Swiss success at Tambo Creek16 May 2013
The 1.8MW high head Tambo Creek plant in Switzerland was connected to the grid by the end of August 2012 after a short construction period of only eight months. Even though it is a classical run-of-river scheme, it features a number of aspects not normally associated with small hydro.
In 2008, the small community of Splügen in the Swiss Canton of Grisons joined forces with the Swiss utility Alpiq EcoPower Switzerland Ltd to develop the untapped hydropower potential of their Tambo Creek in the south eastern part of the country. The two partners established a limited company to jointly finance, build and operate the plant. Entegra Wasserkraft AG of St. Gall, Switzerland, was awarded a contract to carry out all engineering services for the project.
The Tambo Creek small hydro project has been implemented as a high head, run-of-river scheme, showing the following main layout characteristics:
• The flow of the Tambo Creek is diverted by a lateral intake at an altitude of 1890m asl directly into a forebay without storage basin.
• The penstock comprising ductile iron pipes of 0.6m and 0.5m diameter has a length of some 1780m and connects the forebay located just below the Tambo Alp to a single four-jet Pelton turbine at the Hinterrhein River in the valley bottom.
• The unit, operating under a net head of some 400m, has an output of 1.8MW and is expected to generate about 7.2GWh of electric energy in an average hydrological year. This covers the power requirements of about 1750 households in Switzerland.
Right from the start, the developer Alpiq set decisive guidelines on the development strategy for this small scale scheme. In order to meet the highest environmental standards only the river reach in the gorge was to be exploited. In this way, the step-pool system of the creek in its steep rocky gorge would hardly be affected by the water diversion for the hydro development and the ecological conditions of the Tambo Creek in its fragile alpine setting would not be altered. In addition, this limitation to the river reach in the steep gorge made it possible to avoid any adverse visual impact which tourists may be faced with due to the technical structures of the plant and the reduced water flow in the creek.
This development approach was generally accepted by the public including environmental groups: there was no opposition against the project during the approval process in 2009 and 2010.
Weir and intake
The challenges faced by the design team were related to a feasible river diversion from the given intake location just upstream of the steep gorge. Initially, a bottom intake or Tyrolean weir had been planned on this alpine creek with its heavy bed load during floods. However, the head loss associated with a bottom intake made it impossible for the penstock to reach stable ground in the steep slopes of the gorge. Eventually, a side intake on a 3.3m high weir sill had to be selected in order to raise the water level to the elevation required by the alignment of the water conductor system in the gorge.
Flood water spilling over the relatively high weir sill of 15m width would provoke erosion on the river bed. In order to reduce this scouring problem, a stepped spillway was selected whereby the energy of the overflow is partly dissipated in a highly turbulent step flow rather than on the river bed at the foot of the weir. This stepped spillway is the second of its kind to be built in Switzerland.
The gravel trap right after the intake gate features a 400mm diameter vortex tube with a longitudinal slot to continuously exclude gravel of up to 60mm in size during operation.
The sandtrap consists of a single basin 21m in length designed to exclude sediment particles down to 0.2mm in diameter. Sediment flushing from this trap is made by a novel technique designed by HSR, a spin-off company from the University of Applied Sciences of Rapperswil. Similar to the gravel trap, it uses a vortex excluder system which flushes sediment without the need for lowering the water level in the sandtrap and stopping of the turbine.
With these novel sediment excluding systems applied at the gravel and sand traps, the plant can continuously be operated with sediment laden water during floods and snow melt. The alternative layout using two sand traps in parallel which would be flushed alternatingly could not be applied at the Tambo Creek due to insufficient space at the intake location and also for cost reasons.
The penstock pipes are buried on their entire length to avoid freezing in winter and to reduce visual impacts. The penstock alignment was selected in such a way that a cost optimisation covering penstock length, bypassing landslide prone areas and excavation in rock could be achieved. Pipes and concrete for anchor blocks had to be flown in by helicopter as the penstock slope was too steep for any other means of transport.
The upper part of the penstock follows an existing road cut into the steep hillside. Here, some stretches required stabilisation measures consisting of micro piles of up to 6m length drilled through the overburden and into the bedrock. As these piles could not be designed to stabilise the entire hill slope and to stop its slow creep movement, geotechnical monitoring measures have been added in order to identify any larger movement which could disrupt the penstock if left undetected. The micro pile cap made of concrete has been jointed every 6.5 meters. Joint deformation and opening are continuously measured and compared with alarm values. As soon as joint movements exceed pre-set values, the penstock emergency shut-off valve at the forebay is triggered and the penstock is emptied so that no water leakage following a penstock rupture could flow out into the steep hillside and trigger landslides with further damage.
Powerhouse and electromechanical equipment
The powerhouse is located on the banks of the Hinterrhein River. As this area is a potential flood plain with a protected wetland flora, the powerhouse had to be slightly set back from the Hinterrhein in order to avoid the protected area and to blend nicely with the natural environment, especially in winter where a cross country skiing track passes through the area.
The height of the powerhouse was limited to 4m due to aesthetic reasons. As a powerhouse crane would not fit into such a limited head room, the Pelton unit was installed through an opening in the roof using a mobile crane. The same method would have to be applied for repair, overhaul and refurbishment of the unit in the future.
The vertical shaft, four-jet Pelton unit is directly connected to a synchronous generator of 1950kVA capacity which operates at 1000rpm and 690Volts. The generator is water-cooled with a heat exchanger in the tailrace channel. With the use of water instead of air cooling, the ventilation system is reduced to small openings in the powerhouse walls whereby noise levels in the sensitive area outside the powerhouse do not exceed 45dBA even at full load. As the plant is located at the end of a spur line of the grid, it had to be equipped with additional protection units for the special case of an unwanted stand-alone operation of the Tambo plant. This dangerous situation could occur when the spur line becomes disconnected from the national grid and the small hydro plant continuous to cover the entire load connected to the spur line. When the grid comes back and the connection to the spur line is made, phase angle and possibly also voltage and frequency might no longer be synchronous and considerable damage to the electrical equipment would occur.
Economics of the plant
Total investment costs of the plant are close to 9M Swiss francs (US$9.6M). As the service lives of most of the power plant elements are estimated at 40 years, the investment is discounted over the same period even though the water use rights have been granted for 60 years. The plant operator is an employee from the commune of Splügen who looks after the day-to-day operation and maintenance of the plant on a 20% part-time basis. This arrangement not only helps to reduce operational costs but it also brings job opportunities to the remote communities of the Swiss Alps.
The plant currently benefits from the Swiss feed-in tariff system KEV which covers the difference between the volatile market prices of power in Europe and the pre-determined generation costs of a small hydro plant of equivalent size as the Tambo Plant. Only with this stable income stream are investors prepared to take up small hydro development and to increase the share of the renewables in the Swiss power mix which currently consists of approximately 60% hydro and 40% nuclear power.
For the two investors, Alpiq EcoPower Switzerland Ltd. and the community of Splügen, the Tambo development has been regarded as a success. Only a few months after the commissioning of the Plant, they are both convinced: we need more of the same.
There are two new projects of similar size and investment volume in the vicinity of Splügen in the pipeline and the design is well under way. If project approval, equipment tendering and construction go as smoothly and efficiently as with the Tambo Plant, then the two new plants could be on line by 2015.