According to the most recent report of the United States Environmental Protection Agency (EPA), the transportation industry generated 29% of US greenhouse gas emissions. The primary sources of these emissions are the fossil fuels burned by cars, planes, and buses [1]. In 2019, people in the United States bought more than 17 million cars, and this rate is predicted to stay stable [2]. Over the years, these statistics have raised significant concerns, but the absence of viable alternatives has left most people with no choice but to burn fossil fuels. In recent years, some researchers thought electric cars are the alternative solutions. But a recent study showcases that the extra weight of the batteries in electric vehicles causes more particulate matter (PM) emissions than any fossil fuels-powered vehicles [3]. Also, to ensure the supply of a significant amount of electricity to power electric cars, countries have to burn more fossil fuels. Therefore, electric cars cannot be considered the ultimate solution to reduce the carbon footprint [4]. 

Figure 1: Sources of Greenhouse gas [5] 

Many researchers are suggesting the use of renewable energy, such as wind or solar energy, to power cars can be the ultimate solution to the excess greenhouse gas emissions. At this moment, there are not many solar cars on the road, but recently big names in the automobile industry are showing interest in solar cars which shows the potential of this technology. For solar cars to become mainstream, solar panels need to be highly efficient, cost-effective, and mechanically robust. Some big automobile companies are trying to bring hybrid-solar technology to the market first to weigh its potential. For example, Toyota’s Solar Prius is going to have an ultra-thin solar panel that has greater efficiency than regular solar panels. Also, Tesla plans to turn the sunroof into a solar array [6]. 

Although commercial uses of solar cars are gaining popularity now, the idea was born in 1955 at the General Motors Powerama auto show. They displayed a futuristic model of a car run by solar energy, paving the way for vehicles powered by clean energy [7]. Although the idea was born more than 66 years ago, many might ask why there are still no solar cars on the roads yet. To answer this question, one has to consider things like the efficiency of solar panels, dedication toward building a sustainable society, and cost of production, etc. 

Most of the current multi-crystalline solar panel technologies have an efficiency of 11-18%, and the use of mono silicon in solar panels increases efficiency by 4%. The efficiency can reach up to 40% by using materials like gallium arsenide [8]. However, after vigorous calculations, researchers have concluded that to drive 50 miles per day, a car needs 3.44 kW, and most of the current solar panels can produce 345 W/ft² under full sun, and that will need approximately 250 ft² of solar panels in a car. It will not lead to a practical design if it is put only on the roof. Therefore, some automakers are trying to use special glasses that contain photovoltaic cells all over the vehicles [9]. One of the leading solar car producers in Europe, Sona Motors, used 248 solar cells all throughout its roof, hood, trunk, and sides, and each cell is capable of producing 4.84 W under the full sun; the whole car can produce 1.2 kW. This car also includes a 35 kWh battery that can go about 255 km after each charge. If one decided to use only the solar cells to power the cars, it could go up to 34 km/day [9]. 

Another automobile company known as Lightware was able to fit as much energy as Sona by packing the PV cells more densely on the roof and hood. The company claims the car can go 12 km for every hour it spends under the sun. However, it still has a battery capacity of 60 kWh [9]. 

Over the years, the use of solar energy in the car is restricted to solar panels, and the vehicles still heavily depend on batteries. A recent study conducted by Kasti proposes the use of single/double solar trailers in solar cars to make the solar cars rely fully on solar energy [10]. 

Figure 2: A depiction of proposed solar battery car and trailers 

The researcher studied the impact of using different numbers of solar trailers, and how it performs at different speeds.

Figure 3: Average battery power for a car, a car with one trailer and, a car with two trailers versus the nine operating states at the four speeds of 25, 40, 60, and 70 km/h [10]

The study suggests that the addition of solar array trailers makes the car more stable, decreases the total energy consumption, and decreases rolling resistance as speed increases [10].

Figure 4: Energy consumed by the car without solar trailers vs. with solar trailer [10]

The study claims even if the weather is cloudy, the use of a solar trailer reduces energy consumption by 20%. It is important to decrease solar cars’ reliance on batteries, and using the solar trailer is a good way to combat that necessity [10]. Although it might not be the most convenient solution considering the fact that urban parking areas are not spacious enough, and also, it is not possible to be in the well-lit area always.  

To make solar panels more efficient, researchers are trying to find materials that can store more energy in less space. Current silicon-based solar cells have a theoretical efficiency of 29%, whereas researchers predict that one of the new material perovskites, which is a combination of organic and inorganic materials, showcases theoretical efficiency of up to 35% and cut down production costs significantly [11]. Recent studies show that CH₃NH₃PbI₃ and mixed halide perovskite CH₃NH₃PbI₃−xClx can reach the efficiency that can enable solar cells to convert more solar energy into mechanical energy. Typically, using the single junction structure (one p-n junction), the PV cells can achieve an efficiency of 20%. However, efficiency can exceed 29% by using a tandem junction structure (more than two p-n junctions) [12]. A recent study achieved more than 30% efficiency using a single-junction structure by modifying the bond angle between X–Pb–X in CH₃NH₃PbX₃. The lower conduction bands of GaAs dilute s-bands, whereas the lower part of CH₃NH₃PbX₃ conduction bands is mostly Pb p-bands which have two more stable states than s bands which causes less dispersion. Therefore, the density of halide-based perovskite is higher than GaAs, and it has stronger absorption properties, as depicted in figure 5 [13]. 

Figure 5: Comparison between different types of PV cells [13]

Although this solar cell technology seems to have a promising future, there are still some barriers. To commercialize these PV cells, researchers have to make them humidity resistant, improve their life expectancy, and find a substitute for Pb to make the PV cells environment-friendly [14]. 

In recent years, many researchers have put their concentration on solar-electric hybrid cars. Many argue that this idea is more feasible than completely solar cars because of the high production cost and low power range of solar cars. A recent study claimed that the integration of PV cells in solar-hybrid vehicles (SHVs) reduces fuel consumption and thus reduces carbon dioxide emissions. Solar hybrid cars utilize solar panels to capture a significant amount of energy while driving and resting. Moreover, recent studies show that PV cells added to the car are more efficient and economically viable than putting PV cells in buildings [15]. To cultivate the most benefits of solar energy, researchers have to redesign the whole vehicle, or else PV cells will only be used to power some car accessories, and the potential would be wasted. A recent study tried to design an optimum SHV by reducing vehicles’ weight, battery choice, and optimization of solar panels [16]. 

                   Figure 6: Fuel Economy (km/l) on ECE Cycle - HSV vs. Toyota Prius [16] 

In this experiment, HSV-A showcases poor performance due to the poor choice of components and relatively high weight (1950 kg). In both HSV-A and HSV-B Lead-acid battery was used, which increased the weight of the vehicles, whereas, in HSV-C, the use of lithium-ion battery and reduction of weight along with 1.44 m² solar panels provided the optimal condition for the hybrid-solar car. In most solar cars, solar panels are aligned horizontally, which is not an optimal condition to maximize energy from the sun. Hence, the study suggests a self-orienting solar roof (2 axis tracking) can solve the problem and ensure better energy efficiency [16].

Figure 7: Energy collected with various options of solar roof [16]

A moving panel does not provide a practical solution because of the urban road structures and modern car designs. Moving panels would require more energy to move, and there are high chances of facing kinematics constraints which will further prevent it from functioning fully. Therefore, 2-axis solar roofs seem like a reasonable solution [16]. 

Considering the current development and technology, it can be said that a solar car is not the most feasible idea at this moment. Recent studies are showing the increasing demand for electric cars, and countries are investing more in sustainable energy sources to generate electricity. However, it does not change the fact that this technology has a promising future and the potential to reduce greenhouse emissions by a significant amount and make the transportation sectors more economically friendly due to the low production cost of solar panels. However, in the meantime, the lack of efficient PV materials and suboptimal design of solar cars make the idea far-fetched. In the future, if the researchers can figure out an optimal PV material that is sustainable and highly efficient at the same time, which will reduce the dependency on batteries, and also find a design that maximizes solar power, the future can potentially be greener and more technologically advanced. 

About Dr. Raj Shah
Dr. Raj Shah is a Director at Koehler Instrument Company in New York, where he has worked for the last 25 years. He is an elected Fellow by his peers at IChemE, CMI, STLE, AIC, NLGI, INSTMC, 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 https://www.astm.org/DIGITAL_LIBRARY/MNL/SOURCE_PAGES/MNL37-2ND_foreword.pdf

Reference 

1. Sources of Greenhouse Gas Emissions. (n.d.). EPA.

2. Wayland, M. (n.d.). US auto sales fall in 2019 but still top 17 million for fifth consecutive year.

3. Victor R.J.H. Timmers, Peter A.J. Achten. Non-exhaust PM emissions from electric vehicles. https://doi.org/10.1016/j.atmosenv.2016.03.017.

4. Gonçalves, A. (Ed.). (n.d.). Are Electric Cars Really Greener? YouMatter.

5. Emissions Sources (2020). (n.d.). Climate Central.

6. Huntington, S. (n.d.). Will We Ever See Solar Cars in the Future?

7. William Cobb demonstrates first solar-powered car. (n.d.). History.

8. Thilagam, A., Singh, J., Stulik, P., (1998), Optimizing Gallium Arsenide multiple quantum wells as high-performance photovoltaic devices, Solar Energy Materials and Solar Cells, Vol: 50, 1-4, January, 1998 pp. 243-249, Elsevier

9. Zientara, B. (n.d.). Solar Panel Car Roofs: Are they a good idea? https://www.solarreviews.com/blog/solar-panel-car-roofs-are-they-a-good-idea

10. Kasti, N. (2017). Ranges of applicability of a solar-battery car with single and double solar-trailers. Solar Energy, 144, 619–628. https://doi.org/10.1016/j.solener.2017.01.051

11. Gronewold, N. (n.d.). This Material Could Squeeze More Energy from Solar Panels. Scientific American.

12. Snaith, H. (2013). Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. The Journal of Physical Chemistry Letters, 4(21), 3623–3630. https://doi.org/10.1021/jz4020162

13. Yin, W., Shi, T., & Yan, Y. (2014). Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell Performance. Advanced Materials (Weinheim), 26(27), 4653–4658. https://doi.org/10.1002/adma.201306281

14. Ogomi, Y., Morita, A., Tsukamoto, S., Saitho, T., Fujikawa, N., Shen, Q., Toyoda, T., Yoshino, K., Pandey, S., Ma, T., & Hayase, S. (2014). CH3NH3SnxPb(1-x)I3 Perovskite Solar Cells Covering up to 1060 nm. The Journal of Physical Chemistry Letters, 5(6), 1004–1011. https://doi.org/10.1021/jz5002117

15. Coraggio G., Pisanti C., Rizzo G., Sorrentino M. (2010, I), Assessment of benefits obtainable in a Hybrid Solar Vehicle using look-ahead capabilities for incoming solar energy. 

16. Rizzo, Gianfranco & Arsie, Ivan & Sorrentino, Marco. (2010). Hybrid Solar Vehicles. 10.5772/10332.

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

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