Smart charging vital for EVs to rely on solar, says Swiss study

Despite allegations and fears to the contrary, charging electric vehicles primarily or exclusively using residential rooftop solar is well within the realm of plausibility, according to a new Swiss study.

But, doing it with “smart charging”, which controls when the car charges, is the key.

Researchers from public research university ETH Zurich in Switzerland have shown how, with the right combination of residential rooftop solar, smart charging, and battery storage, electric vehicles can be charged almost exclusively using solar energy, and without compromising the power needs of the home.

Focusing on EV owners and rooftop solar in Switzerland – which already sees 5% of its annual electricity consumption supplied from solar and has 70,000 EVs on the roads along with 200,000 hybrids – the researchers analysed charging patterns to determine if residential rooftop solar could supply all charging needs.

Their findings were published in the journal Renewable & Sustainable Energy Reviews.

“The mobility sector is responsible for about a third of greenhouse gases in Switzerland,” said Martin Raubal, professor of geoinformation engineering at ETH Zurich. Electromobility is one way to reduce CO₂ emissions from transport.

“Our results show that owners of electric vehicles can use their cars with no particular restrictions, charging them to a large extent with their own photovoltaic power, even without intermediate storage,” added Henry Martin, a doctoral student at the Institute of Cartography and Geoinformation at ETH Zurich.

ETH Zurich’s researchers analysed the charging and mobility patterns of 78 EV owners, all of whom generally lived in a single-family home and regularly used their vehicles during the day. Ten months worth of charging and mobility data was gathered so as to describe for the researchers when the cars were stationary, moving, and being charged.

Most of the EV owners charged their EVs at home with mains electricity, but the researchers wanted to discover if they would have been able to use their vehicles as flexibility if they had charged them using only rooftop solar power.

Giving each of the EV owners a virtual rooftop solar system for the study – using each EV owner’s specific rooftop measurements and historical weather data with a 30-minute resolution – the researchers were able to calculate the potential solar power yield of each house while taking into account relevant negative factors such as shade from neighbouring buildings and trees and accurate weather data.

Four separate models were created, each of which goes a long way to highlighting the way in which rooftop solar and electric vehicles can be paired to better utilise renewable energy.

The first of the four models changed nothing in the behaviours of the EV owners, charging their EVs at the same historical times over the ten months, with the only difference in the model being the swap from mains power to rooftop solar. The results showed that, on average, only 15% of annual EV charging requirements were met by solar power, with the rest supplied by mains power.

However, with a simple smart change – introducing smart charging that controls when an EV is charged – solar power was able to meet 56% of an EVs charging requirements.

“We were surprised by the high proportion,” says Martin. “Smart charging can significantly increase the household consumption of photovoltaic power – and the vehicle can be used just as flexibly as if it were charged with mains electricity.”

The avenue through which to increase this percentage was clear, then, and the third model increased the level of smart charging and saw solar power supply up to 90% of an EVs charging needs.

Adding battery storage naturally helps drive the amount of solar power able to be used for EV charging to virtually 100%, but the researchers are described as “ambivalent about intermediate storage” due to the fact that “the sustainability balance of the charging system as a whole is affected by the fact that the production of electricity storage units causes relevant quantities of CO₂.”

“Our case study demonstrates the potential of smart charging for a decentralised and grid-friendly supply of renewable energy that has been exploited very little so far,” said Raubal, who will lead his research group in developing machine learning strategies with a view towards better predicting solar PV yield and user behaviour.