Saving Energy on the Line
Equipped with a 400-hp V-8 engine and a six-speed manual transmission, the Chevrolet Corvette gets 18 mpg in city driving and 28 mpg on the highway. That makes the Corvette more fuel efficient than many sport utility vehicles. But, oil executives still get the warm fuzzies whenever they see the sleek sports car go by.
Nonetheless, while the Corvette itself won't win kudos for energy efficiency, the General Motors facility in Bowling Green, KY, where the car is assembled, has been recognized for energy conservation by the U.S. Environmental Protection Agency. This is, in part, because engineers significantly reduced the plant's electric bill simply by improving the lights at the facility, which spans some 1 million square feet. Working with Stones River Electric (Nashville, TN), an energy consulting firm, engineers upgraded more than 11,000 light fixtures at the plant over a 2-month period. Incandescent lamps and T12 fluorescent lamps with magnetic ballasts were replaced with T8 fluorescent lamps and electronic ballasts. In areas of the plant that were over lit, some fixtures were taken out of service.
Not only did the project reduce energy costs by 35 percent, it increased light levels throughout the plant by 10 percent to 15 percent. Maintenance costs also decreased significantly, because lighting materials throughout the plant are standardized and because the new lamps and ballasts last longer than the old ones.
"We have an aggressive program...to reduce energy usage by 25 percent at our North American facilities by the end of 2005," says David Skiven, executive director of GM's Worldwide Facilities Group.
Forced to cut costs in every aspect of their assembly operations, U.S. manufacturers are paying closer attention to their utility bills and taking steps to conserve energy. In some cases, these energy-saving measures can be dramatic. For example, the "living roof" at Ford Motor Co.'s flagship Rouge facility in Dearborn, MI, is covered with earth and planted with ground cover. This keeps the facility cool in summer and warm in winter, which saves Ford thousands of dollars annually in heating and air-conditioning costs. In other cases, as engineers at GM's Bowling Green plant discovered, saving energy can be as simple as changing a light bulb.
According to Wayne Stebbins, principal electrical engineer with consulting firm Perigon Engineering (Matthews, NC), most manufacturers have ample room to save money on their electric bills. A well-run, well-maintained facility where every motor is perfectly matched to its load might be 80 percent efficient, he says. That is, 80 percent of the electricity the plant buys actually does some useful work.
"If you surveyed American manufacturers across the board, I doubt most of them would be that good," Stebbins says. "Most would be in the 60 percent to 70 percent range. Of course, it's very process dependent. A steel mill or foundry might not be 50 percent efficient. A factory that assembles PCs might be 90 percent efficient."
Why aren't assemblers more energy efficient? Stebbins believes manufacturing engineers don't usually consider energy costs when specifying and building an assembly line. "Reliability, throughput and other operational issues come way ahead of electrical costs," he says.
Gary Seibert, general manager of Merritech Assembly and Test Systems (Saginaw, MI), agrees. He says energy costs rarely factor into the design process for multistation automated assembly systems, though the issue sometimes comes up during failure mode effects analyses.
"For the most part, [energy efficiency] is not part of the design intent," he says. "One exception is compressed air consumption, especially in blow off, vacuum and feeder assist applications. In that case, the customer simply asks if that is the best practice or if there are lower cost alternatives."
Part of the problem is that most engineers view energy as a fixed cost rather than a variable cost, "something they can actually impact," says Todd Thornburg, manager of marketing technical services at ComEd (Chicago), the electric company for Northern Illinois. "Assembly plants have improved greatly [in how well they use energy], but there's always room for improvement," he adds. "We've dealt with the largest assembly plants-facilities with their own energy departments-and still managed to find significant savings."
Seeing the Light
Thornburg says there are many low-cost or no-cost actions that engineers can implement quickly and easily to produce immediate energy savings. Such actions include using occupancy sensors to activate lighting; adjusting thermostats for when the plant is unoccupied; maintaining the belts on motor-driven systems; and shutting off lights and equipment that are not in use. Such energy-saving opportunities may seem obvious, but they often aren't apparent to the people who work in a plant every day, says Thornburg, whose department has conducted more than 3,500 energy audits of industrial facilities.
For larger savings, engineers need to take a hard look at the three biggest consumers of electricity in an assembly plant: lighting, compressed air systems and electric motors, especially those associated with the heating, ventilating and air-conditioning (HVAC) system.
Most assembly plants are lit by metal halide lamps or T12 fluorescent lamps with magnetic ballasts. Like the Corvette plant, assemblers can cut their electricity costs by switching to T8 fluorescent lamps with electronic ballasts. Today's fluorescent lamps are so efficient that two T8 lamps can replace four T12s. In addition, T8 or even T5 fluorescent lamps can replace 400-watt metal halide lamps.
"Fluorescent light technology has improved to the point where it can replace these older, larger lamps, even in applications with 25-foot ceilings," says Thornburg. "And, you don't have to wait 5 minutes for the lights to come on, like you do with the older metal halide lamps."
Illuminated exit signs are another easy target. Signs lit by 5-watt LEDs consume much less electricity than those lit by incandescent lamps or even compact fluorescents. Skeptical engineers can do the math: If an existing sign is lit by two 20-watt incandescent lamps, the LED saves 35 watts. An exit sign must be lit all day, every day, or 8,760 hours per year. If electricity costs $0.08 per kilowatt-hour, the LED will save $25 per year, enough to pay back the purchase price and installation cost of the new sign in less than 3 years. When savings from lower maintenance are factored in, the pay back time is even less.
A lighting audit by a certified lighting professional can help assemblers identify opportunities for savings-without leaving workers in the dark.
The Cost of Compressed Air
In many assembly plants, air compressors use more electricity than any other type of equipment. According to the U.S. Department of Energy (DOE), generating compressed air accounts for 10 percent to 30 percent of the electricity consumed by a typical industrial facility. By eliminating inefficiencies in compressed air systems, assemblers can often reduce their electric bills by 20 percent to 50 percent.
The most common source of waste in compressed air systems is leaks, says Clayton Fryer, product champion for IMI Norgren Inc. (Littleton, CO), a supplier of cylinders, valves, grippers and other pneumatic equipment. He estimates that leaks waste as much as 50 percent of a compressor's output. If electricity costs $0.05 per kilowatt-hour, a leak just 1/16 inch in diameter will cost $523 annually in electricity.
For compressors with start-stop controls, an easy way to estimate the amount of leakage in an air system is to run the compressor with all the pneumatic tools and actuators turned off. The compressor will cycle on and off as air pressure escapes through leaks. The percentage of air pressure lost through leaks can then be calculated as:
(T * 100) / (T + t)
where T is the average time, in minutes, that the compressor is pressurizing air, and t is the average time that the compressor is not under load. If the system is well-maintained, this ratio should be less than 10 percent.
Leaks can occur anywhere in a compressed air system, but the most common areas are:
- couplings, hoses and fittings
- pressure regulators
- open condensate traps and shut-off valves
- pipe joints, disconnects and thread sealants.
The best way to find leaks is with an ultrasonic acoustic detector, which senses high-frequency hissing sounds above the range of human hearing. Alternatively, engineers could simply apply soapy water to suspect areas.
"Can you imagine anyone letting fluid leak from a hydraulic system or allowing hydraulic filters to operate on bypass for extended periods of time?" observes Fryer.
Less obvious sources of waste are dirty line filters, and oversized cylinders, conductors and fittings. Fryer recommends changing a filter element when contamination causes a pressure drop of about 10 psig. Pressure drop is the reduction in air pressure from the compressor discharge to the point of use. It occurs as the compressed air travels through the treatment and distribution system. In a well-designed system, the pressure drop should be less than 10 percent of the discharge pressure.
Controlling pressure drops is critical in a compressed air system. That's because engineers often try to overcome excessive pressure drops by increasing the system pressure or adding an extra compressor. Either fix will significantly increase energy consumption. Elevating system pressure will increase waste from leaks and other unregulated air consumption. This added demand at elevated pressure is called "artificial demand," and it substantially increases energy consumption.
"[Engineers should] lower the system pressure to better match what is actually required in the plant," advises Thornburg. "If you reduce your system pressure by just 2 percent, you get a potential 1 percent savings in energy."
Instead of increasing the discharge pressure or adding additional compressors, engineers should reduce pressure drops and add storage areas for compressed air at strategic locations on the line. In addition, pneumatic equipment should be specified and operated at the lowest efficient operating pressure.
"If there's a pressure problem in the plant, the first thing engineers consider is a new compressor," Thornburg points out. "But, whenever you're considering adding a compressor, that's the best time to bring in outside help to look at your system, because most of the time, you don't need a new compressor. The problem is your distribution piping or how you're using compressed air. Even though a new compressor is expensive, the energy cost associated with operating it for 1 year will typically exceed its purchase price."
Caterpillar Inc. (Peoria) faced just such a dilemma 6 years ago. Despite operating eight compressors and renting an additional one in the summer, the company's fuel injector assembly plant in Pontiac, IL, was unable to supply one of its assembly lines with consistent air pressure. As a result, Caterpillar commissioned a comprehensive assessment of the plant's compressed air system by an independent team of experts.
Among other problems, the experts found that parts of the air pipeline were undersized for the system's airflow, and that leaks drained 40 percent of the system's output. The team also discovered that undersized and poorly functioning filters, dryers and air coolers were causing excessive pressure drops throughout the system.
Once the team's recommendations were implemented, the plant's compressed air energy savings totaled $226,000 per year, representing more than 6 percent of the plant's annual energy costs. Moreover, the plant was able to increase production by 18 percent without purchasing additional compressors. Had the plant not increased production, it would have been able to take some of its compressors offline.
The improvement project cost approximately $1,000,000, so the simple payback was 4.4 years. In addition, the project resulted in a 40 percent reduction in compressed air energy costs per unit of production.
The efficiency of electric motors has improved dramatically in the past decade, and assemblers may be able to realize significant savings by replacing old motors with so-called "premium efficient" motors. Thanks to better laminations, more active materials, and low-loss cooling fans, these motors are typically 1 percent to 4 percent more efficient than motors meeting federal minimum efficiency standards, says John Malinowski, product manager for AC and DC motors at Baldor Electric Co. (Fort Smith, AR). However, they also cost 20 percent more than standard motors.
If an increase in efficiency of a few percentage points doesn't seem like much, consider these numbers. According to the DOE, electric motor systems account for almost 70 percent of the electricity consumed by the manufacturing sector. The average motor consumes 50 to 60 times its initial purchase price in electricity in 10 years. The DOE estimates that if every eligible application were equipped with premium efficient motors, U.S. manufacturers would save approximately 4 billion kilowatt-hours per year and $200 million in annual energy expenditures.
"If you pay at least $0.05 per kilowatt-hour for electricity, and the motor runs at least 6,000 hours per year, and it's at least 80 percent loaded, then the payback for a premium efficient motor will be less than 2 years," Stebbins predicts.
The economics of a premium efficient motor depend primarily on its horsepower, efficiency rating, annual operating hours, load factor and the cost of electricity, says Malinowski. A gain in efficiency of 1 percent with a fully loaded, 200-hp motor will yield an annual savings of $537 at an electricity rate of $0.04 per kilowatt-hour. Many motor manufacturers offer free software to help engineers make cost-price comparisons, and many utilities offer rebates and incentives for premium efficient motors.
The best candidates for replacement are large motors-100 hp or more-that drive HVAC systems, chillers, pumps and compressors. "That's because a 1 percent improvement in the efficiency of a 200-hp motor is just a whole lot more dollars than a 1 percent improvement in a 20-hp motor," Stebbins explains. "So, there probably would not be much incentive to replace a single conveyor motor with a premium efficient motor. On the other hand, if you were replacing 50 conveyor motors in the entire plant, and the motor manufacturer gave you a quantity discount, then go for it."
Malinowski advises engineers to survey their plants and decide ahead of time how to deal with worn-out motors. Can the motor be repaired? Can it be downsized? Can it be replaced with a premium-efficient model? When such decisions are made at the last minute, opportunities for saving energy are often sacrificed for the sake of expediency, he says.
In any assembly plant, dramatic energy savings don't happen by accident, and decisions to install energy-efficient lights, motors or other technology should not be made on a piecemeal basis. If assemblers are serious about saving energy, they should consider getting an energy audit.
An energy audit is a study to identify how energy is being used in a facility. The audit can look at a specific area, such as the compressed air system, or it can cover the entire plant. The cost for an energy audit depends on how comprehensive assemblers want it to be. Many utilities offer simple "walk-through" assessments at little or no cost, while a complete plant audit covering gas, water and electricity could cost more than $100,000.
"For a walk-through, we might send one engineer for a half-day," explains Thornburg. "For an investment-grade audit, we may have a team of five to seven engineers spend several days on site, taking measurements.
"Before you start exchanging one technology for another, you really need to take energy measurements. You need to know where you're at before you can determine if you can save anything."
Pneumatic or Electric?
Compared with plantwide equipment, such as lighting and air compressors, the assembly line itself offers few opportunities for energy savings. Most assembly processes just aren't that energy-intensive.
Still, we wondered if a technology as simple and ubiquitous as power tools presented assemblers with any choices regarding energy costs. Considering that 85 percent of U.S. assembly lines install threaded fasteners, the issue is not insignificant. We asked Steve Shepard, technical services manager at power tool supplier AIMCO (Portland, OR), for his thoughts.
"There are three factors in choosing a tool: clutch, power source and control," says Shepard. "Rarely do they overlap. The application determines the clutch, such as a pulse tool or shut-off clutch. The power source is almost always determined by the site circumstances. The control method depends on the application. Safety-critical joints typically require a controlled or transducerized tool that measures torque on every run-down. This type of tool is available in both pneumatic and electric versions.
"Energy costs are primarily a side issue. Electric tools are more efficient than air tools. However, the energy cost savings are often inconsequential compared with the initial investment. For example, in a typical 30 newton-meter application, an air tool consumes about $300 per year in electricity from using compressed air. The same-sized DC electric tool uses about $45 per year in electricity. However, most engineers are not going to spend $8,000 on a DC tool solely to save $250 per year.
"If you look at applications involving screwdrivers, the savings would be incrementally smaller. So for the most part, people choose the power source based on convenience, noise levels and tool availability. Cost-savings on energy are helpful, but not a primary motivation for selection.
"When considering battery-powered tools, one must take weight and ergonomics into account. Because the tool has a self-contained power source, it can weigh 50 percent more than an equivalent air tool. Also, battery disposal is a serious issue. Nickel-cadmium batteries are considered hazardous waste and must be disposed of properly.
"Pneumatic tools with shut-off mechanisms will use less air than tools that run until the operator releases the trigger. A consistent air pressure will also minimize energy usage. A higher pressure than necessary increases the tool's air usage. Air pressure should be regulated to the correct pressure, typically 85 psi. A tool running at 100 psi will use about 20 percent more air.
"Engineers should choose tools based on the principles of productivity, ergonomics, reliability and quality. One must balance all four issues to derive the lowest lifetime cost of power tools."
You Can Put a Price on Air
Compressed air is one of the most expensive sources of energy in an assembly plant. Surprisingly, engineers don't often put a price on it.
The DOE offers the following formula for calculating the cost of compressed air:
Cost = [B * T * R * 0.746 * t * b] / E
where B is the full-load horsepower of the compressor motor; T is the number of hours the motor is operating; R is the price of electricity per kilowatt-hour; 0.746 is a conversion factor between horsepower and kilowatts; t is the percentage of time the motor is running at a particular operating level; b is the percentage of full-load horsepower that the motor delivers at that operating level; and E is the efficiency of the motor at that level.
For example, an assembly plant's compressor operates 6,800 hours annually. Fully loaded, it operates at 95 percent efficiency and produces 215 hp. Unloaded, the motor operates at 90 percent efficiency and generates 25 percent of its full-load horsepower. The motor is fully loaded 85 percent of the time and unloaded the rest of the time. The electric rate is $0.05 per kilowatt-hour.
When fully loaded, the annual operating cost of the compressor is
[215 hp * 6,800 h * $0.05/kWh * 0.746 kW/hp * 0.85 * 1] / 0.95 = $48,792
When unloaded, the annual operating cost of the compressor is
[215 hp * 6,800 h * $0.05/kWh * 0.746 * 0.15 * 0.25] / 0.9 = $2,272
Thus, the annual operating cost of the compressor is $51,064.
Help on the Web
Association of Energy Engineers
Compressed Air Challenge
Electric Motor Management
Energy Efficiency Best Practices
Energy User News
Illuminating Engineering Society of North America
Motor Decisions Matter
National Association of Lighting Management Companies
U.S. Environmental Protection Agency