There are some sad statistics: some 85% of energy consumption in the world is covered by the burning of fossil fuels – oil, gas and coal – and projections suggest that oil and gas reserves will be able to satisfy the needs of humanity for 50 – 60 more years. And then what? Beginning of the crisis is already in view, especially if you compare prices at gas stations today and some 20 years ago.
So, the prices for petrol have been rising, the amount of petrol is decreasing and this trend will proceed… until petrol disappears completely. And what in return? In fact, various chemical energy sources may come into play. Some ideas have already been probed in hybrid cars and electric vehicles. To understand what kind of power supply may be the most promising it is useful to compare them in terms of specific energy (i.e. energy per unit mass), the ability to produce necessary power (i.e. energy per unit time), the number of recharge cycles, working temperature range, etc.
It is widely adopted to estimate the energy in Watt-hours (Wh) and therefore energy density – in Wh/kg. Lithium-ion batteries undoubtedly hold the first place amount commercially available energy storage systems in terms of specific energy – the energy density reaches 150 – 200 Wh / kg which is greater than can be obtained from commonly used lead-acid batteries (30 – 50 Wh/kg). It is true, however, that some energy sources are known to have much higher stored energy. Among them there are fuel cells with specific energy up to 950 Wh / kg and recently emerged lithium-air elements with specific energy up to 1500 Wh / kg (the market is not aware of them yet!).
Of course, the consumer would like to have a cycle life of the accumulator up to hundreds of thousands of recharging cycles. And this is not just a whim. For instance, in modern micro-hybrids equipped with start-stop systems the engine needs to start and stop many times while the car toils its way thought the traffic jams… and no one would want to change the battery every two to three months.
There are other suggestions to improve the batteries when they are used in electric and hybrid vehicles: a good idea would be to increase the horsepower so that the vehicle could accelerate quickly… it is also good to extend the working temperature range… that all would be good… but the nature of the electrochemical processes is a hard thing to change. One of the solutions to many problems with batteries – assembling them together with ultracapacitors.
Ultracapacitors Can Help!
This new name – ultracapacitor (supercapacitor, Electric Double Layer Capacitors, EDLC) – has just recently come to the lexicon of experts in the field of energy storage. Ultracapacitors are produced based on the special nanoporous electrode materials with a huge surface area – on the order of thousands of square meters per gram! Ultracapacitors differ from conventional capacitors in that the former have significantly higher specific energy – two to three orders of magnitude larger. And although batteries are still superior over ultracapacitors in the specific energy characteristics, the latter outperform batteries when it comes to power. Besides, the ultracapacitors can survive enormous number of charge / discharge cycles – up to a million and they can successfully operate at temperatures from -50 to +70 oC.
High power density (due to the low internal resistance), wide operating temperature range and virtually unlimited cycling – these benefits are attractive not only for car manufacturers. In recent years the ultracapacitor began invading various market niches, either as stand-alone systems, or in a combination with batteries. Some examples are:
Already In Use…
Japanese company NEC Tokin paralleled small ultracapacitors with lithium-ion batteries in digital cameras, cell phones and laptops – and increased battery life-time by 1,5 – 2 times. This is not surprising since all the power load peaks (although very short – but still harmful to the batteries) were dumped by the ultracapacitors.
Chinese company Aowei has already been producing electric city-buses for over four years. The only source of energy for such buses is the utlracapacitor module which recharges at every bus-stop. The overall mileage of these buses counts more than 20 million kilometers.
The German company AEG offered the use of ultracapacitors for smoothing peak loads on the batteries in portable electric drill and other consumer electronics devices. Such drills used to get stuck when encountering harder area because the accumulators would not cope with the extra load. Now the problem is solved at the expense of the ultracapacitors taking care of the extra power needed to drill through harder areas.
The company ELITE manufactures ultracapacitors for car audio equipment. Pleasant sounding of low frequencies with overtones can be hardly achieved by using only conventional accumulators. In this situation the ultracapacitors also help.
Very promising example of the Electric Double Layer Capacitors application is their use with batteries for alternative energy sources such as wind turbines and solar panels.