Synergy in the Technical Development of Agriculture and Food Industry (Synergy2011)
Gödöllő, Hungary, 9–15. October 2011.
PERFORMANCE ANALYSIS OF PV/T, FLAT PLATE AND VACUUM TUBE
COLLECTORS
I. Kocsány1, I. Seres1
1
Department of Physics and Process Control, Szent István University
Páter K. u. 1., Gödöllő, H-2103, Hungary
Tel.:+36 28 522-000, E-mail: synergy@gek.szie.hu
Abstract: The examination of behaviour of the vacuum tube, flat plate (installed about ten
years ago) collectors (installed in 2009) and PV/T (installed in 2011) are performed in this
paper. Initially the solar system which was installed (2011) in the Department of Physics
and Process Control, Szent István University is presented. The aim of this work is to study
the behaviour of hybrid collector under different load test. The most important measured
parameters and settings of the experiments are introduced. Based on the measurement data
analysis was elaborated for understanding the behavior of different technologies.
Keywords: solar collector, operation of collectors, analysis, PV/T collector
INTRODUCTION
Among the solar energy application possibilities the solar thermal systems can reduce the consumption of
traditional energy resources. By using less energy obviously leads us to less pollution. Photovoltaic solar energy
is one of the renewable sources. As a result of developing the photovoltaic solar energy system the hybrid
photovoltaic/thermal system found, which combined both systems into one system, (Tripanagnostopoulos,
2007). The solar market has shown an effective 33% growth per year since 1997 until today (Hoffmann, 2006).
At the Department of Physics and Process Control, Faculty of Mechanical Engineering, Szent István University,
Gödöllő various solar applications were installed for educational, demonstrational and research purposes, such as
PV and solar thermal units (Fig.1.), transparent wall insulation and solar dryer unit.
Fig. 1. Solar systems at Department of Physics and Process Control
The examination of the PV/T (installed in 2011) is performed in this paper. Initially the solar-thermal system in
the Department of Physics and Process Control Szent István University is presented. In this paper the flat plate
PV/T collector design and analysis of measurements are performed. The solar energy technology has many
advantages and disadvantages comparing to the conventional energy. At first the potential advantages come
below:
•
low maintenance costs,
•
do not makes noise,
•
do not pollute the environment,
•
do not produce any toxic waste or radioactive material,
•
the system life cycle expectation is between 20 and 30 years,
•
It can be substitute the conventional energy.
The disadvantages:
•
large space needed for separate systems (hot water and electricity production),
•
high cost of the solar system installation,
•
long payback time period.
Structure of hybrid collector (PV/T)
Usually a simple PV module is converted the incoming solar radiation into electricity around 10-15%. The rest,
as unutilized energy is wasted what effects worsen efficiency of solar cells. On a typical summer day when the
ambient temperature is high and the sky is clear the solar panels could be heated to 60 - 70°C. Despite the high
solar radiation because of the degradation of the efficiency, higher power utilization cannot be achieved. The
back of PV/T collector is used as a thermal layer which collecting the heat and lead it away so the solar layer
cools down.
Fig. 2. Structure of flat plate photovoltaic-thermal (PV/T) collector
The construction of the flat plate PV/T collector is shown in Fig.2. A complete flat plate PV/T collector should
have composed of a glass cover, solar cells, insulation, copper splitter and absorber plate underneath. The
absorber plate plays important function in PV/T system. It cools down the PV cell or module, simultaneously
collecting the thermal energy produced in the form of hot water or hot air. PV/T collector can be designed
variety etc. air PV/T collector, water PV/T collector, combination of water/air PV/T collector depending on type
of working fluid used. Aste et al. (2008) noted that, amongst all types of PV/T solar collectors, the most widespread PV/T collector which is working with air. Despite of this type of collector has less application compared
to the water collectors. The collector’s glass is made by low iron and tempered solar glass, which optimally
utilized over the 700 nm wavelength of incoming radiation. The nominal electric power of the PV/T applied in
the experiment is 170 W.
MODELS OF PV/T COLLECTORS
The PV/T system can be segregated into two parts; the thermal solar technology what converted the solar energy
into heat, and the photovoltaic technology which derived from solar cell technology and convert the solar
radiation into electricity (Zondag, 2008). The hybrid technology can reduce the main problem of photovoltaic
systems: the high temperature of the solar cell effects reduction in the efficiency. However high solar radiation,
which is necessary for the solar panels to produce more energy, naturally causes high temperature in the cell. To
solve the problem flat plate hybrid collector has developed, which produces both thermal energy (by cooling
back the module) and electricity at the same time. Many types of research were made to analysis the
characteristic of the solar-thermal equipment some of the models described below.
Numerical models
Chow (2003) developed an explicit dynamic finite difference approach model for a single glazed PV/T collector.
The model can create results for hourly performance analysis, including immediate thermal/electrical benefits
and efficiencies. Zondag et al. (2002) developed a numerical model predicting the performance of PV/T
collectors. The model was included two types of models 3D (three dimensional) dynamic and steady state (3D,
2D and 1D) models. It was concluded that for the determination of the efficiency curves the simple steady state
1D model performs well and dynamical effects is very small.
Analytical models
Sopian et al. (1996) analysed the performance of single and double-pass PV/T air collectors with steady-state
models. The results showed the double-pass PV/T air collectors have significant differences in efficiency
compared to the single-pass ones, thermal and combined efficiencies and packing factor are not directly
proportional. Packing factor is defined as the ratio of the photovoltaic cell area to absorber area.
Florschuetz (1979) developed the Hottel–Willier (1958) analytical model of flat plate collectors to made
analytical calculations on PV/T collectors. Bergene and Lovvik (1995), recommended a detailed model based on
energy transfer analysis and to some extent on the models for flat plate solar collectors presented by Duffie and
Beckman (1991) which predicting the performance of PV/T collectors. The model estimated PV/T efficiency
(thermal + electrical) to be about 60–80%.
Experimental work
Sandnes and Rekstad (2002) developed an analytical model for the hybrid system by modifying the previously
mentioned Hottel -Willier (1958) model. The PV/T collector was constructed by single-crystal silicon PV cells
onto a black plastic absorber. They interpreted that to reduce the heat losses to the environment a glass cover is
needed to the PV/T collector. The energy absorption was also reduced by reflection (around 10%) from the glass.
Fig. 3. Thermal efficiency curves from experimental results for collectors configurations: thermal absorber (T),
photovoltaic/thermal absorber (PV/T) and hybrid absorber with glass cover (PV/Tg). Sandnes and Rekstad
(2002).
Picture above (Fig. 3.) shows the thermal efficiency curves for the three configurations: photovoltaic/thermal
absorber (PV/T), thermal absorber (T) with no glass and photovoltaic/thermal absorber with glass (PV/ Tg). The
graph below (Fig. 4.) shows the simulated cell temperature and photovoltaic power output for the PV/T system,
the PV/Tg system and the PV module without thermal insulation. From this graph it is clearly seems that the
black thermal absorber (T) absorbs more efficiently the incoming radiation, covering the absorber with PV cells
(PV/T) reduces the converted energy and the heat loss coefficient.
Fig. 4. Simulated cell temperature and PV power output for a clear summer day for the PV, PV/T and PV/Tg
configurations. Sandnes and Rekstad (2002).
MEASUREMENTS AND RESULTS
The operation (energy production, efficiency, etc.) of the vacuum tube (VT), PV/T or flat plate (FP) collectors
are highly dependent on the environmental circumstances (irradiation, spectra, ambient temperature). To check
the working of the different types, measurements were carried out.
The inclination angle of the absorber surfaces are tilted 45 degrees and south-facing surface according to the
Hungarian conditions. Due to the difference of geometry, under the same incoming radiation and the same
incidence angle (between the beam radiation on a surface and the normal to the surface) the solar collectors
showed a different efficiency, useful area.
The test parameters of the developed system on the department are recorded with a data logger computer. The
most important measured parameters are:
 The temperature of inlet and outlet solar fluid for each collector type,
 Solar liquid volume flow rate,
 Solar radiation,
 front side temperature of PV/T module,
 Voltage and power (for the PV/T).
The efficiency of flat plate and vacuum tube collectors is different (Fig. 5), caused by several reasons e.g.
different convective heat loss, different shape (active surface), different spectral sensitivity, etc. For the
determination of the useful (active) surface of the collectors a geometrical model was developed. Based on the
measured data, the rate of absorbed radiation is principally depends on the angle of incident radiation.
Fig. 5. Efficiency of vacuum tube and flat plate collector
In the literature it can be read that the efficiency of the PV modules is dependent on the module temperature. The
PV/T module made it possible to check this dependence with the next experiment. The PV/T module was let to
warm up to about 50 °C, and in between the electrical output was measured (voltage and current) on a load.
During the test the hybrid collector worked on a separate liquid circle, where the water was cool down by a
special cooling system (Fig. 6.) to around 10 °C. By this way PV/T module temperature was dropped and the
power change were measured.
Fig. 6. External cooling system
It can be seen from the following diagram (Fig. 7.), that the about 10 °C of drop in the module temperature
caused about 1 percent of efficiency increase. It is important to notice, the PV module was not operated at its
MPP during the measurement.
The dependence of the PV/T efficiency on the module temperature
13,8
13,6
13,4
13,2
13
12,8
y = -0,0593x + 16,269
R² = 0,9606
12,6
12,4
12,2
45
50
55
60
65
70
Fig. 7. Dependence of the PV/T efficiency
CONCLUSIONS
This paper has presented a review of design configurations of flat plate PV/T collector. In the frame work it can
be concluded that the hybrid flat plate collector can be used to overcome the overheat problem of the PV system,
because the thermal collector part is working as a cooling system. Based on measurement data it can be
concluded that about 10 °C temperature drop on the collector cause 1 percent efficiency increase. On the other
part of the paper the comparison of the efficiency of the vacuum tube and flat plate collectors are provided. From
the data it can be seen that the efficiencies are depending on the working circumstances and changing a lot
during the day.
ACKNOWLEDGEMENTS
Research was supported/subsidized by the OTKA K 84150 project.
REFERENCES
Aste N, Chiesa G, Verri F. (2008), Design, development and performance monitoring of a photovoltaic-thermal
(PVT) air collector. Renewable Energy, vol. 33, pp. 914–27.
B. Sandnes, J. Rekstad. (2002), A photovoltaic/thermal (PV/T) collector with a polymer absorber plate.
Experimental study and analytical model. Solar Energy, vol. 72 (1), pp. 63–73
H.C. Hottel, A. Willier. (1958), Evaluation of flat-plate solar collector performance. Transactions of the
Conference on the Use of Solar Energy, vol. 2, University of Arizona Press, Tucson, Arizona,.
Hoffmann W. (2006), PV solar electricity industry: market growth and perspective. Solar Energy Materials and
Solar Cells, vol. 90, pp. 3285–311.
J.A. Duffle, W.A. Beckman. (1991), Solar engineering of Thermal Processes. Second edition. John Wiley and
Sons Inc., New York.
K.S. Sopian, H.T. Yigit, H.T. Liu, S. Kakac, T.N. Veziroglu. (1996), Performance analysis of
photovoltaic/thermal air heaters. Energy Conversion and Management, vol. 37 (11), pp. 1657–1670.
L.W. Florschuetz. (1979), Extension of the Hottel–Whillier model to the analysis of combined
photovoltaic/thermal flat plate collectors. Solar Energy, vol. 22 (4), pp. 361–366.
T. Bergene, O.M. Lovvik. (1995), Model calculations on a flat plate solar heat collector with integrated solar
cells. Solar Energy, vol. 55 (6), pp. 453–462.
Tripanagnostopoulos Y. (2007), Aspects and improvements of hybrid photovoltaic/thermal solar energy systems.
Solar Energy, vol. 81(9), pp. 1117–31.
T.T. Chow. (2003), Performance analysis of photovoltaic–thermal collector by explicit dynamic model. Solar
Energy, vol. 75 (2), pp. 143–152.
Zondag HA. (2008), Flat-plate PV-thermal collectors and systems: a review. Renewable and Sustainable Energy
Reviews, vol. 12, pp. 891–959.
Zondag H.A., D.W. Vries, W.G.J. Van Hendel, R.J.C. Van Zolingen, A.A. Van Steenhoven. (2002), The thermal
and electrical yield of a PV– thermal collector. Solar Energy, vol. 72 (2), pp. 113–128.
Zondag H.A., D.W. Vries, W.G.J. Van Hendel, R.J.C. Van Zolingen, A.A. Van Steenhoven. (2003), The yield of
different combined PV–thermal collector designs, Solar Energy, vol. 74 (3), pp. 253–269.