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Energy 147 (2018) 547e560

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Energy

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Conceptual design and performance evaluation of a hybridconcentrating photovoltaic system in preparation for energy

Hasan Baig a, J. Siviter b, W. Li b, M.C. Paul b, A. Montecucco b, M.H. Rolley c, T.K.N. Sweet c,M. Gao c, P.A. Mullen b, E.F. Fernandez d, G. Han e, D.H. Gregory e, A.R. Knox b,Tapas Mallick a, *

a Environmental and Sustainability Institute, University of Exeter, Penryn, UKb School of Engineering, University of Glasgow, Glasgow, UKc Cardiff School of Engineering, Cardiff University, Cardiff, UKd Centre for Advanced Studies in Energy and Environment, University of Jaen, Jaen, Spaine WestCHEM, School of Chemistry, University of Glasgow, Glasgow, UK

a r t i c l e i n f o

Article history:Received 10 August 2017Received in revised form7 December 2017Accepted 24 December 2017Available online 28 December 2017

Keywords:LCPV/TCPCCCPCHybrid

* Corresponding author.E-mail addresses: h.baig@exeter.ac.uk, mail2ba

mallick@exeter.ac.uk (T. Mallick).

https://doi.org/10.1016/j.energy.2017.12.1270360-5442/© 2018 The Authors. Published by Elsevier

a b s t r a c t

Concentrating sunlight and focussing it on smaller sized solar cells increases the device's power outputper unit active area. However, this process tends to increase the solar cell temperature considerably andhas the potential to compromise system reliability. Adding a heat exchanger system to regulate thistemperature rise, can improve the electrical performance whilst simultaneously providing an additionalsource of low temperature heat. In this study the performance of a low concentrator photovoltaic systemwith thermal (LCPV/T) extraction was conceptualised and evaluated in depth. An experimental analysiswas performed using a first-generation prototype consisting of 5 units of Cross Compound ParabolicConcentrators (CCPC) connected to a heat extraction unit. A bespoke rotating table was used as exper-imental apparatus to effectively evaluate the optical performance of the system, as a function of itsangular positions to replicate the motion of actual sun. Key design performance parameters for the LCPV/T collector are presented and discussed. This work also provides a useful technique to effectively calculatesystem performance, as a function of the orientation-dependant electrical characterisation parametersdata. Finally, a Computational Fluid Dynamics (CFD) model was also applied to investigate the efficacy ofthe heat exchanger and hence estimate the overall co-generation benefit of using such optimisationtechniques on realistic CPV systems. It was highlighted through these simulations that the water flowrate had the potential to be a critical power-generation optimisation criterion for LCPV-T systems. Themaximum power output at normal incidence with concentrators and no water flow was found to be78.4mW. The systemwas found to perform with an average electrical efficiency ranging between 10 and16% when evaluated at five different geographic locations. Experimental analysis of the data obtainedshowed an increase in power of 141% (power ratio 2.41) compared to the analogous non-concentratingcounterpart. For example, in the case of London which receives an annual solar radiation of 1300 kWh/m2 the system is expected to generate 210 kWh/m2. This may reduce further to include losses due totemperature, reflectance/glazing losses, and electrical losses in cabling and inverter by up to 36% leadingto an annual power output of 134 kWh/m2 of module.© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Photovoltaics (PV) cells are manufactured from semi-

ig@gmail.com (H. Baig), t.

Ltd. This is an open access article u

conductors that inherently in their deployment operate on alogical paradox e they need sunlight to generate electricity butsuffer a degradation in performance as they get hotter. HybridPhotovoltaic-Thermal (PV/T) technologies combine a photovoltaiccell, which converts electromagnetic radiation (photons) intoelectricity, with a solar thermal collector that captures theremaining energy and removes waste heat from the PV module.

nder the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

http://creativecommons.org/licenses/by-nc-nd/4.0/mailto:h.baig@exeter.ac.ukmailto:mail2baig@gmail.commailto:t.mallick@exeter.ac.ukmailto:t.mallick@exeter.ac.ukhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.energy.2017.12.127&domain=pdfwww.sciencedirect.com/science/journal/03605442http://www.elsevier.com/locate/energyhttps://doi.org/10.1016/j.energy.2017.12.127http://creativecommons.org/licenses/by-nc-nd/4.0/https://doi.org/10.1016/j.energy.2017.12.127https://doi.org/10.1016/j.energy.2017.12.127

H. Baig et al. / Energy 147 (2018) 547e560548

Such a system can provide both electrical and thermal energy at thesame time whilst also reducing the area required to install twoseparate systems.

A further improvement to these systems can be made whencombined with concentrated sunlight. The solar cell area can beeffectively reducedwithout compromising on the obtained thermaloutput of system [1]. Low concentrator photovoltaic (LCPV) tech-nologies (solar concentration

H. Baig et al. / Energy 147 (2018) 547e560 549

2. System design & manufacture

2.1. System components

(a) Optics

The optical design used in the system was based around thewell-established reflective 3D Crossed Compound ParabolicConcentrator (3DCCPC). The profile of a CPC was swept around asquare to generate the used 3DCCPC profile [6]. The opticalacceptance of this geometry was calculated to be ±30�. The opticalacceptance of the geometry is designed to achieve useful concen-tration without tracking. The acceptance range helps in concen-trating most of the direct sunlight within which occur during the±2 h of the solar noon. Previously [16] similar geometry was used ina building integrated concentrating photovoltaic system to maxi-mise the energy yield. The optic was designed to have an exitaperture of 1 cm2 and had a truncated height of 15.8mm with anentrance aperture sized 3.6 times larger than the exit aperture. Onekey benefit of such a design criterion is that it permits a significantcollection of sunlight throughout the day. Another benefit, was thatin comparison to other high concentration systems which requirevery expensive optics, these type of optics can be manufacturedvery cheaply. A groovewasmade along the base of the concentratorfor creating a robust electrical connection between the adjacentsolar cells within a string as shown in Fig. 2. This groove also helpedin maintaining the vertical alignment of the solar cells, a factorcrucial for facilitating effective optical coupling. Ray tracing is astandard technique for evaluation of the optical performance ofconcentrators. A given number of rays can be traced to representthe solar radiation incident on a concentrating system. In thispresent study, all incident rays were assumed to be parallel andcarry an equal amount of energy. These incident rays are reflectedby the parabolic profile and hence are concentrated on the solar cellsurface as shown in Fig. 3.

Ray-trace simulations were carried out using the APEX® soft-ware package [22]. The optical properties of the reflective materialused for making the 3DCCPC and the spectrum of the incomingsolar radiation were kept in accordance with the experimentalsetup. The light distribution reaching the solar cell after concen-tration under normal incidence showed a non-uniformity [4] at thefour corners of the solar cell. The impact of this was however foundto be negligible on the overall performance of the system. For thisdesign, the cell areawas slightly bigger than the exit aperture of theoptical unit to reduce any ohmic losses. Simulations show that thedesign exhibits a wide range of acceptance angles of ±30� as shownin Fig. 4. The maximum optical efficiency of the unit directly

Fig. 2. A single unit of the concentrating optical element.

depends on the quality of the reflective surface used within themanufacture of the optic. In the present study, we utilised areflective coating comparative to the reflectance achieved bythermal evaporative coatings which have an average reflectance of96%. However, it is important to note that it is subject to the surfacefinish of the substrate.

(b) PV Cell

The laser grooved buried contact (LGBC) process is an effectiveway to manufacture high efficiency solar cells using mono-crystalline silicon. In this process, a laser is used to inscribe groovesinto the silicon cell where fingers for the front contact are depositedby electro-plating technique which reduces the shading lossesoccurring due to the fingers. These cells are suitable for low tomedium concentration systems due to this low contact shading [4].Further details on the technique have been previously described inliterature [23]. In general, smaller cells ensure a reducedmaterial inaddition to facilitating effective heat transfer in the system. In thepresent work, we have utilised solar cells having an active area of1 cm2 as shown in Fig. 5. The LGBC solar cells used in this systemhave been optimized to be used up to 50� .

(c) Copper Tube heat sink

A (13mmx13mm) square shaped copper tube was chosen foruse as the heat exchanger. The tube had a 0.1mm wall thicknesswith capped ends for ease of connection to the coolant network.

2.2. Prototype manufacturing

The optics as previously described, were fabricated using 3Dprinting with a Fused Deposition Modelling (FDM) technique. Inthis process, the CAD design of the component was imported intoassisted software, wherein it is positioned in the 3D co-ordinatespace relative to the 3D printer software datums. The design wasthen digitally “sliced” into layers, which hence described themovement path for the extruder head depositing the thermoplasticmaterial. The process is a simple and an effective way of producingbespoke, high quality optical prototypes of complex geometries andcurvatures. The printed prototype was then sanded to improve itssurface finish. The plastic prototype was then coated with itsreflective coating using thermal evaporation of aluminium. Thereflective coating was further protected using a siloxane filmdeposited using plasma process as shown in Fig. 6.

The copper tube was cleaned prior to assembly to remove anyoxidation layers that were present. The tube was marked as per thesolar cell dimensions, leaving enough space to accommodate thesolar concentrators. Along these marked locations, a thermal ad-hesive (PC 3001 Heraeus) was applied in limited and measuredquantities. The silver-filled epoxy conductive adhesive is solventfree and of the thermosetting type, so requires oven curing at200 �C. The key advantage of using this adhesive is that it has highelectrical and thermal conductivity (>5W/m. K). Any excessspillage of the adhesive along the edges of the solar cell can causeshort-circuiting, emphasising the need of a careful manufactureprocess and appropriate considerations to be taken throughout thedesign stages. A summary of the complete assembly and manu-facture method is shown in Fig. 7.

A quality-control check was performed to ensure there were noshort-circuited solar cells after bonding. A k-type insulation wasplaced between the solar cells to ensure they were adequatelylocated as is crucial for effective optical coupling to the cell activearea. Further the solar cells were hand-soldered to connect the busbars in parallel. The copper tube acted as a rear-side common

Fig. 3. Ray tracing of the optical concentrator and irradiance flux profile on the solar cell.

Fig. 4. Optical efficiency as a function of the incidence angle using ray tracing.

Fig. 5. LGBC solar cell used in the system.

Fig. 6. Optics manufactured using 3d printing and evaporative coating.

H. Baig et al. / Energy 147 (2018) 547e560550

terminal due to thismethod, streamlining themanufacture process.A voltage check was then carried out to ensure that all the solarcells were appropriately connected. Once this is done the 3-d printed optical concentrator was attached on the top of the so-lar cell to complete the assembly of the unit using a thin layer ofSylguard encapsulant. This encapsulation layer protected the solarcell against any environmental damage and hence improved devicerobustness.

3. Experimental tests and multiphysics simulation

This section provides details of the indoor experimental char-acterisation of the concentrator unit evaluating its optical, elec-trical, and thermal performances. The IV characterisation of thesystem is reported based on the angular and intensity variations ofthe incoming irradiance. The performance of the system under thesolar simulator was compared with a non-concentrating counter-part to evaluate the effective optical efficiencies.

3.1. Rotating table

Typically, the testing of PV cells is carried out using standardillumination conditions of AM1.5G at 1000W/m2 and a tempera-ture of 25 �C. However, this limits our understanding when evalu-ating solar concentrator systems. Previously [24], a manuallyoperating inclination table was used to evaluate the performance ofthe concentrating photovoltaic system. However, this limits theangular performance variation in only one direction and is moresuitable for the linear solar concentrators. The incoming solar ra-diation is predominantly a function of the geographical location,day, and the time of the year. To simulate the performance of thissystem a customised system was developed that can virtuallysimulate the sun intensity levels at any point on the earth and atany time or over a period. The system in theory allows for the ac-curate positioning of a PV cell array having an area of 25� 25 cmdirectly beneath a collimated light solar source while altering azi-muth and elevation e as relating to the geographical position andtime required. By applying previously developed solar time andlocation calculations and algorithms, it was possible to develop atest system that could position the PV cell underneath a solarsource to simulate any solar illumination on the earth. The algo-rithm selected for this test system was based on [25] to take thedate, time and desired location on the earth, and then convert thisto the solar azimuth and elevation. The work has been expanded toinclude the elevation however for the purposes of evaluation of thetest system this has been excluded and assumed to be sea level. Thisalgorithm was then converted into a MATLAB script and interfacedinto a test program Graphical User Interface (GUI).

The test system consisted of five axes of movement: x, y, rota-tion, azimuth, and elevation. The solar azimuth and elevation were

Fig. 7. Processes in the prototype manufacture.

Fig. 9. Depiction of solar zenith, solar elevation and solar azimuth.

H. Baig et al. / Energy 147 (2018) 547e560 551

translated to the test system to account for the difference in zenithand azimuth as shown in Fig. 8. A schematic showing these anglesis presented in Fig. 9. The test system operated by the movement ofvarious platforms controlled by stepper motors. The azimuth andelevation were actuated upon set gear ratios that allow for a timeaccuracy of 10 s over a 24 h period. The test system independentlyactuated five stepper motors, three scaled to the gear ratios of theazimuth and elevation axes and two for the x and y positioning.Each stepper motor was individually controlled to allow forsimultaneous movement, hence allowing for quick and near-realtime positioning. The test system was built on an Arduino plat-form consisting of five stepper motor driver controllers and a serialinterface to communicate to a MATLAB based platform. The azi-muth and elevation controls were calculated independently andthen translated to several steps to pulse the stepper motor. Throughappropriate optimisation of the software code, the MATLAB inter-face effectively queued a series for instructions to the test system,allowing for the continuous operation of all axes simultaneouslywithout an additional processing delay. The test system comprisingthe movement of x, y, rotation, azimuth, and elevation axes ‘reset’to allow for accurate positioning from when the system calculatesand moves to a desired position. The ‘reset’ functionality wasachieved using limit detection on the x and y axes, and gyroscopicaccelerometer readings for the elevation and azimuth axes. The 3-axis gyroscope is based on the Mems Sensor L3G4200D [26],reporting the roll (q-axis), pitch (u -axis) and yaw (j -axis) of thesolar tracker table as shown in Fig. 10 for use within the Arduinoalgorithm.

To ensure the accurate positioning of the elevation axis, theangle of the platform was compared automatically, as a feedbackloop to the motor driver controller, with both the number of stepsthe axis moves by and the resultant measured gyroscope angle.Hence any geometrical positioning errors were compensated for byadjusting the respective stepper motor axis to this corrective gy-roscope reading.

Fig. 8. Test system showing each of the axes.

Fig. 10. 3-Axis Gyroscope based on L3G4200D.

The testing methodology used, placed the PV cell on the hori-zontal plane and then measured the I-V characteristics at a givenconstant irradiance. To simulate the movement of the sun over thecourse of the day, the platform repositioned for each successivepoint in time. The desired date, geographical coordinates and time

Fig. 12. Rotating table setup.

H. Baig et al. / Energy 147 (2018) 547e560552

steps were input into the MATLAB GUI and the resultant azimuthand elevation data outputted to the motors to effectively plot thecourse of the sun over the defined time intervals. As part of thetesting for this work the solar tracker was set to calculate the in-cremental number of steps from the present position rather thanperform a ‘reset’ operation each time.

The source for the solar radiation (Class AAA Wacom SolarSimulator, collimation angle 1.43) was also interfaced to the MAT-LAB program, allowing the source radiation is read in as a variableand likewise a parameter that could be adjusted automaticallydepending on the experimentally desired irradiation (Fig. 11). Theonly limitation of the setup is that we cannot control the change inspectrumwhile changing the angle and the light intensity. This maylead to some mismatch with theoretical predictions due to thewavelength dependence of each of the materials used.

3.2. Experimental setup

Detailed parameters can be extracted from the IeV character-istics of a solar cell, and as such this technique is considered as thebest method to evaluate the cell electrical performance. A specialsetup with the rotating table as shown in Fig. 12, was used tochange the angular position of the concentrator unit designed inthis work. A “AAA” class WACOM solar simulator was used as thesource of collimated light [27] at a standard irradiation of 1000W/m2 and AM1.5G spectrum and can be modulated as required.

The IV-curve measurement of the devices was carried out usingan IV curve tracer (EKO Model no MP160). The IV tracer had amaximum rated power measurement for PV devices up to 300W.This instrument can be used for both indoor characterisation of PV/CPV modules, using two different software packages. Importantparameters such as, the maximum power point (MPP), fill factor(FF), open-circuit voltage (Voc), short circuit current (Isc), and shortcircuit current density (Jsc) can be extracted from the IV curve dataobtained during the experiment. To start the experiment, the solarsimulator was switched on for 1 h to warm up thereby establishinga steady energy flux output over the illumination area. The shutterwas kept closed during this time. Once the solar simulator hadwarmed up, the light intensity at the working plane and area wasmeasured using a calibrated solar cell. Both the non-concentrating(bare) and the concentrating devices were placed on the rotatingtable which was then tilted and rotated at angular intervals of 5� torecord the I-V characteristics at each position. The IV-tracer used a4-wire connection, 2 positive and 2 negative terminals to eliminatecable resistance errors during measurement. One pair of positiveand negative connections were used to apply a biased voltage andthe other pair were used to sense outputs from the module as

Fig. 11. Control and interface pogram in MATLAB

shown in Fig. 13. Thermal insulationwas applied around the coppertube to prevent any heat losses from the copper tube and maintaina steady temperature of the device minimise the deviation from theadiabatic boundary conditions set in the thermodynamicmodellingof the system, described later in this paper. Additionally, thermo-couples were added at the inlet and the outlet of the copper tube tomonitor the temperature of the cooling water.

3.3. Models and methods in multiphysics simulations

To carry out an effective optical analysis, the light source wasswitched on and off instantly to carry out the I-V measurements ofthe device. However, in realistic operation conditions, devicestypically operate under transient conditions where the incomingwater temperature and the amount of incoming solar radiationchanges with the time of the day. To assess the steady state per-formance of the system an initial thermal performance analysis wascarried out and linked with a numerical simulation as a basis to befurther extended and predict the device performance in real timeoperation.

In this model, the cooling water was modelled to pass throughthe system over small intervals of time until a steady state wasachieved under a fixed light intensity conditions. During these in-tervals, the temperature of the water entering and leaving the unitwas measured with thermocouples as shown previously in Fig. 13.Since measuring the solar cell was difficult it was important tosimulate the system using finite volume methods. A multiphysicssimulation study was carried out to predict the cell temperature ata certain flow rate under cooling and no-cooling conditions. In thetransient simulations, the measured ambient temperature andwater temperature at the inlet of the heat exchanger were used asinput conditions. The computational domain for the multiphysicssimulations included five CCPC's and their respective PV cells, a13mm� 13 mm squared cross-sectional copper tube with 0.5mmwall thickness, and a Sylguard layer as an optical coupling for thesolar cells and the LCPV optics. The volume of fluid flow through inthe tube and the five internal air bodies corresponding to the fivecavities of CCPC, were also included as shown in Fig. 14.

Within the Multiphysics simulations, the material propertieswere appropriately selected to match with the experimental setupand are listed in Table 1. The multiphysics simulations were carriedout using ANSYS CFX 15.0 in transient mode. In the air fluid domain,the 3D incompressible laminar flowmodel, natural convective heat

Fig. 13. Setup shows the system (a) without the concentrator (b) with the concentrator.

Fig. 14. Computational Domain of the system.

H. Baig et al. / Energy 147 (2018) 547e560 553

transfer and the radiation transportation equations were solved.The Bousinessq approximation was involved in the flow model bymeans of air expansion and the grey radiative implication was heldconsistent through the radiation model. In the CCPC and Sylguardlayer solid domains, the conductive heat transfer and radiationtransportation equations were used to improve themodel accuracy.In the copper tube solid domain, the conductive heat transferequations were selected, while for the fluid domain the forcedconvective heat transfer equations and 3D laminar incompressibleflow models were implemented. Details of these equations can be

Table 1Thermal, optical, and radiative property constants of materials in CCPC module at 25 �C.

Property constant Water Air

Density, kg/m3 997.0 1.185Dynamic viscosity, Pa.s 8.899e-4 1.831e-5Specific capacity, J/(kg K) 4181.7 1004.4Thermal conductivity, W/(m K) 0.069 2.61e-2Refractive index e 1.0Absorption coefficient, 1/m e 1.0e-2Scattering coefficient, 1/m e 0.0Emissivity e eCoefficient of Thermal expansion, 1/K e 3.356e-3

found in the appendix.Simulations were carried out using consistent atmospheric

input conditions recorded from the experiment, of 1000W/m2

incoming solar irradiance and a water flow rate of 0.3lit/min. Bothconvective and radiative heat transfer effects were included in thesimulation, a thermal emissivity 0.06 of aluminiumwas used on allthe surfaces exposed to the ambient air for the CCPCs and Sylgardlayer because the CCPCs are made of aluminium. The emissivity ofSylgard was not found in literature, here we gave to it to be 0.96 ofglass. The free convective heat transfer coefficient of 10W/(m2 K)was applied on all the surfaces exposed to the ambient air as well.In the experiments, an air conditioner was running, and air flowvelocity is around 1m/s, then the convective heat transfer coeffi-cient can be estimated 10W/(m2 K), see Ref. [28].

An adiabatic boundary conditionwas fixed to the two side cross-sections of the copper tube, to replicate the thermal insulation asused during the experiments. Based on the flowrate, the velocitymagnitude (2.95� 10 �2m/s) was applied as a boundary conditionat the inlet surface of the tube. At the outlet of the tube, the waterstatic pressure was set to be 0 Pa, as is consistent with a “return totank” line. No-slip boundary conditions were applied on the wetwalls to simulate the frictional boundary films present in real heatexchangers.

There were a few interfaces having set up between two differentcomputational domains, such as the interface between the air bodyin CCPC cavities and CCPC wall, the interface between the air bodyand PV cell surfaces, the interface between one PV cell surface andone surface of the Sylgard layer and the interface between onesurface of the Sylgard layer and the top surface of the tube.

Sylgard PV cell CCPC Tube

1030 2330 840 8933e e e e

1100 712 903 3850.27 148 0.19 4011.42 4.0 1.373 e2.0 7eþ4 307 e0.0 0.0 0.0 e0.06 e 0.96 ee e e e

H. Baig et al. / Energy 147 (2018) 547e560554

Themultiphysics simulation above was based on a finite volumemethod. The radiative heat transfer for the air bodies, CCPCs, PVcells and Sylgard layer was solved using theMonte Carlo method. Indoing so a set of meshes, with 610,403 nodes and 1,747,568 ele-ments at 0.5mm element size and 0.05mm minimum edge lengthrespectively, were generated using the Meshing module of ANSYS[28]. This used hexahedrons for the heat exchanger tube, tetrahe-drons for the air bodies, CCPCs and Sylgard layer, wedges for thefluid domains and pyramids for the air bodies and CCPCs. Also, a 10-layer boundary layer mesh was patched on the tube wet walls toidentify the fine flow structure near the walls. The mean elementquality, which is based on the ratio of the volume to the sum of thesquare of the edge lengths for the square root of the cube of the sumof the square of the edge lengths for 3D elements, was calculated tobe 0.7295 with a standard deviation 0.1997. Usually, a value of 1indicates a perfect cube or square element while a value of 0 in-dicates that the element has a zero or negative volume. Here theobtained 0.7295 mean element quality suggested that the meshgenerated in a good quality and the result obtained from ANSYSwere a close representative of a realistic system. A part of the meshgenerated from these procedures is shown in Fig. 15.

4. Results & discussion

4.1. Electrical performance

For each set of readings at a given light intensity and inclinationangle, IV measurements were taken for both the non-concentratingand the concentratingmodule. The performance of the CPVmodulewas compared with that of the non-concentrating module for each

Fig. 15. Mesh generated in the computational domain.

desired inclination angle. Fig. 16 shows a set of I-V curves for bothdevices with and without the optical concentrator. As expected, acorrelation was seen regarding how the electrical output droppedwith an increasing incident angle. The short circuit currentremained stable at 400mA for angles of incidence of 0, 10 and 20but dropped as the half acceptance angle of 30� was approached.The maximum power output at the “normal incidence” conditionfor the prototype with concentrators and no water flow was foundto be 78.4mW. The fill factor was recorded to be 72%, which wasslightly lower compared to that of the bare solar cell recorded to be80%, this is primarily an impact of the non-uniformity of the illu-mination on the solar cell.

4.2. Optical performance

The optical efficiency can be defined as the ratio of solar radia-tion received by the absorber to the radiation incident on the inputaperture. The optical efficiency accounts for all possible reflectionand transmission losses on the aperture cover. The impact of thesun position on the optical performance was evaluated from theobtained experimental data, while varying the tilt (altitude) andthe rotation angle (azimuth) using the setup described earlier andas shown in Fig. 17. Using the parameters obtained via electricalcharacterisation of the system, the optical efficiency was computedusing Eq. (1) as shown below, where Isc refers to the short circuitcurrent and CG is the geometric concentration ratio 3.6� .

Optical Efficiency ¼ Iwith concentratorSC

Iwithout concentratorSC*1CG

(1)

The maximum optical efficiency of the system was found to be67% under normal incidence. The optical efficiency remained con-stant within the angular acceptance range of the concentrator andthen as expected from theory was found to drop significantlythereafter as shown in Fig. 18. Experimental analysis of the dataobtained showed an increase in power of 141% (power ratio 2.41)compared to the analogous non-concentrating counterpart asshown in Fig. 19. The reflective losses from the rough parabolicsurface further reduces the overall optical efficiency of the system,and hence highlights an area of prototype performance improve-ment with a different manufacturing procedure.

Using the Matlab curve fitting tool the optical efficiency ob-tained during the experiment was converted into a function ofazimuth (a) and altitude angle using Eq. (2) shown below. A fifth-degree polynomial was obtained with an R2¼ 0.9844. Using thisequation, the optical efficiency for any geographic location canhence be evaluated for any given time and day of the year. Using thegeographic location data, the optimum tilt can be estimated. Usingthis information, the incoming solar radiation and altitude andazimuth angles for any given day and time of the year. Using Eq. (2)the optical efficiency can be calculated and the expected electricaloutput from the device estimated.

Eff ða; hÞ ¼ p00 þ p10*a þ p01*h þ p20*a2̂ þ p11*a*hþ p02*h2̂ þ p30*a3̂ þ p21*a2̂*h þ p12*a*h2̂þ p03*h3̂ þ p40*a4̂ þ p31*a3̂*h þ p22*a2̂*h2̂þ p13*a*h3̂ þ p04*h4̂ þ p50*a5̂ þ p41*a4̂*hþ p32*a3̂*h2̂ þ p23*a2̂*h3̂ þ p14*a*h4̂þ p05*h5̂

(2)

Using the above-mentioned methodology, the monthly average

Fig. 16. IV-characteristics and power curve of the prototype non-concentrating (bare) module and the concentrator module with 1000W/m2 radiation intensity incident at 0� , 10� ,20� and 30� .

Fig. 17. Tilting and rotating the CCPC device.

Fig. 18. Variation of the optical efficiency with the changing tilt and rotation.

Fig. 19. Power ratio of the system.

H. Baig et al. / Energy 147 (2018) 547e560 555

solar insolation and the expected power output per m2 of the solarcell materials used for six different cities namely Madrid, London,Delhi, Vancouver, Ottawa and Riyad has been forecasted in Fig. 20.

The Meteonorm weather data was used to develop a simpleTRNSYS model to evaluate the incident solar radiation and thehourly zenith and azimuth angles on the most optimum tilt planefor a given location as shown in Fig. 20(a). The optical efficiency ofthe CPV/T system was evaluated using Eq. (2), which was thentranslated to the estimation of electricity generation per m2 of thephotovoltaic material. These values were integrated daily to esti-mate the monthly performance as shown in Fig. 20(b). The systemperforms with an average electrical efficiency ranging between 10and 16% at different locations. For example, in the case of Londonwhich receives an annual solar radiation of 1300 kWh/m2 the sys-tem is expected to generate 210 kWh/m2. This may reduce furtherto include losses due to temperature, reflectance/glazing losses,

H. Baig et al. / Energy 147 (2018) 547e560556

and electrical losses in cabling and inverter by up to 36% leading toan annual power output of 134 kWh/m2.

4.3. Thermal performance

Two cases were studied using this model. The first case wascalculating assuming no water flow, i.e., the copper tube is filledwith air, and hence this case corresponds to an experimental sce-nario where there is no water cooling effect. In this case, the waterwas replaced with a body of air which was subject to the same

(a)

(b)

Fig. 20. (a) Average solar insolation at different sites (b) Aver

boundary conditions as the five air bodies in the CCPC cavities. Theinitial temperature of the computational domainwas assignedwiththe experimentally measured temperature of 34 �C, but for the airbodies in the CCPC cavities, the initial temperature was 25.3 �C asdefined by the average ambient temperature recorded during theexperiments. The study was performed using 10-min interval un-der a constant irradiance of 1000W/m2. The predicted numericalresults were found to be within 5% of that obtained during theexperiment e further confirming the efficacy of the built ANSYSmodel. In the second case, is the model assumed water to be

age electricity production per unit silicon solar cell area.

Fig. 21. Monitored ambient temperature and water temperature at the tube inlet, andwere input into CFX as time-dependent boundary conditions (a) and predicted watertemperature in the second case (b).

H. Baig et al. / Energy 147 (2018) 547e560 557

flowing through the copper tube to cool the PV cells. Here, thewater temperature at the tube inlet was used as is shown inFig. 21(a) and applied in the simulation. The initial temperature ofthe tube and water was 21.2 �C, while the initial temperature of PVcells is 23.5 �C, the CCPCs and Sylgard layer were subject to 34 �Cinitial temperature, while the air bodies were in 25.3 �C initialtemperature. The predicted water temperature at the tube outlet ispresented and compared with the measurement in Fig. 21(b). Thepredicted temperature across the tube is 0.15 �C in comparisonwiththe temperature difference of 0.1 �C in the experiment. This sug-gests that the 0.3lit/min water flow rate is so high that the watertemperature remains nearly unchanged when it exits tube outlet,even under the LCPV increased irradiance flux. The mean andinstantaneous cell and water temperature contours during the10min duration, for the both cases (with and without water coolingeffect) are illustrated in Fig. 22. The mean cell temperature wascalculated as the arithmeticmean of all the cell temperatures. In thefirst case, the predicted mean cell temperature rises linearly withtime. In the second case-withwater cooling effect, however, the celltemperature is shown to be constant.

In the first case, the tube is full of air, and as such all the cellsexhibit approximately the same temperature. The air in the middleof the tube was shown to be at a higher temperature than the airthe tube ends. In comparison, for the second case the cell and watertemperatures showed a linear correlation along the length of theexchanger tube. The PV cells were modelled as a series heat sourcealong the flow path. In general, the temperature of the cells wasfound to be higher than the temperature of the water underneathdue to the thermal resistances between the cell and the water. Itwas shown that with a water flow rate of 0.3lit/min and a 21.2 �Cinlet temperature, the cell temperature can be effectively cooled to18 �C within 10min.

Fig. 22. (a) Predicted mean cell temperature as a function of time (b) Temperatur

4.4. System losses and improvements

In the present study, we have evaluated a hybrid concept for alow concentrator photovoltaic system. The optical efficiency of thesystem was found to be lower by ~23% than that predicted by theray trace simulations, due to the rough finishing obtained on the 3Dprinted concentrator. An alternative method is to make use of thereflective films on a curved parabolic surface. However, our expe-rience shows that high humidity present within the system cancause the peeling of the reflective film and make system failuremore likely. Improving the manufacturing of the concentrator, suchas by using injection moulding and evaporative coating wouldimprove the overall performance. The second key factor thataffected the system performance was the solar cell temperature.Whenmounted on a copper tube with no fluid running through thesystem, the solar cell temperature was found to rise due to thethermal resistances of the adhesive. Using a cooling fluid tomaintain the solar cell temperature can be beneficial in improvingthe electricity output through the system whilst simultaneouslyproviding a source of low temperature heat. Controlling the flow-rate of the fluid based on the incoming solar irradiance can prove tobe an effective method for the overall improvement of the device.Although the achieved system performance was lower than ex-pected from the design calculations due to the optical lossesinherent to our available manufacture procedure, the design pre-sented here highlights the advantages and co-generation benefitsthat can be achieved using an integrated PV-T co-generationsystem.

5. Conclusions

In this work a Hybrid LCPV/T prototype system was presented.The system was found to have significant improvement ascompared with its predecessors. Optical ray tracing was carried toevaluate the concentration of sunlight on the solar cell underdifferent angles of incidence. Fabrication of the optical concentratorwas carried out using 3D printing in combination with thermalevaporation techniques for the reflective coating. The bonding ofthe solar cells to a conductive heat exchanger element was carriedout using an oven cured a thermal adhesive between the surfaces.LGBC solar cells with an efficiency of 18% was incorporated in thisdesign under a concentration of 3.6� . Awide range of illuminationintensities and angular positions along both the directions wereexperimentally achieved using bespoke designed experimentalapparatus which gave a better understanding of the variation in theefficiencies with the changing sun position over the whole year. Anequation was developed to predict the optical efficiency for anygiven location as a function of the altitude and the azimuth angles.It was found that the performance of the system was closely

e plot of the system at 10min without water cooling (c) with water cooling.

H. Baig et al. / Energy 147 (2018) 547e560558

dependant on the chosen solar cell technology, the bonding be-tween the solar cell and the heat exchanger and the flowrate of thewater through the tube. The heat exchanger considerably reducedthe temperature of the solar cell with the increase in the electricaloutput of the solar cell be experimentally measure and discussed.Changing the flowrate according to the intensity of the incomingradiation was highlighted through this work as a vital optimisationparameter the overall system output and for LCPV-T systems ingeneral. A scale up device based on this concept will be developedto further validate the concept.

Acknowledgements

The authors gratefully acknowledge the EPSRC Solar Challengeproject SUNTRAP (EP/K022156/1).

Appendix. The governing equations of optics, heat transferand fluid flow

1 Radiation Transport in Medium

The sunlight is electromagnetic wave with a spectrum and cantravel in any kinds of medium and be described by the Maxwell'sequations. A CCPC with PV cell can absorb, emit and scatter thesunlight during its propagation. For a plane-parallel medium, themonochromatic radiation intensity of a sunlight beam obeys thefollowing equation along its travel path s [25],

1lvðsÞ

dIvðr; sÞds

þ Ivbðr; sÞ ¼ SnðsÞ (A1a)

where

SnðsÞ ¼ ð1� unÞInbðTÞ þ14p

un

Z4p

dIvðr; s0ÞFðs,s0ÞdU0 (A1b)

lnðsÞ ¼ anðsÞ þ gnðsÞ (A1c)

un ¼ gnðsÞlnðsÞ (A1d)

InbðTÞ ¼2 mn3

c2�exp

�mnkT

�� 1� (A1e)Here SnðsÞ is the spectral source function, lnðsÞ is the spectral

extinction coefficient, an is the absorption coefficient of medium, gnis the scattering coefficient of medium, unðsÞ is the spectral diffusereflectivity that is a ratio of the scattering coefficient to theextinction coefficient, m and k are the Planck and Boltzmann con-stants respectively, c is the speed of sunlight in the medium, n is thefrequency, T is the absolute temperature, r is the position vector, s isthe direction vector, s is the ray path length, InbðTÞ is the blackbodyemission intensity, Inðr; sÞ is the spectral radiation intensity, U is thesolid angle, and F is the scattering phase function.

In simulations in this paper, we assume the medium to be greyand homogenous without any scattering reflection. Thus, theradiative properties of themedium are independent of frequency orwavelength, path length and gn ¼un ¼ 0. Eq. (A1a) is integratedover all frequencies, yielding

1k

dIðr; sÞds

þ Iðr; sÞ ¼ IbðTÞ (A2a)

with

Iðr; sÞ ¼Z∞

n¼0Inðr; sÞdn (A2b)

Z∞n¼0

InbðTÞdn ¼ IbðTÞ ¼n2sT4

p(A2c)

where a is the average absorption coefficient of themedium, IbðTÞ isthe total blackbody radiation intensity, s is the Stefan-Boltzmannconstant, and n is the refractive index of the medium assumed tobe independent of frequency. The solar radiation transport behav-iour through all the media of the CCPC with solar cell shown inFig. 14 is obtained by solving Eq. (A2a) with the Monte Carlomethod.

2 Radiation Transport on Interface

When a beam of sunlight travels through multiple media, itexperiences the interfaces between any two media. On the in-terfaces the sunlight may be reflected and refracted. Considering abeam of sunlight is incident upon the interface between medium 1and medium 2 as shown Fig. A1. As a result, one beam is reflectedbottom tomedium 1with an angle q1, and is refracted into medium2 with an angle q2. The two comments of polarization of electricalfield of the beam reflected are determined by Fresnel's reflectionequations written as [28].8>>>><>>>>:

Er;⊥Ei;⊥

¼ �sinðq1 � q2Þsinðq1 þ q2Þ

Er;kEi;k

¼ tanðq1 � q2Þtanðq1 þ q2Þ

(A3)

where Ei;⊥ and Ei;k denote the two components of the incidentbeam, one is perpendicular to and the other is parallel to the planeof incidence, likewise, Er;⊥ and Er;k represent those of the reflectedbeam. The ratio of radiation intensity of the reflected beam overthat of the incident beam for the components is defined as

8>>>>>><>>>>>>:

2n;⊥ ¼ Er;⊥Ei;⊥

!2¼ sin

2ðq1 � q2Þsin2ðq1 þ q2Þ

2n;k ¼ Er;kEi;k

!2¼ tan

2ðq1 � q2Þtan2ðq1 þ q2Þ

(A4a)

where 2n;⊥ and 2n;k are the reflectivity of the two components atfrequency n, respectively. In CFX, however, the radiation is consid-ered to be unpolarised and two components are subject to an equalintensity, thus the reflectivity is the average of 2n;⊥ and 2n;k, namely,

2n ¼12

�2n;⊥ þ 2n;k

�¼ 1

2

�sin2ðq1 � q2Þsin2ðq1 þ q2Þ

þ tan2ðq1 � q2Þ

tan2ðq1 þ q2Þ

(A4b)

The ratio of radiation intensity of the refracted beam over that ofthe incident beam can now be expressed as

εn ¼ 1� 2n (A4c)

where εn is the emissivity or absorptivity of a medium. We haveredeemed the grey model above, so that 2n and εn are independentof frequency, and hence denoted by 2 and ε respectively.

The angle of refraction q2 is determined by using the Snell's lawof refraction as below

H. Baig et al. / Energy 147 (2018) 547e560 559

sin q2sin q1

¼ n1n2

(A4d)

in which n1 and n2 are the refractive index of media 1 and 2respectively. Ray trace analysis is performed to track the path of thesunlight beam travelling through the interfaces.

3 Fluid Flow Model.The density of the air in the CCPC cavities varies from the PV cell

surface to the CCPC inlet because of the temperature gradient be-tween them. Consequently, the air will be motion in the cavities bythe gravity. This upward air current can convey the heat generatedfrom the solar cell surface to the outside of CCPC, eventually thispart of heat is dissipated to the environment. It is shown that theReynolds number of the CCPC is less than 100 determined based onthe maximum air velocity at zero incidence, thus suggesting thefilled air flow is laminar. In a stationary reference frame, theinstantaneous continuity, momentum and thermal energy equa-tions can be written as [26]:

vr

vtþ V,�rU!� ¼ 0 (A5a)

v�rU!�vt

þ V,�rU!5U!� ¼ �Vpþ V,tþ F! (A5b)vðreÞvt

þ V,�rU!e� ¼ V,ðlTÞ þ t : VU!þ SE (A5c)where r, U

!, p, t, F

!, e, T , l and SE are the density, velocity, pressure,

shear stress tensor, body force, internal energy, temperature, heatconductivity and energy source of air, respectively. Here the bodyforce F

!takes into account the buoyancy force, i.e.

F!¼

�r� rref

�g! (A5d)

where rref is the reference density of air at a reference temperatureTref ¼ 25 �C. Since the temperature difference across a CCPC issmall, the Boussinesq model is adopted to calculate the densitydifference, r� rref , namely�r� rref

�¼ �rref b

�T � Tref

�(A5e)

where b is the thermal expansion of air, and defined as

b ¼ �1r

vpvT

p

(A5f)

Note that in the Boussinesq model, a constant reference densityrref is applied into all terms in the continuity and momentumequations except in the body force F

!. In addition, the pressure in

themomentum equations excludes the hydrostatic gradient causedby rref . The energy source term SE is considered to be zero.

Since there is no fluid flow inside the solid domains such as theCCPC and tube, sylgard and PV cell layers, the thermal energyequation, Eq. (A5c), is simplified to the following heat transferequation

v�rcpT

�vt

¼ V,ðlVTÞ þ SE (A6)

where r, cp and l are the density, specific heat capacity and thermalconductivity of the solids, respectively; the energy source term SE isstill zero.

The above governing equations are solved sequentially in ANSYS

15.0 CFX under a set of appropriate boundary conditions until asolution convergence is reached.

Fig. A1. A beam with initial intensity components, Ei;k and Ei;⊥ , is reflected andrefracted at the interface between medium 1 and medium 2.

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Conceptual design and performance evaluation of a hybrid concentrating photovoltaic system in preparation for energy1. Introduction2. System design & manufacture2.1. System components2.2. Prototype manufacturing

3. Experimental tests and multiphysics simulation3.1. Rotating table3.2. Experimental setup3.3. Models and methods in multiphysics simulations

4. Results & discussion4.1. Electrical performance4.2. Optical performance4.3. Thermal performance4.4. System losses and improvements

5. ConclusionsAcknowledgementsAppendix. The governing equations of optics, heat transfer and fluid flowReferences