Page 410 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Governmental Facility—Mission Critical 383

The two radial systems assumed the same failure rate of 1.64 failures per year and
the same average hours of downtime per failure of 2.58 hours. From this data, the avail-
ability of the utility was computed as 0.999705338.
Formal reliability studies apply field data on failure rates for every element of a
power system. In this example, all the main elements along the power chain—from util-
ity and normally paralleled 13.8 generator, down through the transformer, breakers,
switches, and every foot of cable—has a computed reliability index. The power chain
model is transferred into software that uses cut-set or Monte Carlo simulation methods
to characterize operational availability.
When the reliability block diagram was built and the numbers run, the utility-only
radial system yielded an average forced hours of downtime per year that was about
twice as large as the radial system with cogeneration. The availability of both systems
were about the same—0.999511730 versus 0.999801235—but the effect of transformer
availability and a generating source at utilization voltage, could be clearly seen. The
transformer is the most critical single point of failure in the IEEE system.
A more rigorous description of the reliability modeling process for critical opera-
tions power systems appears in Ref. 14.

Summary of Reliability Worth
The quantitative assessment methods shown in this chapter’s calculations are idealized
for representative generation technologies with CHP as the core concept. Other distrib-
uted resource technologies, such as fuel cells and batteries, are permitted as prime mov-
ers in Article 708, but beyond the scope of this chapter.
The type and extent of new or upgraded electrical systems must carefully balance
the costs of service interruptions against the capital costs of backup systems. Each facil-
ity, nested within a macrogrid with unique operating characteristics, will require a sep-
arate sensitivity analysis of avoided costs that takes into account the scale and
configuration of the entire emergency management facility real-time infrastructure.
Other considerations include
• Before any jurisdiction decides to build a CHP-based COPS all energy conser-
vation measures should have already been deployed. Remove all inefficient
equipment and occupant behaviors out of baseline energy consumption, first.
• The constant-dollar method enables an intuitive understanding of real cost
trends but will tend to understate the carrying cost of capital and present
investment alternates.
• A small cogenerator may need to meet nonattainment area requirements for
NO under the so-called bubble concept. Planners should examine the emission
x
level of the diesel engine before a CHP retrofit to assess conformity the larger,
local carbon regime.
• Many existing small municipal power plants are idle because of high operating
costs relative to the cost of grid-supplied power. These small plants could be retro-
fitted for CHP-based COP. Municipal utilities also have advantages in financing
because they are tax exempted and so is the interest paid on their obligations.
• An existing legacy oil, coal, or diesel emergency power system could be
retrofitted for cogeneration and still qualify for a federal energy tax credit as
long as on-site energy use is reduced.