Alternative energy based on the use of fuel cells is gaining more and more popularity and is increasingly being used in the automotive, aerospace, and energy industries as well as other sectors of the economy.
What is a Fuel Cell?
Fuel cells (FC) are electrochemical devices which convert the chemical energy of a fuel directly into usable energy – electricity and heat – without combustion. This is quite different from most electricity-generating devices (e.g., steam turbines, gas turbines, reciprocating engines), which first convert the chemical energy of a fuel to thermal energy via combustion, then into mechanical energy, and finally to electricity.
Fuel cells are similar to batteries containing electrodes and electrolytic materials to accomplish the electrochemical production of electricity. Batteries store chemical energy in an electrolyte and convert it to electricity on demand until the chemical energy has been depleted.
Fuel cells do not store chemical energy. Rather, they convert the chemical energy of a fuel into electricity. Thus fuel cells do not need recharging, and can continuously produce electricity as long as fuel and an oxidizer are supplied.
A prototype fuel cell is shown below in Figure 1.
What is the operating principle of a fuel cell?
Today, there are two types of electrolytes used in fuel cells: acid or alkali. The type also depends on the chemical reactions that take place in the element itself.
Hydrogen entering through the anode, in the presence of a catalyst, reacts with hydroxyl ions (OH–), forming water and an electron. At the cathode, oxygen reacts with electrons in the external circuit and water, forming hydroxyl and perhydroxyl ions. The resulting reaction at the cathode allows the balance of matter and charge in the electrolyte to be maintained.
In modern fuel cells with an acidic electrolyte, hydrogen is fed through a hollow anode, entering through small pores in the electrode material, and entering the electrolyte. In the process of chemisorption, hydrogen molecules break down into atoms, which turn into ions with a positive charge, donating one electron. Oxygen is supplied to the cathode and also enters the electrolyte, reacting with hydrogen with the participation of a catalyst. When oxygen combines with hydrogen and electrons in the external circuit, water is formed.
The processes taking place in fuel cells are inherently opposite of electrolysis. During the reactions, part of the energy is converted into heat, and the flow of electrons in the external circuit is a direct current used to do work. Most reactions provide an electromotive force (EMF) of about 1V.
Below, in Figure 2, a scheme of a fuel cell’s operation is shown.
Figure 3 shows the types of fuel cells, reactions, and temperature modes of operation.
- AFC – Alkaline Fuel Cell
- PEMFC – Proton Exchange Membrane Fuel Cell
- PAFC – Phosphoric Acid Fuel Cell
- MCFC – Molten Carbonate Fuel Cell
- SOFC – Solid Oxide Fuel Cell
What approaches or methods can improve the efficiency of fuel cells? What determines the efficiency of a fuel cell?
There are several main factors that affect the efficiency of a fuel cell system:
- Increasing the thermodynamic parameters of the oxidizer and fuel (increasing the temperature and pressure of oxygen/air and fuel) increases the efficiency and efficiency of the fuel cell system;
- Use of new and promising types of fuel cell designs, which affect the type of electrolyte, the parameters of the supplied fuel and oxidizer, and consequently, the temperature regime;
- Application of various types of plates and their configurations, taking into account the peculiarities of the processes in the fuel cell (circular, square, pentagonal, hexagonal plate and etc.);
- Use of various types of channels for the flow of fuel and oxidizer inside the fuel cell, taking into account the peculiarities of the processes in the fuel cell (rectangular, triangular, wavy, offset strip pin, straight/parallel, pin, single/multiple serpentine types and etc.); or
- Use of various types of fuel (H2, CH2OH, CH3OH + H2O, CH4, C3H8O3, organic matter, and others.).
One of the possible options aimed at increasing the energy efficiency and efficiency of the fuel cell is to increase the thermodynamic parameters of the working fluid. The most effective of them is an increase in the pressure of the oxidizer and the supplied fuel. Figure 5 shows the map of a fuel cell with the dependence of polarization curves on current and voltage at different air pressures at a constant temperature, as well as the dependence of power on air pressure at constant temperature. To this end, we will consider the features of the use of an auxiliary device, such as a compressor. A more detailed look at the compressor is in the air supply as the oxidant for the fuel cell.
Figure 4 shows the general map of the fuel cell. The dependence of the polarization curve on voltage and current is presented, as well as the dependence of efficiency and power.
How can the thermodynamic parameters of an oxidizer/fuel in a fuel cell system be improved?
A fuel cell system can use an air compressor to deliver oxygen to the fuel cell stack, and the fuel is usually pressurized in the tanks in such systems.
Fuel cell systems have two types of pressure: the pressure in the tank and the ambient pressure. Increasing the pressure in the battery FC usually leads to greater efficiency, improved specific work in the FC, and higher system response. However, the pressure increase uses part of the power output of the fuel cell and in some cases can lead to a decrease in system efficiency as a whole. Consequently, the compressor used to increase the pressure has a direct impact on the performance of the fuel cell system.
In the next week’s entry, we’re going to take a closer look at how compressors can be used to greatly increase the efficiency of a fuel cell system by supplying more oxygen to the system.