Role of PV-Powered Vehicles in Low-Carbon Society and Some Approaches of High-Efficiency Solar Cell Modules for Cars

Development of highly-efficient photovoltaic (PV) modules and expanding its application fields are significant for the further development of PV technologies and realization of innovative green energy infrastructure based on PV. Especially, development of solar-powered vehicles as a new application is highly desired and very important for this end. This paper presents the impact of PV cell/module conversion efficiency on reduction in CO2 emission and increase in driving range of the electric based vehicles. Our studies show that the utilization of a highly-efficient (higher than 30%) PV module enables the solar-powered vehicle to drive 30 km/day without charging in the case of light weight cars with electric mileage of 17 km/kWh under solar irradiation of 3.7 kWh/m/day, which means that the majority of the family cars in Japan can run only by the sunlight without supplying fossil fuels. Thus, it is essential to develop high-efficiency as well as low-cost solar cells and modules for automotive applications. The analytical results developed by the authors for conversion efficiency potential of various solar cells for choosing candidates of the PV modules for automotive applications are shown. Then we overview the conversion efficiency potential and recent progress of various Si tandem solar cells, such as III-V/Si, II-VI/Si, chalcopyrite/Si, and perovskite/Si tandem solar cells. The III-V/Si tandem solar cells are expected to have a high potential for various applications because of its high conversion efficiency of larger than 36% for dual-junction and 42% for triple-junction solar cells unHow to cite this paper: Yamaguchi, M., Masuda, T., Araki, K., Sato, D., Lee, K.-H., Kojima, N., Takamoto, T., Okumura, K., Satou, A., Yamada, K., Nakado, T., Zushi, Y., Yamazaki, M. and Yamada, H. (2020) Role of PV-Powered Vehicles in Low-Carbon Society and Some Approaches of High-Efficiency Solar Cell Modules for Cars. Energy and Power Engineering, 12, 375-395. https://doi.org/10.4236/epe.2020.126023 Received: May 8, 2020 Accepted: June 27, 2020 Published: June 30, 2020 Copyright © 2020 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access


Introduction
The solar electricity including solar photovoltaics (PV) is expected to contribute to the primary energy with a share of approximately 20% and 70% in 2050 and 2100, respectively. It respectively occupies in the total energy of the world, according to the recommendation (World Energy Vision 2100) by the German Advisory Council on Global Change [1]. According to "Sky Scenario" reported by Shell [2], the cumulative capacity for the power systems based on PV in the world is reached to 22 TW by 2050. However, the number is only 600 GW in 2019. The fact suggests that the importance of further installation of the PV based power systems, and the importance of further development of science and technology and deployment of PV. Especially, development of PV-powered vehicles applications is desirable and very important for creation of new clean energy infrastructure based on PV [3] [4]. In order to realize PV-powered vehicles, development of high-efficiency, low-cost, light weight, 3-dimensional curved and colorful solar cell modules is necessary. This paper especially presents the importance of developing highly-efficient and low-cost solar cells and modules for automotive applications by showing efficiency impact on PV-powered vehicles in Section 2 and cost impact on them in Section 4. This paper also shows analytical results for efficiency potential of various types of solar cells in order to provide knowledge for selecting candidates of high-efficiency solar cell modules for automotive applications as described in Section 3. In Section 5, our approaches to PV-powered vehicle applications by using III-V triple-junction cell modules, static low concentrator III-V triple-junction solar cell modules and III-V/Si partial concentrator solar cell modules, and III-V/Si tandem cells are also reported.

Efficiency Impact on PV-Powered Vehicles
Even in the transport sector, reducing CO 2 emission is a critical challenge for contributing to sustainable development, because the proportion of CO 2 emission from the road transport in the overall energy-related CO 2 emission in Ja-  16.9%, respectively [5]. Figure 1 shows the CO 2 emission per 1 km driving for various types of vehicles in Japan [6]. Although BEV (Battery-powered Electric Vehicle) has advantage for less CO 2 emission compared to ICE (Internal Combustion Engine vehicle), FCV (Fuel Cell-powered vehicle), and HEV (Hybrid Electric Vehicle), it is essential to achieve further reduction in CO 2 emission. Figure 1 shows that the PV can make a significant improvement on CO 2 emission.
According to survey reports [7] [8] of 5000 people and vehicles, effectiveness of the PV-EV (PV-powered EV), a PV-EV with 1 kW rated-power-PV and a 4 kWh rated battery capacity would reduce 12% of CO 2 emission compared to the HEV for 12 years.
In addition, the development of infra-structures such as battery charging stations for BEV and hydrogen stations for FCV is delayed. Figure 2 shows changes in cumulative registration number of Nissan LEAF (BEV) and number of the installed quick chargers [9] [10]. Therefore, development of PV-powered vehicles

Necessity and Selection of High-Efficiency Solar Cell Modules for PV-Powered Vehicles
According to a statistics by Ministry of Japan [11] [12], approximately 70% of   kg, the rate is expected to increase to 17 km/kWh [13]. Divided the range of 30 km by 17 km/kWh, the necessary electricity will be 1.76 kWh/day. Namely, the average annual energy yield that is required for the light-weight family car powered by sunlight will be 642 kWh/year which is not an impossible value as well as a promising when we use a high-efficiency PV with efficiency of larger than 30% enables the society that majority of the family cars run by the solar power and without supplying fossil fuels. Thus, it is important for us to develop high-efficiency and low-cost solar cells and modules for automotive applications. Figure 5 shows the required module efficiency as a function of the solar cell module area for the solar cell modules to achieve 800 W rated-output power [13]. The figure suggests that a promising way to realize the installation of PV on a family car is to create a small size module with 800 W rated output power which can be installed on roof area. The PV module can be realized by using III-V compound multi-junction solar cells which have conversion efficiency larger than 33% under 1 sun illumination.
Therefore it can be said that the development of a highly-efficient PV module with an efficiency of larger 30% is important to realize PV-powered vehicles. Figure 6 shows the recorded conversion efficiency of various types of single-junction solar cells along with their extrapolations [14]. The data were fitted with the Goetzberger function [15]: where η(t) is the time-dependent efficiency, η limit is the practical limiting efficiency, t 0 is the year for which η(t) is zero, t is the calendar year, and c is a characteristic   Table 1. The analysis shows that the improvement of the conversion efficiencies for each cell is converging or will converge soon, which is mainly bounded by the Shockley-Queisser limit [16].
where V oc is the measured open-circuit voltage, k is Boltzmann constant, T is the where n is the number of junction.
The resistance loss of a solar cell is estimated solely from the measured fill factor. The ideal fill factor FF 0 , defined as the fill factor without any resistance loss, is estimated by [25] ( ) where v oc is The measured fill factors can then be related to the series resistance and shunt resistance by the following equation [25]: where r s is the series resistance, and r sh is the shunt resistance normalized to R CH . The characteristic resistance R CH is defined by [25] CH oc sc r is the total normalized resistance defined by In the calculation, highest values [21] obtained were used as J sc .
By assuming no optical loss, we can project the efficiency of various solar cells at different EREs. Figure 7 shows calculated and obtained one-sun efficiencies of   Regarding III-V multi-junction solar cells, efficiency of multi-junction solar cells increases with increase in number of junction as shown in Figure 8. However, it is thought that triple-junction is optimal number of junction because in the case of higher number of multi-junction solar cells of more than quad-junctions, external radiative efficiency (ERE) decreases with increase in number of junction as shown in Figure 8. Fill factor for multi-junction solar cells is also decreases with increase in number of junction as shown in Figure 9.
In addition to high-efficiency and low-cost, development of PV modules with 3-dimesional curvature, color variation and good temperature coefficient is necessary. Figure 10 shows changes in temperature coefficients of various solar cells as a function of open-circuit voltage of solar cells [26] [27] in comparison of calculated values. Calculated values of relative temperature coefficients TC rel. of solar cells were semi-empirical equation [28].
where n is the diode ideality factor. Because the III-V triple-junction solar cells have lower temperature coefficients of 0.09% -0.15%/˚C, compared to that (0.24%/˚C) mono crystal Si solar cells as shown in Figure 10, the higher efficiency solar cells are thought to be attractive for PV-powered vehicle applications.

Cost Impact on PV-Powered Vehicles and Potential of High-Efficiency Low-Cost Si Tandem Solar Cells
Cost reduction of high-efficiency solar cell modules is also very important for PV-powered vehicle applications. Figure 11 shows cost impact on PV-powered vehicle applications estimated from the survey reports [7] [8]. This figure implies that R&D on high-efficiency and low-cost PV is essential in order to introduce PV-powered vehicles as a clean and usable major vehicle for the market.
Although III-V multi-junction solar cells have an extremely high conversion efficiency with efficiencies of 39.2% under 1-sun and 47.1% under concentration, which is suitable for this application, cost reduction is necessary to realize the concept as shown in Figure 5.      Efficiency potential of various Si tandem solar cells is also analyzed [32]. The similar procedure and values described above were used. Figure 14 shows cal-  GaAs nano-wire/Si dual-junction tandem solar cell with an efficiency of 11.4% [35]. Such an efficiency difference is thought to be differences in material quali-   Figure 9.
In this section, recent Japanese activities of III-V/Si tandem solar cells are presented.  with 0.25 cm 2 area by using surface-activated bonding (SAB) [38]. Sharp Corporation has also attained 33.0% efficiency [39] with 3.604 cm 2 InGaP/GaAs/Si triple-junction solar cells by using the mechanical stack. Most recently, AIST has demonstrated a novel interconnect layer made of palladium nanoparticle (PdNP) between III-V and silicon that could be both optically transparent and electrically conductive, allow two-terminal configuration. A 30.8% efficiency [40] under 1-sun with InGaP/AlGaAs/Si triple-junction solar cells fabricated by using PdNP array-mediated bonding was demonstrated. Table 5 shows that the progress of practical phase and development phase PV-powered vehicles [41]. Toyota Motor Co. released Prius equipped with 56 W multi-crystalline Si solar cell modules and new Prius PHV with Si HIT solar cells modules with 180 W into markets in 2010 and 2017, respectively. Other auto companies also have developed PV-powered vehicles, and some of them have announced that they will start selling the PV-powered vehicles in 2020 and 2021. Most recently, Toyota has developed test car by using Sharp's high-efficiency In GaP/GaAs/InGaAs triple-junction solar cell modules (output power of 860 W, average cell efficiency of 34.9%) under the NEDO support [42] as shown in Figure 15. The daily driving range of 44.5 km is expected to be realized by using solar energy. Data collection and analysis are under driving test by using the Toyota Prius PHV test car.

Recent Approaches for PV-Powered Vehicles
We recently proposed a new static low concentrator which does not require tracking system for the automotive application. It is effective to reduce the cost of PV module by utilizing a concentrator because it can be reduced the total solar cell area. Since the installing a tracker on a vehicle roof is difficult, a static concentrator is suitable for the automotive application. Static concentrators that combined with III-V compound solar cells have many other advantages: 1) applicability of mounting on a three-dimensional curved surface, 2) robustness to partial shading, and 3) a shark-skin structure has advantage for aero-dynamics property. A high Voc can be attained thanks to the robustness to partial shading, allows a bypass diode to be equipped with each cell in the gap between cells in the module, and facilitates size reduction of the lens aperture.   Figure 15. Toyota Prius PHV demonstration car using InGaP/GaAs/InGaAs triple-junction solar cell module and characteristics. Figure 16 shows the calculated results of the trends of annual power yield by III-V/Si tandem modules and crystalline Si modules as a function of concentration ratio [43] [44]. The crystalline Si module cannot be attained the target yield even with a considerable (4 m 2 ) roof area., while III-V/Si tandem module and static low concentrator module up to 4-suns concentration factor can generate the target value, which indicating that the cells are promising candidate for automotive applications.
We recently demonstrated aperture efficiency of 27.6% with the static low concentrator module combined with III-V based triple-junction solar cell [43] [44]. Figure 17 shows that the module has 99 solar cells (33 cells were connected in series, and three strings were connected in parallel) which were mounted on the circuit board. The low concentrated lens was aligned on each cell and silicone (sealing resin) was filled in the gap between the cell and lens. The module in which area is 41.2 cm 2 has a conversion efficiency of 27.6% under on-axis 1 sun illumination. The module efficiency can be improved by reducing optical and electrical losses of the concentrator, such as reflection at the interface of Air/PMMA (3.9%), reflection at PMMA/Silicone (0.7%), diode loss (0.5%), voltage mismatch (0.3%), and current mismatch (7.1%). It is expected that the module efficiency is reached to 30% by reducing the mismatching loss and optical losses.

Summary
Development of high-efficiency solar cell modules and new application fields are significant for the further development of photovoltaics (PV) and creation of new clean energy infrastructure based on PV. Especially, development of PV-powered EV applications is desirable and very important for this end. Efficiency impact on reducing CO 2 emission and increase in driving distance in PV-powered vehicles was shown in this paper. Development of high-efficiency solar cell modules with an efficiency of more than 30% is essential for PV-powered vehicle applications. This paper presented analytical results developed by the authors for efficiency In summary, the authors have shown that solar modules for PV-powered vehicles have great ability to reduce CO 2 emission. Development of high efficiency (>30%), colored, and flexible modules is essential. Static concentrator PV, Si tandem PV and colored PV are attractive candidates for future car applications.