ECE 333 Renewable Energy Systems

Lecture 18: Photovoltaic Systems, Utility Rates

Prof. Tom OverbyeDept. of Electrical and Computer Engineering

University of Illinois at Urbana-Champaignoverbye@illinois.edu

Announcements

• 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 – 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

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Bypassdiodes alsopreventoverheatingof shadedcells

Blocking Diodes

• 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

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Some currentwould flowin directionI1

Blocking Diodes

• 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

• 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– 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

7Image: http://www.ecoppia.com/solution/ketura-sun

The below image shows a robot cleaning solar panelsusing a water-free approach

Maximum Power Point Trackers

• Maximum Power Point Trackers (MPPTs) are often a standard part of PV systems, especially grid-connected

• 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

• 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 thatthe output characteristicsof the PV can be specifiedindependently 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

• 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.

• Output voltage is controlled by changing the duty cycle, D, of the switch (which operates at high freq.)– 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 – Duty cycle D is the fraction of time the switch is closed

(net change in current per cycle is zero)

( ) (1 )( ) 0

D is controlled for desirved V

LL

In Out Out

Out In Out

dIL V

dtD V V D V

V DV

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Boost DC-DC Converter

• 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

• Again analysis uses constraint that in steady-state the net current change per switching cycle in the inductor is zero

For a Buck-Boost we get

( )( ) (1 )( ) 0

(1 ) V(1 )

In In Out

InIn Out Out

D V D V V

VV D V

D

V(1 )

InOut

V DD

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MPPTs – Example

• 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

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31.9RR

VP V VR

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MPPTs – Example

• The converter must boost the 17 V PV voltage to the desired 31.9 V

• Solving gives

0

1i

V DV D

31.9 1.8817 1

DD

0.65D

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Grid-Connected Systems

• 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

– 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

• Easily allow expansion• Connections to house distribution panel are simple• Less need for expensive DC cabling

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Basic Voltage-SourcedInverter Operation• Ideally inverter takes a dc input and produces a

constant ac frequency output– Output often doesn’t look like a sine ware– Design goal is to minimize the harmonic content

Figure 6.7 from Elements of Power Electronics by Phil Krein

Filters canbe used toeliminateharmonics

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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

• 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

• Early on electric utilities were recognized as being “natural monopolies” so their rates needed to be set in some sort of public (political) process– 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.

• With 50 states, and thousands of municipals and coops, there are many different rate structures– 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

– 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– 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)

– 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)– 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 27

Time of Usage Rates

• Some utilities, including Ameren, provide the option to have electricity prices vary hourly– Prices are set day ahead

28Image: http://www.citizensutilityboard.org/pdfs/ConsumerInfo/AmerenPowerSmart.pdfPrices: https://www2.ameren.com/RetailEnergy/RealTimePrices

Commercial and Industrial Rates

• Usually commercial and industrial rates also include a "demand charge" that is based on the maximum amount of power used during a time period– 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)

• 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 30

US Average Electricity Rates

• Below graph shows average US rates (from EIA)

31Image: http://www.eia.gov/electricity/data/browser/#/topic/7?geo=g&agg=0,1&endsec=vg

Net Metering for Renewables

• 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

Feed-in Tariffs

• With feed-in tariffs there are two separate meters, one measuring the household consumption and a separate one measuring the PV generation– 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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