphon concept, the highest point of the turbine inlet is located well above headwater level. As a result, no water can flow into the turbine except when needed. Hydraulically, the siphon concept does not change the flow and the geometry.

For turbine overhauls, no drainage will be necessary, which is an attractive feature applicable not only to a small plant but also to larger plants. The turbine gates are superfluous, which allows simplifications in civil construction.

In summary, the siphon concept reduced the total power plant costs by approximately 20 per cent.

Small Hydropower Plant Perlen – Switzerland; A siphoned rebuild 2

Functioning of the siphon concept

1. Evacuation for start up

To start up the turbines, a water-ring vacuum pump evacuates the required quantity of air from the upper space in the spiral casing. After about ten minutes, the first water drops on to the runner and the turbine begins to turn. It sweeps the remaining air and energy production starts within a few seconds.

2. Normal shutdown

The turbines can be stopped by closing the runner blades and a small brake. Thanks to the air-tight spiral casing, the turbine stays filled with water for hours - ready for instant restarting.

3. Air-valves for emergency stop

In the case of an emergency stop, opening the air valve lets the water load in the spiral casing drop within a few seconds to the headwater level; the turbine then runs in air and can be secured by a brake, protecting the generator from overspeeding.

Four key high-performance elements

1. Spiral casing

Thanks to high-precision shuttering, the hydraulic contour of the concrete spiral casing ensures optimal intake flow conditions. As a result, pressure head and kinetic energy are uniformly distributed around the turbine stay-vanes.

2. Turbine runner

The adjustable blades of these classic Kaplan runners are automatically regulated for optimal exploitation of available flow.

3. Generator

The three low speed 24-pole generators are directly driven without transmission, thus reducing noise and maintenance outlay. They are also more efficient than high-speed gear driven generators.

4. Draft tube

Three high-efficiency draft tubes convert the residual kinetic energy into suction head, thus boosting turbine performance.

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Technical Data of the small hydro power plant Perlen

Gross head 2.7 m (range 2 to 3.4 m)

Nominal discharge 3 x 15 m³/s

Max. electric output 3 x 342 kW = 1026 kW

Synchronous generators 3 x 440 kVA / 500 V / 125 rpm

Unit efficiency at nominal load 86 %, cos Φ = 1

Runner-regulated Kaplan units 2100 mm runner dia., 4 blades, CuAl-bronze

Shaft and bearings Vertical shaft, directly coupled generator

Siphon setting 50 cm above headwater level

Mean annual production 7800 MWh

Results and conclusion

The commissioning fulfilled all expectations. The guarantees for efficiencies, performance and sound emissions were achieved immediately and the plant is functioning without any problem. The simple and spacious layout facilitates operation and maintenance.

This reliable, efficient and low cost concept shows that ultra low head plants can be implemented in competition with procurement by large consumers of electricity from the grid at low prices. The necessary factors are cost control concepts, tight planning and appropriate amortization periods.

The innovative concept was supported by the Swiss Federal Office for Energy as a demonstration project. The overall cost of CHF 8 million (about US$ 9 million) for this run-of-river plant (without any canal and intake costs) encourages similar projects to be developed in the future.

Hanspeter Leutwiler graduated as a Mechanical Engineer. His work involves planning and erection of hydro power plants and other services as a consultant in the field of renewable energy.

Pierre-Jacques Frank graduated as a Civil Engineer. His work involves planning of hydro power plants and fish-migration facilities as well as execution of measurements.

ITECO Engineering Ltd., P.O. box, 8910 Affoltern a. A., Switzerland.