Hybrid Storages

Hybrid energy storages combine the advantages of various energy storage technologies increasing the overall value of the hybrid system in comparison to a single storage. Due to the comparatively low energy density of kinetic energy storage systems, the combination with electrochemical energy storages is advantageous in order to achieve a higher total capacity.

In this combined system, the kinetic energy storage plays the role of the short-term energy storage, while the electrochemical storage plays the role of the long-term energy storage. The later results in advantages for the electrochemical energy storage, in terms of service life and dimensioning, since potentially harmful load components are covered by the kinetic energy storage. The kinetic energy storage shows in turn a much higher cycle life.

Structure of the hybrid storage
Structure of the hybrid storage

Essential for the efficient use of hybrid energy storages are appropriate operating strategies, which determine among others the dynamic load sharing between the storages. The IMS employs frequency dependent and rule-based operating strategies for the hybrid energy storages. Operating strategies based on nonlinear optimization and the subsequent training of a neural network are also investigated in simulations. The optimization algorithm aims to minimize the energy losses as well as the aging of the hybrid energy storage.

Aging curves
Aging curves

The economic viability of hybrid energy storage systems is also a research field in the IMS. Regardless of the operating strategy, the hybrid energy storage system has the potential to reduce the cyclic aging of the battery by using the kinetic energy storage. The aging of lithium-ion cells can be estimated using suitable aging models, which distinguish between cyclical and calendrical aging. The hybrid energy storage system shows advantages for applications that exhibit beside smoothly also highly dynamic power profiles. The higher total power in combination with reduced aging justifies the financial burden for the kinetic energy storage. Applications such as primary control reserve and peak shaving in industries confirm the potential of the hybrid storage system.

In addition to stationary applications, hybrid energy storage systems are also suitable for other sectors. Mobility accounts for a large proportion of today's energy consumption and, at the same time, of greenhouse gas emissions. The electrification of the vehicle fleet aims to reduce greenhouse gas emissions. The use of lithium-ion cells as an energy source for electrified motor vehicles is becoming increasingly important today. However, the aging effects of lithium-ion cells still pose challenges. Driving cycles of commercial vehicles, such as those of electric buses or waste disposal vehicles, which shuttle daily in the minute range between the different stations, have been shown to place a strain on lithium-ion batteries. The use of a kinetic energy storage as an additional storage relieves the battery from intensive charging and discharging cycles and consequently increases its service life. At the same time, the necessary for it additional weight plays a minor role in the vehicles mentioned.

The IMS developed and tested experimentally two hybrid energy storage systems. The Hybrid Storage ETA was developed within the scope of the publicly funded project PHI-Factory aiming to increase the energy quality of the Living lab ETA factory as well as to contribute to the power grid balancing. This hybrid system comprises a kinetic energy storage with 1.4 kWh energetic capacity and 120 kW electrical power as well as a lithium-ion battery with 122 kWh energetic capacity and 120 kW electrical power.

The Hybrid Storage SWIVT was developed within the framework of the SWIVT I and SWIVT II projects, which deal with the energy-efficient renovation of an existing urban settlement. Using energy storage units, settlements can reduce their energy costs and provide additional valuable grid services. The hybrid system for the settlement consists of a kinetic energy storage with 2.4 kWh energetic capacity and 100 kW electrical power as well as a lithium-ion battery with 49 kWh energetic capacity and 30 kW electrical power.

KI4ETA – Artificial Intelligence for Energy Technologies and Applications in Manufacturing

SWIVT II – Implementation of an energy-efficient renovation of a mixed existing and new residential estate

STEPS – STorage of Energy & Power Systems

SWIVT I – Conceptual design of an energetic renovation of an existing residential estate

PHI Factory – Grid-adaptive factory management with regard to the interactions of distribution and industrial grid