This post was originally published in 2012. The tips and techniques explained may be outdated.
Almost everyone is familiar with the shiny panels (or photovoltaic modules in technical terms) that can usually be seen on rooftops, yet very few know anything about their corresponding devices in the basement (they’re not always in the basement, but more on that later). These devices are called inverters and they are worth knowing about, not merely because of their distinctive names such as “Sunny Boy” but also due to their critical function in PV plant. So what is the actual purpose of an inverter and what must it be capable of?
1. Power Conversion
An inverter is an essential part of any grid-connected PV plant, which is an environmentally power generation system that uses the photovoltaic effect to convert sunlight into electrical power (but not heat energy, which is solar thermal power). As the name implies, the main task of an inverter is to convert direct current into alternating current. This conversion is necessary since PV modules generate direct current from sunlight while public power distribution grids use alternating current for a variety of reasons. Inverters can function with very little loss of energy – the best devices currently on the market can achieve 99% efficiency. In other words, only 1% of the direct current is lost during conversion, which is a remarkable feat given that modern diesel engines, in comparison, have an efficiency rating of only 45%.
2. Power Optimization
One critical task of the inverter is to make sure that the PV modules are always working at their optimum power levels: Depending on temperature and irradiation intensity, there is always a precise combination of electrical current and voltage that allows a PV module to achieve its maximum power, which is also called its maximum power point or MPP. Since temperature and irradiation intensity are constantly changing (very quickly when clouds pass over), the inverter must continuously monitor the MPP to ensure that the PV modules are supplying as much power as possible. The inverter relies on a operation management system called “MPP Tracker” for this purpose. Some inverters have multiple MPP trackers so that differently aligned subarrays can be operated independently (multiple interconnected PV modules are referred to as a PV array).
3. Monitoring and Protection
The inverter collects data on the energy yields of the PV plant, monitors the electrical activity of the PV array and signals when problems occur. The yield data can be viewed on the device itself or analyzed from another location if suitable communication technology or software and online services are available. The inverter also monitors the power distribution grid it is connected to. If the specified limiting values for voltage and frequencies are not maintained, it will disconnect the PV plant from the grid to ensure safety. However, if there is a slight increase in the frequency, the inverter will reduce its power accordingly to alleviate grid management.
4. Reliable Operation
PV plants are designed to operate for a period of at least twenty years that means their internal components have to be robust and long-lasting. Although one of the biggest advantages of PV plants is their lack of moving parts and hence negligible wear and tear, an inverter is a high-tech device with sensitive electronics inside(?) that must offer reliable operation under the open sky and in all weather conditions. It is therefore critical that the weatherproof enclosure must be able to dissipate heat in a consistent manner. Heat dissipation is ensured by an intelligent cooling concept and an excellent efficiency rating. An inverter with 99% efficiency, for example, will lose only half as much heat as an inverter with 98% efficiency.
5. Promoting the Energy Revolution
PV plants play an important role in the switch to renewable and decentralized sources of power. Even on a cloudy winter day, the PV plants installed in Germany can typically generate just as much power as two large nuclear plants. On a clear summer day, total PV power can equal the power of a dozen nuclear plants. As the link between PV array and power grid, inverters are the central components of any PV plant and are increasingly used for grid management as well. For example, they can adjust the voltage at the grid connection by supplying additional reactive power and support the grid when temporary faults occur (e.g. voltage dips).
Inverters are also important for the topic of “energy storage”. As batteries can only store direct current, the power needs to be converted for each charge and discharge. PV and battery inverters are therefore critical components for creating the smart grid of the future, which can distribute fluctuating amounts of energy in all directions without significant loss and even allows electrical vehicles to be connected.
In the future we will be writing a lot about the subject of smart grids and look forward to your comments, questions and feedback.