Photovoltaic energy generation and storage: structures and topologies
Alternative and renewable energy sources are thriving, and solar is one of them. Despite its fluctuating output in case of bad weather or during the night, photovoltaic (PV) systems are attractive because they are scalable and their cost is decreasing. So, the market for them is growing. But what does a solar power generation system look like? Like so often, the answer is: “it depends”.
Residential installations seldom exceed 10 kW, as self-consumption is the primary focus, and a single home usually does not require more energy at a time. In case of (higher) peak demands and to balance the variable power generation, an energy storage system (ESS) is useful. It stores surplus energy and feeds it back when needed. If the grid fails, it can also act as an emergency power supply. The residential PV installation usually consists of one or more series-connected PV panels, the micro- or string inverter. It perhaps includes an energy storage system as well as a link to an energy management system and a car-charging system. Figure 1 details such an application.
Figure 1: Example of a residential solar power installation. An ESS is optional and can be either DC or AC-coupled.
Commercial PV systems for offices, factories, or large apartment complexes have similar requirements and may include energy storage systems. But, because of their higher energy demand, they are more expensive. This means that the efficiency of the systems and the return on investment (ROI) are key factors. Larger PV systems are arranged in strings, and system voltages are higher, up to 1000 V. Typically, one inverter is allocated for a single or multiple PV strings.
Large commercial and utility installations above about 5 MW may use a string or central three-phase inverter, which feeds the power straight into a transmission grid. Below 10 MW, central inverters have a couple of disadvantages if compared to string inverters. Therefore, the industry tends towards distributed inverter systems. For central PV inverters, too, efficiency and power density are major concerns to reduce system cost and improve the system performance.
The different inverter categories
Until now, we only referenced the three different inverter categories for photovoltaic systems, but we didn’t look into their details. So, here they are:
The microinverters used in residential installations are designed to be flexible and scalable for low-cost and mass production. They can also reach an efficiency of about 96%. For a single PV panel, an isolated DC-DC converter stage feeding an inverter is sufficient and switching frequencies range from 40 kHz to 80 kHz. The conversion topology is frequently a flyback or an LLC DC-DC stage, followed by a bridge inverter for a 110 V/230 V AC output. Silicon MOSFETs or IGBTs are commonly used for it.
String inverters for non-utility installations generate either single- or three-phase AC power. Depending on the output voltages of the PV strings, different implementations are possible, with system power ratings varying from 3 kW up to 350 kW. In general, string inverters have two stages of power conversion. The first stage converts variable DC into fixed DC voltage. Here, a simple 2-level boost topology is preferred for both 600 V and 1000 V PV systems. For the DC-AC stage, innovative topologies like HERIC or H6 are preferred for single-phase outputs – a 3-level topology is preferred for three-phase outputs. Utility-scale applications normally employ central inverters generating three-phase AC outputs. The average power levels are in the megawatts range. Because of the high PV panel voltages, multilevel or parallel inverters with IGBT modules are typically used for the DC-AC stage with either 2-level or 3-level topologies.
Figure 2 below shows the application requirements for the various solar inverter categories:
Figure 2: Details of the various solar inverter categories
What these different applications have in common is that designers strive to minimize possible energy losses and to increase the power density in both the solar power generation system and in the ESS. But, now that we have looked into the basics of such systems, the question arises: “How can we create systems that provide the highest possible efficiency, and which power semiconductors can take us there?” We will give the answer in an upcoming blog post.