integration of solid oxide fuel cells and ... - Ea Energianalyse

4. SYSTEM DESCRIPTION
4.8 Verification of Model
All variables in the model have been assigned units. EES has a feature
to check units automatically. This has been carried out and no unit
problems were found 15 .
4.8.1 Energy balance
The conservation of energy must be fulfilled for all energy systems. In
order to verify the model, an energy balance around the entire system
has been made:
EB tot =∆Ḣ i − ∆Ḣ W GHE X 3,g ,o − Ḣ air,i − Ḣ air,o + ˙Q Chill,EV AP (4.16)
− ˙Q HW + Ḣ w,add +Ẇ BLOW +Ẇ F AN +Ẇ PU MP1
+Ẇ PU MP2 −Ẇ SOFC = 0
where ∆Ḣ W GHE X 3,g ,o is the enthalpy flow of discharged exhaust gas,
Ḣ air,i and Ḣ air,o is the enthalpy flow of the air into the cooling tower
and out of it, and Ḣ w,add is the enthalpy of the fresh water added to the
cooling tower.
4.8.2 Check of heat exchangers
Most of the heat exchangers are defined by a Closest Approach Temperature
Difference. This can however introduce an error if the chosen parameter
for the approach temperature difference is not the closest one - in that
case the real closest approach temperature can then become negative
(whereby ɛ exceeds one).
To avoid this error a variable ”ɛ M AX ” has been introduced. It simply
finds the largest effectiveness of all the heat exchangers in the system.
This way it is not necessary to monitor the effectiveness of all the heat
exchangers during simulation as long as ɛ M AX does not exceed 1,0.
15 Due to a bug in the external function ”CP_LiBrH2O.LIB” (the unit of c p is set
to kJ
kg and not kJ
kgK
), so the unit check feature finds 15 unit problems - all related to
CP_LiBrH2O.LIB. These have been checked manually and no error was found.
100