Performance investigation of a concentrating photovoltaic thermal hybrid solar system combined with thermoelectric generators

Energy Conversion and Management 205 (2020) 112377

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Performance investigation of a concentrating photovoltaic thermal hybrid solar system combined with thermoelectric generators

T

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Afifa Riahia,b, , Abdessalem Ben Haj Alia, Abdelhamid Fadhela, Amenallah Guizania, Moncef Balghouthia a b

Thermal Processes Laboratory, Research and Technology Center of Energy (CRTEn), B.P. 95, 2050 Hammam Lif, Tunisia Faculty of Sciences of Tunis, University of Tunis El-Manar, 2092 Tunis, Tunisia

A R T I C LE I N FO

A B S T R A C T

Keywords: Thermoelectric generator Solar energy Concentrated photovoltaic Hybrid system

Concentrated photovoltaic thermal (CPVT) hybrid solar systems present an attractive technology for the simultaneous production of electrical and thermal energy. Combining thermoelectric generators with CPVT systems is an innovative way to further enhance the solar energy conversion and increase the electric power. In this study, a concentrated photovoltaic thermal (CPVT) and concentrated photovoltaic thermal thermoelectric (CPVT-TE) hybrid solar systems were investigated. A hybrid CPVT and thermoelectric generator unit prototypes were designed, manufactured and experimental tests were carried out. Mathematical models were established to analyze the electrical and thermal performances of the CPVT and CPVT-TE solar systems. The models were validated by means of the data obtained during the experiments. Results show that the electric power output of the CPVT-TE is higher than that of the CPVT and improvements on the electrical efficiency can be achieved through the integration of the thermoelectric generators. The daily electrical efficiency of the CPVT-TE system was improved by 7.46% as compared with the CPVT system, for a sunny day characterized by solar radiation level reaching 935 W/m2 and ambient temperature around 33 °C. This study was also an opportunity to analyze the large-scale application of the CPVT-TE solar system. For a typical year in Tunisia and a solar system aperture area of 39 m2, an extra electric energy of 359 kWh could be generated by the CPVT-TE system due to the integration of the thermoelectric generators. In addition, the analysis proves that the system is able to produce a considerable yearly electric and thermal energy and can save fossil energy and equivalent CO2 emissions.

1. Introduction The global energy demand and consumption is growing and most of resources used for meeting the energy needs are fossil fuels such as coal, natural gas and oil [1]. Fossil fuels are limited and cause greenhouse emissions. Solar energy has been identified as one of the most promising renewable energy source that can reduce the use of fossil fuels and meet the energy demands [2,3]. Among various solar energy technologies, photovoltaic power generators offer an attractive option for harnessing the solar energy resources effectively and they were developed significantly in the recent years [4,5]. Concentrating solar radiation and focussing it onto the photovoltaic (PV) cells, using optical concentrators, allows acquiring more incident irradiations and therefore more output power per unit of PV cells area. With the use of economical optical concentrators, the expensive PV areas are minimized and therefore the overall cost of solar electricity could be reduced [6]. The integration of an efficient heat exchanger device into the

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concentrating photovoltaic systems is compulsory to regulate the elevated operating temperatures and to achieve higher conversion efficiency [7]. A concentrated photovoltaic thermal (CPVT) hybrid solar system is basically an integration of an optical concentrator, a photovoltaic module and a heat removal device and it generates both electrical and thermal energy. The parabolic trough collector is one of the most frequently optical equipment used in solar concentrating applications and it has been commercialized in a wide range. Parabolic trough concentrators offer an effective way on the field of CPVT solar systems [8]. Coventry [9] experimentally studied the performance of a parabolic trough photovoltaic thermal system with monocrystalline PV cells attached to an aluminum receiver. It was reported that the system has an electrical efficiency around 11% and a thermal efficiency of 58% for a direct solar radiation of 1000 W/m2 and ambient temperature of 25 °C. Yongfeng et al. [10] examined the performances of a 2 m2 CPVT system by simulations and experimental tests using various types of PV cells

Corresponding author. E-mail address: afi[email protected] (A. Riahi).

https://doi.org/10.1016/j.enconman.2019.112377 Received 14 October 2019; Received in revised form 1 December 2019; Accepted 2 December 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature A I T Cp k h ṁ P V t

Subscripts

area (m2) direct normal irradiation (W/m2) temperature (K) specific heat (J/kgK) thermal conductivity (W/mK) heat transfer coefficient (W/m2K) mass flow rate (kg/s) power (W) volume (m3) time (s)

pv te h c ele th f ex a i

photovoltaic cell thermoelectric hot cold electrical thermal fluid exchanger ambient insulation

Greek symbols

Abbreviations

α ε η δ ρ

PV TEG CPVT CPVT-TE

absorptance emissivity efficiency (%) thickness (m) density (kg/m3)

photovoltaic cell thermoelectric generator concentrated photovoltaic thermal concentrated photovoltaic thermal thermoelectric

an electrical efficiency of 6% and a thermal efficiency of 44%. In another investigation, the exergetic performance evaluation of the CPVT system was performed by Karathanassis et al. [20]. Valizadeh et al. [21] indicated that the electrical efficiency and thermal efficiency are influenced by the inlet fluid temperature, the receiver dimensions, the collector length and the incident beam radiation. Since a significant range of the solar spectrum is not used in the photovoltaic conversion and dissipated as heat during the operation of solar cells, various ways were applied to improve the photovoltaic systems performances. The proposed techniques mainly focus on the use of effective cooling devices design and innovative heat transfer fluids [22] and PV cells with advanced materials [23]. An innovative technology to improve the performances of photovoltaic systems is to combine the PV cells with thermoelectric modules to further enhance the power conversion efficiency [24]. A thermoelectric module is able to either convert thermal energy into electrical energy through the Seebeck effect or electricity to thermal energy according to Peltier effect. Hence, a thermoelectric module can be used as a thermoelectric generator (TEG) to produce electric power when a temperature difference is available across its junctions or as a thermoelectric cooler (TEC) to generate thermal energy when an electric current is applied [25]. Photovoltaic cells and thermoelectric generators have a common aim of generating electrical power. The combined photovoltaic thermoelectric (PV-TE) hybrid configuration could be a potential system that produces more electricity than the PV only design. There are two common technologies for combining PV cells and TEGs to broaden harvesting of electrical power from solar radiations: spectrum splitting photovoltaic thermoelectric system and integrated photovoltaic thermoelectric system [26]. A spectrum splitting device can divide the sun wavelength into two ranges: the suitable wavelength range for photovoltaic conversion is received by the solar cells, and the remaining range out of the PV working band gap are directed to the TEG modules [27]. The spectrum splitting systems are expensive and complicated especially for the case of large scale applications. In addition, the heat dissipated by the solar cells during the photovoltaic conversion cannot be used by the TEGs, and therefore the combined PV-TE system cannot achieve full solar spectrum utilization. The other type of combination is adding the TEGs to the back side of solar cells. For this configuration, all the energy dissipated by the PV cells could be used by the TEGs for extra electrical power generation [25]. Wu et al. [28] investigated the operating parameters influencing the performance of an integrated concentrated photovoltaic thermoelectric (CPV-TE) system by means of a theoretical model. Based on the report

(crystal silicon, supper cell, and GaAs). Results showed that the series resistance PV cell, temperature and concentrating radiation intensity have a huge impact on the output power of the system. A methodology to characterize and evaluate the CPVT systems was derived by Bernardo et al. [11]. The proposed system is based on a parabolic trough reflector combined with aluminum thermal absorber equipped with monocrystalline silicon cells and measured results showed that the electrical efficiency was 6.4%. The performances of a 10 m2 trough concentrating photovoltaic thermal system were investigated by Li et al. [12]. The experimental results showed that the average electrical efficiencies of the system with three types of solar cell arrays, super cell, GaAs cell and silicon cell were 3.63%, 8.94%, and 3.67% respectively. The thermal efficiencies were around 45.17%, 41.69% and 34.53% for the three PV arrays respectively. Li et al. [13] compared the economic performance of a parabolic trough CPVT with that of a flat-plate PV system. They concluded that the CPVT has promising prospects due to its electricity production cost that can nearly reach the cost of the flat plate PV collector. Moreover, the CPVT system can supply additional useful heat to the users. An additional study that deals with the influence of the solar radiation and the cell temperature on the electric performance of the CPVT system was performed by Li et al. [14]. The obtained results showed that each type of solar cell had its own optimum concentration ratio. The triple junction GaAs cells had good performance characteristics and it operated well at higher concentration ratio. A parabolic trough photovoltaic thermal collector with triple junction cells attached to a triangular linear receiver was studied by Calise et al. [15]. The simulations results showed excellent electrical and thermal efficiencies at high solar radiation. However, the authors pointed out that the system is very expensive due to the use of triple-junction PV cells. Simulations and experimental tests were carried out by Del Col et al. [16] to evaluate the electric and thermal performance of a 6.857 m2 parabolic trough linear concentrating photovoltaic thermal system. The electric and thermal energy of the system were around 500 W and 1250 W respectively during a clear sky day. A CPVT system was designed, simulated and tested by Widyolar et al. [17] with single junction Gallium Arsenide PV cells. A thermal efficiency of 37% and electrical efficiency of 8% were obtained. Yazdanifard et al. [18] investigate the effect of some parameters such as the tube geometry, the concentration ratio and the flow regime on the performance of a parabolic trough concentrating photovoltaic thermal system. Karathanassis et al. [19] designed and tested a novel prototype of parabolic trough CPVT system with three variations of receiver incorporating different PV modules and heat-sink designs. The CPVT system achieves 2

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improvement on the electrical efficiency by placing thermoelectric generators and PV cells on the lateral area of the absorber tube. Gu et al. [36] illustrated that high concentration ratio and small temperature coefficient of PV cells are suitable for designing the CPV-TE hybrid system. Yin et al. [37] found that the output power of the TEG module enables the total output power of the CPV-TE hybrid system to be higher than that of the CPV system. Furthermore, the superiority of the CPV-TE system is more significant as the concentration ratio increases. Experimental realization and modeling of a CPV-TE hybrid system were addressed by Mahmoudinezhad et al. [38] and they indicated that using thermoelectric generator in the hybrid system leads to having more stable overall electric power. Based on the literature survey, most studies have been considered a small-scale concentrating photovoltaic thermoelectric solar system and ideal assumptions for the thermoelectric generators. Therefore, issues regarding their feasibility for a large scale photovoltaic-thermoelectric system are still limited. Also, there are a few research held regarding the integration of thermoelectric generators with parabolic trough concentrating photovoltaic thermal solar systems. In this study, a concentrated photovoltaic thermal (CPVT) and concentrated photovoltaic thermal thermoelectric (CPVT-TE) hybrid solar systems were investigated. A CPVT and thermoelectric generator unit prototypes were designed, built and experimental tests were conducted. Mathematical models were performed and validated against the experimental results. The models were used to evaluate the electrical and thermal performances of the CPVT-TE system and to compare it with the CPVT solar

by Lamba and Kaushik [29], the contribution of the TEG to the total power output of the CPV-TE system is more important at higher concentration ratios. Yin et al. [30] studied a CPV-TE system with three cooling methods, including natural cooling, forced air cooling and water cooling and the results showed a significant superiority of water cooling. Feasibility of the integration of concentrated PV and TEG modules was studied by Rezania and Rosendahl [31] based on a parametric investigation of critical parameters in the hybrid CPV-TE. They concluded that the combined system offers an improved efficiency over PV cell only especially at high sun concentrations. A similar conclusion was highlighted by Li et al. [32], a CPV-TE hybrid system can exhibits higher output electric power than that of the corresponding PV cell system and the enhancement becomes larger as the concentration ratio rises. Kil et al. [24] manufactured a CPV-TE hybrid generator which gives rise to the conversion efficiency larger than the single CPV cell by around 3% at a solar concentration of 50 suns. Yin et al. [33] reported that the one day performance of the combined PV-TE system is higher than the pure PV system when optimizing the concentration ratio and enlarging the thermoelectric figure of merit value. Mohsenzadeh et al. [34] experimentally studied a parabolic trough photovoltaic thermal collector combined with thermoelectric generators. The thermoelectric modules were integrated between silicon PV cells and a triangular channel. The results showed that the daily electrical and thermal efficiencies reached 4.83% and 46.16% respectively. Soltani et al. [35] modelled and simulated a hybrid photovoltaic thermoelectric system integrated with parabolic trough collector. Their results reflected an

Fig. 1. Schematic representation of the hybrid solar system and cross-sectional schematics of the receiver of (a) the CPVT and (b) the CPVT-TE. 3

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insulated connecting pipes. Two types of hybrid solar receivers were designed and manufactured. For the first receiver case, a hybrid CPVT prototype was constructed by attaching a heat exchanger to the rear side of a monocrystalline solar cell as shown in Fig. 5(b). A thermally conductive paste was used to ensure good heat transfer through the interfaces between the cell and the heat exchanger. The second receiver is a thermoelectric generator unit as shown in Fig. 5(c). In this unit, two thermoelectric modules TEC1-12706 were used and they were electrically series connected. The hot sides of the TEGs are bonded with a solar selective absorber plate, which faces the parabolic trough concentrator, and the cold sides are fixed to the heat exchanger. The heat exchanger was fabricated in the mechanical workshop of the CRTEn. The device is equipped with circular multi-channels and it is made of aluminum. The parallel and equal minichannels are arranged at equal spacing throughout the heat exchanger with common inlet and outlet ports. An insulation layer was inserted to reduce the thermal energy losses from the back and the edges surfaces of the heat exchanger device. For each case, the receiver prototype was fixed at the focal line of the parabolic trough collector. The concentrator tracks the sun around its axis structure in the East-West direction and reflects the incident solar radiations onto the receiver. The cooling water was driven from the tank to the heat exchanger using a circulating pump. A variable resistor was employed as an electric load. The solar radiation, ambient temperature and wind speed are recorded by a meteorological station installed at the CRTEn. For all the experimental tests, K-type thermocouples were used to measure the temperature profiles of the solar cell, the hot and cold side of the TEG modules and also the inlet and outlet water. The accuracy of the temperature sensors has a maximum deviation of ± 2.2 °C or ± 0.75%. The instruments are connected to a Keithley 2700 data system acquisition connected to a computer and a LabVIEW program was used for acquiring all the measurements.

system. 2. Systems description The structure of the concentrated photovoltaic thermal (CPVT) and the concentrated photovoltaic thermal thermoelectric (CPVT-TE) solar systems studied in this paper are depicted in Fig. 1. Both systems are mainly composed with a parabolic trough concentrator, a hybrid receiver, a storage tank, a circulation pump, thermally insulated connecting pipes and a sun tracking device. The receiver of the CPVT system comprises an array of photovoltaic cells and a heat exchanger device as shown in Fig. 1(a). The heat exchanger is bounded to the back side of PV cells to extract the dissipated heat and transfers it to the water passing through. The receiver was insulated to limit the back and lateral thermal losses. For the CPVT-TE system, the receiver is an integration of PV cells, thermoelectric generators and the heat exchanger device. The thermoelectric modules were located between the rear side of the solar cells and the heat exchanger as indicated in Fig. 1(b). The solar cells are oriented to the parabolic trough concentrator and arranged along its focal line. The concentrator tracks the sun to collect and concentrate the incident direct solar radiations onto the PV cells all the time throughout the day. During the operation of the hybrid CPVT system, the electric power is generated by the solar cells and the excess heat is removed by the fluid flowing in the heat exchanger and collected as useful thermal energy. When the CPVT-TE system is being operated, the heat dissipated by the solar cells is used by the attached thermoelectric generators to produce extra electric power. The proposed hybrid solar systems were assumed to operate during all the day. The thermal energy captured and stored can be used in medium temperature applications. The characteristics of the proposed solar hybrid systems are given in Table 1. The PV cells used are mono-crystalline silicon type. In addition to its lower cost, silicon solar cells have been proven as more well suited for the concentrating devices with medium concentration ratio (20 < C < 100) than multi-junction cells that require high concentration ratios (C > 200) in order to perform efficiently [9–11]. Among crystalline solar cells, Monocrystalline types can achieve the highest efficiency compared to polycrystalline silicon and amorphous silicon [19]. Concerning the thermoelectric generators, the selected type consists of TEC1-12706 modules. It was the adequate candidate that can be used as a thermoelectric generator due to its availability in the local market and also to its lower cost compared to other thermoelectric modules. Parameters of the parabolic trough collector field installed in the Research and Technology Center of Energy (CRTEn, Northern coast of Tunisia) are used in this study. The field consists of three modules assembled in series and fixed on a metallic rotation axis structure in the East-West direction. Each module is composed of a parabolic curved reflector having a reflectance of 0.89 and a 5.8 m long by 2.26 m aperture width. The optical properties of the parabolic trough concentrator were studied experimentally by Balghouthi et al. [39]. The parabolic trough collector field features a single-axis drive mechanism that allows the reflector to rotate around its focal axis in order to track the sun’s position during the day.

4. Mathematical model and simulation Two models were formulated to simulate the hybrid CPVT and CPVT-TE solar systems. The models were developed by applying the energy balance equations for each component of the solar collectors, including the PV cells, the thermoelectric modules, the heat exchanger, Table 1 Solar system characteristics [39]. Parameter Concentrator Reflection coefficient Intercept factor PV array Absorptance Emissivity Temperature coefficient Reference efficiency Length Width Thermoelectric modules Module dimension TE element number Lenght of TE element Cross sectional area of TE element Seebeck coefficient Thermal conductivity Electrical resistivity Heat exchanger Thermal conductivity Specific heat Density Insulation Thickness Thermal conductivity

3. Experimental setup The outdoor experimental setup was built-up at the Research and Technology Center of Energy (CRTEn) during a summer day in Tunisia. The hybrid solar receiver prototypes were tested under the concentrated solar flux generated by the parabolic trough concentrator installed at the CRTEn and shown in Fig. 2. Figs. 3 and 4 show the test rig and the schematic diagram of the experimental setup of the solar system. The main components are the parabolic trough concentrator, the receiver, the water tank, the circulating pump and the thermally 4

Value

0.89 0.7 0.85 0.95 0.0045 1/K 15% 2m 0.1 m 40 mm × 40 mm × 3.8 mm 127 1.5 mm 1.5 mm × 1.5 mm 2× 10-4 V/K 1.7834 W/mK 9.41× 10-6 Ω m 237 W/mK 903 J/kgK 2700 kg/m3 0.05 m 0.03 W/mK

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Fig. 2. The parabolic trough solar collectors installed at the CRTEn in Tunisia.

the heat transfer fluid and also for the storage tank. These models were used to predict the component temperatures, the electrical and thermal powers and the electrical and thermal efficiencies of the CPVT and CPVT-TE solar systems. The following assumptions were taken into account: -The systems are under dynamic conditions. -The fluid flow rate in the tube is uniform. -The thermo-physical properties of the solid materials of each component in the systems are considered constants. -The thermophysical properties of the heat transfer fluid are assumed dependent of its average temperature. The amount of solar radiations intercepting the surface of the PV cells located along the focal line of the parabolic trough concentrator is

a function of the concentration ratio C, the intensity I of the direct normal irradiation (DNI), reflectance ρref and the intercept factor γ of the concentrator respectively, and the absorptivity αpv and the surface area Apv of PV cells:

Qsol, pv = CIρref γαpv Apv

(1)

The electrical power produced by the PV cells is given by:

Ppv = CIρref γαpv p ηref Apv

(2)

where p is the packing factor and ηref is the PV cell efficiency calculated as [36]:

ηref = ηr (1 − βr (Tpv − Tr ))

Fig. 3. Experimental rig of the solar systems, (a) the CPVT receiver and (b) the thermoelectric generator TEG receiver. 5

(3)

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Fig. 6. Schematic diagram of thermoelectric elements in a TEG.

(4)

Voc = S (Th − Tc )

where S is the Seebeck coefficient which is an indicator of the magnitude of the Seebeck effect exhibited by the thermoelectric material. The heat energy Qh at the hot side of the TEG and the heat energy Qc at the cold side of the TEG are given by [36]:

Qh = S It e Th + Kt e (Th − Tc ) − Qc = S It e Tc + Kte (Th − Tc ) +

1 Rint It2e 2 1 Rint It2e 2

(5)

(6)

where Ite is the output current of the TEG. The internal electrical resistance Rint and the thermal conductance Kte of the TEG are calculated through the material properties and the geometrical parameters of the p- and n-type thermoelectric elements:

Fig. 4. Schematic diagram of the solar system experimental rig.

where Tpv the temperature of the PV cells, ηr is the reference cell efficiency at the reference operating temperature Tr and βr is the reference temperature coefficient which represents the amount of efficiency loss per each temperature degree rise in PV cells. A thermoelectric module is a solid state energy converter and it consists mainly of multiple thermoelectric elements, ceramic plates and conductive tabs as can be seen in Fig. 6. The thermoelectric elements are made from two dissimilar semiconductors materials positive p-type and negative n-type. Good conductive tabs, such as copper, connect the thermoelectric elements electrically in series and thermally in parallel between the two ceramic layers to form a flat array. The ceramic layers act as an electrical insulation material and mechanical substrate. A thermoelectric generator produces electric power if a temperature difference exists between its junctions. The effects occurring in the thermoelectric generator can be classified into contributions from the Seebeck effect, thermal conductance, Joule heating and Thomson effects. The Thomson effect is smaller than the Joule heating and its contribution can be significant under large temperature gradient, therefore its effect is not taken in consideration [40]. The Seebeck effect indicates that by supplying heat at one side of the TEG to reach a hot temperature Th and maintaining the other side at a lower temperature Tc, the resulting temperature difference across the TEG module induces an electrical open circuit voltage Voc given as follows:

ρp lp ρ ln + n ⎞⎟ Rint = n ⎜⎛ An ⎠ ⎝ Ap

(7)

kp Ap ⎞ k A Kt e = n ⎜⎛ n n + ⎟ l lp ⎠ ⎝ n

(8)

where ρp , ρn , kp , kn, lp , ln , Ap and An are the electrical resistivity, the thermal conductance, the length and the cross-sectional area of p- and n-type thermoelectric elements respectively. The n- and p- type thermoelements in the TEGs are identical in dimensions and material properties. The TEG can be electrically modelled as a voltage source Voc in series with an internal resistance Rint. When a set of thermoelectric generators are electrically connected in series, the resulting array of the TEGs can be simplified to a voltage source whose value is the sum of each TEGs’ open-circuit voltages, and an internal resistance equal to the sum of the individual internal resistances [40]. Fig. 7 illustrates the series connection of Nte number of TEGs, each of them represented by a voltage source Voc in series with an internal resistance Rint. When the TEG is connected with an external load Rload, the output voltage Vout, the current Ite and the electrical power Pte generated by the TEG array in the close-circuit are calculated as:

Vout = Nte Voc − Nte Rint Ite

Fig. 5. (a) The heat exchanger, (b) the CPVT receiver and (c) the thermoelectric generator TEG unit. 6

(9)

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For the CPVT-TE solar hybrid receiver, a part from the concentrated solar energy received by the PV cells is transferred by conduction to the hot ceramic layer of the TEGs. This thermal energy is conducted to the cold ceramic layer and then transferred to the heat exchanger. The TEGs convert a portion of the thermal energy received from the PV cells into electrical power by the Seebeck effect. The energy balance for the solar cells is expressed as: Fig. 7. Electrical schematic of an array of TEGs electrically connected in series.

ρpv Vpv Cp, pv

Ite =

Rint

Voc + Rload

−hcv, pv − a Apv (Tpv − Ta)

(11)

dt

ρcr Vcr Cp, cr

ρcr Vcr Cp, cr

(12)

ηele =

Ppv CIApv

(19)

The total power output Ptot of the CPVT-TE system is the sum of the power outputs of PV cells and TEG modules and can be provided by:

Ptot = Ppv + Pt e (13)

̇ p, f = hex − f Aex − f (Tex − Tf ) − mC

ηele =

dTf dx

(14)

Ppv + Pte CIApv

(21)

The thermal efficiency of the CPVT and CPVT-TE system is given as:

-Insulation layer:

dTi = hex − i Ai (Tex − Ti ) − hi − a Ai (Ti − Ta) dt

(20)

The electrical efficiency of the CPVT-TE system is defined by:

-Fluid:

ρi Vi Cp, i

(18)

The heat balance equations for the heat exchanger, water and insulation are similar to Eq. (13)–(15). The electricalηele efficiency of the CPVT system is given as:

dTex dt

dt

dTc = Nte (S Ite Tc + Kte (Th − Tc ) + 0.5Rint It2e ) dt − hcd, cr − ex Ate (Tc − Tex )

= hpv − ex Aex (Tpv − Tex ) − hex − f Aex − f (Tex − Tf ) − hex − i Ai (Tex − Ti )

ρf Vf Cp, f

(17)

The energy balance of the cold ceramic layer of TEGs:

-Heat exchanger:

dTf

dTh = hcd, pv − cr Ate (Tpv − Th) − Nte (S Ite Th + Kte (Th − Tc ) dt − 0.5Rint Ite2 )

= CIρref γαpv Apv − Ppv − hpv − ex A ex (Tpv − Tex ) −hpv − s Apv (Tpv − Ts )

(16)

The energy balance of the hot ceramic layer of the TEGs:

− hpv − a Apv (Tpv − Ta)

ρex Vex Cp, ex

= CIρref γαpv Apv − Ppv − hcd, pv − cr Ate (Tpv − Th) − hr , pv − s Apv (Tpv − Ts )

The solar energy absorbed by the PV cells is equal to the sum of the produced electric power, the heat dissipated to the ambient through convection and radiation and the thermal energy conducted to the heat exchanger. The thermal energy received by the heat exchanger is transferred to the water by convection transfer. The insulation layer exchanges the heat by conduction with the heat exchanger and by convection with the ambient air. The energy balance equations for the various components of the hybrid CPVT solar receiver are given below. - PV cells:

ρpv Vpv Cp, pv

dt

(10)

Pte = Vout Ite

dTpv

dTpv

ηth = (15)

̇ f (Tf , out − Tf , in ) mCp CIApv

Fig. 8. Variations of solar radiation, ambient temperature and the inlet temperature. 7

(22)

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The heat transfer fluid released from the hybrid solar collectors is stored in a storage tank with a global loss coefficient Ut of 1 kJ/hm2K [41]. The modeling of the storage tank was performed according to the mixing zones methodology [42]. Based on this methodology, the storage tank is separated in (n) horizontal sections with equal volume and the temperature Tst,i in each section (i) was considered to be uniform. Every section exchanges heat with the neighbor section and thus temperature stratification is created inside the storage tank. The energy balance in a section (i) is given by:

mst , i Cp, f

dTst , i = mst , i Cp, f (Tst , i − 1 − Tst , i ) − UT Ast , i (Tst , i − Ta) dt

The comparisons of simulated and measured results are reported in Figs. 9 and 10. The results illustrated in Fig. 9 show a good accordance between experimental and simulated values of PV cell temperature and electrical efficiency. The maximum deviation between the simulated and measured data was around 5% and 1.3% for the PV temperature and the electrical efficiency respectively. Fig. 10 shows the simulated and experimental values of outlet water temperature and thermal efficiency during the test day. Comparing the experiment with the simulation, the optimum deviations for the outlet temperature and the thermal efficiency were 1.7% and 10% respectively which seems to be an acceptable error level especially considering various experimental aspects such as the accuracy and the sensitivity of the temperature sensors and also the precision of the concentrating system. Fig. 11 depicts the variation of the solar radiation and ambient temperature during the experimental test of the hybrid TEG unit prototype. The solar radiation ranged between 745 and 911 W/m2 and the ambient temperatures ranged between 24 and 32 °C. Fig. 11 also illustrates the measured values of the temperatures of the hot and cold side of the thermoelectric generators. The experimental values of the electric power produced by the thermoelectric generators during the test and their corresponding simulated values are illustrated in Fig. 12. The values of the electric power deviation ranged between 0.2 and 9.3%. Therefore, both results are in good agreement which indicates the validity of the proposed mathematical model. The electrical power generated by the TEG unit is influenced by the temperature difference through the TEGs which represents an important parameter affecting the power generation by the thermoelectric modules. As can be seen in Fig. 11, the temperature difference across the TEGs showed a rise tendency with the solar radiation increase since a higher solar radiation increases the TEGs hot side temperature. Hence, the higher power is generated when increasing the temperature difference through the thermoelectric generators.

(23)

The two models of the CPVT and CPVT-TE hybrid solar systems were implemented in the Engineering Equation Solver (EES) software. The input parameters for the software program are the geometrical, optical and materials characteristics of the systems and the climatic data namely the direct normal irradiation (DNI), the ambient temperature and the wind speed. The thermophysical properties of water, used as heat transfer fluid, are given by the EES software libraries. 5. Results and discussion 5.1. Model validation To validate the models, the simulation results were compared to the measured data collected during the experimental tests. Simulations of both hybrid receiver prototypes were carried out by considering the same operating conditions as the CRTEn test facility. The congruence between simulation and experimental results was evaluated using the value of the relative error given by:

Err =

Vsim − Vexp Vexp

·100 (24)

where Vsim and Vexp are the simulated and experimental values respectively. The variation of the solar radiation, the ambient temperature and the inlet water temperature during the experimental test of the CPVT system are displayed in Fig. 8. The solar radiation varies between 761 and 917 W/m2 and the ambient temperature ranges between 26 and 33 °C.

5.2. System performance analysis This section reports the simulations results of the electrical and thermal performances of the CPVT and CPVT-TE hybrid solar systems. The two systems are evaluated through analyzing the temperature variations, electric power and thermal energy output as well as

Fig. 9. Comparison of measured and simulated results for the PV cells temperature and electrical efficiency of the CPVT solar system. 8

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Fig. 10. Comparison of measured and simulated results for the outlet water temperature and thermal efficiency of the CPVT solar system.

Fig. 11. Variations of solar radiation, ambient temperature and the temperature of hot and cold sides of TEGs.

935 W/m2 and the maximum ambient temperature was around 33 °C. Fig. 14 illustrates the daily variation of the temperature profiles of PV cells for both cases of CPVT and CPVT-TE solar systems. For the two systems, the PV cells temperature rise during the day due to the increase of solar radiation. A part of the received solar radiation is transformed into electric power by the PV cells, and a part is transformed into thermal energy which rise the cells temperature. The temperatures of solar cells in the CPVT-TE system are higher than those of the cells in the CPVT system during the day. The maximum solar cell’s temperature for the CPVT system was around 56 °C, whereas the temperature of the CPVT-TE system was around 63 °C. This rise in the PV cell’s temperature in the CPVT-TE configuration is linked to the thermal resistance of thermoelectric modules which decreases the cooling effect of the working fluid. Variations of the electrical power output of the PV array for three

electrical and thermal efficiencies. The analyses were performed considering the characteristics of the parabolic trough solar concentrator installed at the Research and Technology Center of Energy (CRTEn) in Tunisia. As mentioned above the characteristics of the hybrid solar collectors are summarized in Table 1. Analyzes were carried out by varying the load resistance (Rload) to find its optimum value for maximum hybrid CPVT-TE system power output and efficiency. Based on these analyzes, the CPVT-TE system shows a maximum efficiency with a load resistance of 1.73 Ω which is different from the value of the internal electrical resistance of the TEG equal to 1.59 Ω. The climatic data such as direct normal irradiation (DNI), ambient temperature and wind speed, were acquired from the high precision meteorological station located in the south of Tunisia [3]. The climatic conditions, shown in Fig. 13, of a sunny day in Tunisia were considered in the simulations. As illustrated, the solar radiation achieved a maximum value of about 9

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Fig. 12. Comparison of measured and simulated results for the electric power generated by the TEG receiver.

Fig. 13. Daily variations of solar radiation, ambient temperature and wind speed during a summer day in Tunisia.

increases with the increase in solar radiation due to higher temperature difference provided between its hot and cold sides. Therefore, the advantage of the CPVT-TE system over the CPVT becomes more important as the solar radiation increases. The electrical efficiency of the CPVT and CPVT-TE systems are illustrated in Fig. 16. Both curves show a decrease tendency with increase in PV cells temperatures due to the negative impact of operating temperature increment on the efficiency of solar cells. An enhancement of around 7.46% in the electrical efficiency is achieved through the CPVT-TE hybrid system as compared with the CPVT system. Fig. 17 presents the variation of the fluid temperatures at the outlet of the receiver and in the storage tank for the CPVT and CPVT-TE systems during the day. It is shown that the temperatures of outlet

cases: PV in the CPVT system, PV in the CPVT-TE and overall output PV +TE in the CPVT-TE system, and also the power output of thermoelectric generators are presented in Fig. 15. The trends of the output power of PV cells were highly correlated with the daily variation of solar radiation for both hybrid solar systems. The thermal resistance caused by the TEG modules increases the temperature of the PV cells, so that the power generated by the solar cells in the CPVT-TE was reduced as compared with the output power of the cells in the CPVT system. On the other hand, the created temperature difference between the hot and cold sides of the TEG generates an additional electrical power which allows compensating the reduced power by the PV cells in the CPVT-TE system. As a result, higher electric power is generated in the CPVT-TE compared to the CPVT system. The power output of TEG modules

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Fig. 14. Variations of PV cells temperature for CPVT and CPVT-TE systems during a summer day.

Fig. 15. Variations of electrical output power of PV cells for three cases: PV in the CPVT system, PV in the CPVT-TE system and overall power PV+TE in the CPVT-TE system and output power of TEGs during a summer day.

focused solar radiation, which occurs around noon. The CPVT exhibits higher useful thermal energy and higher thermal efficiency than the CPVT-TE. This is because a part of the thermal wasted by the PV cells was converted into electricity by the thermoelectric generators. The CPVT and CPVT-TE offers a maximum thermal efficiency of 47.35% and 46.13% respectively. A comparison between the proposed CPVT and CPVT-TE hybrid solar systems and similar systems was performed. The purpose of this comparison is to show the performance superiority of the solar systems

water and water in the storage tank for the CPVT were slightly higher than those of the CPVT-TE system. Both hybrid solar collectors could provide a stored water temperature around 49.19 °C and 48.34 °C for the CPVT and CPVT-TE system respectively which is an acceptable temperature for applications that require medium temperature heat. The thermal efficiency and the amounts of the useful energy gain obtained from the extraction of the thermal energy from the PV cells in the CPVT and CPVT-TE are depicted in Fig. 18. For both solar systems, it is observed that the thermal energy rise with the increment in the 11

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Fig. 16. Variations of electrical efficiencies of the CPVT and CPVT-TE systems during a summer day.

system investigated in [34] which is based on a parabolic trough photovoltaic thermal collector combined with mono-crystalline cells and thermoelectric generators. The electrical efficiency reported in [34] was 4.83% whereas the present CPVT-TE system reached an electrical efficiency of 7.27%. Therefore, the results of the comparisons showed the superiority of the proposed CPVT-TE hybrid solar system.

considered in this study. The analysis was made on hybrid solar systems based on similar design that use parabolic trough concentrator and mono-crystalline silicon cells. The electrical efficiency of the CPVT system studied in [11] was 6.4% and the electrical efficiency found in [19] was 6%. The electrical efficiency of the proposed CPVT system reached 6.76% which is in similar range but higher than that of the systems studied in [11] and [19]. In addition, it can be noticed that the CPVT system studied in this work could provide a stored water temperature around 49 °C which can be used in medium temperature applications. The proposed CPVT-TE system was compared with the

6. Energy saving and CO2 emissions reduction The use of the proposed hybrid solar systems aims to minimize the

Fig. 17. Variations of temperature of the fluid in the storage tank and outlet fluid temperature for CPVT and CPVT-TE systems during a summer day. 12

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Fig. 18. Variations of thermal energy and thermal efficiency of the CPVT and CPVT-TE system during a summer day.

of CPVT-TE system was enhanced by the integration of the TEG modules. The annual electric energy production was increased from 5930 kWh for the CPVT to 6289 kWh for the CPVT-TE. Thus, an extra electric energy of 359 kWh could be generated by the TEGs. It was found that the CPVT has the greater annual useful thermal energy than the CPVTTE. This is explained by the lower values of the useful thermal energy collected when the integration of the TEGs. The monthly distribution of the energy saving of gasoil due to electric energy and useful thermal energy production of the CPVT and CPVT-TE solar systems are illustrated in Fig. 20. The CPVT-TE has a higher annual amount of gasoil due to the electric energy as a direct result of its highest electric output by comparing with the CPVT. The annual fossil energy reduction corresponds to saving 1645 L and 1550 L of gasoil due to the electric output of the CPVT-TE and CPVT respectively. The quantities of CO2 emissions due to the electric energy are higher for the case of the CPVT-TE while those due to the thermal energy are higher for the CPVT system (Fig. 21). The annual avoided amount of CO2 emissions due to electric energy reaches 4277 kg and 4032 kg for the CPVT-TE and CPVT respectively. Both solar systems are able of displacing considerable quantities of primary energy and cutting down CO2 equivalent emissions by producing

fossil energy resources consumption resulting also in a reduction of their associated CO2 emissions. In this section, an assessment is presented to identify the most favourable hybrid solar system in terms of the monthly and the yearly electric and thermal energy production, the quantities of primary energy saved and the amount of avoided CO2 emissions for a typical year in Tunisia. The considered parabolic trough solar concentrator installed at the Research and Technology Center of Energy in Tunisia has a total aperture area of 39 m2. According to the investigation of the National Agency for Energy Management in Tunisia ANME [43], the conversion factor to calculate the quantities of CO2 emitted into the atmosphere is 0.68 kg of CO2 per 1 kWh of electrical energy. Moreover, a quantity of 2.6 kg of CO2 is released by the combustion of 1L of gasoil. The monthly electric and thermal energy production of the CPVT and CPVT-TE solar systems for a typical year in Tunisia are shown in Fig. 19. The electric and thermal energy production have a similar trend of variations throughout the year for both systems. The highest electric and thermal outputs are reached in the summer months (June, July and August). While, the lowest electric and thermal energy productions are obtained in the cold period (November, December and January). Comparing with the CPVT system, the monthly electric energy output

Fig. 19. Monthly distribution of the electric energy (a) and useful thermal energy (b) production of the CPVT and CPVT-TE solar systems. 13

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Fig. 20. Monthly distribution of the energy saving of gasoil due to electric energy (a) and useful thermal energy (b) production of the CPVT and CPVT-TE solar systems.

• The stored water temperature exceeds 48 °C for the CPVT-TE hybrid solar system which is an acceptable range for medium temperature applications. • An annual extra electric energy of 359 kWh could be generated by the CPVT-TE due to the integration of the thermoelectric generators. • From an environmental perspective, the CPVT-TE solar system is able of displacing considerable quantities of primary energy and cutting down CO2 equivalent emissions by producing electrical power and preheated water simultaneously using environmentally friendly and sustainable energy. • The present study is beneficial for the large-scale application analysis of the concentrated photovoltaic thermoelectric hybrid solar systems and to assess its suitability for the provision of electricity and hot water. The improvement of material physical properties of the TEGs is recommended in order to boost its contribution on power generation on the integrated configuration. Flexible thermoelectric materials as well as flexible solar cells can be another option for the next generation of the integrated photovoltaic thermoelectric solar systems.

electrical power and pre-heated water simultaneously using environmentally friendly and sustainable energy. 7. Conclusion A concentrated photovoltaic thermal (CPVT) and concentrated photovoltaic thermal thermoelectric (CPVT-TE) hybrid solar systems were studied in this work. A hybrid CPVT and thermoelectric generator unit prototypes were fabricated and outdoor experiment were conducted. The collected measurements were employed to validate the proposed mathematical models. By means of the developed and validated models, the electrical and thermal performances of the CPVT and CPVT-TE solar systems were analyzed in terms of the operating temperatures, the electrical and thermal powers and the electrical and thermal efficiencies. In addition, the monthly and the yearly electric and thermal energy production, the primary energy saved and avoided CO2 emissions were evaluated for the proposed hybrid solar systems. The main outcomes that may be drawn from the current work are summarized as below. • The integration of the thermoelectric generators with the PV cells improves the output electric power and the electrical efficiency of the CPVT-TE solar system. • The electrical efficiency of the CPVT-TE was enhanced by 7.46% as compared with the CPVT solar system.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 21. Monthly distribution of the avoided CO2 emission due to electric energy (a) and useful thermal energy (b) production of the CPVT and CPVT-TE solar systems. 14

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