2. Theory
This section derives the heat-absorbing and eemitting temperatures
that are used to calculate the maximum power to heat
ratio and power losses of the CHP plant based on ClausiuseRankine
cycle. Central thermal performance parameters of a CHP plant are
also defined in the beginning of this chapter.
The total efficiency of the CHP plant is:
hCHP ¼ Pel þ Q_
Ffuel
; (1)
where Pel is the electrical power output in a generator, Q_ the
useful heat output from the plant process (e.g the heat needed for
district heating) and Ffuel the fuel input into the boiler. The powerto-
heat ratio is an important performance parameter for a CHP
plant and is defined as follows:
a ¼ Pel
Q_
; (2)
For a real CHP plant the power-to-heat ratio can be calculated
using conventional energy and mass balance calculations which are
based on the First Law of Thermodynamics. To calculate the
maximum power-to-heat ratio of the CHP plant the First Lawcan no
longer be used but the calculation must be based on the Second
Law of Thermodynamics and the use of effective heat-absorbing
and -emitting temperatures.
The thermodynamic system can both absorb (Qþ) and emit (Q-)
heat when its state changes from the initial state 1 to the final state
2 as a result of the transformation process w. The net heat Q
received by the system after the process w may be written as follows
[17]:
dQ ¼ dQþ dQ (3a)
Qþ ¼
Z2
1;w
dQþ (3