I personally think that the concept of a fuel cell has a lot of potential. I think that with sufficient research, it could be something that is used in more and more cars and in more places in a positive manner. This, of course, means that researchers would find a way to guarantee that the explosiveness of hydrogen would not be triggered and that there would not be any harmful effects on the environment.
After completing this project I feel that I have a greater appreciation for fuel cells and the technological advancements that are being made in storing electricity. While I believe that fuel cells certainly do have some major advantages, I would personally wait for a few years before the cost of fuel cells decreases and the engineering principles that are in place are shown to contain the explosive nature of hydrogen gas reliably. I would also wait until the technology is easier to use. Right now there are very few options at the pump for filling up hydrogen fuel cell cars. Hopefully this will change soon and we will start to see either an increase in hydrogen fuel cells or the advancements of other, better technologies being implemented in the near future.
Team C Blog: Fuel Cells
Ryan, Taua, Mya
Thermodynamics of Fuel Cells:
First law of Thermodynamics: The first law of thermodynamics refers to the conservation of energy and states that “energy may neither be created nor be destroyed, but may be converted from one form to another”. This statement says that the total amount of energy available on a system doesn’t change, even though the forms in which it’s available may change. The first law of thermodynamics is represented by the following equation:
Where E represents the energy of the system and Q and W represent the heat transferred to the system and the work done by the system, respectively. E is a function independent of the path covered by the system, while Q and W are dependent functions of the path. There are three distinct types of system that can be described as:
- Open system – for those which the mass and energy transfer is allowed
- Closed system – for those which only the energy transfer is allowed
- Isolated system – for those neither mass nor energy are allowed to be transferred from the system surroundings.
In terms of Fuel Cells, the type of system considered is the open system, where there are a flow of mass and energy between the system and its boundaries. In this kind of system, the energy change may be represented by the following equation:
For this equation, the term ΔU represents the internal energy change, ΔE(k) is a mention to the kinetic energy change, ΔE(p) refers to the potential energy change, and Δ(PV) is the pressure-volume work done on the fluid to keep it flowing through the system. Another important thermodynamic property to be introduced is the concept of enthalpy (H), which is the combination of internal energy and the PV work done by the system, as the following equations express:
From the equations of enthalpy and energy change, is possible to stablish an equation of the energy change in an open system, as the following one:
This equation is valid only for steady flow conditions though, those in which ΔE(k) and ΔE(p) are equal to zero. The work obtained in the fuel cell occurs from the transport of electrons over a potential difference, instead of the rotation of a mechanical mean (such as turbine blades). The fuel cell can be represented as a control volume, as shown in the figure below.
Application of the first law of thermodynamics to fuel cells: the operation on a fuel cell is based on the following reaction:
There is a transfer of electrons from on electrode to the other, to make the reaction consummate. The amount of electrons transferred is proportional to the amount of equivalents of chemical change that happens when the fuel is oxidized. The amount of electricity transferred when the reaction is done is given by the equation below:
Where F is the Faraday’s constant. The amount of electricity transferred can be related to the electrical work done by the cell, as shown in the following equation:
Where the parameter E refers to the cell voltage, which correspond to potential difference between the two electrode terminals of the cell. The negative sign is explained by the fact that it’s a certain work being done by the system. Another way of expressing the work is shown in the equation below:
Where the parameter I refers to the current and t the time. Relating the previous equation to the entropy change equation, we have:
The second law of thermodynamics: Even though it’s known that heat can’t flow from a cold body to a hot one, the first law of thermodynamics doesn’t impose any restrictions to the direction of the energy transfer. Smith, Van Ness and Abbot stated about the second law of thermodynamics with the following sentences:
- No apparatus can operate in such a way that its only effect on the system and surroundings is to convert heat absorbed by a system completely to work done by the system (or) it is impossible by a cyclic process to convert the heat absorbed by the system to completely into work done by the system.
- No process is possible that consist solely in the transfer of heat from one temperature level to a higher one.
Therefore, the second law places a limit on the fraction of heat that may be converted to work by a system.
Reversible process and the concept of entropy: Another important point to be discussed in terms of thermodynamics is the reversibility; where a system can be called reversible when it’s and stays in equilibrium with its environment as it passes from its initial state to its final state. This concept of reversibility can be easily visualized for an electrochemical cell, and according to the Gibbs statement:
“If no changes take place in the cell except during the passage of current, and all changes which accompany the current can be reversed by reversing the current, the cell may be called a perfect electrochemical apparatus”.
The concept of entropy is describe by the second law of thermodynamics, which is a measure of the disorder level in a system. The reversible processes can be reached by reducing the finite temperature gradient into an infinitesimal difference. Entropy is based on such reversible heat transfer and it’s defined by the equation below:
And, mathematically, the second law may be expressed as:
And, for reversible processes, the equality applies.
Relation between the second law and the fuel cells, the Gibbs free energy: Starting from the discussion of entropy and enthalpy, for a reversible fuel cell, the term Q may be replaced by:
And, by differentiation, we have:
From the basis that the fuel cell operates reversibly, the work obtained in the process is the maximum possible, with minimal losses. This work may be expressed as a function called Gibbs free energy (G):
Substituting the Gibbs free energy equation on the differentiation of the entropy, in terms of thermodynamic state functions, we have:
From the described equations, there is a new viewpoint about the parameters G, H, and S. Where H can be seen as the total energy possessed by the system, S as the unavailable energy, and G as the maximum useful work, or simply the free energy of a system.
Fuel cells vs. Carnot efficiencies: In a general way, a heat engine is that which takes the heat from a hot source and rejects it to a lower temperature font, doing useful work. The efficiency of this process is limited by the second law of thermodynamics and can be written as:
The efficiency of a fuel cell can be described by the following expression:
Where HHV is the higher heating value.
In a comparison between both efficiencies, we have the following chart:
A hydrogen fuel cell is made by taking a carbon channeled plate and stacking chemically treated paper onto it. Then you add a membrane and another chemically treated piece of paper after. Last, another carbon channeled plate is added. In order to make an engine, you stack cells one on top of other until over a hundred have been stacked together.
The paper and membrane will contain a catalyst. An example of this could be platinum or tungsten sulfide. The catalysts split the hydrogen gas into protons and electrons. The catalysts, also, serve as the reaction site. This is where they take in oxygen gas until it reacts with the generated protons to form water.
To increase the efficiency of the current, the stacked fuel cells are compressed together using a hydraulic press. The compression also forces the rubber seal of each fuel cell to form a sealed container and guarantee no leaks. Leaks can be found by running nitrogen gas through each hydrogen fuel cell after it has been compressed. After the compression, a reinforced steel road is added for security. At this point, a circuit board is installed to measure the voltage of each cell. The electrical circuit board is connected to the fuel cells by a silver containing adhesive to increase conductivity. The fuel cell engine has a hydrogen line, an airline (for reaction), and a water line to allowing cooling to happen.
When this is operating, an engine blower blows in air to the stack of hydrogen cells mentioned above. The fuel cells are injected with hydrogen gas from the fuel tank that enters through the ends. The carbon from the carbon channeled plate conducts electricity and increases the speed of electron transfer through the cells.
As hydrogen enters through the carbon channels, the gas encounters the chemically treated paper that converts the gas into electricity. The hydrogen gas then travels from the chemically treated paper to a membrane that splits the gas into protons and electrons. The oxygen coming into the membrane reacts with the free protons and water is the result. A pump then draws out the water. The electrons that were split previously travel to the end of the stack of fuel cells to the electrical wires.
Applications and Implications
Benefits of using fuel cells:
- Fuel cells have a higher efficiency than diesel or gas engines.
- Fuel cells can decrease or eliminate pollution caused by burning fossil fuels; for hydrogen fuelled fuel cells, the only by-product at point of use is water. If the hydrogen comes from the electrolysis of water driven by renewable energy, then using fuel cells eliminates greenhouse gases over the whole cycle.
- Fuel cells do not need conventional fuels such as oil or gas and can therefore reduce economic dependence on oil producing countries, creating greater energy security for the user nation. Since hydrogen can be produced anywhere where there is water and a source of power, generation of fuel can be distributed and does not have to be grid-dependent. Many times the best option for energy independence is to have many options for energy sources. Fuel cells can help provide another option and lead to greater energy independence.
- Operating times are much longer than with batteries, since doubling the operating time needs only doubling the amount of fuel and not the doubling of the capacity of the unit itself.
- Fuel cells have no “memory effect” when they are getting refueled. The maintenance of fuel cells is simple since there are few moving parts in the system.
A few of the applications of fuel cell technology can fall into three categories: portable power generation, stationary power generation, and power for transportation.
Portable fuel cell applications refer to the applications in which the fuel cell is built into, or used to charge up, products that are designed to be moved. Some examples of these applications include:
Military applications – portable soldier power, skid mounted fuel cell generators etc
Auxiliary Power Units – portable products (torches, vine trimmers etc) – small personal electronics (mp3 players, cameras, laptops, printers, toys
Portable fuel cells are being developed in a wide range of sizes ranging from less than 5 W up to 500 kW.
Fuel cells are being sold commercially for these applications.
- Fuel cells in portable applications offer advantages in that:
- They offer off-grid operation
- longer run-times compared with batteries
- shorter recharging times
- significant weight reduction potential (especially important in military applications)
- convenience, reliability, and lower operating costs also apply
A good image depicting the applications of portable fuel cells is below:
Some applications of transport fuel cells:
- Cars, trucks, regular citizen vehicles.
- Two- and three-wheeler vechicles such as scooters
- Trains and trams
- Some aircraft
- Unmanned aerial vehicles (UAVs) and unmanned undersea vehicles (UUVs)
An image of an actual hydrogen fuel cell car:
Image taken from: caranddriver.com
Stationary fuel cells provide electricity and are not designed to be moved. Fuel cells are also more efficient at generating electricity which gives units overall efficiencies of 80-95%.
Electricity and heat for residential applications
Is hydrogen safe?
Like any other fuel, hydrogen is flammable and has the potential to react violently with oxygen in the air. However, this is also true of gasoline, diesel and natural gas and yet this has not prevented the use of these fuels. The key is having the correct safety features and infrastructure to allow non-hazardous use of potentially hazardous substances. The industry is putting considerable development into designing the correct equipment and procedures for the safe use of hydrogen.
Why not burn hydrogen instead of using it in a fuel cell?
Hydrogen is an extremely clean-burning fuel, and offers a good alternative to gasoline or diesel for internal combustion engines. However, any combustion process will produce small amounts of pollutants whereas a fuel cell has the potential to emit none. This is in addition to the other benefits of fuel cells mentioned above, including more efficient use of any given fuel.