May 2023 Volume 5

ENERGY

Applying the data to the measured throughput of 4753 lbs./hr., the average heating cost of one induction unit comes out to $9,015.81/ month , which equals $18.24/ton of material. Measured Efficiency Next is the calculation of efficiency. Table 3 lists the throughput parameters of the IH evaluated.

InTable 2, the kWdemand charge has the most significant influence on utility cost, comprising 61.31% of the utility bill. Next is the kWhr (energy) charge at 38.39%, and finally, the kVAR (reactive/ lagging load) charge at 0.31%. To explain the impact of demand charges on a utility bill, let us consider the CPP definition of demand, which most electrical utilities adopt. In clause 523.043(d), “Determination of Demand,” it reads: “(d) Determination of Demand. The kilowatt demand shall be determined monthly by demand measurements and shall be the maximum thirty-minute [integrated] kilowatt demand during the month.”

Table 3: IH throughput parameters During steady state operation, electrical efficiency of the power supply (input to coil terminals) was calculated at 92.3%, where:

The power required to heat the material at 100% efficiency in a zero loss (ideal) situation is calculated to be 502 kW. From this, it is found that the total efficiency of the IH system is 63.6%, where:

As described in terms of pounds per kilowatt, an efficiency of 6.01 lbs./kW is obtained, where:

Chart 1A: Induction kilowatt (kW) demand over a shift Chart 1A shows that the demand average is 740 kW and represents the power in kW averaged over the study. At the start of production, the demand was lower, around 700 kW, then increased to 791 kW. According to the CPP rate schedule, the higher demand sets the demand charge in Table 2. Because of the definition of demand, if the IH operates for a mere 30 minutes and then remains off for the remainder of the billing period, the utility bill would still include the 791kW demand charge, amounting to $5,527 regardless of how long the IH operated following the 30-minute period of operation. Suppose the kW demand had increased from 791 kW to 1100 kW for 30 minutes and then reduced to 791 kW. Also, suppose the total kWhr remained the same for the entire month. What would happen? Thanks to the demand billing definition, the demand charge for the whole of the month would be set at the 1100 kW rate. The utility bill would be increased by more than $2,000 for a single 30-minute increase in demand. Chart 2 is a graphical display of how this affects the charges.

is different than the kW meter values

*Special note: the value for used in Table 2. The value of

was measured during one hour of uninterrupted operation and is not averaged against downtime. The kW meter value used for the cost calculation is averaged over 5.65 hours, then extrapolated, accounting for power level adjustments and downtime. In other words, the value kW meter is what the utility meter would have ‘seen’ and billed against. * Discussion From the data, two parallel discussions naturally arise: utility costs and efficiency. These parallels then come back together, leading to the topic of operational efficiency, which is discussed in the second part of this article. Utility Costs Industrial electrical/utility providers use a complex billing structure to determine the cost of electrical energy they supply to customers. The most notable is that the EP ‘rewards’ customers who keep their demand low, i.e., spreading power use over more extended periods.

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