Introduction:

Solar energy has become a key player in the energy sector. Indeed, solar power is the fastest growing renewable energy source, currently composing 4.5 percent of electricity generation across the globe in 20221. The most prevalent solar technology is solar photovoltaic (PV) cells, more commonly known as solar panels. PV cells are composed of semiconductor material, allowing the cell to absorb incoming photons and receive the dislodged electrons from the interaction2. As a single PV cell produces only a negligible amount of electricity from this process, PV panels typically connect hundreds of these cells2, and proper PV arrays consist of a network of connected PV panels2. This array is known as a solar farm, as hundreds of PV panels are aligned in a grid and angled to face the sun as shown in Figure 1.

Figure 1: A solar farm in Copake, New York. Source: https://www.bloomberg.com/opinion/articles/2022-03-20/how-copake-new-york-fought-and-then-accepted-a-solar-farm#xj4y7vzkg 

As the climate change crisis continues to threaten the globe, many countries are scrambling to lessen its effects in any way possible. Fossil fuels and other ecologically damaging fuels are being phased out in favor of renewable alternatives. Thus, solar energy naturally garnered public attention. As more solar panels cropped up, researchers further pondered other markets that solar energy could feasibly intercept.

Inevitably, the transportation industry came into focus. Automobiles are an irreplaceable facet of modern society but depend heavily on fossil fuels. The automobile industry is currently undergoing a revolution, moving to electrification; part of this shift is including a focus on renewable power sources3. Notably, solar-powered cars, then, became an avenue worth pursuing. Solar-powered cars were not entirely new; the first-ever solar-powered car was constructed in 1955 from 12 PV cells and a balsa wood base4; granted this vehicle, the Sunmobile, was a handheld model car. However, actual road vehicles have emerged since then, ranging from the first street worthy car in 1976 to a whole slew of futuristic models currently under development.The concept is simple, integrated solar cells in the vehicle generate power from sunlight and store that power in a battery to power the rest of the vehicle5. However, there was still much to improve, namely in the affordability and efficiency of both the solar panels and the vehicle. In addition, the added weight of the solar panels must also be considered and either lessened or worked around. The past five years have brought myriad innovations to solar energy, and many have directly or indirectly fed into the development of solar cars. 

Perhaps the biggest challenge with solar cars is the amount of energy that can be collected by an automobile, given their size.  Due to safety and durability concerns, the solar panels could only be located on the top of the vehicle, with a maximum of 4 square meters.  In an ideal case (clear skies, equator, stationary vehicle in clear sunlight), the total amount of solar irradiance on the top surface of the car would be 34 kWHr per day.  However, modern PV cells are only 20 percent efficient, resulting in approximately 7 kWHr per day.  For modern electric vehicles, such as a Nissan Leaf, this much electricity would provide a range of 25 miles.  Although a short range, this which would be sufficient for the bulk of drivers. Unfortunately, the solar irradiance depending on the time of year and latitude, can reduce the solar irradiance significantly.  For example, a vehicle in New York in wintertime would only have a range of 4 miles on a clear day, and likely less than a mile on an overcast day.  As such, the vehicles and their powertrains must be incredibly efficient to optimize the use of this energy.

This article catalogs several of these advancements, including those in solar technology and powertrain components, which will support the overall feasibility of solar cars.  Indeed, for solar cars to be feasible, the efficiency and cost of the solar panels must be improved while the powertrain components must be optimized for the use of solar power.

AFPM Motors:

A new type of motor could simplify a solar car’s design with minimal effect on the vehicle’s performance. In 2018, a research article drew focus to the Axial flux permanent magnet (AFPM) machine, a structure consisting of a rotor and a stator, with a gap in between the two such that the force of magnetic flux between the rotor and stator aligns with the axis of rotation6. The resulting motor, shown in Figure 2, is compact while producing high torque7, ideal for solar car models that require a smaller motor without sacrificing performance. Additionally, the article proposed a different design that substituted the traditional ferromagnetic core with additional rotor and stator disks7. This coreless design would reduce loss in torque at the expense of a higher electromagnetic air gap. In comparing these motors, the researchers sought to identify the optimal AFPM motor for a solar vehicle. 

Figure 2: A 3D model of the traditional AFPM motor with 36 stator slots and 12 rotor poles [7]. Source: https://ieeexplore-ieee-org.proxy.library.stonybrook.edu/document/8410487/figures#figures 

In their experiment, the researchers considered two solar car models: a three-wheeled model with one rear driving wheel and a four-wheeled model with two rear driving wheels. The motor is embedded within the driving wheel(s), removing mechanical components and simplifying the overall system7. The two machines considered for use were a conventional single rotor-single stator core model, as seen in Figure 2, and a coreless multidisc configuration with windings concentrated around the air gaps. Then, several thousand design candidates for both types of machines were studied, prioritizing designs with minimal reductions in mass while retaining the required torque. 

Following the study, the researchers concluded that the coreless design would be more suitable for two-wheel drive model cars, and those with a single driving wheel are more suited for a traditional AFPM motor. While the motor is slightly less efficient due to the lower torque limit of the coreless model, the improved mass distribution of the four-wheeled model offsets the limit while allowing for a more compact car and a simpler design. While more testing is ideal for this design, the placement of the motor in the wheel could significantly improve the simplicity of future solar car models, allowing for less costly repairs while remaining efficient.

Solar Glass:

Next, innovations in solar cells have spurred the construction of semitransparent photovoltaics, allowing the construction of see-through solar panels. Organic solar cells (OSCs) utilize organic polymers or other molecules to absorb light and transport electrons. Due to their composition, these cells are lighter and more flexible than traditional solar cells while being cheaper to produce and exhibiting semitransparency8. These properties have made OSCs quite popular for use in solar power. In particular, their semitransparent quality has garnered attention for use in windows, as the material functions as a solar cell while retaining the clarity of the scene when looking through it9.

While OSCs could theoretically function as windows, in practice, the structure underperformed due to low visibility and poor electric transmittance10. To rectify this, a group of researchers in China conducted a study in 2022, printing an OSC but also introducing a pseudo-planar heterojunction (PPHJ) structure to both the donor and acceptor layers to improve average visible transmittance (AVT) and power conversion efficiency (PCE)8. Then, a hydrophobic layer was applied to the glass encapsulation to allow long-term waterproof performance without compromising visibility. The study then compared the experimental OSCs to traditional opaque OSCs, finding that the experimental cells are slightly less efficient than the opaque cells.

Table 1: Photovoltaic parameters of OSCs based on the system under observation. Source: https://onlinelibrary-wiley-com.proxy.library.stonybrook.edu/doi/full/10.1002/adma.202201604#adma202201604-bib-0017 

The results of the experiment are significant, as shown in Table 1. The experimental OSC glass has significantly improved from previous iterations of the same concept, indicating that the PPHJ structure was quite successful. In addition, the hydrophobic layer proved quite successful, allowing the glass to perform optimally under long-term rainfall conditions. The AVT performance is also quite significant, measuring around 20.42%. While the front, back, and front side windows of a car require more clarity, as typical automotive glass operates under a visual clarity of around 70-85%11, back side windows are free to use this material, which would further boost the conversion efficiency of the vehicle as a whole. Further tests are necessary to improve consistency and clarity, but for now, PPHJ layers could become commonplace in future solar glass models. 

MPPT:

To combat issues presented by sudden changes in weather, a research team in Japan published a study detailing the construction of a Maximum Power Point Tracking (MPPT) circuit suitable for installation in solar car models12. In solar cells installed on moving objects, especially fast-moving ones like cars, the power generated by the cells can fluctuate drastically due to changes in sunlight exposure brought on by conditions like changing directions or shadows. This common dilemma for solar energy has been resolved with the Maximum Power Point Tracking (MPPT) algorithm, which extracts the maximum available power from the module and optimizes power generation in suboptimal conditions13. MPPT circuits function well for stationary fixtures, but in motion, sharp fluctuations occur too quickly for any conventional machine to follow. Moreover, for a solar car, solar cells are typically aligned to optimize the car’s aerodynamics, meaning that the solar cells are not aligned, and the array’s characteristics are not uniform.

To solve this, the team aimed to construct an MPPT control circuit suitable for moving objects using analog switches for higher processing speed and two operational amplifiers. The circuit functioned at an efficiency above 98% while operating at an input range of 50 W and above and weighed 170 grams12. Then, eighteen of these circuits were inserted into a solar car model named the 2013 Tokai Challenger, and the car participated in the annual Bridgestone World Solar Challenge (BWSC) held in Australia12. The power generated by the solar cells during the vehicle’s performance was recorded and measured.

The result was a resounding success. The MPPT circuits provided sufficient power to supply the vehicle’s 3000 km circuit, lasting roughly 36 hours and 22 minutes12. For each module of 140-W solar cells, the MPPT circuits only required 0.17% of the power the machine was outputting12. The circuits proved to be a practical low-consumption solution to sudden changes in solar radiation, and the application of MPPT circuits in solar cars appears quite viable. Seeing as this study primarily focused on the implementation of MPPT circuits for a racing car, and thus the resulting construction was required to be lightweight and aerodynamic, future iterations of this technology can be improved on without having to conform to those specifications. The future looks bright for MPPT technology in solar cars.

Carbon-Neutral Vehicles:

In the Netherlands, a student team of researchers at the Eindhoven University of Technology has developed a solar car that will theoretically remain carbon-neutral over its entire lifespan14. This car, named the “Zem,” is a prototype vehicle that sports solar panels on its roof and hood to maximize power conversion, as seen in Figure 3. Additionally, the car contains a unique filter system capable of capturing carbon dioxide, effectively capturing its carbon emissions with those in its surroundings. According to the team’s estimates, Zem can absorb 2 kilograms of CO2, assuming the car travels 20000 miles a year14. Though the amount seems small, the team hopes to implement this technology on a much larger scale. As over a billion passenger cars exist globally15, wide-scale implementation of these vehicles would ideally cut carbon emissions by billions of kilograms.

Figure 3: A photo of the Zem solar car. Solar panels are attached to the sunroof and hood of the vehicle. Source: https://www.tue.nl/en/news-and-events/news-overview/21-07-2022-tue-students-develop-zero-emissions-car-that-actually-captures-co2-from-the-air-as-it 

Moreover, the team ensured that the production cycle of these vehicles also remained carbon-neutral. The car’s chassis and body panels are 3D printed14 to eliminate the production of residual waste, and the circular plastics manufactured can be recycled for other purposes. The car is still a proof of concept, but this technology and the cleaner production cycle of these models show that eco-friendly technology is inevitable. If further experiments prove successful, future models of solar cars, and cars in general, may possess similar carbon-trapping technology.

Solar Micro Cars:

While numerous companies have already unveiled full-size solar car models, one, in particular, has taken the solar car approach towards micro vehicles instead. Dutch company Squad Mobility unveiled plans for its ideal urban vehicle in 201916: a compact solar electric vehicle at an affordable cost. These plans culminated in the EU launch of the Squad Solar City Car in 202217, a two-seater solar car costing a modest 6250 euros in European markets. Weighing in at roughly 350 kg, or 771 pounds, this vehicle is powered by two rear in-wheel motors and four swappable lithium-ion battery packs. While this means that at full charge, the car can only travel up to 100 kilometers at a speed of 45 km/h, the vehicle is meant for shorter trips around the city rather than long distances. Designed to be lightweight and efficient, the attached solar panels can garner an impressive amount of solar energy for the battery packs and are capable of charging the packs while the car is in the sun.

Figure 4: A Squad Solar City Car in public. Due to its small size, it can be cross-parked between larger cars without much hassle. Source: https://electrek.co/2022/05/16/squad-mobility-officially-launches-its-solar-city-car-optimized-for-private-and-shared-ev- 

Squad Mobility has further plans for this vehicle than private ownership, however. In addition to public solar battery racks to readily replace and charge empty battery packs (as seen in Figure 5), the company plans to launch a shared mobility app, allowing users to access publicly available solar cars at their leisure. In addition, plans are underway to launch these vehicles in the US by 2024, though it is classified in the US as a low-speed vehicle (LSV) and thus is more restricted in its use than in the EU18. Still, the widespread availability of the project and the relatively affordable price is monumental for many prospective buyers in urban areas. Though the mileage is disappointing for some, the company is also developing a full-size four-passenger model for those who require a solar car capable of traveling longer distances17. If this project proves successful, these solar-powered microvehicles may become commonplace in urban environments everywhere. Despite their limited capabilities, their price tag is relatively cheap for many, and their compact size ensures that parking and other storage-related issues will be far less likely.

Figure 5: A Squad battery wall. Owners can theoretically exchange their vehicle’s empty battery packs for charged packs here. Source: https://electrek.co/2022/05/16/squad-mobility-officially-launches-its-solar-city-car-optimized-for-private-and-shared-ev-use/

Conclusion:

Solar cars are far from a new concept, and solar technology is further improving. Solar panels are cheaper and more efficient19 and inventions like solar glass ensure that more and more solar panels can function discreetly in public fixtures. Many solar car models have been tested and scheduled for production in 2023, and more are still in development. The improvements to solar car motors, solar cells, maximum power detection systems, and more have advanced the field significantly in the past five years, and the industry has yet to grow. Inevitably, solar panels will be more efficient and more flexible, and solar panels will become an irreplaceable facet of everyday society, providing clean and renewable energy for all.

References:

1. International Energy Agency. “Solar PV.” [Online]. Available: https://www.iea.org/energy-system/renewables/solar-pv  

2. U.S. Energy Information Agency. “Solar Explained: Photovoltaics and Electricity.” [Online]. Available: https://www.eia.gov/energyexplained/solar/photovoltaics-and-electricity.php

3. Möller, T. Schaufuss, P. “Road Mobility.” 2022. [Online]. Available: https://www.mckinsey.com/capabilities/sustainability/our-insights/spotting-green-business-opportunities-in-a-surging-net-zero-world/transition-to-net-zero/road-mobility 

4. Solar Energy Insights. “The History of the Solar-Powered Car.” [Online]. Available: https://solarpower.guide/solar-energy-insights/history-solar-powered-car 

5. Industries Tec. “How [a] Solar Car Works - Advantages & Disadvantages of [a] Solar Car.” 2022. [Online]. Available: https://industriestec.com/how-solar-car-works/

6. Parviainen, A. “Design of Axial-Flux Permanent-Magnet Low-Speed Machines and Performance Comparison Between Radial-Flux and Axial-Flux Machines.” 2005. Diss. Lappeenranta University of Technology. ISBN 952-214-029-5. [Online]. Available: https://web.mit.edu/kirtley/binlustuff/literature/electric%20machine/designOfAxialFluxPMM.pdf

7. Taran, Narges, et al. “Coreless and Conventional Axial Flux Permanent Magnet Motors for Solar Cars.” IEEE Transactions on Industry Applications, vol. 54, no. 6, 2018, pp. 5907–17, https://doi.org/10.1109/TIA.2018.2855123. [Online]. Available: https://ieeexplore-ieee-org.proxy.library.stonybrook.edu/document/8410487 

8. Liu, Li, H., Wu, X., Chen, D. et al. “Pseudo‐Planar Heterojunction Organic Photovoltaics with Optimized Light Utilization for Printable Solar Windows.” 2022. Advanced Materials (Weinheim), 34(23), e2201604–n/a. https://doi.org/10.1002/adma.202201604. [Online]. Available: https://onlinelibrary-wiley-com.proxy.library.stonybrook.edu/doi/full/10.1002/adma.202201604 

9. T. Yan, W. Song, J. Huang, R. Peng, L. Huang, Z. “Highly Efficient Semitransparent Organic Solar Cells with Color Rendering Index Approaching 100.” Ge, Adv. Mater. 2019, 31, 1902210. [Online]. Available: https://onlinelibrary-wiley-com.proxy.library.stonybrook.edu/doi/10.1002/adma.201807159 

10. Y. Li, G. Xu, C. Cui, Y. Li, “Flexible and Semitransparent Organic Solar Cells” Adv. Energy Mater. 2018, 8, 1701791. [Online]. Available: https://onlinelibrary-wiley-com.proxy.library.stonybrook.edu/doi/10.1002/aenm.201701791 

11. Johnson Window Films. “Net VLT and Calculation.” [Online]. Available: https://www.johnsonwindowfilms.com/dealer/articleView.php?ARTICLE_ID=197

12. Kano, Fumihisa, et al. “MPPT Circuit with Analog Control Suitable for Solar Cars.” Electrical Engineering in Japan, vol. 213, no. 1-4, 2020, pp. 43–51, https://doi.org/10.1002/eej.23277. [Online]. Available: https://onlinelibrary-wiley-com.proxy.library.stonybrook.edu/doi/full/10.1002/eej.23277

13. Leonics. “Basics of MPPT Solar Charge Controller.” [Online]. Available: https://www.leonics.com/support/article2_14j/articles2_14j_en.php 

14. Eindhoven University of Technology. “TU/e students develop zero emissions car that actually captures CO₂ from the air as it drives.” 2022. [Online]. Available: https://www.tue.nl/en/news-and-events/news-overview/21-07-2022-tue-students-develop-zero-emissions-car-that-actually-captures-co2-from-the-air-as-it-drives 

15. Which Car. “How Many Cars are There in the World?” 2022. [Online]. Available: https://www.whichcar.com.au/news/how-many-cars-are-there-in-the-world

16. Squad Mobility. “Squad Solar Car.” 2019. [Online]. Available: https://www.squadmobility.com/blog/squad-mobility-news-1/squad-solar-car-1

17. Elektrek. “Squad Mobility Officially Launches its Solar City Car Optimized for Private and Shared EV Use.” 2022. [Online]. Available: https://electrek.co/2022/05/16/squad-mobility-officially-launches-its-solar-city-car-optimized-for-private-and-shared-ev-use/ 

18. Green Car Reports. “$6,250 Squad Solar EV Coming to US - Wherever 25 MPH is Enough.” 2022. [Online]. Available: https://www.greencarreports.com/news/1138101_6-250-squad-solar-ev-coming-to-us-wherever-25-mph-is-enough

19. CNET. “Why the Cost of Solar Panels Will Likely Keep Falling.” 2023. [Online]. Available: https://www.cnet.com/home/energy-and-utilities/why-the-cost-of-solar-panels-will-likely-keep-falling/

About the Authors
Dr. Raj Shah​ is a Director at Koehler Instrument Company in New York, where he has worked for the last 28 years. He is an elected Fellow by his peers at IChemE, CMI, STLE, AIC, NLGI, INSTMC, Institute of Physics, The Energy Institute and  The Royal Society of Chemistry. An ASTM Eagle award recipient, Dr. Shah recently coedited the bestseller, “Fuels and Lubricants handbook”, details of which are available at ASTM’s Long-Awaited Fuels and Lubricants Handbook 2nd Edition Now Available (https://bit.ly/3u2e6GY).He earned his doctorate in Chemical Engineering from The Pennsylvania State University and is a Fellow from The Chartered Management Institute, London. Dr. Shah is also a Chartered Scientist with the Science Council, a Chartered Petroleum Engineer with the Energy Institute and a Chartered Engineer with the Engineering council, UK. Dr. Shah was recently granted the honorific of “Eminent engineer” with Tau beta Pi, the largest engineering society in the USA. He is on the Advisory board of directors at Farmingdale university (Mechanical Technology) , Auburn Univ ( Tribology ), SUNY, Farmingdale, (Engineering Management) and State university of NY, Stony Brook ( Chemical engineering/ Material Science and engineering). An Adjunct Professor at the State University of New York, Stony Brook, in the Department of Material Science and Chemical engineering, Raj also has over 575 publications and has been active in the alternative energy industry for over 3 decades.

Mr. Beau Eng and Ms. Mrinaleni Das are part of a thriving internship program at Koehler Instrument company in Holtsville, and are students of Chemical Engineering at Stony Brook university, Long island, where Dr.’s  Shah and Mittal  are on the External Advisory Board of Directors.

Dr. Vikram Mittal, is an Associate Professor at the United States Military Academy in the Department of Systems Engineering. He earned his doctorate in Mechanical Engineering at the Massachusetts Institute of Technology where he researched the relevancy of the octane number in modern engines. His current research interests include Fuel technologies, system design,  Alternative  energy technologies, model-based systems engineering and modern engine technologies. He has numerous publications in various peer reviewed journals.

The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag

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