If the functions of an electrolyzer and a fuel cell is combined into single unit, which can operate in either mode, a reversible fuel cell is obtained. These reversible fuel cells (RFCs) during the day will operate in the electrolyzer mode (blue operating mode in figure below), converting solar energy into chemical energy (hydrogen), while at night they will switch into their fuel cell mode (shown in red below) 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. One of the key requirements is to develop catalysts that are light, inexpensive and plentyful to minimize the cost and weight of the RFC.
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 the figure above) requires heat, while the fuel cell mode (shown by red in the 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 left in the gas phase and 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.
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 doesn’t need to be fast, but once the RFC is in operation, its time constant is very short—a 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) or by the electricity demand (fuel cell mode). 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 for full automation are in the process of being developed.