Energy Storage in Vehicles

Almost all vehicles store the energy they need on board. The notable exceptions are electric trains, streetcars, and trolley buses which transfer power from electrical conductors along their routes. The mass of the powerplant itself is also significant to mobile applications, and the safety of the technology is even more important for transportation equipment than for stationary applications. Aircraft, which have to lift everything off the ground, are the most demanding type of vehicle.

Motor vehicle design involves many tradeoffs, but power and range are among the most important to the consumer. These parameters are limited by the energy supply stored. Common hydrocarbon transportation fuels all have comparable energy storage performance: gasoline 46.4 MJ/kg or 34.2 MJ/litre, diesel 46.2 MJ/kg or 37.3 MJ/litre, jet A fuel (kerosene) 42.8 MJ/kg or 33 MJ/litre.[1][2] Typical passenger cars have an engine with a maximum power output of order 100 kW (746 W = 1 horsepower) although the average power output during normal use is much lower than that. They also have a fuel tank holding around 50-75 litres of gasoline, and achieve a typical cruising range of perhaps 500 km. The typical motorist’s commute to work is on the order of 10′s of km, with North American suburbanites going much longer distances than their urban cousins anywhere in the world. In that sense, a 500 km range is unnecessary most of the time, but motorists sure appreciate a large cruising range, particularly on long road trips.

Now suppose that manufacturers want to build a vehicle that doesn’t rely on hydrocarbon energy storage. What options to they have? The most talked about alternatives are electricity and hydrogen.

In the case of electric vehicles (EVs), the greatest attraction is that electric motors are ideally suited for propelling vehicles. Vehicles have to start and stop often. Internal combustion engines have poor torque at low speed, which is why most vehicles need transmissions. Electric motors can be built to deliver high torque even at zero speed and can be made reversible, allowing some EV designs to be “direct drive” with no transmission at all, greatly simplifying the mechanical components of the drivetrain. Also, electric motors can be designed to run as generators and recapture energy when braking. This is called “regenerative braking” which both adds a bit of charge to the energy storage system and helps reduce wear on the mechanical brakes. Electric motors are also very efficient. Some can achieve over 90% efficiency, twice what is possible with internal combustion engines.

Unfortunately, the stored energy density of normal battery systems is about two orders of magnitude worse than hydrocarbon fuels. There is, however, a large variety in battery performance depending on chemistry. Lithium-ion are among the best, managing up to 0.72 MJ/kg, only 64 times worse than gasoline, but much better than lead-acid batteries which at 0.14 MJ/kg are 330 times worse than gasoline. Even that is deceptive, as batteries cannot be fully discharged without damaging them, so only at best half the capacity is actually available in practical applications. Lithium is thought to be in adequate supply for the auto industry’s potential needs, with an estimated global reserve of 29 million tonnes.[3] Still, that 64 times worse energy density is going to hurt. Auto makers and buyers must accept much shorter cruising ranges, and often some compromise in performance. Personally, I wonder whether they have thought through the problem of heating cars in cold climates. That consumes a lot of power, power that is readily available as surplus heat in combustion engined cars. But that power is at a premium in an EV, and the situation is aggravated by the fact that many battery types perform poorly in the cold.

What is the likelihood of scientific breakthroughs resulting in dramatically better batteries? Consider that all elements and their physical properties (including density) are known. The elements’ electrochemical properties such as electron affinities and electronegativities (tendency to attract electrons) are known. The effect of the elements’ incorporation into larger molecules is also well understood. As a result, it is possible to calculate an upper limit on the specific energy and energy density of batteries that carry their own reactants- a definite upper bound happens when all the materials in the battery are selected to maximize specific energy or energy density, and there is 0% supporting materials content. A recent battery proposal, as yet unproven, may be pushing the limit of what’s possible, claiming to achieve 97% functional materials with a lithium chemistry and a solid electrolyte.[4] Another approach is to make oxygen one of the reactants in the cells, so that the battery doesn’t have to carry all its chemicals, and have it “breathe” instead. This approach, using a lithium anode – oxygen cathode chemistry, is thought to be capable of 600 mAh/g of specific energy not counting the oxygen[5], which by my calculations translates into a theoretical 6.28 MJ/kg, or only 7-8 times worse than gasoline. However, Li-O batteries do not yet lend themselves to recharging.

It would be unreasonable to expect that electrochemical batteries with dramatically better specific energies or energy densities than these will ever be invented. However, small improvements in energy storage density may still be possible. Efforts to improve other parameters – battery reliability, greater number of charge cycles, deep cycling, rapid charging/discharging, cost, efficient lithium recovery from spent batteries and Li reprocessing into new batteries – these will probably be more productive research directions.

Since most motorists cannot accept a cruising range of 100 km as can be expected from a pure EV, the short-term solution is hybrids. They carry the hydrocarbon fuel and a small internal combustion engine to extend the driving range. The details of how this is done may be revisited in a future post on EV design.

Are there alternatives to electrochemical batteries for electric vehicles?

Some people have touted the possibility of using ultracapacitors instead of batteries. Capacitors and ultracapacitors store electric charge as free electrons instead of binding electrons chemically. Capacitors can be fully charged and discharged very rapidly, with no deterioration in their condition, and with nearly 100% efficiency. However, the energy density achieved so far is poor: as noted above it’s less than 0.02 MJ/kg, or more than two thousand times worse than gasoline. One company, EEStor, has promised ultracapacitors with a phenomenal energy density of up to 10,000 J/cm3 (10 MJ/litre) [6] but the feasibility of such ultracapacitors is being greeted with much skepticism in the scientific community. Ultracapacitors nonetheless have their uses in EVs and hybrid cars, namely for short-term energy storage in the regenerative braking system.

Other people consider hydrogen to be a promising energy storage medium for cars. Molecular hydrogen, or H2, is a gas at room temperature. The most economical way to produce hydrogen is by re-forming hydrocarbons such as natural gas (methane). Hydrogen can be combined with oxygen by burning in a combustion engine or by reacting it with oxygen in a proton exchange membrane (PEM) fuel cell. Fuel cells have achieved an efficiency of about 50%, and the theoretical limit is 83% at a working temperature of 298 Kelvins.[7] Decades of research and hundreds of millions of dollars have gone into PEM R&D, but the technology has yet to be deployed on a large scale. PEMs appear to be too unreliable to use in mass-produced cars. PEM cost is also unclear. Devices actually available for sale cost 3000 $/kW of peak output versus about 50 $/kW for internal combustion engines[8], but other sources claim that the cost of fuel cells can be as low as 61$/kW. PEMs are very fragile, with a life expectancy in a motor vehicle that is a small fraction of the 20 years an engine can last. Therefore if hydrogen is to be used at all, it makes more sense to burn it in an internal combustion engine.[9]

Hydrogen has an extremely high specific energy of 143 MJ/kg, but the energy density is low and depends on the form in which the hydrogen is held: liquid 10.1 MJ/litre, gas at 700 bar pressure manages 5.6 MJ/litre, while gas at 1 atmospheric pressure delivers 0.01079 MJ/litre[1]. A “bar” is a unit of pressure approximately equal to one atmosphere at sea level. I’m not sure how the quoted figures for liquid and compressed gas take into account the latent heat of evaporation and the work done during the expansion of the gas. These factors do affect the energy actually available. Hydrogen is extremely troublesome as a fuel precisely because of its ugly storage properties. It should be obvious that in uncompressed form, it is useless because its density is too low. That means one is tempted to compress it as a gas in a steel gas cylinder. But hydrogen is known to cause metals to deteriorate by a process known as “hydrogen embrittlement”. H2 occasionally dissociates into 2 H atoms, and these are highly reactive. They tie off loose bonds in the surface to which they cling, and over time cause the structure (e.g. the gas cylinder) to become like swiss cheese on an atomic level, and much weaker structurally. Eventually, the weakened pressurized gas cylinder may explode, probably while being refilled or while being bumped in an accident. Neither automakers nor their customers would like to see that happen. Another way of storing hydrogen is by chemical storage in tanks, using metal hydrides, carbon nanotubes, or other stabilizers. This, unfortunately, makes for very heavy tanks because stabilizers hold only 2% of their own weight in hydrogen, or even less.[10] This may be acceptable in stationary applications, but not in motor vehicles. Finally, there is the liquid option. Unfortunately, liquid hydrogen is a cryogenic liquid (meaning, it exists only at extremely low temperatures). In the case of H2, the temperatures at which it is liquid are below −423.17 °F, −252.87°C, or 20.28 Kelvins, just slightly above absolute zero. Hydrogen is very expensive (in energy and dollars) to compress to the liquid state, and it boils off quickly even when kept in special cryogenic tanks. 10% of tank capacity loss per day is a reasonable guess, and that’s after a 10-20 % loss during filling. Distributing hydrogen the way we distribute gasoline or diesel would be a nightmare, and filling tanks would pose great risks of cryogenic burns as well as explosions. While not a truly promising option as a transportation fuel (in my opinion at any rate) hydrogen is sufficiently interesting and often talked about that it warrants a more detailed examination in an future post.

In the case of aircraft fuels, neither hydrogen nor electricity merit serious consideration. Both have an energy storage density that is far too low to be of any use, and in addition hydrogen is too awkward and dangerous to handle for this application.

Given that neither electricity nor hydrogen appear to be the “silver bullet” that will meet the needs of the transportation sector as an across-the-board replacement for fossil hydrocarbons, it is likely that in the distant future we will see vehicle specialization. Small EVs may be used as commuter cars alongside greater use of public transit. More electric commuter rail lines might be built, and even long-range electric rail, an activity in which Europe is already well ahead of North America. Meanwhile long-range road vehicles such as transport trucks and cars meant for highway cruising will continue using hydrocarbon fuels, but not necessarily fossil hydrocarbons. We will need to take a closer look at the “biofuel” and “re-formed” hydrocarbon alternatives. These long-range vehicles may be hybrids.

[4] “The power of the press”, The Economist, January 29, 2011, pgs.77-78
[5] “Batteries that breathe”, IEEE Spectrum, February 2011, pg. 13
[10] “Feathers Hold Hydrogen Promise”, New Scientist, 27 June 2009, pg.19

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