INTERACTION BETWEEN SUNLIGHT AND PV DEVICES

Photovoltaic energyelectricity from the sun

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

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

…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

equivalent circuit

IV curve equation

solar cell modelling

solar cell characteristics

Iscshort circuit current

solar cell characteristics

Vocopen 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

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

pv module losses

due

to

packaging density factor

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

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

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 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

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

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

ηsysbut also

from the pre-conversion efficiency

ηpre

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

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 (%)

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

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

output energy per area

Eenergy delivered by a system with

area A

is defined as

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

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%

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

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]

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

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

PRactual yield/

target yield

is defined as

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

correlation betweenenergy 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

cell

efficiency

power

fill factor

short circuit current

open circuit voltage

NOCT

summary

module ηnom

ηrel

Ppeak = HoAηnom

Pmodule = HoAηnomηrel

summary

system ηsys

ηpre

Psys = Pmoduleηsys