STUDIES ON BATTERIES CONFIGURATIONS FOR ISOLADED POWER GENERATION SYSTEMS ¹

STUDIES ON BATTERIES CONFIGURATIONS FOR ISOLADED POWER GENERATION SYSTEMS  ¹
STUDIES ON BATTERIES CONFIGURATIONS FOR ISOLADED
POWER GENERATION SYSTEMS ¹
Weschenfelder, F.²
Schaeffer, L.³
Abstract
This article is based on the study of the best topology battery to be used in isolated power
generation. An isolated system consists of solar panels, for example, the power generating
source can vary, we can use wind, biomass. Also part of the system charge controllers,
inverters and batteries. Currently, there are several types of batteries such as nickelcadmium, lead-acid, among others. It is necessary to assess the feasibility of each for the
desired application, according to the characteristics of the system being implemented. A
solar generation system has several advantages, among which we can highlight its extreme
simplicity, the absence of any moving mechanical part, its modular feature (from mW to MW),
the short terms of installation, high reliability level systems and their low maintenance.
Furthermore, solar PV systems represent a source silent, non-polluting and renewable
electricity very well suited to integration in urban and isolated places, almost completely
reducing the transmission losses of energy due to the proximity between generation and
consumption.
Keywords: Batteries; Systems isolated power generation, Solar Energy.
1 SENAFOR/2nd International Conference on Materials and Processes for Renewable Energy
2 Master’s Student / UFRGS / [email protected]
3 Teacher and Coordinator Laboratório de Transformação Mecânica – LdTM/ UFRGS.
1 INTRODUCTION
The last decades have been characterized by an important development and
maturation of technologies for using renewable energy sources. Since 1980,
photovoltaic systems, wind and biomass have increased the efficiency and
consequently to become more economically viable. These systems have been
running in various forms and strengths in various parts of the world with great
success. Technologies for use of renewable energies, can no longer be regarded as
unrealistic solutions within the overall context of rational use and energy saving [1].
Photovoltaic solar energy, renewable energy source obtained by converting light
energy into electrical energy, is an option for isolated systems. The isolated systems
are characterized by not connecting to the power grid. The system supplies directly
appliances that use energy and are usually built with a specific purpose and location.
This solution is widely used in remote locations since it is often the most economical
and practical way to obtain electricity at these places. Examples of use are water
pumping systems, electrification of fences, refrigerators to store vaccines, light poles,
signal replicating stations, etc.. The energy produced is stored in batteries that
guarantee supply during periods without sun [2].
The capacity and the electrochemical characteristics of various types of batteries
available in the market show that for a project of this magnitude, knowledge of
batteries is critical to a good choice. For any PV system with batteries, choosing the
type of battery used will dictate the success or failure of the project [3].
2 ISOLATED SYSTEMS
The isolated systems for generating solar photovoltaics, simply, are composed of
four components, Figure 1 shows a schematic of this system.
Font: Macroblock (2010)
Figure 1. Simplified schematic of an isolated system of solar generation.
2.1 Solar Panels
They are the heart of the system and generate the electricity that supplies the
batteries. Has the property of turning solar radiation into electric current. A system
can have only one panel or several panels interconnected as shown in Figure 2 [3].
Font: Solar EnergiesUK
Figure 2. Solar Panels
2.2 Controllers load
They are the heart valve and ensure the correct supply of batteries avoiding overload
and deep discharge, increasing its life [3].
2.3 Inverters
Are the brain of the system and has the function to transform direct current (dc) into
alternating current (AC) and cause the voltage, eg from 12V to 127V. In some cases
it may be connected to another type of generator or the grid itself to supply batteries
[3].
2.4 Batteries
They are the lungs of the system and store electricity for use at times when there is
no power generation and no other sources of energy [3].
3 REQUIRED CHARACTERISTICS
A battery consists of four basic elements: an anode made of material that can
contribute electrons, a cathode that will accept electrons, electrolyte and separator.
The arrangement of these elements is shown in Figure 3, during battery discharge,
the anode contributes electrons for oxidation, which generates positive ions.
Similarly, the cathode generates negative ions in the process of accepting electrons
[4].
A key element of a battery is a separator between the anode and cathode while
permitting free flow of ions, flow of electrons is forced to travel on the external circuit,
completing the circuit. In a rechargeable cell the process is reversible, in the
discharge cathode is the positive terminal and the negative terminal anode. However,
during charging, the reverse occurs, now the positive terminal is the anode and the
cathode negative terminal [4].
Font: (Vehicle Technologies Program)
Figure 3. Principal constituent parts of an electrochemical
3.1 Capacity
The capacity is characterized as the maximum current that a battery can deliver continuously
for an hour, without causing their destruction. Its unit of measurement is given in amperehours (Ah). A battery that can provide two amperes at a time, not necessarily in two hours
provides only one ampere, since the higher its discharging time, the greater its efficiency [5].
3.2 Rate “C”
The rate "C" is a measure of the current loading or unloading, in terms of capacity in one
hour. As an example, a standard AA battery has a capacity at a time of about 500 mAh.
Consequently, a charge rate of 2C is A 1 and a C/10 rate is 50 mA. Manufacturers usually
specify the best discharge current to a particular battery type [5].
3.2 Discharge curve
The discharge curve of a battery can be characterized as the peak voltage value, nominal and
when fully discharged. Generally, the charts provided by manufacturers to discharge curve are
functions of the rate "C", as can be seen in Figure 4 [6].
Font: (Catalog Moura)
Figure 4. Discharge curve for a lead acid battery manufacturer Moura
3.3 Energy Density
Energy density is how much energy a battery can store and provide for the
application with a given battery size — the more energy dense the battery, the less
volume and weight is needed [7].
3.4 Self Discharge
The self-discharge is the loss of stored energy when the battery suffers when it is not
being used (loading or unloading). This effect is caused by the electrochemical
process internal and resembles the effect caused by a small load connected to the
battery, Table 1 shows the monthly discharge rate of some batteries [8].
Table 1: Monthly discharge of some batteries
Battery type
Monthly discharge
Lead-Acid
Nickel - Cadmium
Nickel - Metal Hydride
4% - 6%
15% - 20%
30%
Lithium
2% - 3%
Font: Marques, 2012.
3.5 Life cycle
Is given in numbers of cycles, followed by unloading cargo, that a rechargeable cell
can provide. When considering the life of the battery can distinguish between aging
and wear of the battery of the battery. The aging relates to processes and factors
which tend to limit the duration of the battery physical integrity and their ability to
perform the function predicted. Battery wear refers to processes that tend to limit the
amount of electricity that can be stored or delivered [9].
Corrosion is a major component of the battery aging, this process can be greatly
accelerated by harsh environmental conditions or improper maintenance, but these
can presumably be controlled. Battery wear, moreover, is very much a function of a
history of charging / discharging the battery is in particular subject to a standard of
abuse may cause the battery to fail long before it would be feasible only through
processes aging [9].
4 ANALYSIS OF BATTERIES
4.1 Lithium-ion
Advanced technology that requires a protection circuit built into the battery. Li-Ion
battery is used which states that a battery lighter and more hours of use (better than
NiMH). Approximately 35% lighter than NiCd. State of the art in terms of chemical
composition. Not develop memory effect. No need for conditioners. It provides more
hours of usage and a longer operation than NiMH. Is not suitable for solar since it
has longer charging time, has good performance in low temperatures and is the most
expensive commercial battery [10].
4.2 Nickel Cadmium (NiCd)
The battery Nickel - Cadmium consists of an anode formed by an alloy of cadmium
and iron hydroxide and a cathode (oxide) nickel (III) immersed in an aqueous solution
of potassium hydroxide with a concentration between 20% and 28% by weight . The
main drawback of this technology is its susceptibility to memory effect. Originally, the
term memory effect was coined to describe a memory problem where the cyclic NiCd battery would "remember" the amount of discharge for discharges previous life
and limit the battery recharge. The problem is less prevalent with modern Ni-Cd
batteries, which are designed to prevent memory problems cyclic. Contains elements
toxic to the environment and must be recycled [10].
The battery has a good performance if left in the charger and only used for short
periods of time, which prevents its use in solar systems, as this application will be
carrying throughout that period there brightness [10].
4.3 Nickel-Metal Hydride (NiMH)
The nickel-metal hydride (Ni-MH) can be considered as the successor of nickelcadmium batteries, with the advantage of not containing toxic heavy metals in its
composition, and have greater energy density. Moreover, they are considered more
environmentally correct as they may reduce the problems associated with the
disposal of rechargeable nickel [11].
The metal hydride electrode has a higher energy density than a cadmium electrode,
so the mass of active material for the negative electrode used in a nickel-metal
hydride batteries can be smaller than that used in nickel-cadmium batteries. This also
allows one to use a larger amount of active material for the positive electrode,
resulting in an increased capacity or discharge time for this battery [10].
It has poor charge retention, since it undergoes a process of self-discharge of
approximately 2% per day. It has moderate memory effect. Generally provides more
"run time" (time between each recharging of use), but a lower life cycle (the number
of times that the battery can be charged / discharged) than most NiCd. Not active at
temperatures as low as NiCd. Discharge more quickly when stored for long periods of
time without use. Are more sensitive to damage caused by heat [11].
There are two main factors that hinder the use of this type of battery for isolated
systems, the first is the high cost of the same and the second factor is the high selfdischarge [11].
4.4 Lead acid
The lead-acid batteries were invented in the nineteenth century has as basic
components lead or lead oxide and sulfuric acid. As the battery lead / acid is
discharged, the sulfuric acid is consumed and water is produced. The composition of
sulfuric acid in the electrolyte and its density varies from 40% (m/m), and 1.30 g/cm3,
in the fully loaded state, to about 16% (m/m), and 1.10 g/cm3; in the discharged state
[12].
Among the advantages that make it viable is the application of energy generation can
be mentioned that have inexpensive, available in large quantities and a variety of
sizes and shapes. They have good charge retention for applications in intermittent
loads. Available in the form of free maintenance, low cost compared with other
secondary systems. The cell components are easily recyclable, Figure 5 shows the
components thereof [12].
The limiting this option are the low life cycle (50 - 500 cycles). Energy Density limited
- typically 30-40 Wh / kg, long term storage conditions of discharge can cause
irreversible polarization of the electrodes (sulphation). Difficulty in manufacturing in
small sizes, hydrogen release some models can cause an explosion (flame inhibitors
are installed to prevent this), the release of some toxic gases when present can be
hazardous to health. And, there is heat loss in improperly designed batteries [12].
Font: (Prime Products)
Figure 5. Lead-acid Battery
4.5 Iron - Nickel (Battery Edison)
Edison batteries (Ni-Fe) are rechargeable batteries are designed to store DC power
systems for power generation alone, without a network connection. The optimum
temperature of operation thereof is between -20 ° C and 50 ° C. Unlike lead acid
batteries that have a life cycle of approximately 7 years, nickel-iron systems have an
expected lifetime of 25 years or more and therefore become a choice for
environmentally sensitive off-Grid systems and applications storage of renewable
energy [13].
Fonte: (Alibaba product -131277179)
Figura 6. Edison Battery
The efficiency of loading depends on the temperature at which the cell is exposed,
and also the state of charge that it lies. Figure 7 shows the variation in the efficiency
of this process related to temperature. You can see that the efficiency can be 50%
and reaches 90% if the two parameters are controlled [13].
Font: (Willians, 2011)
Figure 7. Efficiency Battery Edison [13].
Cells nickel-iron (Edison) are designed for a lifetime of 20 years but the temperature
increase of the electrolyte will reduce the useful life. In general, all 9°C of
temperature increase above the normal ambient temperature of 25 °C reduces the
lifetime of the cell Edison 20%. For lead acid batteries, every 9 °C of temperature
increase, a reduction of 50%. Figure 8 shows a graph comparing the expected life at
high temperature, for both types of batteries [13].
Font: (Willians, 2011)
Figure 8. Estimated battery life of Edison and Lead Acid.
5 CONCLUSION
By analyzing the characteristics of all the batteries mentioned in this article, one can
see that the most appropriate for applications in isolated power generation batteries
are lead-acid and nickel-iron batteries (Edison batteries).
Between these two, it is possible to be done for a comparative particular application,
the lead battery takes advantage to present a more consolidated market, have good
charge retention for applications where intermittent loads which makes them very
attractive, since the power generation solar has this feature.
In contrast, Edison battery have a life cycle than the cycle of lead acid batteries,
which can make the systems more affordable cost. Another point in favor of this
configuration is Nickel-iron batteries do not have the lead or cadmium of the leadacid and nickel-cadmium batteries, which makes them a lesser burden on human
and ecological health.
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