ELECTROLYSER

A Technical Appraisal of Electrolyser Characteristics.

1. INTRODUCTION

The excess of electricity generated by the PV array is used to produce hydrogen from water by electrolysis.

With the electrolytic method it is possible to obtain highly purified hydrogen and oxygen appropriately separated in an electrolyser.

2. THERMODYNAMICS OF WATER DECOMPOSITION

Passing electric current through the phase-boundary cathode-electrolyte and anode-electrolyte carries out the electrochemical decomposition of water. Different electrochemical reactions occur at each electrode and water is decomposed into oxygen and hydrogen. The overall reaction for water electrolysis is:

The reaction in the cathode is the reduction (electron acceptance) of water molecules and transformation into hydroxyl ions with the evolution of hydrogen. The overall cathodic reaction is:

At the anode, the oxidation of OH- ions (giving up electrons to the electrode) takes place to form water molecules and oxygen. The overall anodic reaction is:

The ideal minimum energy required is equal to the reaction enthalpy, DH, and it has to be introduced in the system in the form of electrical energy, according to the thermodynamic equation:

The amount of energy that has to be introduced from outside the system is the combination of the free energy, DG, necessary to make the reaction occur, and Q=TDS. If DG is introduced in the form of electricity, then the process that takes place is endothermic, because of the additional amount of energy required is TDS. Above thermoneutral energy, (DG + TDS), the reaction takes place exothermally, and the excess of heat is given off. The following figure shows the relationship of the factors stated above.

Fig. 1. Energy vs. Temperature for electrolyser

The minimum electrical energy needed from theoretical values at T=25 C is 2.94 kWh. If the temperature is increased to 100C, the energy input is reduced to 2.79 kWh. The electrochemical efficiency of electrolysis is calculated from the ratio of the energies theoretically necessary to those that have to be used.

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