With energy savings, there’s intent and then there’s plan. Industrial facilities in the United States show a sustained interest in energy management. That’s the intent: Reduce overall energy usage or sustain usage but increase the amount produced per kW used. The plan? Sometimes that’s a problem.
In manufacturing, a plan will only stick if it has both the wisdom of experience guiding the vision and the ROI numbers to back up the reason-why. But in energy, there just isn’t the body of research out there for an industrial plant manager to use, to set baselines for what “reasonable” energy usage looks like in a manufacturing facility. So, how to assess what portion of current energy usage is reasonable and what is wasteful, or of that wasteful portion, what provides high enough ROI to address?
The ROI under discussion here is the cost/kWh as charged by the utility. Those units carry a different rate depending on time of day and year. Reducing that expense is the savings. The investment is the materials and labor required to change energy consumption. The return is the period of time it takes for the reduced utility bill to pay for the investment. The gravy comes after the expense is paid off.
Returning to the issue of the plan, how then to draw up an ROI estimate when there is no industry standard for reasonable energy usage?
Profiling industrial energy usage
Industrial energy usage varies based on mulitple variables:
- plant age
- load type and size
- operational schedule, both hours/week and intensity of loading
- number of workers
- maintenance philosophy.
The answer is: Don’t try to manage every kW consumed by your facility. This is the “wisdom of experience” part of the equation. Divide the facility into the electrical infrastructure and then key systems.
Energy savings start with two basic tactics: a general inspection of key systems and targeted data gathering, including logging energy usage at the main service entrances and at those key loads. Identify how much a system is specified to consume, determine how much it is currently consuming, and identify wasteful practices, either in the hours and type of operation, or in the equipment/system itself. To achieve the savings, the facility must address the waste, either by changes in operation, in maintenance, or in equipment/controls.
Figure 1. Energy is expressed in real, reactive, and apparent power.
Before we explain how to trace energy consumption, let’s revisit how we define and measure energy.
Energy is expressed in real, reactive, and apparent power (Figure 1).
Energy flow is described by:
- real (P) or active power in Watts (W)
- reactive power (Q) in Volt Ampere reactive (VAR)
- complex power (S) in Volt Ampere (VA)
- apparent power, the magnitude of complex power (VA).
The mathematical relationship of real, reactive, and apparent power can be represented by vectors or expressed using complex numbers, S = P + jQ (where j is the imaginary unit).
Reactive power does not transfer energy — it does not produce work — so it is represented as the imaginary axis of the vector diagram. Real power moves energy, so it is the real axis.
The rate of energy flow in a system is dependent on the load — is it resistive, reactive, or both?
With a purely resistive load, voltage and current reverse polarity at the same time, at every instant the product of voltage and current is positive, and only real power is transferred: work is produced.
If the load is purely reactive, the voltage and current are out of phase, and the product of voltage and current can be positive or negative indicating some portion of the energy is transferred to the load and some portion flows back. The net transfer of energy to the load is zero: no work is produced.
In reality, all loads have a combination of resistance, inductance, and capacitance, creating both real and reactive power in a system. For that reason, electrical systems are designed to tolerate a certain amount of reactive power. The problem comes when too much reactive power is generated. Not only is there not enough real power to produce the required work, but the overall work-generation capacity of the system is compromised. That’s why utilities penalize their customers if their loads produce too much reactive power: it’s waste power that costs money to generate but can’t be used. Most utility bills track VARs (reactive power), and many calculate power factor, where power factor is a rating of how far below 100% real power a system has fallen. Most utilities require their customers to stay above .95 PF.
Tracing energy consumption
The understanding of basic energy components enables an electrician to set up energy logging equipment to measure overall level and quality of consumption and then trace when energy is consumed by what (Figure 2).
Figure 2. Set up energy logging equipment to measure overall level and quality of consumption and then trace when energy is consumed.
Log power at the main and secondary panels and at major loads. Record kW, kWh, and power factor over a representative period of time.