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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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