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ScienceDirect

Procedía Technology 24 (2016) 513 - 522

International Conference on Emerging Trends in Engineering, Science and Technology

(ICETEST - 2015)

Numerical Simulation for Solar Hybrid Photovoltaic Thermal Air Collector

Lippin paulya*, L Rekhab, Christy V Vazhappillya, Melvinraj C Ra

aDepartment of Mechanical Engineering, Jyothi Engineering College, Jyothi hills Thrissur 679531, India bDepartment of Mechanical Engineering, Government Engineering College, Thrissur 680009, India

Abstract

Solar energy is one of the renewable energy sources which have potential for future energy applications. The current well-liked technology converts solar energy into electricity and heat individually. In this paper, an effort is made to simulate and evaluate the overall performance of a hybrid photovoltaic thermal (PV/T) air collector using computational fluid dynamics (CFD) software. The numerical analysis of the flow and heat transfer in hybrid PV/T systems is computationally quite complicated and the number of research works on this topic is quite low. Based on numerical analysis, the performance of a solar hybrid PV/T air collector has been studied. The numerical simulation was done in commercial software ANSYS FLUENT 14.5.0. The electrical energy conversion in solar cell was calculated with user defined function. The numerical results are validated with experimental results from literature. The results show a good agreement between experimental and simulated result for outlet air temperature and PV cell temperature. Using validated model, effect of mass flow rate and duct depth on the performance of solar hybrid PV/T collector has been studied and optimum values are identified. In order to increase the overall performance of a solar hybrid PV/T air collector, a novel design is proposed here. The result shows in the proposed design gives 20% enhancement in overall performance compared to conventional solar hybrid PV/T air collector.

© 2016 PublishedbyElsevierLtd. This is anopen access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICETEST - 2015 Keywords: PV/T air collector; solar radiation; CFD simulation.

* Lippin Pauly. Tel.: +919447054926. E-mail address: lippinpauly@jecc.ac.in

2212-0173 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICETEST - 2015 doi:10.1016/j.protcy.2016.05.088

1. Introduction

Over the past century fossil fuels have been provided most of our energy requirements because these are much cheaper and more convenient than energy from other sources, and until recently environmental pollution has been of slight concern. With the increasing demand of energy, nowadays the world daily oil burning up is 85 million barrels of crude oil. In spite of the well-known consequences of fossil fuel combustion on the ecosystem, this is estimated to increase to 123million barrels per day by the year 2025[1]. This is the main cause for pollution. Many researches towards the solar energy arise all over the world due to the anxiety of this reason. The utmost advantage of solar energy as compared with other forms of energy that is ecological friendly and plentifully available and can be supplied without any ecological pollution. Usually, devices intended for using solar energy fall into two major classes depending on the process of its conversion: either heat or electricity, like thermal collectors and photovoltaic modules correspondingly. Solar thermal energy collectors are particular kind of heat exchangers that convert solar radiation into thermal energy usually through a moving fluid. Photovoltaic (PV) is the most convenient way of utilizing solar energy by directly converting it into electricity. Among these, one that has developed very enormously is the photovoltaic (PV) technology. A photovoltaic system consists of solar cells and auxiliary components. It converts the solar radiation directly into electrical energy. The temperature of PV modules is increased by the absorbed solar radiation that is not converted into electrical energy, causing a decline in their efficiency. For monocrystalline (c-Si) and polycrystalline (pc-Si) silicon solar cells, the efficiency decreases by about 0.45% for each degree rise in temperature. For amorphous silicon (a-Si) cells, the effect is fewer, with a decrease of about 0.25% per degree rise in temperature depending on the module design [2]. So due to the disadvantage of more energy payback period & poor efficiency at high temperatures of a photovoltaic (PV) module, the cost of electricity produced by a PV module is higher than that of electricity produced by fossil fuels. In order to reduce such demerits, a PV module can be incorporated with cooling systems by using ducts or channels under the PV module. So it can also be used for thermal applications such as air/water heating, space heating, solar agricultural drying and can then be more cost effective. Such systems are referred to as hybrid photovoltaic thermal systems. A photovoltaic-thermal hybrid system (PV/T) produces both electricity and heat by means of one incorporated component, in which cells are applied on the thermal absorber.

It was not long before scientists noticed that the PV panels which they used to convert the incident solar irradiation to electricity were able to convert only a relatively small fraction of the irradiation to electricity, while a major portion of the solar energy would convert into heat. Moreover, heat affects the efficiency of PV cells adversely, decreasing their energy generation potential [2]. Thus, scientists began exploring means for decreasing the temperature of PV panels in order to enhance their conversion efficiency. Evans [3] was amongst the first to investigate the possibility of harnessing the heat generated on PV panels as utilizable thermal energy. However, research on the field has been very slow, with studies necessary for the design of a commercial PV/T system taking place primarily during the past decade. Methods to evaluate and optimize the efficiency and performance of PV/T systems have been recommended in most studies, with a few studies investigating their performance with numerical simulations.

Garg and Adhikari [4] have developed a steady state model in order to predict the overall performance of PV/T air heating system with single and double glass configurations. Working with a steady state PV/T model, they pointed out that further than the critical point the single glass cover collects more heat than double glass. They conclude that the parametric studies of PV/T air collector show the effect of collector length, collector area, mass flow rate and duct depth on the overall efficiency of PV/T collector.

Dubey et al [5] carried out an analysis on different configurations of glass-to-glass and glass-to-tedlar PV modules and developed an expression for electrical efficiency as a function of climatic conditions and design parameters. The result shows that glass-to-glass PV module incorporated with air cooling duct have the maximum efficiency and its annual average efficiency is about 10.41%.

By use of validated theoretical and experimental models, Tiwari et al [6] proved the degree of improvement by integrating unglazed PV module with duct air cooling for meteorological conditions of Indian climate. The result also explored to found out an optimal values for mass flow rate of air, duct depth and duct length

Joshi et al [7] have analysed and compared the overall performance of hybrid PV/T air collector with glass-to-tedlar and glass-to-glass configurations. The results showed that the hybrid PV/T air collector with glass to glass configuration has better results than glass to tedlar configurations. The results also explored to found out the effects of different control parameters such as mass flow rate and length of the duct and it showed the parameters have

Nomenclature

A Area of PV module m2

Ac Area of solar cell m2

b Breadth of PV module m

Ca Specific heat of air kJ/kgK

Cf Conversion factor of the thermal power plant

EVA Ethyl Vinyl Acrelate

I(t) Incident solar intensity (a function of time) on the

inclined module surface W/m2

kT Conductive heat transfer coefficient from solar cell to back surface W/m2K

L1 Length of the PV module m

Mass flow rate of air kg/s

PV Photovoltaic

PV/T Photovoltaic thermal

Tf Bulk mean temperature of air K

T L air,in Inlet air temperature K

Tair,out Outlet air temperature K

Ti insulation temperature K

T a Ambient air temperature K

Tb s Back surface temperature of tedlar K

Tc Temperature of solar cell K

Ub Overall heat transfer coefficient from bottom of tedlar to ambient W/m2K

Ut Overall heat transfer coefficient from solar cell

to ambient through glass cover W/m2K

UtT Overall heat transfer coefficient from solar cell to

back surface through tedlar W/m2K

Greek Letters

ac Absorptivity of solar cell

aT Absorptivity of tedlar

ßc Packing factor of solar cell

Solar cell efficiency %

"HE Electrical efficiency %

TIE th Thermal Equivalent efficiency %

Overall efficiency %

Tg Transitivity of glass

significant influence on the overall performance of hybrid PV/T air collector

Tonui and Tripanagnostopoulos [8] suggested a new design with two low cost modifications in traditional type hybrid PV/T air collector system. They designed PV/T air system by adding slender flat metal sheet suspended on the centre of air duct or create an extended surface on the walls of the air duct. The results showed that the modifications are very useful to extract more thermal energy and keep the PV module much cooler to acquire higher overall efficiency. By the use of validated numerical results, they studied the effect of duct depth, channel length and air flow rate on the overall performance of hybrid PV/T air system.

Basavanna et al [9] numerically analysed a steady state solar collector model in order to study the effect of different configurations of collector channel on fluid flow and heat transfer characteristics. The numerical results are validated with experimental results and the results showed that triangular tube configuration extracts more energy due to increased surface area of contact.

Touafek et al [10] developed a new design in order to extract more useful thermal energy and to maintain PV module with in an acceptable temperature level. The results showed that the new design had better thermal and

electrical performance compared to the traditional hybrid PV/T collectors. They obtained a useful thermal energy of about 290W and got a thermal efficiency of around 48 %.

2. Methodology

The aim of this work is to evaluate the thermal performance of a hybrid photovoltaic thermal air collector system. A numerical model was developed to predict the overall performance of solar hybrid PV/T air collector. Parametric studies has been carried out in order to study the effect of operating and design parameters of hybrid PV/T air collector on its overall energy efficiency. Finally, a novel design of hybrid solar collectors for air heating and electrical production is proposed in order to give good thermal and electrical performance compared to traditional hybrid collectors.

2.1. Energy Analysis

In order to write down the energy balance equation for each section of a PV/T air collector system, following assumptions are made:

• The system is in quasi-steady state condition

• Transmitivity of EVA is almost 100%

• The temperature deviation along the thickness is negligible

• The air flow between tedlar and insulation is homogeneous for forced manner of operation.

Fig. 1. (a) cross sectional view for PV/T air collector; (b) thermal resistance circuit diagram.

Figure.1 (a) shows the cross sectional view of a PV/T air collector and (b) shows its corresponding thermal resistant circuit. The energy balance equations for each part of PV/T air collector to compute the thermal parameters and thermal, electrical and overall efficiency of a PV/T air collector are as follows:

• For the PV module

tg[acp + ar( 1 -P)]l(t)bdx = [Ut(Tc - Ta) + UT(TC - Tbs)]bdx + ^cTgpi{t) (1)

[The rate of solar energy available on PV module] =

[The rate of overall heat transfer from top surface of cell to ambient] + [the rate of overall heat transfer from cell to back surface of tedlar] + [the rate of electrical energy produced]

For the back surface

UT(_TC - Tbs)bdx = hT(Tbs - Tf)bdx

[The rate of heat transfer from cell to back surface of tedlar] = [the rate of heat transfer from back surface of tedlar to flowing fluid]

For air flowing below the tedlar

mfCa^dx + U„(Tf - Ta)bdx = hT(Tbs - Tf)bdx

[The rate of heat gain by flowing fluid in duct] + [An overall heat transfer from flowing fluid to ambient] = [the rate of heat transfer from back surface of the tedlar to flowing fluid]

The rate of useful thermal energy can be calculated for the PV/T air system as: Thermal efficiency of PV/T collector is

Where Q is rate of useful energy in watts

TI(t)bL1

In the earlier studies [11] electrical efficiency of a PV module has been calculated from following equation

= vd 1 - 0.0045(7; - 298)] Here we convert the electrical efficiency to thermal equivalent efficiency through the following equation:

HEth — r

The overall efficiency of PV/T collector can be computed by adding the Eqn. (5) and Eqn. (7) as

Vo = VEth + Vth

Root mean square of percentage deviation (e) and linear coefficient of correlation (r) are calculated by using Eqn. (9) and (10) to compare the computer-generated and experimental results.

Where,

= ^exp(i)] x 100

^sim(i)

_N (EXeXp *Xsjm)-(l.XeXp^*(I.Xsim)_

jN*(lXixp)-(IXexp)2*jN*(lXs2im)-(IXsim)2

The linear coefficient of correlation ranges between -1 and 1. The experimental and simulated values are said to be strong positive linear correlation, if 'r' approaches to 1.

2.2. Numerical Analysis

The geometric model of the hybrid PV/T air collector is depicted in Fig. 2. The domain is created in ANSYS Design Modeler 14.5.0 where PV module, duct and insulation are fully modeled. The design parameters are corresponding to experimental system described by Joshi et al [7] in order to validate the simulated outcomes. The operating and design parameters of the PV/T air collector required during validation procedure are described in the Tab. 1. Further information regarding the experimental set up, method and its condition are found in Ref. [7].

Fig. 2. Geometric model of hybrid PV/T air collector.

An unstructured tetrahedral mesh is employed in this simulation. Mesh is refined for optimum value of aspect ratio and skewness. Near to the inner surface of duct wall, a very fine mesh is required to resolve the flow parameters. In ANSYS Meshing, this is achieved with an inflation layer for computational efficiency. The mesh becomes increasingly coarse towards the Centre of duct to save computation time. A mesh independence test is carried out in order to achieve a statistically accurate and converged solution

Table 1. Design and operating parameters of hybrid PV/T air collector system.

Solar PV/T air collector parameters Values

Module type (Monocrystaline silicon) Siemens SP75

Length & width of PV module, L1 & b 1.2 & 0.45

Thickness of glass cover, solar cell, tedlar and 0.003m, 0.0003m,

back insulation 0.0005m, 0.05m

Conductivity of glass cover, solar cell, tedlar 1, 0.036, 0.033,

and back insulation 0.035 [W/mK]

Transmittivity of glass 0.95

Absorbtivity of solar cell & tedlar 0.9 & 0.5

Packing factor of solar cell 0.83

Duct depth 0.05m

Conversion factor, Cf 0.36

Wind speed, ambient temperature and solar Varied as per

intensity operating

condition

The fluid flow and heat transfer characteristics in the PV/T collector has been studied by means of computational fluid dynamics (CFD) calculations through the FLUENT 14.5.0 software. A simplified meshed model which was previously built has been imported to FLUENT software for analysis. The governing equations including energy equation, radiation model and k-s turbulence model have been chosen to test the suitability and the applicability of the model on the flow through the duct. Following boundary conditions are applied on the computational domain, as per physics of the problem. Longitudinal uniform velocity is introduced in the inlet. The outer boundary is an average static reference pressure of 0pa. Top surface of the PV panel is subjected to solar flux as well as convective and radiative losses which are implemented in FLUENT by user defined functions. Bottom surface of the PV/T collector are defined by convective boundary condition. The rest of the outer boundaries are adiabatic boundary conditions. The inner wall of the duct has a no slip boundary condition where velocity increases from zero at the wall surface to maximum at the Centre of the duct. Since the presence of the electrical efficiency of PV module in Eqn. (1) and dependency of electrical efficiency with cell temperature in Eqn. (6), the thermal analysis of hybrid PV/T air collector and its electrical analysis are dependent. The calculation precision of thermal parameters of a PV/T air collector will be improved if the electrical efficiency of PV module is calculated in a precise way. So in order to improve the accuracy of analysis, electrical efficiency is incorporated in the simulation using user defined functions (UDF).

The simulations have been performed in steady state conditions. It is important to be mention that a number of works report only 0.2% of variation between simulations carried out under steady state and under dynamic environment [12]. Solver uses SIMPLE algorithm for pressure velocity coupling in Cartesian coordinate. Equation discretization of the model is achieved by means of the upwind differencing scheme. Advection schemes with different levels of accuracy have been tested and compared for the simulation of hybrid PV/T air collector system. Convergence criteria chosen for the simulation are 10-6 for continuity, x y z-velocities, k, epsilon, P1 and 10-12 for energy.

3. Result and discussion

3.1. Experimental Validation

The experimental results of Joshi et al. [7] for a PV/T air collector which includes the values of the solar radiation intensity, ambient temperature, inlet temperature and outlet temperature of air, solar cell temperature and inlet air velocity have been used to validate the results obtained by simulation model. The computer-generated values of outlet air temperature, solar cell temperature, thermal efficiency, electrical efficiency and overall efficiency in current work have been validated by their equivalent experimental values in Ref. [7].

The comparison between simulated values of solar cell temperature, outlet air temperature and the corresponding experimental data [7] are shown in Fig. 3. In the Figure, the subscript 'sim' indicates simulated values of parameters in the current work and the subscript 'exp' shows the experimental values. According to the figure, it is observed that there is a good agreement between the experimental and simulated values of these parameters. However the agreement in the simulated and experimental values for solar cell temperature is closer in low ambient temperatures. Moreover, the root mean square of percentage deviation and linear coefficient of correlation of these parameters are 0.73%, 0.44% and 0.99, 0.995 respectively. Meanwhile the root mean square of percentage deviation and linear coefficient of correlation for theoretical values of these parameters reported by Joshi et al. [7] are 16.82, 4.66% and 0.95, 0.98 respectively. It is observed that the simulated results are more precise than the theoretical results reported in Ref. [7]

Figure.4. shows the experimental and simulated values of overall efficiency. According to the figure, it is observed that there is a good agreement between the experimental and simulated values of the overall efficiency. The zigzag manner of variation of overall efficiency with respect to time is because of different inlet air velocities (Ref. [7]) at different instance.

outlet air exp oulet air sim

For air outlet temperature

e = 0.44, r = 0.995

9 10 11 12 13 14 15 16 Time (hr)

10 11 12 13 14 15 16 Time (hr)

Fig. 3. Experimental validation of hourly variation of solar cell temperature and outlet air temperature of hybrid PV/T air collector Fig. 4. Experimental validation of hourly variation of overall efficiency of hybrid PV/T air collector

3.2. Parametric Studies

The effect of mass flow rate with respect to overall efficiency of hybrid PV/T air collector keeping the other parameters constant is plotted in the Fig. 5.a. It can be seen from the graph that the overall efficiency increases initially with mass flow rate due to the fact that more air passes through the duct extracts more thermal energy. However at higher mass flow rates, the overall efficiency cannot be increasing linearly as much as expected due to the short flow residence time inside the duct. Hence, the overall efficiency approaches to a constant value at higher mass flow rates.

Figure.5.b shows the effect of duct depth on overall efficiency of the PV/T air collector keeping the mass flow rate constant. It can be seen that increasing the duct depth decreases the overall efficiency as expected. So in order to increase the overall efficiency, the duct depth must be kept as low as possible. But the value of the duct depth is limited by the velocity of air flowing through the duct which is based on characteristics of fan used at the inlet.

0.67 0.66 0.65 0.64 0.63 0.62 0.61 0.6

0.04 0.06 Duct Depth(m)

^ 0.7 >>

(5 0.4 0.3

0.5 1 1.5 2 Mass flow rate (kg/s)

Fig. 5. (a) variation of overall efficiency for different mass flow rates; (b) variation of overall efficiency for different duct depth

3.3. Proposed Model

Based on the previous studies done by Lambarski [13], it can be seen that the electrical efficiency of PV cells is influenced by their temperature and consequently the temperature distribution on the PV panel. In fact when a temperature gradient exists, not all cells may be capable of operating in the same manner at the same time causing parallel series mismatches. In details, cell temperature has only a very small effect in series connection (efficiency loss about 1%). But a substantial effect in parallel connections with about 17% loss. So considering the parametric studies in the current work and the previous studies, a new model is designed by using a variable cross sectional duct and is proposed here in order to reduce the temperature gradient over the panel surface and increase the overall efficiency of solar hybrid PV/T air system. In the new model larger cross sectional area at the inlet (same as the previous simulated model used in this work with duct depth of 0.05m) and smaller cross sectional area at the outlet are considered. Same operating conditions are used to analyse the new model in order compare its overall performance with conventional type of hybrid PV/T collectors.

The temperature distribution over the panel for conventional and novel design of hybrid PV/T air collector is shown in the Fig. 6 (a) and 6 (b). The temperature distribution of air flowing through the duct for conventional and new design is shown in the Fig. 6 (c) and 6 (d). The result shows that the new design has lesser average cell temperature and better uniformity of temperature over the panel. The outlet air temperature increased from 320.6 K to 322.3 K by the new design. The overall efficiency also increased at the rate of 20% due to higher outlet air temperature and decreased average cell temperature. Moreover, actual increase in electrical efficiency of the system will be more than simulated ones due to better uniformity of temperature over the panel, which cannot be able to capture by the simulation. The increase in overall performance of the hybrid PV/T air collector system is because of increase in velocity of air towards the outlet due to the new duct design. Increasing velocity of air towards the outlet makes the system capable of extracting more energy towards the outlet thereby increasing the outlet air temperature and maintaining better uniformity of temperature over the panel surface. Hence the new design gives indeed better thermal and electrical performance compared to conventional hybrid PV/T air collector system.

Fig. 6. (a) Temperature distribution over the panel for conventional model; (b) Temperature distribution over the panel for newly designed model; (c) Temperature distribution of air flowing through for conventional model; (d) Temperature distribution of air for newly designed model;

4. Conclusion

In this paper, the performance evaluation of a hybrid PV/T air collector was carried out. Electrical conversion inside the panel was compensated by user defined functions to increase the calculation precision of PV/T air collector thermal parameters. Finally, validation, numerical simulation and parametric studies were carried out and based on the studies a new duct design is proposed. On the basis of present study, the following conclusions have been drawn:

• The numerical simulation results of this study are in good agreement with the experimental measurements noted in the previous literature. Further, it is observed that the simulation results obtained in this paper is more precise than the theoretical results reported in Ref. [7].

• With increase in mass flow rate, the overall efficiency also increases initially due to the fact that more air passes through the duct extracts more thermal energy and reaches to an approximate constant nature at higher flow rates due to less flow residence time inside the duct.

• While duct depth is increasing, the overall efficiency of a hybrid PV/T air collector decreases.

• Based on the present study, a new design for hybrid PV/T air collector is proposed which gives 20% enhancement in overall performance compared to conventional hybrid PV/T air collectors.

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