LOW COST IMPROVEMENTS TO BUILDING INTEGRATED AIR COOLED
HYBRID PV-THERMAL SYSTEMS
Y. Tripanagnostopoulos, Th. Nousia and M. Souliotis
Department of Physics, University of Patras, Patra 26500, Greece
Tel/Fax : +30 61 997472, e-mail : yiantrip @ physics.upatras.gr
ABSTRACT : Hybrid PV-Thermal systems can be integrated on building façade and inclined roof, instead of plain PV
modules, providing simultaneously electricity and heat. Heat extraction by air circulation is considered a simple PV cooling
mode, by which PV electrical efficiency can be kept at a satisfactory level, building undesirable heating can be avoided during
summer and space heating of building can be achieved during winter. Aiming to low cost improvements of building integrated
air cooled hybrid PV-Thermal systems we tested experimental models consisting of pc-Si PV modules and air channel at PV
rear surface, regarding system inclination, air channel depth, air channel opposite surface emissivity and heat extraction modes.
A water cooled PV module, a PV module with back insulation and a PV module with both surfaces free to ambient were also
tested in order to compare their performance with that of the air cooled PV systems. From the results we estimate that simple
heat extraction modes as the roughened opposite channel surface and the placing of thin metallic sheet inside air channel can be
considered low cost system improvements.
Keywords : Building integration – 1: Thermal performance – 2: Cost reduction - 3
1.
INTRODUCTION
The absorbed and not converted into electricity solar
radiation increases the temperature of building integrated
PV modules, resulting to the reduction of PV efficiency
and to the increase of undesirable heat transfer to the
building, mainly during summer.
Air cooled hybrid PV-Thermal systems usually
consist of PV modules with air channel at their rear
surface. Air of lower temperature than that of PV modules
is circulating in the channel and both PV cooling and
thermal energy output can be achieved. By air cooling, PV
electrical efficiency is kept at a sufficient level and thermal
energy output can be used for building thermal needs.
Several studies are referred to theoretical and
experimental results of hybrid PV-Thermal systems.
Among them S. Hendrie [1] and P.Raghuraman [2] give
results for air cooled systems, by which a maximum
thermal efficiency of about 35% is achieved. A.K.
Bhargava et al [3] and J. Prakash [4] present results
regarding the effect of air mass flow rate, air channel
depth, air channel length and fraction of absorber plate area
covered by solar cells (packing factor) on single pass and
K. Sopian et al [5] on single and double pass hybrid PVThermal system performance.
Modeling and experimental work for the assessment
of the energy efficient impact of PV integration on building
façade is presented by J.J. Bloem and H.A. Ossenbrink [6],
with given results from the analysis of the air flow in the
air gap behind the PV panel in the hybrid façade of the
building by B. Moshfegh et al [7]. In addition, a parametric
study of PV cladding for facade and roof of buildings is
given in the work of B J. Brinkworth et al [8].
In our laboratory we are studying hybrid PV-Thermal
systems with water or air as heat removal fluid. In this
paper we present results from air cooled hybrid PVThermal experimental models, which are used to test some
modes of performance improvement, regarding building
façade or inclined roof integrated systems. The
experimental systems consist of pc-Si PV modules with air
channel at PV rear side, properly constructed giving
flexibility in changing of air channel depth and in placing
some elements inside air channel to test their effect on
system performance.
We used two systems, considering one (PV/S) for
testing of several changes in air channel and the other as
reference, (PV/R) for direct comparison under same
weather and operation conditions. The systems were tested
in vertical and in inclined position with forced air flow. In
addition, we performed tests with three more systems, a
water cooled PV module (PV/W), a PV module with back
thermal insulation (PV/T), and a PV module, which both
surfaces are free to ambient (PV/F), in order to compare
their performance to that of the air cooled PV systems.
Aiming to the achievement of optimized air cooled
hybrid PV-Thermal systems in electrical efficiency,
thermal efficiency and cost, we tested them regarding air
channel depth (5,10,15 cm), air channel opposite surface
emissivity (low ε, high ε) and heat extraction modes (fins,
tubes, flat sheet, etc.).The tests were performed outdoors
and the necessary collected data were used to determine the
electrical and thermal performance of the systems.
2.
EXPERIMENTAL MODELS
The experimental models were consisted of pc-Si PV
modules of small size (1.0 m * 0.45 m). An air channel at
PV rear surface was added in two of the models to form the
air cooled hybrid PV-Thermal systems, PV/S and PV/R.
The reference system PV/R was consisted of air channel
with 15 cm depth.
In Fig. 1 we show the cross section of PV/R model
(which is similar to PV/S). The systems were thermally
insulated and during tests air was circulating through the
formed air channel. In all performed tests the system with
the modifications (PV/S) was tested in comparison with the
reference system (PV/R) for the same operation conditions
(position mode, weather conditions), but with several
changes in the air channel (air channel depth, addition of
fins, tubes, flat sheet).
We used a pyranometer to measure the incoming
solar radiation on PV plane, air flow meter to measure air
flow rate and thermocouples to measure temperature at
several points on the devices (front and rear PV surface,
input and output of air, opposite channel surface, ambient
temperature, etc ).
Figure 1: Cross section of the tested system.
Regarding the additional three experimental PV
systems (PV/W, PV/T, PV/F) the water cooled PV system
consists of a thermally insulated heat exchanger, placed at
the rear surface of the PV module. We used copper tubes in
thermal contact with a flat copper sheet, for the
construction of the heat exchanger and water from mains to
extract heat from PV module.
We used the PV/W system in comparative tests in order
to have the lower limit of PV temperature increase during
operation, as cold water from mains was circulating in the
pipes of the heat exchanger decreasing PV temperature.
System PV/T corresponds to usual building integrated
PV modules (on facade or inclined roof), with significant
increase of PV temperature during operation, because of
the low thermal losses from the rear PV surface.
System PV/F was used as reference PV module,
regarding PV operating temperature, because thermal
losses from both PV surfaces keep PV temperature at a
lower level than that of system PV/T.
3.
Therefore, the fin-plate element of 1.5 cm size was
costructed by using aluminum Π profiles on aluminum
sheet and the fin-plate element of 4 cm size was
constructed by using galvanized iron Π profiles on
galvanized iron sheet. We used the fin-plate elements, with
their metallic surface to test the effect of low ε surface to
system thermal performance and also painted them black,
corresponding to high ε fin surface.
Heat transfer from rear PV surface to circulating air
can be increased by using metallic cylindrical tubes
installed along the air channel and pressed by PV rear
surface and opposite wall surface. In Fig. 3 we show the
cross section of the system, which includes two tubes of 15
cm diameter each, constructed by using thin (0.5 mm)
aluminum sheet. We used two tubes instead of three, in
order to have low weight, cost and pressure drop.
The used fin-plate elements of 1.5 cm and 4 cm size,
as well as the two tubes of 15 cm diameter, result to an
increase of 100% the additional heat exchanging surface
area, considering both PV rear and opposite cavity wall
surface areas, of the air channel without any modifications.
PV
Figure 2: Cross section of the tested system with fin
plate element.
PV
HEAT EXTRACTION MODIFICATIONS
In air cooled hybrid PV-Thermal systems, the heat
extraction from PV modules depends on air channel depth,
air flow mode, air flow rate and other parameters of the
heat exchanging mode. On the other hand, a high efficiency
in heat extraction is usually achieved by a high air flow
rate, which increases the pressure drop of the circulating
air. This effect reduces the net electrical energy gain from
solar cells, as an increase of the electrical input is necessary
for the operation of the air pump.
Aiming to low cost heat extraction modifications we
used fins attached on the opposite air channel (of 15cm
depth) surface, considering it a cheaper mode than placing
them on the PV rear surface. Fins of 1.5 cm and 4 cm of Π
profile were used to form fin-plate elements, with their flat
vertical surfaces along to air flow, increasing the heat
exchanging surface area in the air channel.
In Fig. 2 we show the cross section of the tested
system, with Π profiles for the formation of fins. The finplate elements can be commercially fabricated by an
aluminum industry, mainly for fins of small size, while for
fins of large size we considered that they could be
fabricated with lower cost by using galvanized iron sheets.
Figure 3: Cross section of the tested system with
metallic cylindrical tubes.
Considering that PV modules are installed on building
façade or inclined roof surfaces, a simple mode for PV
cooling by heat extraction increase is to use these surfaces
as opposite surfaces of the formed air channel, which can
be properly roughened. By these roughened walls, a low
cost increase of heat exchanging surface area can be
achieved, as it can be made by using façade or roof
constructive materials. Simulating this type of air channel
modification, we tested the system by using roughened
opposite channel wall surface, estimating that an increase
of about 100% of heat exchanging surface area was
achieved.
Additionally to the above described heat extraction
modifications we used a flat thin metallic sheet placed in
the middle of the air channel and parallel to the PV rear
surface and also the opposite cavity surface. In Fig. 4 we
show the cross section of the tested system with this
modification. We used thin (0.1 mm) aluminum sheet,
which is of lower cost compared to fins and tubes,
regarding same additional heat exchanging surface area
inside air channel. Flat sheet in the middle of the air
channel increases convection heat transfer to the circulating
air, estimating that it also reduces heat transfer to the
opposite channel wall.
A combination of the flat sheet in the middle of the air
channel with roughened opposite channel wall surface can
be also considered a low cost modification, which was
formed for testing.
4.
RESULTS AND DISCUSSION
4.1
System position
The intensity of solar radiation on PV surface is a very
important parameter regarding PV temperature increase.
We tested two main system positions, vertical and inclined,
corresponding to building façade and inclined roof PV
integration modes.
During experiments we recorded solar radiation
intensity Ir of about 400 – 500 Wm-2 for the vertical
position and of about 700 – 900 Wm-2 for the inclined
system position. From the results we observed that for
ambient temperature Ta = 30 o C, the PV temperature TPV
was recorded during noon in the range of 40 – 45 oC for
vertical position and in the range of 50 – 60 oC for the
inclined position.
The corresponding TPV values for the systems PV/W,
PV/T and PV/F were : 35–40 oC , 45–50 oC and 35-40 oC
for vertical position and 40-45 oC ,60-70 oC and 45-55 oC
for inclined position, respectively. These values show that
the addition of heat extraction modes to the decrease of PV
temperature can be considered more significant for the
inclined system position (higher values of Ir and TPV) and
rather less cost effective for vertical system position (lower
values of Ir and TPV).
4.2
Air channel parameters
The air channel depth and the opposite cavity surface
are important in our study. The opposite cavity surface
corresponds to the external surface of the wall façade (or
the external surface of the inclined roof) of the building
and the temperature of it (TW) must be kept as low as
possible during summer to minimize air conditioning needs
of the building.
The channel depth affects not only the pressure drop
of air flow but also the TW , as there is a heat transfer from
PV rear surface to the opposite channel surface. Testing air
channel of system PV/S with depth of 5,10 and 15 cm, we
observed that the rise of PV temperature TPV was lower for
larger channel depth by 1-2 oC for the vertical position and
by 2-3 oC for the inclined position. The same thermal
performance was also observed for opposite cavity surface
temperature TW.
Regarding opposite cavity surface emissivity, we used
in the reference system PV/R aluminum sheet, to
correspond to low emissivity surface, with black painted
opposite surface in the system PV/S to correspond to high
emissivity surface. During all tests and for circulating air
through system channel, the high emissivity opposite
surface resulted to a decrease of TPV by 1-3 oC and almost
same increase of TW, of system PV/S, with higher
temperature value for 15 cm channel depth.
Figure 4: Cross section of tested system with flat
metallic sheet in the middle of air channel.
4.3 Heat extraction modes
PV temperature depends strongly on the ambient
temperature and incoming solar radiation. The results from
the three additional PV systems showed that PV water
cooling (with circulating water of about 22 oC through the
heat exchanger attached on PV rear surface) achieves a
minimum PV temperature of about 35 oC for vertical and
of about 40 oC for inclined position, with Ta=30 oC. It is
obvious that by using air of 25 oC, 30 oC or 35 oC for PV
air cooling, the minimum obtained TPV values cannot be
lower than those by using water, and this determines the
limits of air PV cooling during summer.
The used metallic fins (Fig. 2) of 1.5 cm and 4 cm size,
resulted to increase the heat extraction by the circulating
air. A small decrease of PV temperature by 1-3 °C was
achieved for fins of 1.5 cm size and we also observed a
same temperature increase of TW. For fins of 4 cm size the
corresponding system performance was a TPV decrease of
2-4 °C and almost same increase of TW.
The used tubes (Fig. 3) of 15 cm diameter performed
better than fins regarding TPV, TW and circulating air
heating. A decrease of 2-3 °C for both temperatures was
observed in system PV/S compared to the reference system
PV/R and an additional temperature increase of 1-2 °C for
air heating was achieved, because the exchanger surface
area of tubes was in thermal contact with PV rear surface.
The tested system with roughened channel opposite
surface performed almost similar to system with small size
fins, achieving a TPV decrease of 1-2 oC for wall surface of
high emissivity with same increase of TW.
The most promising modification was the placing of a
thin flat metallic sheet in the middle of the air channel and
along the circulating air (Fig. 4). Wall surface is usually of
high emissivity, so the sheet surface facing the wall can be
of low or high emissivity, but the surface facing the PV
rear surface must be of high emissivity, in order to be
achieved a temperature decrease of PV module.
Test results showed that the used sheet in the middle of
the air channel of PV/S reduces both TPV and TW by about
2-3 oC each and rise circulating air temperature by more
than 1-2 oC, compared to PV/R. The installation of the flat
sheet inside the air channel of the hybrid PV system can be
considered cost effective in material and installation and is
suggested as a low cost improvement of these systems.
A more efficient combination was also tested, which
included the use of the flat sheet with the roughened
opposite air channel surface. By this system a better
thermal performance was achieved with more than 2-3 °C
air temperature increase of heating air and a 2-3 °C
decrease of TPV and TW.
Thermal and electrical system performance
Air cooling of the hybrid PV – Thermal system
results to a higher PV electrical efficiency. The suggested
modification (flat metallic sheet in the middle of the air
channel) increases thermal (nth) and electrical (nel)
efficiency. As an example we present in Table 1 the values
of nth and nel as well as TPV and TW with the measured
values of Ta and Ir for vertical and inclined system
operation. The measured values of the corresponding
parameters of systems PV/W, PV/T and PV/F under same
operating conditions are also presented for comparison.
The results show the better performance of system PV/S
compared to system PV/R. Also we can see that system
PV/W is the most efficient compared to the other tested
systems.
•
•
4.4
Table 1: PV temperature (TPV), wall temperature (TW),
thermal (nth) and electrical (nel) efficiency for
steady state operation in vertical and inclined
position of the systems
vertical
TPV °C
TW °C
nth %
nel %
Ta °C
Ir Wm-2
PV/R
44.9
32.0
35.3
11.2
30.2
481
PV/S
43
30.6
38.3
11.3
30.2
481
PV/W
39.2
51.3
11.6
30.2
481
PV/T
49.5
10.7
30.2
481
PV/F
39.8
11.5
30.2
481
inclined
TPV °C
TW °C
nth %
nel %
Ta °C
Ir Wm-2
PV/R
55.4
37.4
35.5
10.0
32.1
810
PV/S
51.2
35.2
38.6
10.5
32.1
810
PV/W
45.2
53.6
11.1
32.1
810
PV/T
68.3
8.7
32.1
810
PV/F
49.3
10.7
32.1
810
5.
•
•
•
ACKNOWLEDGEMENTS
This paper includes a part of the work of Building
IMPACT Joule project,. JOR3-CT98-0308-DGXIIWSMN., funded by E.C. which is gratefully acknowledged
by the authors.
REFERENCES
[1]
[2]
[3]
[4]
[5]
CONCLUSIONS
The main conclusions from test results are the
following:
•
Air PV cooling has lower cost than water PV cooling,
but it also has less efficiency in electrical and thermal
performance.
•
High air flow rate increases the thermal output of the
air cooled hybrid PV systems, but the electrical input
by the fun (to overcome the increased pressure drop)
reduces the net electrical gain.
•
Wide air channel depth results to a lower pressure
drop, reducing the needed electrical input by the fun,
but decreasing the thermal output of the system.
Opposite air channel surface of high emissivity
increases the thermal and the electrical output of the
system, but does not avoid the undesirable heat
transfer from PV modules to the building.
The use of fin and tubes is effective in thermal and
electrical system performance, but the additional cost
and pressure drop must be also taken under
consideration.
The use of opposite cavity roughened surface is an
improvement of minimum cost, which increases
slightly thermal and electrical system output.
The installation of a thin flat metallic sheet in the
middle of a wide (15 cm) channel and along air flow
is a low cost improvement that increases thermal and
electrical system output.
System improvements in heat extraction are more
effective in the case of inclined position, because solar
radiation intensity has higher values resulting to
higher PV temperature increase.
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