Photovoltaic energy storage

In what photovoltaic installations do we use batteries?

Photovoltaic energy storage serves the purpose of storing excess electricity generated and utilizing it when production is less efficient or unavailable. Photovoltaic batteries can be applied in two types of setups:

Energy storage in off-grid photovoltaic installations

Off-grid photovoltaic systems, which are self-reliant and operate independently of the traditional power grid, are typically employed in small residential communities located in remote areas where connecting to the grid would not be cost-effective.

Energy storage in hybrid photovoltaic installations

Hybrid installations, which combine their own energy storage with a connection to the grid, are quite common. Their key advantage lies in the ability to draw power from the grid when photovoltaic panel generation falls short, while also avoiding the need to feed excess energy back into the grid when it’s not as financially beneficial, as long as their storage capacity permits.

Advantages of storing energy from photovoltaics

Photovoltaic systems paired with energy storage are gaining popularity due to the continuously decreasing installation costs. These systems offer homeowners a sustainable and cost-effective way to generate clean energy and achieve long-term savings.

However, the presence of photovoltaic installations presents challenges for the power grid. In many cases, the electrical grids, particularly in Poland, are in suboptimal condition and have limited capacity for new connections. As a result, individuals interested in installing photovoltaic systems without energy storage often face rejections.

Power grid operators view photovoltaic systems with energy storage as a potential solution to alleviate these common issues. Energy storage can enhance flexibility in connecting distributed generation setups and significantly enhance the overall stability of the power grid.

Construction of a photovoltaic installation with a battery

Construction of an off-grid installation, (source: I. Góralczyk, R. Tytko: Photovoltaics. Photovoltaic and electrical devices, installations, 2015)

A PV installation with energy storage consists of:

• photovoltaic panels
• charge regulator
• energy storage
• inverter
• receiver

The key components that set off-grid solar installations apart from on-grid ones are the charge controller and the energy storage system, typically a rechargeable battery or galvanic cell.

It’s important to note that using a battery solely with photovoltaic panels isn’t sufficient. Directly connecting solar panels to a battery can lead to overcharging during the day and discharge at night due to reverse current flow. Thus, a charge controller is essential in ensuring the proper coordination between the PV panels and the energy storage, safeguarding the battery against overcharging or discharging. Moreover, it can offer valuable information about system performance, battery charge status, and the amount of energy currently received from the solar panels.

Battery as an energy store

Electricity can be stored in various ways, including mechanical, electrochemical, and electrical methods.

Batteries fall into the category of electrochemical storage, specifically secondary cells. These devices operate through reversible chemical reactions occurring within the electrodes submerged in an electrolyte. During the charging process, electrical energy is converted into chemical energy, and during discharge, this chemical energy is transformed back into electrical energy. This enables the storage and retrieval of electricity as needed.

The most important features of a photovoltaic battery:

Battery Capacity [Ah]: This represents the electric charge a battery can store and is typically measured in ampere-hours (Ah). It signifies the rate at which a discharged battery could provide electrical current for one hour.

Electric Capacity of the Battery [Wh]: This denotes the total amount of electricity a battery can deliver, from being fully charged to fully discharged. It is expressed as the power that can be used to discharge a fully charged battery in one hour, measured in watt-hours (Wh).

Cycle [1]: In the context of battery operation, a cycle consists of four fundamental phases: charging, a break, discharging, another break or discharge, another break, and finally charging again.

Charging Cycle: During this phase, the battery acts as a recipient of electrical energy within the system. Inside the battery, electrical energy is converted into chemical energy for storage.

Discharging Cycle: The charged battery serves as a source of electric current, which is consumed by connected devices, resulting in a gradual discharge. It is crucial to protect the battery from deep discharge, which can significantly shorten its lifespan; often, a charge regulator performs this function.

Battery Lifetime (Lifetime) [Calculated in Cycles]: This metric represents the number of operational cycles a battery should endure while maintaining its specified capacity. It measures the expected lifespan of the battery based on its usage and the number of cycles it can withstand before a noticeable decrease in capacity occurs.

Selection of a battery for a photovoltaic installation

The most important thing for those looking to use energy storage is to pick the right system and determine how they’ll use it, like for daily or temporary needs. This way, they can reduce the number of times the system needs to charge and discharge, which is the main factor in shortening the battery’s lifespan. When choosing a battery for solar panels, it’s crucial to remember not to leave the batteries discharged for more than 24 hours, as this can harm them or make them wear out faster.

The most important features of energy storage for photovoltaics

When selecting a battery, the following parameters should be taken into account:

Rated Capacity [Ah]: This is the capacity of new batteries, usually expressed in ampere-hours (Ah), where 1 Ah equals 3600 coulombs. To understand how much current a device can provide and for how long, check the device’s operating specifications. For example, a 100 Ah battery can supply a current of 1 A to devices for 100 hours or 100 A for 1 hour.

Discharge Voltage: This indicates the acceptable voltage range for battery discharge, ensuring the device’s durability is not compromised. It typically depends on the discharge current and is specified for the battery’s rated operating temperature, usually 20°C.

High Operating Temperature: When a self-service battery operates at elevated temperatures, its lifespan significantly decreases. It can reduce by as much as half for every 8°C increase above the rated operating temperature (usually 20°C).

Low Operating Temperature: Operating at low temperatures may lead to a reduction in the battery’s rated capacity. At 0°C, approximately 85% of the rated capacity remains available. At -10°C and -20°C, it’s reduced to 75% and 65% of the capacity, respectively.

Battery Location: This parameter is linked to operating temperature. It’s important to choose a location for the battery that is away from heat sources and allows at least 1.5 cm of space between the battery and other devices. Proper air circulation is necessary, especially for devices with ventilation holes, and natural or forced ventilation should be considered.

Battery Life: The longevity of a battery depends on various factors. Typically, battery durability is measured by the number of work cycles it can withstand. This can be influenced by factors such as temperature, usage patterns, and maintenance practices.

• operating temperature,
• discharge depth,
• discharge current

Working mode ? you should also pay attention to what mode the battery should operate in:

• Buffer: Typically used as a device to stabilize voltage in the network during power failures, such as in an Uninterruptible Power Supply (UPS). Buffer batteries are continuously connected to a charging system and experience periodic charging and discharging cycles. When used correctly, the service life of such a battery is approximately 5 years, provided it operates within the recommended conditions (e.g., at an operating temperature of 20°C and following proper charging voltage guidelines).
• Cyclical: Cyclical batteries are continuously connected to the power network and undergo regular charging and discharging cycles. These batteries are designed for frequent use in applications where they are regularly charged and discharged as part of their normal operation.

Self-Discharge: Self-discharge refers to the natural loss of capacity that occurs in batteries, even when they are not actively in use. The rate at which self-discharge happens depends on the storage conditions, particularly the ambient temperature and humidity. On average, at a temperature of 20°C, batteries tend to self-discharge at a rate of approximately 3% per month. This means that over time, the battery’s stored energy gradually diminishes, making it less effective if left unused for extended periods.

Classic batteries in a PV installation, i.e. electrolyte in liquid form

This category of batteries includes lead-acid batteries, well-known for their use in automobiles. In lead-acid batteries, the electrolyte is a liquid solution of sulfuric acid, and it fills the cell. The electrodes are composed of lead and lead oxide (PbO2 for the anode). When using lead-acid batteries, it’s advisable to use additional equipment, such as gas recombinators. These devices are designed to reduce ventilation requirements and can lead to less frequent service inspections. For off-grid photovoltaic setups, it’s recommended to use classic batteries with a reinforced positive plate. These batteries are advantageous due to their affordability and widespread availability.

Lead-acid batteries are typically deployed for emergency situations when there’s a sudden loss of power. Frequent discharging is not recommended; instead, efforts should be made to minimize discharging. Additionally, they have substantial operational requirements. For instance, a set with a capacity of about 5 kWh in an off-grid system may weigh nearly 150 kilograms and should be placed in a well-ventilated location, such as a basement or garage, to ensure proper airflow.

Gel batteries for photovoltaics

Gel batteries are a type of lead-acid battery where the sulfuric acid electrolyte is in a gel-like form. These batteries offer several advantages, including high charging efficiency and reduced ventilation requirements. They are also known for preventing electrolyte stratification during slow charging, which is crucial in photovoltaic installations.

When choosing batteries for solar panels, it’s advisable to opt for models that support full recovery from a deep discharge state and provide an extended number of deep charge and discharge cycles. This is achieved through the use of reinforced plates in the electrodes. Gel batteries are particularly well-suited for installations with an unstable power supply network.

One of the key benefits of gel batteries is their suitability for pulsed operation. They are less sensitive to slow discharging and recharging or being left partially charged compared to other battery types. This makes them a practical choice for a wide range of consumers.

AGM (Absorbed Glass Mat) batteries in photovoltaic installations

AGM (Absorbent Glass Mat) batteries use a unique design where the electrolyte is held within specialized fiberglass separators positioned between the lead battery plates. This design is particularly well-suited for devices or applications that demand a significant surge of power during startup. AGM batteries are often chosen for their ability to provide high power output when needed, making them suitable for various applications, including those with high initial power requirements.

Advantages of gel and AGM batteries for photovoltaic panels:

• fast charging time
• high efficiency
• long self-discharge time
• work stability
• wide range of temperatures in which they can work
• possible to work in various positions
• eliminating the risk of electrolyte leakage from a mechanically damaged battery

Disadvantages of gel and AGM batteries for photovoltaic panels::

• price ? they are much more expensive than standard batteries with liquid electrolyte
• very short lifespan
• require the use of charge controllers, i.e. voltage stabilizers, ensuring the correct charging process and maintaining appropriate operating conditions and cooperation with the network

Lithium-ion batteries for photovoltaics

A common application of batteries with photovoltaic panels is to store solar energy during the day for use at night, and for this purpose, lithium-ion batteries are highly recommended. The same technology is used in mobile phones and electric cars.

Lithium-ion batteries offer several advantages over other battery types, including their lightweight nature, durability, and ease of use. Although they are currently more expensive than standard batteries, their prices are rapidly decreasing as technology advances. Furthermore, they have a service life of at least several thousand charging cycles, which can be attractive to investors.

It’s important to note that lithium-ion batteries require special protection to ensure their safe operation. Precise control of parameters like charging and discharging voltage and cell temperature is necessary. As a result, lithium-ion energy storage systems are often equipped with an integrated or external Battery Management System (BMS). This BMS allows for the monitoring and control of individual cells or the entire storage system to ensure correct operation and safety, including the ability to shut down individual cells or the entire system in the event of any anomalies.

The future of photovoltaic energy storage

In recent years, there has been a notable shift in how we perceive energy storage. Photovoltaic systems paired with energy storage are no longer solely seen as a backup power source for emergencies. Both individual prosumers and larger power plants have recognized the potential of energy storage as a solution to address various challenges within the power system, particularly those arising from the growing popularity of renewable energy sources.