Energy Conversion and Management 173 (2018) 450–460

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

The theoretical performance evaluation of hybrid PV-TEG system Challa Babu, P. Ponnambalam

⁎

T

School of Electrical Engineering, VIT, Vellore, Tamilnadu, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Photovoltaic Thermoelectric generator Hybrid PV-TEG Overall eﬃciency Parameters estimation

The concept of co-power generation from the excess heat is a rapidly developing technology due to diﬀerent material innovations in the ﬁeld of Thermoelectric Generators (TEGs). The photovoltaic (PV) module subjected to solar irradiation utilizes photon energy but also absorbs the heat energy. The module heat is utilized to generate power by the co-generation devices such as TEG. In this paper, theoretical analysis of hybrid PV-TEG system is analyzed using diﬀerent analytical methods. The performance of PV-TEG system is estimated using mathematical models of PV and TEG using MATLAB/SIMULINK environment. In this work, a novel non-concentrated ﬂat plate PV-TEG conﬁguration is proposed by using the multi crystalline PV connected in rear side with the Bismuth Telluride TEG. The proposed conﬁguration results in production of 5% additional energy with an increase in overall eﬃciency of 6% at STC conditions. The TEG in the conﬁguration provides cooling for the PV system and also contributes energy of 1–3% of PV rating. The eﬃciency of proposed PV-TEG conﬁguration is compared with that of standalone PV by making changes in irradiation and ambient temperature.

1. Introduction The power sources in the world are changing rapidly towards renewable energy. Out of all the renewable sources, the Photovoltaic (PV) gains more attention in the power generation, because of its advantages such as clean-green energy, no mechanical rotation parts and an abundant amount of input energy. But the amount of input gained by the PV sources is not utilized to its maximum. The PV uses only the light emitted energy to produce electrical power and heat pipes use only the thermal energy to produce hot water/air according to the application. The combination of heat and light energy utilization in a single module is initiated by the hybrid PV-Thermal systems [1]. In the PV-Thermal systems, the coolant in the system creates the eﬀects on the semi-conduction material properties in PV [2]. There are several hybrid conﬁgurations in PV-Thermal systems are designed such as hybrid PV with Phase Change Materials (PCM) [3], Nano ﬂuid-based PV-Thermal systems [4] and PV-Thermoelectric Generator (TEG) [5]. Out of all those systems, the PV conjunction with TEG gains more attention in recent years [6]. The recent developments in TEG materials [7] are made co-generation with the PV system for the low-grade heat power sources. The concentrated type PV-TEG conﬁguration are popularly used to capture maximum energy from the both PV and TEG systems [8,9]. But the concentrated type conﬁgurations, increase the level of complexity during the installation and maintenance. The research in ﬂat plate PV system with the TEG is in developing stage as a

⁎

commercial product for domestic users. The performance analysis of PV-Thermal systems is analyzed with respect to diﬀerent measures like Coeﬃcient of Performance (COP), overall eﬃciency and energy & exergy studies. The eﬃciency of the PV system majorly depends on the irradiance and module temperature [10]. The eﬃciency of the TEG majorly depends on the temperature diﬀerence between the two junctions [11]. Hence the overall eﬃciency of hybrid PV-TEG conﬁguration majorly inﬂuenced by the irradiance and temperature. The excess amount of energy generated for the same input irradiance leads to increase in overall eﬃciency of the system [12]. There are diﬀerent hybrid PV-TEG conﬁgurations proposed by the authors in [13–16]. It shows that the hybrid PV-TEG conﬁguration generates additional energy so that the overall eﬃciency increases from 3% to 14% as compared to the standalone PV system. The theoretical performance analysis of PV-TEG conﬁguration by the [17–19] are for the generalized conditions of diﬀerent crystalline PV with TEG systems. In [17], the author made a theoretical analysis of forced convective cooling fed hybrid solar thermoelectric generator and it generates additional energy of 4.7 W with an electrical eﬃciency of 1.2%. The author [20] made a theoretical model of ﬂat plate PV-TEG conﬁguration the electrical eﬃciency of the crystalline silicon PV is 14.03% and TEG with an eﬃciency of 5%. Cotfas et al., made a simulation in LABVIEW environment of ﬂat plate PV-TEG conﬁguration and it achieves power gain of 7% with an overall eﬃciency of 18.93% [21]. Deng et al., simulated the thin ﬁlm solar cells PV modules with heat

Corresponding author. E-mail addresses: [email protected] (C. Babu), [email protected] (P. Ponnambalam).

https://doi.org/10.1016/j.enconman.2018.07.104 Received 27 April 2018; Received in revised form 30 July 2018; Accepted 31 July 2018 Available online 04 August 2018 0196-8904/ © 2018 Elsevier Ltd. All rights reserved.

Energy Conversion and Management 173 (2018) 450–460

C. Babu, P. Ponnambalam

Nomenclature Symbol ζ ZT τα α ρ γ TH TC

PCM PV/T RC TEG PV-TEG DSSC c-Si pc-Si a-Si CIGS GaAs CdTe BIPVT

description (unit) temperature coeﬃcient of PV eﬃciency (%/°C) ﬁgure of merit transmittance-absorption coeﬃcient (–) Seebeck coeﬃcient (V/K) thermal resistivity (Ω-m) thermal conductance (w/K) hot side temperature of TEG (K) cold side temperature of TEG (K)

Abbreviations PV

phase change materials photovoltaic thermal thermal contact resistance thermoelectric generator photovoltaic-thermoelectric generator dye-sensitized solar cell crystalline silicon poly-crystalline silicon amorphous silicon copper indium gallium selenide gallium arsenide cadmium telluride building integrated photovoltaic thermal

photovoltaic

environment is used to simulate the commercial PV and TEG modules and, an optimization technique is used for estimating the unknown parameters of PV. The combined PV-TEG conﬁguration performance is analyzed with respect to the diﬀerent inputs such as irradiance and ambient temperature. This paper is organized as follows; Section 2 describes the mathematical modelling of PV and its thermal behavior, Section 3 describes the mathematical modelling of TEG, Section 4 describes the theoretical performance analysis of PV-TEG conﬁguration with respect to irradiance and temperature and Section 5 describes the results and the discussion.

collectors connected TEG modules integrated to enhance the amount of power generated. The power generated by the hybrid system is twice higher than the single silicon solar cell [22]. Verma et al. implemented simulation models of PV-TEG system in the MATLAB/SIMULINK with two individual MPPT controls to extract the maximum power the hybrid system [23]. Kwan et al. implemented a simulation model of PVTEG with Lock on Mechanism (LOM) maximum power control strategy for the conﬁgurations [24]. Guiqiang et al., made a theoretical analysis of ﬂat plate PV-TEG load electrical resistance with respect to the internal electrical resistance. The crystalline solar cell maximum eﬃciency occurs at TEG internal resistance of 0.47 Ω and load electrical resistance of 0.75 Ω. For the Gallium Arsenide maximum eﬃciency occurs at TEG internal resistance of 2.0 Ω and load electrical resistance of 1.6 Ω [25]. In [22–25], the authors majorly concentrated on the extraction maximum power from the panel by the diﬀerent controls strategies applied on load side of the system. In this work, we made an attempt to analyze the performance of the system with respect to input parameters such as irradiance and temperature for a novel design PVTEG conﬁguration. The amount of power generation is maximized by the system design conﬁgurations parameters such as glazing, PV material, absorber plate and the type of TEG, number of TEGs, and location of TEG [26]. The Maximum power extraction of the any system conﬁguration with a unit time will lead to generate maximum amount power from that system. So that, the maximum power generated with respect to solar inputs has to analyze with respect to overall eﬃciency of the system. The research in the ﬁeld of TEG alloys materials are moving rapidly to generate the maximum amount of power from the low-grade heat sources [27] and the amount of energy contributed by the TEG is increased from 1 to 10% of PV rating [28]. The feasibility of TEG with the PV made the system are more reliable and economical [29]. But some of the research gaps in the PV-TEG conﬁguration are main constrains for the development of commercialized product and are (i) lack of energybased studies with respect to each parameter, (ii) The optimum location and rating of TEG are suitable to integrate PV system, (iii) The optimum number of TEGs to connect for the PV system. This work helps to ﬁll the gap in the energy studies of the proposed conﬁguration with respect to the change in irradiance and ambient temperature. In this paper, we consider a commercial multi crystalline ﬂat plate PV with commercial TEG and integrated it on the rear side of PV made of a copper absorber plate. The mathematical models of PV and TEG modules are implemented in MATLAB/SIMULINK environment. The Fireworks algorithm is used for the estimation of PV parameters, and analytical method used for the estimation of TEG parameters. The combined performance of PV-TEG are analyzed for the change of irradiance and the ambient temperature. The novelty of this work is, it is a proposal a non-concentrated type ﬂat plate PV-TEG conﬁguration integrated with commercial PV and TEG modules. The MATLAB

2. Modelling of PV The performance of the PV-TEG system majorly depends on the PV system. The amount of power generated by the PV will decrease with the increase of heat above the Standard Test Conditions (STC), which will be indicated as a temperature coeﬃcient of power in the manufacturer data sheets. Here we are considering a multi crystalline solar cell and the data sheet shown in Table 1. To understand the accurate behavior of the selected module, we need to simulate the module by estimating the parameters. 2.1. Double diode modelling The mathematical model of PV system can be done for two diﬀerent analogies one is single diode model and another is double diode model. Most of the PV models are single diode models, because, the unknown parameter extraction can be done easily, and it takes less computational time to parameters estimation. The double diode model produces more accurate results than the single diode model by considering the recombination losses of the system [31]. The I-V curves produced by the double diode model more accurate during the low irradiation conditions. The double diode model of the solar cell is shown in Fig. 1. By applying the Kirchhoﬀ Current Law for the equivalent circuit

I = Iph−Id1−Id2−Ip

(1)

Table 1 Datasheet values of Kyocera – KC200GT 215 module [30].

451

Parameter

Value

Open circuit voltage (Voc) Short Circuit current (Isc) Temperature coeﬃcient of voltage Temperature coeﬃcient of current Area of the solar cell (A) Eﬃciency (η) at STC Fill Factor

32.9 V 8.21A −1.23 * 10−1/°C 3.18 * 10−3/°C 2612.5 cm2 17% 0.74

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C. Babu, P. Ponnambalam

RS

Iph

D1

Id2

I

Ip

Rp

D2

The Eqs. (1)–(9) are satisfactory works for standalone PV system. But when it comes to hybrid PV with thermal systems, the amount of energy generated by the PV system depends on the temperature of the module. The amount of energy is generated by the hybrid PV systems without considering the conduction, convection, and radiation heat loss coeﬃcients.

V

Id1

2.2. Thermal analysis of PV

Fig. 1. Double diode model equivalent circuit of a solar cell.

EPV = τα × A mod × G × ηref [1−ζ (T mod −Tref )]

By substituting the diode currents and the current through the parallel to the resistance we get Eq. (2).

V + IRs ⎞ ⎫ V + IRs ⎞ ⎫ V + IRs I = Iph−I01 ⎧exp ⎛ −1 − −1 −I02 ⎧exp ⎛ ⎬ ⎨ ⎨ N V Ns Vt 2 ⎠ ⎬ Rp s t 1 ⎠ ⎝ ⎝ ⎭ ⎭ ⎩ ⎩ ⎜

⎟

⎜

where τα the transmittance-absorption coeﬃcient of PV (0.95) ζ is the temperature coeﬃcient of PV eﬃciency Tref is the reference temperature (25 °C).

⎟

(2)

In the diode currents, I01 & I02 are the diodes reverse saturation currents and the Vt1 & Vt2 are the thermal voltages which are deﬁned by the Eqs. (3) and (4).

Vt1 = a1

KT q

(3)

Vt2 = a2

KT q

(4)

The module temperature is dynamically varied with respect to ambient temperature, wind velocity and the irradiation of the solar energy. There are many correlations proposed in the literature [33]. In this study, we used a correlation with less error to calculate module temperature [34].

To know the eﬀect of each parameter individually, one of the (G or Vw or Ta) parameter is maintained constant and the remaining two are varied (Vw & Ta, G & Ta, G & Vw) from minimum to maximum ranges. The results for module temperature are taken constant of G, Vw and Ta are 1000w/m2, 1 m/s, and 20 °C respectively. The variation for G, Vw and Ta minimum and maximum values are 100–1000w/m2, 1–10 m/s, and 10–50 °C respectively. Fig. 5 clearly shows impact of each parameter on the module temperature. The main objective of all the PV-thermal systems is to extract maximum heat and make it available. But the amount of heat energy available at the absorber plate is around 45% of heat at top of the glass cover of PV. In this paper, we used copper absorber plate as a heat extractor from the solar heat. This heat is the input for the hot junction of Thermoelectric Generator (TEG).

G GS

(5)

In the above equation, Iph-s is the light current at STC and is deﬁned as in Eq. (6) and the KI is the temperature coeﬃcient of current.

Rp + Rs ⎞ Iph − S = ISC − S ⎛⎜ ⎟ ⎝ Rp ⎠

(6)

The unknown parameters (I01, I02, a1, a2, RS, Rp) of the PV are estimated by the diﬀerent approaches like analytical, numerical and evolutionary algorithm techniques. In this work, the unknown parameters of the solar cell estimated by the Fireworks algorithm (FWA) proposed by Rajasekar et al. [32]. The estimated values of the PV cell are presented in Table 2 The estimated parameters are validated with the experimental voltage and current values and are tabulated in Appendix A. The FWA estimates values with an accuracy of 0.014065 and the average Individual Absolute Error (IAE) and Relative Error for the estimation are 0.34995 and 0.207503 respectively. By substituting the values in Eqs. (2)–(6) we get the solar cell current in Eq. (1). The estimated parameters are validated with the manufacturer datasheet I-V curves of PV shown in Fig. 2. The power generated by the PV system is given by,

P mod = FF × Isc − s × Voc

3. Modelling of TEG Thermoelectric are the direct energy conversion solid state devices, which work on the principle of Seebeck eﬀect. TEG combines the properties of Electrical, Thermal, and Semiconductors. According to the Seebeck eﬀect, the diﬀerence between the two junctions (Hot, Cold) leads to generate the potential across the load [35]. The basic structure of TEG shown in Fig. 6. In this process, the hot junction receives heat ﬂux αITh and the cold junction ejects the heat ﬂux of αITc. In TEG electrical power generation, there are three other eﬀects taking place

(7)

1. Joule eﬀect, causing heat because of electric current ﬂow 2. Leakage eﬀect, causing heal loss between the two junctions 3. Thomson eﬀect, causing heat due to the temperature gradient and current

The electrical eﬃciency of the PV system given by,

ηPV =

(11)

T mod = 0.943Ta + 0.0195G−1.528Vw + 0.3529

The light generated current Iph deﬁned in Eq. (5)

Iph = (Iph − S + KI (T −Ts )

(10)

FF × Isc − s × Voc A mod × Gs

(8)

The electrical energy generated by PV system given by,

E = A mod × ηPV × H × PR

Table 2 Estimated values of PV module.

(9)

where, H is the annual average solar irradiation on tilted panels and PR is the Performance Ratio generally varies from 0.5 to 0.9. The power output of PV system is analyzed by making any one parameter constant and another varied from minimum to maximum value. First, ambient temperature (25 °C) is maintained constant and the irradiation is varied from 100 to 1000w/m2 as shown in Fig. 3. Then, irradiance is made constant at 1000w/m2 and the temperature is varied from 10 to 50 °C as shown in Fig. 4. 452

Parameters

Values

I01 I02 a1 a2 RS Rp

1.11E-8 (A) 1.11E-8 (A) 1 1.2 0.303 (Ω) 343.10 (Ω)

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α is the Seebeck coeﬃcient = αp − αn (V/K) ρ is the thermal resistivity (Ω-m) ΔT is the temperature diﬀerence (K) = Th − Tc E is a geometric property of the pellets deﬁned as height divided by the cross section (m/m2). Deﬁning γ = λ/E (w/K) as the thermal conductance of TEG and β = ρE (Ω) as electrical resistance. The Eq. (13). Rewritten as

1 Qh = αITh− βI 2−γ ΔT 2

(14)

Similarly for the cold junction

Fig. 2. Comparison of I-V characteristics of PV.

Qc = αITc +

1 2 βI + γ ΔT 2

(15)

To ﬁnd the power developed by the TEG, we must know the internal parameters (α, β, γ) of TEG. In this paper, we used an analytical method to estimate the parameters of TGM199-1.4-2.0 module [37]. The manufacturer data sheet of TEG module is given in Table 3. The analytical method gives the values of Seebeck coeﬃcient (α), electrical resistance (β), thermal conductance (γ) values are shown in Table 4. The estimated parameters validated with the manufacturer data sheet and is shown in Fig. 7. The potential generated across the load given by

V = α × ΔT

Fig. 3. Output power versus Irradiance.

(16)

The amount of current supplied to the load given by

200

I=

150

α × ΔT RTEG + RL

(17)

The eﬃciency of TEG deﬁned by the ratio of electrical energy generated for the input heat energy.

100 ηTEG =

50

V×O Qh

(18)

3.1. Power developed by TEG

0 10

15

20

25

30

35

40

45

50

The estimated parameters of TEG resemble the properties of real time TEG. The amount of power generated by the TEG depends on the temperature diﬀerence between the two junctions and the connected load. To track the maximum power from the TEG, the load resistance is maintained equal to the internal resistance of TEG [38]. For the selected module, the maximum eﬃciency of 5.3% is observed at the load resistance of 3.7 Ω. From the manufacturer data sheet, the amount of load power versus the temperature diﬀerence shown in Fig. 8.

Fig. 4. Output power versus ambient temperature. 70 60 50

By varying Irradiation

40

By varying Wind velocity

30 20

4. Performance analysis of PV-TEG

By varying Ambient Temperature

The individual theoretical performance of PV and TEG are described in above Sections 2 and 3. In this section, the integration of PV with TEG and its combined performance analysis is done with respect to change of irradiance and temperature and its eﬀects on the overall efﬁciency examined. The existing literature shows that, by combing the TEG to PV produce more output power [39].

10 0

Fig. 5. Module temperature with respect to the change of G, Vw, and Ta.

Among the three eﬀects, Thomson eﬀect shows the less impact on the output of the TEG. For that Thomson eﬀect neglected. The energy equation of TEG given as

ETEG = Qh−Qc

4.1. Integration of PV with TEG The present design model of PV-TEG conﬁguration is shown in Fig. 9. The ﬂat plate single glazed multi crystalline PV layer is placed above the copper absorber plate and TGM199-1.4-2.0 module is connected in between the absorber plate and the heat sink. The feasibility of the PV and TEG are made electrical, thermal characterization and its electrical equivalent circuit shown in Fig. 10. The hybrid PV-TEG performance is simulated at STC conditions. While the integration of PV with TEG there must be some resistance

(12)

The energy at hot junction given by [36]

1 λ ΔT Qh = αITh− ρEI 2 + 2 E

(13)

where 453

Energy Conversion and Management 173 (2018) 450–460

C. Babu, P. Ponnambalam

Copper strips

N

P

N

Hot side

P

N

P

N

P

+

Cold side

Thermoelectric Element

I

V RL Fig. 6. The basic structure of TEG. 8

Table 3 Data sheet of TGM199-1.4–2.0 at Th = 200 °C & Tc = 30 °C. Value

Number of thermocouples Maximum power Maximum current Maximum voltage Resistance Dimensions (L * W * H)

199 7.3 W 2.65 A 11 V 3.7 Ω 40 * 40 * 4.4 mm

7 6

Power Output

Parameter

5 4 3 2 1

Table 4 Analyzed parameters of TEG. Parameter

Value

Seebeck coeﬃcient (α), Electrical resistance (β) Thermal conductance (γ)

162.856 µV/K 0.0104 Ω 0.153 W/K

0 20

40

60

80

100

120

140

160

180

Temperature differance

Fig. 8. Load power versus Temperature diﬀerence.

• The absorber surface temperature is uniformly distributed and constant. • The ﬂat plate collector mounting and its eﬀects are neglected. The primary part of PV functionality is same for temperature below 25 °C, further increase in temperature leads to decrement in the power generated in the PV system. In general, the overall eﬃciency of PV-TEG system depends on the individual eﬃciencies of PV and the eﬃciency of TEG. In this conﬁguration, overall eﬃciency is only the electrical eﬃciency, because we are not extracting thermal energy for storage or utilization. The electrical eﬃciency of PV depends on the output electrical power to the amount of solar energy per unit area. The electrical eﬃciency of TEG depends on the amount of heat energy at the hot junction to the amount of heat rejected at cold junction. In this conﬁguration, the input to the TEG is the absorber plate temperature and output is the electrical energy generated by the TEG.

Fig. 7. Comparison of V-I characteristics of TEG.

4.2. Change of irradiance between the absorber plate and TEG hot surface and is known as thermal contact resistance (Rc). Some of the assumptions considered for this analysis:

The PV-TEG conﬁguration has major input which is the irradiance coming from the sun. The irradiance scatters by the transmissivity, absorptivity, and emissivity of the glazing and its supporting’s. The 454

Energy Conversion and Management 173 (2018) 450–460

C. Babu, P. Ponnambalam

PV cell

Thermo Electric Generator Hot side Glass cover Cold side

Heat Sink Fig. 9. A cross-sectional view of PV-TEG system.

scattered amount of energy is almost 20%, remaining beam energy strikes to a semi conducting PV wafer, which converts that into electrical energy. The electrical eﬃciency is directly proportional to the irradiance because the amount of electrical energy generated is directly related to solar irradiation (G). By adding TEG to the existing PV system, the amount of energy generated increased for the same irradiance. The total energy generated by the PV-TEG in Eq. (19). (19)

The EPV and ETEG are deﬁned in Eqs. (10) and (12). By applying the hybrid conﬁgurations constrains to the equations amount of energy generated by the system is calculated. The TEG contributes about 4% of additional energy to the PV system by the change of irradiance. The comparison of power output generated by the PV-TEG with standalone PV system shown in Fig. 11. The Fig. 11 conveys that the power generated by the PV-TEG is higher than the standalone PV system by 4% at the irradiance 1000 w/ m2. This result also coincides with the work [40] related to the TEG power contribution by the change of irradiance. The change of irradiance directly impacts the amount of power generated by the TEG is around 5% of its rating. In the present work, we integrated the TEG on the rear side of the PV, so that power output of TEG decreased by 1%.

Fig. 11. Comparison of the power output of PV-TEG to the standalone PV.

1000w/m2, wind velocity at 1 m/s for the calculation of absorber plate temperature. By considering the cold junction temperature as 20 °C and this temperature diﬀerence is fed to the TEG, which produces the electrical energy. The amount of power contributed by the TEG in hybrid PV-TEG conﬁguration is shown in Fig. 13. The overall system performance with respect to the ambient temperature is given in Fig. 14. The maximum power generated by the hybrid PV-TEG system is higher than the standalone PV because in hybrid system TEG contributes around 7% for the change of ambient temperature and TEG provides cooling to the PV system. The contribution of TEG power changes with respect to the varying input parameters because as compared to the eﬀect of temperature more impacts is on the amount of energy generation rather than the irradiation.

4.3. Change of temperature Temperature is one of the major factors for this analysis. The ambient temperature of the air considered as the temperature at the top of the glass. The heat transfer in diﬀerent sections of TEG is shown in Fig. 12. The net energy available at the absorber plate is the input for TEG. The energy conversion eﬃciency of TEG is low as compared to other conversion engines. The developments take place in the ﬁeld of materials of TEG made moderately feasible for the PV system now. By applying the diﬀerent ambient temperatures and making irradiance at

4.4. Parameters eﬀect of overall eﬃciency In this conﬁguration the output is only electrical, so that eﬃciency considered the electrical eﬃciency of the system. The electrical eﬃciency of the PV-TEG conﬁguration depends on the amount of

RTEG

RS

Iph

D1

Id2 D2

I

Ip

RC V PV

Id1

Rp

Fig. 10. The electrical equivalent circuit of PV-TEG. 455

VTEG

VL

EPV − TEG = EPV + ETEG

Energy Conversion and Management 173 (2018) 450–460

C. Babu, P. Ponnambalam

Solar heat energy 100% 10%

Heat loss 35%

5%

Solar heat Energy

5%

Absorbed energy

5%

Radiated energy

Collected heat 45% Fig. 12. Heat transfer in PV-TEG.

The Figs. 11, 14 and 15 clearly shows that, the proposed non-concentrated PV-TEG conﬁguration generates the additional energy than the standalone PV system. Because, the TEG absorbs the heat from the PV panel which leads to increase in the PV eﬃciency and TEG also contributes the additional energy with the module heat. The overall electrical eﬃciency of the system is directly related to the amount of energy generated by the system. The proposed PV-TEG conﬁguration improves the amount of energy generated per square meter area. The non-concentrated type PV-TEG conﬁgurations are suitable for the rooftop power generation in distribution energy systems. The amount of thermal energy extracted in concentrated PV-TEG conﬁgurations loses its thermal quality with increase in utilization time. To eliminate the thermal storage direct energy conversion systems such as TEG are more suitable for the small power generating systems. In the non-concentrated type conﬁgurations, there is no reﬂection, optical losses to make concentration at a point.

Fig. 13. TEG power developed.

5. Results and discussion The energy eﬃciency of PV system is degraded because of the excess heat of the panel. The large PV plants provide enough cooling for the whole system in an economical way. But the cooling of PV system is not economical for the loads below 2KW. The proposed system provides suﬃcient heat removal from the PV system and generates electrical energy by the removal heat. The amount of energy generated by the system is directly related to the direct radiation coming from the sun. The total power generated by the PV-TEG system compared with standalone PV system and the power generated increased by 8.3% at the STC conditions. The amount of power generated by the TEG system Fig. 14. Electrical power Vs ambient temperature.

irradiance radiated per unit area to the amount of electrical energy generated [41]. The electrical eﬃciency of PV-TEG system increases because of the heat reduction by the TEG module system. The electrical energy eﬃciency of PV-TEG system is increased by 6% as compared to standalone PV system and is shown in Fig. 15. The overall eﬃciency of the PV-TEG system given as

ηPV − TEG = ηPV + ηTEG (1−ηPV )

(20)

The above equation shows the individual eﬃciencies of PV and TEGs gives the eﬃciency of PV-TEG system. But in the real time scenario, the eﬃciency of the PV system is increased by reduction of module temperature.

Fig. 15. Electrical eﬃciency Vs Irradiance. 456

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the STC input conditions. This will increase in future by the changes in the design conﬁguration with thermal energy storage systems. In the present analysis, multi crystalline PV used as a major electric power source, the electrical eﬃciency of PV system increased by the amount of power generated for the same radiated light energy. The energy

is directly related to the temperature diﬀerence. In this analysis, we assigned a non-practical assumption of TEG cold junction temperature constant one. Because it changes with respect to the ambient temperature and the mounting conditions. The electrical eﬃciency of the overall system is increased by 6% at Table 5 Overall eﬃciency of diﬀerent PV-TEG conﬁgurations. PV type

TEG Type

Mono crystalline

System conﬁguration

Eﬃciency

Remarks

Reference

Bismuth telluride (Bi2TE3)

Overall eﬃciency is increased by 2% than the standalone system

By adding the individual eﬃciencies of PV and TEG improves overall eﬃciency

[42]

Poly crystalline (pc-Si)

Bismuth telluride (Bi2TE3)

8% higher than the standalone PV system

Amorphous silicon (a-Si)

Bismuth telluride (Bi2TE3)

Overall eﬃciency is increased by 2.15% for the ﬁgure of merit of 2.4 as compared to ﬁgure of merit of 0.7

TEG ﬁgure of merit is 2.4 at STC conditions

[44]

Dye-Sensitized Solar Cell (DSSC)

Bismuth telluride (Bi2TE3)

Overall eﬃciency is 13.8% of the system

Fill factor of 0.56 at irradiance of 100 mW/cm2

[45]

Copper Indium Gallium Selenide (CIGS)

Bismuth telluride (Bi2TE3)

Overall eﬃciency is increased by 4% than the standalone system

TEG ﬁgure of merit is 0.001 K−1 at STC conditions

[42]

Cadmium Telluride (CdTe)

Bismuth telluride (Bi2TE3)

Overall eﬃciency of 10% than the standalone system

At maximum temperature of 200 °C at TEG

[42]

Gallium Arsenide (GaAs)

Skutterudite CoSb3

Overall eﬃciency of hybrid PV-TEG is 27.49%

Solar cell operating at 75 °C and TEG temperature diﬀerence 85 °C

[46]

Multi crystalline silicon

Bismuth telluride (Bi2TE3)

Overall eﬃciency increased by 6% than the standalone system

Operating at STC conditions (1000w/m2 & 25 °C)

Proposed model

457

[43]

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Multicrystalline PV-TEG [proposed method] CdTe PV-TEG [42] DSSC PV-TEG [53] CIGS PV-TEG [43] GaAs PV-TEG [52] Polymer PV-TEG [43] amorphous Silicon PV-TEG [42] Poly crystalline silicon PV-TEG [43] Mono crystalline silicon PV-TEG [43] 0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

Fig. 16. Overall eﬃciency of diﬀerent PV-TEG conﬁgurations. (See above-mentioned reference for further information.)

product made up of multi-crystalline silicon PV and Bi2Te3 TEG taken for the analysis. In [42,43,53], authors made theoretical model of different PV material conﬁgurations. By comparing the results with the proposed work, multi crystalline fed PV-TEG conﬁguration produces additional energy of 1.3% with an eﬃciency of 24%. The research and developments in the materials of semi conduction materials made with this PV-TEG conﬁguration will improve feasibility for the domestic consumers. The low-grade heat utilization by the TEG contributes 4 to 7% of energy to the PV system. The crystalline silicon-based PV-TEG conﬁgurations are economical than the DSSC, CdTe and CIGS PV systems. The research progress in the areas of techno economic, Life cycle analysis are still lacking for the diﬀerent PV-TEG conﬁgurations. In future, the hybrid PV-TEG conﬁgurations more feasible for the distribution generation systems.

generated by PV-TEG is increased by about 10% for the same irradiance level for standalone PV system. The TEG contributed about 5% additional energy to the system. The above literature shows the diﬀerent PV-TEG conﬁgurations with respect to diﬀerent PV and TEG materials. The eﬃciency of different conﬁgurations obtained are for the diﬀerent operating conditions. The comparison of proposed conﬁgurations for the diﬀerent conﬁgurations is presented in Table 5. Apart from the CIGS, CdTe and DSSC conﬁgurations, the multi crystalline PV-TEG conﬁguration gives the maximum eﬃciency. In the economical point of view, the multi crystalline solar cell is more economical than the DSSC, CIGS and CdTe [47]. This work conveys that, the eﬃciency of PV system is increased by the reduction of heat from the panel and that heat is used for the cogeneration by the TEG modules. In future, developments in materials of TEG and design conﬁgurations of PV-Thermal systems will improve the reliability and economic aspects of micro grid systems [48]. The eﬃciency of the proposed system coincides with [9]. The results conﬁrm that the eﬃciency of the PV system decreases and TEG eﬃciency increases with the irradiation above the STC. In [49], the results also coincide in the aspect of temperature. The eﬃciency of PV system increases by the reduction of temperature of the TEG system. In [50], author analyzed with indirect coupling of TEG to the PV system, the results convey that the PV-TEG conﬁguration improves overall eﬃciency with respect to temperature and provides additional cooling to PV-TEG conﬁguration by adding 1% eﬃciency to the overall system. The results in [51,25], the authors gave the strength to the proposed model. The prosed conﬁguration gives support to the design of novel PV-TEG systems in future. The overall eﬃciency of any hybrid PV-thermal system conﬁguration mainly depends on the type of PV material. In the current work, we proposed a novel design conﬁguration of PV-TEG to get maximum efﬁciency. The proposed conﬁguration results are compared with other design conﬁgurations presented in Fig. 16. The results show that, the multi crystalline silicon PV-TEG produces the more energy with an overall eﬃciency of 24% at STC conditions. The overall eﬃciency of the PV-TEG conﬁguration is majorly impacted by the PV material. In this work, we considered the commercial

6. Conclusion The theoretical performance analysis of ﬂat plate hybrid PV-TEG system is analyzed with the commercial PV and TEG modules data. The ﬂat plate PV-TEG system produces additional energy of 5% with an increase in eﬃciency of 6%. The power developed by the PV-TEG system is increased from 7 to 8.3% with respect to the irradiance changes from 100 to 1000w/m2. The ambient temperature causes increase in power after the STC temperature for a hybrid PV-TEG, but in the case of standalone PV, the temperature increase above STC temperature leads to decrease in the amount of power developed. This analysis shows that, the eﬃciency of PV-TEG system is not the same as the addition of individual eﬃciency of PV and TEG systems. Because the heat removal from the PV system increases the eﬃciency of PV, the optimal location and number of TEGs will produce maximum energy from the TEG modules. Acknowledgments The authors would like to thank Dr. Rajasekar N, Professor, Solar Energy Research Cell (SERC) for providing the valuable inputs to this research work.

Appendix A. Realization of KC200GT 215 module with FWA

S. no

Vexp

Iexp

Ical

IAE

RE

1 2 3 4 5

0 0.5438 2.0268 3.0155 4.0042

8.1983 8.177 8.1556 8.1556 8.1449

8.2066 8.2066 8.1947 8.1829 8.1711

0.0082 0.0296 0.0392 0.0273 0.0262

0.001 0.0036 0.0048 0.0033 0.0032

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6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

5.2401 7.0198 8.1073 12.0127 13.9901 16.0169 17.5 18.0438 19.0325 21.0099 22.048 24.2232 26.0523 26.9915 28.524 29.0184 30.0565 31.2924 32.0339 32.9237

8.1342 8.1235 8.1128 8.0915 8.0594 8.0487 8.0273 8.0166 7.9959 7.8356 7.5632 7.1124 6.5866 6.0557 5.3658 4.3217 3.6527 2.5352 1.3756 0.1496

8.1592 8.1474 8.1237 8.1 8.0763 8.0526 8.029 8.029 7.9653 7.8596 7.5816 7.1326 6.4988 6.086 5.399 4.3985 3.6359 2.5599 1.3895 0.1492

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0.025 0.0239 0.0108 0.0085 0.0169 0.0039 0.0016 0.0123 0.0306 0.024 0.0184 0.0202 0.0878 0.0303 0.0331 0.0769 0.0168 0.0247 0.0139 0.0004

0.0031 0.0029 0.0013 0.0011 0.0021 0.0005 0.0002 0.0015 0.0038 0.0031 0.0024 0.0028 0.0135 0.005 0.0061 0.0175 0.0046 0.0096 0.01 0.0029

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The theoretical performance evaluation of hybrid PV-TEG system Challa Babu, P. Ponnambalam

⁎

T

School of Electrical Engineering, VIT, Vellore, Tamilnadu, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Photovoltaic Thermoelectric generator Hybrid PV-TEG Overall eﬃciency Parameters estimation

The concept of co-power generation from the excess heat is a rapidly developing technology due to diﬀerent material innovations in the ﬁeld of Thermoelectric Generators (TEGs). The photovoltaic (PV) module subjected to solar irradiation utilizes photon energy but also absorbs the heat energy. The module heat is utilized to generate power by the co-generation devices such as TEG. In this paper, theoretical analysis of hybrid PV-TEG system is analyzed using diﬀerent analytical methods. The performance of PV-TEG system is estimated using mathematical models of PV and TEG using MATLAB/SIMULINK environment. In this work, a novel non-concentrated ﬂat plate PV-TEG conﬁguration is proposed by using the multi crystalline PV connected in rear side with the Bismuth Telluride TEG. The proposed conﬁguration results in production of 5% additional energy with an increase in overall eﬃciency of 6% at STC conditions. The TEG in the conﬁguration provides cooling for the PV system and also contributes energy of 1–3% of PV rating. The eﬃciency of proposed PV-TEG conﬁguration is compared with that of standalone PV by making changes in irradiation and ambient temperature.

1. Introduction The power sources in the world are changing rapidly towards renewable energy. Out of all the renewable sources, the Photovoltaic (PV) gains more attention in the power generation, because of its advantages such as clean-green energy, no mechanical rotation parts and an abundant amount of input energy. But the amount of input gained by the PV sources is not utilized to its maximum. The PV uses only the light emitted energy to produce electrical power and heat pipes use only the thermal energy to produce hot water/air according to the application. The combination of heat and light energy utilization in a single module is initiated by the hybrid PV-Thermal systems [1]. In the PV-Thermal systems, the coolant in the system creates the eﬀects on the semi-conduction material properties in PV [2]. There are several hybrid conﬁgurations in PV-Thermal systems are designed such as hybrid PV with Phase Change Materials (PCM) [3], Nano ﬂuid-based PV-Thermal systems [4] and PV-Thermoelectric Generator (TEG) [5]. Out of all those systems, the PV conjunction with TEG gains more attention in recent years [6]. The recent developments in TEG materials [7] are made co-generation with the PV system for the low-grade heat power sources. The concentrated type PV-TEG conﬁguration are popularly used to capture maximum energy from the both PV and TEG systems [8,9]. But the concentrated type conﬁgurations, increase the level of complexity during the installation and maintenance. The research in ﬂat plate PV system with the TEG is in developing stage as a

⁎

commercial product for domestic users. The performance analysis of PV-Thermal systems is analyzed with respect to diﬀerent measures like Coeﬃcient of Performance (COP), overall eﬃciency and energy & exergy studies. The eﬃciency of the PV system majorly depends on the irradiance and module temperature [10]. The eﬃciency of the TEG majorly depends on the temperature diﬀerence between the two junctions [11]. Hence the overall eﬃciency of hybrid PV-TEG conﬁguration majorly inﬂuenced by the irradiance and temperature. The excess amount of energy generated for the same input irradiance leads to increase in overall eﬃciency of the system [12]. There are diﬀerent hybrid PV-TEG conﬁgurations proposed by the authors in [13–16]. It shows that the hybrid PV-TEG conﬁguration generates additional energy so that the overall eﬃciency increases from 3% to 14% as compared to the standalone PV system. The theoretical performance analysis of PV-TEG conﬁguration by the [17–19] are for the generalized conditions of diﬀerent crystalline PV with TEG systems. In [17], the author made a theoretical analysis of forced convective cooling fed hybrid solar thermoelectric generator and it generates additional energy of 4.7 W with an electrical eﬃciency of 1.2%. The author [20] made a theoretical model of ﬂat plate PV-TEG conﬁguration the electrical eﬃciency of the crystalline silicon PV is 14.03% and TEG with an eﬃciency of 5%. Cotfas et al., made a simulation in LABVIEW environment of ﬂat plate PV-TEG conﬁguration and it achieves power gain of 7% with an overall eﬃciency of 18.93% [21]. Deng et al., simulated the thin ﬁlm solar cells PV modules with heat

Corresponding author. E-mail addresses: [email protected] (C. Babu), [email protected] (P. Ponnambalam).

https://doi.org/10.1016/j.enconman.2018.07.104 Received 27 April 2018; Received in revised form 30 July 2018; Accepted 31 July 2018 Available online 04 August 2018 0196-8904/ © 2018 Elsevier Ltd. All rights reserved.

Energy Conversion and Management 173 (2018) 450–460

C. Babu, P. Ponnambalam

Nomenclature Symbol ζ ZT τα α ρ γ TH TC

PCM PV/T RC TEG PV-TEG DSSC c-Si pc-Si a-Si CIGS GaAs CdTe BIPVT

description (unit) temperature coeﬃcient of PV eﬃciency (%/°C) ﬁgure of merit transmittance-absorption coeﬃcient (–) Seebeck coeﬃcient (V/K) thermal resistivity (Ω-m) thermal conductance (w/K) hot side temperature of TEG (K) cold side temperature of TEG (K)

Abbreviations PV

phase change materials photovoltaic thermal thermal contact resistance thermoelectric generator photovoltaic-thermoelectric generator dye-sensitized solar cell crystalline silicon poly-crystalline silicon amorphous silicon copper indium gallium selenide gallium arsenide cadmium telluride building integrated photovoltaic thermal

photovoltaic

environment is used to simulate the commercial PV and TEG modules and, an optimization technique is used for estimating the unknown parameters of PV. The combined PV-TEG conﬁguration performance is analyzed with respect to the diﬀerent inputs such as irradiance and ambient temperature. This paper is organized as follows; Section 2 describes the mathematical modelling of PV and its thermal behavior, Section 3 describes the mathematical modelling of TEG, Section 4 describes the theoretical performance analysis of PV-TEG conﬁguration with respect to irradiance and temperature and Section 5 describes the results and the discussion.

collectors connected TEG modules integrated to enhance the amount of power generated. The power generated by the hybrid system is twice higher than the single silicon solar cell [22]. Verma et al. implemented simulation models of PV-TEG system in the MATLAB/SIMULINK with two individual MPPT controls to extract the maximum power the hybrid system [23]. Kwan et al. implemented a simulation model of PVTEG with Lock on Mechanism (LOM) maximum power control strategy for the conﬁgurations [24]. Guiqiang et al., made a theoretical analysis of ﬂat plate PV-TEG load electrical resistance with respect to the internal electrical resistance. The crystalline solar cell maximum eﬃciency occurs at TEG internal resistance of 0.47 Ω and load electrical resistance of 0.75 Ω. For the Gallium Arsenide maximum eﬃciency occurs at TEG internal resistance of 2.0 Ω and load electrical resistance of 1.6 Ω [25]. In [22–25], the authors majorly concentrated on the extraction maximum power from the panel by the diﬀerent controls strategies applied on load side of the system. In this work, we made an attempt to analyze the performance of the system with respect to input parameters such as irradiance and temperature for a novel design PVTEG conﬁguration. The amount of power generation is maximized by the system design conﬁgurations parameters such as glazing, PV material, absorber plate and the type of TEG, number of TEGs, and location of TEG [26]. The Maximum power extraction of the any system conﬁguration with a unit time will lead to generate maximum amount power from that system. So that, the maximum power generated with respect to solar inputs has to analyze with respect to overall eﬃciency of the system. The research in the ﬁeld of TEG alloys materials are moving rapidly to generate the maximum amount of power from the low-grade heat sources [27] and the amount of energy contributed by the TEG is increased from 1 to 10% of PV rating [28]. The feasibility of TEG with the PV made the system are more reliable and economical [29]. But some of the research gaps in the PV-TEG conﬁguration are main constrains for the development of commercialized product and are (i) lack of energybased studies with respect to each parameter, (ii) The optimum location and rating of TEG are suitable to integrate PV system, (iii) The optimum number of TEGs to connect for the PV system. This work helps to ﬁll the gap in the energy studies of the proposed conﬁguration with respect to the change in irradiance and ambient temperature. In this paper, we consider a commercial multi crystalline ﬂat plate PV with commercial TEG and integrated it on the rear side of PV made of a copper absorber plate. The mathematical models of PV and TEG modules are implemented in MATLAB/SIMULINK environment. The Fireworks algorithm is used for the estimation of PV parameters, and analytical method used for the estimation of TEG parameters. The combined performance of PV-TEG are analyzed for the change of irradiance and the ambient temperature. The novelty of this work is, it is a proposal a non-concentrated type ﬂat plate PV-TEG conﬁguration integrated with commercial PV and TEG modules. The MATLAB

2. Modelling of PV The performance of the PV-TEG system majorly depends on the PV system. The amount of power generated by the PV will decrease with the increase of heat above the Standard Test Conditions (STC), which will be indicated as a temperature coeﬃcient of power in the manufacturer data sheets. Here we are considering a multi crystalline solar cell and the data sheet shown in Table 1. To understand the accurate behavior of the selected module, we need to simulate the module by estimating the parameters. 2.1. Double diode modelling The mathematical model of PV system can be done for two diﬀerent analogies one is single diode model and another is double diode model. Most of the PV models are single diode models, because, the unknown parameter extraction can be done easily, and it takes less computational time to parameters estimation. The double diode model produces more accurate results than the single diode model by considering the recombination losses of the system [31]. The I-V curves produced by the double diode model more accurate during the low irradiation conditions. The double diode model of the solar cell is shown in Fig. 1. By applying the Kirchhoﬀ Current Law for the equivalent circuit

I = Iph−Id1−Id2−Ip

(1)

Table 1 Datasheet values of Kyocera – KC200GT 215 module [30].

451

Parameter

Value

Open circuit voltage (Voc) Short Circuit current (Isc) Temperature coeﬃcient of voltage Temperature coeﬃcient of current Area of the solar cell (A) Eﬃciency (η) at STC Fill Factor

32.9 V 8.21A −1.23 * 10−1/°C 3.18 * 10−3/°C 2612.5 cm2 17% 0.74

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RS

Iph

D1

Id2

I

Ip

Rp

D2

The Eqs. (1)–(9) are satisfactory works for standalone PV system. But when it comes to hybrid PV with thermal systems, the amount of energy generated by the PV system depends on the temperature of the module. The amount of energy is generated by the hybrid PV systems without considering the conduction, convection, and radiation heat loss coeﬃcients.

V

Id1

2.2. Thermal analysis of PV

Fig. 1. Double diode model equivalent circuit of a solar cell.

EPV = τα × A mod × G × ηref [1−ζ (T mod −Tref )]

By substituting the diode currents and the current through the parallel to the resistance we get Eq. (2).

V + IRs ⎞ ⎫ V + IRs ⎞ ⎫ V + IRs I = Iph−I01 ⎧exp ⎛ −1 − −1 −I02 ⎧exp ⎛ ⎬ ⎨ ⎨ N V Ns Vt 2 ⎠ ⎬ Rp s t 1 ⎠ ⎝ ⎝ ⎭ ⎭ ⎩ ⎩ ⎜

⎟

⎜

where τα the transmittance-absorption coeﬃcient of PV (0.95) ζ is the temperature coeﬃcient of PV eﬃciency Tref is the reference temperature (25 °C).

⎟

(2)

In the diode currents, I01 & I02 are the diodes reverse saturation currents and the Vt1 & Vt2 are the thermal voltages which are deﬁned by the Eqs. (3) and (4).

Vt1 = a1

KT q

(3)

Vt2 = a2

KT q

(4)

The module temperature is dynamically varied with respect to ambient temperature, wind velocity and the irradiation of the solar energy. There are many correlations proposed in the literature [33]. In this study, we used a correlation with less error to calculate module temperature [34].

To know the eﬀect of each parameter individually, one of the (G or Vw or Ta) parameter is maintained constant and the remaining two are varied (Vw & Ta, G & Ta, G & Vw) from minimum to maximum ranges. The results for module temperature are taken constant of G, Vw and Ta are 1000w/m2, 1 m/s, and 20 °C respectively. The variation for G, Vw and Ta minimum and maximum values are 100–1000w/m2, 1–10 m/s, and 10–50 °C respectively. Fig. 5 clearly shows impact of each parameter on the module temperature. The main objective of all the PV-thermal systems is to extract maximum heat and make it available. But the amount of heat energy available at the absorber plate is around 45% of heat at top of the glass cover of PV. In this paper, we used copper absorber plate as a heat extractor from the solar heat. This heat is the input for the hot junction of Thermoelectric Generator (TEG).

G GS

(5)

In the above equation, Iph-s is the light current at STC and is deﬁned as in Eq. (6) and the KI is the temperature coeﬃcient of current.

Rp + Rs ⎞ Iph − S = ISC − S ⎛⎜ ⎟ ⎝ Rp ⎠

(6)

The unknown parameters (I01, I02, a1, a2, RS, Rp) of the PV are estimated by the diﬀerent approaches like analytical, numerical and evolutionary algorithm techniques. In this work, the unknown parameters of the solar cell estimated by the Fireworks algorithm (FWA) proposed by Rajasekar et al. [32]. The estimated values of the PV cell are presented in Table 2 The estimated parameters are validated with the experimental voltage and current values and are tabulated in Appendix A. The FWA estimates values with an accuracy of 0.014065 and the average Individual Absolute Error (IAE) and Relative Error for the estimation are 0.34995 and 0.207503 respectively. By substituting the values in Eqs. (2)–(6) we get the solar cell current in Eq. (1). The estimated parameters are validated with the manufacturer datasheet I-V curves of PV shown in Fig. 2. The power generated by the PV system is given by,

P mod = FF × Isc − s × Voc

3. Modelling of TEG Thermoelectric are the direct energy conversion solid state devices, which work on the principle of Seebeck eﬀect. TEG combines the properties of Electrical, Thermal, and Semiconductors. According to the Seebeck eﬀect, the diﬀerence between the two junctions (Hot, Cold) leads to generate the potential across the load [35]. The basic structure of TEG shown in Fig. 6. In this process, the hot junction receives heat ﬂux αITh and the cold junction ejects the heat ﬂux of αITc. In TEG electrical power generation, there are three other eﬀects taking place

(7)

1. Joule eﬀect, causing heat because of electric current ﬂow 2. Leakage eﬀect, causing heal loss between the two junctions 3. Thomson eﬀect, causing heat due to the temperature gradient and current

The electrical eﬃciency of the PV system given by,

ηPV =

(11)

T mod = 0.943Ta + 0.0195G−1.528Vw + 0.3529

The light generated current Iph deﬁned in Eq. (5)

Iph = (Iph − S + KI (T −Ts )

(10)

FF × Isc − s × Voc A mod × Gs

(8)

The electrical energy generated by PV system given by,

E = A mod × ηPV × H × PR

Table 2 Estimated values of PV module.

(9)

where, H is the annual average solar irradiation on tilted panels and PR is the Performance Ratio generally varies from 0.5 to 0.9. The power output of PV system is analyzed by making any one parameter constant and another varied from minimum to maximum value. First, ambient temperature (25 °C) is maintained constant and the irradiation is varied from 100 to 1000w/m2 as shown in Fig. 3. Then, irradiance is made constant at 1000w/m2 and the temperature is varied from 10 to 50 °C as shown in Fig. 4. 452

Parameters

Values

I01 I02 a1 a2 RS Rp

1.11E-8 (A) 1.11E-8 (A) 1 1.2 0.303 (Ω) 343.10 (Ω)

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α is the Seebeck coeﬃcient = αp − αn (V/K) ρ is the thermal resistivity (Ω-m) ΔT is the temperature diﬀerence (K) = Th − Tc E is a geometric property of the pellets deﬁned as height divided by the cross section (m/m2). Deﬁning γ = λ/E (w/K) as the thermal conductance of TEG and β = ρE (Ω) as electrical resistance. The Eq. (13). Rewritten as

1 Qh = αITh− βI 2−γ ΔT 2

(14)

Similarly for the cold junction

Fig. 2. Comparison of I-V characteristics of PV.

Qc = αITc +

1 2 βI + γ ΔT 2

(15)

To ﬁnd the power developed by the TEG, we must know the internal parameters (α, β, γ) of TEG. In this paper, we used an analytical method to estimate the parameters of TGM199-1.4-2.0 module [37]. The manufacturer data sheet of TEG module is given in Table 3. The analytical method gives the values of Seebeck coeﬃcient (α), electrical resistance (β), thermal conductance (γ) values are shown in Table 4. The estimated parameters validated with the manufacturer data sheet and is shown in Fig. 7. The potential generated across the load given by

V = α × ΔT

Fig. 3. Output power versus Irradiance.

(16)

The amount of current supplied to the load given by

200

I=

150

α × ΔT RTEG + RL

(17)

The eﬃciency of TEG deﬁned by the ratio of electrical energy generated for the input heat energy.

100 ηTEG =

50

V×O Qh

(18)

3.1. Power developed by TEG

0 10

15

20

25

30

35

40

45

50

The estimated parameters of TEG resemble the properties of real time TEG. The amount of power generated by the TEG depends on the temperature diﬀerence between the two junctions and the connected load. To track the maximum power from the TEG, the load resistance is maintained equal to the internal resistance of TEG [38]. For the selected module, the maximum eﬃciency of 5.3% is observed at the load resistance of 3.7 Ω. From the manufacturer data sheet, the amount of load power versus the temperature diﬀerence shown in Fig. 8.

Fig. 4. Output power versus ambient temperature. 70 60 50

By varying Irradiation

40

By varying Wind velocity

30 20

4. Performance analysis of PV-TEG

By varying Ambient Temperature

The individual theoretical performance of PV and TEG are described in above Sections 2 and 3. In this section, the integration of PV with TEG and its combined performance analysis is done with respect to change of irradiance and temperature and its eﬀects on the overall efﬁciency examined. The existing literature shows that, by combing the TEG to PV produce more output power [39].

10 0

Fig. 5. Module temperature with respect to the change of G, Vw, and Ta.

Among the three eﬀects, Thomson eﬀect shows the less impact on the output of the TEG. For that Thomson eﬀect neglected. The energy equation of TEG given as

ETEG = Qh−Qc

4.1. Integration of PV with TEG The present design model of PV-TEG conﬁguration is shown in Fig. 9. The ﬂat plate single glazed multi crystalline PV layer is placed above the copper absorber plate and TGM199-1.4-2.0 module is connected in between the absorber plate and the heat sink. The feasibility of the PV and TEG are made electrical, thermal characterization and its electrical equivalent circuit shown in Fig. 10. The hybrid PV-TEG performance is simulated at STC conditions. While the integration of PV with TEG there must be some resistance

(12)

The energy at hot junction given by [36]

1 λ ΔT Qh = αITh− ρEI 2 + 2 E

(13)

where 453

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Copper strips

N

P

N

Hot side

P

N

P

N

P

+

Cold side

Thermoelectric Element

I

V RL Fig. 6. The basic structure of TEG. 8

Table 3 Data sheet of TGM199-1.4–2.0 at Th = 200 °C & Tc = 30 °C. Value

Number of thermocouples Maximum power Maximum current Maximum voltage Resistance Dimensions (L * W * H)

199 7.3 W 2.65 A 11 V 3.7 Ω 40 * 40 * 4.4 mm

7 6

Power Output

Parameter

5 4 3 2 1

Table 4 Analyzed parameters of TEG. Parameter

Value

Seebeck coeﬃcient (α), Electrical resistance (β) Thermal conductance (γ)

162.856 µV/K 0.0104 Ω 0.153 W/K

0 20

40

60

80

100

120

140

160

180

Temperature differance

Fig. 8. Load power versus Temperature diﬀerence.

• The absorber surface temperature is uniformly distributed and constant. • The ﬂat plate collector mounting and its eﬀects are neglected. The primary part of PV functionality is same for temperature below 25 °C, further increase in temperature leads to decrement in the power generated in the PV system. In general, the overall eﬃciency of PV-TEG system depends on the individual eﬃciencies of PV and the eﬃciency of TEG. In this conﬁguration, overall eﬃciency is only the electrical eﬃciency, because we are not extracting thermal energy for storage or utilization. The electrical eﬃciency of PV depends on the output electrical power to the amount of solar energy per unit area. The electrical eﬃciency of TEG depends on the amount of heat energy at the hot junction to the amount of heat rejected at cold junction. In this conﬁguration, the input to the TEG is the absorber plate temperature and output is the electrical energy generated by the TEG.

Fig. 7. Comparison of V-I characteristics of TEG.

4.2. Change of irradiance between the absorber plate and TEG hot surface and is known as thermal contact resistance (Rc). Some of the assumptions considered for this analysis:

The PV-TEG conﬁguration has major input which is the irradiance coming from the sun. The irradiance scatters by the transmissivity, absorptivity, and emissivity of the glazing and its supporting’s. The 454

Energy Conversion and Management 173 (2018) 450–460

C. Babu, P. Ponnambalam

PV cell

Thermo Electric Generator Hot side Glass cover Cold side

Heat Sink Fig. 9. A cross-sectional view of PV-TEG system.

scattered amount of energy is almost 20%, remaining beam energy strikes to a semi conducting PV wafer, which converts that into electrical energy. The electrical eﬃciency is directly proportional to the irradiance because the amount of electrical energy generated is directly related to solar irradiation (G). By adding TEG to the existing PV system, the amount of energy generated increased for the same irradiance. The total energy generated by the PV-TEG in Eq. (19). (19)

The EPV and ETEG are deﬁned in Eqs. (10) and (12). By applying the hybrid conﬁgurations constrains to the equations amount of energy generated by the system is calculated. The TEG contributes about 4% of additional energy to the PV system by the change of irradiance. The comparison of power output generated by the PV-TEG with standalone PV system shown in Fig. 11. The Fig. 11 conveys that the power generated by the PV-TEG is higher than the standalone PV system by 4% at the irradiance 1000 w/ m2. This result also coincides with the work [40] related to the TEG power contribution by the change of irradiance. The change of irradiance directly impacts the amount of power generated by the TEG is around 5% of its rating. In the present work, we integrated the TEG on the rear side of the PV, so that power output of TEG decreased by 1%.

Fig. 11. Comparison of the power output of PV-TEG to the standalone PV.

1000w/m2, wind velocity at 1 m/s for the calculation of absorber plate temperature. By considering the cold junction temperature as 20 °C and this temperature diﬀerence is fed to the TEG, which produces the electrical energy. The amount of power contributed by the TEG in hybrid PV-TEG conﬁguration is shown in Fig. 13. The overall system performance with respect to the ambient temperature is given in Fig. 14. The maximum power generated by the hybrid PV-TEG system is higher than the standalone PV because in hybrid system TEG contributes around 7% for the change of ambient temperature and TEG provides cooling to the PV system. The contribution of TEG power changes with respect to the varying input parameters because as compared to the eﬀect of temperature more impacts is on the amount of energy generation rather than the irradiation.

4.3. Change of temperature Temperature is one of the major factors for this analysis. The ambient temperature of the air considered as the temperature at the top of the glass. The heat transfer in diﬀerent sections of TEG is shown in Fig. 12. The net energy available at the absorber plate is the input for TEG. The energy conversion eﬃciency of TEG is low as compared to other conversion engines. The developments take place in the ﬁeld of materials of TEG made moderately feasible for the PV system now. By applying the diﬀerent ambient temperatures and making irradiance at

4.4. Parameters eﬀect of overall eﬃciency In this conﬁguration the output is only electrical, so that eﬃciency considered the electrical eﬃciency of the system. The electrical eﬃciency of the PV-TEG conﬁguration depends on the amount of

RTEG

RS

Iph

D1

Id2 D2

I

Ip

RC V PV

Id1

Rp

Fig. 10. The electrical equivalent circuit of PV-TEG. 455

VTEG

VL

EPV − TEG = EPV + ETEG

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Solar heat energy 100% 10%

Heat loss 35%

5%

Solar heat Energy

5%

Absorbed energy

5%

Radiated energy

Collected heat 45% Fig. 12. Heat transfer in PV-TEG.

The Figs. 11, 14 and 15 clearly shows that, the proposed non-concentrated PV-TEG conﬁguration generates the additional energy than the standalone PV system. Because, the TEG absorbs the heat from the PV panel which leads to increase in the PV eﬃciency and TEG also contributes the additional energy with the module heat. The overall electrical eﬃciency of the system is directly related to the amount of energy generated by the system. The proposed PV-TEG conﬁguration improves the amount of energy generated per square meter area. The non-concentrated type PV-TEG conﬁgurations are suitable for the rooftop power generation in distribution energy systems. The amount of thermal energy extracted in concentrated PV-TEG conﬁgurations loses its thermal quality with increase in utilization time. To eliminate the thermal storage direct energy conversion systems such as TEG are more suitable for the small power generating systems. In the non-concentrated type conﬁgurations, there is no reﬂection, optical losses to make concentration at a point.

Fig. 13. TEG power developed.

5. Results and discussion The energy eﬃciency of PV system is degraded because of the excess heat of the panel. The large PV plants provide enough cooling for the whole system in an economical way. But the cooling of PV system is not economical for the loads below 2KW. The proposed system provides suﬃcient heat removal from the PV system and generates electrical energy by the removal heat. The amount of energy generated by the system is directly related to the direct radiation coming from the sun. The total power generated by the PV-TEG system compared with standalone PV system and the power generated increased by 8.3% at the STC conditions. The amount of power generated by the TEG system Fig. 14. Electrical power Vs ambient temperature.

irradiance radiated per unit area to the amount of electrical energy generated [41]. The electrical eﬃciency of PV-TEG system increases because of the heat reduction by the TEG module system. The electrical energy eﬃciency of PV-TEG system is increased by 6% as compared to standalone PV system and is shown in Fig. 15. The overall eﬃciency of the PV-TEG system given as

ηPV − TEG = ηPV + ηTEG (1−ηPV )

(20)

The above equation shows the individual eﬃciencies of PV and TEGs gives the eﬃciency of PV-TEG system. But in the real time scenario, the eﬃciency of the PV system is increased by reduction of module temperature.

Fig. 15. Electrical eﬃciency Vs Irradiance. 456

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C. Babu, P. Ponnambalam

the STC input conditions. This will increase in future by the changes in the design conﬁguration with thermal energy storage systems. In the present analysis, multi crystalline PV used as a major electric power source, the electrical eﬃciency of PV system increased by the amount of power generated for the same radiated light energy. The energy

is directly related to the temperature diﬀerence. In this analysis, we assigned a non-practical assumption of TEG cold junction temperature constant one. Because it changes with respect to the ambient temperature and the mounting conditions. The electrical eﬃciency of the overall system is increased by 6% at Table 5 Overall eﬃciency of diﬀerent PV-TEG conﬁgurations. PV type

TEG Type

Mono crystalline

System conﬁguration

Eﬃciency

Remarks

Reference

Bismuth telluride (Bi2TE3)

Overall eﬃciency is increased by 2% than the standalone system

By adding the individual eﬃciencies of PV and TEG improves overall eﬃciency

[42]

Poly crystalline (pc-Si)

Bismuth telluride (Bi2TE3)

8% higher than the standalone PV system

Amorphous silicon (a-Si)

Bismuth telluride (Bi2TE3)

Overall eﬃciency is increased by 2.15% for the ﬁgure of merit of 2.4 as compared to ﬁgure of merit of 0.7

TEG ﬁgure of merit is 2.4 at STC conditions

[44]

Dye-Sensitized Solar Cell (DSSC)

Bismuth telluride (Bi2TE3)

Overall eﬃciency is 13.8% of the system

Fill factor of 0.56 at irradiance of 100 mW/cm2

[45]

Copper Indium Gallium Selenide (CIGS)

Bismuth telluride (Bi2TE3)

Overall eﬃciency is increased by 4% than the standalone system

TEG ﬁgure of merit is 0.001 K−1 at STC conditions

[42]

Cadmium Telluride (CdTe)

Bismuth telluride (Bi2TE3)

Overall eﬃciency of 10% than the standalone system

At maximum temperature of 200 °C at TEG

[42]

Gallium Arsenide (GaAs)

Skutterudite CoSb3

Overall eﬃciency of hybrid PV-TEG is 27.49%

Solar cell operating at 75 °C and TEG temperature diﬀerence 85 °C

[46]

Multi crystalline silicon

Bismuth telluride (Bi2TE3)

Overall eﬃciency increased by 6% than the standalone system

Operating at STC conditions (1000w/m2 & 25 °C)

Proposed model

457

[43]

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Multicrystalline PV-TEG [proposed method] CdTe PV-TEG [42] DSSC PV-TEG [53] CIGS PV-TEG [43] GaAs PV-TEG [52] Polymer PV-TEG [43] amorphous Silicon PV-TEG [42] Poly crystalline silicon PV-TEG [43] Mono crystalline silicon PV-TEG [43] 0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

Fig. 16. Overall eﬃciency of diﬀerent PV-TEG conﬁgurations. (See above-mentioned reference for further information.)

product made up of multi-crystalline silicon PV and Bi2Te3 TEG taken for the analysis. In [42,43,53], authors made theoretical model of different PV material conﬁgurations. By comparing the results with the proposed work, multi crystalline fed PV-TEG conﬁguration produces additional energy of 1.3% with an eﬃciency of 24%. The research and developments in the materials of semi conduction materials made with this PV-TEG conﬁguration will improve feasibility for the domestic consumers. The low-grade heat utilization by the TEG contributes 4 to 7% of energy to the PV system. The crystalline silicon-based PV-TEG conﬁgurations are economical than the DSSC, CdTe and CIGS PV systems. The research progress in the areas of techno economic, Life cycle analysis are still lacking for the diﬀerent PV-TEG conﬁgurations. In future, the hybrid PV-TEG conﬁgurations more feasible for the distribution generation systems.

generated by PV-TEG is increased by about 10% for the same irradiance level for standalone PV system. The TEG contributed about 5% additional energy to the system. The above literature shows the diﬀerent PV-TEG conﬁgurations with respect to diﬀerent PV and TEG materials. The eﬃciency of different conﬁgurations obtained are for the diﬀerent operating conditions. The comparison of proposed conﬁgurations for the diﬀerent conﬁgurations is presented in Table 5. Apart from the CIGS, CdTe and DSSC conﬁgurations, the multi crystalline PV-TEG conﬁguration gives the maximum eﬃciency. In the economical point of view, the multi crystalline solar cell is more economical than the DSSC, CIGS and CdTe [47]. This work conveys that, the eﬃciency of PV system is increased by the reduction of heat from the panel and that heat is used for the cogeneration by the TEG modules. In future, developments in materials of TEG and design conﬁgurations of PV-Thermal systems will improve the reliability and economic aspects of micro grid systems [48]. The eﬃciency of the proposed system coincides with [9]. The results conﬁrm that the eﬃciency of the PV system decreases and TEG eﬃciency increases with the irradiation above the STC. In [49], the results also coincide in the aspect of temperature. The eﬃciency of PV system increases by the reduction of temperature of the TEG system. In [50], author analyzed with indirect coupling of TEG to the PV system, the results convey that the PV-TEG conﬁguration improves overall eﬃciency with respect to temperature and provides additional cooling to PV-TEG conﬁguration by adding 1% eﬃciency to the overall system. The results in [51,25], the authors gave the strength to the proposed model. The prosed conﬁguration gives support to the design of novel PV-TEG systems in future. The overall eﬃciency of any hybrid PV-thermal system conﬁguration mainly depends on the type of PV material. In the current work, we proposed a novel design conﬁguration of PV-TEG to get maximum efﬁciency. The proposed conﬁguration results are compared with other design conﬁgurations presented in Fig. 16. The results show that, the multi crystalline silicon PV-TEG produces the more energy with an overall eﬃciency of 24% at STC conditions. The overall eﬃciency of the PV-TEG conﬁguration is majorly impacted by the PV material. In this work, we considered the commercial

6. Conclusion The theoretical performance analysis of ﬂat plate hybrid PV-TEG system is analyzed with the commercial PV and TEG modules data. The ﬂat plate PV-TEG system produces additional energy of 5% with an increase in eﬃciency of 6%. The power developed by the PV-TEG system is increased from 7 to 8.3% with respect to the irradiance changes from 100 to 1000w/m2. The ambient temperature causes increase in power after the STC temperature for a hybrid PV-TEG, but in the case of standalone PV, the temperature increase above STC temperature leads to decrease in the amount of power developed. This analysis shows that, the eﬃciency of PV-TEG system is not the same as the addition of individual eﬃciency of PV and TEG systems. Because the heat removal from the PV system increases the eﬃciency of PV, the optimal location and number of TEGs will produce maximum energy from the TEG modules. Acknowledgments The authors would like to thank Dr. Rajasekar N, Professor, Solar Energy Research Cell (SERC) for providing the valuable inputs to this research work.

Appendix A. Realization of KC200GT 215 module with FWA

S. no

Vexp

Iexp

Ical

IAE

RE

1 2 3 4 5

0 0.5438 2.0268 3.0155 4.0042

8.1983 8.177 8.1556 8.1556 8.1449

8.2066 8.2066 8.1947 8.1829 8.1711

0.0082 0.0296 0.0392 0.0273 0.0262

0.001 0.0036 0.0048 0.0033 0.0032

458

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6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

5.2401 7.0198 8.1073 12.0127 13.9901 16.0169 17.5 18.0438 19.0325 21.0099 22.048 24.2232 26.0523 26.9915 28.524 29.0184 30.0565 31.2924 32.0339 32.9237

8.1342 8.1235 8.1128 8.0915 8.0594 8.0487 8.0273 8.0166 7.9959 7.8356 7.5632 7.1124 6.5866 6.0557 5.3658 4.3217 3.6527 2.5352 1.3756 0.1496

8.1592 8.1474 8.1237 8.1 8.0763 8.0526 8.029 8.029 7.9653 7.8596 7.5816 7.1326 6.4988 6.086 5.399 4.3985 3.6359 2.5599 1.3895 0.1492

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0.025 0.0239 0.0108 0.0085 0.0169 0.0039 0.0016 0.0123 0.0306 0.024 0.0184 0.0202 0.0878 0.0303 0.0331 0.0769 0.0168 0.0247 0.0139 0.0004

0.0031 0.0029 0.0013 0.0011 0.0021 0.0005 0.0002 0.0015 0.0038 0.0031 0.0024 0.0028 0.0135 0.005 0.0061 0.0175 0.0046 0.0096 0.01 0.0029

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