As the number of different technologies producing power and providing storage increases, the grid is getting complicated. The best way to make it resilient against outages is therefore changing. The traditional way is to shut down the failing plant, leaving the rest of the grid to cope as best as it can with the change in voltage and frequency. Xi Zhang at the Energy Futures Lab, Imperial College, describes the research looking at multi-energy microgrids: “island” grids that balance the multiple generation and storage technologies locally, not nationally. Integrated into the national grid yet self-contained, they can isolate themselves from catastrophic events. Local backup generation, batteries, electric vehicle storage capacity, thermal storage of buildings/heating/cooling are all considered. Add to that research into optimal dispatch and the strategic curtailment/shifting of non-essential loads. The puzzle is simplified by breaking it down into three elements: energy sources, storage, users. Zhang says load balancing designs are better made at the local level, making them more cost effective too. And as electrification increases the cost of getting it wrong increases too.
We’re expecting to see fundamental transformations in the operational configuration of our urban energy infrastructure over the coming decades. This is due, in part, to the increased urban electric load we anticipate, driven by electrification of the transport sector, cooling/heating technologies, various forms of energy storage technologies, plus advancements in distributed generation and demand-side response.
Of particular interest to us in Project 4 of IDLES is how the resilience of this changing energy system can be protected and improved.
Low-probability, high-impact disruptive events
Although there is not yet a consensus on the exact definition of resilience with respect to the power system, it is widely accepted that resilience can strengthen the immunity of the power system to ‘low-probability, high-impact’ disruptive events. Put simply, it is the ability of the power system to gracefully degrade the magnitude and duration of an outage and rapidly recover to its normal state.
Microgrids: local resilience can beat national
In the future, resilience will not necessarily be delivered through asset redundancy at the national level, as is traditionally the case, but rather through smart control of multi-energy systems at the local district level, including making use of local backup generation and energy storage options. Smart multi-energy microgrids (Fig.1) will constitute the cornerstone of this future urban energy system.
A microgrid is an integrated energy system consisting of energy generation sources, storage options, and energy users. It can be connected to the main electrical grid or may operate in isolation, as an ‘island’, often in rural environments. Those microgrids connected to the main grid can also disconnect and operate independently if necessary, i.e. if there’s a fault within the main grid.
Designing multi-energy microgrids
Microgrid design approaches have largely been addressed for the electricity supply, however there’s a research gap in designing reliable multi-energy microgrids, i.e. those that incorporate power, gas, heat storage, Vehicle-to-Grid (V2G) and other concepts that could play an important role in integrated energy networks. This is a highly challenging research area due to the additional complexity of numerous potential interactions between energy vectors and technologies.
To support such a paradigm shift, a fully intelligent and sophisticated coordination of the system through corrective control actions is required.
Due to the effects of climate change and increased cyber-attacks, over 130 major power outages have occurred across the world in the past two decades. Severe disruptive events cripple the power system, impact on public services and can result in danger to life. The power outage of Friday, 9th August, 2019 is one such recent example.
Due to the increased occurrence and severity of power outages in recent years, more attention has become focussed on improving the resilience performance of the power system.
On the one hand, enhancement of power system resilience could continue to be delivered through the redundancy of infrastructure, however this requires huge investment for what are only low-frequency disruptions, thus significantly reducing the capacity utilisation rate of assets.
On the other hand, when microgrids “island” themselves from the main electrical grid, they can continue to supply the local load using various distributed energy resources (small-scale power generation or storage technologies) coupled via strong links between the different energy vectors, thus mitigating against the adverse effects (e.g. frequency drop) posed by the power imbalance of the main grid.
Improvements in flexibility support decarbonisation
Our recent research has demonstrated that the coordinated operation of different energy vectors via multi-energy microgrids can facilitate cost effective decarbonisation, through providing improved flexibility which supports the local network management and the national system operation.
More research needed
However, the potential benefits of this coordinated operation to enhance the resilience of the urban energy system has not been fully investigated. Understanding these interactions is of huge importance when we consider the future structure of the urban energy network.
In this context, our present work investigates alternative coordination strategies aimed at enhancing resilience of urban energy systems, which includes application of local backup generation, integration of batteries, electric vehicles and inherent thermal storage of buildings to support the local district infrastructure.
Alongside this we are looking at co-optimising operation of local CHP plants and making use of intrinsic thermal storage in the heating/cooling sector. Furthermore, optimal dispatch of local resources (coordinating resources to provide a certain amount of energy/storage while minimising cost) and strategic curtailment/shifting of non-essential loads can significantly enhance the resilience of urban energy systems.
Our next work will try to consider the network impact on resilience delivery between micro-grid communities and investigate how energy systems integration (ESI) can drive cost-effective investment of infrastructure towards a resilient and low-carbon system.
We are currently drafting a conference paper for the IEEE Power & Energy Society General Meeting 2020 to present our findings. This is a huge event within power engineering, usually attracting more than 2000 attendees. It provides an international forum in which to discuss our work and learn about developments from other groups.
One of the key themes will be the Energy Systems Integration (ESI) session, into which our paper will fit. It will be a great platform for our IDLES work and I hope our findings will be of interest to the ESI community, showing the contribution that multi-energy microgrids can make to the resilience of urban energy systems.
Xi Zhang is a Research Associate in the Department of Electrical and Electronic Engineering, Imperial College, UK
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