Hydrogen Energetic Applications

Hydrogen has many potential energy uses, including powering non-polluting vehicles, heating homes, and fueling aircraft. Energetic applications of hydrogen can be classified into two main categories, i.e. direct combustion (e.g. internal combustion engine) and fuel cells.

2.3.3.1 Internal Combustion Engine

An internal combustion engine (ICE) transforms chemical energy contained in a fuel into mechanical energy through combustion in a piston or rotary engine. Modified ICE can utilize hydrogen in place of gasoline. Optimized hydrogen engines can be run at higher compression ratios than those with unleaded gasoline. It makes hydrogen-powered engines 15-25 % more efficient than gasoline engines. Advantages of the ICEs are mainly relatively mature technology, relatively low cost when compared to a fuel cell, and potentially low greenhouse gas emissions. Depending on the source of the hydrogen gas the greenhouse gas emissions will be lower than gasoline. If renewable energy is used to generate the hydrogen they may approach zero. However, the disadvantages of the ICE are: lower efficiency than a fuel cell, and some pollutant emissions from the engine remain (e.g. Nitrogen oxides).

2.3.3.1.1 The Engine

The properties of hydrogen (vid. in Section 2.2.1) that contribute to its use as a combustible fuel are wide range of flammability, low ignition energy, small quenching distance, high autoignition temperature, high flame speed at stoichiometric ratios, high diffusivity, and very low density. The theoretical or stoichiometric combustion of hydrogen and oxygen is given as 2H2 + O2 = 2H2O; where 2 moles of H2 and 1 mole of O2 are needed for complete combustion. Because air is used as the oxidizer instead oxygen, the calculations showed that the stoichiometric or chemically correct air to fuel (A/F) ratio for the complete combustion of hydrogen in air is about 34:1 by mass. This means that for complete combustion, 34 kg of air are required for every kg of hydrogen. This is much higher than the 14.7:1 A/F ratio required for gasoline [46].

Since hydrogen is a gaseous fuel at ambient conditions it displaces more of the combustion chamber than a liquid fuel. Consequently less of the combustion chamber can be occupied by air. At stoichiometric conditions, hydrogen displaces about 30% of the combustion chamber, compared to about 1 to 2% for gasoline. Figure 2.13 compares combustion chamber volumes and energy content for gasoline and hydrogen fuelled engines. Depending on the method used to meter the hydrogen to the engine, the power output compared to a gasoline engine can be anywhere from 85% (intake manifold injection) to 120% (high pressure injection).

Gasoline Hydrogen Flame Speed
Fig. 2.13 Combustion chamber for gasoline and hydrogen fuelled engines [46]

2.3.3.1.2 Thermal Efficiency

The theoretical thermodynamic efficiency of an Otto cycle engine is based on the compression ratio of the engine and the specific-heat ratio of the fuel as shown in the equation [46]:

where

Vr/V2 = the compression ratio; y = ratio of specific heats; and nth = theoretical thermodynamic efficiency

The higher the compression ratio and/or the specific-heat ratio are the higher the indicated thermodynamic efficiency of the engine. The compression ratio limit of an engine is based on the fuel's resistance to knock. A lean hydrogen mixture is less susceptible to knock than conventional gasoline and therefore can tolerate higher compression ratios. The specific-heat ratio is related to the fuel's molecular structure. The less complex the molecular structure, the higher the specific-heat ratio. Hydrogen (y = 1.4) has a much simpler molecular structure than gasoline and therefore its specific-heat ratio is higher than that of conventional gasoline (y = 1.1).

2.3.3.1.3 Emissions

The combustion of hydrogen with air (instead of oxygen), however can also produce oxides of nitrogen (NOx) [46]:

The oxides of nitrogen are created due to the high temperatures generated within the combustion chamber during combustion. This high temperature causes some of the nitrogen in the air to combine with the oxygen in the air. The amount of NOx formed depends on the air/fuel ratio; the engine compression ratio; the engine speed; the ignition timing, and whether thermal dilution is utilized. Compared to fossil fuel ICEs, however, NOx emissions of lean-burn hydrogen ICEs for road vehicles are very low.

2.3.3.1.4 Power Output

The theoretical maximum power output from a hydrogen engine depends on the air/fuel ratio and fuel injection method used [46]. As mentioned before that the stoichiometric air/fuel ratio for hydrogen is 34:1. At this air/fuel ratio, hydrogen will fill 29% of the combustion chamber leaving only 71% for the air. As a result, the energy content of this mixture will be less than it would be if the fuel were gasoline (since gasoline is a liquid, it only occupies a very small volume of the combustion chamber, and thus allows more air to enter).

Since both the carburator and port injection methods mix the fuel and air prior to entering the combustion chamber, these systems limit the maximum theoretical power obtainable to approximately 85% of that of gasoline engines. For direct injection systems, which mix the fuel with the air after the intake valve has closed (and thus the combustion chamber has 100% air), the maximum output of the engine can be approximately 15% higher than that for gasoline engines. Therefore, depending on how the fuel is metered, the maximum output for a hydrogen engine can be either 15% higher or 15% less than that of gasoline if a stoichiometric air/fuel ratio is used [46]. However, at a stoichiometric air/fuel ratio, the combustion temperature is very high and as a result it will form a large amount of nitrogen oxides (NOx), which is a pollutant. Since one of the reasons for using hydrogen is low exhaust emission, hydrogen engines are not normally designed to run at a stoichiometric air/fuel ratio. Typically hydrogen engines are designed to use about twice as much air as theoretically required for complete combustion. At this air/fuel ratio, the formation of NOx is reduced to nearly zero.

Unfortunately, this also reduces the power out-put to about half that of a similarly sized gasoline engine. To make up for the power loss, hydrogen engines are usually larger than gasoline engines, and/or are equipped with turbochargers or superchargers.

2.3.3.1.5 Current Status

A few auto manufacturers have been doing some work in the development of hydrogen-powered vehicles (Ford has recently announced that they have developed a production ready of hydrogen-powered vehicle using an ICE and BMW has completed a world tour displaying a dozen or so hydrogen-powered 750i vehicles). However, it is not likely that any hydrogen-powered vehicles will be available to the public until there is an adequate refuelling infrastructure and trained technicians to repair and maintain these vehicles.

2.3.3.2 Fuel Cells

The fuel cells are devices that convert hydrogen gas into low-voltage, direct current electricity by combining hydrogen and oxygen electrochemically. In a fuel cell, a fuel gas is converted into electrical energy in an electrochemical process. Some fuel cells use methane and a few use liquid fuels such as methanol, but most of the fuel cells use hydrogen as the fuel. Besides, oxygen (usually obtained from air) is also needed by fuel cells. Fuel cells can be made in a wide range of sizes. They can be used to produce small amounts of electricity for portable devices, as well as the very high power electric power stations.

The efficiency of a fuel cell is as high as 75 % [46]. There are no NOx, CO, HC emissions, because hydrogen is not burnt in air. Fuel cells potentially produce low greenhouse gas emissions. Depending on the source of the hydrogen gas the greenhouse gas emissions will be lower than those of a current internal combustion engine. If renewable energy is used to generate the hydrogen greenhouse gas emissions may approach zero. Types of fuel cells have been developed usually classified according to the electrolyte used. In the Figure 2.14, fuel cell types are presented including their operating temperature and anode/cathode reactions.

2.3.3.2.1 Operation Principles

A fuel cell is an energy conversion device that converts the chemical energy of a fuel directly into electricity without any intermediate thermal or mechanical processes. Energy is released whenever a fuel reacts chemically with the oxygen in air. In an internal combustion engine, the reaction occurs combustively and the energy is released in the form of heat, some of which can be used to do useful work by pushing a piston. In a fuel cell, the reaction occurs electrochemically and the energy is released as a combination of low-voltage DC electrical energy and heat. The electrical energy can be used to do useful work directly while the heat is either wasted or used for other purposes.

In a fuel cell, the fuel and the oxidant gases themselves comprise the anode and cathode respectively. Thus, the physical structure of a fuel cell is one where the gases are directed through flow channels to either side of the electrolyte. The electrolyte is the distinguishing feature between different types of fuel cells. Different electrolytes conduct different specific ions. Electrolytes can be liquid or solid; some operate at high temperature, and some at low temperature. Low-temperature fuel cells tend to require a noble metal catalyst, typically platinum, to encourage the electrode reactions whereas high-temperature fuel cells do not. Most fuel cells suitable for automotive applications use a low temperature solid electrolyte that conducts hydrogen ions.

In principle, a fuel cell can operate using a variety of fuels and oxidants. Hydrogen has long been recognized as the most effective fuel for practical fuel cell use since it has higher electrochemical reactivity than other fuels, such as hydrocarbons or alcohols. Even fuel cells that operate directly on fuels other than hydrogen tend to first decompose into hydrogen and other elements before the reaction takes place. Oxygen is the obvious choice of oxidant due to its high reactivity and its abundance in air.

Water

Alkaline Fuel Cell

Proton Exchange Membrane FC PEMFC

Direct Methanol Fuel Cell DMFC

Phosphoric Acid Fuel Cell PAFC

Molten Carbonate Fuel Cell MCFC CC

Solid Oxide Fuel Cell

SOFC

Fuel

Proton Exchange Membrane FC PEMFC

Direct Methanol Fuel Cell DMFC

Phosphoric Acid Fuel Cell PAFC

Molten Carbonate Fuel Cell MCFC CC

80°C

Anode Electrolyte Cathode

Fig. 2.14 Principles and types of fuel cells [33]

Steam

1000°C

Cxygen (Air)

Anode Electrolyte Cathode

Fig. 2.14 Principles and types of fuel cells [33]

2.3.3.2.2 Types of Fuel Cells

Types of fuel cells differ primarily by the type of electrolyte they employ (Figure 2.14). The type of electrolyte, in turn, determines the operating temperature, which varies widely between types. High-temperature fuel cells operate at temperatures higher than 600 °C. These high temperatures permit the spontaneous internal reforming of light hydrocarbon fuels — such as methane — into hydrogen and carbon in the presence of water. This reaction occurs at the anode over a nickel catalyst provided that adequate heat is always available. This is essentially a steam reforming process (see Section 2.3.2.2). Internal reforming eliminates the need for a separate fuel processor, and can use fuels other than pure hydrogen. These significant advantages lead to an increase in overall efficiency by as much as 15%. During the electrochemical reaction that follows, the fuel cell draws on the chemical energy released during the reaction between hydrogen and oxygen to form water, and the reaction between carbon monoxide and oxygen to form carbon dioxide.

High-temperature fuel cells also generate high-grade waste heat, which can be used in downstream processes for co-generation purposes. They react easily and efficiently without an expensive noble metal catalyst, such as platinum. On the other hand, the amount of energy released by the electrochemical reaction degrades as the reaction temperature increases. They also suffer from severe materials problems. Few materials can work for extended periods without degradation within a chemical environment at high temperature. Furthermore, high-temperature operation does not lend itself easily to large-scale operations and is not suitable where quick start-up is required. As a result, current high-temperature fuel cells applications have focused on stationary power plants where the efficiencies of internal reforming and co-generative capabilities outweigh the disadvantages of material breakdown and slow start-up. The most prominent high-temperature fuel cells are molten carbonate and solid oxide.

Low-temperature fuel cells typically operate below 250 °C. These low temperatures do not permit internal reforming, and therefore require an external source of hydrogen. On the other hand, they exhibit quick start-up, suffer fewer materials problems and are easier to handle in vehicle applications. The most prominent low-temperature fuel cells are alkaline; phosphoric acid, and proton exchange membrane (or solid polymer).

2.3.3.2.3 Advantages and Disadvantages of Fuel Cells

Fuel cell systems are usually compared to internal combustion engines and batteries. They offer unique advantages and disadvantages with respect to them, as summarized in Table 2.4.

a. Advantages

Fuel cell systems operate without pollution when operated with pure hydrogen, the only byproducts being pure water and heat. When using hydrogen-rich reformate gas mixtures, some harmful emissions result although they are less than those emitted by an internal combustion engine using conventional fossil fuels. Thermodynamic efficiency of the fuel cell is higher than that of heat engines. Since fuel cells do not use combustion, the efficiency is not linked to their maximum operating temperature. As a result, the efficiency of the power conversion step (the actual electrochemical reaction as opposed to the actual combustion reaction) can be significantly higher. The efficiency characteristics of fuel cells compared with other electric power generating systems are shown in Figure 2.15.

Table 2-4. Advantages and disadvantages of fuel cells [77]

Fuel cell

(°C)

Applications

Advantages

Disadvantages

PEM FC

Solid organic polymer poly-perfluorosulfonic acid

60 -100

Electric utility, portable power, transportation

Solid electrolyte reduces corrosion & management problems Low temperature Quick start up

Low temperature requires expensive catalysts

High sensitivity to fuel impurities

AFC

Aqueous solution of potassium hydroxide soaked in a matrix solution

90-100

military space

Cathode reaction faster in alkaline electrolyte — so high performance

Expensive removal of CO2 from fuel and air streams required

PAFC

Liquid phosphoric acid soaked in a matrix

200

electric utility transportation

• Up to 85 % efficiency in co-generation of electricity and heat

• Impure H2 as fuel

• Low current and power

• Large size/weight

MCFC

Liquid solution of lithium, sodium and/ or potassium carbonates, soaked in a matrix

600 -1000

electric utility

High temperature advantages

High temperature enhances corrosion and breakdown of cell components

SOFC

Solid zirconium oxide to which a small amount of ytrria is added

600 -1000

electric utility

• High temperature advantages*

• Solid electrolyte advantages (see PEM)

High temperature enhances breakdown of cell components

b. Disadvantages b. Disadvantages

Fuel cells require relatively pure fuel, free of specific contaminants [46]. These contaminants include sulfur and carbon compounds, and residual liquid fuels (depending on the type of fuel cell) that can deactivate the fuel cell catalyst effectively destroying its ability to operate. None of these contaminants inhibit combustion in an internal combustion engine. Fuel cell systems are heavy. Fuel cells themselves are not excessively heavy, but the combined weight of the fuel cells, their support systems and their fuel storage is presently greater than that of a comparable internal combustion engine system. Fuel cell systems are generally lighter than comparable battery systems even though the battery systems require less support equipment.

Despite their weight, existing fuel cell prototype vehicles have shown that systems can be made sufficiently compact for automotive use.

^■•-Theoretical Maximum, Hydrogen Fuel Cells

^■•-Theoretical Maximum, Hydrogen Fuel Cells

1 10 100 1,000 10,000 100,000 Power Output kW

Fig. 2.15 Power generating systems efficiency comparison [46] 2.3.3.2.4 Applications

1 10 100 1,000 10,000 100,000 Power Output kW

Fig. 2.15 Power generating systems efficiency comparison [46] 2.3.3.2.4 Applications

Fuel cells are inherently modular and therefore lend themselves to a wide range of applications, from large stationary power plants to small portable power packs.

a. Stationary Power plants

Stationary power plant applications have been demonstrated in a number of pilot projects using a variety of fuel cell technologies over the past decades. The largest power plant to date is the Ballard Generation System's 250 kW natural gas fuelled proton exchange membrane fuel cell power plant currently operating at a number of sites worldwide. Although 250 kW is a small amount of power compared to conventionally powered generating stations, it is adequate to service isolated neighbourhoods or to provide emergency backup power to critical facilities, such as hospitals. Stationary power plants are obvious candidates for operation using conventional fuels, such as natural gas, which can be piped to the power plant and reformed on site. Overall size and warm-up time are less critical issues than in smaller, mobile applications. In addition to the high operating efficiency, low emissions and good transient response characteristic of fuel cell systems, stationary applications also produce copious amounts of hot water and waste heat that can be used directly in the surrounding community, further in-creasing the overall system effectiveness.

b. Traffic Applications

Fuel cells systems are attractive for traffic applications due to their low noise, and zero emissions. Buses are the most commercially advanced of all fuel cell applications to date. For example, a successful demonstration program has been carried out by XCELLSiS Fuel Cell Engines, Inc., with the introduction of three buses each in Vancouver and Chicago. All of these buses use pure hydrogen stored as a high-pressure gas; other demonstration vehicles have used liquid fuels and incorporate on-board reformer systems. Buses are a logical starting point for the introduction of fuel cell technology into the transportation sector for several reasons: they offer a reasonably large platform for system components and fuel storage, they can be fuelled at a central fuelling station, and they are regularly maintained by trained personnel.

Cars represent the ultimate market for fuel cell manufacturers due to the quantities involved worldwide. While cars provide the major stimulus for fuel cell development, as they are a major contributor to air pollution, they also pose some of the greatest challenges to commercialization. These challenges include their relatively small size, the vast fuelling infrastructure required, and the inconsistent maintenance habits of the public at large. In addition, performance and reliability expectations are high, while cost expectations are low. Many major car companies are engaged in automotive fuel cell programs including DaimlerChrysler, Ford, General Motors, Nissan, Mazda, Subaru, Toyota, Honda and Hyundai. Some of these companies have built prototype vehicles using fuel cells with or without auxiliary batteries, and fuelled using either pure (gaseous or liquid) hydrogen or reformate.

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment