Eco-friendliness is the new catchphrase of the automotive industry. Many manufacturers are investing in R&D efforts to utilize pollution-free energy as a power source for their cars.
The decision to install a solar roof on the new Sonata Hybrid, released last July, reflects such a trend. Solar roofs, which absorb the energy from the Sun for use in the vehicle, were first adopted by Toyota on the 2012 Prius. At the time, though, technological limitations in energy efficiency made the energy from the solar roof only useful for peripheral devices like air conditioning rather than for a meaningful increase in driving distance.
More efficient solar roofs that have a real impact on driving distance are currently in use on Japan’s Toyota Prius Prime and the American Karma Revero. Both models are plug-in hybrid types, and the Prius Prime PHEV offers the solar roof as an option (at approx. $2,550) for sale in Japan and Europe, while the Revero includes the function in the North American base model (the list price for the Revero is approx. $110,000-$125,000).
The mechanism behind the roof’s function is simple. When sunlight is absorbed by the batteries on the solar panel, electricity is produced. After being processed through various control mechanisms to increase efficiency, the electricity is stored in both the starter battery and the drive battery. This electricity in the drive battery functions to extend the driving distance, while that in the starter battery reduces the time needed by the alternator to charge the starter, thereby reducing the burden to the engine and improving fuel efficiency.
The Prius’s solar roof follows a different working principle: it stores the energy from the roof onto a separate battery, which then transfers the energy to charge the drive battery. Not only does the system requires an additional battery, but it also loses energy efficiency in the process. The Sonata’s system is more efficient in that regard, as it simultaneously charges both the starter and the drive battery.
The Sonata’s solar roof underwent much research to improve upon its efficiency. First, there is a relationship between energy efficiency and the angle at which the sun’s rays enter the panel. The solar panel, as it is placed on the car’s roof, is parallel to the ground, but research has found that the ideal angle to maximize the efficiency is at 30 degrees to the ground. The car roof is structurally limited to take the 0-degree angle, which explains its lower efficiency compared to the properly angled rooftop panels.
Considering this, though, Hyundai has installed on the solar roof panels high-performance cells with high charging efficiency. The cells on the Sonata’s panels have a 22.8% efficiency rate, roughly 30~50% higher than the rate for the typical cells used on rooftop panels (15~19%).
There is also a relationship between efficiency and external conditions. For example, it may appear that dead leaves or dust on the roof will significantly reduce the production of electricity. But tests have confirmed that such conditions resulted only in a 3~10% loss in production. As long as the roof is not completely covered or tarnished, the charging function remains mostly intact.
The Sonata Hybrid’s solar panels have a capacity of approximately 200W (204 to be exact); that is, panels exposed to the Sun in good sunlight will produce 200Wh of electricity. That is a non-negligible amount: 200W can turn on two 100-watt bulbs or 11 household LED fluorescent lamps (18W each). At this rate, charging for 5.8 hours per day adds 1,300 km per year to the total driving distance. Those who park their cars outside and/or drive more during the day will see even a greater bonus.
Because the efficiency of the solar roof is subject to external conditions (like the weather), its contributions to fuel efficiency were not reflected in the official fuel efficiency measures. But on average on a standard km/ℓ basis, the roof improves fuel efficiency by about 7%.
The calculation used average hours of sunshine data from the Korean Meteorological Administration and KOSTAT(Statistics Korea). There certainly are variations in weather throughout the year, but the average data takes those variations into account. Hours of sunshine have been increasing from 5.5 hours in 2010 to 6.9 hours in 2018, but the calculation used the more conservative figure of 5.8 hours, a 10-year average value.
The solar roof also functions to reduce the incidences of battery discharge. Recent booms in car electronics, such as the black box camera, have increased battery consumption and thus the number of discharge accidents. According to the Korea Insurance Development Institute, a shocking 4 of the 10 on-road emergency services (6,150,000 in total, 2015 data) pertained to battery discharge.
The electrical current charged through the solar roof per day is 81,200 mAh, which far exceeds the 720 mAh needed per day for a standstill car. Moreover, it merely takes the roof an hour to meet the daily energy needs of the black box camera (12,000 mAh). The solar roof definitely reduces the likelihood of discharge accidents: even when the black box camera is on full-time, as long as the car is parked outside, the car will not exhaust its battery. The only situation in which there may be a problem is one in which the car is parked underground long-term.
How eco-friendly is the solar roof? Looking into its impact on carbon emissions answers this question. Reducing carbon dioxide emissions is the biggest challenge facing all car manufacturers. The U.S. and European markets, mainstays of the global automotive market, have put strict regulations on CO2 emissions. Financial ramifications are clear here: the Fiat/Chrysler Group recently made headlines by purchasing carbon credits, and though the specific figures were not released, the Financial Times estimated the purchase fee to be several hundred million euros.
In addition to regulating CO2 emissions, the U.S. government offers “off-cycle credits” to manufacturers who use eco-friendly parts on their models. The credits are given to the technology that, while not reflected on the official fuel efficiency measures, nonetheless improves fuel efficiency or help reduce the emissions of greenhouse gases. Cars released after 2011 are eligible for the credits.
Off-cycle credit is given in units of grams per mile (g/km in Korea), and the car’s CO2 emissions are offset by the total amount of credits accrued by the eligible eco-friendly mechanisms in the vehicle as determined by the EPA (Environmental Protection Agency). For example, high-efficiency lamps are given credits of 0.16 g/mi, while ISG (Idle Stop & Go) is given 1.5 g/mi. The Sonata Hybrid’s solar roof was granted 8.98 g/mi by the EPA, which is the largest off-cycle credit ever given to a single eco-friendly technology. Hyundai’s leading development of the solar roof is expected to be especially meaningful in Europe, where additional taxes will be imposed from 2021 on models with emissions greater than 95 g.
There are not many manufacturers whose mass-produced models can be equipped with solar roofs that extend driving distance. In fact, the Toyota Prius is the only one; the Karma Revero comes with a solar roof on all trims, but its high price at over $110,000 realistically puts the model beyond comparison.
As said, the Prius’s solar cell system transfers the energy from the solar panels to a separate solar battery before charging the drive battery, whereas the Sonata’s system directly charges the batteries from the solar panels. This difference directly translates to sizeable differences in energy efficiency.
Moreover, the Sonata Hybrid’s solar roof has improved its panel output and made the controller more efficient, increasing the daily driving distance by 20% compared to the Prius. Sonata’s driving distance for one day’s charge is 3.6 km, higher than the distance of 2.9 km announced by Toyota.
In addition, the cost of the solar roof for the Sonata Hybrid is about $1,100, while it is 280,000 yen (approx. $2,550) for the Prius. The Sonata’s system is, in other words, the more powerful yet twice as economical system as the Prius’s.
The two solar roofs also differ in structure. The Prius’s solar battery system is without a buttressing frame for the panels; perhaps as a result, the Prius has reportedly failed the roof strength test conducted by the IIHS (Insurance Institute for Highway Safety). The Sonata Hybrid, on the other hand, received a “Good” score on the IIHS’s Roof Crush Test. While meeting the IIHS standards is not a prerequisite for selling cars in North America, it is an important, reputable safety standard for consumers.
Toyota is not offering the solar roof for North American consumers; it is an option for the Japanese and European consumers only. Finally, a difference in the design—the Sonata Hybrid uses a black back sheet that blends in with the cells on the panels, effectively making them less visible. The Prius, on the other hand, uses a white back sheet.
On the whole, the Sonata Hybrid’s solar roof system is not only more efficient but also more economical than the Toyota Prius’s system, though it should be admitted that driving distance alone cannot sufficiently evaluate the entire appeal of the respective systems. But regardless, it is without a doubt that the solar roof system allows Hyundai to move one step closer to the ideal eco-friendly car imagined by all manufacturers. Hyundai and Kia are working to apply the solar roof technology to their other eco-friendly models for a more sustainable future.
HMG Journal Operation Teamgroup@hyundai.com
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