192-198

International Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (October 5-7, 2012)

Punjab Technical University, Jalandhar-Kapurthala Highway, Kapurthala, Punjab-144601 (INDIA) 192

STUDY OF DESICCANT COOLING TECHNOLOGY: AN OVERVIEW

A.K. Asatia, Rakesh Kumar

b,*

aPrincipal, Rayat Bahra college of Engineering and Nano-technology for women, Hoshiarpur, Punjab, India.

E-mail ID: [email protected] bResearch scholar, Punjab Technical University Jalandhar, Punjab, India. E-mail ID:[email protected]

ABSTRACT

Energy conservation has become an area of wide attention due to global energy crisis. Energy consumption of air

conditioning equipments can be reduced with the application of desiccant based cooling technology. This

Technology also improves indoor air quality and brings environmentally friendly products. In hot and humid

regions, air humidity ratio is high and air-conditioning energy consumption is more. Because the huge energy is

used for dehumidification of the air, so this paper analyzes the utilization of different types of desiccant based

cooling technologies. The technology has the potential to significantly affect the air conditioning market and its

energy use. The working of three types of desiccant dehumidification systems; solid desiccant, liquid desiccant and

hybrid desiccant system, is studied in the current paper. After reviewing principle of operation, a comparative study

is presented as conclusions for different types of desiccant based systems.

Keywords: Energy, desiccant, humidity ratio, air conditioning, dehumidification systems.

1. Introduction

Desiccant cooling technology provides a tool for

controlling humidity levels for conditioned air

spaces. Desiccant systems work in conjunction with

conventional air conditioning systems to dehumidify

the air. Desiccant materials are those that attract

moisture due to differences in vapor pressure. Most

people are familiar with desiccants such as silica gel

packages that are included with new electronics or

textile products. Desiccants can be in the form of a

solid or a liquid. People have identified types of desiccants that are appropriate as a component of

commercial heating, ventilation and air conditioning

(HVAC) systems. These desiccants have been

selected based on their ability to hold large quantities

of water, their ability to be reactivated, and cost. In

order to be effective, the desiccant must be capable

of addressing the latent cooling load in a continuous

process. In order to accomplish this, commercial

desiccant systems consist of a process air path and a

reactivation air path. The desiccant that is in the

process air path has been prepared to have a lower vapor pressure than the air passing over it. Thus, the

moisture in the air is transferred onto the desiccant

material.

Humidity control is important in many

engineering applications, such as space air

conditioning (AC), storage warehouses, process

industries and many others. In space AC, where

conventional vapor compression cooling is used,

humidity control is often accomplished by cooling

the air below its dew point. In hot and humid

climates, this method is inefficient, since it should be

followed by reheating the air to a safe temperature

before it is introduced into the conditioned space.

The evaporator in the vapor compression system

operates at a lower temperature than is required to

meet the sensible cooling load, leading to a lower

coefficient of performance and, consequently, higher

energy requirements. The effectiveness of

evaporative cooling relies heavily on the existence of

low relative humidity conditions, being higher as the

relative humidity decreases. The humidity puts an

extra load on the vapor compression AC systems and

renders the evaporative cooling systems ineffective.

Under high humidity conditions, energy efficient

vapor compression systems operate at higher

evaporator temperatures and found unable to

maintain the indoor relative humidity within a

comfortable range. This calls for dehumidifying the air prior to entering the evaporator.

Separating the control of humidity and

temperature by means of desiccants could result in

energy savings, as well as improved humidity control

as explained by Oberg et al. (1998). Desiccants are

chemicals with great affinity to moisture. They may

be used effectively as a supplement to conventional

vapor compression systems to remove the latent part

of the cooling load. Desiccants are efficient in

handling latent loads (reducing the humidity),

whereas the evaporator of the vapor compression

system is efficient in handling the sensible cooling

loads (lowering the air temperature).

As the desiccant vapor pressure increases due to

the presence of the moisture that it has attracted, the

desiccant material is transferred to a reactivation

process. In the reactivation process, hot air is passed

over the desiccant. The vapor pressure of the hot air



is lower than the desiccant surface which forces the

moisture to transfer from the desiccant surface into

the hot air stream. The moist hot air is then exhausted

from the system into the outdoor air. The desiccant

material that has had the trapped moisture removed is

now prepared to attract moisture as it is transferred

back into the process air path. The dry process air

leaving the desiccant is then passed over a

conventional cooling coil which addresses the

sensible cooling work required to meet the air

specification of the conditioned space.

Advances in the last decade have resulted in

successful application of the technology in niche

markets. However, penetration into the air

conditioning mass market requires further

improvement in efficiency and reliability, reduction

in size and cost, and improvement in technology

acceptance by the building community. Investments in further research and development in materials,

components, and systems are needed and justified

considering the potential of the desiccant cooling

technology.

Desiccant technologies should be considered when

• Moisture levels are high

- Latent/total cooling load ratio is >= 30%

- High levels of outdoor air make-up required in

building

- High building occupancy

• Potential costs savings are significant - High electrical demand charges

- Low natural gas rates

- Low cost central steam available

- Heat recovery options available

• Tight control over moisture levels is required

- Hospital operating rooms

- Avionics repair laboratories

- Museums

- Munitions storage

• Moisture is problematic to interior spaces such as:

- Ice Arenas (fogging) - Hospitals (bacteria)

- Hotels/Apartments (moisture damage)

- Food Stores (freezer case moisture)

• Occupant comfort cannot be compromised

2. Benefits of Dehumidification with Desiccants

Desiccant cooling has many benefits including lower

energy consumption, using renewable energy or

waste heat, lower use of CFCs, and improved indoor

air quality. Davanagere et al. (1999) listed the

advantages of using desiccant based systems.

• Increased Comfort: Independent control of

humidity and temperature is possible in desiccant

cooling systems. Desiccant unit controls humidity

and conventional cooling system controls temperature

• Lower Operating Costs: This type of systems

utilize lower cost natural gas for regeneration.

Conventional cooling system operates at a higher

efficiency due to higher suction temperatures.

• Lower Peak Electric Demand: Latent cooling

uses alternate energy sources i.e., natural gas,

steam, heat recovery.

• CFC Free: Desiccant systems do not use CFC's

for moisture removal.

• Improved Indoor Air Quality: indoor quality of the air is improved by using appropriate levels of

fresh air and reduced levels of air borne bacteria.

such systems also uses chemicals with liquid

desiccants for air treatment.

3. Conventional Cooling System A conventional cooling system lowers the

temperature of the air stream as the air passes over a

cooling coil. Energy is removed from the air in the

form of sensible cooling and latent cooling. Sensible

cooling is simply the reduction of the dry bulb

temperature of the air. Latent cooling is the removal

of moisture from the air or dehumidification. Latent

cooling takes place when the air is cooled below the

air dew point. Cooling below the dew point causes

the moisture in the air to condense and leave the air

stream. The air that leaves the cooling coil under these conditions is near saturation. The air is then

mixed or reheated to the desired supply air

temperature. This process is illustrated on the

psychometric chart below.

State 1 to State 2: Air is cooled to the point of

saturation

State 2 to State 3: Further cooling cause’s moisture

to be condensed from the air as the temperature of

the air continues to drop.

Fig 1 Dehumidification and heating Process

State 3 to State 4: The air is then mixed or passes

through reheat to supply air at the desired

temperature.

4. Desiccant Dehumidification Types

Air dehumidification can be achieved by two

methods: (I) cooling the air below its dew point and

removing moisture by condensation, or (2) sorption

by a desiccant material. Desiccants in either solid or



liquid forms have a natural affinity for removing

moisture. As the desiccant removes the moisture

from the air, desiccant releases heat and warms the

air, i.e., latent heat becomes sensible heat. The dried

warm air can then be cooled to desired comfort

conditions by sensible coolers (e.g., evaporator coils,

heat exchangers, or evaporative coolers.). To re-use

the desiccant, it must be regenerated or reactivated

through a process in which moisture is driven off by

heat from an energy source such as electricity, waste

heat, natural gas, or solar energy.

The design and operation of a desiccant system is

based on the desiccant material used to accomplish

the dehumidification. Desiccant materials attract

moisture through the process of either adsorption or

absorption. Adsorption is the process of trapping

moisture within the desiccant material similar to the

way a sponge holds water through capillaries. Most adsorbents are solid materials. Absorption is the

process of trapping moisture through a chemical

process in which the desiccant undergoes a chemical

change. Different materials used in desiccant based

cooling systems are silica gel, lithium chloride,

lithium bromide, activated alumina, titanium silicate,

tri ethylene glycol and calcium chloride etc. More

details about desiccant types, properties and the

regeneration process are given by Kinsara et al.

(1996).

Different types of desiccant cooling systems are Solid desiccant system, liquid desiccant system

hybrid desiccant system and Solar based desiccant

systems. Brief review of each type of desiccant

system is described in the following part of paper.

4.1 Solid Desiccant System For industrial applications, solid desiccant cycles

use dual-column packed-bed dehumidifiers;

however, the most appropriate dehumidifier

configuration for air-conditioning applications is the

rotary wheel (Figure 2). The air to be dehumidified

enters the system, comes into contact with the desiccant wheel, and exits the dehumidifier hot and

dry. The wheel is then rotated so that the desiccant

portion that has picked up moisture is exposed to hot

reactivation air and its moisture removed.

Fig 2. Solid Desiccant System

The air leaving the desiccant is heated because of the

release of heat adsorption; there is a need for cooling

the dried air in cooling applications. This can be

accomplished with a sensible heat exchanger such as

a heat pipe or with a standard vapor-compression

cooling coil.

4.2 Liquid Desiccant System Figure 3 describes the liquid-desiccant

dehumidification system. In a liquid system, there are

two separate chambers; one to perform the

dehumidification and the other to regenerate the

desiccant. The processed air from the

dehumidification chamber enters into the conditioned

space.

Fig 3. Schematic of a Liquid Desiccant System

The desiccant, leaving the dehumidification chamber

containing absorbed moisture, goes through a heat

exchanger and down to the regenerator, where heat is

added to remove the moisture. The liquid desiccant is

pumped continually between the two chambers when dehumidification is needed.

Fig. 4 shows the experimental set-up of a liquid

desiccant system. It is used for both air

dehumidification and desiccant regeneration. In the

air dehumidification mode, the air conditioned to the

required temperature and humidity is introduced into

the absorption tower from the bottom.

Fig 4. Liquid desiccant system used for both air

dehumidification and desiccant regeneration



Desiccant at the required temperature and flow rate is

pumped into the top of the dehumidifier/regenerator

via the rotameter. The desiccant flowing

countercurrent relative to the humid air flow is

distributed over the packing and absorbs moisture as

it comes into contact with the humid air. The diluted

desiccant flows by gravity to the storage tank.

Uniform temperature and concentration of the

desiccant are ensured using an electric heater with

temperature controller as shown in Fig. 3. The flow

rate is monitored using a calibrated rotameter. The

dry bulb and wet bulb air temperatures are monitored

using mercury thermometers located at the inlet

(bottom) and exit (top) of the

dehumidifier/regenerator. Also, two more

thermocouples are used to measure the inlet (tower

top) and exit (tower bottom) desiccant temperatures.

The air flow rate is monitored using a vane type anemometer.

The desiccant regeneration process used in this

work consists of pumping the liquid solution,

collected during the experiments, from the catch tank

to the heater tank where it is heated to about 55oC.

The heated solution is then pumped to the

dehumidifier/regenerator where the desiccant comes

into contact with dry and hot air supplied from the

AC unit. In this mode, very low flow rates for the

desiccant were used while the air flow rate was at its

maximum. The entire process described above may be repeated until the required desiccant initial

concentration is restored.

In order to investigate the performance of packed-

type dehumidifiers, several design parameters that

are expected to affect the performance will be

studied. To predict the interaction of these

parameters, only one parameter at a time will be

allowed to vary over its range, while the other

variables were held constant at their intermediate

levels. The parameters that will be considered in this

work included air flow rate, desiccant flow rate,

desiccant inlet concentration, air inlet temperature, desiccant inlet temperature, air humidity and

desiccant humidity.

The performance of the dehumidifier is evaluated by

calculating the moisture removal rate and the column

effectiveness. The rate of moisture removal from the

air is calculated from Eq. (1)

( ) AGYYm outincond ∗∗−=.

(1)

where G and A are the air flow rate and the column

cross-sectional area respectively.

The dehumidification effectiveness, εy is an air-

side characteristic parameter which is related to the

mass transfer effectiveness during the

dehumidification operation.

The effectiveness is defined as the ratio of the

actual change in moisture content of the air leaving

the absorber to the maximum possible change in

moisture content under a given set of operating

conditions. The dehumidifier effectiveness, εy, is

expressed mathematically as:

equin

outiny

YY

YY

−

−=ε (2)

where Yin and Yout are the absolute humidity of the

air at the inlet and outlet conditions respectively. Yequ

is the absolute humidity of the air, which is at

equilibrium with the desiccant solution at the inlet

concentration and temperature.

4.3 Hybrid Desiccant System In a hybrid system, the desiccant dehumidifier

handles the latent cooling load and VCS/VAS

handles the sensible cooling load. Fig. 5 shows a

hybrid system where the solution chilled by a VCS is

passed through the absorber to simultaneously cool

and dehumidify the air. In contrast, a hybrid solid desiccant system has to first dehumidify the air in a

desiccant wheel and then cool it in a cooling coil. In

both the cases the required chemical

dehumidification would be less than the stand alone

DCS, as no further moisture needs to be added (in an

evaporative cooler) to achieve cooling. Further, as

the cooling coil is maintained at higher apparatus

dew point (ADP) than a stand alone VCS, the overall

COP is also high. Waste heat from condenser may

also be used for regeneration (part or full) of

desiccant. So considerable energy savings can be achieved by using these systems instead of

conventional systems using refrigerated coils alone,

especially for high latent load and low humidity

applications.

Fig 5. Hybrid Desiccant Cooling System



Many studies have been carried out using renewable

energy sources especially solar energy [8, 10] that

tend to increase the energy savings while also

increasing the complexity and cost of the unit. The

performance of the desiccant systems and thus the

actual savings, depends primarily on the performance

of the dehumidifier and the thermodynamic cycle

representing the system configuration, as most of the

other components have evolved to a large extent.

Oberg and Goswami [11] presented a review of

liquid desiccant dehumidification equipment and the

various thermodynamic cycles. Thermodynamic

analysis of several liquid desiccant cycles has been

reported by Jain et al. [12].

4.4 Solar based Desiccant Cooling System According to this principle, author established a

compound solar liquid desiccant air-conditioning system (CSLDAS), and through the numerical

simulation, analyzed the performance and operation

energy consumption. Results indicate that CSLDAS

is more economical and energy-saving.

The composition of CSLDAS is introduced in

Fig.6. In the dehumidification process, use of natural

cold source to cooling and dehumidify fresh air,

reduce dehumidifier air inlet humidity ratio and

temperature, pretreated fresh air direct contact with

desiccant within dehumidifier, the air moisture will

be further absorbed by desiccant, the air is dried, and achieved to the supply air status, later, feed into the

air-conditioning room, to removal indoor latent

cooling load. In this process, air humidity ratio and

desiccant concentration will be down respectively.

Fig 6. Solar based Desiccant cooling system

In the regeneration process, use of indoor return air

as renewable air, and heated by environment heat

(return air heating device), to diminish the relative

humidity of renewable air. In the regenerator,

renewable desiccant would be heated, and renewable

air in direct contact with heated desiccant, mass and

heat transfer will be processed between them, the

heated desiccant moisture was absorbed by

renewable air, the desiccant concentration increasing,

concentrated desiccant would be distributed into

dehumidifier again, spray the fresh air.

5. Application Issues

Desiccant dehumidification is an established

technology that has been used successfully for many

years in institutional and industrial applications.

Commercial applications are now gaining

acceptance. Desiccant systems have been applied

successfully in supermarkets and ice rinks. Hotels

and motels, office buildings, and restaurants provide the next opportunity. Lowering the cost of desiccant

dehumidification systems and improving their

performance will clearly provide more opportunities

for desiccant dehumidification technology.

Currently, a number of cost-effective applications in

the market will result in increased sales during the

next several years; but as in other technologies,

further R&D and demonstration programs will

enhance broader applications of the technology. Low

temperature desiccants can effectively use waste heat

from electric air conditioners and improve their efficiency and effectiveness-an area that utilities and

EPRI need to participate for further development.

Desiccant dehumidification systems as add-on

modules to electrical refrigeration systems could help

solve the challenges facing the HVAC industry in the

1990s: increased ventilation rates, need for improved

indoor air quality and better humidity controls,

phase-out of CFCs, national standards requiring

higher efficiency for cooling systems, and desire for

lowered peak electric demands. These factors, and

the ability for desiccant systems to solve specific

problems, are driving these desiccant technologies to the mainstream of the air-conditioning market.

Table1. Typical Applications and Benefits for Desiccant

Dehumidification

Application Benefits of Desiccant

Dehumidification

Supermarket

• Energy savings through reduced

refrigeration display compressor

loads

• Fewer defrost cycles in

refrigerated display systems

• Eliminates condensation on

display cases

Ice Rinks


latent loads

• Less ice resurfacing

• Eliminates fogging

• Reduced building maintenance

Refrigerated

Warehouse


latent loads

• Eliminates temperature

fluctuations



Reduces workplace hazards

Hospital

Operating

Room

• Eliminates perspiration of

surgeons

• Eliminates fungal amplification in

ductwork

• Eliminates condensation in

operating room

Movie

Theater

• Increased customer comfort

• Allows increased ventilation in response to ASHRAE Standard 62

• Increases useful life of seats and

carpets that are damaged by the

presence of high moisture levels

School

• Reduced health risks associated

with air-borne infectious agents

• Decreased levels of indoor CO2

• Lower energy costs

Fast Food

Restaurant

• Allows increased ventilation in

response to ASHRAE Standard 62


• Lower energy costs

Hotel


• Allows increased ventilation in

response to ASHRAE Standard 62

• Increases useful life of wallpaper,

tapestries and carpets that are

damaged by the presence of high

moisture levels

6. Concluding Remarks

Desiccant dehumidification systems are widely

used in many industries including HVAC, for air-

conditioning needs. These are commonly operated in

a hybrid mode with vapor compression systems, to

offer better indoor air quality (IAQ) and higher

energy efficiencies especially for low sensible heat

ratio and high humidity applications. The comparison

of experimental performance of dehumidifiers

reveals that proper selection of type of column with

its operating parameters including solution to air flow rates, inlet concentration of desiccant and

packing size results in high dehumidification

effectiveness of about 0.9 or more. Internal cooling

could help effectiveness values to exceed 1. A

critical analysis of different empirical

dehumidification effectiveness correlations for

packed bed columns using three desiccant solutions

viz. triethylene glycol, lithium chloride and calcium

chloride has been reported in this paper. The analysis

shows wide variations in effectiveness values

ranging from 10% to 50% or more, with higher deviations occurring for lower ratios of liquid to gas

flow rates. It emerges from the study that there is a

need to develop more comprehensive empirical

models for performance prediction of desiccant

dehumidifiers. Further, standardized testing

procedures need to be evolved to enable easier

comparison of different designs of dehumidifiers.

Overall, the use of liquid desiccant may be

advantageous compared to solid desiccants. Some

advantages of liquid desiccant include a small air

pressure drop and heat fact that a liquid can be

transported directly to the source of regeneration

heat. The ability to pump the liquid desiccant makes

it possible to connect several small desiccant

dehumidifiers to a larger regeneration unit which

would be especially beneficial in large buildings.

Using a liquid desiccant also enables more efficient

heat transfer since highly efficient liquid-liquid heat

exchanger may be employed. Finally, since a liquid

desiccant system does not require simultaneous air

dehumidification and desiccant regeneration, it is

possible to store the dilute liquid until regeneration

heat is available.

Acknowledgement Authors are thankful to the IET Bhaddal

Technical Campus Ropar and Punjab Technical

University for their valuable support to carryout the

research.

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