Prius White Paper
PV Prius
8: Terrestrial PV Systems
8.1 Stand Alone Systems, Including Hybrid Systems

Edward J. Simburger, (310) 336-7126
yerbasec@pacbell.net
Joseph T. Simburger
Edward J. Simburger, Electrical Engineer
Greg Johanson, (805) 497-9808 info@solarelectricalvehicles.com
& Mark Bagnall
Solar Electrical Vehicles

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%.

Description of Solar Module

The solar module is fabricated from molded fiberglass to fit on the roof of a standard Toyota Prius automobile. The solar module includes 146 four-inch- square mono-crystalline cells which are rated at 16% nominal efficiency. The cells generate 3.5 A short circuit and 0.57 VDC open circuit. Using a series connection, the PV Cells, which produce 80 VDC, open circuit, 70 VDC at the peak power point with a string current of 3.25 amperes at AM 1.5 conditions.

Figure 1. Photo of the Prototype PV Prius

An insulating layer of Tedlar is bonded to the fiberglass with a heat-cure adhesive. The solar cells are series connected and then bonded to the Tedlar insulation with EVA encapsulant. Finally, a Tefzel layer is bonded to the top of the cells for ultraviolet and weather protection. Figure 2 provides a diagram showing the individual layers of the solar module.

Figure 2. Solar Module Component Layers

Description of the Interface Electronics

The solar module is connected to a 48 VDC Sealed Lead Acid Battery via a peak power tracking 70 V to 48 V Battery Charger. The Solar Energy stored in the Sealed Lead Acid Battery is in turn delivered to the PV Prius’s primary motive NiMH Battery pack using a DC-DC converter, which steps up the battery voltage from to 48V to 240 V. The Battery Charger and DC-DC converter has a 95% efficiency rating and can provide daily watt-hours and total watt-hour data to the PV Prius’s solar monitoring system. Figure 3 provides a schematic of the electrical components, which are added to a stock Prius to convert it to a PV Prius. Figure 4 shows Lead Acid Battery Box in the trunk of the PV Prius. The Battery Charger and DC-DC converter are contained in a separate compartment in the Battery Box.

Figure 3 Connection Diagram for Solar Components of the PV Prius


Figure 4 Photo of the DC-Dc Converter installed in the PV Prius

Evaluation of Economic Benefits of the PV Prius Over a Stock Prius

The Toyota Prius is rated at 60 miles per gallon city, 51 miles per gallon highway and 55 miles per gallon combined. To evaluate the return on investment for the PV Prius we will assume that the useful life of the vehicle is 100,000 miles and will calculate the economic benefit of the PV Prius based upon this figure. Thus, using the 55 miles per gallon number, the stock Prius will consume 1818 gallons of gasoline during its 100,000 miles of useful life. The PV Prius will consume somewhere between 17% and 29% less gasoline than the stock Prius. This savings would be between 1287 and 1505 gallons of gasoline. Thus, the PV Prius will save between 312 and 530 gallons of gasoline over its service life as compared to a stock Prius.

What if the gasoline consumption estimates provided by Toyota were a bit optimistic and in reality the combined fuel consumption rate was somewhere around 45 miles per gallon. Then the stock Prius would consume 2222 gallons of gasoline during its useful service life. The PV Prius would consume somewhere between 1573 and 1840 gallons of gasoline during its useful service life. That would be a savings between 382 and 648 gallons of gasoline. Thus, if the actual fuel consumption of the Toyota Prius is somewhat less than specified than the savings in gasoline consumption increases for the PV Prius.

The economic value of the PV Prius as compared to a stock Prius is tied to the price of gasoline. Thus, in Figure 5 the potential savings that one could expect from a PV Prius over that of a stock Prius is presented as a function of gasoline prices ranging between $2.00 per gallon and $6.00 per gallon.

Figure 5. Total Savings for a PV Prius as compared to a Stock Prius as a Function of Gasoline Prices

Environmental Benefits for a PV Prius
In addition to the economic benefits described in the previous section there are significant environmental benefits that can be realized by the PV Prius. The primary benefit would be the reduction in Carbon Dioxide emissions. The consumption of one gallon of gasoline will emit to the atmosphere 24 pounds of Carbon Dioxide gas. Using this number one can determine the reduction in the emissions of Carbon Dioxide gas for each of the four cases presented in the previous section. This data is presented in figure 6.

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.

Figure 6 Reduction in the Number of Pounds of Carbon Dioxide emitted to the Atmosphere

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.

Data from the operation of a 6 kW photovoltaic array installed in the Los Angeles area reports total production for the period beginning October 2004 through October 2005 to be 8193 kWh.1 This works out to 22.4 kWh per day. Based upon energy consumption of 175 W/Mile for the Total PV Prius the amount of energy that would be required from the residential photovoltaic array on a daily basis would be 4 kWh. This is only 18% of the average daily production from the 6 kW residential solar array.

Figure 7 provides a comparison of the utility value of the electricity from the residential PV system compared to the total value of fuel that it offsets. The present utility value of the residential PV system electricity connected to the Southern California Edison System is $0.22 per kWh.

This method of operation would result in a vehicle that could truly claim zero emissions, insofar as all of the energy required for its operation would be provided from the Total PV Prius’s own solar array with supplemental energy provided from the residential PV system. Thus, the total reductions in Carbon Dioxide emissions over the service life of the Total PV Prius of 100,000 miles would increase to somewhere between 43,636 pounds at 55 MPG and 53,333 pounds at 45 MPG.

Figure 7. Value of the Gasoline that would be Required during the 100,000 mile Service Life of the Total PV Prius compared to Utility Value Residential PV Electrical Energy

Conclusions

The feasibility of installing an aftermarket photovoltaic module on a Toyota Prius has been shown. The economic return from the conversion of a stock Prius to a PV Prius is dependant upon the nominal daily trip length, the price of gasoline required to operate the gasoline engine, actual fuel efficiency of the gasoline engine, the number of Wh/mile and the number of Wh provided by the solar module.

With further modification of a PV Prius to increase the primary motive battery capacity and the use of electricity from a residential PV system to charge the high-capacity battery a Total PV Prius could be operated totally on energy from the sun!

References

[1] Edward J. Simburger, Joseph T. Simburger, Edward J. Simburger Electrical Engineer, Residential Photovoltaics; An Investment Vehicle for Retiree’s, 4th World Conference on Photovoltaic Energy Conversion, May 8-12, 2006, Waikoloa, Hawaii.

 

A residential or commercial solar electrical system coupled with the SEV system.
  Provides clean renewable solar energy for the home or office while helping your car consume less gasoline and produce fewer greenhouse gases.

 

 

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