CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
Renewable Energy Sources II
Alternatives
Hydroelectricity
OTEC
+
Tidal
+
Waves
Wind
2002
Geothermal
1
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
Renewable Energy
• Note:
– Renewable sources only
8%
– Of these Solar renewable
energy is 96%
– Direct solar energy is
only 1%
2002
DOE Annual Energy Review, 1999
2
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
• Solar energy absorbed in surface layer of ocean - bottom remains cool
3
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
Geothermal & Tidal Energy
The 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.
2002
4
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
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
5
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
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
6
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
–
–
–
–
Renewable and inexpensive
The dammed rivers form recreational lakes
Lake water can be shared with irrigation needs
Lake water can provide drinking water
7
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
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.
8
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
Hydroelectric Energy (5)
Hoover Dam, Nevada and Hydroelectric power station below
9
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
Hydroelectric Energy (6)
Generators coupled to water turbines in a hydro power station
10
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
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
11
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
Wind Energy (2)
• Average wind power density (watts/m2) over the country
– Note coastal regions and plains are best locations for wind energy conversion 12
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
– Dangerous to birds
13
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
Wind Energy (4)
Example of a “Wind Farm” generating electricity
14
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
• At present the conversion of wave energy to supplement
national power needs is experimental
15
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
Ocean Thermal Energy Conversion
OTEC
• 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
2002
16
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
Ocean Thermal Energy Conversion
OTEC (2)
CONTOURS OF OCEAN
TEMPERATURE
DIFFERENCE SURFACE DEEP
Note large area of pacific
with highest temperature
difference
Note favorable locations
are remote from centers
of population
Problem of transporting
energy to the end users
2002
17
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
Ocean Thermal Energy Conversion
OTEC (3)
Schematic diagram of a possible OTEC heat engine
18
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
Ocean Thermal Energy Conversion
OTEC (4)
• 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
2002
– Possible use of hydrogen derived from electrolysis of sea water as
intermediate form of energy
19
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
Ocean Thermal Energy Conversion - OTEC (5)
Artist’s Impression of Full Size OTEC Energy Plant
20
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
– Fossil fuels
– Food
– Biomass fuels
21
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
li ght

O2  CH2O H2O
Basic formation of primitive carbohydrate CO2  2H2O 
Actual carbohydrates have chemical form: Cx(H2O)y
E.g. Glucose C6H12O6 and sucrose C12H22O11
photosynthesis
H

CO

2H
O












O2  CH2 O H2 O
Reversible
reaction
2
2
combust
ion/
decay
2002
22
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
– Methane can be produced by anaerobic fermentation of biomass waste
– Growing plants for methane production is not competitive with natural
gas
23
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
24
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
Distribution of Biomass Energy
Compare with US total annual consumption of 97 QBtu
25
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
26
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
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.
27
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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.
2002
28
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
29
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
Geothermal Energy Plants
30
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
– When sun/moon/earth are aligned extra high tide
– When moon at right angle to sun-earth line extra low tide
31
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
ENERGY
USU 1360
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
2002
• The US has potential sites in Alaska and the Bay of Fundy in
Maine/Canada
32
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
Rance River Tidal Power Plant
33
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
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
34
future.
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
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
35
of the wind speed
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
2002
ENERGY
USU 1360
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
36
CHAPTER 5 RENEWABLE II
ALTERNATIVES
PHYX 1020
USU 1360
Learning Objectives (3)
•
•
•
•
•
•
•
•
2002
ENERGY
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.
37