Solar Power - The Battery
Solar Power - The battery
Solar Power - The battery
The battery hosts a certain reversible chemical reaction that stores electrical
energy that can later be retrieved when needed. Electrical energy is
transformed into chemical energy when the battery is being charged, and the
reverse happens when the battery is discharged.
A battery is formed by a set of elements or cells arranged in series. Leadacid
batteries consist of two submerged lead electrodes in an electrolytic solution
of water and sulfuric acid. A potential difference of about 2 volts takes
place between the electrodes, depending on the instantaneous value of the
charge state of the battery. The most common batteries in photovoltaic solar
applications have a nominal voltage of 12 or 24 volts. A 12 V battery therefore
contains 6 cells in series.
The battery serves two important purposes in a photovoltaic system: to provide
electrical energy to the system when energy is not supplied by the array
of solar panels, and to store excess energy generated by the panels whenever
that energy exceeds the load.
The battery experiences a cyclical process of charging and discharging, depending on the presence or absence of sunlight. During the hours that there is sun, the array of panels produces electrical energy. The energy that is not consumed immediately it is used to
charge the battery. During the hours of absence of sun, any demand of electrical
energy is supplied by the battery, thereby discharging it.
These cycles of charge and discharge occur whenever the energy produced
by the panels does not match the energy required to support the load. When
there is sufficient sun and the load is light, the batteries will charge. Obviously,
the batteries will discharge at night whenever any amount of power is
required.
The batteries will also discharge when the irradiance is insufficient
to cover the requirements of the load (due to the natural variation of climatological
conditions, clouds, dust, etc.)
If the battery does not store enough energy to meet the demand during periods
without sun, the system will be exhausted and will be unavailable for
consumption. On the other hand, the oversizing the system (by adding far too
many panels and batteries) is expensive and inefficient.
When designing a stand-alone system we need to reach a compromise between the cost of
components and the availability of power from the system. One way to do
this is to estimate the required number of days of autonomy. In the case of
a telecommunications system, the number of days of autonomy depends on
its critical function within your network design.
If the equipment is going to serve as repeater and is part of the backbone of your network, you will likely want to design your photovoltaic system with an autonomy of up to 5-7 days.
On the other hand, if the solar system is responsible for a providing energy to client equipment you can probably reduce number of days of autonomy to two or three. In areas with low irradiance, this value may need to be increased even more. In any case, you will always have to find the proper balance between cost and reliability.
Types of batteries
Many different battery technologies exist, and are intended for use in a variety
of different applications. The most suitable type for photovoltaic applications
is the stationary battery, designed to have a fixed location and for scenarios where the power consumption is more or less irregular. "Stationary" batteries can accommodate deep discharge cycles, but they are not designed to produce high currents in brief periods of time.
Stationary batteries can use an electrolyte that is alkaline (such as Nickel- Cadmium) or acidic (such as Lead-Acid). Stationary batteries based on Nickel-Cadmium are recommended for their high reliability and resistance whenever possible. Unfortunately, they tend to be much more expensive and difficult to obtain than sealed lead-acid batteries.
In many cases when it is difficult to find local, good and cheap stationary batteries
(importing batteries is not cheap), you will be forced to use batteries
targeted to the automobile market.
Using car batteries
Automobile batteries are not well suited for photovoltaic applications as they
are designed to provide a substantial current for just few seconds (when
starting then engine) rather than sustaining a low current for long period of
time.
This design characteristic of car batteries (also called traction batteries)
results in an shortened effective life when used in photovoltaic systems.
Traction batteries can be used in small applications where low cost is the
most important consideration, or when other batteries are not available.
Traction batteries are designed for vehicles and electric wheelbarrows. They
are cheaper than stationary batteries and can serve in a photovoltaic installation,
although they require very frequent maintenance. These batteries
should never be deeply discharged, because doing so will greatly reduce
their ability to hold a charge.
A truck battery should not discharged by more than 70% of its total capacity. This means that you can only use a maximum of 30% of a lead-acid battery's nominal capacity before it must be recharged.
You can extend the life of a lead-acid battery by using distilled water. By using
a densimeter or hydrometer, you can measure the density of the battery's
electrolyte.
A typical battery has specific gravity of 1.28. Adding distilled water
and lowering the density to 1.2 can help reduce the anode's corrosion, at a
cost of reducing the overall capacity of the battery. If you adjust the density of
battery electrolyte, you must use distilled water, as tap water or well water
will permanently damage the battery.
States of charge
There are two special state of charge that can take place during the cyclic
charge and discharge of the battery. They should both be avoided in order to
preserve the useful life of the battery.
Overcharge
Overcharge takes place when the battery arrives at the limit of its capacity. If energy is applied to a battery beyond its point of maximum charge, the electrolyte begins to break down. This produces bubbles of oxygen and hydrogen, in a process is known as gasification.
This results in a loss of water, oxidation on the positive electrode, and in extreme cases, a danger of explosion.
On the other hand, the presence of gas avoids the stratification of the
acid. After several continuous cycles of charge and discharge, the acid
tends to concentrate itself at the bottom of the battery thereby reducing
the effective capacity. The process of gasification agitates the electrolyte
and avoids stratification.
Again, it is necessary to find a compromise between the advantages (avoiding
electrolyte stratification) and the disadvantages (losing water and production
of hydrogen). One solution is to allow a slight overcharge condition every
so often. One typical method is to allow a voltage of 2.35 to 2.4 Volts for each
element of the battery every few days, at 25C. The regulator should ensure
a periodical and controlled overcharges.
Overdischarge
In the same way that there is a upper limit, there is also a lower limit to a battery's
state of charge. Discharging beyond that limit will result in deterioration
of the battery. When the effective battery supply is exhausted, the regulator
prevents any more energy from being extracted from the battery. When the
voltage of the battery reaches the minimum limit of 1.85 Volts per cell at
25C, the regulator disconnects the load from the battery.
If the discharge of the battery is very deep and the battery remains discharged
for a long time, three effects take place: the formation of crystallized
sulfate on the battery plates, the loosening of the active material on the battery
plate, and plate buckling. The process of forming stable sulfate crystals
is called hard sulfation. This is particularly negative as it generates big crystals
that do not take part in any chemical reaction and can make your battery
unusable.
by: Yoni Levy
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