Batteries and alternatives for powering portable equipment.

Batteries and alternatives for powering portable equipment

Batteries and alternatives

• Primary (non-rechargeable) cells

• Secondary (rechargeable) cells

• Capacitors (short-term charge storage)

• Fuel cells (like batteries but with external source of fuel)

• Solar cells

• Generators

How batteries work:Work function and contact potential

When two dissimilar metals are connected together, a voltage called the “contact potential” is produced. It’s not easy to observe, because however you connect the wires, the voltages cancel out.Contact potential arises because some metals lose electrons more easily than others. Reactive metals (Lithium, Magnesium, Aluminium, Zinc, Iron) lose electrons easily, while less reactive metals (platinum, rhodium, chromium, copper) do not easily lose electrons. The energy needed to remove an electron from a material is called it’s work function.V

Copper

Copper

Iron

Work function

• The work function for a conductor is a measure of how easily it loses electrons.

• The contact potential is the difference between their work functions – so connecting a platinum wire to a sodium wire would give a contact potential of 5.3 – 2.3 = 3.0V

• The work function falls as the temperature is increased – hot metals lose electrons more easily.

Metal Work function

Sodium 2.3 eV

Lithium 2.3 eV

Zinc 3.1 eV

Calcium 3.2 eV

Cadmium 3.7 eV

Aluminium 4.1 eV

Graphite 4.5 eV

Tungsten 4.5 eV

Nickel 5.2 eV

Platinum 5.3 eV

Metal Work function

Sodium 2.3 eV

Lithium 2.3 eV

Zinc 3.1 eV

Calcium 3.2 eV

Cadmium 3.7 eV

Aluminium 4.1 eV

Graphite 4.5 eV

Tungsten 4.5 eV

Nickel 5.2 eV

Platinum 5.3 eV

Periodic table of the elements

Alkali metals very low work function – easily lose electrons

Halogens easily gain electrons

Transition metals e.g. Zinc have a complicated electronic structure

Simple ‘battery’ cell

• The open circuit cell voltage depends on the reaction potentials at each electrode – usually the work function of the metal.

• The current that a cell can provide is limited by its internal resistance

• This is controlled by physical factors – mainly the surface area of plates, their separation, and the conductivity of the electrolyte.

Alessandro Volta

Used disks of silver (4.4 eV) and zinc (3.1 eV) separated by cardboard soaked in brine to make a “voltaic pile”.Each cell in the pile produced about 1¼ volts. By using a large number of cells he was able to build a battery that would give an electric shock.

1.3V

Primary – or - Secondary

Primary cells• Not rechargeable• Working reaction

uses up the electrode material

• Offer high mass-energy densities

• Long shelf life• Not recycled

Secondary cells• Rechargeable• Use reversible

reactions at both electrodes

• Need charging before use

• Need maintenance• Some use cadmium

which is a poison

Primary Cell

• Zinc - carbon

• Zinc chloride

• Manganese Alkaline

• Lithium

Rechargeable cells

• Lead-acid

• NiCad

• NiMH

• Li-ion

• Mn-Alkaline

Battery types comparedType Charge? volts AA cell

capacitySpecific Energy

@ 10 C

Zinc-carbon no 1.5 V 1.1 Ah 30 Wh / kg

Mn Alkaline P’raps 1.5 V 2.5 Ah 80 Wh/kg

Lead-acid yes 2.2 V n/a 30 Wh / kg

Ni - Cd yes 1.25 V 0.8 Ah 50 Wh / kg

NiMH yes 1.25 V 1.8 Ah 80 Wh/kg

Li ion yes 3.6 V 2.3 Ah 140 Wh / kg

UltraCap yes 2.5V n/a 3 Wh / kg

Other power sources for portable equipment

• Thermoelectric generators

• Fuel cells

• Micro-generators

• Ultra-capacitors

• Solar cells

Specific energy is not the only factor to be considered. Many applications need to use the stored energy in short bursts rather than over long periods of time. Electric vehicles is one example.

Seebeck effect - thermocouples

If two dissimilar wires are connected together as shown their contact potentials will cancel out. However, if one junction is heated the work functions change. Now the contact potentials at the two junctions are different, and a net voltage can be measured. This voltage is proportional to the temperature difference between the two junctions, and can be used as a temperature sensor. V

Hot junction

Cold junction

Copper

Copper

Iron

Peltier effect

If the same thermocouple has a current passed through it the reverse effect occurs. One junction heats up and the other cools down.

It’s important to use good conductors because Joule heating (W = V * I) also occurs and this can easily exceed the Peltier effect cooling.

One junction heats up

One junction cools down

Copper

Copper

Iron

Currentflows

+

Thermopile

By connecting together a lot of short sections of wire or semiconductors we can make a ‘thermopile’. This can be used as a Peltier cooler, or a thermoelectric generator. Materials used for Peltier cooling often have relatively low melting points, but materials for thermoelectric generation must have higher melting points.

Thermogenerator

Three parts:

Burner

Thermopile

Cooling

Thermoelectric generators

• Typically 20 W – 500 W• Thermoelectric generators have no

moving parts• Reliable• Cost-effective• Need a supply of fuel• Not very efficient• Produce exhaust

Fuel cells• Work in basically the same way as primary cells• The electrodes are replaced by hydrogen (fuel)

and oxygen (or air) • The fuel cell does not run down or require

recharging as long as there is a supply of fuel and oxidant.

• Early fuel cells were easily damaged by contamination, but are now more reliable.

• Fuel cells are very efficient (85%) and can be used for electric utility applications as well as mobile power sources.

Hydrogen fuel cell diagram

How a fuel cell works

Hydrogen fuel is fed into the "anode" of the fuel cell. Oxygen (or air) enters the fuel cell through the cathode. Encouraged by a catalyst, the hydrogen atom splits into a proton and an electron, which take different paths to the cathode. The proton passes through the electrolyte. The electrons create a separate current that can be utilized before they return to the cathode, to be reunited with the hydrogen and oxygen in a molecule of water.

Hydrogen economy

• Present fuel cells get hydrogen from hydrocarbon fuels.

• “Molten carbonate” fuel cells can use a wide range of fuels including hydrocarbons but are only suitable for high power outputs – 10kW to 2MW working at 650C

• Proton exchange membrane fuel cells are suitable for applications where fast startup is required. Working at 80C they can generate 50kW – 250kW.

Hydrogen is an obvious alternative to hydrocarbon fuels, such as gasoline. It has many potential uses, is safe to manufacture, and is environmentally friendly. Today many technologies exist that can use hydrogen to power cars, trucks, electrical plants, and buildings - yet the absence of an infrastructure for producing, transporting, and storing large quantities of hydrogen prevents its practical use.

Hydrogen economy

Hydrogen economy - opponentsNot surprisingly the H economy has its opponents,

based on the following:• Generating hydrogen requires energy that is

most easily produced by burning fossil fuels.• Hydrogen is most easily produced by cracking

hydrocarbons. • Mining and producing platinum and similar

metals to be used in fuel cells, electrolysis plant etc. uses vast quantities of fossil fuels.

• The energy payback time for platinum in a fuel cell is about thirty years. (March 24 2003)

Gas turbine generators

Gas turbines are very reliable and efficient engines and well suited to operating at a constant speed as required for generating mains electricity (in MW).

They have been used for many years to provide motive power for ships, and electricity at remote sites such as offshore oil rigs.

Recent developments in materials could enable their efficient use for smaller energy demands (50W).

Miniature Gas-Turbine Power Generator

NASA's Jet Propulsion Laboratory, California

A proposed microelectromechanical system (MEMS) containing a closed-Brayton-cycle turbine would serve as a prototype of electric-power generators for special applications in which high energy densities are required and in which, heretofore, batteries have been used. The system would have a volume of about 6 cm3 and would operate with a thermal efficiency >30 percent, generating up to 50 W of electrical power. The energy density of the proposed system would be about 10 times that of the best battery-based systems now available, and, as such, would be comparable to that of a fuel cell.

Gas turbine for power generation

Two-thirds of the rotational energy produced is required to drive the turbines' air compressor stages in order to maintain sufficient air flow through the turbine to support combustion. The remaining one-third of the rotational energy produced is converted to electrical energy.

The working gas for the turbine would be Xe containing small quantities of CO2, O2, and H2O as gaseous lubricants. The gas would be contained in an enclosed circulation system, within which the pressure would typically range between 5 and 50 atm (between 0.5 and 5 MPa). The heat for the Brayton cycle could be supplied by any of a number of sources, including a solar concentrator or a combustor burning a hydrocarbon or other fuel. The system would include novel heat-transfer and heat-management components. The turbine would be connected to an electric power generator/starter motor.

The system would include a main rotor shaft with gas bearings; the bearing surfaces would be made of a ceramic material coated with nanocrystalline diamond. The shaft could withstand speed of 400,000 rpm or perhaps more. The materials and fabrication techniques would be suitable for mass production.

Miniature Gas-Turbine Power Generator (2)

Diagram of gas turbine generator

Miniature Gas-Turbine Power Generator (3)

The disadvantages of the proposed system are that unlike a battery-based system, it could generate a perceptible amount of sound, and, if it were to burn fuel, then it would also generate exhaust, similarly to other combustion-based power sources.

This work was done by Dean Wiberg, Stephen Vargo, Victor White, and Kirill Shcheglov of Caltech and Philip Muntz of the University of Southern California for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Machinery/Automation category.

UltraCapacitors

• Charge & discharge times of 0.3 – 30 sec• Low specific energy but very high specific

power (4kWh per kg)• Presently expensive but only because of

low volume production• Used to supply short term high currents

e.g. for load stabilizing on electric vehicles, where high discharge demand can damage batteries.

Solar (Photovoltaic or PV) cells

• The earth receives 1368 watts on a square meter surface facing the suns rays at the top of the atmosphere.

• At noon at the equator each square meter receives about 1kW.

• The solar energy reaching the ground in the UK (averaged over day and year) is between 2.4 – 3 kWh per square meter.

What does this mean for a designSolar cells only provide power when it is light – so apparatus that uses them almost always uses the electricity from the solar cell to charge batteriesCommercial solar panels used to recharge 12V lead-acid batteries are rated at 5W/15V for a 330 * 330 mm panel. They cost £40They are said to provide typically 140Wh per week (averaged across day & year). Battery and circuit inefficiency would limit the useful weekly power to 100Wh, or 14Wh per day This would be enough to run a 2W LED lamp during the hours of darkness.

Solar battery example applications

• Operating equipment in remote areas

• Trickle charging batteries to maintain charge

• For maximum benefit solar cells must be placed in direct sunlight and oriented in the direction of the sun at noon – i.e. due south, about 60 degrees from horiziontal

PV summary

• Present day solar cells use semiconductor technology. They are expensive, not very robust.

• Novel types of photovoltaic cells will enable pv to be more widely used, more cost effective and more reliable

• Energy payback times (at full output) are now less than 1 year.

Resources

• www.powerstream.com/

• www.webelements.com/

• www.users.pandora.be/educypedia/

• physics.nist.gov/PhysRefData/