European Journal of Scientific Research - EuroJournals

Photovoltaic-Stand-Alone Hydrogen System 385
higher rate (generating more hydrogen in a given time), more voltage must be applied. For commercial
electrolysis systems that operate at about 1 A/cm³, a voltage of 1.75 V is required. This translates into
about 46.8 kW-hr/kg, which corresponds to an energy efficiency of 70%. Hofmann’s Voltameter, for
that matter, 12V is applied for a current of between 0.1 to 2.0A [2].
Efficiency expressions for solar hydrogen storing system
In some solar photonic processes, solar photons drive a chemical reaction that stores part of the photon
energy as chemical energy-rich product P (e.g. hydrogen). The efficiency η of such a photoprocess is
defined as
η=
∆GpR
EsA
p
where ∆Gp is the standard Gibbs energy for the energy storage reaction generating product P, Rp is the
rate (mol s -1 ) of generation of P in its standard state, Es is the incident solar irradiance (Wm -2 ) and A is
the irradiated area (m -2 ). The water splitting reaction
H2O � H2 + ½ O2
(2)
is thermodynamically a two-electron process per molecule of hydrogen generated, with ∆Go= 237kJ
mol -1 [3].
In practice, efficiencies are measured using equation (1), where ∆Gp for reaction (2) is 237,200
J mol -1 at 298 K. This requires that no matter what the system, the evolved hydrogen must be collected
and measured volumetrically with appropriate corrections for water vapour if present. The factor RH2 in
equation (1) is the mole per second of pure hydrogen gas produced by the system, where the hydrogen
is generated in its standard state.
In equation (1), EsA is the total light power incident on the system. This factor is replaced with
power input of the battery (i.e. current x voltage).
Also, by Gas Equation the mole of hydrogen generated is
PV
nH2 = (3)
RT
where P is the dry pressure (atm) of hydrogen, V is the volume measured in litre of hydrogen gas
generated, R is the molar gas constant (0.082058 L atm K -1 mol -1 ) and T is the absolute temperature in
K [4].
Also if the hydrogen is generated at a pressure P1 lower than 1 atm (Po), a term RTln(Po/P1) is
subtracted from Gp in equation (1).
ηH2=
∆GpRp
− RT ln( Po
/ P1)
EsA
Experimental Setup
A Stand-alone Solar Hydrogen System consists of a photovoltaic module, an alkaline battery, a
datalogger, a Hoffman Voltameter (Hydrogen Unit) and an inverter. The system (Fig. 1) consists of the
physical structure made of Aluminum angle bar built in the form of movable stand with four wheels.
On this structure is mounted the PV panel, wired through a power regulator to a 12V battery. The
structure makes it possible for the system to be moved out into the sun. The battery is charged during
the day, and can be used for hydrogen production, lighting, PC works, etc. The module is fixed
mounted with tilt-angle of 9º to the horizontal. An Inverter capable of converting 12V from the battery
to 220V AC is mounted to the rear of the stand. This inverter connects directly to the battery, not
through the regulator. In the box are the Datalogger, power regulator and a potential divider. The
(1)
(4)