From being a concept known mainly only by a close few even as recently as several years ago, plug-in hybrid electric vehicles (PHEV) are now being seen by an increasing number of transportation technologists and policy-makers as a near-term solution for reducing petroleum consumption and emissions of greenhouse gases.
The just-released Climate Change Technology Program Strategic Plan from the Department of Energy, for example, highlights plug-ins as one of the promising near-term solutions in the transportation sector. (Earlier post.)
The third day of this week’s California Air Resources Board ZEV Symposium was dedicated to PHEVs and the batteries that support them and all-electric vehicles (EVs).
This year marked the first year when I came to this conference and assumed that everyone already had fairly detailed technical knowledge of plug-in hybrid vehicles. It’s also the first year where I won’t spend the first third of a talk explaining what plug-ins are, and the next third why.—Dr. Mark Duvall, technology development manager for electric transportation & specialty vehicles, EPRI
There is a pressing need to do something new in the transportation industry specifically related to addressing our petroleum consumption...I feel that the PHEV is one of those potential options to help us address those issues...ideally one of the near-term solutions.—Tony Markel, senior engineer, NREL
The dramatic improvements in battery technology over the past few years are a major enabler and catalyst for this intensifying focus on PHEVs. Improvements in basic battery chemistry and manufacturing are not sufficient to unleash a flood of PHEVs on the market, however.
There are many ways to design a plug-in—and these have a direct effect on the size and type of battery applied in the vehicles. One of the challenges in the industry now is to determine the expectations of the different stakeholders, and then balance those objectives in such as a way as to give battery manufacturers a target to which they can work.
Charge-depleting vs. charge-sustaining modes. A plug-in battery operates in two modes: charge-depleting and charge-sustaining. Charge sustaining is the mode of operation for batteries in conventional hybrids. The state of charge may fluctuate, but on average it is maintained at a certain level while driving as it supports the engine. Discharge patterns consist of many, relatively shallow discharges.
In charge-depleting mode, the battery draws down its charge to power the vehicle. If the PHEV is operating in all-electric mode, the depletion will be fairly constant, except for bursts of regenerative charging from braking. If the PHEV is operating in blended mode, the state of charge may fluctuate but will, in general, decline until it reaches the point at which it switches over to charge-sustaining mode. Here the discharge patterns combine deep discharges with many more shallow discharges.
All-electric operation vs. blended mode operation. One of the fundamental questions designers must address is the operating strategy for the PHEV and its battery. The two basic options are to run the vehicle as an all-electric vehicle from startup, and then kick over to the engine when the battery reaches its minimum state of charge (SOC) threshold. (At which point the battery switches to charge-sustaining mode.) The other option is to run in blended mode, a charge-depleting strategy in which the engine supplements the battery up to the minimum SOC.
|Cycle life vs. state of charge swing. Click to enlarge. Source: NREL|
The State of Charge (SOC) Window. Regardless of whether the PHEV is designed for all-electric or blended mode, an important parameter for the battery is the SOC Window. The SOC window defines the swing: the upper and lower parameters for the state of charge of the battery, which, in turn can help define the required total kWh capacity of the battery pack for a given application.
Battery cycle life is a strong function of SOC swing. Battery SOC swing is a strong function of driving habits.—Tony Markel
In the absence of large test fleets of PHEVs on the road, simulation has become a tool for helping engineers figure out the likely technical dynamics of the PHEV market. NREL has used real world driving data from St. Louis in its modelling, as described by Markel at the ZEV Symposium.
Among the early findings and insights:
Consumers are likely to experience higher fuel economy than the rated value in daily driving, although a lower all-electric range than rated (due to driving patterns). In other words, as Markel said, “People would be very pleased with these vehicles.”
A point related to the above: vehicles designed with all-electric range likely to operate in a blended mode to meet driver demands.
Fuel savings relative to conventional vehicles are almost entirely distance-dependent. Savings relative to conventional HEVs are distance dependent up to PHEV distance then constant.
PHEVs that saved the most fuel relative to an HEV travel about 25 miles with speeds under 60 mph and light accelerations. PHEVs that saved the least fuel relative to the HEV in the 20-30 mile range had periods of 60+ mph highway driving and the accelerations were significantly more aggressive. (See chart below.)
Reduction of petroleum consumption is not tied to an all-electric range.
PHEV benefit is strongly related to distance and aggressiveness of real-world usage.
|Real world PHEV simulations of acceleration versus velocity. Click to enlarge. Source: NREL|
The decision of whether to support an all-electric or a blended strategy has a direct impact on the battery and motor combination—an all-electric strategy requires a more powerful battery and motor.
The blended strategy, by allowing the engine to switch on to provide additional power, can use a less powerful battery and motor combination. NREL’s simulation work so far finds that both strategies save about the same amount of petroleum.
The blended operation scenario potentially is less expensive, and may be a little more efficient too. We’ll have to see.—Tony Markel
Still, it all comes down to the capabilities of the battery. A special DOE meting on PHEVs in May of this year concluded, not surprisingly, that the success of the technology will be directly related to how good the batteries are.
It is our personal experience that the batteries are very good. That advanced batteries continue to provide remarkable performance improvements across the board. We are cautiously optimistic about the future of batteries.
What we have here is a technology with tremendous potential to improve the sustainability of the transportation sector.
What we know is that there are very promising current durability test data. But simple data. Deep cycling data. What we don’s have as much knowledge on is how those cells will perform specifically under a PHEV duty cycle. We have a large body of knowledge of battery capabilities...but that knowledge is very focused on battery capabilities for pure electric vehicles. There is a need for specific test data on PHEV requirements—and we need to evaluate the latest and most advanced chemistries.—Mark Duvall
In terms of chemistries, lithium-ion increasingly appears to be the current chemistry of choice for PHEVs. Beneath that umbrella, however, there are a large number of options in terms of materials, electrolytes and specific chemistries, all of which must be tailored to meet the particular application requirements.
|Baseline characteristics of batteries|
|Source: Dr. Andrew Burke, UC Davis ITS|
|Sodium Metal Chloride
Very broadly, one main distinction is between energy batteries and power batteries. An energy battery, simply, is designed to optimize the energy density; a power battery is designed to optimize the power density.
Tien Duong, vehicles technologies team lead at the Department of Energy’s FreedomCAR and Vehicle Technologies Program Office, noted during the ZEV Symposium that conventional lithium-ion batteries for power-assist HEVs “are about ready for commercialization.”
Duong said that the DOE’s main focus now is to support the development of novel materials for cathodes, anodes and electrolytes that promises greatly increased power and energy.
For PHEV batteries, I would not rule out NiMH, but I would say that lithium-ion batteries are technically feasible. There are no batteries specifically built for this application that I know of. I would think that there are strong synergies with the development of PHEV, HEV and EV [batteries]. One thing we don’t know is the impact of real world operations during charge-depleting mode.—Tien Duong
The new materials—as represented by A123Systems, but also by companies such as Altair Nanotechnologies, who was also present at the ZEV Symposium—offer designers more power or more energy depending upon the requirements.
A123Systems has a high volume battery product currently designed for power tools. (Hybrids are currently quite a small market for lithium-ion battery manufacturers, who have a much larger opportunity with laptops, cell phones and power tools. Power tools will outnumber hybrids by two or three times as measured by MWh, according to some analysis.)
A123Systems’s current product—the power tool cell—is somewhere in the middle of the range between power and energy. In other words, the A123Systems production cells put together by a PHEV conversion company such as Hybrids Plus in its Boulder PHEV demonstration aren’t really designed for PHEV operation. (Earlier post.)
Power is a function of the diffusion of the ions in the battery, notes Ric Fulop, one of the co-founders of A123Systems. The nature of the electrode materials and the electrolytes can all make a difference in terms of tilting the battery toward the power or the energy ends of the spectrum (or the area within the Ragone plot).
While that customization is possible from the battery manufacturers’ points of view, what they need are targets to hit: the specifications from the automakers.
And one of the best ways to achieve that—aside from the simulation work underway—is to get more trials out on the road that then feed back into shared project knowledge. Similar, for example, to what automakers and energy companies are doing for hydrogen in Europe. (Earlier post.)
So while the conversions of existing hybrids to plug-in operation might not represent the optimal in terms of battery technology or systems design, they do represent an extremely valuable potential source of real world data that needs to be fed back to battery makers and to automakers.