Basic Photovoltaics (PV 101)

Balance of System Components

Basic PV Manual: Chapter 5

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Chapter 5:Balance of System ComponentsAll PV systems consist of modules, but additional components vary with type of system. The balance of system (BOS) components include “everything other than the array” – mounting system, wiring, inverters, charge controllers, batteries, disconnect switches, load centers, etc.

Content outline:

• Inverters

• Charge controllers & batteries

• Maximum Power Point Tracking (MPPT)

• Combiner boxes, disconnect switches, lightning arrestors, load centers & kWh meters

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Inverters• Series (string) inverters• Multiple PV modules connected in series to create high-voltage DC• Utility interactive systems

• Microinverters• Each module gets its own inverter; output connected to AC trunk cable

• Stand-alone inverters• Low voltage DC for battery banks (12, 24, 48 volts)• Can be lower quality than grid-tied inverters, depending on loads

• Multimode inverters• Contain automatic transfer switch to move from grid power to battery

bank (for critical loads) or to store energy “behind the meter”

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Series (String) Inverters• High voltage DC system (250-600 VDC)• 600 volts is the maximum allowed for a residence• 1,000 & 1,500-volt systems used in commercial applications

• PV modules are wired in series and brought to the inverter

• Series strings can be combined in parallel and brought to the inverter or a combiner box

• Allows for smaller conductors from the array (or combiner box) to the inverter

• Some inverters offer multiple MPPT inputs for different strings to maximize production

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Series (String) Inverters• High voltage reduces conductor size

This is why transmission and distribu1on lines are high voltage

• Volts (E) x amps (I) = wa^s (P)

Example:

120 V x 10 A = 1,200 W (1.2 kW)

240 V x 5 A = 1,200 W (1.2 kW)

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Series (String) Inverters

Example: Two identical 2.5 kW PV arrays

System #1 = 24 V (battery-based)

2.5 kW ÷ 24 V = 104 A

4/0 AWG aluminum

System #2 = 300 V (string inverter)

2.5 kW ÷ 300 V = 8.3 A

6 AWG copper

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Combiner Boxes• Combines paralleled strings of PV modules, each

string protected with its own fuse

• If one string has an issue, the rest of the array can produce without interruption

• Only required for arrays with three or more parallel strings

• Located at or near the array

• Years ago were homemade

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Combiner Box

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Maximum Power Point Tracking (MPPT)As the voltage or current varies in a PV system, utility interactive inverters maximize power output by “converting” excess voltage to current through high-speed electronics.

Source: Ecodirect. “MPPT Chart.” ecodirect.com 9

Inverters with Multiple MPPT• Many string inverters come with multiple inputs or

“channels” for MPPT optimization

• Allows for maximum efficiency when there are paralleled strings, especially with different tilt or roof angles

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DC-to-DC Optimizers • Provide MPPT for each PV module

• Correct for soiling, shade, module mismatch, etc.

• Maintain consistent voltage to the inverter

• Provide automatic shutdown for each module whenever inverter (grid) is down

• Good for monitoring

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Microinverters• Mounted on each PV module

• Change DC to AC

• Synchronize with the grid

• Provide anti-islanding protection

• Provide MPPT for each module

• Output plugs into AC trunk cable

• Good for monitoring

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Stand Alone (Off-Grid) Inverters

• System operates as a self-contained electrical system

• Interacts with charge controller & ba^ery bank• Low voltage systems (12, 24, or 48 VDC)• Changes DC into AC, boosts to higher voltage (120 or 240 VAC)

• Hybrid systems are common: mulcple sources can feed a ba^ery bank

• Square wave and modified square wave inverters can be used with certain “simpler” loads

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Stand Alone (Off-Grid) InvertersSINE WAVE INVERTER

• Produces a smooth waveform, similar to grid-quality AC• Recommended for

electronics, or anything that requires a “clean” waveform

MODIFIED SQUARE WAVE INVERTER

• Mimics a sine wave by switching the DC current on-and-off (very quickly)

• Takes “chopped” DC pieces and rearranges them to form a pseudo-sine wave

• Use with caution!

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Hybrid Systems: PV + Wind

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Multimode Inverters• When grid is present and healthy, the inverter is connected

and interactive (running in parallel) with the grid and contains anti-islanding protection

• If the power goes out, the inverter’s internal transfer switch moves your system to battery bank (critical loads)

• When power is restored, the transfer switch resynchronizes and reconnects to the grid

• The grid can be used to exercise/charge the battery bank

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Multimode Inverters & Batteries: New Products

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Grid-Tied InvertersALL grid-ced inverters – string inverters, microinverters, or mulcmode inverters – must be:

• UL-listed (1741) for anc-islanding proteccon

• Pure sine wave inverters

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Battery Types• Flooded lead-acid (deep cycle)

• Sealed lead-acid (deep cycle)

• Lithium-Ion

• Nickel-Cadmium (NiCd)

• Flow batteries

• All are expensive

• All require attention& maintenance

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Batteries

• Most efficient at 77˚ F

• Lose 20% of amp-hour capacity at 32˚ F

• Gain 12% amp-hour capacity at 122˚ F

• Need to be vented

• Need to be properly stored

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BatteriesMaintenance:

• Water batteries every 30 days

• Equalize every 60-90 days

• Inspect & clean bus bars every year

• Check specific gravity

• Clean up spills

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Safety• Venting• Passive (gravity)• Active (fans)• No smoking!

• Spill containment

• Baking soda

• Hydrocaps

• Personal Protective Equipment (PPE) required• Gloves, aprons, eye protection, insulated tools

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Charge Controllers• Protect batteries from overcharging

• Protect batteries from being drained too low (DOD)

• Charge regulation (settings)

• Diversion for dump loads

• Equalizing

• Input voltage can be variable

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Charge ControllersBattery Charging

PWM (Pulse Width Modulation)

• High speed connection and disconnection between batteries and charging source

• A series of short charging pulses are sent to the battery bank, depending on state of charge

• Low charge = long pulses• Full charge = short pulses

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Charge ControllersBattery Charging

PWM (Pulse Width Modulation)

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Maximum Power Point Tracking (MPPT)• Without MPPT, a PV module would “follow” ba^ery voltage

• At Standard Test Condicons, a 75-W PV module may be rated for 17 volts and 4.4 amps

17 V x 4.4 A = 75 W• A ba^ery bank at 10 VDC will be charged slightly above 10 volts

10 V x 4.4 A = 44 W

41% less than nameplate racng...

• With MPPT, the PV module operates at full output (75 W), regardless of ba^ery voltage

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Maximum Power Point TrackingWith MPPT, the PV module operates at full output, regardless of battery voltage

Using a DC to DC converter, unused voltage is converted to current

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Metering• All battery-based systems have a meter that keeps track of

the state of charge (SOC) of the battery bank

• If the resource is reduced, or loads are increased, energy consumption may need to be reduced, based on battery bank’s SOC

• Metering is necessary so you know the health of your battery bank

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Metering• Battery voltage

• Current in & out

• A-H of battery bank used, depth of charge

• Percent of battery bank left, state of charge

• Historical data

• Equalizer reminder

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Disconnect Switches & Lightning Protection

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Load Centers• Utility interactive systems are connected to the grid at or

very near the main service

• PV output can be connected to either SUPPLY-side or LOAD-side of the load center • SUPPLY-side = “before” the utility kWh meter, “before” main breaker• LOAD-side = connected to 2-pole circuit breaker in load center• Depends on existing electrical infrastructure and inverter size desired• Depends on utility interconnection requirements

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Load Centers & kWh Meters

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Chapter 5 Review:Balance of System ComponentsREAD CHAPTER 5 IN THE BASIC PV MANUAL

• Inverters

• Charge controllers & batteries

• Maximum Power Point Tracking (MPPT)

• Combiner boxes, disconnect switches, lightning arrestors, load centers & kWh meters

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