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

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 2 nd 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) 1

Bypass Diode Impact on Module 2 Bypass diodes also prevent overheating of shaded cells

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 3 Some current would flow in direction I 1

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 4

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 5

PV and Dust: Robots Cleaning the Panels 6 Image: The below image shows a robot cleaning solar panels using 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 7

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 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 8

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. 9

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 10

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. 11

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 12

MPPTs – Example A PV module has its maximum power point at V m = 17 V and I m = 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 13

MPPTs – Example The converter must boost the 17 V PV voltage to the desired 31.9 V Solving gives 14

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 15

Components of Grid-Connected PV 16

Individual Inverter Concept Easily allow expansion Connections to house distribution panel are simple Less need for expensive DC cabling 17

Basic Voltage-Sourced Inverter 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 can be used to eliminate harmonics 18

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 19

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 20

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 21

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 22

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: ¢/kWh – Tier 2: ¢/kWh – Tier 3: ¢/kWh – Tier 4: ¢/kWh 23

Eastern Illini Electric Coop Rates 24

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: ¢/kWh – Transmission: ¢/kWh – Generation: 3.767¢/kWh first 1000 kWh, then 2.647¢/kWh – Total: ¢/kWh first 1000 kWh, then ¢/kWh Rate 20 (Electric Heat, Single Phase) – Same categories, base is $50 per month, similar rates in summer (4 months); winter rate >1000 is ¢/kWh – 25

Sample Ameren Bill 26

Time of Usage Rates Some utilities, including Ameren, provide the option to have electricity prices vary hourly – Prices are set day ahead 27 Image: Prices:

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 28

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, $ Winter, $0.114 Summer Small Business: $8.38 monthly, $0.095 Winter, $ Summer, Demand Charge $8/kW Winter, $9.68/kW Summer Large Industrial: $ monthly, $ Winter, $ Summer, Demand Charge: $11.46/kW Winter, $14.63/kW Summer Source: 29

US Average Electricity Rates Below graph shows average US rates (from EIA) 30 Image:

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 31

Net Metering for Renewables 32 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 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 33