ECE 333 Renewable Energy Systems Lecture 20: Photovoltaic Systems Prof. Tom Overbye Dept. of Electrical and Computer Engineering University of Illinois at Urbana-Champaign 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 2 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 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 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 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/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 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 generator9 Principal Components of Grid- Connected System 10 Figure 6.2 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 12 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: Losses from Mismatched Modules Illustrates the impact of slight variations in module I-V curves Only 330 W is possible instead of 360 W 13 This issue can be partially addressed with micro- inverters Peak-Hours Approach The peak-hours approach treats the expected insolation (over days, months, years, etc.) as hours of one sun is 1 kW/m 2 We can say that 5.6 kWh/(m 2 -day) is 5.6 hours of one sun (or peak sun) If we know P ac, computed for one sun, just multiply by hours of peak sun to get kWh 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 15 Note, assumed tilt angle is for a typical residential roof pitch of 4 in 12; this can be higher in snowy areas 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 kWh/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 m 2 Use SunPower 240W modules, with V oc = 48.6 V, I sc = 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 SunPower SPR-5000m inverter Rated efficiency of 95.5%, peak of 96.5% MPP tracking input range of V Maximum of V Need to stay less than 600 V even on cold days, when 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 kWh/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 21 Source: 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 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) 24 https://openpv.nrel.gov/breakeven 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 kW 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 $/kWh? 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 26 These values vary linearly with the assumed PV installed cost 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 27 https://qzprod.files.wordpress.com/2014/11/us-consumer-price-indexes-year-on-year-change-core-cpi-headline-cpi_chartbuilder.png?w=1280