Photovoltaic Systems Engineering SEC598F17 Photovoltaic Systems Engineering Session 11 Storage for PV Systems Batteries – Part 2 September 26, 2017

Session 10 content PV System Storage Components Batteries Construction, types Operation, reliability, failure mechanisms

PV Systems - Batteries

PV Systems - Batteries

PV Systems - Batteries Heat Spring, S+S short course

Step 3: Battery Selection PV Systems - Batteries Step 3: Battery Selection In summary

PV Systems - Batteries The discharging process At the anode (oxidation, loss of electrons) PbSO4 + 5H20 PbO2 + 3H3O+ + HSO4- + 2e- At the cathode (reduction, gain of electrons) PbSO4 + H3O+ + 2e- Pb + H2O + HSO4- Discharging is spontaneous

PV Systems - Batteries The overall redox process: + PbSO4 + 5H20 = PbO2 + 3H3O+ + HSO4- + 2e- + PbSO4 + H3O+ + 2e- = Pb + H2O + HSO4- or 2PbSO4 + 5H20 + H3O+ + 2e- = PbO2 + 3H3O+ + HSO4- + 2e- + Pb + H2O + HSO4- 2PbSO4 + 2H20 = Pb + PbO2 + 2H2SO4

PV Systems - Batteries Lead-Acid Battery During the charging process, especially with excess “overvoltage”, or after the charging is complete, electrolysis of the H2O can take place producing H2 gas! Lead-acid batteries eventually lose the ability to “hold” a charge, generally due to sulfation, the crystallization of PbSO4. At the start of the battery life, the lead sulfate is an amorphous film, easily dissolved, reverting to lead and lead oxide. But crystalline PbSO4 is stable, doesn’t dissolve during recharging. It reduces available Pb and can crack the electrodes. Sulfation can be minimized with careful attention to both the charging and discharging procedures.

PV Systems - Batteries Lead-Acid Battery Lead acid batteries are less prone to electrolysis and sulfation if the charging protocol does not employ a constant rate. The optimized charging profile looks like this: This current profile is produced by a “charge controller”

PV Systems - Batteries Lead Acid Battery Lead acid batteries had to be redesigned for PV system applications. They have been used in the automotive world for decades, and were designed with thin lead plates with high surface area – to produce high surge currents for the starter motor. The high currents actually help reduce sulfation, but the thin plates disintegrate with repeated deep charge and recharge cycles. Lead-acid batteries used in PV systems will generally go through deep cycles, so much thicker lead plates are employed. This reduces the peak currents but also enhances the durability.

PV Systems - Batteries Lead Acid Battery

PV Systems - Batteries Types of lead-acid batteries Flooded Gel Absorption Glass Mat (AGM)

PV Systems - Batteries Lead Acid AGM Battery C.S.Solanki, Solar Photovoltaic Technology and Systems

PV Systems - Batteries Other batteries for PV system applications Nickel Cadmium (NiCd) Nickel Metal Hydride (NiMH) Lithium Ion Lithium Ion Polymer Flow Batteries

PV Systems - Batteries Li-Ion Battery Operation Li-ion batteries store electrical energy in electrodes made of lithium-intercalation (or insertion) compounds On charging, Li+ ions are deintercalated from the layered LiCoO2 cathode, transferred across the electrolyte, and intercalated among the graphite layers in the anode. On discharging, these processes are reversed, with electrons flowing through the external circuit

PV Systems - Batteries Li-Ion Battery X.Yuan et al., Lithium-Ion Batteries

PV Systems - Batteries Li-Ion Battery

PV Systems - Batteries Li-Ion Battery Components Discharge reactions LiCoO2 or LiFePO4 or LiMnBO3 (cathode) Graphite or TiO2 or Bi (anode) Liquid containing a Li salt (LiPF6) (electrolyte) Polymeric membrane or fabric mat (separator) Discharge reactions Anode (oxidation): LixC6  xLi+ + xe- + C6 Cathode (reduction): Li1-xCoO2 + xLi+ + xe-  LiCoO2 Redox: LiC6 + CoO2 = C6 + LiCoO2

PV Systems - Batteries

PV Systems - Batteries

PV Systems - Batteries Li-Ion Battery

PV Systems - Batteries Flow Battery Rechargeability is provided by two chemical components dissolved in Liquids contained within the system and most commonly separated by a membrane. One of the biggest advantages is the ability to be almost instantly recharged by replacing the electrolyte liquid, while simultaneously recovering the spent material for re-energization (like refilling a fuel tank)

PV Systems - Batteries

PV Systems - Batteries C-Rate If a load is connected to a fully charged battery which causes the battery to discharge in N hours, the discharge rate is defined as: C/N The charging rate is defined in the same fashion High discharge rates result in less charge being available for a load High charge rates, a small fraction is used for charging and a larger fraction is dissipated as heat in the battery

PV Systems - Batteries C-Rate M&V, Ch3

PV Systems - Batteries C-Rate M&A, Ch3

PV Systems - Batteries C-Rate

PV Systems - Batteries Performance (Degree of Discharge) C.S.Solanki, Solar Photovoltaic Technology and Systems

PV Systems - Batteries

References for Batteries R.Messenger and A.Abtahi, Photovoltaic Systems Engineering, 4th Ed., CRC Press, Boca Raton, 2017 J.Jung, L.Zhang, J.Zhang, Lead-Acid Battery Technologies, CRC Press, Boca Raton, 2016 X.Yuan, H.Liu, J.Zhang, Lithium-Ion Batteries, CRC Press, Boca Raton, 2012 C.S.Solanki, Solar Photovoltaic Technology and Systems, PHI Publishing, Bombay, India, 2015