PV Power Systems

7/21/2019 PV Power Systems

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Photovoltaic Power Systems -2

Grid connected PV

Professor Chem Nayar

Curtin University of Technology

Perth , Western Australia


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Grid Connect PV Systems

Simplest of systems

No storage

Maximise PV to Grid


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Grid Connect PV Systems

Net Metering single meter runs in both directions

Can also be with two meters : one to measure energy sold

and the other energy bought


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Components of a Grid Connected System


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SPECIFICATION (NOMINAL VALUE)

MODEL : PV-MF130EA2

MAXIMUM SYSTEM VOLTAGE 600 V

MAXIMUM POWER (Pmax) 130 W

OPEN CIRCUIT VOLTAGE (Voc) 24.2 V

SHORT CIRCUIT CURRENT (Isc) 7.39 A

MAXIMUM POWER VOLTAGE (Vmp) 19.2 VMAXIMUM POWER CURRENT (Imp) 6.79 A

PV PANELS


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Sine wave output Low harmonic distortion (lessthan 4%)

Input voltage range 160 350 VDC

Output voltage range 187 253 VAC

Single phase, can operate in

frequency range 50 Hz +/- 6% Power factor > 0.98

High efficiency (more than 90%)

Maximum power point tracking

Mains and solar generator are galvanically

isolated

Disconnect from grid line within 1 cycle in

case of abnormal condition

Computer interface for local and remote

monitoring and data retrieval


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Connecting Solar Panels

Series connection to increase voltageParallel connection for increasing current

Terminology Module

String

Sub array

Array


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PV Array Diagram


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Array of sub arrays


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Blocking Diodes to prevent reverse

current flow

PCU


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Cable Sizing

Size for volt drop Maximum of 5% recommended

Size for current rating

Note that energy can typically feed from boththe array and the power conditioner

Current rating of the cable is the rating of the

protective device, not the PV output Consider cable exposed temperature when

sizing for current rating


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Protection Requirements

Module protection Bypass diodesString protection

Blocking diodes

Fuses

Array protection

Overcurrent protection disconnection


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Australian Requirement

Breaker trip current to be between 1.25 x Isc

2 x Isc

Isc is for the section feeding through thetrip device

Cable is then sized to the breaker

Note some PV manufacturers recommendmaximum fuse ratings for the modules


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Components

Over current protection Must be DC voltage rated

DC arcs are hard to extinguish

Disconnection Distinguish between isolators for breaking

down the array and load break isolators for

disconnection under load Plugs and sockets cant be separated under

load


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Components cont.

Blocking diodes Are not considered a fuse

Cannot be relied upon to block reverse current

Make sure they meet the voltage rating requirementsof the system

They can get hot, keep them cool

Australian requirements include breaker the arrayinto Extra Low Voltage (ELV) sections and being

able to isolate the inverter for removal


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Australian considerations

Australian requirements include breaking the array into ELV sections for safe

install and maintenance

being able to isolate the inverter for saferemoval


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GRID CONNECTED INVERTER SYSTEM

Converts DC current from solar panels to AC current and feed to the

grid . The system uses 50 Hz voltage waveform from grid line as a

reference signal and feed current to the grid line. Before connecting,the inverter will check property of grid line according to followingconditions :-

Voltage level

Frequency range

Phase of signal

If all conditions are within specified range and synchronized withinternal generating frequency, the inverter will be connected to thegrid

In case there is some abnormal condition with the grid, invertershould disconnect itself for both safety to human life and safety to

the system.


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PV/Grid Energy System Inverter

Configurations

Large Single Inverter Type (Central

Inverter)

Multiple Small Inverter Type (StringInverter)

DC Bus (Multi-string Inverter)

AC Module


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PV/Grid Energy System

Inverter Configurations


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Central Inverter Type

Series and Parallelconnection on DC side

All PV panels

connected to single DC

bus

Single Central Inverter

Affected by partial

shading of panels

Only one protection

system required


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Kalbarri PV System

in Western Australia (1995)

10kW 10kW

35kVA

(75kVA)

250Vdc

6.6kV

100kVA

415Vac

10kW10kW 10kW10kW

35kVA

(75kVA)

250Vdc35kVA

(75kVA)

250Vdc

6.6kV

100kVA

415Vac

6.6kV

100kVA

415Vac

6.6kV

100kVA

415Vac


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String Inverter TypeOne inverter per

string

Panels grouped

into smaller

inverter ratedpower of Inverter (

0.7-5kW)

Not so badlyaffected by shading

Not badly affected

by inverter failure


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@ 3.3kW

Grid-Connected PV Inverter (String Type)


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String Inverter Battery Backup

Controller Back up Line

AC Grid Line

DC 48 V

AC Line

DC from PV

160 to 240 V

AC Line

DC 48 V


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Grid-Connected PV System

with Back up Inverter

Kang Som-Mao, Ratchaburi

PV

CONTROLLER -

BATTERY batteries for S-218C

INVERTER APOLLO G304 And S-218C

75 Wp x 42 modules


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DC Linked or Multistring type

Each panel orgroup have a DC-DC step up

converterHigh voltage DC

link feeds

transformer-lessconverter


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DC Linked

#1String HFTboostL

PVC

1S

3S

4S

2S

1D

3D

2D

4D

DCC

HFTboostL

PVC

1

S3

S

4S

2S

1D

3D

2D

4D

DCC

HFTboostL

PVC

1S

3S

4S

2S

1D

3D

2D

4D

DCC

gridi

gridLGridGround

3S

4S

1S

2S

#2String

#3String


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AC Modules

One Inverter perpanel

High volume/ lowcost

Plug-and-play


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Inverter characteristics

Efficiency

Response times

Harmonic output

Fault current contribution

Synchronisation

Frequency control

Power factor

DC injection

Requirement Standard Details


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General AS/NZS 3100 Electrical Safety Requirement

Compatibility with AS 60038 A.C. Voltage and frequency ratings

electrical installation

Power flow direction N/A Power flow between energy source and grid may

be in either direction

Power factor AS 4777.2 Range between 0.8 leading to 0.95 lagging

between all outputs from 20% to 100%

of rated volt-amperes

Harmonic Currents AS 4777.2

Harmonic current shall not exceed the limits in

Table 1.

EMC

Radio

Communications

Act

Voltage fluctuation

AS/NZS

61000.3.3 Rated less that or equal to 16A per a phase

and flicker

AS/NZS

61000.3.5 Rated more than 16A per a phase

Impulse protection IEC 60255-5 Withstand a standard lightning impulse of 0.5J, 5kV

with 1.2/50 waveform

Transient voltage AS 4777.2 Voltage-duration curve derived from

limits measurements taken at a.c. terminal shall

Not exceed the limits listed in Table 2.

Direct current N/A Single-phase inverter: the dc output current of the

injection inverter at the a.c. terminals shall not

exceed 0.5% of its rated output or 5mAwhich ever is greater

Three-phase inverter: the dc output current of the

inverter at the a.c. terminals measured between

any two phases or between any phase and neutral

shall not exceed 0.5% of its rated output or 5m

which ever is greater

Data logging and AS/NZS 60950 Any electronic data logging or communications

communication equipment incorporated in the inverter requires to

devices comply with the appropriated requirements


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DC-AC ELECTRICAL CONVERSION

EFFICIENCY

Efficiency is the most important parameter for grid-connected PV

generation Depends on whether galvanic insulation transformer is used

between the AC on the grid side and the DC generated on the PVside or not.

Transformer can be either 50 Hz LF transformers, or HF

transformers. The presence or absence of LF or HF transformers in the inverters

influences not only the size, weight, ease of installation and materialcosts, but also the earthing and safety measures to be adopted in thePV system, and the control of DC injection feed into the grid.

Inverters with an LF transformer can achieve DC-AC efficiency of92%,while those with an HF transformer typically achieve amaximum efficiency of 94%.


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European EfficiencyNormalized efficiency, E, and is

valid for irradiance levels in centralEurope. It is defined as a function of

the efficiency at defined percentage

values for nominal AC power. This is

shown in the following equation:

E = 0.035% + 0.0610% +0.1320% + 0.130% + 0.4850% +

0.2100%


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94.292.690.892.3E

94.292.890.093.3100

95.093.490.993.850

94.693.192.593.130

94.292.392.091.020

91.588.990.485.810

86.785.184.877.55

Transformerless

LF (new

technology)

LF (old

technology)HF

Efficiency by inverter type (%)

AC power

(% of nominal)

Experimental inverter efficiencies for different string inverters; values used are

representative of state-of-the-art technology

Experimental inverter efficiencies


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MAXIMUM POWER POINT TRACKING EFFICIENCY

The DC power input to an inverter depends on which

point in the current-voltage (I-V) curve of the PV arrayit is working at. Ideally, the inverter should operate atthe maximum power point (MPP) of the PV array. TheMPP is variable throughout the day, mainly as a

function of environmental conditions such asirradiance and temperature, but inverters directlyconnected to PV arrays have an MPP trackingalgorithm to maximize energy transfer. The MPP

tracking efficiency, MPPT, can be defined as the ratioof the energy obtained by the inverter from a PVarray, to the energy obtained with ideal MPP trackingover a defined period of time.

MAXIMUM POWER POINT


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where PDC isthe DC input

power to the

inverter and PMis the power at

MPP

MAXIMUM POWER POINT

TRACKING EFFICIENCY


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Inverters for grid-connected PV

systems must generate energy at adefined quality

The standards (example:

international Standard IEC 61000-

3-2 ) above require a THD of

5% for the harmonic spectra of the

current waveform. nominal.

2

1 1

1

2

1

2

1

100

100

100%

=

=

=

h s

sh

s

ss

s

dis

I

Ix

I

IIx

I

IxTHD

Total Harmonic Distortion

Table 1 - Harmonic current limits [2]

Harmonic order number Limit for each individual harmonic

based on percentage of fundamental

2-9 4%

10-15 2%

16-21 1.50%

22-33 0.60%

Even harmonics 25% of equivalent odd harmonics

Total harmonic distortion (to the 50th harmonic) 5%

AS 4777


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Power Factor

Traditionally poor due to displacement power factor

harmonics

Present technology is very good

Maintain close to unity without great difficulty

Can regulate power factor or reactive powerfor voltage control or power factor correction

applications


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Example :Current THD and power

factor vs AC power


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DC Injection

Is possible if an output transformer is notpresent

Control systems can be added to preventexcessive injection

Is regulated by standards

Limits of 5 mA (0.025% of the rms outputcurrent for a 5 kW system, based on the IEC61000-3-2) or 0.5% (UL1741) are beingadopted in the UK and US respectively


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Synchronisation

Performed automatically

Typically uses zero crossing detection on

the voltage waveform

Can be instantaneous on the next zero

crossing

If phase locked loops are used it could takea up to few seconds


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Frequency Control

Locked to the grid

May have a bias to drift in the event of grid

failure

Lock range may be limited

Germany 49.8Hz - 50.2Hz

Australia 48Hz - 52Hz India 47Hz - 53Hz


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Prevention of Islanding

An island occurs when the invertercontinues to supply power to a portion of

the grid that has become isolated from the

rest of the systemThe power may be unstable during the

island period


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Anti islanding methods

Inverters are required to have measures toprotect against this occurring

Passive methods

Under/Over voltage Under/Over Frequency

Active Methods

Frequency drift

Impedance measurement

Power Shifting


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Earth Leakage Current

In the US, the National Electrical Code, NEC,

requires all PV installations with system

voltages above 50 V DC to be earthed.

Ground fault protection ('GFP') devices areused to measure the earth leakage current, in

order to disconnect from the ground (that is,

unearth the installation), in the case of fault.

Stray leakage currents may be an issue in the

sensitivity of this protection.


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Fault currents

Battery-less systems can only deliverwhat the energy source can deliver

for PV this can be very little to a maximum

of 1.2 times rated current wind is extremely variable

If a battery is present the fault current

contribution is limited by the inverter.

Typically in the range of 100% to 200%


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AC Power Output

The losses in a PV system are due to: Inverter losses

Dust/dirt in the modules

Mismatch in modules

Differences in ambient conditions from

Standard Test Conditions (STC) 1000w/m2,AM 1.5 and 250C.

Pac =Pdc,STCx efficiency


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Mismatch in Arrays


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Mismatch in Arrays


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Mismatch in Arrays


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1. Select the size of the system to be installed2. Select main equipment to be installed, calculate

for matching of spec. of

2.1 PV panel2.2 Grid connected inverter3. Examine location for PV mounting. There should

be no obstruction of sunlight for whole day or at

least 9.00 a.m. to 4.00 p.m.4. Consider for tilt angle of panels according to

latitude of that location5. Select PV mounting structures.

System design


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6. Check ampere capacity of each string of inverter, select sizeof blocking diode to be 30 % larger than string short circuitcurrent with diode max voltage more than 2 times of maxsystem voltage.

8. Select proper wire size so voltage drop for DC side is lessthan 3%8.1 Select wire size between each string to the combiner box

to enable less than 1% voltage drop8.2 Select wire size between the combiner box to control

box / inverter to enable less than 2% voltage drop9. Select proper wire size so voltage drop for AC side is less

than 3%10. Select size of disconnect switch both DC and AC side to

proper rating

System design

Case Study : A PV grid connected system in


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1. Select size of system to be around 3 kWp2. Select main equipments as

2.1 PV panel - Mitsubishi model PV-MF130EA2- 130 Wp / panel- 2 strings with 12 panels in each string- Isc / string = 7.39 amp.- Total PV power = 130 x 24 = 3,120 Wp

- V max = Voc = 24.2 x 12 = 290.4 Vdc- Oper. volt. at max. power = 19.2 x 12 = 230.4 Vdc- Max DC current = Isc x 2 = 7.39 A x 2 = 14.78 Amp

2.2 Grid connected inverter - Leonics G-303M- 2.7 kW output- Max DC voltage = 350 Vdc- Nominal Operating PV voltage = 230 Vdc

3. Location for PV mounting is on the roof deck with no obstruction ofsunlight for whole day

4. Select hot dip galvanized steel for PV mounting with stainless steelnuts & bolts

5. Tilt angle of panels is set to 14 deg. facing south as Bangkok locates atlatitude 13.73 deg. North

Case Study : A PV grid connected system in

Bangkok


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6. Plan to install control box and inverter in training room , 3 rd floor.

7. Selection of blocking diode7.1 Min. device rating (I) = Isc x 1.3

= 7.39 x 1.3 = 9.61 A

7.2 Min. device rating (V) = Voc x 2

= 290.4 x 2 = 580.8 V

Then select blocking diode to be 10 ampere 600 V. for each string.

8. Measure cable length of the system

8.1 Cable length between each string to the combiner box

= 10 meters

Select wire for each string to be 4 sq.mm. to get voltage drop < 1%

Voltage drop in each string = 11,650 x 10 x 7.39 = 0.86 V

Percentage of volt. Drop = 0.86 / 205 = 0.42 %

Case Study : A PV grid connected system in Bangkok

Case Study : A PV grid connected system in Bangkok


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8.2 Cable length between combiner box to control box / inverter is 35 m.Select wire size to be 10 sq.mm. to get voltage drop < 2%

Voltage drop = 3,903 x 35 x 7.39 x 2 = 2.02 V


9. Cable length between Control Box / Inverter to load panel is 12 meters

Select wire size to be 2.5 sq.mm. to get voltage drop < 3%

Voltage drop = 15,695 x 12 x (2,700/238)

= 2.14 V


10. Max DC current = 7.39 x 2 = 14.78 A

Max AC current = 2,700 / 232 = 11.64 A

Select both DC and AC breaker to be 20 A


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Calculate annual energy output

Use data source and get annual daily average energy

available Adjust down for losses

Inverter 7%

Temperature 15%

Cable 3% Dirt 2%

Orientation 1%

Total about 25%-30%

Multiply by the size of the array to get the electrical kWhroutput OR

Use a modelling package


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Verify

Does it fit in the areaDoes it meet budget

Does it produce required kWhr

Is the CO2 offset met

Check it works

Re-size if necessary


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System Acceptance Test1. Sum total module ratings at STC (Standard Test Condition) : Watts STC

2. Estimate inverter AC output to be 70% of Watts STC : Watts AC-estimated

3. Measure real AC output and irradiation, then define

Watts AC-corrected = Real AC output / irradiation x 1000

4. Compare that Watts AC-corrected is more than Watts AC-estimated

Result from the installation

Generating power and irradiation is measured on Mar 26, 2004 at 11.25 p.m.

Watts STC = 130 x 24 = 3,120 Wp

Watts AC-estimated = 3,120 x 0.7 = 2,184 Watts

Watts AC-corrected = 2,010 / 870 x 1000 = 2,310 Watts

4. Watts AC-corrected (2,310) > Watts AC-estimated (2,184)

*** PASS SYSTEM ACCEPTANCE TEST ***


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Generating Power VS Time for 3.12 kWp

Grid Connected inverter at Leo Electronics Co., Ltd. (Apr 1, 2004)

0

500

1000

1500

2000

2500

7 8 9 10 11 12 13 14 15 16 17 18

Time

Generating

Power


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Date Gen. Power Date Gen. Power Date Gen. Power

1/4/2004 14.30 24/3/2004 13.73 16/3/2004 12.85

31/3/2004 12.79 23/3/2004 12.12 15/3/2004 10.88

30/3/2004 12.13 22/3/2004 10.94 14/3/2004 12.53

29/3/2004 12.33 21/3/2004 8.02 13/3/2004 12.02

28/3/2004 13.49 20/3/2004 7.22 12/3/2004 11.67

27/3/2004 13.51 19/3/2004 8.57 11/3/2004 13.21

26/3/2004 13.14 18/3/2004 11.87 10/3/2004 11.34

25/3/2004 13.01 17/3/2004 14.68 9/3/2004 10.15

Max. Generating Power/day 14.68

kWh/day

Min. Generating Power/day 7.22 kWh/day

Average Generating Power/day 11.94 kWh/day

Power generating from Grid Connected System


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Orientation terminology


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Tracking Array

The PV array may either be fixed, sun-tracking

with one axis of rotation, or sun-tracking withtwo axes of rotation.

Generally fixed arrays are used thoughsignificant increase in energy yield is possiblewith single axis tracking with an additional smallgain using duel axis tracking

Trackers

add cost but offset by PV savings require some maintenance

Very good for water pumping applications

Tracking Relative Energy


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g gy

Production

0%

20%

40%

60%

80%

100%

120%

140%

160%

Albany Geraldton Halls Creek

Fixed north facing

at latitude angle

N-S Axis tracker -horizontal

N-S Axis tracker -

Fixed at latitude

angleDual Axis

34o57" 28o48" 18o14"


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Energy from Power of the Sun

0

200

400

600

800

1000

1200

0:00

2:00

4:00

6:00

8:00

10:00

12:00

14:00

16:00

18:00

20:00

22:00

Time

Powe

rEnergy =Power x Time

Area = 7500W.hr

= Area under curve


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Peak Sun Hours

0

200

400

600

800

1000

1200

0:00

2:00

4:00

6:00

8:00

10:00

12:00

14:00

16:00

18:00

20:00

22:00

Equivalent Time at 1 peak sun (1000W/m2)

7.5 hours

Area = 7500W.hr1000W/m2

Solar Irradiance


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0

250

500

750

1000

0:00 6:00 12:00 18:00 0:00

Time

IrradianceS(W/sqm)

18/05/98

A typical sunny day in Perth

0

250

500

750

1000

0:00 6:00 12:00 18:00 0:00

Time

IrradianceS(W

/sqm)

15/05/98

A Typical cloudy day in Perth

Solar Irradiance

Average Daily Solar Radiation Perth


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Average Daily Solar Radiation, Perth



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Use data source and get annual daily average energy

available Adjust down for losses

Inverter 7%

Temperature 15%

Cable 3% Dirt 2%

Orientation 1%

Total about 25%-30%

Multiply by the size of the array to get the electrical kWhroutput OR

Use a modelling package

Verify


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Verify

Does it fit in the area

Does it meet budget

Does it produce required kWhr

Is the CO2 offset met

Check it works

Re-size if necessary

Suboptimal orientation the


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impact

Common in building integrated

applications

Roof may be wrong orientation

Facade may be vertical

Tilt angle may be dictated by aesthetics


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Documents

PV Power Systems