Lead-Acid's Sweet Zone

How to get more energy out of your Solar Batteries & Panels

PresenterMukesh Bhandari COO

AuthorKurtis Kelley

Firefly International EnergyPeoria Illinois

Feb 2014

Lead-Acid's Sweet Zone

How to get more energy out of your Solar Batteries & Panels

Many off-grid installations operate below 50% efficiency

but

Can operate close to 95% efficiency

Lead-Acid's Sweet Zone

Lowers your cost per kWh near 50%.

Lower battery-array spec. amp-hrs needed for same functional cycling capacity

More efficient use of panel solar power generated

Lead-Acid characteristics you need to know

Charge and Discharge SOC changes

Resistance some components increase & others decrease

Chemistry goes through phases between easy and difficult

Secondary Reactions such as gassing can become easier than charging

All lead acid batteries share certain basic attributesSome lead acid have amazing Deep Discharge performance

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.8

1.85

1.9

1.95

2

2.05

2.1

2.15

2.2

HPPC testing - typical lead acid cellRdis

Rreg

Voc

DoD

Re

sis

tance

OC

V (

V)

Resistance vs. State-of-Charge

Charge Resistance (Rreg) and Discharge Resistance (Rdis) vary with the State-of-Charge (SOC)

Rapidly rising charge resistance >80% SOC ( gassing)

Lead-acid batteries

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

SOC

Storage Efficiency vs. SOC

100 90 80 70 60 50 40 30 20 10 00

20

40

60

80

100

Typical lead-acid efficiency vs. SOC

Battery State of Charge (%)

Ro

und t

rip e

ne

rgy e

ffic

iency (

%)

Battery efficiency changes with state-of-charge

The Sweet Zone

Above 80% SOC, battery efficiency is very low& Charge Cycling losses are high.

This is also where most battery systems operate& where losses largely from gassing / electrolysis

Gassing also competes for energy at these higher voltages

Battery Efficiency Degradeswith Cycle Life

Recharge Strategy is Important in Energy Efficiency

- How you control current, voltage, and time have a big impact.

- Float charging is almost never recommended.

Note: Colored bands represent various common charging strategies.

En

erg

y L

os

s

Variables:1. SPSOC – battery setpoint state of charge – 0% to 100%2. PV array size – 0 to 40kW3. back-up Generator size – 0 to 20kW4. Battery storage system size – 0 to 96kWh5. Converter size – 0 to 20kW6. random 25% day-to-day variability allowed in load

Model Variables

The Homer Model, originally developed by NREL, was used to find optimal system within the variable ranges listed.

Every combination was analyzed

How do these Battery attributesaffect System Efficiency?

1. System must meet all loads.

2. Generator operates at 100% efficiency or nothing.

3. Average 30 kWh /day – hourly load data from US home.

4. 38' North Latitude, approximate center of USA

5. One year of hourly data analysis

6. Lead-acid Batteries

7. Generator is cycle charging (CC)

8. 25 year system analysis

Model Assumptions

What is Set-Point-State-of-Charge?(SPSoC)

quick definition- The SPSoC is used to tell the system when the batteries must be

charged.

- The SPSoC requires that the battery State-of-Charge be determined.

- Below the SPSoC, the generator will supply recharge energy if other charging sources are absent.

- Above the SPSoC, the generator will supply recharge only if it can operate near its peak operating efficiency (its maximum load capacity).

0 20 40 60 80 1000

5

10

15

20

25F

req

ue

nc

y (

%) Frequency Histogram

State of Charge (%)

Jan Feb M ar Apr M ay Jun Jul Aug Sep Oct Nov Dec0

20

40

60

80

100

SO

C (

%)

Monthly Statis tics

m ax

dai ly high

m ean

dai ly low

m in

80% SPSoC Battery ArrayUse Summary for

Traditional Lead-Acid

Battery storage system spends most of it's life in higher states of charge

At 80% SPSoC there seems to be sufficient returns to a full charge

Data generated with Homer Legacy software available from Homer Energy, LLC

0 20 40 60 80 1000

2

4

6

8

10F

req

ue

nc

y (

%)

Frequency Histogram

State of Charge (%)

Jan Feb M ar Apr M ay Jun Jul Aug Sep Oct Nov Dec0

20

40

60

80

100

SO

C (

%)

Monthly Statis tics

m ax

dai ly h igh

m ean

dai ly low

m in

20% SPSoC Battery ArrayUse Summary for

Carbon Foam Lead-acid

Battery storage system SOC is a broad zone – much in the Sweet Zone

At 20% SPSoC the system rarely sees a full charge

Data generated with Homer Legacy software available from Homer Energy, LLC

PV Trad PbA from 80-100SOC0

10,000

20,000

30,000

40,000

Ne

t P

res

en

t C

os

t ($

)

Cash Flow Summary

PVGenerator 1Trad PbA from 80-100SOCConverter

PV Generator Firefly Oasis

Converter0

10,000

20,000

30,000

40,000

25 yr. Life costsvs.

Set Point State of Charge (SPSoC)

80% SPSoC

20% SPSoC

Data generated with Homer Legacy software available from Homer Energy, LLC

Trad. PbA

Cost of Energy vs.Battery SPSoC

20 40 60 80 1000.3

0.4

0.5

0.6

0.7

Levelized

Co

st

of

En

erg

y (

$/k

Wh

)

Levelized Cost of Energy vs. Setpoint SOC

Setpoint SOC (%)

FixedOR Solar = 25 %

Levelized Cost of Energy vs. Setpoint SOC

Leveliz

ed C

ost

of

Energ

y ($

/kW

h)

Setpoint SOC (%)

20 40 60 80 100

0.7 0.6 0.5 0.4 0.3

Energy Costs increase as Setpoint SOC increases,representing increasing efficiency losses approaching the 100% SP SOC

Data generated with Homer Legacy software available from Homer Energy, LLC

Set Point State-of-Charge SPSoC

20% 80%

PV (kW) 15 20

Gen (kW) 6 3

Converter (kW) 4 4

Energy Storage Capacity (kWh)

19 77

Initial capital $21,349 $31,499

Diesel (L) 815 496

Gen (hrs) 325 395

Operating cost ($/yr)

$1,936 $3,309

COE ($/kWh) $0.33 $0.53

System Design & Energy costs based on SPSoC

Data generated with Homer Legacy software available from Homer Energy, LLC

How It All Ties Together

Levalized Cost of Energy ($/ kWh)=O∧M Costs+ Re charg eCosts+Discharg eCosts+ InstallationCosts

O∧M Costs⃗ Function of : recombination efficiency ; termin al design ; replacements ; cell equalization

Re chargeCosts=Cost of Grid Energy

Wh Re charge Efficiency

Discharg eCosts=BatteryCosts

Total Energy Discharg ed=

(Battery Capital Cost )∗(Energy Storage System Size )( # cycles)∗(% DoD)∗(Capacity FadeQuotient )∗(Energy Storage System Size )

BatteryCapital Cost=(Pr oductionCost

Gross M arg in

Delivered Energy )=(Pr oductionCost

GrossM arg in

f (η+ ;η− ; ηe ) )InstallationCosts⃗ Function of : power electronics ; HVACCosts ; system volume(Wh/L )

HVAC Costs⃗ Functionof :Whefficiency ; operating temperature ; Wh/L

The total cost of Ownership

Sulfation vs. Overcharge

-The Quandary-

Solution:Operate battery in

PSOC (easy in Firefly –

since no hard sulfation)

Problem:PSOC operation

causes Hard Sulfation

(except in Firefly)

Problem: Frequent recharge

to 100% SOC lowers cycle life & reduces efficiency, increases losses

Solution: Frequent recharge to

100% SOC (bad idea, but due to

poor PSOC in common cells, resort to this

wrongly)

What Really Matters?

Attributes that don’t matter much:

• Whr/kg

• Wh/l

• Cost of battery

• Coulombic efficiency (Ah efficiency)

Attributes that matter a lot:

• Energy efficiency

• Cycle life

• Calendar life

• Maintenance costs ( & cost of ownership)

Okay, it all matters...we're just trying to make a point here.

Firefly Batteries

the ONLY High-Capacity, PSOC, PbA

Battery Technology

Firefly's Carbon Foam Battery:

1. Insensitive to PSOC operation range

great PSOC performance, larger PSOC dynamic range with long life – better efficiency since avoiding the gassing “Knee”

2. No PSOC restriction ( recharge when convenient, not to avoid hard-sulfation issues)

3. No Float Charging (avoid gassing losses)

4. High Useful Capacity ( 50% to 100% larger cap w/o compromising lifetime excessively)

5. Deep Discharge

6. Exceptional Cycle Life

Did you really think that you'd get through the entire presentation without a sales pitch?

Firefly'sPartial-State-of-Charge Battery

All lead-acid batteries are not the same

Thank You

Firefly Oasis Firefly Battery management module