1. Battery Backup PV Systems Design Considerations
Session 18
Battery Backup PV Systems
Design Considerations
November 03, 2015

2. Session 18 content Battery Backup PV Systems Design Basics
Standby Loads
Inverter and Battery Selection
BOS components

3. Learning Outcomes
An examination of the design of a grid-connected battery backup photovoltaic system leading to a recognition of the difficulties in large expansion of this technology

4. Battery Backup PV Systems – The Design Process
Design Steps in a battery backup PV system
Examination of site and estimation of performance
Securing financing
Carrying out PV system engineering and design
Standby loads
Connection to charge controllers and batteries
Altered inverter design
Securing relevant permits
Battery housing
Inverter operation
Construction
Inspection
Connection to the grid
Performance monitoring

5. Battery Backup PV Systems – Engineering Issues
Steps in annual system performance design
Evaluation of solar availability, electrical consumption, essential electrical loads
PV array sizing
Inverter selection
Module selection
Charge controller selection
Battery selection
Balance of system

6. Battery Backup PV Systems – Engineering Issues
The Battery-Backup Grid-Connected PV System resembles the Grid-Connected PV System in that the grid and the PV source work together to supply electrical power to the residence – when the grid is available
When the grid is unavailable, a grid-connected PV system is automatically disconnected from the grid – for safety considerations
But a battery-backup system continues to provide electrical power to the residence through the battery/PV portion of the system

7. Battery Backup PV Systems – Engineering Issues
When there is a utility outage, the inverter disconnects from the grid, but continues to monitor the status of the grid, and automatically reconnects when the grid is stable
The inverter for a battery-backup system is more complicated and serves several purposes:
It supplies AC power to the grid, when available
It serves as the conduit to charge the batteries with grid electricity if the PV system cannot do this
It supplies AC electricity to the “standby loads” in the residence when the grid is down

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

9. Battery Backup PV Systems – Engineering Issues
Block diagram of battery-backup system (ac)

10. Battery Backup PV Systems – Engineering Issues
Step 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

11. Battery Backup PV Systems – Engineering Issues
Step 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 a true electricity storage technology, it is a viable solution

12. Battery Backup PV Systems – Engineering Issues
Step 2: Battery Selection
The battery that has seen the widest application in PV systems is the tried-and-true lead-acid embodiment
However, there are alternative battery technologies poised to change the selection process

13. Battery Backup PV Systems – Engineering Issues
Step 2: Battery Selection
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.

14. Battery Backup PV Systems – Engineering Issues
Step 2: Battery Selection

15. Battery Backup PV Systems – Engineering Issues
Step 2: Battery Selection
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 batteries

16. Battery Backup PV Systems – Engineering Issues
Step 2: Battery Selection
The process used to select the battery has these steps:
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 do not discharge by more than 80%
Multiply by the number of days of backup
Determine the battery array topology – the series/parallel connections

17. Battery Backup PV Systems – Engineering Issues
Step 3: PV array sizing
After the standby loads are calculated, and the battery capacity has been selected, 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

18. Battery Backup PV Systems – Engineering Issues
Step 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

19. Battery Backup PV Systems – Engineering Issues
Step 4: Charge Controller Selection
These features must be met in the choice of charge controller:
Its DC output must match the battery array voltage
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

20. Battery Backup PV Systems – Engineering Issues
Step 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 pass through AC input from the grid to the standby loads and the main panel

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