Optimizing Shredder Operations

Dec 10, 2019, 19:54 PM
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November/December 2019

By Sabine Mueller

Auto shredding plants are energy-intensive operations.Try these strategies to reduce your electricity consumption and your electricity costs.

Feature Graphics 300 x 2004The primary sizing and shredding of scrap metal is a very energy-intensive process, averaging 18 to 35 kilowatt-hours a ton, depending on density. There’s no escaping this substantial energy demand, but a few strategies have the potential to reduce electricity costs and consumption and maximize energy efficiency. Whether these strategies are possible or practical for any specific shredding facility will depend on the shredder’s location, its utility provider, and a variety of operational challenges.

For some of these strategies, it helps to understand a few electrical terms and what role they play in utility rate structures. A kilowatt-hour, or kWh, is the amount of electrical energy consumed when 1,000 watts are used for one hour. If you have a 2,000-kW motor and run it for one hour at its full load, you would consume 2,000 kWh. Power factor is a number you can calculate for any alternating-current induction motor that’s an indicator of its efficiency—how much energy the motor is converting into work. Typically, an AC motor has a power factor of 0.70 to 0.80. Some utilities charge a power-factor penalty if the power factor is below a set limit, which could be 0.80 to 0.96.

The power-factor triangle consists of real power, reactive power, and apparent power. Engineers like to explain the terms using the example of a glass of beer. When you buy a glass of beer, it typically contains some liquid beer and some foam. The liquid beer is the real power—it’s what you want. The foam is the reactive power—it’s not what you want, but it comes with the liquid beer. The total contents of the glass are the apparent power—that’s what you’re paying for, regardless of the ratio of liquid to foam.

Real power is measured in kilowatts, or kW. Reactive power in your electricity supply is the energy you’re receiving that the motor is not using to do real work, like turning the shredder rotor. Instead, it’s creating the magnetic fields in the motor using inductance. It’s measured in kilovolt-amps reactive, or kVAr. Apparent power is the energy the utility needs to supply to you for the motor to do real work. It’s measured in kilovolt-amps, or kVA. The greater your reactive power, the more apparent power you need to do the work. In beer terms, the more foam in your glass, the more glasses of beer you need to purchase to achieve your drinking goal.

The final term to know is demand, which your utility might measure in kilowatts or kilovolt-amps, depending on its rate structure. It calculates your demand as a rolling 15-minute or 30-minute average of the real power or the apparent power. Utilities may charge for your peak kW demand consumed, peak kVA consumed, or both. The important thing to remember about demand is that your peak demand determines the price you pay for the entire billing period, whether you have that demand for 15 minutes or 200 hours in that period.

If this isn’t complicated enough, the typical electric bill has many lines of charges: transmission, distribution, equipment, demand, line losses, and fees related to FERC Order 745 (which compensates providers of demand response programs), renewable energy portfolios, maintenance, sales taxes, and more. To analyze your bill, I suggest you segregate your electric bill into charges that are based on units of kWh and those based on units of kW, kVAr, and kVA. You’re likely to find the most savings by reducing the kW-, kVAr-, and kVA-based charges. You might also be able to reduce your kWh-based charges, but those efforts are likely to offer the least savings. (To see a sample of one utility’s rate schedule for large general service at secondary voltage, go to bit.ly/dominionenergy.) With that background, consider whether these strategies might help you save money and energy in your shredder operations.

Operate when electricity rates are lowest. Utilities vary their rates based on time of day and time of year. Peak times and rates during the summer will differ from peak times and rates during the winter. Some utilities even set peak times related to forecasted temperatures. Consider whether you can adjust your production schedule to run the shredder only in off-peak times to pay the lowest rate. This admittedly can pose challenges. The best rates typically are overnight, which could require running a second shift. And production needs might require you to run during a peak time interval. Unfortunately, even if you only run for 15 minutes during a peak interval, you’ll pay that peak demand charge for the whole month.

Eliminate power-factor penalties. If your utility charges a penalty for your power factor, you may be able to work with an electrical engineer to correct it by installing capacitor banks. Inductance and capacitance are reactive elements that offset one another. If the voltage and current are out of sync in an induction motor, capacitive current can offset the inductive current, reducing the apparent power in the power-factor triangle. Installing power-factor correction will not only reduce or eliminate a kVAr-related penalty, it will also reduce kVA demand slightly.

In addition to fixed capacitor solutions, static VAR compensators can provide fast reactive energy injection to improve voltage regulation and power factor. Utilities initially developed these systems to operate at the grid level, but over time the technology has improved and become more affordable. You may be able to use SVC in conjunction with an energy storage solution such as batteries.

Install automated control systems. Full-box shredding is a term made popular by Trevor Masters, a British shredder system designer. The philosophy is simple: After allowing time for start up, keep the shredding chamber full every minute the shredder is running. This improves efficiency because the metal inside the shredding chamber works in conjunction with the hammers and the rotor to densify the shred. Masters was one of the first shredder operators to develop a control system designed to create consistent density, thus maximizing electrical efficiency. Shredder manufacturers quickly filled the growing demand for automatic control systems for feeding the shredder.

Another approach is to install a demand management control system, which is designed to prevent the shredder from exceeding a predefined level of kilowatt demand during a moving 15- or 30-minute average. The demand management control system might result in some time intervals when the shredder chamber is not full. You should be able to calculate the energy savings associated with reducing demand to determine whether the savings are worth it. Early demand management systems were add-on devices; now that most modern shredders have programmable logic control systems, they often can incorporate demand management into existing controls.

Sign up for a voluntary demand response program. Your utility may offer a discount if you are willing to participate in a voluntary demand response program to shed load when the utility needs power, typically during weather-related or maintenance-related events. Participants receive from one to 24 hours’ notice of an event. The demand response discount varies based on how much load you are willing to shed. If you have a baler and a shredder, perhaps you commit to shutting off the baler but not the shredder, for example. Your utility should be able to tell you how often to expect these requests.

Many people find it difficult to work directly with their utility. Other companies aggregate demand response and sell their aggregated customer VDR levels to the utilities. These aggregators serve as middlemen who take care of the details and the communications about upcoming demand events, and they typically have more flexible VDR rules. Demand response aggregator companies include EnerNoc, Comverge, CPower, Honeywell, and Schneider Electric.

Apply for incentives and rebates. Every day, shredding facilities upgrade and replace items such as air conditioners, lighting, air compressors, motors, pumps, or power-factor-correcting capacitors. Your utility may have incentives that will offset the cost of replacing equipment. The utility may even offer incentives for converting a piece of equipment from diesel power to electric. For example, Commonwealth Edison Co. will pay $40 per unit of horsepower toward the cost of a variable-frequency drive that replaces a traditional starter. I recommend using a VFD for any motor greater than 50 hp that would benefit from running at lower speeds, such as those on fans, pumps, and conveyors. The electrical savings will pay for the VFD in two to three years.

Investigate renewable energy. The return on investment for wind and solar generation equipment has fallen to below 10 years. These technologies will not make a big dent in your electric bill, but they’re a way we all can increase our environmental stewardship. This could be as simple as covering the office or car park roof with solar panels. The energy is stored in batteries, converted to alternating current, and put back into the grid.

Consider new energy storage options. Advances in battery technology have made it possible to install a large bank of batteries which charge during off-peak times, allowing you to use the stored energy to reduce on-peak demand. Much like an SVC, the energy enters the line very quickly. Utilities are implementing energy storage systems at the grid level to manage wind and solar energy, and large energy consumers are now implementing these systems to offset demand charges. If your demand rates are greater than $18 per kilowatt, and you have to run during peak demand intervals, you might want to consider this option. Companies that specialize in energy storage solutions include Stem, Recurrent Energy, ABB, Siemens, Schneider Electric, and NextEra Energy.

Clean-energy initiatives have pushed utilities to develop technologies that in the coming decades will see more widespread use in energy-intensive industries like scrap recycling. As recyclers, we are on the front lines of environmental stewardship, and it is not an easy job. I hope this article provides some insight into understanding electrical costs and managing them.

Sabine Mueller is national engineering manager at Sims Metal Management (Rye, N.Y.) and former president of Mueller Engineering (New Berlin, Wis.).  

Here’s how auto shredding plants can save money by reducing energy consumption and electricity costs.

  • 2019
  • Nov_Dec

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