Electric vehicle
Electric Vehicle Technology Explained, Second EditionTable 3.8
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| 54 Electric Vehicle Technology Explained, Second Edition Table 3.8 Nominal battery parameters for zinc–air batteries Specific energy Wh kg −1 Energy density Wh l −1 Specific power W kg −1 Nominal cell voltage V Amphour efficiency Not applicable Internal resistance Medium Commercially available Very few suppliers Operating temperature Ambient Self-discharge High as electrolyte is left in cell Number of life cycles >2000 Recharge time min, while the fuel is replaced The general characteristics of the battery are given in Table 3.8. A few manufacturers have claimed to produce electrically rechargeable zinc–air batteries, but the number of cycles is usually quite small. The more normal way of recharging is as for the aluminium–air cell, which is by replacing the negative electrodes. The electrolyte, containing the zinc oxide, is also replaced. In principle this could betaken back to a central plant and the zinc recovered, but the infrastructure for doing this would be rather inconvenient. Small zinc–air batteries have been available for many years, and their very high energy density makes them useful in applications such as hearing aids. These devices are usually ‘on’ virtually all the time, and thus the self-discharge is not so much of a problem. Large batteries, with replaceable negative electrodes, are only available with great difficulty, but this is changing, and they show considerable potential for the future. Use of a replaceable fuel has considerable advantages as it avoids the use of recharging points – lorries delivering fuel can simply take the spent fuel back to the reprocessing plant from where they got it in the first place. The high specific energy will also allow reasonable journey times between stops. 3.8 Supercapacitors and Flywheels 3.8.1 Supercapacitors Supercapacitors or ultracapacitors are large capacitors which can be can be used as energy stores. They are devices which have high specific power and low specific energy. Capacitors are devices with two conducting plates which are separated by an insulator. An example is shown in Figure 3.12. ADC voltage is connected across the capacitor, one plate being positive, the other negative. The opposite charges on the plates attract and hence store energy. The charge Q stored in a capacitor of capacitance C farads at a voltage of V volts is given by the equation Q = C × V (3.10) As with flywheels, capacitors can provide large energy storage, although they are more normally used in small sizes as components in electronic circuits. The large energy storing |