EXPERIMENTAL AND COMPARATIVE ANLYSIS ON THE

PERFORMANCE OF MEMBRANE BASED LIQUID DESICCANT AIR

CONDITIONING SYSTEM

M Kum Jaa*

, G. C. Leeb, W. S. Ooi

b, S. Bharath

a, Dubey Swapnil

a, F.H. Choo

a, Anutosh

Chakrabortyb , E.T. Mohan Dass

a, K Zhao

c

aEnergy Research Institute @ NTU, Nanyang Technological University

CleanTech One, Singapore 637141 bSchool of Mechanical and Aerospace Engineering Nanyang Technological University

50 Nanyang Avenue, Singapore 639798 cMemsys Clearwater Pte. Ltd. 82 Toh Guan Road East, #02-11 WaterHub

Singapore 608576.

* Corresponding author, E-mail : mkumja@ntu.edu.sg

Tel : (65) 6592 3588. Fax : (65) 6316 3195

ABSTRACT

For tropical countries such as Singapore, removing latent load from the building and fresh air is

one of primary energy consumption in the building sector. Among of many dehumidification

techniques, which are mainly for removing latent load, membrane based liquid desiccant

dehumidification has recently gained much attention due to its merit of zero desiccant

contamination or carryover, and using waste heat or renewable energy as a driving heat source.

But the mass transfer resistance cross over the membrane between air flow and liquid desiccant

flow deters the dehumidification performance improvement. However, it can be overcome by a

proper selection of optimum operation parameters of regenerator. A series of experiments have

been carried out. The statically analysis of these experiment data are done by using Design

Expert® software to find the optimum operation parameters, heating temperature and desiccant

flow rate of regenerator. A membrane based liquid desiccant air conditioning system (membrane

based LDAC), is composed with a flat sheet membrane-based treble flow dehumidifier unit, a V-

MEMD (vacuum multi-effect-membrane-distillation) system regenerator, and vapour compression

unit. The comparative analysis has been carried out between the membrane based LDAC system

and a conventional vapour compression system and commercial available LDAC system as well.

Keywords

Membrane, dehumidification, liquid desiccant

1. INTRODUCTION

Air-conditioning system plays a vital role in modern society where it has been extensively

employed in many countries due to the rising of global warming. It serves not only for heating and

humidification in colder countries, but also for tropical countries for cooling and dehumidification,

for the purpose of providing thermal comfort and acceptable indoor air quality in a conditioned

space. Its application of usage ranged from domestic to commercial and industrial sectors. With an

improvement in the standard of living over the past decades, there is an increase in population and

economic activities in many parts of the world which lead to an upward trend in the energy

consumption in the air-conditioning system. With a rise in energy consumption, it leads to burning

of more fossil fuels to meet the energy demands, creating more carbon emission, thus contributing

to more global warming.

For tropical countries such as Singapore, due to its hot and humid conditions, and the

increasing in climate mean temperature, it leads to a rise in the energy demand for cooling

(Singapore National Climate Change Secretariat, 2015). The air-conditioning is widely used and

incorporated into almost all buildings, ranging from small capacity unitary air-conditioning system

to large capacity centralised air-conditioning system. The energy consumed by air-conditioning

constitutes up to 50% of the total energy consumption in commercial buildings (Singapore

National Environment Agency, 2010). With an increase in demand of air-conditioning system, the

rising of energy cost and energy consumption over the years, these explain the need and potential

to improve the existing conventional air-conditioning systems through modification in the aim

towards electrical energy savings perspective to minimise the depletion of non-renewable

resources (Singapore National Environment Agency, 2014).

Instead of the inefficient conventional vapour compression air-conditioning system that

purely uses electrical energy for cooling and dehumidification process, and the need to over-cool

and reheat the air, the electrical energy savings can be achieved by adopting a Liquid Desiccant

Air-Conditioning (LDAC) system. The LDAC system has been gaining attentions and regarded as

promising competitors over conventional air-conditioning system as it has the capability to utilize

renewable solar energy or waste heat. It does not rely on cooling of the air to provide

dehumidification, but instead uses concentrated liquid desiccant in removal of moisture in the air.

This results to a diluted liquid desiccant. Through the use of renewable solar energy and waste

heat, it aids in regenerate the liquid desiccant back to the initial concentrated state. The LDAC

process will significantly reduce the amount of electrical energy required for cooling and

dehumidification of air, thus improving the overall energy efficiency of the air conditioning

system, leading to a lower global carbon footprint. In addition, with the usage of liquid desiccant,

it can be served as an efficient energy storage medium in storing the thermal energy for air

dehumidification demands, where it aids in bridging the mismatch between energy supply and

demand. Lastly, it has the capabilities of effectively eliminate microbial growth, viruses and

removing odours without the need of expensive filters.

Both the dehumidifier and regenerator are the most critical components in the LDAC

system. In this paper, the development trends of the dehumidifier are being focused & discussed.

There are two conventional LDAC systems, high-flow LDAC and low-flow internally cooled

LDAC. Kathabar is an example of commercially available high-flow LDAC system widely used in

industrial sector due to its merit in simple design, reliability, durability, low operating costs, and

provide energy efficient air temperature and humidity control. For high-flow LDAC system, the

desiccant flow is first cooled before spraying onto the bed of the dehumidifier, which is an

adiabatic porous packed bed of structured contact media. This packed bed is highly flooded with

liquid desiccant that ensures the complete wetting of the media and to prevent heating of the

desiccant (Lowenstein, 2008). Heat and mass transfer were involved between the process air and

the liquid desiccant in counter flow pattern. Even though externally cooled dehumidifier is cheap

and has simple construction, the relatively high desiccant flow rate constitutes to serious concern

of desiccant carryover problem where liquid desiccant droplets can entrain in the supply air, thus

potentially causes health concerns to humans. On top of that, due to the liquid desiccant highly

corrosive nature, it will cause corrosion to any metallic components. This leads to the mandatory

use of high maintenance droplet filters to protect the air distribution system from corrosive

desiccant droplet carryover that will result to high maintenance cost incurred in the long run.

Furthermore, due to the high flooding rate that constitutes to high pumping power for the

desiccant, all these explained the reason why high-flow LDAC system is not often used in

commercial buildings. Therefore, this highlighted the use of low-flow internally cooled LDAC

over the high-flow LDAC system.

Low-flow LDAC system has more complex dehumidifier design where three distant fluid medium

are involved instead of the two fluid medium (desiccant and air) for the high-flow LDAC system.

The additional third coolant stream (either liquid or refrigerant) is integrated internally within the

dehumidifier, which provides continuous internal cooling of the liquid desiccant to control the

desiccant temperature independently of the amount of water absorbed and to prevent large changes

in its temperature. Apart from the drawbacks, low-flow LDAC system has several advantages

(Kozubal et al., 2014) that are widely regarded as a better alternative solution over the high-flow

LDAC system. The key advantage is the use of low flow rate of liquid desiccant which effectively

reduced the desiccant droplet carryover problem without the use of any droplet filters, thereby

minimising the corrosion to any downstream metallic components and maintenance requirements.

Other advantages such as reducing the air-side pressure drop through the dehumidifier, the ability

of using less energy per unit of water removed and the capability for the system to deliver drier air

than their predecessors due to the integration of cooling into the dehumidifier.

Many researches had been conducted in designing an effective internally cooled

dehumidifier such as using embedded cooling coils. However, many undesirable drawbacks were

encountered such as the incapability of separating between liquid desiccant and process air stream

to achieve zero desiccant crossover, complex installation and construction, and increased

equipment requirement. Thus, this highlighted the use of membranes that are permeable to water

vapour yet impermeable to liquid desiccant, being adapted to the liquid desiccant air

dehumidification system to separate the process air from liquid desiccant in the objective in

achieving zero liquid desiccant carryover.

2. OPTIMUM OPERATION PARAMETER OF V-MEMD SYSTEM REGENERATOR

V-MEMD (vacuum multi-effect-membrane-distillation) system, a patented and product of

memsys® for desalination and water treatment, is a novel product and combined with the

advantages of multi-effect evaporation and the merit of the membrane’s mass transfer process

(Choo. et al., 2014) The proposed set of experiments has considered a wide range of test data to

characterize the output of regenerator system, develop correlations, and optimize the overall

design. The analysis used specialized design of experiments (DOE) software, like, Design Expert®

to optimize the set of experiments. Design expert is more efficient, insightful and less error than

conventional ways. It takes advantage of the advanced theory and facilitates to make experimental

data analysis more accessible with less statistical background.

Figure 1 : Optimum performance ratio and its operation parameters, heating temperature, LiCL

flow rate, and (a) LiCl inlet concentration 31.71% , (b) LiCl inlet concentration 37.74% .

(a)

(b)

This will also help in optimization analysis and to work out the optimal or high efficient

working area. The regenerator experiments are designed with Box-Behnken statistic method

according to response surface methodology (RSM). Design Expert was used to characterize the

output of regenerator system and optimize overall design using set of experiments test data. The

optimized working zone for performance ratio for different feed flow rate and hot water

temperature is shown in Figure 1. The optimized zone for LiCl inlet concentration 31.71% is

representing performance ratio of 0.5 if operated at feed flow rate range of 32 L/min to 60 L/min

and hot water temperature of 54°C to 67°C, see Figure 1 (a). Figure 1 (b) shows that the optimized

zone for LiCl inlet concentration 37.74% is representing performance ratio of 0.5 if operated at

feed flow rate range of 40 L/min to 50 L/min and hot water temperature of 59°C to 67°C.

3. MEMBRANE BASED LDAC SYSTEM

The membrane-based LDAC ( Liquid Desiccant Air Conditioning ) system integrated

with VC (Vapor Compression Cycle) provide cooling and dehumidification within the test bed

area which has a 50m2 floor area office space to be conditioned to achieve thermal comfort and

acceptable air quality. The test bed, schematic diagram and photos are shown in Figure 2, is

composed of 5 systems: namely (1) a flat sheet membrane-based treble flow dehumidifier unit, (2)

V-MEMD (Vacuum Multi-Effect-Membrane-Distillation) system Regenerator, (3) Vapour

Compression Cycle Refrigeration system, (4) Solar Thermal System, (5) Salt Solution Energy

Storage System. In this membrane based LDAC system, see in Figure 2, there are three dampers to

control the OAF (Outside Air Fraction) from 0 % to 100% in the mixing process of 5.OA (Outside

Air) and 4.RA (Return Air). After mixing process the 1.MA ( Mixing Air) is ready to process by

treble flow membrane based flat type liquid dehumidifier. In the dehumidifier, the moisture from

the process is absorbed by the liquid desiccant through the membrane. The moisture (latent load) is

dehumidified until to reach the point 2.DA (Dehumidified Air). The humidity ratio of DA must

be the same with the 3.SA supply air to absorb the room latent load. In this air conditioning

process, the RSHR (Room Sensible Heat Ratio) is assumed at 0.75. All the points of each stage are

shown in psychometric chart (See Figure 3). After dehumidification, the air (2.DA) is cooled down

by fin tube cooling coil unit until to reach the design supply air (3.SA) temperature.

Figure 3 shows the design points of each stages of air processing in membrane based

LDAC system for RSHR 0.75 room loads. The setting point of room temperature and relative

humidity are 24°C and 50% RH (According to Singapore Standard, SS 553:2009, Section 7.1.6.).

The outdoor air is assigned as 32oC dry-bulb and 80% RH (According to National Environment

Agency (NEA) website). In mixing process, fresh air is mixed with return air by controlling the

damper to achieve the required OAF. The mixed air, TMA and RHMA, can be calculated by the

following equations;

(1)

(2)

The energy cooling energy required to remove heat from the process air can be calculated the

following equation;

(3)

The room load designed with RSHR 0.75 and outside air load can be calculated by the following

equations;

= + )

= + )

( )

Figure 2: Schematic diagram and photos of membrane based LDAC system test bed.

5. OA

Fan

4. RA

Room

Dilute Tank

Concen.

Tank

PSLDeh

PCompressor

Fan Coil Unit

PCWDeh

Cold

TANK

PHWCollector

Hot

TAN

K P

HWLoad

Solar Collector

P

HWReg

Distillate Water Regenerator

(V-MEMD)

PSLReg

Dehumidifier

1. MA 2. DA 3. SA

(4)

(5)

Electrical COP of membrane based LDAC system can be defined as the ratio of total

cooling load divided by the all electrical energy input. The following equation is electrical COP

equations.

(6)

The total cooling load is sum of Room Load, QRL, and Outside Air Load, QOA, which can

be calculated by Eq. (4) and (5). For conventional vapour compression cycle, the cooling coil

process load, QCoilLoad, is calculated by the following equation.

(7)

For comparative analysis, electrical power consumption and performance of conventional

Vapour compression cycle, COPCoven, are calculated from the value of the cooling coil process

load, QCoilLoad, and kW/RT which is measured from the typical chiller plant running in NTU

campus. (0.78 kW/RT at 6°C evaporator temperature).

(8)

Figure 3: The design points of each air processing stages of Membrane based LDAC system for

0.75 RSHR.

5.OA

3.SA

4.RA

2.DA

60%OAF, 1.MA

( )

( )

( )

5. EXPERIMENTAL RESULT AND ANALYSIS

The whole day-run experiments have been carried out in numerous days with different

OAF and optimum operating parameter of LiCl flow rate, LiCl inlet concentration, and heating

temperature obtained from the experiments and statistical analysis discussed in section 2. From

these experiment data, the most stable experiment data of 3 Feb 2015 and 5 Feb 2015 was selected

to conduct the energy analysis.

Figure 4: One day long record data of each air processing stage (OA, RA, MA, DA, SA) and other

operating parameters, running at 70% OAF.

Figure 4 shows the experiment results and the design points of each stage for membrane

based LDAC system, which is running at 70% OAF and a day long record data. The experiment

was carried out with the optimum operating parameters. The lower cooling temperature the better

absorption performance of dehumidifier, but the experience from previous running experiments

shows that if cooling water temperature is lower than 14 or 15°C, it can be occurred energy loss by

condensation (water droplet can be found outside of the dehumidifier). Hence, to avoid the energy

loss, the cooling water temperature was set at 16°C. From the preliminary experiments, 31 % LiCl

inlet concentration running with cooling water temperature 16°C can absorb the moisture until the

desired dryness, 0.0071 Kgwv/kgair of Humidity Ratio. The required design dryness to meet

comfort condition is calculated from the psychometric chart, shown in Figure 3. Data sampling

rate is one sample per 10 second to record each stage of process air, and the experiment was

running from 8:52 am to 7:10 pm. Figure 5 shows the recorded experiment data plotting on the

psychometric chart. The data of conventional LDAC system from literature, product of Kathaba,

[8] are also plotted on the psychometric chart for comparative analysis. Energy calculation for

membrane based LDAC system is based on the hourly average process air data. For electrical COP

calculation, the cooling loads (room load QRL and outside air load QOAL) are calculated out by

using the Eq. (4) & (5) and the average data of OA, RA, SA. Figure 6 shows Electrical COP

comparison and electrical energy saving between conventional VC system and membrane based

LDAC system. The experimental data are also compared with the simulation model result.

0

5

10

15

20

25

30

-100%

-80%

-60%

-40%

-20%

0%

20%

40%

60%

80%

100%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 110%

CO

P E

lect

rica

l Po

wer

Ele

ctri

cal P

ow

er

Savi

ng

[ %

]

Out Side Air Fraction [ OAF ]

Electrical PowerSaving Simulation Model Electrical PowerSaving Experiment OAF 25%

Electrical PowerSaving Experiment OAF 70% COP Conventional VC, Simulation Model

COP Membrane LDAC, Simulation Model COP Membrane LDAC, Experiment OAF 70%

COP Membrane LDAC, Experiment OAF 25%

Figure 5 : The recorded experiment data running with 70% OAF , and Kathabar from literature

plotted on the psychometric chart.

Figure 6 : The comparison of electrical energy saving and COP between the model and

experimental result.

The simulation model equations for membrane based dehumidifier and regenerator, which

are empirical equations, are developed from the experiment data using statistical analysis software,

Design Expert®. The simulation results are found in good agreement with the experiment. The

result shows that average 46.7 % of electrical energy saving can be achieved when the return air is

mixing with the outside air in OAF 70%.

5. CONCLUSIONS AND DISCUSSION

Numerous experiments have been conducted with membrane-based flat type treble flow

dehumidifier unit and V-MEMD system regenerator to determine the optimum operation

parameter and to develop the empirical equations for simulation model by using Design Expert®

statistical analysis software. A membrane based LDAC system test bed was built, and run to cool

the 50m2 room spaces. The comparative analysis has been carried out between the membrane

based LDAC system and a conventional vapour compression system and commercial available

LDAC system as well. The experiment and simulation model result shows that the more energy

saving can be achieved by the higher OAF running with optimum operation parameters.

ACKNOWLEDGMENT

The authors would like to thank A* Star, MND, and Building Construction Authority (BCA),

Singapore for funding this research work under the grant (Project No. SERC 112 176 0024).

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