Energy Storage Technologies

Energy storage technologies include:

Utility battery storage.

Ultra capacitors.

Flywheel storage.

Superconducting magnetic storage.

Compressed air energy storage.

Pumped hydropower.

Storage devices helps to power the vehicle taking some the energy requirement off the engine thus allows the engine to run more efficiently, using less fuel and producing fewer emission.

Hybrid vehicles require additional energy storage element to cater to the power flows and load leveling, mainly during start up, acceleration and deceleration.

Power Management diagram below shows how the intended energy storage unit is connected for a combined ultra capacitor and a battery energy storage system for a fuel cell powered hybrid vehicle. The load can be shared by all the sources at any given point of time.

Batteries and Ultra Capacitors

A combination of the two is a best approach, combining excellent power performance of the ultra capacitors with the greater energy storage of a battery providing a fast acting energy storage device.

Batteries and ultra capacitors have different properties which can prove advantageous when combined together.

In order to reduce the cost of the battery, current needs to be decreased and stabilized. Ultra capacitors can supply large burst of current for a short time but cannot store much energy. Conversely a battery can store large energy. Combining both of them can thus yield the peak current characteristics desired by the vehicle. This has several benefits:

Capturing regenerative breaking energy.

Enhancing fuel economy.

Lowering cost per unit power.

Cutting down the dependence on natural oil.

Ultra capacitor/super capacitor

Ultra capacitor is an electro chemical capacitor that has an unusually high energy density when compare to common capacitors. They are of particular interest in automotive application for hybrid vehicles and as supplementary storage for battery electric vehicles.

Ultra Capacitor cell is a laminated type comprising two layers of propylene-polycarbonate, electrolyte impregnated activated carbon electrodes, sandwiched between aluminum collector plated and separated by a separator plate.

The capacitor is a physical storage device versus chemical batteries. It stores energy electro statically by polarizing an electrolytic solution in an electric field and there is no moving part and no chemical change of state (chemical reaction).

Ultra Capacitors are also known as electric double layer capacitors. Each electrode has two layers of charge. It is actually a pair of capacitors in series.Applications:

Automobiles Drive trains applications like recapture of braking energy, launch assist, stop and go systems, power initial acceleration, to operate electrical sub systems.

The best use for U caps are strategies that make engines operate more efficiently (idle off, load leveling)

Powering of the on board electronic features, and back up for memory functions

Used for energy storage for solar panels

To reduce electrical system current, which adversely affect battery life.

Power source for systems that does not require power bus to be active e.g. watches, display light, alarms etc.

Used to provide the increase range and short-term power.

Systems that require uninterrupted energy such as medical systems, computers, telecommunication.

Advantages. Such energy storage has several advantages compared to batteries particularly for automotive applications needing short high power charge and discharge bursts:

Deliver up to ten times the power.

Very high cell voltage possible, voltage determined by circuit application, not limited by the cell chemistry.

Simple charging methods

Very high rate of charge and discharge

Little degradation over hundreds of thousands of cycles

Good reversibility

Low toxicity of material used

High cycle efficiency (95% or more)

Last up to ten times as long.

Operates more reliably in high and low temperature conditions.

Requires far less maintenance.

Reduces environmental issues associated with battery disposal.

Is inherently robust.

Reliable, last longer, more efficient.

Can be charged extremely quickly and can be cycled hundreds of thousands of time.

Provides quick short bursts of power during peak demands offering better response vs a battery, preventing severe drain on battery power. In this way:

Serves as a buffer for rapid charge/ discharge that occurs in urban driving.

Can bring major benefits, especially in battery cycling (Less frequently batteries are cycled smaller they can be).

Can put out more power for a given weight.

Can function at more extreme temperature.

In applications where power determines the size of energy storage device, an ultra capacitor may be a better solution.

Cars which have stop-and- go operation:

Ultra capacitor and a powerful starter motor will instantaneously jolt the engine back to life on signal light.

Helps in regenerative braking- converting into electricity the kinetic energy and storing that electricity in the ultra capacitor.

Disadvantages:

Power density inferior to latest type batteries

More expensive.

Adds weight and bulk to the system

Linear discharge voltage characteristic prevents use of all the available energy in some applications.

Power only available for a very short duration.

The voltage varies with the energy stored. To affectively store and recovers energy requires sophisticated electronic control and switching equipment

High self discharge- the rate is considerably high than that of an electrochemical battery.

Since the capacitor is normally connected in parallel with the battery in these applications, it can only be charged up to the battery upper voltage level and it can only be discharged down to the battery lower discharge level, leaving considerable unusable charge in the capacitors thus limiting its effective or useful energy storage capacity.

Fuel Cell

A fuel cell is an electrochemical energy conversion device. A fuel cell converts the chemicals hydrogen and oxygen into water, and in the process it produces electricity.

It is an electricity producing system that directly converts energy stored in a fuel into electric energy without the process of combustion.

Fuel cells are usually classified by the type of electrolyte they use. Some type of fuel cells work well for use in stationary power generation plants. Others may be useful for small portable applications or for powering cars.

There are several types of fuel cell technologies possible for commercial uses:

Alkaline fuel cell (AFC). It is very susceptible to contamination, so it requires pure hydrogen and oxygen. It is also very expensive, so this type of fuel cell is unlikely to be commercialized.

Phosphoric-Acid Fuel Cell (PAFC). It operates at a high temperature than PEM fuel cell, so it has a longer warm up time. This makes it unsuitable for use in cars but has potential for use in small stationary power generations. Solid Oxide Fuel Cell (SOFC). Best suited for stationary power plants. It operates at a very high temperature (10,0000C), which causes reliability problem, but steam produced by the fuel cell can be channeled into turbines to generate more electricity. Molten Carbonate Fuel Cell (MCFC). They are also suited for stationary power plants but operate at 6000C, do not need exotic materials and makes the design less expensive. Proton Exchange Membrane Fuel Cell (PEMFC). Is one of the most promising technologies that will end up powering cars, buses and may be even houses. It uses one of the simplest reactions of any fuel cell. It operates at a fairly low temperature (800C), which means they warm up quickly and do not require expensive containment structures.There are four basic elements of PEMFC.

The Annode, the negative post of the fuel cell. It conducts the electrons that are feed from the hydrogen molecules so that they can be used in an external circuit.

The Cathode, the positive post of the fuel cell. It conducts the electrons back from the electron circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water. The Electrolyte is the proton exchange membrane. It only conducts positively charged ions but blocks electrons. The Catalyst, it facilitates the reaction of hydrogen and oxygen.Problem with Fuel Cells

Expensive

Hydrogen is difficult to store and use.

High cost of hydrogen production.

Absence of hydrogen infrastructure.

Reliability problems.

Advantages.

Offers a means of making power more efficiently and with less pollution.

Most promising technology that is suitable for transportation applications.

High power density.

Low temperature operation.

Low manufacturing cost.

No moving parts.

Applications.

Base load power plants.

Electric and hybrid vehicles.

Auxiliary power.

Off-grid power supply.

Notebook computers for applications where AC charging may not be available for weeks at a time.

Portable charging docks for small electronics.

Comparison of Powering sources

Fuel Cell I.C. Engine Battery

80% efficient Overall 20% 90%

30-40%

40% for

with reformer

power plant

Motor/ invertor 90% for charging

Efficiency 80%

the car

Overall Motor/Invertor

24-32%

80%

Overall 26%

Efficiency is not the only consideration. The other considerations are:

Is the car quick and easy to refuel?

Can it travel a good distance before refueling?

Is it as fast as the other cars on the road?

How much pollution does it produce?

This list, of course, goes on and on. In the end, the technology that dominates will be a compromise between efficiency and practically.

Supercapacitors, Ultracapacitors or EDLC (Electric Double Layer Capacitors)

They have double layer construction consisting of two carbon electrodes immersed in organic electrolyte. See below:

During charging, the electrically charged ions in the electrolyte migrate towards the electrodes of opposite polarity due to the electric field between the charged electrodes created by the applied voltage. Thus two separate charged layers are produced. Although similar to battery, the double layer capacitor depends on electrostatic action. Since no chemical action is involved the effect is easily reversible and the typical cycle life is hundreds of thousands of cycle.

Hybridizing Energy Storage

(Ucap + Battery)

Advantages:

Reduced battery currents

Reduced battery cycling range

Increased battery cycle/calendar life

Increased combined power and energy capabilities

Lower cooling requirements

Better low temperature performance

Disadvantages:

Complex control strategy

Larger volume and mass

Need for electronics for each system

Increased energy storage cost

Any need for DC/Dc converters adds even more cost and complexity

Strong attributes of Ucaps

Specific use High specific power & efficiency:Engine assist

Efficient and fast charge acceptance:Regen capture

Low resistance

: Lower cooling needs

Quick response

: Supporting engine transient

Long calendar & cycle life: Fewer replacements

High specific power at

: Smaller size and

low temp. (cold start). less expansive

Weak attributes of Ucaps

Low specific energy : Limited durations for power

draw

High self-discharge: Loss of functionality & balance

at atart

Quick voltage variation: More difficult to control

Low energy density : Limited time for running

auxiliaries at idle

High cost per unit energy : Too expensive currentaly