17.4 Energy Management & Optimization

Why Energy Is a Grade IV Topic

23 CCR 3701(c)(4) states that Grade IV examinations shall test knowledge of "supervision and management responsibilities including energy management, safety program development and control, operator training, and budget development and control." Energy is named first.

The reason is scale. Water and wastewater systems are typically the largest single electricity consumer in a municipal government, often 30 to 40 percent of a city's total electric use. Within a water utility, pumping dominates; within a wastewater plant, aeration dominates, commonly 45 to 60 percent of plant electricity.

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Reading the Electric Bill

[!IMPORTANT] Demand charges can approach half the bill. They are set by a single 15-minute interval. If three pumps happen to start in the same 15 minutes on one afternoon in August, that interval can set the demand charge for that month and, under a ratchet, for the following eleven. Staggering equipment starts and interlocking large loads so they cannot run simultaneously is often the single fastest payback in a water utility - it costs a PLC change, not a capital project.

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The Affinity Laws

For a centrifugal pump or blower at variable speed:

$Q \propto N \qquad H \propto N^{2} \qquad P \propto N^{3}$

Running at 80 percent speed uses about half the power. This is why variable frequency drives are the largest single efficiency opportunity at most facilities - but only where the load actually varies and where the system curve is friction-dominated. A VFD on a pump lifting water against a fixed static head has far less benefit, because reducing speed below the static head simply stops the flow.

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Pumping Efficiency

$\text{WHP} = \frac{Q_{\text{gpm}} \times H_{\text{ft}}}{3{,}960} \qquad \text{BHP} = \frac{\text{WHP}}{\text{Pump Efficiency}} \qquad \text{MHP} = \frac{\text{BHP}}{\text{Motor Efficiency}}$

$\text{Wire-to-Water Efficiency} = \text{Pump Efficiency} \times \text{Motor Efficiency}$

Worked example. A pump delivers 1,400 gpm at 185 ft of head, with a pump efficiency of 72 percent and a motor efficiency of 92 percent.

WHP = (1,400 x 185) / 3,960 = 65.4 hp BHP = 65.4 / 0.72 = 90.8 hp MHP = 90.8 / 0.92 = 98.7 hp = 98.7 x 0.746 = 73.6 kW Wire-to-water = 0.72 x 0.92 = 66.2 percent

At 73.6 kW running continuously and $0.16/kWh, that pump costs 73.6 x 24 x 365 x 0.16 = $103,000 per year. A five-point improvement in wire-to-water efficiency is worth roughly $7,800 per year from that one pump.

Where Pumping Efficiency Goes

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Benchmarking

The theoretical minimum energy to lift water is fixed by physics; the gap between that and actual consumption is the efficiency opportunity. Trending kWh/MG for a single pump station month over month detects a worn impeller or a fouled main long before anything fails.

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The Practical Measures, In Order of Payback

1. Stagger equipment starts and prevent coincident peaks. A PLC change; cuts demand charges.
2. Shift pumping to off-peak hours, filling storage at night. Requires storage capacity and a level control strategy, and it also improves tank turnover, which helps water quality.
3. Fix air leaks and change inlet filters at wastewater plants. Free energy.
4. Implement most-open-valve aeration control so blower discharge pressure is only as high as it must be.
5. Clean fine-bubble diffusers on a condition basis using back-pressure trending.
6. Correct power factor with capacitors where the utility applies a penalty.
7. Replace throttling with VFDs or trimmed impellers on variable-load pumps.
8. Trim or replace oversized impellers and right-size motors at replacement.
9. Clean and line tuberculated mains - a C-factor restored from 65 to 130 cuts friction energy by roughly a factor of 3.6.
10. Premium-efficiency motors at replacement, and repair-versus-replace analysis on rewinds, since each rewind typically costs a fraction of a point of efficiency.
11. Reduce real water losses - every gallon leaked was pumped and treated.
12. LED lighting and building controls - small but easy.

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On-Site Generation

Biogas systems require gas conditioning - removal of moisture, hydrogen sulfide, and siloxanes - because untreated biogas destroys engines. Siloxanes, which come from personal care products, form abrasive silica deposits on engine components and are a leading cause of biogas engine failure at plants that skipped conditioning.

[!TIP] Energy management is one of the few places where the operator's daily decisions show up directly on a bill the governing board reads. An operator who documents that changing a pump start strategy cut $40,000 a year off the demand charge has made an argument for the operations budget that no compliance statistic can match.