Energy Matters: The Fixed Usage Trap
November 01, 2009
by Paul H. Stiller, PE, CEM and Jeff Wells
Because of the recent economic downturn, many bakeries are scaling back production and experiencing significant increases in energy cost per unit of production. The cause is clear and preventable, and your company can avoid the fixed usage trap by taking immediate action to reduce the non-variable component of its energy consumption. Failing to take action caused a dramatic increase in energy cost per pound at a large Chicago, IL-based operation this past spring.
This is a common problem and opportunity. In one example, a 47% reduction in output resulted in only an 8% reduction in energy usage. That small reduction in energy, paired with the large production cut, resulted in an increase of 74% in unit energy, or cents per l00 lb produced.
An analysis of the facility’s energy usage determined that 59% of the energy consumed in March was used during plant-idle periods, thus only 41% of the purchased energy was used during production. Imagine the financial result if 59% of labor and material costs were incurred while producing nothing.
The cause is fixed energy usage, which is energy consumption not directly associated with production. Proportional reductions in labor and materials are realized when shifts are eliminated; however, a large fixed energy component causes much higher usage than anticipated as production is reduced.
For example, a presently idle facility in South Carolina is using more than 400,000 kWh per month. The annual cost is nearly $500,000.
ENERGY USAGE AND PRODUCTION
Bakeries are usually designed to operate around the clock. Facility designers focus on meeting utility requirements for continuous production and not the controls necessary to scale back usage when the lines are idle. The significant energy requirement at zero production represents fixed usage.
Labor, material and energy usage must all correlate to production. Labor and material costs often do, and energy is the opportunity.
Many facilities require energy for refrigeration or freezers that will not correlate to production, and in these cases, a fixed component as high as 50% may be acceptable. However, many facilities can make significant improvements.
Most bakeries waste energy in one or more of the following areas:
• Oven heat rejection/building ventilation
• Compressed air
• Space heating and cooling
People need fresh air, and ventilation helps to maintain an acceptable atmosphere. However, excess ventilation serves no useful purpose and can waste energy by heating or cooling large amounts of outside air. The key is to know your ventilation requirements and meet them exactly — no more, no less.
Demand-controlled ventilation is best in office areas. In this case, sensors monitor carbon dioxide (CO2) levels and control makeup air to maintain a maximum allowed CO2 level. Ventilation is also needed in baking areas to limit ambient temperature and to expel smoke or excess moisture.
Ventilation requirements are usually not constant. More or less ventilation is required depending on the level and nature of production activity. Again, the key is to know your ventilation requirements and exactly meet them. You should:
• Make sure exhaust hoods run only when necessary.
• Fix economizer and ventilation controls. Typically, 20 to 30% are bypassed or otherwise not functional.
• Know your ventilation requirement (total cu ft per minute or cu m per hour) under various operating conditions. How many air changes per hour do you need?
• Verify that the requirement is being exactly served — no more, no less.
Air compressors required for production should be turned off when the plant is idle. Too often, large compressors are left running to accommodate a small requirement for maintenance activities. These oversized units mostly blow air out of leaks in the distribution piping and provide air to idle production equipment. As much as 30% of compressor load is not providing any benefit to the plant. Consider purchasing a small compressor to use for maintenance activities when necessary.
What is your dew point requirement? Many compressed air systems waste energy maintaining dew point well below the requirement. We recently visited a food packaging operation near St. Louis, MO, that was wasting $13,900 per year maintaining dew point at -50°F when 0°F was the requirement. Air compressor electric loads are a significant contributor to inflated fixed energy usage and must be addressed.
SPACE HEATING AND COOLING
Make sure thermostats are set back to avoid heating or cooling when not necessary. We have conducted midnight surveys and observed air conditioned offices so cold there is condensate on the windows with not a person in sight to enjoy the comfort. Extra heating and cooling expense can often be avoided by simply monitoring thermostats.
How many orphan light fixtures do you have? Light fixtures are often installed in a grid pattern without regard to requirements. This is a luxury we can no longer afford. The same St. Louis-based food packaging operation mentioned above had more than 300 light fixtures (400 watts each) burning over coolers and other areas where no light is required. The fixed annual cost exceeded $150,000.
Modern lamps, motion sensors and timers make it practical to provide all the required light without wasting energy on light that is not needed.
Take a walk around your facility at night and on the weekend. Note lighting levels and identify areas being over served. Also, do not overlook lighting maintenance. Light levels can often be increased with no additional energy by cleaning fixtures.
During a late-night walk around your facility also look for the following:
• Compressed air flowing to idle production equipment or leaking.
• Unnecessary ventilation.
• Unnecessary heating or cooling.
• Idle production lines with support equipment still running.
In these challenging economic times, all companies are tasked with doing more with less. One of the ways you can do this is by avoiding unnecessary energy expenses. Your goal is to reduce fixed energy cost as much as possible. You will see a dramatic reduction in energy consumed per unit of production during periods of low activity and also enjoy the benefits of lower energy costs at all levels of production. n
Both authors currently work for Summit Energy Services, Louisville, KY, a national energy management company. Mr. Stiller is the director of energy management, and Mr. Wells is a regulated markets analyst and is responsible for research and analysis of natural gas and electric markets in Ohio and Indiana. For additional information, contact email@example.com or call (502) 429-3800.