Extensive development has been achieved in recent years in the field of high capacitance Electrochemical Double-Layer Capacitors (EDLC) [40]. More commonly refereed to as
‘Supercapacitors’ or ‘Ultracapacitors’, these devices are able to operate at power levels high above that of conventional batteries and can store a considerable amount of energy above the energy capacity of conventional capacitors. These devices represent one of the latest innovations in the field of electrical energy storage [41], and lends itself as a significant technology enabler for future electric and hybrid electric vehicles.
As a relatively new energy storage device, EDLC technology warrants a brief historical introduction. The first high capacity electrochemical capacitor device was patented in 1957 (US Patent 2800616) [42]. Developed by Howard Becker of General Electric Company, the device was of a basic construction consisting of porous carbon electrodes. Becker described
the large capacitive phenomena of the device but acknowledged that the exact reason for this exceptionally large capacitance was not fully known at the time. Subsequently, in 1966, Robert Rightmire of Standard Oil Company Cleveland Ohio (SOHIO) introduced a double layer capacitor utilising porous carbon in a non-aqueous electrolyte (US Patent 3288641).
Four years later, Donald Boos, also with SOHIO, patented another device that used a carbon paste soaked in an electrolyte (US Patent 3536963), which SOHIO later put into production. Thus making them the first company to market high capacitance devices.
Between 1975 and 1980, Brian Conway carried out extensive fundamental work on EDLCs and also ruthenium oxide type electrochemical capacitors. A detailed account of this can be found in Conway’s scientific monograph [43]. Conway was also the first to use the term
‘supercapacitor’. However, in 1971, the Nippon Electric Company (NEC) produced the first commercially successful high capacitance device under the same name, ‘supercapacitor’ [44].
The NEC device was however primarily targeted for memory backup applications. Pinnacle Research Institute (PRI) began developing high power EDLCs in 1982, which they called
‘ultracapacitors’ [45]. Intended for critical military applications, the capacitors were designed for utilisation in electromagnetic launchers, missile guidance systems, laser weaponry, arming systems and power conditioners. It was more than a decade later that EDLC devices found presence in vehicular applications [46].
Interestingly enough, it is the advent of EDLCs in vehicle applications that has created a synergistic effect in terms of technology awareness of EDLCs and fuelled a popularity increase in hybrid and electric vehicle. Some conjectures can be made from Figure 2.1 to support the pervious statement. As the histogram shows, the increasing interests in EVs and HEVs coincide with the decade old introduction of EDLCs in vehicle applications. In retrospect, the increasing attention to EV power and energy management can also be linked to the introduction of this technology enabler to the vehicular application domain. Figure 2.4 illustrates the progress of EDLCs development since its inception in 1957.
(a) From US Patent 2800616 (1957) (b) From US Patent 3288641 (1966)
(c) From US Patent 3536963 (1970) (d) Modern ultracapacitor construction (2002) Figure 2.4 Evolution of the EDLC technology
(a) Patent by H.I Becker, (b) Patent by R. A. Rightmire, (c) Patent by D.L. Boos, (d) Modern ultracapacitor (Maxwell-Montena)
The terms ‘Supercapacitor’, ‘Ultracapacitor’ and ‘Electrochemical Double Layer Capacitor’
have been used indiscriminately in literature in reference to high capacitance devices.
Huggins [47] identified this uncertainly in terminology and made a distinction between these type of capacitors in terms of their storage mechanisms and redox pseudo-capacitance.
However, it is generally recognised that these terms are interchangeable depending on the
manufacturer. Throughout the rest of this dissertation, the term ‘Ultracapacitor’ will be adopted for the sole purpose of keeping with consistency when presenting the actual device used in the experimental part of the work. A listing of current manufacturers of these devices and their respective device names are shown in Table 2.1
MANUFACTURER DEVICE NAME CAPACITANCE (F)
AVX Bestcap 0.022 - 0.56
Cap-XX Supercapacitor 0.09 - 2.8
Cooper PowerStor 0.47 - 50
ELNA Dynacap 0.033- 100
ESMA Capacitor modules 100 - 8000
Epcos Ultracapacitor 5 - 5000
Evans Capattery 0.01 - 1.5
Kold Ban KAPower 1000
Maxwell Ultracapacitor 1.8 -2600
NEC Supercapacitor 0.01 - 6.5
Ness EDLC 10 - 3500
Panasonic Gold capacitor 0.1 - 2000
Tavirma Supercapacitor 0.13 - 160
Table 2.1 Manufactures of High Capacitance devices (Extracted from Namisnyk [48])
As one of the key technology enablers for electric vehicles, work on ultracapacitor systems and the associated power and energy management is being carried out worldwide. Research activities supported by the European Community Joule III [13] program specifically titled,
‘Development of Supercapacitors for Electric Vehicles’ began in 1996. In 2002, a worldwide consortium called KiloFarad International (kFi©) [49] was established as a regulatory body.
One of the aims of kFi is to initiate working groups to drive forward the adaptation of ultracapacitor technology in automotive and other applications. The US DoE recognises ultracapacitors as one of the critical technology enablers for the future ‘More Electric Vehicles’ (MEVs), and stipulated the performance expectation of this technology in their 2003 annual report. An excerpt from the report is reproduced in Table 2.2.
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PROPERTY NEAR TERM TARGET ADVANCED TARGET
Energy Stored (Wh) 500 750
Maximum Power (kW) 50 80
Weight (kg) <100 <50
Volume (l) <40 <20
Energy Density (Wh/kg) >5 >15
Maximum usable power density (W/kg) >500 >1600
Round trip efficiency (%) >90 >90
Table 2.2 US DoE target performance specification for ultracapacitors (Extracted from the FY2003 Progress report for Energy Storage Research and Development)
There is substantial evidence in literature to support further development in integration technology of ultracapacitors in electric vehicle power systems [50-54]. However, the obtainable efficiency enhancement has regularly been contested by a cost factor. In their 2002 report, Simpson and Walker [55] reported a lifecycle cost analysis of ultracapacitors in electric vehicles. They stipulated that efficiency gains are marginal compared to the lifecycle cost of ultracapacitors. Burke [56] however reported that due to the high prices of speciality carbons used in manufacturing ultracapacitors, a price reduction by a factor of ten (at 2000 prices) is necessary to justify capital costs. In 2002, Barker [57] stated that ultracapacitors are not ready from a cost perspective and requires a 2-3 cost reduction factor. Barker however concluded that the benefits of ultracapacitors as a leading contender in storage technology would very likely reduce the cost. Arguably, cost will always be an issue to debate. Prices have exponentially dropped since the year 2000, as manufacturers race to reduce the cost per farad. In fact, in 2004, both Miller [58] and Barrade [52] reported a significant cost reduction and projected a further fall towards more favourable cost targets.