Crop description. Camelina sativa L. Crantz—commonly known as gold-of-pleasure and camelina—belongs to the family Cruciferae and grows well in temperate climates (see Fig. 4.16). It is an annual oilseed plant and is cultivated in small amounts in France, and to a lesser

Figure 4.16 Camelina sativa L. Crantz. (Photo courtesy of Prof. Arne Anderberg [http://linna. eus. nrm. se/flora/di/brassica/camel/ camemic. html].)

extent in Holland, Belgium, and Russia. The oil content of camelina seeds ranges from 29.9% to 38.3%. However, it is an underexploited oilseed crop at present. Its fatty acid profile includes oleic acid (14-19.5%), linoleic acid (18.8-24%), linolenic acid (27-34.7%), eicosenoic acid (12-15%), and erucic acid (less than 4%) [133]. Budin et al. have concluded that camelina is a low-input crop possessing a potential for food and nonfood exploitation [133].

Main uses. This crop has recently been rediscovered as an oil crop. At the moment, the feasibility of utilizing oil from this plant is being investi­gated [53, 134]. Oil is used as a luminant and emollient for softening the skin. Fiber is obtained from the stems. Frohlich and Rice have investi­gated production of methyl ester from camelina oil. Biodiesel was pre­pared by means of a single-stage esterification using methanol and KOH [135]. Steinke et al. have developed both alkali-catalyzed and lipase-catalyzed alcoholyses of camelina oil [136, 137].

Both ethanol and methanol, as listed in Table 7.3, have high knock resistance (as the octane numbers are 89 and 92, against 85 for gaso­line), wide ignition limit, high latent heat of vaporization, and nearly

the same specific gravity. All those properties are of great advantage if used in SI engines. Some important advantages of alcohol-fueled engines compared with gasoline engines are listed below:

1. The alcohols (both) have higher heat of vaporization. As the liquid fuel evaporates into the air stream being charged to the engine, a higher heat of vaporization cools the air, allowing more mass to be drawn into the cylinder. This increases the power produced from the given engine size. High latent heat of vaporization leads to higher volumetric efficiency and provides good internal cooling.

2. The high octane number of alcohols compared to petrol means higher compression ratios can be used, which results in higher engine effi­ciency and higher power from the engine.

3. Ethanol burns faster than petrol, allowing more uniform and efficient torque development. Both alcohols have wider flamma­bility limits, which results into a rich air—fuel (A:F) ratio being used when needed to maximize power by injecting more fuel per cycle.

4. Alcohols also have lower exhaust emissions than gasoline engines except for aldehydes. Both alcohols have lower carbon-hydrogen ratio than petrol and diesel, and produce less CO2. For the same power output, CO2 produced by an ethanol-fired engine is about 80% of the petrol engine. Because of high heat of vaporization, the fuels burn at lower flame temperatures than petrol, forming less NOx. The CO percentage in both cases (alcohol and petrol) remains more or less the same.

5. Contamination of water in alcohols is less dangerous than petrol or diesel because alcohols are less toxic to humans and have a recog­nizable taste.

6. The alcohols can also be blended with gasoline to form the so-called gasohol (80% petrol and 20% alcohol), which is widely used in the United States.

7. Ethyl alcohol as a fuel offers great safety due to its low degree of volatility and higher flash point (17°C).

8. The heating value of alcohol is 60% of that of petrol (60% only), and it shows equally good thermal efficiency and lower fuel consumption, because the air required for petrol and alcohol is in the ratio of 15:9 by weight, which is the same as their calorific value, i. e., the same heat is developed per cylinder charge in petrol and alcohol engines. The power per unit volume of cylinder for petrol, ethanol, and methanol are closely similar.

9. In many hot-climate countries, more precautions are often taken for the use of more volatile spirit-based fuels, while alcohol is perfectly safe in the hottest climate.

10. The major problem faced with ethanol is corrosion; special metals should be used for the engine parts to avoid corrosion.

Alcohols are clean-burning, renewable alternative fuels that can come to our rescue to meet the duel challenge of vehicular fuel oil scarcity and fouling of the environment by exhaust emissions.

Alcohols inherently make very poor diesel engine fuels as their cetane number is considerably lower. They can be used in dual-fuel engines or with assisted ignition in diesel engine. In the dual-fuel mode, alcohol is inducted along with air, compressed, and then ignited by a pilot spray of diesel oil.

Phosphoric acid fuel cells (see Fig. 9.8) operate at intermediate tem­peratures (~200°C) and are very well developed and commercially avail­able today. Hundreds of PAFC systems are working around the world in hospitals, hotels, offices, schools, utility power plants, landfills and wastewater treatment plants, and so forth. Most of the PAFC plants are in the 50- to 200-kW capacity ranges, but large plants of 1- and 5-MW capacity have also been built; a demonstration unit has achieved 11 MW of grid-quality ac power [3]. PAFCs generate electricity at more than 40% efficiency and if the steam produced is used for cogeneration, efficien­cies of nearly 85% can be achieved. PAFCs use liquid phosphoric acid as the electrolyte. One of the main advantages to this type of fuel cell, besides high efficiency, is that it does not require pure hydrogen as fuel and can tolerate up to 1.5% CO concentration in fuel, which broadens the choice of fuels that can be used. However, any sulfur compounds present in the fuel have to be removed to a concentration of <0.1 ppmV. Temperatures of about 200°C and acid concentrations of 100% H3PO4 are commonly used, while operating pressure in excess of 8 atm has been used in an 11-MW electric utility demonstration plant [3, 22, 23].

Electrochemistry of PAFCs. The electrochemical reactions occurring in a PAFC are

At the anode:

H2 ^ 2H+ + 2e~

At the cathode:

—O2 + 2H+ + 2e S H2O The overall cell reaction:

2 O2 + H2 s H2O

The fuel cell operates on H2; CO is a poison when present in a concen­tration greater than 0.5%. If a hydrocarbon such as natural gas is used as a fuel, reforming of the fuel by the reaction

CH4 + H2O ^ 3H2 + CO

and shifting of the reformat by the reaction

CO + H2O ^ H2 + CO2

is required to generate the required fuel for the cell.

Electrolyte. The PAFC uses 100% concentrated phosphoric acid (H3PO4) as an electrolyte. The electrolyte assembly is a 0.1- to 0.2-mm-thick matrix made of silicon carbide particles held together with a small amount of PTFE. The pores of the matrix retain the electrolyte (phosphoric acid) by capillary action. At lower temperatures, H3PO4 is a poor ionic con­ductor and CO poisoning of the Pt electrocatalyst in the anode can become severe. There will be some loss of H3PO4 over long periods, depending upon the operating conditions. Hence, as a general rule, suf­ficient acid reserve is kept in the matrix at the beginning.

Electrode. The PAFC (similar to a PEMFC) uses gas diffusion electrodes. Platinum or platinum alloys are used as the catalyst at both electrodes. In the mid-1960s, the conventional porous electrodes were PTFE-bonded Pt black, and the loadings of Pt were about 9 mg/cm2. In recent years, Pt supported on carbon black has replaced Pt black in porous PTFE — bonded electrode structures. Pt loading has also dramatically reduced to about 0.25 mg Pt/cm2 in the anode and about 0.50 mg Pt/cm2 in the cathode. The porous electrodes used in a PAFC consist of a mixture of the electrocatalyst supported on carbon black and a polymeric binder to bind the carbon black particles together to form an integral structure. A porous carbon paper substrate provides structural support for the electrocatalyst layer and also acts as the current collector. The com­posite structure consisting of a carbon black/binder layer onto the carbon paper substrate forms a three-phase interface, with the electrolyte on one side and the reactant gases on the other side of the carbon paper. The stack consists of a repeating arrangement of a bipolar plate, the anode, electrolyte matrix, and cathode.

Hardware. A bipolar plate separates the individual cells and electri­cally connects them in a series in a fuel cell stack. A bipolar plate has a multifunction design; it has to separate the reactant gases in the adja­cent cells in the stack, so it must be impermeable to reactant gases; it must transmit electrons to the next cell (series connection), so it has to be electrically conducting; and it must be heat conducting for proper heat transfer and thermal management of the fuel cell stack. In some designs, gas channels are also provided on the bipolar plates to feed reactant gases to the porous electrodes and to remove the reaction products. Bipolar plates should have very low porosity so as to minimize phosphoric acid absorption. These plates must be stable and corrosion-resistant in the PAFC environment. Bipolar plates are usually made of graphite — resin mixtures that are carbonized and heat treated to 2700°C to increase corrosion resistance. For 100-kW and larger power generation systems, water cooling has to be used and cooling channels are provided in the bipolar plates to cool the stack.

Temperature and humidity management. Temperature and humidity management are essential for proper operation of a PAFC. The PAFC system has to be heated up to 130°C before the cell can start working. At lower temperatures, concentrated phosphoric acid does not get dis­sociated, resulting in a low availability of protons. Also, due to lower vapor pressure of the concentrated acid, the water generated will not come out with the reactant stream and the moisture retention dilutes the acid. This causes an increase in acid volume, which results in acid oozing out through the electrode. With the start of normal cell operation, its tem­perature increases and acid concentration gets back to its normal value that causes acid volume to shrink, resulting in drying of the electrolyte matrix pores if the acid is not replenished. Controlled stack heating at start-up is achieved by using an insertable heater system. During oper­ation, the temperature of the stack is maintained by controlling the air flow in the oxidant channel. At high loading conditions, insertable coolers may be used to remove excess heat from the stack. Large-power PAFC systems use a water-cooling system.

Moisture generated at the cathode dilutes the acid on the cathode side of the electrolyte matrix, causing higher vapor pressure. This results in more moisture out with the oxidant stream. With the movement of pro­tons from anode to cathode, moisture migration takes place at the cath­ode side also. This water evaporation results in an acid concentration gradient from anode to cathode, causing low availability of protons and a lower potential of the cell. Therefore, water management is needed to maintain humidity of the anode stream gas at a sufficient level so that the vapor pressure matches the acid concentration level at the operat­ing temperature.

Performance. For good performance, the normal operating tempera­ture range of a PAFC is 180°C < T < 250°C; below 200°C, the decrease in cell potential is significant. Although an increased temperature increases performance, higher temperatures also result in increased catalyst sin­tering, component corrosion, electrolyte degradation, and evaporation. PAFCs operate in the current density range of 100-400 mA/cm2 at 600-800 mV/cell. Voltage and power limitations result from increased corrosion of platinum and carbon components at cell potentials above approximately 800 mV. Since the freezing point of phosphoric acid is 42°C, the PAFC must be kept above this temperature once commis­sioned to avoid the thermal stresses due to freezing and thawing. Various factors affect the PAFC life. Acid concentration management by proper humidity control is very important to prevent acid loss and performance degradation. A PAFC has a life of 10,000-50,000 h, commercially avail­able (UTC Fuel Cells) PAFC systems operating at 207°C have shown a
life of 40,000 h with reasonable performance (degradation rate AVlifetime (mV) = —2 mV/1000 h) [3, 23].

Refer to Sec. 1.14, Chap. 1, for more details on biomass. Solid products fall under the following categories:

1. Direct outcome of photosynthesis: Products from forest, shrubs, agri­cultures, and aquacultures.

2. Nonphotosynthesis: Mushrooms, animal biomass, indirect from photofixation.

3. Wastes: Forests and agricultural products.

4. Municipal solid wastes: Not all solid biomass may be suitable for dif­ferent end uses, i. e., energy production or energy recovery. For exam­ple, mushrooms are notably useful as food, feed, or fodder, not otherwise. Biomass properties are guidelines to further and more fruitful end uses. The properties depend on the following:

a. Water or moisture content (aqueous/dry)

b. Calorific or combustion value

c. Dry residues/ash content/silicates, and so forth

d. Alkali metal/oxides in the ash

e. Ratio of cellulose/liquid/oils/fats/of other carbonaceous matters

f. Ratio of solid/liquid/volatiles

Direct combustion of biomass for heat generation is the most inefficient technique in energy economy, heat being the most inefficient of all forms of energy. The best way to utilize biomass is to recycle biomass for pro­duction of other or further biomass, namely, agriculture, horticulture, aquaculture, poultry, animal farming, and so forth. Randomness is reduced (low entropy change), and environmental chaos is lessened. Properties (a), (c), and (d) are significant for farming; (b) and (f) are important for hydrolytic processes; and (e) is important for biofuels and biodiesel. All the points are important for fermentations and in biore­fineries. Biorefinery has become a new science and technology harmony for a promising future, which takes care of different aspects of biosafety, minimizes waste, and maximizes energy efficiency. It is a field of engi­neering and technology for the future. Biorefinery is a system similar to that of petroleum in its requirements for producing fuels and chemicals from biomass. A biorefinery is a capital-intensive project and is based on a conversion technology process of biomass. Hence, several technologies— thermochemical, chemical, biochemical, and so forth—are combined to reduce the overall cost. Fernando et al. suggest an integrated biorefinery process from bio-oil produced from pyrolysis of biomas (see Fig. 2.12),

which will not only produce sugar but also different by-products and electricity [24]. The process can produce its own power.

Fermentation is equally important. Anaerobic and restricted aerobic digestion with selected algae species allow us to harvest hydrogen and clean fuels, without much loss of biomass and with the least amount of waste products. In an aerobic process, the process is carried out by oxi­dizing the volatile matter into biodegradable organic fractions of solid waste. Air acts as a source of oxygen, and aerobic bacteria act as a cata­lyst. The change occurring during the process may be represented as

Biomass + O2 (Aerobic bacteria) s CO2 + H2O + Organic manure

Anaerobic digestion is carried out by segregating the nonbiodegrad- ables and the biodegradables at the same time. This may be done man­ually or mechanically. The smaller pieces of inorganic materials like clay and sand may be removed by washing the biomass with water. The washed material is then shredded into a size that will not interfere with mixing and may be more amenable to bacterial action. The shred­ded biomass is then mixed with sufficient quantity of water, and slurry is fed into a digester system. If necessary, nutrients like nitrogen, phos­phorus, and potassium have to be added to the digester. The process involves four groups of bacteria in the digested slurry as follows:

1. Hydrolytic bacteria catabolize carbohydrates, proteins, lipids, and so forth contained in the biomass to fatty acids, H2, and CO2.

2. Hydrogen-producing acetogenic bacteria catabolize certain fatty acids and some neutral end products to acetate, CO2, and H2.

3. Homoacetogenic bacteria synthesize acetate, using H2, CO2, and formate.

4. In the final phase, called the methanogenic phase, methanogenic bacteria cleave acetate to methane and CO2.

Water acts as a catalytic agent in methane formation. Thus water is acted upon by enzymes, itself breaking down to hydrogen and oxygen. Hydrogen is used by microorganisms to reduce CO2 to CH4, while oxygen oxidizes carbon dioxide, i. e., makes it acidic (H2CO3). In simple terms, acetate (in presence of CoI) is simultaneously oxidized to CO2 and reduced to CH4. For details, refer to Chap. 1, methanation, and Baker’s and Ganzalus pathway. Thus, methane-forming bacteria play an impor­tant role in the circulation of substances and energy turnover in nature. They absorb CO, CO2, and H2 to give hydrocarbon and methane and help synthesis of their own cell substances. During anaerobic digestion, gas containing mainly CH4 and CO2 is produced. The gas is known as biogas, which is used for the generation of electricity or fuel. The residual biomass comes out of the digester in the form of a slurry, which is separated into a sludge, which is used as fertilizer and a stream of waste water. Research is ongoing to produce renewable energies from different plant sources, which will necessarily dominate the world’s energy supply in the long-term. Using renewable-energy system technologies will create employment at much higher rates than any other technologies would [1]. There are economic opportunities for industries and craft jobs through production, installation, and maintenance of renewable energy systems.

Since distillation is a highly energy-consuming process, several processes have been developed for purification of ethanol from fer­mentation broth: for example, solvent extraction, CO2 extraction, vapor recompression systems, and low-temperature blending with gasoline [9]. However, these processes are not established in the industrial pro­duction of ethanol.

Crop description. Virola surinamensis and V sebifera (see Fig. 4.25)— commonly known as ucuhuba, ucuiba, ucuba, muscadier porte-suif, and yayamadou—belong to the family Myristicaceae and grow in tropical swampy forests. Major producing countries are Brazil, Costa Rica, Ecuador, French Guiana, and Guyana. A typical tree is of medium height and can produce 60-90 L of oil each year. The seeds contain 65-76% oil. The yellow-brown aromatic oils from both varieties are very similar. Other related species, such as V otoba, which grows in Colombia and Peru, yield a fat similar to ucuuba, which is known as otoba butter or American nutmeg butter. Major fatty acids present in the oil are lauric

acid (15-17.6%), myristic acid (72.9-73.3%), palmitic acid (4.4-5%), and oleic acid (5.1-6.3%) [77, 87].

Main uses. This fat has been used traditionally in candle manufacture. The fat and pulverized kernels find use in traditional medicines. The tree has been proposed as a potential source of isopropyl myristate, which is used in cosmetic manufacture [186]. However, no references related to its use as a raw material to produce biodiesel have been found to date.

Corrosion of the engine parts has been one of the main reasons for not using alcohols as fuels. The problem of corrosion is severe during start­ing and idling; but once the engine starts and gets heated, corrosion does not take place. Severe corrosion is noticed with Zn, Pb, Cu, Mg, and Al. This problem has been solved by using a methanol-resistant filter before the carburetor. Corrosion by methanol has been prevented by using the corrosion inhibitor LZ541 manufactured by M/S Lubrizol India. Being solvent, it swells or softens many parts of plastic or rubber commonly used for gaskets or floats in the carburetor. This is solved by using elas­tomers instead of rubber or plastic. American Motors’ Gremlin model of 1970 has been used continuously for 9 years using pure methanol with­out facing any difficulty of corrosion. Two 1972 Plymouth Valiants have been used for 7 years: one using pure methanol and the other using a methanol blend without any difficulty. None of these vehicles has had a failure of engine components or fuel system components.

Btu British thermal unit. Heat energy necessary to raise the temper­ature of 1 lb of water 1°F.

cal or gcal Calorie or gram calorie. Heat energy required to raise the temperature of 1 mL of water 1°C (from 15 to 16°C).

electron volt 1.6 X 10 12 erg = 1.6 X 10 19 J = 23.06 kcal/mol. Energy gained by an electron passing through a potential of 1 V.

foot • pound ft • lb. Work energy needed to raise 1 lb to a height of 1 ft = 0.138 kg • m.

force Correct force definition can be obtained from the second law of Newton stating that the inertia is disturbed by unbalanced force, which causes acceleration on a body directly proportional to the force (F) and inversely proportional to the mass of the body F = K mf (F = Kmf where m is mass and f is acceleration). If all are reduced to unity, the unit of force becomes pound foot per second per second (poundal) or gram cen­timeter per second per second (dyne).

joule Work energy to raise 1 kg to a height of 10 cm = 0.1 kg • m = 0.74 ft • lb.

joule (electrical) 0.239 cal. Energy developed when 1 C of electrons (10.364 X 10~6 mole) passes through a potential of 1 V.

kcal/einstein Energy of a mole of a photon (einstein) of wavelength (in ^) 28589.7 ^ 1 kcal/mol.

power Rate of doing work, P = W/t.

J/s = W or ft • lb/s

or horsepower = 550 ft • lb/s or 33,000 ft • lb/min.

107 ergs/s = 1 J/s = 0.239 cal/s 550 ft • lb/s = 33,000 ft • lb/min 746 W = 178 cal/s 1000 W = 1.34 hp

3.6 X 106 J = 860 kcal/h = 3413 Btu/h 1.356 W = 0.324 cal

quantum Wavelength ^eV = 1239.8 ^—

work, W Force X distance (both in the same direction on a body).

621D ^ 242He + 211H + 2n + 43.1 Mev

Several designs and modifications are suggested:

2P ^ e+ + y + D D + P ^ T + y 2T ^ He + 2P The fusion reaction, omnipresent in the sun, needs to be tried out: 2H1 ^ e+ + v + H2

where two protons fuse, and deuterium, positron, and neutrino are evolved; energy is evolved in two steps; four protons are annihi­lated for each helium formed. Much of the reaction mechanism is yet unknown, but the model shows great promise.

[2] Geothermal source: Other than volcanic or geyser origin at an 8000-ft depth of the earth’s crust, it is possible to obtain geother­mal steam at 2000oC, which can be used for producing electricity.

Hot dry rock (HDR) remains out of reach at present capability of drilling. But “heat mining,” as estimated by Los Alamos Scientific Laboratory, promises 1.2 cents/MJ compared to 2 cents/MJ from an oil-fired thermal plant ($34/bbl).

[3] Aerodynamic generations: Several models are available. Low — velocity windmills are also being used. Wind is stronger at upper atmosphere; array of floating windmills are also designed.

[4] Hydrodynamics: High hopes are created by some hydroelectric firms, who proclaim that power can be effectively generated by ocean waves and ocean currents.

[5] Magnetohydrodynamic generators: High-temperature com­bustion gas expands through a nozzle where ionized sodium is intro­duced and directed to a magnetic field and a moving conductor cuts the field, and an electromagnetic field (EMF) is produced.

[6] Oil shale and oil sand: Though of limited supply, these have not been fully explored.

[7]

[8]

[9]

A body can do work, or work can be done upon a body; a body of water can turn a turbine, or one may pedal a bike to move it. If work is done on a body, it will possess energy. When energy is possessed by a body, the body can do work.

An agent may do work when it possesses energy, i. e., the amount of work that an agent can do is the amount of energy it possesses. So a body may gain kinetic and potential energy or lose the gained energy by pro­ducing heat or converting it to other forms of work.

Kinetic energy is due to the motion of a body.

Potential energy is due to the position or status of a body.

Frictional or colligative motion energy is produced in a water­fall; heat evolves to overcome a frictional resistance or checks the motion of a body but sets useless motion to others (e. g., rolling of peb­bles in a stream or dust behind a vehicle). Mechanical friction causes a matchstick to ignite.

Units of energy are the same as those of work and are assigned equiv­alent quantities. Some important definitions and units are given in the appendix. Energy content of some common substances are provided in Table 1.1.

1.1.1 Thermodynamics

All three principles of thermodynamics are very much applicable in the area of biological energy and chemical changes related to it. It is worth­while to review a few fundamental points. Chemical reaction can take

place only if the energy status changes, i. e., A will be converted to B only if B has a free energy content less than that of a change in free energy AF that is easy and spontaneous; reactions may be written as

A = B + (-AF) or A = B — AF

or

-AF = Fa

The reaction is called exergonic, or energy is evolved or given out. If AF has a positive expression, the reaction is driven by the input of energy and called endergonic; such reactions are difficult to complete. At equi­librium, AF = 0 (±), a point which may be arrived at by the end of the reaction, or a reaction may be typically of that type (practically sluggish, the progress of the reaction will depend on the change in concentration of reactants, the change of temperature or pressure, etc.).

AF = AF0 + RT ln B/A, where B/Ais the ratio at equilibrium or equilib­rium constant, i. e., Ksq. Then, 0 = AF0 + RT ln B/A or AF0 = — RT ln B/A =

-1363 logio Keq at 25°C. Here, R = 1.987 cal/mol/K, T = (273 + 25) K = 298 K, and ln B/A = 2.303 log10 Keq. This expression can be very useful:

When A and B exist equimolar, then the expression AF = AF0 + RT ln 1 means AF = AF0, and the state is called a standard state.

Chemical conversions and change of state need some other consider­ation in the light of the third law of chemical thermodynamics:

AF = AH — ATS

AH is the change in heat content, T is the absolute temperature at which the reaction occurs, and AS is the change in entropy (change, GR), or degree of disorder in the system, understood as the heat gained isothermally and reversibly per unit rise of temperature at which it happens (unit being calories per kelvin). The absolute value of H and S of a system cannot be directly determined. “Heat content” is also known as “heat content at constant pressure” or “enthalpy.” The third law sug­gests chemical pathway of finding entropy values in absolute terms. The first law of thermodynamics deals with conservation of energy and the second law with the relation between heat and work.

1. Energy cannot be destroyed or created, i. e., the sum of all energies in an isolated system remains constant.

2. All systems tend to approach a state of equilibrium. This means that the entropy change of a system depends only on the initial and final stages of the system, expressed by R. Clausius.

a. The total amount of energy in nature is constant.

b. The total amount of entropy in nature is increasing.

Prospects of ethanol and biodiesel as substitutes for conventional fuels will not be discussed here; these two aspects are presented in sufficient detail in Chaps. 3, 4, 5, and 6. One of the promising approaches for future fuel is, perhaps, hydrogen and methane, both of which could be obtained from living, particularly microbial resources.

Photosynthesis is the main route through which oxidized carbon is reduced and again oxidized back to carbon dioxide for the generation of energy. Based on this principle, we can utilize a few steps from this life chain. This topic could be called biophotolysis—alternatively, photobiolysis.

In the system, direct electron transport from water to hydrogen has not been demonstrated as a technically feasible reaction. For this, con­tinued research is required to elucidate the basic nature of FeS (PEA, ferredoxin, and hydrogenase). This may lead ultimately to the practical feasibility of production of hydrogen (ideally 20 ^L/h). Section 1.16 dis­cusses hydrogen in detail. One inherent problem is the stability of the hydrogenase system because of its sensitivity to molecular oxygen pro­duced during photosynthesis.

However, one may design a two-step or two-compartment system. Reduced Co II is the oxygen-stable electron carrier between photo­synthesis and hydrogenase. A higher ratio of reduced Co II or Co II helps the evolution of hydrogen, in spite of the unfavorable redox potential of the coenzyme. Only Co II (reduced) can be pumped or transported from one stage (compartment) to the other. Photosynthesis and hydrogenase systems have to be encapsulated or immobilized sep­arately in order to retain their respective activity; the two stages or compartments may be connected through fiber filters. An example could be to use appropriate algae to produce reduced organic com­pounds which can be pumped into bath of photosynthetic bacteria of hydrogen fermentation.

One partial modification will be to collect oxygen during the day and hydrogen at night, at the expense of accumulated reduced coenzymes, made operative by anaerobically adapted microalgae or nonheterocystous nitrogen — fixing blue-green algae. For product separation, the enzyme technology or immobilization is inapplicable for biophotolysis. However, there are potential practical applications of immobilized hydrogenase in biochemical hydrogen—oxygen fuel cells. If such enzymes can be immobi­lized on an electrode surface, an inexpensive fuel cell might be developed, which would increase the energy recoverable for hydrogen to save fuels.

Awareness of the limitations due to efficiency, engineering, and the economy of these principles will save disappointment and encourage con­tinued research. Geographical location and frequency of weather change limits the insolation. The best photosynthetic efficiency is only 6% of the total incident solar radiation, i. e., 5 kg/(m2 • yr) of H2 by biophotolysis. Half of this could be a very satisfactory achievement.