While the renewable energy community is generally supportive of PHEVs, they have yet to fully appreciate the new opportunity that this would create for their industry. As the vehicle fleet moves toward at least partial electric drive and grid charging, this creates the opportunity for renewables -- beyond biofuels -- to serve as a source of energy for the transport sector.

Although solar photovoltaics (PV) is not viewed as cost-effective without significant subsidy when compared to grid power, it begins to look interesting when compared to the price of gasoline for transportation. Furthermore, the opportunity that solar hybrids offer is exciting, whereby a new generation of solar PV technology is developed specifically for vehicle integrated (VIPV) applications (see Solar Today, May/June 2006, Letendre, Perez, and Herig).

Growth in VIPV applications could one day mimic the current explosion in new product development for building integrated PV applications. While PV may serve just a small part of a vehicle's overall energy needs, the modest additional costs from a 500 watt VIPV system may be well worth the investment. VIPV should be considered as an avenue to enhance a PHEV's overall efficiency, similar to regenerative breaking.

It is generally understood that PHEVs would charge during the evening and early morning hours when electricity demand is low and there is significant excess capacity available on the network. In some areas, wind resources are strongest at night, thus allowing wind power to serve as an energy source for vehicles. With smart charging, the rate of charging could be adjusted to match the power production from a distant wind farm. I can envision interesting marketing schemes whereby PHEVs are sold in conjunction with a wind power supply contract further enhancing the vehicle's "green" attributes.

The potential of grid connected cars is even more exciting when the notion of vehicle to grid (V2G) is considered. The emerging V2G concept envisions grid connected cars with bi-directional chargers that both accept power from the grid and deliver power to serve different ancillary services markets. Initial economic analyses suggest that V2G capable vehicles could generate significant revenue for vehicle owners by providing specialty grid services such as regulation (frequency response) and spinning reserves. Ratepayers pay over $1 billion for these services each year.

The battery wear and tear from providing these services would be minimal given that regulation services would include both charging and discharging energy from the vehicle's battery pack in response to signals from a grid operator striving to maintain grid frequency at 60 Hz. Spinning reserves is another ancillary service needed to maintain grid reliability; these reserves are infrequently called upon, and when they are dispatched, are only used for a short duration and thus would have minimal impact on the vehicle battery pack.

While a small number of PHEVs have been built and are being tested, there are still hurdles to be overcome, the most important of which is the batteries. The U.S. Department of Energy recently convened experts from across the country to discuss the potential of PHEVs. While batteries were identified as a key technical challenge, the general consensus was that this barrier can be overcome. Additional research and development funding should be directed toward developing advanced batteries for PHEV applications. There is general agreement among electro chemical engineers about the potential that lithium-ion (Li-ion) batteries offer in serving the hybrid vehicle market. In fact, several battery companies are currently developing Li-ion battery packs for vehicle applications.

A more distant potential that grid-connected cars may offer is their role in serving as storage to allow greater penetration of intermittent resources on to the nation's electric grid. I envision a future with millions of vehicles connected to the electric grid providing storage to allow large amounts of wind and solar to become seamlessly integrated in to the electric grid ushering in a truly sustainable energy future. In addition, PHEVs would require less liquid fuels making it more likely that biofuels could meet the challenge of displacing 100% of petroleum as a transport fuel.

While the renewable energy community must address a number of issues, I would urge renewable energy industry associations and advocacy organizations to join the growing chorus directed at the major automobile manufacturers to commit to bringing PHEVs to the market place. Grid-connected cars offer many fascinating opportunities to allow renewables to play a greater role in fueling society's transportation needs.

About the author...

Steven Letendre serves as the Director of Research at the Prometheus Institute for Sustainable Development, located in Cambridge, MA. Dr. Letendre has over ten years of research experience in the field of renewable energy. He has published widely on the topic of solar energy and advanced vehicle concepts. Dr. Letendre has served as a consultant on a range of projects for a variety of organizations including the National Renewable Energy Laboratory, California Air Resources Board, Union of Concerned Scientists, Northeast Sustainable Energy Association, and SUNY Albany's Atmospheric Sciences Research Center. Prior to joining the Prometheus Institute, Dr. Letendre was an Associate Professor of Business and Environmental Studies at Green Mountain College in Poultney, VT. In 1997, he received a Ph.D. from the University of Delaware in Urban Affairs and Public Policy with a concentration in energy policy and economics. In addition, he holds a masters degree in economics from Binghamton University, formerly State University of New York at Binghamton. Prior to pursuing his doctoral studies, Letendre was employed as an energy economist with the Research Triangle Institute in North Carolina.