Power electronics for customized battery energy storage

Degradation of individual battery cells is a driver in BESS losing storage capacity over time. Small differences in quality, which initially seem minimal, gradually increase and since the cells are connected in series, and can only be charged and discharged together, if one fails, it affects performance of the entire string.

With regular balancing coming at the expense of efficiency, systems that do not require an external inverter are attractive.

Such set-ups, known as “multilevel inverter” concepts generate a sine wave by switching the current flow to individual battery cells on or off.

P&e power&energy, a company with years of research experience in the area, is offering to support battery manufacturers in the development of their own multilevel inverter applications.

The company’s founders, Gerold Schulze and Koenraad Muyllaert, researched the concept with the University of Kassel and developed prototypes that have been continuously charged and discharged for two years. In separate tests, their battery cells have completed more than 6,000 cycles using the typical pulse patterns of multilevel power electronics. The cells still display 86% capacity. For an average photovoltaic energy storage system, usually charged once per sunny day, that would correspond to a 24-year service life.

P&e Managing Director Schulze said multilevel operation can extend battery lifespan by up to 50%, adding, “Because each cell is individually controlled, an overall system with 120 or more cells can actually achieve the service life of the individual cells.”

How it works

Each p&e battery cell has an “H-bridge” of four metal-oxide semiconductor field-effect transistor power semiconductors. That enables cells to be switched to voltage-free, bypass, positive, or negative voltage. Each cell is also monitored by a microprocessor and switched by a central control unit. If cells are impaired, they are used less frequently or can be bypassed, in the event of defects. The battery management system (BMS) enables the monitoring of each cell. The high round-trip efficiency of such systems, measured in practice, is more than 93% and the power electronics alone can achieve up to 99.5%.

Since a high voltage simply requires connecting a large number of battery cells together, BESS developed with multilevel technology offer a range of design options. For example, one demonstrator consisting of 480 cells achieved a voltage of 1.5 Kv. Another delivered current of 200 amperes and the two characteristics can be combined in one system.

Schulze said the cost of the power electronics is equivalent to that of conventional systems but the concept offers savings in cell purchase, assembly, BMS, and commissioning costs. The multilevel system enables cells with different properties to be processed, increasing supplier choice, and buyers are not as dependent on quality guarantees as they are with conventional systems.

Since cell prices represent the largest cost item in a BESS, final-system cost savings of 20% are possible, estimated Schulze.

P&e power&energy’s multilevel concept has made the company a finalist in this year’s The smarter E-Award.

In a bid to accelerate the spread of multilevel inverter technology, p&e is offering to contribute its knowhow for the development and construction of new BESS with integrated power electronics for direct-current and alternating-current coupling within the framework of a license agreement, or in the form of project partnerships.

From pv magazine Deutschland.