Energy as a Process Variable
Industrial energy consumption is becoming a larger issue every day, and it should be. For a host of reasons, companies and individuals need to consider how they use energy and make every effort to curtail its use on all sides. Almost 30 years ago, President Jimmy Carter characterized energy independence for the U.S. as the “moral equivalent of war.” While the political implications and motivations around that statement have changed, the importance of reducing energy consumption has not. To some, it can be updated to say that reducing our carbon footprint is the moral equivalent of war. To others, the issue may simply be a desire to cut climbing fuel costs. One way or another, there will be additional demands placed on plant control systems to implement energy reductions.
Macro level: carbon footprints
While it’s difficult to predict the political future, energy conservation and carbon reduction will likely happen for two major reasons: cost cutting for the sake of profitability mixed with government imposed regulations. The current annual emission level of carbon equivalents in the U.S. is 7.2 gigatons, and following current trends that figure is projected to increase to 9.7 gigatons by 2030. There are various legislative proposals to control emission levels, ranging from drastic reductions to simply slowing the growth rate. One mid-range objective is to freeze output slightly below current levels. This means, effectively, an annual reduction of about 3.0 gigatons over the next 20 years.
Reducing emissions to stay at or below current levels will add costs, but some efforts can also save money in the long term. Some industry leaders believe this can work out as a wash, with beneficial cost reductions mitigating or overshadowing the costs of complying with regulations.
“A lot of greenhouse gas emission is linked with energy or fuel consumption,” says Harsh Chitale, vice president of strategy and global marketing for Honeywell Process Solutions. “By reducing fuel or energy consumption, you reduce carbon emission and costs at the same time. Fuel savings pay for the initial investment.”
Honeywell participated in a recent greenhouse gas study released by McKinsey & Company, and has calculated that self-financing projects capable of saving more than they cost will account for approximately 1.7 gigatons of carbon equivalents, or more than half the reductions needed to remain at current levels. Unfortunately, the balance will likely have considerable net costs, but when the two are weighed against each other, the ultimate effect will be cost neutrality if spread over the whole economy. The final outcome at an individual plant will depend on the situation.
Micro level: your plant
While it’s useful to have these macro goals in mind, how can we make this more manageable at a plant level?
While the notion of energy consumption and conservation covers a vast range of areas in process and other manufacturing plants, for purposes of this article we will look primarily at electrical consumption of ancillary equipment using motors. While that may not sound like much, the U.S. Department of Energy (DOE) estimates that motor driven equipment accounts for 64% of electricity consumed in the industrial sector.
The study goes on to say that in the nation’s most energy-intensive industries, motor systems consume approximately 290 billion kWh per year. If that consumption is spread out 24/7/365, it means that at any given moment, more than 33,000 MW (100% of the output of 50-60 typical coal-fired power plants) is running industrial electric motors.
Measure, then control
Electrical consumption is one of the final frontiers as process plant operators look for ways to reduce costs through saving energy. Historically, the power consumed to drive pumps, compressors, blowers, and the like was regarded essentially as a fixed cost. As an example, a pump has a 20 hp fixed speed motor and it’s either running or it’s not, depending on the needs of the process. There isn’t much there to optimize, or so it might appear. Someone might suggest using a higher efficiency motor, but that’s effectively it.
Electrical consumption was typically monitored by the maintenance or larger electrical department if at all, but not considered part of the process, nor was it done in any detail. Moin Shaikh, DCS marketing consultant for Siemens Energy & Automation, points out the problems with that separation: “Companies understand the cost of energy, but don’t know how to pinpoint it within an operation. You have a process control system and energy management system that are completely different or implemented in a separate environment and not tied back into the process. To make that link with the process, it has to be in one system. Then you can tie energy consumption to the process area and have an understanding of where and how much energy is consumed.”
Most people begin by examining the overall facility but don’t dig into smaller units. Shaikh adds, “Once you have a view into your process with smart motor controls and a good energy management system that is in synch with your process, you can look at the steps and know which part of the process is responsible for what energy use. Then you can deep-dive into the process and realize where the savings can be made.”
Fortunately, technology in this area is advancing, and the ability to monitor and control electrical consumption down to the individual motor and drive-system level is available and generally affordable. Motor control devices can incorporate intelligent functionality and report conditions to plant operators and the maintenance department. Typical performance data can include:
Power quality—Voltage fluctuations, harmonics problems, phase problems;
Motor operating conditions and trends—Sensors can monitor and trend winding temperature, power consumption, log starts, etc.;
Load and power situation—Motor loading of strategic equipment can offer a look into a process, such as the power required to deliver a specific flow rate through a pump, trended over time; and
Operating efficiency—For cost savings, the ultimate information is efficiency, which supports operating cost calculations, appropriateness of equipment sizing, potential for additional capacity, etc.
Companies considering adding this capability may find that their existing control-network infrastructure is not up to the task of adding communication from a significant number of motor installations around the plant. Intelligent motor control systems can be retrofitted to existing installations without too much difficulty, and data can be sent back to the DCS via digital communications, such as Modbus or Profibus. This approach reduces the amount of wiring considerably. Wireless options are also available.
More for operators?
Will this information soon begin appearing on the screens in process industry control rooms?
“Not anytime soon,” says Shaikh. “Plants have fewer operators and they often have too many things to look at already from the process controls perspective. You can’t burden them with more things. Electrical performance may come from the same control system as the process, but display on different HMIs.”
There are few processes where an operator will adjust a set point based on electrical consumption. While there are undoubtedly some that exist, such would already have controls in place. For more typical installations, the question becomes: can we reduce electrical power usage and still maintain the processes?
Load shedding capabilities
In some areas, electric utilities can encounter situations where they ask customers to reduce power consumption either due to exceptionally high demand or because generating capacity has gone off line. Requests for load shedding may come with a day’s notice or more if there is a planned generating shutdown, or it may come with mere minutes to spare in the case of a system failure. Companies that can accommodate these requests often receive preferential treatment of some type from the utility, typically as rate reductions or the ability to effectively sell power back to the utility during the shutdown.
“One large issue facing process plants and even large manufacturing plants is demand response,” says Philip Barton, national business development manager for Square-D PowerLogic division. “Utilities and intermediaries have contracts with their large customers for load shedding, and that has provided motivation for many large process plants to take on major energy management projects. Utilities ask customers to shed load either for a price event, or a reliability event. A price event is when the wholesale power price is high, and those tend to have day-ahead warning. Reliability events are when they lose a generator, and these usually have much shorter warning.”
Obviously many processes can’t simply start shutting down sections of the plant on short notice, but when power consumption is well monitored and managed, there may be options available to shut down specific substations. Large process plants with internal co-generating capacity can shift from the grid when circumstances demand it.
Sometimes load shedding may be in the plant’s favor too. “A lot of process manufacturers buy power in bulk, and the power rates are changing all the time,” says Shaikh. “One customer showed me that their rates can change every five minutes. If part of the plant is shut down for maintenance and, through their power management system, they know they’re not going to be using 40 MW, they might be able to sell the power back to the grid. You can call the utility and say, ‘We’re not going to be using 40 MW for the next 16 hours,’ and if the utility can sell that to other customers, it will credit the plant.”
The need for this capability varies with location and seasonality, so this may not offer a significant justification for a project in all areas.
Benchmarking and optimization
Having detailed information regarding energy consumption provides critical information when trying to analyze process performance for benchmarking, optimization, or exploring process modifications. Performing any of these actions without enough data makes for haphazard results.
“Since the 1992 Comprehensive Energy Policy Act (EPact), legislation setting goals to improve energy efficiency and provide incentives with rebates and investments in energy conservation, every electrical device should be investigated on ways to lower overall cost,” says Jeff Otterstein, product manager for Eaton’s CurrentWatch sensor family. “Current sensors can improve awareness of energy consumption and energy cost by early detection of poor performance and real-time monitoring. It’s not difficult to deploy them at strategic points to monitor power consumption of whole process units or even individual pieces of large equipment. The information can be analyzed and used to match costs with operations or determine equipment loading. This data can be very useful for historical trending or identifying where there might be possibilities to control and optimize electrical consumption.”
Clearly the technology exists to reduce and optimize energy use. All that remains is our will to put it in practice.
|Peter Welander is process industries editor. Reach him at PWelander@cfemedia.com .|
Do high-efficiency motors save money?
Jim Clark, ABB’s U.S. manager of low voltage motors, has done the math: “Given that about two thirds of the total electricity consumed is by electric motors, and that electric motors generally consume their own purchase price in energy within the first 30 to 60 days of operation, it follows that making the most efficient use of that energy should be a primary goal. ABB invests a significant amount in R&D to ensure its designs are at the leading edge of electrically efficient performance.”
The cost of a high-efficiency motor will be a larger proportion of it’s total lifetime cost, but the entire cost pie will be smaller. in the long run that’s the real saving.
Is it best to replace an underloaded motor?
So, if a 25 hp motor is doing a job that could be done by a 20 hp motor, or even smaller, is it a good idea to put in a smaller unit more suited to the load? Often the answer is yes, but not always, nor is the difference necessarily large enough to pay for the work involved.
Conventional and high-efficiency motors often have their highest efficiency at approximately 75% of full load, and the curve stays fairly flat between 50% and full load, so as long as a motor is operating at 50% or more, it’s in its best efficiency band.
The DOE advises that to make an informed decision, several key bits of information are required:
Load on the motor;
Operating efficiency at that load point;
Full load speed (rpm); and,
Full load speed of the proposed replacement motor.
From that data it is possible to calculate an actual comparison for a given installation using either a direct kilowatt ratio or voltage compensated slip analysis. The DOE offers the relevant equations through its Motor Challenge program, including a small software package to do the necessary calculations.
Department of Energy initiatives
The DOE has a range of initiatives to help conserve energy use in industrial applications. The Save Energy Now program helps companies analyze and optimize energy use in all forms. A key element of the program includes conducting plant energy assessments to help manufacturing facilities identify immediate opportunities to save energy and money, focusing primarily on energy-intensive systems, including process heating, steam, pumps, fans, and compressed air.
“The Save Energy Now project was launched in 2005 with the objective of helping manufacturing plants find energy saving projects with quick paybacks”, says Paul Scheihing, technology manager, DOE industrial technologies program. “Since then we’ve completed 325 Save Energy Now assessments where $95 million in annual energy cost savings has already been realized and another $270 million in savings is planned or in progress of being implemented. The total potential energy savings identified at the 325 plants is 64 trillion Btu of natural gas, which is equivalent to the natural gas use of almost 1 million typical US homes.”
Turn to the Inside Process section of this issue for an actual Save Energy Now program case study.
Regulating pump output: Valve vs. VFD
“Oversized pumps are a common problem within industry and will continue to be so,” says Dan Kernan, product manager for ITT’s PumpSmart controls division. “The pressure to deliver the required product or process will almost always trump energy efficiency. If you were given the task to design a pump system, the one thing that you do not want to do is choose a pump that is too small. It is much easier to throttle an oversized pump then replace an undersized pump.
“A simple solution to this problem is variable speed pumping, which provides the flexibility to size a pump for a maximum condition, then slow it down under normal operations. This provides a more energy efficient method of control over a much broader range. There are certainly many variables in a pump system that affect the use of variable speed, but more often you will find variable speed pumping to be the most cost effective solution over the life of the pump.”
This approach does indeed save energy, and depending on the application, the difference can be as great as 50-70%. When an output valve or damper is replaced by controlling the speed of the motor, it’s a simple matter to control pump speed to match a desired flow rate. If the pump is regulated by a PID loop, power to the motor is indeed a process variable.
So is there still a place for fixed speed motors? If loading is consistent most of the time and a motor is sized appropriately for the duty, a variable speed drive is probably overkill, but with the cost of VFD units coming down, they are not the luxuries they once were.