Batteries and Inverters in Solar Energy
Edis Osmanbasic posted on February 14, 2018 |
Solar panels converting the sun’s energy into useful electricity. (Image courtesy of Raysonho.)
Solar panels converting the sun’s energy into useful electricity. (Image courtesy of Raysonho.)

There’s evidence to suggest that we could be the last generation powered by fossil fuels. Recent investments in renewable energy sources, such as wind, solar and biomass, have rapidly increased the competitiveness of fossil fuel alternatives. According to many energy consumption statistics, renewables are the fastest-growing energy source for power generation.

There are two main reasons for making investments in renewable (aka green) energy. First, fossil fuels are limited in supply, and they will eventually run out.Second, and more importantly, fossil fuel emissions like carbon dioxide are increasing the planet’s temperature and damaging our ecosystem.

Green energy is the energy of the future, so it’s important to become familiar with the working principles of renewable energy systems.The central feature of these systems is the battery. In this article, we’ll discuss the specifics of the batteries and inverters used in renewable energy systems, particularly those used for solar energy.

Solar Energy and How to Convert It to Electricity

The sun is the source of almost all energy available on Earth. Solar energy is produced from nuclear fusion in the sun’s core, where hydrogen is fused into helium. This releases a huge amount of energy in the form of light and heat, and a small amount of this energy strikes the Earth's surface—up to 1kW/m2, though the actual value depends on location, season, weather conditions and other factors.

Solar energy, which is almost unlimited if we can collect it and convert it to an appropriate form, can be harnessed in the three following ways:

  • Solar thermal collector: absorbs sunlight to collect heat
  • Photovoltaic (PV)cell: converts sunlight directly into electrical energy
  • Concentrated solar power: uses mirrors to concentrate a large area of sunlight onto a small area

There are three main parts of solar energy systems: solar panels, solar charge controllers, and an inverter and battery storage system. Solar energy systems engineers must consider the following parameters: PV cell maximum power, sunlight intensity, angle of the sunlight (PV panel tilt angle), and the amount of sunhours (generally calculated by the amount of sun hours per day).

How Does a Battery Work?

A battery is a portable energy source that converts chemical energy to electrical energy. Simply put, batteries contain three basic parts: the electrodes, the electrolyte and a separator. There are always two electrodes in a battery: the cathode is connected to the positive end, while the anode is connected to the negative end. When the battery powers a load, it discharges, and current flows from the cathode to the anode. When the battery charges, current flows from the anode to the cathode.

Electrodes are immersed in an electrolyte, a liquid or gel substance that contains the electrically charged ions that react with the electrodes. This chemical process causes the battery to generate electricity. The separator physically separates the electrodes. Without it, the electrodes would come into contact and be short-circuited, destroying the battery.

The main parts of a battery: cathode, anode, electrolyte and separator.
The main parts of a battery: cathode, anode, electrolyte and separator.

Batteries provide electricity in the form of direct current (DC), but an inverter can be used to achieve alternating current(AC). The most important parameters of any battery are the following:

  • Nominal cell voltage
  • Nominal capacity
  • Battery type
  • Number of cells in the battery string

 

A battery string in a substation.
A battery string in a substation.

The capacity of a battery indicates how much energy it can store, which can be measured in ampere hours (Ah). This gives an approximate measure of how much current the battery can provide in an hour. If a more accurate definition is required, the battery voltage must be considered, as voltage decreases during discharge. The total energy of a battery can be given in watt-hours (Wh), which are calculated as follows:

where E is the energy in Wh,

Vavg is average voltage over a discharge cycle, and

is the battery capacity in Ah.

It is amazing that batteries are currently available on the market with capacities up to 3000Ah.

Different Battery Types

Batteries can be roughly grouped into two types: regular (nonrechargeable) and rechargeable. This article focuses on the rechargeable batteries used in renewable energy systems. In this battery type, the chemical reaction is reversible, allowing both discharging as well as recharging. There are three main types of rechargeable batteries: lead-acid, nickel-cadmium (NiCd) and lithium-ion.

Types of rechargeable batteries.

Lead-acid batteries, designed as flooded (wet) or valve-regulated lead-acid (VRLA), can be manufactured with a nominal cell voltage of 2V or 12V. Nickel-cadmium battery cells have a nominal voltage of 1.2V. Lastly, the nominal voltage of lithium-ion batteries can range from3.3-3.7V, depending on the chemistry of the cell.

Life Span of a Battery

It’s almost impossible to estimate how long a given battery will last, as there are many factors that influence the life cycle of a battery. These include battery type, the number of charging/recharging cycles, operating conditions like temperature, how fully the battery was discharged, among others.

If all battery systems are properly managed, a battery will die after all the active materials have been consumed or the positive grids have deteriorated due to corrosion, which occurs throughout the battery’s lifetime. Failure to follow appropriate design and system management guidelines will almost always guarantee a battery system’s early failure. There are eight ways to kill a battery:

  • Overcharge
  • Overdischarge
  • Excessive charge rates
  • Excessive discharge rates
  • Improper equalization
  • Too hot or cold operational environment
  • Extended storage period
  • Improper battery for a given application

Solar energy is available during the day, but energy is also necessary during the night. This makes batteries a very important part of the solar energy system, as they can provide constant electrical power whether the energy source is available or not. Solar energy system owners need a reliable battery with a long lifespan—not to mention an affordable price.

It can be hard to find a battery that meets all these requirements.Batteries in solar energy systems require a long lifespan because of the many charge/discharge cycles that occur during the day and night. Since the battery must power appliances during the overnight hours, it should also have a high capacity and be able to operate without damage when it is deeply discharged (this is known as deep-cycle storage capability).

What’s a Good Battery for Solar Energy Systems?

Deep-cycle storage capability is a mandatory feature for batteries in a solar energy system. Lead-acid batteries have this feature, as they can be discharged up to 80 percent of total capacity without any repercussions. Flooded lead-acid batteries are the most commonly used batteries in solar energy systems, as they also have a long lifespan and are cost-effective.

The disadvantage of flooded lead-acid batteries is that they are difficult to maintain, a definite drawback because solar plants are typically installed in terrain that can be challenging to access. The electrolyte of flooded lead-acid batteries evaporates, so these batteries must be refilled. They must also have an exhaust system to prevent hydrogen gas from accumulating to dangerous levels, so the battery station requires ventilation. Another problem with flooded lead-acid batteries is disposal, because of their toxicity. Thankfully, due to battery development in the automotive industry, it’s now possible to recycle these batteries.

Gelled electrolyte VRLA batteries use a silica gel instead of a liquid electrolyte. These batteries cannot be spilled and are very low maintenance. However, because they are quite expensive and have less capacity than other battery types, gelled electrolyte VRLAs are not popular in solar applications.

Absorbed glass mat (AGM) VRLA batteries offer the advantages of gelled electrolyte VRLA batteries while avoiding their disadvantages. As an electrolyte, they use fine fiber boron-silicate glass mat instead of a gel, so they can’t be spilled even when broken. Their self-discharging rate is even better than with flooded lead-acid batteries, and they also better at withstanding temperature variations. However, one clear disadvantage is that AGM batteries are two to three times more expensive than flooded lead-acid batteries.

Nickel-cadmium batteries are not as popular for use in solar energy systems. In recent years,the electric car industry has been focusing on lithium-ion batteries and has invested a lot in their development. Thus, lithium-ion batteries are becoming increasingly popular in renewable energy applications, offering good quality and a long lifespan (approximately five years, according to Tesla).

Lithium-ion batteries are the most common energy storage technology used today. However, these batteries have disadvantages for use in renewable energy systems. They have lower efficiencies, higher self-discharging rates, and worse temperature tolerances than lead-acid batteries. In addition,they are still expensive.

Currently, flooded lead-acid batteries are the most popular batteries used for solar energy applications, and they will probably remain dominant in that space over the next few years.

Inverter Design

The majority of electrical devices use ACvoltage, so inverters are used to convert DC electricity to useable AC.

The main components of inverters are the switching elements, which provide two alternate paths for the load current flow. The switching elements constantly switch, alternating the current direction in each switching sequence. Semiconductor switches (primarily transistors) are typically used as the switching element. In practice, MOSFET transistors are widely used with output power up to a maximum of 5kW. When more power is required, IGBT transistors are generally used.

Basic diagram of a single-phase inverter (left) and three-phase inverter (right).
Basic diagram of a single-phase inverter (left) and three-phase inverter (right).

A simple inverter produces square wave signals. The figure that follows shows the basic operating principle of this inverter (creating the AC square wave signal).

Inverter operation sequence to create a square wave signal.
Inverter operation sequence to create a square wave signal.

This simple inverter can be used in basic applications such as for lights, heaters and so on, but not for most electrical devices, as it produces significant harmonic distortion.

Many different variations of inverters are available on the market—from simple square waves up to pure sine waves.Advanced inverters use many stages to produce a sine wave, and filters are used to produce as pure a sine wave as possible.

Main Inverter Parameters

Single-phase inverters are usually installed in low power systems like houses, while three-phase inverters are generally used in high power solar plants. High power solar plants usually have transformers to increase the network voltage value.

The inverter power is determined by photovoltaic (PV) solar generation power. The voltage and its frequency value should always be stable, and should also be tolerated on the time-limited overload and high inrush current (peak current). The inverter nameplate should have information about the overload power in limited time. The inverter input and output should be galvanic isolated.

An important parameter of inverters is their efficiency. Inverter efficiency represents the inverter’s losses when it converts DC into AC power, and it’s defined as the ratio between useful output power and the input. The inverter withdraws energy from the battery even when it does not supply any AC load, which is why larger applications use “sleep mode” inverters in which a sensor detects if supplying power is required and activates the inverter accordingly.

Inverter technology has progressed enormously. Today, inverters are available with efficiency ratings of 95 percent, and the best solutions on the market can even provide 98 percent efficiency.

Specifics of the Solar Inverter

Photovoltaic solar systems generate DC voltage, and an inverter converts the power to AC voltage. Solar inverters produce a sine wave and are designed for high power—up to hundreds of kilowatts. Unlike simple electronics inverters, solar inverters provide numerous functions in addition to DC-to-AC conversion. They are responsible for energy metering, monitoring, regulation and protection of the solar energy system.

Solar energy systems can be designed as on-grid or off-grid (isolated) systems. Off-grid systems are designed to work independent of the electrical network, while on-grid systems can supply energy to the network. On-grid systems can be set up with or without a battery storage system, which can be used for backup power. Inverters are a mandatory component in both types of systems.

Energy in a solar energy system can flow in different directions. In the case of a simple system in a home application, the users can be supplied only from PV panels. If there is insufficient sunlight, the users can be supplied from both PV panels and batteries, or only from the batteries (e.g., during the night).

An on-grid solar energy system.
An on-grid solar energy system.

A very important function of solar inverters is their role as the solar charge controller. Depending on the energy state of the PV panels, the solar inverter will direct energy to users, battery charging or the network. The charge controller connects the PV panels, batteries and users, but also protects the battery from overvoltage and deep discharging.The battery voltage is adjusted automatically depending on the battery type, condition and temperature, while the user supply voltage must be constant.

Another important task of the solar inverter is providing protection and safety. This is especially important in on-grid systems, as there are strict regulations when a solar system is connected to the network. In this case, the inverter must synchronize the voltage and frequency of the solar plant to the network. It also must synchronize any disconnections from the network during outages, as it can be problematic if the solar system continues to supply energy even though the power grid is no longer in place. This is called islanding,and it can be dangerous to workers who may come in contact with powered circuits. Solar inverters should be able to detect islanding and disconnect the solar system from the circuit—a feature called anti-islanding.

Maximum Power Point Tracking

Maximum power point tracking (MPPT) is a very specific inverter parameter in solar systems. Solar energy systems produce a variable amount of energy depending on the light intensity, and MPPT is a method of finding the point at which the PV panels are supplying maximum power. The goal is to adapt the load characteristic based on the solar panel’s energy state to provide the most highly efficient power transfer from the solar panels.

The maximum power point. (Image courtesy of mysolarshop.)
The maximum power point. (Image courtesy of mysolarshop.)

The Energy of the Future

If we are truly to be the last generation powered by fossil fuels, engineers must have a firm understanding of the technologies used in renewable energy systems. We’ve discussed the specifics of batteries and inverters used in solar energy systems, but many other forms of renewable energy will also be crucial to ending our reliance on fossil fuels.


To learn more about renewable energy, read Renewable Energy Supplied Nearly 25% of Global Electricity Needs in 2015.


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