Exergy based performance analysis of hybrid solar collectors
I. Farkas1* and I. Kocsany2 and I. Seres3
1 Szent Istvan University, Department of Physics and Process Control, Pater K. u. 1., Gödöll , H-2103 Hungary
2 Szent Istvan University, MTDI, Physics and Process Control, Pater K. u. 1., Gödöll , H-2103 Hungary
3 Szent Istvan University, Department of Physics and Process Control, Pater K. u. 1., Gödöll , H-2103 Hungary
*Farkas.Istvan@gek.szie.hu
Abstract
Flat plate collectors and PV modules are the most commonly used equipment of solar energy. The solar market has shown an effective 33% growth per year since 1997 until today. As a result of developing the photovoltaic solar energy system the hybrid photovoltaic/thermal system was developed. This paper introduced the advances and disadvantages of hybrid photovoltaic collector.
After explain about detailed efficiency analysis, the further section is focused on analytical models of water PV/T collector. Initially the solar system which was installed 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 behaviour of different technologies. Preliminary standards and evaluation of performance reviewed. A thorough exergy and energy specifying are studied. In this overview operation and behaviour of water PV/T collector under different circumstances were examined. There are many modulus which influenced the performance of the solar equipment.
Keywords: Thermal exergy efficiency, electrical exergy efficiency, performance of PV/T collector, advances, disadvantages
1. Background of PV/T collector
At the Department of Physics and Process Control, Szent István University, Gödöll various solar applications were installed for educational, demonstrational and research purposes, such as PV and solar thermal units, transparent wall insulation and solar dryer unit. The term PV/T refers to solar thermal collectors that use PV cells as an integral part of the absorber plate [1]. 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. The hybrid collector can reduce the main problem of photovoltaic systems, the high temperature of the solar cell effects reduction in the efficiency. In order to solve that problem and get more efficiently system it is necessary to cool the PV cell and decrease its temperature. To solve the problem -which is not an easy task especially in hot and humid climate areas- flat plate hybrid collector has developed, it is produces both thermal energy (by cooling back the module) and electricity at the same time. Thus, the PV/T solar collector gives opportunity to growing the overall solar efficiency and provides a better way utilizing solar energy.
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. 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, for a PV/T
module, the solar irradiation with the wavelength over the 700 m is absorbed by the PV cells and converted into electricity, during the rest irradiation is transformed into thermal energy.
2. Calculation method
The PV/T, combining PVs into the solar thermal module, indicates a new direction for renewable energy utilizing. The examination of 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 performed.
The solar energy technology has many advantages and disadvantages comparing to the conventional energy. The potential advantages of PV/T collector is the low maintenance costs, 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. On the other hand large space needed for separate systems (hot water and electricity production), high cost of the solar system installation, long payback time period, PV module needs system to hold the cell temperature (i.e. cooling system) are the disadvantages.
The PV/T module can collect solar energy at different brands of the spectrum and lead to energy and exergy efficiency [2]. Sum of the collector’s thermal efficiency and the PVs’ electrical efficiency gives the overall efficiency. Hybrid collector efficiency is defined as the ratios of useful system heat gain and electricity gain to the incident solar irradiation on the collector’s absorbing surface [3], and is written as follow:
e th
o " ! . (1)
The PV/T module can collect solar energy at different brands of the spectrum and lead to energy and exergy efficiency. Sum of the collector’s thermal efficiency ( th) and the PVs’ electrical efficiency ( e) gives the overall efficiency (Eq. 1). Hybrid collector efficiency is defined as the ratios of useful system heat gain and electricity gain to the incident solar irradiation on the collector’s absorbing surface.
2.1. Efficiency of PV/T collector
Efficiency of hybrid collector is given by a thermal ( th) and an electrical part ( e) in this period the separate theory of efficiency determined.
2.1.1. Thermal efficiency
The steady state thermal efficiency ( th) of a traditional flat plate solar collector is calculated by:
I Qu
th " (2)
By calculating the specific heat (Qu) the useful collected heat from solar radiation could be determined as the multiplied by mass flow rate (m), capacity of flowing medium (Cp) and temperature differences of working fluid (Ti – inlet and To- outlet temperature of working fluid).
) ( o i
p
u mC T T
Q " $ # (3)
The difference between the heat losses and the useful solar radiation:
% &
'
L(. p,m a) e(
c
u A I U t t Q
Q " )* # # # (4)
In Eq. 4 a more complex determination of specific heat can be seen where Ac is the surface of the collector,)* is the transmittance-absorption effort of glazing cover, UL is the overall thermal loss coefficient, Qe electrical energy generated from the PVs, tp,m is the absorber plate temperature.
Hottel and Willer developed the equation of flat plate collector [4]. Because it is hard to measure or calculate the temperature of absorber plate as it is a complicated function of incident solar radiation, different collector geometry and attribution of working fluid. In the literature many detailed analysis let us to get information about process of hybrid solar collector. Hottel et al, instead of the mean absorber temperature (tp,m) used the fluid inlet temperature (ti). Afterwards that correlation used in evaluating and designing the different type of solar collectors. It is important to notice that the equations are correlated to the solar collector formation (as cylinder or rectangular shape). Some geometrical parameters in the equations could vary correspondingly if the collector shape is changed meanwhile the basic work principle of the collector remains the same.
% &
'
.L( i a) e(
c R
u F A I U t t Q
Q " )* # # # (5)
The unknown parameters are in the Eq. 5 can be determined as: FR is the heat-removal factor, ta is the ambient temperature.
2.1.2. Electrical efficiency
The ratio of incident solar radiation to measured output power (Po) gives the electrical efficiency ( e) of a PV module is.
c
e IA
P0
" (6)
As it is mentioned the cells’ temperature is influenced the electrical efficiency of the PV module, it is decreasing and this dependence can usually be written as [6]:
% &
'
PV PV rc(
rc
e" 1#+ t #t (7)
Where the rc is the initial electrical efficiency at reference temperature, +PV is the cell efficiency temperature coefficient, the tpv is the PV cell temperature and trc is the reference temperature of PV cell.
% &
800 823 20
218! #
!
" NOCT
K C
T
Tc amb f t . (8)
The generated electrical energy can be calculated as follows:
c e
e P IA
Q " 0" (9)
3. Models of PV/T collectors 3.1. Analytical models
Sopian et al. [7] 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 [8] developed the Hottel–Willier analytical model of flat plate collectors to made analytical calculations on PV/T collectors. Bergene and Lovvik [5], 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 which predicting the performance of PV/T collectors. The model estimated PV/T efficiency (thermal + electrical) to be about 60–80%. Sandnes and Rekstad [9]
developed an analytical model for the hybrid system by modifying the previously mentioned Hottel - Willier 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. 1. Simulated cell temperature and PV power output for a clear summer day for the PV, PV/T and PV/Tg configurations [9]
The graph above (Fig. 1.) show 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 result it is clearly seems that covering the absorber with PV cells (PV/T) reduces the converted energy and the heat loss coefficient.
4. Evaluation of technical performance 4.1. Overall energy efficiency
Overall energy efficiency is the ratio of collected heat and electrical energy to incoming solar radiation on the PV/T absorber. It is calculated from the first law of thermodynamics and gives value of the percentage of the energy converted from the solar radiation. Experiments show that the overall energy efficiency is meanly lean on the thermal energy conversation because the electrical efficiency is much lower [10]. It should be noticed that the overall energy efficiency neglects the difference between heat and electrical energy in terms of the energy quality.
4.2. Overall exergy efficiency
To get a real evaluation of the performance of a PV/T collector, exergy efficiency must be take into account. Using the theory of Carnot cycle difference of energy grades between heat and electricity determined. The overall exergy (eo) of the PV/T could be written as:
% &
I Ie e
e0 " th ! e ,th!,e ",0 , (10)
where: eth - thermal exergy, ee electrical exergy, ,th thermal exergy efficiency, ,e electrical exergy efficiency, ,0 overall exergy efficiency.
The thermal exergy could be further written as:
I I Q
eth " c u " c th ",th . (11)
Eq. 11 shows a new parameter which is c the ideal Carnot efficiency [11]. It can be determined as follow:
% &
--./ 001
2
#
" !
a wm
c K t t
K 293
293 , (12)
where, twm is the final temperature of the work medium.
The electrical exergy is written as:
I I
ee" e ",e . (13)
The overall exergy efficiency could be written as:
e th c
,
e " ! . (15)The exergy efficiency has considered the energy grade difference between the heat and electricity and therefore, is a more rational index to evaluate performance of the PV/T systems.
5. Results and discussion
Performance of solar equipment is depending on the environmental factors, the design of the collector or PV module, materials used in it, etc. The main factors which are impact on the lower thermal efficiency of a hybrid collector when compared with a thermal collector were identified [10]. Among others due to the imperfect adhesive between additional thermal resistance and the encapsulation, the high reflection losses still appeared, energy is transmitted to the electrical production, the lower quality of absorber plate. The lower electrical efficiency when compared to a photovoltaic collector is due to the optical losses in the glass cover. A sensitivity analysis were carried out it can be seen on Fig. 2. The graph shows that the emittance of the solar cells is the most important factor in heat losses.
From the investigation it can be concluded, that the thermal characteristic of the adhesive and working fluid flow rate has a considerable impact on the overall efficiency. It was also verified that the angular dependence of optical properties related to the components of the radiation must be taken into account.
Otherwise there can be an overestimation on electrical and thermal efficiencies in the order of 2%.
Fig. 2. Sensitivity analysis on PV/T [10]
Many factors have effect on performance of a photovoltaic installation, such as: shading, corroded parts, contaminations, accessories failure, objects on solar panels, etc. These factors include cell density, duct depth, length of collector, inlet temperature of working fluid, flow rate, number of covers, absorber to fluid thermal conductance, and in the case of water type PV/T collectors, absorber plate design parameters such as tube spacing, tube diameter and thickness of fin. Based on
observations of experiment and literature it can be concluded that the liquid based photovoltaic thermal collector systems has better indexes in performance than air based systems [12].
Fig. 3. Exergy efficiency of different type of solar collectors [3]
In the literature many detailed analysis found about PV/T collector process of exergy efficiency, which can be seen on Fig. 3. Based on that results it can be concluded that the best combination of these solar systems is the glazed PV/T water collector compared to different type of hybrid collectors [3].
In our department a PV/T module was installed in the solar system in order to study proposes and demonstrational as mentioned earlier. This type of solar equipment is made it possible to check this performance dependence on temperature. During the experiment the hybrid 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 hybrid collector worked on a separate liquid circle, where the water was cool down by a special cooling system to around 10 °C. By this way PV/T module temperature was dropped and the power change were measured.It can be seen from the following diagram (Fig. 4.), 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.
Fig. 4. Rate of the module temperature and useful solar energy of a PV/T collector
dt dT dt c dV dt m dT c
Pthermal " "
3
. (15)By the equation above (Eq. 15) specified thermal performance determined with a comparison of the incident radiation power efficiency. However that value changing when the operating parameters are changing also, but it is characterized the operation of the device. Based on measurements calculation results can be seen on Fig. 5.
Fig. 5. Thermal performance of hybrid and flat plate collectors
A comparison of thermal performance made between the hybrid and flat plate collector. Graph above show that the hybrid collector thermal performance much lower than the other one. However, it should be take into account that the hybrid collector is not specifically designed for domestic hot water. On the oder hand electrical performance of the PV/T collector was compared to a polycrytalline PV module (Fig. 6).
Fig. 6. Electrical performance of hybrid and polycrystalline
5. Conclusions
In this paper, the exergetic performance assessment of a PV/T collector was carried out. A detailed energy and exergy analysis was carried out to get a complete view of the utilized solar power of hybrid solar collectors, thermal and electrical parameters, exergy components and exergy efficiency of different type of PV/T collectors. 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. The best combination of
Performance[W] Performance [W]
these solar systems was introduced. Based on simulation results it can be concluded that glass covered water PV/T collector has the highest exergy efficiency compared to different type of hybrid collectors.
From our examination on hybrid collector, based on measurement data it can be concluded that about 10 °C temperature drop on the collector cause one percent efficiency increase.
Acknowledgement
This work was supported/subsidized by TÁMOP-4.2.2.B-10/1 "Development of a complex educational assistance/support system for talented students and prospective researchers at the Szent István University" project.
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![Fig. 1. Simulated cell temperature and PV power output for a clear summer day for the PV, PV/T and PV/Tg configurations [9]](https://thumb-eu.123doks.com/thumbv2/9dokorg/1119273.78556/4.892.262.636.266.489/fig-simulated-temperature-power-output-clear-summer-configurations.webp)
![Fig. 2. Sensitivity analysis on PV/T [10]](https://thumb-eu.123doks.com/thumbv2/9dokorg/1119273.78556/5.892.255.637.762.952/fig-sensitivity-analysis-pv-t.webp)
