the thermal performance of a solar air heater - Eurotherm 2008

5th European Thermal-Sciences Conference, The Netherlands, 2008
THE THERMAL PERFORMANCE OF
A SOLAR AIR HEATER
J. Deans, A. Weerakoon
The University of Auckland, Auckland, New Zealand
Abstract
The use of commercial roofing panels to harvest the solar radiation received by a building wall is
becoming a common design feature of modern buildings. This energy is used to provide
background air heating within the building and experience has shown that the ambient air which
flows through an array of holes pierced in the roofing panel can extract more than 60% of the
incident solar radiation. The air passing through the panel flows into the air gap behind the panel.
In this heating system the panel and the gap behind it effectively form a single stream heat
exchanger with variable mass flow. The air warmed by the panel is transferred to the building’s
heating system.
The simplicity of the heat exchanger design masks a number of coupled heat transfer processes
which can make the panel design unduly complicated. This study describes the experimental and
analytical investigations which were focussed on the evaluation of these processes. A large solar
heated panel was used in most of the experimental studies while both finite difference and finite
volume models were used in the analysis. The investigation of the heat and momentum flows on
either side of the panel, using the CFX finite volume model, has enabled new insights to be gained
about the heat transfer mechanisms between the panel and the air.
1 Introduction
The use of commercial roofing panels to extract some of the solar energy impinging on a building’s
surface and to use this energy to heat the air circulating within the building has been actively
investigated. One such system, commonly known as unglazed transpired solar collectors was
originally developed by Conserval Inc.,[2002] and marketed by them using the Solarwall name
[Hollick, 1996]. The arrangement of this collector is shown in Figure 1(a) where the solar heated,
metal panel has an array of small holes, pierced through the panel to form a permeable surface. The
air being heated by the panel flows through the holes and into the air gap behind, this air is collected
from a manifold at the top of the air gap and directed towards the suction fan. This fan powers the
complete system by drawing ambient air from the front of the solar collector, through the air gap,
before delivering it to the building’s heating system.
The simplicity of the panel’s design masks a number of complex heat transfer processes and a
summary of these processes are shown in Figure1 (b). Solar radiation heats the dark metal panel
and the low thermal resistance of this panel suggests that energy will be transferred by convection
to the air flowing on both sides of the panel. The air approaching the front of the panel will flow
radially towards each hole and that fraction of the flow which is adjacent to the panel’s outer
surface will be convectively heated. The increasing air flow behind the panel will also be
convectively heated and this transport process will be enhanced by the jets of warm air discharging
from the back of the panel. The main energy losses from this heat exchanger develop at the outer
panel surface and these are surface re-radiation and the surface cooling by wind. The geometry of
the panel profile and the variation in the rates of solar radiation give added complexity to the
analysis of this heat exchanger. The efficiency of energy transfer from the collector to the air
flowing through it is generally high (≈ 60%); Rhee and Edwards [1981] attributed this to the low
convective and radiation loses from the front of the collector.

5th European Thermal-Sciences Conference, The Netherlands, 2008
PANEL
Solar Radiation
Radiation Loss
Wind Loss
AIR GAP
Increasing
Mass Flow
Rate
Internal
Convection
External
Convection
(a) Physical layout.
Figure 1 Arrangement of a transpired solar collector.
(b) Mass and energy flow routes.
The performance of transpired solar collectors has been studied by Kutscher [1993, 1994], Van
Decker [2001], Gunnewiek [1996, 2002] and Arulanandam [1999] who have conducted a wide
range of experimental and computational studies. A major concern with most of these studies is
that the test geometries modelled tend to over simplify the problem and consequently, limit the
solution. The test rig developed by Kutscher, for example, was only a small laboratory based
collector which used a flat panel and no air gap. The exclusion of the air gap from this model
inherently assumes that all of the energy transfer between the panel and the air will take place from
the front of the panel. These studies generally demonstrate that the thermal performance of the
transpired collector is dependent on the diameter of the hole through the panel, the distance between
holes, the panel thickness, the suction flow rate of the air and wind speed. The model developed to
describe the interaction of these parameters assumed that the holes effectively “harvested” the
thermal boundary layer from the outer surface of the panel. Both Gunnewiek [1996, 2002] and Van
Decker [2001] did however recognise that heat transfer from the panel to the air would also develop
within the air gap but neither had the test data required to develop this alternative method.
The models developed by Kutscher [1993, 1994] and Van Decker [2001] do have wide recognition
but Fleck [2002] has observed that “it is quite clear from our observations that such a model is a
poor descriptor of the physical phenomena driving UTSC (Unglazed Transpired Solar Collector)
performance”. The paucity of test information from operational unglazed transpired solar collectors
is surprising since many operating systems have been commissioned, the only studies on large
collectors are those published by Carpenter [1991] Fleck [2002] and Weerakoon 2006]. The test
programme of Weerakoon [2007] examined the performance of transpired solar heated collectors
constructed from standard size roofing panels and was directed towards the determination of the
effects which the main design variables had on the panel’s performance.
2 Experiment set-up
The main test rig was constructed to accommodate a standard 2.0m * 0.8m corrugated metal roofing
panel that could be used as the solar collector. The complete test facility is shown in Figure 2
where the panel was mounted to create a small uniform, sealed gap between the panel and the wall
behind. The air which migrated through the panel flowed up this gap and its depth was normally set
at 20 mm. The orientation of the vertical panel could be changed but was usually north facing and
located on the exposed roof of a 20 m high building.

5th European Thermal-Sciences Conference, The Netherlands, 2008
Radiometer
Space bars to
create cavity
Outlet air manifold
Solar
panel
Weather
shield
Solar
panel
Thermocouple
points
Wheels
Data logger
Back wall
Linear flow element
Ballast
Exhaust air
Blower fan
(a) Front of the test rig.
(b) Schematic rear view of the test rig.
Figure 2 The Test Facility.
The air transferred through the panel was collected in the outlet air manifold shown in Figure 2(b)
before it was ducted to a laminar flow element and then to a suction fan before return to the
environment. A differential pressure transducer within the laminar flow element measured the air
flow rate through the system and calibrated platinum resistance thermometers measured both the
ambient and the manifold outlet air temperatures. The incident solar radiation (İ solar ) on the metal
sheet was measured using an Eppley precision radiometer and the surface temperature of the panel
was monitored using an Agema Thermovision 570 infrared camera.
The transpired solar collectors were constructed using 0.8mm gauge, galvanised steel, roofing
panels. The 1.6 mm diameter holes through the panels were created using a hand drill and the panel
profiles used in this study are shown in Figure 3. The external colours of the panels are described
as iron sand and ivy green; the emissivity value for both panels was determined to be 0.88.
The thermometers were calibrated using a reference thermometer traceable to ITS-90. The error in
the thermometer readings is believed to be less than ±0.05 o C. The calibrated charts supplied by the
manufacturers were used to determine the air flow rate and the level of solar radiation, the
uncertainty in both of these was less than 3%. The uncertainty associated with derived values such
as heat flow rates, is also estimated to be 3%. The accuracy of the infrared camera was ± 1.0 o C, but
its ability to determine temperature gradients was much lower.
Apriori information about the overall heat transfer coefficient for the collector panel enabled its
stabilisation period to be determined and thermal response rate calculated. In most tests the test
period lasted for 60 mins and the results were not accepted during the first 15 minutes. All of the
tests described below were conducted on days when there were few, if any, clouds and between the
hours of 10.00 am and 2.30 pm.
(a) Styleline Panel. .(b) Corrugated Panel. (c) Plane Panel.
Figure 3 Types of collector panel’s.

5th European Thermal-Sciences Conference, The Netherlands, 2008
3 Test Results
The experimental programme was structured to identify the influence which various design
variables have on the collector’s performance. The general results presented in Figures 4 and 5
show the influence of panel porosity, air gap depth and air suction flow rate on either the overall
performance of the collector or the energy transferred to the air. The influence which these
variables have on the on the design of the collector tend to support the argument that it is effectively
a one stream heat exchanger.
The temperature increase for the air stream passing through the panel is shown in Figure 4 where
this increase is measured against the incident solar radiation and the panel’s porosity. These results
were obtained for a range of collector panel porosities with a constant air mass flow rate of 0.029
kg/m 2 s (based on a panel surface area) and with average wind speeds in the range 1.0 to 2 m/s. In
these tests the porosity of the panel was increased by decreasing the pitch between holes. If the
collector efficiency of the panel is defined as the ratio of energy extracted by the air stream and the
radiant energy received by the panel then this value is effectively constant for the range of solar
radiation measured during these tests. The relative unimportance of porosity is significant, since the
studies conducted by Kutscher [1993, 1994] and Van Decker [2001] correlated the size and the
spacing of the holes to the performance of the transpired solar collector.
The effects which the air mass flow rate has on the performance of the plate are shown in Figure 5
where the collector efficiency, increase in air temperature and the depth of the air gap are examined.
These measurements were obtained using either an ivy-green coloured corrugated panel or a plane
ironsand coloured panel. The panel’s performance is measured in Figure 5(a) as either the increase
collector efficiency or a reduction in the air temperature increase when the air mass flow rate
through the collector increases. This relationship evolves because of the reduction in thermal losses
from the collector when the average air temperature and the panel surface temperatures are reduced.
The coupling of these three variables permits the design of the panel to be optimised. Some
confidence in the results in Figure 5(a) can be gained by extrapolating the air mass flow rates down
to the values measured by Fleck (2002) who reported collector efficiencies in the range 28% to 32%
when the air flow rate was 0.01 kg/m 2 s.
A plain perforated panel was used in the tests which examined the influence of air gap depth behind
the panel; this choice was made to ensure a well defined minimum value. The results given in
Figure 5(b) show that thermal efficiency of the collectors used in these tests was not influenced by
changes in the air gap when its depth is increased from 5 mm to 40 mm. Most of the commercial
designs [www.solarwall.com] for transpired solar collectors have air gaps which are in the range 20
mm to 40 mm.
Air Temperature Increase [ o C]
20.0
16.0
12.0
8.0
4.0
Ironsand coloured Styleline panel
Wind speed 1 - 2 m/s
Air Flow rate 0.029 kg/m 2 s
0.0
400.0 500.0 600.0 700.0 800.0
Solar Radiation [W/m 2 ]
0.2% porosity
0.4% porosity
0.8% porosity
Figure 4 Air temperature rise in an iron sand coloured Styleline panel.

5th European Thermal-Sciences Conference, The Netherlands, 2008
Collector Efficiency [%]
100.0
80.0
60.0
40.0
Ivy Green corrugated panel
20.0
0.4% porosity
Wind speed < 1m/s
4.0
Solar radiation 750 - 780
0.0
2
0.0
0.0150 0.0180 0.0210 0.0240 0.0270 0.0300
Mass Flow Rate [kg/m 2 s]
20.0
16.0
12.0
8.0
Increase in Air Temperature [ o C]
70
60
50
40
30
20
10
5 mm gap
20 mm gap
40 mm gap
Plane Panel
Porosity 0.2%
Wind speed < 1 m/s
Solar Radiation 700 W/m 2
0
0.01 0.015 0.02 0.025 0.03
Mass Flow Rate (kg/m 2 s)
(a) Change in collector efficiency and outlet
(b) Change of collector efficiency
air temperature with air flow rate.
with gap depth and air flow rate.
Figure 5 Collector efficiency.
4 Energy Balance for the Collector
The surface temperatures of the panel during each test were obtained from infrared photographs and
Figure 6 shows typical thermal images obtained for an ivy green solar panel with 0.4% porosity.
These images, obtained at three different test conditions, show that the panel’s external surface
temperature is rarely uniform and that it generally decreases with increasing height. It should also
be recalled that the panel has a low thermal resistance and that the mass flow rate up the air gap also
increases with height. A comparison of the photographs given in Figures 6(a) and 6(b) shows that
the panel’s surface temperature increases when the air flow rate is decreased. When the results in
Figures 6(a) and 6(c) are compared it can be concluded that increasing the wind speed over the
panel will also decrease its surface temperature. The results from this comparison confirm that both
the exit air temperature and consequently the panel’s thermal efficiency will decrease with
increasing wind speed. It should be noted from these images, that although the panel receives a
constant heat flux, it is apparent that neither its external surface temperature nor the energy flux
absorbed by the panel is constant. The low thermal resistance of the panel implies that this
approach can also be extended to the air flow in the gap behind the panel and to conclude that
neither constant heat flux nor wall temperature can be assumed in the heating of this air.
Temperature
isotherm
Panel conditions (a) (b) (c)
Air mass flux 0.029 kg/m 2 s 0.019 kg/ m 2 s 0.029 kg/m 2 s
Solar radiation (İ solar ) 780 W/m 2 760 W/m 2 780 W/m 2
Wind speed 1.5 m/s 1.5 m/s 2.5 m/s
Figure 6 Thermal images for an ivy green solar panel.

5th European Thermal-Sciences Conference, The Netherlands, 2008
It has been widely recognised (e.g. Kutscher [1993, 1994]) that the transpired panel and the air gap
behind it form a single stream heat exchanger, with variable mass flow. Weerakoon (2007) has
shown analytically that the air mass flow rate into the air gap behind the panel increases linearly
with height for all of the panel geometries tested. The information presented in Figure 1(b) showed
that the energy transferred to the panel surface is given by the summation of the absorbed solar
energy, the re-radiated energy and the convective losses from the front of the panel. All of the
components in the energy transfer are dependant on the surface temperature and the images
presented above showed that this temperature varied with height. If it is assumed that the energy
absorbed to the panel at any particular height is used to increase the local enthalpy of the air, in the
air gap, it is then possible to generate a stepwise process to calculate the local energy balance and
determine the local air temperature within the air gap. An element in this procedure is shown in
Figure 7a. In developing this procedure the surface of the panel and the volume of the air gap
behind it are sub-divided into a number of horizontal elements and a mass (equation 1) and energy
balance (equation 2) for each element is produced in a stepwise procedure. The mass of ambient air
added to each element is known and the air temperature leaving the last element can be compared
with measured values.
⎛ •
4 4 ⎞
Q element = ⎜ I solar h loss (T s _ element T amb ) (T s _ element Tamb
) ⎟
α −
− − ε σ −
× Aelement
(1)
⎝
⎠
( m + m )C T = m C T + m C T + Q (2)
in
add
p
air
out _ element
in
p
air
in _ element
add
p
air
amb
element
Typical values for the derived temperature profiles for the air, in the air gap, are shown in Figure
7(b) and were derived using equations 1 and 2. Within this calculation all of the initial values were
measured; the surface temperature for each element (T s element ) was obtained using the images from
the infra red camera and the external heat transfer coefficient (h loss ) was determined from an overall
energy balance. The measured temperatures for the air stream at three locations within the air gap
are also shown in this Figure 7(b). It is apparent from these results that the air temperature within
the air gap increases rapidly in the lower part of the gap when the air flow rate is relatively small.
The air flowing up this gap can attain 75% of the exit air temperature within the first 0.3m of its
height. The form of the temperature profile shown in Figure 6(a) demonstrates that the energy
transfer in the upper parts of the air gap is dominated by the mixing of the two air streams.
m in + m add
T out_element
1
m add
T amb
I solar
T s_element
A element
m in
T in_element
0.8
0.6
0.4
0.2
0
Air Flow Rate Insolation
0.013 kg/m 2 s 770 W/m 2
0.019 kg/m 2 s 760 W/m 2
0.023 kg/m 2 s 750 W/m 2
0.029 kg/m 2 s 740 W/m 2
Corrugated panel, 0.4% porosity
Wind speed 2.0 -2.5m/s
0 0.5 1 1.5 2
Distance from bottom of panel (m)
(a) Element from the model.
(b) Calculated and measured airstream
temperatures within the air gap.
Figure 7 Finite difference model.

5th European Thermal-Sciences Conference, The Netherlands, 2008
This conclusion can be further substantiated if the local value for the efficiency (η) of the energy
transfer process in each element is considered. This efficiency is defined as ratio of energy
transferred to that received by radiation in each element and is shown by equation 3
element
η
element
=
(3)
•
I
Q
solar
A
element
The analysis shows that the derived values for the efficiency increases with distance from the base
of the panel for all mass flow rates. This increase when the temperature of the panel decreases and
the temperature of the air increases further suggests that convection is not the dominant energy
transfer process. The importance of the mixing process within the air gap was also recognised by
Belusko and Saman [2006], who studied a modified version of the solar air heater described above.
Weerakoon [2007] has also examined the performance of these collectors using a finite volume
CFD package (ANSYS CFX, version 5.7.1). A range of numerical models which encompassed the
region in front of the panel, the gas space and a panel with 1, 2, 3, 5 or 10 holes were constructed.
The models examined the energy transfer mechanisms which develop with increasing air mass flow
rates through the panel. During the commissioning of these models their prediction were compared
with experimental values and Figure 8(a) shows the comparison of experimental values of collector
efficiency with extrapolated values from the numerical models. The close agreement in these
results gives confidence to the predictions from the numerical models. The information presented
in Figure 8(b) shows the vectorial velocity profiles which develop before and after the panel which
has a 20mm air gap. In this particular condition, the hole is near the base of a panel which has an
equivalent mass flux of 0.0133 kg/m 2 s and the momentum of both the air moving up the gap and
the air leaving the hole are low. Figure 7(b) shows that in these conditions there will be a rapid
increase in the air gap temperature and the results given in Figure 8(b) shows that this increase is
primarily due to the mixing of the two air flows. The localised nature of the air flow entering the
hole is also shown in Figure 8(b); very little of this air has contact with the external wall of the
panel.
70
60
50
40
30
Eperimental
One Hole Model
Two Hole Model
20
Plain Panel
Three Hole Model
No Wind
10 Porosity 0.2%
Solar radiation 555 W/m 2
0
0 0.005 0.01 0.015 0.02 0.025 0.03
Air Mass Flow Rate (kg./m 2 s)
(a) Overall performance.
(b) Air flow on both sides of the panel.
Figure 8 Predictions from the CFD models.

5th European Thermal-Sciences Conference, The Netherlands, 2008
5 Conclusions
This experimental study of a transpired solar collector has shown that mixing of the warm air in the
air gap behind the panel is the controlling heat transfer mechanism. This mechanism was not
considered in earlier studies. The important design variables that influence the performance of the
collectors were shown to be the suction flow rate and the incident solar radiation. The suction flow
rate is the only controllable parameter that can be used to promote either greater energy transfer from
the transpired collector or to obtain a larger exit air temperature rise. The interaction of these
variables showed that the collector effectively performed as a one stream heat exchanger with
variable mass flow. The experimental investigation confirmed that transpired solar collectors have
the potential to transfer about 60% of the incident solar radiation.
References
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