22/05/2015
Proactive Energy Management
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Moving liquid is a key process in many applications, from water and wastewater to commercial buildings to industry to agricultural irrigation. Water and wastewater treatment facilities, for example, must treat and move massive amounts of influent to maintain public health. Meanwhile, commercial buildings need to transport chilled water for their HVAC systems and to provide water for human consumption. And industrial manufacturing facilities need to move water for process applications and washdown procedures, and they may also need to move other liquids, including everything from slurry to melted plastic to chocolate.
The movement of liquid is performed with pumps powered by motors. In an ideal world, pumps and motors would be properly sized to meet the needs of the application. Unfortunately, improper pump and motor sizing is a common occurrence in many fluid-handling applications.
Whether it’s a small five horsepower motor for a chemical-feed pump, or a massive 1,000 hp counterpart for influent pumping in a water and wastewater treatment facility, electricity is needed to power the motors that operate the pumps that move the liquid as necessary. This fact can create challenges for facility owners and managers, who must balance steadily increasing utility rates with the need to cut costs in a sluggish economy.
The good news is that pumps can be a key target for increased energy efficiency, as the latest generation of variable frequency drives (VFDs) enable an active energy management approach. VFDs regulate motor speed and, by extension, pump speed, which is generally a more efficient approach than regulating the flow of liquid with valves. For example, a water and wastewater treatment facility has varying flow amounts throughout the course of a day, and it is more efficient to slow pump motors at times of reduced flow. Otherwise, the plant is wasting energy by manipulating valves in the system while the pump continues to operate at full speed.
Pumps account for the largest share of industrial motor energy use. In addition, VFDs allow facility managers and owners to be proactive about energy usage, rather than reacting to rising utility rates. According to the U.S. Department of Energy’s Energy Information Administration, the per-kilowatt hour cost of electricity rose from 7.6 cents to 9.8 cents from 2004 to 2008 — a 28.8 percent increase. That amount is expected to hold stable from 2010 to 2020. However, an increase to 10.2 percent by 2030 is expected. As pumps are a large energy consumer – nearly 25 percent of electricity in the industrial world, 85 percent in clean water plants – the efficient use of pumps can have a dramatic bottom-line impact.
Moving Liquid
Perhaps it’s no surprise that as the demand for energy increases, so does the call for greater efficiency, and for many reasons. For federal facility managers, meeting energy efficiency mandates imposed by Congress is crucial. The Energy Independence and Security Act of 2007 (EISA 2007), for example, requires all federal government facilities to reduce energy consumption by 3 percent per year through 2015 for a total 30 percent reduction. Another key piece of legislation is the American Recovery and Reinvestment Act of 2009 (ARRA), perhaps better known as the Economic Stimulus Package passed by Congress last year. To apply for ARRA dollars, 20 percent of the money must be used for “green” technologies, or products that promote energy efficiency, including VFDs.(1)
Another key motivator, beyond reducing costs and meeting energy efficiency mandates, is being a good environmental steward. This can be achieved through many means, including certification through the Leadership in Energy & Environmental Design (LEED) Green Building Rating System administered by the U.S. Green Building Council or the U.S. Environmental Protection Agency’s Energy Star program.(2)
Graph showing the energy savings that can be generated via pumps as compared to other technologies. Deployment of VFDs to better manage motors in pump applications is but one piece of a larger energy efficiency puzzle for facility managers, but it’s an important one. According to the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE), the potential electrical usage savings by motors for HVAC and pump applications is vast. The EERE reports that 73 percent of what it defines as energy consumption sectors include various types of pumping.3 That finding is magnified by another Department of Energy report, from the Office of Industrial Technology, which states that pumps account for nearly a quarter (24.8 percent) of industrial motor energy use. Further, a Department of Energy opportunities assessment of the U.S. industrial motor systems market reports that the energy savings potential of 51 hp to 1,000 hp motors is conservatively 6,500 GWh per year.
One of the reasons pump applications are such a viable energy efficiency target is explained by the Affinity Laws of Centrifugal Loads, which dictates that:
• Flow is proportional to motor speed;
• Pressure is proportional to motor speed squared; and
• Power is proportional to motor speed cubed
This means that reducing pump motor speed by 20 percent reduces electrical consumption by almost 50 percent. While substantial, that amount of energy savings is just the beginning. VFDs limit current in-rush to facilitate soft starting, which avoids higher peak demand charges by “ramping up” a motor’s current usage gradually, instead of immediately. Such drives also have inherent power factor correction; because they control a motor’s current usage, they better regulate motor speed and slippage so the induction is reduced, meaning there is less wasted power. VFDs also protect motors and wiring from overload current, which can reduce maintenance and replacement costs.
Some VFDs have built-in power monitoring capabilities as well, or can be connected to a motor management system, which facilitates monitoring and control of individual devices, like motors that power pumps. The predictive logic of motor management systems can alert facility managers to potential problems before they occur, like a sudden increase in motor current. Integrating VFDs or motor management systems into a system-level power monitoring system allows facility managers to monitor all power aspects — from distribution to device.
An Active Approach to Energy Management
The myriad capabilities of VFDs allow facility owners and managers to adopt an active approach to energy management, which should complement passive measures. The latter can include tactics like installing more energy-efficient lighting fixtures and luminaires; increasing insulation; or deploying power factor correction devices. While these measures can translate to up to 15 percent savings, continuous energy improvement over the lifecycle of a facility and changing conditions should be the ultimate goal, which is best facilitated through automation and regulation. This is the hallmark of an active approach, because these factors can be adjusted based on new energy-efficiency opportunities that arise in the future.
For example, an active approach to energy management may include several tactics, such as:
• VFDs that increase or reduce the speeds of motors that power pumps based on demand.
• System-level power monitoring and device-level motor management, which provides energy usage information and alerts about specific devices so potential problems can be proactively addressed and new energy-efficiency opportunities can be captured.
• A lighting control system that automatically turns interior and exterior building lights on and off based on a pre-set schedule, instead of relying on personnel (including the facility manager) to remember.
All told, an active energy management approach can add another 15 percent energy-efficiency increase onto the gains made by passive measures, for a total potential savings of 30 percent. But best of all, active measures can account for new energy-efficiency opportunities that may arise in the future. One recent example is demand response, a scenario where a facility owner or manager would sign an agreement with the local utility to receive a signal from the utility when electrical rates reach a pre-set ceiling, so pre-selected electrical loads can be turned off.
In an active approach, information is crucial, because it can be actionable. For example, a water and wastewater treatment facility manager needs to understand his facility’s kilowatt hours used per million gallons per day. Energy consumption information about a given motor powering a pump can be gathered by a VFD and delivered to an on-site human-machine interface (HMI) or remote Supervisory Control and Data Acquisition (SCADA) system for monitoring by the facility manager. Real-time data allows for adjustments, such as slowing down a VFD to improve pumping efficiency and thus reduce costs. The same is true in an industrial facility. Gathering a baseline snapshot of electrical usage by all pumps can be done seamlessly via VFDs with an integrated control card, which completes those calculations, and a tiny programmable logic controller (PLC), which executes adjustments.
Properly considered and applied, an active approach to energy management can translate to substantial cost savings. For example, the Schneider Electric North American Operating Division instituted an energy action plan in 2004 designed to increase energy efficiency at 26 facilities. In addition to application of measures like power monitoring, lighting control, building control and enterprise energy management, the company deployed VFDs for HVAC and pump applications.
The application of VFDs were a major contributor to substantial energy efficiency results; from 2004 to 2008, the company saved $6.1 million in energy costs and reduced its energy usage by 14 percent, along with avoiding 40,000 tons of carbon dioxide.
Energy Efficiency vs. Management
It’s important to note that there is a difference between energy efficiency and energy management. The former is really the end goal, which can translate to other benefits, like cost savings, LEED certification and good environmental stewardship, as note earlier. Energy management, however, should be considered the actions taken by a facility manager and owner to achieve increased energy efficiency. Deploying VFDs within a facility’s HVAC and pump applications can be the linchpin for an active energy management approach and help achieve as much of the vast energy-efficiency potential as available.
