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

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

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

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

Battery-Backup PV Systems – The Design Process

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

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

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

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

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

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): https://en.wikipedia.org/wiki/Compressed_air_energy_storage

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 = 1 1000 1 kWh V BATT The battery voltage (VBATT) is 2, 6, 12, or 24 V, for lead-acid or Li-ion chemistries

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%)

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

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

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

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

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

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