Drives & Controls October 2022

40 n WATER INDUSTRY October 2022 www.drivesncontrols.com Pump upgrade pays for itself in three months M unicipalities across Europe are looking to cut their energy use and minimise their carbon footprints. Producing biogas from wastewater treatment processes is one way for them to become self-sufficient. For example, the municipality in the German town of Stendal is aiming to make a wastewater treatment plant that it operates entirely energy self-sufficient by 2025. The site has a biogas production facility which provides low-carbon energy from waste. Sludge resulting from treating the wastewater is pumped into storage tanks before being transferred to the biogas plant. The utility assessed various potential enhancements to the site that would reduce its reliance on the electricity grid and increase its use of low-carbon energy. Eccentric screw pumps were quickly identified as a promising technology. These pumps (also known as progressive cavity pumps), are used to transport sludge generated during wastewater treatment. They resist clogging and can handle fluids containing solids. Eccentric screw pumps can operate around-the-clock, supporting continuous wastewater treatment processes. However, continuous operation requires large amounts of energy. Furthermore, the pumps often work at low speeds, which can result in the motors that power them operating inefficiently. The operators of the Stendal plant therefore looked for a way of enhancing the efficiency of the pumps. They decided to fit high-efficiency permanent magnet synchronous motors (PMSMs) to the pumps. A key advantage for PMSMs in such applications is that they can deliver high torque at low speeds with high efficiency. At Stendal, the pumps typically run at around 25Hz to reduce wear and to provide a reserve for large volumes of water. At this frequency, asynchronous motors would need to be oversized to deliver the required torque, resulting in inefficient operation. However, due to their characteristics under partial load, PMSMs can deliver full torque at this speed from smaller motors. High torque is available from start-up too, making these motors ideal for pumping applications. Furthermore, energy savings of at least 30% are possible under partial loads, compared to asynchronous motors. Before committing itself to the change, the Stendal municipality performed some tests. “We operated two identical pumps alternately with a PMSM and an IE3 asynchronous motor for a month each, comparing electricity demand and primary sludge pumping,” explains Micheal Riske, head of Stendal’s wastewater/water department.“The pump with the PMSM installed was found to have consumed 24% less energy.” During the trial, the PMSM-powered pump consumed 495.3kW to pump 2,410m 3 of primary sludge. The IE3 asynchronous motor required 729.3kW to pump 2699.5m 3 of sludge. This equates to 4.866m 3 /kW for the PM motor and 3.7m 3 /kW for the IE3 motor. The PMSM-powered pumps moved almost a third more sludge for the same output. The operator specified Allweiler AE2E eccentric screw pumps coupled to Bauer BG30 helical gears and PMSMs. These were installed in the site’s pump-house cellar. The PMSMs have an efficient rotor design matched to optimised magnets. There are no heat losses from the rotor, and total losses are cut by 25%. According to Bauer, users of these motors typically achieve energy savings of at least 10% compared to other drives. “Another benefit of the PMSM design is that it is very power dense,”says Bauer Gear Motor’s key account manager, Marc Piwko. “This means that we can effectively downsize motors without any loss in performance while improving efficiency. At Stendal, we were able to downsize from the 5.5kW original asynchronous motor to a 3kW. Additionally, the low heat characteristics of PMSMs mean forced cooling fans can be eliminated.” Based on Allweiler’s experience,“it takes no more than six months to achieve return-on- investment on a PMSM,”says Klaus Kaiser, a sales engineer with the company. In fact, the low power consumption of the new pumps at Stendal meant that ROI was achieved after only three months of operation. The installation has helped the municipality to take a large stride towards its goal of switching to low-carbon energy. n A Germanmunicipality has achieved a payback of just three months by installing efficient pumps coupled to permanent magnet synchronous motors at a wastewater treatment plant. The installation is part of a programme tomake the site energy self-sufficient by 2025. The combination of eccentric screw pumps and permanent magnet synchronous motors paid for themselves in three months