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Electrochemical Energy

 

The electrochemical process by which electrical energy is extracted from a mix of chemicals in a galvanic cell is described in detail in the section about Cell Chemistries which also provides links to pages describing the operating characteristics of individual battery types.

 

Primary Batteriea

Detailed considerations of electrochemical theory, battery applications, primary batteries, secondary batteries, battery performance, battery management and battery charging can be found elsewhere on this site.

 

Energy Efficiency

  • Storage Efficiency
  • The storage efficiency of a battery, more commonly known as its energy density, is the amount of energy the battery will store per unit weight. There are wide variations in the energy density depending on the reactivity of the chemicals used in the cell design. Primary batteries only have to be optimised for the discharge process and consequently usually have much higher energy densities than secondary batteries which must be optimised for the charge process as well as the discharge process. Examples are shown in the Battery Cost Table below.

  • Utilisation Efficiency
  • The round trip efficiency of the charge - discharge process of secondary batteries, known as the Coulombic efficiency, is typically around 90%.

     

Energy Costs

Batteries convert chemical energy directly into electrical energy but this is a very expensive way to buy electrical energy. Unfortunately it is the price which must be paid for portability.

  • Primary Batteries

    A typical 1.5 Volt AA alkaline cell retails for around $0.60 and has a rated capacity of 2.8 Ah which corresponds to an energy content of 4.2 Wh. The cost of the energy in a primary cell is therefore about $140 per kWh.

    This is over 1000 times more than the cost of electrical energy supplied by the generating utility to residential (domestic) customers who pay on average $0.10 per kWh (though there could be large variations depending on the location and whether the tariff is for domestic or industrial use).

  • Secondary Batteries
  • Rechargeable batteries only store energy they are not primary energy sources. (That's why they are called secondary cells). Though they are expensive, a high capacity Lead Acid Battery costs $150/kWh and a similar Lithium Battery costs $400/kWh, the cost could be considered to be the capital cost paid for storage capacity. The actual energy stored in the battery is usually purchased from the generating utility for $0.10 per kWh as noted above.

     

  • Battery Price Comparisons

  • To compare the costs of primary and secondary cells we should take into account the number of times the secondary cells can deliver its rated energy content (its cycle life) to calculate a capital cost per cycle and we must add the cost per cycle of the energy used to charge the cells. The table below shows comparative costs of different energy cells.

     

    Corner

    Battery Energy Costs

    Corner

     

    Purchase price

    per kWh

    Energy Density

    Wh/Kg

    Cycle Life

    (Cycles)

    Cost per

    Cycle

    Charging

    Cost / kWh

    Effective Cost

    per kWh

    Alkaline cell

    $140

    175

    1

    $140

    N/A

    $140

    Lead Acid Battery

    $150

    40

    300

    $0.50

    $0.12

    $0.62

    Lithium Battery

    $400

    140

    1000

    $0.40

    $0.12

    $0.52

    Electricity Supply Utility

    $0.10

    N/A

    N/A

    N/A

    N/A

    $0.10

     

    Battery Energy Costs Summary

    • The cost of buying one kWh (a "unit") of electrical energy from a battery source is a thousand times the cost of buying it from a generating utility and if you need the battery as an alternative source of power for AC appliances, the cost of an inverter and its associated efficiency losses of around 10% should also be taken into account.
    • Charging the cell consumes more energy than the 1 kWh that the cell is designed to deliver. This is because around 10% must be allowed for the inefficiency of the charger and another 10% for the coulombic efficiency losses (round trip charge/discharge efficiency loss) of the battery. This adds another $0.02 to the cost of the energy available at the terminals.
    • Expensive Lithium secondary cells provide a lower effective energy cost than the less costly Lead Acid cells because they have a longer cycle life.
    • There is a large weight penalty associated with rechargeable cells compared with single use primary cells which store more energy per unit weight.

     

    Nuclear Batteries

    Nuclear depend on two energy conversions. The batteries use the heat generated by nuclear decay as the energy source in a thermal battery to provide a DC power source.

    See Nuclear Power

     

    Return to Electrical Energy Overview

     

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