Page 318 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Sustaining Operational Ef ficiency of a CHP System 291

and natural gas for supplemental heat elsewhere in the system). The fuel energy may be
based on the lower heating value (LHV) or higher heating value (HHV) of the fuel. By
convention, the gas turbine industry uses the lower heating value to characterize energy
use and calculate efficiencies, while the natural gas distribution and electric power gen-
eration industries use the HHV for sales and to characterize natural gas energy use
(Energy Nexus Group 2002). Use of the LHV for determining energy use or the efficiency
of small turbines and reciprocating engines in CHP systems seems reasonable because
the products of combustion (exhaust) leave the turbine or engine at conditions at which
the water is in vapor phase. For monitoring CHP system performance and detecting
degradation over time, either the LHV or the HHV can be used as long as the use is con-
sistent. For comparisons to benchmarks such as data from manufacturers, care must be
taken to ensure that the LHV or HHV is used consistently in determination of the bench-
mark and in calculations of monitored performance. Furthermore, if condensing HRUs
are used in the system, the HHV should be used in calculations of fuel energy inputs.
Other variables appearing in Eq. (17-54) are defined as follows: Y and Y repre-
elec th,j
sent, respectively, the value per unit of electricity generated (e.g., in $/kWh) and the
value per unit of useful heat (or cooling) provided (e.g., in $/million Btu) by the jth
thermal application technology (e.g., absorption chiller or desiccant unit), and Price
Fuel,j
is the price of the fuel injected at point j in the system, with the discussion of different
fuels versus a single fuel from the immediately preceding paragraph applying. By
accounting for the value of products, this metric represents the value of products per
unit of expenditure on fuel and has units of dollar value of produced energy per dollar
value of fuel consumed. In operating a plant, EUF should be maximized to achieve
VW
the most economic operation. Because generally Y > Y for most thermal applica-
elec th,j
tions, a CHP plant should be operated to maximize electricity production. If, however,
the amount of electricity above on-site requirements cannot be sold to the grid, the
electricity production should follow variations in on-site electric load. Changes in the
value of EUF caused by degradations in CHP system performance would be weighted
VW
by their effects on the value of the energy produced. As a result, faults and performance
degradations having the greatest dollar impacts would be recognized by larger changes
in the EUF .
VW
Other system-level variables that can be separately monitored to provide informa-
tion useful for diagnosing changes in CHP system efficiency and understanding operating
costs are

• Current rate of useful heating or cooling output, Q (kW or Btu/h)
th th
• Current electric power output, W elec (kW)
Fuel ∑
=
• Current total rate of fuel use, Q Q Fuel, j (kW Fuel , MJ Fuel /h, or Btu Fuel /h)
j
=
• Current rate of expenditures on fuel, Cost Fuel ∑ Q Fuel,j Price Fuel,j ($/h)
j
Average values of these metrics over various time intervals can also be constructed
for each of them, for example, average daily useful heat output, daily average hourly
heat output, total daily heat output, and so forth for the other variables.
These indicators of overall system performance are supplemented with the compo-
nent performance indicators to enable system-level and finer resolution performance
monitoring and potentially fault detection and diagnostics in support of condition-
based maintenance of CHP plants.