1. 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

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

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

4. 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

5. 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):

6. 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

7. 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%)

8. 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

9. 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

10. 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

11. 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

12. 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

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

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

15. Battery-Backup 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

16. 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

17. Battery-Backup 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

18. 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:
208 Ah with C/24 rate derates to 186 Ah with C/8 rate

19. 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

20. 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

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

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

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

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

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

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

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