1. Photovoltaic Systems Engineering Session 17 Solar+Storage Systems
SEC598F18
Photovoltaic Systems Engineering
Session 17
Solar+Storage Systems
Grid-tied Systems, part 2
October 31, 2018

2. Session 17 content Battery Enhanced Systems Design Example
Battery Purposing
Inverter and Battery Selection
Required programming

3. Battery-Enhanced PV Systems – Example
Part 1: Standby Loads

4. Battery-Enhanced PV Systems – Example
Part 2: Inverter Selection
Characteristics
Pout = 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

5. Battery-Enhanced 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
wire loss
CC loss

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

7. Battery-Enhanced PV Systems – Example
Part 3: Battery selection, cont.
Choosing 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:
208 Ah with C/24 rate derates to 186 Ah with C/8 rate

8. Battery-Enhanced 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

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

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

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

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

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