III. The Challenge of Mobile Energy Storage
(originally published April 30, 2007)
In my previous two posts, I outlined Ulf Bossel’s Electron Economy concept and the importance of the electric grid as an energy transmission medium for now and the foreseeable future. If we are serious about reducing our carbon emissions, an electron economy, an economy in which most powered devices use electricity, is the only feasible alternative for the next several decades. Cheap, high-capacity, accessible storage of electrical energy has been one of the challenges in the development of the electron economy and again represents a turning point in growing a clean-energy economy in the 21st century. Energy storage in a wide variety of sizes and capacities allows for the convenience of being able to tap into an energy source on demand in any situation or physical location.
Energy storage can be divided into two categories: mobile and stationary storage. In the mobile storage category, moving more of our energy needs into electric-powered devices means finding more ways to use electricity when not connected to the grid or at least having to deal with an electric cord or active connection to the grid. In the stationary storage area, finding economical ways to store electric energy on a mass scale to power the grid or off-grid facilities when the supply of renewable energy dips below demand is also another area of development for a greener economy. I will deal with stationary storage challenges in a future post.
Energy storage or assumptions about energy storage influence decisions people make about which type of energy delivery they prefer, even if it is not often discussed in these terms. Currently, the electron economy is in partial competition for public, political, and investor attention with three other energy delivery systems that we can call the fossil fuel economy, the hydrogen economy, and the biofuel economy. For environmental reasons, the fossil fuel economy is not desirable but from every other perspective it is the one to beat, especially in the area of portable energy storage. Fossil fuels, in particular petroleum derivatives, have a high energy content per unit weight (called “energy density”), have been fairly plentiful, stored as they are underneath the ground, and we have already built a huge industrial and transport infrastructure around them. There are also reserves of petroleum, natural gas, and coal that can be more or less measured, so we can project their presence or absence into the future and can therefore make business or other plans based on them.
Two hopefully greener alternatives, the hydrogen and biofuel economies, are modeled on the fossil fuel economy with regard to the potential for both stationary and portable storage. Hydrogen and biofuels can potentially be stockpiled, though currently they are not produced cleanly in surplus amounts that could allow for their accumulation. Also they have high energy content per unit weight, though hydrogen needs to be compressed into a liquid to have a reasonable energy content per unit volume. They also are liquids or can be made into liquids, so can be dispensed and metered out in a way not unlike the fossil fuel economy.
Electricity is an energy flow or alternatively a state of electrostatic charge, not a substance that can normally be contained or stored. Technically, electric current, the type of electricity that does work is the movement of electric charge along a gradient of electric potential, capitalizing on the characteristics of physical substances called conductors. Storing electricity means storing it as a physical and/or chemical potential that can be rapidly converted to an electric current on demand. The most important areas of energy storage for the growth of the electron economy are battery and ultracapacitor storage but many other forms of energy storage may come into play as technology advances and economic conditions favor carbon neutral solutions.
Batteries store electricity by utilizing the difference in electric potential between two chemical reactions, one at the positive electrode and the other at the negative electrode of each of the constituent cells that together make up the battery. The chemistry and design of batteries determines their energy and power per unit weight and volume. Battery performance continues to progress under pressure primarily from the demands of the portable electronics industry but, increasingly from hybrid electric and now electric-propulsion vehicles.
For portable electricity storage devices one of the greatest challenges is the energy content per unit weight/volume, technically “energy density.” In the early 20th century prior to the systematic exploitation of oil resources, battery electric vehicles and petroleum fueled vehicles were competing on equal footing, as electric vehicles were easier to use and quieter. The electric starter motor, ironically, helped hasten the demise of early battery electric vehicles as internal combustion engines could now be started without a hand crank. Once easily started and with a growing petroleum infrastructure, the higher energy density of petroleum-based fuels allowed drivers a much extended range and convenient fueling.
Though lousy for our environment when burned, petroleum products have enviable energy density per volume or weight: approximately 10,000 Watt-hours/liter or 12,000 Watt-hours/kilogram for gasoline/petrol and slightly higher for diesel. By contrast, lead acid batteries have about 1/250th the energy density, coming in at 40 Watt-hours/liter or 25 Watt-hours/kilogram. Internal combustion engines have about 1/3 to ¼ the energy efficiency of electric motors, but this only makes up some of the difference. Early electric vehicles or for that matter any portable electronic device, that depended on lead acid batteries, would need to make many compromises in terms of range, weight, and internal storage. The pioneering battery electric vehicle, the GM EV-1, in its first generation carried 1200 lbs of lead acid batteries for a range of 80-100 miles. By contrast a full 20-gallon tank of gas weighs approximately 150 lbs.
Despite their weight and bulk per unit energy, rechargeable batteries are quite an efficient means of storing electrical energy with an efficiency of more than 70% and as much as 90%, meaning that less than 30% and more like 20 or 15% of the energy is lost in the round trip.
Modern and Future Rechargeable Batteries
The lead acid battery continues to be useful but in the last 10 years there has been rapid progress in battery chemistry and technology. Most important has been the shift away from lead acid to first nickel metal hydride and now various lithium chemistries. All innovations in rechargeable batteries for powering transport are focusing on safety, reliability and battery life, meaning the number of cycles of charge-recharge a battery can endure before it needs to be overhauled or replaced. Different strategies for increasing the energy content, power and speed of recharge as well as safety, reliability and battery life are the following:
Most notable in the increased battery life and slim size of some of our favorite portable electronic devices (cell phones, Ipods) are the new generation of lithium-ion batteries, which offer high energy densities (130-200 Watt hours/kilogram). This means that lithium batteries contain as much as 5 times the energy per unit weight of a conventional lead acid battery.
For the purposes of the electron economy and transport applications, lithium ion batteries are being used in a new generation of battery electric vehicles, though the expense of these batteries is still high. The Tesla Roadster ($92K MSRP), the first all-electric production vehicle to exceed 200 miles per charge, uses 6,831 lithium ion batteries not unlike those found in a laptop. Tesla has built a complex power management system to ensure the safety and reliability of its battery pack. Various boutique electric car conversion outfits are creating lithium ion versions of their vehicles.
Innovations in the structure of lithium ion batteries promise to increase their reliability and usefulness for electric vehicle and transport applications. Altairnano’s Nanosafe battery uses nanostructured electrodes among other features to lengthen the life of the battery, increase battery safety, widen the operating temperature range, and, the company claims, to recharge within a few minutes. A123 Systems have also developed a nanostructured lithium ion battery with similar benefits.
Lithium Ion Polymer Batteries
Lithium Ion Polymer batteries can be physically shaped and bent according to the requirements of the application and also can have high energy density. While the physical form factor is less important for massive vehicles as opposed to portable electronic devices, lithium ion polymer batteries have been shown to have a high energy density. Electrovaya, a battery manufacturer largely for electronic goods, claims an energy density of 470 Wh/l and 330 Wh/kg for its SuperPolymer batteries.
Nickel-Metal Hydride Batteries
Though now “old news”, nickel metal hydride batteries (NiMH) are still a viable alternative for powering electric vehicles and hybrids. NiMH batteries were used in the final generation of the EV1, increasing the car’s range to 100-130 miles. NiMH batteries have a range of energy densities from 30-80 Watt-hours/kilogram so are more energy dense than conventional lead acid batteries but are considered to have favorable power and durability (many discharge/charge cycles) suitable for hybrid and battery electric vehicle applications. The current Toyota Prius parallel hybrid vehicle, uses an NiMH battery pack that is recharged by its gasoline engine and regenerative braking.
Lead Acid Innovations
Firefly Energy claims to be on a path to raise the energy density of lead acid batteries closer to the their theoretical limit at 170 Watt-hours per kilogram, a 4 fold increase over conventional lead acid batteries. Firefly is using a foam technology to revolutionize the structure of electrodes used in conventional lead acid batteries. Firefly claims their technology will lead to much higher energy and power densities and longer battery life. The continuing advantage for lead acid is that it is the most established rechargeable battery technology and potentially still the cheapest.
Quick-Charge and Energy Density in Battery-powered Transport
Many of the innovations in battery design are currently focused on quick charge, as this area seems to be more tractable to engineering solutions than extending the overall energy content of the battery through the discovery of new fundamental battery chemistries. Redesigning the physical structure of electrodes on a microscopic level, it seems, will lift some of the traditional limitations in energy flow, power and battery life that traditional electrode structures maintained.
If one can recharge a 60 kWh battery pack in 5-10 minutes with a suitably powerful electricity source, this would allow travelers to approach the convenience of fossil fuel powered vehicles that require a similar amount of time to refuel. And, if energy densities remain at or near the 200 Wh/kg level for a lithium ion battery pack, this leads to an approximate 200-250 miles per charge range for a moderately sized vehicle with an 800 to 1000 lb battery pack. While 1000 lbs is heavier than the 200 lbs that a full 20 gallon gas tank weighs, the much smaller size and weight of most electric motors over internal combustion engines will partly compensate for the difference in weight. So a long-distance trip in a purely battery powered vehicle with these newer quick charge batteries would mean approximately 3 hours driving with 10 minute breaks to recharge, as opposed to 6 hours with a similar break for most petroleum powered vehicles.
For the quick charge scenario to work, it would require the development of a new quick charge infrastructure to grow out of or alongside our current or a modernized electric grid. Quick charge would require high voltage fueling stations that can transfer as much as 80 kWh to a vehicle in the period of 5-10 minutes safely and reliably, whether at peak electric power usage times (middle of the day) or at night. This type of power is now available to certain industrial power customers and is therefore it is technically feasible to offer to a broader set of customers. Utilities or their partners would need to build roadside charge depots that would become the fueling stations of the 21st century. These charge stations would have to charge a premium over regular power rates to make it worth their while to build the charge infrastructure as well as perhaps, clean or cleaner electric generation facilities to supply the station with power, especially during the peak usage times.
Quick-charge may leap one hurdle but increases in energy density of batteries will eventually boost electric vehicle range between charges. There are unconfirmed announcements of future rechargeable batteries that use aluminum, which would have an energy density of as much as 1300 Wh/Kg. A theoretical 150 lb battery pack with these characteristics would yield ranges of easily over 500 miles per charge. This remains marketing hype until working prototypes can be demonstrated.
Ultracapacitors or supercapacitors, like regular capacitors, can store electricity as electrostatic charge because of the physical composition and design of the capacitor. Ultracapacitors have much higher charge and discharge rates than electrochemical batteries and are used most frequently now in hybrid electric vehicles to capture the energy of braking and recharge the batteries. Working in tandem with the vehicle’s batteries, ultracapacitors are now used in regenerative braking in hybrid-electric and fully electric vehicles, including the Toyota Prius and the Tesla Roadster.
Currently most existing ultracapacitors have per unit weight much less energy capacity than electrochemical batteries though they have greater power (rate of energy discharge). A number of research labs and at least one private company have been targeting a substantial increase in the energy density of ultracapacitors so they might replace batteries as the primary energy store of electric vehicles. Most prominent and well-funded among the ultracapacitor makers is the secretive Texas company EEStor, backed by the venture capital firm Kleiner Perkins, which claims to be able to deliver this year a Electrical Energy Storage Unit (EESU) that will weigh 100 lbs and store 15 kWh of energy, an energy density of approximately 340 Wh/Kg. The composition of their ultracapacitor is complex but is based on a ceramic made with barium titanate. EEStor is rumored to be making a 52 kWh EESU that would weigh 336 lbs. The EESU is rumored to be able to charge very rapidly, within a few minutes with a sufficiently powerful charging sourced if the cables and connections are cooled. The EESU is also rumored, under mass production conditions, to able to cost a fraction of what lithium ion batteries do, with EEStor claiming from 1/8 to ¼ the price.
EEStor has initially developed an exclusive relationship with ZENN Motor Company, a Toronto company that makes so-called neighborhood electric vehicles that are limited to speeds of 25mph/40kph. They will be delivering their 15 kWh EESU to ZENN later in 2007.
So much controversy and secrecy shrouds EEStor currently that it is difficult to evaluate its claims. If its claims can be substantiated and proven in real world applications, its EESU will revolutionize portable energy storage.
The potential of the EESU would also depend on a quick-charge infrastructure and economy similar to that outlined above under “Batteries”. The demand for high electric power to charge such a large capacity energy storage unit quickly would lead to high voltage/amperage outlets/charging stations along the roadways and in parking facilities as well as potentially at the level of individual households.
Flywheels can capture energy as angular momentum around a fixed axis. Modern flywheels can spin at speeds as high as 100,000 rpm depending on the strength of the materials used. Flywheels are very efficient at capturing energy but tend to bulky as compared to batteries due to the strong containment vessels required to protect people from the potential effects if the flywheel should break. Flywheels have been considered as energy storage devices in hybrid electric vehicles but as yet there are no working prototypes.
Biofuels as Storage in the Electron Economy
Of course biofuels store the sun’s energy and can be converted to electricity using a variety of fuel cells or be combusted to drive a steam turbine that generates electricity. Ulf Bossel in his outline of the electron economy highlights how biofuels will play a role in a largely electric-driven economy. It in fact their ability to be stockpiled as well as the relative low cost and current availability of the infrastructure (agriculture) that makes them attractive. They do however rely on the relatively inefficient use of land and the sun which wind and solar use much more effectively to capture energy. Algal and cellulosic biofuel production may make them more attractive but these are still unproven solutions to our new energy crisis.
Hydrogen as Storage in the Electron Economy
The much-vaunted Hydrogen Economy, in the ideal version at least, relies on renewably generated electricity to split water into hydrogen and oxygen gas, the hydrogen is stored, and, at the point of use, converted into electricity by a Proton Exchange Membrane (PEM) fuel cell. In the favored scenario for hydrogen as a clean energy solution, Hydrogen should be considered as an electric energy storage medium rather than an energy source in itself. Unfortunately the round trip efficiency of this cycle is approx. 25%, so 75% of the energy is lost as compared to approximately 20% losses in recharging and discharging a battery, a technology that is readily available to us. In a future of superabundant renewable energy, hydrogen may very well become the storage medium of choice but currently it falls short in important ways to batteries and, perhaps, ultracapacitors.
To move beyond our current crisis and to pay attention to the actual technological facts at hand, hydrogen is over-hyped and the much more workable solutions encapsulated in the Electron Economy idea need to be brought home to investors, government and consumers.
Future posts will highlight how innovations in energy storage will lead to new economic models and opportunities.