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The paper discusses the use of a dual energy storage system based on batteries and supercapacitors in hybrid electric vehicles (HEV). The battery has a large energy density, enabling an all-electric driving range of 100 km, while the supercapacitor has a large power density and provides peak power during acceleration and regenerative breaking. The paper discusses the benefits and drawbacks of both storage systems and the specific requirements imposed by the hybrid drive train. Coupling such a HEV to the grid allows interaction between grid and HEV, providing the grid with a controllable load. Depending on the communication between the hybrid fleet and the grid, this load can be controlled by adjusting the electricity price in order to allow a higher penetration of intermittent renewable energy sources such as wind parks in the grid and if the communication allows the transmission system operator to reduce the load imposed on the grid by the hybrid fleet, the hybrid fleet can become part of the secondary frequency control reserve. In case of sudden demand or supply fluctuations, the hybrid fleet can assist in primary control of the grid. Due to the dual energy storage system the HEVs can also provide fast load tracking to keep the voltage in microgrids at the desired set point. An experimental setup with a battery, grid coupling and induction machine proves the feasibility of the concept.

Any capacitor large enough to run the bus, would presently be many times the size of the actual bus, thus the capacitors would need to be in a string of trailers behind the bus to power it and leave room for the passengers. If such a capacitor-powered bus was ever designed, then consider how damaging the energy stored in the capacitor/s, if a sudden short circuit developed - it would demolish several city blocks in the resultant explosion. By comparison, energy stored in chemical form is far safer, small volume for large energy storage, easy to control.

Such a capacitor gauge could become a common sight on the dashboards of the future. A capacitor can discharge and recharge far faster than a battery, making it ideal both for generating bursts of speed and for soaking up the energy collected by regenerative braking. AFS Trinity, a company based in Washington state, has turned that insight into a piece of equipment that it has fitted into an otherwise standard production model as an experiment. The result—the XH-150—was unveiled at this year's Detroit motor show.

We have previously discussed a secretive Texas company called EEStor, who are reported to be working on a new type of ultracapacitor that can hold 10x the energy in 1/10th the weight of typical batteries, at a fraction of the cost. They have an agreement to produce caps for Zenn electric cars but to date have not shown any prototypes. This has led some to suspect EEStor as not having the technology they report. Today, however, Lockheed Martin, the major U.S. military equipment manufacturer has announced a partnership agreement with EEStor to develop energy applications.

Here are a couple of pieces that appeared recently in Technology Review: one relates to EEStor's recent announcement with Lockheed Martin and what it means for the company, while the other is a look at a new type of hybrid lead-acid battery with an integrated supercapacitor that claims to last four times longer than conventional lead-acid systems and to perform just as well as nickel-metal hydride systems -- but at a fraction of the cost.

ScienceDaily (Jan. 18, 2008) — The odometer of a low emission hybrid electric test vehicle recently reached 100,000 miles as the car circled a track in the UK using the power of an advanced CSIRO battery system. The UltraBattery combines a supercapacitor and a lead acid battery in a single unit, creating a hybrid car battery that lasts longer, costs less and is more powerful than current technologies used in hybrid electric vehicles (HEVs).

Ultracapacitors' Rapid Charge/Discharge Capabilities Boost Acceleration and Increase Braking Energy Recuperation Efficiency, Reducing Fuel Consumption and Emissions