In isolated electricity supply facilities, we need to store the solar energy captured during the hours of solar radiation in order to be able to cover the supply during the hours when there is none (daily cycle and seasonal cycle). For this, the installation of an electric energy accumulator is necessary.
Solar energy accumulators must have sufficient capacity to ensure electricity supply during cloudy periods. These are electrochemical systems based on reversible chemical reactions that take place inside them.
Usually, autonomous solar power systems, in addition to photovoltaic electric accumulators, are accompanied by thermal energy storage. In this installation, solar panels are also used to obtain hot water for domestic use in water heaters and heating systems.
In solar systems connected to the electricity grid, solar accumulators can also be used to save energy and reduce the amount of the electricity bill.
Main parameters of an electric accumulator
The main parameters of solar batteries are:
1. Accumulator capacity
Capacity is the maximum amount of electricity it 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 usable capacity.
The units of the battery capacity are ampere hours (Ah). Therefore, we use the following notations C5, C25, and C100 to express the discharge time represented in hours, respectively 5, 25, or 100 (C5 = discharge in 5 hours).
Lithium batteries (lithium-ion) and AGM batteries are one of those with the highest capacity.
2. Depth of discharge
The depth of discharge is the percentage of the maximum capacity of the accumulator that can be extracted from the battery under normal conditions. It is a variable term that depends a lot on the accumulator type and influences its useful life.
3. Shelf life
Lifespan is usually measured in cycles (rather than years), so a cycle is a complete charge-discharge process (until the recommended depth of discharge is reached). Assuming an average cycle of one cycle per day and a well-maintained accumulator, it should last a minimum of 10-15 years.
Self-discharge is a phenomenon whereby a battery, 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 for energy storage according to their use:
Monoblock batteries: these batteries are commonly used in small installations.
Stationary batteries: they are usually in a fixed place and provide electrical current permanently or sporadically for various purposes. However, they are not asked to give high-intensity values in a short time.
Starting accumulators are responsible for producing electrical energy with high current intensity values for short periods.
Traction accumulators are responsible for providing current to small electric vehicles; therefore, relatively high current intensities are required for a few hours.
For solar installations with photovoltaic solar panels, preferably use stationary accumulators.
Regarding the characteristics of the electrolyte, we have the following types of electric accumulators:
Acid (lead-acid batteries, Pb-Sb, Pb-Cd). This type of battery is also known as deep-cycle because it is designed to be regularly deeply discharged using most of its capacity.
What function does an electric accumulator have?
The primary functions of solar accumulators are:
Store the energy production when solar panels produce electricity, and it is not needed.
Supply energy in the absence of radiation. During nights, cloudy days, in the daily cycle and in the seasonal cycle, the energy supply must be ensured for the use of electric radiators or other devices.
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 need to be better for the operation of some devices.
Supply instantaneous power, or for a limited time, more significant than what the field of panels could generate even in the best of cases. This is the case with starting motors such as, for example, the compressor motor of a refrigerator.
Characteristics of a solar accumulator
For the selection of a battery, at least it is necessary to know the following:
Battery type with nominal voltage, dimensions, weight, etc.
Discharge capacities C20, C50, C100 with the corresponding shear stress values.
Working temperature range.
Maximum discharge depth.
Maximum daily cycle allowed.
Maximum working time at 50% load and with a 10% cycle.
Capacity variation as a function of temperature.
Final voltages according to the discharge rate.
Maximum charging voltage as a function of temperature and charging regime.
Density according to the state of charge.
The behavior of a storage battery in a PV solar energy system
The voltage at the battery terminals depends on the following factors:
1. Level or state of charge
The voltage at the battery terminals decreases when it is discharged and increases when it is charged until it reaches a maximum. When it is discharged, before it is completely discharged, a lower limit voltage value is reached below which the battery may not recover if it continues discharging.
On the other hand, care should be taken to charge the battery appropriately. If the solar panels continue to send current to the battery, chemical reactions continue to take place that harm and shorten the useful life of the solar accumulator.
2. Upload or download speed
If a battery is charged, the voltage across its terminals is greater than the drop in internal electrical resistance. However, when discharged, it happens the other way around: the small voltage drop across the internal resistance causes the potential difference across the terminals to be somewhat lower than measured.
As the internal reactions that take place in a battery are chemical in nature, the temperature has a decisive influence on these reactions. Thus, the recommended final voltage to reach the entire charge state should be higher the lower the temperature, because chemical reactions have more difficulties taking place and, therefore, need more energy for the process to be carried out.
This fact is essential since, depending on the place where the installation is, the value of the applied voltage must be corrected according to the temperature to which the battery is subjected.