2-1 a positive charge, resulting in a photovoltage.

2-1 Overview

One way of utilizing the energy of the sun is
to generate electricity directly from sun light by the PVs. The photovoltaic
effect is defined as the generation of the electromotive force as a result of
the absorption of ionizing radiation 4. Groups of PV cells are electrically
configured into modules and arrays, which can be used to charge batteries,
operate motors, and to power any number of electrical loads. With appropriate
power conversion equipment, PVs can produce alternating current compatible with
any conventional appliances, and can be operated in parallel with the utility
grid.

2-2 History of Photovoltaic:

The first conventional PV cells were produced
in the late 1950s, and throughout the 1960s were principally used to provide
electrical power for earth-orbiting satellites. In the 1970s, improvements in
manufacturing, performance and quality of PV modules helped to reduce costs and
opened up a number of opportunities for powering remote terrestrial
applications, including battery charging for navigational aids, signals,
telecommunications equipment and low power needs. In the 1980s, PV became a
popular power source for consumer electronic devices, including calculators,
watches, radios, lanterns and other small battery charging applications.
Following the energy crises of the 1970s, significant efforts also began to
develop PV power systems for residential and commercial uses both for
stand-alone, remote power as well as for utility-connected applications. During
the same period, international applications for PVs to power rural health
clinics, refrigeration, water pumping, telecommunications, and off-grid
households increased dramatically, and remain a major portion of the present
world market for PV products 5,6.

2-3 Principles of PV- Cells Operation:

PV cells are made of semiconductor materials
that can generate electricity electromagnetically when exposed to sunlight. If
a minority electron-hole pair generated by absorption of photons in the
semiconductor material (the holes in n-regions, and the electrons in p-region)
diffuses into a boundary region in which there is an electric filed, the
electron will be accelerated into the n-region, and the hole into the p-region.
This causes the n-region to accumulate a negative charge and the p-region
builds up a positive charge, resulting in a photovoltage. If there is a closed
external circuit, a photocurrent and photovoltage can be measured by the
external resistance. The work of PV cells as shown in Fig.2.1 (a) and (b). The
balance of electrons and holes can be shifted in a silicon crystal lattice by
doping it with other atoms. Atoms with one more valance electron than silicon
are used to produce n-type semiconductor materials. Atoms with one less valence
electron result in p-type material. Once the n- and p-type semiconductor
materials are reached, the process will produce potovoltage. The most common
materials used informing n- and p-type semiconductor materials are phosphorus
and boron1. A p-n junction can be formed either through a high temperature
diffusion process or an ion implantation process. Diffusion can be made either
from a vapor phase or a solid phase. Crystalline silicon and amorphous silicon
are the most dominant semiconductor materials for commercial PV cells 1.

Fig.2.1
(a) and (b): PV cell operation diagram;3.

 

2-3-1 Equivalent Circuit of Solar
Cell:

The
equivalent circuit of a solar cell consists of a constant current source Isc, a
nonlinear function (diode) impedance Dj, a shunt resistance Rsh due to leakage near
the edge and corner of the cell, a series resistance Rs due to the resistance
of the cell material, the resistance encountered when electrons travel along
the thin top sheet of n- or p-type doped material, the contact resistance, and
finally a resistive load RL 7. Fig.2.2 shows the equivalent circuit.

Fig.
2-2 Equivalent circuit of a solar cell including a resistive, load;7

2-3-2 Current
–Voltage Characteristic:

When the
solar cell exposed to light a constant current Isc is generated which, causes a
current IL to flow in the load RL. Fig.2.3 shows an I-V characteristic together
with the power curve. At zero voltage, the current flow Ij through the junction
is zero and IL = Isc (short circuit current). For small increase in voltage Ij
remains effectively zero and the slope of I-V curve depends only on the cell
shunt resistance. If Rsh infinite, the curve would be horizontal in this region
1.

Fig.2.3
Current-voltage characteristic together with the power curve of solar cell; 1

At a certain potential however the junction
begins to conduct current, increase exponentially with voltage, causing IL to
decrease rapidly. At Voc (open circuit voltage), Ij effectively equals Isc and
no current flows through the load. In the region of knee of the curve to Voc,
the slope of the I-V curve is governed by Rs, high values of Rs leads to steep
slopes. The power delivered to the load at any point on the I-V curve is IxV.

–  Efficiency and fill factor:

Efficiency of the solar cell is defined as:

Where:

Pout = the electrical output power
of the cell.

Pin = the input power of the cell.

Ps = the solar radiation level per
unit area.

As = the active cell area.

The maximum cell efficiency can be defined as:

Where:

Imp = Current at maximum power
point.

Vmp = Voltage at maximum power
point.

To optimize the cell efficiency, one has
optimized Imp and Vmp. The maximum voltage and current achievable are Voc and
Isc.

–         
Define
the fill factor (FF):

The fill factor is a practical quantity to use
when one wishes to compare the different solar cells under the same conditions.

2-4 Solar irradiation:

The solar radiation of  PVs depends critically on the spectral
distribution of the radiation coming from the sun 14. To good approximation,
the sun acts as a perfect emitter of radiation (black body) at a temperature
close to 5800 k. In general, the total power from a radiant source falling on a
unit area is called irradiance 1.

2-4-1 Calculation of Average Power
for One PV Module:

The electrical power generated and terminal
voltage of PV module depends on solar irradiance and ambient temperature. The
equivalent electrical circuit describing the solar cells module used in the analysis
is shown in Fig.2.2.15 The circuit consists of a light dependent current
source and a group of resistances, including internal shunt resistance, Rsh,
and series resistance, Rs. The series resistance should be as low as possible,
but the shunt resistance should be very high, so that most of the available
current can be delivered to the load. The mathematical equation describing the
I-V characteristics of a PV solar cells module are given by 16:

Where:

I(t) : The hourly output current,
Amp.

V(t) : The hourly output voltage,
Volt.

A : The ideality factor for p-n junction.

T(t) : The hourly temperature, Kelvin.

KB : The Boltzman’s constant in Joules per
Kelvin, 1.38*10-23 J/k.

q : The charge of the electron in Coulombs,
1.6*10-19 C.

Io(t) : The hourly reverse saturation current,
Amp. This current varies with temperature as follows 16:

Iph(t) : The hourly generated current of solar
cells module. This current varies with temperature according to the following
equation 16:

Where:

Tr : The reference temperature,
oK.

Ego : The band-gap energy of the
semiconductor used in solar cells module.

KI : The short circuit current
temperature coefficient.

Ior : The saturation current at Tr
, Amp.

HT(t) : The average hourly
radiation on the tilted surface, kW/m2.

Isc : PV cell short-circuit
current at 250 C and 100 mW/cm2.

The hourly output of the solar cells module
can be calculated by the following equation 20:

From the above equations, it can be concluded
that the output current and power of a PV module are affected by solar
insolation and operating cell temperature.

2-5 PV Cells, Modules and Arrays:

PV cells are connected in series and / or
parallel to produce the required voltage and current levels to form PV module,
the inter connection of modules on a support structure forms what called a PV
array as shown in Fig.2.7.

Fig.2.7 PV Cells,
Modules and Arrays.

The PV modules represent the basic
construction unit of the PV generator. The modules are connected in series to
form strings where the number of series modules determined by the selected DC
bus voltage, as shown Fig.2.8, and the number of parallel strings is given by
the required load current, as shown Fig.2.9.1

Fig.2.8 Series module.

Fig.2.9 Parallel
module.

2-6 Balance Of System equipment:

In addition to the PV modules, there is
balance-of-system (BOS) equipment needed to operate the PVs. This includes
battery charge controllers, batteries, inverters (for loads requiring
alternating current), wires, conduit, earthling, fuses, safety disconnects,
outlets, metal structures for supporting the modules, and any additional
components that are part of the PVs 8.

2-6-1 Charge Controller:

The charge controller regulates the flow of
electricity from the PV modules to the battery as will as the connected load.
The controller keeps the battery fully charged without overcharging it. When
the controller senses that the battery is fully charged. It stop the flow of
charge from the modules to the battery. Many controller also sense when the
batteries are over loaded, and automatically disconnects parts of the load
until sufficient charge is restored to the batteries. This last feature can
greatly extend the battery’s lifetime. Charge controller costs generally
depending on the ampere capacity at which PVs will operate and the monitoring
features required 9.

2-6-2 Battery:

The battery stores electricity for use at
night or for meeting loads demand during the day when the modules are not
generating sufficient power to meet load requirements. To provide electricity
over long periods, PVs require deep-cycle batteries. These batteries are
designed to gradually discharge and recharge 80% of their capacity hundreds of
times. Automotive batteries are shallow-cycle batteries and should not be used
in PVs because they are designed to discharge only about 20% of their capacity,
the climatic conditions in which it will operate, how frequently it will
receive maintenance, and the types of chemicals it uses to store and release
electricity. A PVs may have to be sized to store a sufficient amount of power
in the batteries to meet power demand during several days of cloudy weather,
this is known as days of autonomy 8,17.

2-6-3 Inverter:

To power an AC equipment inverter
is required, which changes the DC electricity produced by PV modules and stored
in batteries into AC electricity. Different types of inverters produce a
different quality of electricity. For example, lights, television, and power
tools can operate on lower-quality electricity, but computers, laser printers,
and other sophisticated electronic equipment require the highest-quality
electricity. So, matching the power quality required by the loads with the
power quality produced by the inverter is important 9.

Inverters cost for most stand-alone
applications is affected by several factors, including the quality of the
electricity it needs to produce, whether the incoming DC voltage is 12, 24, 36,
or 48 volts, the AC power required, the amount of extra surge power the AC
loads need for short periods, and whether the inverter has any additional
features such as meters and indicator lights 17.

2-7 Types of PVs:

PV power system is generally
classified according to their functional and operational requirements and how
the equipment is connected to other power source and electrical loads. The two
principle classifications are either stand alone or grid connected system 1.

2-7-1 Stand Alone PVs:

SAS are designed to operate
independent of the electric utility grid in its simple form, it consists of the
array supplying the load directly, as shown in Fig.2.10 such system can be used
for battery charging via charge controller or for water pumping, where the
storage medium is a storage tank.

 

Fig. 2-14 stand-alone
AC system with battery and back-up generator

 

2-8 PVs Market Overview:

In 2004 more than 2700 MWp of PV
were installed worldwide and its applications as shown in Fig.2.15. Japan has
the highest installed capacity followed by Germany and USA. These three
countries represents about tow third global PV capacity. Market grow rate in
the last 10 years were between 20% and 40%, and in recent decades, there was a
price reduction of 20% when the market volume doubled. As a result of this,
photovoltaic prices dropped by about 50% every decade. It is not sure how long
this price reduction process will continue. However PVs have the potential to
become comparative even with conventional grid-connected systems, in the few
decades 11.

 

Fig.2.15 World PV modules
production, consumer and commercial (MW) 11

 

The cost of a PV module is
measured in dollars per peak-watt $/Wp, where “peak watt” is defined
as the power of full sunlight at sea level on a clear day. Modules are rated
using standard test conditions which is 1000 W/m2, an air mass of 1.5 at 25 oC.

Thus PV module “cost
reduction” is the result of either a decrease in manufacturing cost or an
improvement in module efficiency. Crystalline silicon PV module process has
decreased from $51/Wp to approximately $3.50/Wp in 2002 11.