Water Online

January 2017

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

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By Jeff Knapp and Linda Reekie W ater systems have an untapped potential to recover energy using in-line hydroelectric generation and to reduce net energy consumption, greenhouse gas emissions, and operating costs. The Water Research Foundation (WRF) co-funded a project with Halifax Water, Nova Scotia, Canada to investigate the installation of a hydrokinetic in-line energy recovery turbine generator within its water system to determine the system's benefits and operational characteristics. The journey began in earnest in 2010, when Halifax Water began investigating the feasibility of energy recovery instead of traditional pressure reduction using pressure-reducing valves (PRVs) in its water distribution system. A driver was the launch of the new provincial renewable energy initiative called the community feed-in tariff (COMFIT) program, which offered preferential energy rates to renewable energy generation projects that led to the reduction of greenhouse gas emissions in Nova Scotia. The approved COMFIT rate of $0.14/kWh for run-of-river hydro allowed the Halifax Water project to proceed based on sound financial and performance estimates and a reasonable return on investment (ROI). Without this preferential energy rate, the project's rate of ROI would not have been as attractive and may have prevented the project from moving forward. Halifax Water retained a consultant to conduct a preliminary study of the potential for energy recovery using an in-line turbine (ILT) from Halifax Water's control chambers that used PRVs or flow control valves (FCVs) for downstream pressure and flow control. Several potentially viable sites were identified. The selected site, the Orchard Control Chamber, was thought to be the best initial site for research and development of a prototype system because of its relatively stable but significant diurnal flows and level of pressure reduction. Furthermore, the Orchard Control Chamber supplies water to two large reservoirs that provide significant hydraulic cushion for pressure transients that could result from the operation of the ILT, thus presenting a low risk because the potential failure of any prototype turbine generators would have a minimal impact. The Orchard Control Chamber was also in close proximity to an easily accessible point of interconnection with the electrical grid. Two types of turbines were considered to recover energy from the differential pressure and flow inside of the pressure-controlled water supply system: a Francis turbine and a reverse-acting pump, or pump-as-turbine (PAT). A fixed-geometry PAT was selected as the preferred technology given the relatively stable diurnal flows expected through the Orchard Chamber. The flow characteristics of this type of turbine were determined to be ideally suited to the application, with runaway flows and heads being limited by the internal resistance of the fixed impeller and volute geometry of the turbine, and its ability to control flow to the turbine through the inlet control valve. The PAT had a number of other advantages over other types of hydro turbines, including: • Simple design and ease of application • Similar operational and maintenance considerations to regular pumps • Availability for a large range of heads and flows • Availability in a large number of standard sizes • Lower cost • Availability of spare parts • Ease of installation • Ease of integration within an existing system • Direct coupling of turbine/generator resulting in lower friction loss, longer bearing life, and less maintenance The turbine and generator were selected with a turbine size based on the average diurnal flows of 3.9 cubic feet per second and a head of 130 feet. From the project outset, the objective was to develop, install, and commission a small in-line recovery system that could be used in place of, or operated in parallel to, an existing PRV system, and operate effectively and within a pressure-controlled municipal water distribution system. A critical factor was the ability of any in-line energy recovery system to control upstream and downstream pressure transients in the water distribution system because of the lack of tolerance of some of the older sections of Halifax Water's distribution system for significant pressure or flow transients. A portion of the Orchard research project was also to investigate how pressure transients could be controlled and/or eliminated. Data was collected to allow Halifax Water to identify both normal and upset operating conditions created by both the water supply system and the operation of the ILT and bypass PRV system. The data was collected to allow Halifax Water to correlate downstream effects (pressure/flow transients) with the operation of the ILT and the bypass PRV system and identify detrimental conditions. The project involved the development, design, installation, and commissioning of the Orchard Control Chamber ILT for energy recovery. It also looked at the operational characteristics and effects 32 wateronline.com n Water Innovations In-Line Turbines Harness Energy For Water Utilities Halifax Water and the Water Research Foundation's study finds that turbine generators can do more than just replace the work of pressure- reducing valves (PRVs).

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