September 2021

32 n ENERGY EFFICIENCY September 2021 www.drivesncontrols.com VSDs boost efficiency at Jersey desalination plant by up to 38% S ince 1999, Jersey Water has operated a desalination plant in an abandoned quarry at La Rosière in the south-west corner of the largest of the Channel Islands. The plant replaced an earlier multi-stage flash (MSF) distillation plant that had reached the end of its life. A deep-sea water intake allows seawater to be collected at any time, despite tide levels that can vary by up to 12m. The seawater is pumped into in a former quarry, where it is stored, allowing unwanted debris to settle. The water is then transferred via a filtration system to a dual reverse osmosis (RO) plant capable of producing 5,400m 3 of fresh water every day. Almost half (45%) of the filtered seawater is converted into fresh water, with the remaining brine being returned to the sea. Water is fed to each RO system by a VSD- controlled multi-stage centrifugal pump, at a pressure of 56 bar. The seawater is ejected from the RO membranes under high pressure and used to operate an energy recovery system, in series with high-pressure pumps, recovering energy that would otherwise be lost. The two RO systems can be operated independently or together. After the “new” plant had been in service for almost two decades, Jersey Water decided it was time for a major upgrade to help meet the growing demand for water on the island as a result of population growth and climate change. The upgrade would increase the plant’s fresh water production capacity from 6 to 10.8 megalitres per day – enough to meet around half of the island’s daily demand for water, as well as improving its operating efficiency. The upgrade included installing a large number of new pumps, almost all of which are controlled by AC drives. After investigating the possible options and going through a competitive tendering process, Jersey Water chose Danfoss’VLT Aqua drives which are dedicated to water industry applications. High pressure The largest of the new drives (630kW) control two high-pressure pumps with 600kW motors operating at 690V, that are fed from a custom 11kV/690V transformer. Other drives operate from 400V supplies and include three 160kW models and nine with ratings from 18.5–55kW. Danfoss also supplied two 110kW energy-VLT soft-starters for the submersible seawater intake pumps. Jersey Water’s chief engineer and water supply manager, Malcolm Berridge, says that a key requirement was that the drives should produce the lowest possible level of harmonics in the supply system. The utility’s previous experience with drives had shown that some VSDs can distort the electrical supply and affect nearby instrumentation and computer equipment. Although the Danfoss drives offers good harmonic performance without external filters, Jersey Water decided that, because minimising harmonics was such an important requirement, it would still be desirable to install filters as a precautionary measure. Accordingly, Danfoss supplied active harmonic filters for the largest drives. Because the drives and soft-starters are located close to the sea, Jersey Water specified conformally-coated printed circuit boards to protect against corrosion. As part of the project, an existing motor control centre was decommissioned and removed, and the new drives had to fit into the limited space this made available. Space was saved by using IP54 versions of the drives that do not need additional protective enclosures. Berridge reports that commissioning of the new drives “was fast and straightforward,” thanks to their intuitive user interface and dedicated pump functions. “The upgraded plant has now been in service for many months, and the drives have fully lived up to expectations in terms of performance and reliability,” he says. “They are also making a big contribution to the enhanced energy efficiency of the plant, which has increased by between 36% and 38%.” n Jersey Water recently upgraded its La Rosière desalination plant, including the installation of several drive-controlled pumps that have boosted the plant’s efficiency by up to 38%.