The right size for future electric grids?

Interconnected electricity grids can be a variety of sizes but they all do the same thing: balance supply and demand at all times.

Supply and demand should be finely balanced so the pulse of the grid, be it 50 Hz or 60 Hz, stays healthy. Too much demand and hundreds or thousands of spinning machines supplying the grid will labour under the effort, slowing down and dropping frequency. Too little demand and the machines will race. The grid frequency must stay within tightly controlled bounds to maintain system stability. Some national grids have experienced widespread blackouts that demonstrate both the importance and the difficulty of keeping electricity grids running.

There isn’t an established ideal size for an electric grid. The geographic extent over which balancing is done varies worldwide, from independent grids of smaller countries, to vast interconnected mesh networks of Europe and the United States, to rapidly expanding networks of China and India, to relatively linear coastal networks of which Australia’s is the longest. Deregulated markets and vertically integrated utilities alike manage the dispatch of generation to ensure N-1 redundancy across their jurisdiction.

Now the balancing game is changing as renewable energy sources are introduced to mitigate climate change and attempt sustainability. The natural variability of some renewable sources suggests that geographical extent will be important. Wind, solar, and ocean energy from different sites is not independent but strongly correlated with the movement of weather systems, so significant peaks and troughs remain even after combining the potential electricity supply from these sources over a wide area. Global climate datasets and modelling show that gathering renewable energy on an intercontinental scale can reduce the risk of coincident loss of supply to an acceptable level – and such large networks can be contemplated with advances in ultra-high-voltage direct-current (UHVDC) transmission technology. Advocates for pan-European and pan-Asian networks are undertaking serious studies of their technical and economic viability.

But would this be a sensible investment? The alternative approach is to deploy energy storage facilities at strategic supply-side and demand-side locations. Remote and island power systems present an extreme case where energy storage and fuel-based reserve generation is the only way to achieve high levels of renewable energy supply. The technical maturity and supply chain for grid-scale energy storage is rapidly making it a main-stream technology. Future grid planning can consider energy storage investments alongside transmission expansion to achieve an ideal match of technology to renewable resource and demand characteristics.

This consideration must account for:
• Renewable energy resource diversity over the geographical region
• Requirements for fuel-based generation plant to provide reserve capacity
• Transmission infrastructure requirements for system adequacy
• Demand growth and variability and the distribution infrastructure serving it
• Energy storage providing reserve capacity for both generation and transmission

Issue of national strategic importance will be teased out through such analysis. Should nations like Australia and China create renewable energy supply zones to harness the best resources in the sparsely populated interior or, in Australia’s case, on the distant coast? Or should they use resources of lesser quality that are more conveniently located near population centres and transmission corridors? As with the mining industry, the renewable resources in some favoured locations will increase in value until it is economically attractive the harvest them, and investment will follow. The grid will grow and find new operating modes to reflect additional sources of variability and increased flexibility in each nation’s energy mix.

In Australia, such renewable energy supply zones are likely be placed near large mining loads to offset transported fuels and reduce transmission requirements. The potential to export electrical energy and natural gas to other Asian countries should also be considered. As technology develops and regional demand for clean energy increases, the offshore market may prove more attractive than supplying domestic demand. Ultimately, international connectivity will enable efficient trading so that each nation can focus on developing its best energy resources, benefiting the whole region.

Geoff James, Principal Research Scientist, Energy Technology, CSIRO Australia