Monday, April 30, 2012

Mandatory Stop/Start by 2014

Canada's set to force all automakers to add stop/start technology to all cars starting on 2014. Stop/Start technology is a fuel-saving feature used in many large cities for buses and tractor trailers. They stop the engine from running when at red lights or stopped for an extended period of time. The engine will automatically start if it detects a lift off the brake pedal and press on the gas pedal.

But there is one problem. Most regular lead-acid batteries wear out after 1 year of this type of use. The solution: UltraCapacitors. In 2000, you could buy a 3Kilo-Farad UltraCapacitor for $2000. Now they cost $50. I say you could get away with using 4 of them to generate 12 kF of stored energy, literally enough juice to weld solid steel beams together that are 1 inch thick.

Supercapacitors have several disadvantages and advantages relative to batteries:

Disadvantages

  • The amount of energy stored per unit weight is generally lower than that of an electrochemical battery (3–5 W·h/kg for a standard ultracapacitor, although 85 W.h/kg has been achieved in the lab[4] as of 2010 compared to 30–40 W·h/kg for a lead acid battery, 100-250 W·h/kg for a lithium-ion battery and about 1/1,000th the volumetric energy density of gasoline.
  • Has the highest dielectric absorption of any type of capacitor.
  • High self-discharge – the rate is considerably higher than that of an electrochemical battery.
  • Low maximum voltage – series connections are needed to obtain higher voltages, and voltage balancing may be required.
  • Unlike practical batteries, the voltage across any capacitor, including EDLCs, drops significantly as it discharges. Effective storage and recovery of energy requires complex electronic control and switching equipment, with consequent energy loss. A detailed paper on a multi-voltage 5.3 W EDLC power supply for medical equipment discusses design principles in detail. It uses a total of 55 F of capacitance, charges in about 150 seconds, and runs for about 60 seconds. The circuit uses switch-mode voltage regulators followed by linear regulators for clean and stable power, reducing efficiency to about 70%. The authors discuss the types of switching regulator available, buck, boost, and buck-boost, and conclude that for the widely varying voltage across an EDLC buck-boost is best, boost second-best, and buck unsuitable[12].
  • Very low internal resistance allows extremely rapid discharge when shorted, resulting in a spark hazard similar to any other capacitor of similar voltage and capacitance (generally much higher than electrochemical cells).

Advantages

  • Long life, with little degradation over hundreds of thousands of charge cycles. Due to the capacitor's high number of charge-discharge cycles (millions or more compared to 200 to 1000 for most commercially available rechargeable batteries) it will last for the entire lifetime of most devices, which makes the device environmentally friendly. Rechargeable batteries wear out typically over a few years, and their highly reactive chemical electrolytes present a disposal and safety hazard. Battery lifetime can be optimised by charging only under favorable conditions, at an ideal rate and, for some chemistries, as infrequently as possible. EDLCs can help in conjunction with batteries by acting as a charge conditioner, storing energy from other sources for load balancing purposes and then using any excess energy to charge the batteries at a suitable time.
  • Low cost per cycle
  • Good reversibility
  • Very high rates of charge and discharge.
  • Extremely low internal resistance (ESR) and consequent high cycle efficiency (95% or more) and extremely low heating levels
  • High output power
  • High specific power. According to ITS (Institute of Transportation Studies, Davis, California) test results, the specific power of electric double-layer capacitors can exceed 6 kW/kg at 95% efficiency[13]
  • Improved safety, no corrosive electrolyte and low toxicity of materials.
  • Simple charge methods—no full-charge detection is needed; no danger of overcharging.
  • When used in conjunction with rechargeable batteries, in some applications the EDLC can supply energy for a short time, reducing battery cycling duty and extending life

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