[advances in applied microbiology] advances in applied microbiology volume 2 volume 2 || biological...

Biological Transformation of Solar Energy

WILLIAM J . OSWALD AND CLARENCE G . GOLUEKE Division of Sanitary Engineering and School of Public Health.

University of California. Berkeley. California

Sanitary Engineering Research Laboratory. University of California. Berkeley. California

I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 A . Sources of Energy., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 B . Processes for Fixing Solar Energy ............................ 224

I1 . Production of Algae in Waste Waters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 A . Historical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 B . Algal Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 C . Nutrition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 D . Pond Design and Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 E . Separation of the Algae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 F . Quantity of Algae Produced., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 G . Storage of Algae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

I11 . Production of Methane Through Digestion of Algae., . . . . . . . . . . . . . . . . . . . 244 A . Historical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 B . Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 C . Quality and Quantity of Gas Produced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 D . Characteristics of Digested Algal Sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 E . Operational and Design Criteria for Algal Digestion . . . . . . . . . . . . . . . . . . 246 F . Energy Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 G . Gas Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249

IV . Power Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 A . Type of Plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 B . Quantity Produced . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 C . Plant Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252

V . Estimation of Cost of Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 A . Cost of the Plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 B . Capacity and Power Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

VI . Discussion and Future Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 VII . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261

1 . Introduction This article presents an evaluation of a microbiological process which

converts solar energy to electrical power through algal photosynthesis, methane fermentation of algae, and thermal combustion of methane . Al- though proved to be technically feasible in the laboratory (Golueke e t al., 1957; Golueke and Oswald, 1959a), the process as dealt with in this paper is tentative due to the fact that no pilot-scale installation has been studied .

223

224 WILLIAM J. OSWALD AND CLARENCE G. GOLUEKE

A. SOURCES OF ENERGY From the time of his origin, man has depended almost entirely upon

photosynthesis as his key to solar energy. Each year photosynthesis yields an abundance of energy in the form of food, fuel, and fiber, but man, because of his demands for a fantastic and ever-increasing amount of power, is forced by economic pressure to exploit photosynthates stored in the earth eons ago in the form of the fossil fuels, coal, oil, and natural gas. Inevitably, the conveniently accessible fossil fuel reserves are destined to become depleted, and eventually man will be compelled to meet his energy needs by resorting to other sources of energy.

It has come to be generally accepted that, with exhaustion of accessible fossil fuels, future energy needs will be met through nuclear fission, by solving the problems of obtaining energy from nuclear fusion, and through the development of practical methods of fixing the almost inexhaustible supply of solar energy. Factors concerned with economics and convenience should determine which of these methods will be used to meet a given need for energy.

It has been envisaged that the production of power through fission processes will become economically competitive with conventional thermal power plants within 5 years (Ullman, 1958). Deterrent factors to universal adaptation of fission energy are the high capital costs, necessary isolation of the plants with attendant problems of maintenance and power distribution, the need for a highly developed technology and heavy in- dustry and a highly trained personnel, the expense of supplying adequate plant and personnel safety standards, and the problem of the development and maintenance of rigid standards for atomic waste disposal.

At this point in history, many systems for developing fusion energy have been proposed and tested, but none has produced net power, hence experience is not adequate to warrant any speculation as to the economics of fusion power.

B. PROCESSES FOR FIXING SOLAR ENERGY Thus we conclude that new processes for the fixation of solar energy

deserve our continued attention and evaluation. Many physical, chemical, and biological processes for fixing solar energy were reviewed in 1955 at the World Symposium on Solar Energy at Phoenix and the Conference on Solar Energy a t Tucson, Arizona. Discussion at the Symposium and the Conference indicated that of the processes already in development, except in isolated and limited applications in unusual locations, most were not economically feasible, mainly because of high capital costs for collector surfaces. For example, although flat plate collectors probably are the most

BIOLOGICAL TRANSFORMATION OF SOLAR ENERGY 225

economical of the physical sources of solar energy presently available, published estimates (Hobson, 1956) indicate that power produced by plate collectors would cost about 50 mills/kw.-hr., five to ten times that of conventional power in the U. s. 1. Solar Energy Fixation b y Algae

At the time of the Symposium, only passing thought was given to the production of power from solar energy by means of algae, because it was believed that the cost of producing algae was too high, and because direct combustion of algae was the only certain means of releasing the energy contained in the algal cells. At the time, no information was available on methane fermentation by the anaerobic digestion of algae. Daniels (1956), in discussing the general prospects of solar energy, noted that, although mass culture of algae could produce tentimes as much organic matter as could conventional agriculture, the need for heavy capital investment for water tanks, carbon dioxide enrichment, fertilizers, cooling equipment, and harvesting machinery, would bring the cost of algae to $0.25 per pound. Such a price would rule out the use of algae for anything but food. The high protein content of algae would make direct combustion an unruly and odorous process.

In the Tucson Conference there were reports of the growth of algae in sewage which indicated that the cost of such algae might be as low as a few cents per pound (Gotaas and Oswald, 1955). At the same time, it was not only proposed that the cost of algal production might be greatly reduced, but also proposed that, through methane fermentation, algae might be used for hydrocarbon synthesis (Meier, 1955). For many years, fermentation of organic wastes has been a well-established practice in sewage treatment plants throughout the world. However, methane fermentation of digestible waste organic matter other than sewage sludge for power production is limited to small farm operations, especially in rural French and German communities. It cannot be made feasible on a significantly large scale, because the diffuse distribution of farm wastes available for fermentation necessitates the scouring of areas so extensive that a point is reached at which the energy expended in collection exceeds the yield.

2. Integrated Algal Bacterial System

Results of studies on the mass culture of algae led the authors to believe that the possibility of fixing sunlight energy by means of high-rate algal ponds merited appraisal, because with such ponds areal yields are increased tenfold, and, as a result, the major problem of procuring sufficient low-cost organic material is minimized. Experiments a t Richmond, Cali- fornia, with a group of 7000-gallon ponds, and a 1.6-acre pond a t Concord,


California, demonstrated that there existed many feasible methods for reducing the cost of algal production, and that algae could probably be produced for digestion a t less than $0.01 per pound in open, shallow, and sewage-fertilized ponds. Hence, the economic feasibility of utilizing algae for the fixation of solar energy appeared within the realm of possibility. These results and the abundant supply of algae from the pilot plants prompted the authors to begin studies on thc methane fermentation of algae by anaerobic bacteria.

The early studies showed that, although the production of methane from algae grown on a community’s organic wastes may be technically and economically feasible, some source of nutrient for the algae other than domestic sewage would have to be found to enable the process to supply a signifi- cant fraction, if not all, of the power requirements of a community. This led to the concept of introducing digester residue influents into the algal culture, thereby recycling fertility elements and thus increasing the energy-fixing capacity of the ponds. Studies of the practicability of recycling digester residues were undertaken, and i t was found to be feasible on a laboratory scale (Golueke and Oswald, 1959a), Thus a promising method

MUNICIPAL, MAKE-UP WATER SOLAR ENERGY AGRICULTURAL 8 DOMESTIC WASTES BIOLOGICAL OXIDATION

PHOTOSYNTHETIC REDUCTION CULTURE

PHYSICAL a !- SEPARATION - 1 1 L

7 SOLIDS

WATER RECARBONATION

COAGULANT 8 EXCESS

---+ WATER FLOCCULATION

GAS

jPJ STORAGE J DEWATERING

STORAGE

4 POWER I FIG. 1. Schematic diagram of system for the biological conversion of solar energy

to electrical power.


for producing electrical power frorn solar energy became available for evaluation.

The complete power-producing system as proposed by the authors involves a series of unit processes which are illustrated in Fig. 1. As the figure indicates, the principal features of the system are integration of the algal growth process with the gas-producing process in such a manner that fertility elements and water are repeatedly recycled from one process to the other; and integration of the power plant with the growth system SO

that heat and carbon dioxide may be recovered and recycled for use in the growth processes.

An evaluation of the large scale potentialities of the system entails a detailed consideration of the following:

1. Production of algae in waste waters 2. Methane fermentation of algae by anaerobic bacteria 3. Integration of the algal production and methane fermentation sys-

4. Economic evaluation of the process, through computations of the tems into a unified process

potential quantities and cost of power.

II. Production of Algae in Waste Waters

A. HISTORICAL

The culture of algae a t the extremely low cost required to make economically feasible the production of power through algal digestion necessitates highly specialized pond design and operation critera. The development of these criteria was accomplished in a series of studies in progress a t the University over a period of 10 years. Work on the growth of algae was initiated a t the University in 1950 with an investigation of the role of algae in sewage oxidation ponds (Ludwig et aE., 1950). Findings resulting from these studies demonstrated the possibility of producing an animal feedstuff on a practical scale in the form of algae on waste waters, and led to the conduct of pilot plant investigations on the subject. Until 1958, the pilot plant studies were concerned primarily with the development of design criteria for sewage treatment (Oswald and Gotaas, 1957; Oswald et aE., 1957). In 1958, the scope of the studies was expanded to include the production of algae (Oswald et al., 1959).

Information gained from the laboratory and pilot plant studies a t the University, together with that from the numerous excellent studies of algal cultivation in inorganic media carried on a t other laboratories (Bur- lew, 1953; Tamiya, 1957), make it possible to define a number of clearly essential and fundamental operational criteria for the culture of algae in

228 WILLIAM J. OSWALD AND CLARENCE G . GOLUEKE

organic wastes. These criteria will be discussed on the basis of algal species, nutrient supply, pond design and operation, and algal separation.

B. ALGAL SPECIES Fortunately, the groups of algae most commonly encountered in the

ponds, namely unicellular or nonfilamentous members of the Chlorophy- ceae, are also the types most suited to economical large-scale culture. Cultures of such organisms are characterized by high growth rates and photosynthetic efficiency. Their small size (3 to 10 microns) and low specific gravity normally permit them to remain in disperse suspension, so that the individual cell is completely surrounded by light and nutrient.

Since no attempt is made to maintain pure cultures, i t is not uncommon to find a mixture of several algal species in a pond culture, although frequently one or another may predominate. During the 10 years of studies a t Richmond and a t Concord, California, species of Scenedesmus, Chlo- rella, Chlorogonium, Chlamydomonas, Euglena, and Microactinium have been observed. Nevertheless, under normal conditions, Chlorella and Sce- nedesmus, either individually or collectively, constitute from 95 to 99% of the algal population.

Special inoculation or seeding of desirable algal species normally is not required; and it would be futile to inoculate the pond with groups not adapted to the highly specific ecological conditions encountered in the ponds, Of course, a heavy inoculation with suitable strains of algae developed under specific conditions will accelerate the development of a culture.

C. NUTRITION The biological conversion of solar energy to methane involves use of

organic waste materials as the ultimate and major source of nutrient for the system, because the cost of any but small supplementary quantities of synthetic media would render the process economically unfeasible.

The groups of algae most commonly encountered in the ponds cannot readily utilize many of the complex organic constituents of waste waters directly as nutrient, and, therefore, it is necessary to culture them in symbiosis with naturally occurring bacteria, which breakdown the organic matter to simple substances immediately available to the algae. These bacteria have not been identified, but observation of their gross morphology and growth characteristics indicates that they are similar to those found in the flocculent particles of activated sludge. Such particles are composed of a gelatinous matrix, in which filamentous and unicellular bacteria are imbedded, and on which protozoa sometimes crawl and feed.

The role of the symbionts in the algal-bacterial symbiosis is as follows:


aerobic bacteria (including the actinomycetes) and, to a negligible extent, protozoa, utilizing oxygen released by the algae as a result of photosynthesis, decompose complex organic materials to COz , NH3, simple amino acids, phosphates, and numerous other compounds. The algal members of the symbiosis assimilate these nutrients and in the presence of sunlight fix them into new energy-rich cellular material and, in so doing, release oxygen.

Because all organic material is subject to eventual bacterial attack, and since organic sediments may remain in the ponds indefinitely, theoretically very few limitations exist as to the type of waste which can be used. In practice, however, wastes which break down fairly rapidly, and which can be placed in suspension, should predominate. Wastes such as domestic sewage, ground garbage, slaughter house wastes, animal feeding-pen wastes, sewage sludge, and the residue from the digestion of algae, all have been found to serve as excellent sources of nitrogen and other nutrients for algal growth.

We have found that, other factors being equal, the growth of algae on wastes in symbiosis with bacteria exceeds that on synthetic media, both in rate and in extent. This is to be expected because the principal products of waste decomposition, such as ammonia and amino acids, are released in a form in which they can be used directly by the algae for their growth. Moreover, since the wastes are originally organic in nature, the necessary major nutrients and trace elements are present in quantities precisely suited to the needs of the algae. The nutritional advantage coming from the use of organic wastes is diminished only slightly by the absorption of light by the bacteria because most of the bacteria are present in flocculent particles which accumulate and are permitted to remain most of the time on the bottom of the pond.

The nutrient strength of a waste usually is evaluated in terms of the biochemical oxygen demand or B.O.D., which is a measure of the quantity of oxygen required to carry out the aerobic bacterial oxidation of the bio- logically available organic material in wastes under specific conditions of time and temperature (Standard Methods for the Examination of Water, Sewage, and Industrial Wastes, 1955).

Our studies demonstrate that in steady state continuous cultures under specific conditions of light intensity, temperature, and other factors, there is an optimum B.O.D. for algal growth and that, up to this optimum, algal growth increases almost linearly with increased B.O.D. A decrease in concentration of algae occurs a t B.O.D. concentrations in excess of this optimum, probably because strong wastes contain excess colloidal material and bacteria which remain in suspension and thus decrease the energy available for algal growth.


The economics of the process also preclude enrichment of the pond with CO, obtained from sources other than the over-all system itself, or with elaborate GO,-distributing equipment, or covering the pond to prevent the introduced CO, from escaping. I n the system developed by the Uni- versity of California, part of the CO, comes into the pond by way of dif- fusion a t the surface because the normally high pH of the pond liquid promotes absorption of atmospheric COZ. Much CO, also comes from the metabolic activity of the bacteria which make up the bottom sludge. Carbon dioxide released from the digester and formed by the combustion of methane is returned to the pond in the manner shown in Fig. 1. The normally high pH of the pond liquor enhances absorption of CO, from the combustion gases by the liquor.

D. POND DESIGN AND OPERATION

1. Detention Period

mathematically as The hydraulic detention period, D , for production of algae is defined

D = V / & (1) in which V is the volume of culture in the pond, and Q is the volume of fluid introduced daily. When the detention period is short and the pond has an impermeable lining, factors such as evaporation, precipitation, and infiltration, are usually negligible, and the quantity of liquid discharged to the separator approximates that of the pond influent.

The type of relationship to be expected between detention period, yield, efficiency, and the concentration of a culture a t equilibrium is exemplified by the curves in Fig. 2, which were derived from data obtained in experiments with Chlorella pyrenoidosa grown in continuous indoor culture units under controlled environmental conditions (Oswald, 1957). Although the curves given in Fig. 2 are based upon data obtained with a single species of algae under controlled conditions in the laboratory, the results of cx- periments with outdoor pilot plant ponds indicate that the same relation- ships hold in outdoor cultures.

As the figure indicates, the concentration of a culture increases almost linearly with increase in detention period from a minimuin 0.5 day’s detention period to a concentration of about 250 mg./liter a t 1.5 days’ detention period. Above 250 mg./liter the increase in concentration with detention period is proportionately less. The length of a detention period a t which the concentration of a culture approaches zero is determined by the generation time of the algal species involved. As the detention period exceeds generation time, more time is allowed for the cells to multiply and accumulate, and, consequently, the concentration of the culture increases

BIOLOGICAL TRANSFORMATION OF SOLAR ENERGY 23 1

2 3 5 6 7 8 0 0

DETENTION PERIOD IN CONTINUOUS CULTURE, DAYS

FIG. 2. Relationship between detention period, yield, efficiency, and the concentration of a chemostatted culture of Chlorella pyrenoidosa.

in direct proportion to increase in detention period until some factor other than time limits growth. The increase in culture concentration with detention period then slowly declines as the influences of limiting factors other than time become more pronounced.

Culture yield reflects the productivity of a culture per unit of volume when it is computed according to the equation

Y = C,/D

in which Y is yield in milligrams per liter per day, C., the concentration of the culture in milligrams per liter, and D , the detention period in days. In the example given in Fig. 2, maximum yield occurred a t a 1.25-day detention period. At detention periods longer than 1.25 days, the yield decreased slowly, and was only one-half its maximum value when the culture was operated on a detention period of 8 days.

Over-all light energy conversion efficiency, F , is a measure of culture productivity per unit of area, and is computed from the quantity of energy fixed in algal cells per unit of time, H , and the quantity of light energy absorbed during the same time, H', and may be expressed as

F = H/H' (3)

As is shown in Fig. 2, efficiency increasc>s linearly with increase in de- tetion period from 0 a t 0.5 day's detention period to a maximum of 5.5% a t 1.5 days' detention period, beyond which efficiency gradually

232 WILLIAM J . OSWALD AND CLARENCE G . GOLUEKE

declines with increase in detention period. Factors such as an insufficient supply of light received by the individual cells because of mutual shading, or deprivation of nutrient because of increased competition accompanying increase in culture concentration, account for the gradual decline in efficiency after the maximum is reached.

Maximum combined volumetric and areal yield can be determined from the efficiency and dry weight of a culture. In a series of comparable cultures having various detention periods the product of these two factors attains a maximum. Experiments conducted in our laboratory and with outdoor pilot plant ponds show that maximum values are attained a t a detention period of about 2 days. Were it not for the cost of harvesting algae, this would be the average detention period to select for algal production. However, since the cost of harvesting algae must be considered, and since the cost per unit weight of algae harvested must decrease with increase in concentration of algae, a 2-day detention period is less than optimum. Our experiments have shown that when the cost of harvesting is included a 3-day detention period is more favorable. There is the possibility, however, that in regions where light is abundant the detention period for maximum efficiency and yield may be a fraction of a day less than those we have found to be optimum in Richmond or Concord, Cali- fornia.

2. Pond Depth and Sunlight Energy

Many studies have firmly established that the efficiency of the conversion of light energy into the chemical energy of algal cells increases as the intensity of light is decreased, until a point is reached below which efficiency declines linearly (Rabinowich, 1951 ; Oswald, 1957; Golueke and Oswald, 1959b). The linear decline continues until light intensity is so low that the resultant cell synthesis is less than the cellular respiration which occurs a t the same time, and a negative efficiency results. This latter intensity, termed the compensation intensity (Myers, 1956), varies with temperature and nutrient, and with the physiological state of the cells, and indeed, even with the intensity of the previous illumination.

The effect of light intensity upon culture concentration and over-all conversion efficiency is illustrated in Fig. 3, in which is shown the influence of light intensity upon the over-all conversion efficiency of C. pyrenoidosa cultured in sewage under laboratory conditions (Oswald, 1957). Accord- ing to the figure, culture concentration increased with increasing intensity between 0 and 30 cal./liter/min., and gradually declined as the flux was increased beyond this point. That the drop in concentration may have been due to some physical damage to the cells when the light energy input exceeded 30 cal./liter/min., is indicated by a corresponding decline in chlorophyll content of the cells observed in the experiment. According to

BIOLOGICAL TRANSFORMA'TION OF SOLAR ENERGY 233

I 19

7oc -

- 8

I- z

- 7 w V n W a

z Cell concentration

n

a w >

2- a n W

APPLIED LIGHT ENERGY FLUX. CALORIES PER LITER PER MINUE

FIG. 3. The effect of light intensity upon the over-all conversion efficiency, concentration of a chemostatted culture of Chlorella pyrenoidosa.

I 19

7oc -

- 8

I- z

- 7 w V n W a

z Cell concentration

n

a w >

2- a n W

APPLIED LIGHT ENERGY FLUX. CALORIES PER LITER PER MINUE

FIG. 3. The effect of light intensity upon the over-all conversion efficiency, yield, concentration of a chemostatted culture of Chlorella pyrenoidosa.

yield, , and

Fig. 3, the over-all conversion efficiency of C. pyrenoidosa increased linearly from 0.5% to 8% as the light, energy flux was increased from 1.8 cal./liter/niin. to 4 cal./liter/min. As the light energy input was increased beyond 4 cal./liter/min., however, efficiency decreased rapidly, perhaps as a result of light saturation.

The maximum value for the product of light energy flux and conversion efficiency, which indicates the productivity of a culture, was attained at a light energy flux of 17.5 cal./liter/min. with C. pyrenoidosa. This value is the same as that of the energy flux into a pond 8 inches deep during a clear day on the fall equinox in the middle latitudes.

Energy flux received by a given volume of culture is determined by the depth of the culture. Experimental evidence (Burlew, 1953; Oswald and Gotaas, 1957) shows that the depth to which light penetrates an algal culture can be fairly accurately represented by Eq. (4), which is a modi- fication of the Beer-Lambert law

(4)


in which d is the depth of light penetration, I, is the surface light intensity, I is the light intensity a t depth d , C, is the algal cell concentration, and a is a crude absorption coefficient. In mixed cultures of algae and bacteria, a usually has a value of about 1.5 X when d is expressed in centimeters, I, and I in foot-candles, and C, in parts per million. Since a increases in magnitude with increased chlorophyll concentration, and since the chlorophyll content of individual algal cells increases with decrease in the average light intensity to which they are exposed, values for a in Eq. (4) tend to be higher than 1.5 x in ponds deeper than 8 inches. As a result, a t a given concentration of algae and light intensity, the extent of light penetration into a culture grown in a deep pond would be less than that into the same culture in a shallow pond.

Practical experience with outdoor ponds indicates that 8 inches is an optimum pond depth for productivity (Oswald et al., 1958). Experiments with a laboratory culture of C. pyrenoidosa showed that the lowest intensity a t which a measurable over-all efficiency was attained was 30 ft.- candles. I n an outdoor pond, dalyight will penetrate 12 inches into a culture having a concentration of 140 p.p.ni., and only 4.5 inches into a culture having a concentration of 340 p.p.ni., before i t is diminished to 30 ft.- candles. In an outdoor pond, daylight will penetrate 12 inches into a cul- the higher concentration, theoretically 4.5 inches should be the optimum pond depth, a depth less than that indicated by actual experience. The difference between the laboratory-based values and those proven optimum by experience is explained by the fact that continuous mixing maintains a constant homogeneity in laboratory cultures, while the intermittent mixing carried out in outdoor ponds results in a lower concentration of cells in the upper strata than in the culture as a whole during the greater part of each day.

3. Mixing

The necessity for having both a bacterial and an algal phase in the ponds imposes mixing as an essential operational procedure. Upon enter- ing an aerobic pond, the organic fraction of the wastes is adsorbed by bacteria, which, as noted previously, form flocculent particles that settle to form a sludge layer on the bottom of the pond. Since the bacteria contained in this layer rapidly deplete their oxygen supply, normal aerobic oxidation would cease were not the oxygen released by the algae made available to them through mixing. The occurrence of anaerobic conditions in the sludge layer would seriously interfere with the operation of the pond, since most of the organic breakdown takes place in this layer, as is evidenced by the fact that 95% of the biochemical oxygen demand which

BIOLOGICAL TRANSFORMA'I!ION OF SOLAR ENERGY 235

enters a pond disappears from the supernatant and a major fraction of the 95% reappears in the sludge layer (Oswald et al., 1959).

I n an undisturbed pond, the formation of excess oxygen occurs only in the upper strata, while the lower layers suffer from a lack of oxygen because of the limited depth to which light penetrates a dense algal culture. Furthermore, such nutrients as are released remain either in, or in close proximity to, the bottom layer and, consequently, are unavailable to the surface algae. However, if the ponds are adequately mixed at sufficiently frequent intervals, nutrients are made available to the algae, while oxygen and raw nutrients are made accessible to the bacteria.

If allowed to remain undisturbed, the bacterial sludge layer a t the pond bottom will begin to show signs of anaerobiosis within 12 hours after mixing, a fact which is demonstrated by darkening of the sludge and a rapid decrease in the oxidation-reduction potential. Production of hydrogen sul- fide with its typical vile odor begins within a few days after mixing ceases (Oswald et al., 1959).

Complete homogenization of the supernatant liquid and the bottom sludge can be attained only by mechanical means. Wind mixing is in- effective, because normally it is not sufficiently violent, nor is it dependable. Experiments have shown that it is necessary to move the pond liquid a t velocities of a t least 1 ft./sec. to resuspend sludge and settled algae and bring about complete homogeneity. This is essentially the velocity known to suspend and transport organic sediments in a flowing stream.

Although mixing is essential for the successful operation of an algal pond, continuous mixing is highly detrimental, because penetration of light into the culture would be reduced tto only 1 or 2 nim. by the continual dispersion throughout the culture of the bacterial floc particles. As a con- sequence, the supply of light to the algal cells would be inadequate, and their photosynthetic activity and concomitant production of oxygen less- ened. Since oxygen cannot diffuse from the air into the culture with a rapidity sufficient to meet the demand for aerobic decomposition, and the supply of photosynthetic oxygen is sharply reduced through continuous mixing, a semiaerobic condition would develop under which neither the algae nor the aerobic bacteria could prosper. On the other hand, if mixing is completely withheld, conditions become such that most of the algae and other suspended solids are coagulated and settle to the bottom of the pond as floc particles which become covered with the descending sludge so that light is excluded and photosynthesis ceases.

A combination of light and pH effects results in a theoretical advantage in mixing ponds once during the night and a second time during the afternoon. Obviously, at night the amount of light received by the algal


cells is not affected by the light-obscuring effects of the bacterial sludge, so that nocturnal mixing is preferred. Mixing during the afternoon lowers the pH of the ponds which, especially on sunny days, becomes excessively high for bacterial activity. Mixing twice daily also ensures a relatively continuous supply of nutrient and oxygen to the bacteria, and of nutrient and CO, to the algae. A program of mixing for a period of 2 to 4 hours beginning at midnight and again for a half-hour period beginning at 1 : 00 P.M. is indicated.

Our experience demonstrates that, when done only once each day, mixing for several hours between midnight and sunrise is preferable to mixing during the periods from noon to sunset.

4. Pond Design

As is indicated by the previous discussion, an ideal production pond should have a liquid depth of 8 inches, a detention period of 3 days, and should be mixed twice daily by circulating the pond liquid a t a velocity of 1 ft./sec. A design of a pond embodying these characteristics is shown in Fig. 4.

Arrangement of a pond into a number of channels of equal width by means of the elongated baffles shown in Fig. 4 permits the location of pumps a t one end and mixing the pond contents by circulating the pond liquid. The elongated baffles serve to direct the flow of the pond liquid and prevent short circuiting. Channels should be as wide as hydraulic efficiency permits, to keep the cost of the channel dividers a t a minimum. Estimates are that 200 feet is an appropriate channel width for large ponds.

A pond should be equipped with a smooth, impervious, and stable lining to reduce friction losses and turbidity during mixing, to prevent loss of water through infiltration, and to prevent the growth of water weeds. Water weeds impair the hydrodynamic properties of the pond, decrease the availability of light, and encourage the breeding of flies and mosquitoes.

MIXING PUMPS-

I N F L U I

EFFLUENl c

FIG.

'I

4. Diagram of a high-rate oxidation pond for algal production.


The necessity of maintaining a shallow pond depth places an upper limit on channel length and determines the size of the pumping units. In- dividual pumping units which discharge about 3 cu. ft./sec. appear to be optimum. Aligned along the return chamber as shown in Fig. 4, the pumps discharge into the upper reach of the pond in parallel.

Mixing-pump discharge transitions are designed to increase the depth of flow a t the point of the pump discharge to such a depth that a sufficient hydraulic gradient is established to bring about the required mixing velocity of about 1 ft./sec. in the body of the liquid. This is efficiently accomplished by converting pump kinetic energy into liquid depth by means of a suppressed hydraulic jump.

A downstream velocity not exceeding one and one-half the average velocity is required to avoid excessive loss of energy in mixing, and consequently, the downstream depth is not permitted to fall below one-half the average depth. During mixing, therefore, the depth a t the upper end of the pond reach (al) is (d + d / 3 ) ; and a t the lower end of the reach the depth (a2) is (d - d / 2 ) . The change in depth, Ad, therefore, may be expressed as

Ad = dl -- d2 15)

or, substituting values for dl and dz , in terms of the average depth, d,

Ad = ( d +d /3 ) - (d - d / 2 ) = 5/6d (6)

(7)

The change in specific energy, Ah, then is

( d + d / 3 + V12/2g - d /2 - VZ2/2g) = Ah

in which V is velocity in feet per second, Ah is in feet, d is depth in feet, and g the acceleration due to gravity in feet per second per second. Ac- cording to Manning’s equation, the slope of the energy grade line along a reach of pond during mixing is

in which S is slope of the energy grade, n is the Manning roughness coefficient; R is the hydraulic radius, and V the velocity. The length of the reach is

Ah (nV /1.486R2I3)

L = A h / S =

in which L is length of reach in feet, and h, n, V , and R are defined above. Several important factors influence the slope of the energy grade line

and hence Ah. They are important because they determine the channel length, once depth and velocity are established. For wide flow channels, the


TJ

YI'D 2 2 8 , , , , , , , , I , , ,

Mean velocity - I ft./sec

a = 00.008 ' 0.dlO ' o.& 0.0:4 ' O.& ' O.dlB ' ;).&o MANNING'S n VALUE

FIG. 5. The influence of Manning's n value and depth on total channel length

value of R becomes numerically identical to the depth. Thus, a channel 100 feet wide and 8 inches in depth has a hydraulic radius of approximately 0.67 feet.

Manning's n value is used to designate the degree of smoothness of channel surfaces, and, as such, i t constitutes an independent variable because of its influence on the channel length. In Fig. 5 is demonstrated the influence of Manning's n value and depth on the total channel length through which a depth of moving water greater than one-half the static water depth is maintained. Smooth linings permit greater channel lengths, as is shown in the figure and, hence, decrease pumping costs.

Manning's n value also is important in determining the amount of power used by the pumps during the mixing operation, as is shown in Fig. 6. As the figure demonstrates, assuming a motor efficiency of 70% and a pumping efficiency of SO%, the maximum power requirement is about 10 kw.-hr./ acre a t an n value of 0.020, and about 5 kw.-hr./acre a t an n value of 0.014.

Manning's n values for various types of pond lining materials have been established from experience with drainage design in highway practice and are published in the U. S. Bureau of Public Roads (1951).

Characteristic n values are as follows: smooth asphalt, 0.013; rough asphalt, 0.016; float finished concrete, 0.014 ; broom-finished concrete, 0.016 ; rough concrete, 0.020; vinyl, 0.009. Through selection of pond lining ma-


14

12

(3

z4 x I 0 z x 2

C (

FIG. 6. Power reqL and depth.

ii

I I

Change in depth : (d t02 ) f t Averoge velocity: I O f t /set Mixing time = 3 5 hr

- Motor efficiency=70% Pumping efficiency = 60%

per doy

I I I 1 I

308 0010 0012 0014 0 016 0018 0020 MANNING? n VALUE

red to mix an acre of pond as a function of Manning’s n value

terials, mixing velocity and pond depth are established, which combined with channel width, determine the total area of a given pond. 5. Pond Operation

When a pond receives its very first loading of sewage, development of the algal bloom usually occurs within 1 to 3 weeks. Discharge of any effluent should be delayed until the bloom develops. This initial delay period may be shortened by a mass inoculation of the pond with algal types suited to the specific environment. Once a bloom has developed and algae have been cultured in a pond, no further inoculations are necessary.

When the bloom has developed, the pond is placed upon the selected detention period, and the effluent is discharged through a separator in which the algae are removed. The supernatant from the separator is passed through a chamber to be impregnated with CO, and is then returned to the pond (cf. Fig. 1) .

E. SEPARATION OF THE ALGAE

The problem of separating algae from pond effluent in an energy conversion system differs from that in food production and waste treatment in that the permissible monetary expenditure is less, and all of the algae need not be removed from the effluent because it is returned directly to the pond (cf. Fig. 1). Moreover there need be no concern regarding the quality or purity of algae produced for digestion.

240 WILLIAM J. OSWALD AND CLARENCE G. GOLUEKI?

1. Methods

Experience has shown that, among the methods of removing algae from effluent, two are most promising, namely, centrifugation and coagulation (Golueke and Gotaas, 1957, 1958; Oswald e t al., 1959). Of the two methods, coagulation is by far the more economical a t present. The cost of centrifuges and the power to drive them is excessive with equipment presently available, but centrifugation would become economically feasible in the energy conversion process if centrifuges 6 to 8 feet in diameter and designed for power recovery were available. Filtration is not practical because the algae clog the filters unless an excessively large amount of filter aid is used (Gloyna and Hermann, 1955; Golueke and Gotaas, 1957).

In algal production, coagulation can be induced by the addition of reagents such as aluminum sulfate (alum), lime, or organic cationic floc- culents, or through suitable alterations in the chemical and physical con- stitution of the pond culture.

2. Chemical Coagulation

When coagulation is induced by the addition of aluminum sulfate, the pH of the culture is first lowered to about 7.0 with sulfuric acid and finally reduced to 6.5 with aluminum sulfate. The addition of the reagents is followed by a brief period of rapid mixing and then by a 3- to 5-minute period of gentle stirring to develop floc particles of sufficient size and den- sity to permit rapid sedimentation and subsequent removal of the flocculated material. The same procedure is followed when lime is employed, except that no acid is used and the pH of the culture is raised to 11.3 with the addition of lime. Although the cost of separating algae by means of chemical flocculation is low, nevertheless, when the product is used solely for power production, the expenditure for the necessary reagents may constitute too large a fraction of the total power cost. Consequently, a more economical method of separation is ordinarily required.

3. Autoflocculation

A natural separation process well within the economic demands of the energy conversion system, was observed during the course of laboratory experiments a t Richmond, California, and in experiments with outdoor pilot plant ponds a t Richmond and a t Concord, California (Oswald e t al., 1959). It was found that under certain conditions algae clumped together or flocculated and settled to the bottom of the pond. Investigation showed that this flocculation always occurred during the afternoon of sunny days, and when the temperature of the ponds had increased several degrees above the morning level. Moreover, it was noted that the pH of the media increased to 10 or 11, probably as a result of a shortage of CO, brought about


by its high rate of consumption by the algae. This phenomenon of self- separation by the algae was termed “autoflocculation.”

Analyses of the flocculated material showed that its concentration of magnesium, ammonium, and phosphate ions greatly exceeded that of the supernatant, as much as threefold for magnesium, and fifteenfold for phosphate. Inasmuch as the solubility of Mg(OH)z and of the magnesium phosphate compounds, as well as of the ammonium organic phosphate com- plexes decreases with increased pH and temperature, the autoflocculation observed on the occasions described above probably was the result of the formation of floc particles consisting of coagulated compounds and algae entrapped in the particles.

Since autoflocculation occurs most rapidly in shallow ponds and rarely in ponds deeper than 2 feet, design conditions to take advantage of the phenomenon require that the culture be placed in an autoflocculation pond 3 to 6 inches deep during the sunlight hours. After autoflocculation, the supernatant would be decanted for return to the pond, while the flocculated algal material would be drawn into a secondary decanting pond for further concentration. Experimental results to date indicate that autoflocculation is a dependable and extremely low cost process for concentrating algal sludges in preparation for digestion.

F. QUANTITY OF ALGAE PRODUCED

The amount of power produced by a pond digester system is determined directly by the quantity of algae made available in the growth process. In simplest terms, the quantity of algae varies as a function of the quantity of available solar energy and the photosynthetic efficiency, thus

W = kFS, (10)

in which W is the quantity of algae produced per acre per day, k is a constant to convert calories per centimeter squared to the desired units for W , F is the conversion efficiency expressed as a whole number, and S, the quantity of available solar energy in Langleysl per day. If W is to be expressed in pounds of algae per acre per day, the value of k is 0.15.

1. Available Solar Energy

The quantity of available solar energy varies with time, locality, and climate, and, therefore, information on the amount is an essential part of the design of a solar energy conversion plant. A table of solar energies based partially upon theoretical values and partially upon observations was published by Oswald and Gotaas (1957). An abstract of this table is presented in Table I. In the table is predicted the maximum monthly quantities

Gram calories per centimeter squared.


Feb.

266 219

244 184

213 140

176 96

130 53

70 19

32 4

TABLE I

WHICH WILL PENETRATE A SMOOTH HORIZONTAL WATER SURFACE AS A

FUNCTION OF LATITUDE AND MONTH

PROBABLE VALUES OF VISIBLE LIGHT ENERGY I N LANGLEYS PER DAY

Mar.

271 206

264 193

246 168

218 134

181 85

141 58

107 33

_ _ _ _

Lati- tude

0

10

20

30

40

50

60

max.b min.*

max. min.

max. min .

max. min.

max. min.

max. min.

max. min . -

~

Tan.

255 210

223 179

183 134

136 76

80 30

28 10

7 2 -

-

Lpr .

266 188

271 183

27 1 170

26 1 151

24 1 125

210 97

196 79

-

-

249 182

270 192

284 184

290 184

286 162

27 1 144

249 132 -

Month

une

236 103

262 129

284 148

296 163

298 173

297 176

294 174

-

-

-

Julj

238 137

265 158

282 172

289 178

288 172

280 155

268 144

-

-

__

iug.

252 167

266 176

272 177

271 166

258 147

236 125

205 100 -

-

jept

269 207

266 196

252 176

23 1 147

203 112

166 73

126 38

-

-

~

Oct.

265 203

248 181

224 150

182 113

152 72

100 40

43 26 -

-

TOV

256 202

228 176

190 138

148 90

95 42

40 15

10 3

-

-

258 195

225 162

182 120

126 70

66 24

26 7

5 1 -

a S = Average available solar energy. Correction for cloudiness: S, = P (max.-

b Maximum assumes sunshine during the entire day; minimum assumes continuous min.) + min.; in which P is the fraction of total possible time there is sunshine.

overcast.

of solar energy in the visible spectrum which will penetrate a smooth water surface. Although the maximum and minimum vales given in the table may be exceeded on some days, they are the true average maxima or minima for the month. Specific values for a given locality may be obtained by assuming that maximum values correspond to continuous daily sunshine and minimum values correspond to almost continual overcast, and in- terpolating between maximum and minimum values on the basis of the percentage time the sun is shining to arrive a t a specific value of S, , as is indicated in the table.

2. Conversion Efficiency

The conversion efficiency to be expected under various conditions and the factors which influence i t have been discussed previously. I n outdoor

BIOLOGICAL TRANSFORMATION O F SOLAR ENERGY 243

TABLE I1 EFFICIENCY ATTAINED I N EXPERIMENTAL PONDS AT VARIOUS DEPTHS,

DETENTION PERIODS, TEMPERATURES, A N D SEASONS

Detention Tempera- (Langleys/ Depth period ture, "C.

(days) (average)

s,

day 1 a Efficiency

(%I (inches) Month

(aver age)

1 49 10.6 7 11 6.2 1 49 14.1 4 11 9.8 2 113 7.1 2.5 11 8.8 2 113 7.1 5.5 11 3.5 3 164 3.7 4 14.5 1.8 5 260 8 2 17 8.5 6 232 8 1 20 3.5 7 207 12 4 18 5.0 8 194 12 3 19 6.0 8 194 12 1 19 7.7 9 152 7.1 1 19 4.2 9 152 16 3 19 7.7

10 122 10.6 5.5 19 4.2 10 122 14.1 5.5 19 3.8 11 66 10.6 2.5 16 4.0 12 58 14.1 7 13 8.3

5 Observed visible light energy available to the algal cells.

ponds, conversion efficiency varies from moment to moment because the degree of turbidity, the quantity of available nutrients, light intensity, and temperatures are continually changing. The transient efficiencies which result from these changes can be characterized by an over-all average efficiency computed by correlating algal growth with the sunlight energy received by the culture. Some average efficiencies based upon observations made over a number of years and for a wide variety of environmental conditions are listed in Table 11. As the table indicates, the range in efficiency extended from about 2 t o 10%. Highest efficiencies were obtained during periods of mild cloudy weather and in the deeper ponds, while lowest values frequently occurred during periods of brilliant sunshine and in very shallow ponds. Excluding values of conversion efficiency for ponds having detention periods less than 2 or greater than 5 days, and depth less than 8 inches or more than 14, the average conversion efficiency is about 6.2%.

Efficiencies higher than 6.2% will be obtained in the energy conversion system described in Fig. 1 because CO, , which is produced in the digester, together with the C 0 2 produced from burning methane, is returned to the culture solution. Experience with C02-enrichment in cultures indicates

244 WILLIAM J . OSWALD AND CLARENCE G. GOLUEKE

that growth may be more than doubled under otherwise identical conditions. Inasmuch as the Table I1 efficiencies were obtained without SUP-

plementary COP , efficiencies between 6 and 10% of visible sunlight may be attained when COP-enrichment is applied.

G. STORAGE OF ALGAE Since the peak demand for power usually occurs when the quantity of

solar energy is at its minimum, and the greatest production of algae occurs during the time of maximum sunlight energy, some method of storage of energy is required. Storage in the form of methane gas is expensive because of the large volume of the container required; whereas storage in the form of dried algae necessitates only stock-piling in the open or in simple sheds. Experience gained during experiments conducted a t Concord, Cali- fornia, (Oswald e t al., 1959) demonstrates that algal sludge having a solids content as low as 1.0 to 2.0% of the total wet weight may be easily dewatered and dried on simple sand beds. About 2 square feet of bed area is required per pound of algae (dry weight). A maximum of 10 days is required for the algal product to be air-dried sufficiently for an indefinite period of storage without decomposition, as long as i t is protected from rain and excessive moisture. The air-dried algae readily digests upon being resuspended and introduced into the digester. Thus, by air-drying and stock-piling a portion of the algal yield, peak requirements for winter power could be balanced against peak summer pond production.

111. Production of Methane Through Digestion of Algae

I n the proposed system, the final step in the transformation of solar energy to methane is the anaerobic fermentation of algal cellular material by bacteria in a process known as digestion, in specially designed tanks termed digesters.

A. HISTORICAL The production of methane through anaerobic fermentation of organic

matter occurs naturally in marshes, lakes, and streams, and has been the subject of much study from the time B6champ (1868) postulated the biological formation for methane, until the present, when Barker (1949, 1956) and Stadtman and Barker (1951) identified the organisms involved and traced their metabolic pathways. The production of methane as a by-product of the anaerobic biological breakdown of settled sewage solids has been a common practice in sewage treatment for many years (Eddy, 1925), and has received much study (Walraven, 1932; Buswell and Hat- field, 1939; Buswell and Mueller, 1952; Golueke, 1958). During World War 11, digester gas was used to propel automobiles in Germany (Imhoff,


1946). Since 1945, methane has been produced for domestic use from the digestion of animal wastes in rural districts of France and Germany (Gotaas, 1956). Consequently, much information on fundamental and practical aspects of digestion was available a t the time we began our studies on digestion. However, our experiments soon demonstrated that the digestion of algae differed in several important aspects from that of urban or agricultural wastes (Golueke et al., 1957).

B. ORGANISMS The complexity of algal cellular material necessitates the activities of

a succession of many groups of bacteria in the conversion of the material to methane.

The diversity of the organisms involved makes their classification difficult, and, consequently, little has been done in this respect. Moreover, very little is known about the initial steps in the breakdown of the cells. Eventually, however, COz and simple volatile or fatty acids such as pro- pionic, acetic, butyric, and valeric acids are formed which serve as sub- strates for the methane-producing organisms. Because the methane organisms are specific with respect to substrate, several groups undoubtedly are present, since there is a variety of volatile acids.

Gross observations made on the morphology of the bacteria found in samples of digested sludge showed that no one morphological type pre- dominated, and all forms from coccal to rod-shaped forms were observed. No protozoa were seen.

C. QUALITY AND QUANTITY OF GAS PRODUCED

As a result of the digestion process, 50 to 60% of the volatile matter in the introduced algal slurry ends as gas. In terms of volume, from 6 to 10 cu. ft. of gas are produced for each pound of volatile matter introduced, and from 14.1 to 16 cu. ft. per pound of volatile matter destroyed. The principal constituents of the gas produced are GOz, CHI , and Hz , of which 30 to 32% is COZ ,61 to 63% is CH, , and from 1.0 to 2.5% is hydrogen. The remaining fraction consists of nitrogen, HzS, and other gases. Combustible gases, therefore, comprise from 62.5 to 65.5% of the total gas produced. Thus, from 4.3 to 6.6 cu. ft. of combustible gas are produced per pound of volatile matter introduced, or from 2.4 to 3.6 CU. ft. per pound of algae (dry weight).

D. CHARACTERISTICS OF DIGESTED ALGAL SLUDQE

Sludge obtained from the digestion of raw sewage sludge differs from that of algal digesters in that the former is a gritty material and has a tarry odor, while the latter contains very little granular or flocculent ma-


terial, and is highly colloidal in nature. Because it is hydrophilic, digested algal sludge dewaters very slowly. The difference in physical characteristics between the two sludges probably stems from differences in the nature of the material used as a nutrient source. The predominantly pro- teinaceous composition of the algal cells tends to give rise to a highly colloidal sludge rather than a flocculent material easily separated from water.

The nitrogen content of the sludge obtained in our studies varied from 8.1 to 6.6%, depending upon the temperature a t which the algae were digested. From 61 to 67% of the nitrogen was in the form of ammonia- nitrogen. Since this latter form of nitrogen is directly available to the algae, i t is recovered upon return to the algal pond (cf. Fig. 1).

E. OPERATIONAL AND DESIGN CRITERIA FOR ALGAL DIGESTION As in the production of algae, the economical production of methane

through digestion requires specialized design and operational criteria. Among these are detention period, loading rate, temperature, and the actual design of the digester.

1. Detent ion Period

The rate of destruction of the volatile matter introduced into a digester determines the length of the detention period. Obviously, the longer the detention period, the more extensive the breakdown of organic matter and the greater the production of gas per unit of volatile matter introduced. However, a point is reached a t which the small amount of additional gas obtained does not warrant the added digestion time because of the required increase in the volume of the digester. On the other hand, if detention period is too short, the volume of culture removed each day de- pletes the total bacterial population to such an extent that an insufficient number of organisms is available for breaking down the incoming material.

Our studies have shown that gas production drops rapidly when the detention period is less than ll days and becomes negligible a t 3 days. It remains the same at detention periods of 11 to 30 days, as does the volatile acid content and physical characteristics of the digesting sludge. The optimum period probably is within the range 11 to 20 days.

2. Loading R a t e

Loading refers to the total amount of volatile solids given a digester each day, since this is the fraction subject to decomposition. Studies show that gas production remains approximately the same a t loading rates from 0.09 pound of volatile matter/cu. ft. of culture/day to 0.18 pound/cu. ft./day. Sustained studies were not made of higher loadings, and, hence,


the upper limit of loading for algal digesters is not known a t present. However, experience with conventional raw sewage sludge digestion indicates that, as daily loading approaches 0.25 lb./volatiles/cu. ft./day, gas production drops.

Varying the solids concentration of the slurry fed the digester within the range studied, namely, 1.5 to 5% of the wet weight, had no effect on the rate of breakdownof volatile solids or production of gas.

3. Temperature

Algal cells are not immediately killed a t the time of introduction into a digester operated a t temperature less than 40°C. despite the adverse conditions encountered there. In fact, intact cells were detected after periods of exposure to digester conditions as long as l week. However, the organisms are rapidly killed when the temperature of the digester is raised to 45°C. or higher. Since bacteria do not enter the intact walls of living cells, rapid digestion demands operating temperatures higher than the thermal death point of the algal cells used as feed. Consequently, the optimum temperature range for most rapid breakdown of volatile matter and greatest yield of gas is higher than that for the digestion of agricultural and sewage wastes.

Our studies have shown that gas production from algal digestion a t 45 and 50°C. was uniformly greater than a t 35"C., whereas the rate is uniform at all three temperatures with digestion of raw sewage wastes (Garber, 1954; Golueke, 1958). Temperature had no effect on the composition of the gas. Daily production of gas ranged from an average a t 35°C. of 6.5 cu. ft./lb. of volatile matter introduced to 8 cu. ft./lb. a t 50°C. The gas produced per pound of volatile matter destroyed was the same regardless of temperature, however. The lower yield of gas per pound of volatile matter introduced a t 35°C. most likely was due to the less extensive breakdown of organic matter, since the yield per pound of volatile matter destroyed was not affected by temperature. The less extensive breakdown at the lower temperature is also indicated by the higher nitrogen content (8%) of the sludge as contrasted with the average of 6.6% in the 50°C. sludge, as well as the higher volatile acid content, namely, 2300 to 4080 p.p.m. (as acetic acid) a t 35"C., and 354 p.p.m. to 1250 p.p.m. a t 50°C.

4. Digester Type A heated digester with floating gas-cover and gas recirculation of the

type commonly used in the U. S. today probably is best suited for the energy conversion system. Although other types of digesters may be made which are much less expensive, they are less effective in output and more difficult to control. Construction, operation, and maintenance of the con-


ventional type of digester is a well-established art, and reliable data are available on costs for construction, operation, and maintenance.

5. Digester Size

In a full energy conversion system, a digester of sufficient size must be designed to take the average load of cell material from the ponds together with the new sludge separated from the daily increment of sewage. Since the volume of the daily increment of sewage sludge will constitute only from 2 to 5% that of the algae added each day when the system is operating a t full capacity, sewage sludge may be neglected in designing the digesters, and only the daily input of algae need be considered.

The expected yield of algae available as feed for the digester each day is easily predicted. According to Eq. (10) , the production of algae may be expressed as a function of the available sunlight energy and the light conversion efficiency. In estimating peak plant capacity, it should be re- membered that maximum production of algae takes place when maximum sunlight energy and maximum efficiency occur simultaneously. Since maximum visible sunlight energy is 298 caI./cnx2/day (cf. Table I) , and maximum conversion efficiency is about 10% (cf. Table 11), maximum yield would be 298 x 10 x 0.15, or 443 lb./acre/day. When such high yields are obtained, one-half the harvested crop would be stored. Only when the yield decreased to about 200 lb./acre/day would the full quantity of algae be placed in the digester. Inasmuch as our experience indicates an allow- able digester loading of 0.18 lb./day/cu. ft., the digester size requirement is 1100 cu. ft./acre. If allowance is made for about 18% additional capacity, a value of about 1300 cu. ft./acre is obtained. The latter capacity per acre is the recommended one.

Since the digesting culture should be maintained at 43 to 45°C. to obtain the desired rate of decomposition, provision must be made for heating the digester. According to Imhoff and Fair (1940) , one-half of the waste heat from the combustion of the gas for power production is available for heating the digester. Calculations show that about one-half the waste heat from power production is more than adequate for maintaining a temperature of 45 to 50°C. in digesters of one million or more cubic feet capacity ; consequently, the minimum feasible heated-digester size for the solar energy conversion system will be from one to one and one-half million cubic feet capacity.

If methane were to be converted to liquid fuel for use outside the plant instead of being used for power generation, as has been proposed by Meier (1955) , use of a heat-pump would be indicated for heating the digesters. This method of digester heating has been investigated by Smith and Morris (1953). The heat-pump would extract heat from the warm pond

249 BIOLOGICAL TRANSFORMATION OF SOLAR ENERGY

water during the day and concentrate it in the digester. However, 25% of the total gross energy yield of the plant would be required to operate the heat-pump. Hence, production of electrical power appears more feasible.

F. ENERGY CONVERSION

The fuel value of methane is 896 B.T.U./cu. ft. under standard conditions and equals the heat liberated in combustion minus the heat of con- densation of water vapor, the gas being saturated with water vapor (Im- hoff and Fair, 1940). I n experiments in which digester loadings of 0.18 pound volatile matter/cu. ft./day were used, daily gas production averaged 8 cu. ft. of 62% methane, with a heat value of 4400 B.T.U./lb. of volatiles introduced; and on several occasions daily production rose to 8.5 cu. ft. of gas, with a heat value of 4600 B.T.U./lb. of volatile matter introduced (Golueke and Oswald, 1959a). On one or two occasions after the digester had received a loading of 0.025 pound of volatiles/cu. ft., a yield of 10 cu. ft./lb. of volatiles was obtained, and the energy yield was 6250 B.T.U./lb. of algae introduced. The hydrogen (2%) and traces of HzS contained in the gas also have high heat values. Since the total fuel value of algae is 10,000 B.T.U./lb. of volatile matter, energy recovered in the form of methane, therefore, usually varied from 45 to 65%, although on several occasions an energy recovery as high as 75% was obtained. It is highly unlikely that energy recovery would exceed 85% at any loading rate.

I n Fig. 7 is summarized our experience on the conversion of algal energy to the energy of methane in a laboratory digester. The solid line repre- sents conversions actually attained with the laboratory apparatus, while the dotted line indicates conversions to be expected in a digester in which the total daily loading is added in such a manner that the digester receives volatile matter continually throughout the day. Assuming that 50 to 60% of the algal volatile matter is converted to methane, then gas production should range from 1000 to 2000 cu. ft. of gas/acre/day.

G. GAS STORAGE

To meet variations in hourly power demand, gas storage capacity must equal about one-half the average daily gas production, i.e., about 1000 cu. ft./acre. I n alternative and commonly used methods of storage, the gas is held at low, intermediate, or high pressures. Each of the methods has unique advantages for a given purpose. Reynolds (1956) has shown that one of the most economical methods of storing gas is to keep it under 600 Ib./cu. ft. pressure in buried 36-inch steel pipes, because in this manner fabrication, land, and maintenance costs are minimized. Reynolds’ method requires about % h.p./1000 cu. ft. of gas compressor capacity and about

250

o.250

- . + u 0.20-

(r W a

5 0.15

1

- G 9 ?j 0.10

?

3

d

l.7

+, z 0.05 a

0 0

WILLIAM J. OSWALD AND CLARENCE G. GOLUEKE

20 40 60 80 100 , I f , t / I , ,

, . \ -. -- '. ' \ '

\$ digesters

-

-Methane I

- I

I

Digesters - fed doily

I

\

\

\ \ . \ '.'

2 6 8 10 " I " " "

PERCENT ENERGY RECOVERY

ENERGY- BTU PER LB. OF VOLATILE MATTER INTRODUCED x I O - ~

FIG. 7. The relation between daily loading of a digester and energy recovery in the form of methane.

2 kw.-hr. of power/1000 cu. ft. of gas compressed. If digesters with floating covers are used, capacity for low pressure gas storage is also available under the cover.

IV. Power Production

A. TYPE OF PLANT No details are presented in this paper concerning the type of generator

plant in which the methane will be converted to electrical energy. It is assumed that the plant will be a steam or "thermal" plant of conventional design, such as is widely used today to convert coal, natural gas, or fuel oil to electrical energy. Since the plant will be operated on water-washed methane, a clean, high-energy fuel, construction and operation of the plant will probably be within the lower range of published costs, and the percentage conversion of heat energy to electrical energy will probably be high.

B. QUANTITY PRODUCED On the basis of 5700 B.T.U. of methane produced per pound of algal

volatile matter introduced to the digester, and a power plant conversion of 8500 B.T.U./kw.-hr. (Ayers and Scarlott, 1952), gross power production


will be 0.67 kw.-hr./lb. of volatile matter introduced to the digester. From the yield in electricity per pound of algal volatile matter, the line electrical power output of a plant may bc predicted by using average solar energy data from Table I and Eq. (11) :

Po = 0.10 F S,

in which F is the photosynthetic efficiency, Po is the daily power in kw.-hr., and S , is the available solar energy. Since 8, values are daily averages on a monthly basis, the total power for each month is obtained by multiplying the values obtained from Table I by the number of days in the month. Thus, monthly power P, is

P, = 3 F S,

(11)

(12) and the annual power is

in which Pa’ is the gross annual line power in kw.-hr./acre and F and S, are as defined previously.

The power requirements for plant operation must be subtracted from the gross annual power to obtain a net power yield. Assuming that the average daily internal power requirement for mixing the cultures, operating the separation plant and digester is 10 kw.-hr./acre/day, the net annual yield ( P A ) in kw.-hr./acre is

PA = Pal -- 3650

In Table I11 are listed the estimated values of net annual yield a t various latitudes and photosynthetic efficiencies.

(14)

TABLE 111 THEORETICAL MAXIMUM ANNUAL LINE POWER (Pa)

I N KILOWATT-HOURS PER ACRE PER YEAR

Collector efficiencya (%)

North latitude (degrees)

0 10 20 30 40 50

1 2 4 6 8

10 12

5,825 15,300 34,250 52,900 72,150 91,100

110,050

5,565 5,070 4,375 3,295 14,780 13,830 12,400 10,240 33,150 31,230 28,450 24,130 51,550 48,760 44,500 38,020 70,070 66,110 60,550 51,810 88,500 83,750 76,660 65,800

106,400 101,030 92,650 79,690

2,035 7,720

19,090 30,460 41,830 53,200 64,570

a Efficiency in fixing visible sunlight energy available to the algal cells.


TABLE IV THEORETICAL MAXIMUM AVERAGE ANNUAL POWER CAPACITY (C,) OF

SOLAR COLLECTOR PONDS IN KILOWATTS PER ACRE^

North latitude (degrees)

0 10 20 30 40 50

Collector pond efficiency (%) -~

1 1 . 3 1.2 1 .1 1 . 0 0.73 0.45 2 3 . 4 3 . 3 3 .1 2 .8 2 .3 1.7 4 7.6 7 .4 6 . 9 6 . 3 5 .4 4 . 2 6 11.7 11.4 10.8 9 .9 8 . 5 6 . 7 8 16.1 15.7 14.7 13.5 11.5 9 . 3

10 20.2 19.6 18.6 17.0 14.6 11.8 12 24.4 23.6 22.4 20.5 17.8 14.4

a Based on 4,500 kw.-hr. per annum per kilowatt of capacity.

C. PLANT CAPACITY

Since the demand for power varies with time of day as well as time of year, and since storage of electricity is uneconomical, i t is necessary to have an installed power plant with capacity in excess of the average output. On the other hand, inasmuch as a half-day or more of gas storage is included in the pond-digester design and because algae are easily stored, the rated capacity of the pond-digester system is considered to be the same as the installed capacity of the steam plant it would serve. Since, according to the Bureau of Power, Federal Power Commission (1957), each kilowatt of installed generator capacity produces about 4500 kw./ year, the annual output of the pond-digester system from Table I11 may be converted to per-acre capacity through use of Eq. (15)

C , = P~/4500 (15)

in which C , is defined as average capacity per acre of the pond-digester system and PA is the net annual power yield.

Estimated values for pond power-production capacity on a per-acre basis for various latitudes and efficiencies are tabulated in Table IV.

V. Estimation of Cost of Power

A. COST OF THE PLANT

Sufficient evidence and information are available on which to base an estimate of the cost of the electrical energy produced by the solar energy conversion process described in this paper. In Fig. 8, curves are given which show the cost of the algal and the sedimentation ponds and of the digester as a function of pond size in acres, and digester and sedimentation tank size in cubic feet. In estimating the costs, it was assumed that


channel dividers, pump chamber:' pumps and transitions

n- --

$ 1 ' I I- I I I 1 1 1 1 1 I I I I I 1 1 l l 1 I I

20 30 40 50 100 200 300 500 1000 2000 3000

SEDIMENTATION TANKS AND DIGESTER VOLUME IN CUBIC FEET x lo3

8 0. I 0 lo POND SlZE IN ACRES

FIO. 8. Cost of algal pond, sedimentation pond, and digester as a function of pond size in acres.

the algal growth ponds would be lined with asphaltic concrete, asphalt, or plastic, and would have the design features shown in Fig. 4. Sedimen- tation tanks in which autoflocculated algae accumulate before transfer to the digester would be of similar design and cost to those employed for sewage treatment. The cost of the digesters is also based upon digester costs for sewage treatment. Details of the computations as well as the over-all costs are presented in Tables V and VI.

B. CAPACITY AND POWER COSTS

I . Capital Investment

Capital investment in power facilities is usually evaluated on the basis of plant costs in dollars per kilowatt of capacity. Tables V and VI show that the expenditure involved in the construction, maintenance, and operation of the collector ponds and digesters would be approximately $4400/acre in a 2000-acre installation. Inasmuch as per-acre capacity is a variable, capital costs also are variable, the approximate relationship being


TABLE V ESTIMATED COST OF COLLECTOR POND INSTALLATION

Dollars per acre of pond (prorated)

~~

Fixed cost Algal growth ponds and autocoagulation ponds Sedimentation tanks Algal drying beds and removal equipmenta

2700 250 100

Annual fixed costs Depreciation and interest (8%) Depreciation and interest (10%) Depreciation and interest (12%)

Operating costs Personnel: 12 men a t $6,00O/yr. (av.) b*c

Make-up or dilution water: 5 ft/acre/year a t $10 Coagulant: &?/ton algaed Make-up nutrients: $l/ton algae Predator control Pond repairs and maintenance

Total operating costs

3050

246 305 366

36 50a 60' 30a 4a

40

220

-

~ ~

a These costs will vary with plant output. * Digester and pond operators belong t o the same crew and, hence, are interchange-

c Includes fringe benefits. able.

Will not be required when algae are harvested by autoflocculation.

in which D is the capital investment in dollars/kw. of installed capacity, C , is the plant capacity and 4400 is the fixed cost in dollars per acre.

In Fig. 9 is shown the relationship between plant costs and plant capacity for capacities as high as 30 kw./acre computed as shown in Eq. (16). For a complete plant, it would be necessary to add to the values obtained from Fig. 9 the capital cost of the generator station, which should be approximately $150/kw. Thus, for example, a pond-digester fuel system with a capacity of 15 kw./acre would entail an expenditure of $295/kw., while the fuel system plus power plant would cost $445/kw.

2. Cost of Power In the pond-digester fuel system under study, the cost of power is a

function of the per cent rate of interest and depreciation applied to the


TABLE VI ESTIMATED COST OF DIGESTER INSTALLATION

Dollars/acre of pond (prorated) Item

Fixed Costa Algal dry storage and handling equipment, Central facilities: meters and controlso Digesters, complete Gas compressors and storage facilities

50 100

1050 150

Annual fixed costs Depreciation and interest (8%) Depreciation and interest (10%) Depreciation and interest (12%)

Operating costs Personnel: 5 men at $6000/yr. (av.)**c Maintenance and repairs

1350

108 135 162

15 50

65

a Portions of these facilities may be combined with the power plant. b Digester and pond operators belong t o the same crew and, therefore, are inter-

changeable. Includes fringe benefits.

fixed costs, the operational costs, and the per-acre capacity of the plant. The relationship is shown in Eq. (17) for a 2000-acre plant

44R + 285 4.5ca

M = __-

in which M is expressed in mills per kilowatt-hour of line power, R is the rate of depreciation plus interest expressed as a whole number, 44 is the fixed cost per unit of depreciation and interest, and 285 is the estimated cost of operation and maintenance. C, is the variable capacity in kilowatts per acre of collector pond, 4.5 is the capacity factor divided by 1000 to convert dollars to mills. In Fig. 10 is shown the power costs in mills per kilowatt as a function of collector pond capacity a t various rates of depreciation plus interest.

To determine the ultimate cost of line power, generator costs must be added to the values obtained from Fig. 10. For example, according to Fig. 10, a plant operating a t a capacity of 15 kw./acre would produce methane


FIG. 9. FueI system capacity cost as a function of coIlection pond capacity a t various rates of depreciation plus interest.

a t 11.5 mills/kw.-hr., assuming interest and depreciation combined are 10% per annum. If generator plant costs are 4.5 mills/kw.-hr., the total cost of power would be 16 mills/kw.-hr.

By using the cost data in Fig. 10 and capacity data from Table IV, the cost of generating power a t a given geographical Iocation can be estimated. For example, from Table IV, a pond-digester system located a t 20" north latitude which attains 6% light conversion efficiency will have an average annual capacity of 10.8 kw./acre, and, from Fig. 10, will produce methane for 15 mills/kw.-hr. of line power when the interest and depreciation rate is 10%. I n general, it appears that only ponds located near the equator and which attain efficiencies of 6 to 7% could produce power within the range of 10 to 20 mills/kw.-hr., as is required to compete effectively with other sources of energy.

An estimate of the costs involved in a typical solar energy fuel system is given in Table VII, in which is also included for comparison the cost of energy obtained with a conventional steam plant and from nuclear fission. The proposed Rural Cooperative Power Association atomic energy plant a t Elk River, Minnesota, was chosen in this comparison because i t


- - - -

a W e

5

R :merest t depreciotion expressed I

(1s a whole number 10 - 9 -

8 -

7 -

6 -

5 -

2 3 5 10 15 20 30 40 C o - K W PER ACRE

FIG. 10. Fuel system power costs as a function of collector pond capacity a t various rates of depreciation plus interest.

was expected to have a capacity of 22,000 kw., approximately that of our hypothetical solar power plant (Ullman, 1958). In arriving a t the estimate given in Table VII for the cost of power from the solar plant, the entire cost of pond and digester including operation and maintenance was at- tributed to fuel, while fixed charges and operation and maintenance for the generator plant were taken as essentially the same as those for a conventional steam plant, the assumption being that the two thermal plants would be equivalent except for fuel costs. In each case the depreciation and interest was assumed to be 10%.

These costs for power in Fig. 10 do not' include the savings resulting from the sewage treatment accomplished as a part of the process. Although it is not possible to give a definite value for the worth of the sewage treatment, a per capita savings of a t least 10 mills/day is a conservative estimate. This may represent a savings of 1 to 2 mills/kw.-hr., depending upon the per capita power consumption.


TABLE VII POWER COSTS OF VARIOUS POWER PRODUCING SYSTEMS

Item

Costs in 1958 mills/kw.-hr.

Conventional0 Nuclearb Solar’ steam power power

Fixed charges 2.43 12.1 2.5 Fuel 1.62 3.5 13.3 Operation and Maintenance 0.45 1.1 0.5

4.50 16.7 16.2 -~ ~ ~ ~ ~ ~ ~ ~ ~~~ ~

Ullman (1958). b Proposed atomic energy plant for the Rural Cooperative Power Association,

Assumes 6.2% photosynthetic efficiency, 12 kw./acre; 2000 acre-plant = 24,000 Elk River, Minnesota, 22,000 kw. (Ullman, 1958).

kw.; and interest and depreciation at 10%.

VI. Discussion and Future Considerations

The methods of insuring an adequate supply of organic material for conversion to methane proposed in this article requires some elaboration. Two extremes are to be considered. I n the first extreme, direct fermentation of all of the community’s organic wastes without growing algae would produce sufficient methane to supply only from 2 to 5% of that community’s electrical power requirements. In the second extreme, through growing algae, the same products of fermentation may be employed over and over again together with each day’s new supply of wastes to produce methane. If there were no losses of fertility elements and this cycle could be continued indefinitely, the collector ponds would have to be increased in volume and area until, ultimately, they would cover the land and produce many times more power than needed by the community. A practical plant would operate somewhere between these two extremes, its ultimate size and output limited not by requirements but by the rate of daily loss in nutrient from the system with relation to the daily increment in nutrient. Although such losses can be maintained a t low levels, certain in- herent difficulties do exist, however, which will make losses inevitable. For example, uptake of COB from the atmosphere is enhanced by increase in pond pH (alkaline absorption), but ammonia tends to escape from solution a t high pH levels; thus, losses of ammonia are to be expected. If the losses in ammonia could be maintained below 10% per pass, the use of small amounts of supplementary inorganic fertilizers to increase algal production probably would be economically feasible. Studies indicate that unless a pond is enriched with COz , or the ammonia-nitrogen content of


the pond is less than 6 to 10 p.p.m. in the autumn months in the middle latitudes and 10 to 30 p.p.m. during the summer months, nitrogen added over and above that contained in the wastes is lost, either through volatili- zation or with the effluent. It is hoped that reuse of CO, as discussed previously will minimize losses of both COz and ammonia,

A number of opportunities exist for reducing the cost of power production of the solar plant to levels lower than indicated in Fig. 10 and Table VII. For example, with respect to thermal power-generating plants, in the future, combined steam and gas turbine cycles are expected to bring about improved efficiencies, and possibly yields as high as 7000 to 7500 B.T.U./ kw.-hr. could be obtained from the methane. If this increase were aug- mented by an 80% efficiency in converting algal energy to methane in the digester, the output energy should be increased from the present 0.67 to 1 kw.-hr./lb. of algal volatile matter introduced into the digester. Thus, the output in power from 1 acre of algal culture could be increased 25% and unit power cost proportionally decmased with but little added expense.

Digester costs may be greatly decreased from those given in Fig. 8. Di- gesters may be constructed simply as paved ponds in areas where the mean temperature is high. Rigid molded plastic covers having numerous cells of small dimension (honeycomblike) could be used for gas collection in such paved digester ponds, thus decreasing the cost of digesters to about $0.50/cu. ft. or less. Through these savings, the cost of the digestion step of the fuel system would be decreased over-all by about 2.5 mills/kw.-hr.

As the factors influencing photosynthetic efficiency become better known, engineering steps may be taken to increase efficiencies to the levels routinely attained in the laboratory, namely, 10 to IS%, which would make possible a savings of 4 mills/kw.-hr. in fuel cost. It therefore seems plausible to assume that continued research and development could reduce the cost of power produced in a pond-digester system to 10 mills/ kw.-hr. of line power or less.

Costs were computed in this study on the basis of material and labor in the United States, hence it should be expected that costs would be some- what lower in most other countries. However, any savings in labor in underdeveloped countries might be offset by higher costs of material and higher interest rates, so that each country would require an independent evaluation.

Finally, it should be repeated that the costs given here apply mainly to a 2000-acre system having a capacity of 20,000 to 25,000 kw. Smaller plants would have higher unit costs, while material savings would probably result from constructing plants in the 100,000- t o 500,000-kw. range, although availability and transportation costs for organic wastes would probably limit the ultimate size of such plants. In a study of feasibility i t


must be recognized that transportation of organic wastes away from communities is an expense which should be met regardless of whether or not the waste is utilized.

Taxes and insurance (other than that for the workers) have been pur- posefully omitted from cost estimates, because it is apparent that in most cases power production by this method would have to be a public enter- prise in which taxes would be omitted and insurance greatly reduced. Land cost has also been omitted. It is recognized, of course, that the system as shown is highly dependent upon the availability of inexpensive land, but the consideration of the use of land for the system involves extremely complex factors. Land required normally for waste disposal, land in the approaches to jet airports, or land in frequently flooded areas such as tidal flats, could be used for ponds at a low cost. The possibility that ponds of the type described could be located in lands reserved for “green belts” needed to produce oxygen and alleviate air pollution in the city of the future merits study.

Ordinary reasoning will show that power production from solar energy by the system outlined cannot compete economically on a short-term basis with power produced from fossil fuels, when fossil fuels are available in present quantities. The proposed plant must produce its own fuel in addition to producing power, whereas a plant using fossil fuel need only produce power. In comparison with large nuclear fission plants the process seems expensive. Nevertheless the requirements for a pond-digester plant are much simpler than the requirements for a nuclear reactor. Hence, in special areas of the world biological transformation of solar energy should be seriously studied as a possible alternative to other forms of energy.

VII. Summary A description of a hypothetical solar energy conversion plant is given,

in which an algal culture pond, algal digester, and a thermal power generator are combined to transform solar energy into electrical power. Descrip- tion, specifications, and cost estimates are given for designing, maintaining, and operating each of the units. The cost of line power as a function of latitude and photosynthetic efficiency is estimated.

According to the data and information presented, the cost of line power in the lower latitudes of the earth would be from 15 to 20 mills/kw.-hr. This compares favorably with the estimated 16.7 mills/kw.-hr. for a fission plant of equivalent capacity, but is about three times the cost of present-day thermal power in the U. S.

By increasing the efficiency of the solar energy collecting ponds and im- provements in digester design, power costs could be decreased to the extent that solar energy would probably compare favorably in cost with other sources of energy in special areas of the world.

BIOLOGICAL TRANSFORMATION OF SOLAR ENERGY 26 1

ACKNOWLEDGMENT

This research was supported in part by a research grant from the National Insti- tutes of Health, United States Public Health Service.

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