Future Options for Lightweight Photovoltaic Modules in Electrical Passenger Cars

sustainability

Review Future Options for Lightweight Photovoltaic Modules in Electrical Passenger Cars

Sehyeon Kim 1, Markus Holz 2 , Soojin Park 3, Yongbeum Yoon 3, Eunchel Cho 1,* and Junsin Yi 1,*

1 Department of Electrical and Computer Engineering, Sungkyunkwan University, Suwon 16419, Korea; [email protected] 2 Department of Economics Program Director Logistic & Air Trafﬁc Management, Anhalt of University of Applied Science, 06406 Bernburg, Germany; [email protected] 3 KEPCO International Nuclear Graduate School, Ulsan 45014, Korea; [email protected] (S.P.); [email protected] (Y.Y.) * Correspondence: [email protected] (E.C.); [email protected] (J.Y.); Tel.: +82-31-290-7689 (E.C. & J.Y.)

Abstract: Twenty-three percent of carbon emissions come from fossil fuels used in transportation. Electric vehicles are suggested as alternatives to fossil-fueled vehicles. Cars having vehicle integrated photovoltaics (VIPV) on the roof have recently been launched, aiming to increase fuel efficiency and increase maximum mileage by supplying electricity to the vehicle when needed. VIPV needs to be light in terms of efficiency. The use of polymeric materials, made of low-iron tempered glass on the front that contributes significantly to the module’s weight, is required instead. The use of a sandwich structure with polymer material achieves nine times stiffer rigidity than an aluminum sheet of the same weight. It can be used with a weight that is half that of glass through the lightweight and light-transmitting polymer material on the front side. The concentrator photovoltaic module structure

is used to compensate for various angles of incidence on a moving car, and it is advantageous because it is easy to apply and has a low weight owing to its excellent ﬂexibility. It is possible to reduce the

Citation: Kim, S.; Holz, M.; Park, S.; weight from 20 kg to less than 10 kg by limiting the use of glass. Yoon, Y.; Cho, E.; Yi, J. Future Options for Lightweight Photovoltaic Keywords: photovoltaics; VIPV; energy; electrical car; lightweight module carbon emission Modules in Electrical Passenger Cars. Sustainability 2021, 13, 2532. https://doi.org/10.3390/su13052532 1. Introduction Academic Editor: Domenico Mazzeo Significant efforts have been made to preserve the environment worldwide. Re- searchers have tried to reduce greenhouse gases (GHG) that cause global warming and Received: 22 January 2021 climate climate anomalies. Many countries have signed agreements, such as the Kyoto Pro- Accepted: 20 February 2021 tocol and the Paris Climate Agreement, to reduce carbon dioxide emissions, which account Published: 26 February 2021 for 77% of GHG [1]. The Paris Climate Agreement aimed to reduce carbon emissions by 37% by 2030. Transportation accounts for 23% of the world’s CO2 emissions, mostly from Publisher’s Note: MDPI stays neutral fossil fuel combustion [2]. The root cause of carbon emissions from transportation is an with regard to jurisdictional claims in increase in vehicle use, population, and gross domestic product (GDP) per capita [3]. published maps and institutional affil- The vehicles’ fuel consumption has been reduced by improving their efficiency by iations. means such as reducing the weight of vehicles and improving engine performance. How- ever, this is not an effective way to reduce CO2 emissions. As a fundamental measure, the use of electric vehicles, not fossil fuel vehicles, should be emphasized. Electric vehicles use electric energy for power and do not emit pollutants. The disadvantage of electric vehicles Copyright: © 2021 by the authors. is that the refueling (charging) time is comparably longer than that of fossil fuel vehicles. A Licensee MDPI, Basel, Switzerland. fossil fuel vehicle takes up to 5 min to refuel, but an electric vehicle can take up to 30 min This article is an open access article to recharge 80% of the battery capacity under normal charging conditions [4]. The use of distributed under the terms and photovoltaic modules helps alleviate the battery capacity constraints in electric vehicles conditions of the Creative Commons and will increase the electricity consumption efficiency in the future. As a solar module Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ directly produces electricity using the photoelectric effect of silicon, unlike other renewable 4.0/). energy sources, it does not need a mechanical motor. This makes it suitable for automobiles

Sustainability 2021, 13, 2532. https://doi.org/10.3390/su13052532 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, x FOR PEER REVIEW 2 of 7

Sustainability 2021, 13, 2532 a solar module directly produces electricity using the photoelectric effect of silicon, unlike2 of 7 other renewable energy sources, it does not need a mechanical motor. This makes it suitable for automobiles because the solar module does not generate noise and vibration dur- becauseing power the solargeneration, module is does simple not to generate install and noise is and inexpensive vibration [5] during. A photovo power generation,ltaic module iscan simple installed to install on the and roof is inexpensive of a vehicle [ 5and]. A can photovoltaic be charged module simply can by installedexposing on the the module roof ofto a sunlight. vehicle and can be charged simply by exposing the module to sunlight. CarsCars that that include include solar solar modules modules are are expected expected to to be be released released soon soon in in the the market. market. Cars Cars withwith photovoltaic-equipped photovoltaic-equipped solar solar roofs roof includes include the the Hyundai Hyunda Motorsi Motors Sonata Sonata model model and theand Toyotathe Toyota Prius Prius model. model. In 2014, In a2014, concept a concept design design using solarusing and solar wind andpower wind power was introduced was intro- byduced Mercedes-Benz, by Mercedes and-Benz, the Dutch and the Lightyear Dutch conceptLightyear car concept was capable car was of driving capable 400–800 of driving km through400–800 solar km through charging solar with charging the goal ofwith launch the goal in 2021 of launch [6]. Australia’s in 2021 [6] EVX. Australia venture’ aimss EVX toventure drive up aims to 400to drive km on up cloudy to 400 dayskm on or cloudy nights, days while or Hanergynights, while in China Hanergy is launching in China a is conceptlaunching car witha concept a goal car of with six hours’ a goal driving of six hours’ time. Conceptdriving time. cars are Concept appearing, cars are and appearing, a model thatand is a actually model that a solar-roof-type is actually a solar module-roof applied-type module to a passenger applied carto a as passenger an option car is alsoas an availableoption is [ 7also]. available [7]. FigureFigure1 shows1 shows the the Hyundai Hyundai Motors’ Motors’ Sonata Sonata model, model, which which is is not not designed designed to to install install additionaladditional modules modules on on the the vehicle vehicle roof. roof. As As it it performs performs the the role role of of the the roof roof simultaneously simultaneously withwith power power generation, generation, the the use use of of the the module module does does not not cause cause air air resistance resistance and and design design problems.problems. Solar Solar roofs roofs provide provide fuel fuel efﬁciency efficiency improvement improvement and and auxiliary auxiliary power power source. source. WhenWhen installing installing a a solar solar module, module, it it should should be be lightweight lightweight because because increasing increasing the the weight weight reducesreduces the the fuel fuel efﬁciency efficiency of of the the car. car. It It may may be be installed installed only only on on the the roof roof but but can can also also be be installedinstalled on on the the bonnet bonnet or or the the side side of of the the vehicle vehicle in in the the future future [8 [8]]. .

FigureFigure 1. Design 1. Design of photovoltaicof photovoltaic mounted mounted roof roof sonata sonata of of Hyundai Hyundai Motors Motors [9 [9]].. In In the the above, above, thethe copyrightcopyright of Hyundai （ Motor’s photographsMotor's photographs is not copyrighted is not copyrighted when used forwhen academic used for purposes. academic (purposes.https://www.hyundai.news/eu/brand/https://www.hyundai.news/eu/brand/hyundai-launches-first-car-with-solar-roof-charging-system/） hyundai-launches-ﬁrst-car-with-solar-roof-charging-system/), accessed on 25 February 2021.

2.2. Effects Effects of of Applying Applying Solar Solar Modules Modules to to Passenger Passenger Cars Cars Currently-released car models with solar roofs include the Toyota’s Prius with a 180 Currently-released car models with solar roofs include the Toyota’s Prius with a 180W Wmodule module and and Hyundai Hyundai Sonata Sonata with with a 210 a 210W module. W module. When When charging charging for 5 for h, the 5 h, maxi- the maximummum amount amount of energy of energy that thatcan be can obtained be obtained by using by using the module the module during during a day ais day1 kWh. is 1H kWh.owever, However, the energy the energy production production from the from PV themodule PV module depends depends on many on factor manys, factors,i.e., loca- i.e.,tion location (latitude), (latitude), temperature temperature of the module, of the module, tilt angle, tilt angle,shading shading (in the (incities). the cities). FigureFigure2 shows2 shows a grapha graph of of the the module module efﬁciency efficiency and and the the area area according according to to the the fuel fuel efﬁciencyefficiency of of an an electric electric vehicle. vehicle. As As the the weight weight of of the the vehicle vehicle gradually gradually becomes becomes lighter, lighter, thethe area area of of the the module module for for driving driving a a day day with with the the solar solar module module decreases. decreases. With With 20 20 percent percent ofof the the modules, modules, a a 600 600 kg kg car car would would need need an an area area of of 3.8 3.8 m m2 whereas2 whereas a a 1410 1410 kg kg car car would would needneed an an area area of of 8 8 m m2.2 Today’s. Today’s electric electric vehicles vehicles can can move move approximately approximately 6 6 km km on on average average withwith 1 1 kWh kWh of of electricity. electricity. If If the the weight weight of of an an electric electric vehicle vehicle can can be be reduced reduced from from 1410 1410 to to 600600 kg, kg, it it can can travel travel 17 17 km km with with 1 1 kWh kWh of of energy energy [10 [10]]. When. When 2 2 kWh kWh of of energy energy is is generated, generated, aa 600 600 kg kg car car can can drive drive 34 34 km. km. In In order order to to get get 2 2 kWh kWh of of energy energy per per day, day, a a module module of of 800 800 W W mustmust be be installed installed in in the the vehicle. vehicle. The The area area for for installing installing the the 800 800 W W module module is is 3.6 3.6 m m2,2 which, which is more than 2.5 m2 combined with the roof and hood, so it needs to be installed on the side of the vehicle [11]. If you include the side of the vehicle, it is 4.5 m2. Sustainability 2021, 13, x FOR PEER REVIEW 3 of 7

Sustainability 2021, 13, x FOR PEER REVIEW 3 of 7

is more than 2.5 m2 combined with the roof and hood, so it needs to be installed on the Sustainability 2021, 13, 2532 2 3 of 7 isside more of thethan vehicle 2.5 m 2[11] combined. If you includewith the the roof side and of hood,the vehicle, so it needsit is 4.5 to m be. installed on the side of the vehicle [11]. If you include the side of the vehicle, it is 4.5 m2.

Figure 2. Module efficiency and area according to electricity efficiency. Figure 2. Module efficiency and area according to electricity efficiency. Figure 2. Module efficiency and area according to electricity efficiency. The average driving distance in Korea is 38.5 km, and the average daily drive dis- The average driving distance in Korea is 38.5 km, and the average daily drive distance tanceThe of averageurban drivers driving in distanceJapan is in24 Korea km. In is Germany,38.5 km, and the theaverage average mil eagedaily perdrive year dis- is of14,000 urban km, drivers which in means Japan isthat 24 a km. carIn travels Germany, about the 38average km per mileageday [8,12 per–14] year. This is 14,000means km,that whichtance of means urban that drivers a car travelsin Japan about is 24 38 km. km In per Germany, day [8,12 the–14 ].average This means mileage that per with year solar is 14,000with solar km, whichmodules, means city thatdrivers a car do travels not need about to recharge.38 km per day [8,12–14]. This means that modules,Because city driverselectric dovehicles not need do not to recharge. have an engine, the most common method is to re- with Becausesolar modules, electric city vehicles drivers do do not not have need anto recharge. engine, the most common method is to duce the weight of the vehicle as a method to improve fuel economy, except for aerody- reduceBecause the weight electric of vehicles the vehicle do asnot a have method an engine, to improve the most fuel economy,common method except for is to aero- re- namic design and battery efficiency. Commercial silicon-based modules weigh 20 kg, and dynamicduce the weight design andof the battery vehicle efficiency. as a method Commercial to improve silicon-based fuel economy, modules except weigh for aerody- 20 kg, lightweight modules weigh approximately 10 kg [15]. The weight of the panoramic sun- andnamic lightweight design and modules battery efficiency. weigh approximately Commercial 10 silicon kg [15-based]. The modules weight ofweigh the panoramic 20 kg, and roof used on the roof of a passenger car is 60–80 kg, and the solar module used for the sunrooflightweight used modules on the roof weigh of a approximately passenger car is10 60–80 kg [15] kg,. The and weight the solar of module the panoramic used for sun- the Prius is approximately 30 kg. By contrast, reducing the weight of a fossil-fueled passenger Priusroof used is approximately on the roof of 30 a kg. passenger By contrast, car reducingis 60–80 kg, the and weight the ofsolar a fossil-fueled module used passenger for the car by 10 kg would improve fuel economy by 2.8% [16]. In the case of an electric vehicle, carPrius by is 10 approximately kg would improve 30 kg. fuel By contrast, economy reducing by 2.8% [ the16]. weight In the caseof a fossil of an- electricfueled passenger vehicle, it it is expected that it will reach 10 km/kWh if the weight becomes lighter by 400 kg. iscar expected by 10 kg that would it will improve reach 10fuel km/kWh economy if theby 2.8% weight [16] becomes. In the case lighter of an by electric 400 kg. vehicle, it is expected that it will reach 10 km/kWh if the weight becomes lighter by 400 kg. 3.3. MethodsMethods ofof ReducingReducing thethe WeightWeight of of Solar Solar Modules Modules for for Passenger Passenger Cars Cars 3. MethodsTheThe weightweight of Reducing ofof thethe solarsolar the moduleWmoduleeight for offor Solar aa passengerpassenger Modules carcar for isis important Passengerimportant because becCarsause it it is is related related toto thetheThe fuelfuel weight economy of the of of solarthe the car. car.module The The roof for roof ashape passenger shape of ofthe thecar passenger is passenger important car caris bec not isause flat not andit flat is relatedis and aero- is aerodynamicallytodynamically the fuel economy curved. curved. of Figure the car. Figure 3 shows The3 showsroof the shape area the and of area the curvature and passenger curvature of the car passenger ofis not the flat passenger carand roof is aero- car[17]. 2 roofdynamicallyThe [flat17]. roof The curved. area flat of roof Figurethe areapassenger 3 ofshows the car passengerthe is area 2 m and, which car curvature is is 2 mnot2, ofwide, which the passengerand is notthe wide,applicable car roof and [17] area the. applicableThecan flatbe installed roof area area can with of be the a installed module passenger withof 200car a W moduleis 2standard m2, which of 200 [18 Wis]. not standard wide, [and18]. the applicable area can be installed with a module of 200 W standard [18].

FigureFigure 3. 3.Passenger Passenger carcar roofroof areaarea and and curvature. curvature. Figure 3. Passenger car roof area and curvature. TheThe generalgeneral structure of of a a silicon silicon-based-based module module is isshown shown in inFigure Figure 4. Low4. Low-iron-iron tem- tempered glass is used on the front of the solar cell to protect the solar cell and transmit as peredThe glass general is used structure on the of front a silicon of the-based solar module cell to protect is shown the in solar Figure cell 4. and Low transmit-iron tem- as much light as possible. Ethylene vinyl acetate (EVA) and Tedlar polyester were used as peredmuch glasslight isas usedpossible. on the Ethylene front of vinyl the solaracetate cell (EVA) to protect and Tedlarthe solar polyester cell and were transmit used asas the laminating material, and aluminum was used as the frame [19]. However, PV modules muchthe laminating light as possible. material, Ethylene and aluminum vinyl acetate was used (EVA) as the and frame Tedlar [19] polyester. However, were PV modulesused as integrated into passenger cars must be light and ﬂexible for better performance. The glass the laminating material, and aluminum was used as the frame [19]. However, PV modules accounts for half of the solar module weight. Modules for photovoltaic mounted vehicles mainly used transparent plastic materials or glass ﬁber reinforced polymer composite materials instead of the tempered glass [20]. Sustainability 2021, 13, x FOR PEER REVIEW 4 of 7

integrated into passenger cars must be light and flexible for better performance. The glass accounts for half of the solar module weight. Modules for photovoltaic mounted vehicles Sustainability 2021, 13, 2532 4 of 7 mainly used transparent plastic materials or glass fiber reinforced polymer composite materials instead of the tempered glass [20].

FigureFigure 4. 4. CommercialCommercial view view of of solar solar panel panel construction construction..

MaterialsMaterials used used to to replace replace glass glass are are polycarbonate polycarbonate (PC) (PC) and and polymethyl polymethyl methacrylate methacrylate (PMMA).(PMMA). Table Table 11 showsshows the the density density and and thermal thermal conductivity conductivity properties properties of of glass, glass, PMMA, PMMA, andand PC. PC. PMMA PMMA and and PC PC have have a adensity density half half th thatat of of glass, glass, which which can can significantly signiﬁcantly reduce reduce thethe weight weight of of the the module. module.

TableTable 1. 1. CharacteristicsCharacteristics of of materials. materials.

MaterialsMaterials DensityDensity (g/cm (g/cm3)3) ThermalThermal C Conductivityonductivity (W/mk) (W/mk) GlassGlass 2.552.55 0.8 0.8 1 PMMAPMMA 1.191.19 0.1670.167–0.25–0.25 1 PCPC 1.21.2 0.19 0.19–0.22–0.22

Because its thermal conductivity is about 0.2 W/mk, which islower than that of glass, Because its thermal conductivity is about 0.2 W/mk, which islower than that of it is possible to minimize the influence of solar heat on the cell characteristics. A method glass, it is possible to minimize the influence of solar heat on the cell characteristics. A of using a polymer protective film is also presented. The polymer protective film is 32 method of using a polymer protective film is also presented. The polymer protective film times lighter than glass, 16 times thinner than glass, and has high transmittance; therefore, is 32 times lighter than glass, 16 times thinner than glass, and has high transmittance; it appears to be an effective replacement material [21]. However, this polymer protective therefore, it appears to be an effective replacement material [21]. However, this polymer film does not pass the international standard IEC61215. It lacks mechanical rigidity. Com- protective film does not pass the international standard IEC61215. It lacks mechanical plementing the mechanical stiffness is a very effective way to reduce weight. rigidity. Complementing the mechanical stiffness is a very effective way to reduce weight. Figure 5 shows the structure using ethylene tetrafluoroethylene (ETFE) with the front Figure5 shows the structure using ethylene tetrafluoroethylene (ETFE) with the front glassglass and and frame frame removed removed [22] [22. ].ETFE ETFE is a is fluorine a fluorine-based-based plastic plastic that that is widely is widely applied applied in thein automotive the automotive industry. industry. ETFE ETFE offers offers a variety a variety of properties, of properties, including including resistance resistance to radi- to ation and outdoor climatic conditions, excellent electrical resistance, stiffness, and impact Sustainability 2021, 13, x FOR PEER REVIEWradiation and outdoor climatic conditions, excellent electrical resistance, stiffness, and 5 of 7 stimpactrength strength[23]. The [ 23backside]. The backside uses the usesaluminum the aluminum honeycomb honeycomb core structure core structure instead insteadof the backof the sheet back to sheet ensure to ensurethe rigidity the rigidity of the module. of the module.

Figure 5. Sandwich honeycomb core structure with ETFE. Figure 5. Sandwich honeycomb core structure with ETFE.

Figure 6 shows the structure of the honeycomb sandwich. The structure consists of three components: skin, core, and adhesive. The skin is attached to the core through the adhesive. The sandwich structure is similar to that of the I-shaped steel. The main differ- ence is that the core of the sandwich is made of a material different from the skin, and the stress is not concentrated in a specific area but is distributed through the skin.

Figure 6. Sandwich structure of honeycomb core.

Therefore, the adhesive bond between the skin and the core must be strong enough to transfer stress from one skin to another through the sandwich core without peeling. If the material is selected properly, it can be made almost 37 times stiffer and more than nine times stronger than conventional aluminum sheets while weighing almost the same [24]. This can be used to create a much lighter material. Instead of using ETFE, glass fiber reinforced polymer (GFRP) can be used to reduce the module weight to 5–7 kg [25]. GFRP is also based on the honeycomb core structure. Both methods have passed the IEC 61215 standard, and the module’s weight was reduced to 6 kg/m2. It is a structure for building- integrated photovoltaics (BIPV) owing to its sufficient mechanical strength, but it can be used for vehicle-integrated photovoltaics (VIPVs). Figure 7 shows the structure of the concentrator photovoltaic (CPV) [26]. The silicon lens used on the front makes it easier to respond to the incident angle of sunlight, which is difficult to cope with in the city. In urban areas, the angle of incident light is not constant due to tall buildings and street trees. The influence of the incident light angle is corrected through the convex lens. In addition, one size is advantageous for bending and can be installed in a larger area. Sustainability 2021, 13, x FOR PEER REVIEW 5 of 7

Sustainability 2021, 13, 2532 5 of 7 Figure 5. Sandwich honeycomb core structure with ETFE.

FigureFigure 66 showsshows the the structure structure of of the the honeycomb honeycomb sandwich. sandwich. The The structure structure consists consists of threeof three components: components: skin, skin, core, core, and andadhesive. adhesive. The skin The skinis attached is attached to the to core the corethrough through the adhesive.the adhesive. The sandwich The sandwich structure structure is similar is similar to that toof thatthe I of-shaped the I-shaped steel. The steel. main The differ- main encedifference is that isthe that core the of core the ofsandwich the sandwich is made is madeof a material of a material different different from fromthe skin, the skin,and the and stressthe stress is not is concentrated not concentrated in a specific in a speciﬁc area areabut is but distributed is distributed through through the skin. the skin.

FigureFigure 6. 6. SandwichSandwich structure structure of of honeycomb honeycomb core core..

Therefore,Therefore, the the adhesive adhesive bond bond between the skin andand thethe corecore mustmust be be strong strong enough enough to totransfer transfer stress stress from from one one skin skin to to another another through through the the sandwich sandwich core core without without peeling. peeling. If theIf thematerial material isselected is selected properly, properly, it can it can be made be made almost almost 37 times 37 times stiffer stiffer and moreand more than than nine nine times timesstronger stronger than than conventional conventional aluminum aluminum sheets sheets while while weighing weighing almost almost the samethe same [24]. [24] This. Thiscan can be used be used to create to create a much a much lighter lighter material. material. Instead Instead of using of using ETFE, ETFE, glass glass fiber fiber reinforced rein- forcedpolymer polymer (GFRP) (GFRP) can be can used be used to reduce to reduce the module the module weight weight to 5–7 to kg5–7 [ 25kg]. [25] GFRP. GFRP is also is alsobased based on theon honeycombthe honeycomb core structure.core structure. Both methodsBoth methods have passed have thepassed IEC the 61215 IEC standard, 61215 2 standard,and the module’s and the module’s weight was weight reduced was toreduced 6 kg/m to. 6 It kg/m is a structure2. It is a structure for building-integrated for building- integratedphotovoltaics photovoltaics (BIPV) owing (BIPV) to owing its sufficient to its sufficient mechanical mechanical strength, strength, but it can but be it used can be for vehicle-integrated photovoltaics (VIPVs). used for vehicle-integrated photovoltaics (VIPVs). Figure7 shows the structure of the concentrator photovoltaic (CPV) [ 26]. The silicon Figure 7 shows the structure of the concentrator photovoltaic (CPV) [26]. The silicon lens used on the front makes it easier to respond to the incident angle of sunlight, which is lens used on the front makes it easier to respond to the incident angle of sunlight, which difficult to cope with in the city. In urban areas, the angle of incident light is not constant is difficult to cope with in the city. In urban areas, the angle of incident light is not constant due to tall buildings and street trees. The influence of the incident light angle is corrected due to tall buildings and street trees. The influence of the incident light angle is corrected Sustainability 2021, 13, x FOR PEER REVIEWthrough the convex lens. In addition, one size is advantageous for bending and6 of can 7 be through the convex lens. In addition, one size is advantageous for bending and can be installed in a larger area. installed in a larger area.

FigureFigure 7. Concentrator 7. Concentrator photovoltaic photovoltaic structure structure0.0. 4. Conclusions 4. Conclusions This study investigated solar modules suitable for electric passenger cars. Since Korea hasThis a relativelystudy investigated small land solar area, modules solar modules suitable must for beelectric used passenger in the form cars. of VIPV. Since For Ko- this, rea wehas conducted a relatively a small review land on lighteningarea, solar themodules vehicle. must The be electric used in vehicles the form currently of VIPV. available For this,have we conducted a power efﬁciency a review ofon 6 lightening km/kWh, the so vehicle. solar modules The electric cannot vehicles be completely currently exempted avail- ablefrom have the a power requirement efficiency for charging. of 6 km/kWh, However, so solar with modules only 10 cannot km/kWh, be completely the 500 W ex- solar emptedmodule from can the generate requirem aent driving for charging. distance However, of 20 km owingwith only to roof 10 km/kWh, and hood the installations. 500 W solar module can generate a driving distance of 20 km owing to roof and hood installations. Because the average daily mileage of a car is less than 40 km, it is possible to generate enough energy to cover as much as 50% of the driven distance. To improve the fuel economy of an electric vehicle, the most obvious way is to reduce the weight. If the weight can be reduced to 600 kg, an additional fuel economy of 17 km/kWh can be obtained, freeing the driver from the burden of charging. Therefore, the weight of the solar module plays an important role. To reduce the weight of solar modules, a method to replace the low-iron tempered glass used as a finishing material has been proposed. The method providing the most weight reduction uses the polymer film, but the IEC61215 test has not been performed, so it appears that the mechanical properties are insufficient. It is believed that if this part is supplemented, it is possible to create a module with simple yet excellent characteristics. Alternatively, CPV may be the best fit. In the case of automobiles, because it is difficult to change the incidence angle, the use of CPV minimizes the effect of the angle and enables stable power generation. In addition, it can be installed according to the shape of a curved car, so it can use a wide area, and its weight is lighter than 5 kg/m2; thus, it is possible to reduce weight. The market for electric vehicles continues to grow and so does the use of solar modules. Many cars that use solar modules are expected to be released in the future. Increased research on automotive solar modules may lead to increase in the number of cars using them.

Author Contributions: Kim, S. and Cho, E.-C. are the main author; methodology, Holz, M.; validation, Cho, E.-C. and Yi, J.; formal analysis, Park, S. and Yoon, Y.-B.; writing—original draft preparation, Kim, S. and Cho, E.-C..; writing—review and editing, Cho, E.-C..; supervision, Yi, J.; project administration, Yoon, Y.-B.; funding acquisition, Yi, J. All authors have read and agreed to the published version of the manuscript Funding: Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Korean Ministry of Trade, Industry and Energy (MOTIE). Institutional Review Board Statement: Not applicable Informed Consent Statement: Not applicable Data Availability Statement: Not applicable Acknowledgments: This research was supported by grants from the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean Ministry of Trade, Industry and Energy (MOTIE) (Project No. 20194010000090 and 20193010014850). Sustainability 2021, 13, 2532 6 of 7

Because the average daily mileage of a car is less than 40 km, it is possible to generate enough energy to cover as much as 50% of the driven distance. To improve the fuel economy of an electric vehicle, the most obvious way is to reduce the weight. If the weight can be reduced to 600 kg, an additional fuel economy of 17 km/kWh can be obtained, freeing the driver from the burden of charging. Therefore, the weight of the solar module plays an important role. To reduce the weight of solar modules, a method to replace the low-iron tempered glass used as a finishing material has been proposed. The method providing the most weight reduction uses the polymer film, but the IEC61215 test has not been performed, so it appears that the mechanical properties are insufficient. It is believed that if this part is supplemented, it is possible to create a module with simple yet excellent characteristics. Alternatively, CPV may be the best fit. In the case of automobiles, because it is difficult to change the incidence angle, the use of CPV minimizes the effect of the angle and enables stable power generation. In addition, it can be installed according to the shape of a curved car, so it can use a wide area, and its weight is lighter than 5 kg/m2; thus, it is possible to reduce weight. The market for electric vehicles continues to grow and so does the use of solar modules. Many cars that use solar modules are expected to be released in the future. Increased research on automotive solar modules may lead to increase in the number of cars using them.

Author Contributions: S.K. and E.C. are the main author; methodology, M.H.; validation, E.C. and J.Y.; formal analysis, S.P. and Y.Y.; writing—original draft preparation, S.K. and E.C.; writing—review and editing, E.C.; supervision, J.Y.; project administration, Y.Y.; funding acquisition, J.Y. All authors have read and agreed to the published version of the manuscript. Funding: Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Korean Ministry of Trade, Industry and Energy (MOTIE). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: This research was supported by grants from the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korean Ministry of Trade, Industry and Energy (MOTIE) (Project No. 20194010000090 and 20193010014850). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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