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Solar powered Prius
Solar powered Prius

A presentation at the Green Car Congress details how a Canadian engineer named Steve Lapp has created his own version of a prototype Photovoltaic Prius. The car is a 2001 Toyota Prius modified with roof-top solar panels and an additional battery storage system to supplement the charge in the original equipment NiMH batteries. The car transport is still a rough prototype a demonstration of concept but even with the limitations of the systems, he says he has achieved an initial 10 percent fuel efficiency improvement from 4.5 l/100km (52 mpg US) to 4.0 l/100km (59 mpg US).

Steve Lapp’s PV Prius is still a rough prototype a demonstration of concept but even with the limitations of the systems, he has achieved an initial 10% fuel efficiency improvement from 4.5 l/100km (52 mpg US) to 4.0 l/100km (59 mpg US). Click below for more on this story.

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From the original description of the plan:

…fact that [current Toyota hybrids] can run on electrical energy alone, with their gasoline engines off, offers the opportunity to provide them with more electricity and therefore drive further with the gasoline engine off.

Electricity can be provided from the electrical grid by charging an onboard battery, and depending on where that electricity comes from, it will have various emissions associated with it. [The plug-in concept.] However if it is provided from renewable energy sources, such as photovoltaic panels, then it is green.

This begs the question of why not put the PV panels directly on a hybrid car and generate electricity onboard while the car is parked outside, or even while driving. The general reaction of people to this idea is that there could not be enough energy striking the roof of a car to provide enough power supply to drive any meaningful distance.

This is where the incredible efficiency of the hybrid car must be taken into account. To drive a hybrid car about 1 km, takes about the same electricity as to light a 150 watt bulb for one hour! The point is not to drive the car using only solar power, but to effectively use solar power to improve gasoline fuel efficiency.

How much gasoline can this photovoltaic hybrid car save? Well let’s look at the energy available from the sun on the roof of the car. For June and July in Kingston Ontario, about 6 kWh of energy from the sun strikes each square meter of horizontal surface. If we install 2 square meters of photovoltaic panels on the car and we collect 10% of the energy from the sun as electricity (well within present PV efficiency), we can theoretically go about 8 km each day on just the sun’s energy. If we drive 24 km on a sunny day, that is enough to reduce our gasoline consumption by 33%. This would take the Prius from 5.0 l/100km to 3.3 l/100km.

The PV Prius uses a 12-volt PV source with a small lead acid battery and battery voltage controller, inverted to 120 VAC, transformed to 345 VAC, then rectified with current control to nominal 300 VDC. The charge from the PV batteries flows into the Prius hybrid battery when the ignition is on.

The 300 VDC output of the solar subsystem is attached to the switched side of the original Prius battery, so the PV battery cannot recharge the NiMH while the ignition is off. The PV system can inject a maximum of up to 2 amps continuously into the battery while the ignition is on.

Lapp’s modelling predicts a 10%-20% fuel efficiency improvement for the 270 watts of PV (to be bumped up to 360 watts with the additional of a fourth panel), so the 10% on the first trip with little optimization was a pleasant surprise.

He is working with between two to six 20 Ah sealed lead acid batteries, experimenting to discover useful amount of buffer storage, given typical solar and driving conditions.

The decision not to charge the hybrid when the car is off was a pragmatic choice, given the financial and time constraints of his project. Among other issues, there would need to be a thorough analysis to determined the optimal PV-NiMH energy flow/charge relationship.

Resources:
For the Green Car Congress, see the following link: www.greencarcongress.com

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