Renewable Energy Sources II Alternatives

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ENERGYPHYX 1020 USU 1360C

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Renewable Energy Sources IIAlternatives

Hydroelectricity

Wind

Geothermal

OTEC

+Tidal

+Waves

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Renewable Energy• Note:

– Renewable sources only 8%

– Of these Solar renewable energy is 96%

– Direct solar energy is only 1%

DOE Annual Energy Review, 1999

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Indirect Solar energy• Hydroelectricity

– Gravitational potential energy of water• Solar energy lifts water by evaporation and convection

• Wind– Kinetic energy of moving air

• Solar energy changes atmospheric density differentially and buoyancy creates horizontal pressure differences

• Biomass– Chemical energy of carbohydrates

• Carbohydrates created from solar energy by photosynthesis

• Waves– Potential and kinetic energy of moving water

• Solar energy creates wind - friction of wind on water creates waves

• Ocean temperature gradient– Heat energy in ocean water

• Solar energy absorbed in surface layer of ocean - bottom remains cool

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Geothermal & Tidal EnergyThe Non-Solar Alternatives

• Two energy sources are not a result of the presence of solar energy at the earth.

– Geothermal Energy• Heat energy from earth’s core.• Results from mass energy converted to heat energy in radioactive decay.

– Tidal Energy• Gravitational potential energy of water.• Results from gravitational energy of the interaction of earth, moon and sun.

• Note the sources of all of our energy except tidal are nuclear reactions since solar energy derives from nuclear fusion in its interior

– But, gravitational energy contributes to hydroelectric energy and wave energy.

– Also gravitational energy contributes to some wind fields - e.g. on/off shore winds and the earth’s rotation controls wind direction in large size wind systems through Coreolis forces.

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Hydroelectric Energy (1)• Energy from flowing water has been used for about 2000 years and

converted to mechanical energy of rotating water wheels.– This rotational energy was used for grinding, sawing, hammering

• The water flowed because of the conversion of the gravitational potential energy of a stored volume of water to kinetic energy at the place where the device to convert the kinetic energy of the flowing water to mechanical energy was located.

• The water acquired the potential energy by being lifted from oceans by evaporation and convection resulting from its absorption of solar energy and transportation in the form of clouds to elevated land areas.

• The elevation of the surface of the water above the place where its kinetic energy is converted is called the HEAD (h)– The potential energy of the head of water = mgh– Since g = 9.8 ms-2, the energy stored is 9.8 J/kg/m– Reservoirs have millions of kg and h is many meters

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Hydroelectric Energy (2)• The circulation of water from the oceans to the land back to the oceans is

called the Hydrologic Cycle• It is driven by solar energy producing evaporation, gravitational

convection and large scale wind systems.• Hydro-energy schemes tap into this cycle effectively intercepting some of

the solar energy

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Hydroelectric Energy (3)• Hydroelectric power

– Water is stored in a dammed river to provide a head of water– The water is run down to water turbines either below the dam or further down

the valley– The turbines rotate generators and produce electrical energy

• Hydropower is often in rainy, mountainous areas without high population density so the power has to be transmitted long distances.– Also found in mountainous desert areas when a large river is fed from distant,

rainy/snowy mountains (e.g. Colorado River)

• Hydropower produces low pollution– Downstream river temperatures– Erosion– Changes in flora and fauna

• Hydropower has advantages– Renewable and inexpensive– The dammed rivers form recreational lakes– Lake water can be shared with irrigation needs– Lake water can provide drinking water

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Hydroelectric Energy (4)• Growth in

hydropower from 1950-75.

• No significant capacity added since.

• Fluctuations due to rainfall variability.

• Large and small (e.g. Logan) plants included

• Percentage decrease because of increase in total electrical energy use.

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Hydroelectric Energy (5)

Hoover Dam, Nevada and Hydroelectric power station below

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Hydroelectric Energy (6)

Generators coupled to water turbines in a hydro power station

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Wind Energy (1)• The kinetic energy of the moving air mass called wind has been converted

to mechanical energy for 100s of years in sailing boats.• This technology peaked in the 19th century with the transoceanic sailing

ships which used the wind to provide them with 10,000 hp.• Also stationary machines to harness wind energy have been used for

centuries in the form of windmills• Windmill technology has now been refined and these devices are again

being used to convert wind energy - now to electrical energy• About 2% of the solar energy reaching the earth’s upper atmosphere is

converted into the kinetic energy of wind• The power (P) of a wind moving at v m/s is

– P = 6.1 x 10-4 v3 kW/m2 (note dependence on cube of wind speed and the area is the area traced out by the windmill blades)

– In theory only 59% of this power can be extracted by a windmill– Modern windmills can achieve 50-70% of this maximum power

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Wind Energy (2)

• Average wind power density (watts/m2) over the country– Note coastal regions and plains are best locations for wind energy conversion

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Wind Energy (3)• Wind powered electricity generation

– Large modern propeller-type windmills are coupled directly to generators mounted on the mast

– The trend is to use collections of smaller capacity wind generators than few large ones - the concept is called a “wind farm”

• Power capacity in 50 to 600kW range for each windmill• Typical average annual energy output 100 - 500 kWh/m2

– Presently wind energy supplies only 1% of the 8% renewable energy• Amounts to <2% of renewable electrical energy generated

– To equal total electrical generating capacity of US would require a 15m rotor diameter windmill every 200m in regions with 300W/m2 or more.

– Suffers from same problems as direct solar energy - variable wind speed which are not very predictable (c.f. sun and clouds).

• However this is helped by the national grid distribution system.

– Produces visual and local noise pollution

– Dangerous to birds

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Wind Energy (4)

Example of a “Wind Farm” generating electricity

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Water Wave Energy• The effect of the wind blowing across large areas of water sets the water

into motion in the form of surface waves.

• The water motion has kinetic and potential energy gained from the kinetic energy of the wind.

• Devices to convert this energy into forms we need are unusual partly because of the low frequency of the waves.– Promising approaches are to use the waves to compress air or pump water into an

elevated reservoir either of which could then rotate a turbine and thence an electrical generator.

– Many 1000s of buoys are in use in which the wave energy is converted to electrical energy to power the navigational aids on the buoy

• The power available is described in kW/m of along the wave

• In favorable locations this can be ~50kW/m

• At present the conversion of wave energy to supplement national power needs is experimental

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Ocean Thermal Energy ConversionOTEC

• Solar energy heats the surface of the oceans, but no solar energy penetrates deep in the oceans which stay at ~5°C

• The surface temperatures vary from 15 to 24°above the deep temperature.

• Thus the surface is a very large heat source and the depths form a very large heat sink.

• This temperature difference can be used as the source and sink for a heat engine.

• The heat engine can be used to drive a turbine and produce electricity.

• This is the basis of the OTEC project.– Pilot plants have been built– Large commercial plants have been designed

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Ocean Thermal Energy ConversionOTEC (2)

Note large area of pacific with highest temperature difference

Note favorable locations are remote from centers of population

CONTOURS OF OCEAN TEMPERATURE DIFFERENCE SURFACE - DEEP

Problem of transporting energy to the end users

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Schematic diagram of a possible OTEC heat engine

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• Thermodynamic efficiency– Th = 29°C = 273 + 29 = 302K; Tc = 5°C = 273 + 5 = 278K– Eff = (1-Tc/Th) = (1- 278/302) ~ 0.08 or 8%

• This is low because of the small temperature difference– But acceptable because the energy is renewable

• Environmental impact– Large structure - shipping hazard– Will cool ocean surface

• Impact on weather? (remember El Niño)

• Too far for cable transmission of electricity– Possible use of hydrogen derived from electrolysis of sea water as

intermediate form of energy

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Ocean Thermal Energy Conversion - OTEC (5)

Artist’s Impression of Full Size OTEC Energy Plant

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Energy from Biomass

• The conversion of energy that results in chemical energy from biomass results from the interaction of solar electromagnetic radiation with plant tissue.

• Solar radiant energy causes chemical reactions in plants in which cause water and carbon dioxide to combine to form carbohydrates and in the process release oxygen.

• This results in growth or mass increase of the plant material.

• This process is called PHOTOSYNTHESIS

• This is a very important process for our energy supply– Fossil fuels– Food– Biomass fuels

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Plant Level Photosynthesis• Light Reaction

– Light photons produce oxygen from water and energize other molecules making them more active

• Dark Reaction– Activated molecules

make carbohydrates from carbon dioxide

CO2 +2H2Olight ⏐ → ⏐ ⏐ O2 +CH2O[ ]+H2O

ΔH +CO2 +2H2O combustion/decay← ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ ⏐ photosynthesis ⏐ → ⏐ ⏐ ⏐ ⏐ ⏐ O2 + CH2O[ ]+H2O

Basic formation of primitive carbohydrate

Reversible reaction

Actual carbohydrates have chemical form: Cx(H2O)y E.g. Glucose C6H12O6 and sucrose C12H22O11

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Uses of Biomass as an Energy Source• Direct burning

– Renewable biomass• Primarily wood used for residential and industrial heating

– Municipal solid waste• 73% of solid waste comes from biomass• About 16% of solid municipal waste is incinerated and the heat energy used to produce

electricity

• Liquid fuel– Ethanol can be produced by fermentation of grain or sugar cane

• 95% of ethanol is produced from grain• Currently used as a 10% supplement to gasoline - called gasohol

• Gaseous fuel– Methane can be produced by anaerobic fermentation of biomass waste– Growing plants for methane production is not competitive with natural gas

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Biomass Energy Budget• Note that the production of liquid and gaseous fuel from biomass needs

fossil fuel energy in the processes

• Also in the large scale production of biomass other forms of energy are used for– Ground preparation– Harvesting– Transportation– Processing– Production of fertilizers

• Efficiency = Biomass energy / Energy supplied– This varies with crop and cultivation methods– Even though manpower is low input, typically yields are lower for this mode of

cultivation

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Distribution of Biomass Energy

Compare with US total annual consumption of 97 QBtu

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Geothermal Energy (1)

• Geothermal energy is the first of two viable energy sources that are non-solar in origin.

• The core of the earth is at a temperature of ~4000°C below 6400km depth.– This elevated temperature is maintained by the radioactive decay of nuclei

in the earth’s core.• The surface is ~20°C, so there is a flow of heat energy to the

surface by thermal conduction.• The total power flowing through the earth’s crust is 32 x 1012 W• This is a large amount of power, but averaged over the surface

area of the earth it is only 1/16 W/m2.• However this is an average, and there are places where much

higher power per m2 flows to the surface.• This allows geothermal energy to be a significant energy source

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Geothermal Energy (2)

• Geothermal energy arises from locations where the earth’s crust is thin and the heat flow from the interior is much greater then the average.

• These sites have been known for thousands of years and have supplied heat energy for bathing and space heating.

• More recently the heat energy has been used to drive turbines and generators and be converted to electrical energy.

• Since the heat flow is determined by the conductivity of the earth’s crust, care has to be taken not to extract heat energy faster than it can be supplied from the core of the earth.

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Categories of Geothermal Energy• Hot water reservoirs

– Mainly used for space heating• Natural steam reservoirs

– Fairly rare occurrences - steam pressure can drive turbines.• Geopressured reservoirs

– Heat energy in brine also saturated with dissolved natural gas• Normal geothermal gradient

– Anywhere with normal crust thickness has a temperature gradient of 30°C/km. A 20,000 foot well results in a temperature difference of ~200°C. Technology to use this not yet developed.

• Hot dry rock– Locations with thinner crust leading to temperature gradients of

~40°C/km. In principle could be used for stem generation with external water supply

• Molten magma– Molten rock emerging to the surface - no technology yet.

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US Geothermal Energy Resources

• The resources should be viewed in the light of the total annual US energy consumption of ~96QBtu per year

• Low pollution control costs• High maintenance costs due to corrosive nature of geothermal steam

and water

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Geothermal Energy Plants

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Tidal Energy

• While geothermal energy is non-solar in origin, it is a result of nuclear reactions as is solar energy.

• Tidal energy results from the gravitational forces between the earth and the sun and moon.

• These forces produce two bulges in the level of the sea which stay aligned with and anti-aligned with the net force direction while the earth rotates under them.– This results in a tidal surge that moves around the earth separated by about

12 hours

• The average tidal range (height of the bulge) in the US is 2 - 18 ft– When sun/moon/earth are aligned extra high tide– When moon at right angle to sun-earth line extra low tide

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Tidal Energy (2)• In long tapered river estuaries with containing cliffs the tidal bulge

height gets amplified and can reach much higher levels– At a location in northern France the range is 18-44 ft (Rance River)

• The tidal surge flows through a dam while the tide is rising, then entry ports are closed and the water remains trapped as the tide recedes

• This produces a low head hydroelectric system– The Rance river plant is the largest tidal power plant with an annual electricity

production of 540 million kWh

– The turbines can also act as pumps driven by electrical energy during low demand periods, thereby storing energy for release during peak periods

• The US has potential sites in Alaska and the Bay of Fundy in Maine/Canada

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Rance River Tidal Power Plant

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Summary of Alternate Energy Capabilities

• 1998 average summer generating capacity was 782,000MW• Note that even if the expectations on the capacities of various alternate energy sources

were met we would still not be close to the present peak power generating capacity.• Alternates are some years away from peak power capacity by which time demand will

have grown.• The bottom line is that alternative technologies other than renewable energy need to

be developed in the very near future.

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Learning Objectives (1)• Understand the meaning of indirect alternate solar energy sources• Know the five energy sources from indirect solar energy• Know the two energy sources which are not solar related• Know the basis of hydroelectric energy• Know what is meant by the Hydrologic Cycle• Know how hydroelectric energy plants are implemented• Know that hydroelectricity supplies about 10% of our electrical

energy needs• Be aware of the environmental impact of hydroelectricity energy

conversion• Know that wind energy is is the kinetic energy of moving air

masses• Be aware of the windmill as a means of converting the wind energy

to rotational kinetic energy• Know that the power from a windmill increases as the cube of the

wind speed

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Learning Objectives (2)• Be aware of the advance in technology from the older windmills

to the propeller types used in wind farms.• Be aware of the environmental impact of wind power generation

systems• Be aware that wind energy can be converted to wave energy in

large areas of water forming another potential source of energy.• Understand that there is a temperature gradient from deep in the

ocean to its surface.• Be aware of the OTEC project to use this temperature gradient

and the heat stored in the surface of the ocean to make heat engines

• Know what is meant by photosynthesis• Know the three methods by which biomass can be used as an

energy source• Be aware of the biomass energy budget as a result of increased

mechanization.• Understand that waste material is made up of large quantities of

biomass

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Learning Objectives (3)• Know what is meant by geothermal energy• Know the non-solar source of geothermal energy• Know the conditions for geothermal energy to be exploitable• Understand that there are different categories of geothermal

energy• Know what causes the tides and their characteristics• Understand the favorable land features necessary to increase

the gravitational potential energy of the tidal surge.• Know the general arrangement to utilize the energy of the

tidal surge and convert it to electrical energy.• Be aware that the present and predicted energy available

from renewable sources is a long way from meeting our energy needs.

Documents

Renewable Energy Sources II Alternatives