interaction between sunlight and pv devices
DESCRIPTION
Photovoltaic energy electricity from the sun. interaction between sunlight and pv devices. motivation. to evaluate the energy produced by the pv system. …how the pv cell can generate power. this process requires. solar cell modelling. solar cell characteristics. - PowerPoint PPT PresentationTRANSCRIPT
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INTERACTION BETWEEN SUNLIGHT AND PV DEVICES
Photovoltaic energyelectricity from the sun
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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
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to evaluate the energy produced by the pv system
incoming light
pv cell
it is
needed
to know how any pv device works
it is crucial
to study the interaction between the sunlight and the pv device
namely
the working principle of a solar cell
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…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
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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
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equivalent circuit
IV curve equation
solar cell modelling
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solar cell characteristics
Iscshort circuit current
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solar cell characteristics
Vocopen circuit voltage
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solar cell characteristics
FF fill factor
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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:
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pv module
consist of a transparent top surface
a rear layer
a frame around the outer edge
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module equation
pv module modelling
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
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pv module losses
due
to
packaging density factor
the interconnection of mismatched solar cells
the temperature of the module
failure modes of modules
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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
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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)
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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
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heat loss in module
due
to
conduction
convection
radiation
the operating temperature of a module is an equilibrium between the heat generated by the module and the heat loss to the surrounding environment
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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
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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
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nominal operating cell temperature (NOCT)
an approximate expression for calculating the cell temperature is given by
S = insolation in mW/cm2
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module efficiencies
ηnom
nominal module efficiencyis the efficiency that is measured under standard testing conditions
ηrel
relative module efficiencyis the efficiency that is observed when the conditions differ from the standard testing conditionthis factor is dependant on changes in temperature, intensity of the incoming light and ratio of diffuse radiation to direct radiation
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module output power
Ppeakthe peak power of the module is related to the module area A and nominal efficiency by:Ppeak = HoAηnom
Pmodulewhen 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
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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
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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
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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
ηsysbut also
from the pre-conversion efficiency
ηpre
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pv system related efficiencies
ηsysthe system efficiency reflects electrical losses caused by wiring, inverter and transformer and considers the module efficiency
ηprethe 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
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pv system performance
may be defined by any one, or a combination (performance ratio), of the
following performance criteria
output powerpower is typically in units of
watts (W)
output energyis typical in units of watt-hours (Wh)
conversion efficiency (%)
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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
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output energy
output energy
indicates the amount of energy (watt-hour or Wh) produced during a certain period of time
the parameters areoutput 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 asoutput energy per area
output energy per rated power
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output energy per area
Eenergy delivered by a system with
area A
is defined as
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energy output per rated power
Ethe energy yield is expressed in terms of the peak power of
the module, which is independent from the area of
the module
is defined as
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conversion efficiency
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%
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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
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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?
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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
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energy of incoming light
H0 = 1,000 W/m2
Hthe yearly sum of global irradiation that hits the
module
is specific to the location
should be obtained from databases,
measurements, or - in the first instance - from an
irradiance map
it is measured in [kWh/m2]
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if we defined target and actual yields as
target yield
theoretical annual energy production on the DC side of the module
taking into account only the energy of the incoming light and module's nominal efficiency
actual yield
annual energy production delivered at AC
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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
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performance ratio
PRactual yield/
target yield
is defined as
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we can define the performance ratio
is independent from the irradiation
useful to compare systems
it takes into account all pre-conversion losses
inverter losses
thermal losses
conduction losses
PR
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correlation betweenenergy yield and performance ratio
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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
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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
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energy losses diagram
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summary
solar
cell
efficiency
power
fill factor
short circuit current
open circuit voltage
NOCT
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summary
module ηnom
ηrel
Ppeak = HoAηnom
Pmodule = HoAηnomηrel
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summary
system ηsys
ηpre
Psys = Pmoduleηsys