Renewable energy company Pacific Hydro not only celebrated its 20th anniversary in 2012 but also celebrated ten years of working in Chile. Following the acquisition of the Coya and Pangal hydro power plants in 2004, the company acquired the water rights of the upper Cachapoal River basin in the country's Sixth Region. The 11MW Chacayes run-of-river hydro is Pacific Hydro's first project in this basin which will lead to the eventual development of some 600MW of hydropower.
Located in a dramatic setting in the Andes Mountains, Chacayes is approximately 10km upstream of the Coya hydro project and 90km south of the capital Santiago. It was built under an EPC contract with Constructora Astaldi Fe Grande Cachapoal Ltda (a consortium made up of Astaldi SPa from Italy and Chilean construction company Fe Grande). The project presented a number of logistical and commercial challenges as the development is located in a seismically active region of Chile, with one of the two diversion intakes located in a national reserve. Careful attention also had to be given to the visual impacts on this unique development.
Chacayes was inaugurated in October 2011. Its clean energy is needed to meet the 6% annual growth in the Chilean electricity market and to reduce reliance on energy from imported fossil fuels. Chacayes will deliver an average of 558GWh per annum - enough electricity to supply more than 300,000 Chilean homes - to Chilectra, the largest electricity distributor in the country, under a long-term power purchase agreement. Its operations will also abate approximately 357,000 tonnes of greenhouse gas emissions every year.
The main components of the Chacayes project are:
• Cipreses diversion on the Río Cipreses and diversion tunnel (2.5km) to upstream of the Chacayes headworks.
• Chacayes headworks on the Río Cachapoal.
• Conveyance system including two free flow tunnels.
• Chupallal regulating pond.
• Low and high pressure tunnels.
• Drop shaft and surge shaft.
• Surface powerhouse with two Francis units.
• Connection to Coya water conveyance system.
• A 220kV, 17km long transmission line.
• Cipreses diversion intake.
The Cipreses diversion is located on the Cipreses River within the boundaries of the Reserva Nacional Río Los Cipreses and the project design had to take this into account. As a result, it was decided to utilise a 20m long, 3m high rubber dam for impoundment, as this provided a low profile structure to minimise the visual impact in the area. It also provides for fail-safe flood passage as the rubber dam automatically deflates when overtopped.
The diversion weir is constructed of conventional concrete, with a layer of granite paving stones provided to resist abrasion from the large volume of bed-load material transported during floods and the snow-melt runoff. An access gallery is provided below the body of the weir to provide access between the two banks of the river. In addition a degraveller/sediment sluice is provided on the right bank to allow flushing of sediments from in front of the intake structure.
The intake, located underground again to reduce visual impact, consists of two openings with an intake gate controlling flows into the 2500m long free-flow diversion tunnel. The tunnel discharges into an open canal that in turn discharges into the headpond above the Chacayes diversion intake structure. The 5m diameter tunnel was constructed using a TBM. This method was selected by the EPC contractor in order to accelerate construction and provide access (via the excavated tunnel) to the Cipreses intake, as no construction road access was allowed through the National Reserve. It also meant that there was no drill and blast tunnelling within the reserve.
Chacayes diversion Intake
The Chacayes headworks is located on the Cachapoal River and is designed to capture a flow of up to 72.5m3/sec. It comprises a diversion weir, intake and desander. The diversion weir has five main bays equipped with radial gates and a degraveller bay located in front of the intake for flushing of bed load. The intake has four gates equipped with trashracks leading to a four-bay desander, designed to remove suspended sediment down to 0.2mm. The desander discharges into the conveyance system.
Water conveyance system
The water conveyance system (excluding the pressure tunnels) consists of a combination of some 6km of open canal, four culverts under major stream crossings and two tunnels. The canal is trapezoidal in cross section with a bottom width of 3m and a water depth of approximately 5m. The culverts allow the crossing of major streams over the canal that, due to heavy bed load, cannot be diverted under the water conveyance system. The culverts are double-box reinforced concrete free flow structures and the two free flow tunnels, La Isla and Peralitas, are 7m horseshoe shaped, 285m and 945m long respectively.
The water conveyance system flows into the Chupallal reregulation reservoir which provides the storage required by the project for potential peaking operation and to earn firm capacity benefits. It also acts a de-silter to remove some of the fine sediments.
The pond has a live storage of 840,000m3 with a surface area of approximately 18ha. The embankments are composed of selected fill excavated from the pond area with proper compaction to facilitate the installation of an impervious lining consisting of a sandwich of 1.5 mm HDPE geomembrane, geotextile and geodrain. Sediment settling out in the pond is removed by dredging and discharged back to the river.
Pressure tunnels
The pressure tunnel consists of a 1992m long low pressure tunnel, a 100m high surge shaft, a 77m deep drop shaft and a 368m long steel lined high pressure tunnel leading to a surface powerhouse.
Two significant underground issues had to be addressed. One was hydrojacking where there was insufficient confining pressure surrounding the tunnel to contain the water pressure that could then jack open the rock with a resulting leakage. This was addressed through in situ testing to determine the confining pressure and the design providing a steel liner from the powerhouse to the bottom of the drop shaft, and from there to just upstream of the surge shaft by an impermeable membrane sandwiched between the initial shotcrete support and the concrete liner.
Another phenomenon observed in all of the tunnels, but particularly the power tunnel, was the existence of swelling minerals in the rock mass. The initial geotechnical investigation did not identify this issue and extensive testing using ethylene glycol had to be done in the tunnel to identify the areas most susceptible to swelling. The observed swelling pressures of up to 1.2 MPa resulted in the need for a heavily reinforced concrete or fibre reinforced shotcrete liner in the affected areas.
At the downstream end of the high pressure tunnel, the tunnel bifurcates into two penstocks which carry the flow to each of the two units in the powerhouse. Each of the penstocks has turbine inlet valves.
Powerhouse
With dimensions of 56m x 21m the surface powerhouse contains the two vertical Francis turbines supplied by Andritz Hydro, each with a rated output of 55.4MW at a net head of 168.6m and a rated discharge of 36.25m3/sec. The runners, wicket gates and discharge rings are of a modern design and have a ceramic coating on the runners for maximum resistance to sediment erosion and cavitation.
To maximise plant and unit reliability, each generator has a dedicated 13.9/110/220kV main transformer rated to handle the maximum output capability. The power is evacuated through a conventional air-insulated switchyard connected to a double circuit 220kV transmission line.
Turbined water discharge from the tailwater bay (22.5m3/sec) is conveyed across the river through a steel aqueduct to feed the existing Coya plant water conveyance canal, with the remainder being spilled back into the river.
Environmental Initiative of the Year 2011
Pacific Hydro adopted several measures in the design process that ensured there was minimal impact during construction of Chacayes 111MW run-of-river hydro plant, which is surrounded by woods and mountains. This was acknowledged by the International Tunnelling Society which granted Chacayes with the environmental initiative of the year award in 2011.
Recognised measures included the:
• Use of a TBM for the excavation of the Cipreses tunnel.
• The rubber dam at the Cipreses intake to produce a low profile and risk free diversion.
• Construction of a 20km long, 66-kV line to supply all the construction power requirements instead of diesel generators.
Pacific Hydro had undertaken a continuous dialogue with communities in order to avoid or mitigate social impacts and also to manage any issues in neighbouring rural and urban areas through the implementation of a community management plan. Equal importance was given to monitoring of the native fauna in the project surroundings, with special emphasis on the endangered species such as the Andean puma, Torrent duck, Tricahue parrots and small Bagre catfish.
One of the highlights of the project was the development of a tree nursery fully managed by community members for reforestation, which created local training and employment opportunities and allowed the production of approximately 400,000 native plants and trees. These were used in the reforestation of 160 hectares. Also, more than 30km of local roads were widened and upgraded, as well as the widening of an existing bridge to improve the connectivity of the region.
Since 2007, Pacific Hydro has been running the Creciendo Juntos Community Fund, which has financed over 80 projects submitted by the nearby communities. The company also supports training courses to improve the skills of members of the local communities and has developed educational publications based on information collected by its environmental team.
Clean Development Mechanism accreditation
The United Nations Framework Convention on Climate Change approved the registration of Chacayes under the Kyoto Protocol's Clean Development Mechanism at the end of 2012. The project underwent a two year process in order to get this registration which requires compliance with several requirements such as verifying that reducing emissions is essential for the viability of the project, field visits and audits by international auditors. The accreditation allows Pacific Hydro to issue and trade in carbon credits from Chacayes.
Matthew Forward is General Manager Group Engineering Services at Pacific Hydro and can be reached at mforwood@pacifichydro.cl
