Energy Engineering: Power Pursuit
March 01, 2009
by Paul H. Stiller, PE, CEM
Energy prices are likely to continue to escalate for the foreseeable future. Bakeries and other food processing plants that effectively manage energy will have a competitive advantage as utilities become a larger portion of conversion cost. The question many operators ask is, “How do power-quality devices fit into my overall energy reduction strategy?”
Electric power-quality products such as filters, surge suppressors and capacitors can effectively solve common electric equipment problems. They protect against damage caused by voltage sags, surges, harmonics and other transients. Bakeries that experience these problems should consider the many power-quality solutions that are available.
However, most power-quality devices do not save significant amounts of energy. If your primary goal is to improve energy management and reduce usage, you should direct your attention and limited resources to areas more likely to yield significant results. It is important, none the less, to understand the role power quality devices play in your operation.
POWER QUALITY/ENERGY SAVINGS.
As noted, power-quality solutions generally do not significantly reduce energy cost. Devices such as capacitors, filters, zig-zag reactors, soft starters and surge suppressors solve reliability and equipment availability problems. The benefits can be critical if you have these problems. While there may be a small energy-saving benefit, it is generally insignificant in evaluating the investment and should be ignored.
Ideal electric power systems are balanced with 50 to 60 Hz sine-wave voltage and current. There are no interruptions or momentary deviations from these conditions, and voltage level is always within nominal range. Of course, the real world is far from ideal, and modern bakeries face a variety of power-quality defects. These may include voltage sags and surges, harmonic distortion, unbalance, momentary interruptions or excessive devices can play in your overall energy management plan and you are comfortable with the expected ROI levels, then you can look in other areas for quicker and stronger returns on your energy management dollar.
There is often a line item on electric bills based on peak demand. If your peak usage during the billing period is 1,415 kW and your supplier charges $12 per kW, the demand charge for that month would be $16,980. Of course, this is in addition to charges for energy, kilowatt hours (kWh), taxes and other fixed charges.
Demand charges allow suppliers to justify investment in generators and a distribution network capable of delivering peak load, even if the bakery only uses that amount of power for a small portion of the month.
Bakers should obtain and analyze demand interval data (see “Interval Data Chart,” below), which shows plant load over time. Reviewing electric bills helps clarify how demand impacts cost. If you have significant demand charges, you should make sure only essential equipment is running during peak times. It is common for non-essential equipment to be running and unnecessarily increasing your electric bill.
During peak-load periods, demand and spot price for electricity are high. Anyone with the capacity to generate electricity can sell it at an exceptionally good price. Also, companies willing to reduce usage during this time can “free up” generation capacity and, thus, enable their suppliers to sell that capacity for a higher price.
These arrangements are called “demand response” reactive power demand (low power factor).
Collectively known as power-quality issues, these defects can be addressed by energy providers, consultants and equipment vendors. Again, the decision to invest should be based on anticipated savings from improved reliability and overall equipment effectiveness — not energy savings.
Unless you install an electric generator such as solar panels or another source of real kilowatt (kW) power, the best you can do is reduce losses. Returning to an ideal scenario, the goal would be to eliminate all losses and inefficiencies in electric power systems, but that’s not practical. Superconductivity is the only known technology capable of reducing losses to essentially zero.
In real-world factories, electric power distribution losses average 1 to 4%. Therefore, a liberal estimate of potential savings from any loss-reduction device is perhaps 1 to 2%. This is a practical limit.
Consider a device that reduces distribution losses from 4% to 3%. There are such devices, and this is possible.Your investment payback or ROI calculations should anticipate a 1-percentage point reduction in losses. However, these opportunities rarely clear the hurdle rate or minimum payback requirements of most companies, and as a result, the investments do not receive funding. There are usually much better places to invest for energy savings.
Note that this scenario describes the reduction of losses by 25% (from 4% to 3%). This is very different than a 25% reduction in energy consumption. So as the usage and financial impact is considered, it is important to clearly understand which “reduction” is being referenced.
Once you have a solid handle on the role power-quality programs. In exchange for an agreement to limit usage during certain future events, the utility customer is paid what can be a very substantial participation fee. This fee is paid whether or not the customer is actually called upon to reduce load.
Demand response programs are not offered everywhere, but it could prove beneficial to investigate the options available to your facility.
POWER FACTOR CORRECTION.
Motors and other inductive loads require two types of electric power: real power measured in kW and reactive power measured in kilovolt-ampere reactive (kVAR). Real power translates to shaft torque, speed and heat — it does all the useful work. Reactive power is required to create the magnetic fields that allow the motor to function. Reactive power does no useful work.
Your facility needs both types of power. Most energy suppliers include a certain amount of reactive power based on how much real power you purchase. If you need more reactive power, you pay for it on your electric bill. However, does your facility need more reactive power than comes with the real power? If so, how much are you paying for the extra reactive power?
The price for reactive power varies over a wide range, from 15c up to $263 per kVAR. This wide range results from the variety of methods used by different utility companies to calculate the charge.
A generator or other power source supplies real power for the facility, but you can supply your own reactive power with a capacitor bank. The installed cost of a capacitor bank ranges from $25 to $100 per kVAR. It is possible that the installation of a capacitor bank could have a simple payback of just a few months.
THE ULTIMATE SAVINGS.
Of course, the cheapest kWh is the one you don’t purchase. While attention should be given to demand management, demand response and power-factor correction, consider the old standby — turn off equipment that is not needed.
One simple approach to finding these areas of opportunity is to walk around your bakery at a time of low production. Observe lighting, compressed air, heating/ ventilation, cooling and production equipment. See if you can spot any opportunities to reduce electric load. If your bakery is like most, the opportunity will be there, and it could be significant. A 10 to 20% reduction in electric load during times of low production is not unrealistic. The money you would spend on this unnecessary electricity falls directly to your bottom line without investment or production risk.
Paul H. Stiller, PE, CEM, has more than 30 years of industrial energy management experience. He is presently the director, energy management at Summit Energy Services, Louisville, KY. He conducts energy management workshops in North America and Europe. Mr. Stiller is a licensed professional engineer in Ohio and a certified energy manager. He holds six US patents related to his work.