Photovoltaic Systems Engineering Session 22 Solar+Storage Systems SEC598F17 Photovoltaic Systems Engineering Session 22 Solar+Storage Systems Battery Backup Systems, part 2 November 07, 2017

Session 22 content Battery Backup Systems Design Basics Standby Loads Inverter and Battery Selection BOS components Required programming Expansion considerations

Battery Backup Systems – Engineering Issues Block diagram of battery-backup system (dc)

Battery Backup Systems – Engineering Issues Part 1: Load Determination What are the essential electrical loads in the residence? YES NO Refrigerator Electric water heater TV/Radio Electric clothes dryer Computer Electric stove/oven Fans Central Air Conditioning Electric Circuits (lights, chargers) Standby loads

Battery-Backup PV Systems – Engineering Issues Part 2: Battery Selection The battery remains the most common technological approach for storing energy in PV and other electrical systems. It is by no means an ideal solution, but in the absence of another viable storage technology, it is a useful solution There are some credible research efforts to replace or supplement battery storage, include residential scale compressed air (CAES): https://en.wikipedia.org/wiki/Compressed_air_energy_storage

Battery-Backup PV Systems – Engineering Issues Part 2: Battery Selection, cont. As before, the key factor in battery selection is the battery energy capacity. Although the energy capacity should be expressed in kWh (power x time), it is common to define it through (current x time) ampere-hours (Ah): The battery voltage (VBATT) is 6, 12, or 24 V, for lead-acid or Li-ion chemistries

Battery-Backup PV Systems – Engineering Issues Part 2: Battery Selection, cont. Determine the standby loads in kWh/day Determine the battery (array) energy EBATT = Eloads Floss Floss is a number larger than 1 that accounts for wire and inverter losses, typically about 1.04 Convert the battery energy to Amp-hours by dividing by the battery voltage Multiply the battery Amp-hours by 1.25, which is a factor to ensure that the batteries retain a state of charge greater than 20% (or depth of discharge doesn’t exceed 80%)

Battery-Backup PV Systems – Engineering Issues Part 2: Battery Selection, cont. Multiply by appropriate temperature factor Capacity decreases with decreasing temperature Multiply by the number of days of backup Different consideration than for stand-alone system Determine the battery array topology – the series/parallel connections Rule-of-thumb: No more than four parallel strings of batteries. This provides more closely balanced currents

Battery-Backup PV Systems – Engineering Issues Part 3: PV array sizing After the standby loads are calculated, and the battery capacity has been selected, then the PV system array size can be determined Let QBATT be the battery array capacity in Ah (per day) The PV array “capacity” is then defined as QPV = QBATT/hBATT where the factor hBATT is the battery efficiency, typically about 0.9 The PV array capacity must be multiplied by the battery array voltage to yield the required PV array energy (per day): PVWatts can then be used to find the PV array power (in kW) needed to produce the required energy to recharge the battery array PVWatts

Battery-Backup PV Systems – Engineering Issues Part 4: Charge Controller Selection The charge controller is another essential electronic component in any PV system that employs battery storage The charge controller carries out some important functions: It accepts the DC power from the PV array employing the MPPT process It directs the DC power to the battery array matching the optimal charging procedure, as needed It directs the DC power to the inverter for the AC standby loads or grid connection

Battery-Backup PV Systems – Engineering Issues Part 4: Charge Controller Selection, cont. These features must be met in the choice of charge controller: Its DC voltage output must match the battery charging protocol Its DC output must supply enough current to recharge the battery array in one day Its DC current directed to the inverter must not exceed the allowable inverter input There are quite a few Charge Controller manufacturers with very high quality products

Battery-Backup PV Systems – Engineering Issues Part 5: Inverter Selection The inverter in a battery-backup PV system is quite different from the inverter used in a grid-tied system It can accept DC input (through the charge controller) and deliver AC output to both the standby loads and the main panel (and grid) It can accept DC input (through the charge controller) and deliver AC output to the standby loads while disconnected from the main panel (and grid) It can be disconnected from the DC input and allow AC input from the grid to pass through to the standby loads and the main panel

Battery-Backup Grid-Connected PV Systems Inverter bypass box Ground fault breaker box

Battery-Backup PV Systems – Example Part 1: Standby Loads

Battery-Backup PV Systems – Example Part 2: Inverter Selection Characteristics Pout = 3600W @ 120V This power level is less than the highest possible load (5044W), so at some point the inverter will shut down until the load drops. In other words, not all the loads could be operated simultaneously Ipass = 60A (continuous) The load current maximum is 42.4A, and if it is viewed as a continuous flow, then the design current is: Idesign = 1.25 * 42.4A = 53A < 60A

Battery-Backup PV Systems – Example Part 3: Battery selection Daily load -> Eday = 7.35kWh/day Energy supplied to inverter: Esupply = 7.35kWh/(0.98*0.94) = 8.0kWh Charge to be supplied by batteries: Qsupply = Esupply/Vbatt = 8000Wh/(48V) = 166Ah To account for 80% depth of discharge QF = Qsupply/DoD = 166Ah/0.8 = 208Ah

Battery-Backup PV Systems – Example Part 3: Battery selection, cont. Autonomy: Battery choice: Vbatt = 6V Qbatt = 200-250Ah Number of batteries: Eight 6V batteries in series yields 48V outage t < 1 day Grid on Grid on

Battery-Backup PV Systems – Example Part 3: Battery selection, cont. If we chose Pb-acid batteries: Flooded batteries are lower cost, but typically have lower DoD allowances AGM and Sealed batteries are more expensive, can discharge faster, but have fewer O&M troubles A faster discharge rate reduces capacity: 6V @ 208 Ah with C/24 rate derates to 6V @ 186 Ah with C/8 rate

Battery-Backup PV Systems – Example Part 4: Array sizing On a daily basis, charge to be supplied by the array: Qarray = Qsupply/hbatt = 166 Ah/0.9 = 185 Ah This charge must be supplied at 48V Earray = Qarray * Vbatt = 185 Ah * 48 V = 8.88 kWh To account for array losses Earray, daily = Earray /(hmismatchhwirehMPPT) = 8.88 kWh/(0.85*0.98*0.98) = 10.9 kWh

Battery-Backup PV Systems – Example Part 4: Array sizing, cont. On a monthly basis Earray, monthly = Edaily*30 = 327 kWh Using PVWatts A system of 4kW would be perfectly suitable Upon reconsidering A system of 4kW would over-produce most of the year, so it is prudent to calculate a size that match the need in the month most likely to have power failures So, 4kW can be reduced to 2.8kW

Battery-Backup PV Systems – Example Part 5: Charge Controller and Inverter considerations PV Battery Charging

Battery-Backup PV Systems – Example Part 5: Charge Controller and Inverter considerations PV Battery Charging Complete

Battery-Backup PV Systems – Example Part 5: Charge Controller and Inverter considerations No PV

Battery-Backup PV Systems – Example Part 5: Charge Controller and Inverter considerations No Grid

Battery-Backup PV Systems – Example Part 5: Charge Controller and Inverter programming

Battery-Backup PV Systems – Example Part 5: Charge Controller and Inverter programming

Battery-Backup PV Systems – Example Part 5: Charge Controller and Inverter programming