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By Robert Rapier on May 28, 2008 with no responses

The Solar-Powered Prius


Solar-Powered Prius (Source: http://www.solarelectricalvehicles.com/)

At the end of my recent essay on Nissan’s electric car announcement, I wrote “For my next calculation, I need to see how much power I could generate by putting a solar panel on the roof of my electric car and letting it recharge all day.”

In response, a reader wrote and told me that a feasibility study has been done for this on a Toyota Prius. The paper was the source of the above picture of the prototype:

Prius White Paper

From the paper:

Abstract

The major automobile manufacturers are producing hybrid automobiles, which are part electric and part gasoline powered. Could these automobiles take another step and obtain some of their fuel from the sun?

Solar Electrical Vehicles has developed a prototype PV Prius to help answer that question. The PV Prius is fitted with a custom molded fiberglass photovoltaic module as shown in Figure 1. Solar Electrical Vehicles has applied for a patent on the PV Prius solar system.

The photovoltaic module is rated at 215 watts at AM 1.5. The module is connected to a DC-DC converter and peak power tracker. The output of the converter is directly connected to the primary motive NiMh battery.

The daily power production available for charging the Prius primary motive battery is estimated to be between 850 and 1,300 watt-hours. The car uses 150-175 watt-hours per mile. Thus, the expected range per day that the PV Prius would have on solar power alone would be between 5 and 8 miles. Based upon a nominal daily trip length of 28 miles the gasoline consumption of the PV Prius would be reduced by 17% to 29%.

The following section was of particular interest to me:

Can a PV Prius obtain all of its fuel from Solar?

The answer to this question is a definite yes providing that the stock Prius, in addition to having the solar modifications described in the previous section, increase the size of the secondary battery and the DC-DC converter used to deliver solar energy to the NiMH battery. Using a maximum depth of discharge of 50% to provide some reserve power and extend the cycle life of the enhanced Lead Acid battery, the capacity would have to be increased from its present 3 kWh rating to 8 kWh.

In addition, the 48 to 240 V DC-DC converter capacity would need to be increased to at least 2000 watts. With this battery capacity, increased energy from a residential photovoltaic array could be used to recharge the battery at night when the car is parked in the garage. This complete system is the Total PV Prius.

How can this be? How can you recharge your Total PV Prius at night parked in the garage? The answer to that is net metering with your local electric utility. That is if you live in a region of the country which net metering is offered for residences with a grid connected photovoltaic array, then the owner of the Total PV Prius would be able to supply energy to the utility grid during the day light hours and have it returned to him in the evening. While the energy returned to the homeowner may be produced using fossil fuels, the energy supplied to the utility during the daylight hours would have reduced the use of fossil fuels by an equivalent amount.

Some of you solar guys take a crack at that, and let’s discuss what some of the hurdles might be. The economic analysis isn’t all that promising, as the expected gasoline savings over the lifetime of the vehicle is estimated at somewhere between 300 and 600 gallons. That’s not going to warrant much capital on a purely economic evaluation. No cost estimates are given for the required modifications, but I am guessing the cost is more than can be justified by the gasoline savings.

It’s a start, anyway. Keep in mind that this is a Prius, and a lighter car can have a much greater range. The economics may look a lot better if the range is higher, and so is your daily commute.