Psce 2011 Article final

A. Equations and Premises 
The fuel utilization efficiency (FUE) equation in which the 
methodology is based is shown in (1). 
(1) 
where: 
W = Overall Useful Mechanical/Electrical Work 
Q = Overall Useful Heat 
E = Overall Fuel Energy, based on LHV 
It is important to state here that the purpose of this work is 
not to find the best equation that can summarize the overall 
energy efficiency of a cogeneration plant, but to establish 
practices that could be considered acceptable for most of the 
generation agents interested in supply side energy efficiency 
management, based in a known equation that resume the main 
terms related to the energy transformation within the process. 
An interesting argument towards this statement is the fact that 
even with differences between efficiency related equations in 
PURPA, European Commission and Brazilian laws, all of 
them orbit around electric, thermal and fuel energy terms and 
if any generation agent would requested to submit data to the 
regulator in its country, the furnished values for each term 
would be the same and the only difference would be in the 
final numbers after manipulating the same sort of data.
The typical PFD for a biomass cogeneration plant is shown 
in Fig. 1 below. 
Fig. 1. Typical Process Flow Diagram (PFD) for a biomass cogeneration 
plant. 
A cogeneration plant is a complex system whose operation 
varies in a range of possible stability conditions with variable 
performance along time. In order to follow this performance 
variability of the plant, it is important to establish for the 
purposes of this work that each component of the FUE 
equation represents a sum of partial energy flows during a 
period of time. Thus, the chosen dimension for each term is 
kWh/h. Although the dimension kWh/h could represent a 
power unit, it is considered here as the accumulated amount of 
transferred energy for each term during a time base of one 
hour. This way, the FUE equation can be rewritten based on 
the transferred energy flows for the cogeneration system 
volume control as in (2).
(2) 
where: 
= Useful Mechanical/Electrical transferred Work, in kWh/h 
= Useful transferred Heat, in kWh/h 
= Fuel transferred Energy, in kWh/h 
Hence, the detailing of each term from (2) is as follows. 
Equation (3) details the sum of useful mechanical or electrical 
transferred work. 
(3) 
where: 
TGX
= Electric Energy per turbine generator, in kWh/h 
CHP Aux.
= Electricity consumption in cogeneration 
auxiliary systems, in kWh/h 
TX
= Mechanic Energy per turbine driven machines such 
as make-up pumps, etc, in kWh/h 
Equation (4) details the sum of useful transferred heat. 
(4) 

---

4 
where: 
ES
= Escape Steam Enthalpy, in kWh/h. 
EC
= Escape Condensed Enthalpy, in kWh/h. 
Losses
= Heat transfer losses from escape steam to condensed 
water, in kWh/h. 
Simplifying from the first law of thermodynamics, the heat 
transfer represented by each term described in (4) can be 
obtained by the application of (5) as shown below [10]. 
(5) 
where: 
= Mass Flow, in kg/h. 
h = Specific Enthalpy, in kJ/kg. 
1kWh = 3600kJ 
Equation (6) details the transferred fuel energy. 
(6) 
where: 
= Mass Flow, in kg/h. 
LHV = Lower Heating Value, in kJ/kg. 
1kWh = 3600kJ 
The specific enthalpy of steam or condensate in each part 
of the flow is given by combined known values of the pair 
temperature and pressure within the physical state of water. 
B. Calculation 
The calculation of overall energy efficiency of a CHP plant 
shall be made during its design phase. The value of each of the 
terms detailed above is supposed to be known during basic 
design of a cogeneration. For cogeneration engineering sector, 
it is usual to find these data published at the mass and energy 
balance report or at the basic process flow diagrams for steam, 
water and fuel. 
Table I shows an example of calculation based on values 
obtained during the basic design phase of a sugarcane bagasse 
cogeneration plant. 
C. Measurement 
Far more challenging than design phase predicted FUE 
calculation is the measurement of operational parameters that 
make possible to check the overall energy efficiency of a 
plant. 
The here proposed methodology establishes general and 
specific premises in order to obtain measurements to directly 
achieve terms of above defined equations. 
1)
 
General Premises 
In order to have a standard measurement process, it is 
important to establish the following premises. 
Premise 1 – the time base for integration of the 
accumulated energy amounts is 1 hour. 
TABLE
I 
E
XAMPLE OF 
FUE
CALCULATION DURING DESIGN PHASE OF A GREENFIELD 
SUGARCANE ETHANOL PLANT