SOLAR ENERGY, PHOTOSYNTHESIS, AND SYNFUELS

 

A. Photosynthesis: Photosynthetic bacteria and green plants use energy from sunlight to add hydrogen atoms to carbon dioxide to make sugars in photosynthesis. The overall process can be summarized as follows:

 

Chlorophyll and Enzymes

Carbon dioxide + water + light glucose + oxygen

or,

 

6CO2 + 6H2O + 48 to 60 quanta at 680-700 nm C6H12O6 + 6O2

 

The minimum of 48 quanta at 680 to 700 nanometers provide about 48 times 40,000 calories per mole, or about 1,920,000 calories of energy of which 686,000 calories are captured in the sugar, glucose, giving a maximum efficiency of about 36% compared to about 15% for photocells. This sugar may be stored as starches or used to make structural cellulose as in plant fibers and wood. We should try to balance photosynthetic and oxidation reactions of food and fuels on Earth to survive in the long term.

 

B. Aerobic Respiration: In the presence of oxygen, sugars and starches are usually oxidized back to carbon dioxide and water by bacteria, plants, and animals:

 

Enzymes

C6H12O6 + 6O2 6CO2 + 6H2O + 686,000 calories per mole of glucose

 

However, only about 360,000 calories are captured as useful energy in 36 ATP, giving an overall efficiency of about 52% for aerobic respiration of glucose.

 

C. Anaerobic Respiration and Fermentations: In the absence of oxygen, for example, in mud, deep water, and deep underground, some bacteria and fungi are able to obtain energy by removing some of the hydrogens from the sugars producing alcohols, etc. This is because in the absence of oxygen, the hydrogens from glucose cannot be transferred to oxygen, but must be added back to one of the carbon containing products. The most common example is the fermentation of glucose to ethyl alcohol:

 

Glucose + 2ADP 2 Pyruvate + 2ATP 2 Acetaldehyde +2CO2 + 4H

 

2 Acetaldehyde +2CO2 + 4H 2 Ethanol + 2CO2

 

In summary: Glucose + 2ADP 2ATP + 2 Ethanol + 2CO2

 

In this case, only about 20,000 calories of useful energy are obtained per mole of glucose oxidized giving an overall efficiency of about 20,000/686,000 or 2.9%. This explains why oxygen is so important to most bacteria, plants, and animals. The critical step for anaerobic respiration is the removal of a pair of hydrogens from the critical hydrogen carrier (NADH + H+) so the process can continue in the absence of oxygen. In alcohol fermentation, the hydrogens are used to reduce acetaldehyde to ethanol regenerating NAD+. In other fermentations, other alcohols, acids (e.g., lactic), and even hydrogen gas, H2, are produced if the necessary enzymes are present in the cells.

 

D. Synfuels and Organic Feedstocks: It is obvious that if the necessary enzymes can be found, usually in bacteria or fungi, we can convert sugars and sugar derivatives, such as cellulose, to liquid fuels such as ethanol, organic feedstocks, and perhaps even as hydrogen gas. However, such a system must contain stable populations of bacteria, fungi, plants, and animals to survive over a long period of time. We will probably develop a mixture of photosynthetic organisms, photocells, nuclear fusion, and fermentations to produce electricity, liquid and gaseous fuels and feedstocks in the absence of fossil fuels.

 

E. Is there enough sunlight to drive all this? The Earth intercepts 1367.6 W/m2 of sunlight. The average radius of the Earth is 6368 km. Thus, the Earth potentially has access to 1367.6x3.1416x(6368x1000)2 = 1.742x1017 Watts of sunlight continuously. This is equivalent to 1.742x1017x365x24 = 1.526x1021 Watt hours per year. Since 1.0 KWH = 3412 Btu, this equals 1.526x1018 KWHx3412 Btu/KWH, or 5.207x1021 Btu, or 5.207*106x1015 = 5.207x106 Quadrillion Btu, or 5,207,000 Quad Btu per year of solar energy available above the Earths atmosphere.

 

Suppose 1% can be captured on land and photocells are 15% efficient, we could still obtain 7810 Quad Btu per year. However, remember that the Earth and all plants and animals must have part of this energy for humans to survive. Humans currently use about 446 Quad Btu per year excluding food and our use is increasing at 6.3 Quad/year. The cost of energy is likely to increase by a factor of 3 to 5 as we run out of fossil fuels made by bacteria and plants in the past. However, we can support a limited number of humans for thousands of years by using plants and photocells to capture solar energy and converting some of the sugar and other carbohydrates to liquid fuels such as ethanol. Note that the US is currently using 100 times as much energy for other purposes as it uses for food. Our food, organic fuels, organic feedstocks, solar, wind, and tidal energy all depend upon the Sun.

 

F. The transition from fossil to renewable energy over the next 100 years: Most fossil fuels will probably be burned by 2100. The burning of coal is particularly controversial because burning coal produces much more carbon dioxide (H/C ratio less than 2.3) than natural gas (mostly methane, H/C = 4.0). If glucose is converted to ethanol, all of the carbon in glucose will end up as carbon dioxide giving an overall H/C = 2.0 which is about the same as for coal. The higher the ratio of hydrogens to carbon the more energy available per CO2 produced. When any carbon based fuel is burned almost all of the Hs end up in water and the Cs end up in carbon dioxide. Reducing the increase in global warming by capturing carbon dioxide is going to be a major factor in the cost and use of coal fired power plants and ethanol plants if we wish to reduce CO2 as quickly as possible. No one knows how effective carbon sequestration can be but pilot plants are being constructed to find out.

 

Converting glucose in corn to ethanol (and carbon dioxide) is not a good long term idea because corn is a major human and animal food and requires a lot of water to grow. Ethanol fuels will increase energy independence a little, but they will not slow global warming very much, at least in the near term. Hybrid cars can reduce fuel consumption, but if plugged into electrical outlets to recharge, more carbon dioxide will be produced at any fossil fueled power plant.

 

Energy independence and reduced carbon dioxide emissions are desirable, but they must be considered along with conservation, efficiency, and cost to create a workable transition to a long term solution using nuclear fusion (not fission), geothermal, and minimal carbon-based fuels and feedstocks.

 

G. Long term prospects: Nuclear fusion in the Sun (and perhaps on Earth) and geothermal are our only long term energy options. We need to reduce our current population from 6.5 billion until we reach a GDP per capita of about $20,000/person/year (in constant purchasing power) without increasing energy consumption. Increasing energy efficiency and reducing the human population are the only ways to keep from increasing energy consumption while increasing the average GDP per capita. We then need to keep our energy consumption and population balanced at about $20,000/person/year so everyone has the opportunity to develop individual potentials. Quality of life should be more important than numbers living when considering billions of humans with a varied gene pool which has survived thousands of years and can survive many more if we act responsibly!

 

Acting responsibly will include maintaining a fairly stable human population, a fairly high GDP per capita, and a fairly constant atmospheric temperature to prevent excessive global warming or future ice ages. The carbon dioxide we are planning to store in the near future may be needed to avoid future ice ages. If we can manage the Earth to this extent, we might be capable of finding, reaching, terra-forming, and settling another planet. At present, we have not proven that we are capable of any of these steps. We are grossly overconfident of our current abilities to establish and maintain ourselves over long periods of time. It is time to back off and demonstrate that we can actually create a stable, peaceful Earth.

 

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