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Hybrid energy storage systems

Capacity optimization and environmental implication of hybrid energy storage systems in renewable power systems

Time: Wed 2022-12-14 10.00

Location: Kollegiesalen, Brinellvägen 8, Stockholm

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Language: English

Subject area: Electrical Engineering

Doctoral student: Yang Jiao , Elektroteknisk teori och konstruktion

Opponent: Professor Alfred Rufer, Ecole Polytechnique Fédérale de Lausanne

Supervisor: Associate professor Daniel Månsson, Elektroteknisk teori och konstruktion

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QC 20221117


Electricity production contributes a large share of greenhouse gas emissions that lead to climate change. At present, power generation in many parts of the world still heavily depends on fossil fuels. To prevent global warming, the electricity sector is supposed to transition from the current fuel mix to renewable energy sources. Renewable power systems, which are sustainable and environmentally friendly, have the potential to satisfy our electricity consumption. Wind power and solar power increased rapidly in the past decade, however, they only accounted for about 10% of the global annual electricity production in 2021. The main barrier to wind and solar power is variability, which brings a severe challenge to conventional power systems. The power grid needs to constantly maintain the supply-demand balance due to a lack of energy storage. 

Energy storage affects every aspect of power systems from generation to consumption and brings great benefits to renewable energy resources and power grids, such as energy time-shifting, improving power quality, and voltage regulation. However, none of the energy storage technologies can satisfy the diverse and even multiple needs of power systems. Therefore, the hybrid energy storage system is a promising solution. 

This thesis discusses hybrid energy storage systems from two aspects to make better use of them in renewable power systems: capacity optimization and environmental implication. Firstly, capacity optimization is a significant concern for hybrid energy storage systems. To seek the optimal capacity of a hybrid energy storage system, this thesis explores the energy exchange between the individual energy storage devices within the system. It leads to oversized capacity and increased loss. Hence, an improved low-pass filter controller that contains the power direction control strategy and the state-of-charge control strategy is presented in this thesis. The improved controller effectively eliminates the unnecessary energy exchange to ensure the minimized capacity of the system and improves round-trip energy efficiency. In addition, an alternative controller with a variable time constant is presented to utilize the secondary energy storage device more properly in hybrid energy storage systems. Moreover, despite helping the integration of variable renewable energy, energy storage systems still have greenhouse gas emissions from cradle to grave. This thesis presents a consequential life cycle assessment approach and evaluates the life cycle greenhouse gas emissions from hybrid energy storage systems in renewable power systems. Different combinations are compared to seek the appropriate combination and forecast the potential of energy storage to achieve a 100% renewable power system with low emissions.