Evaluation of Scc

8/14/2019 Evaluation of Scc

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First Annual Civil Engineering Conference: CIVIL 2009

ESTIMATION OF SERVICE LIFE

FOR A SOLAR CHIMNEY-

COLLECTOR SYSTEM

A A Adedeji

Department of Civil Engineering, University

of Ilorin, Ilorin, [email protected] [email protected]
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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INTRODUCTION AND OVERALL AIMS

In order to construct and find the service life time of a solarchimney-collector used for enhancing ventilation in a room,the following objectives were put in place:

Internal microclimatic condition of the specimenroom was evaluated by measuring thetemperature of solar chimney-collector and theroom.

Appropriate properties of materials werecollected for the solar chimney-collector and thespecimen one-room building.

Applied test-retest reliability technique to accessservice life of the SCC unit.


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Stack Ventilation Principle

Stack ventilation is caused by stack pressure or buoyancyat an opening due to variation in air density as a result ofdifference in temperature across the opening.

The same principle has been applied for opening atdifferent height, where the difference in pressure betweenthem is due to the vertical gradient (windward and leeward

sides respectively).

Figure 1: Convectional Stack Ventilation Movement


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Advantages

Potential benefits regarding natural ventilation and use of solarchimney-collector include (Olayioye,2003):

Improved ventilation rates on still, hotdays

Reduced reliance on wind and winddriven ventilation Improved control of air flow though a

building envelopes

Greater choice of air intake (i.e. leewardside of building) Improved air quality especially in urban

areas

Increased night time ventilation rates


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Advantages contd.

Allow ventilation of narrow, small spaceswith minimal exposure to externalelements

Improved passive cooling during warmseason (mostly on still, hot days)

Enhanced performance of thermal mass(cooling, cool storage)

Improved thermal comfort (improved airflow control, reduced draughts)

It requires no major maintenance once itis installed.


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Limitations of stack driven

ventilation

Lower magnitude compared to windventilation

Relies on temperature differences

(inside/outside) Design restrictions (height, location of

apertures) and may incur extra costs(ventilator stacks, taller spaces)

The quality of air it introduces inbuildings may be polluted especially inindustrial areas


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METHODOLOGY

Construction

Pyrex glass was used for the modelbuilding wall with aluminium roofing sheet.For the SCC, ply wood used as aninsulator, transparent glass, aluminiummetal sheet (coated black) and the pipe(used as air outlet). Also used arechloroform and araldite (as adhesive). SeeFigure 2.

Properties of glass and aluminium also areshown in Table 1. Figures 3a and 3b showone-room building attached with SCC.


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Figure 2 Solar collector


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Table 1 Properties of plain glass and absorber

0.78-2.70.88Aluminium(absorber)

-0.922001.1Glass (Plain

glass)

AbsorbtivityReflectivityDensity

(Kg/m3)

Specific heat

(Kj/KgoC)Materials

Source: Olayioye, (2003)


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Solar

Chimney-

collector unit

Model room

Raft foundation

Digital multi-

meter

Figure 3a Chimney-collector attached to the room


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Vent pipe

Solar chimney

collector unit

Model room

Foundation

Figure 3b Solar chimney attached with the building


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Temperature Measurement

Digital multi-meters (Figure 4) were used to

measure the ambient, chimney chamber and

the rooms when windows were closed and

opened each for 5 weeks, between 10.00Hrs to 15.00 Hrs each day. The average

results are shown respectively in Figure 5.

Figure 4 Digital multi-meter


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0

5

1015

20

25

3035

40

45

windowsclosed

windowsopened

Dry(Ambient) C

Dry(Room) C

Dry(Chimney

before) C

Dry(After) C

R.H%

Figure 5 Average values of the ambient, room and chimney

temperatures and the Relative Humidity (%) for the rooms closed

and opened windows


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Reliability Analysis and Application

Constant failure rate (CFR) is employedhere for easy algebraic manipulation(Adedeji 2008, Leitch 1988, Trochim 2006)so that reliability of the components:

R = e-t/m (1)where t = period of time, m = expected averagenumbers of break down=2 assuming that all otheparts of the solar collector remain stable except

glass and fading of the black colour.

Average temperature intensity/week(window closed only) is shown in Table 2.


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3.000.8612.037.074.6298.5636325

3.90.9210.236.738.11123.9041344

2.570.5713.255.925.34185.8634323

2.160.5418.561.076.03118.2740332

1.860.632051.198.5692.9337321

CRCRCRCTRT

Rate per week

(d)

Temperature

intensity (Ri)

%

Energy (Q)

(W)

Average

temper-

ature (Tav

)oC

Week

RT, CT = Room and chimney temperatures respectively

Table 2 Average temperature /week (window

closed)


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The mean time to failure (MTTF) is theaverage functioning (without a failure)

period for an item or average life cycle ofa number of items is,

MTTF = 1/d (2)

Where d= temperature intensity/week.The lifetime failure probability P

lifein the

objective function which is thenminimized as,

Clife.min = Ci (t) + CfPlife (3)

Where Plife

= 1 - (R(t)) (4)

Ci, Cf = initial and life time (final) costs

resp.


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Results of Analysis and Discussion

Cost analysis The cost of constructing a standard room size of 3.6m x

3.6m x 3.0m using pyrex glass for the wall and the solarchimney-collector attached is about N 373,598.00

Projecting the amount for the same room size for

sandcrete block walls attached with the solar chimney-collector costs N2, 213,127.00. (June, 2009).

Reliability analysisResults showed that: at the maximum MTTF of 0.303 andthe lifetime failure probability Plife = 0.288.

for a closed window room, the average Reliability = 0.746, while the reliability of 0.711 was recorded for a room with

window opened.

At the reliability of0.746 for Plife = 0.254, servicelife of the Chimney-collector is estimated to be50yrs


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Contributors

D D Olatubosun - Designer/Builder H. K Yusuf GAs Application Analyst T A Owolabi Reliability Analyst


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Evaluation of Scc