Transcript Photovoltaic (PV) Systems ME 252 Thermal-Fluid Systems

Photovoltaic (PV)
Systems
ME 252
Thermal-Fluid Systems
G. Kallio
Introduction
• Electricity produced from PV
modules must be properly controlled,
stored, converted, and distributed to
be useful
• PV systems are generally divided into
two major categories:
– Stand-alone systems for remote power
– Grid-connected systems for interacting
with the utility grid
• PV system loads may be DC or AC;
here, it is assumed that the loads are
solely AC
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Stand-Alone PV System
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Grid-Connected PV
System
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PV Components
• PV array
• Primary Balance of System (BOS)
– Charge controller
– Battery Bank
– Inverter
• Electrical BOS
– Disconnect & protection devices
– Grounding equipment
• Mechanical BOS
– Array support & alignment structures
– Enclosures
– Ventilation
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The PV Cell
• Photons dislodge electrons from the silicon
atoms which move as a current under the
influence of the electric field set up by the
the P-N junction
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I-V Characteristic
• Isc = short-circuit current
• Voc = open-circuit voltage
• PV cell can be considered a constantcurrent device for a given solar irradiation
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Maximum Power Point
• The maximum power point (MPP) is
where the cell operates at maximum
efficiency and corresponds to a load that
produces the highest IV product
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Effect of Irradiation
• Irradiation has a large effect on current but
little effect on voltage; maximum solar
irradation is about 1.0 kW/m2 without
concentration
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PV Configurations
• PV cell – thin, semiconductor wafer that converts
sunlight to DC current
• PV module – series and parallel cell circuits sealed
in a protective laminate
• PV panel – two or more modules assembled as a
pre-wired, field-installable unit
• PV array – complete power-generating unit that
consists of the required number of modules/panels
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Batteries
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Batteries, cont.
• Batteries are used in stand-alone PV
systems since energy supply  demand
• Functions
– Store electrical energy as it is produced
– Supply power to loads at stable voltages
– Supply power to loads at high operating
currents
• Primary Types
– Flooded lead-acid
– Captive electrolyte lead-acid
– Nickel-cadmium
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Charge Controllers
• Primary Function: maintain batteries at
highest level of charge while protecting
against overcharging by the PV array and
from overdischarge by the load
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Charge Controllers, cont.
• Seven Important Functions
–
–
–
–
–
Prevents battery overcharge
Prevents battery overdischarge
Provides load control
Provide system status information
Interfaces with and controls backup
energy sources
– Diverts PV power to auxiliary load
– Serves as a wiring hub
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Charge Controllers, cont.
• Two methods of charge control
– Shunt regulation
– Series regulation
• Three-stage charge control
– Bulk:: maximum preset charging
current, battery voltage increases
– Absorption: battery voltage held
constant, current decreases
– Float: voltage constant: small amounts
of current as needed
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Inverters
• Inverters convert battery DC power
 AC load power
• Often incorporate equipment to
charge batteries from AC source
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