Solar Hybrid Vehicle Cost and Economics

The question of whether solar vehicles are viable is not about dependency on oil or being green, it’s about cost and the premium people are willing to pay above a straight economic payback. The cost of adding solar to cars is based on data from adding PVs to buildings since vehicle PV information is not available. Table 1 compares the costs and payback from both building and solar hybrid vehicles. The calculation needs the capital cost of the PV system and the displaced fuel cost. A PV panel currently costs about $5.41 per Watt . Installation on the vehicle is assumed to cost $1 per Watt, but there is no information available on this. Included in the building installation cost of $3 per Watt is a DC to AC converter. This puts the total PV system cost on a building in the range of $8 to $10 per Watt, which is a pretty standard range. The cost of adding a solar PV panel to a Toyota Prius would be about $962. Federal or state incentives could reduce the cost of the solar PV system and a 50% reduction is incorporated in the table. This incentive is included to demonstrate the benefits if solar hybrid incentives were enacted since there have often been incentives for building solar systems or the wind production tax credit.

Using a Toyota Prius for this example, a Prius can travel roughly 7 miles per kiloWatt-hour (kWh) of electricity (a kiloWatt is 1,000 Watts). A PV panel on a building can achieve a capacity factor of about 25% based on a California Energy Commission presentation . (Capacity factor is the ratio of actual generation divided by potential generation if the system ran at full output for every hour.) A Prius would be able to travel about 2,300 miles per year on solar power with a 150 Watt PV panel, a 25% capacity factor and 7 miles per kWh. Miles on solar will vary with location and will probably be in the range of 1,500 miles to 2,500 miles. Solar hybrids are more viable in southern, sunny areas than northern areas or cloudy areas. Solar hybrids are not for everyone.

In Table 1, both building and car systems are the same size for ease of comparison. In reality, a residential building PV system would be about 10 times larger. If the building application were full sized, all of the costs and savings would be scaled up, but the payback period would be exactly the same.

Three scenarios are shown in Table 1 for gasoline prices because of the uncertainty and volatility in gas prices: $2, $3 and $4 per gallon. The mileage estimate of 44 miles per gallon (mpg) on gasoline is Consumer Report’s estimate for a Prius based on what a typical person would achieve in a mix of city and highway conditions. The U.S. EPA’s official mileage numbers of 60 mpg city and 51 mpg highway are generally optimistic. Assuming 12,000 miles per year spread evenly every day (32.9 miles per day), 10-miles of plug-in power every day, and the 2,300 miles on solar per year, Consumer Report’s mileage estimate would go from 44 mpg for a standard Prius to 87 mpg for a solar hybrid Prius. The EPA mileage numbers would go from 60 mpg city/51 mpg highway for the standard Prius to 119 mpg city/101 mpg highway for the solar hybrid Prius. These calculations do not account for any change in gasoline mpg due to the weight of the batteries or wind resistance from the solar PV system.

The economics of these solar PV alternatives can be compared in terms of fuel savings and payback. Table 1 shows that a PV panel on a building would save $32.19 per year in electricity. By contrast, the same sized PV panel on a solar hybrid would save $104.52, $156,78, or $209.05 per year at $2, $3, or $4 per gallon gasoline, respectively. Payback is the number of years of savings required to return the original purchase cost. As shown in Table 1, the solar hybrid vehicle systems requires from 4.6 years to 2.3 years for the savings in gasoline to offset the cost of the PV system, depending on the cost of gasoline. It requires 19.6 years for the electricity savings to repay the cost of a PV system installed on a building even when the initial cost is reduced by 50% via an incentive. Adding solar PV system to a hybrid vehicle is considerably more cost effective than adding a solar PV system to a building.

The attractiveness of solar power, or hybrid cars for that matter, is not strictly about cost savings. The question is whether consumers are willing to pay a premium to obtain additional benefits, such as solar power. Some consumers are willing to pay the premium for solar power on a building even though it requires many years to recover the initial cost. Consumer Repots found that only two of the hybrids currently on the market returned the additional purchase cost within five years (the Consumer Reports study considered all aspects of hybrid ownership and not just the limited scope evaluated here). Even though the gasoline savings may not justify the premium on the purchase price of a current hybrid vehicle, 205,000 buyers were willing to pay the premium for a hybrid car in 2005 because that is the number that was sold. With similar economics and very attractive environmental and societal benefits, solar hybrid vehicles would appear to be saleable. The cost premium for adding solar PV to a hybrid is reasonable, and the environmental and societal benefits are significant. The evidence from sales of solar systems on buildings and current hybrid cars indicates that many customers would be willing to pay the additional $1,000 of so for the solar PV system on a vehicle (and even less after incentives). Solar hybrids are likely to be even more marketable than conventional hybrids because the solar system makes a statement and would offer greater marketing cachet than the techy hybrid system.

Producing solar hybrids would also help bring down the cost of solar PV systems. Government incentives for solar systems on buildings is justified, in part, as a means of bringing down the cost of solar systems in the near future. Developing both building and vehicular solar systems would bring the cost down faster. It is likely that as the cost of solar PV systems come down with increasing research, development and production, solar hybrid vehicles will become fully economic before solar systems for buildings do because of the relative economics of electricity and gasoline.

Solar hybrid vehicles could have a major impact on America’s dependence on oil if they were widely adopted. The U.S. currently uses about 10 million barrels of oil per day for transportation. Solar hybrid vehicles are not for everyone, but if half the vehicles in the country converted to solar hybrids, solar would displace one million barrels per day of oil (assuming about 20% of driving was on solar). Overall, this half of vehicle fleet would reduce oil consumption by about 3.4 million barrels per day (if half the country’s cars went the current Corporate Average Fuel Economy guideline of 27.5 mpg to the overall mileage of 87 mpg for a solar hybrid shown above).

Challenges to Solar Hybrid Development

The first challenge to solar hybrids is that there does not appear to be any research and development currently being undertaken on solar hybrid vehicles, despite the promising economics and environmental and societal benefits. Additional research on solar hybrids is needed. The values shown in Table 1 are rough and need to be confirmed with real world experience. The annual capacity factor of solar systems on cars needs to be determined: cars might be able to attain higher capacity factors than fixed building solar systems by ‘repositioning the solar array’ after lunch (parking facing east in the morning and west in the afternoon). Means of installing solar PV cells to vehicles needs to be developed as well as control systems for solar PV panels on vehicles. There is much to be researched, but serious research does not appear to be going on. An Internet search on solar cars reveals virtually nothing but a few solar nuts attaching standard PV panels to the Prius. The National Renewable Energy Laboratory (NREL) is tasked with researching renewable energy, yet emails to the NREL showed that solar hybrid vehicles are not under investigation there. A quick review of the administration’s proposed budget failed to find a single dollar of funding for solar hybrid vehicles in almost 2.8 trillion dollars of federal government expenditures proposed for fiscal year 2007.

Solar hybrid vehicles need to utilize the larger battery of a plug-in hybrid vehicle, which has both benefits and challenges. The benefits of adding solar can be combined with the benefits of a plug-in hybrid vehicle. If a plug-in hybrid had a battery that provided 10 miles of driving on a charge, then adding solar panels capable of providing 10 miles a day of travel would enable the vehicle to travel a total of 20 miles on electric and solar. If the plug-in hybrid had sufficient battery storage for 20 mile of driving, then this range could be extended to 30 miles with the type of solar panel described above. However, the costs of these forms of hybrid also need to be combined. The calculations in Table 1 are for only the incremental cost of the solar PV system added to a plug-in hybrid vehicle so as to focus on only the solar costs and benefits. A challenge of a solar hybrid system is that it combines the cost of hybrid technology with the cost of plug-in hybrid technology with the cost of the solar system. All of these costs are coming down, but the combined cost could be a difficult hurdle to overcome.

The factors limiting plug-in hybrid development are also limiting solar hybrid development, and commercialization of plug-in hybrids appears to be stymied by battery technology. Most existing hybrids use nickel-metal halide batteries. Nickel-metal halide batteries are heavy, however. Plug-in developers appear to be waiting for a breakthrough in lithium-ion batteries. Lithium-ion batteries are expensive; Prius conversions to plug-in hybrid using lithium-ion batteries cost $12,000.

Solar PV technology may be an alternative to bigger batteries. If solar PV panels cost less than batteries, then instead of making a plug-in hybrid with a bigger battery, use solar. Instead of a plug-in hybrid with a 20 mile battery range, it would be less expensive to make a solar hybrid with 10 miles of battery range and 10 miles of solar range. The battery size can be determined based on the amount of power the solar system provides in a day.

Solar PV panels have also improved in recent years but additional improvement may be needed for some vehicles. A few years ago, most solar panels were too weak in order for one powerful enough to recharge the car’s batteries to fit on the roof of the vehicle. Now, or in the near future, car-roof-sized PV panels will be able to provide significant range in many cases. Some vehicles may still be limited by their ability to fit a sufficiently powerful PV system on the roof. As PV panels continue to be improved over time, solar PV panels will be able to provide additional range on a day’s recharging. Improvements in PV technology will mean that more vehicles will be able to utilize solar power, range will be extended, and costs will come down.

Rapid Technological Change and Unlimited Potential

Solar hybrid technology is undergoing rapid change. Five years ago, solar vehicles were pie-in-the-sky, Buck Rodgers type of technology. Then the hybrid drive system was developed, and the revolution started. With hybrid drive systems, it became practical to combine gasoline and electric propulsion systems in vehicles. Electric vehicles are no longer limited to the distance that can be provided by batteries; the gasoline motor (or, preferably, a flex-fuel engine) can be used as a back-up. In addition to this paradigm shift in automotive technology, solar PV panels became more efficient and are now powerful enough to recharge car batteries in a reasonable amount of time. The recent increases in the cost of gasoline have also improved the economics of solar hybrid vehicles. Solar hybrids are now viable and economic. As shown using the rough values in this article, solar PV panels added to hybrid cars and displacing gasoline are more economic than solar PV panels on buildings that displace electricity. Solar hybrid technology warrants further research to more accurately determine its economics and to overcome its challenges. Incentives for solar hybrid vehicles are also warranted as a way to overcome the challenges facing this technology and as a means of bringing down the cost of solar panels. Solar hybrids vehicles offer unlimited potential.

References
1. Retail PV costs from May, 2006 survey from www.solarbuzz.com

2. The California Car Initiative, www.cal-cars.org, email dated 4/28/2006 from calcars-news@yahoogroups.com. Reported 146 Watts-hours per mile. Inverting and changing decimal places gives 6.85 miles per kiloWatt-hour. This was rounded to 7 miles per kilowatt-hour assuming that a factory production product would perform better than a home conversion.

3. Miller, Sandy, Emerging Renewables Program and Performance Incentives Update, Solar Forum, Sept, 12, 2005

4. Consumer Reports, The dollars & sense of hybrids, http://www.consumerreports.org/cro/cars/new-cars/high-cost-of-hybrid-vehicles-406/overview.htm

5. http://www.whitehouse.gov/omb/budget/fy2007/budget.html

6. http://www.edrivesystems.com/faq.html

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