Experimental Study of Thermoelectric Assisted Indirect Evaporative Cooling System

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Experimental study of thermoelectric assisted indirect evaporative cooling system

ABSTRACT

Evaporative cooling is being used widely to improve the indoor conditions due to its energy efficient and environment friendly temperature relief. However, it is not easy to control the temperature and humidity ratio for the product air owing to the uncertainty of the outdoor conditions. This paper presents a new experimental model utilizing combined evaporative cooling with thermoelectric (TEC) chips. Specifically, thermoelectric cooling modules are sandwiched after channels of a flat plate cross flow indirect evaporative cooler. An experimental test rig as well as modeling of the combined system are investigated for the sake of subsequent development. The influences of main operating and geometrical parameters on the system’s performance are analyzed in detail. Results are expected to motivate the evaporative cooling technology and make it more feasible for the hot-humid area.

 

TABLE OF CONTENTS

APPROVAL

ABSTRACT

TABLE OF CONTENTS

LIST OF FIGURES

TABLES

LIST OF ACRONYMS

Chapter 1.   INTRODUCTION

1.1. Indirect-Direct Evaporative Cooling system

1.2. Thermoelectric

1.3. Theory for Evaporative Cooling System

1.3.1. Indirect evaporative cooling (IEC):

1.3.2. The regenerative indirect evaporative (R-IEC)

1.3.3. Dew point indirect evaporative cooling (D-IEC)

1.3.4. Maisotsenko indirect evaporative cooling (M-IEC)

1.4. The combined indirect/direct evaporative cooling systems

Chapter 2. Literature review:

2.1. Studies on Indirect/Direct Evaporative System

2.2. Evaporation Enhancement Studies

Chapter 3. Mathematical modeling of the system

Chapter 4. System description

4.1. The project processes

Chapter 5. Results and discussion

Chapter 6. CONCLUSION

REFERENCES

Appendix A.

The comfort temperature zone

Types of Moisture Absorbers

LIST OF FIGURES

Figure 1. Working principle scheme of IEC………………………………………………….4                 

Figure 2. The process of IEC in psychrometric chart……………………….

Figure 3.working principle scheme of R-IEC……………………………..

Figure 4.the process of R-IEC in Psychrometric chart………………………

Figure 5.working principle scheme of D-IEC    ……………………………………………..6              

 Figure 6.the process of D-IEC in Psychrometric chart………………………

Figure 9.working principle scheme of M-IEC……………………………..

Figure 10.the process of M-IEC in Psychrometric chart……………………..

Figure 11. Case generated wet air to the atmosphere………………………

Figure 12.  Case 2 generated wet air to the mixing unit……………………..

Figure 14. Schematic diagram for the hybrid cooler and Cell element applied for numerical simulation.

Figure 15. Front picture for the model show the gap between the channels………..

Figure 16.scheme shows the model and air flow………………………….

Figure 17. A. Real picture shows the four bolts and the thermelectric………………….15

                 B. All the system…………………………………………

Figure 18. Comparison between ideal COP and volume flow rate………………

Figure 19. Comparison between actual COP and volume flow rate……………..

Figure 20. Comparison between efficiency and volume flow rate……………….

Figure 21. COP of DEC and efficiency At 3 volume flow rate………………….

Figure 22. COP of IEC and efficiency At 3 volume flow rate…………………..

Figure 23. COP of TEC and efficiency At 3 volume flow rate………………….

Figure 24. the human comfort zone……………………………………

TABLES

Table 1. List of simply

Table 2. Project fixed parameters

Table 3. First experiment

Table 4. Second experiment

Table 5. Third experiment

Table 6. Moisture Absorbers

LIST OF ACRONYMS

ECS

Evaporative cooling system

IDECS

Indirectdirect Evaporative cooling system

DECS

Direct evaporative cooling system

IECS

Indirect evaporative cooling system

R-IEC

The regenerative indirect evaporative

D-IEC

Dew point indirect evaporative cooling

M-IEC

Maisotsenko indirect evaporative cooling

TEC

Thermoelectric cooler

COP

Coefficient of Performance

DP

Dew pint temperature

Chapter 1.                

INTRODUCTION

Evaporative cooling system (ECS) technologies are being used increasingly in residential and commercial applications worldwide. ECS technologies-which rely on water as a coolant rather than on chemical refrigerants have important environmental benefits. This paper introduces the technical aspects of ECS, reviews ECS scope of application, and investigates the specific climatic conditions under which EC can be used most effectively in industrialized and developing countries

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1.1.        Indirect-Direct Evaporative Cooling system

Under the right conditions and applications, ECS can provide excellent cooling and ventilation with minimal energy consumption using water as the working fluid. Policymakers in particular should become better informed about ECS because it is very helpful in the potential for mitigating problems of peak electricity demand during the hot season in many countries. The viability of using ECS will depend on the particular application and on the local climatic conditions. For example, for comfort cooling, ECS is most suited to dry regions, although technical improvements such as indirect/direct evaporative cooling system (IDECS) and desiccant-assisted systems widen the zone of applicability. On the other hand, some commercial applications of ECS are suitable even in humid climates.

Indirect/Direct evaporative cooling system IDECS is described as a direct contact type or indirect contact type. Direct evaporative cooling (DECS) keeps the primary air in direct contact with water, thus, with the reduction of air temperature and the air humidity increases, which often makes the people uncomfortable. At indirect evaporative cooling (IECS), the primary air flows through a wet pad zone, which has thermal conductive surface that get cold from the other side. The secondary air flows through the dry side of the cold plate, which add more cooling to the air. IECS systems can decrease the primary air temperature below its wet bulb temperature, even close to dew point temperature without adding moisture into it.

In general, several sectors have significant reasons for considering employing IDECS technologies:

  • Governments: cost savings from reduced electrical consumption can be realized directly by incorporating IDECS technology into buildings and other installations. In addition, government planners should encourage use of IDECS technologies that will save consumers money, reduce overall electrical demand, reduce pollution emissions.
  • Consumers who use IDECS at home can save money on cooling costs. The typical capital, installation, and operation costs are significantly low for IDECS technologies comparing with the conventional cooling system, i.e vapor compression system. Moreover, IDECS technology is simple enough so that most homeowners can maintain their own units.

The main disadvantage of the evaporative cooling system is that it depends on ambient air condition (temperature and humidity). In humid regions, cooling potential of IECS systems is limited because of the high humidity of ambient air.

In order to enhance IECS systems cooling performance, many researchers have made a theoretical and experimental studied on IECS technology. Researches focused on the theoretical analysis and mathematical model development of IECS heat exchangers and systems, optimization of the geometrical configuration of IECS heat exchangers, lab onsite testing of the heat exchanger and whole IECS system. While, some investigators to made a hybrid system combining IECS technology with other cooling system. Usually, the IEC system is in a joint operation with other cooling devices, such as two or three stages of IECS/DECS system, IECS/DECS/ direct-Direct evaporative cooling system. These multi-stage cooling systems could decrease the primary air temperature.

1.2.        Thermoelectric

In 1821, Thomas Johann Seebeck discovered that a thermal gradient formed between two dissimilar conductors can produce electricity, which is known now as thermoelectric. In 1834, Jean Charles Athanase Peltier discovered the reverse effect attained while running an electric current through the junction of two dissimilar conductors could, depending on the direction of the current, cause it to act as a heater or cooler.

Thermoelectric is one of the most popular cooling devices. It has two main jobs: first, converts heat directly into electrical energy through something called the Seebeckeffect, second, convert the electrical energy into heat. Thermoelectric consists of three major components: thermoelectric materials and thermoelectric modules.

A. Thermoelectric materials generate power directly from heat by converting temperature differences into electric voltage. These materials must have both high electrical conductivity and low thermal conductivity to be good thermoelectric materials. Having low thermal conductivity ensures that when one side is made hot, the other side stays cold, which helps to generate a large voltage while in a temperature gradient. The measure of the magnitude of electrons flow in response to a temperature difference across that material is given by the Seebeck coefficient (S).

B. Thermoelectric module consists of two dissimilar thermoelectric materials joined at their ends: an n-type (negatively charged), and a p-type (positively charged) semiconductor. A direct electric current will flow in the circuit when there is a temperature difference between the ends of the materials. In application, thermoelectric modules in power generation work in very tough mechanical and thermal conditions. As they operate in a very high temperature gradient, the modules are subject to large thermally induced stresses and strains for long periods of time. They also are subject to mechanical fatigue caused by large number of thermal cycles. Thus, the junctions and materials must be selected so that they survive these tough mechanical and thermal conditions. Also, the module must be designed such that the two thermoelectric materials are thermally in parallel, but electrically in series. The efficiency of a thermoelectric module is greatly affected by the geometry of its design.

1.3.        Theory of Indirect Evaporative Cooling System

1.3.1.  Indirect evaporative cooling (IECS):

The working principle scheme of the IEC equipment is presented on the left side of figure 1. The warm primary air (1) is flowing inside the dry channels and transfers heat through the heat surface to the wet channels. At the outlet, the primary air (2) will have a lower temperature as at inlet, due to the transferred heat.

The secondary air (3) is flowing inside the wet channels together with the water. The behavior of the air and water in the wet channel is similar to the DECS process. The water as latent heat absorbs the heat transferred through the surface between the dry and wet channels and a corresponding part of the water is evaporated being embedded by diffusion into the secondary air, increasing the moisture content of this air. If the temperature at the end attains to saturated level, the secondary air will be one of the following three cases: 

  1. Lower than the WB temperature of the secondary air at the inlet (no saturation)
  2. Equal with the WB temperature of the secondary air at the inlet (saturation is reach at the outlet).
  3. Higher than the WB temperature of the secondary air at the inlet (saturation before the outlet).

Figure 1. Working principle scheme of IEC                   Figure 2. The process of IEC in psychrometric chart

1.3.2.  The regenerative indirect evaporative (R-IEC)

The regenerative indirect evaporative cooling (R-IEC) was developed to decrease the primary air temperature at the outlet, below the WB temperature of the secondary air at the inlet. The warm primary air (1) is flowing inside the dry channels and transfers heat through the heat surface to the wet channels. At its outlet, the primary air (2) will have a lower temperature than at inlet. A part of the outlet primary air is used as secondary air being introduced in the wet channels.

Figure 3. Working principle scheme of R-IEC                                         Figure 4. The process of R-IEC in

                                                                                                                        Psychrometric chart

1.3.3.  Dew point indirect evaporative cooling (D-IEC)

The dew point indirect evaporative cooling (D-IEC) was developed to decrease the primary air temperature near the limit of the dew point (DP) temperature of the primary air at the inlet. The D-IEC consists of multiple stages of R-IEC equipment.

The working principle of D-IEC equipment, with two stages of R-IEC is presented in figure 5 and the corresponding working process is presented in figure 6. The warm primary air (1) is flowing inside the dry channels and transfers heat through the heat surface to the wet channels.

At outlet, the primary air (2a) will have a lower temperature. A part of the outlet primary air of the first stage is used as secondary air of the first stage being introduced in the wet channels. The rest of the outlet primary air of the first stage is used as primary air of the second stage. The working process inside the wet channels of both stages is similar with the one described in the paragraph referring the classic IEC with the difference that in this case the secondary air is always much cooler.

 

Figure. Working principle scheme of D-IEC               Figure 6. The process of D-IEC in Psychrometric chart

1.3.4.  Maisotsenko indirect evaporative cooling (M-IEC)

The indirect evaporative cooling system, developed by Valerij Maisotsenko is representing an alternative possibility for cooling the primary air near the DP temperature of the inlet air. After the name of its inventor, the system was named M-IEC. The M-IEC has two types of dry channels, one for the primary air and one for the secondary air. The main characteristic of the system is that secondary air has multiple passages from its dry channels into the wet channels. The primary air is simply flowing into the dedicated dry channels.

The M-IEC are realized mainly from horizontal plates. The working principle scheme and a simplified flow scheme are presented in figure 7 while the corresponding working process is presented in figure 8.  The warm outside primary air (1) is flowing inside the dedicated dry channels and transfers heat through the heating surface to the wet channels. At the outlet, the primary air (2) will have a temperature near the DP temperature of the primary air at inlet. The secondary air is the same outside air (1) and also flows inside dedicated dry channels, but having multiple passages into the wet channels were evaporated water is embedded by diffusion as moisture into the secondary air. It can be considered that in each section of the equipment, the secondary air is constantly increasing its moisture until the outlet (3).

Figure 7. Working principle scheme of M-IEC

Figure8. The process of M-IEC in Psychrometric chart

1.4.          The combined indirect/direct evaporative cooling systems

The dry and wet channels form a pair arranged to encounter each other as shown in Fig9. The dry and wet channels are separated by thin flat plate in which a pad covers the inner surface of the wet channel to allow the evaporative cooling. The air stream is first delivered to the dry channels and then diverted at the end of the dry channel into the wet channel that cooled airstream sensibly by vaporizing water on the wet surface. The product air can be the outlet from the dry channel , Fig9. or the outlet from a mixing unit as shown in Fig10. For both cases portion of the outlet air is needed to directed to the wet channel to generate the cooling thrown the evaporating cooling. In case 1, the generated air from the wet channel is through to the atmosphere while in the second case, the generated air from the wet channel is mixed with the outlet air from the dry channel. Both cases will be tested for best optimisation for the product air.

Figure 9. Case generated wet air to the atmosphere

Figure 10.  Case 2 generated wet air to the mixing unit

Thermoelectric is attached to the dry channel, so the cold side will enhance the cooling while the hot side will enhance the evaporation of the water, as shown in Fig13.  Moist absorber can also be used to absorb humidity for the entrance air.

Chapter 2.                 Literature review:

The present chapter reviews various studies carried out related to the evaporative cooling system recently and the past. The cooling efficiency of the evaporative cooling system is found to be increased by imparting the following three methodologies in the system, viz., (i) Combining direct and indirect evaporative cooling systems; and (ii) Evaporation enhancement studies; and (iii) Utilization the thermoelectric.

2.1.         Studies on Indirect/Direct Evaporative System

Datta et al (1987) have experimentally studied an 8.5 ton indirect-direct evaporative cooling system and reported that such a system provides a relief cooling rather than comfort cooling. The room could be maintained at 4-5 ºC above the inlet wet bulb temperature using such a cooler. A facility of using indirect-direct evaporative cooling for residential use in arid regions of Palestine is attempted by Navon and Arkin (1994). Such a system is shown to provide a higher level of thermal comfort where external humidity is around 80 %.

El-Dessouky et al (2000) have developed a membrane air dryer and coupled with a conventional direct/indirect evaporative cooler. As the membrane drier, removes the moisture from the incoming air, the air can be cooled to lower temperature by the subsequent evaporative cooler. Using such a system, reasonable cooling has been obtained. When such a system is combined with Mechanical Vapor Compression system to achieve perfect thermal conditions, about 50 % savings in electricity are obtained.

Gomez et al (2005) have developed a ceramic evaporative cooling system which acts as a semi-indirect cooler. The water cooled in a cooling tower is passed through the annulus passage of the ceramic tube. The out side air is passed through the central region. Chilled water evaporates by seeping through pores. Such a system permits the recirculation of indoor air, which is not possible in the conventional evaporative cooling system. Use of such a system is experimentally demonstrated and 5-12 ºC drop in temperatures is obtained under various conditions.

Jain (2007) has developed and tested a two-stage evaporative cooler. Such a cooler could provide necessary comfort even though outside humidity is higher. The two-stage cooler is found to provide 20 % better cooling when compared to single stage cooler. A novel dew point evaporative cooling system for sensible cooling of ventilation air has been developed by Riangvilaikul and Kumar (2009) and tested experimentally. Wet bulb effectiveness of 92-114 % and the dew point effectiveness 58-84 % are reported. Heidarinejad et al (2010) have tested a ground assisted hybrid evaporative cooling system. The ground-coupled circuit provides necessary pre-cooling effects. Simulation studies have shown that such a hybrid system can provide cooling effectiveness of 100-110 %.

2.2.        Evaporation Enhancement Studies

Giabaklou and Ballinger (1996) have attempted to study the effectiveness of a passive evaporative cooling system employing natural ventilation. The front faces of a building are provided with water guide filaments, where in water flows from the top to bottom by gravity. The incoming air gets cooled and goes inside the building. Such a system is found to reduce the temperature of incoming air by 9.9 ºC, averaged over a day. Giabaklou (2003) has extended the study the using Fanger PMV (Predicted Mean Vote) methodology. It shows that such a system can improve PMV significantly when the number of air changes per hour is higher.

Kant and Mullick (2003) have studied on thermal comfort in a room with exposed roof using the evaporative cooling system. Hourly values of temperature and humidity are computed and compared with the values that are obtained during the unexposed condition. The levels of thermal sensation, which could be obtained with a direct evaporative cooler, are computed.

Kittas et al (2003) have investigated the temperature and humidity gradients during summer in a greenhouse equipped with a ventilated cooling-pad system and half shade plastic roof. The cooling performance up to 80 % is reported. The temperature of the greenhouse is lowered by 10 ºC than the outside air.

2.3.        Studies on Thermoelectric

P. Phaga et al. (2012) has been investigated low cost thermoelectric generator (LCTEG) and high performance to reduce costs of production. The thermoelectric power generation composed of small n-type (n-CaMnO3) and p-type (pCa3Co4O9) of 31 couples/ in2 and the use of thin copper plate and silver paint as electrodes. It was found that the mean voltage is ~121.7 mV, current is ~0.0121mA; power is ~1.47 μW

Chapter 3.                 Mathematical modeling of the system

In this study, the presented model consists of the repetition of a dry channel and a wet channel pair arranged to come into contact with each other as shown in Fig. 14. The dry and wet channels separated by a thin aluminium flat plate, and a thin water film covers the inner surface of the wet channel. A portion of the airstream diverted at the end of the dry channel into the wet channel that cooled airstream sensibly by vaporizing water on the wet surface. To analyze the heat and mass transfer analysis, the suggested model is based on the following assumptions:

  1. Saturated air and water film in counter direction.
  2. Two-dimensional flow.
  3.  Steady and incompressible flow.
  4. The air flow through a system’s channels is fully developed.
  5. The shape of channels is uniform through the cooler.
  6. Water flow in a closed cycle.
  7. The system is well insulated, and there is no heat exchange with the surrounding.

Figure 14. Schematic diagram for the hybrid cooler and Cell element applied for numerical simulation.

Table 1. List of simply

simply

Definition

qd

Heat transfer of dry channel

md

The air flow rate through the dry channel

cpd

Is the specific heat at constant pressure of the dry air

ψ

Wettability factor

Udw

The overall heat transfer coefficient from the dry air channel to the water film

Uds

The overall heat transfer coefficient from the dry air channel to the saturated air

dqs

Sensible heat

dqss

The sensible heat between wet air and water film

dqsl

The latent heat between wet air and water film

mw

The mass flow rate for the evaporative

ωs

The humidity ratio

es

The enthalpy of saturated air

Nu

The Nusselt number

Kair

The thermal conductivity of air

Dh

The hydraulic diameter

TEC

Thermoelectric cooler

α

Seebeck coefficient

Z

Figure of Merit

R

Electrical resistivity

k

Thermal conductivity

Pin

Power in

The direct and indirect evaporative cooling systems 

For the air stream in the dry channel, the heat transfer rate:

-dqd=md cpd dTd     

The rate of sensitive heat transfer from the dry to the water film can be expressed as [1]

dqd = U dw (Td – Ts) ψ

A

U dw =1R

   ,     R=1hd+twallkwall+1hw

The rate of sensitive heat transfer from the dry air channel to the saturated air:

dqs = U ds (Td – Ts) (1ψ)

A

U ds =1R

   ,     R=1hd+twallkwall+1hs

The energy balance for the dry channel along the y direction for the control volume

dTddy=dx(md cpm)UdwTdTwψ+U ds (Td – Ts)(1ψ)

Sensible heat for the wet side is:

dqs=dqss+dqsl

dqss=hs(TwTs) ψ

A

dqsl=dmw esTw

dmw= ms dωs

msdes=hsTwTsψA+ms dωs es(Tw)

The energy balance equation at the wall separating the wet and dry channel:

mw dew+dmw ew=dqd+msdes(Ts)

ew=cp Tw

dTw=dqd+msdes(Ts)cpw Twdmwmw cpw

The applied boundary conditions:

Td=Tw

  at       y=0

ωd=ωs

  at       y=0

Tin=Td

  at       y=L

ωin=ωd

  at       y=0

Heat transfer coefficients:

h=Nu kairDh

Dh=4Acp

         [2]

The TEC Equations

V=αT

Q̇Peltier=πABI

Q̇Thomson=αTI

Q̇=IR2= 

Peltier Cooling =12I2RLeAe

Qḣ=Pin+Qċ

Pin=QḣQċ

COP=QċPin

The TEC Design Equations

Qċ=ZhAc(TcTc)

z=α2kR

Qċ=nαTcI12I2RLeAekAeLeThTc

Qḣ=ZhAh(ThTh)

Qḣ=nαThI+12I2RLeAekAeLeThTc

Chapter 4.                 System description

Table 2. Project fixed parameters

Parameters

specification

Wall material

Stainless steel sheets

Wall thickness

1mm

Dry channel (length x width)

300 x 300mm

Wet channel (length x width)

300 x 300mm

Number of wet channels

3

Number of dry channels

2

Number of blowers

2

Number of thermoelectric

14

By using Ten 1 mm thickness sheets of stainless steel and two 5mm thickness pieces of wood,  we installed it together by four screws from the corners, leaving 2 cm gap between each two plates and every gap between the two plates we consider it as a channel as shown in Fig15. The cross sectional area of the channel will be 2 cm high and 30cm as a width. At the wet channels there is a pad fixed in the plat (wall channel).  For thermoelectric we put it between two sheet plates as sandwich the hot side facing the wet channel and the cool side facing the  dry channel as shown in Fig16. Fig17 shows the four bolts and thermoelectric and all the system.

Figure 15. Front picture for the model show the gap between the channels.

A schematic diagram of a thermoelectric assisted IEC system is shown in Fig16, flat plate cross flow IEC with TEC. Primary air gets cooled as it flows through the dry channel, while its humidity ratio is constant. Water is pumped to wet channels, and it flows down along the wet plat channels after being distributed by nozzles. As it flows down, water absorbs the heat released by the hot sides of the TEC, and part of the water evaporates into secondary air.

Fig. 16 depicts the detailed structure of the thermoelectric/indirect evaporative cooler. The TEC modules are inserted inside between dry and wet channels. Cold sides of TEC are connected with the dry channel of the cooler (also known as primary air channel), and hot sides of the TEC are connected with the wet channel (also known as secondary air channel). By introducing TEC, this system could further decrease the temperature of the primary air, and then improve the cooling effectiveness of indirect evaporative coolers, especially in humid or mild climate regions.

Figure 16.scheme shows the model and air flow

Figure 17. A. Real picture shows the four bolts and the thermelectric. B. All the system

4.1.        The project processes

  1. Direct evaporative cooling test:

The test rig is tested for the evaporative cooling for different outside environment: the outlets air properties are recorded such as temperature, humidity ratio and water consumption for different flow rate.

  1. Indirect/direct evaporating cooling test:

The indirect channel is activated so the outside air is entering first the dry channel, then is directed to the wet channel. The properties of both dry channel and wet channels are recorded. The percentage of the dry channel product air that supplies the wet channels varies from 0 to 100% by 10% increment.

  1. Mixing unit

Depending on the results from part B, mixing of primary and secondary air may be achieved through mixing unit.

D. Enhancing the system using thermoelectric

Thermoelectric is embedded into the flat plate. Different arrangements of thermoelectric are used to cover the flat plat eternally for best performance.

Equipment needed:

  • Centrifugal fan
  • Flat plate: Stainless steel
  • Pad: moister barrier
  • Circulating Pump
  • Spray arrangement for water

Measuring tools:

Temperature, flow rate, humidity ratio

Data acquisition system

Chapter 5.                 Results and discussion

The power used to operate the device

  • Pump: voltage=220V, power=18W, hmax=2m, output=1000L/h
  • Blower: voltage=220V, power=500W, n=13000rpm, mdot=2.3m3/min
  • Thermoelectric Cooler: power=16.4W, Vmax=16.4V,1A power for 14 chips 229.6W
  •  

Table 3. First experiment

Mass flow rate

volume flow rate

Velocity (m/s)

Power (W)

38.1

31.10

5183.67

817.20

Results

DEC

IEC

TEC

Temp. In

33

33

33

Temp. out

25.3

29

27

Humidity in

32%

32%

32%

Humidity out

60%

32%

32%

Qc

293.37

152.4

228.6

COP Ideal

4.285

8.25

5.5

COP Actual

0.359

0.186

0.280

η

0.084

0.023

0.051

Table 4.Second experiment

Mass flow rate

volume flow rate

Velocity (m/s)

Power (W)

29.5

24.08

4013.61

817.20

Results

DEC

IEC

TEC

Temp. In

33

33

33

Temp. out

23

29.3

26.5

Humidity in

32%

32%

32%

Humidity out

65%

32%

32%

Qc

381

140.97

247.65

COP Ideal

3.3

8.919

5.077

COP Actual

0.466

0.173

0.303

η

0.141

0.019

0.060

Table 5. Third experiment

Mass flow rate

volume flow rate

velocity (m/s)

power (W)

20.7

16.90

2816.33

817.20

Results

DEC

IEC

TEC

Temp. In

33

33

33

Temp. out

22

27

25

Humidity in

32%

32%

32%

Humidity out

66%

32%

32%

Qc

419.1

228.6

304.8

COP Ideal

3

5.5

4.125

COP Actual

0.513

0.280

0.373

η

0.171

0.051

0.090

Figure 18. Comparison between ideal COP and volume flow rate

Figure 19. Comparison between actual COP and volume flow rate

Figure 20. Comparison between efficiency and volume flow rate

Figure 21. COP of DEC and efficiency At 3 volume flow rate

Figure 22. COP of IEC and efficiency At 3 volume flow rate

Figure 23. COP of TEC and efficiency At 3 volume flow rate

Discussion

Three cases direct evaporative cooling DEC, indirect evaporative cooling IEC and the last case is evaporative cooling with the assisted thermoelectric TEC have been used. The ideal coefficient of performance increases with increasing volume flow rate in the three cases (direct, indirect and with the assisted thermoelectric slides) as shown in Fig16. While the actual coefficient of performance decreased with increasing volume flow rate in the three cases (direct, indirect and with the assisted thermoelectric slides) as shown in Fig19. Efficiency decreases with increasing volume flow rate in all three cases (direct and indirect and with the assisted thermoelectric slides) as shown in Fig 20.  Efficiency also increases with the increase in the actual coefficient of performance in direct evaporative cooling as shown in Fig 21.  Efficiency also increases with the increase in the actual coefficient of performance in indirect evaporative cooling as shown in Fig 22. Efficiency also increases with the increase in the actual coefficient of performance in evaporative cooling with the assisted thermoelectric slides as shown in Fig 23. If the velocity is increased, the kinetic energy and the heat will increase. Since temperature is the measure of heat, the temperature increases as the velocity increases.

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Chapter 6.                 CONCLUSION

A novel thermoelectric assisted IEC system is proposed in this paper by sandwiching thermoelectric cooling modules containing (Mr.Satayu Travadi 2014)adjacent channels of a flat plate cross-flow indirect evaporative cooler operated in the regenerative mode. The mathematical model of this novel system is developed and the performance of the system is theoretically investigated. The highlighted results can be concluded as follows:

 (1) From the analytical results, the proposed system could cool the primary air to a temperature much lower than inlet air wet bulb temperature, even dew-point temperature by appropriately selecting the number and working electric current of TEC modules.

(2) Influences of the inlet mass flow rate, temperature and humidity ratio of primary air, mp, T and W and COP of the proposed system are also evaluated. It is found that with the increase of mp, increase of Tp, and decrease of W, COP increases. However, there always exists an optimal mass flow rate ratio corresponding to a maximum COP under various conditions, and the optimal value of x changes little with varying I and n.

(3) Finally, optimal widths of primary air channel and secondary air channel resulting in maximum COPs could always be obtained. The values of maximum COP and optimal channel widths all decrease with the increases of number and electric current of TEC modules. Optimal value of primary air channel is higher than that of secondary air channel.

In general, the developed model for the novel system and obtained results could provide theoretical guidance for designing and optimizing of this kind of thermoelectric assisted IEC systems for air conditioning applications..

REFERENCES

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  • Bogdan Porumba, Paula Ungureşana, Lucian Fechete Tutunarua, Alexandru Şerbanb, Mugur Bălana. 18-20 November 2015. A review of indirect evaporative cooling technology. Romania : University of Braşov, Str. Universității 1, Brașov 500068.
  • Cuce PM, Riffat S. 2016. A state of the art review of evaporative cooling systems for. Renew Sustain Energy Rev 54:1240e9.
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Appendix A.  


The comfort temperature zone

For evaporative air-conditioning, it is more reliable to consider a comfort zone bounded by relative humidity and extended to take into account the cooling effect of increased airflow, as shown in Figure 24

Figure 24. the human comfort zone

The actual comfort depends on the flowing factors:

  • Saturation effectiveness of the evaporative air-condition
  • Heat absorption of the space to be cooled
  • Heat generation in the space
  • Sizing of the EAC unit
  • Proper installation and airflow
  • Activity of the occupant

Types of Moisture Absorbers

1. Silica Gel

Silica gel has a very strong absorbing quality when in room temperature. However, it may give up the water it holds when exposed to temperatures near or higher than 40 degrees Celsius. These moisture absorbers are usually only used in products that are placed in room temperature.

2. Clay Absorbers

There is one disadvantage to using clay absorbers, though. Its moisture absorption property is very low as compared to the other two types. For heavily humid areas, clay absorbers are not recommended.

3. Calcium Chloride

a mixture of chlorine and calcium. It has a very strong moisture absorbent property that makes it an ideal candidate for high humidity moisture absorption projects. Calcium chloride is usually used in shipping goods and does a good job in keeping these goods dry during the whole duration of the trip.

Table 6. Moisture Absorbers

Availability

Effectiveness

price

Absorbers

Available on the Internet

Effective indoor

Expensive

 (for larger areas)

2.3 dollar’s for 1 gram

Silica Gel

Available in nature

Less effective and cause problems

Inexpensive

Clay Absorbers

The composite can be made, and its composition materials are available and easy to obtain

The most effective and usually used in the open

Inexpensive

Calcium Chloride

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