For Further Research

Further research is underway to develop enhanced models which consider:

• Demand profile flexibility. Other load-duration shapes with different underlying frequency distributions (e.g. triangular, normal and auto-correlated loads).

• Economies of Scale. The fact that larger units have better fuel-to-electricity efficiencies (lower heat rates) and lower per unit installed cost ($/kW).

EXAMPLE. A manufacturing plant operates 7500 hours per year and has a fairly constant electrical (billing) peak demand every month (See Figure 1). The actual load, however, varies widely between a minimum of 2000 kW and a maximum of 5300 kW (See Figure 2). The demand charge is $10/kW/month and the energy charge is $0.05/kWh. The installed cost of a diesel generator set, the auxiliary electrical switch gear and peak-shaving controls is about $300 per kW. Alternatively, the plant can lease a PSG for $50/kW/yr. The operation and maintenance cost (including diesel fuel) is $0.10/kWh.

Assuming the plant leases the PSG, estimate (1) the optimal PSG size, (2) the annual savings and (3) the PSG annual operation time.

1) The optimal generator size is calculated using equation [9]

2) Using a commercially available PSG of size g* = 600 kW, the potential annual savings are estimated using equation [7]

AW = g2 • T • Ce/[2(Du - D1)] + 12 • g • Cd

= 6002 x 7500 x 0.05/(2(5300 - 2000)) + 12 x 600 x 10

- ($50 + 0.10 x 600 x 7500/(2 (5300 - 2000))) 600 = $20,455 + $72,000 - $70,909

3) The expected annual operating time for the PSG is estimated using Equation [6]

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