Ushering in An Era of Solar-Powered Mobility
In 2003, the author presented the vehicle integrated photovoltaic (VIPV) concept to an American audience at the annual meeting of the American Solar Energy Society. The paper titled, Vehicle integrated PV: A clean and secure fuel for hybrid electric vehicles argued that HEVs create an opportunity for PV to serve as an energy source for the transport sector.
Until recently, PV has not been considered a viable energy source for vehicles. Some experiments were conducted using PV for electric vehicle (EV) charging, but efforts to commercialize have stalled due to the perceived lack of market acceptance for these types of vehicles. Other efforts to deploy PV for transportation have taken place at a variety of university research centers, where teams of students and faculty build vehicles powered solely from solar. These vehicles are designed and built to compete in solar car races such as the World Solar Challenge, which began in Australia in 1987. These vehicles were never intended for commercial production, the futuristic look and design of these experimental vehicles would not likely appeal to mass markets.
Since the 2003 conference, the author learned of a variety of projects to advance the VIPV concept. Researchers at the University of Queensland in Australia are developing a commuter hybrid vehicle with PV integrated in to the body panels. An engineer in Canada installed a 270 watt solar array on the roof of his Toyota Prius, increasing the mileage by approximately 10%. Even the major auto manufacturers are eyeing the VIPV opportunity, with both Ford, and its close corporate partner Mazda, displayed hybrid vehicles with modest amounts of VIPV at recent auto shows. The author produced a second article on the topic highlighting recent VIPV activities, which appeared in the May/June 2006 edition of Solar Today.
In October of this year, the French specialty vehicle manufacturer Venturi Automobiles announced plans to offer the first commercially available solar hybrid sports car called the Astrolab. The company also produces an urban electric commuter vehicle called the Eclectic. The 3-seater vehicle has solar PV integrated on to the roof of the vehicle. Venturi claims that this is the first energy-autonomous vehicle available to the public.
Recently, Taiwan's PV cell manufacturer E-Ton Solar announced a joint venture with several partners, including Yulon Nissan Motor Co., Ltd. to develop PV products for the car market. The joint venture began with the manufacturing of PV modules for car sunroofs.
Design Considerations for Solar Hybrids
Given current HEV designs, VIPV could serve to enhance the overall efficiency of the vehicle, but only provide a small portion of the vehicle's energy requirements. In this context, VIPV is similar to regenerative breaking, which, through converting the kinetic energy lost in breaking to electrical energy, serves to enhance the overall efficiency of an HEV. A number of design and engineering considerations could serve to increase PV's role in fuelling a new generation of solar hybrid vehicles
The key parameters dictating VIPV's ability to displace gasoline for transportation are the quantity of PV in watts integrated on to the body panels and the efficiency of the vehicle drivetrain. The amount of PV that can be integrated on to a vehicle is a function of the available space and the efficiency of the PV technology deployed. Venturi Automobile's Astrolab mentioned above contains 3.6 m2 of PV integrated on to the vehicle. Measurements of the available surface area of a number of conventional vehicles suggest available surface areas of between 3.5 m2 to 5.5 m2 (Letendre et al., 2006). Figure 2 in the paper (download below) indicates potential PV in watts for three different scenarios of available surface by PV conversion efficiencies.
As Figure 2 illustrates, the sunlight to conversion efficiency of the PV technology deployed in VIPV applications is an important parameter. While flat plate silicon PV has high conversion efficiencies, thin film PV may be better suited for VIPV applications. Again referring back to Venturi Automobile's Astrolab, the vehicle uses high efficiency monocrystalline PV cells to achieve 600 watts of PV on the available 3.6 m2 of surface area. 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. One US company, DayStar Technologies, is nearing commercial-scale production of a CIGS PV product on flexible steel. Generally, the US is leading in the development of the next generation PV technology, which should be predominantly flexible thin films.
It should be noted that the onboard PV capacity may not necessarily be constrained by the available surface area on the vehicle's body panels, but flexible PV could be used to design retractable solar shades that could be deployed when the vehicle is parked to provide additional PV capacity for daytime charging.
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 all electric efficiencies in the 156 watt-hours per kilometer range. Figure 3 illustrates solar miles for a 500 watt VIPV system in a region with an average of 4 sun hours per day for total annual PV generation of 710 kWh.
Advances in the use of lightweight materials for vehicles will serve to increase the potential solar miles delivered from a VIPV system. However, even today's commercially available hybrid can benefit from VIPV. Initial VIPV applications will provide incremental improvements in vehicle efficiency, but the future potential is much greater. The Leonardo Project, sponsored by the European Commission, aims to train a new generation of engineers in sustainable transportation focused initially on designing and building a solar hybrid. This project, and other like it, will serve to advance knowledge on these concepts and ultimately achieve advanced designs that dramatically improve existing technologies and approaches.
Battery storage devices are a critical enabling technology for the solar hybrid revolution. While many advances have been made in battery technology, reductions in price and improvements in performance are needed to produce commercially viable solar hybrid vehicles.
A promising new battery technology was unveiled at the September 2006 California Air Resources Board Zero Emission Vehicles Symposium. Nevada-based Altairnano announced a new lithium ion battery system called NanoSafe™, which replaces graphite as the electrode materials with nano-titanate materials (www.altairnano.com). The company claims that this new materials solve the thermal runaway problem with conventional lithium ion batteries, and offer significant improvements in cycle life and delivers optimum energy/power balance in the high power region, which is critical for hybrid and electric vehicle applications.
Download Complete Ushering in An Era of Solar-Powered Mobility paper here.
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