A programme of modernisation investment is being rolled out at hydro plants owned wholly or partly by Norwegian utility EB, reports Patrick Reynolds
Known for its global status in hydropower use and nous, Norway has extensive water resources, integrated catchment management, a large body of established assets with myriad needs in maintenance, and also a big, strategic plan to become a "green battery" for Europe’s grid network in the era of increased used of intermittent renewables.
But within that mature picture there are a few gaps that offer the possibility of capacity upgrades at existing sites, either within footprints or calling for new powerhouse and other infrastructure construction to exploit some of the untapped potential at key locations in catchments.
One such expansion project is the redevelopment programme at the Embretsfoss site, which is co-owned by local utilities EB and E-CO. Lying almost 50km west of Oslo, the complex is upstream of Drammen and Hokksund. The main part of the redevelopment is the new Embretsfoss 4 powerhouse, which is being build in the riverbed immediately downstream of the dam, on the opposite side from the existing plants. A new dam is also being constructed.
The Embretsfoss redevelopment is also among four modernisation or enhancement projects that EB has, or recently had, in the construction phase, including: Kaggefoss, on a tributary in the Drammen catchment; and also Gamlebrofoss and Mykstufoss, to the west in another but very long catchment that, eventually, runs down to Larvik.
EB – portfolio and plans
EB’s asset base is 24 wholly or partly-owned hydropower plants. The utility wholly owns 15 plants with total capacity of 430MW and producing about 2.1TWh on average annually, and partly owns nine stations (operating six) plants that have 85MW in combined capacity and generate about 700GWh/year.
In addition, it owns a 25% share in Norsk Gronnkraft AS, which is one of the country’s largest small hydro investors. The company has about 25 plants of less than 10MW capacity. EB also has some involvement with wind power as well as undertaking power grid and district heating infrastructure work, contracting services, and also retailing electricity and broadband services.
The hydropower assets are primarily run-of-river plants which operate more or less continuously throughout the year, which is excellent for production – operating hours at each facility is, therefore, about 8700 hours with allowance for around two days stoppage for planned maintenance. This high operational level and large flows are key to the relatively high output per installed MW in the wholly-owned portfolio.
Challenges and Opportunities
But there are challenges and risks, potentially, including there being only one turbine at most of the plants, and the average age of the units is more than 50 years. However, there are also opportunities with untapped hydropower resources within the catchments.
EB notes that the normal lifetime of "heavy components", such as a runner, rotor, stator or transforner is expected at around 30-40 years, and they are of such importance that the consequence of breakdown of any one could be significant production losses running into months. It notes further that while lifetime of mechanical components, such as turbines, can be stretched it would necessitate more frequent stoppages for inspections plus increased maintenance, which overall reduced efficiency.
In contrast, EB says, there is more certainty around the lifetime of electrical components, such as the generator and transformer. It ads that this is due to aging and deterioration of insulation materials as well as wear.
The circumstances for control equipment lifetime is different again with technology leaping forward, resulting in technologies, systems or key components becoming outdated (15-20 years) significantly faster than mechanical and electrical equipment. Further complications include, as technology and production moves on to other systems, there being more difficulty in accessing spare parts, limited knowledge remaining of the older systems, and possibily more support required to maintain old equipment.
Consequently, the asset base has been undergoing extensive, plant-specfic analyses to evaluate the needs, risks and also the possibilities for modernisation. The analyses has helped to develop the large programme of investment works planned for the rest of this decade across the portfolio.
Investing for the Future
For the investment plans, EB and some co-owners of certain assets have developed a refurbishment, upgrade and redevelopment strategy across many of the plants to enhance safety, improve efficiency, increase output and extend the operating lives of the assets before being caught unstuck by aging plant failure and leaving hydro potential unexploited.
The 2012-2021 investment programme has a budget of approximately NKr2 billion (US$349 million), and is majority funded by EB and E-CO (co-owner at Embretsfoss). At present, approximately 50% of the budget relates to projects either committed or under construction. The total investment is expected to add approximately 250GWh of electricity production annually, and also ensure the plants have useful working lives for another 30-50 years.
The four plants currently being upgraded are expected to bring a total of up to about 140GWh extra in electricity production per year, chiefly from Embretsfoss 4 (E4) – 120GWh/year – with contributions each of 5GWh/yr – 10GWh/yr from the Kaggefoss and Mykstufoss projects when completed over 2014-15.
Approval is awaited for a plan to gain a further 6GWh/yr at the Hellefoss Kraft AS facility, downstream of Embretsfoss 4 and near Hokksund. Studies and design work are underway for many further projects at other plants in the portfolio from 2015 and to be executed, it is planned, in a rolling programme into the early 2020s. In the near-term, the projects include extra generation at Gravfoss I, Begna and Henfoss – all upstream of Embretsfoss 4.
Further ahead in the programme, moderniation works are planned at plants including Dovikfoss, Djupdal, Hofsfoss, Pikerfoss and also Embretsfoss 3 (E3).
Redevelopment at Embretsfoss
Embretsfoss is on the lower section of the Drammen River ("Drammenselva"), which has one of the largest catchments in the country – approximately 17,000km2. At the site, the concrete dam was built in stages in the first half of last century and has almost no storage capacity. It has two flap gates, a sector gate plus a fixed spillway on each bank.
The first hydro plant at the site – Embretsfoss 1 (E1), on the east bank – was commissioned in 1916 with 1.2MW capacity. Five years later Embretsfoss 2 (E2) was commissioned at 9MW (3 x 3MW Francis) on the west bank, and in 1955 the turbine runners were replaced.
Shortly before the runner replacement at E2, on the same bank – next door, in fact – construction was completed for the E3 powerhouse while E1 was decommissioned. E3 was installed with a single 18.5MW Kaplan turbine. Taking a design flow of 125m3/s and operating under a gross head of 16.3m, the E3 powerhouse generates about 65 GWh of electricity per year. The station can, however, take flow of up to 150m3/s at 25% overload during periods of water surplus.
Under the redevelopment plan, the E2 powerhouse is to be closed down and E3 switched to low base load/flood peaking operation after E4 is commissioned next year. The changes will boost overall output at the site by about a third to approximately 330GWh of electricity annually, and all achieved within existing licensing. Then, in about nine years or so, E3 will undergo a large refurbishment.
Birger Holt, EB’s head of planning and development, says: ‘It is not urgent to modernise E3 before the 2020s since it will be very smooth operated after E4 is commissioned. E4 will have almost sufficient capacity to handle the water itself.’
The redevelopment work at the site is currently one of Norway’s largest ongoing power projects, and has a budget of approximately NKr735M (US$128M) plus facilitation costs. The works also include an upgrade to the existing, Class-1 category dam to pass higher estimated flood flows, as calculated in 2007.
Existing flood diversion capacity is approximately 1900m3/sec at flood level 33.8m asl with gate capacity of 650m3/sec at maximum operating headwater of 30.23m asl. The revised flood estimate is 2660m3/sec at flood level of 35.86m asl, calculated as the 1-in-500 year flood event. The need to pass larger floods calls for more larger gate size and fixed spillway capacity.
"Since the flood gates make up a significant part of the dam, both size- and cost-wise, it was found more cost efficient to build a new dam downstream of the existing dam," Holt says. "The approach of constructing a new dam also ‘made it easier to maintain normal production in the existing powerhouses.
"An additional important function of the flood gates are to control and maintain stable headwater level when having surplus water," Holt explains. He adds that due to experience on a previous project the design choice was for a pair of 20m wide by 7m high radial gates to be installed in the new dam.
The new dam construction does not affect E2 and E3, which have separate headraces tunnels leading from intakes just upstream of the existing dam.
As the anchor piece of the latest redevelopment of the Embretsfoss site – almost 60 years after the first enlargement, in the 1950s – the E4 powerhouse is being constructed on the east bank, slightly downstream from the original dam but next to the new structure.
The surface powerhouse, founded deep in the rock mass of the river bed, is being equipped with a single 52.5MW Kaplan unit. Under the same gross head as the other plants across the site, and with a design flow of 340m3/sec, the new station is expected to produce about 270GWh of electricity annually.
"Two smaller units instead of a single unit were considered," Holt says, "but this is a more costly solution, and the existing unit at E3 will be in operation for many years with significant capacity."
Board approval for E4 was obtained at the end of 2009 and build contractor AF Gruppen began construction on site the following February. Project and construction management services are provided by EB, with project services from Holte Consulting. Planning was performed by Sweco-Norge.
By mid-2011 the site was well established with a major excavation blasted in the bedrock in front of the east side of the dam. The building foundation with the Kaplan draft tube was constructed in the excavation and from there the building would be constructed upwards and finish above the surface.
But last Summer saw unusually high river flows which quickly arrived and then persisted, causing difficulties for the construction work, some programme changes and extra costs, but there was good flexibility and support from suppliers, EB notes. However, despite the difficulties, which Holt says has been the ‘main challenge for the project,’ the scheme is still on schedule.
Explaining further, Holt says that normal flows at the site in Summer are around 170m3/sec-200m3/sec but the flows that were experienced just over a year ago were about four times that rate – and they were maintained through most of early June to early October. It was the worst sice records began more than 100 years ago, he says. The peak in flow was more than 1000m3/sec, which was within the temporary flood discharge capacity of 1400m3/sec.
:This special Summer flood caused some shift from Summer work to Winter work without delaying the project," Holt says.
In total, the construction works have called for 40,000m3 of concrete and 1200 tonnes of steel plus 35,000m2 of formwork. Construction challenges further included some complicated, precise geometry to be achieved for the reinforced concrete in the rockbed excavation for the spiral drum (lower cone and inner part) and suction tube (entirely in concrete; no no steel lining).
EB notes that the maximum reinforced concrete sections to be constructed were each 3000m3 in volume for the reprofiled dam sill and roof of the spiral drum (which was 2m thick), respectively. The utility adds that the concrete mix design was continuously adjusted
The electro-mechanical equipment, including the control systems, are being supplied by Voith Hydro, which was awarded the contract at the beginning of 2010. The transformers are coming from Etra/Energia and the machine hall crane is supplied by Kone Cranes. The hydraulic gates have been manufactured for the project by Lysaker & Thorrud.
Now in the closing stages of the project construction phase with major E&M installation work having started early this Summer, the powerhouse has only a few months to go until trial operations begin around March 2013.