Future Roles of Milli-, Micro-, and Nano- Grids
Although it has slowed considerably, consumption of electricity continues to grow in developed economies. Further, there are some unknowns which might accelerate this growth, such as electrification of vehicle fleets and geothermal heat pump space and water heating. Most analysts anticipate that distributed energy resources (DER) will provide a large share of the expanded generation capacity required to meet this seemingly inexorably increasing electricity demand. Further, given the urgency of tackling the climate change problem, most of the added assets must be carbon free renewables or nuclear, end-use efficiency improvements, or highly efficient fossil-fired technologies. In developed economies worldwide, the current power delivery paradigm has been in place for more than a century, i.e. since the emergence of polyphase AC systems around the turn of the last century. A key feature of this structure is that, in principle, universal service is delivered at a consistent level of power quality and reliability (PQR) throughout large regions. This paper describes a future possible structure for the electricity generation and delivery system that leaves the existing high voltage meshed grid paradigm in place, but involves radical reorganization of parts of the distribution network and customer sites. Managing a much more diverse dispersed system poses major challenges to the current centralized grid paradigm, particularly since many of these assets are small to tiny by macrogrid standards and they may ultimately number in the millions. They are also not ones that centralized control can rely upon to function in traditionally dependable ways, e.g. renewable generation can be highly variable and changes in output of generators are not independent. Although most involved in the industry agree that a paradigm shift is both necessary and desirable to manage the new system, the nature of the future system remains quite unclear. In the possible structure described here, the traditional grid, or macrogrid, remains similar at the high voltage meshed level. Three new entities are added more locally: community grids or milligrids that operate a segment of the existing distribution system, microgrids which are akin to current customer sites but which have autonomous control, and nanogrids, such as telecom or Ethernet networks that currently distribute power to many low-power devices. The latter exist currently in the local electrical systems but are not typically considered a part of the traditional electricity supply system. Because all these new entities exhibit some localized control, providing appropriate local heterogeneous PQR becomes a possibility. These new grid concepts enable a more "bottom-up" approach to electricity distribution, in contrast to the historic "top-down" model. The future will almost certainly include a mix of the two, but the balance among them and the interface (if any) between them is unclear.