Page 338 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Sustaining CHP Operations 311

Another metric that is important is the amount of avoided greenhouse gas (GHG)
produced based on the fuel saved, which is equal to the calculated amount of fuel that
would have been used in the BAU case minus the fuel that is used by the CHP plant.
The BAU amount of fuel can be calculated by the following formula:

BAU fuel consumption = power produced/local grid generation effi ciency
+ the sum of all heat recovered/heat production effi ciency
Typical grid generation efficiency is about 32 percent and typical natural gas–fired
boiler efficiency is about 80 percent. Given the amount of fuel saved, the amount of CO
2
eliminated can be calculated as described in Chap. 7.
Finally, it should be noted that no single metric can be used to accurately model
CHP plant operations, and each metric has its limitations. For example, heat rate can
only be used to approximate electric power generation efficiency, but cannot account
for heat recovery. Similarly, CHP efficiency while capturing the overall thermodynamic
process and fuel utilization efficiency cannot account for the added value of the gener-
ated electricity and recovered heat. The EUF does not account for labor costs, debt
VW
service, and reserve fund costs. ROI calculations do not account for positive externali-
ties such as pollution reduction. Each metric does provide important feedback, which
when trended and taken together with other metrics can provide important plant oper-
ating guidance.

Benchmarking
Benchmarking, when employed together with the above-described metrics, allows
CHP plant personnel to compare their CHP plant cost to generate a kilowatthour or a
pound of steam (with other similar CHP facilities) as well as their energy use per unit
area, which can depend on the type of facility, facility construction, facility location,
season, weather, and occupancy schedule. Benchmarking can sometimes be mislead-
ing. For example, a CHP facility with a favorable low energy use per square foot com-
pared to other CHP facilities with higher values may be the result of a more benign
climate compared to others located in more extreme microclimates, and/or the result of
shorter operating hours than other facilities in the CHP plant group being compared
against.

Maintaining an Issues Log
In order to maintain or sustain efficient operations, CHP plant management should set
up formal procedures to capture and document all relevant operational issues, changes
to controls, and any plant operating strategies. The issues log (which can either be elec-
tronic, hard copy, or both) should also provide a chronological format to record all trou-
ble calls, equipment trips, alarms, likely cause(s), and resolution(s). The log book should
also document any systems and/or equipment, and/or controls that have been placed
in hand, bypassed, and/or overridden as well as any equipment that is out of commis-
sion. The log should also provide space to record operator-requested changes to pro-
gramming/operating strategies, the reason for the change, and the resolution of the
request. Finally, and importantly, operators must be given a way to provide suggestions
for improvement so that CHP plant operations can remain optimized.