13
CHAPTER 2
LITERATURE REVIEW
2.1 INTRODUCTION
Absorption systems have been found in literature for more than 100
years. A detailed review of the literature on absorption systems has been
made by Liao (2004). Hence, the focus is made on the major and minor areas
of research such as absorption cycles and air-cooled systems respectively, in
this work.
Table 2.1 Research focus of absorption technology (Liao 2004)
Focus Area Contribution (%)
Absorption Additives 7 %
Absorption Cycles 46 %
Air-Cooled Systems 2 %
Alternative Working Fluids 6 %
Combined Heat and Power 4 %
Component Design 8 %
Economic Assessment 4 %
Heat and Mass Transfer 7 %
Modeling and Simulation 9 %
Properties 7 %
14
Table 2.1 shows the categories of research areas pertaining to absorption
technology.
VARS are classified into water-lithium bromide and ammonia-
water systems, based on the working fluids used as mentioned in the earlier
section. Water-lithium bromide systems are employed for air-conditioning
applications, and ammonia-water systems for industrial cooling/cold storage
(Ataer and Gö üs 1991) applications. The following section highlights the
literature review of the ammonia-water system, with regard to the
improvement in the system performance and also on the type of cooling
medium used, such as water, or air cooled systems.
The literature review that has been extensively done is classified into:
(a) Working fluid
(b) Theoretical studies and
(c) Experimental studies on the air-cooled GAX based vapour
absorption refrigeration systems.
2.2 WORKING FLUID
The performance of the absorption system depends on the
thermodynamic and chemical properties of the working fluids (Perez-Blanco
1984, Eisa and Holland 1987, Narodoslawsky et al 1988). Also, both the first
cost and the operating cost of an absorption system depends primarily on the
working fluid properties. Some of the desirable properties (Holmberg and
Berntsson 1990) of the working fluids are furnished below:
(a) The refrigerant should have high latent heat of vaporization.
(b) The difference in the boiling point between the pure
refrigerant and the mixture should be as large as possible.
15
(c) There should be high affinity between the refrigerant and the
absorbent.
(d) The mixture should be chemically stable, non-explosive and
non toxic.
(e) Both the refrigerant and the absorbent should be non-
corrosive, environment friendly and low cost.
(f) The transport properties such as viscosity, thermal
conductivity, etc that influence heat and mass transfer, should
be favourable.
A survey of different and alternative working fluids shows that the
most common working fluids are ammonia-water and water-lithium bromide
(Marcriss et al 1988). The reason why none of the alternative working fluids
have gained a market foothold is because the proposed alternatives may
address one or two drawbacks of the conventional working fluids, while
contributing to several of their drawbacks. Among the common working
fluids, ammonia-water is the most suitable one for the air-cooled system and
GAX operation, as discussed in the previous chapter.
2.2.1 Properties of ammonia and water
Ammonia is a colourless, alkaline gas at ambient temperature and
pressure, with a distinct pungent odour, (McKee and Wolf 1963) and is highly
soluble in water. Ammonia and Water are highly polar substances and have
the hydrogen bonding. Major properties of ammonia and water are given in
Table 2.2.
16
Table 2.2 Key properties of ammonia and water
Ammonia Water
Molecular weight (kg/kmol) 17 18
Boiling point at 1 bar (°C) - 33.2 100
Freezing point at 1 bar (°C) - 77.6 0
Critical pressure (bar) 113.5 221.2
Critical temperature (°C) 132.5 374.3
The required thermodynamic and transport properties of the
ammonia-water mixture that have been presented and discussed by various
researchers are explained as under:
Ziegler and Trepp (1984) presented a correlation used to calculate
the equilibrium properties of ammonia-water mixtures for a pressure and
temperature range of upto 50 bar and 500 K respectively. The equations of
state used are based on those of Schulz. The values of the specific volume,
vapor pressure, enthalpies and equilibrium constants for mixtures are
compared with the experimental data and the results are presented in the form
of vapor pressure and enthalpy concentration diagrams.
Renon et al (1986) formulated a cubic equation of state for the
ammonia-water vapour-liquid equilibria. The mixing parameters were
obtained from the data of Guillevic et al (1985), Pawlikowski et al (1982) and
Scatchard et al (1947). The parameters can be used to design a heat pump
easily, using the Aspen plus flow sheeting program.
17
Park and Sonntag (1990) calculated the thermodynamic properties
such as the bubble point temperature, dew point temperature, enthalpy and
entropy of ammonia-water mixtures, based on the generalized equation of
state. They compared the calculated thermodynamic properties with those of
the previous data in order to investigate the accuracy of the new approach.
The range of pressure and temperature is extended upto 20 MPa and 650 K
respectively, with an appropriate concentration of ammonia. For the
superheated region, the temperature range is easily extended to the limit of
chemical stability in principle.
Ibrahim and Klein (1993) formulated an equation of state for the
ammonia-water mixtures in the pressure range between 0.2 and 110 bar and
temperature range between 230 and 600 K. Different equations are used to
calculate the properties of the liquid and vapor phases. In the vapour phase,
the mixture is assumed to behave as an ideal solution, while in the liquid
phase, Gibbs excess energy is used to allow any departure from the ideal
solution behavior. The accuracy of the results showed a good agreement
between the computed properties and the experimental data.
Patek and Klomfar (1995) proposed a set of five equations for fast
calculations of the selected thermodynamic properties of ammonia-water. The
equations are suitable for the industrial design of the absorption refrigeration
system. The equations are constructed by fitting critically assessed
experimental data using simple functional forms, that cover the region in
which the absorption systems usually operate. The enthalpy of the gas phase
is calculated in the ideal mixture approximation and the results are presented
in the form of an enthalpy concentration diagram.
18
Sun (1997) presented the thermodynamic properties of water-lithium
bromide and ammonia-water mixtures. The results can be used to select the
operating conditions for absorption systems, and to realize automatic control
for the operation of these systems at optimum conditions.
Sun (1998) formulated the thermodynamic properties of ammonia-
water, ammonia-sodium thiocyanate and ammonia-lithium nitrate. The
thermodynamic properties were expressed in the form of polynomial
equations.
Tillner-Roth and Friend (1998) developed a thermodynamic model
based on the fundamental equation of state for the Helmholtz free energy of
the ammonia-water mixture. The model covered the entire two phase region
between the solid-liquid-vapour boundary and the critical locus. Experimental
data in the single phase region are restricted to sub critical temperatures, for
the liquid below 420 K and 40 MPa and for the vapour for pressures below 10
MPa. No experimental data are available for super critical temperatures. The
authors concluded that more experimental data are needed in order to verify
the reliability of the calculated thermodynamic properties, especially in the
single phase region.
Soleimani Alamdari (2007) presented a set of five simple and
explicit functions for the determination of the vapour-liquid equilibrium
properties of the ammonia-water mixture. The functions are constructed by
the least square method of curve fitting, using the valid available data in the
literature. The presented functions are valid upto a temperature and pressure
of 140°C and 100 bar respectively. A reasonable accuracy has been observed
when the obtained results are compared with those of other correlations in the
literature.
19
The required thermodynamic and transport properties of the
ammonia-water mixture for the theoretical analysis and the design of the
vapour absorption system have been obtained from the literature cited as
above.
2.3 THEORETICAL STUDIES
To analyze the performance of the vapour absorption refrigeration
system, several researchers have done simulation studies based on the
thermodynamic analysis. The influence of various operating parameters on
the system performance has been inferred, as these inferences are essential for
the design of the vapour absorption refrigeration system. A review of the
theoretical studies on the air-cooled and GAX based systems are furnished in
the subsequent sections.
2.3.1 Air-cooled systems
Oh et al (1994) simulated an air-cooled gas fired double effect
parallel flow water-lithium bromide absorption heat pump of capacity 7 kW.
The performance of the absorption heat pump in the cooling mode was
investigated by simulation to obtain the system characteristics, depending on
the temperature of the air at the inlet of the absorber, the working fluid
concentration, the ratio of the mass flow rate of the solution into the first
generator to the total mass flow rate of the solution from the absorber, and the
leaving temperature differences of the heat exchanging components. It was
observed that an increase in the inlet air temperature of above 37°C, decreases
the COP, and also causes a corrosion problem, due to the high temperature of
the first generator. A reasonable agreement was obtained when the predicted
results were compared with the measured data for the same design conditions.
It was suggested that for an optimum design, the leaving temperature
20
difference at the absorber, evaporator and condenser and the second generator
need to be maintained at -2°C, 2°C and less than 7°C respectively.
Kim et al (1999) analyzed the performance of the air-cooled system
for various operating conditions, using three different working fluids, namely,
LiBr + H2N(CH2)2OH + H2O, LiBr + HO (CH2)3OH + H2O, and LiBr +
(HOCH2CH2)2NH + H2O. All the three new working fluids were safe in
operation even at high sink temperatures, and served as alternatives to the
conventional water based absorption systems.
Lee et al (2000) investigated the performance of an air-cooled
double effect series flow absorption system, by using a new working fluid
H20+LiBr+LiI+LiNO3+LiCl. A cycle simulation was carried out for the
system and the thermodynamic design data were calculated at various
operating conditions. The performance of the new working fluid was found to
have a low crystallization temperature, reasonable COP, and was applicable
for air-cooled operation even at higher absorber temperature.
Salim (2001) performed simulation studies on an air-cooled water-
lithium bromide absorption system of capacity 7 kW using ABSIM software,
but no experimental validation was done.
Alva and Gonzalez (2002) investigated the technical feasibility of
air-cooled solar based absorption systems of capacity 10.5, 14 and 17.5 kW.
Simulations were conducted to evaluate the system's performance when
subjected to dynamic cooling loads. Within the computer model, heat and
mass balances were conducted on each component of the system, including
the solar collectors, thermal storage tank, the air-cooled condenser, and the
air-cooled absorber. The heat input to the absorption system generator was
supplied by an array of flat plate collectors that were coupled to a thermal
21
storage tank. The performance was compared with that of the water cooled
absorption system and no experimental validation was done.
Kim and Machielsen (2002) compared the performance and the
costs of different air-cooled solar vapour absorption refrigeration systems.
The single regenerative cycle showed a significant performance, but its
application is limited due to the high initial cost. Compared to the single
effect system of the same cooling capacity, the half effect system required
about 40% more heat exchange surface and 10 to 60% more collector area,
depending on the collector type. Ammonia-lithium nitrate systems showed
better results when different working fluids were used, and compared. The
authors suggested to develop low cost half effect absorption system to bring
down the initial cost. They concluded that the ammonia-sodium thiocyanate
system was not favourable due to an excessive pumping power requirement.
Izquierdo et al (2004) studied the performance of a double stage
air-cooled water-lithium bromide absorption cycle to produce cooling at 5°C,
operated by solar energy. When compared to that of the single stage cycle, the
double stage system allowed the use of condensation temperatures higher by
13°C than those of the former. The performance analysis showed that the
single effect system cannot be operated above 40°C of condensation
temperature, and crystallization does not occur in the double effect system
even if the condensation temperature is above 50°C.
Wang et al (2007) carried out a thermodynamic analysis of a gas
fired air-cooled adiabatic absorption refrigeration system of capacity 16 kW
using water-lithium bromide as the working fluid. The system had two new
features, such as the waste heat recovery of condensed water and an adiabatic
absorber with an air cooler, which enhances the effective heat and mass
transfer. They found that the outdoor air temperature had a great influence on
22
the cooling capacity and the COP of the system. The results indicate that the
reduction in the solution distribution ratio helped the cycle to operate in
vacuum, and to obtain a higher COP when the outdoor air temperature was
higher than that in normal operating conditions. However, a low solution
distribution ratio also increases the risk of crystallization in the high
temperature heat exchanger, as the air temperature decreases. They suggested
that a proper control of the solution distribution ratio is crucial to have high
efficiency and ensure reliability in operation.
Kim and Ferreira (2009) investigated an air-cooled absorption
chiller operated by flat collectors. The cycle used diluted water-lithium
bromide as the working fluid so that the crystallization issue is less, even
when the ambient conditions are high. The direct and indirect air-cooled
chillers considered in the study delivered chilled water at 5.7°C and 7.8°C,
with a COP of 0.36 and 0.38 respectively, at 35°C ambient condition and for a
heat source temperature of 90°C.
2.3.2 GAX based systems
Kandlikar (1982) proposed an effective method of utilizing the heat
of absorption to improve the system performance. The performance of the
proposed system with a heat recovery absorber was found to be 10% higher
than that of the conventional ammonia-water absorption refrigeration system.
The improvement in the COP of the system also reduced the cost by the
effective utilization of solar energy which decreased the collector area. The
possibility of incorporating an air-cooled condenser and absorber has been
greatly improved. The author concluded that a detailed analysis has to be
done in order to arrive at optimum design conditions.
23
Kaushik and Rajesh Kumar (1985) showed that an absorber heat
recovery cycle yields a higher COP compared to the conventional cycle at
higher generator temperatures. Water-lithium bromide was used as the
working fluid pair. The analysis was carried out to produce evaporator
temperatures of 5°C and 10°C with similar condenser and absorber
temperatures of 30°C. They revealed that the addition of the absorber heat
exchanger does not increase the heat exchanger area, because it reduces the
size of the absorber and generator. Even at higher generator temperatures, a
higher COP has been obtained, unlike the conventional water-lithium bromide
and ammonia-water cycles. However, the system was restricted to a limited
range of operating conditions, due to the crystallization problem associated
with the working fluid pair.
Scharfe et al (1986) analyzed the advantages and the limitations of
a GAX cycle. An equation for the heat of desorption was derived and it
showed that at any temperature interval, the heat requirement of the generator
is higher than the amount of heat supplied by the absorber.
Herold et al (1991) proposed a branched GAX cycle which
addresses the main problem in the standard GAX cycle. It was found that the
heat available in the absorber at each temperature level was not sufficient to
meet the heat requirement of that temperature level in the generator. The
branched GAX cycle provided a better match between the hot and cold sides
of the GAX heat exchanger, by increasing the solution flow rate in the high
temperature end of the absorber. The authors analyzed and presented the
results of the performance evaluation in both the heating and cooling modes
over a range of typical ambient conditions. The performance of the branched
GAX cycle was higher than that of the simple GAX by 20%. However,
24
compared to the simple GAX cycle, the cost of the branched GAX cycle
increased due to the additional solution pump.
Mcgahey and Christensen (1993) investigated a natural gas fired
GAX absorption heat pump, by using a modular steady state simulation,
which was used for commercial applications. An enhanced version of the
simulation model developed by the Oak Ridge National Laboratory (ORNL)
was used to model the complete absorption system, including an indoor gas
fired generator and an outdoor air-to-hydronic heat exchanger. The model was
used to map the heat pump performance for outdoor air temperatures between
-18°C and 46°C. The simulation model was used to optimize the system,
based on the performance of the UA values of the heat exchangers.
Groll and Radermacher (1994) simulated an ammonia-water
desorber absorber heat exchange (DAHX) cycle, and reported that the internal
heat exchange between the desorber and the absorber resulted in a very low
pressure ratio of about 70% lesser, when compared to that of the conventional
ammonia refrigeration system, and up to 62% lesser than that of the R-22
system. The reduction in the pressure ratio leads to an energy savings in the
DAHX cycle because of a more efficient compression process. The cooling
COP of the DAHX cycle was 10% higher than that of the conventional
ammonia-water refrigeration system, and 26% higher than that of an R-22
system for the same operating conditions. The results showed that the COP
was highly dependent on the logarithmic mean temperature difference of the
internal desorber / absorber heat exchange.
Rane and Erickson (1995) presented a patented three pressure GAX
cycle, which addresses the problem that at a high temperature lift greater than
60°C, there is no GAX temperature overlap. At these higher temperature lifts,
the three pressure GAX cycle gives a better COP compared to that of the
25
conventional single effect system, by tapping into the lost availability in the
externally cooled absorber and rectifier. It was found that, for a temperature
lift of 85°C, the analyzed GAX cycle prevents 10% reduction in the COP due
to the rectification losses in a conventional cycle.
Grossman et al (1995) investigated the performance of a GAX heat
pump for both the heating and cooling modes by using Absorption Simulation
(ABSIM) software (Grossman et al 1991). The simulation was carried out
over a wide range of ambient conditions, and the performance of the system
was calculated along with the internal flows and concentrations of the
solution and the refrigerant. It was inferred that the rectifier could produce
distilled refrigerant vapor with 99% concentration over the entire range of the
heat rejection temperatures. By increasing the rectifier temperature, there was
a marginal increase in the COP of the system, but the water content in the
refrigerant increased, which led to a larger temperature glide in the
evaporator. The influence of some of the design parameters, such as the flow
rate in the GAX heat transfer loop, and the refrigerant flow control was also
investigated. A cooling COP of 1.0 and a heating COP of 2.0 were obtained.
Hanna et al (1995) analyzed the GAX system as shown in
Figure 2.1, by employing the pinch point analysis technique. This technique
has been commonly used in chemical process industries, where internal heat
recovery plays a major role from the process design point of view. The study
focused mainly on the processes in the cycle, and the advantage is the manner
in which one could view the details of the internal processes of the cycle. The
graphical method of the pinch point analysis showed the importance of
internal heat recovery for cycle efficiency. By knowing the closeness of the
state points of the heat recovery processes, an economic trade off of the cycle
components was achieved.
26
Figure 2.1 Schematic of the GAX heat pump (Hanna et al 1995)
Ozaki et al (1995) developed computer codes for cycle simulation,
to study the performance of the absorption heat pump. Several factors, which
influence the COP of the absorption cycle, such as the evaporating
temperature, condensing temperature, and efficiency of the heat recovery heat
exchangers were considered. It was found that 5K decrease of temperature
difference of the GAX heat exchanger increased the cooling by 5.5%, and
there was a maximum heating COP at 4K of temperature difference of the
GAX heat exchanger.
Staicovici (1995) introduced poly-branched regenerative GAX
advanced cycles, which combine the advantages of the GAX, branched GAX,
regenerative GAX and regenerative GAX with rectification heat recovery.
Figure 2.2 show the schematic of a multi branched GAX cooling system. The
27
results highlighted the use of high solubility combinations at elevated
temperatures, as well as increasing the boiling temperature and the number of
stages. Compared to a double effect cycle, a two stage poly branched
regenerative cycle with rectification heat recovery was simple in construction,
and its thermal performance was 40% better. A three stage poly branched
regenerative GAX cycle had a COP 1.3 to 1.9 times higher and 70 to 80%
Carnot cooling efficiency for lifts (temperature difference between the
condenser and the evaporator) varying between 68 and 47°C. The
polybranched regenerative cycles were capable of producing both cold and
hot water at 50-70°C.
Figure 2.2 Schematic of a multi branched GAX cooling system
(Staicovici 1995)
Kang et al (1996) established a theoretical model for the design of
rectifier in a GAX absorption heat pump and considered three different rectifier
configurations namely vertical fluted tube, confined cross flow with fluted tube
and coiled smooth tube, for the study. The study revealed that a minimum
temperature difference between the interface and the bulk regions and a high heat
transfer coefficient in the vapour region reduces the size of the rectifier.
28
Ozaki et al (1996) compared the performances of the ammonia-
water advanced cycles, a GAX cycle, a hybrid cycle (a combination of a basic
cycle and a mechanical compressor) and a GAX hybrid cycle. The GAX hybrid
cycle was found to be more efficient in the cooling mode, and the difference
between the condensing and evaporating temperature influenced the cooling
COP. In the heating mode no cycle had any advantage over the others.
Erickson and Anand (1996) developed a vapour exchange GAX
cycle (VXGAX) similar to the branched GAX cycle. It was a three pressure
cycle that incorporated the heat of absorption into both the high pressure and
intermediate pressure generators. The performance of the new cycle was
better than that of the conventional cycle. The economic analysis indicates
that the VX GAX cycle provides commercially viable industrial refrigeration
operated by prime fuel or waste heat.
Erickson and Tang (1996) investigated double lift waste heat GAX
cycles (semi GAX cycles) that utilize the internal heat exchange between the
intermediate pressure absorber and the high pressure generator. A computer
modeling was done in order to identify the key performance parameters. An
increase of 20% in the COP was obtained for the semi GAX cycle when
compared to the conventional double lift cycle, and it require less total heat
duty, implying lower first cost.
Garimella et al (1996) studied the performance of a GAX heat
pump in both the heating and cooling modes by using ABSIM software. The
variables that affect the system performance were systematically investigated
over a wide range of ambient temperatures. The system cooling COP at the
rating point was maximized by varying the UA values of the heat exchanger.
The decrease in the GAX overlap at low ambient temperature, and the
corresponding transformation into the absorber heat exchange cycle was also
modeled, and its performance were investigated. Also, the role of an
additional solution – solution heat exchanger at low ambient temperature in
29
enhancing the system COP was quantified. The results showed that the COP
of the cooling and heating modes was 0.925 and 1.51 respectively, for an
ambient of 35°C and 8°C. In the cold ambient heating mode, the liquid heat
exchanger introduced between the solution heated desorber and the solution
cooled desorber offered significant performance benefits.
Saghiruddin and Siddiqui (1996) analyzed the economic aspect and
the performance study of the absorber heat recovery cycle as shown in
Figure 2.3, using ammonia-water, ammonia-lithium nitrate and ammonia-
sodium thiocyanate. The performance of the system improved by about 20 to
30 % in the ammonia-water mixture and by 30 to 35% in the ammonia lithium
nitrate and ammonia-sodium thiocyanate mixtures. Also, there was a
considerable reduction in the energy costs also by about 10 to 25% in the
Figure 2.3 Schematic of an absorption refrigeration system with theheat recovery absorber (Saghiruddin and Siddiqui 1996)
ammonia-water system, and around 20 to 30% in the ammonia-lithium nitrate
and ammonia-sodium thiocyanate systems. The operating cost of the system was
the lowest when bio gas was used as the heat source compared to liquefied
petroleum gas.
30
Zaltash and Grossman (1996) demonstrated the potential of using
ternary fluid mixtures for the advanced cycle by comparing the performance
of the simple GAX and the branched GAX cycle using ammonia-water and
ammonia-water-lithium bromide mixture. ABSIM was used to investigate the
potential of combining the above advanced cycles with the ternary fluids. The
performance parameters of the cycles, including the COP and heat duties,
were investigated as functions of the different operating parameters in the
cooling mode for both the ammonia-water binary and the ammonia-water-
lithium bromide ternary mixtures. The high performance potential of GAX
and branched GAX cycles using the ammonia-water-lithium bromide ternary
fluid mixture was achieved, especially at a higher generator temperature of
around 200°C. The cooling COPs have been improved by 21% over the COP
achieved with the conventional ammonia-water binary mixture.
Engler et al (1997) performed the simulation of a gas fired
ammonia-water GAX system using an ABSIM modular program. They
analyzed different configurations, such as the conventional single effect cycle,
the simple GAX cycle, the branched GAX cycle and the absorber heat
exchange cycle. Each configuration was formed on the basis of the previous
one by adding one or two components at each stage resulting in increased
complexity, but in improved performance. The influence of the components
added at each stage on the cycle, and the effects of the important operating
parameters in the heating and cooling modes were investigated over a wide
range of conditions. The COP ranged from about 0.5 for the conventional
single effect cycle to about 1.1 for the branched GAX cycle.
Potnis et al (1997) simulated an ammonia-water GAX system for
simultaneous heat and mass transfer, for coexisting liquid film absorption and
flow boiling desorption. The simulated temperature profiles were found to be
31
close to the experimentally obtained profiles for different vapour and liquid
flow rates. Also, the simulated values of the absorption side vapour phase
flow rates were close to those of the experiment boundary condition. The
simulation of the coupled heat and mass transfer processes could predict the
pinch point, appropriate sizing of the equipment, the COP of the system as
well as the vapour and liquid flow rates. The flow-boiling-side heat transfer
coefficients were found to be an order of magnitude higher than the
absorption-side liquid-phase heat transfer coefficients and the absorption-side
vapour-phase heat transfer and mass transfer coefficients were found to be
about an order of magnitude lower than those of the liquid phase.
Kang et al (1999) developed an advanced GAX cycle, namely,
Type A, B and C, as shown in Figure 2.4, to utilize the waste heat, and
studied the parametric analysis of the effects of the waste heat source
temperature and the outlet temperature of the gas-fired generator. In the Type
A cycle, the solution heated desorber of the standard GAX is replaced by a
waste heat exchanger; an extra heat exchanger is added to the standard GAX
in type B, and in type C the solution heated desorber is placed between the
rectifier and the GAX desorber to transfer the extra heat of the strong solution
to the desorber column. It was found that the effect of the waste heat
temperature on the performance of the system was negligible for a given gas
fired generator outlet temperature. The corrosion problem in the standard
GAX cycle at temperatures higher than 200°C could be solved by employing
the GAX cycle utilizing the waste heat (WGAX). The GAX effect was
dominant for a temperature lower than 181°C, while the effect of exergy loss
was dominant for a temperature higher than 181°C. Type A was better from the
view point of the GAX effect, whereas, Type B was better from the point of
view of the exergy loss. A comparison of the type B and type C cycles showed
that the solution heated desorber should be placed below the GAX desorber to
improve the cycle performance. It was recommended that sub cooling is
necessary to improve the COP in WGAX systems.
32
Figure 2.4 Schematic of the WGAX cycle (Kang et al 1999)
Kang and Kashiwagi (2000) compared the performance of an
ammonia-water GAX system (PGAX) and a single effect cycle for panel heating
(PSE) applications. Due to the internal heat recovery in the GAX component, the
COP of the PGAX was higher than that of the PSE. It was reported that the
performance of a hydronically cooled absorber was more sensitive to the coolant
temperature than that of the solution cooled absorber, and therefore, the effect of
the overall conductance (UA) ratio on the total COP of the PGAX was higher
than that of the PSE. The panel heating COP was more significantly affected by
the UA variation of the absorber than the space heating COP. The parametric
study revealed that the UA ratio could be used to select absorbers for heating
capacities. The stream from the hydronically cooled absorber is split into the
rectifier and the solution heat exchanger based on the split ratio, and the optimum
UA value occurred for a split ratio of 0.87.
33
Anand and Erickson (2001) examined the characterization of the
absorption cycle performance in terms of cycle lift (difference between the
condenser and evaporator temperatures) and revealed that the absorption cycle
performance was dependent on the absorption compressor and absorber
temperature. Further, the variation in the absorber temperature affected the
overall system performance. An effective lift was defined for each cycle to
incorporate the influence of the absorber temperature. Variations of the
effective lift curve, such as those observed for the basic GAX and VXGAX
cycles at low lifts, indicate the limitations in the performance of the system.
The performance data of the actual system were presented for the VX GAX
cycle heat pump, and the concept of the effective lift was validated.
Velázquez and Best (2002) reported the thermodynamic analysis
of a 10.6 kW air-cooled GAX system operated with hybrid natural gas and
solar energy, as shown in Figure 2.5. The COP was found to be 0.86 for
cooling and 1.86 for heating together with total internal heat recovery of
16.9 kW, at the generator and evaporator temperatures of 200°C and 4°C
respectively. The efficiency of the system decreases as the temperature lift
(temperature difference between the condenser and evaporator) increases.
The mass flow of the analyzed cycle was compared with that of the basic cycle,
showing 73% and 62% less for the circulation ratio and flow ratio respectively.
The system was found to be an excellent option for air conditioning purposes,
where the temperature lift was small. The proposed methodology allowed to
find the best working condition for a particular design.
34
Figure 2.5 Schematic of the solar GAX cycle using ammonia-watersolution pair (Velazquez and Best 2002)
Kang et al (2004) developed four different advanced hybrid
GAX cycles and carried out a parametric analysis. The development of the
four cycles was: Type A for performance improvement, Type B for low
temperature applications, Type C for the reduction of the generator
temperature and Type D for hot water temperature applications. The
schematic of the hybrid GAX cycle is shown in Figure 2.6. A compressor has
been placed between the evaporator and the absorber in Type A and Type B,
and between the desorber and the condenser in Type C and Type D. The
improvement in the COP of Type A was 24% higher than that of the simple
GAX for the same operating conditions. In Type B, it was observed that at an
evaporation temperature of -80°C the COP was 0.3. The maximum generator
temperature that could be reduced was 164 °C, and eventually this eradicates
the corrosion problem, that occurs at temperatures above 200°C in the simple
GAX. In Type D, the highest hot water temperature that could be obtained
was 106°C, which can be subsequently used for space heating and panel or
floor heating applications.
35
Figure 2.6 Schematic of the HGAX cycle (Kang et al 2004)
Sabir et al (2004) analyzed the performance of a novel GAX -
Resorption heat driven refrigeration cycle. The novel system was as simple as
that of a single effect cycle and the performance was found to be sensitive to
the inlet temperature of the cooling / chilled water. The COP of the system
was better than that of the conventional single effect vapour absorption and
resorption cycles, but less than that of the GAX cycles. However, it was
anticipated that, a wide range of water temperatures, and mass and heat
transfer effectiveness would result in a better performance than that of a
simple GAX system.
Ramesh Kumar and Udayakumar (2007) simulated a GAX
absorption compression cycle operated with the ammonia-water working fluid
pair. The degassing range (difference between the mass fraction of the weak
and strong solutions) of the cycle was optimized for the maximum COP and
the effect of the absorber pressure on the component heat duties was
investigated. It was found that the maximum COP occurs at an optimum
degassing range of about 0.4 kg of ammonia per kg of strong solution. The
hybrid GAX cycle showed an increase of 30% COP compared to that of the
36
simple GAX cycle. It was reported that the required COP of the hybrid cycle
could be attained in the lower degassing ranges, and it can be operated by
utilizing low temperature energy sources.
Zheng et al (2007) simulated a single stage ammonia absorption
system and a GAX cycle, and reported that the COP and the exergy efficiency
of the latter were 31% and 78% respectively higher than those of the former,
for the heat source temperatures of tH = 120°C, tM = 25°C and tL = 5°C. Based
on the concept of exergy coupling, the absorption cycle was divided into the
heat pump and heat engine sub-cycles. By means of the energy grade factor-
enthalpy diagram, the thermodynamic analyses of the two frameworks were
studied , which showed that the exergy demand of the heat pump sub-cycle in
the GAX cycle was the same as that of a single stage cycle. Also, the energy
grade factor-enthalpy diagram, clearly illustrated the level of the energy
quality at each state of the cycle and the energy load caused in the process. A
reduction in the external heat loss, external exergy loss and the internal
exergy loss of the heat engine sub-cycle, reduced the energy consumption,
and an increased benefit was obtained from the overall cycle.
Park et al (2008) developed an ammonia GAX absorption cycle that
could supply both chilled and hot water, using a single hardware. The effect of
the outlet temperature of the hot water, and the split ratio (the ratio of the
solution flow rate into a hydronically cooled absorber to the total flow rate of
the weak solution from the GAX section of the absorber) of the solution on the
cooling and heating COP were investigated. It was inferred that when the
system was operated at a full cooling load and the temperature of the hot water
was 55°C, the cooling COP of the three modes was 60%, 42% and 87%
respectively. Mode 1 gave a better result from the hot water supply point of
view. However, during summer, when the cooling mode is the primary purpose
rather than the hot water supply, case 3 was the most desirable. It was
recommended that the optimum UA values of the solution cooled absorber and
hydronically cooled absorber for mode 3 should be less than those of mode 1.
37
Ramesh Kumar and Udayakumar (2008) studied the effect of the
compressor pressure ratio on a 3.5 kW ammonia-water GAX absorption
compression cooler. The effects of the generator, sink and evaporator
temperatures on the performance of the cycle as a function of the pressure
ratio were studied. The COP increased with an increase in the low side
pressure ratio and generator temperature, and decreased with an increase in
the absorber and condenser temperatures. The low side pressure ratio of the
cycle was optimized for the optimum COP. The optimum COP corresponding
to the optimum pressure ratio was found to be independent of the sink and
evaporator temperatures, for a given value of the generator and approach
temperatures. The performance of the analyzed cycle was nearly 25% higher
than that of the standard GAX cycle.
Ramesh Kumar and Udayakumar (2008a) carried out simulation studies
based on approach temperature on a 3.5 kW GAX absorption compression cooler
and compared its performance with that of a simple GAX cycle. Three working
fluids namely ammonia-water, ammonia-sodium thiocycnate and ammonia-
lithium nitrate were used. The GAX absorption compression cycle showed a
better performance than that of a simple GAX cycle.
Ramesh Kumar et al (2009) reported the heat transfer modeling of an
11.5 kW GAX absorption compression cooler as shown in Figure 2.7. The effect
of the UA value of each component of the cycle on the COP and the cooling
capacity was analyzed, and it was found that the UA value of the absorber and
generator had a significant impact on the cycle performance. At an operating
condition of the minimum UA value of all the heat exchanging components, the
maximum COP of the system was found to be around 1.2. The variation in the
temperature of the cooling medium in the absorber was found to have a greater
effect on the COP and capacity of the system, than a variation in that of the
condenser. The effects of the mass flow rate and the inlet temperatures of the hot
fluid, chilled water and cooling water were also investigated.
38
Figure 2.7 Schematic of the GAX absorption compression cooler
(Ramesh Kumar et al 2009)
Velázquez et al (2010) presented a numerical simulation of the
solar GAX cycle of cooling capacity 10.6 kW, as shown in Figure 2.8. A
linear fresnel reflector concentrator (LFRC) was used as an ammonia vapour
generator. A mathematical modeling, considering the geometrical, optical,
thermal and fluid dynamic aspects of the LFRC was carried out. The COP of
the solar GAX cycle and the efficiency of the LFRC were about 0.85 and 0.60
respectively. The numerical result also revealed that the availability of the
solar beam radiation had a negligible effect on the COP of the system but a
significant effect on the capacity of the LFRC and the refrigeration cycle. The
study also revealed the technical feasibility of using an LFRC as a generator
in the solar GAX cycle.
39
Figure 2.8 Schematic of the ammonia-water solar GAX refrigeration
system with LFRC (Velázquez et al 2010)
Yari et al (2011) compared the energy and exergy analyses of GAX
and GAX hybrid absorption refrigeration cycles. It is inferred from the
parametric analyses that the generator temperature has more influence on the
second law efficiency that that on the COP of the cycles. The generator,
absorber and the expansion valve contributes to the highest and the lowest
exergy destruction respectively, in both the cycles.
2.4 EXPERIMENTAL STUDIES
The experimental studies on the air-cooled and GAX based systems
carried out by various researchers are explained in detail in the next section.
40
2.4.1 Air-cooled systems
Kurosawa and Fujimaki (1989) conducted experimental studies on
an air-cooled double effect gas fired 70 kW absorption chiller, using water-
lithium bromide as the working fluid. Even though the ambient temperature
reached 34°C, the system operated normally. When the ambient temperature
reached its peak, the temperatures of the solution at the absorber inlet and
outlet were 56°C and 30°C respectively. The temperatures of the cooling
medium at the inlet and outlet were 34.1°C and 44°C respectively. Since the
outlet temperature of the absorber was found to be lower than the outlet
temperature of the cooling medium, it indicates that the counter flow heat
exchange was conducted smoothly.
Castro et al (2002) developed a 3 kW water-lithium bromide
vapour absorption refrigeration system and compared the performance of the
system theoretically and experimentally, for hot water temperatures ranging
from 80 to 95°C. The initial results indicate discrepancies between the cooling
capacity and the COP, due to the poor performance of the absorber.
Incomplete wetness had been observed in the horizontal tube heat exchangers
(generator and evaporator) with the consequent reduction of the useful heat
exchange area.
Castro et al (2008) presented the recent developments in the design
of air-cooled hot water driven water-lithium bromide absorption chiller. From
the view point of methodology, the main novelty was the systematic
application of mathematical models for the design and prediction of the
thermal behavior of the new prototype. Due to the cycle configuration (single
effect), the temperature lifts between the evaporator and condenser are
41
limited, if the hot water temperature is restricted to 95°C in order to avoid
crystallization. The results showed that an evaporator temperature of 5 to 6°C
could be reached. A maximum COP of 0.65 has been obtained, and an
acceptable agreement between the experimental COP and the theoretical one,
especially with the primary fluid data, and under conditions far from the
minimum driving temperature at the generator was observed.
2.4.2 GAX based systems
Erickson et al (1996) reported the results of a gas-fired branched
GAX cycle heat pump as shown in Figure 2.9. A novel thermosypon cooled
absorber that was used, eliminated the need for the outdoor hydronic loop and
reduced the cost by 10%. The inference from the results highlighted that a
Figure 2.9 Schematic of the branched GAX prototype (Erickson et al1996)
42
cooling COP of 1.06 was obtained at a temperature effective lift (the
difference between the evaporator temperature and the average of the
condenser and absorber temperatures) of 38.9 °C and 14.7 kW cooling
capacity. For the same lift, the cycle cooling COP in the branched GAX was
1.04, and at an ambient condition of 35° C, a cooling load of 15.7 kW was
achieved at a cooling COP of 0.95. The performance of the branched GAX
was marginally lower than that of the GAX cycle due to the sub-cooling of
the absorbent liquid at the top and bottom of the GAX component.
Zhou and Radermacher (1997) experimentally investigated the
vapor compression cycle with a solution circuit and desorber/absorber heat
exchanger. The working fluid employed was an ammonia-water mixture. For
a temperature lift between 60 °C and 80 °C, a COP in the range of 1.2 to 1.8
was obtained and the cooling capacity range was found to be between 7 and
12 kW. The results were compared with those of a single stage and two stage
cycle which revealed, that the two stage cycle had the highest temperature lift
and lowest cooling COP. However, the single stage cycle had the highest
cooling COP but the lowest temperature lift. The experimental results showed
that when a bypass was introduced between the outlet of absorber I and the
inlet of generator II, the sub-cooling decreased and the variation in the
cooling COP was between 1% and 3%, and the temperature lift increased by a
maximum of 6 °C.
Ng et al (1998) examined a 7 kW gas fired ammonia-water
absorption chiller with a generator heat exchanger, an absorber heat exchanger
and a regenerative GAX configuration. The COP of the system was about 0.8 at
an operating condition of a generator temperature of 200°C, condenser
temperature of 44°C, absorber temperature of 41°C and evaporator temperature
of 5°C. The study revealed the significance of the process average temperature
in the analysis of the chiller. From the thermodynamic considerations, a proper
43
process average temperature has been derived for the non-isothermal processes
occurring in the absorption chiller, and quantified the inaccuracies that derived
from an incorrect process average temperature, in predicting chiller
performance and in estimating optimal operating conditions.
Priedeman and Christensen (1999) presented a general ammonia-
water absorption heat pump cycle of capacities 10.5, 11.5 and 17.5 kW that
was modeled and tested. The experimental results were used to calibrate both
the cycle simulation and the component simulations, yielding computer
design routines that could accurately predict the component and cycle
performance. The modeling incorporated a heat loss from the gas-fired
generator and pressure drops in both the evaporator and absorber. The
experimental findings of the 17.5 kW capacity chiller were found to be in
close agreement with the simulation results.
Priedeman et al (2001) tested a gas operated, 17.5 kW ammonia-
water GAX system under steady state operation at 35°C ambient conditions.
The COP of the system was found to be 0.68 at a full load condition.
Simulation was also carried out, and the results of the experimentation were
found to be in close proximity with the simulation results. Lower burner
generator efficiency, a pressure drop between the evaporator and pump inlet,
and less heat recovery in the GAX resulted in a lower performance of the
system, and improvements in these conditions could result in achieving the
target COP of 0.70 and delivering chilled water at the required temperature of
7°C.
Gomez et al (2008) evaluated an indirect thermal oil fired, 10.5 kW
GAX cycle. A simulation model was developed, calibrated and validated with
experimental findings in order to predict the performance of the system
outside the design parameters. The COP of the system was found to be 0.58
with a generation temperature of 192 °C. An internal heat recovery of
44
approximately 55% of the total heat supplied to the generator was obtained. A
maximum cooling load of 7.1 kW was obtained. The experimental results
were lower than the design values due to a lower range of the operating
temperature of the heating oil and the low performance of the absorber. The
performance of the GAX absorption system integrated to a micro gas turbine
as a cogeneration system, was also simulated, and it was found that the
overall efficiency of the cogeneration system varied between 29% and 49%
for cooling loads of 5 kW and 20 kW respectively.
Saravanan et al (2008) evaluated the performance of a biomass
heated GAX vapour absorption refrigeration system of 140 kW cooling
capacity, used for milk chilling operation. The evaporator was operated
between -2°C and 0°C. The GAX component recovers about 28 kW of
internal heat to attain a cooling capacity of 130 kW. An actual and real COP
of 0.58 and 0.52 were obtained for a generator temperature of 120°C and a
sink temperature of 30°C. Compared to the single effect absorption system,
the COP of the GAX system was found to be 30% higher. A saving of about
70% of electrical energy was obtained, compared to that of a vapour
compression system of the same capacity. The reduction in the emission was
about 586 tonnes of CO2 per year.
García-Arellano et al (2010) presented the dynamic analysis of a
shell and tube evaporator of an air-cooled absorption refrigeration GAX
system, to reach an operational stabilization. The dynamic analysis of the
evaporator was carried out using mathematical tools. The experimental results
showed that the evaporator can be simulated by means of a linear model. The
comparison between the theoretical and experimental results was found to be
in close agreement. The results obtained between the heat load and the
refrigerant mass flow rate could be used to design a control algorithm in order
to obtain a complete autonomy system. Also, it will maintain the COP of the
system closer to the design values.
45
2.5 CONCLUSION
In this chapter, the works carried out by various researchers on
working fluids, theoretical studies and experimental investigations on the air-
cooled GAX based vapour absorption refrigeration systems are reviewed in
detail. The following conclusions are drawn from the literature review.
1. The performance of the heat recovery absorber cycle is 10%
higher than that of a conventional single effect ammonia-
water system.
2. The GAX cycle is an elegant way of improving the
performance of a conventional absorption system. The COP of
the GAX cycle can be enhanced upto 40% than that of the
conventional single effect system for the same operating
conditions.
3. The GAX cycle has the ability to operate over a wide
concentration difference with respect to the ammonia-water
mixture, which cannot be realized in the water-lithium
bromide system.
4. In addition to ammonia-water, a few other absorbents for
ammonia have been studied. Studies on the GAX cycles with
respect to the different absorbents for ammonia need further
investigations to exactly predict the best one.
5. A majority of the results that are available in the literature for
the air-cooled GAX absorption systems are theoretical
simulations, and the experimental findings are limited.
It is inferred from the literature review that a significant amount of
work has been done on cycle modifications, to improve the performance of
the system. In the present work, the conventional ammonia-water cycle is
modified by incorporating GAX arrangements both on the low and high
46
pressure side, condensate pre-cooler, solution cooler and an additional
solution heat exchanger to recover the energy from the exhaust gas. The GAX
arrangement at the high pressure side, namely, the high pressure GAX
(HPGAX), heats the weak solution by the refrigerant vapour from the
generator and thereby the refrigerant vapour is purified. This eliminates the
necessity of a separate reflux cooler / rectifier as in the case of a conventional
NH3-H2O system. The GAX arrangement at the low pressure side, namely the
low pressure GAX (LPGAX), reduces the absorber heat load by absorbing a
partial amount of refrigerant vapour, by splitting the refrigerant vapour from
the evaporator by a factor, termed as split factor (Z), which is defined as the
ratio of the mass flow rate of the refrigerant to the absorber to the total mass
flow rate of the refrigerant in the cycle. These two new concepts are
employed in the air-cooled GAX system and hence, it requires indepth
analysis.
It is also inferred from Table 2.1, that the research work on absorption
systems with the air-cooled concept is meagre. Due to the increasing energy
costs (Sieres et al 2008), a concern for the environment and the unavailability
of commercial products with the air-cooled concept, the present research work
is aimed at developing an air-cooled GAX based vapour absorption
refrigeration system, using the conventional working fluid ammonia-water,
suitable for cold storage and other industrial applications.
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