In the autonomous electricity supply facilities, it is necessary to store the energy captured during the hours of solar radiation in order to cover supply during the hours when there is no (daily cycle and seasonal cycle).
- Electric batteries have a very important and fundamental to the proper functioning and duration of a solar photovoltaic installation function.
- They must have sufficient capacity to ensure supply of electricity during periods of clouds (autonomy of installation).
- It is reversible electrochemical systems based on chemical reactions taking place inside.
We can distinguish different types of accumulators according to their use:
- stationary batteries: usually in a fixed place and provide electrical power permanently or sporadically for various purposes. At no time, however, they are asked to give values for high currents in short times.
- Starter batteries: are responsible for producing electricity with intensity values high current for short times, for example, in cars every time you start up, or when an engine is started.The plates of the electrodes of these batteries are thicker than those of the stationary and life expectancy is shorter because of the "hard" conditions.
- Traction accumulators: are responsible for providing power to small electric vehicles and therefore asked relatively high intensities of current during periods of some hours ..
For solar photovoltaic, stationary batteries used preferably.
As for the characteristics of the electrolyte, we have type:
- Acid (lead-acid, Pb-Sb, Pb-Cd).
- Alkaline (nickel-cadmium).
The main parameters of an accumulator of electrical energy are:
- Capacity: Maximum amount of electricity that can be stored. In practice, to avoid irreversible damage to the battery, it can only provide a portion of the total capacity, we call useful capacity. The useful capacity depends on the type of battery and working conditions, but usually values from 30% to over 90% (good quality alkaline batteries) of the maximum capacity. The amount of electricity that can provide a battery also depends on the time of discharge, so the capacity will be greater the more slowly the shock. The battery capacity is expressed in ampere hours (Ah). With C5, C25 notation C100 represents time in hours discharge, respectively 5, 25 or 100 (C5 = download in 5 hours). These values give us the number of hours during which theoretically could have determined a current from the battery.
- Depth of discharge: is the percentage of maximum battery capacity that can be drawn from the battery under normal conditions. It is a very variable term which depends on the type of accumulator and that influences their life.
- Lifespan: it is usually measured in cycles (rather than years), so that a cycle is a complete charge-discharge process (up to the recommended depth of discharge). Assuming an average cycle of one cycle per day and a storage well maintained, should last a minimum of 10 years.
- Self-discharge: a phenomenon in which an accumulator, for various reasons, is discharged slowly but continuously even when not connected to an external circuit.
Functions electric accumulator
The basic functions of the batteries in solar installations are:
- Supply energy in the absence of radiation: night and day with clouds in the daily cycle and seasonal cycle.
- Maintain a stable voltage level in the installation: the voltage at the output of the modules varies according to the incident radiation, which may not be very good for the operation of some devices.
- Provide a snapshot, or for a limited time, superior power to which the field of panels could generate even in the best of cases. It is the case of engine starting, for example, the compressor motor of a refrigerator.
As we said, the most used in photovoltaic solar installations are stationary lead-acid type.
Types of batteries
Among lead-acid batteries that are in the market, we distinguish three types:
- Accumulators compact, monobloc (similar to type boot). Regular use in small installations (use on weekends ...).
- stationary batteries: built with separate cups, plates and bars with tubular low antimony.These are ideal for solar photovoltaic installations, as they have been designed to slowly discharge and recharge when available energy.
- Traction batteries: designed for moving vehicles and electric trucks; They are cheaper than stationary and can give good service in photovoltaic solar installations, whenever you need to be aware that more frequent maintenance.
It is important to know that when the battery is connected to the photovoltaic modules , the battery voltage determines the operating voltage of the modules. Thus, the operation curve of the modules have an operating point conditioned by the accumulator and not the reverse, so the intensity value giving the module is adjusted according to the voltage of the battery connected.
Although batteries typically are identified by their nominal value voltage, actually, the voltage of each cell or vessel varies depending on the state of charge. This value fluctuates between 1.85 V values (downloaded) and 2.4 V (loaded) approximately according to the type and manufacturer.
In an accumulator consists of 6 vessels (12 V nominal), the margin of fluctuation is 10.5 to 14.4 V.
Keep in mind that, normally, in an installation of photovoltaic solar energy , the voltage of the modules will be similar to the battery (except in cases where the regulator has follower of maximum power point modules). This implies that the modules work at below maximum power voltages and therefore at a temperature below the maximum possible power.
So when choosing the right storage for a solar photovoltaic installation, the choice will always be a compromise between economy and suitability, respecting minimum standards of quality in terms of reliability and durability.
In any case, for the correct selection of the appropriate storage tank it will be necessary to have the characteristics with performance curves.
For the selection of a battery, it is at least necessary to know:
- Type battery with nominal voltage, dimensions, weight ...
- Discharge capacities C20, C50, C100 with the corresponding values of shear stress.
- Working temperature range.
- maximum depth of discharge.
- Value of self-discharge.
- maximum permitted daily cycle.
- maximum working time by 50% charge and with a duty cycle of 10%.
- Load performance.
- Capacity variation as a function of temperature.
- end voltages according to the discharge rate.
- maximum charging voltage depending on the temperature and loading rate.
- Density as the state of charge.
Behavior of a storage battery in a solar photovoltaic
The voltage at the terminals of the battery depends on the following factors:
- Level or charging status
The voltage at the terminals of the battery decreases when discharged and increases when loaded to a maximum (p. Eg., 14 V batteries 12 V). When discharged, before doing the whole, it comes down to a value lower voltage limit below which the battery can not be recovered if it continues to discharge. For a typical lead-acid 12 V, this value is 10 V. In cases of battery lead-acid, avoid the effect of sulfation that occurs when you get to a high state of depth of discharge and thus it remains a while. Lead sulfate begins a process of irreversible descristal·lització, blocks the reaction of charge and makes the battery behaves as if it had lost some of its capacity, so must be replaced by another. Furthermore, care must be taken not to overcharge the battery, because in these conditions, if the panels are sending current to the battery, are still producing chemical reactions in the electrolyte and starts producing oxygen and hydrogen gas, which harms and shortens the life. Some manufacturers incorporate a recuperative plugs, through "catalysis" recombine oxygen and hydrogen returning water in the cells. But the best way to prevent gasification is a charge controller.
- Speed loading or unloading
If a battery is charged, the voltage across its terminals is higher than if desconnectéssim the charging current so that the internal resistance of the battery produces an internal voltage drop. When you download the opposite is true: the small voltage drop across the internal resistance causes the potential difference at the terminals is somewhat lower than the measure.
As the internal reactions that occur in nature are chemical battery, temperature has a decisive influence on these reactions. Thus, the final voltage recommended to achieve the status of full load should be higher the lower the temperature, because the chemical reactions are more difficult to take place and therefore need more energy to make the process run. This is important as depending on where the facility, must correct the value of the applied voltage depending on the temperature at which the battery is subjected. This conditions the room of the batteries , as discussed below.
On the other hand, we must bear in mind that:
- By increasing the temperature, the reactions are accelerated and therefore, the life decreases.
- At low temperatures, the lifespan increases, but the risk of freezing run, unrecoverable which can cause damage to the battery. Thus, to anticipate this fact, it should adjust the room batteries with moderate temperatures.
In a normal battery acid (Pb-sulfuric acid), the acid concentration is 40% and, under these conditions, the freezing point is -60 ° C. When the battery is discharged, decreases as the electrolyte concentration increases the freezing point, reaching the limit of the freezing point of 0 ° C when the electrolyte concentration is zero (water). Then the battery can definitely spoil (high mountain facilities). This phenomenon reaffirms the need to have the room batteries as isolated as possible from the cold.
Last review: April 7, 2016