Week 2 Challenge

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

COMPARING DIFFERENT TYPES OF FUEL CELLS AND THEIR PURPOSE/APPLICATIONS.

Fuel cells are classified primarily by the kind of electrolyte they employ. This classification determines the kind of electro-chemical reactions that take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required, and other factors. These characteristics, in turn, affect the applications for which these cells are most suitable. There are several types of fuel cells currently under development, each with its own advantages, limitations, and potential applications. Some of them are:

• Polymer electrolyte membrane fuel cells
• Direct methanol fuel cells
• Alkaline fuel cells
• Solid oxide fuel cells

Polymer electrolyte membrane fuel cells

Polymer electrolyte membrane (PEM) fuel cells—also called proton exchange membrane fuel cells— use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum or platinum alloy catalyst. They need only hydrogen, oxygen from the air, and water to operate. They are typically fueled with pure hydrogen supplied from storage tanks or reformers.

PEM fuel cells deliver high power density and offer the advantages of low weight and volume compared with other fuel cells.

Alkaline fuel cells

Alkaline fuel cells (AFCs) 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.

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.

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.

It is similar to the PEM cell in that it uses a polymer membrane as an electrolyte. However, the platinum-ruthenium catalyst on the DMFC anode is able to draw the hydrogen from liquid methanol, eliminating the need for a fuel reformer. Therefore pure methanol can be used as fuel, hence the name.

Solid oxide fuel cells

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%.

Fuel cell type	Common electrolyte	Operating temperature	Typical stack size	Electrical efficiency (lhv)	Applications	Advantages	Challenges
Polymer electrolyte membrane (PEMFC)	Perfluorosulfonic acid	<120°C	<1 kW–100 kW	60% direct H2; 40% reformed fuel	Backup power Portable power Distributed generation Transportation Specialty vehicles	Solid electrolyte reduces corrosion and electrolyte management problems Low temperature Quick start-up and load following	Expensive catalysts Sensitive to fuel impurities
Alkaline (AFC)	Aqueous potassium hydroxide soaked in a porous matrix, or alkaline polymer membrane	<100°C	1–100 kW	60%	Military Space Backup power Transportation	Wider range of stable materials allows lower cost components Low temperature Quick start-up	Sensitive to CO2 in fuel and air Electrolyte management (aqueous) Electrolyte conductivity (polymer)
Solid oxide (SOFC)	Yttria stabilized zirconia	500°–1,000°C	1 kW–2 MW	60%	Auxiliary power Electric utility Distributed generation	High efficiency Fuel flexibility Solid electrolyte Suitable for CHP Hybrid/gas turbine cycle	High temperature corrosion and breakdown of cell components Long start-up time Limited number of shutdowns
Direct methanol (DMFC)	Proton exchange membrane	45-130 C	1-100 W	20-30%	Portable fuel cell applications such as cell phones or laptop computers.	No fuel storage problems Methanol has a higher energy density than hydrogen Reduced system complexity	Suitable for apliication up to 100 watts Complex stack structure Anode kinetics Cross over Noble catalyst required

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.

ENERGY DENSITY Vs. POWER DENSITY PLOT OF VARIOUS ENERGY STORAGE SYSTEMS DEVICES LIKE LITHIUM ION, LEAD ACID, NICKEL CADMIUM, ULTRACAPACITOR, FUEL CELL.

These energy storage technologies could be mainly categorised into four types: electrochemical devices (e.g., batteries, fuel cells), electrostatic devices (e.g., supercapacitors), electro-mechanical devices (e.g., flywheels), and electromagnetic devices (e.g., superconducting magnetic energy storage (SMES) systems). The energy density and power density are the two key features that could be used to characterise an energy storage system. The energy density is defined as the energy stored per-unit weight, hence energy density signifies the relative size of the storage system. The power density is defined as the amount of power that could be obtained for a per-unit weight. Figure below illustrates the power and energy density characteristics of different energy storage technologies suitable for ship microgrids.

Battery Energy Storage Systems (Lead-acid, li-ion, NiMH or NiCad)

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

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

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.