ECE 333 Renewable Energy Systems Lecture 20 Photovoltaic

ECE 333 Renewable Energy Systems Lecture 20: Photovoltaic Systems Prof. Tom Overbye Dept. of Electrical and Computer Engineering University of Illinois at Urbana-Champaign overbye@illinois. edu

Announcements • • • 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 today. 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

Buck Converters and Computers Trend is towards lower voltages; current designs can be >80% efficient at less than 1 V; data centers use about 2. 5% of total US electricity consumption in 2013 2

Improved • One solution to the low voltage supply efficiency problem is to stack the processors 3

Contrasting Electric Rates: PGE • • Example of rates increasing with demand Rates are set based on usage – – • Tier 1: < 365 k. Wh Tier 2: From 365 to 475 k. Wh Tier 3: From 475 to 730 k. Wh Tier 4: Greater than 730 k. Wh Rates – – Tier 1: 12. 85 ¢/k. Wh Tier 2: 14. 90 ¢/k. Wh Tier 3: 29. 56 ¢/k. Wh Tier 4: 33. 56 ¢/k. Wh 4

Contrasting Electric Rates: Eastern Illini Electric Coop • Rate 1 (General Service, Single Phase) – – – – • Base charge $40 per month All following charges per Delivery: 3. 767¢/k. Wh first 1000 k. Wh, then 1. 767¢/k. Wh Energy: 3. 432 ¢/k. Wh Transmission: 1. 433 ¢/k. Wh Generation: 3. 767¢/k. Wh first 1000 k. Wh, then 2. 647¢/k. Wh Total: 12. 399 ¢/k. Wh first 1000 k. Wh, then 9. 279 ¢/k. Wh 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 ¢/k. Wh 5

Four Ways to Connect PV Systems • • Grid-connected system without storage Stand-alone system Pumping Microgrid 6

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. 7

Grid-Connected System • This is a quite common system; the PCU includes the MPPT as well as the inverter; an alterative is to have microinverters on the individual modules

Interfacing with the Utility • • • Net metering – customer only pays for the amount of energy that the PV system is unable to supply Good grid-connect inverters have efficiencies above 90% In the event of an outage, the PV system must quickly and automatically disconnect from the grid, though it could be used with a standby generator http: //www. pasolar. ncat. org/lesson 05. php 9

Principal Components of Grid. Connected System Figure 6. 2 10

Predicting Performance • • • PV modules are rated under one sun at 25 C based on their maximum dc output What is delivered ac is this value times a derate factor Table 6. 2 lists varies components of this derating, which include the inverter efficiency, nameplate rating (not all modules off the assembly line produced rated), wiring losses, isolation transformers, soiling (dirt, snow), module mismatch, aging – – Aging is expected to be about 0. 5% per year (crystal silicon) Actual output might be 80% of rated in full sun 11

Temperature Related Derating • Reduction in efficiency appears to be close to linear over a wide range of temperatures A ballpark figure is around 0. 3% per degree C; if the cell temperature is below 25 C then the output can be above the tested value. Note this is the cell temperature, not ambient. Image: http: //energy-models. com/basic-photovoltaic-principles-and-methods 12

Losses from Mismatched Modules • • Illustrates the impact of slight variations in module I -V curves Only 330 W is possible instead of 360 W This issue can be partially addressed with microinverters 13

“Peak-Hours” Approach • • The peak-hours approach treats the expected insolation (over days, months, years, etc. ) as hours of one sun is 1 k. W/m 2 We can say that 5. 6 k. Wh/(m 2 -day) is 5. 6 hours of one sun (or “peak sun”) If we know Pac, computed for one sun, just multiply by hours of peak sun to get k. Wh If we assume the average PV system efficiency over a day is the same as the efficiency at one sun, then 14

Capacity Factor of PV Note, assumed tilt angle is for a typical residential roof pitch of 4 in 12; this can be higher in snowy areas 15

Practical Considerations • How much solar can be produced depends on the available area – • Also with net metering there can be limitations on total production not exceeding household consumption Maximum voltages and currents also need to be considered – – National Electric Code restricts residential wiring to no more than 600 V; usually 48 V is considered "safe" Wire size increases with current 16

Example 6. 4 • Size a system in Silicon Valley to supply 5000 k. Wh/yr using a 4 in 12 roof pitch – Assume an average daily insolation is 5. 32 hours of peak sun and a total derate of 0. 75 • This requires a system with a dc capacity of • • Assuming 19% efficiency this requires 18. 05 m 2 Use Sun. Power 240 W modules, with Voc = 48. 6 V, Isc= 6. 3 A; this will require 3. 42/0. 24 = 14. 3 modules 17

Example 6. 4, cont. • • If using a single inverter, then how the panels are configured depends on the inverter Example uses a Sun. Power SPR-5000 m inverter – – – • Rated efficiency of 95. 5%, peak of 96. 5% MPP tracking input range of 250 -480 V Maximum of 250 -600 V Need to stay less than 600 V even on cold days, when http: //www. stellarsolar. net/downloads/SPR_2007 -08 -02 SPR-5000 m-SPR-6000 m-SPR-7000 m. Data. Sheet. pdf 18

Example 6. 4, cont • And • This gives constraints that • Also, look at high temperature conditions • • Hence So two strings of seven modules would be good, giving 4893 k. Wh/day 19

Rooftop PV and Fires • Key issue is electric shock for firefighters – – • • Electric shock is not unique to PV systems, but with voltages up to 600 V dc they can be quite dangerous Other issues are slipping hazards, increased roof load causing a collapse during a fire, burning hazardous materials, and battery hazards such as flammable gas Firefighters are already familiar with disconnecting electric and can usually hear if there is a backup generator; in contrast a solar system may be silently energized Issues need to be considered when fighting a fire 20

PV System Economics • PV total system costs have been decreasing, with the key metric $/W of peak dc power; historical US data is shown below Source: www. nrel. gov/docs/fy 13 osti/56776. pdf 21

Update to Data Pricing • A good source for current prices is NREL's Open PV project, which provides customer reported prices – Openpv. nrel. gov; they say $4. 53/W in 2013 22

NREL Open PV Illinois Data 23

Breakeven Prices • Breakeven analysis looks at the required PV price for the cost of a solar system to match electricity purchased from the grid – The value depends upon lots of assumptions, such as the location, size of the system, system parameters (tilt, derate, annual degradation, electricity prices, tax credits, financing, etc) https: //openpv. nrel. gov/breakeven 24

Amortizing PV Costs • Simple payback is the easiest analysis, which assumes there is no cost for money and no inflation. Annual cost is just total cost divided by lifetime – • This can give a quick ballpark figure Example: Assume 5 k. W system with a capacity factor of 18%, an installed cost of $ 5/W (after tax credits), and a lifetime of 20 years with no maintenance costs. What is $/k. Wh? 25

Amortizing PV Costs • • More detailed analysis uses the capital recovery factor using an assumed discount rate Redo the previous example using a discount rate of 5% per year These values vary linearly with the assumed PV installed cost 26

Complications • • • "It's tough to make predictions, especially about the future", Yogi Berra (a baseball player/coach appearing as a player or coach in 21 world series) There is uncertainly about the rate of electric rate inflation, and the decreasing costs of solar panels Also, how long will you own the house, how is PV included in home's value https: //qzprod. files. wordpress. com/2014/11/us-consumer-price-indexes-year-on-year-change-core-cpi-headline-cpi_chartbuilder. png? w=1280 27