Bangladesh outlines plan for up to 40 GW of

renewables in 2041
The most ambitious scenario outlined in a new, draft solar energy strategy for Bangladesh envisages almost 40
GW (1 GW=109 Watt or 1 billion watt) of renewable energy generation capacity in 2041.The 20-year National
Solar Energy Action Plan drawn up by the government states a business-as-usual scenario will result in only 8
GW of clean energy capacity by that date and a medium implementation forecast would drive 25 GW of
projects.The government has set a target to generate 2000, MW of electrical power from renewable energy by
2021. Currently, the total electricity generation from such sources is 404 MW. The new target of renewable
energy would be 10% of the total electricity generation in 2021 and would increase to 20% percent by 2030.

Basic Definitions

Latitude & Longitude

Latitude is the distance in degrees North or South of the equator. With the equator being 0 and the poles at 90
deg. Latitude lines are parallel to the equator.
Longitude run from pole to pole. The prime meridian is 0 and runs through Greenwich England they are
measured in degrees east and west of the Prime Meridian. Up to 180 East or 180 west. Longitude is determined
by the time difference between your position and GMT Greenwich Mean Time.
Altitude is your height above sea level (or some other base, such as an airport runway). This is usually
determined with an altimeter which works the same as Latitude & Longitude

figure-1
 Figure-2

Figure-3
Parallels of latitude:

These lines circle the earth from north pole and south pole. Equator is the longest latitude which divides the
earth between northern hemisphere and southern hemisphere. These are measured in degrees.
Equator is latitude of 0°. The latitudes in the north area numbered with °N with it. The latitudes in the south are
numbered with °S with it. Latitudes are from 0° to 90° N and 0° to 90°S. Latitudes are not of the same length.
Other important latitudes are Tropic of cancer at 23.5°N, Tropic of Capricorn at 23.5°S,Article circle at 66.5°N,
Antarctic circle 66.5°S, North pole 90°N, South pole at 90°S.
Meridians of longitude:
These lines are drawn joining north pole and south pole. Unlike latitudes, longitudes are of same length.
These are measured in °W or °E.
Prime meridian is the most important longitude at 0°. This longitude divides the earth in eastern
hemisphere and western hemisphere. The lines lying on the east of prime meridian are named as °E and those
to the west of prime meridian are measured as °W. There are 360 longitudes. But 180°E and 180°W are not the
same lines.
Longitudes along with helping in finding location also helps to know the time zone. For each meridian you
travel, the time differs by 4 minutes.

Figure-4

The latitude and longitude of Bangladesh is:

24° 0' 0" N / 90° 0' 0" E
Figure-5
 World Map with latitude & Longitude

SOLAR RADIATION
Some of the solar radiation entering the earth's atmosphere is absorbed and scattered. Direct beam radiation
comes in a direct line from the sun. Diffuse radiation is scattered out of the direct beam by molecules, aerosols,
and clouds. The sum of the direct beam, diffuse, and ground and surroundings reflected radiation arriving at the
surface is called total or global solar radiation.
GHI is the total amount of solar energy incident on a horizontal surface. DNI is the amount of radiation incident
on a surface that is always kept perpendicular (normal) to the direct solar beam. Part of the solar radiation that
arrives on a horizontal surface, called diffuse horizontal irradiance (DHI), is due to scattering of sunlight in the
atmosphere and reaches the horizontal surface from all directions of the sky.
GHI is the important parameter for photovoltaic applications (PV), while DNI is
the most important parameter for CSP plants and Concentrating Photovoltaic
(CPV) plants.
DNI: Direct Normal Radiation. DHI: Diffuse Horizontal radiation. GHI: Global horizontal Radiation.
Solar angles.
DNI is the amount of solar radiation received per unit area by a surface that is held perpendicular to the rays
coming directly in a straight line from the sun.
DHI: is the amount of radiation per unit area by a surface that doesn't arrive in a direct path from the sun.
GHI: is the total amount of radiation received above a surface horizontal to ground. This is of particular interest
for a solar PV.
GHI= (DNI x CosθZ) + DHI. Here ϴZ is the zenith angle.
 Figure-6

Using Solar Angles to Predict the Sun's Location

Below is an overview of the angles involved in calculating the amount of solar radiation that a PV panel
receives at any given time (also see Figure 3). The angle at which the sun hits a PV panel is the basis for
understanding how to design the most efficient PV array for a specific location. This is one of the first topics
presented in solar engineering textbooks.
Zenith Angle, Θz: This is the angle between the line that points to the sun and the vertical — basically, this is
just where the sun is in the sky. At sunrise and sunset this angle is 90º.
Solar Altitude Angle, αs: This is the angle between the line that points to the sun and the horizontal. It is the
complement of the zenith angle. At sunrise and sunset this angle is 0º.
Solar Azimuth Angle, γs: This is the angle between the line that points to the sun and south. Angles to the east
are negative. Angles to the west are positive. This angle is 0º at solar noon. It is probably close to -90º at sunrise
and 90º at sunset, depending on the season. This angle is only measured in the horizontal plane; in other
words, it neglects the height of the sun.
Angle of Incidence, θ: This is the angle between the line that points to the sun and the angle that points straight
out of a PV panel (this is also called the line that is normal to the surface of the panel). This is the most
important angle. Solar panels are the most efficient when pointing at the sun, so engineers want to minimize this
angle at all times. To know this angle, you must know all of the angles listed and described next.
Hour Angle, ω: This is based on the sun's angular displacement, east or west, of the local meridian (the line the
local time zone is based on). The earth rotates 15º per hour so at 11am, the hour angle is -15º and at 1pm it is
15º.
Surface Azimuth Angle, γ: This is the angle between the line that points straight out of a PV panel and south. It
is only measured in the horizontal plane. Again, east is negative and west is positive. If a panel pointed directly
south, this angle would be 0º.
Collector Slope, β: This is the angle between the plane of the solar collector and the horizontal. If a panel is
lying flat, then it is 0º. As you tip it up, this angle increases. It does not matter which direction the panel faces.
Declination, δ: This is the angle between the line that points to the sun from the equator and the line that points
straight out from the equator (at solar noon). North is positive and south is negative. This angle varies from
23.45 to -23.45 throughout the year, which is related to why we have seasons.
Solar Tracking Systems
Now let's talk about how to apply all of this information. Figure 1 shows a small portion of North America's
largest solar PV power plant (as of 2010). The 14-megawatt power plant is located at Nellis Air Force Base in
Nevada and is expected to provide more than 30 million kilowatt-hours of electricity each year. A typical
compact fluorescent lamp (CFL) uses 15 watts, so when the sun is shining, this plant could power almost a
million CFLs. At this site, 72,000 PV panels are placed across 140 acres of land. This power plant has more
than just PV panels. Look at Figure 1 to see if you notice anything else that might affect the efficiency of the
panels.

Figure-6

Fig.7

https://youtu.be/hGPKie9QOBI How to measure Solar Angle. Click the link on the left.
 Figure-8

Figure-9
Air Mass:

Air mass is defined as the thickness through which sun's rays travel through atmosphere. In the space, air mass
is zero. Since particles in the atmosphere absorb and scatter light rays, the more atmosphere solar radiation
passes through on its way to us, the less solar energy we can expect to get.
The Air Mass is defined as:
AM=1/cos(θz), where θz is the angle from the vertical (zenith angle). When the sun is directly overhead, the
Air Mass is 1.
What Does Air Mass Have To Do With Sunlight?

The atmosphere can be divided into large volumes with common properties. Air masses are large portions of
gasses and particles that have similar temperature, chemistry and pressure. These common physical properties
affect how solar intensity decreases as it travels through the atmosphere. When it comes to solar simulation, Air
Mass (AM) is a fundamental concept that we need to consider if we want to mimic, as closely as possible, the
solar radiation as experienced on the surface of the earth.

The solar spectrum is generated by the sun’s surface, which has a temperature of about 5800 K. The sun’s
emitted spectrum, as seen from space, is equivalent to the electromagnetic radiation emitted by a black body at
that temperature. The light from the sun travels 150 million km and arrives at Earth’s orbit. Because it has
traveled through the vacuum of space, there has been no change in the spectrum, and this is what we call Air
Mass 0, indicating that the sunlight has not interacted with any of the earth’s atmosphere.

However, when the solar spectrum is measured on Earth, it is shifted slightly from black body radiation due to
the scattering of blue light and the absorption of red light by the earth’s atmosphere. The more atmosphere
sunlight passes through, the greater the attenuation.

Air mass is defined as the path length of the direct sunbeam through the atmosphere expressed as a ratio
relative to the sun at the zenith (a zenith is an imaginary point directly above a particular location) above
a sea-level location. In this case, the sun’s direct radiation passes vertically through the atmosphere in the
shortest possible path. This is known as Air Mass 1.
The spectrum generated by sunlight at Air Mass 1 is commonly known as “Air Mass 1 Global” (AM1G)
radiation spectrum, meaning “one atmosphere”. The spectrum generated by sunlight at AM1 (at 0° from the
zenith) to AM1.1 (at 25° from the zenith) is a useful range for estimating the performance of solar cells in
equatorial and tropical regions. Because it passes through no air mass, the extraterrestrial spectrum is called the
“Air Mass 0” (AM0) spectrum.

A first-order approximation for air mass is AM~1/cos z where z is the zenith angle in degrees. This
approximation is reasonably accurate up to around 75 degrees, but overestimates air mass at high zenith angles,
because the atmosphere has a maximum height. There are a number of approximations that provide a better
estimate of air mass across the full range of zenith angles, some even including the effects of atmospheric
refraction (for example, in the open ocean, the sun is visible before it has risen above the horizon, because light
has been bent by the atmosphere around the curve of the earth).

Now that we understand what AM0 and AM1 are let’s take a deeper look at AM1.5, which is the air mass
chosen as a standard spectra to represent sunlight.

Because most major population centers of the world (Europe, China, Japan, the United States, northern
India, southern Africa and Australia) lie in mid-latitudes, an AM number that represents mid-latitudes is
the most commonly used to characterize the performance of solar cells. AM1.5 atmosphere thickness
represents a zenith angle of z=48.2°.
During the summer months, the AM number for mid-latitudes is less than 1.5, and higher figures apply
during the morning and evening. Therefore, AM1.5 is a useful representation of the atmosphere
thickness as a yearly average for mid-latitudes. This air mass of 1.5 was selected as the standard spectra
in the 1970s for standardization purposes based on a solar radiance analysis in the United States.

Other AM values are used to approximate sunlight at regions other than mid-latitudes or at higher elevations.
AM2 and 3 (z=60° and z=70° respectively) for example, is useful to determine the solar performance of some
devices (e.g. solar cells) at higher latitudes such as those in northern Europe. An AM value of 40 is typically
regarded as being the air mass value of the horizontal direction (z=90°) at the equator.

Standard spectrums include AM0, AM1.5G, AM1.5D (direct radiation that does not include scattering). These
are defined by ASTM and other standards bodies in an effort to provide standard test conditions so that
experiments and results can be compared, and get a reasonable approximation for real-world performance.

What is the Solar Spectrum?

Solar spectrum is defined as the electromagnetic spectral distribution emitted by the sun or received by a
collector or instrument on Earth.

The sun radiates solar energy or sunlight by electromagnetic waves over a range of wavelengths known as the
Solar Spectrum.

The Sun emits radiation from X-rays to radio waves, but the surface of the earth receives mainly wavelengths
between 350 nm and 400 nm. The region visible to humans is restricted to 400 nm to 700 nm, approximately
43% of the total energy.
Standardized Solar Spectrum and Solar Irradiation
The standard spectrum outside the Earth's atmosphere is called AM0, because at no stage does the light pass
through the atmosphere. This spectrum is typically used to predict the expected performance of cells in space.

The efficiency of a solar cell is sensitive to variations in both the power and the spectrum of the incident light.
To facilitate an accurate comparison between solar cells measured at different times and locations, a standard
spectrum and power density has been defined for both radiation outside the Earth's atmosphere and at the
Earth's surface.
The standard spectrum at the Earth's surface is called AM1.5G, (the G stands for global and includes
both direct and diffuse radiation) or AM1.5D (which includes direct radiation only). The intensity of AM1.5D
radiation can be approximated by reducing the AM0 spectrum by 28% (18% due to absorption and 10%
to scattering). The global spectrum is 10% higher than the direct spectrum. These calculations give
approximately 970 W/m2 for AM1.5G. However, the standard AM1.5G spectrum has been normalized to
give 1kW/m2 due to the convenience of the round number and the fact that there are inherently
variations in incident solar radiation.
Sunlight energy that reaches the ground is around 4% ultraviolet, 43% visible light, and 53% infrared.
Solar panels mostly convert visible light into electrical energy, and they also can make use of almost half
the infrared energy. But solar panels only use a small portion of ultraviolet.

How does a solar cell respond differently to different wavelengths of light?

The shorter the wavelength of incident light, the higher the frequency of the light and the more energy
possessed by ejected electrons. In the same way, photovoltaic cells are sensitive to wavelength and respond
better to sunlight in some parts of the spectrum than others.

Will solar lights charge on cloudy days?

Although the direct sunlight will be blocked on cloudy days, the solar-based lights are still receiving a charge.
The clouds are diffusing the strength of the sunlight but solar irradiance is still transmitted from the sun to the
earth. So, the solar power system still charges during cloud cover.
Can you charge solar lights without sun?

You can charge solar lights without sunlight while placing solar panels directly underneath a household light to
charge them speedily. Place the solar lights close to the artificial lighting or incandescent bulb to charge solar
lights without the presence of sunlight.

Manufacture of crystalline solar panels

 Basic Structure of a cell

Photovoltaic cells, modules, panels and arrays

Solar Panel Size Comparison

 Residential Panels Commercial Panels
# of Solar Cells 60 72
Average Length (inches) 65 78
Average Width (inches) 39 39

Manufacturers Weight (60-Cell Residential

Panels)
SolarWorld 40 – 47 lbs
LG 38 lbs
Canadian Solar 40 – 51 lbs
Hyundai 38 – 41 lbs
Hanwha SolarOne 40 – 42 lbs
Hanwha Q CELLS 41 lbs
Trina 41 – 50 lbs
SunPower 33 – 41 lbs
Axitec 39 – 41 lbs
Kyocera 42 – 44 lbs

Types of solar panels / Cells.

Depending on the alignment of molecules & purity of cells, Solar Cells are classified as:
(i) Monocrystalline Silicon solar cells
(ii) Polycrystalline (or multi-crystalline) Solar Cells
(iii) Amorphous / Thin film solar cells.
(iv) Hybrid Silicon solar cells.
(i) Monocrystalline Cells:
This is the most effective and commonly used solar cells with 18-20% efficiency. They require less space (due
to high efficiency) and can field four times more power than that of thin film cells. They also last longer and
perform better at low lights.
Disadvantage is the comparatively high cost. Also badly effected by shade and dirt and such shade & dust may
break the circuit to disrupt the continuity of supply.
 Picture of Mono crystalline cells & panel & Picture of Polycrystalline cells & panel

(ii) Polycrystalline Cells:

With an efficiency of 15-18%, polycrystalline cells are preferred due to lesser cost. They are made from a
member of smaller silicon crystals that are melted together and then recrystallized. The process to create them is
simpler and less wasteful than mono crystalline.

The main disadvantage is the lesser efficiency.

(iii) Amorphous / Thin film solar cells:
At 10% efficiency, it is the least efficient cell with the cheapest option. They work well in low light even is
moon light. They are made from noncrystalline silicon that can be transferred in a thin film onto another
material such as glass. The main advantage is that it can be mass produced at a much cheaper cost but is more
suitable for situations where space is not a big issue.
The main disadvantage for thin film is that it degrades quicker
than crystalline cells.
 Picture of Thin Film cell & panel
(iv) Hybrid Silicon Solar Cells:
With efficiency 22%, hybrid solar cells are made from a mix of amorphous & monocrystalline cells to generate
maximum efficiency presently at research stage.

Picture of Hybrid Silicon solar cell & Solar panels.

Mono Crystalline Poly Crystalline

 Amorphous or Thin Film Hybrid Silicon

Different parts of Solar Panel

PV Wafer /Cell Ribbon PV Glass

Encapsulant and Back Sheet Aluminum Frame Junction Box and Cable

Encapsulant (EVA):
In the solar industry, the most common encapsulation is with cross-linkable ethylene vinyl acetate (EVA).
With the help of a lamination machine, the cells are laminated between films of EVA in a vacuum, which is
under compression. Encapsulant sheets play an important role in preventing water and dirt from infiltrating into
solar modules as well as protecting the cell by softening the shocks and vibrations to the cell. They should
possess certain properties like excellent durability, adhesive bonding to the cell and glass, excellent optical
transmission and transparency and flexibility. Cells are encapsulated before being laminated with glass and the
backsheet.
This procedure is conducted under temperatures of up to 150°C. One of the disadvantages of EVA films is that
it is not UV-resistant and therefore protective front glass is required for the UV screening. Also with the help of
the EVA, the solar cells ‘are floating’ between the glass and backsheet, helping to soften shocks and vibrations
and therefore protecting the solar cells and its circuits. Ethylene vinyl acetate (EVA) properties
Durability
Quality EVA film is known for its excellent durability, also in difficult weather circumstances, such as high
temperature and high humidity.
Bonding
Under the right circumstances, EVA film will have excellent adhesive bonding to solar glass (NOT standard
glass, solar glass has a rough surface). Also EVA bonds very well to the backsheet.
Optical
EVA is known for its excellent transparency. This means that the optical transmission is acceptable and doesn’t
block too much of the sunshine trying to reach the solar cells.
Nowadays, several manufacturers in Asia use a transparent backing, which has transparency between the cells
as a result.
This type of module is known as semi-transparent.
Tedlar is the brand name of the US-American chemical company DuPont and refers to Polyvinyl fluoride (PVF), a
thermoplastic fluoro polymer material which features high weather-resistance and inherent strength, has low
permeability of moisture, vapor, oil and can be used in a wide temperature range of between -70°C to +110°C.
Why Solar panels are rated as Watt Peak or WP?
The output of solar panel depends on the surroundings. So, to make
comparison of different solar panels, the term WP is used. For any panel,
output power or watt peak is specified on the basis of the following three
parameters:
i) solar irradiance 1000W/m2
ii) Cell temperature 25oC
iii)AM=1.5
SUN SIMULATOR
 Effect of temperature on I-V Characteristics of solar Cell

As can be seen, V-I characteristics of solar cell vary under different temperature. ... The parameter that most
affected by temperature is Voc. According to equation 2, the open-circuit voltage decreases with temperature
due to the temperature dependence of the reverse saturation current (Io).
SOLAR CELLS OPERATION
The photovoltaic (PV) effect is the direct conversion of light into electricity in solar cells. When solar cells are
exposed to sunlight, electrons excite from the valence band to the conduction band creating charged particles
called holes. In one PV cell, the upper layer is crystalline silicon doped with phosphorus with 5 valence
electrons while the lower layer is doped with boron, which has 3 valence electrons. By bringing N and P type
silicon (semiconductors) together, a p-n junction serves for creating an electric field within the solar cells,
which is able to separate electrons and hole and if the incident photon is energetic enough to dislodge a valance
electron, the electron will jump to the conduction band and initiate a current coming out from the solar cells
through the contacts . Figure 1
shows this process.

A. I-V Characteristics of Solar Cells

The equivalent circuit of the solar cells is combination of a current source (light generated current) and a diode.
Solar cells behave similarly to diodes and thus the electrical characteristics of solar cells represented by using
current-voltage curves (I-V curve). Figure 2 shows the I†•V characteristics and the equivalent circuit of
solar
the maximum output power represents the largest rectangle area under the I-V curve. The basic equation for
solar cells is follow:

As at VOC, I=0. So, IL/Io=e (voc/vT) from which, equation 2 is derived.

Where k is the Boltzman constant, T is the temperature in terms of Kelvin, q is Electric charge, V is output
voltage of solar cell, IL is light generated current, and Io is the reverse saturation current.
n: diode ideal factor. For an ideak diode, n=1.
Isc represents the short circuit current, at which the current is at maximum and where voltage is zero. Voc is
open circuit voltage, at which the voltage is maximum and where current is zero. The maximum power Pmax
produced by a solar cell is reached when the product I-V is maximum. This can be shown graphically (fig. 2.b)
where the maximum output power represents the largest rectangle area under the I-V curve.

The above graph shows the current-voltage ( I-V ) characteristics of a typical silicon PV cell operating under
normal conditions. The power delivered by a solar cell is the product of current and voltage ( I x V ). If the
multiplication is done, point for point, for all voltages from short-circuit to open-circuit conditions, the power
curve above is obtained for a given radiation level.

With the solar cell open-circuited, that is not connected to any load, the current will be at its minimum (zero)
and the voltage across the cell is at its maximum, known as the solar cells open circuit voltage, or Voc. At the
other extreme, when the solar cell is short circuited, that is the positive and negative leads connected together,
the voltage across the cell is at its minimum (zero) but the current flowing out of the cell reaches its maximum,
known as the solar cells short circuit current, or Isc.
Then the span of the solar cell I-V characteristics curve ranges from the short circuit current ( Isc ) at zero
output volts, to zero current at the full open circuit voltage ( Voc ). In other words, the maximum voltage
available from a cell is at open circuit, and the maximum current at closed circuit. Of course, neither of these
two conditions generates any electrical power, but there must be a point somewhere in between were the solar
cell generates maximum power.

However, there is one particular combination of current and voltage for which the power reaches its maximum
value, at Imp and Vmp. In other words, the point at which the cell generates maximum electrical power and this
is shown at the top right area of the green rectangle. This is the “maximum power point” or MPP. Therefore the
ideal operation of a photovoltaic cell (or panel) is defined to be at the maximum power point.

The maximum power point (MPP) of a solar cell is positioned near the bend in the I-V characteristics curve.
The corresponding values of Vmp and Imp can be estimated from the open circuit voltage and the short circuit
current: Vmp ≅ (0.8–0.90)Voc and Imp ≅ (0.85–0.95)Isc. Since solar cell output voltage and current both
depend on temperature, the actual output power will vary with changes in ambient temperature.

So far we have looked at Solar Cell I-V Characteristic Curve for a single solar cell or panel. But many
photovoltaic arrays are made up of smaller PV panels connected together. Then the I-V curve of a PV array is
just a scaled up version of the single solar cell I-V characteristic curve as shown.

Solar Panel I-V Characteristic Curves

Photovoltaic panels can be wired or connected together in either series or parallel combinations, or both to
increase the voltage or current capacity of the solar array. If the array panels are connected together in a series
combination, then the voltage increases and if connected together in parallel then the current increases. The
electrical power in Watts, generated by these different photovoltaic combinations will still be the product of the
voltage times the current, ( P = V x I ). However the solar panels are connected together, the upper right hand
corner will always be the maximum power point (MPP) of the array.
The Electrical Characteristics of a Photovoltaic Array
The electrical characteristics of a photovoltaic array are summarised in the relationship between the output
current and voltage. The amount and intensity of solar insolation (solar irradiance) controls the amount of
output current ( I ), and the operating temperature of the solar cells affects the output voltage ( V ) of the PV
array. Solar cell I-V characteristic curves that summarise the relationship between the current and voltage are
generally provided by the panels manufacturer and are given as:

Solar Array Parameters

• VOC = open-circuit voltage: – This is the maximum voltage that the array provides when the terminals
are not connected to any load (an open circuit condition). This value is much higher than Vmp which
relates to the operation of the PV array which is fixed by the load. This value depends upon the number
of PV panels connected together in series.
• ISC = short-circuit current – The maximum current provided by the PV array when the output connectors
are shorted together (a short circuit condition). This value is much higher than Imp which relates to the
normal operating circuit current.
• MPP = maximum power point – This relates to the point where the power supplied by the array that is
connected to the load (batteries, inverters) is at its maximum value, where MPP = Imp x Vmp. The
maximum power point of a photovoltaic array is measured in Watts (W) or peak Watts (Wp).
• FF = fill factor – The fill factor is the relationship between the maximum power that the array can
actually provide under normal operating conditions and the product of the open-circuit voltage times the
short-circuit current, ( Voc x Isc ) This fill factor value gives an idea of the quality of the array and
the closer the fill factor is to 1 (unity), the more power the array can provide. Typical values are
between 0.7 and 0.8.
• %eff = percent efficiency – The efficiency of a photovoltaic array is the ratio between the maximum
electrical power that the array can produce compared to the amount of solar irradiance hitting the array.
The efficiency of a typical solar array is normally low at around 15-17%, depending on the type of cells
(monocrystalline, polycrystalline) being used.

Equivalent Circuit of Solar Cell:

An ideal solar cell may be modelled by a current source in parallel with a diode; in practice no solar cell
is ideal, so a shunt resistance and a series resistance component are added to the model. The resulting
equivalent circuit of a solar cell is shown on the left.
Diodes in Photovoltaic Arrays
The PN-junction diode acts like solid state one-way electrical valve that only allows electrical current to flow
through themselves in one direction only. The advantage of this is that diodes can be used to block the flow of
electric current from other parts of an electrical solar circuit. When used with a photovoltaic solar panel, these
types of silicon diodes are generally referred to as Blocking Diodes.
Bypass Diodes are used in parallel with either a single or a number of photovoltaic solar cells to prevent the
current(s) flowing from good, well-exposed to sunlight solar cells overheating and burning out weaker or
partially shaded solar cells by providing a current path around the bad cell. Blocking diodes are used differently
than bypass diodes.
Bypass diodes in solar panels are connected in “parallel” with a photovoltaic cell or panel to shunt the current
around it, whereas blocking diodes are connected in “series” with the PV panels to prevent current flowing back
into them. Blocking diodes are therefore different than bypass diodes although in most cases the diode is
physically the same, but they are installed differently and serve a different purpose. Consider our photovoltaic
solar array below.
 https://youtu.be/ZAZSkZgVROI

Video showing function of By pass diode

diodes are devices that allow current to flow in one direction only. The diodes coloured green above are “bypass
diodes”, one in parallel with each solar panel to provide a low resistance path. Bypass diodes in solar panels and
arrays need to be able to safely carry this short circuit current.
The two diodes colored red are referred to as the “blocking diodes”, one in series with each series branch.
Blocking diodes are different than bypass diodes, but in most cases the two diodes are physically the same.
However, they are installed differently and serves a different purpose.
These blocking diodes, also called a series diode or isolation diode, ensure that the electrical current only flows
in one direction “OUT” of the series array to the external load, controller or batteries.
The reason for this is to prevent the current generated by the other parallel connected PV panels in the same
array flowing back through a weaker (shaded) network and also to prevent the fully charged batteries from
discharging or draining back through the array at night. So when multiple solar panels are connected in parallel,
blocking diodes should be used in each parallel connected branch.
Generally speaking, blocking diodes are used in PV arrays when there are two or more parallel branches or
there is a possibility that some of the array will become partially shaded during the day as the sun moves across
the sky. The size and type of blocking diode used depends upon the type of photovoltaic array.
Two types of diodes are available as bypass diodes in solar panels and arrays: the PN-junction silicon diode and
the Schottky barrier diode. Both are available with a wide range of current ratings. The Schottky barrier diode
has a much lower forward voltage drop of about 0.4 volts as opposed to the PN diodes 0.7 volt drop for a silicon
device.
This lower voltage drop allows a savings of one full PV cell in each series branch of the solar array therefore,
the array is more efficient since less power is dissipated in the blocking diode. Most manufacturers include both
blocking diodes and bypass diodes in their solar panels simplifying the design.

MPP = maximum power point – This relates to the point where the power supplied by the array that is
connected to the load (batteries, inverters) is at its maximum value, where MPP = Imp x Vmp. The maximum
power point of a photovoltaic array is measured in Watts (W) or peak

FF = fill factor – The fill factor is the ratio between the product of current and voltage at maximum
power point to the product of short circuit current and open circuit voltage of solar cell.

Fill Factor, FF = Pmax/ Isc. Voc

= Vmax. Imax/ Isc.Voc

This fill factor value gives an idea of the quality of the array and the closer the fill factor is to 1 (unity),
the more power the array can provide. Typical values are between 0.7 and 0.8.

%eff = percent efficiency – The efficiency of a photovoltaic array is the ratio between the maximum
electrical power that the array can produce compared to the amount of solar irradiance hitting the array. The
efficiency of a typical solar array is normally low at around 10-12%, depending on the type of cells
(monocrystalline, polycrystalline, amorphous or thin film) being used.

Equivalent circuit of a solar cell

The equivalent circuit of a solar cell to understand the electronic behavior of a solar cell, it is useful to create a
model which is electrically equivalent, and is based on discrete ideal electrical components whose behavior is
well defined. An ideal solar cell may be modelled by a current source in parallel with a diode; in practice no
solar cell is ideal, so a shunt resistance and a series resistance component are added to the model.[4] The
resulting equivalent circuit of a solar cell is shown

At VOC, I=0. So, 0=IL-I0 (eV/VT-1) or, IL/Io=eVoc/VT or, VOC/VT=ln(IL/Io), VOC=VTln(IL/Io) volts
Effect of various parameters on the V-I characteristics of a solar cell:
Effect of temperature: Temperature affects the characteristic equation in two ways: directly, via T in
the exponential term, and indirectly via its effect on I0. While increasing T reduces the magnitude of the
exponent in the characteristic equation, the value of I0 increases exponentially with T. The net effect is to reduce
VOC (the open-circuit voltage) linearly with increasing temperature. The magnitude of this reduction is inversely
proportional to VOC; that is, cells with higher values of VOC suffer smaller reductions in voltage with increasing
temperature. On the other hand, the amount of photogenerated current IL increases slightly with increasing
temperature because of an increase in the number of thermally generated carriers in the cell. However, since the
change in voltage is much stronger than the change in current, the overall effect on efficiency tends to be similar
to that on voltage.

Effect of Series Resistance: As series resistance increases, the voltage drop between the junction
voltage and the terminal voltage becomes greater for the same current. The result is that the current-controlled
portion of the I-V curve begins to sag toward the origin, producing a significant decrease in the terminal voltage
and a slight reduction in ISC, the short-circuit current. Very high values of RS will also produce a significant
reduction in ISC; in these regimes, series resistance dominates and the behavior of the solar cell resembles that of
a resistor. Losses caused by series resistance are in a first approximation given by P loss=VRsI=I2RS and increase
quadratic ally with photo-current. Series resistance losses are therefore most important at high illumination
intensities.

Effect of Shunt Resistance: As shunt resistance decreases, the current diverted through the shunt
resistor increases for a given level of junction voltage. The result is that the voltage-controlled portion of the I-
V curve begins to sag far from the origin, producing a significant decrease in the terminal current I and a slight
reduction in VOC. Very low values of RSH will produce a significant reduction in VOC. Much as in the case of a
high series resistance, a badly shunted solar cell will take on operating characteristics similar to those of a
resistor. These effects are shown for crystalline silicon solar cells in the I-V curves displayed in the figure to the
right.

Diodes in Photovoltaic Arrays

A diode is a two terminal semiconductor device, which allows electrical current to flow in one direction. This is
due to the fact that it offers low (ideally zero) resistance to current in one direction and at the same time, offers
high (ideally infinite) resistance to the current in the opposite direction. This property of a diode is extensively
used in the photovoltaic industry. A blocking diode and bypass diode are commonly used in solar energy
systems and solar panels.

Blocking Diode:

A blocking diode allows the flow of current from a solar panel to the battery but prevents/blocks the flow of
current from battery to solar panel thereby preventing the battery from discharging.

Blocking diode configuration

Figure 2 shows the simple working of a blocking diode. Electricity flows from high potential to low potential.
 Figure 2: Blocking diode in solar system

In this setup, during the day the solar panel (at high potential) produces electricity and charges the battery (at
low potential). During night, when the panel is not producing any electricity (low potential), the battery is at a
higher potential. There is a possibility of the current flowing from the battery to the solar panel, thereby
discharging the battery overnight. To prevent this from happening, a blocking diode is installed. It allows the
current to flow from the panel to the battery but blocks the flow in opposite direction. It is always installed in
series with the solar panel.

Bypass Diode:

A bypass diode is used in case one of the panels of a multi panel string is faulty, it bypasses the faulty panel by
providing current an alternative path to flow and thereby maintains the continuity of power production.

Bypass diode configuration

Figure 3 shows the simple working of a bypass diode. In this setup, one of the solar panel is faulty and is not
producing any current.
 Figure 3: Bypass diode in solar system

The bypass diode in this case provides an alternate path for the current to flow and completes the circuit. It
also prevents the current from other panels which are working (at high potential) to flow back to the faulty
panel (at low potential).Thus even when a panel is faulty, the bypass diode still makes the whole solar system
run and produce electricity at a lower rate. The bypass diodes should be installed in parallel to the panel.

Why solar cells have low efficiency?

Solar cells have low efficiency only 15% which is only about 1/6th of the sunlight striking the cell generates
electricity. The low efficiency is due to the following major losses-

i) When the photons of sunlight strikes the solar cell, some of them (30%) gets reflected and thus
produce loss.

ii) Photons of quantum energy less than bandgap energy can not produce electricity. This energy is
converted into thermal energy and lost.

iii) Photovoltaic cell is directly exposed to the sun. As the temperature rises, the leakage across the cell
increases. Consequently, output power reduces.
Link for solution of Problems on Photovoltaics__Stanford University

http://web.stanford.edu/class/ee293b/EE293B/Welcome_files/FEP14_SOL.pdf

In a photodiode (or some other photodetector), the quantum efficiency can be defined as the fraction of incident
(or alternatively, of absorbed) photons which contribute to the external photocurrent.

The "quantum efficiency" (Q.E.) is the ratio of the number of carriers collected by the solar cell to the number
of photons of a given energy incident on the solar cell. The quantum efficiency may be given either as a
function of wavelength or as energy.

E=hf, multiplying both sides by ϕ (photons incident per sec), we get Eϕ=hfϕ. Or Eϕ=Energy per sec=P
Watt. So, P=hfϕ

**When quantum efficiency is 100%, then one photon of light emits one electron-hole pair. If photon falling on
a surface is ϕ per m2 per sec, and if A is the area of the photocell, then the current generated is: Isc= ϕqA

What is Black body radiation?

Blackbody is a surface that absorbs all radiant energy falling on it. The term arises because incident visible
light will be absorbed rather than reflected, and therefore the surface will appear black. The concept of such a
perfect absorber of energy is extremely useful in the study of radiation phenomena. It then emits thermal
radiation in a continuous spectrum according to its temperature.

Example : A solar cell (0.9 cm2) receives solar radiation with photons of 1.8 eV energy having
an intensity of 0.9 mW/cm2. Measurements show open circuit voltage of 0.6 V/cm2, short
circuit current of 10 mA/cm2 and the maximum current is 50% of the short circuit current. The
efficiency of cell is 25%. Calculate the maximum voltage that the cell can give and find the fill
factor.

Solution:

We know,

Efficiency,

Ƞ = Pmax/pin

= Vmax Imax/ Pin

Or, Vmax =Pin. Ƞ/ Imax

=(0.9x10-3 x0.25)/ 5x10-3

=0.045 V/cm2

Again,
 Fill Factor = Pmax / (Voc. Ioc)

= (Imax*Vmax)/ (VocxIoc)

= (5x10-3x 0.045)/ (0.6x10x10-3)= 0.0375

#2. The short circuit photo currents of two diodes are 0.482A & 0.202A respectively. Find the open ckt
voltage. Io =10-8A , T=300k.

Voc =KT/q lnIs/Io , KT/q=0.026v

For diode 1 , Voc1=〖KT/q) ln 0.482/(10-8) =0.46V

For diode2 , Voc2=KT/q ln 0.202/(10-8) =0.437V

Calculation of Vm & Im.

Voltage at maximum power output Vm is calculated from the following equation

ISC/Io = (1+Vm/VT)xeVm/VT

For the diodes in the previous problem, Isc/Io=4.82x107 for diode 1 and 2.02x107 for diode 2

Putting the value of Isc/Io in eqn 1,we get Vm1 =0.388 V & Vm2 =0.367 V

Also Im =Isc –Io [e(KT/q) Vm-1]=0.482-10-8 [e^ (0.026x0.388)- 1]=0.402A

& [0.202-10-8 [e^(0.026x0.367)-1]=0.189A

Pout1 =0.452*0.388=0.175w Pout2 =0.189*0.367=0.069w

3. The sun radiates like a 6000K black body and when the power density of such radiation is
1000w ⁄m2, the total flux is 4.46x1021 photons per m2 per sec. almost exactly half of these photons have
energy equal or larger than 1.1eV. Consider a small silicon photodiode with a 10 by 10 cm area . When
2 v of reversed bias is applied, the resulting current is 30nA.This is, of course, the reverse saturation
current, Io.

When the photodiode is short circuited & exposed to black body radiation with a power density of
1000w/m2, a short circuit current, Is circulates.

Assuming 100% quantum efficiency, what is the value of this current?

ISC =qɸA=1.6x10-19 x0.5x4.46x10 21x10-2 =3.57A

What is the open circuit voltage of the photodiode at 300 k under the above illumination?

Voc =KT/q ln Iv/Io=0.026 ln 3.57/(30x10-9)=0.484V.

 Treat the photo diode of this problem as an ideal structure and assuming 100% quantum
efficiency.

4. A photo diode has an area of 1cm by 1 cm & is illuminated by monochromic light (one color light)
with a wavelength of 780 nm & with a power density of 1000w/m2. At 300k, the open ckt voltage is
0.683V.

What is its reverse saturation current, Io?

f=c/λ =(3x108)/(780x10-9)=385x1012 Hz

P=1000w/m2 x0.01x.01 m2 =0.1 W

At 300k , kT/q =0.0258 V Voc =KT/q ln IL/Io,

ISC/Io=exp Voc/VT=exp0.683/0.0258=314x109
The photon flux is
ɸ=P/hf=0.1/(6.36x10-34 x385x1012)=392x1015 photons per sec per m2
With 100% quantum efficiency, each photon causes 1 free electron to appear:
IL =q ɸ=1.6x10-19 x392x1015=0.0268A per unit area
Io=0.0268/(314x109 )=0.2x10-12 A.

Prob 14.8 What is the short-circuit current delivered by a 10 cm by 10 cm photocell (with 100% quantum efficiency)
illuminated by monochromatic light of 400 nm wavelength with a power density of 1000 W/m2 .
..................................................................................................................... The frequency that corresponds to light with
400 nm wavelength is f = c/λ = 3 × 108 /400 × 10−9 = 750 × 1012 Hz. The corresponding photon energy is Wph = hf =
6.62 × 10−34 × 750 × 1012 = 497 × 10−21 J. The light power density, P, is the product of the photon flux (# of photons
per second per m2 ) times the energy, Wph, of each photon: P = φWph, hence φ = P Wph = 1000 497 × 10−21 = 2.0 ×
1021 photons m−2 s −1 . Each photon liberates 1 electron (100% quantum efficiency), thus 2.0× 1021 electrons m−2 s −1
are circulated. Since the area is 10−2 m2 , the current is I = 2.0 × 1021 × 10−2 × 1.6 × 10−19 = 3.22 A. The short-circuit
current delivered by the photocell is 3.2 A. 0100202 Solution of Problem 14.
Battaries:

Lithium Ion Batteries: Lithium Ion batteries make use of lithium ion (Li-Ion) in order to store electricity.
The Anode is usually made up of Carbon (or any other similar nonmetal), Cathode is made of any metal oxide
and electrolyte is generally made up of lithium salt solution. In the case of lithium-ion batteries, the electrolyte
is a salt solution that contains lithium ions—hence the name.

Discharging Mechanism of Lithium Ion Battery: When a battery is placed in a device, the
positively charged lithium ions are attracted to and move towards the cathode. Once it is bombarded with these
ions, the cathode becomes more positively charged than the anode, and this attracts negatively charged
electrons. As the electrons start moving toward the cathode, we force them to go through our device and use the
energy of the electrons “flowing” toward the cathode to generate power. Thus, the battery gets discharged.

Charging Mechanism of Lithium Ion Battery: Lithium-ion batteries are great because they
are rechargeable. When the battery is connected to a charger, the lithium ions move in the opposite direction as
before. As they move from the cathode to the anode, the battery is restored for another use.
Advantages: Lithium ion batteries can produce a lot more electrical power per unit of weight than other
batteries. This means that lithium-ion batteries can store the same amount of power as other batteries, but
accomplish this in a lighter and smaller package. Besides this some advantages are given below

a) High specific energy and high load capabilities with Power Cells
b) Long cycle and extend shelf-life;
c) maintenance-free
d) High capacity,
e) Simple charge algorithm and reasonably short charge times
Low self-discharge (less than half that of NiCd and NiMH)

Disadvantages:
a) Requires protection circuit to prevent thermal runaway if stressed.
b) Degrades at high temperature and when stored at high voltage.
c) No rapid charge possible at freezing temperatures (<0°C, <32°F).
d) Transportation regulations required when shipping in larger quantities.

Some of the attributes of lithium ion batteries are listed below;

• Specific Energy: 100: 265W-h/kg

• Energy Density: 250: 693 W-h/L
• Specific Power: 250: 340 W/kg
• Charge/discharge percentage: 80-90%
• Cycle Durability: 400: 1200 cycles
• Nominal cell voltage: NMC 3.6/3.85V

Discover why lithium-ion is a superior battery system.

Pioneering work of the lithium battery began in 1912 under G.N. Lewis, but it was not until the early 1970s that
the first non-rechargeable lithium batteries became commercially available. Attempts to develop rechargeable
lithium batteries followed in the 1980s but failed because of instabilities in the metallic lithium used as anode
material. (The metal-lithium battery uses lithium as anode; Li-ion uses graphite as anode and active materials in
the cathode.)
Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest specific
energy per weight. Rechargeable batteries with lithium metal on the anode could provide extraordinarily high
energy densities; however, it was discovered in the mid-1980s that cycling produced unwanted dendrites on the
anode. These growth particles penetrate the separator and cause an electrical short. The cell temperature would
rise quickly and approach the melting point of lithium, causing thermal runaway, also known as “venting with
flame.” A large number of rechargeable metallic lithium batteries sent to Japan were recalled in 1991 after a
battery in a mobile phone released flaming gases and inflicted burns to a man’s face.

The inherent instability of lithium metal, especially during charging, shifted research to a non-metallic solution
using lithium ions. In 1991, Sony commercialized the first Li ion, and today this chemistry has become the most
promising and fastest growing battery on the market. Although lower in specific energy than lithium-metal, Li
ion is safe, provided the voltage and currents limits are being respected. (See BU-304a: Safety Concerns with
Li-ion.)

Credit for inventing the lithium-cobalt-oxide battery should go to John B. Goodenough (1922). It is said that
during the developments, a graduate student employed by Nippon Telephone & Telegraph (NTT) worked with
Goodenough in the USA. Shortly after the breakthrough, the student traveled back to Japan, taking the
discovery with him. Then in 1991, Sony announced an international patent on a lithium-cobalt-oxide cathode.
Years of litigation ensued, but Sony was able to keep the patent and Goodenough received nothing for his
efforts. In recognition of the contributions made in Li-ion developments, the U.S. National Academy of
Engineering awarded Goodenough and other contributors the Charles Stark Draper Prize in 2014. In 2015,
Israel awarded Goodenough a $1 million prize, which he will donate to the Texas Materials Institute to assist in
materials research.

Li-ion is a low-maintenance battery, an advantage that most other chemistries cannot claim. The battery has no
memory and does not need exercising (deliberate full discharge) to keep it in good shape. Self-discharge is less
than half that of nickel-based systems and this helps the fuel gauge applications. The nominal cell voltage of
3.60V can directly power mobile phones, tablets and digital cameras, offering simplifications and cost
reductions over multi-cell designs. The drawbacks are the need for protection circuits to prevent abuse, as well
as high price.

Types of Lithium-ion Batteries

Lithium-ion uses a cathode (positive electrode), an anode (negative electrode) and electrolyte as conductor.
(The anode of a discharging battery is negative and the cathode positive (see BU-104b: Battery Building
Blocks). The cathode is metal oxide and the anode consists of porous carbon. During discharge, the ions flow
from the anode to the cathode through the electrolyte and separator; charge reverses the direction and the ions
flow from the cathode to the anode. Figure 1 illustrates the process.

Ion flow in lithium-ion battery

Figure 1: Ion flow in lithium-ion battery.

When the cell charges and discharges, ions shuttle between cathode (positive electrode) and anode (negative
electrode). On discharge, the anode undergoes oxidation, or loss of electrons, and the cathode sees a reduction,
or a gain of electrons. Charge reverses the movement.

Li ion batteries come in many varieties but all have one thing in common – the “lithium-ion” catchword.
Although strikingly similar at first glance, these batteries vary in performance and the choice of active materials
gives them unique personalities. (See BU-205: Types of Li-ion-ion.)

Most Li-ion batteries share a similar design consisting of a metal oxide positive electrode (cathode) that is
coated onto an aluminum current collector, a negative electrode (anode) made from carbon/graphite coated on a
copper current collector, a separator and electrolyte made of lithium salt in an organic solvent. Table 3
summarizes the advantages and limitations of Li-ion.

Advantages

High specific energy and high load capabilities with Power Cells

Long cycle and extend shelf-life; maintenance-free

High capacity, low internal resistance, good coulombic efficiency

Simple charge algorithm and reasonably short charge times

Low self-discharge (less than half that of NiCd and NiMH)

Limitations

Requires protection circuit to prevent thermal runaway if stressed

Degrades at high temperature and when stored at high voltage

No rapid charge possible at freezing temperatures (<0°C, <32°F)

Transportation regulations required when shipping in larger quantities

Lead Acid Battery

The battery which uses sponge lead and lead peroxide for the conversion of the chemical energy into
electrical power, such type of battery is called a lead acid battery. The lead acid battery is most commonly used
in the power stations and substations because it has higher cell voltage and lower cost. The main active material
required to construct lead acid battery are
 1. sponge lead

2. Lead peroxide and

3. Dilute H2SO4.

Discharging Process of Lead Acid Battery

The lead acid storage batteries are formed by placing lead peroxide plate and sponge lead plate in dilute
H2SO4 and a load is connected externally between this plates. In diluted H2SO4 the molecules are split into H+
ion and SO4- ion. The hydrogen ions when reach at PbO2 plate they receives electrons from it and become
hydrogen atom which again attacks PbO2 and form PbO and H2O. This PbO reacts with H2SO4 and forms
PbSO4 and water.

The each sulphate ion (SO4—) moves towards the cathode and reaching the pure lead plate they give up the two
extra electrons becomes radical SO4. As the radical SO4 cannot exist alone it will attack the metallic lead
cathode and form lead sulphate according to the chemical equation. As positive H+ ion electrons takes electrons
from PbO2 and negative SO4- ion gives electrons to Pb plate there would be inequality of electrons between
these plates. Hence there would be a flow of current through the external loads between these plates for
balancing these inequality of electrons. This process is called discharging of lead acid battery.
Charging Process of Lead Acid Battery

For recharging, the anode and cathode are connected to the positive and the negative terminal of the DC
supply mains. During discharging the density of H2SO4 falls but there still H2SO4 exist in the solution. The
molecules of the sulfuric acid break up into ions of 2H+ and SO4—. The hydrogen ions being positively charged
moved towards the cathodes and receive two electrons from there and form a hydrogen atom. The hydrogen
atom reacts with lead sulphate cathode forming lead and sulfuric acid according to the chemical equation.

SO4— ion moves to the anode, gives up its two additional electrons becomes radical SO4, react with the lead
sulphate anode and form leads peroxide and lead sulphuric acid according to the chemical equation. Thus lead
acid battery becomes ready for discharging.

PbSO4 + 2H2+ SO4= PbO2 + H2SO4

Advantages
1. Low cost, Reliable and Robust.
2. Tolerant to abuse, tolerant to overcharging.
3. Low internal impedance, can deliver very high currents.
4. Wide range of sizes and capacities available.

Disadvantages:

1. Very heavy and bulky.

2. Typical coulombic charge efficiency only 70% but can be as high as 85% to 90% for special designs.
3. Danger of overheating during charging
4. Not suitable for fast charging
Key Battery Parameters

Voltage: The nominal voltage of a lead battery cell is Vcell=2V. Higher voltages can be obtained by
series connecting various cells. The most common nominal battery voltage is 12V, followed by 6 or 4V. Most
large batteries are composed of 2Vcells. Series connecting cells or batteries has no effect on their capacity.

Capacity (C): Battery capacity C, which means a battery’s charge storage capacity Q in ampere-
hours (Ah), is determined by discharge time, discharge current and operating temperature. Hence battery
capacity and discharge current are often indicated in conjunction with a subscript for discharge time in hours.
Hence C10 means battery capacity C for a discharge time of 10h, while I10 means the volume of discharge
current that flows during this period. The higher the discharge current, the lower the battery capacity. C 10 is
normally used to designate a battery’s nominal capacity.

Energy Efficiency/Watt Hour (Wh) Efficiency ȠWh :

Battery’s energy or watt hour efficiency is always somewhat lower than its Ah efficiency, and for lead batteries
is usually between 70 and 85%.

Depth of Discharge, tZ : Depth of discharge simply refers to the degree to which a battery is discharged
in relation to its total capacity. Depth of discharge is the ratio of discharged battery capacity QE to nominal
battery capacity C10 and thus indicates the depth of battery discharge during a cycle relative to nominal
capacity C10.

Depth of Discharge= QE/C10

In the interest of maximizing battery life span, in some batteries the allowable depth of discharge is limited (e.g.
for certain lead–calcium batteries it is limited to tZ = 50

When a battery is discharged, the amount of energy taken out will determine the depth at which it was discharged. For
example if you have a 100 amp hour battery and use 50 amp hours you have discharged the battery 50% which means the
depth of discharge is 50%. If you took the same battery and discharged it only 20 amp hours or 20% of the battery, your
depth of discharge will be 20%. This is an important number to keep in mind, because depending on which type of battery
you are using, the number of cycles will be vary based on your depth of discharge.

Most lead acid batteries experience significantly reduced cycle life if they are discharged more than 50%, which can result
in less than 300 total cycles. Conversely LIFEPO4 (lithium iron phosphate) batteries can be continually discharged to
100% DOD and there is no long term effect. You can expect to get 3000 cycles or more at this depth of discharge.

Figure 1 compares the characteristics of the six most commonly used rechargeable battery systems in terms of energy
density, cycle life, exercise requirements and cost. The figures are based on average ratings of commercially available
batteries at the time of publication.
C10 vs C20
It’s a fact that all C rated batteries have the same capacity, i.e., 150AH on specific load conditions. In simple
words, we can say that the integer mentioned in the type of the battery represents an attribute of the battery. For
example, at C10, the battery will last for about 10 hours (load-15A), and it should not be discharged within 10
hours. If acted otherwise, the battery life decreases. By the same token, a battery at C20 will last for 20 hours
(load-7.5A) with the condition that it should not be discharged within 20 hours or its fate will remain similar to
that of C10 rated battery. The faster the battery gets discharged, the lesser energy you get out of it. C10 rating is
known to be FAST DISCHARGE whereas a C20 rating is attributed as MEDIUM DISCHARGE.

C10 rated batteries are always recommended for solar and industrial purposes with the best charging and
discharging rates. As the high load uses battery power, it is capable of delivering more energy in a short time.
C20 rated batteries, however, are not preferred in this case due to the excessive current drawn than it is obliged
to supply which will in turn reduce its life cycle.

Car Batteries and solar Batteries

Although car batteries and solar batteries are similar, they aren’t the same. Each battery type has a different
purpose. While a car battery helps start the engine of a car, it won’t meet the needs of a solar-powered home.
For instance, solar batteries are deep cycle batteries, which means the battery can discharge over long periods
until it has almost nothing left to give, and then it recharges for further use. Two types of deep cycle batteries
are used most for solar. These include lithium-ion and lead-acid batteries.

Lithium-ion Batteries and Solar

Currently, lithium-ion solar batteries are taking the lead in solar and home battery use. Lithium-ion batteries are
lighter, have better discharge depth, and superior round-trip efficiency, and a greater lifespan.

Lithium-ion batteries use lithium as the electrolyte that creates an electricity-generating chemical reaction.
Lithium is a lightweight metal, and an electric current can easily pass through it, which makes it an ideal option
for home solar batteries.