Saving Energy on Conveyor Belts PDF Print E-mail
Written by Western Area Power Administration   
Wednesday, 09 June 2010 15:07
(original article available here)

Technical help

Case example

July 2003

Question:

What can be done to save energy on conveyor belt systems? One of our utility's customers operates outdoor conveyors to move gravel. Each conveyor section is about 50 ft. long. The motors on the conveyor are sized for starting torque and are thus running at only about 30% load under normal operating conditions. Is this sacrificing efficiency?

Answer:

The quick answer to your basic question is there is probably not as much efficiency lost as you think in running the motors. These may be NEMA Design C (high starting torque) motors. According to the inventory data in MotorMaster+ software, the efficiency loss at 25% of full load compared to fully loaded is about 2-8%, typically around 4%. Most motors peak in efficiency around 75% of full load, and maintain near-peak efficiency down to about 40-50% load before they begin to drop off significantly.

This often surprises people since many motor-driven systems—such as pumps, fans, and compressors—are much more efficient if they can be run at a near-constant speed and nearly fully loaded. The motors themselves, however, have primarily friction and winding losses that stay fairly constant throughout the normal range of their performance curves. It is only when the load is small enough that these losses begin to have a larger relative impact that you get significant losses in efficiency. So if the size of motor that is installed is required to start the conveyor or for safety in high-load conditions, then 30% load during normal operation is not disastrous. However, 50% or higher would be better from an efficiency standpoint. If you don’t need the extra power at any time, then it would definitely be worth changing the motors for smaller ones. Even at a 4% savings, that difference can often pay for itself in a fairly short period of time, since the cost to operate a motor is many times the cost to purchase one. In fact, if you wait until they need replacing anyway, then you will get an immediate payback, since a smaller motor will cost less.

Bear in mind that motors can operate up to their "service factor," which is typically about 15% above their rated horsepower, for short periods of time. So if the extra power is needed just for occasional short periods, such as for starting, you can probably get away with a smaller motor on that basis.

Other than making sure that a motor is NEMA Premium Efficient, if it is running most of the time (or evaluated using MotorMaster+ for the most economic efficiency to use if it runs less than full time), and sized so that it has enough torque for starting and high-load conditions, but is running above about 40% of full load during normal operation, there is not too much you can do with motors on a conveyor system to increase efficiency.

If there are times when you would like the conveyor to run slower than full speed, you may want to consider speed control—either two-speed motors or adjustable speed drives.

If you decide the motors need to be the size they are for other reasons, or if they spend much of their time running unloaded or underloaded, you may want to consider installing a motor voltage controller, sometimes referred to as Nola controllers. However, be sure to check out the projected savings and costs carefully. Even under the conditions you describe, savings will probably be minimal (contrary to the claims of manufacturers). Be aware that many companies sell these devices and most try to sell them for inappropriate applications. You will not save 10% or more in any application where the motor is more than 50% loaded most of the time. It only makes sense where motors spend large amounts of time unloaded or partially loaded. I would stay away from any company that tells you that you will save money with their device in any application. They may even demonstrate the savings to you by demonstrating their lack of understanding of the effect of power factor.

Other Resources

I had our Energy Library do a search for literature on conveyor systems. They found lots of good information which I think will be of interest to you, discussing many other aspects of conveyor systems in addition to motors.

System Design
  • "Energy Efficient Materials Handling," Jim Whalen, Modern Materials Handling, Jan. 2002. Has a good discussion about choosing, using, and maintaining motors and related components in a conveyor system.
  • "Automated Everything," Rodney Garrett, Pit and Quarry, Jan. 2001.This is essentially a case study of a quarry that installed new automatic controls on their conveyor system, saving significantly on their operation cost while increasing production 15%.
  • "Conveyor Drives: Reliability Begins with Design," Jim Ehler, Maintenance Technology, Jan. 2001. Covers the basic factors a designer should consider when designing a good conveyor system.
  • "Something Old, Something New," Rodney Garrett, Pit and Quarry, Oct. 2002. Another success story of new equipment retrofits in a quarry, including crushing and screening equipment.
  • "Computer Simulation Solves Conveyor Problems," Mark Alspaugh, et. al., Coal Age, Jan. 2002. Discusses a method of computer modeling called Discrete Element Modeling for modeling conveyor systems.
  • "Right Conveyor, Right App," David Maloney, Modern Materials Handling, Sep. 2002.
  • "Standardized Savings," Mark Kuhar, Pit and Quarry, Oct. 2002. Discusses the value of using standardized components in design in order to expedite and save costs in maintenance.
  • "Ceramic Lagging Cures Belt Slippage Ills," Power Engineering, Oct. 2002.
  • "Buckle Down to Business," Russell Carter, Coal Age, Aug. 2002. Discusses many conveyor add-ons that are available.
Motorized Pulleys

It sounds like your client is operating in a fairly harsh outdoor environment. If so, you may want to explore motorized pulleys, which will protect the motors from the elements.

  • "Revving Up with Motorized Pulleys," Michael Gawinski, Pit and Quarry, March 2002.
  • "Internally Powered Motorized Pulleys Growing as Option," Michael Gawinski, AFE Facilities Engineering, July/Aug. 2001.
  • "Drum Motors Run Continuously, Despite Dust," ad from Modern Materials Handling, Oct. 2002.
Maintenance
  • "A Systematic Approach to Material Handling Maintenance," Roland Newhouse, Material Handling Management, July 2002.
  • "Get a Handle on Rip-Repair Fasteners," Kevin Finnegan, Pit and Quarry, March 2001.
  • "Chain Belt Tensioners," Mark McCullough, Pit and Quarry, March 2001.
  • "Conveyor Performance Impacts Mine Productivity," Coal Age, Nov. 2001. Discusses both design and maintenance in optimizing a conveyor system.
Association

Your client is no doubt familiar with the Conveyor Equipment Manufacturers Association.


Question:

Do you have information on the energy balance of biofuels—that is, how much energy does it take to produce a gallon of biofuel versus how much energy you get out of it?

Answer:

The question you ask is an interesting one, and the answer is not only complex, but controversial. Much research has been done on this topic with a wide range of results, depending on assumptions made, source of data, and thoroughness. The three main biofuels are ethanol, biodiesel, and methanol. Since ethanol is the most ubiquitous of these three, and the most hotly debated, I will cover it first then look at the other two.

Ethanol and Energy Balance

The bottom line of most of the studies on biofuels relating to this question is what they call “energy balance,” or energy output : input ratio, which is the ratio of the energy output of the fuel to the energy input in growing the crops, producing the fuel, and transportation and delivery. The other way to quote the result is the “net energy value” (NEV), which is the net energy value in each gallon of the fuel in Btus. Since a gallon of ethanol delivers 77,000 Btus, an energy balance of 1.25 (the number most often quoted by the ethanol industry) would mean that the input energy to produce that gallon would be 61,600 Btus. I find the energy balance method more meaningful and will use that in most cases.

Some studies put the energy balance for ethanol as high as 2.62, meaning that you get over two-and-a-half times as much energy out of the ethanol fuel as you put into it to produce it. Others, most notably David Pimentel, a professor at Cornell University, claim that producing ethanol actually takes more energy than the finished ethanol can deliver. In fact, he claims that it takes 70% more energy to produce a gallon of ethanol than it provides, for an energy balance of 0.59. The numbers most often promulgated by ethanol industry groups such as the National Corn Growers association, the National Ethanol Vehicle Coalition, and the American Coalition for Ethanol range from an energy balance of 1.23 to 1.79. I asked the American Coalition for Ethanol this same question last week, and they sent me a study that I've attached, which concludes that the energy balance of ethanol is about 1.79.

Though several studies have shown an energy balance of less than 1.0 for ethanol, the ones that have caused the most controversy are those published by David Pimentel. He has been studying this issue for over 20 years, and began publishing his findings of negative net energy value for ethanol at least as early as 1991. His most recent article, published in 2001 in the Encyclopedia of Physical Sciences and Technology, has evoked several responses from the industry. One of his consistent points is that ethanol is only produced in the quantities that it is because of heavy federal subsidies. He contends that because the process is inherently uneconomic, it will continue to be heavily subsidized if the industry is to survive. At this point, I do not have a copy of that article, but his main points are summarized in a news release from Cornell University. An interview of him on the topic is titled Straight talk about corn-based ethanol. Another article on this topic:

Information Distortion and Competitive Remedies in Government Transfer Programs: The Case of Ethanol, by Ronald N. Johnson
Here are some documents from the USDA defending their position:

Another article arguing that ethanol makes a positive energy contribution is "How Much Energy Does It Take to Make a Gallon of Ethanol?" by David Lorenz and David Morris.

Note that most of these studies assume that corn is being grown specifically for ethanol production. If only the byproducts of other food processing, such as corn, are used, the net energy gain is much more positive, and it avoids the question about the ethics of using food to produce energy.

Biodiesel

As with ethanol, estimates of the energy balance for biodiesel vary tremendously, but generally with much higher numbers than for ethanol. According to Douglas G. Tiffany at the College of Agricultural, Food, and Environmental Sciences (at the University of Minnesota, I believe), the energy balance of biodiesel is 3.24, compared to 1.25 for ethanol. (Biodiesel: A Policy Choice for Minnesota, November 2001.)

The National Biodiesel Board has on their website the Executive Summary of a report called, "How Much Energy Does It Take to Make a Gallon Of Soydiesel?" The report does not give authors or a date, but it was posted on the site in May 2003. They conclude that the energy balance for biodiesel is 1.44, not including a credit for byproducts. Including the byproducts credit raises the balance to 2.51. Using state-of-the-art methods and best results gives an energy balance of 4.1.

The Office of Energy Efficiency and Renewable Energy’s Biofuels program uses the energy balance number of 3.37.

Despite these generally more positive numbers for biodiesel compared to ethanol, you may not be surprised to find out that David Pimentel, in some of the studies cited above, finds that the energy balance for biodiesel, too, is less than 1.0. He typically does not give credit for byproducts, which is one of the reasons that his numbers are generally more negative than others. The way he justifies this omission is that he is most concerned with the theoretical arguments about supplying a substantial percentage of our energy needs with biofuels. In those quantities, we would be producing more of the byproducts than we could economically use, so their value is much lower than currently.

Methanol

Methanol is not nearly as commonly used as a fuel as either ethanol or biodiesel, so it is not as important in the total energy picture, and it is consequently more difficult to find information on. Methanol is mostly produced from wood products, thus its common name, “wood alcohol.” When it is produced from timber byproducts, it can make a lot of sense. If trees (biomass) are grown specifically to produce methane, the equation changes significantly. As with the other fuels, David Pimentel finds that from purpose-grown biomass, the energy balance for methanol is less than 1.0.

Other Issues

Besides the energy balance question per se, the question about the value of biofuels is often framed in four issues: sustainability, energy security, greenhouse gases, and the question about the implications of using agriculture (food-producing resources) for energy. Addressing each of these issues in depth is beyond the scope of this response, but they are all worth exploring in deciding the issue, and at least partial information is available on all these issues in the above resources.

Enclosures

  1. "Feasibility of Large-Scale Biofuel Production," David Pimentel, et. al., Bioscience, October 1997.
  2. "Allocation Procedure in Ethanol Production System from Corn Grain," Kim and Dale, Online First, Michigan State University, May 2002.
 

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