As the cost of energy and water and wastewater services continues to rise, operators are looking for ways to reduce the costs of these basic, yet crucial utilities. Proposed energy-efficiency standards from the U.S. Department of Energy (DOE) and other federal measures could help.
On the water side, the DOE has a mandate to develop and enforce national minimum energy efficiency standards for any product if significant savings can be realized in a cost-justified and technically feasible manner. During the past year, the DOE and stakeholders have been negotiating a regulation for pump efficiency. That rule, now in the Notice of Proposed Rulemaking (NOPR) phase, would apply only to clean water pumps of these types:
- End suction close coupled
- End suction frame mounted/own bearings
- In-line
- Radially split, multi-stage, vertical, in-line, diffuser casing
- Vertical turbine submersible
With the massive infrastructure of existing water distribution systems, the ability to increase energy efficiency comes down to a few items, including pumps and the motors that operate them. Utilities are incentivizing facilities to install variable frequency drives (VFDs) onto their motors and pumps to reduce the electrical surges caused by starting and stopping motors/pumps – which benefits the facility through reduced water hammer and lower maintenance costs.
By increasing the efficiency of the pump itself as well as the motor, accomplished by selecting a premium efficiency motor and operating that pump/motor system at constant pressure or constant flow, a VFD can help reduce the energy consumed. But how do we know we will save energy?
This energy consumption is due to the affinity laws; the three affinity laws describe the relationship between speed of the pump (n), flow (Q), pressure head (H) and electrical power consumption (P):
These calculations show that the power to run a pump is relative to the cube of the speed. If a pumped is slowed by 20 percent, it will experience a nearly 50 percent reduction in power or energy consumption. Even small decreases in pump speed can result in energy savings. It is important to note the maximum speed of the pump may not be the highest efficiency that it can pump when looking at gallons per kWh. It may be experienced at a lower than full speed flow.
But energy consumption may not be the only issue that variable frequency drives may help to resolve. What about non-revenue water – since we can now better regulate pressure, why not reduce our system pressure?
Before VFDs were commonly used, water distribution systems typically incorporated pressure switches to control pumps. When the system hit a low pressure point, the pump turned on; when it hit a higher pressure point, the pump shut off. There might have been two pumps: a smaller one for slow night hours and a larger unit for the higher daily usages. This type of control causes pressure fluctuations, but is often common practice. With a VFD, pressure is maintained at a constant set point and, as pressure decreases, the motor/pump increases speed to maintain pressure. By reducing and maintaining a constant pressure of the system, water leakage can be reduced. For example a single 1/4-inch hole in a distribution pipe can leak as much as 4 million gallons per year at 50 PSI or over 5 million gallons per year at 80 PSI. By keeping the pressure more constant, non-revenue water losses can be reduced.
But, while there are opportunities to reduce energy consumption and losses in water distribution system which could lead to higher revenue and lower operating costs, there are additional savings that may not be initially realized:
- Protection of pumps and assets
- Reduction of maintenance cost
- Limiting the risk for bacteria/contamination of tap water
- Limiting the risk for road breaks
- Reducing pipe repair cost
- Extended service life of network
- Postponing investment in system upgrades
- Improved control performance
- Increased redundancy
- Reduced load on supervisory control and data acquisition (SCADA) system because of dedicated VFD software features
Similarly, on the wastewater side of the system, there are more processes and, as a result, a greater number of motors and pumps. Since wastewater pumps operate at different conditions from clean water pumps, they have not been included in the DOE rule, although that does not mean the DOE could not develop a rule for wastewater pumps in the future. However, the DOE’s schedule for efficiency rulemakings under the present Administration appears full and a federal initiative in the near term seems unlikely.
On the other hand, the U.S. Environmental Protection Agency (EPA), through its Clean Air Act (section 111d), will require states to develop compliance plans, which could include state energy efficiency plans. Wastewater treatment plants could be included in these regulations. While states are unlikely to require a minimum efficiency specifically for wastewater pumps, the EPA could incentivize plants to reduce energy use.
So how can wastewater operations work to increase efficiency and reduce costs today? Adding a VFD to the motor and pump can make it more efficient if it is controlled properly, similar to clean water applications. Utility rebates can help with the acquisition and installation costs. Further, the resulting lower energy bill can help to reduce the operating costs, but it’s important to be smart in the control of these systems.
For smaller facilities, using a larger SCADA system may be out of the question to operate the entire system. But today’s VFDs typically include integral controllers that can be used instead to maintain level, pressure, flow, dissolved oxygen (DO), turbidity, or other monitored processes. Simply by providing an analog signal to the VFD from the monitoring equipment, the VFD has the ability to maintain a constant process output. Internal communication modules transmit data to other areas, allowing end users to monitor the current status.
Integral cascade controllers can operate multiple pumps simultaneously allowing for better overall control. When used in a variable pumping system, where pumps of different flow capabilities are installed and staggered to meet flow requirements, VFDs can increase system efficiency. This type of control, Master/follower, can offer the most efficient operation of a pumping system and the highest system efficiency.
In almost any size system, the use of VFDs on pump motors means greater flexibility and better process control. By reducing the surges that a plant may see during an event or normal loading, VFDs allow a plant to more closely handle waste influx by regulating the flows into the plant. VFDs can assist the existing SCADA or overseeing system.
Drives can also offer features – such as a deragging function – that help maintain pump efficiency. This feature will rotate the impeller backward (on a pump that can be operated backward) to dislodge solids, stringy materials or other debris, which can help the impeller pump at optimum performance.
In addition to deragging capabilities, drives designed specifically for the water/wastewater market tend to offer additional features that facilitate motor/pump operation, including initial ramp and/or check valve ramping, flow counting, flow confirmation, and no-flow or low-flow/dry pump protection. Additionally, analog and digital output cards, communication cards and specific application cards are available to assist in control functions.
As the water and wastewater industry continues to modernize and expand – and issues such as reducing energy consumption and water loss become more important and regulated – VFDs will become more commonplace in many applications.
This article from Jeffrey Bergman, Global Key Account Manager at Danfoss Drives, was first published in Pumps & Systems: http://www.pumpsandsystems.com/pumps/september-2015-energy-efficiency-water-wastewater
