Chapter 22: Sustainable Energy

In this chapter, the following topics will be covered:

• Different systems for capturing solar energy

• How photovoltaic collectors generate electricity

• Diminishing fuelwood in less-developed countries

• The use of dung, crop residues, energy crops, and peat as potential

energy sources

• The use of hydropower, wind, and geothermal energy as source of

power

• The use of tidal and wave energy and ocean thermal gradient as a

source of electrical energy

Conservation

• Conservation is the best way to avoid energy shortages andrelieve environmental and health effects of our current energy technologies.

Utilization Efficiencies

• Our current energy technologies are extremely inefficient and muchpotential energy in fuels is wasted.

• Many conservation techniques are simple and cost effective.

• Many improvements in domestic energy efficiency have been made.

- The average home today uses half the fuel of a house built in 1974.- Changes include improvements in insulation, placement of homes, window construction, etc.

• More efficient industrial practices have also helped conserve energy.

Energy Conversion Efficiencies

• Energy efficiency is a measure of energy produced compared toenergy consumed.

• Net energy yield is based on the total useful energy produced duringthe lifetime of an entire energy system minus the energy required to make useful energy available.

Table 22.1 -- Typical net efficiencies of energy-conversion devices Electric Power Plants Yield (%)

Hydroelectric (best case) 90 Combined cycle-steam 90 Fuel cell (hydrogen) 80 Coal-fired generator 38 Oil-burning generator 38

Nuclear generator 30 Photovoltaic generation 10 Transportation Pipeline (gas) 90 Pipeline (liquid) 70 Waterway (no current) 65 Diesel-electric train 40 Diesel-engine automobile 35 Gas-engine automobile 30 Jet-engine airplane 10 Space heating Electric resistance 99 High-efficiency gas furnace 90 Typical gas furnace 70 Efficient wood stove 65 Typical wood stove 40 Open fireplace -10 Lighting Sodium vapor light 60 Fluorescent bulb 25 Incandescent bulb 5 Gas flame 1

Table 22.2 -- Typical net useful energy yields Energy Source Yield/Cost (Ratio)

Nonrenewable sources Coal (space or process heat) 20/1 Natural gas (as heat source) 10/1 Gasoline and fuel oil 7/1 Coal gasification (combined cycle) 5/1 Oil shale (as liquid fuel) 1/1 Nuclear (excluding waste disposal) 2/1

Renewable sources Hydroelectric (best case) 20/1 Wind (electric generation) 2/1 Biomass methane 2/1 Solar electric (10% efficient) 1/1 Solar electric (20% efficient) 2/1

Negawatt Programs

• Utility companies are finding it much less expensive to financeconservation projects than to build new power plants. Rather than buy megawatts of new generating capacity, power companies are investing in "megawatts" of demand avoidance.

Cogeneration

• Cogeneration is the simultaneous production of both electricity andsteam in the same plant.

• By producing two types of useful energy, efficiency is increasedfrom 30% to 80-90%.

Tapping Solar Energy

A Vast Resource

• The amount of solar energy reaching the earth's surface isapproximately 10,000 times all the commercial energy used each year.

• However, until recently, this energy has come in a form too diffuseand low intensity to be used except for environmental heating and photosynthesis.

Passive Solar Heat

• Passive heat absorption is using natural materials or absorptivestructures with no moving parts to simply gather and hold heat.

• A modern adaptation of this principle is the greenhouse.

Active Solar Heat

• Active solar systems pump a heat absorbing fluid through asmall collector to gather heat.

• In countries where fuels are expensive, up to 70% of domestichot water comes from solar collectors.

• Eutectic (phase-changing) chemicals store a large amount ofenergy in a small volume.

High-Temperature Solar Energy

• Parabolic mirrors are curved reflecting surfaces that collect lightand focus it into a concentrated point. By focusing these mirrors, energy can be harnessed and used to produce power.

• Parabolic mirrors are also being tested as possible devices for homecooking in tropical areas where fuel is scarce but sunshine is plentiful.

Promoting Renewable Energy

• In the United States, policies that promote energy conservation and alternative energy sources are being included in utility restructuring.

Photovoltaic Solar Energy

• Photovoltaic cells capture solar energy and convert it directly toelectrical current.

• These handcrafted single-crystal cells were too expensive for anypractical use until the recently.

• Research on photovoltaic cells has dramatically increased theirefficiency, and they are now a very promising alternative source of energy.

Storing Electrical Energy

• Electrical energy is difficult and expensive to store.

• All batteries have drawbacks, from lifespan to the amount of energythey may store.

• New strategies for storing electrical energy include storing it an oneform and converting it back to electrical energy or to use it in electrolytic processes.

Fuel Cells

• Fuel cells are devices that use ongoing electrochemical reactions toproduce an electrical current.

• The fuel cell provides direct-current electricity as long as it issupplied with hydrogen and oxygen.

• A fuel cell run on pure oxygen and hydrogen produces no wasteexcept drinkable water and radiant heat.

Fuel Cell Types

• Proton Exchange Membrane (PEM) is the design of fuel cell beingdeveloped for use in automobiles. These cells are lightweight and operate at a relatively low temperature.

• Phosphoric acid fuel cells are most common for stationary electricalgeneration. These have higher efficiency than PEM cells but are heavier and larger.

• Because they operate at very high temperatures, carbonate fuel cellsare difficult to use.

• Solid oxide are the least developed of all fuel cells but have thehighest efficiency.

Energy from Biomass • Plants capture about 0.1% of all solar energy that reaches the earth's

surface. • The magnitude of this resource is difficult to measure. Most experts

estimate useful biomass production to be 15-20 times the amount we currently get from commercial energy sources.

Burning Biomass• Wood provides less than 1% of the energy in the United States but is

very important in less developed countries. • Inefficient burning of wood can cause a great deal of pollution. • Highly efficient and clean-burning woodstoves are available but

expensive. • Woodchips, sawdust, and other plant materials are being used in

some places in the United States andEurope as a substitute for coal and oil in industrial boilers.

Fuelwood Crisis in Less-Developed Countries

• About 40% of the world's population depend on firewood andcharcoal as their primary energy source.

• As firewood becomes scarce, more and more hours are spentsearching for fuel rather than other productive activities.

• In larger cities of underdeveloped countries, fuelwood can beruinously expensive.

• About half of all wood harvested each year world-wide is used asfuel.

• In countries where fuel is scarce, desperate people will often chopdown anything that will burn, leading to severe deforestation in some parts of the world.

Dung and Methane as Fuels

• Where wood and other fuels are in short supply, people often dry andburn animal manure.

• This can intensify food shortages in some areas because the fertilizeris not being returned to the soil.

• Also, when burned in open fires, more than 90% of the potential heatand most of the nutrients are lost.

• Methane gas is the main component of natural gas. It is produced byanaerobic decomposition of any moist, organic material.

• Burning methane produced from manure produces more heat thanburning the dung itself, and the sludge left over can be a rich fertilizer.

• Methane is a clean fuel that burns efficiently. It is produced in a lowtechnology, low capital process. Although this technology is well developed, its use could be much more widespread.

Alcohol from Biomass

• Ethanol and methanol are produced by anaerobic digestion of plantmaterials with high sugar content.

• Gasohol is a mixture of gasoline and ethanol and burns with betteroctane ratings that gasoline.

• Ethanol production could be a solution to grain surpluses and bring ahigher price for grain crops than the food market offers.

• Methanol burns at a lower temperature than gasoline or diesel. • Both methanol and ethanol make good fuels for fuel cells.

Crop Residues, Energy Crops, and Peat

• Crop residues can be used as a fuel source but are expensive togather and are often better left on the ground as soil protection.

• Some crops are grown specifically as an energy source. However,some of these might be put to better use in other products or the cultivation of them may have negative ecological impacts.

• Peat bogs are also being considered as a source of energy butdisturbing them could also have negative environmental impacts.

Energy from the Earth’s Forces

• Winds, waves, tides, and thermal gradients are renewable energy sources that might be valuable in some areas.

Hydropower

• Falling water has been used as an energy source since ancient times. • Water power is about 25% of total electrical generation • Currently, we use only about 10% of the available water power. • Most of the hydroelectric power produced is generated by huge

dams. Damming freeflowing rivers has many drawbacks including health risks, ecological impacts, and impedement of river traffic.

• Low-head hydropower is generated by smaller dams and causesmuch less of a disturbance on rivers.

• Some high-efficiency turbines can operate on run of the river flow. • Micro-hydro generators are small generators able to power single

homes by being submerged in a moving current.

Wind Energy

• Wind power has the capacity to produce 50 times the world nucleargenerating capacity.

• Like solar power, wind power is a limitless resource and does littleenvironmental damage.

• By the middle of the next century, Shell Oil suggests that half of allthe world's energy could be wind and solar generated.

• Wind farms are large concentrations of wind generators producingcommercial electricity.

• Wind farms do have a few problems, including bird kills anddisruption of the natural beauty.

Table 22.4 -- Jobs and land required for alternative energy sources

Technology Land Use Jobs m2 per gigawatt-hour # per terawatt-hour

for 30 years per year ------------------------------------------------------------------------------------- Coal 3,642 116 Photovoltaic 3,237 175 Solar Thermal 3,561 248 Wind 1,335 542 -------------------------------------------------------------------------------------

Geothermal Energy

• The earth's internal temperature can provide a useful source ofenergy in some places.

• Geothermal energy has been used in electric power production,industrial processing, space heating, agriculture, and aquaculture.

• Geothermal generators have a long lifespan, no mining ortransportation of fuels, and little waste disposal.

Tidal and Wave Energy

• A tidal station works like a hydroelectric dam with spinningturbines as water flows through them.

• Environmental impacts of large scale projects to harness tidal andwave energy are a major concern.

Ocean Thermal Electric Conversion

• A closed-cycle ocean thermal electric conversion (OTEC) systemuses heat from sun warmed upper ocean layers to evaporate a working fluid. The pressure of the formed gas is used to spin turbines.

• An open cycle system uses seawater itself as the working fluid. • OTEC systems require a temperature difference of about 20

degrees between ocean layers to work effectively. • Disadvantages of OTEC include energy cost of pumping seawater,

saltwater corrosion of pipes and equipment, vulnerability to storms, and ecological destabilization.

Summary

• Several sustainable energy sources could reduce or eliminate ourdependence on fossil fuels and nuclear energy.

• Both ancient (water power, wind power) and exciting newtechnologies (fuel cells, parabolic mirrors) are being developed as possible alternative energy sources.

• Biomass may also produce some useable alternative energy sources,but many biomass products could better be used elsewhere.

• The major problem with alternative energy sources is that they arestill experimental and too expensive to be feasible sources of power.

• Although conventional and alternative energy sources offer manyattractive possibilities, conservation is the least expensive and easiest solution to energy shortages.