Old and new at Waldeck

15 April 2011

With the liberalisation of the electricity market and the rapid expansion of wind power, increasing use is being made of pumped storage power plants to provide balancing power. This is creating a favourable climate for investment in the preservation of existing and the construction of new plants of this type. Martin Rau from E.ON Wasserkraft explains how the combined refurbishment and rebuild project at Waldeck pumped storage plant in Germany has helped to secure efficient plant operation for another generation

With the liberalisation of the energy market and the rapid growth of wind power in Germany, pumped storage power plants (PSPs) are gaining in importance for the supply of balancing power and energy. Since the year 2000 the development of PSPs has acquired even greater significance as a result of two changing boundary conditions; the liberalisation of the energy market in 1998 and the rapid expansion of wind power since the renewable energy law came into force in 2000.

The consequential increase in the intrinsic value of pumped storage power plants is creating a favourable climate for investments in the refurbishment of existing plants and the development of new build hydropower plants of this type. At the pumped storage power plant of Waldeck I, the existing plant was refurbished and a new pressurised duct power plant was constructed in parallel next to the existing machine hall under a preservation order. Both schemes were carried out with the power plant still operating at part load, so thorough planning and scheduling of refurbishment and new build activities was essential for the success of the project.

Waldeck I PSP was commissioned in 1932 as part of a hydropower development at Edersee in Hesse in central Germany. The Edersee hydropower complex comprises two pumped storage power plants, Waldeck I (140MW), Waldeck II (440MW), and the Hemfurth storage power plant (20MW).

Waldeck I PSP had reached the end of its life after some 75 years of operation of four horizontal pumped storage units. Several alternatives for modernisation were considered and evaluated, with the final decision going for a combination of modernisation and new built with maximum utilisation of the existing structures. In the final concept which is now approaching commissioning, the existing upper basin and penstocks were refurbished; a new pump turbine was constructed as a pressurised duct power plant connecting to one of the penstocks, and the efficiency of two existing Francis units was improved while all pumps in the old powerhouse were decommissioned. A particular challenge of the planning and construction work was to maintain operation of the old Waldeck I power plant during implementation.

This present paper provides a general outline of the changing significance and mode of operation of pumped storage power plants in liberalised energy markets, and describes the function of the whole Edersee hydropower complex. The concept used for Waldeck I – utilising and refurbishing existing assets while constructing a new pressurised duct powerhouse with an installed rating of 70MW in the limited space available within the plant perimeter – is also explained in detail. Special focus is given to challenges during the planning and implementation of all the works, since the existing power plant was in part load operation throughout the construction period of about three years. This paper shares the lessons learned from the start of the feasibility study in mid 2004 to commissioning in the spring of 2009. The concept of refurbishing old units while simultaneously constructing a new plant has increased the circulation efficiency considerably.

A changing climate

A little over 30 pumped storage power plants with a total output of approximately 6500MW are currently in operation in Germany, and almost all of these were built by the end of the 1970s. The construction of pumped storage projects subsequently stagnated because the nationally controlled electricity tariffs offered insufficient variation between day-time and night-time hours. It was not until the year 2000 that pumped storage began to be attractive once more. Two developments in particular have helped assure the commercial viability of PSP plants:

• Liberalisation of the electricity market in 1998.

• Huge expansion in wind power since 2000 due to the Renewable Energy Law (EEG).

The liberalisation of the electricity market coupled with free commodity trading changed the value of traded electricity in line with the particular demand situation. Today, PSPs have significantly more operating hours and a greater number of operating mode changes. Whereas these power plants used to run an average of one pump cycle and two turbine cycles per day, they now feed into the power grid more frequently and often for shorter periods as well. The price differences with their considerable hourly fluctuations, their day-time peaks and night-time troughs and their general dips at weekends, enable once again to calculate revenue and project promising investments in the preservation of existing and the construction of new PSPs.

Modernisation and new build

The Waldeck I pumped storage power plant is located in Northern Hesse right next to the Edersee Lake. The old power plant with its four horizontal units and a total installed capacity of 140MW was commissioned in 1932 and forms part of the Edersee power plant complex operated by E.ON Wasserkraft (EWK). In immediate proximity to the Waldeck I PSP, EWK operates the Waldeck II cavern PSP (460MW) that was commissioned in 1975, plus the Hemfurth storage power plant (20MW) located on the Edertal dam.

The Waldeck I PSP consists of the upper basin with a net capacity of 695,000 m³, two inclined penstocks with an inside diameter of 2.2-2.6m and the machine hall that is right on the bank of the Affolder Lake.

With Waldeck I PSP having reached the end of its technical life, in 2004 EWK began to look at what measures would be needed to ensure safe and reliable operation for a further 30 years. It was obvious from the outset that modernisation would only be possible by making optimum use of the existing plant components and infrastructure. Initial investigations indicated a number of boundary conditions for consideration:

• The upper basin was damaged and needed a new seal on the water side. Extending the upper basin was not an option for economic and permit-related reasons.

• The two existing inclined penstocks had been renewed in 1978-9 and could be re-used without any additional refurbishment work.

• The replacement of identical units would not be economically viable, so a new power plant had to be built if operations were to continue.

• Building a new cavern power plant could not be an option if the existing penstsocks were to be retained, instead only a shaft power house would be possible on the existing site.

• Continued operation of the old pumps was technically impossible. However two of the turbines in the old plant could be refurbished and used for ancillary services, thereby significantly improving the project's financial viability.

• The plant shutdown should be kept as brief as possible in the interests of economics of the overall project.

EWK together with lahmeyer International subsequently adopted the following modernization concept:

• Construction of a new pump turbine with approximately 70MW capacity as a shaft power plant on the existing site, improving the circulation efficiency from 63% to 75%.

• Retaining the existing upper basin and penstocks as well as rehabilitating the upper basin.

• Refurbishing two Francis turbines and decommissioning all four pumps and two turbines in the existing old plant.

Design competition and contract award

The entire construction contract was awarded to a consortium as an EPC (engineering, procurement and construction) contractor. An EU-wide prequalification procedure was held in early 2005 on the basis of a functional specification. This was followed by a design competition with three consortia. Each consortium developed a design planning which was negotiated and agreed with EWK over several rounds of talks. Whereas functional commissioning of the new-build work was relatively straightforward, the functional award of a contract for the rehabilitation proved to be challenging as the different bidders had different ideas about the extent of rehabilitation that would be necessary. For the scope of works for rehabilitation therefore, the contract described service items and defined quantities with unit prices. Rehabilitating the upper basin on the other hand was also awarded on a functional lump sum basis as the bidders would be responsible for both planning and execution.

Each consortium submitted a bid following the design planning phase. The bids were evaluated as much on plant downtimes as on the cost of the new build and refurbishment work. It was apparent that the rehabilitation works to the upper basin were time-critical and could only be carried out during the period March-September.

The contract to build the new power plant was eventually awarded to the Bilfinger Berger AG/voith-siemens Hydro consortium at the end of 2005.

New build shaft power plant

The biggest challenge for 2006 – the first year of construction – was to build the powerhouse shaft, involving the excavation of a 25 x 17m pit almost 40m deep with just minimal clearance from existing structures. Close to the surface the excavation pit was established by a grouted anchored bore pile wall. In the underlying rock levels, the pit was secured using SN anchors plus a reinforced 15cm thick shotcrete shell. Adjacent to the existing lower basin the pit was secured with a box cofferdam.

To seal the subsoil, the existing sandstones and clays were consolidated with cement injections. Owing to the presence of a geological fault, it became clear that the existing rock was far weaker than the geotechnical site investigations had predicted, so the strength of the rock was improved with further injections to ensure the stability of the existing plant. A total of 10,000m were drilled and 670 tonnes of cement injection were grouted.

In the excavation pit of the shaft powerhouse the rock was carefully loosened with a hydraulic digger, so protecting the existing plant from the excessive shock and vibration that would be caused by blasting. The excavation work was monitored with the help of a comprehensive observation measuring programme. The measured parameters that were recorded on the installed sliding deformeters, extensometers, measurement levels and groundwater levels were all within the calculated design limits. Consequently the operation of the old plant was not affected by the construction works of the new build.

Concreting the shaft commenced in January 2007. The 1m thick walls of the shaft were of impermeable concrete. Jacking formwork was used for the construction of the shaft's outer walls. Finally, the new turbine hall was built within around three months from prefabricated concrete sections in early 2008.

The new pumped storage set consists of a reversible Francis pump turbine with a vertical shaft and an 80MVA motor generator. The main assembly commenced in early April 2008 once the machine hall crane became ready for use in the new power plant.

Repairs to the old plant

The operation of the old plant was contractually agreed with the appointed consortium as a separate item. Two of the existing machine units continued operating until the end of the first year of construction - the end of December 2006 - then had to be finally decommissioned. Once the penstock connections for these two units had been dismantled, the existing penstocks were connected up to the new shaft power house with an inclined shaft. The other two units from the old plant remained in operation for roughly another year to the end of March 2008.

The first of April 2008 saw the start of extensive rehabilitation works to all components of the old plant that would be needed for continued future operation, and by the end of 2008 work was simultaneously ongoing on all three construction sites - shaft power house, old power plant and upper basin.

New seals were installed on the upper basin with the aim of achieving a watertight basin. A two-layer bituminous concrete seal was applied to the basin invert. The up to 17m high sides of the basin wall were sealed with a waterproofing PVC geomembrane system (carpi). Both seals were constructed over an underlying drain system to allow the condition of the seals to be reliably monitored and assessed in operation by measuring seepage water. A fibreglass cable was also installed in the transition zone between the membrane and the bituminous concrete with which leaks at the membrane can be located.

A trial filling of the basin proved that the membrane and bituminous concrete had been constructed according to quality standards. However it appeared that there were leaks in the intake area where only minor repairs had been carried out to the concrete. With the approach of winter threatening to halt work for a long period of time, EWK quickly decided to reconstruct the intake area. This work was carried out between October and December 2008 and was completed just in time before the onset of a lengthy period of ice and snow.

In addition to the construction work, rehabilitation works were also carried out to two turbines that formed part of the old plant. These two units will continue to run for around 15 years supplying power for station services. The two turbines were stripped down and overhauled partly on site and partly at the factory. The extent of the work could only be assessed accurately once the units had been dismantled. The originally planned package of rehabilitation measures would have to be constantly updated, eg on one unit the wicket gates were found to have significant damage and had to be replaced. Fortunately a set of spare wicket gates was still available and this was installed following modifications.

Finally, we should record that the planning of the rehabilitation to the old plant while the plant was in operation was not without its difficulties. It was not until the old plant had been completely shut down that the full extent of the rehabilitation works could be finally determined. Even then, refurbishment works were subject to constant review while the work was ongoing. This was the only way in which the assigned timeframe and budget could be achieved.

After some five years spent in project planning and construction, the modernisation and new build of the Waldeck I PSP was finally completed, with commissioning of the plant at the beginning of March 2009.

When it came to modernising the Waldeck I plant, E.ON Wasserkraft opted for a concept which allowed for the continued use of essential existing assets as well as the construction of a new build power plant utilising existing plant components of the existing plant. In this way the site could be secured for a further generation while guaranteeing efficient continued operation.

Martin Rau is a senior project manager at E.ON Wasserkraft GmbH, Luitpoldstraße 27, 84034 Landshut, Germany

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