1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN IN –
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH 0976
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME
ENGINEERING AND TECHNOLOGY (IJARET)
ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 4, Issue 7, November-December 2013, pp. 01-09
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IJARET
©IAEME
EXERGY ANALYSIS OF A SINGLE-ENDED GLASS DIRECT FLOW
EVACUATED TUBE SOLAR COLLECTOR
Hamza Al-Tahaineh1, Rebhi Damseh2
1,2
Department of Mechanical Engineering, A-Huson University College,
Al Balqa Applied University, Irbid, Jordan.
ABSTRACT
Exergy analysis for a single ended glass evacuated tube solar collector system was carried out
in this investigation. The second law of thermodynamics was used to obtain the net exergy, exergy
destructed, and exergetic efficiency of the Evacuated Tube Solar Collector (ETSC) system.
According to the mean solar insolation in Jordan and assumptions of calculation in specific region
around the year, the results obtained show an exegetic efficiency of 65.88 % which seems to have a
steady value despite the increase in the temperature difference of water in and out of the collector.
Keywords: Second Law of Thermodynamics, Exergy, Evacuated Tubes, Solar Systems.
INTRODUCTION
Evacuated tube solar collectors have been commercially available for over 20 years;
however, until recently they have not provided any real competition to flat plate collectors. In order
to investigate the flow structure and heat transfer within the tube, extensive experimental
Investigations have been done on cylindrical open thermosyphon with various tube aspect ratios,
heating schemes and Rayleigh numbers. Extensive numerical modeling has been done for a number
of Years. A numerical model of the inclined open thermosyphon has been developed using a finite
difference algorithm to solve the vorticity vector potential form of the Navier-Stokes equations the
geometry considered was an open cylinder, inclined at 45° to the vertical. Steady flow is simulated at
various combinations of Rayleigh number, aspect ratio and mode of heating. Two heating schemes
were used, uniform wall heating and differential wall heating [1-3].
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2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME
S.K. Tyagi et al 2007 evaluated the exergetic performance of concentrating type solar
collector and the parametric study was made using hourly solar radiation. The exergy output is
optimized with respect to the inlet fluid temperature and the corresponding efficiencies were
computed. The performance parameters were found to be the increasing functions of the
concentration ratio but the optimal inlet temperature and exergetic efficiency at high solar intensity
are found to be the decreasing functions of the concentration ration [4].
I. Jafari et al 2011 investigated energy and exergy of air-water combined solar collector
which is called dual purpose solar collector (DPSC). Analysis is performed for triangle channels.
Parameters like the air flow rate and water inlet temperature are studied. Results are shown that
DPSC has better energy and exergy efficiency than single collector. In addition, the triangle passage
with water inlet temperature of 60 oC has shown better exergy and energy efficiency [5].
Michel Pons 2012 investigates the main types of exergy losses that can be identified in solar
collector systems in order to minimize the source of exergy losses and maximize the solar energy
benefits [6].
The objective of the present investigation is to analyze the evacuated tube solar system from
the second law of thermodynamics point of view in order to improve the system performance by
investigating the operating conditions where the exergy destruction become minimum and the
exergetic efficiency maximum.
EXERGY ANALYSIS OF EVACUATED TUBE SOLAR COLLECTOR
Exergy is the maximum amount of work that can be obtained from a stream of matter, heat or
work as it comes into equilibrium with a reference environment. The term "exergy" or absolute
energy efficiency is also used to define the combination of energy quantity (which is conserved
according to the first law of thermodynamics) and energy quality (which is consumed according to
the second law of thermodynamics).Thus, (Exergy = Energy Quantity × Energy Quality).
The general rate form of exergy balance equation is given by:
•
•
•
X 42 4
out
1in − X 3
Rate of net Exergy transfer
through the collector
•
− 1destroyed = ∆ X system
X 4
42 3 1 4
42 3
Rate of exergy
destructio n
(1)
Rate of change
of exergy
The exergy carried by the evacuated tube is given by the following relation:
•
•
(2)
X in = η col Q
Where:
•
X in : The rate of exergy transfer to the collector by heat (W)
ηcol : Collector efficiency.
The exergy destroyed is another expression for the system irreversibility (I) which is the
difference between the heat input and the useful heat obtained by the solar collector ; i.e.: I = X
destroyed . System irreversibility which could be also expressed as the system heat losses and it is
divided to the tank heat loss and tube heat loss.
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3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME
For a real process the exergy input always exceeds the exergy output, this unbalance is due to
unbala
irreversibilities (called exergy destruction Xdesroyed). The exergy output consists of the utilized output
and the non-utilized exergy of waste output. This latter pan we entitle the exergy waste Xdesteryed. It is
utilized
very important to distinguish between exergy destruction caused by irreversibilities and exergy waste
due to unused exergy flow to the environment both represent exergy losses, but irrever
irreversibilities have,
by definition-no exergy and no environment effects[7].
no
effects
The exergy destruction (system irreversibility, ) is related to the entropy generation by
system irreversibility
by:
•
•
•
I = X destroyed = To S gen
(3)
Where To is the environment temperature and Sgen is the entropy generation and governed by the
following equation:
•
•
S gen
Q  Tsur 
1 −
 (W K )
=
Tsur  Tsys 


(4)
Where:
•
Q : Useful energy gain from the collector (W).
Tsur: surrounding temperature (equal ambient temperature, Ta= 20 oC).
Tsys: system temperature.
Substituting equations (2), (3) into equation (1) yields in:
•
η col Q
13
2
Rate of net Exergy transfer
by heat
•
•
− To S gen = ∆ X system
1 4
42 3
1 3
2
Rate of change
Rate of exergy
destructio n
(5)
of exergy
Where the first component of the left hand side of equation (5) is the efficiency of the
collector
which was modeled experimentally by Budihardjo as a function of ambient
temperature (Ta), average film temperature of inlet and outlet water temperatures of the tube
,
and global solar irradiance at the collector plane (G) as a second order equation [3]:
(
ηcol
(T
= 0.58 − 0.9271
f
− Ta
G
) − 0.0067 (T
f
− Ta
)
2
(6)
G
TUBE EXERGETIC EFFICIENCY
Exergy efficiency of the solar collector can be defined as the ratio of increased mass exergy
to the exergy of the solar radiation, in other word; it is a ratio of the useful exergy delivered to the
exergy absorbed by the solar collector [7,8].
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4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME
The final expression for exergy balance in the solar collector will be:
•
•

 T 
T 
&
I = Q1 − sur  − mc p (Tout − Tin ) − Tsur ln out 
T 
 T 
sys 
 in 


(7)
The exegetic efficiency (ηП) of an evacuated tube solar collector system is given by the
following relation [7,8]:
•
•
ηΠ =1 −
X
destroyed
•
X
in
= 1−
T sur S gen

 1 − T sur

T sys

(8)
 •
Q


Where:
Xdestroyed: Exergy destructed or destroyed.
INVESTIGATION APPARATUS AND SETUP
The results of the current study was obtained by investigated a 20 single-ended evacuated
tubes with specifications shown in table (1). The tubes were connected directly to a horizontal
storage tank mounted over a diffuse reflector plate, Collector inclination: 45º, Tube aspect ratio
(length/diameter):1500/34, Absorber diameter: 37 mm, Inter-tube spacing: 70 mm.
Each evacuated tube consists of two glass tubes made from extremely strong borosilicate
glass. The outer tube has very low reflectivity and very high transmisivity that radiation can pass
through. The inner tube has a layer of selective coating that maximizes absorption of solar energy
and minimizes the reflection, thereby locking the heat. The ends of the tubes connected to the copper
header are fused together and a vacuum is created between them.
Figure 1: Evacuated tubes solar collector connected directly to a horizontal storage tank
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5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME
Table 1 Evacuated Tube Basic Specifications
Length
1500 mm
Outer tube diameter
47 mm
Inner tube diameter
37 mm
Glass thickness
1.6 mm
Thermal expansion
3.3x10-6 oC
Material
Borosilicate Glass 3.3
Absorptive Coating
Graded Al-N/Al
Absorptance
93%
Emittance
7% (100oC)
Vacuum
P<0.005 Pa
Stagnation Temperature
>200oC
Heat Loss Coeff.
<0.8W/ ( m2 oC )
Tube Life
>15 years
RESULTS AND DISCUSSION
To analyze the thermal data, a simplified model was proposed, based on the following
assumption: Ambient air temperature 20 ºC, hot water supply to the household 70°C. The hot water
is defined that water having a temperature equal to 40oC or exceeds. The convention is to rise the
cold water temperature in the water heating systems 50oC, i.e. if the cold water temperature 5oC (like
in winter) it will rise to 55oC, while the cold water temperature will not exceed 20oC the decision to
rise its temperature 50oC to become 70oC was determined to avoid the formation of Calcium
sedimentations [1].
Figure 2: Sunshine and solar radiation in Amman [8]
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6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME
Figure (2) show the amount of incident radiation at the location of investigation, 32o north
latitude, around the year. The peak insolation was found to be at June with a maximum value of solar
insolation 28.32 (MJ/m².day) while the minimum was found to be 9.87 (MJ/m².day) at December.
The average values and trend of solar insolation were found to be constant for different years.
1000
900
X_in
X_destoyed
800
E
xerg (W
y )
700
600
500
400
300
200
100
0
0
10
20
30
40
50
60
Temperature Difference (Tout-Tin)
Figure 3: Variation of Exergy Input and Exergy Destructed with Temperature difference
The net useful exergy is the difference between transfer exergy (as input exergy, Xin) and the
exergy destructed due to irreversibility and entropy generation (Sgen). Figure (3) show that the net
useful exergy decreases with increase in water temperature difference (Tout-Tin) and this is due to
increase in entropy generation with temperature since the amount of heat transfer to the surrounding
(ܳሶ ) will increase.
Figure 4: Variation of thermal and exergetic efficiencies with collector temperature difference
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7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME
Figure (4) show that while the thermal efficiency of the collector under specified condition
decrease with temperature difference, the exergetic efficiency start to increase until it reach a steady
value (o.66) at a temperature difference ሺܶ௢௨௧ െ ܶ௜௡ ሻ ؆ 50 Ԩ after which the exergetic efficiency
become almost constant. This behavior means that the exergy destruction starts to decrease with
temperature difference until it reach its lowest value after which no more destruction in exergy. The
exergetic efficiency was found to be constant around the year for the same region and the same
temperature difference and its value around (0.66).
0.60
Present Work
0.58
Gang Pei Work (2012)
ETC Therm Efficiency
al
0.56
0.54
0.52
0.50
0.48
0.46
0.44
0
10
20
30
40
50
60
70
80
Temperature Difference (Tout-Tin)
Figure 5: Comparison of ETC thermal efficiency of present work with Gang work [9]
0.80
0.70
E e e E ie c
x rg tic ffic n y
0.60
0.50
0.40
0.30
0.20
Present Work
Gang Pei (2012)
0.10
0.00
0
10
20
30
40
50
60
70
80
Temperature Difference (Tout-Tin)
Figure 6: Comparison of exergetic efficiency of present work with Gang work [9]
From figures (5) and (6), when comparing the results of present investigation with Gang
result [9], it was found that while the thermal efficiency of the both ETC’s show the same trend there
was some difference in the exergetic efficiency at low temperature differences and this may be
explained by higher loss in exergy in gang model which was avoided in the present model .As the
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8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME
temperature difference increase above 50 oC the exergetic efficiency of present investigation show
good agreement with Gang. This means that despite the model and conditions used for investigation
of ETC the exergetic efficiency was found to be at its maximum steady value at a temperature
difference above 50 oC and all ETC show higher destruction in exergy at lower temperatures.
CONCLUSIONS
Carrying out a detailed exergy analysis for a single ended glass evacuated tube solar collector
system to show the effect of temperature difference (Tout-Tin) of the collector on the net exergy,
exergy destructed, and exergetic efficiency of the Evacuated Tube Solar Collector (ETSC) system.
The analysis was carried out based on the mean solar insolation in Jordan and assumptions of
calculation in specific region around the year. Based on the results of the analysis carried out, one
can conclude the following:
•
•
•
•
The exergetic efficiency of the ETC seems to be steady with temperature difference
especially at higher values while the thermal efficiency decreases with increasing temperature
difference.
Most of the system exergy destroyed were from the tubes since it has high heat loss
coefficient (~0.8 W/m2.K). For larger number of tubes the losses will be bigger.
The exergy destroyed increases when the temperature difference between the system and the
surrounding increases i.e. when (Sgen) increases.
The ETC show good exergetic efficiencies at higher temperature difference, i.e. at higher
energy collected and stored through the system.
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[6]
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