In autonomous electricity supply installations, it is necessary to store the energy captured during the hours of solar radiation in order to be able to cover the supply during the hours when there is no (daily cycle and seasonal cycle).
- Electric accumulators have a very important and fundamental function in the good operation and duration of a photovoltaic solar installation.
- They must have sufficient capacity to ensure electricity supply during periods of cloud (autonomy of the installation).
- These are electrochemical systems based on reversible chemical reactions that take place inside.
What are the main parameters of an electric accumulator?
The main parameters of an electric energy accumulator are:
- Capacity: the maximum capacity you can store.
- Discharge depth
- Shelf life
Capacity is the maximum amount of electricity you can store. In practice, and to avoid irreversible damage to the battery, it can only provide a part of the total capacity, which we call useful capacity.
The useful capacity depends on the type of accumulator and the working conditions, but it usually has values from 30% to more than 90% (in good quality alkaline batteries) of the maximum capacity. The amount of electricity that an accumulator can provide also depends on the discharge time, so the capacity will be greater the slower the discharge occurs.
The battery capacity is expressed in amp hours (Ah). The notation C5, C25, C100 represents the discharge time in hours, respectively 5, 25 or 100 (C5 = discharge in 5 hours). These values give us the number of hours during which theoretically we could have a certain current intensity coming from the accumulator.
The depth of discharge is the percentage over the maximum capacity of the accumulator that can be removed from the battery under normal conditions. It is a very variable term that depends a lot on the type of accumulator and that influences its useful life.
What is the useful life of an energy accumulator?
Service life is typically measured in cycles (rather than years), so a cycle is a complete charge-discharge process (up to the recommended discharge depth). If we assume an average cycle of one cycle per day and a well-maintained accumulator, it should last a minimum of 10 years.
Self-discharge: it is a phenomenon whereby an accumulator, for various reasons, discharges slowly but continuously even though it is not connected to an external circuit.
What are the types of electric accumulators?
We can differentiate different types of accumulators according to their use:
- Stationary accumulators: they are usually in a fixed place and provide electrical current permanently or sporadically for various purposes. At no time, however, are they asked to give high intensity values in short times.
- Starter accumulators: they are responsible for producing electrical energy with high current intensity values for short times, for example, in cars every time they are started, or when an engine is started. The electrode plates of these accumulators are thicker than those of stationary ones and their useful life is shorter due to "harsh" working conditions.
- Traction accumulators: they are responsible for supplying current to small electric vehicles and, therefore, they are asked for relatively high current intensities for periods of a few hours.
For photovoltaic solar installations, preferably use stationary accumulators.
Regarding the characteristics of the electrolyte, we have the following types of electric accumulators:
- Acid (lead-acid, Pb-Sb, Pb-Cd).
- Alkaline (nickel-cadmium).
What is the function of an electric accumulator?
The basic functions of accumulators in solar installations are:
- Supply energy in the absence of radiation: nights and days with clouds, in the daily cycle and in the seasonal cycle.
- Maintain a stable level of voltage in the installation: the voltage at the output of the modules varies depending on the incident radiation, which may not be very good for the operation of some devices.
- Provide instantaneous power, or for a limited time, greater than the field of panels could generate even in the best of cases. This is the case of engine starting, such as the compressor motor of a refrigerator.
As we have said, the most used in photovoltaic solar installations are those of the stationary type of lead-acid.
What types of electric energy accumulators are there?
Among the lead-acid accumulators on the market, we differentiate three types:
- Compact accumulators, monoblock type: (similar to starter type). Of habitual use in small installations (use on weekends ...).
- Stationary accumulators: built with independent vessels, tubular plates and bars with low antimony content. These are ideal for photovoltaic solar installations, as they have been designed to be able to slowly discharge and recharge them when energy is available.
- Traction accumulators: designed to move vehicles and electric trucks; They are cheaper than stationary and can give good service in photovoltaic solar installations, provided that they need more frequent maintenance.
It is important to know that when the accumulator is connected to the photovoltaic modules, the accumulator voltage determines the operating voltage of the modules. Thus, the operating curve of the modules will have an operating point conditioned by the accumulator and not the other way around, so the value of the intensity given by the module is adjusted according to the voltage of the connected accumulator.
Although accumulators are normally identified by their nominal voltage value, in reality, the voltage of each cell or vessel varies depending on the state of charge. This value fluctuates between approximately 1.85 V (unloaded) and 2.4 V (charged), depending on the type and manufacturer.
In an accumulator consisting of 6 vessels (nominal 12 V), the fluctuation margin ranges from 10.5 to 14.4 V.
It should be borne in mind that, normally, in a photovoltaic solar energy installation, the voltage of the modules will be similar to that of the battery (except in cases where the regulator has a follower of the maximum power point of the modules). This fact implies that the modules work at voltages lower than the maximum power and, therefore, at a power lower than the maximum possible.
Thus, when choosing the right accumulator for a photovoltaic solar installation, the choice will always be a compromise between economy and suitability, respecting minimum quality in terms of reliability and duration.
In any case, for the correct selection of the appropriate accumulator, it will be necessary to have the characteristics with the operating curves.
What are the characteristics of a battery?
For the selection of a battery, at least it is necessary to know:
- Battery type with nominal voltage, dimensions, weight ...
- Discharge capacities C20, C50, C100 with corresponding values of cut-off voltage.
- Working temperature range.
- Maximum discharge depth.
- Self-discharge value.
- Maximum daily cycle allowed.
- Maximum working time at 50% load and with a cycle of 10%.
- Charging performance.
- Variation of capacity depending on temperature.
- Final voltages according to the discharge regime.
- Maximum charging voltage depending on temperature and charging regime.
- Freezing temperature.
- Density according to the state of charge.
Behavior of an accumulator battery in a photovoltaic solar energy installation
The voltage at the battery terminals depends on the following factors:
- Level or state of charge
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Level or state of charge
The voltage across the battery terminals decreases when discharging and increases when charging to a maximum (eg 14 V on 12 V batteries). When discharging, before fully discharging, a lower limit voltage value is reached below which the battery may not recover if continued discharging.
For a typical 12V lead-acid battery, this value is 10 V. In lead-acid batteries, the sulfation effect that occurs when a high discharge depth state is reached should be avoided. and it stays like that for a while. Lead sulfate begins a process of irreversible decrystallization. It blocks the charging reaction and makes the battery behave as if it had lost part of its capacity, so it must be replaced by another.
On the other hand, care must be taken not to overload the battery, since under these conditions, if the panels continue to supply current to the battery, chemical reactions of the electrolyte continue to occur and it begins to produce gaseous oxygen and hydrogen, which damages and shortens shelf life. Some manufacturers incorporate recovery plugs that, through "catalysis", recombine oxygen and hydrogen, returning the water in the cells. But the best way to prevent gassing is a charge regulator.
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If a battery is charged, the voltage at its terminals is higher than if the charging current is disconnected so that the internal resistance of the battery produces an internal voltage drop. When discharged it happens the other way round: the small voltage drop in the internal resistance makes the potential difference at the terminals somewhat less than measured.
Since the internal reactions that take place in a battery are chemical in nature, temperature has a decisive influence on these reactions. Thus, the recommended final voltage to reach the state of full charge should be higher the lower the temperature, because chemical reactions have more difficulties to take place and, therefore, require more energy for the process to run.
This fact is important, since depending on where the installation is, the value of the applied voltage must be corrected depending on the temperature to which the battery is subjected. This conditions the battery room, as we will see later.
What other elements should be considered?
On the other hand, it must be taken into account that:
- As the temperature increases, the reactions accelerate and, therefore, the useful life decreases.
- By lowering the temperature, the service life increases, but there is a risk of freezing, which can cause unrecoverable damage to the battery. Therefore, to foresee this fact, the battery room should be adapted with moderate temperatures.
In a normal acid battery (Pb-sulfuric acid), the acid concentration is 40% and, in these conditions, the freezing point is -60 degrees Celsius. When the battery discharges, as the electrolyte concentration decreases, the freezing point increases, reaching the freezing point limit of 0ºC when the electrolyte concentration is zero (water).
Then the battery can be permanently damaged (high mountain installations). This phenomenon reaffirms the need to keep the battery room as isolated from the cold as possible.