Lead-Acid's Sweet Zone How to get more energy out of your Solar - - PowerPoint PPT Presentation

Lead-Acid's Sweet Zone How to get more energy out of your Solar Batteries & Panels

Presenter Mukesh Bhandari COO

Author Kurtis Kelley

Firefly International Energy Peoria 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 attributes Some lead acid have amazing Deep Discharge performance

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

Rdis Rreg Voc DoD Resistance OCV (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 20 40 60 80 100

Typical lead-acid efficiency vs. SOC

Battery State of Charge (%) Round trip energy efficiency (%)

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 Degrades with 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.

E n e r g y L

• s

s

Variables:

• 1. SPSOC – battery setpoint state of charge – 0% to 100%
• 2. PV array size

– 0 to 40kW

• 3. back-up Generator size

– 0 to 20kW

• 4. Battery storage system size

– 0 to 96kWh

• 5. Converter size

– 0 to 20kW

• 6. 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 attributes affect 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
• ther charging sources are absent.
• Above the SPSoC, the generator will supply recharge only if it can
• perate near its peak operating efficiency (its maximum load

capacity).

20 40 60 80 100 5 10 15 20 25

F re que nc y (% )

Frequency Histogram

State of Charge (%)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 20 40 60 80 100

SOC (%) Monthly Statistics

max daily high mean daily low min

80% SPSoC Battery Array Use 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

20 40 60 80 100 2 4 6 8 10

Fre que nc y (% )

Frequency Histogram

State of Charge (%)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 20 40 60 80 100

SOC (%) Monthly Statistics

max daily high mean daily low min

20% SPSoC Battery Array Use 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 80-100SOC 10,000 20,000 30,000 40,000 Net Present Cost ($)

Cash Flow Summary

PV Generator 1 Trad P bA from 80-100SOC Converter

PV Generator Firefly Oasis Converter

10,000 20,000 30,000 40,000

25 yr. Life costs vs. 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 80 100 0.3 0.4 0.5 0.6 0.7 Levelized Cost of Energy ($/kWh) Levelized Cost of Energy vs. Setpoint SOC Setpoint SOC (%) Fixed OR Solar = 25 %

Levelized Cost of Energy vs. Setpoint SOC Levelized Cost of Energy ($/kWh) 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+ RechargeCosts+Discharg eCosts+ InstallationCosts O∧M Costs⃗

Functionof :recombination efficiency; ter min 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 StorageSystem Size)

BatteryCapital Cost=( Pr oductionCost Gross M argin Delivered Energy

)=(

Pr oductionCost Gross M argin f (η+ ;η−;ηe)

)

InstallationCosts⃗

Functionof : power electronics; HVACCosts; systemvolume(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's Partial-State-of-Charge Battery

All lead-acid batteries are not the same

Thank You

Firefly Oasis Firefly Battery management module