Oversized PV arrays and Battery Days of Autonomy in Stand ... · and reduce the battery bank’s...

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Training • Consulting • Engineering • Publications

Oversized PV arrays and Battery Days of

Autonomy in Stand-Alone Power systems

Geoff Stapleton

Managing Director, GSES


INTRODUCTION AND BACKGROUND

creating sustainable change through education, communication and leadership © 2010 GSES P/LSustainable change through education, engineering and communication © 2019 GSES P/Lcreating sustainable change through education, communication and leadership © 2010 GSES P/LSustainable change through education, engineering and communication © 2019 GSES P/L

Introduction

• The presentation is based on a Tech Info released by GSES in March 2019.

• Batteries are the heart of an off grid solar system—

• it does not matter how many modules you have on the roof—

• if the battery is not sized correctly and if the customer loses power at 10pm with 15 minutes to go in their favourite football game, TV show etc

• YOU AS THE DESIGNER INSTALLER WILL BE PHONED BY AN UNHAPPY CUSTOMER AT 10PM


Autonomy

• AS/NZS4509.2:2010 as “The number of days of

operation of the power system without energy input from

generators before exceeding the design maximum depth

of discharge of the battery”.

• The minimum number of days of autonomy for

sustainable system function is generally recommended

anywhere from 2 to 5 days, depending on a number of

variables.


However…….

the price of photovoltaic solar modules has dropped greatly

in recent years, and it is now far more economical to

purchase additional PV generation capacity than battery

storage capacity.

It is known that even on cloudy days, some solar irradiation

is still available. THEREFORE

Is it reasonable to install an oversized solar PV array

and reduce the battery bank’s days of autonomy, while

maintaining quality outcomes for a stand-alone power

system?


Background

After the launch of the solar accreditation program in 1993

by Solar Energy Industry Association of Australia (SEIAA)

to address poor designs, a design guideline was developed

and included in the original training courses conducted by

SEIAA.

This guideline recommended 5 days of autonomy with a

maximum depth of discharge of 70%,


5 days allowed

• It allowed for a number of cloudy days before the system

owner might need to start a generator.

• It provided a daily depth of discharge of less than 20%

and resulted in the lead acid battery having a high cycle

life.


AS/NZS4509

• The first version of AS/NZS 4509.2, released in 2002,

– has 3 to 5 days autonomy for systems with manual genset control

– 5 days autonomy for PV systems with no genset.

• AS/NZS 4509.2:2010.

– typical days of autonomy to be 2 days for systems with automatic start generator

– 2 to 3 days for systems with manual start, with the number of days determined in consultation with the user.

– For systems without generators the days of autonomy should be 4 or 5 days but with consideration provided for local weather conditions such as the possibility of multiple days of low solar irradiance


Oversupply co-efficient

• AS/NZS 4509.2:2002 introduced an oversupply co-

efficient, i.e. a capacity oversize factor, for stand-alone

systems without a back-up generation set.

• a substantial decrease in charging efficiency of the lead

acid battery is experienced when the battery is

approaching 100% state of charge and particularly when

equalisation charge is required.

• The typical oversupply co-efficient for stand-alone

systems with PV arrays, this value is 1.3 to 2.0.


THE STUDY


Solar Resource Analysis

• Daily irradiation figures can be obtained for each day

over the previous 20 to 30 years for most weather

stations from the Bureau of Meteorology

(http://www.bom.gov.au/climate/data/

Following sites were used:

– Bairnsdale, Victoria

– Parkes, New South wales

– Cairns, Queensland

– Alice Springs, Northern Territory; and

– Darwin, Northern Territory.

http://www.bom.gov.au/climate/data/


ANAYLSIS METHOD


Irradiation Used for Sizing PV

Array

• For each site it was assumed the daily energy

requirement was fixed.

• The average monthly irradiation figures for the site were

determined and the worst month was selected in

accordance to the requirements of AS/NZS4509.2:2010.

• The size of the PV array (in kW) was then calculated

based on the design principles of AS/NZS4509.2:2010

using typical assumptions for equipment efficiencies


System Blackout

• A program was developed that used the actual historical

irradiation for each day to determine whether the

available usable load energy in the battery (from the

preceding day) plus the available energy from the solar

array was sufficient to provide the required daily energy

each day.

• If not, then it was deemed a blackout for that day.


Battery Efficiency

• the average battery efficiency was applied and no

allowance was provided for the fact that an oversize is

required (if there is no generator available) for lead acid

batteries as defined in AS/NZS4509.2 to ensure

equalisation of the battery is achieved.


Tables

For each site tables were produced for:

• 1 day of autonomy;

• 2 days of autonomy;

• 3 days of autonomy;

• 4 days of autonomy; and

• 5 days of autonomy.


Data in Tables

For no (zero %) PV array oversizing and various

percentages of oversizing, the tables provided:

• average number of blackouts each month and each

year;

• number of years, out of 28, the system would have had

blackouts; and

• the highest and lowest number of blackouts determined

in a year.


Module Degradation

• A factor not considered in the analysis is the fact that the

output of the array degrades over time.

• This could be ½ to 1 per cent per year depending on the

quality of the module.

• The efficiency and the usable capacity of the battery will

typically decrease with time.

• As the system ages the number of blackouts per year

will increase


Cairns July 2010 2 Days

Autonomy- Raw data

PSH

Load energy supplied by solar

Daily Load energy

Stored Enery at end of day

16-Jul 4.72 5.37 5.00 0.3717-Jul 5.16 5.88 5.00 1.2418-Jul 3.75 4.27 5.00 0.5119-Jul 6.27 7.13 5.00 2.6420-Jul 6.16 7.01 5.00 4.6521-Jul 4.65 5.29 5.00 4.9422-Jul 2.41 2.74 5.00 2.6823-Jul 4.44 5.05 5.00 2.7324-Jul 2.27 2.59 5.00 0.3225-Jul 1.82 2.08 5.00 0.0026-Jul 2.17 2.47 5.00 0.0027-Jul 3.65 4.15 5.00 0.0028-Jul 5.72 6.50 5.00 1.5029-Jul 5.68 6.46 5.00 2.9730-Jul 5.41 6.15 5.00 4.1231-Jul 4.48 5.09 5.00 4.21

PSH

Load energy supplied by solar

Daily Load energy

Stored Enery at end of day

10.001-Jul 5.65 6.42 5.00 10.002-Jul 3.34 3.80 5.00 8.803-Jul 3.72 4.23 5.00 8.034-Jul 4.06 4.62 5.00 7.655-Jul 2.00 2.27 5.00 4.926-Jul 5.10 5.80 5.00 5.727-Jul 3.89 4.43 5.00 5.158-Jul 3.48 3.96 5.00 4.109-Jul 3.55 4.03 5.00 3.14

10-Jul 3.10 3.53 5.00 1.6611-Jul 4.51 5.13 5.00 1.7912-Jul 3.75 4.27 5.00 1.0613-Jul 3.44 3.92 5.00 0.0014-Jul 4.61 5.25 5.00 0.2515-Jul 4.06 4.62 5.00 0.00


FINDINGS


Lead Acid Vs Li Ion

• The estimate frequency of blackouts is unchanged

whether the batteries are lead acid or Lithium-Ion.

• The fact that the Lithium-Ion batteries are more efficient

than lead acid batteries resulted in a smaller PV array for

a system using Lithium-Ion batteries compared with that

using lead acid batteries.


However….

• at least 30% of the oversize might be required for lead

acid batteries to ensure effective charging.

• Therefore the tables in the tech info could be applied to

Lithium-Ion batteries directly,

• When deciding on oversize percentage and days of

autonomy for lead acid batteries then an extra 30% (as

per AS/NZS4509) should be added unless there is a

back-up generator and the user is prepared to use it.


Cycle life vs Depth of

Discharge- Lead Acid


Days of autonomy vs Cycle

Life (Lead Acid Battery)

Days of AutonomyDaily Depth of

DischargeCycle Life

1 70% Approx. 2300 cycles

2 35% 5000 cycles

3 23.3% >5800 cycles

4 17.5% >5800 cycles

5 14% >5800 cycles


Temperature Effect


Li Ion

• Lithium-Ion many of those batteries are rated at 6,000 –

10,000 cycles for the usable energy (maximum depth of

discharge) of 80 to 100% of their energy capacity.

• So, 1-day autonomy would still result in a battery cycling

frequently and to a high DoD.

•


Li Ion Cycle Life Vs DOD

and Temperature Effect


FINDINGS


Average Yearly Number of Blackouts vs

Days of Autonomy – no oversize

SiteDays of Autonomy

1 day 2 days 3 days 4 days 5 days

Alice Springs

22.17 12.03 7.38 4.62 2.66

Bairnsdale 23.55 9.38 4.31 1.97 1.14

Cairns 41.62 25.10 16.72 12.00 8.76

Darwin 19.97 12.83 9.28 6.34 4.55

Parkes 20.90 9.21 5.24 2.90 1.76


Possibility of Blackouts in a year vs Days of

Autonomy-No Oversize

SiteDays of Autonomy


Alice Springs

100% 96.4% 78.6% 60.7% 42.9%

Bairnsdale 100% 100% 71.4% 35.7% 17.9%

Cairns 100% 100% 96.4% 82.1% 75.0%

Darwin 100% 96.4% 89.3% 78.6% 53.6%

Parkes 100% 96.4% 67.9% 42.9% 21.4%


Bairnsdale Average Number of Blackouts vs

Days of Autonomy _No oversize

Month PSHDays of Autonomy


January 5.09 1.59 0.41 - - -

February 4.93 0.97 0.10 - - -

March 4.81 1.38 0.34 0.17 0.03 -

April 4.39 1.55 0.48 0.10 - -

May 3.74 3.41 0.83 0.10 - -

June 3.47 5.52 3.41 1.97 1.00 0.55

July 3.75 3.45 2.69 1.66 0.93 0.59

August 4.30 1.07 0.31 0.14 -

September 4.55 0.76 0.14 0.07 - -

October 4.77 0.79 0.03 - - -

November 4.79 1.17 0.14 - - -

December 4.89 1.90 0.48 0.10 - -

Total 4.46 23.55 9.38 4.31 1.97 1.14

Total number of years with blackout out of 28

28 28 20 10 5

Maximum instances of blackout within a year

40 23 18 12 9

Minimum instances of blackout within a year

11 1 0 0 0


Darwin Average Number of Blackouts vs Days

of Autonomy No Oversize

Month PSHDays of Autonomy


January 5.06 6.45 4.59 3.48 2.69 2.03

February 5.34 5.17 3.55 2.69 2.03 1.55

March 6.02 2.93 1.93 1.41 0.97 0.52

April 6.50 0.69 0.17 0.03 - -

May 6.36 - - - - -

June 6.24 0.28 0.03 - - -

July 6.45 - - - - -

August 6.92 - - - - -

September 6.91 0.03 - - - -

October 6.72 - - - - -

November 6.16 0.24 0.10 0.07 - -

December 5.39 4.17 2.45 1.59 0.66 0.45

Total per year

6.17 19.97 12.83 9.28 6.34 4.55

Total Number of years with blackout out of 28

28 27 25 22 15

Maximum instances of blackout within a year

42 32 27 25 20

Minimum instances of blackout within a year

5 0 0 0 0


Bairnsdale: 1-day of Autonomy

Average Number of blackout days

Oversizing of array

No Oversize

10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

January 1.59 1.14 0.90 0.66 0.52 0.52 0.34 0.24 0.24 0.21 0.17

February 0.97 0.79 0.69 0.59 0.45 0.28 0.21 0.17 0.17 0.17 0.14

March 1.38 0.86 0.52 0.31 0.24 0.17 0.14 0.10 0.07 0.07 0.07

April 1.55 0.83 0.55 0.41 0.41 0.31 0.31 0.24 0.24 0.17 0.17

May 3.41 1.76 0.62 0.24 0.17 0.10 0.10 0.10 - - -

June 5.52 2.38 1.66 1.24 0.90 0.66 0.55 0.34 0.24 0.21 0.21

July 3.45 1.45 0.86 0.62 0.41 0.28 0.17 0.14 0.14 0.07 0.07August 1.07 0.62 0.34 0.24 0.17 0.17 0.14 0.07 0.03 0.03 0.03

September 0.76 0.45 0.41 0.31 0.24 0.21 0.21 0.17 0.14 0.07 0.07

October 0.79 0.45 0.41 0.24 0.21 0.14 0.07 0.03 0.03 0.03 -

November 1.17 0.97 0.83 0.48 0.41 0.31 0.21 0.17 0.14 0.14 0.14

December 1.90 1.24 1.03 0.83 0.69 0.52 0.34 0.14 0.10 0.10 0.07

Total per year 23.55 12.93 8.83 6.17 4.83 3.66 2.79 1.93 1.55 1.28 1.14


Bairnsdale: 2-days of Autonomy


Oversizing of array

No Oversize

10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

January 0.41 0.14 0.07 0.03 0.03 0.03 - - - - -

February 0.10 0.03 0.03 0.03 - - - - - - -

March 0.34 0.17 0.17 0.03 0.03 0.03 0.03 - - - -

April 0.48 0.10 0.10 0.07 0.03 0.03 0.03 0.03 0.03 - -

May 0.83 0.07 - - - - - - - - -

June 3.41 0.90 0.41 0.28 0.17 0.07 0.07 0.07 0.07 0.07 0.07

July 2.69 0.41 0.14 0.10 - - - - - - -August 0.31 0.14 0.07 - - - - - - - -

September 0.14 0.10 0.10 0.07 0.07 0.03 - - - - -

October 0.03 - - - - - - - - - -

November 0.14 0.03 - - - - - - - - -

December 0.48 0.24 0.14 0.03 - - - - - - -

Total per year 9.38 2.34 1.24 0.66 0.34 0.21 0.14 0.10 0.10 0.07 0.07


Bairnsdale: 3-days of Autonomy


Oversizing of array

No Oversize

10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

January - - - - - - - - - - -

February - - - - - - - - - - -

March 0.17 0.07 - - - - - - - - -

April 0.10 - - - - - - - - - -

May 0.10 - - - - - - - - - -

June 1.97 0.34 0.21 0.03 0.03 0.03 0.03 0.03 0.03 - -

July 1.66 0.07 - - - - - - - - -August 0.14 - - - - - - - - - -

September 0.07 - - - - - - - - - -

October - - - - - - - - - - -

November - - - - - - - - - - -

December 0.10 - - - - - - - - - -

Total per year 4.31 0.48 0.21 0.03 0.03 0.03 0.03 0.03 0.03 - -


Bairnsdale:

Average Number of blackout days -1 day autonomy

Oversizing of arrayNo

Oversize10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Summary of BlackoutsNumber of years out of 28 with blackouts

28 19 16 12 8 5 3 2 2 1 1

Maximum instances of blackouts in a year

23 8 5 3 2 2 2 2 2 2 2

Minimum instances of blackouts in a year

1 0 0 0 0 0 0 0 0 0 0

3 days autonomyNumber of years out of 28 with blackouts

20 5 3 1 1 1 1 1 1 0 0


18 5 4 1 1 1 1 1 1 0 0

2 days autonomyNumber of years out of 28 with blackouts

28 19 16 12 8 5 3 2 2 1 1


23 8 5 3 2 2 2 2 2 2 2

Minimum instances of blackouts in a year

1 0 0 0 0 0 0 0 0 0 0


2 vs 3 days

• The current version of AS/NZS4509 allows 2-3 days of

autonomy if there is a manual generator available.

However, it also states that it can be less than 3 days if

the generator is automatic,


Bairnsdale 2 vs 3 days

Assuming 3 days autonomy for a manual generator there is :

• 71% (20/28) possibility of having a blackout each year; and

• an average of 4.31 blackouts per year

With 2 days of autonomy and a 10% array oversize results in

slightly better figures than that for 3 days of autonomy and no

oversizing, :




Darwin 2 days vs 3 days

3 days of autonomy results in:



2 days of autonomy and a 10% array oversize would result in:

• a higher possibility of 96% (27/28) for having a blackout each year; and

• a lower average of 7.66 blackouts per year

Increasing the oversize of array at Darwin to 20%, sizing to 2 days of autonomy, would decreases the chance of blackout to 86% (24/28) and even lower average of 5.34.


3 Days of Autonomy (no oversizing) vs

Oversizing for 2 days autonomy with

oversize

Location

3 days of autonomy 2 days of autonomy

Possibility of Blackout

Average Yearly Number of Blackouts

10% Oversize 20% Oversize 30% Oversize







Alice Springs

78.6% 7.38 82.1% 5.3 71.4% 3.45 57.1% 2.48

Bairnsdale 71.4% 4.31 67.9% 2.34 57.1% 1.24 42.9% 0.66Cairns 96.4% 16.72 96.4% 11.83 85.7% 7.14 82.1% 4.66Darwin 89.3% 9.28 96.4% 7.66 85.7% 5.34 78.6% 4.07Parkes 67.9% 5.24 75.0% 3.28 50.% 1.76 42.9% 0.97


5 Days of Autonomy (no oversizing) vs

Oversizing for 2 days autonomy

5 days of Autonomy 2 days of autonomy

LocationPossibility

of BlackoutAverage Yearly

Number of BlackoutsOversize



Alice Springs

42.9% 2.6650%60%

53.6%39.3%

1.591.24

Bairnsdale

17.9% 1.1450%60%

17.9%10.7%

0.210.14

Cairns 75% 8.7640%50%

78.6%71.4%

3.412.38

Darwin 53.6% 4.5550%60%

57.1%50%

2.171.66

Parkes 21.4% 1.7660%70%

25.0%17.9.%

0.310.24


OTHER TECHNICAL CONSIDERATIONS


Battery discharge current

The battery bank must also be able to provide the:

• maximum demand ; and

• the surge demand.


Lead Acid Battery

• the battery capacity must be based on the typical

discharge current for the battery.

• The effective available amp-hours from a lead acid

battery is dependent on the discharge rate.

• A battery being discharged at the 1-hour discharge rate

has a lower amp-hour capacity than when the same

battery is discharged at the 100-hour rate.

• Reducing the number of days of autonomy will result in a

battery typically being operated at higher discharge rates

and hence the lower capacity at this higher discharge

rate must be applied.


Battery charge current

• All batteries also have a maximum charge current

• When oversizing the array, it is important that the

available charging current from the array is not greater

than the maximum charging current of the selected

battery.


Lead Acid Battery

Charge current from Array

Oversize Array Lead Acid (A) Li-Ion (A)

0% 42.80 35.8610% 47.08 39.4420% 51.36 43.0330% 55.64 46.6140% 59.92 50.2050% 64.21 53.7860% 68.49 57.3770% 72.77 60.9680% 77.05 64.5490% 81.33 68.13

100% 85.61 71.71


Maximum Charge Currents for Lead Acid

batteries (0.1C10)

Lead Acid Maximum Charge current 10A/100Ah

Days of Autonomy Batt AhMax Charge current (A)

1 160 16.0

2 320 32.0

3 480 48.0

4 640 64.0

5 800 80.0

Assuming a maximum depth of discharge of 70% for the lead acid battery,

• without oversizing of the array, the battery bank needs to have at least 3 days of autonomy in order to have a maximum charge current from the array less than the 0.1C10 charge current rating.

• The maximum array oversizing even with 3 days autonomy is 10%. • 5 days autonomy –allows 100%noversize


Maximum Charge Currents for Lead Acid

batteries (0.35C10)

Lead Acid Maximum Charge current 35A/100Ah

Days of Autonomy Batt AhMax Charge current (A)

1 160 56.0

2 320 112.0

3 480 168.0

4 640 224.0

5 800 280.0

Assuming a maximum depth of discharge of 70% for the lead acid battery,

with 1-day autonomy the maximum oversize is 30%. All other days of autonomy can accept charge currents from 100% oversize array.


maximum Charge currents of Lithium Ion

batteriesBased on the usable energy of the Lithium-Ion battery being 90% of the rating and that the Lithium-Ion battery is able to have a maximum charge current equivalent to its watt-hour rating

Lithium-Ion Battery Maximum Charge current

Days of AutonomyBattery

WhMax Charge current

(A)

1 5,974 124.5

2 11,947 248.9

3 17,921 373.4

4 23,895 497.8

5 29,869 622.3

Lithium-Ion battery, even at 1-days autonomy, could accept the charge current from the 100% oversized array


CONCLUSIONS


Blackouts and Days of

Autonomy

• A reduction of days of autonomy within a system will

significantly increase the number of days of blackouts in

a year.

• Regardless of the days of autonomy designed into a

stand-alone system, customers need to be aware of the

frequency they are likely to experience loss of power

and/or how often they will need operation of a generating

set to provide back-up power as a result of the system

design.


Days of Autonomy vs

Oversizing

This study showed:

• reducing the designed days of autonomy from 3 days to

2 days, the PV array needs to be oversized by 10%-20%

to maintain outcomes for the system equivalent to or

better than 3 days of autonomy.

• The PV array needs to be oversized by between 50%

and 70% to maintain system outcomes equivalent to or

better than 5 days of autonomy.

• But remember charge/discharge current restraints.


THANKYOU

www.gses.com.au

[email protected]

http://www.gses.com.au/

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