1

FLORIDA SOLAR ENERGY CENTERA Research Institute of the University of Central Florida

How does Solar Energy Work

Florida Solar Energy Center1679 Clearlake Road

Cocoa, Florida , USA, 32922

October 3, 1998 2

Objectives

Understand the variables affecting the amount of solar energy received on a given surface Differentiate between solar irradiance (power) and solar insolation (energy)Demonstrate how solar radiation and weather data are used in sizing photovoltaic systemsAssess site specific issues such as array location, orientation and shading, roof condition, safety hazards, and other requirements for PV installations.

October 3, 1998 3

Solar Spectral Data and PV Device Response

0

400

800

1200

1600

2000

0 0.5 1 1.5 2 2.5 3

Wavelength (micron)

Irrad

ianc

e (W

/m2)

0

0.2

0.4

0.6

0.8

1

Qua

ntum

Effi

cien

cy (%

)

ASTM892 ASTM891 Cz-Si

October 3, 1998 4

Factors Affecting the Solar Radiation Received on a Surface

Geometric effects:rotation of the earth about a tilted axis and earth’s orbit around the sunOrientation of the surface with respect to the sun’s rays

Atmospheric effects:scattering and absorption by atmospheric constituentseffects vary significantly with altitude, latitude, time of day and year, and local weather conditions

Shading effects:Objects shading the sun from the array

October 3, 1998 5

Earth’s Rotation and Declination

Sun

Earth’s Rotational AxisArctic Circle 66.55 oN

Tropic of Cancer 23.45 oN

Equator

Tropic of Capricorn 23.45 oS

Antarctic Circle 66.55 oS

EquatorialPlane

Solar Declination (*)

Ecliptic Plane

October 3, 1998 6

Earth’s Orbit Around the Sun

1

2

3

4

Earth’s Orbit Around the Sun(counter- clockwise)

Sun

Earth’s axis ofrotation

EclipticPlane

Vernal Equinox: March 21Declination = 0o

Autumnal Equinox: September 23Declination = 0o

Perhelion -January 2

Aphelion - July 296 million miles

(1.017 AU)90 million miles

(0.983 AU)

Winter Solstice: December 22Declination = - 23.45o

Summer Solstice: June 22Declination = +23.45o

October 3, 1998 7

Sun Paths for 30o N Latitude

1

2 4

3

NorthPointO

East

West

Zenith

June 22: 12 noon

September 23 and March 21: 12 noon

December 21: 12 noon

10 am

8 am2 pm

4 pm

4 pm

4 pm

8 am

8 am2 pm

2 pm

10 am

10 am

South

October 3, 1998 8

Atmospheric Effects

Results in the scattering, attenuation and absorption of direct solar radiation received outside the earth’s atmosphereEffects vary significantly with altitude, latitude, time of day and year, and local weather conditions.

October 3, 1998 9

Effect of Air Mass

Sun at noon

Atmosphere

Sun at mid-morning ormid-afternoon

1.5 Air Mass( AM 1.5 )

One Air Mass(AM 1 )

Earth

Atmospheric path length affects the amount and spectral content of solar radiation.PV module performance is rated under AM 1.5 spectral distribution

October 3, 1998 10

Sun Position - Definitions

NorthPoint O

East

West

Zenith90 deg altitude

South0 deg azimuth

Altitude Angle (")

Azimuth Angle (R)

October 3, 1998 11

Solar Irradiance (Power)

Solar irradiance is the radiant power per unit area, commonly expressed in units of kW/m2, or W/m2

Outside the earth’s atmosphere, the sun’s power is relatively constant, equal to 1.36 kW/m2 and is referred to as the Solar Constant.Typical peak terrestrial irradiance values are approximately 1 kW/m2 (1000 watts/m2) for surfaces normal to the sun’s rays under clear sky conditions.1 kW/m2 is also used as the peak rating condition for PV module performance.

October 3, 1998 12

Peak Solar Irradiance on a Surface

Sunlight intensity measured in watts per square meter

With sun directly over head and PV flat on the ground

1 mOne

SquareMeter = 1000 watts = 1 kilowatt

1 m

October 3, 1998 13

Solar Insolation (Energy)

Solar irradiance (power) summed over time equals solar insolation (energy)Solar Insolation is the radiant energy per unit area, and is expressed in units of kWh/(m2-day)Peak Sun Hours (PSH) is the amount of solar energy received on a surface, and is equivalent to the number of hours that the solar irradiance would be at a peak level of 1 kW/m2, or the equivalent number of hours per day that a PV array will operate at peak output levels.

October 3, 1998 14

Solar Insolation(Peak Sun Hours)

Sola

r Irr

adia

nce

(W/m

2 )

Time of Day (hrs)

1000 W/m2

Sunrise Noon Sunset

peak sun hours

Solar insolation

Solar irradianceArea of box equalsarea under curve

500 W/m2

October 3, 1998 15

Array Orientation

North

East

West

Zenith

Array azimuthangle (R)

Array tilt angle (")

Array surface

South

Surfacenormal

Solar incidenceangle (()

October 3, 1998 16

Array Orientation – Tilt Angle

Optimal performance of PV arrays is achieved by facing the array south (north in the southern hemisphere), and at a tilt angle from horizontal using the guidelines below:

Application Best Array Tilt AngleMaximum Annual Energy 90% of LatitudeProductionWinter Peak Load Latitude plus 15 degreesSummer Peak Load Latitude minus 15 degrees

October 3, 1998 17

Array Orientation – Azimuth

Optimal energy performance of PV arrays is achieved by facing the array due south. In most cases, latitude-tilt surfaces with azimuth orientations of +/- 45 degrees from due south will receive 75-80 percent of solar energy on south-facing surfaces.Where magnetic declination is significant,adjust compass readings for due north south by adding magnetic declination:

See: http://geomag.usgs.gov/For example, a magnetic compass needle in central California will point 15 degrees east of true north, and a compass in New York will point 15 degrees west of true north.

October 3, 1998 18

U.S. Magnetic Declination

October 3, 1998 19

U.S. Solar Radiation DataJune

October 3, 1998 20

U.S. Solar Radiation DataDecember

October 3, 1998 21

Peak Sun-Hour Data

Minimum daily Peak Sun-hours during hurricane season.

October 3, 1998 22

U.S. Solar Radiation Data Availability

National Renewable Energy Laboratory - Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors:

http://rredc.nrel.gov/solar/pubs/redbook/Other solar resource data is available from:

http://rredc.nrel.gov/solar/pubs/

October 3, 1998 23

Solar Radiation Measurement

PSP

PV - PSP

October 3, 1998 24

Solar Radiation Measurement

Solar Meter: See: http://www.solaqua.com/daysolmet.html

October 3, 1998 25

What is Required for Energy Estimation

While there are several methods to determine PV system performance, the following parameters are required

Nominal Array SizePeak Sun-hours - Sunlight Energy incident on the PV arrayArray TiltArray Azimuth Angle

October 3, 1998 26

The PV Installation Process

Conducting a Site Survey

Installing the System

Selecting a System

System Checkout & Inspection

October 3, 1998 27

Solar Photovoltaic System (SPS)

Solar Photovoltaic System (690.2)The total components and subsystems that, in combination, convert solar energy into electrical energy suitable for connection to autilization load.

loadutilization

energysource power

conditioning

energyconversion

InverterCharger

Controller

PV Arrayenergydistribution

loadcenter

batteryenergystorage

electricutilitynetwork

October 3, 1998 28

Solar Photovoltaic System (SPS) Components

PV Array: An electrical assembly of photovoltaic modules that convert sunlight to DC electricity.Inverter: A device that converts DC power from batteries or PV arrays into utility-grade AC power.Energy Storage: Electrical or other storage devices sometimes used to store energy produced by PV arrays for later consumption.System Charge Control: A device used to protect batteries from overcharge and overdischarge, sometimes provide load control functions.Load: Energy consuming electrical appliances served by the system.Balance of System (BOS) Components: Other equipment required to control, conduct, protect and distribute power in the system.

October 3, 1998 29

Solar Cell

Solar Cell (690.2)The basic photovoltaic device that generates dc electricity when exposed to light. A typical silicon solar cell produces about 0.5 volt and up to 6 amps and 3 watts for larger area cells.

electrical load

typical crystalline silicon photovoltaic cell

(-)

(+)

phosphorous-doped (N-type) silicon layer ~ 0.3 x 10-6 mboron-doped (P-type)

silicon layer ~ 250 x 10-6 m

dc current flow

sun

October 3, 1998 30

Photovoltaic Modules

Module (690.2)A complete, environmentally protected unit consisting of solar cells, optics, and other components, exclusive of tracker, designed to generate dc power when expose to sunlight.

60 watt polycrystallinemodule

75 watt crystallinemodule

12 watt thin filmmodule

October 3, 1998 31

Photovoltaic Modules and Arrays

Typical PV modules range in size from around 0.5 m2 to over 3 m2 surface area, with peak power output of 50 to 300 watts dc. Area power densities range from 80-120 W/m2. Most commercially available crystalline and multi-crystalline PV modules have 36 cells in series, and have open-circuit voltages of 20-22 volts dc, and designed for battery charging applications. Most listed modules can be connected in series up to 600 volts DC.Some thin-film modules have open circuit voltages as high as 100 volts dc, and may use multiple parallel module connections per source circuit.

October 3, 1998 32

Definitions: Photovoltaic Cells, Modules, Panels and Arrays

cell module

panelarray

October 3, 1998 33

Typical Module Label Required by National Electrical Code

Siemens Solar IndustriesCamarillo, CA 93011

MODEL M55PHOTOVOLTAIC MODULEAT 1000 W/M2 SOLAR IRRADIANCEAND 25oC CELL TEMPERATURE 30B9 LISTED

MAX. POWER SHORT CKT. RATED 53 WATTS 3.35 A 3.05 A

MAX. SYST. OPEN CKT. V. OPEN CKT. RATED 600 VOLTS 21.7 V 17.4 V

FIRE RATING SERIES FUSE CLASS C 5 A

FIELD WIRING BYPASS DIODECOPPER ONLY, 14 AWG MIN. INSTALLATION GUIDEINSULATED FOR 75 C MIN. 233-701500-20 MADE IN U.S.A.

October 3, 1998 34

Identification of Photovoltaic System Components

Photovoltaic source circuits

Solar cells

Blocking diodes

Module

Panel

Array

Fuses

Photovoltaicoutput circuits

Adapted from NEC 2002Figure 690.1(A)

October 3, 1998 35

PV System Charge Controllers

October 3, 1998 36

Battery Charging

Battery charging modes:Bulk or normal chargingFinishing chargeEqualizing charge

Temperature compensationminimizes excessive charge and electrolyte when hot,improves capacity when cold.

October 3, 1998 37

Inverters for PV Systems

Inverter (690.2)Equipment that is used to

change dc input to ac output, and may also function as a battery charger for systems using storage.Inverters for PV systems in

sizes from 100 watts to custom designs of up to 1 MW or moreDC operating voltages of 12

volts up to 600 volts, with AC outputs from 120 V single phase to 480 V three phase.

October 3, 1998 38

PV Inverter Classifications

Stand-Alone InvertersInverter in solar PV systems that operate and supply power independent of the electrical production and distribution network, typically operate from storage batteries.

Utility-Interactive or Grid-Connected InvertersInverters in solar PV system that operate in parallel with and may deliver power to an electrical production and distribution network, may be connected to PV arrays or batteries.

Bi-Modal InvertersCan operate either in interactive or stand-alone mode, but not simultaneously, typically use batteries.

October 3, 1998 39

Alternating Current Waveforms

square wavesine wave

quasi-sine wave

Time

Am

plitu

de

One Cycle

October 3, 1998 40

Inverter Efficiency vs. Load

Output Power Level

Effic

ienc

y

October 3, 1998 41

Batteries for PV Systems

Storage Batteries (690.71, 480)Batteries are used in some PV systems to store energy produced by the PV array and supply it to electrical loads as needed.Charge control is required in most cases to protect batteries from overcharge by PV array, and overdischarge from loads.

October 3, 1998 42

Secondary Battery Types and Characteristics

BATTERY TYPE Cost Deep CyclePerformance

Maintenance

FLOODED LEAD-ACIDLead-Antimony low good highLead-Calcium Open Vent low poor mediumLead-Calcium Sealed Vent low poor lowLead Antimony/Calcium Hybrid medium good medium

CAPTIVE ELECTROLYTE LEAD-ACID(VRLA)

Gelled medium fair lowAbsorbed Glass Mat medium fair low

October 3, 1998 43

Rack-Mounted PV Arrays

October 3, 1998 44

Tracking Array Configurations

October 3, 1998 45

Standoff-Mounted Arrays

Above and parallel to roof slopePromotes array coolingCan Reduce heat gain into buildings

October 3, 1998 46

Commercial BIPV System

Image courtesy of PowerLight Corp

October 3, 1998 47

Direct-Coupled Stand-Alone Systems

Simplest type of stand-alone PV system, common applications include water pumps and fans.DC load is directly connected to a PV array, no energy storage.No overcurrent device typically required.

PV Array DC Load

October 3, 1998 48

Stand-Alone PV System with Battery Storage

PV array charges battery which supplies power to DC electrical loads as needed.Without charge control, battery is susceptible to overcharge and overdischarge.Charge control may only be eliminated under special circumstances the load is well defined and the battery is oversized.

PV Array Battery DC Load

October 3, 1998 49

Stand-Alone PV System with Batteries and Charge Control

Charge control is required whenever the load is variable and the battery is not oversized.Protects the battery from overcharge and overdischarge, and may provide load control functions.

DC LoadPV Array

Battery

ChargeController

October 3, 1998 50

Stand-Alone PV System with AC and DC Loads

DC LoadPV Array

Battery

ChargeController

Inverter/Charger

AC Load AC Source(Charger Only)

October 3, 1998 51

Stand-Alone PV Hybrid System

DC LoadPV Array

Battery

ChargeController

Inverter

AC LoadEngine-generator, wind turbine or grid backup

Rectifier

ChargerDC Bus

AC Bus

October 3, 1998 52

Basic Utility-Interactive or Grid-Connected PV System

DistributionPanel

PV Array Inverter/PowerConditioner

AC Loads

ElectricUtility

October 3, 1998 53

Utility-Interactive PV SystemNo Battery Storage – Dual Metering

PV Array

InverterCustomerElectrical

Panel

CustomerElectrical Loads

Customer-supplied meter for PV generation(recommended)

OutdoorDisconnect

Customer-supplied visible break, lockable PV disconnect

(utility may require)

PV kWh

To UtilitykWh

Electric UtilityNetwork

Customer

Utility

From UtilitykWh

* Arrows indicate directions of power flows Two, unidirectional revenue meters, or single bi-directional recording meter (utility-supplied)

October 3, 1998 54

Utility-Interactive PV System with Energy Storage

PV Array

Inverter/Charger

BatteryStorage

Critical LoadSub Panel

BackupAC Loads

Main Panel

PrimaryAC Loads

ElectricUtility

Bypass circuit

* Arrows indicate directions of power flows

October 3, 1998 55

Utility-Interactive PV System with Battery Storage – Net Metering

PV Array

Battery Storage

CustomerMain Electrical

Panel

Customer PrimaryElectrical Loads

Customer-supplied sub metering(recommended)

OutdoorDisconnect

Customer-supplied visible break, lockable PV disconnect

(utility may require)

To MainkWh

ElectricUtility

Network

Utility

Inverter Bypass CircuitTransfer Switch

Utility NetkWh

CustomerBackup Load

Customer Sub Panel

Inverter/Charger

Customer

FromMain kWh

FromMain kWh

* Arrows indicate directions of power flowsStandard, bi-directional revenue meter (utility-supplied)

October 3, 1998 56

Utility-Interactive PV System with Battery Storage

Source: Xantrex/Trace Engineering

October 3, 1998 57

Watts Electrical Schematic

1

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

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+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

EXTERNAL GFDI MAY BE

REQUIRED (690.5)

15A FUSEDCOMBINERS

40A BREAKERS

PV MODULES

40A CHARGECONTROLLERS

+

-

120V OUTPUT

+ - GND

NEUTRAL BUS

+

-

+

-

+

-

POSITIVEBUS

120VEMERGENCY

LOADSSUBPANEL

GROUNDINGELECTRODE

[250.64]

UTILITYCONNECTION

AC LOADCENTERUTILITY

DISCONNECT

INTERCONNECTPER 690.64

2

3

BATT

ERY

BO

X

4

2

4

250A

80A4 4

56

6

3

7

7

50A

60A

1 2 #10 USE-2 W/ #10 BARE EQUIPMENT GROUND, NOT INCONDUIT (NOTE: SOME JURISDICTIONS REQUIRE A #6BARE GROUND

2 #10 USE-2 TRANSITIONS TO #8 THWN-2 IN METALJUNCTION BOX; METAL CONDUIT CONNECTSCONSECUTIVE JUNCTION BOXES

3 6 #8 THWN-2 IN METAL CONDUIT TO COMBINERS

4 2 #6 THHN IN CONDUIT

ROOFTOP MODULES & CONDUITALL OTHER EQUIPMENT

ASSUMED AMBIENTTEMPERATURES:

65˚C30˚C

ALL TERMINAL TEMPERATURERATINGS ASSUMED TO BE 75˚C,

EXCEPT FOR 90˚C MODULETERMINALS

7

5 2 #4 THHN IN CONDUIT

6 2 #4/0 THHN IN CONDUIT

#10/

#6 G

RO

UN

D

#8 THWN-2 IN METAL CONDUIT

#6 T

HH

N

#6 THHN

#4 T

HH

N

#4/0 THHN

7 2 #6 THHN IN CONDUIT

#6 THHN

#6 THHN

#10

US

E-2

INVERTER

October 3, 1998 58

October 3, 1998 59

Zero Energy Homes

October 3, 1998 60

Energy Estimation Software

Selected PV Software ProgramsMaui Solar PV Design Pro

http://www.mauisolarsoftware.comPV CAD: http://www.iset.uni-kassel.dePV F-Chart: http://www.fchart.com/pvfchart/pvfchart.htmlPV Sol: http://www.valentin.de/englisch/startseite-e.htmPVSYST 3.21: http://www.pvsyst.comKeryChip: http://www.kerychip.dk/eng.htmHOMER: http://analysis.nrel.gov/homer/PV Watts: http://www.pvwatts.org

October 3, 1998 61

Solar Hot Water Systems

Differential controller operated Direct pumped systemPhotovoltaic operated systemIndirect pumped systemDrain back systemIntegral collector storage (ICS) systemThermosiphon system

October 3, 1998 62

Types of Solar Water Heating SystemsPassive

Direct

Indirect

Active

October 3, 1998 63

Typical Thermal System Diagram

October 3, 1998 64

Solar Water Heat – Large Fields

Martin County Correctional InstituteTwo fields, each:

70 flat plate collectorsTwo 3000 gallon tanksThree pump loops

October 3, 1998 65

Large Utility Scale Power

October 3, 1998 66

Large Scale Power – CSP Dish

UNLV PilotPlanned: 1-MW, >40 units near Las Vegas

October 3, 1998 67

Audubon Society Nature Center, LA

100 % Solar Thermal ACHeat, DHW w/800 SF array25 kWp PV for pumps, fans, electrical

October 3, 1998 68

Other - Transpired Collector wall

October 3, 1998 69

Many designs and proven performancePersonal cookers to Village cookers

October 3, 1998 70

Solar Thermal Shower

October 3, 1998 71

Summary

Identified factors affecting the amount of solar energy received at a given locationDefine solar irradiance and insolationDiscussed criteria for orienting PV arraysDemonstrated use of solar radiation data in estimating the performance of PV systemsIdentified considerations for PV installation site surveys