What is a solar inverter and how does it work?
With the increased interest in renewable energy sources across the globe, the interest in solar systems has also shot up at amazing rates. The ability of the sun to power an entire home is a huge environmental plus (think of all the electricity saved!) and is a great use of a resource that we have many hours of each and every week.
But, when it comes to solar systems, do you actually know what they are made up of and how they work? Solar systems consist of solar panels, (or photovoltaic (PV) panels), a solar inverter (super important) and a rack to keep everything in place. They may also contain a battery, depending on the system and an electric meter, and the amount and type of panels for each system will depend on the energy output needed. Considering how important and beneficial solar systems are, we thought we would put together a little information regarding solar inverters, how they work and what to look for in a good solar inverter, given their importance. If you have any questions after you have read this article, give Fallon Solutions a call at 1300 762 260.
How does a solar inverter work?
A solar inverter works by taking in the variable direct current, or ‘DC’ output, from your solar panels and transforming it into alternating 120V/240V current, or ‘AC’ output. The appliances in your home run on AC, not DC, which is why the solar inverter must change the DC output that is collected by your solar panels.
To be a little more technical, the sun shines down on your solar panels (or photovoltaic (PV) cells), which are made of semiconductor layers of crystalline silicon or gallium arsenide. These layers are a combo of both positive and negative layers, which are connected by a junction. When the sun shines, the semiconductor layers absorb the light and send the energy to the PV cell. This energy runs around and bumps electrons lose, and they move between the positive and negative layers, producing an electric current known as direct current (DC). Once this energy is produced, it is either stored in a battery for later use or sent directly to an inverter (this depends on the type of system you have).
When the energy gets sent to the inverter, it is in DC format but your home requires AC. The inverter grabs the energy and runs it through a transformer, which then spits out an AC output. The inverter, in essence, ‘tricks’ the transformer into thinking that the DC is actually AC, by forcing it to act in a way like AC – the inverter runs the DC through two or more transistors that turn on and off super fast and feed two varying sides of the transformer.
Types of solar inverters
Now you know what a solar inverter is and how it works, it’s time to look at the different types of inverters. There are 5 different kinds of solar inverters, all with varying benefits :
A battery inverter is the best option if you are needing to retrospectively fit a battery into your solar system, or are wanting to keep your battery separate from your solar panels and run through a different inverter. A battery inverter converts your battery power into 230V AC and feeds it into your switchboard (instead of grid power) wherever possible.
A central inverter is huge and is used for systems requiring hundreds of kilowatts (or even sometimes megawatts) of volume. They aren’t for residential use and resemble a large metal cabinet, with each ‘cabinet’ being able to handle around 500kW of power. They are generally used commercially for large-scale installations, or for utility-scale solar farms.
Hybrid inverters, otherwise known as ‘multi-mode inverters’, are pretty uncommon in Australia and allow you to connect batteries to your solar system. It engages with the connected batteries through ‘DC coupling’ (when both the solar and batteries use one inverter and the DC from the solar panels charges the batteries via a DC charger) and its electronics organise the charging and discharging of the battery.
As their name suggests, microinverters are super small (the size of a book!) and the ratio of solar panels to microinverters is 1:1. The benefit of a microinverter, among others, is that they optimise each solar panel individually, which offers more energy (especially in shady conditions).
Last but not least, there are string inverters. String inverters are the most common inverter option for residential use, and there is usually 1 string inverter per solar installation. They are known as ‘string inverters’ due to the fact that a string of solar panels is connected to them.
New hybrid inverter for rooftop PV arrays from SMA
SMA’s new hybrid inverter reaches a maximum efficiency of 98.2% and a maximum European efficiency of 97.5%. It is compatible with DC-coupled high-voltage lithium-ion batteries from leading suppliers, according to the manufacturer.
Image: SMA Solar Technology
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German inverter manufacturer SMA Solar Technology AG has launched a new hybrid inverter for residential and commercial rooftop PV installations.
The Sunny Tripower Smart Energy is a compact two-in-one device for the operation of heat pumps and electric charging stations. It can be connected via integrated interfaces. The DC-coupled hybrid inverter is available in four power classes: 5kW, 6kW, 8kW, and 10kW. It can be used with a maximum PV array power ranging from 7.5kW to 15kW, with a maximum input voltage of 1,000V.
The transformerless inverter measures 500 mm x 598 mm x 173 mm and weighs 30 kg. It also features natural convection cooling and IP65-rated protection.
The device can reach maximum efficiency of 98.2% and maximum European efficiency of 97.5%. It is compatible with DC-coupled, high-voltage lithium-ion batteries from leading suppliers, according to the manufacturer.
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“With its fast charging and discharging capability, the three-phase hybrid inverter ensures maximum battery utilization even in changeable weather,” SMA said. “The integrated automatic backup power supply ensures that consumers in the household can continue to operate in the event of grid failures.”
The manufacturer also said that the inverter has a charging capacity of up to 30A, and noted that it is suitable for new and existing PV systems.
The device can be easily installed in just a few simple steps. It can also be commissioned intuitively via the SMA Installer App 360° without having to open the hybrid inverter. SMA’s own “Shade Fix” software ensures that the photovoltaic yields are maximized, even in light shading. In addition, the devices are automatically monitored.
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Emiliano joined pv magazine in March 2017. He has been reporting on solar and renewable energy since 2009.
Square Wave Solar Power Inverter
The Square Wave Solar Power Inverter is the simplest and the least expensive type of inverter available. It is generally not used commercially due to its low quality of output power and very large harmonics. Square wave inverters equipped with thyristor output stages chop and invert (hence the name Inverter) the DC input positive power to generate a square wave alternating positive to negative AC output signal that is later filtered to approximate a sine wave and eliminate undesired harmonics.
Cheaper square wave inverters may also use push-pull transistor circuits with step-up transformers to produce the required output voltage. Square wave inverters are really only used in small stand alone PV systems that will run simple things like lighting or hand tools with universal motors with no problem – but not much else.
Modified Sine Wave Solar Power Inverter
The Modified Sine Wave Solar Power Inverter also called a quasi-sine wave inverter, is basically a modified square wave inverter which produces a square wave output with low harmonic distortion and a small “OFF” time between the positive and negative half cycles as the inverter switches polarity.
Modified sine wave inverters are suitable for most types of electrical and electronic loads, and are a popular type of inverter on the consumer market today due to their good conversion efficiency, relatively low cost, and can be used in solar installations where waveform shape is not too important.
However, modified sine wave inverters may not allow printers, copiers, light dimmers, rechargeable and variable tools to operate correctly due to the switching action of the inverters output stage. Also some audio amplifiers and radios may produce a low frequency background buzz due to the inverters output switching components.
Sine Wave Solar Power Inverter
The Sine Wave Solar Power Inverter produces a high quality AC voltage waveform with low total harmonic distortion (THD) similar to what you get from your local electricity or utility company. They are used when there is a need for clean sine wave outputs to operate sensitive devices such as electronic equipment, printers, copiers, and stereos, etc.
Pure or true sine wave power inverters are required as standard by most electrical utility companies as part of a grid connected PV solar system. Sine wave inverters have a much higher cost than the previous types for the same wattage due mainly to their better internal electronic circuits. Their output voltage can be controlled either in square wave mode or in pulse width modulated (PWM) mode.
In PWM pure sine wave circuits, the output voltage and frequency are controlled by varying the duty cycle of the high frequency pulses. Chopped voltage then passes through an output LC low pass filter network to produce a clean sinusoidal output.
This allows the output voltage and frequency to be well controlled, ensuring that any AC load within the inverters power limits will operate properly. Pure sine wave inverters are generally not suitable for home solar power as their cost is too expensive and are inefficient, instead “stepped” sine wave inverters are used.
Only the top end high quality inverters actually supply a pure sine wave as their output waveform. Also some of the higher end pure sine wave inverters have solar tracking circuits or maximum power point tracking (MPPT) built-in, in order to optimally operate a motorised solar tracking PV array.
Most sine wave models available on the market produce a variation on the modified sine wave inverter above. Their voltage output is not a pure sinusoid but they can operate almost anything that can be connected to the local grid. When it comes to efficiency, sine wave inverters perform better than pure sinusoidal inverters.
Solar Power Inverter Selection
After the solar PV panels themselves, Solar Power Inverters are the next most important part of a grid connected PV system and therefore the DC input power rating of the inverter should be selected to match the PV panel or array.
Generally, power inverters are selected for a particular solar system based on the maximum load, the maximum surge required, AC output voltage required, input battery voltage and any optional features needed. The size of an inverter is measured by its maximum continuous output in watts and this rating must be larger than the total wattage of all of the AC loads connected at the same time.
Also electrical appliances such as washing machines, dries, fridge’s and freezers which use electric motors require more power to start themselves than they require to run.
This high starting power consumption can be more than twice the normal power consumption so this must be considered when sizing an inverters wattage. Most power inverters are capable of delivering three to five times their rated wattage for short term surges and overload conditions.
Lets assume that we have calculated the total AC power consumption of our home and that we would need a 2500W or 2.5kW solar inverter. The PV solar panels we are interested in are 24 volt monocrystalline silicon panels rated at 140 Watts peak. Then dividing 2500W by 140 watts means that 18 PV panels will be needed, yielding 2520 watts in total.
But how do we connect these 18 panels to the inverter. We know from previous tutorials that PV solar panels can be connected together just like batteries, and in a series combination the voltage adds, with a constant current through each panel, and in a parallel combination the current adds with a constant voltage across each panel.
We first need to calculate how many modules can be connected together in a series branch. The datasheet for our inverter tells us that the maximum power point tracking (MPPT) input voltage is between 175 and 480 volts at a maximum current of 15 amps. The open circuit voltage ( Voc ) of each 24 volt PV panel at 25 o C is given as 36.8 Volts.
Then we can see that the maximum number of panels we can connect together in a single series branch is calculated as: 480/36.8 = 13 panels. Likewise, the minimum number of 24 volt PV panels required to keep the MPPT tracking voltage above the minimum 175 volts is there calculated as: 175/24 = 7.3, or 8 panels.
Then to keep within the inverters input voltage limits for our simple example so that the PV array voltage is not lower than 175V or greater than 480V requires an array branch of between 8 and 13 solar PV panels. Since our calculated array consists of 18 panels, two branches or strings of 9, 24 volt PV panels each is acceptable. The short circuit current, Isc of our 24 volt monocrystalline silicon panels is given as 5.8 amps. Two branches will therefore give a total maximum current of 11.6 amps, well within the inverters specification.
So to determine the number of panels that can be connected on one series PV branch, check that the sum of the open circuit voltage of all the panels does not exceed the maximum power point tracking DC input voltage and that the minimum number of panels in the series branch does not fall below the minimum MPPT voltage not forgetting that the voltage in a series branch varies up and down with temperature. Also check that the short circuit current ( Isc ) of the array is less than the maximum DC input current of the power inverter.
In the next tutorial about Solar Power, we will look at the advantage of using Deep Cycle Batteries compared to normal automotive batteries and how to connect them into our home solar power system for use in a stand alone or grid tied PV system.
How Long Do Solar Panels Last? Lifespan of Each Type, Batteries,
Solar Panel Connector Types Ranked, When to Use Each (And When Not To)
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