Solar in the conduction band, and P-type having

Solar
Photovoltaic system complexity varies from as simple as pocket calculators and
as complicated as space station power suppliers specially, because of the
phenomenon of photovoltaic effect, the conversion from solar energy to direct
current electricity in certain types of semiconductors. To understand the
mechanism of solar cells one needs to have knowledge of photons and solar
radiation, semiconductor structure, conversion of solar energy and light
energy, chemical and electrical energy.

This
chapter will be focusing on the mechanism within the solar photovoltaic cell
principles and its conversion of energy to produce electricity, the structure
of solar cell will also be covered.

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2.1     Photovoltaic Theoretical Background

          In semiconductor materials, conduction is dependent on
the energy band called band gap. There exist one excited band and one lower band.
In the normal state, exited band contains minimum number of electrons and lower
band which is also called valence band contains maximum possible number of
electrons, when energized, electrons move from valence band to excited state
where they become free to move hence resulting in easy conduction of the
material. The free electrons in the conduction band are separated from the
lower state is measured in electron volts (eV). This energy in diodes is
provided by electricity to make the diode conduct and is quite opposite in the
photovoltaic cell in which it is provided by photons (particles of light). When
electrons gain enough energy in the valence band they move up to the conduction
band and when the move from conduction band to the valence band the lose energy
equivalent to the difference of energy band. Process of movement of electrons
from valence band to conduction band is called excitation as it gains energy
and when electrons move back from conduction band to valence band is called
recombination as it losses energy.

Figure 1. Excitement energy levels in
intrinsic and doped semiconductor. Modified from Nipun (2015) 1

 

2.1.1  Doping

it is the process to make semiconductor
materials to conduct in more proper way, by adding some impurities. As
semiconductor materials from fourth group are mixed with 3rd and 5th
group elements making P-type and N-type semiconductors respectively, figure 1
shows N-type, a semiconductor doped with 5th group element has
excessive number of electrons in the conduction band, and P-type having minimum
number of electrons there.

2.1.2  PN-Junction

When
both N-type and P-type elements are combined, electrons and holes diffuse at
the center making a region called depletion region when electrons from N-type
burst into the P-type and hole from P-type to the N-type. This process is well
explained in the figure 2.

Figure 2. Depletion region and electric
field E created by diffusions of electrons and holes in P-N junction. Modified
from Wikimedia (2016) 3.

 

          With the creation of depletion region,
it becomes hard to conduct, in photovoltaic cell when exposed to light,
receives energy that is needed to make it conduct and depletion region also
known as PN-Junction contracts as show in next figure.

 

 

P

type

N

type

E

photons

load

I

 

Figure
3. Occurrence of electric current I when an external circuit was connected to a
P-N junction. Modified from Apec (2007) 4.

 

In photovoltaic cell, as
the light energy is received at PN-Junction, current starts flowing in the
reverse direction, which is called photovoltaic effect which manipulates the
solar power using semiconductor properties.

 

 

2.1.3  Solar Cell

          As the generic semiconductor doping is done by adding
impurities into the 4th group of elements of 3rd and 5th
group elements. But famous solar cell materials are Silicon (Si), Gallium
arsenide (GaAs), Cadmium selenide (CdSe) have spectrum properties according to
the wavelengths of the solar radiation received on the surface of earth. By
doping the different elements and making multiple junctions increased range of
sun light can be used to make solar cell more efficient.

 

 

 

 

72

93

44

34

79

18

23

18

17

11

9

76

8

67

8

61

7

13

5

51

0

1000

2000

3000

4000

5000

6000

7000

8000

0

0.5

1

1.5

2

2.5

InSb

InAs

Ge

GaSb

Si

InP

GaAs

CdTe

CdSe

GaP

Wavelength nm

Band Gaps eV

Figure 4. Band gap levels of typical
semiconducting materials and corresponding light’s wavelength values. Data
gathered from Georgia State University 5 and Michigan State University 6.

Among
the materials shown about, most famous is Silicon for its availability, found
from the silica sand and purified either in form of monocrystalline or
polycrystalline. Si for its cheapness is mostly used and its pn-junction
behavior as well as it is conducted at mid-range radiation from sun. c-Si solar
cells have efficiency averaging over 15%. Which means they are capable of
converting over 15% of solar energy in electrical. Typically, each solar cell
is made up of a square having an area of approximately 1 cm2 produces
power of about 1.5 W of the voltage 0.6 V, which is too below the least
required to operate for applications. Therefore, they are combined in series to
make a proper standard of voltage. Different sizes and standards are designed
by manufacturers according to the applications. Each photovoltaic system
contains Solar module which is heart of solar PV system.

2.1.4  Solar Photovoltaic System

          Basically, solar system contains,

·       
Photovoltaic cell

·       
Battery backup

·       
Control unit or
solar module

·       
Load

·       
Main supply unit

PV
cell

Battery backup

Load

Control Unit

 

 

 

 

 

Main supply

                                                                             Figure
5.

Typical
Block diagram of solar

                                                                             Photovoltaic
system

          The PV cell is the main component of
the system which converts solar energy into electrical energy, which is usually
a series of solar modules mounted on solar panels for larger output power. It
converts light into direct current, DC power which can be utilized directly for
low power applications and even can be utilized for high power applications by using
inverter and storing the power into the battery for backup.

          Due to the serial connection of
photodiodes, if any of the module is stops conducting in case it is shaded and
other is producing energy, power will start flowing within the shaded solar module
and causing over heat and may damage the solar panel, so for its safety this
problem is prevented by using bypass diodes, even blocking diode is used so
that battery does not drain its power when solar panel is shaded.

          The solar panel is supported by a
mechanical structure or placed such a way that it receives maximum possible
radiations coming onto the surface of earth, or a solar tracking system is
designed.

 

2.2     Solar Module’s Performance and Solar
Tracking System

 

2.2.1  Performance by Fixed Mounting

          For PV module which catches solar radiation, solar
radiation that is reached on the surface of earth has three components.

·       
Direct beam, that
reaches on the surface without scattering

·       
Diffuse radiation,
that scatters while entering earth’s atmosphere

·       
Radiation that
reflects from surface of earth

Solar
radiation reaching on the surface of earth consist of 80-90% component of
direct beam in sunny weather. It is the major source of energy for PV modules,
so it needs to be aligned with the direct beam of solar radiation for maximum
possible duration. This process can be well defined by assuming an angle j,
which is the angle between solar radiation’s direct beam and surface of PV
module. The area that collects the solar radiation direction beam is proportional
to the sine(j), hence the power collected will be calculated as;

P
= P(max) * sin(j)                                                 
(1)

Where
P(max) is maximum power achieved by PV module when angle between surface of PV
module and direct beam radiation is 90°, and the loss of power can be
calculated as using equation (1),

P(loss)
=

 = 1 – sin(j)                               (2)

According to equation (2), as the solar direct beam
goes parallel to surface of the PV module, P is lost directly, as the radiation
is more misaligned with direct beam the more energy is lost. This relationship
is illustrated in figure 6.

Figure 6. Relationship between power loss and
misalignment of direct beam

The graph of figure 6 shows power loss in accordance
with misalignment if the angle of misaligned is assumed to be i, then the
equation (1) and equation (2) will be function of cosine. In that manner as the
angle of cosine increases the power loss increases. As it can be seen that if
the angle of misalignment is 30° then the power loss is about 15%. Power loss
increases gradually if the angle of misalignment is further increased. By some
calculation we can obtain the values of output power of PV module on certain
location of the earth, let’s say sun rises at 05:30 and sets at 19:30,
calculating the output power from the solar power calculator database online we
get a graph assuming weather is sunny. By using equation (1) and giving time of
sunrise and sunset for a particular location we get approximation of output
power of a fixed mounted PV module.

Figure
7. Approximation of power output (red line) compared to maximum output (blue
line) for a fix mounted solar module. Data gathered from NOAA Solar Calculator
7.

 

It can be seen in figure 7, that from 11:00 to 16:00,
power output is merely over 80% from a fixed mounted PV module. Which is only 5
hours of the day. For the typical 12 hours of summer day in Pakistan, 5 hours
of 80% efficiency from a PV module is certainly not a good way to gather solar
energy.

However, figure 7 does not reflect the whole story, is
based on assumption but somehow it does explain that fixed mounted Solar Panel
is not a solution for maximum power conversion of solar energy despite in the
ideal weather conditions.

In reality, there are multiple problems that caused
solar radiations fade that are cloud shades, seasonal angles changes and
limited time of daylight. In spite of mechanical stability and simplicity fixed
mounted solar panel can not exploit maximum possible conversion. Therefore, not
suitable for important projects and high capacity. There
is a need of better solutions for mounting the system. We will be subsequently
discussing about current solutions of solar tracker for better harvesting of
solar power. This subtopic contains useful information for the design and
creation of the solar tracking system in this project.

 

2.2.2   Enhancement Using Tracking Systems

 

As discussed above, fixed
mounted panels have many power losses for the working of PV module. Solar
tracker systems are solution decreasing the angle of misalignment between solar
radiation’s direct beam and panel. Using different means of mechanical
structures, solar trackers can rotate the panels to the optimized position
throughout the whole operation of the system. Comparing solar tracker system
with fixed mounted panel below,

 

Advantages

Disadvantages

Higher overall
efficiency
Higher accuracy
Longer active
functioning time
Better lifetime for
solar cells
Applicability for
different applications

More complicated
design
Higher cost
Worse tolerance
against weather condition
Consumption of energy
(active trackers)

Table
1. Advantages and disadvantages of trackers over fixed mounts

 

Solar
trackers can be categorized in two types, one being the active and other is
passive.

Passive
trackers are designed by flued that vaporizes and expands along both ways when
heated by sun. Flued is filled in canisters connected to a tube along both
sides. Different expansions of the flued according the heat of the sun
according to its position occur. Balancing weight of the solar panel on both
sides, causing it rotate towards the direction of the sun. Passive trackers are
less favorable and efficient because of its complex design and low accuracy.

Active trackers on the other hand, utilize motor
system to control the movement of panels in single or dual-axis, by observing
the Sun’s position using photo sensors. The operation of this type of trackers
is managed by controller or computer. Active trackers normally cost more to the
system, but provide the best accuracy and efficiency compared to the other
solutions. A dual-axis tracker (DAT) can provide additional 40% of solar energy
over the year, compared to normal fixed mounting system.

 

2.3.2  Active
Solar Trackers

  

Among
the introduced solar tracking systems, active solar tracker is the chosen topic
of research for this project, because of its extensive utilization of
electrical and electronic knowledges. It is also the most implemented solution
for capturing the sunlight of PV systems. Together with better manufacturing
technologies of PV materials, enhancing the operation of active solar trackers
is the most efficient way to better exploit the immense energy amount of the
Sun.

Based
on rotation of solar modules, active solar trackers can be categorized into two
main types: single-axis and dual-axis. In single-axis trackers (SAT), solar PV
panels are rotated about a single axis that normally aligns with the North meridian.
SATs can be configured in a number or ways according to the position of the
axis with respect to the ground: 

 

·        
Tilted single-axis tracker (TSAT) –     Horizontal single-axis tracker (HSAT) –           Vertical single-axis tracker
(VSAT). 

 

SATs allow the solar
modules to rotate between east-west directions according to the

Sun’s
positions. SATs provide reasonably good balance between flexibility, simplicity
and performance. Different configurations of SAT are illustrated in figure 8.

Figure 8. Typical
configurations for active solar tracking systems: (1) TSAT (2) HSAT (3) VSAT
(4) TTDAT (5) HDAT (6) AADAT. Reprinted from Juda (2013) 8

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