1. Photovoltaic Systems Engineering Session 16 Solar+Storage Systems
SEC598F18
Photovoltaic Systems Engineering
Session 16
Solar+Storage Systems
Grid-tied Systems, part 1
October 29, 2018

2. Session 16 content Solar + Battery PV Systems Design Basics
Standby Loads
Inverter and Battery Selection
BOS components
Required programming
Expansion considerations

3. Learning Outcomes
An examination of the design of a grid-connected battery-enhanced photovoltaic system leading to a recognition of the complexities in solar + storage technology

4. Battery-Enhanced PV Systems – The Design Process
Design Steps in a battery-enhanced 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 – The Design Process

6. Battery-Enhanced PV Systems – The Design Process
Step 3: System Engineering and 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

7. Battery-Enhanced PV Systems – Engineering Issues
The Battery-Enhanced 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

8. Battery-Enhanced PV Systems – Engineering Issues
When there is a utility outage, the inverter in a conventional grid-tied system 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

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

10. Battery-Enhanced PV 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

11. Battery-Enhanced 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):

12. Battery-Enhanced PV Systems – Engineering Issues
Part 2: Battery Selection, cont.
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):
1 Ah = 1 Wh V BATT = kWh V BATT
The battery voltage (VBATT) is 2, 6, 12, or 24 V, for lead-acid or Li-ion chemistries

13. Battery-Enhanced 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%)

14. Battery-Enhanced 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
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

15. Battery-Enhanced 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

16. Battery-Enhanced 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 (usually) 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

17. Battery-Enhanced 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

18. 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

19. Battery-Enhaced Grid-Connected PV Systems
Inverter bypass box
Ground fault breaker box