Desalination 209 (2007) 238–243

The Ninth Arab International Conference on Solar Energy (AICSE-9), Kingdom of Bahrain

Exergy analysis of evaporative cooling for reducing energy use in a Malaysian building

B.N. Taufiq*, H.H. Masjuki, T.M.I. Mahlia, M.A. Amalina, M.S. Faizul, R. Saidur

Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

Tel. +60 3 7967 6842; Fax +60 3 7959 5317; email: taufiq@um.edu.my

Abstract

To reduce energy consumption in the buildings and minimize negative environmental impacts, it is necessaryto determine the operating condition that provides optimum system performance. Entropy or exergy analysis canbe used as a reliable tool for analyzing energy consumption and environmental impact. This paper describes themodelling and optimization analysis for cooling system in the building. Exergy technique has been used toevaluate overall and component efficiencies and to identify thermodynamic losses. The method is well suited foranalyzing thermodynamic model and identified exergy losses of air conditioning application in a building.

Keywords: Energy consumption; Building; Exergy; Optimization; Air conditioning

1. Introduction

The main contributor to increasing atmo-spheric carbon dioxide (CO2) concentration isthe combustion of fossil fuels from electricitygeneration, commercial and domestic uses. Thedemand for energy is expected to grow rapidlyin developed countries as well as in the develop-ing countries as they attempt to obtain a higherstandard living. This increase energy demand

and consequently increase carbon dioxide con-centration in the atmosphere.

Energy consumed in HVAC accounts forapproximately 20% of the total energy con-sumption nowadays. Effective use of energy isimportant. Evaporative cooling uses recoveredenergy for air conditioning. Evaluation of evap-orative cooling performance is very important inimproving the use of energy and the indoor airquality [1].

Like other developing countries with hot andhumid climates, Malaysia has been experiencing*Corresponding author.

doi:10.1016/j.desal.2007.04.0330011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.

B.N. Taufiq et al. / Desalination 209 (2007) 238–243 239

dramatic growth in the number of use of airconditioners, and the usage will be higher in thefuture. In the HVAC applications, evaporativecooling may be utilized to reduce energy con-sumption or to replace conventional refrigera-tion system.

Evaporative cooling is a technology thatcan substantially reduce the cooling energyrequirement in the building. There are threetypes of evaporative cooling process: direct,indirect and indirect/direct. In direct evapora-tive cooling processes, the air is brought intodirect contact with water in the direct evapora-tive cooler. Indirect evaporative cooling isachieved by sensibly cooling a primary airstream through heat exchanger. Heat is trans-ferred to a secondary fluid on the cold side ofthe heat exchanger, which ultimately rejectsheat to the atmosphere by evaporation effect.The cold-side fluid may be air, water, or in thecase of a heat pipe-refrigerant [2].

Energy and exergy methods are usuallyused to study energy conversion processes.The exergy method, known as the second lawanalysis, calculates the exergy loss caused byirreversibility, which is an important thermo-dynamic property which measures the usefulwork that can be produced by a substance orthe amount of work needed to complete a pro-cess [3]. Unlike energy, exergy is a measureof the quality or grade of energy and it can bedestroyed in the thermal system. Analysisof exergy losses provides information as towhere the real inefficiencies in a system lie.The second law of thermodynamics uses anexergy balance for the analysis and design ofthermal systems.

In this paper, the use of the concept of exergyin the assessment of air-conditioning applica-tions will be applied to optimize the system. Theconcepts of physical exergy and chemicalexergy are an important role in assessing thetrue thermodynamic merit of air conditioningapplications.

2. Data input

Malaysia, being an equatorial country, has auniform temperature throughout the year. Therewas no large variation in temperature through-out the country. Several studies show that ambi-ent temperature has major contribution ofenergy consumption of air conditioning system.The data represent the average humidity andtemperature for 10 years from 1972 to 1998 inMalaysia is collected for this study. However,because of commercial building is occupiedfrom 8 p.m. to 5 a.m., therefore only the tem-peratures when the building is occupied areused for this study. The average hourly temper-ature and relative humidity in Malaysia show inTable 1 [4].

For the purpose of selecting the temperatureof air entering indoor side, the comfort range andthe effective temperature for the population in theparticular country and region were considered.The classification was made on the effective tem-perature and comfort range for a hot climatecountry (population) like Malaysia. In fact, to setan effective temperature for all human being isimpossible because an optimum and acceptablelevel of comfort range for acclimatized AsianAfricans was found to be higher compared to the

Table 1Records of daily average temperature and relativehumidity of Malaysia [3]

Hour Temperature (°C) RH (%)

8 25.63 92.2 9 27.52 82.0

10 29.21 74.6 11 30.39 70.1 12 30.01 67.5 13 31.23 67.0 14 30.98 68.5 15 30.21 71.8 16 29.2 75.6 17 28.09 79.3 Average 29.25 74.86

240 B.N. Taufiq et al. / Desalination 209 (2007) 238–243

higher population of North Amaerica and Europe[5]. The comfort range and effective temperaturefor hot climate have been discussed in the Usermanual fro the ASEAN climatic atlas and thecompendium of climatic statistics. The result istabulated in Table 2 [6].

3. Methodology

A reversible thermodynamic process can bereserved without leaving any trace on thesurroundings. That is, both the system and thesurroundings are returned to their initial states atthe end of the reverse process. This is possibleonly if the net heat and network exchangebetween the system and surrounding is zero. Allreal processes in nature are irreversible.

Exergy measures the ability of energy towork. Exergy can be transferred by three means:exergy transfer associated with work, exergytransfer associated with heat transfer, andexergy transfer associated with the matter enter-ing and exiting a control volume. All suchexergy transfers are evaluated relative to theenvironment used to define exergy. Exergy isalso destroyed by irreversibilities within the sys-tem or the control volume [7]. For fluid flow,the exergy represents the work that can beproduced in a reversible process that is designedto bring it in a state of equilibrium with the envi-ronment. The exergy balance for the controlvolume may be represented as follows:

(1)

The physical exergy is the work obtainableby taking the substance through reversibleprocess from its initial state to the state of theenvironment. The specific exergy is written as:

(2)

where h0 and s0, respectively, the specificenthalpy and entropy at the restricted dead state.The moist air encountered in air conditioningapplications may assumed from the definition ofthe physical flow exergy applied to an ideal mixtureand its specific total flow exergy per mole ofhumid air mixture may be represented as follows:

(3)

where the mass ratio called specific humidity orhumidity ratio, w, and the mole fraction ratio, w−,are defined as follows:

(4)

(5)

The specific total flow of dry air can becalculated as follows:

(6)

Table 2Comfort range and effective temperature for hot climate

Comfort range Effective temperature °F (°C)

Above acceptable Above 76 (above 24.5) Upper acceptable 73–76 (22.8–24.5) Optimum 69–73 (20.6–22.8) Lower acceptable 66–69 (18.9–20.6) Below acceptable Below 66 (below 18.9)

m e m e m e STa t,a w t,w a t gen+ = + 0

e h h T s sph = − − −( ) ( )0 0 0

e c c TT

T

T

T

R TP

P

R T

t p,a p,v

a

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⎣⎢

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+ +

+

( ) ln

( ) ln

ω

ω

00 0

00

1

1

111

1 0

+( ) ++

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⎤

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ωω

ω ωω

ln lno

ω =m

mv

a

ω =x

xv

a

e c TT

T

T

T

R TP

PR T

t,a p a

a 00

a 0 0

= − −⎡

⎣⎢

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+ + +[ ]

, ln

ln ln

00 0

1

1 ω

B.N. Taufiq et al. / Desalination 209 (2007) 238–243 241

The total flow exergy of liquid water can beapproximated by using the properties of respec-tive neighboring states on the two-phase domeof mollier chart [8]:

(7)

The schematic diagram of an ideal processof evaporative cooling is presented in Fig. 1.Such process will allow the water to diffuseinto the unsaturated air reversibly and will reachthe saturated state. Usually, the atmosphericstate (T0, p0, w) will be considered as the deadstate. The exergy for water is

(8)

(9)

where Pw,0 is the water vapor pressure of theunsaturated atmospheric air and Pw,0s is the satu-rated water vapor pressure at T0.

The exergetic efficiency provides a truemeasure of the performance of a system fromthe thermodynamic viewpoint. For a controlvolume at steady state whose exergy rate bal-ances can be estimated by:

(10)

The exergetic efficiency can be representedas follow:

(11)

where is the rates at which the fuel is sup-plied and denotes the product is gener-ated. and denotes the rates of exergydestruction and exergy loss, respectively. Therate of exergy destruction per kg of dry air is,therefore

(12)

Then, the exergy efficiency of the evapora-tive cooler can be defined in the term of:

(13)

In HVAC, the energy flows also need tobe properly classified in order to get a properevaluation of exergy product and exergy supply.The input electricity or mechanical work isobviously classified into exergy supply. Thereduction of exergy of the secondary flowstreams that is used to heat or cool, or humiditythe primary flow streams should also be classi-fied into exergy supply. The relevant energyflow can be regarded as the source energy flow.While the increase of exergy of primary flowstreams is classified into exergy product, therelevant energy flow is regarded as serviceenergy flow because it is provided to serve theneed for other systems [9].

4. Results and discussion

The conventional air conditioning is usuallyused to cool or dehumidify the hot moist air.However, in evaporative cooling, air is cooled by

e h T h T T s T s T

P P T v T R T

t,w f g f g

sat f v

≅ − − −

+ −[ ] −

( ) ( ) ( ( ) ( ))

( ) ( )

0 0 0

0 llnφ0

Unsaturated atmosphericair (T0, P0, ω0), 1 kg

dry air + ω0 kgwater vapor

Water (ω0s – ω0) kg,at T0, P0

Saturated atmosphericair (T0, P0, ω0), 1 kg

dry air + ω0 kgwater vapor

Environment T0

System

W

Q

Fig. 1. Schematic of an ideal process.

e R Tt,w v = − 0 0lnφ

φ =P

Pw,0

w,0s

E E E EF P D L= + +

ε = = −+E

E

E E

EP

F

D L

F

1

EF

EP

EDEL

T S

me e e0 gen

at,a t,w t= + −ω

εω

=+e

e et

t,a t,w

242 B.N. Taufiq et al. / Desalination 209 (2007) 238–243

humidifying the primary or secondary air. Thedirect evaporative cooling process is used as anexample in this study. The atmospheric state isselected as the dead state for calculating theexergy of moist air. The choice of P0 = 1 atm isquite obvious, however there is a lack of a con-vention for the selection of T0 and f0. The proper-ties of humid air as the a perfect gas mixture ofdry air and water vapor is tabulated in Table 3 [8].

Fig. 2 shows the correlation between relativehumidity and exergy efficiency. The studyfound that typically, as the relative humidity isincreased, exergy efficiency will be increased.In direct evaporative cooling process, the mix-ing of fresh and return air causes the destructionof the exergy of fluid stream. However, theenthalpy of the moist air approximately remainsconstant because of the adiabatic evaporation.

And, the pressure of the air decrease because ofthe flow resistance. The correlation betweenambient temperature and exergy efficiency ispresented in Fig. 3.

This study also has shown that an effective-ness of evaporative cooling heat exchanges hasgreat importance in improving the performanceof evaporative cooling that applied in coolingsystem.

5. Conclusions

The exergy analysis of an evaporativecooling applied in Malaysian building has beenpresented through the exergy balance of an opensystem. The evaporative cooling is a feasibletechnology that can reduce mechanical coolingand energy requirement in air conditioningapplication that will effect to decrease emissionsfrom electricity generation in Malaysia. Thus,further work should be conducted to improvethe effectiveness of the evaporative cooling.

Nomenclature

cp specific isobaric heat capacity (kJ/kg K)

e specific exergy (kJ/kg) E exergy (kJ)

Table 3Ideal gas constants of dry air and water vapor

Dry air Water vapor

Ra = 0.287 kJ/(kg K) Rv = 0.4615 kJ/(kg K) Cp,a = 1.003 kJ/(kg K) Cp,a = 1.872 kJ/(kg K) Ma = 28.97 kg/kmol Ma = 18.015 kg/kmol R−a = 8.314 kJ/(kmol K) R−a = 8.314 kJ/(kmol K) c−p,a = 29.057 kJ/kmol K) c−p,v = 33.724 kJ/(kmol K)

Fig. 2. Exergy efficiency of the evaporative cooling pro-cess as function of relative humidity with T0 = 29.25°C.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.2 0.4 0.6 0.8 1Relative humidity

Exe

rgy

effic

ienc

y

Fig. 3. Exergy efficiency of the evaporative coolingprocess as function of ambient temperature withRH = 70%.

0.3

0.32

0.34

0.36

0.38

0.4

0.42

0.44

20 21 22 23 24 25 26 27 28 29 30Ambient Temperature (°C)

Exe

rgy

effi

cien

cy

B.N. Taufiq et al. / Desalination 209 (2007) 238–243 243

h specific enthalpy (kJ/kg) mass flow rate (kg/s)

P pressure (Pa) Q heat load (kJ)R specific ideal gas constant (kJ/kg K) S entropy (kJ/kg K) s specific entropy (kJ/kg K) T temperature (°C, K) W work (kJ/kg)f relative humidity (%) e exergy efficiency w specific humidity (%) w- mole fraction ratio

Subscripts 0 ambient state, dead state a dry air ch chemical f saturated liquid g saturated vapor gen generation i input o output p potential ph physical or thermochemical t totalv water vaporw water

References

[1] W.J.R. Kenneth, Advance thermodynamics forengineer, McGraw-Hill, New York, 1995.

[2] W.K. Brown, Application of evaporative coolingto large HVAC Systems, ASHRAE Transactions,(1996).

[3] M.M. Talbi, B. Agnew, Exergy analysis: anabsorption refrigerator using lithium bromide andwater as the working fluids, Appl. Therm. Eng. J.,20 (2000) 619–630 .

[4] Malaysian Meteorological Service, Annual sum-mary of meteorological observation 1972–1998,Malaysian Meteorological Service, KualaLumpur, Malaysia, (1998).

[5] M. Ilyas, Energy conservation in tropical air con-ditioning: environmental variables, Energy Con-serv. Manage., 22 (1982) 227–230.

[6] ASEAN Secretariat, The Asean user’s manual forthe asean climatic atlas and compendium ofclimatic statistics, Asean committee on scienceand technology, Jakarta.

[7] M.J. Moran and G. Tsatsaronis, EngineeringThermodynamics, The CRC Handbook of Ther-mal Engineering, Boca Raton, UK, 2000.

[8] A. Bejan, Advanced engineering thermodynamics,Wiley, New York, 1988.

[9] R. Chengqin, L. Nianping and T. Guangfa, Princi-ples of exergy analysis in HVAC and evaluation ofevaporative cooling schemes, Building Environ.,37 (2002) 1045–1055.

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