Environmental Physics
Chapter 12:
Electricity from Solar, Wind, and Hydro
Copyright © 2008 by DBS
Introduction
Introduction
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Renewable energy sources (RES) provide around 8 % of the world’s energy
Wind energy is the fastest growing energy resource, followed by photovoltaics
Studies suggest renewables could rise to 30-40 % share by 2050
Solar derived
RES
radiant
wind
waves
hydro
biomass
geothermal
tidal
Fundamental Sources of Energy
FUSION
(SOLAR)
FISSION
Fossil fuels
Nuclear energy
Wind
(man-made)
Waves
Geothermal
Biomass
(natural)
Hydro
Radiant
GRAVITATIONAL(
PE/KE earthmoon-sun)
Tides
Wood and
agricultural wastes
3 % of total
energy use
Figure 6.1: U.S. renewable energy consumption (by source), 2003.
Fig. 6-1, p. 162
Introduction
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US < 1 % wind power
Denmark provides 19 % of its electricity via wind
Spain and Portugal – 9 %
German and Ireland – 6 %
http://en.wikipedia.org/wiki/Wind_power
Introduction
Photovoltaic (PV) generation:
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Solar cells remain expensive, growing market so costs expected to decline
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Economic parameters: Cost per Watt and cost per kWh
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Cost in 2003 $0.25-0.30 per kWh (3 x usual fossil-fuel $0.08/kWh)
Figure 12.1: U.S. and non-U.S. PV production by year. Cost of PV cells has
decreased significantly over the years and world production has risen above 550 MW.
Recently Stephen O’Rourke, a research analyst from Deutsche Bank estimated what year electricity
generated by solar panels would achieve “grid parity”. Grid parity is reached when the cost price of
PV generated electricity starts to compete with the local retail price of the centrally generated grid
electricity.
Introduction
Photovoltaic (PV) generation:
Grid elec. 0.09 $/kWh (and rising)
vs. 0.3 $/kWh PV (falling)
The life cycle greenhouse gas emissions of PV electricity is 20 to 30 times less than fossil fuel fired power
plants. Therefore, if PV electricity continues to increase at it current pace it will substantially contribute to a
sustainable society in the years to come.
Introduction
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Applications:
– Utility-scale power plants
– Lighting
– Communications
– Water pumping
– Battery charging
– Vaccine refrigeration
Figure 12.2: PV modules can power vaccine refrigeration units in remote locations.
Solar Cell Principles
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Movie from NOVA DVD
Solar Cell Principles
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‘Photoelectric effect’
When light of a certain frequency strikes a metallic surface electrons are emitted
Discovered in 1887 by Hertz, explained by Einstein
Figure 12.3: Apparatus for observation of the photoelectric effect. Light hits the
metal plate (in the evacuated tube) and electrons are emitted.
Solar Cell Principles
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Energy of each photon depends on its frequency,
E = hf
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(where h = Planck’s constant)
An e- in a metal atom is able to ‘capture’ a photon and obtain the energy necessary to escape if
the energy of the photon exceeds the binding energy of the e- in the metal
24-19
Solar Cell Principles
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Solar cells use a variant of the photoelectric effect, but not totally ejecting electrons out of the
material
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In semiconductors, light of even relatively low energy, such as visible photons, can kick e- out of
the valence band and into the higher-energy conduction band, where they can be harnessed,
creating electric current at a voltage related to the band gap energy.
http://academics.rmu.edu/faculty/short/envs1160/envs1160-demos/PV-Cell.mov
Solar Cell Principles
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PV cell consists of 2 layers of treated crystalline silicon
“doping” adds impurities to make Si a better conductor
– n-type (negative) semi-conductor - adding phosphorus adds extra e– p-type (positive) semi-conductor - adding boron produces ‘holes’ in the crystal
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p-n junction formed
Figure 12.4: Solar cell construction.
Solar Cell Principles
The "extra" electrons in the phosphorus-doped top layer naturally move into the boron-doped bottom
layer—a process that occurs during manufacture in a fraction of a second and only very close to the
junction
Solar Cell Principles
Electric Field Formation:
Once the bottom layer has gained extra electrons, it becomes negatively charged at the junction; at
the same time, the top layer has gained a positive charge there. Now the cell is ready for the sun.
As sunlight hits the cell, its photons begin "knocking loose" electrons in both silicon layers.
The electric field pushes electrons that reach the junction towards the top silicon layer.
Solar Cell Principles
This force essentially slingshots the electrons out of the cell to the metal conductor strips, generating electricity.
Solar Cell Principles
Single-Crystal Silicon parameters:
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p-n junction produces 0.5 V Direct Current (DC)
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Typical cell produces 100 mA/cm2 under bright sun (1000 W/m2)
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10 cm diameter cell (100 cm2) produces around 1 W under bright sun
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Typical efficiency 10-15 %
– energy loss due to EM radiation not being energetic enough to move e- from valence to
conduction band
– Energy loss due to EM radiation being too energetic
– Reflection from cell surface
Multi-layered cells increase efficiency
Mirrors/lenses
Figure 12.5: Multilayered solar cell. A second thin film is used so the stack can respond to a broader
spectrum of light, thus increasing its efficiency.
Question
A typical 10 cm2 solar cell produces a voltage of 0.5 V and a current of 2.5 A in full
sunlight (1,000 W m-2). What is the power produced?
P = V I = 0.5 V x 2.5 A = 1.25 W
Solar Cell Principles
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1987: GM Sunraycer won the 1st World Solar Challenge Race across Australia (1877 mi)
8600 cells, 8 m2
Average output 1000 W
Cruising speed 42 mph, top speed 68 mph
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Similar US race from 1990
N. American Solar Challenge
Improvements in efficiency of cells
require rule changes year to year
Cell Manufacture
Monocrystalline Si (14-18 % efficient)
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Czochralski process: Seed of crystalline Si in pure molten Si and slowly drawn out into a boule
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Sliced into wafers 0.5 mm thick and doped with P and B
Polycrystalline Si (12-14 % efficient)
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Block of Si cooled and solidified
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Consist of many grains of single crystal Si that are randomly packed
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Ribbon Silicon: Flat thin film drawn from molten Si (reduces waste from sawing ingots)
Amorphous Si (5-6 % efficient)
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Non-crystalline, disordered atomic structure
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Thin layers deposited on glass or metal and cut with laser
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Cheaper to produce but efficiency drops with time
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Materials other than Si include: gallium arsenide,
cadmium telluride, cadmium sulfide,
copper indium gallium diselenide
PV Systems and Economics
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Single solar cells are connected electrically as modules
Modules are connected to form arrays
e.g.6 panels producing 47 W each produce 6 x 47 = 282 W
$300 module x 6 = $1800 array producing power at $6.40 per peak Watt
Figure 12.7: This 282-watt, 12 V DC PV unit replaced a noisy, high-maintenance diesel
generator. It is used for communication in remote areas by the Wycliff Bible Translators.
PV Systems and Economics
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Wired in series voltages add and current is the same for each cell
Wired in parallel voltages are the same and currents is the sum of 3 cells
A module will have various series/parallel connections to provide correct I and V
Figure 12.8: Solar cells can be wired in series (a) or parallel (b) to provide increased
voltage or current, respectively.
PV Systems and Economics
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Since output is DC amps need to convert to AC using an inverter
Figure 12.9: Photovoltaic system for a residential dwelling.
PV Systems and Economics
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Barely enough juice to run your lights, TV and a computer…
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What about larger devices requiring over 1 kW? Oven, AC, washing machine, toaster etc.?
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PV power plants require lots of space
PV Systems and Economics
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Espenhain, Germany
5 MW - Largest PV plant in 2003
Largest are all in Spain…
Figure 12.10: 5 MW German PV installation.
PV Systems and Economics
Olmedilla PV Plant
Spain
60 MW
http://en.wikipedia.org/wiki/Photovoltaic_power_stations
No Google Earth image yet!
Alamosa PV Plant
Colorado
8.2 MW
http://www.youtube.com/watch?v=ZGzfbV8bz_o
Table 6.3 not
useful here!
PV Systems and Economics
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Since solar cell output power depends on multiple factors, such as the sun's incidence angle, for
comparison purposes between different cells and panels, the measure of peak Watts (W p) is used
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Peak power at latitude 0-50 degrees on a south facing surface is 1000 W/m2
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Average insolation per day can be written as so many hours per day of 1000 W/m2 insolation
Yearly average US insolation:1500 Btu/ft2.d x 1 Wh/m2.d
0.317 Btu/ft2.d
= 4700 Wh/m2.d
= 4700 Wh/m2.d x (1 peak W / 1000 W/m2) = 4.7 hours of “peak watts”/d
An 80 W module produces 80 W for each peak
Watt of exposure
80 W x 4.7 h (of Wp)/d = 376 Wh/d = 0.376 kWh/d
Small house uses ~ 20 kWh/d and 1 kWh/d would
need 3 modules
PV-powered EV recharging station at the
University of South Florida.
p. 399
PV Systems and Economics
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Pumping water question on p400
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Lifting 60 m3 of water 5 m in 8 hours requires 4 x 40 W solar modules
Water pump powered by a PV unit.
End
• Review
Wind Energy
Wind Energy
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Fastest growing form of energy
DOE: Annual Report on US Wind Power
Installation, Cost , and Performance Trends: 2006
http://www.energy.gov/pricestrends/5091.htm
Wind Energy
Wind Energy
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Environmental impact almost insignificant
Main drawback is often quoted to be
– Aesthetics
– Noise
– bird deaths
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Bird deaths caused by collisions with vehicles and
windows cause far more deaths than wind turbines
Data from Erickson et al, 2005
DOE: Annual Report on US Wind Power
Installation, Cost , and Performance Trends: 2006
http://www.energy.gov/pricestrends/5091.htm
Wind Energy
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A
Wind Energy
DOE: Annual Report on US Wind Power
Installation, Cost , and Performance Trends: 2006
http://www.energy.gov/pricestrends/5091.htm
Wind Energy
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Current wind energy industry began after 1973 energy crisis with R&D by NASA and the DOE
Figure 12.11: NASA experimental wind-turbine generator in Sandusky, Ohio. This first large-scale unit
had an output of 100 kW in 18-mph winds. It was put into operation in 1975. Blades were 125 ft in
diameter.
Wind Energy
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Commercial efforts to build more efficient turbines helped by:
– Energy tax credits
– New technology
– Public Utilities Regulatory Policies Act (PURPA)
1998-2006
600 %
Development slows in 1980’s due to drop
in price of oil, more natural gas usage,
expiration of tax credits
1998-2003
345 %
Wind Energy
DOE: Annual Report on US Wind Power
Installation, Cost , and Performance Trends: 2006
http://www.energy.gov/pricestrends/5091.htm
Wind Energy
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More than 30,000 turbines worldwide (2003 figure) > 75,000 MW (2006 figures)
Denmark supplies > 21 % of its electricity with the wind
DOE: Annual Report on US Wind Power
Installation, Cost , and Performance Trends: 2006
http://www.energy.gov/pricestrends/5091.htm
Wind Energy
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Fastest growing area is offshore wind – higher wind speeds, less turbulence
None currently in US
Figure 12.12: Off-shore wind park in harbor of Copenhagen, Denmark. Capacity 40 MW.
Wind Energy
DOE: Annual Report on US Wind Power
Installation, Cost , and Performance Trends: 2006
http://www.energy.gov/pricestrends/5091.htm
Wind Energy
Wind Energy Systems
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Blades, tail, shaft, gear box, generator, tower,
transmission system
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DC output can be stored in batteries
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Synchronous inverter is used to convert
DC to AC at 60 Hz – can be sold to grid
Figure 12.14: Residential wind energy system.
Wind Energy
Wind Power: The amount of air passing through
a cylinder of area A over a distance d = v x t,
(where v = velocity and t = time)
Mass of air (kg) = Volume x Density
= swept area (m2) x distance (m) x Density (kg/m3)
= Adρ
Where A = area of circle = π r2
Wind Energy (J)
= Kinetic Energy
= ½ mv2 = ½ (Adρ)v2 = 1/2 Aρ(vt)v2 = ½ ρv3πr2 x t
Energy = Power x Time
Wind power = Energy / Time = ½ ρv3π r2
Where r = radius (m), ρ = 1.125 kg/m3.
Wind Energy
Betz Limit
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Betz was a German physicist who in 1919 concluded that no wind turbine can convert more than
16/27 (59.3%) of the kinetic energy of the wind into mechanical energy turning a rotor
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This limit has nothing to do with inefficiencies in the generator, but in the very nature of wind
turbines themselves
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Wind turbines extract energy by slowing down the wind. For a wind turbine to be 100% efficient it
would need to stop 100% of the wind - but then the rotor would have to be a solid disk and it
would not turn and no kinetic energy would be converted
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On the other extreme, if you had a wind turbine with just one rotor blade, most of the wind passing
through the area swept by the turbine blade would miss the blade completely and so the kinetic
energy would be kept by the wind
Wind Energy
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Using the theoretical wind power equation we get the following power:
Wind Energy
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Siting is important
– Since power increases with cube of velocity
– Higher wind speeds with taller towers
– Can be large variations in wind velocity over small areas
Wind Energy
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Turbines are classified by their orientation
– Horizontal axis
– Vertical axis
Most common type is horizontal axis with vertical blades
Figure 12.15: Three types of horizontal axis windmills: (a) “Dutch” type windmill. Thousands were used
for centuries in Holland, but few are in use today. They had small efficiencies (7%) and output (10 hp).
(b) “American multivane” windmill. Dependable and able to operate in winds with small velocities.
Extremely important during the past century for lifting water.
(c) Two-bladed wind turbine: prototype of many in use today.
Wind Energy
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Vertical axis – e.g. Darrieus “Eggbeater” rotor
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Uses aerofoils, not propellers
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Do not have to shift with changes in wind direction
(useful where wind direction is highly variable)
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Gearbox and generator at ground and not on tower
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Problems:
– Difficult to raise their height
– Difficulty self starting
– Unstable in high winds
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Also Giromills and Savonius type turbines
Figure 12.16: Darrieus rotor (250 kW) in Altamont Pass, California wind farm.
Giromill
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e.g. Wind Spire
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1.2 kW 30 foot tower
http://www.youtube.com/watch?v=VxoA8rnJj0U
Savonius
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e.g. HelixWind
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2 or 4 kW
http://crispyneurons.com/wiki/HelixWind
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Attey giromill rooftop turbine
http://www.youtube.com/watch?v=WZ5kX5Yw4eY
Wind Energy
Figure 12.17: Horizontal- and vertical-axis wind turbine configurations.
Wind Energy
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Wind turbine is rated at an output (Watts) for a given wind speed
If rated output is achieved in 20 mph winds, higher winds require changes to blade pitch to
prevent damage to the generator
e.g. turbine rated as 10 kW for 20 mph winds, most of the time a lower output is delivered
If wind speed is only 10 mph, power = (10/20)3 x 10 kW = 1.25 kW
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On average house require 600 kWh/month or 20 kWh/d
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Electricity generated depends on turbine output and the wind velocity profile
(no. hrs wind at given velocity)
– Because of v3 term we cant use average velocity
– Could calculate power at given wind speed and multiply by each time interval wind is blowing
at that speed…too tedious!
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Use an empirical relationship: can obtain 70 kWh/month per rated kW at 25 mph speeds
So a 8.5 kW turbine will produce 8.5 x 70 = 600 kWh/month
Wind Energy
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Winds are intermittent
If energy cannot be sold to the grid it must be stored
Batteries
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10 x 12V DC lead acid batteries would provide 120 V (mains voltage)
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Ten 200 A-hr 12 V batteries have a storage capacity of 10 x 200 A-hrs x 12 V = 24,000 Whrs = 24
kWh (1-2 days supply)
Reservoirs
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Energy may be stored by pumping water into a reservoir, can be used for hydropower
Wind Energy
Wind Energy
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Horse Hollow Wind Energy Center, YX
Largest wind farm in the world
735.5 MW
291 x 1.5 MW General Electric and 130 x 2.3 MW Siemens turbines
http://en.wikipedia.org/wiki/List_of_wind_farms
Class 4 and above needed
Source: DOE
Wind Power
Figure 12.18: Map of U.S. wind energy potential.
http://navigator.awstruewind.com
http://www.pawindmap.org/index.htm
http://www.actionpa.org/cleanenergy/wind.html
End
• Review
Hydropower
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Energy produced from potential energy (mgh) of the water
Supplies 19% worlds electricity
Poor environmental record
– Dam construction floods crop lands
Head height
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Micro hydro-power is becoming increasingly popular (small stream)
Question
Derive an equation for power generated from a hydroelectric station relating power to rate
of fluid flow, φ (=v/t) (m3 s-1).
E = Pt = mgh
P = mgh
t
Let rate of fluid flow = φ = v/t, and ρ = m/v
P = mgh = ρvh
t
t
P = ρ φ gh = 1000 φ gh
Power in kW = φ gh
(since ρ = 1000 kg m-3)
Hydropower
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d
Hydropower
Figure 12.19: Three Gorges Dam on the Yangtze River. This is China’s largest construction project
since the nineteenth century. When completed in 2009, it will provide 10% of China’s electricity.
Figure 12.21: Models of waterwheels or turbines
Impulse Type: (a) Breast Wheel , (b) Overshoot Wheel,
Reaction Type: (c) Francis Turbine, (d) Kaplan or Propeller Turbine
Fig. 12-21b, p. 416
Hydropower
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Micro-Hydropower
– Smaller-scale (low head units)
– Heads of as little as 2-3 m can be used
– May be as small as 200-500 W (primarily used for battery charging)
– Cheaper and more reliable than photovoltaics
Low-head hydroelectric installation.
Pelton turbine
Question
Calculate the electricity generated by a micro hydroelectric station operating at a head of
10 m with a flow of 0.3 m3 s-1. The efficiency of the station is 50%.
P = φ gh x efficiency
P = 0.3 m3 s-1 x 10 m s-2 x 10 m x 0.5 = 15 kW
http://www.itdg.org/?id=micro_hydro_faq
Solar Thermal Electric Facilities
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Use concentrating collectors to focus sunlight for production of high temperature fluids
Used for electric power production, industrial process heat, metallurgy etc.
Figure 12.22: Three types of concentrating collector systems.
Helioststs = highly reflective mirrors
Solar Thermal Electric Facilities
Parabolic Trough Systems
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Concentrate sunlight onto fluid filled tube 400 °C (752 ° F)
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Heat exchanger produces steam used to generate power
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Efficiency ~25 %
Concentrating Solar Thermal
Photo: Schott Glass
73
Solar Thermal Electric Facilities
Figure 12.23: Solar Electric Generating System (SEGS), Kramer Junction,
California, provides 165 MW from concentrating collectors shown here.
Solar Thermal Electric Facilities
Central Receivers (Power Towers)
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Use heliostats to track the sun and reflect energy onto a central receiver
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Unlike trough systems as they can track both N-S and E-W
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Prototype plants no longer in use in USA
Solar Thermal Electric Facilities
Central Receivers (Power Towers)
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Solar one (USA) – pilot project
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Solar two used molten salt to store energy (helped with cloudy periods and nights)
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Decommissioned in 1999
Figure 12.24: The Solar One (and later Solar Two) 10-MW plant in Barstow, California, (Mojave desert) served
as a test facility from 1982 until 1999. Its 1926 moveable mirrors were used to heat a fluid at the top of the
tower, which was used to generate electrical power.
Solar Thermal Electric Facilities
Solar Thermal Electric Facilities
Central Receivers (Power Towers)
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PS-10 - Euope’s first commercial power tower
PS10 near Seville Spain (11 MW)
BBC News:
http://www.youtube.com/watch?v=0OkqJw1oTMk
Summary