The efficiency of single-cycle fuel cells range from 47% to 50%; the efficiency of combined-cycle fuel cells is about 60%, and if the generated heat is also recovered (in the form of hot water or steam), their total efficiency could be around 80%. (The efficiency of gasoline engines is around 25%, of nuclear power plants about 35% and of sub-critical fossil fuel power around plants 37%.)
In the PEM design, the membrane electrode assembly consists of the anode and the cathode that are provided with very thin layers of catalyst bonded to either side of a proton exchange membrane. With the help of this platinum catalyst, the hydrogen at the anode splits into a proton and an electron while oxygen enters at the cathode. When hydrogen reaches the catalyst layer, it separates into protons (hydrogen ions) and electrons. The protons (See Figure) pass through the electrolyte while the free electrons are conducted in the form of a usable electric current- through an external circuit.
At the cathode, the electrons combine with the oxygen in the air and with the hydrogen protons that migrate through the proton exchange membrane to produce water and heat. Air flows through the channels to the cathode. The electrolyte can be a solid polymer, while the electrodes can be made of porous carbon and the catalyst of platinum. To obtain the desired amount of electrical power, individual fuel cells are combined to form fuel cell stacks. Increasing the number of cells in a stack increases the voltage, while increasing the surface area of the cells increases the current. It takes only a few seconds for cold fuel cells to start producing electricity.
The Reversible Fuel Cell (RFC)
In my solar-hydrogen demonstration power plant design, the functions of the electrolyzers and of the fuel cells are combined into single units, which can operate in either mode. These reversible fuel cells (RFCs) during the day will operate in the electrolyzer mode (See Figure Here), converting solar energy into chemical energy (hydrogen), while at night they will switch into their fuel cell mode and will convert the chemical energy stored in hydrogen back into electricity.
By keeping the pressures identical on the two sides of the membrane, these dual-purpose cells can be made light and thin. They also will use only half as much expensive platinum catalyst, and therefore, be much less expensive.
It takes the same amount of energy to split water into hydrogen and oxygen as the energy obtained when hydrogen is oxidized into water. The only difference is that electrolysis increases the entropy, and, therefore, not all the energy needs to be supplied in the form of solar electricity because the environment contributes 48.7 kJ/mole of thermal energy. Inversely, when the RFC is operated in fuel-cell mode, part of the energy in the hydrogen fuel is released as heat. Therefore, the electrolysis mode of operation (Shown in blue in Figure) requires heat, and the fuel cell mode (Shown in red in Figure) releases heat.
In a solar-hydrogen power plant, when excess solar energy is available, the RFC is switched into the electrolyzer mode to split water into hydrogen and oxygen. The hydrogen is collected and is either liquefied or compressed to high pressure (about 1,000 bars = 15,000 psig) and sent to storage.
Whenever solar electricity is insufficient, the RFC is switched into the fuel-cell mode in which the oxidation of one mole of hydrogen will generate 237.1 kJ/mole of electrical energy plus 48.7 kJ/mole of thermal energy. This waste heat also can be used for heating buildings or for preheating boiler feed water.
Controlling the RFC
The role of process control is critical in operating the RFCs. The complexity of the control challenge can be appreciated if we view a stack of 400 RFC cells as 400 pumps operating in parallel, and we realize that switching the RFC from one mode to the other is like switching a chemical reactor from one product to another. Fortunately, the switchover doesnt need to be fast, but once the RFC is in operation, its time constant is very shorta matter of seconds.
In addition to the electric controls that connect the RFC to the grid and convert the direct current to alternating current, a massive quantity of measurements and control algorithms are required. These include the switching between the heating and cooling modes as the RFC operation is reversed. These loops require high rangeability and fast, accurate temperature controls in both modes. The pressures of the oxygen and hydrogen streams entering (in the fuel-cell mode) or leaving the RFC (in the electrolyzer mode) also must be controlled carefully. The oxygen and hydrogen pressures also require accurate controls, because these pressures have to be identical, so that the proton exchange membrane (PEM) diaphragms of the fuel cells will not be exposed to excessive pressure differences.
In addition, the loads (the rates of hydrogen or electricity generation) need to be controlled. These loads are either determined by the availability of excess solar electricity (electrolyzer mode: blue, Figure 1) or by the electricity demand (Fuel-cell mode: red, see figure). Maintaining the load in both modes requires fast and accurate flow controls.
In the fuel-cell mode, the hydrogen fuel flow has to be controlled, while in the electrolyzer mode, the flow of the distilled water supply is one of the key manipulated variables. Controls are also needed to direct the generated distilled water to its destination (FC mode) and to send the generated oxygen to its destination (electrolyzer mode). The destination for distilled water can be the drinking water system, while the destination for the generated oxygen can be the air supply to a fired heater or boiler, if such a unit exists on the site (to increase efficiency by increasing the oxygen concentration of the air).
The instruments used will have to be mass-produced, miniaturized, accurate and inexpensive. The control algorithms have to be state-of-the-art (because my system is not yet patented, here I am not describing the algorithms).
In short, process control will play a key role in the transition from the present fossil/nuclear economy to the inexhaustible and free solar-hydrogen economy of the future. I am sure that our process control profession will meet these challenges, and thereby will not only play a key role in this third industrial revolution, but also will gain the respect it deserves as the most important field of engineering.