There is an advantage for having a bank of capacitors to act as a short term storage in conjunction with the use of a battery as a more longer term form of electrical storage. The capacitors could be charged up very quickly, in seconds or minutes, and then the energy in those capacitors could be used to charge up the battery cells which typically would take hours. A bank of capacitors could be used to charge (and maybe discharge) a number of battery cells. Rather than having to charge a battery with a continuous supply of electricity it could be charged piecemeal over time incrementally. It it were well designed the charging process could be optimised for the particular battery. This approach may be useful for electric vehicles in certain circumstance, depending on the storage capacity of the bank of capacitors. It may also be useful for portable energy units, perhaps in a similar way to a Jerry can of petrol, except with a much lower energy density. Being able to at least partially charge such a portable device in a very short amount of time would be a significant improvement. As stated before, capacitors can not hold that much energy but they can be charged and discharged very quickly and very efficiently. Perhaps to fully charge a battery a series of energy packets (for want of a better term) where the associated capacitors are fully charged might be needed. This would still leave the portable battery pack free to be moved around while it is being charged with the energy in the capacitors. Perhaps a bank of storage units with cascading levels of battery charge could be optimal. It all depends on the design and physical possibilities that are viable.
From 24 November 2005 Lighter Energy Storage?
Nearly three years ago…
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And in yesterday’s Age an article by Al Gore: The challenge for America’s leaders
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22 July
One idea for these kinds of storage units would be to set a weight and rough size for the unit, for example weighing maybe 20kg and being relatively easy for a fit person to carry like a tin of petrol, and then see what the best design given these kinds of constraints would produce. Rechargeable batteries would be likely take up most of the weight while, perhaps as a rough rule of thumb, the banks of capacitors could maybe be about 1 to 1.5 times the storage capacity of the batteries. There would need to be controlling circuitry to maintain voltage levels while the capacitors discharge and to optimally charge and discharge the batteries. Then there would have to be the interfaces of the unit with the outside world to consider, including when the unit is plugged into the mains, inductive charging if feasible, options for linking up units and directly using the stored energy to power appliances or EVs. Slight cooling might also preserve the charge in the batteries a little longer. The energy density for such units could be compared by testing how much energy such units can hold for a certain amount of time, if people agreed to constraints on the weight and volume of these units. That stored energy would include the storage capacity of the fully charged batteries as well as the storage capacity that could be held in the capacitors after the batteries are fully charged. What kinds of parameters would make such units useful?
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23 July
Imagine driving an EV to a service station, paying with a credit card through a wireless Internet connection, moving the vehicle over to a designated energy download zone where the energy just purchased could be transferred to the EV’s storage units in a matter of minutes, and then driving off. Perhaps the batteries could be managed so that they independently go through gentle cycles of fully charging and discharging in turn. The batteries would age and hold less energy with over time and may have to be replaced. The number of independent storage units and how they are configured might become a way to characterise EVs. The storage units will take up a lot of space. As stated in a previous post, the wide scale uptake of plug-in electric cars would be an environmental disaster if the addition amounts of energy needed by these vehicles was not met through newly installed renewable energy sources. Rolling out wind and solar renewable energy sources would have to be done in tandem with selling EVs. I think there may have to be regulatory requirements that balance plug-in EV use with installations of renewable energy sources. Without such measures the use of coal in power stations may accelerate with the wide uptake of hybrid and the plug-in EVs, and electricity prices may become inflated for the whole network – a situation which most people, although clearly not all, would find unacceptable.
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25 July
Another point is worth mentioning again and that is the fact that stored electrical energy is of higher quality than stores of thermal or thermodynamic fuels. The energy density of a tin of petrol would be much more than that of an electrical storage unit as outlined above. A large percentage of the energy in the thermodynamic fuel, however, will be lost in a heat engine that changes the chemical energy into another form.
The efficiency of various heat engines proposed or used today ranges from 3 percent [1](97 percent waste heat) for the OTEC ocean power proposal through 25 percent for most automotive engines, to 45 percent for a super critical coal plant, to about 60 percent for a steam-cooled combined cycle gas turbine.
While petrol powered car engines can have an efficiency of around 25 percent, electric motors can have efficiencies above eighty percent. There are obviously also many more things that could be done with electrical energy and one of the main purposes of burning fuels is to run generators anyway. This is all very basic but it is worth noting that a raw comparison of the energy densities of fuels as compared with electrical storage units may be misleading to an extent. Of the energy contained in a can of petrol only about a quarter will translate into useful work, while you could count on at least three quarters of the energy in an electrical storage unit being put to good use. Measures of energy density alone do not take into account the quality of the stored energy. You wouldn’t consider a plate piled high with fried potato wedges and topped with a liberal amount of lumpy gravey as equivalent to a three course meal in a nice restaurant, or most people wouldn’t anyway.
Perhaps there could be a metric to compare energy densities that is weighted with regard to the quality of the energy and the efficiency with which that stored energy can be changed into useful work. As a rough rule of thumb, going off the percentages in the last paragraph, energy densities of stored electrical units could be scaled by a factor of three to four when compared with fuels. There would probably be better metrics to develop and use. The point is that by comparing raw energy densities with alternatives, fuels will appear to be better than they are in practice, basically because most of the stored energy in fuels can only be converted into thermal energy. That’s a point about the efficiency of heat engines.
Another way to think about it the next time you fill a tank of petrol at a bowser is that, yes, this is really expensive and yes, all of this fuel will be burnt with CO2 being one by-product that will effect global warming, and you know what – a full 75 percent of the energy in this fuel going into this tank will go to waste as nothing more than hot air and heat. Three quarters of the energy will be wasted no matter what because it is a heat engine.
With renewable sources the processes for extracting energy from sunlight or wind typically have low efficiencies as well. Once that sunlight or wind is converted into electrical energy, however, the electrical energy can be converted into other types of energy – for example accelerating an EV – at a very high level of efficiency. Large amounts of electrical energy can also be moved around a grid efficiently and cheaply. This contrasts with the logistics of having to cart physical fuels around in tankers to every petrol station and then having to carry the weight of the fuel in the vehicle. Fuels are wasteful at every stage of the fuel cycle. With a transport system of Electric Vehicles the inefficiencies are only in the stages before electrical energy has been produced, not throughout the whole system.
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27 July
But wait, there’s more…
You also have to consider the equipment needed to extract the useful work out of the stored energy. With petrol as the energy storage medium you need a motor engine to turn (a small fraction of) that stored energy in the fuel into rotational (mechanical) energy that can accelerate the car. Motor engines are heavy and have many components, subsystems and moving parts. An electric motor, by contrast, is relatively lighter and compact and simpler in design. They are also much more efficient. With electric vehicles you could also have one electric motor per wheel and the motor/generator could be configured so that regenerative braking can also be used. This is already incorporated in current hybrid electric vehicles.
The point here is that due to the weight, size and needs of a petrol engine, the chassis of the whole vehicle has to be designed to deal with the structural constraints that having an on-board petrol engine involves. Cars without an engine but with electric motors as part of the wheels’ systems could be designed and built lighter than petrol motor vehicles. Lighter composite materials rather than steal could make up the bulk of the mass for EV cars. It would also depend on developing light weight high capacity batteries.
Passenger safety is obviously a major concern. I don’t know whether lighter, perhaps more elastic, vehicles would protect passengers better than the heavy inelastic cars we have now. Perhaps in a high power collision EVs could be designed to cushion the passengers with airbags and then more or less bounce around till most of the energy has been dissipated. It would work if all the other vehicles responded in similar way – but it might not work if a lighter vehicle fails to bounce out of the path of a heavy vehicle. It would not work at slower speeds when pedestrians might be near. In that case the surface of the EV might best be highly resistive to the road surface. Perhaps the chassis could be elastic with the outer panels being more grainy. It might all sound a bit unusual now. You could imagine the outer panels of EVs being replaceable and available with custom designs, if that takes off as a fashion statement.
The energy needed to accelerate anything is directly proportional to the mass of that object. Lighter EV cars would need less energy than current petrol engine cars, including current electric hybrids, to travel the same distances in similar amounts of time. Regenerative braking also reduces the energy storage requirments for an electric vehicle. Not only is a petrol engine inefficient in that about 75 percent of the energy in petrol is lost as waste heat, but every time brakes are used in a petrol engine car (apart from in hybrids) that twenty five percent of the energy in the petrol that accelerated the vehicle is irretrievably lost as heat in the car’s brakes. In the stop and start traffic conditions of major cities the efficiency of petrol engine cars must be reduced to incredible low – probably single digit – percentages. Petrol powered cars are incredibly wasteful. And with regard to stopping at traffic lights, electric motors do not need to be idled. A stationary EV will require no more energy than whatever the passengers use for comfort like air conditioning, media, etc.
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Continuing the Discussion