The concept of running a centrifugal pump in reverse rotation mode has been recognised by pump manufacturers for many years and within the water supply industry this concept has been exploited to a limited degree as a means of generating power in locations where it is considered too expensive to purchase a hydro turbine. It has not gone unnoticed by water suppliers, operators of small hydropower plants and pump manufacturers that running pumps as turbines (PaTs) is an efficient method of generating energy as well as recovering energy and contributing to energy savings. In the current world economic climate where reducing energy costs is becoming a high priority it is not surprising that PaTs are starting to create significant interest.
KSB Aktiengesellschaft is one of a group of pump manufacturers that is active in investing resources in PaTs and the company has recorded considerable success with its solutions in several parts of the world. Applications where KSB already has pump units on PaTs duties include small hydropower systems (<10 MW), major water transport systems, reverse osmosis and industrial systems where the technology can be employed as an alternative to throttling devices.
“KSB has been active in supplying volute casing and ring-section pumps for PaTs duties over several years, mostly into the small end of the hydropower market, “says Sander Klos, KSB’s senior project manager, Small Hydropower. “In the small end market power requirements are typically up to100 kW, but the investment costs are relatively high for conventional hydropower units and the payback period can be as high as 15 years. With our 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.”
He continues: “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. Not only that, PaTs can also have a role to play in locations where the power supply is readily available and reliable. KSB is now supplying systems for very large water supply infrastructures which traditionally have been the domain of conventional hydroturbines. Here the power being generated is contributing to the overall running costs of the water pumping stations and putting power back into the national grid.”
Identifying the potential for running pumps as turbines originated through users in the water industry wanting 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.
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. 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. The operating range for ring section and volute casing pumps is illustrated in Figure 1.
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 50 Hz the pump as turbine would have to run at a rate of ~1515 rpm (a little over-speed). 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. This mechanical solution to providing energy has been successfully supplied by KSB to locations in many parts of the world where a power supply is not available for operating water distribution/transfer pumps but there is sufficient energy in head of water to drive the PaT.
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.
Because of its exposure in all sectors of the global water supply industry, KSB has products that have been proven over many years. Pumps that the water industry will be more than familiar with are the Omega, Etanorm, Multitec and RDL ranges and it is these products that KSB is advocating for hydropower duties for heads above 10 m. Where the head is below 10m and flow rates are high, then axial flow pumps provide the most effective solution. The scope and operating capabilities of these pumps has enabled KSB to establish a business sector dedicated to designing PaT modules which can be used in single and multiple configurations (see figure 2).
The capability to operate multiple PaT modules is highly relevant for locations where the 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.
According to KSB, running several units 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 that conventional turbines.
Pumps used as turbines
Energy efficient in forward and reverse mode
Are you looking for an economical way to generate energy with low investment and operating costs? Or do you need a standalone plant for a self-sufficient energy supply? Or perhaps you are even thinking of a small pump storage power plant?
ANDRITZ pump operating as a turbine
When running forward, an ANDRITZ centrifugal pump contributes to energy savings and conservation. By operating in backward mode, the pump becomes a mini-turbine that can generate a very healthy ROI by recovering and generating electrical power. ANDRITZ pump-turbines are an economical alternative, compared to the cost of buying and operating a conventional turbine. They operate, for example, as recovery turbines in pulp and paper mills, in small hydropower plants, and supply energy to mountain refuges and forest lodges. Thereby, the transport various media ranging from drinking water, residual and waste water as well as pulp suspensions.
The pump-turbine for energy recovery is best applied in situations where high-pressure is needed in one part of the process, but no longer required afterwards. Typically in these installations, the pressure is reduced with a relief valve and the energy stored in the high-pressure fluid is lost. The ANDRITZ pump-turbine efficiently recovers this “lost” energy and generates low-cost power from it. For more information on our pump-turbines please have a look at our brochures in the sidebar or contact us directly.
- Energy savings
- Lower acquisition costs
- Lower operating costs
- Excellent operating availability
- Wear-resistant execution
- Easy maintenance
- Impeller with large free passage
Fields of application
- Small hydropower plants with grid feed-in
- As pressure reducing plants
- For use of residual water with energy utilization
- As energy recovery turbines, e.g. in the pulp and paper industry
- As stand-alone plant for a self-sufficient energy supply
- In pump storage power plants to generate and store energy
Electricity from your own plant
This self-sufficiency is offered by ANDRITZ mini-turbine plants, either for personal or for industrial use. Pumps as turbines offer considerable advantages for mini power plants thanks to the combination of cost-effectiveness, operating reliability, and efficiency. The plant has a compact design, suitable for isolated operation and for supplying to an existing power network.
Proven open impeller design
Due to the open impeller design, the mini-turbine – a reverse-running centrifugal pump – handles not only drinking water, but also residual and waste water, as well as pulp suspensions. The design is characterized by open channels and a wear-resistant design that makes the pump insensitive to contaminants when used as a turbine.
Example of energy recovery in the paper industry
An example is the pump-turbine operating in the waste water system of a German paper mill. The total shaft output of the pump-turbine amounts to 127 kW. There is no need for a generator. The turbine part contributes 53 kW to this output. The result is the recovery of energy, with more than 42% being efficiently re-used. The pressurized waste water enriched with air is relieved in the feed pipe to the micro-flotation stage. The excess pressure is absorbed by the pump-turbine and is passed on to the booster pump and used to drive the motor. This provides direct support to the pump drive and increases the overall efficiency of the process.
ANDRITZ pumps used as turbines
|Design:||single-stage and multi-stage; single-flow or double suction; open or closed impeller|
|Head:||up to 80 m|
|Flow rates:||up to 6 m³/s|
|Output:||up to 2 MW|