An increasing number of countries released incentives to spur the integration of PV and storage. In China, energy storage is compulsive in many of the renewable energy projects, aiming to meet the demand of frequency regulation and grid stabilization. For markets like the US and Europe, the solar-plus-storage applications are highly recognized as ancillary services or a buffer in the large-scale solar and wind integration in the existing power system.
DC-coupled and AC-coupled solar-plus-storage solutions
The first step to any successful solar-plus-storage solutions is to figure out what requirements you need and which technologies are able to meet these requirements at the highest efficiency. The DC-coupled and AC-coupled designs are undoubtedly typical solutions.
The AC-coupled design means that an energy storage system connects to a solar system via AC side. As we know, the electricity from a solar system is generated in the form of DC which is then turned into the AC by PV inverter. The energy storage system is generally better suited to flexibly participate in a wider array of ancillary services and can be connected to the low voltage side or the high voltage side, ideal for the large-scale solar-plus-storage project.
Respectively, DC-coupled systems can have a small but significant cost advantage over AC-coupling, depending on system size and characteristics, with the primary benefits including 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 be easily added to the existing solar system without reconstructing the entire system.
Both types can leverage the ITC (Investment Tax Credit) and benefit from shared installation and operational costs.
System integration technology is critical to the stable operation of solar-plus-storage project, while 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 the low price competition.
“The most critical aspect of energy storage system integration is the safety of battery system,” commented a spokesperson from the energy storage industry. A qualified system protection design can monitor the operating status of cells, modules, battery clusters, battery management system (BMS) in real-time, enables pre-alarm and faults localization. If a fault occurs, it can protect the system by fast-breaking and anti-arc protection.
Otherwise, minor faults can easily evolve into 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, air conditioning power configuration should strictly be controlled, or it may lead to the uneven temperature of lithium-ion batteries and hence aggravate battery instability.
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, creating smooth communication among hardware, software and systems. There exist a variety of devices from different manufactures, 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 PCS when dispatching the output, to improve the safety and ROI of the whole hybrid plant. So only an experienced company which excels at both energy storage and PV industry can integrate an efficient system.
In addition, efficient energy management is also crucial to safe operation and maximum yields of 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 the 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.
Sungrow is among a few of the suppliers having expertise on EMS. The EMS developed by the company can be applied in a multitude of scenarios covering power generation, transmission, distribution and utilization, supporting multi-functional digital energy management.
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 marketplace will see the integrated energy storage system which is more cost-optimized, safer and more efficient, getting rid of the instability and intermittent constraints of renewable energy.