Hydrogen energy storage system for nanogrid

As the impact of human activities on global climate is becoming increasingly clear, the electrical energy generation sector is undergoing a massive transformation to accommodate more renewable energy generation, and so reduce its carbon footprint. This transformation requires a major restructuring of traditional electrical grid infrastructures from a centralized generation construct, to a distributed generation concept where local energy sources and loads are linked together at the distribution level. Various terminologies have been used to differentiate between these new structures and conventional grid systems, such as mini-grid, microgrid and nanogrid. Each has a similar general concept one or more renewable energy sources, energy storage systems and loads linked together to create a local cluster which is coordinated and controlled to operate either autonomously and or in conjunction with a utility grid with their differentiation determined mostly by size and geographic scale. Mini-grids and microgrids typically meet the needs of a small cluster of premises, or larger scale institutions such as schools, hospitals, community centres and high rise buildings. More recently, the term nanogrid has been coined to describe a scaled down microgrid that serves a single residential customer, often with a low power single phase supply.<br><br> One central issue with the integration of renewable generation into an electrical grid network is the intermittency of renewable energy sources such as PV and wind. Since these sources cannot supply power according to load demand, there will always be a mismatch between supply and demand. Hence a renewable based microgrid system requires some form of energy storage, to store energy when the available generated power exceeds the load demand, and to release this energy when the renewable energy is inadequate to meet the load demand. When the microgrid is operating in grid-connected mode, such a storage system can help residential customers to time-shift their energy usage and reduce their electricity bills. When the microgrid is operating autonomously during times when the grid is not available, a storage system is essential to allow the microgrid to operate. The size and capability of the storage system becomes increasingly important as the size of the microgrid reduces, the renewable energy becomes more intermittent since the sources are in the same geographical location, and load diversity reduces, with higher transient power peaks, as individual equipment is turned off and on.<br><br> Battery storage systems are a popular solution for small scale microgrids and nanogrids. However, it is important to adequately size the battery system to cope with worst case loss of renewable generation conditions over several days. Furthermore, as the battery energy storage capacity increases, its power rating often similar increases, which can lead to a substantial capital investment for a small scale system. In contrast, the energy capacity of a hydrogen storage system (HESS), comprised of an electrolyser, a hydrogen storage tank and a fuel-cell, is readily increased by simply using a larger tank, without requiring a corresponding increase in the rating of either the electrolyser or the fuel cell. Hence in a residential nanogrid with rooftop PV generation, a HESS presents a green and attractive solution to bottle surplus sunshine for future use.<br><br> However, the use of a HESS in a nanogrid does create some particular challenges, particularly if it is a single phase nanogrid that is operating in islanded autonomous mode. For such a small scale system, the capital investment cost needs to be kept to a minimum, which means the question of how to interconnect the PV inverter, the HESS electrolyser and the HESS fuel cell must be carefully considered. Furthermore, fuel cells can be damaged if their output current slews too rapidly, which means that a transient energy buffer is required to accommodate rapid load changes. In addition, the double frequency DC bus power ripple created by a single phase AC inverter is hazardous to fuel cell reliability and longevity. Finally of course, the overall issue of energy balance and management has to be managed efficiently and effectively.<br><br> This thesis addresses these issues for a HESS system that supports a single phase nanogrid structure. The thesis explores how best to interface the HESS components to the nanogrid electrical structure, and proposes novel strategies to manage the power slew rate limitation of the HESS fuel cell and to avoid the single phase double frequency power ripple impacting on the fuel cell. Finally, a simple overall energy management strategy is proposed to balance the nanogrid load requirements, PV renewable energy availability and HESS storage operation. The theoretical developments presented in this work have been validated using simulation and experimental investigations, and the contributions have been published in three international conference papers.<br><br>