Vehicle Integrated Photovoltaics: Exploring the Potential

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1 Abstract Vehicle Integrated Photovoltaics: Exploring the Potential Steven E. Letendre, PhD Associate Professor Green Mountain College, Poultney, VT Phone, Fax, As the vehicle fleet transitions to electric drive, a new market for photovoltaic (PV) technology emerges in vehicle integrated applications VIPV. This paper explores the VIPV opportunity and reports on recent activity in this area. A brief overview of past efforts to utilize solar as a source of energy for transportation is presented. Specific design considerations are discusses, along with an assessment of current and emerging photovoltaic technologies and their use in VIPV application. And finally, commercial development efforts are described, along with an assessment of the future VIPV market opportunity. The paper concludes that electric drive vehicles offer an exciting opportunity to create a multi-mw market for photovoltaics, while at the same time offering a clean and renewable fuel source for light vehicles. Keywords: electric drive, solar, photovoltaics (PV), and vehicle integrated PV (VIPV) 1. Introduction The world faces critical energy challenges. The transportation sector s exclusive reliance on oil for gasoline and diesel fuel creates significant risks. Concerns over the reality of a changing climate resulting from the combustion of fossil fuels and the pending decline in global oil extraction necessitate a rapid change in the way we power our vehicles. While not addressed in this paper, curbing demand for mobility must be part of the solution. Total vehicle miles traveled (VMT) in the US continue to rise. Figure 1 presents total and per capita VMT in the US from 1960 to During this time period, per capita income in nominal dollars increased from $13,800 to $38,856. This suggests that demand for mobility is linked to income levels. Figure 1: US per capita VMT and total VMT ,000 3,500,000 10,000 3,000,000 per capita VMT 8,000 6,000 4,000 2,000 Per Capita VMT VMT 2,500,000 2,000,000 1,500,000 1,000, ,000 Total VMT (millions) Year Source: US Bureau of Transportation Statistics

2 As demand for mobility in the developed world is reaching an apex, millions living in the developing world are just beginning to reach a level of affluence that allows for car ownership [1]. Recent data suggests that China is rapidly expanding its automobile manufacturing capabilities; annual passenger car production grew from 100,000 vehicles in 1991 to 2.3 million in 2004 a 28 fold increase [2]. The environment simply cannot accommodate a level of car ownership and use in the developing world like that found in wealthy nations. However, these new consumers cannot be denied the ability to meet their growing demand for mobility. New technologies must play a role in helping to minimize the environmental impacts of a growing global fleet of vehicles. An emerging consensus is forming around the central role that electric drive will play in future vehicle designs. Initially in the form of hybrid electric vehicles, dramatic improvements in efficiency can be achieved using a hybrid drivetrain that use both an engine and electric motor to propel a vehicle [3]. Hybrid electric vehicles are becoming increasingly popular with consumers. In 2006, 1.5 percent of all new vehicles sold in the US were hybrids ( Many view the evolution of today s hybrid electric vehicles to allow charging from the grid to be desirable, indeed inevitable. These so-called plug-in hybrid electric vehicles (PHEVs) allow consumers greater fuel diversity through substituting gasoline purchased at the pump with electricity from the grid. In a 2004 report prepared by the US-based Center for Energy and Climate Solutions for the National Commission on Energy Policy concluded, We believe that the most plausible vehicle of the future is a plug-in hybrid running on a combination of lowcarbon electricity and a low-carbon biomass-derived fuel [4]. In the past year several major automobile manufacturers have announced near-term plans to build the next generation of hybrid electric vehicles that allow grid charging. General Motors Corporation announced plans to offer two PHEV options, one being a version of its Saturn Vue SUV and the other a new model referred to as the Volt. Very recently, Toyota announced that it would be testing several PHEVs based on the Prius platform in Japan and the US. Ford Motor Company and the electric utility company Southern California Edison also recently announced plans to test PHEV versions of the Ford Escape. In addition, there are several pure electric vehicle developers that have plans to offer products in the next 12 months. These include Tesla Motors with its two-seater all electric sports car and Phoenix Motors Cars, which is producing and marketing an all electric four-door truck for fleet applications. 2. Plugging transportation into the sun Using solar energy to power a vehicle is not a new concept. Albeit extremely limited in scope, two approaches have been used in the past. University engineering teams have been building and designing solar-powered cars for decades to compete in anyone of several solar car challenges, such as the American Solar Challenge. In 2005, this particular event attracts over two dozen university teams to compete in a 2,500 mile race, from Austin, TX to Calgary, Alberta. Vehicles designed for these competitions are not intended for broad commercial applications, but rather serve to challenge engineering students to design and build extremely efficient vehicles that rely exclusively on energy from the sun. In addition to these solar car challenges, a number of solar and electric vehicle enthusiasts have engineered one-off systems to allow charging of their vehicles using stationary photovoltaic panels. The likely emergence of electric vehicles (EVs) and PHEVs for a broad consumer base creates a new commercial-scale opportunity for solar to serve the energy needs of the transport sector both in stationary and vehicle integrated applications.

3 Picture 1: Winners circle for the 2005 American Solar Challenge Source: Solar versus gasoline Electricity generated from photovoltaic (PV) panels can be less expensive as a transportation fuel as compared to gasoline even at current module prices. PV panels convert sunlight directly into electricity without fuel costs or any moving parts. The levelized cost of this energy (LCOE) stated in $/kwh is a function of system costs and annual insolation levels a measure of solar radiation. To compare the cost of solar PV energy as a fuel, a mid-sided PHEV sedan is assumed that obtains 3.49 miles per kwh [5]. Thus the per mile cost of a solar-powered mile is simply the LCOE divided by Table 1 provides the cost to travel one mile on solar power based on various PV LCOEs and the per mile cost of travel using gasoline given different fuel prices and assuming a vehicle efficiency of 30 mpg. Table 1: The per mile costs of travel solar vs. gasoline PV LCOE $0.15/kWh $0.25/kWh $0.35/kWh $0.45/kWh $0.55/kWh cost per solar mile $0.04/mile $0.07/mile $0.10/mile $0.13/mile $0.16/mile $/gallon gasoline $2.00 $2.50 $3.00 $3.50 $4.00 cost per gasoline mile $0.07 $0.08 $0.10 $0.12 $0.13 At current gasoline prices of around $3.00 per gallon, the fuel cost per mile is $0.10. This is equal to or greater then the solar-powered cost per mile when the LCOE for PV are $0.35/kWh and below. As a result, in some locations PV represents a lower fuel cost when compared to gasoline at $3.00 per gallon. In the next few years it is expected that the cost of solar panels will continue their long-term downward trend. The US Department of Energy s Solar American Initiative has as its goal to reduce the LCOE of PV to $0.10 or less by At this point, PV will offer a clear cost advantage over gasoline as a fuel for light vehicles in full range of geographic locations. 2.2 VIPV design considerations Debate on the merits of vehicle integrated PV (VIPV) have considered the trade-offs between using a stationary PV array to charge an EV or PHEV versus a vehicle integration approach. It has been argued that a given investment in solar is best when the system is optimized for total energy production. With a stationary PV array, the system is oriented for maximum solar gain given a fixed angle or tracking

4 system. In VIPV applications, the ability to optimize the system is compromised given the design of the vehicle s exterior and the variability in available parking locations with unobstructed exposure to the sun. Additional research is needed to understand the true penalty of a VIPV system relative to a stationary array. A VIPV approach does not necessarily preclude using a stationary PV array to charge an EV or PHEV but does offers several advantages. Current commercially available hybrid vehicles do not provide the opportunity to charge from the grid, and thus would not be able to charge from a stationary PV array. Current generation hybrids could implement a VIPV strategy today and begin to offset gasoline with solar-generated electricity. Vehicles are predominantly used in a commute pattern and thus would be away from the home where the PV array would likely be sited during periods of peak solar insolation. The VIPV allows opportunity charging during hours when the vehicle is parked away from an electrical outlet. In addition, in emergency situations where fuel supplies become disrupted or the electric grid fails, a VIPV system would provide some level of mobility, albeit rather limited. Initially, solar generated electricity would supplement gasoline or diesel fuel and grid power as an energy source for EV or PHEVs. Solar technology should be considered in the same light as regenerative breaking, which serves to enhance the vehicle s overall efficiency, not as a primary energy source for transportation. For example, the Canadian engineer Steve Lapp augmented his 2001 Toyota Prius with 270 watts of solar and claims that the system achieved a 10 percent improvement in the vehicle s efficiency. In the future, however, with advances in solar cell technology and ultra efficient vehicles, solar power could supply a significant portion of the energy used in a light vehicle. The key parameters dictating VIPV s ability to displace gasoline for transportation are the quantity of PV in watts integrated onto the body panels and the efficiency of the vehicle drivetrain. The amount of PV that can be integrated onto a vehicle is a function of the available space and the efficiency of the PV technology deployed. Figure 2 illustrates the potential size of a VIPV system given available surface area and PV conversion efficiencies. Figure 2: VIPV watts potential: surface area vs. PV sunlight to electricity conversion efficiency 900 watts VIPV sqr. ft. 35 sqr. ft. 45 sqr. ft. 5% 6% 7% 8% 9% 10% 11% 12% 13% 14% 15% 16% 17% 18% 19% 20% PV Conversion Efficiency The efficiency of the vehicle drivetrain determines the number of solar miles obtained from any given VIPV system. Current hybrids, like the Toyota Prius, have an all electric efficiency of approximately 3.5 miles per kwh. Figure 3 illustrates annual solar miles delivered from a 400 watt VIPV system assuming a 15 percent capacity factor, or annual production of 525 kwh. Referring back to Figure 2 above, a 400 watt VIPV system is plausible assuming a 35 square feet of available roof, trunk, and hood space on the vehicle and module efficiency of 12 percent, well within what is commercially available today. The allelectric efficiencies for the SUV, mid-sized sedan, and compact sedan were obtained from analyses

5 conducted by the Electric Power Research Institute and are 2.7 miles/kwh, 3.49 miles/kwh and 4.0 miles/kwh respectively. An all-electric efficiency of 6 miles/kwh was assumed for a future light-weight, low drag vehicle. While this represents a stretch goal, this is a reasonable target for a future vehicle design given development given new light-weight composite materials being developed for vehicles. Assuming 12,000 miles of travel per year, the VIPV system for this possible future vehicle would provide energy for 25 percent of the total miles driven annually. Even with performance characteristics with today s technology, a 400 watt VIPV on a compact sedan could power 16 percent of annual miles driven. Figure 3: Solar miles from 400 watt VIPV system by vehicle type 3,500 3,000 2,500 Solar Miles 2,000 1,500 1, SUV Mid-Size Sedan Compact Sedan Future Ultra Light Weight, Low Drag Sedan 2.3 PV technology for VIPV applications Global production of PV cells in 2006 is estimated to be 2.5 GW, with flat plate crystalline silicon technology representing over 90 percent of global production [6]. The most efficient solar cells use monocrystalline technology, with commercially available modules offering sunlight to electricity conversion efficiencies in the mid-teens. Initially, this would be the best technology to deploy in VIPV applications given the severe area constraints on a vehicle. Thin film photovoltaic technology currently has a low market share, but represents the fastest growing segment of the PV sector, with global production increasing by 58 percent from 2005 to While thin films are just 6 percent of total global production, some estimates suggest a market share of 20 percent as early as 2010 [7]. The efficiencies of thin films are lower than those of flat plate silicon cells, but efficiencies are improving. Thin film technology may emerge, however, as a good match for VIPV applications. Thin films modules are produced by depositing a thin layer of photovoltaic materials on a substrate, such as flexible steel. This may offer significant manufacturing advantages for VIPV systems in that the vehicle s body panels themselves could serve as the substrate. The thin film materials could be deposited directly onto the body panels during manufacturing and thus offer significant economic and production efficiency advantages. Copper indium gallium diselenide (CIGS) solar cells, which are not yet fully commercial, offer both advantages of flexibility like other thin film PV technologies, but with much higher conversion efficiencies. A number of companies are planning large scale production of CIGS including a division of Honda, which explicitly stated in a company press release its intent to produce PV products for vehicles. 3. VIPV commercial developments VIPV is slowly moving beyond the concept phase to commercial development. Although solar is not an option offered by the major hybrid vehicles manufacturers today, there are VIPV retrofit kits available today. In addition, several boutique vehicle manufacturers have begun to market solar hybrids and EVs in

6 small numbers. One indication that the major auto manufacturers are eyeing the VIPV opportunity was Ford Motor Company s debut of hybrid electric vehicles with VIPV called the Reflex at the 2006 North American International Auto Show. The vehicle featured solar panels integrated into the headlights and taillights to charge the hybrid electric battery. Ford s close corporate partner Mazda also unveiled a concept car with VIPV called Senku, which is Japanese for pioneer, at the 2005 Tokyo Motor Show. Picture 2: Ford Reflex Source: Solar Electric Vehicles, based in Westlake Village, CA offers a VIPV installation kit for Toyota Prius vehicles model years 2004 to The VIPV retrofit kit includes a 215 watt monocrystalline PV array, additional battery storage capacity, and the necessary power electronics. The company claims that the kit will increase the fuel economy of a Prius by between 17 and 29 percent. With the federal solar tax credit Solar Electric Vehicles predicts a payback of between 2 and 3 years. Picture 3: Solar Prius Source: The French company Venturi Motors has fully embraced the VIPV opportunity and has begun marketing the first commercially available solar electric hybrid car in the world. The company claims that the Astrolab can go 74 mph and has a range of 68 miles; the vehicle uses high efficient solar cells to fully integrate 600 watts of PV on the body s 38 square feet of surface area. The company also offers a neighborhood electric vehicle with VIPV. In addition ZAP, an electric vehicle manufacturer, is also now offering a VIPV options on several of its models.

7 Picture 4: Venturi Motors Astrolab Sources: Picture 5: Zap Xebra Xero Truck Sources: In addition to interest in VIPV on the vehicle manufacturer side, there is also an indication of interest on the solar cell manufacturing side. E-ton Solar Tech Co., Ltd., the solar-panel production arm of the Taiwanese company the E-Ton Group, recently announced that it would collaborate with several other companies to establish a joint venture called Gloria Solar Co. Ltd. to develop and manufacture automotive solar-powered sunroofs. The solar-powered sunroofs will be produced at E-Ton's existing plant with plans to begin delivering products in the first quarter of Conclusions Recent trends suggest that electric drive will play an increasingly important role in future vehicle design. This creates an exciting opportunity for direct solar to play a role in powering vehicles. Initially, PV could serve to enhance the overall efficiency of a vehicle in the range of percent. In the future,

8 however, light-weight vehicles and high efficiency PV cells could allow PV to provide a significant amount of drive energy. One scenario discussed above suggests that a 400 watt VIPV system with an ultra efficient vehicle could provide as much as 25 percent of total vehicle miles traveled annually, assuming 12,000 miles driven per year. VIPV is moving beyond the concept phase with a number of commercial products emerging. Conversion kits are available for commercially available hybrids and boutique EV and PHEV manufacturers are offering solar as an option. Even the one of the large automobile manufacturers has experimented with VIPV in concept car designs. On the solar industry side, one company is pursuing a business plan to specifically produce VIPV products for after market conversions and OEMs. The VIPV market could represent a large market for solar. It is projected that by the end of 2007 approximately 300,000 hybrid electric vehicles will be sold in the US. If each of these vehicles had a 400 watt VIPV system, this would represent a PV market of 120 MW. In comparison, the grid-connected market in 2006 in the US was estimated to be just over 100 MW. Furthermore, according to a forecast by J.D. Power and Associates the market for hybrid electric vehicles in 2011 will reach 854,000 units. This would represent a PV market potential of 341 MW, assuming again a 400 watt VIPV system on each vehicle. This represents 14 percent of total global cell production in References [1] Myers, N. & J. Kent. The new consumers: The influence of affluence on the environment. Island Press, Washington, DC, [2] Worldwatch Institute. Vital signs : The trends that are shaping our future. W.W. Norton & Company, Inc., New York, NY, [3] Romm, J. & Frank, A. Hybrid vehicles. Scientific America. April [4] The Center for Energy and Climate Solutions. The car and fuel of the future: A technology and policy overview. Prepared for the National Commission on Energy Policy, Washington, DC, [5]. Electric Power Research Institute. Comparing the benefits and impacts of hybrid electric vehicle options. EPRI, , [6] Prometheus Institute. PVNews. Cambridge, MA, April, [7] Prometheus Institute. PVNews. Cambridge, MA, August, 2006.