Energy II: Renewable Energy Sources

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Unit Organization:

• Introduction
• Hydropower (all forms)
• Hydropower from Dams
• Alternative Hydropower
• Tidal and Wave Power
• Biomass
• Solar
• Wind
• Geothermal
• Energy Storage
• Energy Conservation

Reading:

Textbook: Chapter 8

Renewable Energy Sources

Renewable means that the energy available from a renewable source is not depleted by using the source

• Hydropower (including waves, currents, and tides)
• Biofuels (including both energy crops and burning plant material for heat and light)
• Geothermal Power
• Solar
• Wind

Four things to remember about renewable (alternative) energy sources

• There are always environmental costs but the costs vary greatly among alternative sources
• Alternative sources can be either hard or soft technologies and we will discuss both
• All of these technologies are underutilized for various reasons
• The potential for energy production varies greatly among renewable sources

World Renewable Energy Production in 2008 (Wikipedia, but checked with other sources)

• Solar PV refers to electricity from photovoltaic panels and Solar Steam refers to electricity generation by concentrating solar energy to boil water
• Biomass Power refers mostly to electricity from biogenic natural gas but also includes any liquid fuel production apart from ethanol and biodiesel
• Biomass sources include biomass heating, ethanol, biomass power, and biodiesel

Renewables are becoming important as a means for economies to develop

• China, India and Brazil, some of the most dynamic and growing economies have all invested in renewable energy (although the type of energy production varies among countries)
• US Public support for energy companies has been around for many years
  • The current debate is not over should there be support, but which parts of the industry should get the support and how much
    • From the last lecture, we know that renewable energy gets only about 18 % of the US investment
    • Fusion, which has yet to generate enough electricity to power a light bulb, gets almost as much as all renewables

US Renewable Production

• Note that the US has no appreciable contribution from ocean power, wood refers to wood used to produce electricity, that solar here includes both solar categories above, and that biomass in this table is ethanol, biogenic natural gas, and biodiesel
• The US produces, as a percentage of its total, about half of the total renewable energy as the world in general
• much of the difference results from the fact that the US does no heat with biomass

Footprints:

• New energy sources start with heavy economic disadvantages and many governments choose to invest in new technology before it is truly competitive
• Renewable energy gets investment by national governments
• China invests most, US second so investment is not limited to developed countries

Hydropower

The book restricts this topic to electricity produced by dams but I will include all water-based power generation

• Water powered man’s industry thousands of years ago in the form of a Water Wheel
  • Water wheels and turbines work on the same principle
  • blades are pushed by moving water and the blades turn the shaft to which they are attached
  • water wheels used by Romans and Greeks 2000 years ago to grind corn
  • by the 1700’s, factories were powered by water wheels
• in 1872, hydropower for electricity production began in Appleton, Wisconsin at 12.5 kilowatts
  • that’s 125 100-watt incandescent light bulbs

Hydropower may come from

• Dams (gravity supplies the force to move the water)
• Tides (gravity of the moon and sun supply the force to move the water)
• Waves (wind supplies the force to move the water)
• Streams or Rivers (gravity supplies the force to move the water)
• Temperature or Salinity Gradients (potential energy in the gradients generate electricity)

Hydropower from Dams

Dams are built for several purposes:;;

• Generating electricity
• Flood control
• Water Storage
• Recreation

In the US (according to the Army Corps of Engineers)

• There are over 79,000 dams (and many more too small to make the list
• There are 2,540 dams that generate electricity
• There may be as many as 5000 sites for new dams that produce hydropower with the capability of doubling the total electricity
• This could double the electricity generated by dams
  • Currently, about 7% of electricity is hydro, so the potential is only 14% of our electricity demand TODAY and as demand goes up, hydropower could supply less and less of that demand
    • We will not solve the energy crisis with dams

Dams can be removed if their costs are greater than their benefits

American Rivers lists 925 removals over the last 100 years

Tennessee ranks 7th in hydropower production (WA and OR are top 2)

• The history of dam building in TN is interesting and is reflected in many current debates over the role of the federal government in our society (hint for paper topic) and involves the Tennessee Valley Authority (TVA)

Update on the Three Gorges Dam

• on the Yangtze River in China (largest hydropower project ever)
• Project is essentially complete
• Will produce 22,500 megawatts of power (US produces 74,000 megawatts total hydropower)
• Dam is 1.5 miles wide
• Flooded 400 miles of river
• Displaced 1,300,000 people

Impacts of the dam

• Running at full capacity, it
  • Reduces coal consumption by 31 million tonnes per year (tonne means metric ton, 1,000 kg or about 2,200 lbs)
  • Avoids 100 million tonnes of greenhouse gas emissions (CO2)
  • Avoids the release of
    • 1,000,000 tonnes of sulfur dioxide
    • 370,000 tonnes of nitric oxide
    • 10,000 tonnes of carbon monoxide
    • a large amount of mercury
• It saves the energy needed to mine, wash, and transport the 31 million tonnes of coal from northern China.
• Agriculture upstream deposits lots of silt into the Yangtze
  • Much will sediment out in reservoir
  • Loss of silt may cause delta of Yangtze to erode (site of Shanghai)
• Dam is built on a fault line

Advantages of Hydropower from Dams:

• efficiency
• low cost (after initial investment)
• carbon-neutral (after initial investment)

Disadvantages of Hydropower from Dams:

• Initial investment very costly
• Flooding of land adjacent to the river
• Reservoirs (the lakes created by dams) fill up due to sedimentation from incoming water (both river silt and runoff from land next to reservoir)
• Reservoirs affect water quality
• During dry periods, downstream flow may be severely reduced
• Interruption of river migration
  • Anadromous fish grow to adulthood in the ocean but migrate into freshwater to breed and the young fish return to the ocean (salmon, shad herring, some sturgeons)
  • Catadromous fish grow to adulthood in freshwater and migrate to the ocean to breed (the young or larval forms migrate back into freshwater) - freshwater eels are catadromous
• new dams often displace people
• if few people live there, the area flooded is often a valued natural or scenic area
• limited number of places to put dams

The future of hydropower from dams:

• smaller facilities may minimize environmental impacts
• new technology that allows for electricity generation without dams that sluice water past turbines
  • Diversion or Run –of-the-River facilities
    • Topology of the river must be right
  • Small turbines with no dam
    • No reservoir needed
    • Animal migration impact reduced
    • No change to sediment load
    • No water storage or flood control
  • Adding generating capacity to existing dam
  • Upgrading existing plants to increase their capacity through improved technology
• Worldwide, hydropower from freshwater is likely to increase over the next several decades but it is unlikely to contribute as much energy as the fossil fuels have or to meet increases in demand as more countries develop.

Alternative Hydropower 

Ocean Energy is the other source of Hydropower

• Commercial ocean energy developments have been based on both the movement of water and on differences in salinity or temperature.
• Both wave action and tidal flows contain tremendous amounts of kinetic energy and are being used to generate electricity.
• Gradients in salinity (where fresh meets salt water in estuaries) or temperature (ocean surface versus deeper, 4°C water) can generate electricity

Potential of Alternative Hydropower

• DOE Reports assessing the national potential for hydropower conclude that there is great potential for Wave and Tidal Energy Production near U.S. Coasts (January 18, 2012)
  • waves and tidal currents off the nation's coasts could contribute significantly to the United States' total annual electricity production
    • provide clean, renewable energy to coastal cities and communities.
  • water power, including conventional hydropower and wave, tidal, and other water power resources, can potentially provide 15% of our nation's electricity by 2030
    • The United States uses about 4,000 teraWatt hours (TW) of electricity per year.
    • DOE estimates that the maximum theoretical electric generation that could be produced from waves and tidal currents is approximately 1,420 TW per year, approximately one-third of the nation's total annual electricity usage.
    • Note that not all of the resource potential identified in these assessments can realistically be developed
  • Expansion from the 6% of the nation's electricity already generated from renewable hydropower resources is very likely
  • Most US coastal areas have potential to generate electricity from both tides and waves
    • Exceptions are the Gulf Coast and the southern Atlantic coast, where waves are less energetic

Ocean Thermal Energy Conversion (OTEC)

• Potential energy available is large but technology is not ready
• There are two ways of generating electricity from the ocean's temperature gradient
  • Closed OTEC systems
    • Hot surface water used to heat colder water
    • Cold water is held in a rigid container
    • Heating causes expansion of cold water and puts pressure on container
    • Pressure forces heated water through a turbine, generating electricity
  • Open OTEC Systems
  • Hot, surface water used to evaporate (boil) water or another "working fluid" and create steam
  • Steam drives turbine
  • Cold, subsurface water used to condense steam so it can be recycled and heated again
    • Sometimes the “working fluid” is ammonia or a hydrocarbon
    • If water is used, the condensed water is suitable for drinking or irrigation
• Potential is largest where the surface water is warmest, which means the tropics and, especially, the hot areas that determine ENSO
• Can be based on land, floating offshore or on shallow ocean bottom (continental shelf)
  • First thermal plant was built in Cuba in 1934 (22 kW) but all built since then have been for research

Tidal and Wave Power

Tides are the distortion in shape that result from the gravitational attraction between objects and they are complex (below is a bit of a tidal primer)

• We normally use the term to refer to the distortions of the Earth and Sun' effect on one another and Earth and Moon's effect
• Tidal forces are produced by the (slight) differences in the Moon's (or Sun's) gravitational effect in different places on the Earth
  • The Moon's effect on the Earth is greater than the Sun's tidal effect because, even though the Sun is much more massive than the Moon, it is also much farther away
    • The force of gravity declines as the square of the distance and the differences in the gravitational effects that produce tides actually decline as the cube of the distance
• Tidal forces are experienced by all parts of the Earth (inside and outside) but not equally over all parts of the Earth
  • As compression and expansion forces on solid rock
  • Water, of course, is a non-compressible fluid, so ocean water moves when the tidal force is applied
• Most coastlines get two tides a day
  • The differences in tidal force produce a net force in the direction of the Moon on the side of the Earth facing the moon
    • Because the direction of the force changes as you move closer to the line passing through the center of the Moon and Earth, the net effect on water produces a bulge with its peak directly under the Moon
  • However, the tidal forces produce a net force in the opposite direction on the opposite side of the Earth
    • This produces a second bulge with its peak also on the line between the center of the two bodies
  • Tidal bulges remain where the Moon-Earth axis is, and the rotating Earth slides under the bulges
    • The tidal bulges are at "fixed" points in the Earth-Moon system but the Earth is also rotating underneath the fixed bulges, so it takes about 12 hours for a point on the Earth under one bulge to rotate to be under the other bulge
• The time between high tides is not exactly 12 hours
• The Earth-Moon system is rotating (in the same direction as the Earth's spin rotation) while the Earth rotates
• By the time the Earth's spin takes a point on its surface from under one bulge to under the other, the bulge on the other side has moved due to the Earth-Moon system's rotation
  • The motion of the bulge means that it takes a little over 12 hours for the Earth's rotation to move a point on its surface from one tidal bulge to the opposite bulge (it takes about 12 hours and a little over 24 minutes
• So, if high tide is at noon today, then the next high tide is at 24 minutes past midnight  and the one after that is at 12:49 tomorrow
• There is great variation in tidal height and periodicity in different places
• Imagine the Earth as a smooth ball with a uniform layer of water covering it
  • Under these conditions, the tides would be very uniform but a bit delayed (the friction between the water and the bottom of the ocean would create a bit of drag)
• However, there are continents in the way and they have irregular shapes and the water sloshes against them
  • In a smaller basin (like the Caribbean Sea) the sloshing can set up standing waves that affect tide height and timing
• Complex basin morphology and location on Earth (tidal forces change direction, remember) can both
  • Dampen and increase tidal height (up to 45 feet in the Bay of Fundy, in Canada just north of Maine and 40 feet in Rance, France)
  • Make one tide a day higher than the other up to the point that one high tide is canceled out (some places have only 1 tide a day)
• Interaction between the tidal forces caused by the Sun and Moon causes variation in the high tide mark over the course of a lunar month
  • Highest tide is the Spring Tide, when Moon and Sun align
  • Lowest high tide is the Neap Tide, when Moon and Sun are at right angles

Tides can be used to drive underwater Turbines attached to vanes

• These images are from a Marine Energy Park being built by the United Kingdom between Bristol and the Isle of Scilly
• Planned capacity by 2050 is 27 gigawatt

Tidal Barrages dam the tides both coming and going

• A barrage is a way to use the difference in wave height between low and high tide.
  • Essentially a dam along an estuary
  • As the water level is lower on the ocean side of the barrage, the water wants to return to equilibrium.
  • By forcing the water to flow past a turbine, energy is generated when the tide is low and water moves from the “basin” or estuary side to towards the ocean.
  • Opposite happens when tide is coming in and the Ocean side is higher
• The use of tides to power water mills (tidal mills) to grind flour or power a manufacturing process is 2000 years old but the Rance Tidal Barrage in northern France (Brittany) is the first to generate electricity
  • Completed in 1966
  • 240 mW peak generation capacity
  • 8 m to 14 m tides
• the number of sites which are capable of supporting a Tidal Barrage is limited.
  • All sites are in estuaries where the daily tidal change is large
• Tidal barrages have many environmental impacts on estuaries (recall that estuaries are vital areas for many fisheries)
  • Changes pattern of flow and silt deposition in the estuary
  • Changes salinity and temperature gradient within the estuary

T-shaped barrages proposed method of using tides along coast

• Most tides don’t move perpendicular to the coast but at an angle so that they may be said to move along the coast
• T- shaped Barrages that stick out from the coast trap the tide on one side
• As level rises on one side, water flows through barrage sluices and drives turbines
• Not yet built but may greatly expand the number of sites where tidal energy can be harnessed

Sluices (Gated Channels) that narrow the water flow work without the barrage

• A sluice allows water to flow along a path that gets ever narrower
  • Water must flow faster and faster and can be made to turn a turbine
• Less environmental impact than a barrage system
• Can be placed in tandem so that total energy potential is large
• Can be used in rivers as well
• 500 kW units are already operating in Humber Estuary in England

Environmental impacts

• fish and invertebrates can be killed by turbine
• changes flow pattern in the estuary or river

Hydro Energy Barrels (HEBs)

• It’s a waterwheel driven by tide
• Can also be used in streams or rivers
• Micro-power system for individual uses
• Can also make use of wave action

Wave Power

Waves are powered by winds and are available 24 hours of the day

• As wind varies, so do the height and frequency of waves
  • Both affect the power available
• Have to construct equipment to withstand extreme events
• Lots of energy but spread out over large area
• Several different approaches to generating electricity have been devised and most are being used, even if only as demonstrations
  • Some are on land, most are offshore
  • the kinetic energy of the wave is used to turn a turbine that spins the shaft of a generator

Compressed Air Systems

• Waves push water under lip of capture chamber
• Extra water rushing in compresses air which is pushed through the fans of a turbine
• As water recedes, air pressure is reduced and the air flows through the turbine in the opposite direction
• Sites for this sort of generation are limited but environmental impacts are very low

Wave "Worms" ?  Electric Eels?

• Waves bend these segmented "worms" - long buoyant tubes
• Bending segments pull and push pistons on adjacent segments
• Pistons pump up hydraulic pressure and the pressure forces fluid through a turbine

Waves can fill onshore reservoirs

• Waves break over a weir and water remains above sea-level
  • Reservoirs are stored (potential) energy that can be released by allowing gravity to pull the water back to the level of the ocean
  • This flow can turn a turbine and generate electricity
• The same idea can fill smaller, offshore reservoirs

Buoys move up and down with waves

• The vertical motion can be used to pull a coil of wire through a magnetic field
  • This motion will generate electricity
• Lots of buoys needed to generate significant power but each buoy is inexpensive and easy to maintain
  • Many buoy designs are in production
• Low environmental impact

Oyster Power

• As “shell” of oyster is moved by waves, water is pumped onto land, where it can be forced through a turbine or stored in a reservoir for later electric generation
• Oysters are large and hard to position offshore but rather simple devices with few moving parts
  • One oyster produces 315 kW but the second generation design should produce 2.4 mW
• Little environmental impact (mostly the permanent disruption of the sea floor)
  • Underwater “noise” may impede communication between individual fish or marine mammals

Osmosis Power

• Salinity Gradient can power by osmosis
  • Remember, osmosis is the flow of water across a membrane from where the water is in higher concentration (and solutes are in lower concentration) to the side with lower water concentration (and, therefore, higher solute concentration
  • Osmosis is powered by diffusion and proceeds in the same direction (from high concentration to low concentration)
• Osmotic pressure (as water crosses from fresh water side of a membrane to the salty side) can be used to build pressure to turn a turbine and generate electricity
• In late 2009, a 1 megawatt pilot plant opened in Norway
  • Some of the electricity generated is used to pump the water into the plant
  • The surface of the membranes in the plant are measured in square kilometers

Biomass

• Biomass provides as much as 15% of all energy consumed worldwide, although only about 3% of energy demands in the US
  • In some developing countries, biomass provides 90% of the energy consumed.
• Heat generated by the direct combustion of wood or organic wastes is the most common use of biomass energy.
• Biomass is also used to generate electricity
• Biomass-derived fuels, like ethanol and biodiesel, are another way this resource is used.

Major sources of biomass energy include:

• Wastes (construction, home, commercial)
• Sewage (both biogenic natural gas and combusted sewage solids)
• Wastes (industrial) – animal fats, vegetable oil, paper mill waste, various other sources
• Standing forests and tree plantations
• Energy crops (beet or cane sugar, corn sugar and oil, soybean or rapeseed oil, palm oil)

Biomass can be used in several ways:

• Direct combustion
• Thermochemical conversion to methanol or syngas
• Biochemical conversion to ethanol or biogas

Concerns about the use of biomass to generate electricity include:

• It is not renewable if it is harvested faster than it grows
• Some biomass conversion processes are very inefficient
• Biomass combustion can be a major contributor to air pollution

Ethanol from corn as an example of biomass energy and can serve as an example of the how complex the issue of bioenergy may be

• Highly federally subsidized industry, so ethanol gas mixtures are cheaper at the pump only because of government support (say thanks to those who don't drive when filling up!)
• Touted as lowering carbon emissions
  • Carbon in ethanol from corn was taken from the atmosphere, so when the ethanol is burned, it replaces the carbon taken, making it carbon neutral
  • Every gallon of corn ethanol burned is less gasoline burned
• Impact on carbon emissions less than expected
  • Carbon used to produce corn (making pesticides and fertilizers, transporting corn)
  • Conversion of forest to agriculture and pasture releases carbon to atmosphere
• Impact on those depending on corn as food
  • 20% of corn crop in US is used to make biofuel (ethanol)
  • Corn prices have doubled world-wide

Solar Power

• Much of the energy we use at present is indirect solar energy.
  • All fossil fuel’s energy comes from solar energy
• While people currently consume about 13 terawatts of energy per year, the sun delivers about 80,000 terawatts to the Earth per yr.
  • 1⁄4 of total is usable
  • Direct solar energy is dilute, diffuse, and intermittent.
• The large amount of solar energy available could potentially solve many of our current energy problems.
  • Solar's 23,000 tW (teraWatts)/year of energy is greater than all of the energy in the known coal , oil, natural gas, and uranium reserves total
• However, that energy is never very dense anywhere, so it must be harvested from a very large surface area
  • Solar energy is not limited by available energy but by our ability to harvest the energy to do work for us.

Solar energy is not uniformly distributed

• Both latitude and climate effect the amount of energy that gets to the ground
• In the US, the southwest is the place

What is the current state of solar hard tech? - here are the largest power plants as of 2009

• The average capacity of coal-fired plants in 2009 was about 236 MW
• Largest Solar Thermal Power Plants in Operation
  • “Solar Energy Systems” in the Mojave Desert CA, USA — 354 MW
  • “Solnova Solar Power Station” in Seville, Spain — 150 MW
• Largest Solar Photovoltaic (PV) Power Plants in the World
• “Sarnia Photovoltaic Power Plant” in Canada — 97 MW
• “Montalto di Castro Photovoltaic Power Station” in Italy — 84.2 MW
• “Finsterwalde Solar Park” in Germany — 80.7 MW
• “Ohotnikovo Solar Park” in Ukraine — 80 MW

Note the the PV systems are found in colder regions – PV gets more efficient at lower temperatures

As of 2012, some regions have begun to seriously exploit their solar resources for hard tech power production

• Germany is the leader (54% of total power production in a country not famous for its sunshine)
• Italy and Spain are  becoming committed (40% and 15%)
• The US is undercommitted (1%)
  • Some states are committing (NJ and CA at 5% and AZ at 6%)

Challenges for Solar Power

• Intermittency is important (cloudy days, nighttime)
  • Must be able to supply energy continuously
  • Storage for night time
  • Other energy sources may be needed
• Distribution and transmission due to disparity of insolation
  • Southwest gets more sun, but population density is higher in northern US
• Price
  • Cost is mostly for manufacture and installation and loss due to longer transmission
  • PV panels do have finite useful life times but last over 20 years, so costs are spread out over a long time

Ways of using Solar Energy

• Passive Solar Energy is using the Sun to heat houses, offices and stores.
  • Passive means that there are no special systems or moving parts, the design of the building makes the energy available (very soft technology)
• Active Solar includes:
  • Soft Tech
    • Solar Panels that produce hot fluids for building heating.
    • Photovoltaic panels that generate electricity for local consumption.
  • Hard Tech (large Solar grid electricity generation)
    • Photovoltaic (PV) farms with thousands of panels in a single place
    • Solar Concentration Plants where hundreds of mirrors concentrate sunlight and heat a fluid to boiling

Passive Solar

• Passive solar design employs:
  • Solar orientation
  • Thermal mass to moderate energy release
  • Effective insulation
• Passive Solar Energy is using the Sun to heat houses, offices and stores.
  • Passive Solar depends on design for a particular climate
• Orientation of the building and placement of windows
• Natural movement of air through building
• Takes advantage of the changing angle of the Sun as seasons change
  • Overhanging eaves shade in summer but not in winter
  • Sunlight enters through glass and is absorbed by interior of building
• Glass prevents loss of re-radiated energy (as infrared radiation) through greenhouse effect
  • Energy is stored in a “thermal mass” to slowly be released during the night
  • Mass may be concrete, concrete block, stone, or a tank full of water

Active Solar

• Modern active solar technology encompasses two themes:
  • Trapping solar heat and light for useful purposes
  • Direct or indirect conversion to electricity
• Sunlight can be either direct or diffuse.
  • Some technologies require direct solar energy (Thermal) while others (PV) can use either.
• Active solar collectors can be used for space heating in buildings where passive design is not practical or was not used (retrofits)
• Active Solar includes:
  • Solar Panels that produce hot fluids for building heating.or just hot water for home use
  • This is not a passive system as the fluid in panels is circulated
  • Panels heat fluid, which can be used to heat water or entire building
  • Fluid might be air or water
• Not a common usage in the US but, worldwide, a common technology
  • China leads the way, with 2/3 of the installed capacity and Europe has another 1/8

Solar Thermal

• Environments with large amounts of direct solar radiation can also use focusing collectors distributed in large arrays or as power towers.
• The high temperatures produced by these collectors can be used for process heat or to generate electricity.
• By concentrating sunlight, water or another material can be heated and the increase in pressure used to turn a turbine and generate electricity

Photovoltaic Panels

• Photovoltaics use semiconductor technology to generate electricity for direct use or to charge hydrogen fuel cells.
• Photovoltaic panels can be soft tech (generate electricity for local consumption) or
• Hard Tech (large Solar grid electricity generation)
• PV farms

How Photovoltaic Cells work

The Band Gap and Semiconductors

• When electrons in a substance absorb a photon, they may have enough energy to leave the nucleus they are moving around
  • Valence electrons (the outer electrons) have low energies and are trapped by the nucleus but, because they are the outer electrons, they have higher energies than other electrons in that atom
    • The energies of the valence electrons span the Valence Band of energies
  • Higher energy electrons can move from atom to atom (those electrons are in the Conducting Band)
  • The valence bands and conducting bands vary in different substances
    • Metals (like copper) may have valence bands that overlap with conducting bands (that’s why they lose electrons so easily and make good conductors)
  • Other substances may have a gap between the V and C bands, referred to as the Band Gap
    • The larger the band gap, the less likely an electron will get enough energy to leave, and the better insulator the material will be
    • Semiconductors have narrow band gaps that can often be jumped by the absorption of a light photon

Silicon Semiconductors and Doping

• Pure, crystalline silicon is a semiconductor
  • It’s band gap will be affected by the impurities in the crystal, so that different formulations will require more or less energy to jump the band gap
  • Thus, layers of differently doped silicon will absorb different parts of the visible spectrum
  • This means the cell will use more of the available energy
• Photovoltaic cells are semi-conductors, like some of the components of your computer
  • Semi-conductors are neither good conductors nor insulators, but in-between
  • Silicon is a semiconductor that can be grown as a very uniform crystal
    If some improprieties (other elements) are included in the crystal (Doping the crystal), useful electric properties are obtained
  • Depending on the type of impurity, either unpaired, easy to free electrons or positions in atoms that can be filled by electrons (called “holes”) result

P-N Junctions allow a current to flow

• Some doped silicon has a natural excess of free electrons (n-type, for negative)
• Other doped silicon will have places (called holes) where electrons can fit into atoms (p-type, for positive)
• When two semiconductors of opposite types are layered next to one another, electrons can flow from the n-type to fill holes in the p-type, producing a layer between the two types
  • This is a layer of neutral charge and is called the depletion region because the excess charge is depleted in the region
• When a voltage is applied to opposite ends of the crystal, two possibilities arise with different outcomes
  • If the n-type is the positive side of the circuit, electrons will be drawn to the far edge of the crystal and holes will move to the opposite, nEGATIVE side of the electric circuit, increasing the size of the depletion region
    • In this case, the circuit can’t be completed and no current flows
  • If the n-type gets the negative side, its electrons are pushed toward the depletion zone (and holes toward the zone as well)
    • Now electrons cross the depletion zone and current flows
  • So, the P-N junction acts as a one-way gate, allowing current to pass in only one direction

Photovoltaic Effect

• With this background we are now ready to understand how photovoltaic cells generate electricity
• When photons are absorbed by electrons in the depletion zone, they are freed to move (and have the energy to do so) and the leave a hole behind
• The one-way nature of the PN junction means the electrons can only leave the silicon in one direction, which causes a one-way flow of electrons (thus an electric current is created)
• The opposite effect (applying a charge across the junction) results in a photon being freed (which means the semiconductor emits light - the basis of the LED

Environmental Impact of Solar Energy Production

Positive

• Sunlight is a fairly perfect abundant, renewable energy resource
• Post construction, no air or water pollution is produced
• Dispersed nature of resource can be matched by dispersed generation of power

Negative

• Large tracts of solar power plants can be disruptive to ecosystems like the desert biota.
  • 1 km2 per 20-60 mW (Union of Concerned Scientists)
• Production and deployment of solar cell arrays use fossil fuels and other poisonous chemicals.
  • Doping of silicon can involve arsenic, cadmium, and other harmful metals and involve toxic organic solvents

Future of Solar Energy

• Costs are dropping and technology is improving, so that solar panels may be a much more important source of electricity than they are today

Wind Power

Origins

• Wind power is also sun power, as the sun powers the wind
• Wind turbines were invented in1891 and generated commercial electricity by 1887

Current Status

• In 2005 (when the textbook came out), wind power generates less than 1.5-2% of the world’s electricity and is now between 2.5 and 3%
  • Its use is growing rapidly and has averaged 25% per year.
  • The cost of wind-derived electricity has dropped dramatically in the last decade.
• Land Based Wind farms now generate electricity at competitive prices.
• In the U.S., as much as 20% of electricity demand could be met by wind power.
  • This figure is not the theoretical maximum, as there is enough wind energy to power all of the US or the world, but practical considerations limit the estimates.
    • Transmission capability, storage capability and capacity (next slide) are all limiting factors
    • Improving technology has increased both the theoretical maximum and the practical expectation
• By 2012, wind power's potential is starting to make an impact
  • Denmark (close to Germany - see solar) - 30%
  • Portugal, Spain, Ireland, and Germany - 11% to 17%
• US still underexploiting at 3% but
  • IA and SD near 25%
  • ND and MN at 15%
  • KS, IA, OK, and OR at 11%

Capacity:

• The potential for power production from any source will never reach 100% of its capacity
  • Capacity factors are the % of total possible power that is actually produced
  • Each source has different variables that affect capacity
• Capacity for wind is reduced by Intermittency and Variability
  • Intermittency refers to those times when insufficient wind to turn the turbine
  • Variability refers to the fact that, when the wind is sufficient to turn the blades, it does not always blow at the optimum speed to maximize power output

Sources of Wind and Wind Power

• Most wind power is over the ocean (specifically, the northern and southern westerlies
  • This favors countries with coastlines, especially at the appropriate latitudes
  • North America, North Africa, Europe, northern Asia, eastern Asia, all have potential for wind power from their interior regions
  • South America, Sub-Saharan Africa, Australia (note they are at the same latitude) all have less potential for wind power from their interiors
• World-wide generation of wind power has grown exponentially (we know this can only happen for a limited time!)
  • In 14 years, capacity has grown over 30x
  • As developing countries invest, actual capacity has exceeded predictions
• In the US, potential for wind power generation is not uniformly distributed
  • Wind power potential must consider not just the presence of wind but its constancy and both the cost and remoteness of the land needed
  • The Midwest has the greatest land-based potential
  • Map is too coarse-scaled to show good local potential but all states have areas suitable for wind power
• Offshore production potential is high due to constancy of winds
  • Cost of generation is greater offshore due to higher construction, maintenance, and transmission costs
  • Transmission is an extra cost when the source is far from the consumer and many wind power sites are far from population centers

Price of Wind Power

• Price of land-based wind-generated electricity has fallen to near-parity with fossil fuels in some areas
  • Offshore generation costs are still above fossil-fuel generated costs
  • Remember that these costs do not reflect any of the environmental or health costs
  • Remember that these costs are somewhat distorted by unequal subsidies for competing technologies
• Wind power has become competitive even in though fossil fuels have been more heavily subsidized by government (local, state and federal)
• The many factors that affect power prices have lead to many, very different price estimates
  • Some include potential capacity, some do not
  • Some consider subsidies, some do not
  • Almost none of the price estimates include health and environment
• At one time, the US lead the world in wind power but that has changed
  • China has become the #1 installer of wind turbines and is now the largest wind energy producer
    • China installs half of the new turbines each year world-wide
  • India has also become a major installer

Variables affecting Wind Power (from Genstan Total Alternative Power)

• The power generated by wind turbines depends on wind speed but a maximum power limits the utility of high speed winds
• Actual power curves in wind tunnel tests closely match theoretical predictions

Wind Speed

• Theoretically, faster wind carries more power
• In fact, limitations of the mechanism mean that there is an optimal wind speed that maximizes power generation

Rotor Size

• Larger rotors, more power
• Expense goes up with power
• Large rotors can be in excess of 60m (120 m diameter) and generate 7.5 mW (with 10 mW turbines proposed

Tower Height

• Winds are faster the higher you go
• The higher you go, the more people can see the tower, which means that it impacts more people

Intermittency and Variation

• Power demand and power generation do not always occur at the same time
  • The grid allows redistribution of power, which mitigates the effect of both problems but does not solve them
• Power from other sources may be necessary if demand can't be satisfied
• Power storage is another mitigation strategy
  • Small scale storage
    • batteries
  • Large scale storage
  • Fill reservoirs with water pumped by excess wind power production when production exceeds demand
    • Then generate electricity using hydropower (very efficient means of generation)
  • Use excess electricity to generate hydrogen for fuel cells and run the cells when wind power can't meet demand

Noise

• Wind turbines make noise but not much in comparison to everyday sounds
• However, turbine noise is constant when the wind is blowing

Designs (from Wikipedia)

• Three basic designs
  • Vertical axis with large panels to catch wind (Savonius)
  • Vertical axis with blades
  • Horizontal axis with blades (Giromill/Darrieus)
• HAWT (Horizontal axis Wind Turbine)
  • Must be pointed into wind
  • All word done turns shaft in the same direction (not true of VAWT) so these are the most efficient designs
  • Largest is 135 m (413 ft) tall with rotor diameter of 126 m and generates 7.8 mW
  • Best design when winds come from one direction most of the time
  • During high winds, feathering and brakes stop blades from rotating
• VAWT (Vertical-axis Wind Turbine)
  • Don’t have to be pointed into wind
    • Respond instantly to changes in wind direction
  • As each blade rotates, part of the rotation is into the wind, so some power is lost to this design feature
  • Largest has 110 m blades and generates 3.8 mW max.
  • Vertical axis designs rotate more slowly than HWAT at a given wind speed
    • This means that the force of the wind is greater on the mechanism, which increases stress and wear
• Innovation continues
  • Students designed a traffic sign that generates power
  • Floating offshore towers are much cheaper than those sitting on the ocean floor

Environmental Concerns

• Positives
  • Fossil fuel footprint is small
    • Manufacturing carbon production is small compared to fossil fuel plants
    • Auxiliary generators that kick in when wind is not sufficient increase footprint
  • Construction cost is small compared to fossil fuel plants
  • Reliable technology
• Negatives
• Intermittency
• Capacity for production will not meet total US demand
• Unsightliness and noise
• Danger to birds
• Wind farms take up lots of land because there must be lots of space between rows of towers
  • Turbulence behind tower greatly reduces power of next turbine
  • Land-use cost is mitigated if the land between towers can be used
    • Orchard land, fields or pasture lands can extend to tower base
• Soft Tech Applications
• Much of wind power is generated by hard-tech wind farms
• There are many VAWT and HAWT systems designed for small scale, local production

Geothermal Power

• The energy from Earth’s hot interior can be used to heat buildings directly or to generate electricity.
• Geothermal Deposits are regions close to the surface that are hot enough to exploit for power generation or to use the heat as heat

Four basic types of geothermal deposit

• Hydrothermal reservoirs
  • Areas where surface water percolates down to hot regions through cracks in rock
  • Hot water is turned into steam, which is used to drive turbines and generate electricity
  • Can be tapped through closed-loop or open-loop systems (different environmental impacts)
• Geopressured brines
  • Salt water with methane dissolved in it (so much that it is considered a gas source)
  • Heated over boiling due to pressure (300 – 400 °F)
  • 10,000 to 20,000 feet down (thus the pressure)
  • Can be used to heat water (geothermal) or the hydraulic pressure can be used directly
  • Found off of the TX and LA coasts
• Hot Rock and Magma
  • No small or large scale plants use these heat reservoirs as of yet
  • Up until now, all large scale energy production has used permeable rock that already has water in it
  • Here, the energy is tapped by pumping cold water down a well into close proximity to the heat source
  • Water must fracture rock so that it can flow to the second well hole and move back to the surface (as hot, not cold water)

Geography of the Resource

• As expected, geothermal sites correlate with edges of tectonic plates and hot spots
• In the US, most of the currently sites are in the west

Geothermal Heating and Cooling

• Soft-Tech Geothermal
• As you go underground, the temperature increases, but from a starting point that is below room temperature
• Single building systems pump either air or water down to the appropriate level and return it at the desired temperature
  • Can heat home and provide hot water
  • Can also air-condition house during hot weather

Geothermal Environmental Impacts

Positive

• Not air polluting (except for construction)
• Constant energy source

Negative

• Potential is not enough to supply total energy demand
• May require large quantities of water
• Possible ground subsidence
• Water may dissolve hazardous substances and bring them to the surface
• Steam venting may release H₂S, NH₃, CH₄ and CO₂l
  • A Binary system is a closed-loop system, which separates the fluids that absorb the heat from those that generate the electricity

Energy Storage

• A major obstacle to the widespread use of alternative energy is the problem of energy storage and transportation.
  • Most electricity is used as soon as it is generated
  • Intermittency and Variation may mean that storage is necessary on a large scale
• Fossil Fuels are naturally stored
• Batteries are a possible option, but large-scale storage has not been attempted
  • Batteries are often bulky compared to the amount of energy they store.
  • Batteries produce hazardous waste and chemicals.
• Small-scale battery storage is useful for "smoothing" electricity generation for the grid
• Car batteries are an option if electric cars become common
• Alternatives to Batteries
  • Hydroelectric Pumping
    • Pump water during periods of excess generation, use hydrostatic head to turn a turbine and generate electricity when demand is large
  • 22 gigawatts of storage already exist, built to store excess energy from slow-to-adjust nuclear plants
  • New facilities are expensive and difficult to get environmental approval for
  • Thermal Storage
    • Store energy as heat by heating fluids and use heat to generate electricity
    • Short term storage, mainly envisioned as means for storing excess solar energy
  • Compressed Air
    • Excess energy used to pump air into underground caverns, often deep, where the air is naturally heated and more energy is added
    • Hot air turns turbines when released
    • Well tested in Europe and, close by, Alabama
  • Fuel cells are similar to batteries but use hydrogen gas to store energy.
    • Use excess energy to generate H₂ - the energy is stored in the H₂ produced (usually from splitting water)
    • When you need to use the stored energy, stored H₂ is used by Fuel Cells to generate electricity directly, not using combustion
    • Fuel Cells produce electrons on one side and absorb the on the other, so that an electric current is established in wires connecting the two sides
    • Splitting H₂ to produce H⁺ ions produces electrons
    • Combining the H⁺ ions with O₂ and the electrons produced by the splitting form water, the "waste" from fuel cells
    • Fuel cells may be used for tasks like powering electric vehicles.

Energy Conservation

• The simplest and cheapest way of stretching our energy resources and mitigating energy-related problems is through energy conservation and energy efficiency.
• Energy intensity is the ratio of energy consumption to economic output
  • A useful index of overall efficiency
  • Can be lowered while maintaining strong economic growth and high standards of living
  • Many developed economies have become more intense and are getting more for each watt used

Saving Energy

Energy can be saved by:

• Not converting it from one form to another unnecessarily
• Increasing the efficiency of energy conversions
• Not transporting energy unnecessarily
• Increasing the efficiency with which energy is transported
• Encouraging conservation
• Requiring new construction and manufacturing to be efficient

Encouraging Energy Savings

• Voluntary versus Mandatory Measures
  • Governments can mandate and/or encourage energy efficiencies.
  • Voluntary measures can be encouraged through education, advertising, and with positive examples in the popular culture.
  • Industry can also promote efficiency; for example, utility companies promoting energy efficiency to consumers, and adopting demand-size management strategies.
  • Government can also encourage efficiency through tax breaks and subsidies.

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Last updated April 16, 2013