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The technical stuff

Solar inverters: hybrid, series or optimisers

The solar inverter is an often overlooked component of your solar PV system. But, it’s just as important as your solar panels.

A solar inverter completes several jobs. Its main task is to convert the electricity produced by your solar panels from direct current (DC) to alternating current (AC).

Without an inverter, your system doesn’t work.

The Basics

What is a solar inverter?

A solar inverter’s main function, as mentioned, is to convert electricity from DC to AC. This is so the electricity can be used with appliances connected to the National Grid.

A solar inverter also has some other special functions, specifically to improve its use in a PV array. These include voltage management and maximum power point tracking, for example.

There are several different types, and even more brands to choose from.

The types

What is the best solar inverter?

Series

A series solar inverter is the most basic type of inverter. “Series” indicates that the solar panels are not only wired in a series circuit (which most are), but they are acting in this fashion too.

In a series circuit, the solar panels are connected by a single conductive path. The same current flows through each component, but the total voltage is dropped to the lowest reading in the circuit.

In a PV array, if one solar panel is producing less than the others, this output is applied to each component.

Hybrid

A hybrid solar inverter operates exactly the same as a series inverter, but it is also battery-ready. If you were to have battery storage in addition to your solar panels, it would require its own inverter and charge controller.

A hybrid solar inverter replaces the need for an extra inverter and charge controller.

This is a cost-effective solution for those who want both solar panels and battery storage.

Optimisers

A power optimised solar inverter is the most advanced type of inverter. It consists of two components: a traditional inverter and power optimisers.

The inverter will complete one job — converting the electricity from DC to AC. The power optimisers will complete everything else, including tracking the maximum power point of each panel.

This means that even though the system is wired in a series circuit, the system isn’t restricted by the lowest performing panel.

By having a power optimised system, you can increase your system’s generation and therefore your earnings and savings too. This is we call it the inverter upgrade.

You can also have micro-inverters, which is the same as power optimisers but without the need for a traditional inverter.

THE ISSUES

Optimisers > series

Variable degradation

Solar panels degrade over time, which means that their performance reduces as the panel ages. Even though panel manufacturers’ quote a product guarantee of around 25 years, they also predict the estimated decline in performance through a performance warranty.

Panel companies are only comfortable offering this guarantee because of a 2012 NREL study (“Photovoltaic Degradation Rates—An Analytical Review”) that found solar panels degrade about 0.5% to 3% each year, barring any equipment issues.

The reason why performance declines can be due to many naturally occurring factors, including but not limited to:

Natural cell degradation through UV and weather exposure:

• Thermal cycling can cause solder bond failures and cracks in solar cells
• Damp heat has been associated with delamination of encapsulants and corrosion of cells
• Humidity freezing can cause junction box adhesion to fail
• UV exposure contributes to discolouration and back-sheet degradation

However, panel degradation is a complex matter, and whilst these factors can and will have an adverse effect of a panel’s performance. There could be issues in the design, manufacturing or installation processes.

Manufacturers are continually testing and refining every part of the manufacturing process, all the way down to the encapsulants and adhesion materials, to try to slow degradation rates.

In tandem with this, PV system technology can suffer from something known as Potential Induced Degradation (PID).  

One of the culprits of this is the increasingly popular ‘transformerless’ inverters that allow different modules, and different parts of that module (ie individual cells and the frame) to perform at different voltage levels within a system, which can allow electrical current to leak and modules to lose their peak performance.

Often, simply negatively grounding a system removes this issue, but transformerless inverters are ungrounded.

When electrical current leaks, sodium ions in the glass move toward the solar cell or the frame, depending on how the system is grounded.

Frameless modules can help reduce the PID possibility (since there’s no metal frame to disrupt voltages). And many module manufacturers take extra steps to ensure modules are PID-free now. It’s important for installers to know what products they’re combining into a full system to know if something besides the panel may contribute to degradation.

Cheaper panels and less material
A few years ago, as module companies started to compete to lower their prices, they made their frames thinner to reduce the aluminium being used which unfortunately meant they can bend. Bent frames can strain the whole panel, allowing seepage of water under the frame, that when freezes, expands, bending the frame further. This can be especially bad as panels get thinner and less mechanically robust.

More, thinner busbars
Solar panels sometimes fail because of busbar solder bond failures. With the trend of more busbars on solar cells, you would think there is a higher chance of solder bond failures. That’s not entirely true.

Cells can easily break. If you have a big ribbon with a big solder bond, it puts more local stress on the cell and causes them to be more likely to break. By reducing the size of those solder bonds, you can reduce the amount of stress at the point where that ribbon gets connected to the cells.

With more busbars and more solder bonds, there is a higher probability of solder bond failure. But the importance of one solder bond failure goes down when there are more busbars to pick up the slack. Also, more busbars across a solar cell can decrease the chance of full cell breakage.

Flexible panels and installation
As module companies decrease their costs, they may turn to ultra-thin solar cells that use less silicon. Thinner solar panels are more flexible and not as rigid as older module models, which makes installation a delicate process.

Hand-to-hand transport can affect a module, especially if installers are carrying modules on top of their hardhats. That flexing and bouncing up and down can take a real toll and lead to microcracks in the cells. Dropping a module and the biggest no-no—standing or walking on top of solar modules.

What can we do?
Not all new technologies are bad, nor are all modules destined for failure. And although the types of problems may be changing, panel warranties are increasing and system lifespans are getting longer.

Smart buying and installation of solar panels and other project components can help mitigate degradation. Using trusted products and installing them with care will ensure a solar system will perform at its best—with no more than 3% power loss each year.

Power tolerances

A power or manufacturer’s tolerance is the quoted range of the amount of power a solar panel can produce.

The wattage (amps x volts) of a panel can vary but it is issued a nameplate wattage. What a manufacturer will show in its specification of the product is a power tolerance and that is what the panel could produce in its given range. This is all completed under Standard Test Conditions (STC) by the manufacturer.

Example:

A 100 watt (W) solar panel could have a power tolerance of +/- 10%. This means the panel could produce as high as 110 W or as low as 90 W.

Solar PV systems are typically wired in either a series or a parallel circuit, or even a combination of the two. In this particular scenario, it is important to highlight the relationship between the series circuit and the quoted power tolerance of the panels.

The series circuit is a closed continuous loop which the current travels through. If it is broken at any part then the whole circuit would be incomplete and the current would stop flowing. In extreme cases, that means if one solar panel was to stop working then every solar panel in the circuit would also stop working. In regards to the power tolerance, if one solar panel was operating at the lower end of its power tolerance spectrum, the energy production of the rest of the panels in the circuit would also be restricted to that lower-performing panel.

Thermal mismatch

Thermal mismatch happens when PV cells are affected by heat negatively. It may be surprising to know that heat can actually cause damage to PV cells because of the actual functionality of solar PV modules. A manufacturer will only design a module to withstand so much heat therefore if it were to operate at higher temperatures then the module would underperform or even stop working. However, if the module temperature were to drop then the efficiency can increase.

A PV module will consist of a number of PV cells – 60 is often an industry-standard but different models will have more or less. With many older models, it is easy to see each PV cell as they are usually in a rectangular shape on the laminated side of the panel. The specification sheet of the model will detail the PV module temperature coefficient. This means the temperature for the PV module to operate in will be listed but also the decrease/increase in efficiency for every degrees celsius (°C) it goes above/below that temperature.

Typically, the two types of PV modules which are installed are either manufactured with monocrystalline silicon or polycrystalline silicon. However, thin-film PV modules are becoming more popular. The monocrystalline module is of purer quality silicon but doesn’t work as efficiently as a polycrystalline module in higher temperatures. The monocrystalline PV module is the most common type installed in the UK.

In standard test conditions (STC) the testing of a PV module is carried out at 25°C. A solar PV module will also have a specified ‘Module Efficiency’ which is portrayed as a percentage (%). The ‘Module Efficiency’ is the amount of solar irradiance which is converted into direct current (DC). So, the temperature coefficient is the increase/decrease of efficiency in % per °C.

Each PV cell is unique and will perform differently to the other PV cells in their module. Depending on a number of factors, PV cells may be at different temperatures. The issue is that PV cells are installed in a series circuit. So, if one PV cell were to be affected negatively then it may affect the other cells in its group (bypass diodes segregate cells in groups of 20[1]) or the entire module and then possibly the whole array. Cases such as these are commonly referred to as “hot spots” in the industry.

Examples which can cause thermal mismatch include:

● Dissipated power to one area
● Lack of air to cool the PV modules in the middle of an array
● Cell mismatch (cells of varying current production connected in series)
● Cell damage

In the UK, the temperature doesn’t increase over 25 °c very often but as the PV cells absorb solar irradiance, the module can heat up to temperatures higher than the surrounding air temperature.

For STC, an irradiance of 1000 watts (W) per square metre is produced. This stimulates peak sunshine on the tested PV module which would be directly facing the sun without clouds. In real-life conditions, weather like this will occur during Summer. This is the most important season for any investor as it will be when their PV system produces the most for the year. So, if their PV system were to suffer from a decreased efficiency during this season then it lowers their return on investment (ROI). It also increases the amount of energy required from the grid and in turn increases energy costs.

The specification sheet will also show test results of the PV module in a nominal operating cell temperature (NOCT). This is when the manufacturer creates conditions which are similar to real climate. This data shows that the PV module will typically operate at a higher temperature in lower insolation conditions and perform at lower ratings than STC.

If PV cells suffer from high temperatures over their lifespan then it will cause degradation and have a long term negative effect resulting in lower efficiency and lower power output. Affected PV cells may also have a shorter lifespan than the other cells in the module. There are cases when irreversible damage is caused to PV cells.

Thermal mismatch not only affects your production and ROI. It also increases the hazard of a potential electrical fire. As with any electric equipment, a PV module has a threshold for temperatures which are too high for it to operate in. The temperature should be spread across each of the PV cells so the module can complete its task safely but this isn’t always the case.

There are a number of things an installer can do to ensure the prevention of thermal mismatch in a solar PV system and mitigate the effect of any thermal mismatch already present

● Installing the PV modules a safe distance (1–2 inches) from the roof to allow for effective air cooling
● Wiring the panels in parallel (using SolarEdge or Enphase technology) to ensure that the lower-performing panel affected by thermal mismatch does not affect the performance of the others in an array
● Live monitoring for each PV module to help identify potential issues with panels
● Correct and safe wiring
● Use of high-quality panels with robust warranties

An investor should also aim to maintain a regular cleaning cycle for the solar PV system. If there is any concern that there may be damage then an infrared thermal inspection can be completed to see if a PV module is suffering from the thermal mismatch.

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Solarplants Ltd is authorised and regulated by the Financial Conduct Authority under number 738735. We are a credit broker not a lender and have a facility with one lender. Solar Plants is a trading name of Solarplants Ltd.

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