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PV System Components
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
PV system components
Key issues in components:
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Power loss in each component
Implication of components on maintenance, life time of system
Matching of components
Effect on cost of adding component/s
PV system components:
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PV modules
Batteries
Power conditioning
Loads
Balance of systems
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Batteries
• Batteries are required for the following reasons:
– Load profile doesn’t match solar radiation profile
– Mitigating effect of variability in solar radiation
• Have a major impact on the performance of PV systems,
altering the maintenance, design, reliability and safety. They
are a large portion of the total cost of a system.
• The key issues for batteries:
– Why and how battery voltage and capacity change with system and
battery parameters
– Operating constraints on batteries; constraints of state of charge,
charge/discharge rate, temperature
– Lifetime and safety issues with batteries
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Batteries
Battery basics:
• Redox reactions
• Components of batteries
• Equilibrium voltage – Nernst equation
• Ideal battery capacity – calculations
• Non-equilibrium voltage and capacity: Effect of reaction
rates
– Polarization: Activation, Mass transport, Resistance
– Effect of charging, discharging, age
• Time dependent effects in batteries i.e. aging
• General safety issues with batteries
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Batteries
Battery characteristics
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Charging/discharging parameters: State of Charge (SOC), Depth of
Discharge (DOD), discharge rate
Capacity
Efficiency
Energy, volumetric and power density
Other electrical parameters
Lead Acid batteries
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Operation of lead acid batteries
Lead acid battery characteristics
Issues specific to lead acid batteries
Potential problems
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Batteries
Lead acid batteries configurations
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Electrode materials: addition of small amounts of other materials
affects gassing, charging, sulfation etc..
Electrolytes: Gelled or Absorptive Glass Mat (AGM) batteries
Battery housing: flooded, sealed, valve-regulated
Types of lead-acid batteries
Other battery systems
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Nickel-cadmium
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Chemistry basics
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Chemistry basics
• Mole, molar mass
– Mole is unit of amount of a substance i.e. one mole of Iron contains
the same number of atoms as one mole of gold etc..
– Molar mass is how much one mole of an element weighs in
grammes, so if the molar mass of Silver (Ag) is 107.87 it means
107.87 grammes of Ag contains one mole of Ag atoms
• Molarity
– Concentration of a solution expressed as moles per litre
• Balancing chemical equations
– Need to make sure we are losing elements or gaining them
– Can also figure out which reactions should take place and which
shouldn’t
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Chemistry Basics
• Oxidation refers to when an element or molecule loses
electrons i.e. increase in oxidation state
• Reduction refers to when an element or molecule gains
electrons i.e. decrease in oxidation state
• The oxidation and reduction reactions are paired in what is
called reduction-oxidation or redox reactions
• Redox reaction represents the transfer of electrons (and
hence charge) from one material to another
• Also tell us if a reaction will occur spontaneously
• Redox reactions are therefore the basis for batteries
• Redox reactions typically take place in close proximity to
each other
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery basics
• Redox reaction; physically separated
• Oxidation reaction generates electron at surface of anode;
electron transported to cathode; ion in electrolyte transported
to anode; voltage due to chemical potential difference
between anode and cathode
Components
• Electrodes (anode, cathode)
• Electrolyte
• Electron transport (wire)
• Ion transport (electrolyte)
• Housing
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery basics
• Primary battery is one that only discharges – cannot recharge
• Secondary battery can be re-charged
• Anode and cathode collectively
referred to as electrodes
• Oxidation takes place at anode
• Reduction takes place at cathode
• Need to make electrical contact
to both electrodes
• Solution surrounding electrodes
is referred to as the electrolyte
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery basics
• Primary battery is one that only discharges – cannot recharge
• Secondary battery can be re-charged
• Need to be able to transport ions
to maintain charge throughout the
reaction
• Require a conductive medium
• Conceptually use a salt bridge
• Practically, it makes more sense
to use a common electrolyte for
both half-reactions
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Redox reactions
• Redox reactions are paired reduction/oxidation reactions
where electrons are transferred between the reactions
• The transferred electrons come about due to changes in the
valence energy of the materials in the reactions
• Reduction reaction: acquires electrons in order to lower the
valence state of a material
• Oxidation reaction: electrons emitted to increase the valence
state of a material
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Redox reactions
• Overall redox reaction:
• Example
• Example
Balance equation and identify the elements that are oxidized
and reduced respectively
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Redox reactions
• Often helps to separately write oxidation and reduction
reactions (“half-cell reactions)
• Important to figure out these half-cell reactions, some
elements can have multiple oxidation states (think Fe)
• Look up tables exist for these reactions – very useful, we
will see why in a second
• What about the other stuff like SO42- ions? They don’t
change oxidation state, so don’t care about them
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Electrochemical Potential
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Electrochemical potential is a measure of the average energy of the
valence states of a material – analogous to the Fermi energy in
metals/semiconductors
Lowest energy state is a full outer valence band, typically meaning 8
electrons
– Valence changes that get you closer to the ideal state are spontaneous
occurring if an electron is available or an extra electron can be taken by another
material
– Spontaneous changes defined as being positive
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Electrochemical Potential
• A material has a certain electrochemical potential which
depends on the electronic configuration of the compound
• In going from one form to another, the compound changes
it’s electrochemical potential, dependant on the change in
valence state of compounds
• We can measure the change in electrochemical potential and
under standard conditions (25 C, 1 M solutions, compared to
H2), the change in electrochemical potential can be
measured, and is given in a table
– Large positive number indicates a good acceptor of electrons: good
oxidizing agent, it gets reduced in a reaction
– Large negative number indicates a good source of electrons: good
reducing agent, it gets oxidized in a reaction
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Table
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Table
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Electrochemical potential
• Table of electrochemical potentials gives direction of
spontaneous reactions and standard potential of a halfreaction
• The standard potential of a full reaction is simply the sum of
the potentials for the two half-reactions
– An overall positive standard potential indicates the reaction will
proceed spontaneously
– A negative overall standard potential indicates the reaction will not
proceed spontaneously
– Reversing the direction of a (half-)reaction reverses the sign of the
standard potential
– When combining the standard potentials we do not multiply the
potentials in any way even if we have done so to balance the
equation: the standard potential is always the same
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Electrochemical potential
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Electrochemical potentials given for reduction reactions
So if negative number means reducing agent (it is oxidized)
Reversing reaction direction reverses sign of potential
Electrochemical potential for reaction is sum of those for two
half-reactions
• Example:
Zn and Cu; from standard potentials table
Nett reaction
Standard potential for reaction is 1.1 V
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Electrochemical potential
• Example
Will the following reaction proceed?
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Electrochemical potential
• Example
Will the following reaction proceed?
From standard potentials table
The total standard potential for the reaction is -1.43 V
Since it is negative the reaction will not proceed
spontaneously
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Electrochemical potential
• Example
Find the potential for the following reaction
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Electrochemical potential
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Example
Find the potential for the following reaction
Zn(s) + Fe 2+ (aq) → Zn 2+ (aq) + Fe(s)
From the standard potential table we have
Fe 2+ + 2e − → Fe − 0.41V
However we also have
Zn 2+ + 2e − → Zn − 0.76V
Want it the other way around
Zn → Zn 2+ + 2e −
0.76V
So total standard potential is 0.35 V. It is positive, so it proceeds
spontaneously
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
How many electrons do we get?
• We are interested in the current from a battery, as well as it’s
voltage, so important to determine number of electrons
required by reaction
• Definition: 1 Farad(ay) = 1 mole of electrons = 6.022 x 1023
electrons = 96,485 C
• So for example in the following reaction
So for 1 mole of O2 gas we need 4 x 96,485 = 36,000 C of
electrons
Under standard conditions 1 mole of gas is 22.4 litres, so we
have 5.8 x 10-5 litres per C
Now, 1 A = 1 C per second. So we get 5 x 10-5 l/s of O2 for 1 A
of current flow
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
How many electrons do we get?
• Battery capacity
Capacity(Ah) = n.F.
1 hour
3600 sec
Energy Capacity = Ah × Battery Voltage
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Nernst equation
• In reality the chemical potential for a set of reactions also
depends on the concentration in equilibrium of the products
and reactants
• Given by the Nernst equation
RT
0.0592
0
0
E=E −
ln Q = E −
ln Q
nF
n
R is gas constant 8.314 Jmol-1K-1
T is temperature
F is Faraday constant
E0 is standard potential of reaction
n moles of electrons transferred in reaction
Q is reaction quotient
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Nernst equation
• Reaction quotient defined for the following reaction
aA + bB → cC + dD
as
[C ]c [ D]d
Q=
[ A]a [ B ]b
This can allow us to find the voltage for a battery under a
wide range of conditions. But remember it is for when the
battery is in equilibrium
Q depends on concentration of the products and the
reactants. Pure liquids or solids are taken as 1. Liquids in
solution are given by their molarity and gases by their
pressure
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Nernst equation
• Example
A copper-zinc cell has E0 = 1.101 V. If the reaction is
done in a cell with 5.0 Zn2+ and 0.40 M Cu2+ at 25 C
what is the cell voltage?
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Nernst equation
• Example
A copper-zinc cell has E0 = 1.101 V. If the reaction is
done in a cell with 5.0 Zn2+ and 0.40 M Cu2+ at 25 C
what is the cell voltage?
First the equation
Now find Q
Q = [Zn2+]/[Cu2+]= 5.0/0.4 = 16.7
Two electrons transferred in reaction so n = 2
E = 1.065 V
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Non-ideal effects
• Nernst equation holds for the battery being in equilibrium, if
non-equilibrium, the voltage is different due to polarization
• There are three main reasons for this
– Activation over-voltage
– Mass transport over-voltage
– Resistive effects
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Non-ideal effects
• Over-voltage: need a higher voltage to re-charge the battery
than what was obtained when discharging battery
• Resistive Drops: voltage drops at all parts of the battery,
including the surface, meaning higher voltages at terminals
than at battery itself (when charging)
– Non-uniform resistances can cause uneven charging in series
connected systems
• Mass transport effects (involving the surface): Products and
reactants need to diffuse away from or towards the surface
– Regions may have locally a different concentration, causing a nonuniform voltage
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Non-ideal effects
• Activation effect due to reversibility of reactions: most
reactions are not completely reversible due to the formation
of intermediate products in the reactions
– Intermediate product may involve a higher energy state
– A fraction of the initial reactants
will have this higher energy state
– To reverse the reaction, need to
apply a higher potential
– Electrolysis of water has a high
over potential
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery systems
• Key characteristics of battery systems
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Volumetric/Gravimetric energy density
Depth of discharge
Charging regimes
Efficiency
Voltage
Cost
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery Characteristics
• Battery capacity
– Measured in Amp-hours rather than conventional Watt-hours due to
variation in battery voltage
– Battery capacity depends on age of battery and the history
• Depth of discharge
• Battery lifetime: number
of discharges
• Battery voltage and
variation of voltage
• Effect of temperature
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Lead-acid batteries
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For secondary batteries (re-chargeable) the most common type is leadacid
Reaction and standard potential:
Overall
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PbO 2 + Pb + H 2SO 4 ⎯discharge
⎯⎯
⎯→ 2PbSO 4 + 2H 2 O
←⎯charge
⎯⎯⎯⎯
Electrodes are lead oxide and ‘spongy’ or porous lead
Electrolyte is sulfuric acid
Standard potential 2.041 V
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Lead-acid batteries
• Some issues
– Depth of discharge, lifetime and number of cycles
– Reduced life due to sulfation of the battery and shedding of the
plate material
– Gassing of battery: high voltages or fast charging leads to
electrolysis of water
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Lead-acid batteries
• Issues in lead-acid batteries
– Stratification of the electrolyte: electrolyte cannot be
readily mixed
– Physical damage to electrodes due to soft electrode
material
– Spillage of sulfuric acid
• Safety issues particularly for transport
– Temperature dependence of the battery
• Freezing of battery at low temperatures
• Increasing temperature increases the voltage; must be
taken into account for system design particularly the
charging part of the system
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Lead-acid batteries
• Batteries have modified electrode materials, geometries and
electrolyte composition and volume to tailor battery to each
application
• Electrode composition and geometry
– Trace amounts of materials (Antimony, Calcium) added to lead plates
to increase hardness, however, does affect gassing and depth of
discharge of battery
• Electrolyte
– Increasing volume of electrolyte: less sensitive to losses of water, but
makes the battery heavier
– Captive electrolyte: gelled or absorptive glass mat: can be either
shallow or deep discharge, can be sealed
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Lead-acid batteries
• Flooded batteries
– Can add water, check status of electrolyte etc.
– Depending on configuration, can be deep or shallow cycle
• Sealed and valve regulated batteries
– Often are gelled batteries
– Need to control charging regime carefully to prevent generation
of hydrogen
• Can also be classified by use: starting, lighting, ignition
(SLI); Motive power or traction; Marine or RV batteries;
Stationary; Deep cycle
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Power Conditioning
• Power requirements and matching to PV module
• Functions of power conditioning:
– Keep the module operating at it’s maximum power
point (maximum power point trackers)
– Optimizing charging of the batteries to prevent
damage to the batteries (battery charger)
– Prevent discharging of batteries through the solar cell
module (blocking diode)
– Converting from DC to AC (inverter) for applications
or feeding the grid
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Blocking diode
• Must be sized to handle array
current
• Introduces voltage drop that
can be significant for lower
voltage systems e.g. for a 12 V
system it can be ~5%
• Higher voltage system (also
power) typically incorporate the
blocking diode in the charge
controller for the battery
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Maximum power point tracking
• Function of MPPT is to maximize the power generated by the PV module
by ensuring that it operates at it maximum power point
• Recall the characteristic resistance is the resistance corresponding to the
maximum power point. A MPPT tries to maintain the load seen as this
resistance
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Maximum power point tracking
• Constant power curves give I and V for a constant power
• Operating point of the solar panel is given by the intersection
of load line and IV curve
• From constant power curve can determine the amount of
power lost when operating away from mpp
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Maximum power point tracking
• MPPT allows the PV module to operate at its mpp even as
light intensity changes or effective load changes
• MPPT consists of two components: a DC-DC converter and
a circuit which searches for the mpp of the PV module and
ensures the module is biased at mpp
MPPT have
efficiencies > 90%
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery Charging
• Batteries are very sensitive to charging parameters and the
lifetime of batteries can be dramatically altered by improper
battery charging or leaving the battery at low charge
• Function of battery charger to provide appropriate charging
conditions for the battery, particularly by preventing either
over-charging or excessive discharging of the battery
• The type of battery regulator used must be matched to the
battery used and a battery charger designed for one type of
battery will not be effective for another type of battery
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery Charging
• Issues in battery charging
– If charging current is too high, towards the end of the charging
cycle tend to get a lot of electrolysis of the water, resulting in
gassing rather than storage
– Abrupt stopping of the charging, however, results in the batteries
not reaching their fully charged state, if this happens regularly
the battery capacity is reduced
– Solution is to taper the current as battery approaches full charge
– Charging too slowly also has disadvantages:
• Low charging current increases likelihood that power from
solar module is used inefficiently
• Want to fully charge the batteries whilst sun light is available
to us
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery Charging
• Multi-stage battery charging
– Maintenance charge during float operation: keeping the battery
fully charged by continuous low levels of current or by short
bursts of power. Maintenance charging also called float or trickle
charging, is typically used from mid-day into late afternoon when
the batteries may be fully charged and:
– Boost charging: used only in open, flooded lead-acid batteries to
prevent stratification of electrolyte. If the electrolyte is gelled or
absorbed in a AGM, boost charging should not be used
• Pulsed charging: battery is given short pulses of charge
followed by a resting period. Allows charging to higher
capacities whilst minimizing outgassing
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery Charging
• Types of charging regimes
– Self-regulation
• Self-regulation does not employ a separate regulator
but rather relies on the fact that as the battery voltage
increases, the current available from a PV panel
connected in parallel is decreased
• Only really works if the load, solar insolation and
module temperature don’t vary too much
– For example, in hot weather when the PV panel
experiences lower voltage the battery may be
under charged, while under cooler ambient
conditions module voltage will be higher and may
over-charge the battery
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery Charging
– Shunt regulator
• When voltage reaches certain level shunt regulator is switched on
and current diverted through it
• Since shunt dissipates the power from the modules only used for
small system where shunt can handle the array current
– Series regulator
• When a voltage is reached the series switch opens
• Doesn’t allow reduced currents as voltage rises, hence not
optimal charging
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Battery Charging
• Pulse Width Modulation Circuits
– A PWM circuit is used to provide a variable voltage, which allows
greater control over types of charging methods
– Requires more complex circuitry than shunt or series-switch
circuits
• Amp-hour meters and controllers
– Battery charge measures and controls the amps flowing into and
out of the batteries and continuously determines the state-ofcharge of the batteries
– Allows potentially more precise information about the BSOC but
requires periodic calibrations
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Inverters
• Function of inverter is to convert low voltage DC power from
PV array to high voltage AC power as required by many
loads
• Characteristics of inverters
– Power quality
• Waveform shape
• Total harmonic distortion
– Input and output voltage ratings (input determined by battery voltage,
output by that required by loads, typically 120 or 240 V)
– Power and surge power ratings
– Inverter efficiency
– Connection and interaction of inverter with other AC components
• Other inverters
• Grid power
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Inverters
• Basic topology: DC to AC conversion and AC voltage
transformer
– 2 basic approaches:
• High frequency transformer for AC voltage conversion followed by
high frequency to grid frequency conversion
• Direct transformer for AC voltage conversion to grid frequency
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Inverter
• Waveform
– Square wave output: simple, reliable
but very poor approximation to sine
wave
– Modified square (quasi-sine) wave:
• RMS and peak voltage are close
to that for a sine wave
• Still very harmonic content and
DC component in waveform
• Relatively simple and
inexpensive
– Square and modified square wave
not suited to sensitive loads
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Inverter
• Sine wave inverter
– Reproduces sine wave of grid supply
– Suitable for all loads
– Can interact with other AC systems including generators and
grid
– Power output measured by total harmonic distortion (THD)
– More complex circuitry
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Inverters
• Inverter efficiency
– Different types of inverters have substantially different performances
away from rated power and will be matched to different load profiles
– Best matched inverter depends on load characteristics; how high are
the peaks; how long do they last; is there a light load for a large
portion of the day; are there any seasonal variations in the daily load
profile?
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Inverters
• Surge ratings of inverters
– Inverter power loss is caused mainly by current flow
– Below the absolute current limit of the switching transistors, the
current is limited by the temperature of the semiconductor junction
– Junction temperature depends on ambient temperature, power loss
and time
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Inverters
• Other inverter characteristics
– Parallelable:
• Inverters can be connected in parallel, to efficiently provide a
greater power range than is possible with a single inverter
• Important when there is a large variation in the load which
persists for substantial times
– Bi-directional inverter:
• Inverter can both provide power for the battery for charging
and source power from the battery
• Bi-directional inverters used when there are other power
sources in the system, and these can be used to charge
batteries
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Inverters
• Two main types: stand-alone or grid-tied
• Grid-tied inverters can include batteries but many don’t
– If no batteries inverter can be used as MPPT but the input voltage
may vary more than when batteries are present
– If batteries, then grid can be considered another AC power source
• Regulatory constraints for grid-tied inverters: grid-tied
inverters must meet specification set by utility
– Typically includes specification on power quality
– Islanding: if power failure occurs when PV array is producing power,
a grid-tied inverter may produce an island of live grid
• Dangerous for workers on the lines
• Must have provision to isolate the system from the grid when the
grid power goes down
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Balance of System
• Anything else needed to complete the system, including:
– Module mounting:
• Mounting may be fixed or tracking
• Tracking: module tracks the sun to give ~20% extra power over a
year
– One axis tracking: tracks sun along one axis, usually eastwest
– Two axis tracking: module tracks sun in both east-west and
north-south directions. Usually not used in conventional PV
systems. Used in concentrator systems.
– Wiring, cabling
– Housing for batteries, electronics
– Land
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Loads
• Types of loads
– Heating: water heating, cooking, small appliances
– Motor/magnetic components: refrigeration and many
other applications
– Lighting
– Electronic circuits: TV, radio, clocks
– Combination electronic/motor
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Loads
• In determining load need to characterize:
– Power used by load
– AC/DC and variation possibility i.e. can we use a DC fridge
– Power quality required; harmonic content, DC offset, peak voltage
levels
– Variation of load in seasons
– Peak power used by entire load
– Variation of input voltage and current with time (i.e. surge current)
– Is load critical (Availability required >>99% ?)
• Efficiency of loads
– In general efficiency of appliances has been improving over the last
5 years, particularly for motors (20-30% improvement)
– Need life cycle costing to determine if it makes sense to replace
appliance with newer, more efficient version (DC maybe?)
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Loads
• Phantom Loads
– Phantom loads are loads which, though typically quite small and not
counted in load calculations have appreciable (and often unaccounted)
power drain on the system
– Inverter efficiencies are typically not constant with power level (highest
when closest to their rated power), so continuous small loads appear
much larger
− Due to low inverter efficiency a
20 W load will ‘look’ like a 200 W
load
− An increasing number of loads
have cts power requirements,
typically anything with a clock,
remote control, indicator lights
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Loads
• Heating loads
– Generally energy intensive, and often can be accommodated in other
fashions e.g. water heating, house heating, some cooking
– Small heating loads usually not worth converting
– Key design principle is to eliminate as many heating loads as
possible primarily water heating and electric range
• Electronic loads
– Examples include VCR, TV circuitry etc.
– May be damaged by peak voltages outside of range of utility voltage
– Appliance may use power signal not only as power source but also as
timing signal
• Timing signal can be triggered from rising or falling voltage level
• Noise, fluctuations, changes in slope of voltage waveform can
alter triggering of signal disrupting operation; means additonal
requirements for the quality of the output waveform
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner
Summary
• We have seen the major issues in batteries dealing with how
they operate by reduction-oxidation reactions and what
some major degradation and failure modes are
• These issues help guide in deciding the best charging
regimes to be used in systems that use batteries to reduce
variability in output
• Looked at the role of inverters for systems connected to AC
appliances or connected to the grid
• Finally we looked at the role of the load itself in determining
how efficiently a system can operate
ELEG620: Solar Electric Systems University of Delaware, ECE Spring 2009 S. Bremner