Articles & Case Studies

Geislingen Energy Recovery Plant uses KSB Omega pump for power generation

Posted: Monday 12th July 2010

Moving drinking water around a distribution network covering more than 770km is energy demanding and it is inevitable that a proportion of the energy costs incurred end up in customers’ water bills. In Baden-Württemberg and Bavaria, Germany, Zweckverband Landeswasserversorgung (ZV-LW) delivers drinking water to over 250 cities and communities with a combined total population of around 3million. It is a huge undertaking and one that requires not just attention to the pipeline and pumping station infrastructures, but also the energy costs involved.

The main sources of the water supply are the Langenau Groundwater reservoirs and water extraction from the River Donau near Leipheim in the south east of the region and it is from here that two main pipelines run northwest towards reservoirs near Stuttgart. The many cities and communities in the region receive their water supplies through a network smaller pipelines and reservoirs branching off the two main routes (Fig. 1). The two main pipelines start at an altitude of 455m and take different routes and their capacities also differ, with the southern route which runs through mountainous terrain having the higher volume capacity. The southern route is more costly since the water must be pumped about 50m higher than on the other route. From the top of the southern route there is a requirement to pump water to an altitude of 859m. Such demands result in high energy bills.

One of the most important components of the southern route is the energy recovery plant at Geislingen, which has used a Francis Turbine to produce power since 1974. Until 2008 the power which it generated was sold to the public electrical grid, the objective being to earn the maximum amount of money and profit from the potential energy of the water and to keep water transport costs to their customers to a minimum. Keeping costs down was originally achieved by filling the water towers along the network at night when electricity was cheaper and using the turbine during the day to generate electricity through the geodetic height difference.

In recent years deregulation of electricity market in Germany has resulted in a single charge band throughout the day, thereby making filling the water towers at night unnecessary. It has also removed any profitability for ZV-LW from selling power to the producers, so the Company has turned to using the electricity which it produces at its Geislingen energy recovery plant and pumping station entirely for its own purposes. In 2007 the Geislingen energy recovery plant was given the additional task of supplying the total energy requirements for the nearby Amstetten water tower, which had been expanded to accommodate additional pumping capacity. Here additional pumps were introduced to transfer drinking water over the Schwabischen Alb to the water tower at Heuberg, 895m above sea level.

In the case of a turbine failure or maintenance, no energy can be sold to the energy supplier and ZV-LW had to purchase electricity at a very high cost. Therefore, in order to operate the Amstetten pumping station independently from the energy supplier, ZV-LW decided to install a feasible pump as turbine. At the same time, based on extensive experience with using KSB centrifugal pumps as turbines (PaTs), a decision was taken by ZV-LW to consider the feasibility and cost-effectiveness of this method of generating power. ZV-LW has employed KSB’s Etanorm and Multitec pumps for generating electricity elsewhere along its water supply network, having demonstrated that this method is an efficient way of generating energy and contributing to energy savings.

Having investigated the costs of installing a second Francis Turbine capable of generating 1200kW, it was decided that the costs were prohibitive as the investment would not amortise when compared with the lost energy costs resulting from a non-functioning turbine. A KSB Omega 600kW axially split, single stage volute casing pump (Fig.2) was selected to operate as a PaT because the investment and installation costs amounted approximately one quarter of the Francis Turbine. A further advantage of this approach was that when required, the pump could be used in a conventional mode as a pump to provide back-up for transporting water within the pipe network.

Running a centrifugal pump in reverse rotation mode is far from being a new concept. The potential for running pumps as turbines originated when the water industry wanted to know what the impact would be on their systems should a pump stop and then go into reverse operation. This was a scenario which was raised with KSB and in response its hydraulics engineering department was brought in to examine this issue and to calculate the turbine performance curves of pumps when running in reverse. They discovered that the behaviour of the pump running as a turbine is very good as the energy output could be higher than the energy input used to run it as a pump.

Operating a centrifugal pump as a turbine involves the liquid flowing from the discharge outlet to the suction inlet so that the impeller turns in the opposite direction. Should the head of pressure be high enough to overcome the breakaway torque of both the impeller and shaft, the torque can be employed to drive a generator, so the pump operates as a turbine. Hydraulically, the pump in turbine mode can handle a higher volume of water than when in conventional pumping mode. There is a higher flow inside the pump and this means that the amount of energy that comes out is higher.

With the pump running in reverse, the shaft torque can be utilised in a number of ways. When attached to a generator it is the mains frequency that determines the speed. Incorporating frequency inverters and appropriate mains feed circuitry creates greater speed variations without adding very much cost to the package. Quite clearly this is a very cost-effective way of generating power even when compared to the higher efficiencies of a conventional turbine.

At the Geislingen energy recovery plant and pumping station, there is a requirement to generate electricity 24 hours a day, without any interruptions. To meet this requirement, it is necessary to run both the turbine and the KSB Omega pump as a turbine, either in unison or individually as demand dictates (Fig.3). Because the water flow through the station varies throughout the day and also according to the seasons, ZV-LW is in able to switch between the two systems in order to optimise the efficiency of the plant.

During the summer the water flow is higher than at other times of the year. Also during the daytime hours the water flow is higher than at night. By calculating the water flow patterns it has been possible to determine the most efficient time of the 24 hour period to use the Francis Turbine and the PaT. When water flows are at their highest, the Francis Turbine is an efficient method of generation power and when flows are lower greater efficiencies are achieved by switching over to the PaT. However, the overriding objective is to generate optimum power at all times.

The KSB Omega pump operating as a turbine attains its optimum power generating performance of 600kW when the water flow is at 9520/lt/sec. Together with the performance of the Francis Turbine, the KSB Omega PaT ensures that there is sufficient power to support the pumps and infrastructure at the Geislingen plant and the Amstetten water tower without there being a requirement to draw power from the national grid.

Since the decision was taken by ZW-LV to use the KSB Omega PaT at Geislingen, the PaT has also been used to operate as a traditional pump during extended periods of interruption along the southern pipeline route between Langenau and Amstetten. Based on their experience gained at Geislingen with what is the most powerful pump used as a PaT, coupled with its use for emergency pumping duties, ZW-LV is planning to install a further pump to run as a PaT at its energy recovery station at SGA Aufhausen on the northern pipeline route.

Tel: +49 6233 86-3702
www.ksb.com




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