Week 2 Challenge

Question 1:
Compare four different types of fuel cells and state their applications.

Answers:

1.Solid Oxide Fuel cell:

Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. SOFCs are around 60% efficient at converting fuel to electricity. In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 85%.

SOFCs operate at very high temperatures—as high as 1,000°C (1,830°F). High-temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system.

SOFCs are also the most sulfur-resistant fuel cell type; they can tolerate several orders of magnitude more sulfur than other cell types can. In addition, they are not poisoned by carbon monoxide, which can even be used as fuel. This property allows SOFCs to use natural gas, biogas, and gases made from coal. High-temperature operation has disadvantages. It results in a slow startup and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation. The high operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology.

Scientists are currently exploring the potential for developing lower-temperature SOFCs operating at or below 700°C that have fewer durability problems and cost less. Lower-temperature SOFCs have not yet matched the performance of the higher temperature systems, however, and stack materials that will function in this lower temperature range are still under developmen

Current technology employs several ceramic materials for the active fuel cell components. The anode is typically constructed from an electronically conducting nickel/yttria-stabilised zirconia cermet (Ni/YSZ). The cathode is based on a mixed conducting perovskite, lanthanum manganate (LaMnO3). Yttria-stabilised zirconia (YSZ) is used as the oxygen ion conducting electrolyte. To generate a suitable voltage, fuel cells in the same stack are interconnect with a doped lanthanum chromate (eg La0.8Ca0.2CrO3) joining the anodes and cathodes of adjacent units.

Anode reaction,

H2 + O2- -> H2O +2e- 

CO + O2- -> CO2 + 2e- 

Cathode reaction,

O2 + 4e- -> 2O2-

Overall reaction,

H2 + O2 + CO ->H2O + CO2

                SOFC fuel cell manufacturer, has developed a method of reducing the operating temperature of their SOFC system to 500–600 degrees Celsius. They replaced the commonly used YSZ electrolyte with a CGO (cerium gadolinium oxide) electrolyte. The lower operating temperature allows them to use stainless steel instead of ceramic as the cell substrate, which reduces cost and start-up time of the system.

Efficiency: 60%

Advantages:

• High efficiency
• Fuel flexibility
• Solid electrolyte
• Suitable for CHP
• Hybrid/gas turbine cycle

Shortcomings:

• High temperature corrosion and breakdown of cell components
• Long start-up time
• Limited number of shutdowns

Applications:

• Auxiliary power
• Electric utility
• Distributed generation

2.Alkaline fuel cells

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water on-board spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. In recent years, novel AFCs that use a polymer membrane as the electrolyte have been developed. These fuel cells are closely related to conventional PEM fuel cells, except that they use an alkaline membrane instead of an acid membrane. The high performance of AFCs is due to the rate at which electro-chemical reactions take place in the cell. They have also demonstrated efficiencies above 60% in space applications.

A key challenge for this fuel cell type is that it is susceptible to poisoning by carbon dioxide (CO2). In fact, even the small amount of CO2 in the air can dramatically affect cell performance and durability due to carbonate formation. Alkaline cells with liquid electrolytes can be run in a recirculating mode, which allows for electrolyte regeneration to help reduce the effects of carbonate formation in the electrolyte, but the recirculating mode introduces issues with shunt currents. The liquid electrolyte systems also suffer from additional concerns including wettability, increased corrosion, and difficulties handling differential pressures. Alkaline membrane fuel cells (AMFCs) address these concerns and have lower susceptibility to CO2 poisoning than liquid-electrolyte AFCs do. However, CO2 still affects performance, and performance and durability of the AMFCs still lag that of PEMFCs. AMFCs are being considered for applications in the W to kW scale. Challenges for AMFCs include tolerance to carbon dioxide, membrane conductivity and durability, higher temperature operation, water management, power density, and anode electrocatalysis

Anode reaction,

H2 + 2OH- -> 2H2O + 2e-

Cathode reaction,

½O2 + H2O + 2e- -> 2OH-

Overall reaction,

H2 + ½O2 -> H2O.

Efficiency: 60%

Advantages:

• Wider range of stable materials allows lower cost components
• Low temperature
• Quick start-up

Shortcomings:

• Sensitive to CO2 in fuel and air
• Electrolyte management (aqueous)
• Electrolyte conductivity (polymer)

Applications:

• Military
• Space
• Backup power
• Transportation

3.Direct methanol fuel cells

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is usually mixed with water and fed directly to the fuel cell anode.

Direct methanol fuel cells do not have many of the fuel storage problems typical of some fuel cell systems because methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to the public using our current infrastructure because it is a liquid, like gasoline.

Anode reaction,

CH3OH + H2O -> CO2 + 6H+ + 6e-

Cathode reaction:

3(½O2) + 6H+ + 6e- ->3H2O

Overall reaction:

CH3OH + H2O+ 3(½O2) -> CO2 + 3H2O

Shortcomings:

• Methanol is miscible in water, so some of it is liable to cross the water-saturated membrane and cause corrosion and exhaust gas problems on the cathode side

4.Proton Exchange Membrane Fuel cell:

               In Proton exchange membrane fuel cell, the proton conducting polymer membrane called nafion is used. It basically requires hydrogen and oxygen as its inputs, though the oxidant may also be ambient air, and these gases must be humidified. It operates at a low temperature because of the limitations imposed by the thermal properties of the membrane itself. The operating temperatures are around 90 °C. The PEMFC can be contaminated by carbon monoxide, reduce the. It requires cooling and management of the exhaust water in order to function properly. The main focus of current designs is transport applications, as there are advantages to having a solid electrolyte for safety, and the heat produced by the fuel cell is not adequate for any form of cogeneration. Daimler-Benz has taken a high profile in developing cars powered by Ballard fuel cells, while Toyota has recently presented a vehicle that is using a fuel cell of their own design.

                   

Anode reaction,

H2 -> 2H+ + 2e-

Cathode reaction,

½O2 +2H+ + 2e- → H2O

Overall reaction,

H2 + ½O2 -> H2O

Efficiency: 60%

Advantages:

• Solid electrolyte reduces corrosion and electrolyte management problems
• Low temperature
• Quick start-up and load following

Shortcomings:

• Expensive due to the use of platinum
• Requires water management, the membrane must be evaporated precisely at the same rate that is produced.
• The same temperature must be maintained throughout the cell in order to prevent destruction of the cell through thermal loading.
• Automotive fuel cells require a 5,000-hour lifespan under extreme temperatures while current service life is 2,500 hours.

Applications:

• Backup power
• Portable power
• Distributed generation
• Transportation
• Specialty vehicles

Question 2:
Using a plot with energy density and power density on the X and Y axis, arrange various battery types like lithium ion, lead acid, Nickel Cadmium, Ultracapacitor, fuel cell.

Answwers:

 

The Ragone plot is very useful for estimating energy storage performance but ignores the critical features, such as cycle life, cost estimation, and safety for comparison between supercapacitors and batteries

The lead-acid, Li-ion, NiCad and NiMH are the most commonly used battery types in battery energy storage systems. However, the lead-acid batteries are rarely being used in energy storage system applications mainly due to their low energy density and low power density. Similarly, NiCad batteries also have low energy density and low power density and thus rarely being used in battery energy storage systems .

The most commonly used battery technology is the Li-Ion technology, which has high power density and high energy density in comparison to the other battery technologies available in the market. The Li-Ion batteries are maintenance free and also have the highest lifetime (at 80% depth-of-discharge (DoD)) over other battery types available in the market.

Supercapacitors (also known as ultracapacitor) have the capability to release or absorb a large amount of power instantaneously than battery energy storage systems. In supercapacitors energy is stored in an electrostatic field and hence the charging and discharging cycles can be repeated without any limit and also it could be charged within a very short time span as oppose to batteries. The supercapacitor structure is different to a conventional capacitor due to its electrostatic double-layer and thinner carbon electrodes. These properties increase the capacitance of supercapacitor compared to conventional capacitor.

Fuel cell stores high energy because it uses fuel to produce electricity. Fuel cells work like batteries, but they do not run down or need recharging. They produce electricity and heat as long as fuel is supplied. A fuel cell consists of two electrodes—a negative electrode (or anode) and a positive electrode (or cathode)—sandwiched around an electrolyte. A fuel, such as hydrogen, is fed to the anode, and air is fed to the cathode. In a hydrogen fuel cell, a catalyst at the anode separates hydrogen molecules into protons and electrons, which take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity. The protons migrate through the electrolyte to the cathode, where they unite with oxygen and the electrons to produce water and heat. 

 Thus from the graph we found that the lithium ion leas acid Nickel Cadmium batteries have low Power density and moderate energy density.Ultracapacitor or super capacitor have moderate power density and moderate enegry density,the fuel cell have low power density and high energy density