CHAPTER
ONE
INTRODUCTION
1.0 Background of the study
A charge controller is an essential
part of any alternative energy system. In its simplest form, a charge
controller's job is to make sure the power (such as a solar panel) 'plays nice'
with the load (such as a battery). The simplest implementation of this is a
single diode placed in between a solar panel and battery. This ensures that the
battery does not discharge into solar panel at night. A more sophisticated
implementation will be adding the ability for the charge controller to
disconnect the solar panel when the batteries are fully charged in order to
prevent over-charging damage to the batteries (James and Dunlop, 1991).
The current version of the open source
free charge controller is a converter for charging batteries, A bulk converter
steps down voltage from a higher voltage level to a lower voltage level. In
this case, it would step voltage down from the 18volts of a solar panel to the
12volts of a battery (Harrington and Dunlop, 1992). Since the converter is
software controlled, it can be programmed to charge any battery chemistry,
change it drive frequency to achieve maximum conversion efficiency as well as
implement MPPT to allow a solar panel to deliver maximum power, all without any
changes to hardware (Robert and Isaac, 2007). A charge controller also called
charge regulator or battery regulator limits the rate at which electric current
is added to or drawn from electric batteries. It prevents over charging and may
prevent over voltage, which can reduce battery performance or life span, or may
pose a safety risk. It may also prevent completely draining (deep discharging)
a battery, or perform controlled discharges, depending on the battery
technology, to protect battery life.
The term charge controller or charge
regulator may refer to either a stand-alone device, or control circuitry
integrated within a battery pack, battery-power device, or recharger (Dunlop,
1991; Harrington and Dunlop, 1992)
Basically,
there are four types of charge controllers. These are namely :-
I.
Series
charge controller or series regulator
II. Shunt
charge controller or shunt regulator
III. Pulse
width modulated charge controller (MPPT)
IV. Maximum
power point tracker (MPPT)
A
series charger controller disables further current flow into batteries when
they are full. A shunt charge controller diverts excess electricity to an
auxiliary or shunt load as electric heater, when batteries are full (Harrington
and Dunlop, 1992).
Pulse width modulated (PWM) and maximum power
point tracker technologies adjust charging rate depending on the battery
voltage level to allow charging closer to its maximum capacity. Charge
controller may also monitor battery temperature to prevent over-heating. Some
charge controller systems also display and transmit data to remote displays and
data logging to track electric flow over time (Sanjit, 1980; James and Dunlop,
1991; Robert and Isaac, 2007).
The primary function of a charge
controller in a stand-alone PV system is to maintain the battery at highest
possible state of charge while protecting it from over charge by the array of
solar panels and from over discharge by the loads, Although some PV system can
be effectively designed without the use of a charge control, any system that
has unpredictable loads, user intervention, optimized or undersized battery
storage (to minimize initial cost) typically requires a battery charge
controller (James and Dunlop, 1991). The algorithm or control strategy of a
battery charge controller determines the effectiveness of battery charging and
PV array utilization, and most importantly the ability of the system to meet
the load demands. Additional features such as temperature compensation, alarms,
meters, remote voltage sense leads and special algorithm law enhance the
ability of a charge controller to maintain the health and extend the lifespan
of battery, as well as providing an indication of operational status to the
system caretaker (James and Dunlop, 1991; Harrington and Dunlop, 1992).
1.1 AIM AND OBJECTIVES
The project
is aimed at the following:-
I. Preventing
battery overheating to limit the energy supplied to the battery by the PV array
when the battery becomes fully charged.
II. Preventing
battery undercharge to disconnect the battery from electrical loads when the
battery reaches a low state of charge.
III. Providing
load control functions to automatically connect and disconnect an electrical load
at specified time, for example operating a lighting load from sunset to
sunrise.
IV. Designing
a control algorithm for charge controller to determine which particular
algorithm will be suitable and efficient for charge regulation (Stevens, 1999,
Robert and Isaac 2007)
V. Knowing
how to design, select and match guidelines for battery application and charge
control requirements in PV systems.
1.2 JUSTIFICATION OF THE STUDY
Photovoltaic systems remain the best alternative to the
power supply problem in Nigeria today. And the important of a charge controller
in a stand-alone photovoltaic system cannot be over emphasized as mentioned
earlier.
But the efficiency of a charge controller clearly depends of
the regulation technique that is used. Modern charge controllers employ the
dynamic potentials of pulse width modulation in tracking the maximum power of
the battery bank, through the voltage regulation set points. This method
provides for a range of voltages through which charge disconnection and
reconnection occurs.
However, the type of charge controller described above makes
use of several integrated circuits (ICs) to generate the pulses. This is
usually complex and expensive to realize.
In this project work, a single chip of IC 555 timer is used
to generate the pulses, bearing in mind its basic function as a multivibrator,
its availability and low cost.
1.3 RESEARCH
METHODOLOGY
The project begins with the sourcing of materials and
textbooks in the library and through the internet, to understand what a charge
controller is and how it works including it's basic functions.
The research continued with an inquiry into the various
types of charge controllers with their respective charge regulation techniques
and designs which includes series design, shunt design, pulse width modulation
and maximum power point tracking.
Thereafter, the design of the project was done to determine
the appropriate components to be used. Thus, linear circuit theorems were
applied for the mathematical analysis of various voltages, currents and
resistances at set points. For a 24v, 0.96Kw charge controller, it will handle
a maximum current, I as follows:
From P = V2/R,
where P represents total power,
V
represents voltage and
R represents circuit resistance,
If P = 0.96kw and V = 24v,
then 0.96 = (24)2/R
or R = 24x24/960 = 0.6 ohms
By ohm's law, V = IR,
where
I represents the maximum current, it implies that
I = V/R = 24/0.6 = 40A
Therefore, the maximum short circuit current that can be
managed by the charge controller is 40 amperes. Other theorems such as
The venin's and Norton's theorems are also applied in addition to the modeled
equations to calculate and determine the values of components used.
After the designing, the components are sourced according to
the design specifications made by calculation. Thereafter, the components are
mounted by carefully following the schematic diagram for the project.
The project work is tested to examine how it functions, and
a charge controller algorithm is designed to operate the charge controller
according to the voltage regulation set points.
The last stage of the project work deals with conclusion,
where emphasis is laid on problems encountered during the process of carrying
out the project work and suggested solutions as well as recommendations on the
need to encourage charge control in solar energy systems.
1.4 EXPECTED FINDINGS
At the end of the project it is expected that:-
I. In
a PV system, the battery system should be electrically designed for optimal
performance and safety
II. Adequate
knowledge of different types and classification of battery charge controller
will be acquired.
III. Different
design and hardware assembly for the various types of charge controller can be
achieved.
IV. There
is new technique in getting the maximum power by using the pulse width
modulation technique; the PIC micro controller performs this function
primarily. But it is believed that 555timer IC can also perform this function due to its
characteristics (Sanjit, 1980; Lam, 1995)
1.5 EXPECTED CONTRIBUTION TO KNOWLEDGE
I. The
project work will give an insight of the common terminologies associated with
battery charge controller in PV systems.
II. Detailed
understanding of the actual modes operations of different charge controllers in
PV systems will be enhanced.
III. The
effect of charge rate, charge regulation algorithm and set points on battery
performance and life in PV systems will be clearly understood (Robert and
Isaac, 2007).
The importance of equalization for batteries in
PV systems will be known in addition to knowing the types of equalization and
conditions necessary for adoption of
battery equalization (Lam, 1995)
