Page 337 - Sustainable On-Site CHP Systems Design, Construction, and Operations

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The total net amount of money saved by employing CHP versus the conventional
BAU case can offer a realistic estimate of CHP plant’s financial performance and is
determined by calculating and comparing the cost of electricity and local boiler con-
sumed natural gas (NG), if purchased from the local utility, versus the cost of on-site
CHP electric power generation and recoverable waste heat utilization. Cost compari-
sons may be over any time period from 15 minutes, hourly, daily, monthly, to annually,
as appropriate.
Another important metric to calculate and monitor is the overall CHP plant thermo-
dynamic efficiency (or fuel utilization efficiency), which is equal to the quantity of net
CHP plant power output (gross power output less parasitic electric loads needed to
operate the CHP plant) plus net CHP plant thermal output (gross thermal output less
parasitic thermal loads needed to operate the CHP plant) divided by the total quantity
of CHP plant fuel input, all in consistent units. By monitoring CHP efficiency, operating
personnel can have the benefit of feedback regarding operating conditions and strate-
gies, can see trends develop, and can try to maximize CHP operating efficiency, which
minimizes fuel consumption and provides both financial and environmental benefits.
Note that CHP efficiency can be calculated over any time period.
Another important efficiency metric is heat rate, or the amount of fuel required to
produce one unit of power (Btu/kWh). A lower heat rate indicates a more electrically
efficient machine, and given the heat rate, the electric power generation efficiency
(another efficiency metric) can easily be calculated, monitored and trended, and can
provide key feedback regarding the efficiency of prime movers. Note that just because
the facility has achieved a low heat rate (i.e., high electrical power generation efficiency)
does not mean that the CHP facility has achieved a high overall plant thermodynamic
efficiency; and a high heat rate does not mean that the CHP plant has a low overall
thermodynamic efficiency since the CHP plant could recover and use a large percent-
age of the waste heat.
One challenge when considering overall CHP efficiency is that it treats the elec-
tric power and the thermal output equally. However, electric energy typically has a
higher exergy value than thermal energy. As discussed in Chap. 17, another impor-
tant metric that can provide a useful performance indicator regarding CHP plant
operation is the value-weighted energy utilization factor (EUF ). EUF is equal to
VW VW
quantity of the value of the net power produced plus the value of the thermal energy
recovered divided by the cost of the fuel input. The EUF represents the marginal
VW
value–to-cost ratio and should be greater than 1. A EUF less than 1 indicates that
VW
the CHP plant costs more to fuel than the corresponding value of the heat and ther-
mal energy recovered. The value of the power produced is equal to the net kilowat-
thours generated multiplied the cost per kilowatthour. While the value of the
generated steam or hot water is equal to the net thermal output times the cost per
unit of heat (e.g., per Btu). The cost to produce CHP-related services is calculated as
apart of determining CHP unit production costs discussed above and in further sec-
tions below. Typically, the goal of CHP plant personnel is to maximize the EUF
VW
wherever possible.
CHP electrical effectiveness, which equals the net power output divided by the dif-
ference between the fuel input and the total recovered heat provides another metric that
recognizes the value of CHP plant electric power output. The more heat that is recov-
ered for a given power output, the closer CHP electrical effectiveness approaches a
value of 1.0 as all energy not converted to power is recovered and beneficially used.