Solar PV Cells, Module and Array

Solar PV: Cells, Modules, System
design
31-08-2016 IEC-803 ENERGY BASICS BY DR N R KIDWAI, INTEGRAL UNIVERSITY 1

Solar Energy
The sun is a sphere of intensely hot
gaseous matter with a diameter of 1.39
x 109 m
The sun is about 1.5 x 108 km away
from earth,
as thermal radiation travels with the
speed of light in a vacuum (300,000
km/s), after leaving the sun solar
energy reaches our planet in 8 min and
20 s.

Solar Irradiation
•Sun is a radiating body at 6000oC
•Solar power falling earth’s atmosphere is 175 PW
•Max. output is in visible range (within dashed lines)
•Certain wavelengths are scattered & absorbed by
air, moisture & aerosols present in atmosphere.
•This effect varies with thickness of atmosphere the
light must penetrate called as Air Mass (AM)
•When sun is directly overhead, light would pass
through 1 AM. At lower angles distance might be 2
or more times.
•For measurement purpose AM1.5 is the standard.
Spectral Distribution of Sun

Solar Photo voltaic Cell
 1839 photovoltaic effect in selenium in 1839. Silicon PV cells developed in 1958
 Solar cell is the primary device for Solar Photovoltaic Systems. Pure silicon with high
crystal quality is needed to make solar cells. To enable silicon material to generate
energy, impurities, the doping atoms, are introduced into crystal lattice.
 When solar cell is exposed to light, photons are absorbed by the electrons. The input
of energy breaks electron bonds.
 Light knocks loose electrons from silicon atoms
 Freed electrons have extra energy or ‘Voltage’. Internal electric field pushes electrons
to front of cell
 Electric current flows on to other cells or to the load. Cells never ‘Run out’ of
electrons

Solar Photo voltaic cell
Silicon solar cell comprises of two differently doped silicon layers. The layer that faces
the sun’s light is negatively doped with phosphorous. The layer below is positively doped
with boron.
To take power from the solar cell, metallic contacts need to be fitted on the front and
back of the cell. Contacts are usually in the form of a thin grid structure. Antireflective
coating of silicon nitride or titanium oxide is applied into the front face to reduce light
reflection. The front layer must let the light to enter maximum possible extent.
When light falls on solar cell, charge carriers separate and current flows through load.

Solar Photo voltaic cell
 Photo voltaic addition in Cells/ Modules- In each cell, electron gains about one volt
when they are energized and ionized by photons. I
 n passing through the p/n junction, they lose about one half volt through collisions &
accelerations, so electrons are left with only one half volt.
 The process continues & as a net result electrons carry one half volt all the time.
 Solar cells can be combined in series parallel to increase voltage and current.
 When cells are connected in series, the current flow through each cell is same and the
resulting voltage is the sum of the voltages of each cell.
 When cells are connected in parallel, the voltage across each cell is the same and
currents add to produce a final current.

Solar Modules
MODULES are produced by connecting 30-36 cells in series, which generates 15-18 volts,
enough to charge a 12V battery.
Like solar cells, solar modules can also be connected in series and parallel to increase
voltage & current.
For higher voltages, MODULES are interconnected to form PANEL.
PANELS are interconnected in parallel to form ARRAYS to achieve higher current.

Solar Modules
• Series connected cells
increase voltage potential
• Parallel connected cells
increase current potential

Combining Cells & Modules

Combining Cells & Modules
Electrical parameters:
Short circuit current - Isc
Open circuit voltage- Voc
Current at maximum power point – Imp-
Used as the rated current of the device
Voltage at maximum power point- Vmp-
Used as the rated voltage of the device
Maximum Power (Pmax)
Fill factor- It is a indication of solar panel
conversion efficiency.
area A
area B
Isc
Im
p
Pma
x
Vmp Voc
Cell Voltage in V
Fill
Factor =
Area B
Area A

Effect of Light intensity
Effects of Light Intensity: Change of
intensity of light changes, the number of
photons entering PV devices that
proportionally changes the number of
electrons released.
Isc is directly proportional to light
intensity, and voltage varies more slowly
in a logarithmic relationship.
Internationally accepted standard for
light intensity is 1000 watts/ m2. It is
called as one SUN or peak irradiance.
Voc drops
slowly with
lower
irradiance
Imp
Voc
Lower irradiance
reduces current
I

Effect of Temperature
Effects of Temperature: Increase in cell
temperature, reduces the voltage available at
most current. This change in voltage is
directly proportional to temperature.
Isc rises slightly as temperature goes up.
The power reduction factor in maximum
power is a general factor to use to estimate
the effect of temperature on output power. It
is generally about -0.45% to -0.5%.
A standard cell temperature of 25deg c has
been accepted internationally.
Higher temperature
reduces voltage
I

Solar radiation : Seasonal variation
Seasonal variation results due to two reasons
1. The Earth's orbit around the Sun is not circular
but elliptical, meaning that it is closest to the
Sun in late Summer and farthest away in late
Winter. However, this has only a slight effect on
the intensity of solar radiation.
2. More important is the axial tilt of the Earth at
23.45°.
Both variations mean that the path of the Sun
through the sky changes significantly throughout
the year effecting the intensity of incident solar
radiation

Solar radiation : Effect of Atmosphere
Reduction of solar radiation due to absorption, scattering
and reflection in the atmosphere;
Change of spectral content of the solar radiation due to
greater absorption or scattering of some wavelengths;
the introduction of a diffuse or indirect component into the
solar radiation
local variations in the atmosphere (such as water vapor,
clouds and pollution) have additional effects on the incident
power, spectrum and directionality.

Solar radiation : Effect of Atmosphere
gasses, like ozone (O3), CO2, and H2O vapour ,
have very high absorption of photons having
energies close to bond energies of these gases,
results in troughs in the spectral radiation curve
Light is absorbed as it passes through the
atmosphere and at the same time it is subject to
scattering (Ex Rayleigh scattering effect)
Scattered light is undirected, and so it appears to
be coming from any region of the sky. This light is
called "diffuse" light
Effect of clouds and other local variations in the
atmosphere

THREE RULES TO GET THE BEST OUTPUT FROM SOLAR PANELS
Rule 1
There should be no shade on the panel between 09:00 and 15:00.
Rule 2
Tilt the panel at an angle equal to the latitude of the site, though it should never
be tilted less than 5 degrees from horizontal. The panel should face north for sites
south of the equator and it should face south for sites north of the equator.
Rule 3
Mount the panel at least 10cm above other surfaces so air can easily cool the back
of the panel.

SPV Standalone System

Type of Solar PV Systems
Simple One/ Two Module DC Systems- Rural home/ street lighting systems. Modules
connected to a simple low cost battery through a simple charge regulator.
Large DC Systems- Centralized home lighting systems. Multiple modules with higher
capacity charge controller and single deep cyclic battery used.
AC/DC Power Systems- Here additionally, DC to AC inverter is also used to operate
appliances working on AC

Type of Solar PV Systems
SPV-Generator Hybrid Systems- These Systems are combination of photovoltaic and
diesel systems and offer best that both have to offer. Such systems consist of large
arrays of modules, computerized system controller, battery bank for handling load
requirement and DG set etc.
Grid-Connected Systems- These systems are designed to work with grid. Solar arrays
and grid, both, are connected to normal distribution box through the system controller.
Excess power produced during the day time is fed into system controller and changed
into pure sinusoidal AC power that is synchronized with the grid frequency.

• Interface and Control of Backup Energy Sources
• Divert PV Energy to an Auxiliary Load
• Serve as Wiring Centre.
Charge regulators are required to control storage & distribution of solar energy
and to also protect battery from being overcharged by array & over discharged
by the loads. The array is connected to batteries & loads through charge
regulator. In some designs, it may also provide status information to the user.
Other functions of the charge regulator and system control are as under.
Charge Regulator & System Controls

Switching Control of Charge Regulator is done at four basic control set points
Voltage regulation set point (VR) & Array Reconnect Voltage refers to the voltage
set points at which the array is connected & disconnected from the battery to
prevent over-charge.
The low voltage load disconnect (LVD) and load reconnect voltage (LRV) refers to
the voltage set point at which load is connected or disconnected to prevent
battery over-discharge.
Charge Regulator & System Controls (contd..)

Simple Series Configuration
Without Over discharge Protection
With Overdischarge Protection
PV Array
Charge
Regulator
Battery Load
Charge
Regulator
Battery
LoadPV Array

Batteries used in PV Systems
Batteries are used to store the energy produced in excess during the day time
and operate the load when the PV array can not supply enough power. It also
allows loads to operate during extended periods of cloudy or overcast weather.
Primary functions of batteries are as under.
• Energy Storage capacity and Autonomy
• Voltage & Current Stabilization
• Supply Surge Currents

Batteries used in PV Systems (Contd.)
Batteries are used to store the energy produced in excess during the day time and
operate the load when the PV array can not supply enough power. It also allows
loads to operate during extended periods of cloudy or overcast weather.
• The battery storage capacity is generally sized to meet daily electrical loads
for specified number of days without input from PV array for a specified
autonomy period i.e. days of storage. The greater the design autonomy
period, the larger the battery capacity required for a given load.

• Voltage & Current Stabilization – Another purpose of batteries in standalone
system is to stablise or level out the potential wide variations in voltage and
current that may occur in PV system. Battery also allow the loads to operate
within prescribed voltage and current range, as well as ensure that the PV array
is operated near its maximum power voltage.
• Supply Surge Currents – To supply surge or high peak operating currents to
electrical loads.

Battery Types & Classification
• Lead-Antimony Batteries
• Lead-Calcium batteries
• Flooded Lead-Calcium, Open Vent
• Flooded Lead-Calcium, Sealed Vent
• Captive Electrolyte Lead-Acid Batteries

•Battery Charge (Coulombic) Efficiency : the ratio of Ampere hours withdrawn from a
battery during discharge to the Ampere hours provided to re-charge the battery.
•State of Charge (SOC) : the amount of energy in a battery at a particular time. Expressed
as a %ge of energy stored compared to fully charge battery.
•Depth of Discharge (DOD) : %ge of capacity that has been withdrawn compared to fully
charged battery. (Depth of discharge and State of Charge adds to 100% of battery)
•Allowable depth of discharge : The maximum %ge of full-rated capacity that can be
withdrawn from a battery.
Allowable DOD is a seasonal deficit, resulting from low insolation/ temperature and/or
excessive load usage. Depending upon the type of batteries used, the designed allowable
DOD may be as high as 80%. It is related to autonomy, in terms of capacity required to
operate the system loads for a given number of days without energy from the PV array.

Balancing the opposites – System design involves compromise between competing
and desirable requirements. Choices are to be made between type, size of
equipment, location, redundancy, protection, level of safety, amount of complexity,
cost etc. Other important factors are client’s budget, remoteness of the site, nature
of the load and future growth aspects.
Considerations

Considerations (Contd.)
• Required details about PV Application-
o Load requirements
o Load profile
o Surges
o Power quality
o DC or AC
o Critical Loads
o Ease of access to site.
• Data collection about climate-
Latitude, longitude
Insolation
Temperature
Variability of weather
• Information about the user
Budget
Reliability of their data
Level of technical skill
Future growth possibilities
Aesthetic Considerations

System Design Details
Factors involved-
• Load characteristic Variation & Estimation
• Battery Bank Sizing
• Array Sizing
• Charge Controller & Wiring Design
• Load – Total daily load demand is calculated by multiplying each load
demand times the time that the load operates in a typical 24-hours
period. DC loads are estimated by using amp-hours. While AC loads are
estimated by using watt-hours.

DC Loads Qty Amps Hours /day Daily Demand (Ah)
X X
X X
X X
X X
X X
Total DC Loads (Ah)-
Daily Load Form

DC Loads Qty Amps Hours /day Daily Demand (Ah)
X X
X X
X X
AC Sub-Total (Wh)
Continuous Watts = _____
Surge Est = _____
Inverter choice. : ____________________
[ ] / [ ] / [ ] + [ ] = ________
AC Sub-Total Efficiency Input Voltage DC Loads Daily Load(Ah)
Contd.

• Battery Bank Sizing –
For this, required parameters are:
– Number of reserve days
– Daily Load
– Battery Efficiency
– Temperature derating
– Rate factor
– Maximum depth of discharge
Number of days of Reserve x Daily load
Battery Capacity = ---------------------------------
Battery Efficiency x Temp.Derate Factor x Rate Factor

ARRAY SIZING
Module daily output =Max. peak current(Imp) x
sun availability in peak hour
Nominal system voltage
Number of series modules = -----------------------------
Nominal module voltage
Daily load (AH)
Number of parallel modules = ---------------------------------------------
Coulombic Efficiency X Module output X Derating factor

• Charge Regulator Size –
It is in accordance with the total current from an array which is given by the no of
modules or strings in parallel, multiplied by module short circuit current and
safety factor. The formula is as under.
Regulator Size = No. of parallel modules x Isc x 1.2 (safety factor)

The summarized steps:
Step 1- Determination of daily/ weekly/ seasonal total load. Consideration of other
key design inputs such as operating voltage, insolation and autonomy.
Step 2- Determination of the battery bank size. This includes battery efficiency,
no.of batteries, battery configuration etc.
Step 3- Selection of solar photovoltaic array, Estimation of maximum current &
voltage. Determination of number of modules, module configuration etc.
Step 4- Charge controller selection

Solar PV Cells, Module and Array

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