Environmental Engineering Reference
In-Depth Information
power, i.e. energy per second. The hydrogen fuel cell vehicle has been under devel-
opment for many years and is planned to be commercialized soon, opening the way
to a hydrogen society in the future.
Wider use of renewable energies also requires an appropriate backup energy
source that can compensate for periodic shortages in the renewable energy supply.
Such backup electricity may be generated using fossil fuels, but for localized or dis-
tributed energy systems, fuel cell CHP (cogeneration of heat and power) technology
(which was proposed decades ago) is now emerging as a high-efficiency new tech-
nology using fuel cells. Fuel cells may also be used as a larger-scale centralized en-
ergy system as a part of a 'combined cycle' system with gas and/or steam turbines,
which can achieve the highest ever efficiency of any generator using fossil fuels.
In the following sections, the theoretical background and the technologies of fuel
cells will be described in further detail.
4.2
The Principle of Fuel Cell Power Generation
4.2.1
Comparison of 'Normal' Combustion with Fuel Cells
As the historical means for extracting the chemical energy from fuel, mankind has
taken advantage of combustion whereby the chemical bond changes associated with
the reaction of the fuel with oxygen emit heat (heat of reaction), as a result of differ-
ences in binding energies. In thermal power generation, the heat produced is used to
drive a turbine and generate electricity.
In contrast, in the fuel cell, the fuel and oxygen are separated by a membrane
with ionic conductivity and they react through ions passing through the membrane.
However, with this passage, electrons are accumulated on one electrode so if the
electrodes are not connected to an external circuit, the reaction stops. In order for
the reaction to continue, there is a need for electrons to flow between the electrodes
through an external circuit. The force to proceed with the reaction is translated into
the electrical force (voltage) which drives the discharge of electricity to the external
circuit; thus as the reaction proceeds, energy can be taken out as electric power
(Fig.
4.2
).
This situation can be compared with a system used for lifting luggage by con-
necting to a pulley weight (Fig.
4.3
). Due to gravity, the weight tries to descend (it
tries to cause a reaction just as in the fuel cell), and gravity is converted to a force
to lift the luggage (equivalent to the fuel cell voltage). In theory, the weight can lift
luggage up to the same weight. Therefore, the energy of the weight can be converted
most effectively when the luggage is just as heavy as the weight. In reality there is
a frictional resistance, so if the weight is only a little heavier than the luggage, the
pulley does not move. When the weight is much heavier than the luggage however,
it is lifted quickly, but, the energy conversion efficiency is small and excess energy
is released as heat.
