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Materials for the Environment – Energy Storage
Piggybanks for Power
Whether at base or peak load, high-performance energy storage devices and smart energy management systems guarantee optimal power supplies in vehicles.
If electrical energy is to be optimally used, it needs to be temporarily stored. And that’s the case whether we’re talking about cars, buses, streetcars, subway systems or power distribution networks. In road vehicles, electronic components are taking over more and more functions, partly as driver assistance systems, and partly to save energy—particularly in hybrid vehicles that combine an electric motor with a combustion engine. The electric motor serves as either a fully fledged second drive (in a full hybrid), as an auxiliary drive to provide a boost when starting and passing (in a mild hybrid), or as an assistant when the vehicle has to stop and restart frequently (in the start-stop hybrid).
To meet the needs of a growing number of functions, vehicles needs a high-performance energy storage device. Batteries, however, are heavy and their energy density is low. 1 kg of diesel contains 10,000 Wh, while a lead-acid accumulator manages just 30 to 50 Wh/kg. Batteries’ power density is low too, reaching a maximum of 300 W/kg. For an electric car to accelerate as rapidly as a 90 kW gasoline-engine vehicle, it would need a 300-kg lead-acid battery in the trunk. That’s why most of today’s hybrid vehicles employ nickel-metal hydride batteries with a capacity of 60 to 80 Wh/kg. Lithium-ion or lithium-polymer batteries are even more powerful, with 90 to 150 Wh/kg. Alongside storage capacity, the service life of an accumulator is also limited. A lead-acid battery is good for a maximum of around 1,000 charge-discharge cycles. Nickel-metal hydride or lithium-ion batteries last considerably longer.
Accumulators must be charged slowly to avoid damage. But vehicles, in particular, are associated with many applications that need a fast charging capability—for example, when braking energy is harnessed in cars or streetcars. With this in mind, Siemens is promoting the use of double layer capacitors, or so-called supercaps—devices that store electrical energy by separating the charges as soon as a voltage is applied. Supercaps offer capacitances of 300 to 10,000 F. Charge separation takes place at the boundary layer between a solid body and a liquid. High capacitances are achieved by ensuring that the charges are separated by a distance of only atomic dimensions, and by the use of porous graphite electrodes with a large specific surface area.
Supercaps have low energy densities—3 to 5 Wh/kg—but extremely high power densities of 2,000 to 10,000 W/kg. They can be charged within a few seconds, and at a million or so charge-discharge cycles, their service life is extremely long. This is due to the fact that the charge separation processes occurring within them are purely physical in nature. They can take up and release large quantities of energy extremely quickly. This makes it possible to use an electric motor in a hybrid vehicle, streetcar, or locomotive as a generator that recovers braking energy. This regenerated energy is stored in supercaps and re-used when the vehicle accelerates again. The resulting advantage is fuel and energy savings of between five and 25 %, depending on the driving cycle. The capacitor packs can either be carried in the vehicle itself or permanently built into segments of subway lines.
Such a setup has already been tested in several subway systems—for example, in Madrid, Cologne, Dresden, Bochum and Beijing. Supercaps could also be used in energy distribution applications, as power supply networks are constantly subject to load variations to which heavy turbines cannot react quickly enough. Power utilities could use flexible energy stores such as supercaps to balance out load peaks and troughs.
"In ten years, vehicles with these new storage systems might be as commonplace as today’s vehicles with their trusty lead-acid batteries," says Dr. Manfred Waidhas, project head for Electrochemical Energy Storage at Siemens Corporate Technology. Mild or start-stop hybrid vehicles can get by with the limited energy density of the supercaps.
"Ensuring a supply of electrical energy is becoming increasingly important," says Horst Gering, head of the Battery and Energy Management department at Siemens VDO. "This is especially true where safety is concerned, for example, with electric braking or steering." In such systems, it is necessary to constantly monitor the state of the energy store. With this in mind, Siemens has developed BMS (Battery Monitoring System). Here, using supercaps, internal resistance and capacitance are determined in order to evaluate how much current the energy store can provide for specific tasks. Where accumulators are involved, sensors can also determine battery aging and charge state. The energy management system then determines when the store needs to be charged so that it always remains within optimal working parameters—and how much current can be made available to which devices. After all, in some cases, there may not be sufficient power available if many devices are active simultaneously. Siemens has christened the algorithm for this process "Power Trader."
"It’s like having a virtual stock market regulating energy use," says Gering. "Power Trader calculates the supply—in this case, the amount of energy available from the generator—and sets an electricity price according to demand. If demand rises, so does the price. Safety-relevant systems such as electric brakes are set so that no price is too expensive. Comfort systems, on the other hand, purchase less power until the price has come down to a given level. In extreme cases, they will even switch off."
Accumulators such as lead-acid, nickel-metal hydride and lithium-ion batteries have a service life of between three and ten years, on average. They function on electrochemical principles. Charging the battery converts electrical energy into chemical energy. When an electrical device is connected, chemical energy is converted back into electrical energy. Energy stores such as double layer capacitors, in contrast, store energy electrostatically. They last almost indefinitely and exhibit high power densities. However, their energy densities are low. For this reason, their primary use is to cover peak loads such as engine starts or acceleration in hybrid applications.