Boosting energy efficiency: The role of energy storage systems in photovoltaic integration

As the world moves towards more sustainable and renewable energy sources, solar power has emerged as a key player in the energy market. Solar photovoltaic (PV) systems are being widely adopted by homeowners, businesses, and utilities for their ability to generate clean energy while decreasing the reliance on fossil fuels and reducing electricity bills. However, one of the challenges with solar energy is its intermittent nature. The sun doesn’t always shine and accordingly the energy production can be inconsistent. The need for solar inverter with high efficiency, improved power density and higher power handling capabilities continues to scale up. This is where integrating energy storage systems (ESS) with solar inverters becomes a game-changer and powerful solution for ensuring a consistent and reliable energy supply. As technology continues to improve and costs decrease, the adoption of solar-plus-storage systems is expected to grow, paving the way for a more sustainable and resilient energy future. This article explores the benefits, types, and topology considerations for integrating energy storage with PV systems in residential and commercial installations.

Understanding Solar Inverters and Energy Storage

Solar inverters are the heart of a solar PV system. They convert the direct current (DC) electricity generated by solar panels into alternating current (AC) electricity which can then fed into the grid. In addition to conversion, solar inverters manage the energy flow, optimize system performance, and provide safety mechanisms to protect the entire PV system.

Energy storage systems (ESS) are technologies that store energy for later use to help balance supply and demand and enhance grid reliability. These systems can store energy in various forms, such as electrical, chemical, mechanical, and thermal. There are several types of ESS and below are the most common methods:

• Battery Energy Storage Systems (BESS) such as “Lithium-ion Batteries” are widely used due to their high energy density, efficiency, and declining costs. Common in grid storage and electric vehicles.
• Mechanical Storage Systems such as “Pumped Hydro Storage” is the most established large-scale storage technology. It involves the movement of water between two reservoirs at different elevations. This type offers the highest capacity form of energy storage.
• Thermal Storage Systems “Molten Salt Storage” is used in solar thermal power plants to store heat and generate electricity when needed. They are used in commercial applications for short-term energy storage.

In Solar PV systems, BESS of Lithium-ion Batteries are often used to store surplus electricity produced by solar panels. This stored energy can be used during periods when solar generation is low (nighttime and cloudy days) or during peak demand periods, ensuring a steady and reliable power supply.

The Benefits of Integrating Energy Storage with Solar Inverters

Integrating ESS with solar inverters gives energy independence and reliability. By storing excess solar energy, users can reduce their dependence on the grid and ensure a consistent power supply even during outages or periods of low solar generation. This permits the usage of stored solar energy during the time of peak demands or when electricity prices are higher, resulting in reducing utility costs and minimalizing the strain on the electrical infrastructure. Furthermore, energy storage systems can contribute to frequency regulation services by stabilizing the grid frequency and improve the overall grid performance.

Energy Storage Systems Segmentation

Energy storage systems have a wide range of where it can be applied. The ESS segmentation is split by Front-of-the-meter (FTM) and behind-the-meter (BTM). FTM ESS is usually linked to high power systems above 5 MW of Energy. A bulky stationary ESS is being utilized here, starting from the generation phase either in combination with PV utility scale systems or wind systems, moving to the transmission phase, and ending with the distribution phase with. On the right-hand side is the BTM ESS. In this segment, the energy storage systems are in combination with residential and commercial PV systems in the range of few kilowatts to 5 megawatts.

Types of Solar Inverters

The String inverters work by adding solar panels together with strings. The combined DC power from the panels is sent to a single inverter which converts it to AC. They are commonly used in residential, commercial, and utility-scale installation. String inverters generate single or three-phase AC at high power levels up to 200kW. The panel voltages are around 600 V followed by a DC-DC boost converter to provide the DC link voltage for a single-phase inverter. For three-phase inverters, the panel voltage of 1000 to 1500 Volt DC with a boost converter are being used. String inverters are cost-effective and relatively simple to install and maintain. The problem may occur if one panel in the string is shaded or underperforming then the performance of the entire system can be affected.

Conversely, Micro inverters couple each panel with its individual micro inverter and convert DC to AC at the panel level. These systems are wired in parallel versus being wired in a series like string inverters. Hence, if one panel is shaded or underperforming, it does not impact the output of the other panels. The typical power of micro inverters is from 200W up to 1.5kW with a PV array voltage from 40 to 80V. This inverter type is ideal for residential systems where panels may face different directions. The advantages of micro inverters are that they maximize the output of each panel independently. Therefore, the impact of shading or panel mismatches can be minimized. Additionally, micro inverters offer a detailed monitoring of each panel for better maintenance and performance tracking. The main disadvantage is the high initial cost compared to string inverters.

Integrating energy storage systems with solar PV panels results in Hybrid Inverter. This type of inverters works both ways, the generated solar DC power is converted directly to AC or to be stored before the conversion to AC. Hybrid inverters are optimizing energy use and storage by managing the flow of electricity between the solar panels, batteries, and the grid. They can be configured to prioritize battery charging, grid interaction, or self-consumption based on user preferences and utility rates.

Energy Storage Coupling Systems

There are two different approaches to integrate battery storage with solar PV systems. The AC-coupled ESS and DC-coupled ESS. Each has its own advantages and disadvantages depending on the specific application, system configuration, and user needs. The key distinction between an AC-coupled and DC-coupled systems lies in the journey that the electricity takes once generated from the solar panels.

In an AC-coupled system, the solar PV system and the battery storage system are connected through their respective inverters to the AC grid. The solar panels generate DC power which is converted to AC by a solar inverter. On the other path, the battery storage system is typically equipped with its own bidirectional DC-DC and inverter stages for charging and discharging to the AC grid.

On the contrary, in the DC-coupled system, the solar panels and the battery storage share a common DC bus and mainly use a single inverter to convert the DC power to AC for grid or household use. Solar panels can be used to charge the batteries directly and then the stored DC power is converted to AC through a hybrid inverter when needed.

Power Topologies for Solar String Inverters and ESS

Various power topologies can be utilized to design the DC/DC converter and DC/AC inverter stages. Different topologies offer distinct advantages and are chosen based on power requirements, efficiency, cost, and complexity. Here are some of the most common power topologies:

In the first converter stage, the Maximum Power Point Tracking or (MPPT) performs the functions of translating the string voltage to a level suitable for the inverter. Typically, 400V for single phase and 800V for three phases. The MPPT power optimizer DC-DC stage is designed to maximize the energy output from a solar PV system by individually optimizing the performance of each solar panel in the array. It adjusts the output of the panel to its optimal power point before sending the energy to the inverter stage. This optimization is crucial since the power output of a solar panel can vary due to changes in sunlight intensity, shading, temperature, and panel mismatch.

The current trend is towards increasing the DC-link voltage to 1000V or 1500V, to reduce power losses in the system as well as allowing more panels to be added in series. By increasing the maximum DC Voltage of a solar inverter to 1500V or beyond, the PV power plants become more cost-effective. The Typical topologies for this stage are Interleaved boost converter, and Phase-Shift-Full-Bridge (PSFB), and LLC converter.

The second converter stage is the bidirectional DC-DC. This stage is used for charging or storing energy in the battery, and discharge or releasing this energy when needed. Typical isolated topologies are CLLLC and DAB.

The inverter power stage performs the function of converting the DC link voltage to AC voltage for the grid. The Common topologies include two-level B6 and H-bridge, and three-level ANPC and HERIC. Multilevel inverter topologies have become popular in medium and high-power applications. The Benefits of using three-level inverter topologies are:

1. Reducing power dissipation which leads to a smaller heat sink.
2. Minimizing current ripple so filtering is easier due to lower harmonic content.
3. Significantly lower conducted EMI.

Let’s have closer look on the most common topologies for DC/DC stage. The selection of the power switches in the secondary side depends on the battery voltage. For instance, in residential energy storage systems, 48V battery packs are often used whereas the commercial segment is more in the domain of 400V batteries.

The ZVS Phase-Shift-Full-Bridge DC-DC Converter
The Zero voltage switching (ZVS) Phase-Shift-Full-Bridge topology is recommended in a 400V DC-link setup with 650V silicon carbide (SiC) MOSFETs for switches Q1 to Q4 for achieving high efficiency and high-power density. The switches are controlled with a phase-shift technique that allows the switches to turn on when the voltage across them is zero. This significantly reduces the switching losses and electromagnetic interference (EMI) as well as reducing the stress on the semiconductor devices. Furthermore, 650 SiC-diodes are the right choice for D1and D2 on the primary side. In case of 800V DC-link setup, then 1200V SiC-MOSFETs and SiC-Diodes need to be selected. On the secondary side for switches Q5 to Q8, the selection of power switches depends on the battery voltage.

The CLLC DC-DC Converter
One of the most common bidirectional DC-DC topologies is the CLLC converter. It utilizes two inductors (L) and two capacitors (C) in a resonant tank circuit. The arrangement typically looks like an "LLC" resonant tank mirrored on both the primary and secondary sides. SiC-MOSFETs are being used for switches Q1 to Q4, while Silicon (Si) MOSFETs are selected for Q5 to Q8. The CLLC design achieves ZVS for the primary side switches which help reduce switching losses and improve efficiency. It can achieve zero current switching (ZCS) on the secondary side for further enhancing efficiency by minimizing switching losses during turn-off. CLLC converter requires precise control to manage the resonant frequency and switching sequences effectively.

The DAB DC-DC Converter
The DAB converter consists of two active full-bridge circuits on the primary and secondary sides, connected by a high-frequency transformer. Like CLLC topology, both bridges are composed of active switches that allow for bidirectional power flow. Typically, SiC-MOSFETs are utilized for switches Q1 to Q4 and Si-MOSFETs for Q5 to Q8. DAB converter requires sophisticated control algorithms to manage the phase shift between the bridges precisely.

The ANPC DC-AC Inverter
Further exploring the inverter stage, the Active Neutral Point Clamped (ANPC) topology is an advanced inverter configuration. It builds upon the conventional Neutral Point Clamped (NPC) topology by adding active switches which help reduce both conduction and switching losses. The ANPC inverter can produce multiple voltage levels which minimizes the voltage stress on each component, and accordingly a smoother AC output with lower total harmonic distortion can be achieved. Switches Q1 to Q4 operate at the line frequency while Q5 and Q6 modulate at 50 kHz or even higher. In ANPC, all power switches can be rated 600- or 650-volt breakdown voltage. By using SiC-MOSFETs for switches Q5 and Q6, an increase in efficiency and power density can be realised. Advanced control algorithms are required for the ANPC inverter. This topology is more complex to design, and control compared to topologies like the H-Bridge.

The H4 Bridge DC-AC Inverter
H-bridge topology is popular due to its simplicity, efficiency, and versatility as it consists of four switching elements. 650V SiC-MOSFET or GaN-HEMT (Gallium nitride high electron mobility transistors) are commonly used for the fast-switching line Q3 and Q4, while for Q1 and Q2 Si-MOSFETs with fast body diode is the right choice. The main drawback of this two-level operation is that it involves a relatively large output filter since it regenerates energy back during freewheeling to the DC capacitor.

The HERIC DC-AC Inverter
HERIC (Highly Efficient and Reliable Inverter Concept) topology is particularly notable for its high efficiency and superior performance in converting DC to AC. In this configuration, two anti-parallel switches Q5 and Q6 are added to the conventional H-bridge inverter to decouple the AC side from PV modules at a nil stage. Six switches conform this topology, in which the four on the H-bridge (Q1 to Q4) switch at high frequency and the two external one’s switches at grid frequency. Q5 and Q6 switches pass the freewheel current via the shortest route during the period when the output voltage of the H-bridge inverter is zero. The main advantage of HERIC inverter is that only two switches operate simultaneously in all operating modes.

Wide bandgap (WBG) devices provide clear benefits for bidirectional DC-DC converter and DC-AC inverter topologies. SiC and GaN devices have very low reverse recovery charges (Qrr) or even no body diode which eliminates the hard commutation or reverse recovery losses.

Installation and Maintenance Considerations

Proper sizing of both the solar PV system and the energy storage system is crucial for optimal performance. This involves calculating the energy needs, solar panel output, and the required battery capacity. Oversizing or under sizing can lead to inefficiencies and higher costs. The compatibility of the solar inverter and battery storage system is vital. Some manufacturers offer integrated solutions that simplify installation and operation. Compatibility also extends to software and monitoring systems that manage the overall energy flow and performance.

Conclusion

Integrating energy storage with solar PV systems represents a significant advancement in the way we harness and utilize solar energy. Providing a reliable and consistent power supply reduces the dependence on the grid and maximizes the use of solar energy. These systems offer numerous economic and environmental benefits. SiC and GaN power devices help enable bidirectional flow for synchronous rectification topologies while achieving high efficiency and high-power density. Arrow Electronics has always been centred around promoting energy efficiency and we are eager to contribute to this discussion by demonstrating the clear advantages of opting the 650V, 1200V, and 2200V SiC devices with reference boards that ease design effort and shortened times to market.