environmental physics chapter 12: electricity from solar, wind, and hydro copyright © 2008 by dbs

Environmental Physics

Chapter 12:Electricity from Solar, Wind, and Hydro

Copyright © 2008 by DBS

Introduction

Introduction

• 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 GRAVITATIONAL(PE/KE earth-

moon-sun)

Fossil fuels

Wind

Waves

Biomass

Hydro

Radiant

Nuclear energy

(man-made)

Geothermal

(natural)

Tides

Fig. 6-1, p. 162

3 % of total energy use

Wood and agricultural wastes

Figure 6.1: U.S. renewable energy consumption (by source), 2003.

Introduction

• 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:

• Solar cells remain expensive, growing market so costs expected to decline

• Economic parameters: Cost per Watt and cost per kWh

• 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.

Introduction

Photovoltaic (PV) generation:

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.

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.

Grid elec. 0.09 $/kWh (and rising) vs. 0.3 $/kWh PV (falling)

Introduction

• 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

• Movie from NOVA DVD


• ‘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.


• Energy of each photon depends on its frequency,

E = hf (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 cells use a variant of the photoelectric effect, but not totally ejecting electrons out of the material

• 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

http://academics.rmu.edu/faculty/short/envs1160/envs1160-demos/PV-Cell.mov


• 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

• p-n junction formed

Figure 12.4: Solar cell construction.


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

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.

This force essentially slingshots the electrons out of the cell to the metal conductor strips, generating electricity.


Single-Crystal Silicon parameters:

• p-n junction produces 0.5 V Direct Current (DC)

• Typical cell produces 100 mA/cm2 under bright sun (1000 W/m2)

• 10 cm diameter cell (100 cm2) produces around 1 W under bright sun

• 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


• 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

• Similar US race from 1990

• N. American Solar Challenge

• Improvements in efficiency of cellsrequire rule changes year to year

Cell Manufacture

Monocrystalline Si (14-18 % efficient)

• Czochralski process: Seed of crystalline Si in pure molten Si and slowly drawn out into a boule

• Sliced into wafers 0.5 mm thick and doped with P and B

Polycrystalline Si (12-14 % efficient)

• Block of Si cooled and solidified

• Consist of many grains of single crystal Si that are randomly packed

• Ribbon Silicon: Flat thin film drawn from molten Si (reduces waste from sawing ingots)

Amorphous Si (5-6 % efficient)

• Non-crystalline, disordered atomic structure

• Thin layers deposited on glass or metal and cut with laser

• Cheaper to produce but efficiency drops with time

• Materials other than Si include: gallium arsenide, cadmium telluride, cadmium sulfide, copper indium gallium diselenide

PV Systems and Economics

• 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.


• 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.


• Since output is DC amps need to convert to AC using an inverter

Figure 12.9: Photovoltaic system for a residential dwelling.


• Barely enough juice to run your lights, TV and a computer…

• What about larger devices requiring over 1 kW? Oven, AC, washing machine, toaster etc.?

• PV power plants require lots of space


• Espenhain, Germany• 5 MW - Largest PV plant in 2003• Largest are all in Spain…

Figure 12.10: 5 MW German PV installation.


http://en.wikipedia.org/wiki/Photovoltaic_power_stations

Olmedilla PV PlantSpain60 MW

http://en.wikipedia.org/wiki/Photovoltaic_power_stations

http://www.youtube.com/watch?v=ZGzfbV8bz_o

Alamosa PV PlantColorado8.2 MW

No Google Earth image yet!


• 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 (Wp) is used

• Peak power at latitude 0-50 degrees on a south facing surface is 1000 W/m2

• 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 = 4700 Wh/m2.d

0.317 Btu/ft2.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

Table 6.3 not useful here!

p. 399

PV-powered EV recharging station at the University of South Florida.


• Pumping water question on p400

• 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

• Fastest growing form of energy

DOE: Annual Report on US Wind PowerInstallation, Cost , and Performance Trends: 2006http://www.energy.gov/pricestrends/5091.htm

Wind Energy

Wind Energy

• Environmental impact almost insignificant

• Main drawback is often quoted to be

– Aesthetics

– Noise

– bird deaths

• Bird deaths caused by collisions with vehicles and windows cause far more deaths than wind turbines

Data from Erickson et al, 2005

Wind Energy

• A


Wind Energy


Wind Energy

• 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

• Commercial efforts to build more efficient turbines helped by:

– Energy tax credits

– New technology

– Public Utilities Regulatory Policies Act (PURPA)

Development slows in 1980’s due to drop in price of oil, more natural gas usage, expiration of tax credits

1998-2003345 %

1998-2006600 %

Wind Energy


Wind Energy

• More than 30,000 turbines worldwide (2003 figure) > 75,000 MW (2006 figures)

• Denmark supplies > 21 % of its electricity with the wind


Wind Energy

• 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


Wind Energy

Wind Energy Systems

• Blades, tail, shaft, gear box, generator, tower, transmission system

• DC output can be stored in batteries

• 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

• 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

• This limit has nothing to do with inefficiencies in the generator, but in the very nature of wind turbines themselves

• 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

• 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

• Using the theoretical wind power equation we get the following power:

Wind Energy

• 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

• 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

• Vertical axis – e.g. Darrieus “Eggbeater” rotor

• Uses aerofoils, not propellers

• Do not have to shift with changes in wind direction(useful where wind direction is highly variable)

• Gearbox and generator at ground and not on tower

• Problems:

– Difficult to raise their height

– Difficulty self starting

– Unstable in high winds

• Also Giromills and Savonius type turbines

Figure 12.16: Darrieus rotor (250 kW) in Altamont Pass, California wind farm.

Giromill

• e.g. Wind Spire

• 1.2 kW 30 foot tower

http://www.youtube.com/watch?v=VxoA8rnJj0U

Savonius

• e.g. HelixWind

• 2 or 4 kW

http://crispyneurons.com/wiki/HelixWind

• 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

• 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

• On average house require 600 kWh/month or 20 kWh/d

• 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!

• 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

• Winds are intermittent

• If energy cannot be sold to the grid it must be stored

Batteries

• 10 x 12V DC lead acid batteries would provide 120 V (mains voltage)

• 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

• Energy may be stored by pumping water into a reservoir, can be used for hydropower

Wind Energy

Wind Energy

• 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

Wind PowerClass 4 and above neededSource: DOE

Figure 12.18: Map of U.S. wind energy potential.

http

://n

avig

ator

.aw

stru

ewin

d.co

m

http://www.pawindmap.org/index.htmhttp://www.actionpa.org/cleanenergy/wind.html

End

• Review

Hydropower

• Energy produced from potential energy (mgh) of the water• Supplies 19% worlds electricity• Poor environmental record

– Dam construction floods crop lands

• Micro hydro-power is becoming increasingly popular (small stream)

Head height

Question

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 (since ρ = 1000 kg m-3)

Power in kW = φ gh

Derive an equation for power generated from a hydroelectric station relating power to rate of fluid flow, φ (=v/t) (m3 s-1).

Hydropower

• 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.

Fig. 12-21b, p. 416

Figure 12.21: Models of waterwheels or turbinesImpulse Type: (a) Breast Wheel , (b) Overshoot Wheel,Reaction Type: (c) Francis Turbine, (d) Kaplan or Propeller Turbine

Hydropower

• 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

P = φ gh x efficiency

P = 0.3 m3 s-1 x 10 m s-2 x 10 m x 0.5 = 15 kW

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%.

http://www.itdg.org/?id=micro_hydro_faq

Solar Thermal Electric Facilities

• 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


Parabolic Trough Systems

• Concentrate sunlight onto fluid filled tube 400 °C (752 ° F)

• Heat exchanger produces steam used to generate power

• Efficiency ~25 %

73

Concentrating Solar Thermal

Photo: Schott Glass


Figure 12.23: Solar Electric Generating System (SEGS), Kramer Junction, California, provides 165 MW from concentrating collectors shown here.


Central Receivers (Power Towers)

• Use heliostats to track the sun and reflect energy onto a central receiver

• Unlike trough systems as they can track both N-S and E-W

• Prototype plants no longer in use in USA



• Solar one (USA) – pilot project

• Solar two used molten salt to store energy (helped with cloudy periods and nights)

• 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.



• PS-10 - Euope’s first commercial power tower

PS10 near Seville Spain (11 MW)

BBC News:http://www.youtube.com/watch?v=0OkqJw1oTMk

Summary

environmental physics chapter 12: electricity from solar, wind, and hydro copyright © 2008 by dbs

Documents