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  Fuelcell: introduction
 

Introduction l

Direct Methanol l

Alkaline l

Proton Exchange Memb. l

Solid Oxide l

Molten Carbonate l

Phosphoric Acid l

   

 

Fuel cells are electrochemical devices similar to batteries that directly convert chemical energy of a fuel into electrical energy and heat. However, unlike batteries, they oxidise externally supplied fuel.

Each fuel cell comprises an electrolyte – an ionic conductor – and two electrodes (the positive anode and negative cathode) which are essentially electronic conductors.

The nature of the ion transfer depends on the specific fuel used and type of cell; the figure to the right represents the behaviour of hydrogen in a PEMFC.

Operation of a Proton Exchange Membrane Fuel Cell (PEMFC)

The fuel (e.g. hydrogen , in the case presented above) is fed to the anode of the cell where it splits into its constituents: a proton and an electron; the former passes through the electrolyte and the latter is forced around an external circuit where it drives a load. The proton and electron combine with oxygen from the air at the cathode, producing pure water and a small amount of heat.

Since the fuel is not burned in a flame but oxidised electrochemically, fuel cells are not constrained by the fundamental law that governs heat engines, the so-called Carnot limit, which specifies the maximum theoretical efficiency that a heat engine can reach.

Fuel cell’s efficiency increases with part-load, as can be seen on a typical performance curve of a PEMFC in the Figure below.

Characteristics of a PEM fuel cell (Kartha and Grimes, Physics Today 11 (1994), p. 54, Fig. 3). (a) The voltage as a function of the current density, commonly referred to as the performance curve. The efficiency is proportional to the voltage; it is indicated on the secondary vertical axis. (b) The power density as a function of the current density. (c) The efficiency as a function of the power density. Note that the efficiency increases with part-load. The dotted line corresponds to the regime above maximum power. Curves (b) and (c) have been calculated from (a). (d) The efficiency of a complete fuel cell system in a vehicle, as a function of power load (projection by General Motors - Energy Economist December 1994). The same is shown for an ICE. The vertical dotted lines indicate average loads in a car (left) and a bus or truck (right). The curve in (d) does not refer to the same fuel cell as in (a) to (c).

There are several different types of fuel cells (FC):

  • proton exchange membrane fuel cell, PEMFC (also known as solid polymer, SPFC)
  • direct methanol (DMFC; this is essentially a SPFC that runs on methanol rather than hydrogen)
  • phosphoric acid (PAFC)
  • molten carbonate (MCFC)
  • solid oxide (SOFC)
  • alkaline (AFC).

They differ in their operating characteristics, temperatures, power densities and therefore in their most suitable end uses. The following table summarises some characteristics of these fuel cell types and their most suitable uses.

Fuel cell type

Operating temperature [°C]

Module size range [kW]

Suitable applications

Domestic Power

Small-scale power

Large-scale cogeneration

Transport

Battery replacement

AFC

60 – 90

<1 – 200

PEMFC

80 – 100

<1 – 500

PAFC

200

5 – 500

NA

MCFC

650

250 – 5000

SOFC

800 – 1000

5 – 5000

NA

For further information contact David Hart (email: firstname.lastname@e4tech.com) from E4tech

 

 
 
 
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