How is the photovoltaic energy storage system chosen?

Nowadays, in the energy sector, storage technology is undoubtedly the hottest topic.

Several provinces including Shandong, Shanxi, Xinjiang, Inner Mongolia, Anhui, and Tibet have issued documents requiring photovoltaic and wind power stations to install energy storage systems.

Although the energy sector has long acknowledged that "energy storage is a crucial method to address the intermittency and volatility of new energy sources such as photovoltaics and wind power, promoting consumption and reducing curtailment," the impending era of full parity has further highlighted this advantage. However, due to its technological and cost limitations, it has always been somewhat disregarded. Today, with official endorsement, energy storage has finally gained recognition.

But for energy storage to make a grand transition from being an added bonus to becoming a market necessity, it requires not only clearer and stronger policy support but also the advancement through technological and product innovation to drive the development of the photovoltaic storage industry itself. How should the plan be selected? How can integration achieve optimal results? What challenges does integrated technology face? These questions all need to be addressed.

I． What are the typical system solutions?

Currently, the main photovoltaic-storage integration solutions on the market include AC side coupling and DC side coupling.

AC side coupling refers to the connection of photovoltaics and energy storage on the AC side, where the energy storage system can be connected to the low voltage side or collectively to 10 kV ~35kV busbars. This solution is suitable for large-scale photovoltaic storage stations, with centralized layout of energy storage systems, facilitating operation management and grid dispatching.

DC side coupling means that the energy storage system is connected to the DC side, resulting in fewer power conversion stages between the two systems, lower energy loss, and less equipment investment. In this solution, the photovoltaic inverter needs to reserve an energy storage interface.

II. How can we achieve synergy where 1+1 > 2?

The integration plan is in place, but achieving a synergistic effect where 1+1 > 2 is no easy task.

The technology for integrating photovoltaics with energy storage is even more complex. The integrated system needs to ensure the safe and stable operation of photovoltaic panels, energy storage, and the power grid, requiring the removal of barriers between hardware, software, and system levels.

There are numerous devices in a photovoltaic-storage integrated system, and it's necessary to solve the compatibility issues of interfaces between different hardware and software. Devices often come from different manufacturers, which increases the difficulty and cost of power station design, equipment procurement, operation, and maintenance. Most importantly, communication interface solutions vary among different devices, requiring integrators to have a thorough understanding of various protocols and interfaces.

Therefore, the integration of photovoltaics and energy storage is not a simple physical combination of photovoltaic and energy storage devices. It requires deep technical integration to achieve the synergistic effect where 1+1 > 2. This significantly tests the integrative capabilities of integrators.

III. The Chaos of Industry Integration Brought by Low-Price Competition

In the construction of photovoltaic storage power stations, system integration is crucial, yet the domestic integration field faces numerous challenges.

On one hand, there are few companies with comprehensive capabilities in photovoltaic storage system integration. Whether it's technology integration or business model integration, China's energy storage sector is still in the early stages of industrial development. Many companies have strong capabilities in individual areas such as photovoltaic inverters, energy storage batteries, PCS, and EMS, but those capable of complete photovoltaic storage system integration are rare.

On the other hand, low-price bidding is becoming increasingly intense, constraining companies with low costs. Currently, in the domestic new energy sector, the winning bid price for energy storage has dropped from 2.15 yuan/Wh (EPC price) to 1.699 yuan/Wh (EPC price). If equipped according to full capacity and cycle life requirements, this price is well below the industry-recognized cost price.

Different scenarios have varying requirements for energy storage systems, and there are no unified standards for system design and cost, creating a flexible space that, under the pressure of uneven industry integration capabilities and low pricing, can easily evolve into a gray area.

"Nowadays, when companies tender, batteries usually meet the 6000-cycle standard. There is no unified assessment standard in the industry, and some manufacturers participate in project bids with batteries that have a cycle life of less than 3000 times at a low price. Naturally, we cannot compete with them on price," said a veteran in the energy storage sector with resignation.

"Of course, the most critical aspect of energy storage system integration is the safety management of the DC side, which is the safety management of the battery system. This requires a very comprehensive system protection design," the person continued. Cells, modules, battery clusters, and battery system management are interconnected at four levels. A good system protection design can provide real-time knowledge of their operational status, enable fault warnings, and achieve graded protection and rapid coordinated protection if a fault occurs.

Otherwise, minor faults can easily escalate into major problems. Over thirty fire incidents in South Korea in recent years were largely caused by electrical system design flaws and inadequate protection systems.

The challenges do not end here; there is also the issue of battery lifespan, which inevitably brings us to the design of the energy storage thermal control system. Rigorous thermal simulation and experimental verification, the air duct design of the energy storage container, air conditioning power configuration, etc., if these aspects are not strictly controlled and designed, can easily lead to uneven temperatures within the container's lithium batteries, exacerbating cell instability.

I have encountered a 4-hour energy storage system where the temperature difference between cells during operation reached 22°C, not only severely affecting battery lifespan but also increasing the operational risk of the energy storage power station.

IV. How to manage energy storage systems efficiently?

From selecting solutions to integrating systems, the safe operation and optimal profitability of photovoltaic-storage power stations throughout their lifecycle are closely related to the management of the entire energy storage system.

Compared to the traditional economic dispatching mode of power stations, photovoltaic-storage power generation systems need to fully consider the effective management of internal batteries and inverters during dispatching to enhance the safety and economy of the entire photovoltaic-storage power station's operation.

At this point, it is essential to mention the importance of the smart brain of photovoltaic-storage power stations—the EMS (Energy Management System). How does energy storage coordinate with photovoltaic systems and the grid? How much electricity should the battery itself be charged with, how to charge it, and how to ensure safety? All these require a set of intelligent and efficient EMS for comprehensive management.

Taking smoothing the fluctuations of photovoltaic systems as an example, the energy storage system can control the output smoothness based on the photovoltaic power generation, setting smoothness parameters. The EMS uses these smoothness parameters as control targets to rapidly charge and discharge the energy storage system, keeping the power output of the generation system within a set range of variation rates.

Currently, a mature practice in the industry is that the intelligent EMS, based on photovoltaic power prediction and millisecond-level response characteristics of energy storage, achieves smooth control over photovoltaic systems, reducing impacts on the grid and enhancing the stability and reliability of grid operations. At the same time, by establishing a millisecond-level rapid linkage mechanism between BMS, PCS, and EMS at various levels, it maximizes the protection of batteries and the overall system's safety.

Moreover, advanced intelligent EMS can also achieve multi-energy digital integrated management, covering the full scenario of generation, transmission, distribution, and usage.