Transcript Lecture #18 - Course Website Directory

ECE 333
Renewable Energy Systems
Lecture 18: Photovoltaic Systems,
Utility Rates
Prof. Tom Overbye
Dept. of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
[email protected]
Announcements
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Quiz on HW 7 today
HW 8 is 5.4, 5.6, 5.11, 5.13, 6.5, 6.19; it should be done
before the 2nd exam but need not be turned in and there
is no quiz on April 9.
Read Chapter 6, Appendix A
Exam 2 is on Thursday April 16); closed book, closed
notes; you may bring in standard calculators and two 8.5
by 11 inch handwritten note sheets
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In ECEB 3002 (last name starting A through J) or in
ECEB 3017 (last name starting K through Z)
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Bypass Diode Impact on Module
Bypass
diodes also
prevent
overheating
of shaded
cells
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Blocking Diodes
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Consider strings wired in parallel, where one string is in
the shade
We want to prevent current from being drawn instead of
supplied by that string
Some
current
would flow
in direction
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Blocking Diodes
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Solution – blocking diode at the top of each string
Forward biased during normal operation, reverse
biased when the string is shaded
Since they are conducting during normal operation,
they cause an output voltage drop of ~0.6 V
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PV and Dust
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Dust can degrade the performance of a PV system,
sometimes rather significantly (> 15%)
How much dust settles on a PV panel depends on a
number of characteristics
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Amount of dust in the environment, humidity, wind, rain, tilt
of the panel, panel surface finish, how often it is cleaned
Dust attracts dust!
Dust can be reduced by 1) manual cleaning but this
requires water and can be time consuming, 2) surface
treatments, 3) electrostatic charge using surface
material to repel dust, 4) robots
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PV and Dust: Robots Cleaning the
Panels
The below image shows a robot cleaning solar panels
using a water-free approach
Image: http://www.ecoppia.com/solution/ketura-sun
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Maximum Power Point Trackers
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Maximum Power Point Trackers (MPPTs) are often
a standard part of PV systems, especially gridconnected
Idea is to keep the operating point near the knee of
the PV system’s I-V curve
Buck-boost converter – DC to DC converter, can
either “buck” (lower) or “boost” (raise) the voltage
Varying the duty cycle of a buck-boost converter
can be done such that the PV system will deliver the
maximum power to the load
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DC-DC Converters
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PV operational goal is often to operate at the
maximum power point. This requires that the
apparent load resistance vary as the operating
conditions vary.
We want a design such that
the output characteristics
of the PV can be specified
independently from the load,
ideally with 100% efficiency
Several dc-dc converter topologies: Buck, Boost,
Buck-Boost; we’ll cover the Buck and the Boost
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Buck DC-DC Converter
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The buck converter always decreases the voltage.
Converters make use of inductors and capacitors as
energy storage devices
Basic circuit topology: assume the capacitor is large
so the output voltage stays relatively constant.
Assume diode is ideal.
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Buck DC-DC Converter, cont.
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Output voltage is controlled by changing the duty
cycle, D, of the switch (which operates at high freq.)
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Example switches are a insulated-gate bipolar transistor
(IGBT) or a silicon controlled rectifier (SCR)
When the switch is closed the current in the inductor
increases, then decreases when it is open
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Duty cycle D is the fraction of time the switch is closed
dI L
L
 VL (net change in current per cycle is zero)
dt
D(VIn  VOut )  (1  D)(VOut )  0
VOut  DVIn
D is controlled for desirved VOut
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Boost DC-DC Converter
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Used when output voltage is above input voltage
Assume L and C are sufficiently large so we can
treat L as a current source, and C as a voltage source
When switch closed, diode is reverse biased, so
inductor current increases. When open, inductor
drives current into the diode.
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Boost DC-DC Converter
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Again analysis uses constraint that in steady-state
the net current change per switching cycle in the
inductor is zero
( D)(VIn )  (1  D)(VIn  VOut )  0
VIn  (1  D)VOut  VOut
For a Buck-Boost we get
VOut
VIn

(1  D)
 VIn D

(1  D )
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MPPTs – Example
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A PV module has its maximum power point at Vm =
17 V and Im = 6A.
What duty cycle should its MPPT have if the module
is delivering power to a 10Ω resistance?
Max power delivered by the PVs is 17V*6A = 102W
VR 2
P
R
VR  31.9V
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MPPTs – Example
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The converter must boost the 17 V PV voltage to the
desired 31.9 V
31.9  D 
V0
 D 

 
  1.88

17  1  D 
Vi
 1 D 
Solving gives D  0.65
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Grid-Connected Systems
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Can have a combiner box and a single inverter or
small inverters for each panel
Individual inverters make the system modular
Inverter sends AC power to utility service panel
Power conditioning unit (PCU) may include
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MPPT
Ground-fault circuit interrupter (GFCI)
Circuitry to disconnect from grid if utility loses power
Battery bank to provide back-up power
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Components of Grid-Connected PV
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Individual Inverter Concept
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Easily allow expansion
Connections to house distribution panel are simple
Less need for expensive DC cabling
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Basic Voltage-Sourced
Inverter Operation
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Ideally inverter takes a dc input and produces a
constant ac frequency output
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Output often doesn’t look like a sine ware
Design goal is to minimize the harmonic content
Filters can
be used to
eliminate
harmonics
Figure 6.7 from
Elements of Power
Electronics by Phil
Krein
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Stand-Alone PV Systems
•When the grid isn’t nearby, the extra cost and
complexity of a stand-alone power system can be worth
the benefits
•System may include batteries and a backup generator
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Stand-Alone PV - Considerations
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PV System design begins with an estimate of the
loads that need to be served by the PV system
Tradeoffs between more expensive, efficient
appliances and size of PVs and battery system needed
Should you use more DC loads to avoid inverter
inefficiencies or use more AC loads for convenience?
What fraction of the full load should the backup
generator supply?
Power consumed while devices are off
Inrush current used to start major appliances
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Electric Utility Rates
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Early on electric utilities were recognized as being
“natural monopolies” so their rates needed to be set in
some sort of public (political) process
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Also involved an “obligation to serve”
Three main types of utilities: Investor Owned (IOUs),
Municipals (owned by city) or Coops (owned by
members). Rates for IOUs are set through a process
that involves state regulators.
Initially bill was based on the number of light bulbs,
later replaced by electric meters
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Electric Utility Rates, Cont.
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With 50 states, and thousands of municipals and coops,
there are many different rate structures
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Simplest is flat rate per kWh used with many complications
possible: fixed charges, increasing or decreases rates based
on amount used, seasonal and time-of-day rates, real-time
(hourly) pricing, capacity charges, minimum power factor
charges; different for residential, commercial, industrial
In many locations energy might be supplied by a third
party resulting in a transmission and distribution
charges plus an energy charge. Taxes may abound!!
Which is best? Incremental rates important when
considering renewable additions
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Example PGE Rates
(from Section 6.4.4), 2012
• Example of rates increasing with demand
• Rates are set based on usage
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Tier 1: < 365 kWh
Tier 2: From 365 to 475 kWh
Tier 3: From 475 to 730 kWh
Tier 4: Greater than 730 kWh
Rates
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Tier 1: 12.85 ¢/kWh
Tier 2: 14.90 ¢/kWh
Tier 3: 29.56 ¢/kWh
Tier 4: 33.56 ¢/kWh
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Eastern Illini Electric Coop Rates
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Eastern Illini Electric Coop Rates,
2015
• Rate 1 (General Service, Single Phase)
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Base charge $40 per month
All following charges per
Delivery: 3.767¢/kWh first 1000 kWh, then 1.767¢/kWh
Energy: 3.432 ¢/kWh
Transmission: 1.433 ¢/kWh
Generation: 3.767¢/kWh first 1000 kWh, then 2.647¢/kWh
Total: 12.399 ¢/kWh first 1000 kWh, then 9.279 ¢/kWh
Rate 20 (Electric Heat, Single Phase)
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Same categories, base is $50 per month, similar rates in
summer (4 months); winter rate >1000 is 7.346 ¢/kWh
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Sample Ameren Bill
https://www.ameren.com/illinois/csc/bill-sample-2
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Time of Usage Rates
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Some utilities, including Ameren, provide the option to
have electricity prices vary hourly
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Prices are set day ahead
Prices: https://www2.ameren.com/RetailEnergy/RealTimePrices
Image: http://www.citizensutilityboard.org/pdfs/ConsumerInfo/AmerenPowerSmart.pdf
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Commercial and Industrial Rates
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Usually commercial and industrial rates also include a
"demand charge" that is based on the maximum amount
of power used during a time period
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Demand charge value is usually measured over some time
period, such as 15 minutes
It can also be time dependent, such as different values for
summer and winter
Because of the demand charge, the rates paid by
commercial and industrial users are lower
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CWLP Rates (Springfield, IL) (2012)
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Residential, general: $4.67 monthly, $0.0988/kWh Winter,
$0.114/kWh Summer
– Some seniors (62 up) get about a 10% discount
Residential, all electric: $4.67 monthly, $0.0895 Winter, $0.114
Summer
Small Business: $8.38 monthly, $0.095 Winter, $0.1034
Summer, Demand Charge $8/kW Winter, $9.68/kW Summer
Large Industrial: $577.13 monthly, $0.0722 Winter, $0.0781
Summer, Demand Charge: $11.46/kW Winter, $14.63/kW
Summer
Source: http://www.cwlp.com/customer/rates/elecres.html
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US Average Electricity Rates
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Below graph shows average US rates (from EIA)
Image: http://www.eia.gov/electricity/data/browser/#/topic/7?geo=g&agg=0,1&endsec=vg
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Net Metering for Renewables
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Issue with small scale renewables is whether the
generated electricity is sold back to the utility at the
retail rate or the lower wholesale (avoided cost rate)
Net metering allows the customer to offset their own
electric usage, and sometimes sell power back to the
utility at the specified retail rate (meter runs
backwards)
Requirements for net metering by IOUs are often set
by the states; municipals and coops are self-governing
Potential concern about utilities providing services
with costs born by the other customers
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Net Metering for Renewables
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Below graph shows a typical residential situation with
a single meter – note, meter is running backwards when
the PV power is greater than local usage
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Feed-in Tariffs
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With feed-in tariffs there are two separate meters, one
measuring the household consumption and a separate
one measuring the PV generation
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This allows the possibility that PV generation can be
purchased at quite high rates
Feed-in tariffs started in Europe, but are now used some in the
US (website http://www.pv-tech.org/tariff_watch/list
summarizes them)
Current ones in Germany are $0.19/kWh for less than 10 kW,
$0.18 for between 10 and 40 kW; on Hawaii it is
$0.21.8/KWh for less than 20 kW OV
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