The idea of a solar-powered car has drawn another attempt from an optimistic manufacturer. Toyota has announced an optional 180 W Panasonic HIT (heterojunction with intrinsic thin layer) module for the roof of its plug-in hybrid electric vehicle (PHEV), the 2017 Prius Prime. This comes as Toyota’s second attempt; the 2010 Prius infamously could not connect its rooftop panels to the drive battery without strangely broadcasting radio signals, so the 50W panel only powered a fan to cool the interior. With more than triple the original wattage and new optimism from Toyota, the new module is intended to charge the drive battery and power unspecified car accessories. Toyota estimates that the module will add about 3.7 miles daily to the PHEV’s current range (25 miles electric, 615 miles gasoline).
Testing the numbers behind this claim indicate that the additional range would be achieved only if the vehicle was operating at peak wattage for about six hours (assuming approximately 3.5 miles range/kWh). This means the vehicle is parked in direct sun, without considering losses such as from clouding or roundtrip battery efficiency. The Toyota estimate of approximately 1.1 kWh of daily generation represents a unrealistic “best-case” model; given that solar PV in the U.S. typically reaches a capacity factor of around 25%, it becomes likely that average daily generation would be about a quarter of that value, which amounts to about .3 kWh or 1.1 miles of added range. This could be a significant factor if charging is during workdays for commuters travelling to urban areas with potentially limited solar resource. This could further impact geographical roll-out for Toyota: The company may limit itself initially to offering the solar roof in regions with high solar resource.
The Panasonic module may actually decrease Prius range. The additional weight of the panels will reduce mileage efficiency: according to Lux’s lightweighting models this makes the panels net negative for a journey more than around 180 miles. The other use for the module in powering car accessories may also introduce perverse results. Based on the sizing of the module, it most appropriately powers a car stereo or charging outlet. However, the module’s value is maximized by being placed in the sun as often as possible, which introduces a new heating load for the vehicle. However, the Panasonic module undershoots an air-conditioning unit wattage of typically around 3 kW by a full order of magnitude and introduces another unintended consequence. A Prius Prime owner that hopes to use a standard air conditioning system to cool down a vehicle that has been charging in the sun all day should think again: using AC for more than 20 minutes daily would deplete stored generation from Panasonic’s module even in optimal charging conditions.
Furthermore, Toyota is not the first manufacturer to try to make solar work for EVs, and likely not the last. Recent years have seen misses from BMW, Ford and Fisker, as well as a line of concept vehicles from Hanergy, who claims approximately 10 times more solar generation from its panels than Toyota’s system but at a likely cost of tens of thousands of dollars added to the price tag. In contrast, Panasonic’s proposed HIT module would likely only add a cost of around $200 to $300. Toyota has adopted a “why-not” mentality; module costs have dropped by about 50% since the first solar Prius in 2010, and if the unintended effects are ignored, the minimal gains of car-roof solar seem justifiable by cost. However, Panasonic also produces cells for the Prius Prime battery system, and Lux estimates just over $300/kWh for PHEV. If the module cost is redirected to additional battery storage (such as the various storage-sizing options for the Tesla Model S), it would likely mean about the same increase in range as for the solar module. Panasonic has effectively added unreliable energy storage at the same cost as reliable energy storage. The addition of a solar roof for Toyota’s Prius Prime represents a marginal cost for a marginal benefit in the best case, and a likely perverse effect in others.
Panasonic’s tightening relationship with both Tesla’s energy storage and solar module production has sparked speculation that it may partner to offer a similar car-roof option with Tesla’s upcoming Model 3. Tesla CEO Elon Musk addressed these rumors, indicating that Tesla will “probably offer [this] as an option”. As a purely electric vehicle, Tesla’s Model 3 offers a slightly different use case for a solar module than Toyota’s PHEV, and the argument for or against car-roof solar panels becomes slightly murkier. For example, Tesla already allows for a glass roof Model 3 option, meaning a negligible additional weight for a solar module, and possibly less difficulty in incorporating a larger area and wattage module. However even if the entire roof area of the Model 3 were used for Panasonic’s HIT module, the wattage would be less than a third of the demand of a standard vehicle air conditioning system, and a day of optimal generation would extend Model 3 range in the best case by only around 10%. However if Tesla does choose to offer a solar roof for its cars as well as its homes, cost constraints will likely limit module sizing, and the negligible characteristics of the Prius modules are a more likely outcome. Readers should view Toyota’s second attempt at a solar-powered vehicle as an ultimately meaningless addition, and should regard a follow-up by Tesla as the same: solar panels to adorn the roof of a vehicle provide no disruptive value proposition.
By: Max Halik