Using pumps as turbines to generate power

29 September 2011, source Water & Wastewater Treatment

Has the time come for the wider use of pumps-as-turbines? On behalf of KSB, Bryan Orchard reports on the subject

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It is well documented within the pump industry that running a centrifugal pump in reverse rotation mode will, in certain conditions, create a turbine effect. Within the water industry this has been exploited to a limited degree to generate power in locations where a hydro turbine is considered too expensive.

Reducing energy costs is becoming a high priority, and pumps-as-turbines (PaTs) are generating significant interest from water companies, hydro operators and pump manufacturers. Identifying the potential of PaTs originated through water industry users wanting to know what the impact would be on their systems should a pump stop and then go into reverse operation.

Pump manufacturer KSB asked its hydraulics engineering department to examine this issue and to calculate the turbine performance curves of pumps when running in reverse.

They discovered that the behaviour of a pump running as a turbine is very good because the energy output can be higher than the energy input used to run it as a pump.

For locations where there is a relatively constant water supply, but power supplies are unreliable or even non-existent, PaTs are a simple and economic approach to generating power. One example involves the unique water supply system on the Indonesian island of Java.

Turbine driven
Designed by scientists at the Karlsruhe Institute of Technology, and commissioned in March 2010, the combined PaT modules draw their drive energy from a stream running 100m below the ground. Since the rural region has no power grid and diesel-driven pump sets are too expensive to operate, engineers had to come up with an unusual technical solution.

They worked out that by damming the stream, enough electricity can be generated to drive the pumps. So, rather than using expensive and high-maintenance turbines, the drive energy is now generated by reverse running PaTs. They drive several high-pressure pumps via a mechanical coupling, which in turn, transport the water to a tank above.

Depending on the season, the 75,000 local population now have over 50l/d of drinking water per head. Previously, they had to make do with just 10l as they had no access to this source of drinking water, which used to flow unused into the Indian Ocean.

PaTs can also be used where the power supply is readily available and reliable. Systems are now being used for large sites which have traditionally used conventional hydroturbines.

The power being generated is contributing to the overall running costs of pumping stations and, in certain locations, putting power back into the national grid.

Hydraulically, the PaT 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.

Efficiency
An added bonus is that when it is in reverse operation and running as a turbine the pump runs more efficiently than in conventional mode. 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.

In such a scenario, to generate a frequency of 50Hz, the pump would have to turn at an exact rate of 1,500rpm.

Incorporating frequency inverters and appropriate mains feed circuitry would create greater speed variation, without adding very much cost to the package. It can be seen, therefore, that this is a very cost-effective way of generating power even when compared to the higher efficiencies of a conventional turbine.

An alternative application would be to connect a PaT directly to a driven machine, another pump being an example, which does not have either a motor or generator to fix the speed.

In order for this type of unit to operate effectively both the PaT and pump must have 'equal output' at both ends of their shafts, which means that they must be rigidly connected to each other by couplings or a gearbox. If the PaT does not have sufficient strength, then the pump to which it is connected will not be able to provide the necessary discharge head. On the other hand, if it is too strong, the PaT will either waste energy or overload the pump.

Applications where KSB already has pump units on PaT duties include small hydropower systems (<10MW), major water transport systems, reverse osmosis and industrial systems where the technology can be employed as an alternative to throttling devices.

"In the small end market, power requirements are typically up to 100kW," says Matthias Cordier, KSB's senior project manager, small power. "But the investment costs are relatively high for conventional hydropower units and the payback period can be as high as 15 years.

With a PaTs solution, the payback period can be as short as three years and there is the added benefit that pumps are less complicated to operate than turbines."

The capability to operate multiple PaT modules is relevant for locations where water supply can fluctuate. Unlike conventional turbines, PaTs do not have adjustable guide vanes for adapting to fluctuations in the water supply and this is perceived as a drawback to their use.

By employing a number of differently sized units to distribute the total volume of water available, this difficulty can be overcome. This requires only the minimum of control and although this type of arrangement does diminish the cost advantage of the PaT over the use of a single turbine, it remains advantageous in terms of appropriate technology.

Quite simply, pumps are easier to operate and maintain than conventional turbines.