If a mixture of water and alcohol is boiled, the percentage of alcohol to water is greater in vapor than in liquid. Therefore, by repeated distil­lation and condensation, the alcoholic strength of the distillate can be increased until it contains 97.6% alcohol. There are different methods of distillation, but they are not discussed here, as ethanol production is our prime concern.

Alkaline-electrolyte fuel cells (see Fig. 9.7) are one of the most developed fuel cell technologies. They have been in use since the mid-1960s for Apollo and space shuttle programs [3, 6, 18, 19]. The AFCs onboard these spacecraft provide electrical power as well as drinking water. AFCs are among the most efficient electricity-generating fuel cells with an efficiency of nearly 70%. The electrolyte used in the AFC is an alka­line solution in which an OH ion can move freely across the electrolyte.

Electrochemistry of AFCs. The electrolyte used in the AFC is an aqueous (water-based) solution of potassium hydroxide (KOH) retained in a porous stabilized matrix. The concentration of KOH can be varied with the fuel cell operating temperature, which ranges from 65 to 220°C.

The charge carrier for an AFC is the hydroxyl ion (OH-) that migrates from the cathode to the anode, where they react with hydrogen to pro­duce water and electrons. Water formed at the anode migrates back to the cathode to regenerate hydroxyl ions.

Anode reaction: 2H2 + 4OH — ^ 4H2O + 4e-

Cathode reaction: O2 + 2H2O + 4e — ^ 4OH-

Hydroxyl ions are the conducting species in the electrolyte.

Overall cell reaction: 2H2 + O2 ^ 2H2O + heat + electricity

In many cell designs, the electrolyte is circulated (mobile electrolyte) so that heat can be removed and water eliminated by evaporation. Since KOH has the highest conductance among the alkaline hydroxides, it is the preferred electrolyte.

Electrolyte. Concentrated KOH (85 wt.%) is used in cells designed for operation at a high temperature (~260°C). For lower temperature (<120oC) operation, less concentrated KOH (35-50 wt.%) is used. The electrolyte is retained in a matrix (usually asbestos), and a wide range of electrocatalysts can be used (e. g., Ni, Ag, metal oxides, and noble metals). A major advantage of the AFC is the lower activation polar­ization at the cathode, resulting in a higher operating voltage (0.875 V). Another advantage of the AFC is the use of inexpensive electrolyte materials. The electrolyte is replenished through a reservoir on the anode side. The typical performance of this AFC cell is 0.85 V at a cur­rent density of 150 mA/cm2. The AFCs used in the space shuttle orbiter have a rectangular cross-section and weigh 91 kg. They operate at an average power of 7 kW with a peak power rating of 12 kW at 27.5 V. A disadvantage of the AFC is that it is very sensitive to CO2 present in the fuel or air. The alkaline electrolyte reacts with CO2 and severely degrades the fuel cell performance, limiting their application to closed environments, such as space and undersea vehicles, as these cells work well only with pure hydrogen and oxygen as fuel.

Electrodes. A significant cost advantage of alkaline fuel cells is that both anode and cathode reactions can be effectively catalyzed with non­precious, relatively inexpensive metals. The most important character­istics of the catalyst structure are high electronic conductivity and stability (mechanical, chemical, and electrochemical). Both metallic (typ­ically hydrophobic) and carbon-based (typically hydrophilic) electrode structures with multilayers and optimized porosity characteristics for the flow of liquid electrolytes and gases (H2 and O2) have been developed. The kinetics of oxygen reduction in alkaline electrolytes is much faster than in acid media; hence AFCs can use low-level Pt catalysts (about 20% Pt, compared with PEMFCs) on a large surface carbon support [20].

Performance. The AFC development has gone through many changes since 1960. To meet the requirements for space applications, the early AFCs were operated at relatively high temperatures and pressures. Now the focus of the technology is to develop low-cost components for AFCs operating at near-ambient temperature and pressure, with air as the oxi­dant for terrestrial applications. This has resulted in lower perform­ance. The reversible cell potential for an H2 and O2 fuel cell decreases by 0.49 mV/°C under standard conditions. An increase in operating tem­perature reduces activation polarization, mass transfer polarization, and ohmic losses, thereby improving cell performance. Alkaline cells operated at low temperatures (~70°C) show reasonable performance.

Pure hydrogen and oxygen are required in order to operate an AFC. Reformed H2 or air containing even trace amounts of CO2 dramatically affects its performance and lifetime. There is a drastic loss in performance when using hydrogen-rich fuels containing even a small amount of CO2 from reformed hydrocarbon fuels and also from the presence of CO2 in the air (~350 ppm CO2 in ambient air). The CO2 reacts with OH (CO2 + 2OH ^ CO32~ + H2O), thereby decreasing their concentration and thus reducing the reaction kinetics. Other ill effects of the presence of CO2 are:

■ Increase in electrolyte viscosity, resulting in lower diffusion rate and

lower limiting currents.

■ Deposition of carbonate salts in the pores of the porous electrode.

■ Reduction in oxygen solubility.

■ Reduction in electrolyte conductivity.

A higher concentration of KOH decreases the life of O2 electrodes when operating with air containing CO2. However, operation at higher tem­peratures is beneficial because it increases the solubility of CO2 in the elec­trolyte. The operational life of air electrodes polytetrafluoroethylene [PTFE] bonded carbon electrodes on porous nickel substrates) at a current density of 65 mA/cm2 in 9-N KOH at 65°C ranges from 4000 to 5500 h with CO2-free air, and their life decreases to 1600-3400 h when air (350-ppm CO2) is used. For large-scale utility applications, operating times >40,000 h are required, which is a very significant hurdle to com­mercialization of AFC devices for stationary electric power generation.

Another problem with the AFC is that the electrodes and catalysts degrade more on no-load or light-load operation than on a loaded condition, because the high open-circuit voltage causes faster carbon oxidation processes and catalyst changes. The AFC with immobilized KOH electrolyte suffers much more from this as the electrolyte has to stay in the cells caus­ing residual carbonate accumulation, separator deterioration, and gas cross leakage during storage or unloaded periods if careful maintenance is not carried out. In circulating an electrolyte-type AFC, the electrolyte is emp­tied from the cell during nonoperating periods. Shutting off the H2 elec­trodes from air establishes an inert atmosphere. This shutdown also eliminates all parasitic currents and increases life expectancy. The exchangeability of the KOH in a circulating electrolyte-type AFC offers the possibility to operate on air without complete removal of the CO2 [20, 21].

K. B. De

1.1 Energy (Yesterday, Today, and Tomorrow)

Today’s energy concept needs to be modified and should be presented as an integrated management-oriented approach. For example, the problem of nutrition of human population and livestock is also an important item in the energy inventory. So the per capita energy requirement will include 2000 kcal of the basal requirement in the form of nutrients; amount of energy required to produce that amount of food; energy required to preserve the food; energy required to collect the daily requirement of 600-800 L of water; energy required for washing, cleaning, and bathing; and energy required for lights, fans, air conditioners, and transport.

Today’s energy concept should also include the awareness that heat is a wasteful form of energy, always downhill, and hence efficiency is at the most 30-40% and that of an automobile is as low as 15-20%. Even if we go modern, a solar photovoltaic panel has an efficiency of 8%, a solar thermal power plant has 15%, and from sunshine to electricity through biomass is only 1%.

In order to establish innovative technologies for highly effective uti­lization of solar light energy, fundamental research is being conducted in the following areas:

1. Dye-sensitized solar cells: New types of dye-sensitized solar cells
mimicking the active sites of the natural photosynthesis system.

2. Artificial photosynthesis: Hydrogen production from water, using metal oxide semiconductor photocatalyst systems and effective fixa­tion of CO2 by metal or metal complex catalysts.

Estimated contribution of renewable energy resources in the United States by AD 2000, excluding hydro — and geothermal energy, amounts to approximately 5% of the estimated total consumption of 100 quads. Tropical countries, namely, India, receive 1648-2108 kWh/m2 of solar energy in different parts with 250-300 days of sunshine, most of which is unutilized.

While shifting our attention from today into the future, we should look at some discussions that took place in the 12th Congress of World Energy, a conference held in New Delhi, during September 18-23, 1983, the main themes of which were management, policy, development, and quality of life. There were four divisions, and each of these divisions had four sections containing 157 technical papers. In the concluding session, Dr. J. S. Foster, chairman of the program committee, on behalf of the International Executive Council, gave a summary.

1. Innovation: Commenting on innovation, a report from Israel nar­rated absorption refrigeration, and Austria reported on a thermal power plant, investigating a treble Rankine cycle using three sepa­rate working fluid loops. Brazil reported methane from urban refuge and collaborative international efforts on controlled nuclear fusion were highlighted.

2. Self-Reliance: Self-reliance has been well emphasized.

3. Diversification: Diversification in national or regional supply ensures a robust energy structure, reducing vulnerability to vagaries of nature, resource, or market fluctuations.

4. Dependence: Dependence on fossil fuels can be reduced with proper substitution by biogas, solar, wind, and nuclear powers.

5. Efficiency and conservation: Waste heat recovery, cogeneration, and recycling of energy were in the technological aspect. Public and social consciousness through education is the other aspect.

6. Development: International cooperation and development assis­tance should involve mainly (a) financial resources, (b) technology transfers, and (c) transfer of managerial and engineering skills.

7. Care of the environment: Pollutions from fossil fuels, nuclear reac­tors, and effects on forests and vegetation from dams are to be studied along with future expansion schemes.

8. Quality of life: Indiscriminate and unplanned use of energy may lead to negative and harmful impacts. Need for energy education

and man power training with an integrated approach has been rec­ommended.

9. Urgency:

a. World population will reach 10 billion in 2020.

b. Half of the population will have only 20 GJ/yr.

c. The other half in the industrialized countries will use 15 times as much, i. e., 300 GJ/yr.

d. “Firewood crisis” has changed to “firewood catastrophe” and the forest cover is diminishing globally at the rate of 250,000 km2/yr. Energy administration in developing countries depends mainly on three denominators:

(1) Growth rate of population

(2) Energy self-reliant populations growing in size but lowest rate

(3) Rural population largest in size, but lowest rate of energy consumption in most countries

Recommendations of new and alternative energy resources are available. The emphasis is on nonfossil and renewable resources, namely, biogas and biomass, solar active (photovoltaic), solar passive (preheating of water), wind, minihydroelectric, and minitidal resources. The major attempts for conservation include: conservation side legis­lation, education, awareness, management, and forecasting.

In Europe and South America, biomass and its modification have been given a lot of attention as a substitute to fossil fuels. The primary material, of course, is the waste of different plant and vegetable origins. The conversions are to biogas, alcohol, and manure. Proper selection of waste material may lead to optimal production of the right transform. Advanced countries, point out that electrical power is attractive in many respects and that the search for renewable and infinite resources to produce and supplement electrical energy should continue. Hydropower, solar energy, wind, solid waste, biomass, geothermal energy, ocean tidal power, and ocean thermal gradients are a few resources that need atten­tion. In fact, many institutions and organizations have created demon­stration models for these.

In the United Kingdom, the emphasis seems to be more on proper selection of local conditions and availability of the resources. Biomass and biogas need collection, transport, and processing to be properly useful for energy generation. Setting up aerogenerators, wind pumps, and solar heating will depend on available and favorable conditions and the proper location. If successfully implemented, they can reduce the local demand or share the load of a national power grid. The other resources that remain to be developed and commercialized are listed in the fol­lowing discussion.

1. Fusion of thermonuclear devices (an application of plasma physics):

21D + [1] [2]1D ^ [3]2He + n + 3.2 Mev 21D + 31T ^ [4] [5]2He + n + 17.6 Mev 21D + 21D ^ 31T + 11H + 4.0 Mev [6]3Li + n ^ 42He + 31T + 4.8 Mev 21D + 31T ^ 42He + n + 17.6 Mev 21D + 63Li ^ 242He + 22.4 Mev 21D + 32He3 ^ 42He4 + 11H + 18.3 Mev

7. Coal conversion: Many models for fluidization and gasification of coal are available.

8. Black box or hydrogen fuel cell: Usually, these use hydrogen as input fuel based on reverse hydrolysis (see last part of Sec. 1.6):

At anode: H2 ^ 2H+ + 2e~

At cathode: O2 + 4H+ + 4e~ ^ 2H2O

9. Hydrogen as fuel: Hydrogen as fuel is gaining popularity. The most common sources are from (a) excess of nuclear energy, (b) windmills, (c) hydroelectric power, (d) biological sources to some extent, (e) fuel cell (see Sec. 1.6), and (f) microbial hydrogen pro­duction (see Sec. 1.16).

10. Biological energy: A number of biological energy transformation principles, very attractive, remain at the conceptual state.

In Chap. 1, gasification (pyrolysis) of biomass, biogas, gobargas, hydro­gen, and biohydrogen were discussed in detail.

2.6.1 Liquid products

An important renewable energy resource for transportation purposes is liquid fuel based on plant oils. However, pure plant oils are generally not suitable for use in modern diesel engines. This can be overcome by the process of transesterification. The resultant fatty-acid methyl esters have properties similar to those of diesel and are commonly called biodiesel. Biodiesel presents several advantages, such as better CO2 balance than diesel, low soot content, reduced hydrocarbon emissions, and low carcinogenic potential [20]. The specification standards for the European Union (EU) and the United States are EN14214 and ASTM D6751, respectively. The EU directive established a minimum content of 2% and 5.75% biodiesel for all petrol and diesel used in transport by December 31, 2005, and December 31, 2010, respectively. Biodiesel refers to the pure oil before blending with diesel fuel. Biodiesel blends are represented as “BXX,” with “XX” representing the percentage of biodiesel component in the blend (National Biodiesel Board, 2005) [21]. In the biomass-to-liquid conversion processes, biomass is broken down into a gaseous constituent and a solid constituent by low-temperature gasification. The next step involves production of synthetic gas, which is converted into fuel (termed SunFuel) by the Fischer-Tropsch synthe­sis process, with downstream fuel optimization by hydrogen after treat­ment [22]. Ethanol has already been introduced in countries such as Brazil, the United States, and some European countries. In Brazil, it is currently produced from sugar and, in the United States, from starch at competitive prices. Ethanol is currently produced from sugarcane and starch-containing materials, where the conversion of starch to ethanol includes a liquefaction step (to make the starch soluble) and a hydrolysis step (to produce glucose). There are generally two types of processes for production of bioethanol: the lignocellulosic process and the starch process. Unlike the starch-based process, the lignocellulosic process has not been as widely adopted due to techno-economic reasons.

High ethanol yield requires complete hydrolysis of both cellulosic and hemicellulose with a minimum of sugar dehydration, followed by effi­cient fermentation of all sugars in the biomass. Certain advantages of using lignocellulose-based liquid biofuels are that they are evenly dis­tributed across the globe and hence are readily available, less expensive compared to agricultural feedstock, produced at a lower cost, and have low net greenhouse gas emissions. Enzymatic processes (essentially using bacteria, yeasts, or filamentous fungi) have been considered for lignocellulosic processes. The enzymatic process when coupled with the fermentation process is known as simultaneous saccharification and fermentation. This has proved to be efficient in the fermentation of hexose and pentose sugars [23]. Genencor International (www. genen- cor. com/) and Novozymes, Inc., (www. novozymes. com) have been awarded $17 million each by the U. S. Department of Energy with a goal to reduce the enzyme cost tenfold (www. eere. energy. gov/). The Iogen Corp. (www. iogen. ca/) demo-plant is the only one that produces bioethanol from lignocellulose, using the enzymatic hydrolysis process. This plant is known to handle about 40 ton/day of wheat, oat, barley, and straw and is designed to produce up to 3 ML/yr of cellulose ethanol. Refer to Chap. 3 for bioethanol preparation, Chap. 6 for boidiesel processing, and Chap. 7 for ethanol and methanol used in engines.

Mash is usually centrifuged or settled in order to separate the micro­bial biomass from the liquid and then sent to the ethanol recovery system. Distillation is typically used for the separation of ethanol, alde­hydes, fusel oil, and stillage [9]. Ethanol is readily concentrated from mash by distillation, since the volatility of ethanol in a dilute solution is much higher than the volatility of water. Therefore, ethanol is sepa­rated from the rest of the materials and water by distillation. However, ethanol and water form an azeotrope at 95.57 wt% ethanol (89 mol% ethanol) with a minimum boiling point of 78.15°C. This mixture behaves as a single component in a simple distillation, and no further enrichment than 95.57 wt% of ethanol can be achieved by simple distil­lation [9, 47, 81]. Various industrial distillation systems for ethanol purifi­cation are (a) simple two-column systems, (b) three- or four-column barbet systems, (c) three-column Othmer system, (d) vacuum rectification, (e) vapor recompression, (f) multieffect distillation, and (g) six-column reagent alcohol system [9, 47]. These methods are reviewed by Kosaric [9]. The following parameters should be considered for selection of the industrial distillation systems:

■ Energy consumption (e. g., steam consumption or cooling water con­sumption per kilogram of ethanol produced).

■ Quality of ethanol (complete separation of fusel oil and light compo­nents).

■ How to deal with the problem associated with clogging of the first dis­tillation column and its reboiler because of precipitation or formation of solids. Special design and use of a vacuum may be applied for over­coming the problem in the column. Using open steam instead of appli­cation of a reboiler can prevent clogging of the reboiler, in spite of the increase in amount of wastewater.

■ Simplicity in controlling the system.

■ Simplicity in opening column parts and cleaning the columns.

Of course, lower capital investment is also one of the main parameters in the selection of distillation systems.

Ethanol is present in the market with different degrees of purity. The majority of ethanol is 190 proof (95% or 92.4%, minimum) used for sol­vent, pharmaceutical, cosmetic, and chemical applications. Technical — grade ethanol, containing up to 5% volatile organic aldehyde, ester, and sometimes methanol, is used for industrial solvents and chemical syn­theses. High-purity 200 proof (99.85%) anhydrous ethanol is produced for special chemical applications. For fuel use in mixture with gasoline (gasohol), nearly anhydrous (99.2%) ethanol, but with higher available levels of organic impurities, is used [47].

A simple two-column system is described here, while other systems are presented in the literature (e. g., [9, 47]). Simple one — or two — column systems with only a stripping and rectification section are usually used to produce lower-quality industrial alcohol and azeotrope alcohol for further dehydration to fuel grade. The simplest continu­ous ethanol distillation system consists of stripping and rectification sections, either together in one column or separated into two columns (see Fig. 3.10).

The mash produced is pumped into a continuous distillation process, where steam is used to heat the mash to its boiling point in the stripper column. The ethanol-enriched vapors pass through a rectifying column and are condensed and removed from the top of the rectifier at around 95% ethanol. The ethanol-stripped stillage falls to the bottom of the stripper column and is pumped to a stillage tank. Aldehydes are drawn

Stillage <

Figure 3.10 Two-column system for distillation of ethanol.

from the head vapor, condensed, and partly used as reflux. Fusel oil is taken out from several plates of the rectifying section [9, 47, 82].

With efficient distillation, the stillage should contain less than 0.1% ethanol since the presence of ethanol significantly increases the chem­ical oxygen demand (COD) of wastewater. For each 1% ethanol left in the stillage, the COD of the stillage is incremented by more than 20 g/L. Due to the potential impact of residual ethanol content, therefore, proper control over distillation can greatly affect the COD of stillage [82].

Crop description. Aleurites fordii (Vernicia fordii) and A. montana— commonly known as the tung tree, Chinese wood, Abrasin, and Mu (see Fig. 4.24)—belong to the family Euphorbiaceae and grow well in cold cli­mates, but will survive in subtropical conditions (A. fordii). A. montana prefers a tropical climate. Major producers are China, Argentina, Paraguay, Brazil, and the United States. The nut of this deciduous tree contains an oil-rich kernel. The oil content of the air-dried fruit lies between 15% and 20% [77]. Major fatty acid composition of oil includes

palmitic acid (5.5%), oleic acid (4.0%), linoleic acid (8.5%), and eleostearic acid (82%) [77].

Main uses. Tung oil is used in paints, varnishes, and so forth. It is also used in the production of linoleum, resins, and chemical coatings. It has been used in motor fuels in China [77]. The seed cake after oil extraction is used as a fertilizer and cannot be used for animal feed as it contains a toxic protein [75]. No references about its use as a raw mate­rial to produce biodiesel have been found to date.

Methanol has high latent heat; therefore, some provision must be pro­vided to heat the intake manifold, because cold starting problems are often caused by A:F vapor mixture being outside the flammability range. Specially, methanol in its pure form is much more inferior to petrol for cold starting. Cold starting more or less becomes impossible with methanol when the ambient temperature falls below system on chip (SOC). Figure 7.12 shows the modification that is provided to avoid the difficulty of cold starting. By preheating, methanol dissociates into CO + 2H2 to obtain gaseous H2, which gives a broad flammability limit. While cranking the engine, a rich gaseous A:F mixture of methanol is collected near the spark plug, which enables good starting of the engine.

The major applications for fuel cells are as stationary electric power plants (including cogeneration units), as a transportation power source

for vehicles, and as portable power sources, besides an electric power source for space vehicles or other closed environments.

Stationary power applications are very favorable for fuel cell systems. Stationary applications mostly require continuous operation, so start­up time is not a very important constraint. Thus, high-temperature fuel cells such as the MCFC and SOFC systems are also suitable for this application in addition to the PAFC and PEMFC systems. The fuel source for stationary applications is most likely to be natural gas, which is relatively easy to reform in the internal reformer of high-temperature fuel cells or in the external reformer for low-temperature fuel cells. An advantage of using natural gas is that the distribution infrastructure for natural gas already exists. Promising applications for stationary fuel cell systems include premium power systems (high-quality uninterruptible/ back-up power supply systems); high-efficiency cogeneration (heat and electricity) systems for residences, commercial buildings, hospitals, and

industrial facilities; and distributed power generation systems for util­ities. Although some demonstration and commercial stationary fuel cell power plants in sizes from a few kilowatts to 11 MW are in operation, widespread commercialization can be expected only if their installation cost drops down from the present cost of $4000/kW to about $400-700/kW (or about $1000/kW for some premium applications).

The recent surge of interest in fuel cell technology is because of its potential use in transportation applications, including personal vehicles. This development is being sponsored by various governments in North America, Europe, and Japan, as well as by major automobile manufac­turers worldwide, who have invested several billion dollars with the goal of producing a high-efficiency and low-emission fuel cell power plant at a cost that is competitive with the existing internal combustion engines. With hydrogen as the onboard fuel, such vehicles would be zero-emission vehicles. With fuels other than hydrogen, an appropriate fuel processor to convert the fuel to hydrogen will be needed. Fuel cell — powered vehicles offer the advantages of electric drive and low mainte­nance, because of the few critical moving parts. The major activity in transportation fuel cell development has focused on the polymer elec­trolyte fuel cell (PEFC), and many of the technical objectives related to the fuel cell stack have been met or are close to being met. The current development efforts are focused on decreasing cost and resolving issues related to fuel supply and system integration.

Besides exotic areas of applications such as space vehicles or sub­marines, another very promising area of application for fuel cells is portable power systems. Portable power systems are small, lightweight systems that power portable devices (e. g., computers, laptops, cellular phones, and entertainment electronic devices), camping and recreational vehicles, military applications in the field, and so forth. These devices need power in the range of a few watts to a few hundred watts. Fuel cell systems based on DMFC or PEMFC technology are well suited for many of these applications. The convenience of transporting and storing liquid methanol makes DMFC systems very attractive for this application. A small container of methanol or a cylinder of compressed hydrogen can be used as a fuel supply. When the fuel is depleted, a new fuel container may be installed in its place after removing the old one.

In recent years, there has been a lot of interest in electric power gen­eration using renewable energy sources such as wind energy, solar energy, and tidal energy. A major problem with these energy sources is that all are intermittent in nature. Combining the renewable energy — based power generation system with a fuel cell system would solve this problem to a great extent. A hybrid wind/solar energy-fuel cell system can use wind/solar power for generating hydrogen using the electroly­sis of water, and store it in cylinders at high pressure. This hydrogen can then be used as the fuel for the fuel cell stack. The stored hydrogen can also be used to fuel the fuel cell vehicles and so forth. In a grid — connected wind/solar energy—hydrogen system, wind/solar power whenever available provides electricity for hydrogen production. The grid power is used during off-peak periods for low-cost electricity and hydrogen production; whereas during peak-demand periods or no/low wind/solar energy periods, the fuel cell can generate electricity using the stored hydrogen. These hybrid systems could be configured in several ways.

9.4 Conclusion

Fuel cell systems are one of the most promising technologies to meet our future power generation requirements. Fuel cell systems provide a very clean and efficient technology for electrical and automotive power sys­tems. With cogeneration efficiencies higher than 80%, fuel cells prom­ise to reduce primary energy use and environmental impact. Fuel cells are a very good alternative for rural energy needs, especially in remote places where there are no existing power grids or power supply is unre­liable. The application of fuel cells into the transportation sector will reduce greenhouse emissions considerably; if fuels from renewable energy sources are used, it would nearly eliminate greenhouse gas emis­sions. Utility companies are beginning to locate small, energy-saving power generators closer to loads to overcome right of way problems and transmission line costs. The modular design of fuel cells suits this dis­tributed generation strategy very nicely as new modular units can be added when the demand increases. This reduces the financial risk for utility planners. Biofuel cells are very attractive for implant devices as they can use glucose in blood to power these devices, eliminating the need for surgery for maintenance and battery replacement. Use of digester gas as a fuel in biofuel cells makes them very attractive for power generation from garbage and other organic waste. This will also help in waste disposal, a big problem in the agriculture and food industry.

All fuel cell technologies (PEMFC, DMFC, AFC, PAFC, MCFC, SOFC, and MFC) discussed in this chapter are in a very advanced stage of development and are very near to commercialization. Although a number of demonstration units of different types of fuel cells are oper­ating all over the world and many PAFC and AFC units have been com­mercially sold and are successfully operating, fuel cells are still awaiting widespread commercialization due to their high cost and limitation in the choice of the fuel used. These barriers will be overcome in the next few years, and fuel cells will become a preferred power source with widespread applications.

Small vesicles, called chromatophores, can be isolated from the membranes of photosynthetic bacteria, which exhibit two types of electron transfer chains resembling mitochondria and chloroplasts. Chroma-tophores supported on artificial membranes permit the generation of 200 mV on illumination. The salt-bacteria (Halobacterium halobium) contain a simple protein-vitamin A aldehyde, known as bacteriorhodopsin, when supported on artificial membranes that generate 250 mV on illumina­tion. This system is simpler than its counterpart. There is a probabil­ity that the entire system may be successfully synthesized or assembled. Solar photocells made of bacteriorhodopsin show great promise.

There are several possible ways to hydrolyze lignocellulose (see Fig. 3.5). The most commonly applied methods can be classified into two groups: chemical hydrolysis and enzymatic hydrolysis. In addition, there are

some other hydrolysis methods in which no chemicals or enzymes are applied. For instance, lignocellulose may be hydrolyzed by y-ray or electron-beam irradiation, or microwave irradiation. However, those processes are commercially unimportant [15].