The long read: Storing up the future

From pv magazine, October edition

Recent years have seen a major shift towards self-consumption in the residential market. This has stemmed from the decrease of feed-in tariffs, technological advancements in smart buildings fuelling higher self-consumption rates, and grid feed-in to improve the new prosumer model.

Storage technology will be the game changer for energy systems of the future, drastically reshaping the architecture of transmission grids to unleash the full potential of distributed generation by renewables. By 2025, annual grid-connected energy storage will grow to 9 GW with more than 50% behind-the-meter (source: 2018 IHS Markit).

Topology

The topology used in residential solar solutions has also shifted. Two main architectures can be used for energy storage solutions with PV installations – DC coupled and AC coupled – as installers work to introduce new systems via DC technology and retrofit on legacy AC installations.

Current market analysis is showing a 50/50 split between DC and AC topologies in storage with both offering strengths and weaknesses depending on the project and installation legacy.

In DC coupled solutions, the battery charger is located between the booster (when required) and inverter stage (DC link). The DC link voltage range is typically 350-600 V for single-phase applications and the battery energy is converted only once from DC to AC. This enables a more logical way to integrate the battery as it delivers greater efficiency and output. It tends to be reserved for new installations usually with high voltage batteries.

In AC coupled topology, the battery is charged, or discharged, through a battery inverter directly connected to the grid. This means that the AC current generated by the solar inverter will be converted into DC current to be stored in the battery and converted a second time when the battery is discharged. This topology and setup make it suitable for retrofit PV installations.

Although simple to set up, an AC topology does result in lower efficiency than a DC system as the PV energy needs multiple conversion stages.

In DC coupled topology, to maximize efficiency, the voltage level difference between the DC link and the battery should not exceed a ratio of 3:1. For example, a battery to be connected to a 450 V DC link should not provide a minimum voltage below 150 V. The choice of a high voltage battery itself provides greater efficiency than 48 V low voltage batteries.

When considering integration with new photovoltaic plants, DC coupling is the natural solution to add storage capacity, giving added value to the end user from the highest efficiency and greater integration through all-in-one solutions.

Solar storage solutions

All-in-one, bi-directional architectures offer comprehensive technology and flexibility without compromising on efficiency levels within the storage system.

To give an example, ABB’s REACT 2 system is normally coupled with the PV plant on the DC side, but also offers the flexibility to be used as an AC coupled system, to be integrated in any existing PV plant, whether at home or commercially, thanks to bidirectional inverter topology. This makes it ideal for both new systems or the retrofitting of existing photovoltaic installations.

ABB’s REACT 2 new residential solar inverter features a high-voltage 200 V Li-ion battery with modular storage capacity of up to 12 kWh. This delivers up to 10% more system efficiency compared to lower voltage alternatives (48 V) and achieves up to 90% energy self-reliance for residential PV projects.

Digitalization trends

Within the residential storage market, digitalization trends from blockchain energy models and even analysis of data and insights are making storage more accessible and connected from the point of generation on the PV installation through to the point of power within the home.

Through digitalization and data availability on smart devices, including tablets and phones, home owners can increase control over their electricity usage, avoiding consumption peaks by spreading the load to keep within their generating capacity.

Energy transactions have historically been based on a multi-tiered and complex supply and demand business matrix between producers, transmission system operators, distribution system operators, utilities, and consumers.

Yet through the emergence of digitalization, the uptake in combining renewable energy sources and storage – such as rooftop solar connecting to the grid – is offering potential for a more decentralized and simplified energy management architecture.

Blockchain technology, coupled with smart metering technology, will take the prosumer model one step further. It will allow prosumers to not only choose when and how to use the energy they have stored and produced, but also to trade surpluses with peers through a token system, thereby creating a sustainable energy sharing economy.

Summary

With the conversion of digitalization, renewables, and storage, the solar industry is now well placed to manage the complexity and demanding performance of modern energy systems and respond to increasing consumer demand for greater control over energy usage. With the right inverter technology, the installer can enable today’s technologies, but deliver a system for tomorrow.