desiccant cooling sys paper

8/9/2019 Desiccant Cooling Sys Paper

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ABSTRACT

Researchers, designers and manufacturers of refrigeration and air-conditioning systems andequipment today are focusing on new and alternative technologies in view of the phasing out of CFCs.

This has brought about a revolution in HVAC industry with the advent of new replacementtechnologies that operate independent of refrigerants.

Desiccant-based and desiccant assisted air-conditioning systems are currently emerging from a 60-

year industrial heritage and expanding into commercial applications. Today, both the user and the

consultant opt to adopt non-conventional approach case by case rather than sticking to conventionalmethod of designing systems.

Effort at all levels is being made to identify new slots / areas where desiccant cooling system provides

immediate applicability i.e. stand alone as well as provides economical and efficient alternative, by way of

lowering TR (Reduce impact of CFCs).

This paper deals with options and opportunities of viable applications of desiccant augmentedcooling systems e.g. Stand Alone HVAC systems, industrial ventilation, Integration of Desiccant

Cooling in commercial HVAC system, Application of a Desiccant Cooling System to super markets,

Integrated Desiccant Cold Air Distribution Systems (IDCAD), Desiccant Dehumidifiers in ice arenas,Clean Room Application, etc.

Energy shortage is something which we live with day in and day out. This energy shortage isexpected to increase as more and more population shifts to air conditioned buildings.

This presents the air conditioning industry with several challenges. Among these are demands forincreased energy efficiency and improved indoor air quality, growing concern for improved comfort and

environmental control, increased ventilation requirements, phase-out of chlorofluorocarbons (CFCs),

and rising peak demand charges.

TOUGH ISSUES FACING THE HVAC&R INDUSTRY

The air-conditioning industry, worldwide, has been battling to meet new economic,environmental, and regulatory challenges: improved ventilation-rate standards, upgradedindoor air quality demands, reduced levels of gaseous emissions, phase-out of CFC

refrigerant, and peak electric demand. To meet these challenges, the industry and thegovernment in the West, specially in the United States are focusing on alternative and newtechnologies like desiccant cooling and dehumidification technology.

New approaches and options to air conditioning such as Desiccant cooling anddehumidification is providing important advantages in solving many of these problems.However, these options have to be evaluated to resolve economic, environmental, andregulatory issues.

DESICCANT COOLING :DESICCANT VENTILATION

COMES OF AGE EMERGING OPPORTUNITIES

AND DESICCANT COOLING SYSTEMS

JAGDEEP SINGHArctic India Engineering Pvt. Ltd.


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Desiccant technology has become an important alternative amongst the options available tothe industry for space-conditioning. In many cooling applications, desiccant cooling unitsprovide advantages over the more common vapour-compression and absorption units. Forexample, desiccant systems do not need ozone-depleting refrigerants and are very effective attreating the large humidity loads resulting from ventilation air. Also, they use natural gas,

solar thermal energy, or waste heat, thus lowering peak electric demand.As a result, the use of desiccant cooling and dehumidification systems for building comfortconditioning has increased steadily during the past several years in the West.

Recent advances in adsorptive materials, in conjunction with dehumidifier designinnovations, are making the technology increasingly attractive.

SAVING ENERGY AND THUS, THE ENVIRONMENT

Desiccant cooling systems are energy efficient and environmentally benign.

In humid regions, desiccant dehumidification can reduce electricity demand considerably byproviding a drier, more comfortable, and cleaner indoor environment with a lower energybill. Desiccant systems allow more fresh air into buildings, thus improving indoor air quality

without using more energy.Desiccant systems also displace chlorofluorocarbon-based cooling equipment, the emissionsfrom which contribute to the depletion of the Earth's ozone layer.

Desiccant dehumidification technology provides a method of drying air before it enters aconditioned space. When combined with conventional vapour compression systems,desiccant dehumidification systems are a cost-effective means of supplying cool, dry, filteredair.

In the last decade, desiccant dehumidification technology has emerged as an alternative or asa supplement to conventional vapour compression systems for cooling and conditioning air incommercial and institutional buildings. A typical hybrid system combines a desiccant systemwith a conventional vapour compression cooling system.

Desiccant-based systems are cost-effective because they use low-grade thermal sources toremove moisture from the air. In general, the benefits of desiccant-based systems are greaterwhere the thermal energy required for regenerating the desiccant is readily available, theelectricity price is high, and the latent load fraction is high (>25%).

If there is no difference in energy costs, the factors that influence the economy includeclimate conditions (humidity levels) and high outdoor-air requirements. In other situations,the important variables that drive the economics should be carefully evaluated. There are,however, a few applications where the technology's benefits have been so extensivelydemonstrated that no detailed analysis is required: storage spaces, ice arenas, mostsupermarket applications, military commissaries, hospital operating rooms, and as an add-on

to existing air conditioning systems with inadequate dehumidification capacity.DESICCANT TECHNOLOGY

There are three basic types of gas cooling technologies: absorption system, gas engine chillersystems and desiccant cooling cycle and the regenerative cycle.

In the dehumidification/cooling cycle the moist (humid) return air from the storage area andsome makeup air from outside is filtered and then passed through the very slowly rotatingfluted desiccant media [rotor/wheel] which adsorbs moisture.


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This air comes out of thewheel/rotor as warm dryair which is then passedthrough the air handlingunit to be cooled and

distributed back to thestorage area. Theregenerative cycle isbasically regenerating themoisture laden section ofthe desiccant wheel sothat as it rotates back intothe supply air stream it isready to extract moisturefrom the process air. Airis taken from outside and

heated by gas to be hotenough to remove themoisture from the wheel,resulting in warm moist

air that is returned back outside.

Using psychrometrics the properties of the air can be followed through the system. In theconventional system it is seen that the air is cooled to the Apparatus Dew Point (ADP)temperature so that the moisture can be removed, then the air is reheated to the desiredtemperature resulting in wasted energy where as the Desiccant based systems removes thelatent load from the HVAC system thus saving energy.

One must understand that both the latent load an important role in designing of an air-

conditioning system. The advantage of the desiccant system is that it removes the latent loadby the use of a lower costenergy alternative, i.e. gas.

As the process air isalready been dry onlysensible cooling isrequired, so the evaporatorcoils can be operated athigher temperatures. Thus,hybrid systems with acombination of the

desiccant wheel and theexisting air conditioningsystem operates on lessenergy input than theconventional system.

DESICCANT BASICS

A desiccant material

Basic Air Desiccation Process withHeat Exchange

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Comparison of Desiccant and Vapor Compression

Dehumidification Processes

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naturally attracts moisture from gases and liquids. The material becomes saturated asmoisture is adsorbed or collects on the surface; but when heated, the desiccant dries outorregeneratesand can be used again. Conventional solid desiccants include silica gel,activated alumina, lithium chloride salt, and molecular sieves. New solid desiccant materialslike zeolites are designed to be more effective for cooling applications. Liquid desiccants

include lithium chloride, lithium bromide, calcium chloride, and triethylene glycol solutions.In a dehumidifier, the desiccant removes moisture from the air, which releases heat and raisesthe air temperature. The air is then cooled by heat-recovery units and cooling devices such asindirect evaporative coolers or the cooling coils of a conventional air conditioner.

In a stand-alone desiccant cooling system, air is first dried, and then cooled by a heatexchanger clubbed with evaporative coolers. This system is free of ozone-depleting CFC andHCFC refrigerants. In most systems, a wheel containing desiccants continuouslydehumidifies outside air entering the cooling unit. The desiccant is then regenerated bythermal energy supplied by natural gas, waste heat, or the sun. A desiccant system can alsosupplementa conventional air-conditioning system; the desiccant removes the humidity loadwhile the evaporator of the air conditioner lowers the air temperature.

DESICCANT COOLING SYSTEM OPERATION

The dehumidifier is the heart of adesiccant cooling system. Itefficiently removes the moisture(latent load) from the process air;the temperature (sensible load) ofthe dried air is then reduced tothe desired comfort conditions bysensible coolers (i.e., rotary heatexchangers, direct and indirectevaporative coolers, coolingcoils). The latent and sensibleloads are handled moreefficiently than in vaporcompression cooling equipmentbecause the components areoptimized to independentlyremove these separate loads. Thedesiccant in the dehumidifier isregenerated (reactivated) whenheat is applied to release themoisture, which is exhaustedoutdoors. The heat forregeneration can be providedfrom a number of energy sources

such as natural gas, waste heat, solar, and off-peak electricity.

The process air to be dried is passed through a fluted desiccant media. Water vapour isadsorbed into the fluted desiccant media driven by the vapour pressure differential betweenthe process air and the desiccant surface. When the desiccant is nearly saturated, hot air is

Ventilation Mode Desiccant Cooling

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passed through the bed to release the moisture. The desiccant media typically takes the formof a rotor (wheel) so that it may be conveniently rotated between the process andregeneration airstreams.

Desiccant cooling systems are energy efficient, cost effective, and environmentally safe.They can be used as stand-alone systems or with conventional air-conditioning to improve

the indoor air quality of all types of conditioned spaces. In such systems, the desiccantremoves moisture from the air, which releases heat and increases the air temperature. The dryair is cooled using either evaporative cooling or the cooling coils of a conventional airconditioner. The adsorbed moisture in the desiccant is then removed (the desiccant isregenerated to its original dry state) using thermal energy supplied by natural gas, electricity,waste heat, or the sun. Commercially available desiccants include silica gel, activatedalumina, natural and synthetic zeolites, titanium silicate, lithium chloride, and syntheticpolymers.

ADOPTING DESICCANT COOLING OPTIONS

Conventional vapour-compression cooling systems are not designed to handle temperatureand humidity loads separately. Asa result, oversized compressorsare often installed to dehumidifythe incoming air. Also, vapor-compression systems are oftenoperated for long cycles and atlow temperatures to meethumidity requirements, whichreduces their efficiency andrequires reheating the dry, coldair to achieve some degree ofcomfort. Both methods areexpensive.

Desiccant systems, however, cansupplement conventional airconditioners. Working inconjunction, the systems canhandle temperature and humidityloads separately and moreefficiently.

HVAC designers and consultantscan thus design systems which

will reduce compressor size andeliminate excess chiller capacity.

Another emerging application of the desiccant cooling systems is the increasing use ofcombined cooling, heating, and power (BCHP) systems for buildings. Facilities with BCHPsystems recover waste heat from generators, turbines, or engines and use it for heating andcooling to maximize overall facility efficiency. Desiccant systems use these large quantitiesof low-temperature heat to provide dry, cool, comfortable fresh air to building occupants.

Total Ventilation Mode Desiccant Cooling

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FROSTLESS FREEZERS AND HEALTHY AIR

Desiccant technologies can be used to great advantage in many applications like supermarkets, ice arenas, etc. Though,desiccant systems are not yet the all-in-all solution for all cooling and

humidity problems but certain areasare particularly well suited fordesiccant applications, based on bothtechnical merits and cost.

Many supermarkets in the US usethem to reduce frost buildup onrefrigerated cases and frozenproducts, extending product shelflife, as well as the intervals betweenexpensive, energy-consuming defrostcycles. Desiccants contribute to a

drier, cleaner, more comfortableenvironment in stores of all kinds.For example, supermarkets mustcontrol moisture levels in freezerdisplay cases, but antisweat heatersconsume considerable energy. Acommercial desiccantdehumidification system has helpedmany supermarkets save on energycosts by controlling humidity

independent of temperature. The desiccant system reduces frost build-up on frozen products

and refrigerated cases. Additionally, the system extends product shelf life and contributes toa drier, more comfortable store environment for shoppers and employees.

The American Hotel and Motel Association has reported that repair of mold and mildewdamage to wallpaper, paint, carpet, and other materials caused by high humidity levels couldcost hotels and motels $68 million annually.

Desiccant systems can lower the humidity levels in these spaces, saving millions of dollars inunnecessary repairs. Desiccant systems also improve indoor air quality, improve ventilationrates, and remove air pollutants and odorsbenefits demonstrated in two hotels in Florida.Similarly restaurants, schools, and hospitals can use desiccants to remove moisture, odors,and pollutants for a healthier, more comfortable and productive environment. New buildingstandards (e.g., ASHRAE 62R) that require more outside air for ventilation are also

increasing demand for dehumidification products.

Highlights

* Independent humidity control.

* Improved presentation of frozen foods and refrigerated products.

* Lower temperature in refrigeration cabinets.

Currently in the United States, desiccant cooling and dehumidification systems are beingused successfully in industrial and many commercial applications with technology

Recirculation Mode Desiccant Cooling

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advancements which has helped to improve the performance, reliability, and cost-effectiveness of desiccant equipment.

Because desiccant systems perform differently from vapour-compression systems,performance-rating procedures, test methods, and standards are being developed to permitHVAC-system designers to compare various desiccant systems with competing technologies.

Separate procedures are being developed by the ASHRAE(American Society of Heating,Refrigerating, and Air-Conditioning Engineers, Inc.), and the ARI(Air-Conditioning andRefrigeration Institute) to revise conventional comfort standards based primarily ontemperature to include humidity and indoor air quality.

GENERATING AWARENESS IN THE COUNTRY

The need of the day is to generate awareness, identify barriers to wider acceptance andfinding solutions through commercialization of the technology in the country.

Collaboration between government and industry is necessary to develop cost-effective,marketable systems that are energy efficient and environmentally safe. To make desiccantsystems more marketable, the industry along with the government must work as partners to

*Adopt advanced desiccant materials, components available in the country todesign systems with lower costs and improved performance

* Demonstrate desiccant systems in real-world environments

* Develop markets for desiccant systems that will improve indoor air quality andreduce environmental pollution.

The information and data generated through this collaboration should be disseminated to theHVAC industry, the desiccant community, architects, engineers, builders, utilities, and otherend users through industry and professional society meetings, publications, conferences, andworkshops.

In conclusion, the main goals of a Desiccant Cooling Program in India should typically focus

on* Reduction of energy consumption

* Reduction of carbon dioxide emissions

* Initiate training programs to educate design, engineering, and architectural firmsand natural gas end users about the desiccant technology and its benefits.

* Convert designers and consultants of conventional air-conditioning systems to optfor Desiccant based Systems

desiccant cooling sys paper

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