Analytical methods for energy storage design in hybrid renewable systems

Thesis

The need for storage design stems from rising greenhouse gas emissions, a key contributor to climate change. A significant portion of global greenhouse gas emissions originates from the energy sector due to fossil fuel usage. An alternative to fossil fuels is renewable energy. Renewable generation produces no emissions during operation and have lower life-cycle emissions compared to fossil fuel power plants. However, renewables are weather-dependent, leading to intermittent and non-dispatchable generation. This intermittency induces instability in the electricity system, while the lack of dispatchability results in curtailed generation and unmet demand. These challenges can be effectively managed through energy storage. Storage captures surplus energy during periods of high generation, and releases that energy during low generation, resulting in a more consistent generation output that can match the demand. Proper storage design ensures that storage can effectively manage renewable intermittency and provide dispatchability.

The thesis explores storage design, encompassing storage sizing, storage and solar sizing, and storage sizing and placement. First, an analytical method is proposed to size storage based on its largest cumulative charge or discharge. The method is applied to two case studies. The first study demonstrates that optimally sized storage does not have wasted capacity due to over-sizing, nor does it cause energy deficits due to under-sizing. The second study finds that increasing the storage size has diminishing returns on the additional storage energy provided to the system. Second, the thesis proposes a hybridized techno-economic method to size solar photovoltaic and lithium battery storage. The hybridized method uses an analytical method to define the size search space, and then employs an enumerative approach to explore the search space to find the lowest-cost system sizing. The method is applied to a solar-battery microgrid case study, which finds that solar and storage will take up a greater portion of the energy system as their costs come down, but the electricity grid remains essential for providing cost-effective flexibility. Third, the thesis proposes an analytical method for sizing and placing energy storage. The method uses optimal power flow to decide the power dispatch at each location. Then, the analytical method is employed to size and place storage based on the power dispatch. The method is applied to a case study on a village with wind and solar generation. The study finds that storage tends to be placed near large generation, large demand, or lines with high power flow. These works contribute to the purpose of the thesis, which is to gain a better understanding of storage design in the context of hybrid renewable systems.

Authors: Ren, HK

• DOI: 10.5287/ora-pv4pzx5g7
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: optimization; placement; renewable energy; sizing; energy storage
• Subjects: Energy Systems