Energy Storage: From Gasoline to Ultracapacitors

Energy cannot be created or destroyed.  Scientists have accepted this theory of conservation of energy for ages[1].  But this theory seems to be in juxtaposition with the conventional thought of energy being a scarce resource.  If energy cannot be created or destroyed, why are we always scrambling to find new sources of energy?  While energy cannot be created or destroyed, it can change form.  And there are only certain forms of energy that we can practically harness for use.  One of those forms is potential energy[2].  How we work with potential energy, transport it and harness its potential, is an area of significant evolution in science.

Gasoline

Various types of instruments that we refer to as fuel contain chemical potential energy in molecular bonds.  For the purpose of this discussion we will focus on gasoline.  When we burn fuel in an automotive engine we are transforming the chemical potential energy stored in molecular bonds in the gasoline into kinetic energy, and using that kinetic energy to move the car in the direction that we want to travel[3].   This kinetic energy is later converted to other forms of energy.  For example friction between the tires and the road, or between the brake pads and brake rotors when the driver applies the brakes, converts much of the kinetic energy to thermal energy, or heat.  While the energy has not been lost by our universe, it has been converted to a heat in a fashion that makes it essentially unusable to us.  If the driver wishes to continue his journey, more fuel must be converted to kinetic energy to continue to propel the vehicle.

Fossil fuels contain incredible amounts of potential energy.  In this respect, we could place fossil fuels into a larger category of instruments that we will refer to as energy storage instruments.  Gasoline stores energy in chemical bonds ready to be converted to a usable form at our request.  Gasoline is a very good means of storing energy.  It contains incredible amounts of energy in a compact package.  Further, its ingredients contain this energy when they are extracted from the earth; fossil fuels do not need to be “charged” with energy, they come pre-energized.  The fact that gasoline (1) comes pre-energized and (2) contains so much energy in such a small package, has made it a favorite means of portable potential energy in our world.  The problem with automotive gasoline as a means of energy storage is that we do not have practical means of recharging it.  That is, we cannot efficiently create gasoline.  We can efficiently extract its ingredients from the earth, efficiently store it and efficiently convert its potential energy to a usable form of energy[4].  But once we convert its potential energy to a usable form, all that is left of the gasoline is exhaust.

Batteries

Batteries also store energy chemically, but in a different way than gasoline.  Without getting into the nitty gritty batteryscience of how batteries store energy, a primary advantage of many types of batteries is that they can be recharged.  However, these batteries must all be charged with energy in the form of electrical energy.  This requirement can create inefficiencies.  For example, to the extent that the batteries are recharged using electricity from fossil fuel burning electrical plants, the energy captured in electrical form from that plant represents approximately only 40% of the energy in the fuel[5].  This is due to inefficiencies in the process of converting chemical potential energy in fossil fuel to electrical energy.

But the advantages inherent in the ability of batteries to be recharged can help make machines more efficient by capturing energy that would otherwise be lost.  For example, hybrid vehicles use batteries (and capacitors, which will be discussed shortly) in a variety of ways to extract the most kinetic energy out of each gallon of gas.  The hybrid’s internal combustion engine converts the fuel into kinetic energy.  However, when you apply the brakes on a hybrid, instead of clamping brake pads on rotors and thus converting virtually all of this kinetic energy to heat, the hybrid uses regenerative braking to recapture some of that kinetic energy and put it back into the battery as electrical potential energy.  This energy can be reused later by using power from the batteries to spin an electric motor, thereby once again converting this energy to kinetic energy.

However, the chemical construction of batteries creates some limitations.  Batteries can only endure a certain number of charge cycles before they lose their ability to hold a charge.  Additionally, the chemicals in batteries have a limit on the speed with which they can absorb a charge or expel a discharge.

Capacitors

Capacitors are another means of storing electrical energy.  They work by storing energy between two conducting plates separated by a dielectric, non-conducting substance.  The dielectric prevents the charge from moving directly between the plates, instead forcing the charge to travel to whatever device is connected to the plates; for example a light bulb.  As opposed to batteries that store energy in a chemical form, capacitors store energy in electrostatic form.  When the capacitor is charged, the charged plates attract each other and an electrical field is present between them.  To some extent when we see a bolt of lightning, we are watching a capacitor at work.  There are two conducting plates: (1) the ground and (2) the clouds.  And they are separated by a dielectric non-conducting substance:  air.  But for our purposes, capacitors are usually much smaller and contain much smaller charges.

Capacitors have the following advantages over batteries:

  1. High Charge and Discharge Rate: Capacitors can absorb and expel charge very quickly.  For this reason, they are very useful in applications that require a fast burst of energy like a camera flash.  The ability of capacitors to act rapidly also makes them useful for smoothing out electrical currents that contain erratic voltage levels.
  2. Longevity: Since, unlike batteries, capacitors contain no chemical reactions, they can endure significantly more charge and discharge cycles than batteries before they start to degrade.  Capacitors can endure roughly a thousand times more charge cycles than batteries.[6]

Both of the above applications make capacitors uniquely suited for certain applications.  For example, regenerative braking systems in hybrid automobiles often make use of capacitors because they can (1) absorb the sudden high electrical current that occurs when the regenerative braking is engaged and (2) can handle the many charges and discharges that occur in the system.

However, capacitors can only store energy on the surface of the conducting plates.  The electrical storage capacity, or capacitance, of a capacitor is thus proportional to the surface area of the plates[7].  As a result, they tend to have low energy density, or low energy capacity for their size.  This makes them impractical for applications that involve large amounts of energy and limited space.

Ultracapacitors

Ultracapacitors (also referred to as supercapacitors or electric double-layer capacitors) modify the conventional construction of capacitors to yield a higher energy density.  The effect that gives rise to this revised construction was discovered by engineers at General Electric in 1957.

Instead of utilizing flat plates like conventional capacitors, ultracapacitors utilize two layers of porous electrodes, suspended in an electrolyte solution, separated by an extremely thin dielectric.  Ultracapacitors thus have two features that serve to increase the energy density:

  1. The thinness of the dielectric makes the unit as a whole more compact.
  2. Since the electrodes are porous, instead of smooth plates in conventional capacitors, they have increased surface area.  This increased surface area increases the capacitance without increasing the size of the entire unit.

Ultracapacitors combine the advantages of conventional capacitors (ability to endure many charge cycles and high charge/discharge rate) with high energy density.  They are thus able to provide the benefits of a capacitor in a more compact package.

Energy Density

Energy Density is arguably the most important required improvement for future success of ultracapacitors.  Currently, ultracapacitors are not yet compact enough to be a practical universal replacement for batteries.  However, if the energy density is high enough, ultracapacitors could replace batteries in numerous applications.

The following compares the energy density of some modern ultracapacitors and other forms of energy storage.

Device

Energy Density in WH/kg

(Watt Hours per Kilogram)

Existing commercially available ultracapacitors

0.5 to 10 WH/kg

Experimental ultracapacitors from MIT LEES

30 to 60 WH/kg

Experimental ultracapacitors from EEStor

200 to 300 WH/kg

Lead-acid battery

30 to 40 WH/kg

Lithium-ion battery

120 WH/kg

Automobile gasoline

12,000 WH/kg

Much work is being done to improve the energy density of modern ultracapacitors.  For example, to the extent that future technology may allow the porosity of the electrodes to be increased, the surface area of the electrodes could be increased and thus the energy density of ultracapacitors could increase.

Power Density

In addition to energy density, another measure of the performance of an energy storage device is power density.  Power density is a combination of the energy density and the rate at which power can be expelled by the device.  Capacitors (ultracapacitors included) charge and discharge at a rate several orders of magnitude greater than batteries.  As such, while capacitors might have lower energy density, their power density is many times greater than batteries.

Self Discharge

One of the challenges facing capacitors is their propensity to self discharge.   When a capacitor (or ultracapacitor) is charged, and then left in an open circuit state, it tends to lose charge through the dielectric and the surrounding air.  This propensity to self discharge reduces the practicality of capacitors in certain applications, such as a battery in a cell phone or as a replacement for batteries in a hybrid vehicle as the primary non-gasoline source of power.  Modern ultracapacitors will discharge to 50% of their original charge in approximately 30 to 40 days.  By comparison, a nickel-based battery will lose only 10% of charge during that same time period.  However, ultracapacitor research may produce capacitors with lower self discharge rates.

Voltage Delivery

Batteries tend to deliver fairly stable voltages as they discharge.  By comparison,  the voltage delivered by an ultracapacitor decreases linearly with discharge.   The graphs below illustrate the difference between ultracapacitor and battery voltage during charge/discharge.

capbatvolt1

During discharge an ultracapacitor will tend to fall below a given threshold voltage quicker that its battery counterpart.  A DC-DC converter can be used to stabilize the voltage output, but comes at a cost of additional weight and complexity and reduced efficiency.

The Future

Ultracapacitors offer significant possibilities for the future of energy storage.   Capacitors are already utilized in large scale applications such as hybrid vehicles.  They could also have significant applications in storing electricity produced from wind, solar and other sources.  Operators of fuel burning power plants can choose to easily control how fast fuel is converted to electrical energy, thereby throttling production to meet the demand of end users.  However, electrical output from many alternative energy sources such as solar, wind and certain types of hydro electric plants cannot be as easily throttled.  Capacitors, with their ability to quickly absorb and expel charges, can serve to smooth out power flow from these kinds of alternative energy facilities; absorbing energy when power production exceeds power demand, and expelling energy when power demand exceeds production.  As the technology improves, we may see ultracapacitors with higher densities and lower leakage opening the door for ultracapacitors to begin to replace batteries as the conventional method of providing for portable storage of electricity.  Ultracapacitors would offer:

  1. Faster charge times.
  2. Longer life due to greater endurance of charge/discharge cycles.
  3. Greater environmental friendliness due to reduction of toxic chemicals and less waste due to longer life.
  4. Increased safety due to significantly lower propensity to heat during use.

Will ultracapacitors ultimately replace the age old and proven battery technology?  Ultracapacitor research may provide an answer to that question in the coming years.  Until then we can dream of the day when we can charge our cell phones and laptop batteries in a matter of minutes and batteries outlast the devices that they power.


[1] Einstein’s special theory of relativity modifies the theory of conservation of energy slightly.  But the special theory of relativity only becomes relevant in certain special cases, such as when dealing with speeds approaching the speed of light.

[2] Potential energy can come in various forms including gravitational, electrical, chemical, elastic and thermal.

[3] A good part of the energy released in the combustion is also converted to heat; a form of energy that generally goes unused by the automobile.  Some of this heat energy could be used to heat the passenger compartment, but it generally does not contribute to forward motion of the automobile.

[4] Generally kinetic energy, or in some applications heat.

[5] For example, a fuel cell vehicle operates at about 72% efficiency between the potential energy in the battery and the kinetic energy ultimately experienced by the vehicle.  However taking into account the 60% loss in converting energy in fossil fuels to electrical energy, and approximately 10% loss in charging, fuel cell vehicles are only 26% efficient when powered by electricity harnessed from fossil fuels.

[6] Batteries can endure somewhere around 200 to 1000 charge cycles while capacitors can endure between 500,000 and millions of charge cycles.

[7] The formula for calculating capacitance of a capacitor is: C = eA/d Where: C = capacitance,e= permittivity of the dielectric, A = plate area and d = plate separation.

2 thoughts on “Energy Storage: From Gasoline to Ultracapacitors”

  1. Jacob:I am very impressed. I had no idea you were so scientifically inclined. I found this more helpful than Wikipedia in terms of understanding ultra capacitors. So many hidden talents. I like your blog. See you Tuesday,Irwin

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