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 Resistance vs. State-of-Charge Lead-acid batteries Charge Resistance (Rreg) and Discharge Resistance (Rdis) vary with the State-of-Charge (SOC) Rapidly rising charge resistance >80% SOC ( gassing) Rdis Rreg Voc 2.2 HPPC testing - typical lead acid cell 1.4 2.15 1.2 2.1 2.05 0.8 2 0.6 1.95 0.4 1.9 0.2 0 0% 100% 1.85 10% 90% 20% 80% 30% 70% 40% 60% 50% 50% DoD SOC 60% 40% 70% 30% 80% 20% 90% 10% 1.8 100% 0% OCV (V) Resistance 1 Storage Efficiency vs. SOC Battery efficiency changes with state-of-charge 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 Round trip energy efficiency (%) Typical lead-acid efficiency vs. SOC 100 80 60 40 20 0 100 The Sweet Zone Gassing also competes for energy at these higher voltages 90 80 70 60 50 40 Battery State of Charge (%) 30 20 10 0 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. Energy Loss Note: Colored bands represent various common charging strategies. How do these Battery attributes affect System Efficiency? Model Variables The Homer Model, originally developed by NREL, was used to find optimal system within the variable ranges listed. Every combination was analyzed 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 Assumptions 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 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). F r e q u e n c y (% ) 80% SPSoC Battery Array Use Summary for Traditional Lead-Acid Fr e que ncy His togr am 25 20 15 10 5 0 0 20 40 60 State of Charge (%) 80 100 Battery storage system spends most of it's life in higher states of charge M onthly Statis tics SOC (%) 100 80 m ax daily hi gh 60 m ean 40 daily low m in 20 0 Jan Feb M ar Apr M ay Jun Ju l Aug Sep Oct Nov Dec At 80% SPSoC there seems to be sufficient returns to a full charge Data generated with Homer Legacy software available from Homer Energy, LLC F re qu e nc y (% ) 20% SPSoC Battery Array Use Summary for Carbon Foam Lead-acid Fr e que ncy His togr am 10 8 6 4 2 0 0 20 40 60 State of Charge (%) 80 100 Battery storage system SOC is a broad zone – much in the Sweet Zone M onthly Statis tics SOC (%) 100 80 m ax daily high 60 m ean 40 daily low m in 20 0 Jan Feb M ar Apr M ay Ju n Jul Aug Sep Oct Nov Dec At 20% SPSoC the system rarely sees a full charge Data generated with Homer Legacy software available from Homer Energy, LLC 25 yr. Life costs vs. Set Point State of Charge (SPSoC) Cash Flow Summary 80% SPSoC Net Present Cost ($) 40,000 PV Generator 1 Trad PbA from 80-100SOC Converter 30,000 20,000 10,000 0 PV Trad PbA from 80-100SOC Trad. PbA 40,000 30,000 20% SPSoC 20,000 10,000 0 PV Generator Firefly Oasis Converter Data generated with Homer Legacy software available from Homer Energy, LLC Cost of Energy vs. Battery SPSoC Levelized Cost of Energy vs. Setpoint SOC 0.7 Levelized Cost of Energy vs. Setpoint SOC 0.7 Fixed 0.6 0.5 0.6 Levelized Cost of Energy ($/kWh) Levelized Cost of Energy ($/kWh) OR Solar = 25 % 0.5 0.4 0.4 0.3 0.3 Energy Costs increase as Setpoint SOC increases, representing increasing efficiency losses approaching the 100% SP SOC 20 20 40 40 60 Se tpoint SOC (%) 60 80 80 100 100 Setpoint SOC (%) Data generated with Homer Legacy software available from Homer Energy, LLC System Design & Energy costs based on SPSoC 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 $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 Initial capital Data generated with Homer Legacy software available from Homer Energy, LLC How It All Ties Together The total cost of Ownership Levalized Cost of Energy ( $/ kWh)=O∧M Costs+ Re charg e Costs+Discharg eCosts+ InstallationCosts O∧ M Costs⃗ Function of : recombination efficiency ; ter min al design; replacements ; cell equalization Re ch argeCosts= Cost of Grid Energy Wh Re ch arge Efficiency Discharg eCosts= Battery Costs ( Battery Capital Cost )∗( Energy Storage System Size ) = Total Energy Discharg ed ( # cycles )∗( % DoD )∗( Capacity Fade Quotient )∗( Energy Storage System Size ) ( Battery Capital Cost= Pr oductionCost )( Pr oduction Cost Gross M argin Gross M argin = Delivered Energy f (η+ ;η− ; ηe ) Installation Costs⃗ Function of : power electronics ; HVAC Costs ; system volume(Wh/ L ) HVAC Costs⃗ Functionof : Whefficiency ; operating temperature ; Wh/ L ) Sulfation vs. Overcharge -The QuandaryProblem: PSOC operation causes Hard Sulfation (except in Firefly) Solution: Frequent recharge to 100% SOC Solution: Operate battery in PSOC (easy in Firefly – since no hard sulfation) (bad idea, but due to Problem: Frequent recharge to 100% SOC lowers cycle life & reduces efficiency, increases losses 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 Partial-State-of-Charge Battery All lead-acid batteries are not the same 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 Oasis Firefly Battery management module Thank You