Energy Storage for Automotive Propulsion - Part I

Abstract

In this world of $2.30+/gallon gasoline (1.20/liter in Europe, ¥130/liter in Japan), electric-powered vehicles are not unusual, they are in demand. Countries have differing needs and cultures that dictate common modes of transportation. This paper will explore the energy storage technology and demography of vehicles that derive their propulsion from batteries (including supercapacitors). The future of the EV/HEV battery industry from both the business and technological perspectives will be examined, including emerging markets and possible chemistries.

Introduction

Electric vehicles are not new. In fact, they pre-date Henry Ford’s first internal combustion car by nearly 60 years, to 1834. But cheap Texas crude was discovered in the early 1900s, and internal combustion (IC) vehicles became grandfathered into society. Today, politics and public consumption have caused fuel prices to escalate, and global consumers are demanding alternative energy sources for their beloved automobiles. The automotive industry has responded by initiating development into several areas: more efficient IC engines, electric vehicles, hybrid electric vehicles, fuel cells, and niche technologies such as solar power and flywheels.

This essay will focus on stored energy (batteries, supercaps) rather than generated power (IC engines, fuel cells) and how such technology is inserting itself into public and, to a lesser extent, military transportation. The expectation for HEVs is concentrated in three areas: by 2013, sales are estimated at 2 million in the U.S., 660,000 in Japan, and 300,000 in Europe, while EV sales will be mostly in Europe (up to 100,000). There are several sub-areas to investigate: EV versus HEV, pros and cons of different storage systems, demographic demands, next-generation chemistry and engineering, and, of course, cost. Be assured that one size will not fit all.

Hybrids Versus Pure EVs

A decade ago, USABC was all about electric vehicles, California was mandating zero emission vehicles (ZEV), and Li-ion batteries were still the new kids on the block. But gasoline prices more than doubled in those 10 years, and reality struck the EV supporters: rechargeable batteries could not provide the range that most drivers demand. USABC morphed into the Freedom Car program, while hybrid vehicles became the new hot item of the automotive industry, environmentalists, and high-tech consumers.

North Americans (and Australians) love their wide-open spaces and the cars they drive to get them there. Mr. and Ms. Consumer want vehicles with 300-400 mile (500-650 kilometer) driving ranges. Yes, there are many Mr. and Ms. Commuters who need only 100 miles (160km) of available electrons, but is there an electrical outlet available in your work or mall parking area? The infrastructure for EV recharging is woefully weak away from home: even in Pacific Rim countries and Europe, areas attuned to EVs, it’s generally localized to a few trial cities.

Stated another way, batteries for EVs require high energy, long discharge life, and readily available electrical outlets. Typical EV usage is two to four hours daily at moderate speeds with night-time battery recharges. This pattern suggests slow traffic in high population density areas (Europe, Japan, Korea, U.S. East and West Coasts), but is also reflective of service vehicles (local delivery, forklifts, golf carts). Further, EVs must be light: it takes a lot of energy to operate a two-ton van, which limits battery working time.

These constraints, however, permit a generic EV description in its optimum environment. For consumers, a two-passenger auto for 125 mile (200 km) round trips, with few amenities (remember, power accessories, air conditioning, entertainment, etc., all use energy). For commercial and military entities, small vehicles carrying a moderate load (say up to 1 ton) that return periodically to a central maintenance location. Niche markets, to be sure, but substantial niche markets. In fact, service EVs will probably always outnumber EVs for personal use.

Hybrid vehicles are the middle ground between draconian ZEV regulations and the real world of superhighway traffic, an uncomfortable combination of high speeds and stop-and-go driving. Batteries get the HEV moving and provide a power boost when the auto accelerates, but the heavy propulsion work is accomplished by an efficient IC engine. Further, the battery recharges while the engine is running the car, so there’s no need for electrical outlets at every parking spot. This HEV scenario is far more practical than pure EV applications, and requires a smaller, less expensive battery. As a bonus, fuel usage and emissions are lowered dramatically relative to IC vehicles.

What are the attributes of an HEV? A year ago, I would have suggested that most HEVs will be small-to-medium-sized sedans, but there are now several successful hybrid SUVs on the market. Thus, passenger vehicle size does not appear to be a constraint. Clearly, power capability is a must: these vehicles are expected to behave much like IC cars, with substantial acceleration available on demand. And HEVs have a driving range up to 400 miles (650 km), which is competitive with IC autos. Finally, HEVs have low operating costs: only the OPEC nations lack incentive to adopt this technology.

And the eventual winner will be (no surprise!) hybrids over EVs, and in high population density, industrialized areas, IC cars. While zero emissions is an admirable goal, the inconveniences of EVs overshadow their environmental attributes. Hybrids meet the desires of more drivers than EVs ever will, and as the HEV market expands, it will become increasingly competitive with IC-powered vehicles. Heightened production portends lower prices, another powerful selling point.

Table 1, below, is derived from a 2004 report by Mathey and Van Autenboer, and compares EV and HEV battery production data and estimates. Note the projected 45% reduction in energy and power costs, but more striking is the predicted scarcity of EVs versus HEVs in 2012. Also, these figures omit today’s infant markets in China and India, representing nearly 2.5 billion people that are striving for the same technological levels as fully industrialized countries.

Another niche market is two- and three-wheeled EVs, variously called electric bikes, mopeds, e-scooters, and electric motorcycles. These devices are most popular in Asia and Europe, although bikes with two-stroke gasoline engines are the bigger sellers, due to more affordable pricing. This EV segment is dominated by lead-acid and NiMH batteries, typically 42-48V units producing up to 360Wh of power. ITRI/Taiwan energy and power specifications for Li-ion bike batteries are 65Wh/kg and 1kW/kg, respectively. Only a few Li-ion battery-powered motorcycles have entered the market, although these are high-end EVs priced two to five times greater than their smaller brethren. Taiwan manufactures about 400,000 e-bikes per year, Japan about 200,000, a number roughly matched by combined sales in Italy and the Netherlands. World production is less than 10% of the 8.5 million IC motorbikes sold annually. Equal-performing motorbikes are always cheaper if gasoline-powered, thus insuring that e-bikes will remain in the minority for at least the next several years.

Today’s Technologies

This section is a comparison of rechargeable energy storage devices with various chemistries. Although flywheels, fuel cells, zinc-air, molten salt, and a handful of other battery types generally fit the title category, only those technologies with significant current or predicted market impact are considered here. Table 2 exhibits pros and cons of commercial EV/HEV energy storage hardware from the perspectives of the OEM and the consumer.

Table 2 permits an evaluation of the future of these various energy storage units. For example, consumers are notoriously lax on maintenance, which bodes ill for PbH+ and NiMH battery growth. In fact, a 2005 industry report by Saxman & Colson-Inam predicts that Li-ion batteries will outsell automotive NiMH cells within three years, with a total market of ~$300 million, even though NiMH is today’s dominant technology. NiMH cells are widely viewed as near the pinnacle of their performance, with little room for advancement, and they are intolerant of overcharge and high temperature. Ovonics, however, holder of many patents in the field, claims on-going development of units with up to 20% improvements in power and energy.

And what about lead-acid batteries? The advances with thin-film PbH+ cells are impressive, but low energy and cycle life performances are not persuading car manufacturers. Further, auto companies look for minimal weight in auto components, not the strong point of either PbH+ or NiMH batteries. Firefly Energy’s recent invention of a light-weight, high-power foamed graphite/PbH+ cell, however, may change the way the auto industry perceives Pb technology. Details are presented in the next section.

Although not included in Table 2, Zebra batteries are viable in heavy vehicles with relatively short ranges, such as buses or local delivery vans. Producing up to 120Wh/kg, these units feature Na/NiCl2 chemistry and operate at 270-350°C. Heavy insulation and safety concerns preclude their application in passenger cars. Lyons, France, initiated an experimental program with Zebra-powered buses in November 2004.

Supercapacitors (also called ultracapacitors) are loosely defined as short-term power storage hardware, where static electricity is stored on electrode plates. These plates are most commonly high surface area carbon separated by an organic electrolyte (aqueous electrolytes limit supercaps to 1V), and can be charged and discharged very rapidly. Self-discharge is very high, capacity is relatively low (Power Systems claims up to 6.3Wh/kg), voltage decreases linearly with discharge, and operating temperatures as low as 40°C shorten the lifespan. As pointed out in a 2004 Frost & Sullivan report, the specialty electrode carbon is unique to the capacitor industry, with few suppliers and high prices. Carbon accounts for as much as two-thirds of the supercap cost, and the device is three to five times as expensive as a battery system. These traits dictate that supercaps will not be the primary energy source in EV/HEVs but are well suited for power assist (up to 5kW/kg) and regenerative braking. Supercaps are seen as battery adjuncts for HEVs or high efficiency IC autos.

Li-ion and Li-polymer batteries may contain any of several different electrode materials, hence the relatively wide range of electrochemical properties. Also, this field is undergoing rapid advances in stability, rate capability, and longevity. Table 3 describes the various chemistries and performance of lithium rechargeables that are at least in advanced development. By focusing on high power versus high energy units, a clearer picture of preferred usage emerges.

This technical article will be continued in the May and June 2006 editions of ABT.

[W.F. (Rick) Howard has more than 15 years experience in battery materials R&D, and consults for the battery, chemistry and nanotechnology industries. He may be contacted at rikhoward@aol.com.]