CHAPTER 6ENERGY RELATIONSHIPS

From Green Chemistry and the Ten Commandments of Sustainability, Stanley E. Manahan, ChemChar Research,

Inc., 2006manahans@missouri.edu

Energy

Energy is the capacity to do work or to transfer heat.

Heat is the form of energy that flows from a warmer to a colder object and is due to the motion of atoms and molecules.

The internal combustion piston engine used in automobiles (below) converts chemical energy (in fuel) to heat energy, then to mechanical energy in rotation of a crankshaft. Fuel is taken in with air or injected near the top of the compression stroke.Airin Exhaustgases out

Intake Compression Ignition/Power Exhaust

Spark plug

Energy

Conversion of chemical energy to heat energy

C16H34 + 33O2 16CO2 + 17H2O + heat energy (6.1.1)

In a piston engine, the hot gases in the engine’s cylinders push the pistons down, some of this heat energy is converted to mechanical energy.

The standard unit of energy is the joule, abbreviated J.• 4.184 J of heat energy will raise the temperature of 1 g of liquid water

by 1˚ C.

• This amount of heat is equal to 1 calorie of energy (1 cal = 4.184 J)

Thermodynamics

Thermodynamics is the science that deals with energy in its various forms and with work.

The first law of thermodynamics (law of conservation of energy) states that energy is neither created nor destroyed.

• Concentrated, useful energy in the form of hydrocarbon fuel becomes dissipated as a slight warming of the surroundings—no use.

The first law of thermodynamics must always be kept in mind in the practice of green chemistry.

• Green chemistry requires the most efficient use of energy as energy goes through a system

• If enough energy is available, almost anything can be accomplished.

6.2. RADIANT ENERGY FROM THE SUN

Solar flux, 1,340 watts per square meter

Solar energy from thermonuclear fusion of hydrogen in the sun:

1 m1 m1,340 watts

4 1H 2He + 2 +1e + energy41 0

Solar Energy

Most of the solar energy that actually reaches Earth’s surface does so as visible light and infrared radiation.

• Forms of electromagnetic radiation, which includes ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves. AmplitudeWavelengthShorter wavelength,higher frequency,greater energy

Wavelength (, Greek lambda), amplitude, and frequency (, “nu”)

= c (6.2.2)

• in meters (m) • is in cycles per second (s-1), hertz, Hz

The wavelength is the distance required for one complete cycle and is the number of cycles per unit time.

Electromagnetic Radiation

Energy, E, is associated with electromagnetic radiation.• Packets of energy called quanta

• According to the quantum theory, electromagnetic radiation, can be absorbed or emitted only in discrete quanta, also called photons.

• Photons have energy, E

• E = h where h is Planck’s constant, 6.63 x 10-34 J-s (joule x second).

Outbound energy from Earth is in the infrared region above 700 nanometers (nm 1 x 10-9 meters).• Reabsorbed by greenhouse gases, such as carbon dioxide and

methane, delaying its eventual exit from Earth.

• Beneficial greenhouse effect

• Detrimental in excess

Electromagnetic Radiation (Cont.)

Ultraviolet radiation below 400 nm cannot be seen.

• Ultraviolet photons are sufficiently energetic to “excite” the valence electrons of molecules to higher levels.

• Excited molecules can split apart to form very reactive species resulting in photochemical reactions.

Photochemical Energy

Direct photochemical energy

• Warmth from sunlight • Photovoltaic cells generate electricity

Indirect photochemical energy

• Chemical energy from photosynthesis using solar energy, h:

6CO2 + 6H2O C6H12O6 (biomass) + 6O2

• Biomass energy utilized by organisms in aerobic respiration

• C6H12O6 + 6O2 6CO2 + 6H2O + energy

• Biomass energy in fossil fuels

• Indirect solar energy in movement of wind and water

• Wind electrical generators

• Hydroelectric power

6.3. STORAGE AND RELEASE OF ENERGY BY CHEMICALS

Chemical potential energy to heat energy

CH4 + 2O2 CO2 + 2H2O + energy (6.3.1)

Energy is released because of bond energy in chemical bonds.

Calculate the energy released in kilojoules (kJ) when 1 mol of CH4 reacts with 2 mols of O2 to produce1 mol of CO2 and 2 mols of H2O (next slide):

Chemical Energy Release Calculation

In the products:

Total bond energy in reactants = 1644 kJ + 988 kJ = 2632 kJThe difference in bond energies between products and reactants is 3434 kJ - 2632 kJ = 902 kJ

OCO O HH

O HHC=O 799 kJ/mol O-H 459 kJ/mol

H C HH

HOO OO+

C-H 411 kJ/mol O=O 494 kJ/mol

1 mol CO2 ×2 =mol C O×799 kJ=1598kJmol CO2 =mol C O2 mol H2 O×2 -mol O H×459 kJ=1836kJmol H2O -mol O H

Total bond energy in products = 1598 kJ + 1836 kJ = 3434 kJIn the reactants:

Bond Energy Calculation (Cont.)

From the preceding slide, based upon considerations of bond energy, alone, the energy released when 1 mole of CH4 reacts with 2 moles of O2 to produce 1 mole of CO2 and 2 moles of H2O, is 902 kJ.

The reaction is an exothermic reaction in which heat energy is released, so it is denoted as negative, -902 kJ.

6.4. ENERGY SOURCES

Until about 1800, virtually all the energy used in the world was from biomass sources.

• Energy for heating from burning wood

• People and goods moved on land, as well as cultivation of soil, mostly by means of humans, horses, and oxen walking

• The energy above from biomassA significant amount of energy for transportation was provided by wind, which drove sailing boats and ships.

All the above energy was from renewable sources.

The Development of Fossil Fuel Energy Sources

The use of coal for energy grew spectacularly during the 1800s and by the end of that century coal had become the predominant source of energy in the United States, England, Europe, and other countries that had readily accessible coal resources.

Major shift from renewable biomass energy sources to coal, a depletable resource that had to be dug from the ground.

By 1950 petroleum had surpassed coal as a source of energy in the U.S.

By 1950 natural gas had become a significant source of energy lagging behind petroleum in its rate of development.

Other Contributors to Energy

Hydroelectric power had become significant by 1900, and retained a significant share of energy production through the 1900s.

By around 1975, nuclear energy had become a significant source of electricity and now is several percent of world energy.

Miscellaneous sources including geothermal and, more recently, solar and wind energy now make contributions to total energy supply.

Biomass still contributes a little to the total of the sources of energy used.

Current Sources of Energy

Figure 6.5. U.S. (left) and world (right) sources of energy.

Fossil fuel sources account for the vast majority of energy used.

• Coal • Petroleum • Natural Gas

Two problems with fossil fuels

• Running out (petroleum peaked around 2000)

• Greenhouse gas source from CO2

Carbon Dioxide Emissions from Fossil Fuels

Variable amounts of CO2 added by burning various fossil fuels (greater the H2O/CO2 ratio, less CO2 per unit energy produced)• Natural gas: CH4 + 2O2 CO2 + 2H2O + energy

1 CO2 for each 2 2H2Os

• Petroleum: CH2 + 3/2O2 CO2 + H2O + energy

1 CO2 for each H2O

• Coal: CH0.8 + 1.2O2 CO2 + 0.4H2O + energy

1 CO2 for each 0.4 H2O

6.5. CONVERSIONS BETWEEN FORMS OF ENERGY

The most abundant sources of energy are usually not directly useful and must be converted to other forms.

Much of what is done with energy involves converting it from one form to another.

To get energy from fission of uranium1. Enrich in the isotope whose nucleus can undergo fission

2. Place this isotope in a nuclear reactor where fission occurs, converting the nuclear energy to heat

3. Use this heat to produce steam

4. Run the steam through a turbine to produce mechanical energy

5. Couple the turbine to a generator to convert its mechanical energy to electrical energy.

Energy Conversion Efficiencies

Energy Conversion Efficiencies

The preceding slide illustrates major forms of energy and conversions between them.

Higher peak temperature means higher efficiency

Reason for high efficiency of diesel engines

• Vast differences in conversion efficiencies

• Photosynthesis is less than about 0.5% efficient in converting light energy to chemical energy, leaving room for substantial improvement, such as by genetically engineered plants.

• Fluorescent bulbs are 5-6 times more efficient than incandescent bulbs in converting electrical energy to light.

The Carnot equation describes efficiency of converting heat energy to mechanical energy:Percent efficiency = T1 - T2 ×100T1

Superheatedsteam in, T1

Cooler steamout, T2Turbine vanes

Rotating shaft connectedto electrical generator

6.6. GREEN ENGINEERING AND ENERGY CONVERSION EFFICIENCY

Early fossil-fueled electrical power generating plants from around 1900 were only about 4% efficient in converting chemical energy to electrical energy; modern ones exceed 40%.

Change from steam locomotives to efficient diesel locomotives during the 1940s and 1950s resulted in an approximately four-fold increase in the energy-use efficiency of rail transport.

Improvements such as these due to

• Improved materials that can tolerate higher peak temperatures

• Advances in engineering

• Now, computerized control

Combined Power Cycles for Overall Energy Efficiency

District heating, is commonly practiced in Europe and can save large amounts of fuel otherwise required for heating.

ElectricityElectricityAirFuelCombustionturbineGeneratorBoilerExhaustgas

SteamturbineGeneratorHeated buildings

6.7. CONVERSION OF CHEMICAL ENERGYConversion of chemical energy from one form to more useful formGeneration of hydrogen gas from fossil fuels is an important chemical energy conversion process that may become much more widely practiced as fuel cells, which use elemental hydrogen as a fuel, come into more common useCoal gasification burns part of the carbon in the coal,

C(coal) + O2 CO2 + heat (6.7.1)leaving a solid residue of very hot carbon from the unburned coal. This material reacts with water in steam,

C(hot) + H2O H2 + CO (6.7.2)

to generate elemental H2 and CO in a reaction that absorbs heat. The CO can be reacted with more steam over an appropriate catalyst,

CO + H2O H2 + CO2 (6.7.3)

Used for well more than a century in the coal gasification industry.

Coal Gasification

Before natural gas came into common use, steam blown over heated carbon was used to generate a synthesis gas mixture of H2 and CO that was piped into homes and burned for lighting and cooking.• In addition to forming treacherous explosive mixtures with air, it

was lethal to inhale because of the toxic carbon monoxide.

Coal gasification may have a future for the generation of elemental hydrogen for use in fuel cells.

• By using pure oxygen as an oxidant, it produces greenhouse gas carbon dioxide in a concentrated form that can be pumped underground or otherwise prevented from getting into the atmosphere, a process called carbon sequestration.

The synthesis gas mixture of H2 and CO2 is a good raw material for making other chemicals, including methanol or hydrocarbons that can be used as gasoline or diesel fuel.

Methanol: CO + H2 H3COH + CO2 (6.7.4)

6.8. RENEWABLE ENERGY SOURCESRenewable energy resources do not pollute and never run out.

Solar Energy is the best—when the sun shines

Sunshine offers widespread availability, an unlimited supply, and zero cost up to the point of collection.

• Does not cause air, heat, or water pollution.

• Intense and widely available in many parts of the world.

• At a collection efficiency of 10%, approximately one-tenth of the area of Arizona would suffice to meet U.S. energy needs.

Solar EnergyUtilization of solar energy

• Solar heating, solar-heated houses, and solar water heaters

• Solar boilers that generate steam from sunlight reflected from mirrors

• Direct conversion to electricity in photovoltaic cells (next slide), around 12–15% efficient generating electricity at a cost 4–5 times that of electricity generated in fossil fuel power plants and could be used for up to 15% of electricity on existing power grids.

• Means of storing solar generated electricity include batteries (small scale), extremely high-temperature/high-pressure supercritical water stored deep underground, or mechanical energy stored in the extremely rapid rotation of flywheels.

• Generation of hydrogen gas usable in fuel cells by electrolysis of water

2H2O + electrical energy 2H2(g) + O2(g) (6.8.1)

Solar Voltaic Cells

Donor layerAcceptor layer

Si Si SiSi B SiSi Si SiSiSi Si SiSi As SiSi Si SiSiPositive hole fromboron atomExtra electronfrom arsenic

e-

Sun

e-

World wind electricity generating capacity exceeded 60,000 megawatts in 2006 up from less than 5,000 mw in 1995 (1,000 mw is the size of a large conventional coal-fired or nuclear power plant)

Wind Energy

The Growing Success of Wind Power

Production of biomass from photosynthesis could in principle provide all needed carbonaceous fibers and materials and a large percentage of liquid fuels

Photosynthesis is only about 0.4% efficient in converting solar energy to chemical energy

Utilization of biomass energy does not add any net carbon dioxide to the atmosphere

Wood is the most commonly used cooking fuel in many societies

Finland gets about 15% of its energy from wood

Oil seeds can provide direct sources of hydrocarbons, such as those that can be used in diesel fuel

Biomass Energy

Ethanol, C2H6O, is produced by fermentation of sugars by yeasts

• Used to supplement and substitute for gasoline

In the U.S., most of the raw material for ethanol production is sugar derived from corn grain

Little net energy is obtained from ethanol made from grain

Sugarcane returns much more energy than grain by ethanol fermentation and has enabled Brazil to become largely independent of imported petroleum for fuel

Efforts are underway to obtain sugars to make ethanol from cellulose sources, such as wheat straw

Biodiesel fuel is made by converting long-chain organic acids from oil-bearing grains to liquid esters that can fuel diesel engines

Oils for biodiesel fuel production include rapeseed, sunflower, soybean, palm, coconut, jatropha

• Rapeseed predominant in Europe • Soybean predominant in U.S.

Coconut and jatropha are attractive because they grow in the tropics and come from perennial plants

Ethanol and Biodiesel Fuels from Biomass Sources

Lignocellulose in stalks, straw, leaves, and corncobs provides much more potential fuel than grain-derived fuels such as ethanol and biodiesel fuel

Crop byproduct biomass is a large potential source of lignocellulose fuel

A much larger potential source is from dedicated crops

• Hybrid poplars that can re-grow from stumps after harvesting

• Switchgrass • Sawgrass

Lignocellulose can be burned directly for energy

Lignocellulose can be partially burned to generate heat, then reacted with steam to produce CO and H2

Mixtures of CO and H2 can be reacted together to produce methane (CH4) as well as hydrocarbons including gasoline, diesel fuel, and jet fuel

• Byproduct CO2 from these processes can be sequestered

Potential of Lignocellulose Fuels

Biodegradable biomass, represented {CH2O}, can be fermented in the absence of oxygen to produce biogas methane, the gas present in natural gas

2{CH2O} CH4 + CO2

• Generates gas from sewage sludge

• Biogas from livestock wastes

• Potential from biomass produced for fermentation

Biogas

Geothermal Energy

Geothermal energy from hot steam underground is used to generate electricity in Iceland, Japan, Russia, New Zealand, the Phillipines, at Larderello, Italy, and at the Geysers in northern California.

Dry steam is best, but the steam is often mixed with superheated water.

Toxic hydrogen sulfide from underground sources can cause pollution problems.

Injecting water into hot, dry underground water formations to produce steam has the potential to increase resources of geothermal energy ten-fold.

Nuclear energy is generated by the splitting of nuclei of atoms of fissionable uranium-235 (only 7.1% of natural uranium) or plutonium from unfissionable uranium-238 when it absorbs neutrons

18.12. Nuclear Energy

Generation of Electricity from Controlled Nuclear Fission

Advantages:

• No greenhouse gas emissions

• Reliable, steady source of power (with modern design)

• Relative abundance of fuel (uranium)

• Passive stability and simplicity of latest designs

Disadvantages:

• Generation of radioactive byproducts

• Need to decommission radioactive old reactors

• Possible catastrophic failure (Chernobyl)

Advantages and Disadvantages of Nuclear Energy

Nuclear fusion energy is that released when two nuclei fuse together, such as one of deuterium and one of tritium (both forms of hydrogen):

Nuclear Fusion Energy

Despite its promise of a low-pollution process with a virtually inexhaustible source of fuel, a practical nuclear fusion reactor has not been developed, and probably will not be