EE462L, Spring 2014 PV Arrays (Solar Panels)

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

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EE462L, Spring 2014 PV Arrays (Solar Panels)

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Presentation Transcript

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

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)

I-V Curve

Isc

Pmax at approx. 30V

Isc

Pmax 0.7 • Voc • Isc

Voc

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)

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.

Where is the Sun?

Series of equations to get zenith and azimuth angles – see pp. 5-7 in lab doc.

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

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

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.

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

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.

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

85W each

150W

80W each

85W each

Panels Atop ENS

Disconnected

Weather Forecast

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

Panel Pairs Connected to Power Lab

Voltage at Panels

Voltage at Lab Bench

Panel Current

Use these two

• 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

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

clock noon

solar noon

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

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

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)