Since 23% of all CO2 emissions arise from transport it is reasonable too ask whether this could be displaced using solar power? An even more ambitious question is whether some of that power could be supplied from the surface of the vehicle itself?
The short answer to both questions is yes, in 2016 Toyota made a technological demonstrations of a modified Prius vehicle that self-charges using the sunlight that falls onto the vehicle. Their first demonstration used conventional, low-cost crystalline silicon solar cells and resulted in 30km/day autonomous driving. A later demonstration used state-of-the-art III-V multi-junction solar cells that delivered up to 50km/day autonomous range. Since most passenger car journeys are around 20-30km per day, this suggests that electric vehicles with photovoltaic solar cells integrated into the roof could significantly reduce the frequency which the vehicle is charged from the electrical grid.
Some interesting question arises from this: could we ever do better than a few tens of km per day from sunlight? What about other, larger vehicles that have a greater surface area for photovoltaic power generation? The graph below shows an estimate for the power that can be generated in a sunny climate (Sydney, Australia) using conventional 22% crystalline silicon photovoltaic technology from the available area (x-axis) and drive efficiency (y-axis).
Small passenger electric vehicles typically have a few sq.m of practical area available for photovoltaic solar cells and a drive efficiency of 10km/kWh giving 20km per day solar range. Electric buses and trucks have a much larger area available for photovoltaics, up to 50 sq.m but drive efficiencies closer to 1km/kWh which coincidently also results in 20km per day.
Increasing the solar cell efficiency means more electricity is generated from the same area, so doubling the solar cell efficiency would double the range. The absolute physical limit for solar cell efficiency is 87%, with 47.1% demonstrated to date.
Increasing vehicle efficiency is also possible. Toyota's solar Prius had a drive efficiency of 17km/kWh and UNSW's own solar race car team 'SunSwift' achieve 33km/kWh in their solar cruiser vehicle. As an example of just how high the vehicle efficiency can get, Duke University have set a world record of 1283 km/kWh with their Eta vehicle. The day to day practicality of this vehicle leaves much to be desired, but as shown in the chart above, for all practical purposes, it can be powered entirely from sunlight.
To explore the opportunity for integrating photovoltaic power generation into electric vehicles, UNSW held an event on the 11th February 2020. A video recording is available of the event: Making Solar Powered Transport a Reality
Press coverage of the event includes:
- Powered by the sun … with a little help from the grid: the road is clear for solar vehicles, Ecogeneration 21 Feb 2020
- 'Fuel falls from the sky': The solar revolution coming down the road, Sydney Morning Herald, 9 Feb 2020
Vehicle integrated photovoltaics is a growing area of research at UNSW and is now recognised by the International Energy Agency in their Photovoltaic Power Systems programme.