Accepted Manuscript

Selection of wetted media for pre-cooling of air entering natural draft dry cooling

towers

Suoying He, Yi Xu, Guanhong Zhang, Kamel Hooman, Ming Gao

PII: S1359-4311(16)32018-X

DOI: http://dx.doi.org/10.1016/j.applthermaleng.2016.11.179

Reference: ATE 9590

To appear in: Applied Thermal Engineering

Received Date: 30 September 2016

Revised Date: 18 November 2016

Accepted Date: 23 November 2016

Please cite this article as: S. He, Y. Xu, G. Zhang, K. Hooman, M. Gao, Selection of wetted media for pre-cooling

of air entering natural draft dry cooling towers, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/

j.applthermaleng.2016.11.179

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Selection of wetted media for pre-cooling of air entering natural draft dry cooling towers

Suoying Hea,*,suoyinghe@hotmail.com,s.he@sdu.edu.cn, Yi Xua, Guanhong Zhanga, Kamel Hoomanb, Ming

Gaoa,*,gm@sdu.edu.cn

aSchool of Energy and Power Engineering, Shandong University, Jinan 250061, China

bSchool of Mechanical and Mining Engineering, The University of Queensland, QLD 4072, Australia

*Corresponding authors.

Highlights

Four wetted media were comparatively studied for future pre-cooling design.

Characteristics of wetted media suitable for pre-cooling were summarized.

Media with middle efficiencies and pressure drops are superior for pre-cooling.

Cellulose7060 is most promising for pre-cooling the 120m-high NDDCT.

Abstract

The performance of natural draft dry cooling towers (NDDCTs) can be improved by wetted-medium

evaporative pre-cooling. However, the pre-cooling benefit is season dependent and is significantly affected

by wetted media. To further investigate the effect of wetted medium type on pre-cooling performance, the

current study simulates a pre-cooled NDDCT using two typical types of wetted media, i.e., film

(Cellulose7060 and PVC1200) and trickle (Trickle125 and Trickle100). A MATLAB program was

developed and was validated against published data. The model was then used to carry out case study of a

120m-high NDDCT pre-cooled using the selected wetted media with fixed thickness of 300mm. Both the

medium performance and pre-cooled NDDCT performance were compared to recommend the

characteristics of wetted media promising for evaporative pre-cooling. Simulation finds that the media with

high or low cooling efficiencies and pressure drops are not promising for the studied pre-cooling case while

those media with middle cooling efficiencies and pressure drops intend to produce much performance

enhancement and Cellulose7060 is a representative of such kind of medium. For the 120m-high NDDCT,

the wetted-medium pre-cooling can raise the heat rejection rate of the tower without pre-cooling from

45MW to 92 MW, 76 MW, 68 MW and 65MW by Cellulose7060, PVC1200, Trickle125 and Trickle100,

respectively at ambient temperature of 50℃ and humidity of 20%.

Keywords: natural draft cooling tower, wetted medium, air mass flow rate, heat rejection rate

2

Nomenclature

A Area, m2

cp Specific heat, J/(kg K)

FT Temperature correction factor

g Gravitational acceleration,

m/s2

H Height, m

K Loss coefficient

l Medium thickness, m

le Medium geometric length, m

m

Mass flow rate, kg/s

∆p Pressure drop, Pa

Q Heat transfer rate, W

q

Volumetric flow rate, m3/s

T Temperature, K

u Velocity, m/s

U Overall heat transfer

coefficient, W/(m2 K)

NDDCT Natural draft dry cooling

tower

Greek Symbols

∆ Difference

Cooling efficiency, %

Density, kg/m3

Subscripts

1,2,3,4,5,6 Positions within or around the

towers

a Dry air or air side

a, b, c, d, n, β Constants

av Air-water vapor mixture

ct Separation and redirection of

flow at the lower edge of the

tower shell

ctc Contraction

cte Expansion at the heat

exchanger

he Heat exchanger

lm Logarithmic mean

medium Wetted medium

w Water

wb Wet bulb

wi Water inlet

wo Water outlet

to Kinetic energy at the outlet of

the tower

3

1. Introduction

Renewable energy has been an important political topic for a long time. One of the most promising ways to

use solar energy is concentrated solar power (CSP). CSP technologies generate electricity by concentrating

the solar radiation beam onto a small area, where a heat transfer fluid is heated up and this energy is

ultimately transferred to the steam. Electricity is then generated by an electric generator which is driven by

a steam turbine with the efficiency limited by the Carnot cycle [1]. Considerable research efforts have been

conducted to improve the efficiency of the CSP system and make the cost of electricity comparable to that

of the conventional fossil-fuel power plant, e.g., thermal energy storage [2, 3], thermophotovoltaic heat

engine [4] and even the natural draft dry cooling towers (NDDCTs) [5]. The current study focuses on the

performance improvement of NDDCTs which may offer the only effective alternative as most CSPs are

located in arid or semi-arid areas.

The major drawback of NDDCTs is their low performance, especially at hot days. Previous studies find

that the performance of NDDCTs can be improved by wetted-medium evaporative pre-cooling when the

ambient air is hot and dry [6-8]. The concept of wetted-medium pre-cooling of a NDDCT is illustrated in

Fig. 1. There are two water systems in the pre-cooled NDDCT. One system is the hot water from the

thermal power plants, which is to be cooled down. The other water system is wetted-medium evaporative

pre-cooling system, in which a small amount of water is distributed over the top of the wetted media by

water distributor and drips down by gravity and capillarity to wet the media uniformly. The excess water is

collected at the bottom of the media and recirculated by the water pump. Air is driven by buoyancy to pass

through the media to form a cross-flow heat and mass transfer process. The water deposited on the media

evaporates which absorbs heat from the air stream and thus cools the air. The pre-cooled air then flows

through the heat exchanger bundles to cool the tube-side fluid coming from thermal power plants. The heat

exchanger bundles are laid out horizontally at the lower end of the tower. In Fig. 1, the entire cylindrical

shell inlet area of the tower is covered with wetted media. The evaporative pre-cooling system only

operates at high ambient temperatures to assist dry cooling.

4

According to He et al. [6], the pre-cooling benefit is however season dependent and the pre-cooling

enhancement can go up to 46% (where the pre-cooling enhancement is defined as ηenhancement=(Qpre-NDDCT - Q

NDDCT)/QNDDCT×100%, Q is the tower heat rejection rate). What significantly affects the pre-cooling

performance is the wetted medium. In reference [7], the pre-cooling using three film media was studied and

concluded that there are critical temperatures below which the tower performance does not benefit but is

hindered by wetted-medium pre-cooling. The critical temperatures depend significantly on wetted medium

and only one film medium was proved to be useful in the studied pre-cooling case.

The wetted medium is the most important element in a direct evaporative cooling system [9]. Wetted media

fulfil two main functions. First, they provide large contact surface areas for heat and mass exchange

between water and air flows. Second, they delay the fall of water, ensuring that the exchange process lasts

longer [10, 11]. A number of studies have been carried out to better understand the performance of wetted

media. ANSI and ASABE [12] recommended the air speeds and minimum water flows for aspen fiber and

corrugated cellulose media. Franco et al. [10] studied the influence of water and air flows on the

performance of cellulose media. Dowdy et al. [13, 14] experimentally determined the heat and mass

transfer coefficients in aspen media and rigid impregnated cellulose media. Liao et al. [15, 16] carried out

the same analyses for fine and coarse fabric PVC sponge media, nonwoven fabric and coir fiber media. A

comprehensive experimental study of cellulose pads was carried out by Malli et al. [17] to investigate the

effects of medium type, medium thickness and air velocity on evaporative cooling performance (including

pressure drop, humidity variation, evaporated water and cooling efficiency). The procedure for testing

evaporative cooling media was given by Koca et al. [18]. The flows in corrugated evaporative cooling

media were simulated by Beshkani, Hosseini, Tavakoli and Franco et al. [19-22]. The flow and heat

transfer through a porous medium imbedded inside a channel with permeable walls were investigated

using two-equation energy model by Seyed Moein et al. [23]. To sum up, there is no dearth of published

data concerning different types of wetted media, either through experimental study or theoretical and

numerical approaches.

Generally, the wetted media fall into three main categories: splash, film and trickle [24, 25]. Splash fills

require large volumes to break up the water flow, and therefore, large spaces and more water pump

power are needed. Splash fills also tend to produce more water drift than other fill types, especially at high

5

air speeds. Therefore, they are not good option for evaporative pre-cooling of NDDCTs. Film fills have the

disadvantages of relatively high air-side pressure drops and also high fouling risks but they have good heat

transfer characteristics. Trickle fills combine the good aspects of splash and film: they offer larger surface

areas than splash fills and lower pressure drops than film fills; the large openings of trickle fills relieve the

fouling problems of film fills, and the water drift is reduced by their compact structures when compared

with splash fills [9, 25]. Based on the above literature review, it seems that film and trickle media have the

potential for evaporative pre-cooling of NDDCTs. To do further comparison of their pre-cooling

performance, the present study chooses two film and two trickle media from literature review to investigate

their pre-cooling performance and therefore give suggestions for future pre-cooling design. In terms of

optimization of wetted-medium pre-cooling of air entering NDDCTs, the authors are doing economic

analysis of pre-cooled NDDCTs to include all the influencing parameters (including the incremental

equipment cost, the operation/maintenance cost, the consumptive evaporative water use, the effect of

wetted-medium pre-cooling on turbine power output and the water pumping power, etc.). Further study will

be carried out in the future for efficiency improvement by referring to parametric optimization

methodology [26-28] and the results will be reported in the future study.

This paper simulates the pre-cooled NDDCT using four wetted media (i.e., Cellulose7060, PVC1200,

Trickle125 and Trickle100) based on a MATLAB program. Two typical types of wetted media with the

potential of evaporative pre-cooling are selected through literature review and are comparatively studied.

Meanwhile, the characteristics of wetted media promising for evaporative pre-cooling of NDDCTs are

summarized. The present work can be distinguished from previous studies due to (1) the wetted media

designed for wet cooling tower use (PVC1200, Trickle125 and Trickle100) are studied so as to fulfil the

purpose of evaporative pre-cooling of air; (2) the ranges of wetted-medium cooling efficiency and pressure

drop promising for the pre-cooling enhancement of a 120m-high NDDCT are reported, which gives

suggestions for future pre-cooling design.

6

2. Mathematical models

2.1 Mathematical model of the pre-cooled NDDCT

The operation of a pre-cooled NDDCT is coupled by energy and draft equations [7, 8]. The energy equation

of a pre-cooled NDDCT as shown in Fig. 1 is,

( )4 3 ( )1

m c T Tav pav a aQ m c T Tw pw wowi (1)

and the heat transfer rate can be expressed in terms of the overall heat transfer coefficient given by Eq.(2).

( ) ( )4 32

ln ( ) / ( )4 3

UAF T T T TwowiT a aQ UAF TT lm

T T T Twowi a a

(2)

The draft equation balances the buoyancy against the total flow resistances, i.e.,

( ) ( ) / 2 51 4 4 3p g H H H flow resistancesa av av

(3)

=flow resistances p p p p p pct ctc cte tomedium he (4)

The flow resistances in Eq.(4) are coupled to the loss coefficients by 2

2av avp K u . The flow

resistances include the pressure drop across wetted medium pmedium

, loss at tower supports (this loss is

already accounted for by the pressure drop across wetted medium), loss due to the separation and

redirection of flow at the lower edge of the tower shell pct , loss at heat exchanger supports (this loss is

negligible), contraction loss pctc , the frictional loss of heat exchanger phe

, and expansion loss at the

heat exchanger pcte , loss in kinetic energy at the outlet of the tower pto . The losses can be evaluated

using empirical relations reported in reference [29], except for the loss through wetted medium which is

reported in Section 2.2.

The mathematical model of a NDDCT is similar to that of the pre-cooled NDDCT, except for the

calculations of Ta3 in Eq.(1) and Eq.(2) and the flow resistances in Eq.(4). In the NDDCT, the Ta3 is close to

the air temperatures at elevations (1) and (2) in Fig. 1 while significant difference exists in the pre-cooled

7

NDDCT due to the evaporative pre-cooling. There is no loss contribution by wetted medium to the flow

resistances in Eq.(4) in the NDDCT.

2.2 Mathematical model of wetted medium

To simulate the pre-cooled cooling tower using the four selected wetted media, the medium performance is

one of the key issues. The pressure drop introduced by wetted media is of critical importance in NDDCT

applications. It is critical because the pressure drop will affect the air mass flow rate passing through the

tower. The cooling efficiency is a key factor in determining the performance of direct evaporative cooling

system. It determines how close the air gets to the state of saturation [30]. The definition of cooling

efficiency can be expressed as 2 3 2 2

( ) ( ) 100%a a a wb

T T T T . Based on experimental studies by He et

al. [31, 32], the pressure drop data of Cellulose7060, PVC1200, Trickle125 and Trickle100 are correlated

using Eq.(5).

2

12

b

e w a a

a

l q up a c

l q

(5)

The constants in Eq.(5), a, b and c were determined by means of non-linear regression of the test results

(All the experimental data of pressure drop in references [31, 32] were used for analysis). The regression

results are summarized in Table 1.

The cooling efficiency data of the four media can be correlated using Eq.(6).

1 expd

n

a

l

u

(6)

The constants in Eq.(6), , d and n were determined by means of non-linear regression of the test results

(All the experimental data of cooling efficiency in references [31, 32] were used for analysis). The

regression results are listed in Table 2.

The following simulations will use the above correlations of cooling efficiency and pressure drop to model

the performance of wetted media.

8

2.3 Iterative algorithm

The heat and mass transfer in the pre-cooled NDDCT is coupled with air flow through the tower while the

air flow is determined by balancing the energy and draft equations through an iterative process [6]. The

fixed variables during iteration are tower specifications and heat exchanger specifications, hot water flow

rate, hot water inlet temperature, medium thickness and ambient humidity. The iteration variables are air

temperature exiting the heat exchanger and cooling efficiency. The iteration procedure is summarized as

follows.

In the iterative process, the performance of wetted-medium evaporative cooling is characterized by the

cooling efficiency (Eq.(6)) and pressure drop (Eq.(5)). Since the cooling efficiency and pressure drop of the

wetted medium depend on air velocity, a preliminary air mass flow rate ma0 is calculated in Step1 by

neglecting all the flow resistances other than the loss due to the heat exchanger bundles (Initially assume

Ta4=Twi). Step2, a new air mass flow rate ma1 is then calculated by considering all the losses including the

loss due to the wetted media. Step3, the new ma1 is used to calculate wetted-medium evaporative cooling.

The air temperature Ta3 after cooling is calculated from 2 3 2 2

( ) ( ) 100%a a a wb

T T T T . Air humidity

after cooling is calculated according to ASHRAE Handbook [33] (assuming air wet-bulb temperature is

almost constant during evaporative cooling). In Step4, Step1 and Step2 are repeated using the new

temperature and humidity calculated in Step3. After calculations, a new air mass flow rate ma2 is obtained.

Then, recalculation of wetted-media evaporative cooling uses the new air mass flow rate ma2 (i.e., repeating

Step3). Step5 is to make sure that the cooling efficiency and pressure drop calculated in Step3 and Step4

are consistent, which is achieved by a period of iteration of Step4 (Step5 is essentially to make sure the air

temperature and humidity after pre-cooling is the closest values). After that, a closer air mass flow rate ma

is found. In Step6, the energy equations are balanced by a period of iteration. If the air mass flow rate ma

found in Step5 cannot satisfy the energy equations, the air temperature leaving the heat exchanger Ta4 is

reduced step by step to do iterations of Step1-Step5.

Finally, a value of air mass flow rate can be found that satisfies both the energy and draft equations. The

values of hot water outlet temperature, air temperature leaving the heat exchanger, air mass flow rate, heat

rejection rate, cooling efficiency and pressure drop of wetted media when used to pre-cool the NDDCTs

9

can be found simultaneously. For each ambient temperature (Ta1), the iteration time to obtain balanced

energy and draft equations is around 10 seconds.

The iterative algorithms of the NDDCT and the pre-cooled NDDCT are similar with only small difference.

For the iteration process of the NDDCT, there is no need to consider the cooling efficiency and pressure

drop associated with wetted-medium pre-cooling. Meanwhile, the iteration process is simplified as the Step

3 to Step 5 are omitted.

2.4 Model Validation

The heat and mass transfer in the pre-cooled tower is coupled with air flow through the tower while the air

flow is determined by balancing the energy and draft equations through an iterative process. Since there is

no wetted-medium pre-cooling in use so far in NDDCTs, it was difficult to do a rigorous comparison with

measured pre-cooled NDDCT performance. However, the MATLAB code of NDDCT without pre-cooling

was compared with the case study reported by Kröger [29] and found good agreement. This validated

MATLAB code was then adapted to simulate the operation of the proposed tower with and without pre-

cooling. The thermodynamic parameters were calculated according to references [29, 33]. The

meteorological effects were considered according to Kröger’s book [29]. The wetted medium performance

was calculated according to Section 2.2.

2.5 Case Study

The proposed cooling tower has designed conditions as listed in Table 3. The heat exchangers are extruded

bimetallic finned tubes. The NDDCT and the pre-cooled NDDCT have the same tower design and heat

exchanger design: the heat exchanger bundles are laid out horizontally at the lower end of the tower and are

arranged in the form of A-frame placed in a radial pattern.

3. Results and discussions

For evaporative pre-cooling system, the cooling efficiency is acknowledged to be nearly constant when the

supplied water flow rate is enough to fully wet the media (fully wetted medium means there is no streaking

and dry area in the medium) [31, 32]. The water flow rates used in the current simulation are enough to

fully wet the media, i.e., 62, 39, 10 and 10 l/min/m2 for cellulose7060, PVC1200, Trickle125 and

10

Trickle100, respectively. The ambient temperature ranges from 10 to 50℃ while the ambient humidity is

fixed at 20% during simulation. The wetted medium performance and the tower performance comparison

are reported in the following sections.

3.1 Wetted medium performance

The wetted medium performance is coupled with the tower performance but will be reported here for

reference. The cooling efficiency and pressure drop of the wetted media when used to pre-cool the

NDDCTs at medium thicknesses of 300 mm are presented in Fig. 2.

As demonstrated in Fig. 2, for the same medium thickness, Cellulose7060 provides the highest cooling

efficiencies and pressure drops, followed by PVC1200, Trickle125 and Trickle100. With the increase in

ambient air temperature, the cooling efficiency increases but the pressure drop across the media decreases.

This can be explained by the fact that the air velocity passing through the media by natural draft is

decreased (see Fig. 3a). With the increase in ambient air temperature, the air density difference between the

outside and inside of the tower at the mean heat exchanger elevation will decrease, and thus the driving

buoyancy force that causes the air flow will decrease, lowering the air velocity. The lowered air velocity

can provide more time for heat and mass transfer between the water and air, improving the cooling

efficiency. On the contrary, the pressure drop will decrease with the air velocity decreased [10, 15, 17, 18].

3.2 The performance comparison of pre-cooled NDDCTs

In this section, the performance comparisons of the NDDCTs pre-cooled using the four selected wetted

media at medium thicknesses of 300 mm are presented. In a NDDCT, the air mass flow rate that satisfies

the energy and the draft equations is significantly important. The tower heat rejection rate, also known as

the cooling capacity, indicates the rate at which the cooling tower can dispose of heat. The changes in the

air mass flow rate and the heat rejection rate of the towers, with and without pre-cooling are compared in

Fig. 3.

11

The air mass flow rate decreases with the increase in ambient temperature, which is because the driving

force (the buoyancy, which is due to the air density difference resulting from the temperature difference

between the tower inside and outside air) is decreased at higher ambient temperatures. This applies to the

towers with or without pre-cooling. Fig. 3a also illustrates that the incorporation of pre-cooling into the

proposed NDDCT causes a further reduction in the air mass flow rate due to the additional pressure drop

associated with wetted media. Since Trickle100 has the lowest pressure drop when compared with other

media (as shown in Fig. 2b), the tower pre-cooled with this medium offers the highest air mass flow rate

amongst the pre-cooled towers.

In terms of heat rejection rate, although pre-cooling can decrease the inlet air temperature and thus

improves heat rejection rate through Eq.(1), the additional pressure drop causes the drop in air mass flow

rate and therefore impairs heat rejection rate. That is why the heat rejection rate of the pre-cooled NDDCT

can be either higher or lower than that of the tower without pre-cooling. As shown in Fig. 3b that the tower

performance is improved by pre-cooling only when the ambient temperatures are high enough. For the

same medium thickness, the tower performance improvement by the Cellulose7060 at high ambient

temperatures is the highest one, followed by PVC1200, Trickle125 and Trickle100 (Trickle125 and

Trickle100 have almost the same improvement). This is interesting as in reference [7], the authors found

that Cellulose7060 is the most promising medium when compared with Cellulose7090 and Cellulose5090.

Take a deep look at the medium performance, generally, for the proposed NDDCT and within the ambient

temperature range of 10−50℃, the pressure drop and cooling efficiency ranges of the media are

summarized in Table 4 (The data in Table 4 is beyond the simulation results reported in the current study)

[7, 31, 32].

In Table 4, the pressure drop in Pa depends on air velocity, medium thickness and to some extent on the

water flow rate. The cooling efficiency is dependent on the air velocity and the medium thickness [31, 32].

The Cellulose7060 has the lowest cooling efficiency and pressure drop when compared with Cellulose7090

and Cellulose5090, while it provides the highest cooling efficiency and pressure drop amongst PVC1200,

Trickle125 and Trickle100. From the comparative study of the current study and reference [7], the

Cellulose7060 is most promising for the pre-cooling enhancement of the studied NDDCT. This means that

the media with high or low cooling efficiencies and pressure drops are not promising for the pre-cooling

12

application of the studied NDDCT. Those media with middle cooling efficiencies and pressure drops intend

to produce much performance enhancement of the studied NDDCT and Cellulose7060 is a representative

of such kind of media.

For the pre-cooling application of NDDCTs, the cooling efficiency intends to improve the tower heat

rejection performance by reducing the tower inlet air temperature while the extra pressure drop in reverse

impairs the tower heat rejection performance through its contribution to the flow resistances (thereby

reducing the air mass flow rate passing through the tower, e.g., Fig. 3a). Since the NDDCT is operated by

the balance of both energy and draft equations, the cooling efficiency and the extra pressure drop are all

crucial to the tower heat rejection performance. The media with high cooling efficiencies provide more

cooling but also come with large resistances. The media with low pressure drops have the advantage of

small resistances but produce less cooling. The middle option may offer a good alternative. This trade-off

between cooling efficiency and pressure drop can be reflected as Fig. 4.

Overall, the Cellulose7060 with the pressure drops of 28.6−272.1 Pa/m and the cooling efficiency range of

44.7−88.5% is most promising for the pre-cooling enhancement of the studied 120m-high NDDCT.

4. Conclusions

The current study simulates pre-cooling of a 120m-high NDDCT using four selected wetted media based

on a MATLAB program. The simulation finds that: (1) the media with high or low cooling efficiencies and

pressure drops are not promising for the pre-cooling application of the 120m-high NDDCT while those

media with middle cooling efficiencies and pressure drops intend to produce much performance

enhancement; (2) Cellulose7060 with the pressure drops of 28.6−272.1 Pa/m and the cooling efficiency

range of 44.7−88.5% is most promising for the pre-cooling enhancement of the 120m-high NDDCT; (3) the

critical temperatures below which the tower performance does not benefit but is hindered by wetted-

medium pre-cooling are 20℃, 26℃, 26℃ and 24℃ for Cellulose7060, PVC1200, Trickle125 and Trickle100,

respectively at ambient humidity of 20%; (4) the wetted-medium pre-cooling can raise the heat rejection

rate of the 120m-high tower without pre-cooling from 45MW to 92 MW, 76 MW, 68 MW and 65MW by

13

Cellulose7060, PVC1200, Trickle125 and Trickle100, respectively at extreme climate of ambient

temperature 50℃ and humidity 20%.

Acknowledgements

The project was supported by “The Fundamental Research Funds of Shandong University” (Grant No.

2016TB010). The financial supports from Postdoctoral Science Foundation of Shandong Province (Grant

No. 201602018), National Natural Science Foundation of China (Grant No. 51606112) and Shandong

Natural Science Foundation (Grant No. ZR2016EEM35) are gratefully acknowledged.

References

[1] M. Liu, N.H. Steven Tay, S. Bell, M. Belusko, R. Jacob, G. Will, W. Saman, F. Bruno. Review on

concentrating solar power plants and new developments in high temperature thermal energy storage technologies. Renewable and Sustainable Energy Reviews, 53 (2016) 1411-1432.

[2] L.F. Cabeza, A. Solé, X. Fontanet, C. Barreneche, A. Jové, M. Gallas, C. Prieto, A.I. Fernández.

Thermochemical energy storage by consecutive reactions for higher efficient concentrated solar power

plants (CSP): Proof of concept. Applied Energy, Article in Press (2016),

http://dx.doi.org/10.1016/j.apenergy.2016.10.093.

[3] Y. Jemmal, N. Zari, M. Maaroufi. Thermophysical and chemical analysis of gneiss rock as low cost

candidate material for thermal energy storage in concentrated solar power plants. Solar Energy

Materials and Solar Cells, 157 (2016) 377-382.

[4] H.R. Seyf, A. Henry. Thermophotovoltaics: a potential pathway to high efficiency concentrated solar

power. Energy & Environmental Science, 9 (2016) 2654-2665.

[5] X. Li, Z. Guan, H. Gurgenci, Y. Lu, S. He. Simulation of the UQ Gatton natural draft dry cooling tower. Applied Thermal Engineering, 105 (2016) 1013-1020.

[6] S. He, H. Gurgenci, Z. Guan, K. Hooman, Z. Zou, F. Sun. Comparative study on the performance of

natural draft dry, pre-cooled and wet cooling towers. Applied Thermal Engineering, 99 (2016) 103-113.

[7] S. He, H. Gurgenci, Z. Guan, A.M. Alkhedhair. Pre-cooling with Munters media to improve the

performance of Natural Draft Dry Cooling Towers. Applied Thermal Engineering, 53 (2013) 67-77.

[8] S. He, Z. Guan, H. Gurgenci, I. Jahn, Y. Lu, A.M. Alkhedhair. Influence of ambient conditions and

water flow on the performance of pre-cooled natural draft dry cooling towers. Applied Thermal

Engineering, 66 (2014) 621-631.

[9] S. He, H. Gurgenci, Z. Guan, X. Huang, M. Lucas. A review of wetted media with potential application

in the pre-cooling of natural draft dry cooling towers. Renewable and Sustainable Energy Reviews, 44

(2015) 407-422.

[10] A. Franco, D.L. Valera, A. Madueno, A. Pena. Influence of water and air flow on the performance of cellulose evaporative cooling pads used in Mediterranean greenhouse. Transactions of the ASABE, 53

(2010) 565−576.

[11] E. Elsarrag. Experimental study and predictions of an inclined draft ceramic tile packing cooling tower.

Energy Conversion and Management, 47 (2006) 2034−2043.

[12] ASABE standards (ANSI/ASAE EP406.4). Heating, ventilating and cooling greenhouses. American

Society of Agricultural and Biological Engineers, St. Joseph, Michigan, USA, 2008.

[13] J.A. Dowdy, R.L. Reid, E.T. Handy. Experimental determination of heat and mass transfer coefficients

in aspen pads. ASHRAE Transactions, 92 (1986) 60–70.

[14] J.A. Dowdy, N.S. Karabash, Experimental determination of heat and mass transfer coefficients in rigid

impregnated cellulose evaporative media, ASHRAE Transactions, 93 (1987) 382−395.

[15] C.M. Liao, K.H. Chiu. Wind tunnel modeling the system performance of alternative evaporative cooling pads in Taiwan region. Building and Environment, 37 (2002) 177−187.

14

[16] C.M. Liao, S. Singh, T.S. Wang. Characterizing the performance of alternative evaporative cooling

pad media in thermal environmental control applications. Journal of Environmental Science and

Health, Part A: Toxic/Hazardous Substances and Environmental Engineering , 33 (1998) 1391–1417.

[17] A. Malli, H.R. Seyf, M. Layeghi, S. Sharifian, H. Behravesh. Investigating the performance of

cellulosic evaporative cooling pads. Energy Conversion and Management, 52 (2011) 2598-2603.

[18] R.W. Koca, W.C. Hughes, L.L. Christianson. Evaporative cooling pads: test procedure and evaluation. Applied Engineering in Agriculture, 7 (1991) 485−490.

[19] A. Franco, D.L. Valera, A. Pena, A.M. Perez. Aerodynamic analysis and CFD simulation of several

cellulose evaporative cooling pads used in Mediterranean greenhouses. Computers and Electronics in

Agriculture, 76 (2011) 218-230.

[20] R. Hosseini, A. Beshkani, M. Soltani. Performance improvement of gas turbines of Fars (Iran)

combined cycle power plant by intake air cooling using a media evaporative cooler. Energy

Conversion and Management, 48 (2007) 1055-1064.

[21] A. Beshkani, R. Hosseini. Numerical modeling of rigid media evaporativecooler. Applied Thermal

Engineering, 26 (2006) 636-643.

[22] E. Tavakoli, R. Hosseini. Numerical analysis of 3D cross flow between corrugated parallel plates in

evaporative coolers. Energy Conversion and Management, 52 (2011) 884-892.

[23] S.M. Rassoulinejad-Mousavi, H.R. Seyf, S. Abbasbandy. Heat Transfer through a Porous Saturated Channel with Permeable Walls using Two-Equation Energy Model. Journal of Porous Media, 16

(2013) 241-254.

[24] A.K.M. Mohiuddin, K. Kant. Knowledge base for the systematic design of wet cooling towers, Part II:

Fill and other design parameters. International Journal of Refrigeration, 19 (1996) 52–60.

[25] D.G. Kröger. Air-Cooled Heat Exchangers and Cooling Towers: Thermal-flow performance

evaluation and design, vol. 1. PennWell Corp., Tulsa, Oklahoma, USA, 2004.

[26] M.M. Rashidi, M. Ali, N. Freidoonimehr, F. Nazari. Parametric analysis and optimization of entropy

generation in unsteady MHD flow over a stretching rotating disk using artificial neural network and

particle swarm optimization algorithm. Energy, 55 (2013) 497-510.

[27] M.M. Rashidi, N. Galanis, F. Nazari, A. Basiri Parsa, L. Shamekhi. Parametric analysis and

optimization of regenerative Clausius and organic Rankine cycles with two feedwater heaters using artificial bees colony and artificial neural network. Energy, 36 (2011) 5728-5740.

[28] M.M. Rashidi, O. Anwar Be´g, A. Basiri Parsa, F. Nazari. Analysis and optimization of a transcritical

power cycle with regenerator using artificial neural networks and genetic algorithms. Proceedings of

the Institution of Mechanical Engineers Part A Journal of Power & Energy, 225 (2011) 701-717.

[29] D.G. Kröger. Air-Cooled Heat Exchangers and Cooling Towers: Thermal-flow performance

evaluation and design, vol. 2, PennWell Corp., Tulsa, Oklahoma, USA, 2004.

[30] J.R. Watt, W.K. Brown. Evaporative Air Conditioning Handbook, Fairmont Press, Inc., Lilburn,

Georgia, USA, 1997.

[31] S. He, Z. Guan, H. Gurgenci, K. Hooman, Y. Lu, A.M. Alkhedhair. Experimental study on the

applications of two trickle media for the inlet air pre-cooling of Natural Draft Dry Cooling Towers.

Energy Conversion and Management, 89 (2014) 644−654.

[32] S. He, Z. Guan, H. Gurgenci, K. Hooman, Y. Lu, A.M. Alkhedhair. Experimental study of film media used for evaporative pre-cooling of air. Energy Conversion and Management, 87 (2014) 874-884.

[33] ASHRAE. ASHRAE Handbook-Fundamentals, I-P ed., American Society of Heating, Refrigerating

and Air-Conditioning Engineers, Inc., Atlanta, USA, 2009.

15

Fig. 1. Wetted-medium pre-cooling of air entering a natural draft dry cooling tower [9].

Fig. 2. The (a) cooling efficiency and (b) pressure drop of wetted media when used to pre-cool the NDDCTs.

Fig. 3. The (a) air mass flow rate and (b) heat rejection rate of the NDDCTs.

Fig. 4. The trade-off between cooling efficiency and pressure drop of wetted media (1−0.30m Cellulose7060; 2−0.15m

Cellulose7060; 3−0.15m Cellulose7090; 4−0.30m PVC1200; 5−0.15m Cellulose5090; 6−0.30m Trickle125; 7−0.30m

Trickle100) [7].

Table 1. Constants in Eq.(5).

Description a b c R2

Cellulose7060 0.124 -1.038 1825 0.979

PVC1200 0.566 -0.664 3191 0.995

Trickle125 0.131 -1.105 1657 0.976

Trickle100 0.568 -0.648 151 0.973

Table 2. Constants in Eq.(6).

Description d n R2

Cellulose7060 6.785 0.952 0.232 0.993

PVC1200 3.869 1.348 0.585 0.948

Trickle125 1.610 0.976 0.422 0.975

Trickle100 1.540 1.149 0.317 0.949

16

Table 3. Proposed cooling tower design.

Design condition NDDCT Pre-cooled NDDCT

Tower height, m 120 120

Tower inlet diameter, m 83 83

Tower outlet diameter, m 58 58

Finned tube heat exchanger, Apex angle of A-frame 2=61.50 2=61.5

0

Heat exchanger tubes 4 rows, 2 passes 4 rows, 2 passes

Wetted medium thickness, m − 0.3

Designed ambient pressure, kPa 101.325 101.325

Designed ambient temperature, oC 20 20

Designed ambient humidity, % 30 30

Designed water mass flow rate, kg/s 4390 4390

Designed water inlet temperature, oC 60 60

Designed heat rejection rate, MW 296 Depends on wetted medium type

Designed air mass flow rate, kg/s 10912 Depends on wetted medium type

Designed water outlet temperature, oC 43.9 Depends on wetted medium type

Table 4. Pressure drop and cooling efficiency comparisons [7, 31, 32].

Level Medium type Pressure drop, Pa/m Pressure drop, Pa Cooling efficiency, %

Low

PVC1200 22.8−246.8 4.5−43.2 8.8−50.3

Trickle125 17.4−124.1 4.3−48.9 19.1−50.4

Trickle100 12.0−109.6 3.9−33.5 15.8−43.9

Middle Cellulose7060 28.6−272.1 4.2−61.2 44.7−88.5

High Cellulose7090 69.3−561.0 9.9−83.8 64.1−92.4

Cellulose5090 110.3−964.0 12.1−98.3 71.2−95.9

5

Ground

Heat exchanger

Buoyancy

Air inlet

Air outlet

Wetted media

Air inlet

Tower shell

6

2

4

Tower supports

Pre-cooling system

Air flow

Water flow from

water pump

Wetted medium pieces

3

1

10 15 20 25 30 35 40 45 5020

30

40

50

60

70

80

90

Co

olin

g e

ffic

ien

cy,

%

Ambient temperature, oC

Cellulose7060

PVC1200

Trickle125

Trickle100

(a)

10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

Pre

ssu

re d

rop

, P

aAmbient temperature,

oC

Cellulose7060

PVC1200

Trickle125

Trickle100

(b)

10 15 20 25 30 35 40 45 500

2

4

6

8

10

12

14

16

Air

ma

ss f

low

ra

te,

10

3kg

/s

Ambient temperature, oC

No pre-cooling

Cellulose7060

PVC1200

Trickle125

Trickle100

(a)

10 15 20 25 30 35 40 45 500

50

100

150

200

250

300

350

400

450

He

at

reje

ctio

n r

ate

, M

WAmbient temperature,

oC

No pre-cooling

Cellulose7060

PVC1200

Trickle125

Trickle100

(b)

20 40 60 80 10040

50

60

70

80

90

100

7766

55

22

1

Cooling efficiencyHe

at re

jectio

n r

ate

, M

W

Cooling efficiency, %

Benchmark: heat rejection rate of the tower without pre-cooling

1

0 5 10 15 20

44 33

Pressure drop

Pressure drop, Pa

Middle cooling efficiencies

Middle pressure drops

(e.g., 1, 2-Cellulose7060)

High cooling efficiencies

High pressure drops

(e.g., 3-Cellulose7090

and 5-Celluloe5090)

Pre

-co

olin

g e

nh

an

ce

me

nt

of

ND

DC

Ts

Wetted-medium performance

Low cooling efficiencies

Low pressure drops

e.g., 4-PVC1200, 6-Trickle125

and 7-Trickle100

Cooling efficiency, pressure drop

Middle cooling efficiencies

Middle pressure drops

(e.g., Cellulose7060)

High cooling efficiencies

High pressure drops

(e.g.,Cellulose7090

and Celluloe5090)

Pre

-coo

ling e

nha

nce

ment o

f N

DD

CT

s

Wetted-medium performance

Low cooling efficiencies

Low pressure drops

(e.g., PVC1200, Trickle125

and Trickle100)