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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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