Development of the solar-powered electric vehicles (SEV) [1] - [11] is desirable and very important in order to create a new clean energy society where the carbon neutrality is realized. Even in automobile sector, reducing CO₂ emission is a critical challenge for contributing to sustainable development. Although BEV (battery-powered electric vehicle) has an advantage for less CO₂ emission compared to the other vehicles, further reduction in CO₂ emission is necessary since the cars still emit a large amount of CO₂ for the driving. On the other hand, the calculated results show that the vehicle integrated photovoltaics (VIPV) can make a significant reduction in CO₂ emission from 50% to 75% [5] . In order to enhance recognizing the SEV as major clean vehicles and to create a clean energy society based on PV, clarifying values of SEV and development of high-efficiency, low-cost, lightweight, 3-demensional curved applicable and colorful VIPV modules and other technologies are necessary.

This paper presents a topical review for the VIPV and shows effectiveness of high-efficiency VIPV modules upon driving distance, reduction in CO₂ emission and charging cost saving of EV. This paper overviews development of high-efficiency VIPV modules and discusses about reliability, partial shading, curved surface, and color variation of VIPV modules. Perspective for VIPV modules is also presented.

Vehicle-integrated photovoltaics (VIPV) are classified into the following three types depending on how the power generated by the VIPV panels is used. 1) a system to operate the ventilation fan, 2) a system to supply power to the auxiliary battery, and 3) a system to charge the main battery for driving. Figure 1 shows the classification and history of commercially available vehicles equipped with PV panels.

Vehicles that fall under 1) have a long history, and examples include the Mazda Sentia (released in 1991) and the Audi A6 and A8 (released in 1993). These vehicles were equipped with PV panels on their sunroofs with a rated output of around 10 W, and the power generated by the PV panels was primarily used to drive a fan to ventilate the interior of the vehicle.

In the 1990s, many European car manufacturers, including Volkswagen and Mer cedes, sold similar systems. Toyota Motor Corporation’s third-generation Prius, released in 2009, was also equipped with a similar solar power generation system.

Vehicles that fall under 2) include Nissan Motor’s Leaf (released in 2010) and Xpeng P5 (released in 2021). In these vehicles, the power generated by the onboard PV panels is charged into an auxiliary battery (lead-acid battery) and used to operate things such as navigation and air conditioning.

In this way, VIPV panels have a long history, but until recently, vehicles that use the power generated by VIPV panels for driving energy had not been sold. The Prius PHV, which went on sale in 2016, was the world’s first mass-produced system for using an VIPV module to charge a high-voltage drive battery and use it as driving energy. The Prius PHV is equipped with an VIPV panel with a rated output of 180 W, and it is calculated that the EV driving distance by generating electricity from the PV panel is up to 6.1 km/day in Japan. Following the Prius PHV, models such as the Karma Revero and Hyundai’s Sonata HV, which can run on the power generated by onboard PV panels, have been released in the United States and other countries. Additionally, in Japan, the bZ4X and Soltera, which are BEVs equipped with PV panels, have been started selling since 2022.

Recently, longer driving of more than 25 km/day by using solar energy has been demonstrated with Toyota Prius, Nissan EV test cars [6] by using high-efficiency VIPV modules such as III-V 3-junction solar cell modules with module efficiency of more than 30%, Sono Motors Sion [9] and Lighyear One [10] by using Si back contact solar cell modules with module efficiency of more than 20% for proof of concept. However, in order to realize longer driving of more than 30 km/day average, development of high-efficiency VIPV modules with module efficiency of more than 35% [12] is necessary as shown below.

The effectiveness of the introduction of high-efficiency VIPV modules into EVs upon reduction in CO₂ emission was reported by the authors [13] . The VIPV-EV installed with the higher efficiency VIPV modules have a greater ability of reduction in CO₂ emission compared to EV. Annual CO₂ emission reduction of EV with electric mileage EM of 10 km/kWh in Japan is estimated to be 188 kg CO₂-eq/year (46% CO₂ reduction), 255 kg CO₂-eq/year (63% reduction) and 295 kg CO₂-eq/year (73% CO₂ reduction) for VIPV module efficiency of 20%, 30% and 40%, respectively. The experimental results of the Toyota demonstration car which showed that the annual CO₂ emission reduction by 62% [14] for a passenger car confirmed the validity of the calculated results [13] . From the view point of CO₂ emission reduction, the VIPV usage with the high conversion efficiency (>30%) is very effective for heavy vehicles which generally have low electric mileage such as vans, trucks and trailers [15] .

Figure 2 shows calculated results for charging electricity cost saving of EV by usage of VIPV as a function of electric millage EM by using cumulative frequency [16] for daily mileage of Japanese passenger cars. The results show effectiveness of high-efficiency VIPV modules for charging electricity cost saving of EV.

Potential of daily driving distance DD of vehicles powered by various solar cell modules with various module efficiency η(%) was calculated by using Equation

(1) and assuming the following conditions: 4 kWh/m²/day for solar irradiation (SI), 3 m² for installation area (A) of solar cell modules, 0.739 [1] for system efficiency (SE) of PV systems including convertor and 9.35 km/kWh for electric mileage (EM) of vehicle (case of Toyota Prius demonstration car [6] ).

DD[km/day] = SI[kWh/m²/day] * SE * η[%] * A[m²] * EM[km/kWh], (1)

Figure 3 shows estimated daily driving distance of vehicles powered by various solar cell modules of current module efficiency [17] in comparison with actual driving data for the PV driving range of the Toyota Prius demonstration car [6] and Sono Motor Sion [9] . Although our new world record efficiency 33.7% InGaP/GaAs/Si 3-junctuon solar cell module [18] has PV driving potential of about 28 km/day, further efficiency improvements of solar cell modules with an efficiency of more than 36% is very important to realize longer PV driving range of more than 30 km/day under solar irradiation of 4 kWh/m²/day as shown in Figure 3 .

Reliabilities of the VIPV as the power sources and power devices are examined, tested, and qualified by both IEC and ISO standards. VIPV is one of the photovoltaic devices and also one of automobile components. The reference standards for design qualification for PV are the IEC61215 series, and the vehicle standards are the ISO16750 series. Both electrical device standards and automotive standards in AND (electrical device standards and automotive standards) conditions should be examined. VIPV differs from standard PV modules in several points, so the standard testing and rating methods cannot be applied, mainly by the shading impacts, curved surface, partial shading, dynamic shading, and environment ( Figure 6 [35] ).

The expectations and problems for VIPV and Solar Electric Vehicles (SEV) were summarized as a to-do list in 2018 [36] , which is still unchanged. High efficiency is important because it is directly linked to cruising distance. Since it is essential to follow a three-dimensional curved surface, that is, to cover the cell three-dimensionally, the development of such solar cells and PV modules is expected. A PV module (3D-curved PV module) with durability and vibration resistance is required to withstand the car environment [37] .

Most formulas, equations, modeling, and measurement methods for photovoltaic devices are based on the assumption that photovoltaic devices are flat, static (with no movement), and illuminated uniform sunlight [35] [38] . It is a reasonable assumption for typical applications such as rooftop solar panels, but it cannot be applied to vehicle photovoltaic devices. For example, one of the essential equations that (Power output) = (Power conversion efficiency) × (Area of the device) × (Solar irradiance) is not true to the car applications in a physically and mathematically rigorous manner. Deviation of photovoltaic devices on vehicles is sometimes called “strange behaviors.” We may need a general computation method for general curved surfaces and nonuniform shading (aperture) environments. Such computation requires a shading (aperture) matrix instead of shading ratio or angle (scalar), to a tensor form (4-Tensor) for the angular response to the incident light instead of the Lambertian curve, differential geometry description using vector expression of a unit element, instead of cosine loss by angles of the PV panel ( Table 2 ). Even the coordinate system may shift to a local coordinate system with 3D rotation instead of absolute ground coordinates.

Shortly, when EVs become mainstream, VIPV is expected to solve the grid burden problems and lack of charging facilities. Of course, it also contributes to reducing CO₂ emissions. Above all, we hope you can enjoy freedom in many senses: freedom from refueling, battery charging, and carbon-free.

One of the killer applications of the SEV is the energy source in a disaster zone [35] . SEVs save energy through vehicle-integrated photovoltaics (VIPV) and make it possible to donate excess energy voluntarily, thus maintaining facility resilience.

Other promising applications are for heavy-duty vehicles (HDVs). The PV will be valid for saving the fuels of HDV by taking over the energy consumption by alternators and compressors [15] [39] [40] . However, the amount of fuel saving is not precise, varying by the driving modes, driving zones, types of HDVs, and driving habits. An accurate and precise advantage is essential for the logistic companies’ decision to invest PV on their HDVs. An R&D project is wanted to investigate the energy yield of PV on the HDVs and clarify the fuel-saving amount in various situations.

This paper presented a topical review for the vehicle-integrated photovoltaics (VIPV). In this paper, impacts of high-efficiency solar cell modules on increases in vehicle driving distance, reducing CO₂ emission, and charging cost saving of EV powered by VIPV modules were shown. This paper overviewed development of high-efficiency VIPV modules and discussed about reliability, partial shading 3-dimensional curvature and color variations of VIPV modules. Analytical results for some power losses of solar-powered electric vehicles (SEV) by using test driving data by Toyota Motor and Nissan Motor test cars installed with high-efficiency III-V compound 3-junction solar cell modules with a module efficiency of more than 30% were shown in this paper. This paper also presented prospects for VIPV modules.

The authors express sincere thanks to the NEDO for supporting R&D and to Mr. K. Okumura, Mr. T. Mabuchi, Mr. A. Satou, Mr. T. Nakado and Mr. K. Yamada, Toyota Motor, Dr. T. Tanimoto, Mr. Y. Tomit and Dr. Y. Zushi, Nissan Motor, Dr. T. Takamoto, Sharp and the other members of Toyota Tech. Inst. Toyota Motor Co., Nissan Motor Co., Sharp Co. and Univ. Miyazaki, and members of NEDO’s “VIPV Strategy Committee”.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

We expect that all authors will have reviewed, discussed, and agreed to their individual contributions ahead of this time. Contributions will be published before the references section, and they should accurately reflect contributions to the work. The following statements should be used Conceptualization, M.Y. and T.M., Methodology, M.Y. and T.M., Validation, Y.O., K.A. N.K. and M.Y., Formal Analysis, Y.O., K.A. N.K. and M.Y., Module installation, Y.O., Investigation, all, Data Curation, all, Writing Original Draft Preparation, all, Writing Review & Editing, M.Y., Visualization, T.M., Supervision, Y.O. and M.Y.; Project Administration, Y.O. and K.N.; Funding Acquisition, Y.O. and K.N.

The research received funding from the NEDO (New Energy and Industrial Technology Development Organization), grant number 20000938-0.