EE462L, Spring 2014 PV Arrays (Solar Panels)

1. EE462L, Spring 2014PV Arrays (Solar Panels)

2. Diode current 5 External circuit – Diode Amps (e.g. , battery, BV V - Isc A ( e 1 ) lights) + I 0 Diode Volts 0.0 0.6 Electrical Properties of a Solar Cell Photons External circuit n - type – Junction (e.g. , battery, V lights) + p - type I

3. I-V Curve

4. 36 Cells in Series Make a 12V-Class Panel (Voc  19V) 9 cells x 4 cells is a common configuration • Two 12V-Class Panels in Series Make a 24V-Class Array (Voc  38V)

5. I-V Curve Isc Pmax at approx. 30V Isc Pmax 0.7 • Voc • Isc Voc

6. Pmax The Maximum Power Point P=0 at short circuit P=0 at open circuit On a good solar day in Austin, you get about 1kWh per square meter of solar panels (corresponds to about 150W rated)

7. Earth’s Poles • Magnetic poles: Created by Earth’s magnetic field • Can be located with a compass • They move along Earth’s surface! • Celestial poles: Created by Earth’s rotation. • They are two imaginary stationary points in the sky. • Important for PV system applications. Geological Survey of Canada

8. Where is the Sun? Series of equations to get zenith and azimuth angles – see pp. 5-7 in lab doc.

9. Solar Noon

10. Sun Moves Throughout the Year June 21 December 21

11. Sun Moves from Summer to Winter Jun Sep Dec

12. Sun Moves From Winter to Summer Jun Mar Dec

13. Panel Orientation is Important Edge of PV module Austin’s Latitude: 30o 30o June 21 Tropic of Cancer Latitude 23.45o 23.45o March 21 September 21 23.45o Equator December 21 Tropic of Capricorn Latitude -23.45o Earth’s surface

14. Panel Orientation is Important • Best all-year tilt = Latitude • Best winter tilt = Latitude + 15° • Best summer tilt = Latitude – 15°

15. Solar Radiation Monitors Rotating Shadowband Pyranometers Measure GH and DH NREL Sci Tec Two-Axis Tracker Measures DN, GH, and DH GH (Global Horizontal W/m2): Sensor points straight up, sees entire sky, including sun disk DN (Direct Normal W/m2): Tracking device points toward sun and sees only the sun disk DH (Diffuse Horizontal W/m2): Once per minute, band quickly swings over, shadow falls on sensor. Then, sensor sees entire sky, less sun disk.

16. Keep Solar Radiation Monitor Lenses Clean!

17. Computing Incident Power GH: Measured sky on horizontal sensor (includes disk of sun) Direct normal (DN), global horizontal (GH), and diffuse horizontal (DH), all in W/m2, are the three important components of solar radiation. DN can be estimated from GH and DH. (GH − DH): Est. disk of sun component on horizontal sensor DH: Measured sky on shadowed horizontal sensor (excludes disk of sun) Est. disk of sun component on sensor pointed toward sun DN: Est. total sky on sensor pointed toward sun

18. Computing Incident Power, cont. The angle of incidence is the angle between the sun’s rays and a vector normal to the panel surface (0° means that the sun’s rays are perpendicular to the panel surface) Series of equations to get angle of incidence – see pp. 11-12 in lab doc.

19. Computing Incident Power, cont. The incident solar radiation, in kW, on a panel surface is approximated by Measured sky on shadowed horizontal sensor (excludes disk of sun) Est. disk of sun component on sensor pointed toward sun About 14% is converted to electricity Est. disk of sun component on panel surface Est. Watts on panel surface Multiply by surface area

20. 85W each 150W 80W each 85W each Panels Atop ENS Disconnected

21. Weather Forecast http://www.nws.noaa.gov/forecasts/graphical/sectors/southplains.php#tabs

22. Panel Pairs Connected to Power Lab Voltage at Panels Voltage at Lab Bench Panel Current Use these two

23. Use a Variable Power Resistor to Sweep thePanel I-V Curve

24. Record, Plot, and Visually Inspect the I-V Data Points as You Take Them • Take the open circuit voltage reading with no load connected • Adjust the power resistor, backing down in integer volts in two volt steps (e.g. 38V, 36V, 34V, … ) until about 25V, while taking the current readings • At about 25V, continue to back down in integer volts, but in five volt steps, while taking the current readings Reminder - Hand plot as you take your data points • Take the short circuit current and panel voltage reading

25. Use the Excel Solver to Curve Fit Your Measurements

26. Automated way to get I-V curve: • Suddenly connect panel to large discharged C (like 5 or 10 of the DBR C’s), • Capture I and V data points on a scope, save to a floppy, and read the file with Excel, • Replot I versus V, • Replot P versus time to get max P

27. Solar Radiation in Texas

29. Multiply by panel efficiency, e.g. 0.14, to get electrical output

30. clock noon solar noon

31. Solar analysis of Sept. 25, 2006. Assume panels are at 30º tilt, 180º azimuth. Incident kWH on 1m2 panel (approx. 150W rated) is 7.02kWH. Multiplying by 0.14 efficiency yields 0.98 kWH. That corresponds to about 6.6kWH per 1kW rated of solar panels (1000*0.98/150). Thus, if a (non-air conditioned) house consumes 20 kWH per day, then about 3kW of panels are needed. Using $2.5 per W, which inflates to about $7.0 per W with mounting and electronics, then the 3 kW of panels cost about $21K. Consider an average price of electricity for residential users of 11 cents/kWH (TX is about average). So cost of electricity each day is about $2.1. Hence, it will take close to 3 years to pay the solar panels

32. In recent years, financial incentives have acted like catalysts to increase PV power penetration and to bring solar panels costs down

33. Other factors affecting PV use effectiveness and return of investment: - Air conditioner impact - PV panel orientation (SW is better during the summer because it tends to maximize generation when air conditioner consumption is maximum)

34. Practice Problem