Absorption cooling systems use thermal power as a driving force to produce
cooling. Although, these systems have a lower coefficient of performance compared
to the vapour compression systems, absorption cooling systems have been gaining
a considerable attention in recent years due to their driving forces which,
could be from industrial waste heat, renewable energy, or other heat sources
for which the cost of supply is negligible in many cases. Furthermore, absorption
systems use environmental friendly working fluids which do not contribute on
ozone depletion and global warming (Gomri, 2010). The
most common challenge for modeling commercial absorption chiller is the limitation
of configuration data which are confidential to the chiller manufacturer. Many
researchers studied the performance of absorption chiller based on second law
analysis. Most of the previous works dealt with fixed capacity of the machine
and no study in literature for second law analysis under partial load conditions.
Kaushik and Arora (2009) developed a model for water-lithium
bromide absorption systems that investigates exergy and energy under single
and series flow double effect configurations.
Their findings pointed out that Coefficient of Performance (COP) lies in the
range of 0.6-0.75 and 1-1.28 for single and double effect systems, respectively.
Irreversibility is the highest in the absorber in both systems when compared
to other system components.
Gebreslassie et al. (2010) conducted an exergy
analysis for single, double, triple and half effect absorption cycles which
use Water-Lithium bromide as a working fluid. In their study, they only considered
the unavoidable exergy destruction and they concluded that the COP increases
while increasing the cycle effect from double lift to triple. However, there
was no significant effect on exergetic efficiency at various configurations.
For exergy destruction rates, the effect of the heat source temperature has
similar effect for same type of components. Nevertheless, the quantitative contributions
were dependent on the flow configuration and cycle type. The absorbers and generators
have the largest exergy destruction.
Sencan et al. (2005) reported exergy analysis
of a single-effect lithium bromide/water absorption system for cooling and heating
applications. They found out that the heat loads and exergy losses from the
condenser and evaporator were less than those of the absorber and generator.
This happened because of the heat of mixing in the solution which could not
be present in pure fluids. The results show that the cooling and heating COP
of the system increase slightly when increasing the heat source temperature.
However, they reported that increasing the heat source temperature decreased
the exergetic efficiency for both cooling and heating applications.
Kilic and Kaynakli (2007) developed a mathematical
model based on the first and the second law of thermodynamics to analyze the
performance of a single-stage water-lithium bromide absorption refrigeration
system. The model used to assess the system performance, exergy loss of each
component and total exergy loss of all the system components. Their study showed
that increasing the generator and evaporator temperatures increased the performance
of the absorption refrigeration system. However, the performance was decreased
while increasing the temperature of absorber and condenser. The generator had
the highest exergy loss irrespective of operating conditions, indicating that
its the most essential component of the cycle. Most of the previous works
dealt with fixed capacity of the machine and to the best of our knowledge, no
study in literature has been reported that considers the exergy analysis of
absorption chillers under partial load conditions. Hence, our objective in this
study is to develop thermodynamic model for a commercial absorption chiller
and evaluate the chiller performance based on second low analysis under different
Description of double- effect absorption chiller cycle: Figure
1 shows the schematic of a double-effect steam absorption chiller. The circulation
of the solution inside the system is assured by the solution pump. The weak
solution (in LiBr) from absorber is subjected to flow to the high and low temperature
generators (HTG and LTG, respectively) through low and high temperature solution
heat exchangers (LTX and HTX, respectively). At HTG, the solution is heated
by steam that circulates in the tube side, as a results the refrigerant vapor
(water) is generated. The vapor from HTG is used as heat input for the LTG.
Once this vapor is condensed, it is throttled to the condenser pressure. The
vapor generated by LTG is also condensed in the condenser (Gordon
and Ng, 2000).
The strong solutions leaving the HTG and LTG are combined at LTX for transferring
heat to the weak solution that came from the absorber and then the combined
stream enters the absorber. At the absorber spraying nozzles, the strong solution
pressure drops to the absorber pressure. The refrigerant vapor produced in HTG
and LTG is condensed in condenser and throttled to the evaporator pressure.
||Schematic presention of a double-effect steam absorption chiller,
HTG: High temperature generator, LTG: Low temperature generator, CON: Condenser,
EVP: Evaporator, ABS: Absorber, HTX: High temperature solution heat exchanger,
LTX: Low temperature solution heat exchanger, 1, 2, 3
23: State points
of entering and leaving streams
In evaporator, liquid refrigerant is evaporated taking its latent heat from
chilled water that circulates in the tube side and produce the cooling effect.
Then the refrigerant that vapors in the evaporator is absorbed in the absorber
(Ahmed and Ul-Haq Gilani, 2011).
MATERIALS AND METHODS
A computer code for the simulation of a double-effect absorption chiller has
been written using Matlab software. The model is based on mass and energy balances,
heat transfer equations and equation of state. For thermal properties of LiBr
solutions and steam used in the calculations are obtained from Herold
et al. (1996), Chua et al. (2000),
Talbi and Agnew (2000) and (Kreith
and Goswami, 2007). The initial conditions consist of steam inlet pressure,
cooling and chilled water inlet temperatures and flow rates and ambient conditions.
By applying mass balance, energy balance, heat transfer equations and equation
of state for the LiBr-H2O solution for each component of double effect
steam absorption chiller, the following set of equations are obtained.
The following assumptions were employed to properly represent the absorption
||The solution leaving the absorber is saturated
||The enthalpy change across the pump is very small and can be omitted
||The temperatures of the vapor generated in generators are equal to the
average temperature of the entering and leaving temperatures of the solutions
from which it generated
||The water entering the evaporator is saturated
||Expansions are isenthalpic
||The strong solution entering the absorber is saturated
||The simulation is under steady state
Mass balance: For each chiller component, the steady state total mass
balance equation for refrigerant, chilled water, cooling water and lithium bromide
can be expressed as:
where, m is the mass flow rate and in and out are the stream entering and leaving
each of the chiller components.
The mass balance of LiBr-H2O streams:
where, x is the concentration of LiBr solution.
Energy balance: The energy balance for each chiller components is expressed
||The quantity of heat transfer to or from the system
||Enthalpy of each stream
For the absorption process in the absorber:
Heat transfer equations: The log mean temperature difference method
(LMTD) is used to evaluate the product of overall heat transfer coefficient
U and heat transfer surface area A:
The log mean temperature difference is expressed as:
Exergy analysis: The exergy method, known as the second law analysis,
calculates the exergy loss caused by irreversibility which is an important thermodynamic
property that evaluates the useful work that, can be produced by a substance
or the amount of work needed to complete a process. The exergy analysis is a
useful tool for thermodynamic analysis of energy-conversion systems.
The exergy content of a pure substance is generally given by:
||Exergy (kJ kg-1)
||Entropy (kJ kg-1 K-1)
|S0 and h0
||Enthalpy and entropy at environmental temperature T0, respectively
The exergy of the solution is calculated by:
The exergy loss in each component under steady state condition and neglecting
the potential and kinetic energies is calculated by:
The first term on the right-hand side is the sum of the exergy input; the second
is the sum of exergy output while the third term is the exergy of heat Q which
is transferred at constant temperature T. The last term is mechanical work transfer
to or from the system.
The total exergy loss (ΔψT) of the system is the sum of
exergy loss in each component:
The exergetic efficiency τ is given by:
||High temperature generator
||Low temperature generator
||High temperature solution heat exchanger
||Low temperature solution heat exchanger
RESULTS AND DISCUSSION
The mathematical equations that govern the operation of the steam absorption
chiller are developed. A computer program which is written in MATLAB environment
was developed. Simulation was performed in order to get the various state points
properties such as, temperature, concentration, mass flow rate, entropy and
exergy. Table 1 shows the properties at each state point at
the design condition. The exergy flow has a maximum value at state point 22
which is the input heat source (saturation steam) and followed by the state
point 7 where the strong solution left the HTG. The exergy loss and exergetic
efficiency are calculated based on the previous state points properties.
The exergy destructions for various components are plotted versus heat load
starting from minimum heat load till full load (20-100%). The high temperature
generator has the greatest exergy destruction followed by the absorber and high
temperature solution heat exchanger as shown in Fig. 2.
||Exergy loss under various load conditions, HTG: High temperature
generator, LTG: Low temperature generator, CON: Condenser, EVP: Evaporator,
ABS: Absorber, HTX: High temperature solution heat exchanger, LTX: Low temperature
solution heat exchanger
|| Properties of state point values of double-effect steam absorption
chiller at design condition
||Exergy efficiency and COP under various load conditions
The exergy destruction in the first two is due to heat of mixing and separation
in the absorber and first generator, respectively. For high temperature solution
heat exchanger, the exergy loss is due to large terminals temperature difference.
For single and series flow double effect lithium bromide-water absorption systems,
it is found that the highest exergy loss occurs in the absorber (Kaushik
and Arora, 2009; Gebreslassie et al., 2010).
For double effect parallel flow chillers, (Gebreslassie
et al., 2010) also found that the highest exergy loss occurs in the
high temperature generator which well matches our results.
Figure 3 shows the exergetic efficiency (EX.eff) and COP
under different load conditions. The exergetic efficiency decreases with increasing
of the load factor while the COP increasing with the load factor till reached
its highest value. This behaviour is mainly due to the increase in the heat
input to the system which leads to increase the cooling effect in the evaporator
and finally it increases the COP. But due to the rapid increase in heat input,
the heat transferred in all components of the system is increased. The aforementioned
increase in heat input result in an increase in irreversibility and thus reduces
the exergetic efficiency. This result is in a very good agreement with the research
work done by Sencan et al. (2005).
Figure 4 presents the variation of exergy loss of the main
component in the system for different heat load. It is obvious from this figure
that the exergy loss is increasing with the increasing of the load factor and
it is increasing with load factor for all components. The first generator has
the greatest exergy loss that, increasing with load factor. After 60 % load
the increase in exergy loss is lower than that for load less than 60%. The absorber
has the second higher exergy loss and its increases proportionally with load
||Total exergy loss under various load conditions, HTG: High
temperature generator, LTG: Low temperature generator, CON: Condenser, EVP:
Evaporator, ABS: Absorber, HTX: High temperature solution heat exchanger,
LTX: Low temperature solution heat exchanger
||Total exergy loss and exergetic efficiency under various load
The total exergy loss is increasing with the load factor while the exergetic
efficiency is decreasing as illustrated in Fig. 5. This manner
is mainly due to increasing the heat input with respect to the load factor which
leads to increasing the heat transferred in all the system components. Therefore,
the increase in heat transfer results in an increase in the exergy loss in all
components. Hence, it decreases the exergetic efficiency.
The model that represents the operation of the steam absorption chiller is
developed. The properties of refrigerant and solution at each state point are
calculated (temperature, enthalpy, concentration, flow rate, entropy and exergy).
The exergy destruction in different system components is evaluated. The first
generator has the greatest exergy destruction followed by the absorber and high
temperature solution heat exchanger. Further, the performance of the system
is evaluated by means of Coefficient of Performance (COP) and exergetic efficiency
(second law efficiency). This model can be used as a tool for thermo-economic
analysis of the absorption system.
The authors would like to acknowledge Universiti Teknologi PETRONAS for providing
the financial support and facilities.