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Sustainable Energy: 

Sustainable Energy Chapter 22


Outline: Conservation Cogeneration Tapping Solar Energy Passive vs. Active High Temperature Solar Energy Photovoltaic Cells Fuel Cells Energy From Biomass Energy From Earth’s Forces


CONSERVATION Utilization Efficiencies Most potential energy in fuel is lost as waste heat. In response to 1970’s oil prices, average US automobile gas-mileage increased from 13 mpg in 1975 to 28.8 mpg in 1988. Falling fuel prices of the 1980’s discouraged further conservation.

Utilization Efficiencies: 

Utilization Efficiencies Today’s average new home uses half the fuel required in a house built in 1974. Reducing air infiltration is usually the cheapest, quickest, and most effective way of saving household energy. According to new national standards: New washing machines will have to use 35% less water in 2007. Will cut US water use by 40 trillion liters annually.

Energy Conversion Efficiencies: 

Energy Conversion Efficiencies Energy Efficiency is a measure of energy produced compared to energy consumed. Thermal conversion machines can turn no more than 40% of energy in primary fuel into electricity or mechanical power due to waste heat. Fuel cells can theoretically approach 80% efficiency using hydrogen or methane.

Energy Conversion Efficiencies: 

Energy Conversion Efficiencies Net Energy Yield - Based on total useful energy produced during the lifetime of an entire energy system, minus the energy required to make useful energy available. Expressed as ratio between output of useful energy and energy costs.

Negawatt Programs: 

Negawatt Programs It is much less expensive to finance conservation projects than to build new power plants. Power companies investing in negawatts of demand avoidance. Conservation costs on average $350/kw Nuclear Power Plant: $3,000 - $8,000/kw Coal Power Plant: $1,000/kw


Cogeneration Cogeneration - Simultaneous production of both electricity and steam, or hot water, in the same plant. Increases net energy yield from 30-35% to 80-90%. In 1900, half of electricity generated in US came from plants also providing industrial steam or district heating. By 1970’s cogeneration had fallen to less than 5% of power supplies.


TAPPING SOLAR ENERGY A Vast Resource Average amount of solar energy arriving on top of the atmosphere is 1,330 watts per square meter. Amount reaching the earth’s surface is 10,000 times more than all commercial energy used annually. Until recently, this energy source has been too diffuse and low intensity to capitalize for electricity.

Average Daily Solar Radiation: 

Average Daily Solar Radiation

Solar Energy: 

Solar Energy Passive Solar Heat - Using absorptive structures with no moving parts to gather and hold heat. Greenhouse Design Active Solar Heat - Generally pump heat- absorbing medium through a collector, rather than passively collecting heat in a stationary object. Water heating consumes 15% of US domestic energy budget.

Solar Energy: 

Solar Energy Eutectic Chemicals are also used to store large amounts of energy in a small volume. Heating melts the chemicals and cooling returns them to a solid state. Most do not swell when they solidify and undergo phase changes at higher temperatures than water and ice. More convenient for heat storage.


HIGH TEMPERATURE SOLAR ENERGY Parabolic mirrors are curved reflective surfaces that collect light and focus it onto a concentrated point. Two techniques: Long curved mirrors focused on a central tube containing a heat-absorbing fluid. Small mirrors arranged in concentric rings around a tall central tower track the sun and focus light on a heat absorber on top of the tower where molten salt is heated to drive a steam-turbine electric generator.

Promoting Renewable Energy: 

Promoting Renewable Energy Proposed Energy Conservation Policies: Distributional Surcharges Small fee levied on all utility customers. Renewable Portfolio Suppliers must get minimum percentage of power from renewable sources. Green Pricing Allows utilities to profit from conservation programs and charge premium prices for renewable energy.

Photovoltaic Solar Energy: 

Photovoltaic Solar Energy Photovoltaic cells capture solar energy and convert it directly to electrical current by separating electrons from parent atoms and accelerating them across a one-way electrostatic barrier. Bell Laboratories - 1954 1958 - $2,000 / watt 1970 - $100 / watt 2001 - $5 / watt

Energy Costs: 

Energy Costs

Photovoltaic Cells: 

Photovoltaic Cells During the past 25 years, efficiency of energy capture by photovoltaic cells has increased from less than 1% of incident light to more than 10% in field conditions. Invention of amorphous silicon collectors has allowed production of lightweight, cheaper cells. Currently $100 million annual market.

Storing Electrical Energy: 

Storing Electrical Energy Electrical energy storage is difficult and expensive. Lead-acid batteries are heavy and have low energy density. Metal-gas batteries are inexpensive and have high energy densities, but short lives. Alkali-metal batteries have high storage capacity, but are more expensive. Lithium batteries have very long lives, and store large amounts of energy, but are very expensive.


FUEL CELLS Fuel Cells - Use on-going electrochemical reactions to produce electric current. Positive electrode (cathode) and negative electrode (anode) separated by electrolyte which allows charged atoms to pass, but is impermeable to electrons. Electrons pass through external circuit, and generate electrical current.

Fuel Cells: 

Fuel Cells

Fuel Cells: 

Fuel Cells Fuel cells provide direct-current electricity as long as supplied with hydrogen and oxygen. Hydrogen can be supplied as pure gas, or a reformer can be used to strip hydrogen from other fuels. Fuel cells run on pure oxygen and hydrogen produce no waste products except drinkable water and radiant heat. Reformer releases some pollutants, but far below conventional fuel levels.

Fuel Cells: 

Fuel Cells Typical fuel cell efficiency is 40-45%. Current is proportional to the size of the electrodes, while voltage is limited to about 1.23 volts/cell. Fuel cells can be stacked together until the desired power level is achieved.

Fuel Cell Types: 

Fuel Cell Types Proton Exchange Membrane - Design being developed for use in automobiles. Lightweight and operate at low temps. Efficiency typically less than 40%. Phosphoric Acid - Most common fuel design for stationary electrical generation.

Fuel Cell Types: 

Fuel Cell Types Carbonite - Uses inexpensive nickel catalyst, and operates at 650o C. Good heat cogeneration, but difficult to operate due to the extreme heat. Solid Oxide - Uses coated zirconium ceramic as electrolyte. High operating temperatures, but highest efficiency of any design. Still in experimental stage.


ENERGY FROM BIOMASS Plants capture about 0.1% of all solar energy that reaches the earth’s surface. About half the energy used in metabolism. Useful biomass production estimated at 15 - 20 times the amount currently obtained from all commercial energy sources. Renewable energy resources account for 18% of total world energy use, and biomass makes of three-quarters of that supply.

Burning Biomass: 

Burning Biomass Wood provides less than 1% of US energy, but provides up to 95% in poorer countries. 1,500 million cubic meters of fuelwood collected in the world annually. Inefficient burning of wood produces smoke laden with fine ash and soot and hazardous amounts of carbon monoxide (CO) and hydrocarbons. Produces few sulfur gases, and burns at lower temperature than coal.

Fuelwood Crisis: 

Fuelwood Crisis About 40% of world population depends on firewood and charcoal as their primary energy source. Of these, three-quarters do not have an adequate supply. Problem intensifies as less developed countries continue to grow. For urban dwellers, the opportunity to scavenge wood is generally nonexistent.

Fuelwood Crisis: 

Fuelwood Crisis Currently, about half of worldwide annual wood harvest is used as fuel. Eighty-five percent of fuelwood harvested in developing countries. By 2025, worldwide demand for fuelwood is expected to be twice current harvest rates while supplies will have remained relatively static.

Wood Harvest: 

Wood Harvest


Dung Where other fuel is in short supply, people often dry and burn animal dung. Not returning animal dung to land as fertilizer reduces crop production and food supplies. When burned in open fires, 90% of potential heat and most of the nutrients are lost.


Methane Methane is main component of natural gas. Produced by anaerobic decomposition. Burning methane produced from manure provides more heat than burning dung itself, and left-over sludge from bacterial digestion is a nutrient-rich fertilizer. Methane is clean, efficient fuel. Municipal landfills contribute as much as 20% of annual output of methane to the atmosphere.

Anaerobic Fermentation: 

Anaerobic Fermentation

Wind Energy: 

Wind Energy Estimated 20 million MW of wind power could be commercially tapped worldwide. Fifty times current nuclear generation. Typically operate at 35% efficiency under field conditions. When conditions are favorable (min. 24 km/hr) electric prices typically run as low as 3 cents / KWH. Standard modern turbine uses only two or three blades in order to operate better at high wind speeds.

Wind Energy: 

Wind Energy Wind Farms - Large concentrations of wind generators producing commercial electricity. Negative Impacts: Interrupt view in remote places Destroy sense of isolation Potential bird kills

Geothermal Energy: 

Geothermal Energy High-pressure, high-temperature steam fields exist below the earth’s surface. Recently, geothermal energy has been used in electric power production, industrial processing, space heating, agriculture, and aquaculture. Have long life span, no mining needs, and little waste disposal. Potential danger of noxious gases and noise problems from steam valves.

Tidal and Wave Energy: 

Tidal and Wave Energy Ocean tides and waves contain enormous amounts of energy that can be harnessed. Tidal Station - Tide flows through turbines, creating electricity. Requires a high tide / low-tide differential of several meters. Main worries are saltwater flooding behind the dam and heavy siltation. Stormy coasts with strongest waves are often far from major population centers.

Tidal Power: 

Tidal Power

Ocean Thermal Electric Conversion: 

Ocean Thermal Electric Conversion Heat from sun-warmed upper ocean layers is used to evaporate a working fluid, such as ammonia, which has a low boiling point. Gas pressure spins electrical turbines. Need temperature differential of about 20o C between warm upper layers and cooling water.


Summary: Conservation Cogeneration Tapping Solar Energy Passive vs. Active High Temperature Solar Energy Photovoltaic Cells Fuel Cells Energy From Biomass Energy From Earth’s Forces

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