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5 GENERAL DISCUSSION
The absorbtion refrigeration system differs from the more prevalent
compression chillers in that its cooling performance is driven by heat energy,
rather than by mechanical energy (heat-operated cycle). This cooling system is
regarded as environmentally friendly method due to the uses of safe refrigerant
and absorbent substances i.e. LiBr-H2O and NH3-H2O, respectively. The other
advantage of this method is the utilization of heat energy for energy source. Heat
is the end product of energy conversion which is considered as low grade energy
and can be produced from solar energy, biomass, animal husbandry and
agricultural waste and also industrial waste.
In absorbtion refrigeration system, heat is used for regeneration process
which specifically used to separate the refrigerant from the absorbent. The
separatedrefrigerant in vapor phaseis condensed into liquid refrigerant in
condenser. The liquid refrigerant is then evaporated in evaporator by absorbing
heat from the environment which produces cooling effect. The generated water
vapor is then absorbed by high concentration of LiBr-H2O in absorber.
Regeneration process in generator requires higher temperature than 85°C (Ma, et
al., 1998).In addition, traditional single-effect of LiBr-H2O can’t operate
effectively when driven by heat resource below 80°C because of low COP
(Vargas et al., 2009). Gu et al. (2006) used a solar lithium bromide absorbtion
chiller system whose temperature scope was from 80 to 93°C. The average COP
of the system was 0.725. Sumathy et al. (2002) introduced a twostage lithium
bromide absorbtion refrigeration system which could apply low temperature
heater from 70 to 85°C. However, the COP was only 0.39. Kim et al. (2008)
added that the large contact area needed to separate water vapor from aqueous
lithium bromide solution makes traditional generator too bulky. Therefore,
membrane technology is introduced to deal with the uses of high temperature for
refrigerant and absorbent separation.
However, the application of membrane technology in absorbtion
refrigeration system has been previously reported by several researchers although
still focus on the membrane performance. Riffat and Su (1998) used centrifuge
reverse osmosis (RO) membrane in a refrigeration system to reduce the utilization
of high pressure pump. The research found that rate higher than 10.000 rpm at r =
50 mm was required in order to obtain 64% of solution concentration. The
disadvantage of this system was the uses of high velocity which corresponds to
the increasing of mechanical energy used in the system. Wang Z.S. et al., (2009)
used membrane distillation technology based on PVDF-hollow fiber module for
LiBr-H2O separation process. In this research, several parameters i.e. feed flux,
temperature in lumen side and vacuum pressure in shell lumen were observed. It
was found that the increasing of feed temperature and feed flux will increase the
water vapor permeation flux. Ahmed H. and Peter S., (2009) conducted an
experiment to analyze the effect of membrane characteristic towards the
absorbtion process in absorber. A good absorbtion performance was obtained by
membrane characteristic which has high permeability upon water vapor, uses high
pressure hydrophobic membrane to avoid membrane pore wetness, and no water
vapor capillary condensation to avoid membrane pore block. While the problem
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which associates with low value of COP in absorbtion refrigeration system should
analyze the effectiveness of energy utilization and exergy analysis.
The study of water vapor absorbtion by LiBr-H2O solution was to
determine the influence of concentration, temperature and water vapor pressure
toward water vapor absorbtion process. This result was used to determine the
optimum condition of absorber to absorb water vapor. The concentration value of
end product produced from separation process was very important factor in
determining the absorbtion performance. The result discussed in Chapter 2
mentioned that higherconcentration of LiBr-H2O solution increased the
absorbtion rate. In addition, higher concentration reduced water vapor pressure
which then increased the rate of absorbtion process. The temperature influenced
the pressure of absorbent and water vapor. The increasing of temperature
increased the pressure of water vapor and absorbent which tend toreducethe water
vapor absorbtion rate. Similar condition also occurred for relative humidity. The
increasing of relative humidity increased the water vapor volume contained in the
air. Higher rate of water vapor absorbtion is a response of higher relative humidity.
An equation to predict the amount of water vapor pressure was developed from
water vapor absorbtion rate which a function of temperature, concentration and
relative humidity.
The separation process of LiBr-H2O solution was conducted to produce pure
refrigerant to reduce the uses of heat energy in traditional absorbtion chiller
system. This process was performed by applying reverse osmosis (RO) membrane
and vacuum membrane distillation (VMD). The experiment which used RO
membrane Dow Filmtech TW30-1812-50 module generated big effect on the
purity ofthe permeate and retentate. This was indicated by rejection value. Table
5.1.shows the data on separation experiment using RO membrane under different
concentration and pressure operation.
Tabel 5.1. Change of concentration during separation process using RO
membrane
Concentration (% w/w)
Initial Retentate
30
31.2
30
31.6
30
32
25
25.4
25
26
25
26.2
20
23
20
22.4
20
22.2
Permeat
9
9.4
10
9
8.4
8.2
7.4
7.6
8.2
According to Table 5.1, it can be seen that the permeate still contained
LiBr compound. The condition of salt compound that could diffuse the membrane
indicated that RO membrane had small rejection value. In other words, it could be
said that increasing pressure would reduce the performance of a membrane to
reject certain compound. The result showed that RO membrane Dow Filmtech
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TW30-1812-50 was not suitable for LiBr-H2O separation. This suggested that
other RO membrane with better selectivity performance for LiBr-H2O separation
should be found.
Separation process using VMD was conducted under the solution’s boiling
temperature. The experiment resulted pure H2O which was indicated by the
permeate value of 1.330oBrix. The highest flux permeate reached up to 930.068
(kg/m2h). 10-3which was obtained at the initial process i.e. concentration 47.5%,
temperature 80°C and flow rate 1.9 L/min. The retentat concentration was
48.404%.
Regarding the permeate generated from RO and VMD method, it can be
concluded that VMD resulted better performance for LiBr-H2O separation and
produced pure refrigerant. According to the retentate value, it also demonstrated
that VMD resulted better performance than RO membrane in producing strong
solution. Taken retentate and permeate value together, it suggested that VMD
could produce better performance for absorbtion process. This condition would
influence cooling temperature and cooling time.
The determination of water vapor absorbtion in absorber was conducted
using equations cited in Chapter 2. The result of separation process shows that
higher concentration would increase the absorbtion rate. However, some
improvements of membrane application still need to be taken place. An
improvement of RO membrane which specifically designed for LiBr-H2O
separation could be an interesting study. The design should consider the amount
of pressure and rejection factor. Meanwhile, the direct application of VMD on
absorbtion refrigeration system should consider the temperature, concentration
and flow rate in order to obtain good cooling effect. Fouling factor should also be
considered as it can reduce the permeate produced during separation process.
Form the economic point of view, RO membrane has lower cost than VMD due to
the elimination of condenser unit. Condenser unit is not used in the system based
RO membrane as the absence of phase changes of weak solution before entering
membrane and strong solution after passing the membrane.
The analysis on exergy efficiency in absorbtion refrigeration system using
RO membrane showed that evaporator unit had the smallest efficiency compared
to membrane and absorber unit. In the application of VMD, the average value of
exergy destroy was small which tended to increase the efficiency value. From the
experiments, the efficiency energy at temperature 80°C was 98% while remain
was 99%. According to the result of separation process, VMD application had
higher performance than RO membrane. In the application of RO membrane, the
presence of salt compound caused low cooling effect as discussed in Chapter 3.
This condition could be improved by adjusting the design of RO membrane and
using similar concentration applied in traditional absorbtion refrigeration system.
Other consideration was the pressure leaving the RO membrane. The decrease of
this pressure will decrease the chilling temperature.