Cooling of Remote Sites Telecom Shelters

8/10/2019 Cooling of Remote Sites Telecom Shelters

1/10

c ARISER Vol. 3 No. 4 (2007) 144-153

Engineering: Ener

Arab Research Institute in Sciences & Engineering

http://www.arabrise.org

Online Publishing Group ISSN 1994-3253

Cooling of Remote Sites Telecommunication Shelters

Abdelkader DARWICHE and Samivullah SHAIK

Saudi Consolidated Engineering Company Khatib and Alami Khobar, Saudi Arabia

[email protected]; [email protected]

Received 10 November 2007; Accepted 28 November 2007

Telecommunication equipment is often housed inside shelters at remote sites far off from conventional

power lines. Controlling the temperature inside the shelter is a high priority for improving the performance

of the electronic equipment. The equipment dissipates heat during its operation in addition to the absorbed

heat from surroundings with high ambient temperatures especially in desert environment. This poses a great

challenge to the HVAC engineer. This article reviews various techniques of air conditioning these shelters

such as exhaust fans, vortex coolers, Peltier coolers and passive cooling systems (free convection and

phase-change material). The paper then moves on to recent alternatives made possible by new technological

developments such as Direct Current (DC) powered air conditioner units. The paper will also give headlines

on sizing the batteries and the solar panels to meet the cooling load.

Keywords: Air conditioning, Telecommunication Shelters, Temperature.

Contents

1 Introduction . . . . . . . . . . . . . . . . . 144

2 Load Calculation . . . . . . . . . . . . . . 1443 Ventilation by Exhaust Fans. . . . . . . . . 145

4 Vortex Coolers. . . . . . . . . . . . . . . . 145

5 Peltiler Cooler . . . . . . . . . . . . . . . . 146

6 Passive Cooling Systems . . . . . . . . . . 146

6.1 Natural (Free) Convection . . . . . . 146

6.2 Phase Change Materials (PCMs) . . . 148

7 DC Powered Air Conditionning System . . 1498 Example on Sizing and Selecting Equipment

for a DC Powered Air Conditionning System 150

9 Cconclusion . . . . . . . . . . . . . . . . . 151

1 Introduction

Historically, telecommunication equipment has always been considered as sensitive equipment and accordingly

housed in controlled enclosures or shelters the cooling of which has been satisfactorily implemented using tradi-

tional methods. However, in many new systems being developed and deployed today, such as broadband, ISDN,

cellular/wireless and cable Internet services, heat dissipation densities have been increasing considerably, raising

the possibility of heat-related failure{Marongiu [1]}. These heat density trends are reviewed in a recent white pa-

per{[2]}and a dedicated book by the American Society of Heating, Refrigerating and Air-Conditioning Engineers{ASHRAE [3]}.

On the other hand, pipe lines and supply networks often run through remote areas like deserts in which no elec-

trical power is available. Communication along the pipeline or between the networks necessitates having telecom

repeater or relay stations in such remote locations. The daily air temperatures exceeding 50C in middle eastern

deserts, which combined with internal heat generation from the telecom equipment make the inside telecom shelter

prone to temperature as high as 80C.In these conditions, electronic equipment will not function reliably.Note: This paper is actively being considered for publication in the ASHRAE journal. It is presented here with

approval from the authors for informational and educational purposes only. Copyright ASHRAE.

2 Load Calculation

The air conditioning system design needs to be tailored to the type of environmental conditions the shelter will beplaced in as well as the heat generation by the equipment installed inside the shelter. As with other applications,

two indoor parameters should be considered: temperature and humidity.


2/10


3/10

146 A. Darwiche & S. Shaik

FIG . 1: Vortex cooler [5].

5 Peltiler Cooler

Peltier coolers [6] use the Peltier effect to create a heat flux between the junctions of two different types of materi-

als. It is basically a solid-state active heat pump which transfers heat from one side of the device to the other side

against the temperature gradient (from cold to hot), with consumption of electrical energy. Such an instrument is

also called a Peltier device, Peltier diode, Peltier heat pump, solid state refrigerator or thermoelectric cooler (TEC).

Because heating can be achieved more easily and economically by many other methods, Peltier devices are mostly

used for cooling. Although, when a single device is to be used for heating and cooling, a Peltier device may be

desirable. Simply connecting it to a DC voltage will cause one side to freeze, whilst the other side heats up. The

effectiveness of the pump at moving the heat away from the cold side is totally dependent upon how well the heat

from the hot side can be removed. There are no moving parts and such a device is maintenance free.

Due to the relatively low efficiency, thermoelectric cooling is generally only used in environments where the

solid state nature outweighs the poor efficiency. Thermoelectric junctions are generally only around 5 10% asefficient as the ideal refrigerator (Carnot cycle), compared with 40 60% achieved by conventional compressioncycle systems (reverse Rankine systems like a compressor) [6].

Like Vortex cooler, Peltier coolers are better candidates for spot cooling and are seldom used to cool an entire

shelter.

6 Passive Cooling Systems

Passive methods include primarily natural (free) convection and phase-changing materials (PCMs)

6.1 Natural (Free) Convection

This system is by far the most currently installed system in the Middle East although expectations are that it will

be replaced with more advanced systems that will be discussed later in this paper.

This technique does not require any power source as it functions only on natural convection and energy storage

capacity. This system as shown in Fig. 2 basically consists of housings with insulation, sun shade, external and

internal heat exchangers, heat storage tank and conveyance of the medium (mainly water), which must be designed

for the specific application. The heat dissipated by the electronic equipment is absorbed by the internal heat ex-

changer and transferred to the water inside the tank by thermo siphon phenomenon. At night, heat is dissipated

by the same phenomenon to the outside as illustrated in Fig. 3(a & b). The temperature inside the shelter can be

maintained from 15Fto 20Fbelow ambient.

The free convection passive cooling system basically levels off the temperature inside the shelter by temperature

dampening. Fig. 4 shows the expected inside temperature of a passively cooled shelter located in a typical Middle

Eastern desert environment compared to the outdoor air. The tank and heat exchangers temperatures are also

shown on the same Figure. [7].

The main advantage of this system is that no energy is required to run the system. In addition, it is highlyreliable since no moving mechanical parts. The main disadvantages are the huge space required to install this sys-

tem and the higher initial cost associated. Furthermore, this system performance is highly dependent on having


4/10

Cooling of Remote Sites Telecommunication Shelters 147

T a n k

I n t e r n a l H e a t

E x c h a n g e r s

E x t e r n a l H e a t

E x c h a n g e r s

H e a t

S t o r a g e

FIG . 2: Principle of free convection passive cooling system

T a n k

C o l d n e s s f r o m t h e

t a n k c o o l s t h e

i n t e r i o r

O u t s i d e h o t t e r t h a n

i n s i d e : c i r c u i t

s w i t c h e d o f f

T a n k

C o l d w a t e r f r o m t h e

t a n k c o o l s t h e

i n t e r i o r

T h e c o o l n e s s o f

t h e n i g h t c o o l s

t h e t a n k

a. Day b. Night

FIG . 3: Free convection passive cooling system during day and night.

25

30

35

40

45

50

Time (hr)

Temperature(0C)

Environment

Heat Exchanger Outside

Tank

Heat Exchanger Inside

Air in shelter

0.00 6.00 PM 0.00 6.00 AM 0.00 6.00 PM 0.00 6.00 AM6.00 AM

FIG . 4: Temperature levels for a typical passive cooling system.


5/10


high temperature difference between day and night (typical for desert environment). It is not well suited for en-

vironments in which the temperatures do not vary much over the day. In such places, certain manufacturers have

added small Direct Current operated pumps to help the free convection. While such a measure negates on themain advantage of this system (no moving parts), it allows to extend the application of this system to more mild

environments.

6.2 Phase Change Materials (PCMs)

Currently, phase change materials (PCMs) are used in tropical regions in telecom shelters as the main cooling

source or as a back-up cooling systems. In case of a power failure to conventional cooling systems, PCMs minimize

use of Diesel generators, and this can translate into enormous savings across thousands of telecom sites [8].

Phase Change Materials (PCMs) [9] are latent thermal storage materials. They use chemical bonds to store

and release heat. The thermal energy transfer occurs when a material changes from a solid to a liquid or from a

liquid to a solid form. This is called a change in state or phase. Initially, these solid-liquid PCMs perform like

conventional storage materials; their temperature rises as they absorb solar heat. Unlike conventional heat storage

materials, when PCMs reach the temperature at which they change phase (their melting point), they absorb largeamounts of heat without getting hotter. When the ambient temperature in the space around the PCM material

drops, the PCM solidifies, releasing its stored latent heat. PCMs absorb and emit heat while maintaining a nearly

constant temperature. Within the human comfort and electronic-equipment tolerance range of 20Cto 35C,latentthermal storage materials are very effective. The main applications for PCMs are when space restrictions limit

larger thermal storage units in direct gain or sunspace passive solar systems. The most commonly used PCMs are

salt hydrides, fatty acids and esters, and various paraffins (such as octadecane). Recently also ionic liquids were

investigated as novel PCMs [8].

FIG . 5: An enclosure with PCM during daylight hours [1].

Phase change materials perform best in small containers, which therefore are usually divided in cells. The cells

are shallow to reduce static head - based on the principle of shallow container geometry. The packaging material


6/10


should conduct heat well; and it should be durable enough to withstand frequent changes in the storage materials

volume as phase changes occur. It should also restrict the passage of water through the walls, so the materials will

not dry out (or water-out, if the material is hydroscopic). Packaging must also resist leakage and corrosion. Steeland polyethylene are common packaging materials [8].

7 DC Powered Air Conditionning System

Air conditioning the telecommunication shelter to inside temperature around the human comfort levels (i.e 75F)

had always been a challenge for HVAC designers and an invaluable wish for telecom engineers. Under such con-

ditions, protection and reliable operation of the telecom equipment will be optimized like never before. Unfortu-

nately, air conditioning the shelter to such temperatures entailed prohibitive cost by necessitating the installation of

standard active (compressor type) air conditioning units (operating on AC power) and feeding them from batteries

through electrical inverters. With an obvious unfavorable life cycle cost, this option was seldom if ever opted for

by any shelter HVAC designer.

However, recent technological development made high efficiency air conditioning units operating on Direct

Current (DC) power available. In a normal AC air conditioner the compressor is a piston type or rotary compressorwith a volumetric efficiency of 50% driven by an electric motor with an efficiency of around 50%. The overall

efficiency is 50% 50%=25%.In the new DC powered AC units, the efficiency of the smaller models compressor is 60%, and the electric

motor 85% resulting in a total efficiency of 51% which is about two times higher than conventional units. On

larger models, the compressor efficiency is 95% and the motor 85% resulting in an overall efficiency of about

80% which is three times higher than conventional units [10]. Fig 6 shows one DC powered air conditioning units

installed inside a remote telecom shelter.

FIG . 6: DC powered air conditionning unit installed in a telecom shelter [10].

These units are usually available to operate on either 24V or 48V. Combined with a battery system chargedthrough solar panels, these units can now be used to air condition remote telecom shelters to even human comfort

zone.One of the disadvantages of this system is that the foot print area required to install the solar panels is larger than

conventional systems. However, in remote locations, land is a readily available luxury. Another disadvantage is that


7/10


this system contains moving parts which will require some type of maintenance. However, these disadvantages are

outweighed by the cooling level; this system is able to offer and hence, is being increasingly opted for by HVAC

designers.

8 Example on Sizing and Selecting Equipment for a DC Powered Air Conditionning System

A DC powered air conditioning system is planned for a remote shelter in a desert environment. The shelter total

floor area is about 284 ft2. Below are the steps that were followed during sizing of the system.

1. The outdoor/indoor environmental conditions were obtained from published data.

a. Outdoor Conditions: 1150FDBT

b. Indoor Conditions: 72.50FDBT

2. The shelters floor plan and elevations were obtained from the architect.

3. The heat generation from the installed equipment was obtained from the equipment manufacturer.

4. Commercial HVAC load calculation software was used to estimate the required cooling capacity for each hour.The software output indicated that the shelter has a maximum cooling of load of 11,800Btu/hrat 1500hrsfora typical summer day shown in the Tab. 1.

5. Manufacturers catalogue indicates that at the design outdoor conditions, the DC powered air conditioning unit

will deliver 6,500Btu/hr.Hence two units will be required.6. The total energy required during summer day is required in order to size the batteries and the solar array. The

power consumption of the selected unit depends highly on the outside temperature which varies widely during

the day. The temperature profile for a typical summer day was obtained from the computer software used in

step 4 is shown in Fig. 7.

0

2 0

4 0

6 0

8 0

1 0 0

1 2 0

1 4 0

0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 1 2 2 2 3

T i m e ( h r )

T

e

m

p

e

r

a

t

u

r

e

0

F

FIG . 7: Outdoor temperature profile for a typical summer day.

7. In accordance with the variation of the cooling capacity and the full load power consumption for the selected

unit, the cooling capacity and power curves are obtained from the manufacturer and are given as:

Cooling Capacity(Btu/Hr) = (0.0012T2 0.3485T+ 31.484) 1000 (1)

Power(Watts) = (0.0015T2 + 0.4259T 13.672) 54 (2)

whereTis the outside temperature in F.


8/10


8. For each hour of a typical summer day, the required cooling capacity was obtained from the commercial

software and shown in Tab. 1. The unit full load power consumption was obtained from equation (2) and was

indexed with the same percentage as the unit loading.9. For example, at 8:00 a.m. (from Tab. 1), the selected two units will be able to deliver 19,700 Btu/hr (Equation

1). Accordingly, the units loading factor is (7,600/19,700)or 38.5%.The units full load power consumptionis 644.13 W per unit (Equation 2) so the actual power consumption at 8:00 am is 644.3 0.385 or 248.5W perunit.

10. Applying the procedure in Step 7 at the 24 hours of the day yielded a power consumption of 19.407 KWh per

day.

11. Assuming 7 hours of sun-hours per day and 100W per each solar panel,

The total solar array capacity required is 19407/7=2772.5W Assuming 10% cable losses, net solar array capacity required is 3049W

The number of panels required is 3049/100=31 panels

12. Using the daily energy consumption obtained earlier and using a 48VDC voltage,

The daily current consumption is 19407.8/48=404 Ah per day Assuming a 10% safety factor, the daily current consumption is 444 Ah per day

Assuming a 5 days autonomy, the total required current is 444 Ah x 5 Days = 2220 Ah

Using 12V/230 Ah batteries, 4 nos. are required in series to obtain 48V and 2220/230=9.65 (say 10) arerequired to provide the current so a total of 40 batteries are required.

9 Cconclusion

Designing the cooling system for a remote telecom shelter presents a great challenge to the designer. In addition,

with the increasing heat densities, the performance requirements and capacities of the air conditioning system has

become more demanding. Many conventional ways have existed to overcome some of these challenges. Moreover,

recent technological innovations have empowered the HVAC designer with a broader range of equipment selection

with larger capacities. Tab. 2 summarizes the advantages and disadvantages of each of the currently available

system.

References

1. M.J. Marongiu. Managing thermal conditions in

enclosures.MJM Engineering Co.

2. Heat density trends in Data Processing, Com-

puter Systems, a Telecommunications, (2005-

2010) White paper, Uptime Institute Inc.

3. ASHRAE Technical Committee 9.9. (2005) Data-

com Equipment Power Trends and Cooling Appli-

cations. ASHRAE.

4. ASHRAE Fundamentals Handbook (2005).

5. Vortex tube, http://en.wikipedia.org/

wiki/vortex_tube

6. Thermoelectric Cooling, http://en.

wikipedia.org/wiki/Thermoelectric_

cooling

7. Passive Cooling Systems,www.intertec.info

8. Phase Change Material, http://en.

wikipedia.org/wiki/Thermoelectric_

cooling

9. Telecom Shelters PCMs, www.pcmenergy.

com.

10. www.dcairco.com
http://en.wikipedia.org/wiki/vortex_tubehttp://en.wikipedia.org/wiki/vortex_tubehttp://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://www.intertec.info/http://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://www.pcmenergy.com./http://www.pcmenergy.com./http://www.dcairco.com/http://www.dcairco.com/http://www.pcmenergy.com./http://www.pcmenergy.com./http://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://www.intertec.info/http://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://en.wikipedia.org/wiki/Thermoelectric_coolinghttp://en.wikipedia.org/wiki/vortex_tubehttp://en.wikipedia.org/wiki/vortex_tube


9/10


TAB . 1: Hourly cooling load variation and power consumption for the selected ewuipement far a typical summer day.

Hour of

the day

Outdoor T

(

F)

Actual

CoolingLoad

(MBH)

Cooling

Load of 2 units as

per Eq. (1)

(MBH)

Units

LoadingFactor

LF =

Actual CL

/ CL as per

Eq.(1)x100

Full Load

PowerConsump-

tion as per

Eq. (2)

(Watts/unit)

Actual

PowerCon-

sump-

tion

(Watts/unit)

Total

PowerCon-

sumption

(Watts)

0 87.1 7.7 19.50 39.49% 650.39 256.84 513.68

1 85.4 7.8 19.98 39.04% 635.05 247.92 495.84

2 83.7 7.1 20.47 34.68% 619.23 214.73 429.46

3 82.4 6.8 20.86 32.59% 606.83 197.79 395.58

4 81.3 6.8 21.20 32.08% 596.11 191.23 382.46

5 81 7 21.29 32.88% 593.16 195.03 390.06

6 81.7 6.9 21.07 32.74% 600.03 196.45 392.91

7 83.4 6.8 20.56 33.07% 616.39 203.83 407.66

8 86.4 7.6 19.70 38.59% 644.13 248.56 497.12

9 90.9 7.9 18.47 42.76% 683.00 292.08 584.15

10 96 8.7 17.21 50.56% 723.08 365.61 731.22

11 101.7 9.7 15.94 60.86% 762.90 464.30 928.61

12 107.2 10.3 14.86 69.30% 796.32 551.89 1103.77

13 111.3 11.2 14.15 79.13% 818.05 647.31 1294.61

14 114 11.6 13.73 84.47% 830.88 701.86 1403.71

15 115 11.8 13.59 86.86% 835.33 725.57 1451.14

16 114 11.6 13.73 84.47% 830.88 701.86 1403.71

17 111.6 11 14.11 77.98% 819.54 639.09 1278.19

18 107.9 10.8 14.74 73.29% 800.23 586.51 1173.01

19 103.4 10.3 15.59 66.07% 773.75 511.20 1022.41

20 99 9.2 16.52 55.69% 744.69 414.73 829.47

21 95.3 9 17.37 51.80% 717.83 371.87 743.74

22 91.9 8.3 18.22 45.57% 691.19 314.95 629.90

23 89.2 8 18.92 42.28% 668.70 282.70 565.39

Total (Wh per day) 19047.795


10/10


TAB . 2: Comparison of various HVAC systems

S.No System Advantages Disadvantages

1 Exhaust Fans Low cost option Temperature above ambientIngress of outdoor contami-

nants

2 Vortex cooler - No moving parts Low efficiency

- Reliable Limited in Size

Depends on availability of

compressed air

3 Peltier Cooler - No moving parts - Low efficiency

- Reliable - Limited in Size

- Used primary in spot cool-

ing

4 Natural convection-Passive

Cooled systems

No moving parts - Large space required

- High initial cost

5 Phase Change Materials-

Passive Cooled Systems

- No moving parts - May not function properly

if not correctly packed

- Less space required

6 DC powered Air Condition-

ers

- Higher cooling capacity

than any other system

- Moving parts, some kind of

maintenance is required

- Possibility to cool shelter

to human comfort level

-Large foot print required

for the solar panels

Documents

Cooling of Remote Sites Telecom Shelters