This presentation may good for solar engineering.

1. Chapter 4: performance of Solar
Collector
1
Kandahar University
Engineering Faculty
Energy Department
Instructor: Senior teaching asst. Eng Agha Mohammad
Prepared by: Zamir Fatemi

2. Table of content
1. Introduction
2. Collector Thermal Efficiency
3. Collector Incidence Angle Modifier
4. Concentrating Collector Acceptance Angle
5. Collector Time constant
6. Dynamic System Test Method
7. Collector Test Results And Preliminary Collector
Selection
8. Quality Test Methods
2

3. Introduction
 The thermal performance of solar collectors can
be determined by the detailed analysis of the
optical and thermal characteristics of the
collector materials and collector design.
 To perform the required tests accurately and
consistently, a test ring is required. Two such
rings can be used closed and open loop collector
test rings
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4. Cont. …
4

5. Cont.
5

6. Cont.
 Important parameters :
1. Global solar irradiance at the collector plane, Gt.
2. Diffuse solar irradiance at the collector aperture.
3. Air speed above the collector aperture.
4. Ambient air temperature, Ta.
5. Fluid temperature at the collector inlet, Ti.
6. Fluid temperature at the collector outlet, To.
7. Fluid flow rate, m.
6

7. Cont.
 In addition, the gross collector aperture area, Aa,
is required to be measured with certain accuracy.
 The collector efficiency, based on the gross
collector aperture area is given by:
7

8. 4.1 Collector Thermal Efficiency
 The collector performance test is performed
under steady-state conditions, whit steady
radiant energy falling on the collector surface, a
steady fluid flow rate, and constant wind speed
and ambient temperature.
 The useful energy gain from the collector is
calculated from
 We know from last chapter
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9. Cont.
 The thermal efficiency is obtained by dividing Qu
by the energy input (AaGt):
 The beam radiation is normal incidence, thus the
is used.
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10. Cont.
 For concentrating collectors, the following
equations from Chapter 3 can be used for the
useful energy collected and collector efficiency:
 Notice that, in this case, Gt is replaced by GB,
since concentrating collectors can utilize only
beam radiation (Kalogirou, 2004).
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11. Cont.
11
FR , and UL are nearly constant under
steady state

12. Cont.
Stagnation point: is Intersection with the
horizontal axis.
And
Low radiation level
High temperature of fluid inlet
That heat losses equal solar absorption
In such case the collector delivers no useful heat
 And occur when no fluid flows.
12

13. Cont.
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14. Cont.
 The heat loss coefficient, UL, is not constant but
is a function of the collector inlet and ambient
temperatures. Therefore,
 Applying above Eq in last Eqs, we have the
following.
For flat-plate collectors.
and for concentrating collectors.
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15. Cont.
 Therefore the efficiency can be written as:
 For flat-plate collectors.
and for concentrating collectors.
if
thus
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16. Cont.
4.1.1 Effect of Flow Rate
When the flow rate is changed during the use from
test the FR is:
F´ is assumed constant.
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17. Cont.
4.1.2 Collector in series:
 If N panels of the same type are connected in
series and the flow is N times that of the single
panel flow is used during the testing then the
single panel performance data can be applied K
is:
 The correlation factor is:
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18. Cont.
EXAMPLE 4.1
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19. Cont.
4.1.3 Standard Requirements
Here, the various requirements of the ISO
standards for both glazed and unglazed collectors
are presented.
Glazed collectors
according to ISO 9806-1:1994,certain environmental
conditions are required (ISO, 1994):
1. Solar radiation greater than 800 W/m2.
2. Wind speed must be maintained between 2 and 4
m/s. If the natural wind is less than 2 m/s, an
artificial wind generator must be used.
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20. Cont.…
3. Angle of incidence of direct radiation is within ±2% of the
normal incident angle.
4. Fluid flow rate should be set at 0.02 kg/s-m2 and the fluid flow
must be stable within ±1% during each test but may vary up to
±10% between different tests.
5. To minimize measurement errors, a temperature rise of 1.5K
must be produced so that a point is valid.
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21. Cont…
Unglazed collectors:
The same requirements are used like for a glazed
collector.
 Using a solar simulator
For countries which don’t have stubble whether they
use solar simulator. They are two types:
Point source large area multiple
lamps
The simulator characteristics required are
1. Mean irradiance 50 W/m2
2. Radiation at any point must not differ 15% from
the mean radiation.
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22. Cont.
3. The spectral distribution of 0.3 and 3 µm
equivalent to air mass 1.5.
4. Thermal irradiance should be less than 50 W/m2.
5. As in multiple lamp simulators, the spectral
characteristics of the lamp array change with time,
and as the lamps are replaced, the characteristics
of the simulator must be determined on a regular
basis.
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23. 4.2 Collector incidence angle
modifier
 The incidence angle modifier, Kθ, is defined as the
ratio of (τα) at some incidence angle θ to (τα)n at
normal incidence.
 4.2.1 Flat plate collector:
If we plot the incidence angle modifier against
,It is observed that a straight line is obtained, which
can be described by the following expression:
23

24. Cont.
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25. Cont.
 4.2.2 Concentrating Collectors
For concentrating collectors the Kθ when the fluid inlet
teprature is equal with ambient air temperature is:
And efficiency is
where is the measured efficiency at the
desired incident angle and,
with being the normal optical efficiency
25

26. Cont.
26

27. 4.3 Concentrating collector acceptance
angle
collector acceptance angle is the range of
incidence angles (as measured from the normal to
the tracking axis) in which the efficiency factor
varies by no more than 2% from the value of
normal incidence.
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28. 4.4 Collector time constant
 The time constant of a collector is the time
required for the fluid leaving the collector to reach
63.2% of its ultimate steady value after a step
change in incident radiation.
28

29. Cont.
29

30. Cont.
30

31. 4.5 Dynamic system test method
 For location that do not have steady environmental
conditions for long periods of time, the transient or
dynamic system test method can be used.
 In the dynamic system test method test period is
much shorter and can be conducted at any time of
the year under variable weather conditions.
 The test data are measured every 5-10 min.
 For a glazed collector the :
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32. 4.6 Collector test results and
preliminary collector selection
 Collector testing is required to evaluate the
performance of solar collectors and compare
different collectors to select the most appropriate
one for a specific application.
 Collector efficiency curves may be used for
preliminary collector selection.
 Efficiency curves illustrate only the instantaneous
performance of a collector.
 The collector performance equations can also be
used to estimate the daily energy output from the
collector.
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33. Cont. …
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34. Cont.
To select a collector :
 Define the application (water heating, space
heating and etc…)
 Define the thermal properties of applied area and
application (Tin, Tou, Ts, especially the highest
and lowest points and m)
 Select economically
 Select less area collector and lightweight
 Essay understood buy user and essay respire
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35. 4.7 Quality test methods
 The constructed materials of the collector should
be able to withstand
Effect of circulating fluid (corrosion, scale deposits,
etc.)
Adverse effects of the sun’s ultraviolet radiation
Cyclic thermal operation many times a day
Extreme operating conditions (freezing,
overheating, thermal shocks, external impact due to
hail or vandalism, and pressure fluctuations)
For quality tests, the operate the temperature
is stagnation temperature.
35

36. Cont.
1. Internal pressure test
2. High temperature resistance test
3. Exposure test
4. External thermal shock test
5. Internal thermal shock test
6. Rain penetration
7. Freezing test
8. Impact resistance test
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37. Cont.
4.7.1 Internal pressure test:
 The absorber is pressure tested to assess the extent
to which it can withstand the pressures it might meet
in service.
 The test pressure should be 1.5 times the maximum
collector operating pressure specified by the
manufacturer and should be maintained for at least
one hour.
 For air-heating collectors, the test pressure is 1.2
times the maximum collector operating pressure
difference above or below atmospheric pressure, as
specified by the manufacturer, maintained for 10 min.
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38. Cont.
38
4.7.2 High temperature resistance test:
This test is for withstand at high irradiance levels without
failures such as glass breakage, collapse of plastic
cover, melting of plastic absorber, or significant deposits
on the collector cover from out-gassing of the collector
material.
The temperature equal to stagnation temperature. The test
is for a minimum of one hour after a steady state is reached.

39. Cont.
4.7.3 Exposure test:
 The exposure test provides a low-cost indication
of the aging effects that are likely to occur during
a longer period of natural aging.
 An empty collector is mounted outdoors and all of
its fluid pipes are sealed to prevent cooling by
natural circulation of air except one pipe, which is
left open to permit free expansion of air in the
absorber. the collector is exposed until at least 30
d (which need not be consecutive) have passed
with the minimum irradiation shown in Table 4.7.
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40. 4.7.5 External Thermal Shock Test
Collectors from time to time may be exposed to
sudden rainstorms on hot, sunny days, causing a
severe external thermal shock. This test is intended
to assess the capability of a collector to withstand
such thermal shocks without a failure.
This test is under high level of solar irradiance for a
period of 15 minute.
An empty collector is used as in previous test.
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41. Cont.
4.7.5 Internal Thermal Shock Test
 Some times the collectors sudden intake of cold
heat transfer in hot sunny day, and cussing a
severe internal thermal shock.
 This test is used to assess the capability of a
collectors to withstand such thermal shock with
out familiar. An empty collector is used as in
previous test. And the same reference conditions
given in table 4.7 can be used.
 The heat transfer fluid must have a temperature
less then 25C.
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42. 4.7.6 Rain Penetration
 This test is intended to assess the extent to which
collectors are substantially resistant to rain
penetration.
 The collector must be sprayed on all sides using
spray nozzles or showers for a test period of 4 h.
 For this test, the inlet and outlet fluid pipes of the
collector must be sealed, and they must be
placed in a test rig at the shallowest angle to the
horizontal recommended by the manufacturer.
Otherwise is 45° to the horizontal or less.
 For collectors that can be weighed, weighing
must be done before and after the test. For
collectors that cannot be weighed, the penetration
of water into the collector can be determined only
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43. Cont.
4.7.7 Freezing
 This test is intended to assess the extent to which
water-heating collectors that are claimed to be
freeze resistant can withstand freezing and freeze-
thaw cycles.
 Shallowest angle is which recommended by
manufacture otherwise is 30.
1. Filled with water, kept at operation pressure for 10
minutes and then drained.
2. The content of the observer are maintained at -20
±2°C for 30 min.
3. And raised to above 10°C during thawing cycle for43

44. Cont.
4.7.8 Impact Resistance Test
 This is an optional test which is intended to
assess the extent to which withstand the effects
of heavy impacts.
 The collector is mounted either vertically or
horizontally.
 A 150g steel ball is used simulate the heavy
impact.
 The point of impact must be no more than 5 cm
from the edge of the collector cover and no more
than 10 cm from the corner of the collector cover,
the steel ball is dropped 10 times.44

45. Thank
you!!!45