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(a) (b)
(c) (d)
Fig. 4.27. Turbo-charger characteristics due to a step change in
m
f: (a) compressor and turbine power, (b) turbo-charger rotational speed, (c) compressor and turbine isentropicefficiencies, (d) compressor and turbine gas mass flow rate.
APPLICATION OF THE LIBRARIES OF MODELS
167 power systems models can be built up recurring to the base components available in the ‘State Determined’ and ‘Not State Determined’ libraries.
Fig. 4.28. The custom ‘Energy Systems library’ accessible from the ‘Simulink® Library Browser’: detail of the ‘Complete Power System’ sub-library.
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5
C ASE S TUDY :
A C OMBINED MCI-ORC P OWER U NIT
In this section a practical application of the library of components created and presented in the previous Chapters is proposed. A combined MCI-ORC power unit will be presented and analyzed as a possible solution to enhance the performances of a stationary MCI, in those cases when no heat is usefully recovered. Few applications of such systems exist and the study presented is aimed both at proposing possible solutions for energy optimization and at display the capabilities in system design of the developed libraries of models.
A first and second principle analysis is first presented in Par.5.1 to show how, the application of a properly designed Organic Rankine Cycle, can turn to be a feasible solution to recovery part of the heat discharged by a stationary ICE converting it into useful work that can be easily dispatched as electric power. Different system designs are proposed demonstrating that in some cases, and with some kind of organic fluids, the overall electrical efficiency can be raised of above 10%, meaning a significant energy optimization.
In Par. 5.2 the thermodynamic analysis is extended to more advanced configurations that may be adopted to couple ORCs to ICEs, demonstrating that under certain circumstances the ORC could be employed to reduce the exergy content of the high temperature source of the heat that is released by the ICE (i.e. the hot exhaust gases) still leaving an abundant thermal flux at lower temperature (i.e. the heat discharged by the ORC) that could be employed for cogeneration purposes when the heat is required at medium-low temperatures, as in the cases of building heating applications. The overall effect of the ORC in this case would be that of increasing the second principle efficiency of the thermal unit, the price being the heat to be available at lower temperatures.
A complete dynamic model of the entire combined unit is then presented in Par. 5.3, recurring to the models presented in the previous Chapters, providing a complete tool that can be used to further analyze, also in off design and unsteady operating conditions, the proposed system. The complete model hence can actually constitute a virtual test bench that can represent the behaviour of the designed unit, allowing for analyzing different lay-outs and configurations.
C HAPTER
F IVE
CHAPTER FIVE
172
Nomenclature
B Exergy [kJ]
B Exergy Flow [kW]
H Entalpy [kJ]
H Entalpy Flow [kW]
P Power [kW]
Q Heat [kJ]
Q Heat Flow [kW]
T Temperature [K]
V Volume [m3]
V Volume flow rate [m3/s]
b Specific Exergy [kJ/kg]
cp
Specific heat at constant pressure [kJ/kg K]
e Air excess coefficient [-]
h Specific Enthalpy [kJ/kg]
m Mass [kg]
m Mass flow rate [kg/s]
p Pressure [kPa]
s Specific Entropy [kJ/kg K]
v Specific volume [m3/kg]
w Specific Work [kJ/kg]
Greek symbols
α Air fuel ratio [-]
η Efficiency [-]
ηORC Organic Rankine Cycle efficiency [-]
ηb Exergy efficiency [-]
ηg Global Efficiency [-]
Φ Heat Availability [-]
ρ Density
Ψ Irreversibility factor [-]
Abbreviations and subscripts C Condensation CC Combined Cycle
EX Heat Exchanger
ICE Internal Combustion Engine ORC Organic Rankine Cycle
P Pump
PP Pinch Point R Regenerated T Turbine a Available appr Approach cond Condenser
crit Critical
d Direct engine gas/organic fluid heat exchange
dead Dead State e Engine ex Heat Exchanger
f Fluid fin Final
g Gas
htf Heat Transfer Fluid in Inlet
out Outlet tf Transfer Fluid w Water