Photovoltaic energy
electricity from the sun
INTERACTION BETWEEN
SUNLIGHT AND PV DEVICES
motivation
to evaluate the performance of a pv system it is needed to know
energy produced by the pv system
how the system works
how the system components work
how the module works
how the solar cell works
to evaluate the energy produced by the pv system
to know
how any
needed pv device
works
it is
it is
crucial
incoming
light
to study the interaction
between the sunlight
and the pv device
namely
the working principle
of a solar cell
pv cell
…how the pv cell can generate power
light shining on the solar cell produces both a current and a voltage to generate electric power
basic working steps
the generation of light-generated carriers
the collection of the light-generated carries
to generate a current
the generation of a large voltage across the
solar cell
the dissipation of power in the load and in
parasitic resistances
this process requires
a material in which the absorption of light raises an electron to a
higher energy state
the movement of this higher energy electron from the solar cell
into an external circuit
the electron energy dissipation in the external circuit and returns to
the solar cell
a variety of materials
and processes can
potentially satisfy the
requirements for pv
energy conversion
but in practice nearly all photovoltaic energy conversion uses
semiconductor materials in the form of a p-n junction
solar cell modelling
equivalent circuit
IV curve equation
solar cell characteristics
Isc
short circuit current
solar cell characteristics
Voc
open circuit voltage
solar cell characteristics
FF fill factor
solar cell characteristics
η
efficiency
the efficiency of a solar cell is determined as the
fraction of incident power which is converted to
electricity and is defined as:
pv module
consist of
a transparent top surface
a rear layer
a frame around the outer edge
pv module modelling
module equation
N
is the number of cells in series
M
is the number of cells in parallel
IT
is the total current from the circuit
VT
is the total voltage from the circuit
I0
is the saturation current from a single
solar cell
IL
is the short-circuit current from a single
solar cell
n
is the ideality factor of a single solar cell
q, k, T
are constants
pv module losses
packaging density factor
due
to
the interconnection of mismatched
solar cells
the temperature of the module
failure modes of modules
packaging density factor
refers to the area of the module that is
covered with solar cells compared to
that which is blank
affects the output power of the module
as well as its operating temperature
depends on the shape of the solar cells
used
sparsely packed cells in a module with a white rear
surface can also provide marginal increases in output
via the "zero depth concentrator" effect
some of the light striking regions of the module
between cells and cell contacts is scattered and
channelled to active regions of the module
mismatch for cells connected in series
an easy method of calculating
the combined short-circuit
current of series connected
mismatched cells
the current at the point of
intersection represents the
short-circuit current of the
series combination (ie. V1+V2=0)
mismatch for cells connected in parallel
an easy method of calculating the combined open circuit voltage (Voc)
of mismatched cells in parallel
the curve for one of the cells is reflected in the voltage axis so that the
intersection point (where I1+I2=0) is the Voc of the parallel configuration
heat loss in module
the operating temperature of a module is an equilibrium between the heat
generated by the module and the heat loss to the surrounding environment
due
to
conduction
convection
radiation
nominal operating cell temperature
a module will be typically
rated at 25 °C under 1
kW/m2
when operating in the
field, modules typically
operate at higher
temperatures and at
somewhat lower
insolation conditions
in order to determine the
power output of the solar
cell, it is important to
determine the expected
operating temperature of
the module
nominal operating cell temperature (NOCT)
NOCT is defined as
the temperature reached by open circuited cells in a module under special conditions
irradiance on
cell surface =
800 W/m2
air
temperature =
20°C
wind velocity
= 1 m/s
mounting =
open back
side
nominal operating cell temperature (NOCT)
an approximate expression for calculating the cell temperature is given by
S = insolation in mW/cm2
module efficiencies
ηnom
nominal module efficiency
ηrel
relative module efficiency
is the efficiency that is measured under standard testing
conditions
is the efficiency that is observed when the conditions differ from
the standard testing condition
this factor is dependant on changes in temperature, intensity of
the incoming light and ratio of diffuse radiation to direct radiation
module output power
Ppeak
the peak power of the module is related to the
module area A and nominal efficiency by:
Ppeak = HoAηnom
Pmodule
when the conditions differ from the standard testing
condition, the nominal module efficiency must be
multiplied by a relative module efficiency, and the
instantaneous power supplied by the module is:
Pmodule = HoAηnomηrel
H0 = solar constant, insolation in W/m2
pv system
is made up of several solar cells
an individual cell is usually small, typically producing only a small amount of
power
to boost the power output of cells, they are connected together to form
larger units called modules
modules, in turn, can be connected to form even larger units called arrays,
which can be interconnected to produce more power, and so on…
because of this modularity, systems can be designed to meet any electrical
requirement, no matter how large or how small
pv system
by themselves, modules or arrays do not represent an entire system
systems also include
structures that point them toward the sun and components that take the
direct-current electricity produced by modules and "condition" that
electricity, usually by converting it to alternate-current electricity
systems may also include batteries
these items are referred to as the balance of system (BOS) components
pv system
combining array with BOS components creates an entire PV system
the performance of the system is therefore dependent on the
performance of its components
ηsys
but also
from the pre-conversion efficiency
ηpre
pv system related efficiencies
ηsys
the system efficiency reflects electrical losses
caused by wiring, inverter and transformer
and considers the module efficiency
ηpre
the pre-conversion efficiency reflects the
losses incurred before the beam hits the
actual semiconductor material, caused by
shading, dirt, snow and reflection off the glass
pv system performance
may be defined by any one, or a
combination (performance ratio), of the
following performance criteria
output
power power is
typically in units of
watts (W)
output energy is typical in
units of watt-hours (Wh)
conversion
efficiency (%)
output power
output power
is the power (in watts) available at the power
regulator
specified either as peak power or average
power produced during one day
Psys
the system's installed capacity
Psys = Pmoduleηsys
output energy
indicates the amount of energy (watt-hour or Wh) produced during a
certain period of time
the parameters are
output per unit of array area (Wh/m2)
output per unit of array mass (Wh/kg)
output per unit of array cost (Wh/$)
can be defined as
output energy per area
output energy per rated power
output energy per area
is defined as
E
energy delivered
by a system with
area A
energy output per rated power
is defined as
E
the energy yield is expressed in
terms of the peak power of the
module, which is independent
from the area of the module
conversion efficiency
is defined as energy output from array / energy
input from sun x 100%
it is often given as a power efficiency:
power output from array / power input from
sun x 100%
standards
groups are working on standards and
performance criteria for pv systems
to ensure the
consistency and quality
of photovoltaic systems
and increase consumer
confidence in system
performance
energy yield and performance ratio
for investors and operators
alike, there are two
fundamental questions
how much
electricity does the
system generate?
how will does the
system perform?
having already defined the energy yield as
E
energy yield per area
energy yield per rated power
where
H and H0 represents the energy of the incoming light
energy of incoming light
is specific to the location
H
the yearly sum of global
irradiation that hits the
module
should be obtained from
databases, measurements,
or - in the first instance from an irradiance map
it is measured in [kWh/m2]
H0 = 1,000 W/m2
if we defined target and actual yields as
target yield
actual yield
theoretical annual energy
production on the DC side of
the module
annual energy
production delivered at
AC
taking into account only the
energy of the incoming light
and module's nominal
efficiency
we can define the performance ratio
performance
ratio, often called
quality factor
is the ratio
between actual
yield and the
target yield
actual yield/
target yield
performance ratio
is defined as
PR
actual yield/
target yield
we can define the performance ratio
is independent useful to compare
from the
systems
irradiation
PR
it takes into
account all
preconversion
losses
inverter losses
thermal losses
conduction losses
correlation between
energy yield and performance ratio
energy losses
sometimes it is more intuitive to think in terms
of energy losses that occur at every step of the
way rather than component efficiencies
both concepts are the same,
as losses = 1 - efficiency,
both expressed in
percentage terms
starting with the intensity of the
incoming light (i.e. the energy
that is actually available to the
system), there are three major
blocks of energy losses
energy losses
pre-photovoltaic
losses
attenuation of the
incoming light through
shading, dirt, snow and
reflection before it hits
the photovoltaic material
module and
thermal losses
reflecting the efficiency
and temperature
dependence of the solar
module
system losses
reflecting losses in the
electrical components
including wiring,
inverters and
transformers
energy losses diagram
summary
solar
efficiency
power
cell
fill factor
short circuit current
open circuit voltage
NOCT
summary
module
ηnom
ηrel
Ppeak = HoAηnom
Pmodule = HoAηnomηrel
summary
system
ηsys
ηpre
Psys = Pmoduleηsys