ECE 333
Renewable Energy Systems
Lecture 24: Tidal, Geothermal,
Concentrating Solar, Biomass Power
Prof. Tom Overbye
Dept. of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
overbye@illinois.edu
Announcements
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Read Chapter 9
Quiz today on HW 9
Homework to do before final: 8.1, 8.4, 8.6, 9.11
Final Exam is Friday May 8, 7 to 10pm
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A to J in ECEB 3017
K to Z in ECEB 1013
Rooms given on-line are correct!
Comprehensive, with more emphasis on material since last
test; same procedure as per other exams, except you may bring
in three handwritten note sheets
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Tidal Power
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Tides are caused mostly by gravitational forces exerted
by the moon (partially by the sun – 45% of moon)
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There should be two high tides, and two low per day, but what
actually occurs is impacted by land masses
Also there is also monthly variation caused by the lunar cycle
Tidal heights can variety quite widely with the region
with Bay of Fundy in
Canada have tides of
16.3 m; other places
have tides of less than
one meter; quite predictable and reliable
Image: http://oceanservice.noaa.gov/facts/highesttide.html
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Tidal Power
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Tidal power can be extracted either by building dams,
and is hence equivalent to traditional hydro, or by
placing stream (not steam!) turbines in the tidal flow
World’s largest tidal power station
had been in La Rance, France, built
from 1960-66, with a capacity of
240 MW
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It uses a 330 meter long dam, built in
front of a 22 square km basin. Tidal
differences average about 8 meters.
Largest is now 254 MW Sihwa Lake Tidal Power,
South Korea. Uses a 30 square km basin
Photo source: http://www.rise.org.au/info/Tech/tidal/image019.jpg
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Tidal Power
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A 1320 MW stations is under construction in
Incheon, Korea (158 km2)
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It is expected to be operational in 2017
Tidal power tends to have
low capacity factors,
perhaps 20 to 30%
Changing tides can
have environmental
impacts and affect
other uses
Image: pds.joins.com/jmnet/koreajoongangdaily/_data/photo/2009/07/13214733.jpg
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Tidal Power, Stream Turbines
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Tidal stream generators (TSGs) function much like
underwater wind turbines, but since water is 800 times
denser than water, they are obviously not as large.
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P   Av3 Water density is 1025 kg/m3
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If the turbine is open (not in a duct) then there is no
pressure change, and the Betz limit applies; with enclosed
hydro there is a duct and an associated pressure drop, so
efficiency is much higher than 59.3%
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Tidal Power, Stream Turbines
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Commercial solutions are now becoming available,
including the Alstom Oceade18 1.4 MW, 3 blade
machine with an 18 m rotor diameter
Four of these are
being installed for a test
project in France,
operational in 2017
http://www.alstom.com/products-services/product-catalogue/power-generation/renewable-energy/ocean-energy/tidal-energy/tidal-power/
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Geothermal Power: Heat Engines
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Most electricity is generated by converting heat into
mechanical energy to electricity
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Coal, gas, nuclear, solar thermal, geothermal
Thermal efficiency is defined as

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Net work output W

Total heat input QH
Net work output is difference
between heat in and heat out
QH  W  QC
Q  QC
Q
 H
 1 C
QH
QH
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Entropy
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Entropy is used to measure the amount of thermal
energy unavailable for conversion to other forms
Second law of thermodynamics says that network
entropy in universe is increasing
Q
S 
T
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Therefore in moving heat energy from high to low
temperature reservoir
QC QH
TC

 max  1 
TC TH
TH
Temperature values should
with respect to absolute
zero (e.g., in kelvin)
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Geothermal Power
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Geothermal power creates electricity by leveraging
temperature differences between surface and below the
surface
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Should not be confused with ground source heat pumps,
which are also sometimes called geothermal systems
In most areas of world heat differences are not
sufficient to be practical (temperature increases only
by 30º C on average per km)
Some areas have rocks close to the surface
Liquid is used to transfer the heat energy
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Geothermal Power Technologies
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Various technologies are used for moving the heat
from underground to the surface; there are few
locations in which dry steam is available
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Dry steam has no micro drops of liquid water in it
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Geothermal Power in US
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Geothermal is a constant source of energy with high
capacity factors
In 2014 the US got 0.4% of its total electricity from
geothermal (close to what we got from solar!)
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Geothermal has grown by less than 10% in 25 years
Total US geothermal capacity is 3400 MW, with 80% in
California, 15% in Nevada
Details on current research in geothermal is available at
energy.gov/sites/prod/files/2015/03/f20/GTO_2014_An
nual-web_0.pdf
Source: energy.gov/sites/prod/files/2015/03/f20/GTO_2014_Annual-web_0.pdf
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Geothermal Worldwide
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Worldwide geothermal capacity is slowly growing
Geothermal dominates in Iceland, with it having 20
high temperature steam fields
The Rift valley in Africa could provide 15 GW of
geothermal capacity
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Source: /geo-energy.org/events/2014%20Annual%20US%20&%20Global%20Geothermal%20Power%20Production%20Report%20Final.pdf
Concentrating Solar (Solar Thermal)
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An alternative to solar PV is concentrating solar power
(CSP) plants
Similar to fossil, nuclear and geothermal, CSP uses a
heat process to convert solar energy into electricity
Much of what was talked about for the sun as a
resource for solar PV applies, except CSP can only
leverage direct beam insolation
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Tracking must be used
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CSP Energy Storage
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Because CSP is a thermal process, with some heat
storage, it is less intermittent than solar PV
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Thermal capacity can be greater than electrical to allow heat
storage for use after the sun has set (and when electricity
prices may still be high)
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Major CSP Installations
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Over the last few years the number of CSP has
increased rapidly, particularly in the US
In contrast,
worldwide
solar PV
capacity
is greater
than
150 GW
http://www.renewableenergyworld.com/rea/news/article/2009/05/global-concentrated-solar-power-industry-to-reach-25-gw-by-2020
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World's Largest CSP: Ivanpah
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The world's largest CSP went on-line in 2014, located
in Ivanpah, CA (in the desert right by Nevada
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Total capacity is 392 MW on 3500 acres
Developed with $1.6 billion load guarantee from US DOE
Sun is concentrated
on boilers located
on top of 459 foot
towers
Most of the power
is sold to PGE and
SCE under
long-term contracts
http://energy.gov/articles/celebrating-completion-worlds-largest-concentrating-solar-power-plant
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Ivanpah and Birds
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A concern with Ivanpah is close to the tower focus area
the temperatures can get quite high (93 C on the outer
mirrors, 500 C by the tower); birds flying in this area
can quickly get burned
According to a 2014 IEEE Spectrum article, researchers
from the US Fish and Wildlife Service (USFWS) found
141 bird carcasses during their visit, and "At one point,
the researchers describe watching a bird fly over the
heliostat array, ignite, lose and regain altitude and alight
on a perch on the other side."
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Concentrating Solar Technologies
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There are four main technologies used for CSP
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Parabolic Trough
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In a parabolic trough, a heat conducting fluid (such as
synthetic oil) runs down the middle of the tube, with
heat eventually transferred to a convention steam turbine
Image: www.seia.org/policy/solar-technology/concentrating-solar-power
This the most
established
technology and
has the most
installation capacity;
Nine of the top ten
CSP installations (in
terms of capacity)
use parabolic
troughs
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Parabolic Trough System Layout
Image source: NREL
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World's Largest Parabolic Trough
CSP
• The largest parabolic trough CSP is the Solar Energy
Generating Systems (SEGS), located in Mojave Desert
in California
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Has a capacity of 354 MW and covers 1600 acres
SEGS was
built in stages
from 1984 to
1990; its average
capacity factor
is about 21%
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Central Receiver
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In a central receiver system, such as Ivanpah, computer
controlled mirrors, called heliostats, reflect the sunlight
onto a boiler that produces super-heated steam, which
is then used in a conventional turbine
Higher temperatures are
possible compared to
a tough design
Image: www.ivanpahsolar.com/about
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Parabolic Dish
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This design uses a large parabolic dish to concentrate
sunlight on a solar receiver that contains a Sterling engine
Sterling engines differ from
internal combustion engines
in that with a Sterling engine
the combustion is external,
allowing it to run on any heat
source; invented in 1816
System can be air cooled, resulting in little water
requirements
Sterling Engine Systems filed for bankruptcy in 2011
Image://en.wikipedia.org/wiki/Solar_thermal_energy#/media/File:SolarStirlingEngine.jpg
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Biomass -- Wood
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Wood has long been used as a electric fuel source,
primarily wood waste
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In 2014 the US got about 1% of its electricity from wood,
more than two times the total from solar PV and CSP
Significant growth in wood generation is not expected
Largest wood-fired
plant in the US is the
100 MW Nacogdoches
Generating Facility in
Sacul, Texas
http://www.power-eng.com/content/dam/pe/print-articles/2012/oct/f6-Photo-4-1210pe.jpg
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Biomass – Future Possibilities for
Electricity
• Newer crops are being considered for future biomass,
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including various grasses (such as Miscanthus and
Switch Grass) along with algae
Potential uses include both fuel to
create electricity (primarily the
grasses), and conversion to liquid
fuels such as ethanol.
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On campus 320 acres in the South
Farms are devoted to biofuel
research
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Biomass – Future Possibilities for
Electricity
• A full consideration of biomass is beyond the scope of
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ECE 333 since it gets into agricultural economics issues
Miscanthus can be harvested at rates of about 15 tons
per acre in Illinois. Once established it does not need to
be replanted. This allows the energy potential of about
225 Mbtu per acre; income depends on energy price,
say $2/Mbtu = $450 per acre. For comparison corn can
yield up to 200 bushels per acre at say $5.50/bushel =
$1100 per acre
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Energy yield is about 225/15 = 15 Mbtu/ton similar to coal
Not a native species so containment could be an issue
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A controversial fuel: Arundo Donax
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Arundo (known as “giant reed”) is a plant that can
produce up to 20 dry tons per acre, growing up to
30 feet high in one year
But it is also banned in three
states, including California
Oregon is considering
requiring a bond to grow it
Characteristics of good fuels
include not requiring annual plantings, growing
quickly with few pests, low water requirements.
But these also mean they may get out of control!
Image source: http://cabiinvasives.wordpress.com/2010/10/18/are-we-fuelling-future-invasions/
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Cofiring
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Burn biomass and coal
Modified conventional steam-cycle plants
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Retrofit costs range from $150 to $300 per kW (versus
around $1000 to $2000 for the original construction)
Allows use of biomass in plants with higher
efficiencies
Reduces overall emissions; net CO2 emissions are
considered reduced since the CO2 stored in the
biomass recently came from the atmosphere.
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Gas Turbines and Biomass
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Cannot run directly on biomass without causing
damage
Gassify the fuel first and clean the gas before
combustion
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Biomass is heated to about 400º C, vaporizing volatile
components and the water, producing a product called
syngas, which is mostly H2, CO, CH4 CO2 and N2.
Coal-integrated gasifier/gas turbine (CIG/GT) systems
Biomass-integrated gasifier/gas turbine (BIG/GT)
systems
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Biomass and Transportation
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A key issue associated with biomass is the
transportation costs – these grasses are quite bulky.
A rough estimate of the cost per ton for transportation
is about $1 + 0.1 * round trip distance in miles +
harvesting costs of about $22 per ton. So if the power
plant is 50 miles distant, total cost would be $33 per
ton, or about $33/ton/(15 Mbtu/ton) = $2.2 per Mbtu
Total US corn planting is about 87 million acres,
which if planted in grass could yield about 20 quad of
energy.
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Won’t meet all our needs but could play a major role
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Predicted Growth in US Biomass
Electric Generation
The assumed
growth rate
for biomass
electricity is
about 3.8%
per year. Note
total renewables
by 2040 are
still only about
16% of the total
(850 out of 5056
TWh).
Source: DOE EIA Annual Energy Outlook 2015
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