Abstract: Because of importance of energy, optimization of
power generation systems is necessary. Gas turbine cycles are
suitable manner for fast power generation, but their efficiency is
partly low. In order to achieving higher efficiencies, some
propositions are preferred such as recovery of heat from exhaust
gases in a regenerator, utilization of intercooler in a multistage
compressor, steam injection to combustion chamber and etc.
However thermodynamic optimization of gas turbine cycle, even
with above components, is necessary. In this article multi-objective
genetic algorithms are employed for Pareto approach optimization of
Regenerative-Intercooling-Gas Turbine (RIGT) cycle. In the multiobjective
optimization a number of conflicting objective functions
are to be optimized simultaneously. The important objective
functions that have been considered for optimization are entropy
generation of RIGT cycle (Ns) derives using Exergy Analysis and
Gouy-Stodola theorem, thermal efficiency and the net output power
of RIGT Cycle. These objectives are usually conflicting with each
other. The design variables consist of thermodynamic parameters
such as compressor pressure ratio (Rp), excess air in combustion
(EA), turbine inlet temperature (TIT) and inlet air temperature (T0).
At the first stage single objective optimization has been investigated
and the method of Non-dominated Sorting Genetic Algorithm
(NSGA-II) has been used for multi-objective optimization.
Optimization procedures are performed for two and three objective
functions and the results are compared for RIGT Cycle. In order to
investigate the optimal thermodynamic behavior of two objectives,
different set, each including two objectives of output parameters, are
considered individually. For each set Pareto front are depicted. The
sets of selected decision variables based on this Pareto front, will
cause the best possible combination of corresponding objective
functions. There is no superiority for the points on the Pareto front
figure, but they are superior to any other point. In the case of three
objective optimization the results are given in tables.
Abstract: This paper unifies power optimization approaches in
various energy converters, such as: thermal, solar, chemical, and
electrochemical engines, in particular fuel cells. Thermodynamics
leads to converter-s efficiency and limiting power. Efficiency
equations serve to solve problems of upgrading and downgrading of
resources. While optimization of steady systems applies the
differential calculus and Lagrange multipliers, dynamic optimization
involves variational calculus and dynamic programming. In reacting
systems chemical affinity constitutes a prevailing component of an
overall efficiency, thus the power is analyzed in terms of an active
part of chemical affinity. The main novelty of the present paper in the
energy yield context consists in showing that the generalized heat
flux Q (involving the traditional heat flux q plus the product of
temperature and the sum products of partial entropies and fluxes of
species) plays in complex cases (solar, chemical and electrochemical)
the same role as the traditional heat q in pure heat engines.
The presented methodology is also applied to power limits in fuel
cells as to systems which are electrochemical flow engines propelled
by chemical reactions. The performance of fuel cells is determined by
magnitudes and directions of participating streams and mechanism of
electric current generation. Voltage lowering below the reversible
voltage is a proper measure of cells imperfection. The voltage losses,
called polarization, include the contributions of three main sources:
activation, ohmic and concentration. Examples show power maxima
in fuel cells and prove the relevance of the extension of the thermal
machine theory to chemical and electrochemical systems. The main
novelty of the present paper in the FC context consists in introducing
an effective or reduced Gibbs free energy change between products p
and reactants s which take into account the decrease of voltage and
power caused by the incomplete conversion of the overall reaction.
Abstract: An actual power plant, which is the power plant of Iron and Steel Factory at Misurata city in Libya , has been modeled using Matlab in order to compare its results to the actual results of the actual cycle. This paper concentrates on two factors:
a- The comparison between exergy losses in the actual cycle and the modeled cycle.
b- The effect of extracting pressure on temperature water at boiler inlet.
Closed heat exchangers used in this plant have been substituted by open heat exchangers in the current study of the modeled power plant and the required changes in the pressure have been considered. In the following investigation the two points mentioned above are taken in consideration.
Abstract: This paper presents the exergy analysis of a
desalination unit using humidification-dehumidification process.
Here, this unit is considered as a thermal system with three main
components, which are the heating unit by using a solar collector, the
evaporator or the humidifier, and the condenser or the dehumidifier.
In these components the exergy is a measure of the quality or grade
of energy and it can be destroyed in them. According to the second
law of thermodynamics this destroyed part is due to irreversibilities
which must be determined to obtain the exergetic efficiency of the
system.
In the current paper a computer program has been developed using
visual basic to determine the exergy destruction and the exergetic
efficiencies of the components of the desalination unit at variable
operation conditions such as feed water temperature, outlet air
temperature, air to feed water mass ratio and salinity, in addition to
cooling water mass flow rate and inlet temperature, as well as
quantity of solar irradiance.
The results obtained indicate that the exergy efficiency of the
humidifier increases by increasing the mass ratio and decreasing the
outlet air temperature. In the other hand the exergy efficiency of the
condenser increases with the increase of this ratio and also with the
increase of the outlet air temperature.
Abstract: Organic Flash Cycle (OFC) has potential of improving
efficiency for recovery of low temperature heat sources mainly due to
reducing temperature mismatch in the heat exchanger. In this work
exergetical performance analysis of ORC is conducted for recovery of
low grade heat source. Effects of system parameters such as flash
evaporation temperature or heating temperature are theoretically
investigated on the exergy destructions (anergies) at various
components of the system as well as exergy efficiency. Results show
that exergy efficiency has a peak with respect to the flash temperature,
and the optimum flash temperature increases with the heating
temperature. The component where the largest exergy destruction
occurs varies with the flash temperature or heating temperature.
Abstract: In this paper, an attempt has been made to obtain nonsensitive
solutions in the multi-objective optimization of a
photovoltaic/thermal (PV/T) air collector. The selected objective
functions are overall energy efficiency and exergy efficiency.
Improved thermal, electrical and exergy models are used to calculate
the thermal and electrical parameters, overall energy efficiency,
exergy components and exergy efficiency of a typical PV/T air
collector. A computer simulation program is also developed. The
results of numerical simulation are in good agreement with the
experimental measurements noted in the previous literature. Finally,
multi-objective optimization has been carried out under given
climatic, operating and design parameters. The optimized ranges of
inlet air velocity, duct depth and the objective functions in optimal
Pareto front have been obtained. Furthermore, non-sensitive solutions
from energy or exergy point of view in the results of multi-objective
optimization have been shown.
Abstract: In this study thermodynamic performance analysis of a
combined organic Rankine cycle and ejector refrigeration cycle is
carried out for use of low-grade heat source in the form of sensible
energy. Special attention is paid to the effects of system parameters
including the turbine inlet temperature and turbine inlet pressure on the
characteristics of the system such as ratios of mass flow rate, net work
production, and refrigeration capacity as well as the coefficient of
performance and exergy efficiency of the system. Results show that
for a given source the coefficient of performance increases with
increasing of the turbine inlet pressure. However, the exergy
efficiency has an optimal condition with respect to the turbine inlet
pressure.