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Energy Glossary

Battery

A battery is a device which stores chemical energy. The chemical energy can be released as electrical energy.

Basic setup of a battery

Batteries contain the same basic parts:

  1. a system of two chemical elements / compounds
  2. electrodes for the transportation of electrons to the outside of the battery
  3. a separator / membrane between two compartments (depending on battery type)
  4. a case

==PICTODO: setup of battery, OPT: 2, 3 examples==

Primary and secondary batteries

Batteries types are dividet in two different species:

  • Primary batteries:
    Batteries which are produced by the use of chemicals and used only one times by supplying an electrical divice. Think of these batteries as " one way batteries".
  • Secondary or rechargeable batteries:
    Batteries which are produced by the use of chemicals and which can be charged/discharged for several times - namely 100 - 1000 cycles.

Primary batteries are cheaper than rechargeable batteries of the same energy density and special types of primary batteries have high specific energy densities of roughly 700 watt hours per kilogram. (Compare to gasoline which has approximately 12 000 watt hours per kilogramm.) Primary batteries are the solution for devices with very low energy consumption, eg. watches which run meny years with one battery. Another field of application is the supply of devices where a minimum weight is a crucial point, e.g. an initital satellite power supply to unfold the solar panels - after the initial phase rechargeable batteries in combination with photovoltaic cells are responsible for long term power supply.

The best secondary batteries have lower specific energy densities of roughly 350 watt hours per kilogram. But for many applications they are the solution with lower total cost. Especially for mobile solutions like cell phones, mobile computers and in the near future for cars lithium ion batteries are a well adapted solution for power supply. Secondary batteries are called rechargeable batteries or accumulators.

Battery vs. capacitor

Electrical energy can also be stored by capacitors. The main difference is that in a battery the voltage is defined by the compounds used to release the electrical energy. During discharge the voltage decreases only slowly until 80-90 percent of its capacity are used up. A capacitors voltage drops during discharge following an exponential law. Hence a battery is much easier to use in many devices.

==PICTODO Entladung Batterie, Kondensator==

Rechargeable batteries - the limitations

Limited energy density:
The energy density of batteries ist strongly limited by the choice of chemical elements / compounds: Per Atom only a limited number of electrons can be stored. Even the output voltage of a battery limited by the choosen chemistry. This explains that a lead accumulator has a much lower specific energy density (per kilogram) than a lithium ion rechargeable battery. Additionally the lead battery has only 2 volts per cell, the lithium ion battery allows ca. 3.7 volts per cell.

Limited resources / raw materials:
Depending on the used compounds the raw materials might be limited. This affects the pricing and the availability of a battery type. Thus the price of lithium ion rechargeable batteries for electric vehicles might be dominated by the price of the raw materials on the long run. It is recommended to find alternative electrochemically active elements / compounds which are available in vast quantities.

Limitations by the construction:
Each battery needs membranes between the two reactands and structures for the electrodes. The thinner the membrane and the leighter / smaller the electrode structures the higher the specific energy density of a battery. On the other hand membranes and electrodes have to be stable enough to withstand hundreds or thousands of charge / discharge cycles.

Limitations during battery cell manufacturing:
The methods of battery manufacturing has to obeye environmental regulations, economical boundaries and a good quality control. An integrated management of cell production, cell use and cell recycling is mandatory to regain the ressources used for the initially produced cells.

Battery research has the goal to optimize all the listed parameters at the same time. Up to now usable elements / compounds are strongly limited by the laws / facts of the (electro)chemistry. E.g. aluminium is a very interesting element to produce lightweight batteries of high capacity - but many side reactions degrade the electrodes of such (experimental) batteries after a few cycles and destroy the battery.

Future of batteries

  • Fuel cells as refillable primary batteries:
    Fuel cells can be seen as a primary battery which can be refilled. The fuel cell is the reaction chamber consisting of tho electrodes and a separating layer. The chemical is pumped into the reaction chamber. In ordinary primary batteries the chemicals are placed around the electrodes. Fuel cells are an interesting alternative to batteries except for the delicate storage of hydrogen which is used in nearly all modern fuel cell. An interesting development ist the direct methanol fuel cell (DMFC): Methanol is liquid under normal environmental conditions an can be used directly as fuel ("direct methanol").
  • Flow cells - rechargeable batteries with separated chemical storage / electrodes:
    These are investigated, e.g. the so-called vanadium redox battery. These batteries have liquid electrolytes which were pumped through a reaction cell containing the electrodes. The capacity is defined by simple (more or less) electrolyte tanks, the available maximum electrical power is designed by the dimension of the expensive reaction cell. The specific energy density of the above mentioned vanadium redox battery is roughly 20 watt hours per kilogram and much lower than that of a lithium ion rechargeable battery. But the scalability of the vanadium redox battery is much easier and the prices per watt hour stored might be much lower because vanadium can be gained at lower costs and the reaction cell is easier to build.
  • Carbon nanostructure batteries (name given by the author of that article):
    Another interesting element is carbon which is lightweight (12 grams per mol) and available. Nanotubes and other nano-shaped configurations of carbon atoms have interesting properties. Nanotechnology might find ways to shape carbon-compounds to build batteries with higher energy densities at reasonable prices.
  • Capacitors instead of batteries:
  • An alternative to batteries are (ultra) capacitors which do not have the limitations of batteries. They store the electrical energy as an electrical charge on two electrodes separated by an insulating layer. The maximum voltage of capacitors are only limited by the insulation between the electrodes. The area of the electrodes defines the capacity of a capacitor. Both values are limited by the degree of insulation and the package density of the electrode surfaces. A disadvantage of capacitors is the variyng output voltage (see above) but the so-colled switching power supplies might help to solve this issue. These power supplies convert variyng voltages into a single voltage, e.g. the power supply of a mobile computer (100 ... 240 volts ? 19 volts). The capacitor output voltage can be stabilized by such devices and make them a usable power storage device.

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