THERMAL INVESTIGATION ON OPEN CYCLE DESICCANT COOLING AIR CONDITIONING

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International Journal of Research and Innovation (IJRI)

THERMAL INVESTIGATION ON OPEN CYCLE DESICCANT COOLING AIR CONDITIONING

D.V.N.PRASAD1,S.N.CH.DATTU.V2, V.V.Kamesh3

1 Research Scholar,Department Of Thermal Engineering, Aditya Engineering College, Surampalem, Andra Pradesh, India.2 Assistant Professor,Department Of Mechanical Engineering, Aditya Engineering College, Surampalem, Andra Pradesh, India.3 Associate Professor , Department Of Mechanical Engineering, Aditya Engineering College, Surampalem, Andra Pradesh, India.

*Corresponding Author:

D.V.N.PRASADResearch Scholar,Department Of Thermal Engineering, Aditya Engineering College, Surampalem, Andra Pradesh, India.

Published: January 02, 2015Review Type: peer reviewedVolume: II, Issue : II

Citation: D.V.N.PRASAD, THERMAL INVESTIGATION ON OPEN CYCLE DESICCANT COOLING AIR CONDITIONING

INTRODUCTION

Around the world, Increase of the energy consumption and desire to prevent further increased global warming has set a major focus on developing energy efficient and environmentally friendly system solutions. In the summer season especially, air conditioning systems represents a growing market in commercial and residential build-ings. Two of the main reasons behind this are that the demands for acceptable living standards are increasing as well as the comfort demands of the occupants. The air conditioning unit covers both temperature and humidity control, which leads to traditional vapor compression re-frigerating systems requires hue amount of power as well as exhausting a lot of usable waste heat.

Traditional vapor compression air-conditioning systems usually decreases the temperature of the air to below dew point temperature to be capable to deal with both latent &

sensible heat loads. This results in a problem concerning large energy consumption when the system is satisfying temperature and humidity requirement of conventional room. And also it uses CFC and HCFC which are harmful for our environment. Utilization of innovative and clean energy sources has lead technology research in new direc-tions. One attractive alternative to traditional vapor com-pression air-conditioning are a desiccant cooling systems where solid desiccant wheels are used to dehumidify the air. Usually evaporative cooling ensures that the air tem-perature is decreased to acceptable indoor standards.

The energy required cooling the air from the inlet temper-ature to the required condensation temperature and the reheating energy represents an energy surplus to achieve the required conditions. To avoid this energy surplus, the processes involved should be changed, handling the la-tent and sensible load separately. The Sensible Heat Ratio (SHR) is the ratio between the sensible load and the total load inside the conditioned space, and it is considered an important index to assess the convenience of this decou-pling.

Desiccant Cooling (DEC) systems are capable of decou-pling the latent and sensible loads, promising to introduce energy savings that are assessed to increase for decreas-ing SHR, since the sum of the surplus energy required by cooling-based dehumidification systems increases dramatically, [3]. Therefore, DEC systems are considered an interesting solution to reduce the energy consumption and emissions in respect to conventional HVAC systems. Moreover, DEC systems can be used to reduce the re-quired ventilation rate when its minimum value is set to

Abstract

In hot and humid countries like India, Air-conditioning systems of solid desiccant dehumidification based on direct evaporative cooling can be an effective alternative to the existing vapor compression refrigeration air conditioning due to its various advantages in, decreasing latent load of air, environmentally friendly, no pollutants in the process air, decreeing power utilization and finally the equipment cost is much lower. This project first deeply explains about recent researches and developments in solid desiccant dehumidification combined with direct evaporative cooling technologies. A basic description of the principle operation for solid desiccants and different types of desiccant materials is given first. Next, solid desiccant dehumidification system design and working process is included.

Desiccant dehumidifying wheel is the heart of the Desiccant wheel dehumidifying air conditioning. Rotor speed should not be very slow so that desiccant material sector does not get filled up before it moves to the regeneration side. Rotor speed should not be very high so that any sector does not move to the other side before it gets completely with moisture. For best performance, the wheel must be rotated at an optimum speed. A software developed by a USA based manufac-turer Novel Aire Technologies is utilized for determining optimum speed of the desiccant dehumidification wheel.

Also a desiccant wheel with honey comb matrix by GI sheet coated with silica gel powder adhered using glue, was uti-lized for the better dehudification performance. Temperatures and humidity at the inlet and outlet of the wheel were measured by wet bulb thermometer. Slow Speed of the rotor was measured by using stopwatch expressed in rotations per hour (rph). Fan speed regulator is used to get different air velocities. Velocity of air flow was measured by hot wire anemometer. Optimum speed was determined from experimental results which gives the best air conditioning perfor-mance to the conditioning room.


1401-1402

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International Journal of Research and Innovation (IJRI) obtain a sufficient indoor air quality, and not to satisfy the indoor sensible and latent loads. In fact, desiccant dehumidifiers can be used for air cleaning purposes since they are able to absorb other compounds more than wa-ter vapor, [4]. Cleaner supply air streams imply a smaller amount of ventilation air to reach the desired indoor air quality.

DEC systems are thermally driven, making renewable en-ergy sources (as solar, biomass, geothermal) suitable op-tions to provide at least part of the required thermal ener-gy in the system. The recovery of waste heat produced by closed systems is also a very interesting thermal source for DEC systems. All these options decrease the opera-tional costs of the system, making it more attractive.

WORKING PRINCIPLES OF DESICCANT COOLING

Why desiccant cooling system?

Desiccant systems can produce the following benefits over the traditional air conditioning systems:

• Independent control of latent loads in the ventilation air.• Eliminate condensation on cooling coils and drip pans, and reduce humidity levels in ducts. This will virtually eliminate the growth of mold, mildew, and bacteria. The combination can reduce maintenance and help avoid in-door air quality problems.• Lower humidity levels in occupied spaces provides equivalent comfort levels at higher ambient temperatures .This could allow chilled water set-points to be raised and there-by save energy and reduce system operating costs.• Reduce the mechanical cooling load, permitting the use of smaller chillers and possibly even smaller ducting in new construction. These construction cost offsets should be factored into any economic evaluation.• More freshness because of no ‘recirculation’, which is common in conventional vapor compression air condi-tioning systems.• Very low dew points can be achieved without potential freeze-up.• Moderate cost of operation for the dew points achieved.Heatless type can be designed to operate pneumatically for remote, mobile or hazardous locations. NEED OF DEICCANT COOLING SYSTEM:

Increase of the energy consumption around the world, as well as the desire to prevent further increased global warming, has set a major focus on developing energy ef-ficient and environmentally friendly system solutions. In the summer season especially, air conditioning systems represents a growing market in commercial and residen-tial buildings. Two of the main reasons are that the de-mands for acceptable living standards are increasing as well as the comfort demands of the occupants. The air-conditioning unit covers both temperature and humidity control, which leads to conventional vapor compression cooling systems consuming large amounts of electrical energy as well as exhausting a lot of usable waste heat. In the USA, two-thirds of the energy used in buildings and industrial facilities are for heating needs. In China, the national annual energy consumption for heating is about 130 million ton standard coal, which makes up 10% of the total energy consumption.

Overview of solar assisted systems installed in Europe.

THE ADSORPTION PRINCIPLE:

The ability to adsorb and accumulate water is a feature that almost all materials possess, but some materials have a significantly larger capacity. This is the case for the commercial desiccants used in dehumidification pro-cesses. To understand how a desiccant material work, the principles of adsorption needs to be explained. Adsorp-tion is defined as selective binding of a substance by an-other solid substance. The solid substance will in the case of the system described in this thesis be the desiccant wheel, where the desiccants act as the binding substance and are capable of adsorbing large amounts of water mol-ecules into pores on the surface. The forces which primar-ily are responsible for the adsorption processes arise from interactions of the electric field at the surface of the solid substance with the water molecules.

DESICCANT DEHUMIDIFICATION:

The desiccant material used in a desiccant dehumidi-fier can operate in solid or liquid state. In case of solid state, the dehumidifier is either operating in the form of a slowly rotating desiccant wheel or a periodically regener-ated adsorbent bed. In case of liquid state, the dehumidi-fier is the equipment inside which the liquid desiccant is brought into contact with the process air stream.

Functioning scheme of a desiccant wheel.

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Possible geometries of the flutes.

Desiccant wheel

The desiccant wheel adsorbs water from the process air onto its surface and releases the water to the regenera-tion air. It is the high temperature of the regeneration air which causes the desiccant wheel to desorbing. This sensible heat from the regeneration side of the desiccant wheel, as well as the latent heat from the adsorption pro-cess, raises the temperature of the process air interact-ing with the wheel. After the regeneration air has gained the water molecules released from the desiccant, the now cooled and humidified regeneration air usually gets dis-charged to the surrounding environment.

Schematic on the basis of desiccant wheel function

THE SYSTEM PROCESS EXPLANATION

The main components of a desiccant cooling system are shown in figure. The basic process in providing condi-tioned air may be described as follows:

Atmospheric air enters the slowly rotating desiccant wheel and is dehumidified by adsorption of water. Since the air is heated up by releasing latent heat. This dehumidified air is allowed to flow over the heat exchanger (water to air Heat exchanger), where its temperature is significantly falls down. This cool and dehumidified air is supplied to room.

Schematic of the desiccant cooling system

The exhaust air stream of the room is made to flow through divergent portion and then through heater, where it gets heated up temperature ranges from 120 to 150 degree centigrade for regenerating the sorption wheel and to allow a continuous operation of the dehumidifica-tion process.

Assembly of desiccant cooling system.

(a) represents evaporative cooling system to cool water for heat exchanger (radiator). Evaporative cooling system consists of convergent portion. From the portion water will flow down and evaporates with air.

(b) represents fan, which is used to draw the air from room.

(c) represents transformers, which are used to run the fan (inlet and out let). These Transformers reduce the voltage from 240V to 12V and hence power will be reduced.

(d) represents radiator. It is the main component, which cools the air from desiccant wheel and supplies the air to room.

Explanation of the stages occurring in the desiccant cooling system:

Stage 1: Atmospheric air enters the slowly rotating desic-cant wheel. Here the atmospheric air gets dehumidified by releasing latent heat. Due to this the temperature of air increases and relative humidity decreases.Stage 2: The dehumidified air is passed over the heat ex-changer, where its temperature is falls down. This cool and dehumidified air is supplied to the room.Stage 3: The Exhaust air stream is allowed to flow through convergent portion, where the evaporation cooling takes

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International Journal of Research and Innovation (IJRI) place. This air stream is heated for getting temperature ranges required for regenerating desiccant wheel.

Psychometric representation of desiccant cooling system process

EXPERIMENTAL VALUES AND CALCULATIONS

•Condition of air at inlet to wheel: DBT = 29○C WBT = 25○C RH = 72% Specific humidity = 18.2g/kg of da

At rotor speed of 10rph:

1) At air inlet velocity = 1.5m/s•Condition of air at outlet of wheel : DBT = 45o C

2) At air inlet velocity = 2.0m/s•Condition of air at outlet of wheel : DBT = 43.5o C

3) At air inlet velocity = 2.5m/s• Condition of air at outlet of wheel : DBT = 42o C




1) At air inlet velocity = 1.5m/s• Condition of air at outlet of wheel : DBT = 46.5o C


3)At air inlet velocity = 2.5m/s• Condition of air at outlet of wheel : DBT = 43o C






3)At air inlet velocity = 2.5m/s• Condition of air at outlet of wheel : DBT = 45.5o C

4)At air inlet velocity = 3.0m/s• Condition of air at outlet of wheel : DBT = 43o C5)At air inlet velocity = 3.5m/s

• Condition of air at outlet of wheel : DBT = 42o C







Results of the software:

speed =10rph

Sno inlet ve-locity of process air

pro-cessed removed mois-ture

Process out let temp

Reacti-vation outlet mois-ture

Reacti-vation out let temper-ature

1 1.5 7.1 53 21 53

2 2 6.7 51 22.2 49

3 2.5 6.2 49.5 23 47

4 3 5.8 47.5 23.7 45

5 3.5 5.3 46 24 44

6 4 4.8 44.6 24.5 43

7 4.5 4.5 43.5 24.8 42.5

8 5 4.1 42.4 25 42

9 5.5 3.8 41.5 25.2 41.5

10 6 3.6 40.7 25.4 41

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Speed=20rph






1 1.5 6.7 54 21 52

2 2 6.6 52 22.2 47

3 2.5 6.2 50 23.2 45

4 3 5.7 48.2 23.7 44

5 3.5 5.3 46.5 23.9 43

6 4 4.8 45 24.7 42

7 4.5 4.5 43.7 25 41

8 5 4.1 42.7 25.2 40

9 5.5 3.8 41.5 25.2 39

10 6 3.5 40.8 25.3 38

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International Journal of Research and Innovation (IJRI) Speed=30rph






1 1.5 6.5 55 20.7 50

2 2 6.5 53 22 46.5

3 2.5 6.1 51 23 45

4 3 5.75 49 23.7 43

5 3.5 5.3 47 24 42

6 4 4.9 45.5 24.5 41

7 4.5 4.5 44.2 24.7 40.2

8 5 4.2 43 25 40

9 5.5 3.8 42 25.2 39

10 6 3.5 41 25.6 38

Speed=40rph






1 1.5 6.2 56 20.6 51

2 2 6.3 53.5 21.9 46

3 2.5 6.1 51.2 22.9 44

4 3 5.65 49 23.5 43

5 3.5 5.2 47.5 24 42

6 4 4.8 45.7 24.5 41

7 4.5 4.5 42 24.8 40.2

8 5 4.1 43 24.9 40

9 5.5 3.8 42 25.1 39

10 6 3.6 41.3 25.2 38

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RESULT AND DISCUSSION

From the trend of the graphs drawn with the results from software in between inlet air velocity and air outlet tem-perature for different rotational speeds, the optimum speed for the desiccant wheel used is 30rph.

From the experimental results, maximum temperature of outlet air is observed with rotational speed of 30rph for most of the air velocities, which is an indication of maxi-mum moisture adsorption.

REFERENCES

1. Ying Sheng , Yufeng Zhang , Na Deng , Lei Fang , Jin-zhe Ne , Lijun Ma , Experimental analysis on performance of high temperature heat pump and desiccant wheel sys-tem , Journal of Energy and Buildings 66(2013)505-513.

2. Ahmad A. Pesara, A Review of desiccant dehumidifica-tion technology (http://www.doe.gov/bridgr/home.html), June 2-3, 1993.

3. Mario EI Hourani , Kamel Ghali , Nesreen Ghaddar , Ef-fective desiccant dehumidification system with two-stage evaporative cooling for hot and humid climates , Journal of Energy and Buildings 68(2014) 329-338.

4. T.S. Ge , Y.J. Dai, R.Z. Wang, Y.Li , Feasible study of a self-cooled solid desiccant cooling system based on desic-cant coated heat exchanger , Journal of Applied Thermal Engineering 58(2013) 281e290. 5. S.P Halliday , Dr. CB Beggs and Dr. P.A sleigh , The potential for solar desiccant cooling in the UK , Dublin 2000 paper.

6. K.Sopian , Abduljalil A.Al-abidi , Historical review of liquid desiccant evaporation cooling technology , Journal of Energy and Buildings 67 (2013)22-33.

7. Qing Cheng , Xiao-Song Zhang , A solar desiccant pre-treatment electrodialysis regeneration system for liquid desiccant air-conditioning system , Journal of Energy and Buildings 67 (2013)434-444.

8. Qing Cheng , Xiao-Song Zhang , Xiu-wei Li , Perfor-mance analysis of a new desiccant pre-treatment electro-dialysis regeneration system for liquid desiccant.

9. Robert van Zyl Pr Eng (a)Professor Brain warwicker FCIBSE, MASH RAE(b),(Desiccants the future).

10. D. La, Y. Dai, H. Li, Y. Li, J. K. Kiplagat and R. Wang, "Experimental investigation and theoretical analysis of solar heating and humidification system with desiccant rotor," SJTU, Shanghai, 2010.

11. H.-M. Henning, "Solar assisted air conditioning of buildings – an overview," Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, 2006.

12. A. A. Pesaran, T. R. Penney and A. W. Czanderna, "Desiccant Cooling: State-of-the-Art Aessment," National Renewable Energy Labaratory, Colorado, 1992.

Author

D.V.N.PRASAD,Research Scholar,Department Of Thermal Engineering, Aditya Engineering College, Surampalem, Andra Pradesh, India.

S.N.CH.DATTU.V,Assistant Professor,Department Of Mechanical Engineering, Aditya Engineering College, Surampalem, Andra Pradesh, India.

V.V.KameshAssociate Professor , Department Of Mechanical Engineering, Aditya Engineering College, Surampalem, Andra Pradesh, India.

THERMAL INVESTIGATION ON OPEN CYCLE DESICCANT COOLING AIR CONDITIONING

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