Fall 2001

Issue Focus: Alternative Power Sources

This page presents all the articles in the Fall 2001 issue of Energy Matters, the BestPractices quarterly of the U.S. Department of Energy's Industrial Technologies Program.

In This Issue

• Increased Efficiency through Waste Heat Recovery
• Industrial Manufacturers Help Develop Green Power Market
• Black Liquor Gasification Expected to Yield Energy, Environmental, and Economic Benefits
• The Power of the Wind
• Performance Optimization Tips—Measuring the Heart Rate of Motor Systems: Electric Current
• Assessment of Compressed Air Market

Increased Efficiency through Waste Heat Recovery

By Richard L. Bennett

Many industrial heating processes generate large amounts of waste energy that simply pass out the stacks and into the atmosphere. When energy is abundant and cheap, no one seems to notice, but when supplies get pinched and prices climb, people begin to realize just how much of their fuel dollar goes sailing into the blue.

Techniques for Heat Recovery

Stack exhaust losses are part of all fuel-fired processes, and they increase with the exhaust temperature and the amount of excess air the exhaust contains. At stack gas temperatures greater than 1,000°F, the heat that is carried away is likely to be the single biggest loss in the process. Above 1,800°F, stack losses will consume at least 50% of the total fuel input to the process. Waste heat recovery offers a great opportunity to put some of this energy to work, reducing energy consumption and emissions and increasing productivity. There are several techniques for heat recovery, all based on intercepting the waste gases before they leave the process, extracting some of the heat they contain, and recycling that heat.

Direct heat recovery to the product. This is the most efficient method. It takes advantage of the fact that even in the highest temperature processes, the product or charge enters the process at ambient temperature. If exhaust gases leaving the high temperature portion of the process can be brought into contact with a relatively cool incoming load, energy will be transferred to the load, preheating it and reducing the energy that finally escapes with the exhaust.

More often, heat is transferred to a surrogate medium, like combustion air to the burner system. This reduces the amount of purchased fuel required to sustain the process. Figure 1 shows how preheating combustion air affects available heat, which is the thermal efficiency of the combustion process itself.

Figure 1. Effects of preheating combustion air on available heat.

Recuperators. A Recuperator (Figure 2) is a gas-to-gas heat exchanger placed on the stack of the furnace. There are numerous designs, but all rely on tubes or plates to transfer heat from the outgoing exhaust gas to the incoming combustion air, while keeping the two streams from mixing. They are the most widely used heat recovery devices.

Figure 2. Recuperators transfer heat from outgoing gas to incoming combustion air without allowing streams to mix.

Regenerators. These are essentially rechargeable storage batteries for heat. A regenerator is an insulated container filled with metal or ceramic shapes capable of absorbing and storing relatively large amounts of thermal energy. During part of the operating cycle, process exhaust gases flow through the regenerator, heating the storage medium. After a while, the medium becomes fully charged, so the exhaust flow is shut off and cold combustion air is admitted to the unit. As it passes through, the air extracts heat from the storage medium, increasing in temperature before it enters the burners. Eventually, the heat stored in the medium is drawn down to the point where it's necessary to recharge the regenerator. At that point, the combustion airflow is shut off and the exhaust gases return to the unit. This cycle repeats as long as the process continues to operate.

Obviously, if the process is to operate without interruption, at least two regenerators (and their associated burners) are required—one to provide energy to the combustion air while the other is recharging. It is much like a using a cordless power tool—to use it continuously, you must have at least two batteries to swap out between the tool and the recharger.

The fundamental difference between recuperators and regenerators is the way they keep the exhaust gases and combustion air from cross-contaminating each other. Recuperators separate the gas streams with a physical barrier so they can operate continuously. Regenerators operate intermittently, keeping the streams separated by time.

Waste Heat Boilers. Here is an option for plants that require a source of steam or hot water. They are similar to conventional boilers with one exception—they are heated by the exhaust gas stream from a process furnace instead of their own burners.

Making the Choice

How do you decide which recovery technique is right for your operation? In some instances, more than one might fill the bill, but here are some basic points that factor into the selection process.

• Direct heat recovery to the product has the highest potential efficiency, because it doesn't require any "carrier" to return the energy to the product. However, it does require a furnace or oven configuration that permits routing the stream of exhaust gases counterflow to incoming product or materials. This usually rules out most batch-type heating equipment.

• Recuperators are available in the widest range of sizes, configurations, and temperature ranges, and they don't require elaborate combustion control systems. However, they must be protected against overheating damage on high-temperature processes and may not be suitable for some corrosive or dirty exhaust gases.

• Regenerators can operate at temperatures beyond the range of recuperators and at higher efficiency ratings. They are highly resistant to corrosion and fouling, but because of their back-and-forth switching, they require more complex, expensive flow control systems than recuperators do.

• Waste heat boilers may be the answer for plants seeking more steam capacity, but keep in mind the boiler generates steam only when the process is running. Where this is a concern, boilers with auxiliary burners may be the answer.

Industrial Manufacturers Help Develop Green Power Market

Some industrial operations not only work to improve their bottom lines and increase energy efficiency now, but also look ahead to using clean and renewable (or "green") sources of power, such as wind, solar, landfill gas, and fuel cells. Among the companies exploring these possibilities are Cargill Dow LLC, Alcoa Inc., and DuPont. Working with seven other companies within the Green Power Market Development Group (the Group), they are developing strategies to reduce green power costs, reduce market barriers, and help articulate the business case for green energy use. Other member companies of the Group are General Motors, IBM, Delphi Automotive, Interface, Johnson & Johnson, Pitney Bowes, and Kinko's.

The Group, which is organized by the World Resources Institute and Business for Social Responsibility, acknowledges that green power has substantial challenges to overcome. As corporations work to optimize shareholder value, energy purchases are often made on the basis of price alone. However, not all of green power's attributes have monetary value, and there are some good business reasons for purchasing green power, according to the Group, which says that green power purchases can:

• Protect against volatile fluctuations in fossil fuel prices by providing an alternative to traditional power sources
• Provide financial value from avoided emissions
• Help corporations build leadership and trust in the public eye, while differentiating themselves from the competition.

One of the Group's goals is to work toward a sustainable energy future. Similarly, ITP and BestPractices are facilitating a sustainable U.S. industry by helping manufacturers boost energy efficiency and improve productivity. Learn more by logging on to the ITP Web site at http://www.eere.energy.gov/industry/ and the BestPractices Web site. To learn more about the Green Power Market Development Group, log on to www.thegreenpowergroup.org.

Black Liquor Gasification Expected to Yield Energy, Environmental, and Economic Benefits

Benefits

• Reduces NO_X, SO₂, CO, VOC, and particulate emissions
• Expected air emission reduction of 90%
• Replaces existing smelters, eliminating threat of smelt-water explosions
• Reduces use of nonrenewable (fossil) fuels
• Increases energy efficiency
• Decreases capital and operating costs
• Provides hydrogen-rich, clean-burning fuel

Georgia-Pacific (G-P) and ITP have teamed up to study and demonstrate black liquor gasification, which is expected to reduce air emissions by 90%, reduce operating costs, and increase energy efficiency at the G-P containerboard mill in Big Island, Virginia. It will be the first full-scale black liquor gasification system used in the commercial pulp and paper industry. G-P and DOE will share the project cost of approximately $85 million.

The system will replace two 50-year-old smelters and will provide the entire chemical recovery capacity for the G-P mill. It has potential for industry-wide applications to replace Tomlinson recovery boilers, which are the energy-intensive industry standard. The process is suitable for all pulping processes—carbonate, kraft, sulfite, nonwood, and others. Although the technology initially requires a higher capital investment, it will provide capital returns from reduced energy demands and help the forest products industry meet increasingly stringent Environmental Protection Agency (EPA) regulations.

Promising New Technology

Black liquor is a spent product of the chemical pulping/digesting process and a source of energy for the papermaking industry. Black liquor gasification, the conversion of leftover black liquor into a clean-burning fuel for use in burners, boilers, and gas turbines, is a promising new technology for reducing air emissions and increasing energy efficiency in the pulping process. The G-P project will employ a reactor in which tubes, heated by pulses of fired gas, are immersed in a mixture of sodium carbonate and spent black liquor. The pulsing enhances a heat exchange between the tubes and the mixture, which promotes the chemical reactions that produce the fuel. This process will treat all of the 400,000 pounds of black liquor solids that the mill produces each day.

The process differs from other technologies because it does not require partial oxidation of the liquor inside the gasifier. Its lower temperature allows the gasifier to convert black liquor organic material to gas at temperatures well below those required for smelt formation, eliminating the danger of smelt-water explosions in the recovery boiler. This equipment will maximize the recovery of energy and chemicals while producing a medium Btu fuel gas (200 to 300 Btu/scf [standard cubic feet]).

Demonstration Phases

The project is being conducted in two phases. Phase I focuses on validating the process design and solving any technology gaps. Phase II will focus on completing the engineering and construction and functional operation of the new system.

During Phase I, G-P and ITP conducted an engineering study to define the scope of a full-scale demonstration of the technology. Process study areas include the reformer pressure vessel design and refractory system, the product gas clean-up, the pulse heater pressure design, the system start-up methodology, the liquor storage capacity, and the flare system. In addition, the Big Island mill signed an agreement with EPA to install and demonstrate the system under flexible regulatory terms. Phase I is nearing completion, and Phase II, which encompasses gasifier construction and demonstration, will begin shortly. Site preparation work and demolition of existing structures is already underway.

For more information on the G-P black liquor gasification project, visit the ITP Forest Products Portfolio or contact Bob Gemmer, DOE program manager, at bob.gemmer@ee.doe.gov.

The Power of the Wind

By Lynda Butek, with guest editors Robert Thresher and Kathleen O'Dell, National Wind Technology Center, National Renewable Energy Laboratory, Golden, CO

For centuries, man has harnessed the power of the wind and put it to work. Throughout Europe, Asia, and the Americas, farmers used wind power to pump water and mill grains as far back as 300 B.C. The first turbines were developed in Denmark in the 1890s, and by 1900, small wind systems were generating direct current power for many rural homes and farms in the United States. With the advent of rural electric co-ops in the 1930s, small wind turbines were no longer used to generate electricity; however, faced with the ever-increasing cost of electric power, today we are again turning to the power of the wind to generate electricity for our homes and businesses.

Most states have enough wind to power wind turbines, and 37 states have wind resources that would support utility-scale wind power plants. The wind resource in the Great Plains, if properly developed, could supply a significant portion of this country's electricity needs. With today's wind turbine technology, the United States could supply 20% of its electricity needs from wind alone, and with cost-effective storage, wind could supply a much higher percentage.

Modern Wind Turbines

Today's wind turbines come in a variety of sizes and power ratings. Small turbines rated at 100 kilowatts (kW) and less can be used in applications such as supplementing power supplies for single-family homes and small businesses, water pumping, or communications. Large, utility-scale turbines rated as high as 2 megawatts (MW) are commonly grouped to form "wind farms" or wind power plants that are connected to the utility grid to provide power for hundreds of homes. A single 750-kW turbine can provide enough electricity to power approximately 250 average homes.

Like the windmills of old, modern wind turbines are mounted on tall towers to take advantage of the best wind resources. Utility-scale turbines are mounted on towers up to 200 feet high. Most of the turbines used today look much like a child's whirligig with two or more (commonly three) large propeller-like blades mounted on a shaft to form a rotor. The blades act much like airplane wings. When the wind blows, a pocket of low-pressure air forms on the downwind side of the blade pulling the blade toward it, causing lift. This lift force causes the rotor to spin, which turns the shaft that spins a generator to produce electricity.

In addition, wind turbines contain a speed control system or brake. Although wind is a natural part of our environment, too much of a good thing (high gusting or turbulent winds) can cause runaway generators that can overload and overheat if they are not controlled or braked. The margin of error between a full-loaded machine and one that is dangerously overloaded can be as little as 10%. Even though controls cause a loss of overall efficiency, they are necessary for safe operation of the units.

The obvious advantages of wind energy are that the fuel is free, renewable, and clean. Unlike conventional power plants, wind plants emit no pollutants or greenhouse gases. According to a study conducted by the University of California at Irvine, every 500 MW of wind generating capacity can reduce emissions of carbon dioxide, the leading greenhouse gas, by more than half a million tons annually, sulfur dioxide by 637 tons, nitrogen oxides by 1,496 tons, and particulate matter by 17 tons.

Barriers to Wind Power

Although wind energy was the fastest growing energy technology during the 1990s, before it can be developed to its full potential, researchers must find ways to overcome some barriers. One of those barriers is the cost of wind energy production. In 1980, wind energy cost as much as 30 cents per kilowatt-hour (kWh) to produce. Joint research by DOE and members of industry has helped decrease that cost by more than 80% to 4 cents per kWh. To compete with conventional fuels, the cost must be lowered further. Industry members expect to reduce the cost of production an additional 30% with continued research and the introduction of more advanced, efficient turbine designs.

Another barrier to wind energy development is that wind is a fluctuating resource. When the wind blows, it produces electricity. When the wind stops that production stops. This creates an intermittent energy supply that may be difficult to integrate into the utility grids. In addition, good wind resources are often located in remote locations far from major population centers and transmission lines. By studying the nation's wind resources, its characterization, wind forecasting, the nature of current wind farms, and their impacts on the utility grids, researchers are identifying the best areas for development and demonstrating how wind energy can be integrated into the generation mix.

Several environmental groups have voiced concerns about the impacts wind turbines may have on avian populations, especially the raptor populations. In response to their concerns, DOE has worked with industry members to conduct studies on how birds interact with wind turbines. Their studies show that with proper wind farm siting, impacts on avian populations can be greatly reduced.

Despite the barriers facing the wind energy industry today, wind energy's potential to meet this nation's growing electricity needs remains immense. With continued research and development, barriers to wind energy development will be removed, allowing the technology to become a major player in the energy industry so that it can help stabilize energy supplies and pave the way to a cleaner energy future.

To learn more about wind technology research, its potential, and current applications, please visit the National Wind Technology Center Web site at www.nrel.gov/wind/, or DOE's Wind Energy Program Web site at http://www.eere.energy.gov/wind/

Performance Optimization Tips—Measuring the Heart Rate of Motor Systems: Electric Current

By Don Casada, Diagnostic Solutions LLC, Knoxville, TN

Don continues his series on field measurements.

The measurement of electric motor current is used in a variety of ways in industrial settings. Protective devices, such as fuses and thermal overload relays, work because of the heating effect of current, but current measurements usually rely on the magnetic field generated by the current passing through a conductor. Current transformers (CT)¹ are used to estimate this current. However, other devices, such as Hall-effect probes are also used.

Operations and maintenance personnel use current as an indicator of properly operating equipment, and use procedures or log sheets to specify "normal" current ranges. A newer use of current is to diagnose equipment health. Commercially available systems alert users to abnormal conditions, such as very lightly loaded motors (which might mean, for example, that a pump is running dry). More sophisticated techniques use information available in the motor current frequency spectrum to help evaluate the health of the motor and the device it drives.

For those of us doing energy work, current measurements often help estimate the motor load. While we're fundamentally interested in measuring input power, which requires current measurement, current alone can provide us with a means of estimating power, even when we don't have or can't use a portable power meter. For example, ITP's Pumping System Analysis Tool (PSAT)² uses average motor performance characteristics from the MotorMaster+ motor manufacturers database to estimate electric input power from current measurements³.

Protective and operations support functions depend on permanently installed CTs. For energy measurements, we often resort to using temporary, clamp-on CTs, such as those shown in Figure 1. If a motor has permanently installed CTs (and you trust the indicator), those can be used.

Practical Considerations for Clamp-on CTs

It should go without saying that use of accurate test equipment is of fundamental importance. But, there are several important considerations that are specific to the field use of temporary, clamp-on CTs.

1. Make sure the jaws close properly. This is essential to completing the magnetic circuit of the CT. If there are tight clearances where the CT is used, the jaws can bind partially open, even when hand tension is released. The indicated current may be considerably in error. Figure 2 illustrates the effect when a fixed load current of approximately 100 amps was monitored with a) the jaws of the CT fully closed and b) a gap of 0.04 inches (less than the thickness of a dime) separating the jaw faces.

   To ensure the jaws are fully closed, wiggle the probe a bit, making sure it moves freely and is not bound by adjacent wires or other obstructions. At higher current levels, a magnetic "buzz" created by a slight jaw separation can be heard and felt (through gloves, of course).

   

   Figure 2.Current indicated by current transducer with fully closed jaws (left) and 40-mil gap (right) with constant load (1 millivolt/amp scaling).

2. When possible, measure and average all three phases. This precaution applies to both permanent and temporary CTs. A small unbalance in the supply voltage can result in a large current unbalance among the three phases. As a rule, a 1% unbalance in voltage will result in roughly a 7% unbalance in current. Even in the presence of a balanced power supply, there may be current unbalance on the order of 5%.

3. Use properly sized CTs. Like most other measurement devices, CTs lose accuracy when operated at a fraction of their rated range. For example, using a 2,000-amp CT with 0.5% of full span accuracy (which is excellent) to measure a 20-amp current may result in a 50% measurement error.

4. Average current on fluctuating loads. Many motor loads are fluctuating in nature. The current for belt-driven equipment, for example, tends to fluctuate at belt- and sheave-pass frequencies. Some current monitoring devices grab a very short sample (a few cycles, or milliseconds) of data and display a fixed result. Other devices continuously update the data, but the fluctuations make it difficult to pin down.

   A more representative measurement can be obtained on a multimeter with a min/max averaging feature. The multimeter shown in Figure 2 has this feature (note the MIN MAX button near the bottom of the picture). This feature is also helpful in averaging other system parameters that tend to fluctuate, such as pressure. If no such function is available, several samples can be statistically averaged. A computer-based data acquisition system or data logger simplifies the collection and analysis of many (and/or longer duration) samples.

5. Make sure the current measured is really the motor current. Power factor-correcting capacitor banks are often used with induction motors. When capacitors are used, particular care must be exercised in selecting the measurement location. The current from the line to the combination of the motor and the capacitor bank will be less than the motor current. This seemingly contradictory behavior is real, and it occurs because the current to the capacitor bank will lead the voltage by 90°, while the current to the motor will lag voltage by a variable amount, depending on load.

   

   Figure 3. Motor measurements upstream and downstream of a paralleled capacitor bank.

6. If the current will be used to estimate motor load, such as when it is an input to PSAT, the current going to the motor should be measured, not the incoming current from the line. Figure 3 illustrates the error that can occur when this is not done. The current to the motor is 22% greater than the incoming line's current to the motor and paralleled capacitor bank.

7. Take safety precautions. This is absolutely the most important consideration⁴. A 50% error in the current measurement because of failure to follow previously mentioned precautions or other mistakes could result in a poor diagnosis of equipment health or a loss in company profits. But, failure to exercise proper safety precautions during testing (such as wearing insulated gloves) could result in a poor diagnosis for your health or the company's loss being you. Be careful out there!

E-mail Don Casada at doncasada@diagsol.com.

¹ CT usually refers to current transformer. However, this discussion equally pertains to Hall-effect probes; for purposes of this article, CT stands for the broader term, current transducer.

² PSAT and MotorMaster+ are available free from the BestPractices Web site.

³ Author's note: In my experience, the PSAT estimate of electric power (made from measured current and voltage) usually agrees with the measured power within a few percent.

⁴ From strictly a safety perspective, I definitely prefer an instrument whose indication is NOT built into the CT. That way, I don't have to stick my nose into the electrical cabinet to get my reading.

Assessment of Compressed Air Market

Fully 70% of all manufacturing facilities in the United States use compressed air in their production processes. In fact, compressed air systems account for 10% of all electricity use and roughly 16% of the U.S. manufacturing industry's motor system use. However, more than 50% of industrial plant air systems could be optimized for large energy savings—with relatively small project costs.

ITP has recently released the Assessment of the Market for Compressed Air Efficiency Services, a comprehensive report of the compressed air market for services that lead to compressed air system energy efficiency. The assessment discusses key findings about supply and demand side views of compressed air system efficiency. This report is now available online in a PDF format at the BestPractices Technical Publications Web page. Order the report in print from the EERE Information Center by email, or by calling 1-877-EERE-INF (877-337-3463).

NOTICE: This online publication was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.