ECE 310 by hilen

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									ECE 333
Green Electric Energy

Lecture 10
Electric Power Operations

Professor Tom Overbye
Department of Electrical and Computer Engineering

Announcements
• • • •
Be reading Chapter 4 First exam is Oct 8 in class (as specified on syllabus Homework 4 is due now Homework 5 is 4.2, 4.4, 4.5, special problems 3 and 4; it is due on Thursday Oct 1.

Special Problem 3: As presented in class, explain how the area control error is calculated (note the definition presented in class is a simplification of what occurs in practice). Special Problem 4: Briefly discuss the advantages and disadvantages of one method presented in class for charging for electric power transfers.

In the News: DOE Secretrary Chu Presentation at Grid Week, 9/21/09

DOE Secretary Chu “Grid Week” Presentation, Sept 21, 2009, Slide 11

In the News: DOE Secretrary Chu Presentation at Grid Week, 9/21/09
Note, in 2007 total electric generation in the US was 4,156 billion kWh

DOE Secretary Chu “Grid Week” Presentation, Sept 21, 2009, Slide 5

Pricing Electricity
• • • • •
Cost to supply electricity to bus is called the locational marginal price (LMP) Presently PJM and MISO post LMPs on the web In an ideal electricity market with no transmission limitations the LMPs are equal Transmission constraints can segment a market, resulting in differing LMP Determination of LMPs requires the solution on an Optimal Power Flow (OPF)

Three Bus Case LMPs: Line Limit NOT Enforced
Gen 2’s cost is $12 per MWh
Bus 2
60 MW 60 MW

Bus 1 10.00 $/MWh

0 MW

10.00 $/MWh
120 MW

120%

180 MW

0 MW
60 MW

Gen 1’s cost is $10 per MWh

Total Cost 1800 $/hr

120%
60 MW

120 MW

Bus 3

10.00 $/MWh
180 MW

0 MW

Line from Bus 1 to Bus 3 is over-loaded; all buses have same marginal cost

Three Bus Case LMPS: Line Limits Enforced
Bus 2
20 MW 20 MW

Bus 1 10.00 $/MWh

60 MW

12.00 $/MWh
100 MW

80%

100%

120 MW

0 MW
80 MW

Total Cost 1921 $/hr

80%
80 MW

100%

100 MW

Bus 3

14.01 $/MWh
180 MW

0 MW

Line from 1 to 3 is no longer overloaded, but now the marginal cost of electricity at 3 is $14 / MWh

Generation Supply Curve
As the load goes up so does the price
80

Price ($ / MWh)

60

40

20

Base Load Coal and Nuclear Generation
0 10000

Natural Gas Generation

0 20000 Generation (MW) 30000 40000

Renewable Sources Such as Wind Have Low Marginal Cost, but they are Intermittent

MISO LMPs on Feb 24, 2009 (8:35am)

Prices were < $-30/MWh in Minnesota (paid to use electricity) Available on-line at www.midwestmarket.org

Frequency Control
•
Steady-state operation only occurs when the total generation exactly matches the total load plus the total losses
– –

too much generation causes the system frequency to increase too little generation causes the system frequency to decrease (e.g., loss of a generator)

•

AGC is used to control system frequency

April 23, 2002 Frequency Response Following Loss of 2600 MW

Distributed Generation (DG)
• • • •
Small-scale, up to about 50 MW Includes renewable and non-renewable sources May be isolated from the grid or grid-connected Near the end user

Integrated Generation, Transmission, Buildings, Vehicles
Renewables

Grid
kWh kWh Smart meters PHEV

Vehicle-to-Grid Heat kWh

N. Gas

Combined Heat and Power (CHP)

Source: Masters

Pluggable Hybrid Electric Vehicles (PHEVs) as Distributed Generation • Can charge at night
when electricity is cheap

Source: http://www.popularmechanics.com/automotive/new_cars/4215489.html

•

Can provide services back to the grid
Source: www.calcars.org

DG Technologies
• • • •
Microturbines Reciprocating Internal Combustion Engines Stirling-Cycle Engine Concentrating Solar Power (CSP)
– – –

Solar Dish/Sterling Parabolic Troughs Solar Central Receiver

• • •

Biomass Micro-Hydro Fuel Cells

Reasons for Distributed Generation
• • • • •
Good for remote locations Renewable resources Reduced emissions Can use the waste heat Can sell power back to the grid

Terminology
• •
Cogeneration and Combined Heat and Power (CHP)
–

capturing and using waste heat while generating electricity

•

When fuel is burned one product is water; if water vapor exits stack then its energy is lost (about 1060 Btu per pound of water vapor) Heat of Combustion for fuels
– – –

Higher Heating Value (HHV) – gross heat, accounts for latent heat in water vapor Lower Heating Value (LHV) – net heat, assumes latent heat in water vapor is not recovered Both are used - Conversion factors (LHV/HHV) in Table 4.2

HHV and LHV Efficiency
•
Find LHV efficiency or HHV efficiency from the heat rate:

HHV( LHV )

3412 Btu/kWh  Heat Rate (Btu/kWh) HHV( LHV )

(3.16)

•

Convert to get the other efficiency:
HHV
 LHV   LHV   HHV   (4.1)

Note the LHV is less than the HHV

Microturbines
•
Small gas turbines, 500 W to 100s kW Only one moving part Combined heat and power High overall efficiency 80% CHP
Efficiency 230 kW fuel 120 kW hot water output

• • •

65 kW electrical output

45 kW waste heat

Capstone 65 kW Microturbine
Source: http://www.capstoneturbine.com

Microturbines
1. 2.
Incoming air is compressed Moves into cool side of recuperator & is heated Mixes with fuel in combustion chamber Expansion of hot gases spins shaft Exhaust leaves

3. 4.
5.

Figure 4.1

Reciprocating Internal Combustion Engines (ICEs)
• • • • • • • •
Piston-driven Make up a large fraction of the DGs and CHP today From 0.5 kW to 6.5 MW Electrical efficiencies ~37-40% Can run on gasoline, natural gas, kerosene, propane, fuel oil, alcohol, and more Relatively clean for burning natural gas Most are four-stroke engines Waste heat for cogeneration

Four-Stroke Engines
1. 2. 3. 4.
Intake Compression Power Exhaust

Figure 4.3

Two-Stroke Engines
• • • • •
A compression stroke and a power stroke Intake and exhaust open at end of power stroke, close at start of compression stroke Greater power for their size Less efficient Produce higher emissions

Spark-Ignition (Otto-cycle)
• • •
Easily ignitable fuels like gasoline and propane Air-fuel mixture enters cylinder during intake Combustion initiated by externally-timed spark

Compression-Ignition (Diesel-cycle)
• • • •
Diesel or fuel oil Fuels not premixed with air Fuel injected under high pressure into cylinder towards end of compression cycle Increase in pressure causes temperature to rise until spontaneous combustion occurs, initiates power stroke

Diesel Engines
• • • •
More sudden, explosive ignition – must be built stronger and heavier Higher efficiencies Require more maintenance Higher emissions

Charged Aspiration
• • • •
Increases efficiency of ICEs Pressurize air before it enters the cylinder Turbocharger or supercharger Able to lower combustion temperature and lower emissions

Advanced Reciprocating Engines Systems (ARES) Project
• •
US Department of Energy Goals
– – –

Check it out online: http://www.eere.energy.gov/de/gas_fired/

–
–

50% (LHV) electrical efficiency by 2010 Available, reliable, and maintainable Reduce NOX emissions Fuel flexibility Lower cost

Source: http://www.ornl.gov/sci/de_materials/documents/posters/ARESOverview.pdf

Stirling Engines
• • •
An external combustion engine Energy is supplied to working fluid from a source outside the engine Poor-quality steam engines used to explode, and Stirling engines operate at low pressures Used extensively until early 1900s Now – can convert concentrated sunlight into electricity

• •

Stirling Engines
• • • •
Two pistons in same cylinder- left side hot, right side cold Regenerator – short term energy storage device between the pistons Working fluid permanently contained in the cylinder Four states, four transitions

Stirling Engines – State 1
•
State 1
– – –

Cool gas Max volume Min pressure

•

1 to 2
–
–

Cold piston moves left Gas compresses
Figure 4.6

Stirling Engines – State 2
•
State 2
–

–

Compressed gas rejects heat to cold sink Min volume Both pistons move left Gas flows through regenerator & warms up

•

2 to 3
– –

Figure 4.6

Stirling Engines – State 3
•
State 3
– – –

Hot gas Min volume Max pressure

•

3 to 4
–
–

Gas heats Hot gas drives hot piston to left in power stroke
Figure 4.6

Stirling Engines – State 4
• •
State 4
– –

Hot gas Max volume Both pistons move right Gas flows through regenerator & cools off Back to State 1

4 to 1
– –

–

Figure 4.6

Stirling Engines
• • • •
Efficiency ~ less than 30% Less than 1 kW to ~25 kW Inherently quiet Cogeneration possible with cooling water for the cold sink

Concentrating Solar Power Technologies (CSP)
• • •
Basic idea: Convert sunlight into thermal energy, use that energy to get electricity Concentration is needed to get a hot enough temperature Three successfully demonstrated technologies:
–
– –

Parabolic Trough Solar Central Receiver Solar Dish/ Sterling

•

This is a different topic than photovoltaic (PV) cells which we’ll cover later

Solar Dish/ Sterling
• • • •
Multiple mirrors that approximate a parabolic dish Receiver – absorbs solar energy & converts to heat Heat is delivered to Stirling engine Average efficiencies >20%

Source: http://www.eere.energy.gov/de/csp.html

Solar Dish/ Stirling
• •
25 kW system in Phoenix, AZ Developed by SAIC and STM Corp

Stirling engine, generator, and cooling fan

Source:http://commons.wikimedia.org

Parabolic Troughs
• • • •
Receivers are tubes - Heat collection elements (HCE) Heat transfer fluid circulates in the tubes Delivers collected energy to steam turbine/generator Parabolic mirrors rotate east to west to track the sun

Source: http://www.eere.energy.gov/de/csp.html

Source: http://www.nrel.gov/csp/troughnet/solar_field.html

Parabolic Troughs - SEGS

Source: http://www.flagsol.com/SEGS_tech.htm

• • •

Mojave Desert, California Aerial view of the five 30MW parabolic trough plants Solar Electric Generation System Source: http://www.flagsol.com/SEGS_tech.htm (SEGS)

Solar Central Receiver
• • •
Also called Power Towers Heliostats – computer controlled mirrors Reflect sunlight onto receiver

Source: http://www.eere.energy.gov/de/csp.html

Solar Central Receiver – Solar Two
• • •
10 MW Two-tank, molten-salt thermal storage system Barstow, CA

Source: http://www.trec-uk.org.uk/csp.htm

Supplementing CSP
• •
Hybrid Systems
–

Conventional generation as a backup Effectively makes solar power dispatchable Storage is still a largely unsolved issue

Thermal Energy Storage
– –

CSP Thermal Energy Storage
• • •
SEGS I (operated 1985-1999) – two tank energy storage system – mineral oil heat transfer fluid to store energy German Aerospace Center – High-temperature concrete or ceramics – Pipes are embedded, transfer energy to media Solar Two – Molten-Salt Heat Transfer Fluid

CSP Comparisons
• • •
All use mirrored surfaces to concentrate sunlight onto a receiver to run a heat engine All can be hybridized with auxiliary fuel sources Higher temperature -> higher efficiency
Annual Measured Efficiency Dish Stirling Parabolic Troughs Solar Central Receiver 21% 14% 16% Required Acres/MW 4 5 8 “Suns” of concentration 3000 100 1000

Biomass
• • • • • • •
Use energy stored in plant material 14 GW around the world, half in US 2/3 of biomass in US is cogeneration Little to no fuel cost High transportation costs Low efficiencies, <20% Leads to expensive electricity

Gas Turbines and Biomass
• • • •
Cannot run directly on biomass without causing damage Gassify the fuel first and clean the gas before combustion Coal-integrated gasifier/gas turbine (CIG/GT) systems Biomass-integrated gasifier/gas turbine (BIG/GT) systems

Cofiring
• • • •
Burn biomass and coal Modified conventional steam-cycle plants Allows use of biomass in plants with higher efficiencies Reduces overall emissions

Biomass plant in Robbins, IL
• •
GE is converting the plant to generate power from 3’’ wood chips made from scrap lumber Photos from PES field trip last year

Biomass plant in Robbins, IL


								
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