It pays to understand how storage is integrated into energy systems when selecting a battery system, write Mina Zhang and Echo Chen of Sungrow.

Given the increasing concerns about global warming and environmental impacts of burning fossil fuels, policymakers are paying more attention to energy storage solutions than ever before. Apart from having the ability to address the intermittent supply of renewable energy, energy storage assets can also offer a multitude of uses, including frequency regulation and ancillary services.

An increasing number of countries have released incentives to spur the integration of PV and storage. In China, energy storage is compulsory in many renewable energy projects, so that demands of frequency regulation and grid stabilization may be satisfied. For markets such as the US and Europe, solar-plus-storage applications are recognised as ancillary services or a buffer in the large-scale integration of solar and wind into the existing power system.

DC- and AC-coupled solar-plus-storage solutions

The first step to any successful solar-plus-storage solution is to figure out what requirements are needed and which technologies are able to meet these requirements at the highest efficiency. The DC-coupled and AC-coupled designs are typical solutions.

The AC-coupled design means that an energy storage system connects to a solar system via the AC side. As we know, the electricity from a solar system is generated in the form of DC which is then converted to AC by the PV inverter.

Respectively, DC-coupled systems can have a small but significant cost advantage over AC-coupling, depending on system size and characteristics. The primary benefits include the reduction of power conversion equipment required and the ability to recapture DC energy that would otherwise be clipped by the inverters. The pre-assembled storage interface on the PV inverter is essential, which enables an energy storage system to be easily added to the existing solar system without reconstructing the entire system.

Industry chaos

System integration technology is critical to the stable operation of a solar-plus-storage project, but there are huge challenges. Few companies have proven solar-plus-storage system integration capabilities, though many companies do well in individual fields such as PV inverters, batteries, power conversion system (PCS) and energy management system (EMS).

Given the condition that different applications put different requirements on energy storage systems, there is no unified standard for energy storage system design and cost management. This variability leads to a grey zone due to the uneven integration capabilities and low price competition.

The most critical aspect of energy storage system integration is the safety of battery system. A qualified system protection design can monitor the operating status of cells, modules, battery clusters and the battery management system (BMS) in real-time to sound the alarm and locate where the problem is.

If a fault occurs, it can protect the system by fast-breaking and implementing anti-arc protection. Otherwise, minor faults can easily become major problems. Most of the 30 fire accidents that occurred in South Korea in recent years were caused by defects in the electrical system design and failure of the protection system.

The challenges are also related to battery life, in that the temperature control system for energy storage is paramount. Particularly, the thermal simulation and experimental verification, air duct design of storage container and air-conditioning power configuration should be strictly controlled, or it may lead to uneven temperatures within lithium-ion batteries and aggravate battery instability.

Sungrow principal engineer system solution centre Chen Wei at the company’s manufacturing plant in Hefei, China.

The efficient integration: 1+1>2

The solar-plus-storage solution relies on deep analysis and technology integration of the whole system instead of a mechanical combination of two systems, so as to achieve a leap forward of “1+1>2” in terms of efficiency and performance.

Technically, the hybrid system needs to ensure the stable operation of PV, energy storage and the grid, to create smooth communication among hardware, software and systems. There is a variety of devices from different manufacturers, so the compatibility becomes a major challenge for the integrator who needs to be adept at all the protocols.

Particularly, the solar-plus-storage system needs to fully consider the management of batteries and power conversion system when dispatching the output, to improve the safety and ROI of the whole hybrid plant.

Efficient energy management is also crucial to safe operation and to maximise yields for the whole system. As the brain of the solar-plus-storage plant, EMS can control the charge and discharge of the batteries and instruct how the energy storage system cooperates with the PV system and the grid.

The intelligent EMS can manage different manufacturers’ equipment in a unified interface through an intelligent control unit. Based on PV power prediction and millisecond response characteristics of energy storage it can control the PV system smoothly and improve grid stability in return. Meanwhile, a millisecond-level rapid linkage mechanism bridged among PCS, BMS and EMS can ensure the safety of the battery and the entire system.

There’s no doubt that the advancement of technology allows more integration between renewable energy and storage, indicating a mature multiple-energy era is imminent. The energy market will benefit as integrated energy storage systems that are more cost-optimised, safer and more efficient do away with the instability and intermittent constraints associated with renewable energy.

Sungrow is a Chinese inverter maker with expertise in energy management systems. Its EMS solutions can be applied in power generation, transmission, distribution and utilization and support multi-functional digital energy management.

Mina Zhang and Echo Chen are communications executives at Sungrow.