1 EE462L, Spring 2014 PV Arrays (Solar Panels)

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

3 I-V Curve

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

5 I-V Curve I sc V oc I sc P max at approx. 30V P max 0.7 V oc I sc

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

7 Earths Poles Magnetic poles: Created by Earths magnetic field Can be located with a compass They move along Earths surface! Celestial poles: Created by Earths 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 Dec Sep

12 Sun Moves From Winter to Summer Dec Jun Mar

13 Panel Orientation is Important June 21 December 21 March 21 September 21 Equator Tropic of Cancer Latitude o Tropic of Capricorn Latitude o Austins Latitude: 30 o o 30 o Edge of PV module Earths 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/m 2 ): Sensor points straight up, sees entire sky, including sun disk DH (Diffuse Horizontal W/m 2 ): Once per minute, band quickly swings over, shadow falls on sensor. Then, sensor sees entire sky, less sun disk. DN (Direct Normal W/m 2 ): Tracking device points toward sun and sees only the sun disk

16 Keep Solar Radiation Monitor Lenses Clean!

17 Computing Incident Power Direct normal (DN), global horizontal (GH), and diffuse horizontal (DH), all in W/m 2, are the three important components of solar radiation. DN can be estimated from GH and DH. DH: Measured sky on shadowed horizontal sensor (excludes disk of sun) GH: Measured sky on horizontal sensor (includes disk of sun) (GH DH): Est. disk of sun component on horizontal sensor 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 suns rays and a vector normal to the panel surface (0° means that the suns rays are perpendicular to the panel surface) Series of equations to get angle of incidence – see pp in lab doc.

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

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

21 Weather Forecast

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 the Panel 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 Take the short circuit current and panel voltage reading Reminder - Hand plot as you take your data points

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 Cs), 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

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

30 clock noon solar noon

31 Solar analysis of Sept. 25, Assume panels are at 30º tilt, 180º azimuth. Incident kWH on 1m 2 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

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