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Long-distance Interconnects and their Contribution to Grid Balance

In this previous post, we looked at  how much of the current electricity demand could be met directly if the entire production were based on solar and wind. The observation was that depending on the country, about 70-80% could be covered in real-time by scaling up an optimal mix based on the current solar & wind generation capacity. This would leave about 20-30% of unmatched load at some times and the same amount of surplus at other times.

In this post, we looked at how storage and in particular under-utilized car batteries could be used to help balance the grid between times of over and under-production.

Another potential approach of increasing the balance between supply and demand would be to improve geographic diversity across wind or solar generation beyond the synoptic scale of common weather pattern which is in the order of a thousand or more kilometers.

The image above shows the site of the Laufenburg substation, where in 1958 the national grids of France, Germany and Switzerland were interconnected for the first time, setting the foundation for todays continental European synchronous grid spanning from Portugal to Ukraine and from Scandinavia to Turkey or Marocco.

While the synchronous interconnection of AC high voltage transmission grids greatly increases the efficiency and resilience for each of the participating transmission system operators most of the energy continues to flow mostly over short distances between neighboring zones.

Technological breakthroughs in high-voltage DC (HVDC) switching and conversion systems have brought HVDC interconnects from being an expensive niche solution to a core technology of new high-capacity and long-distance super-grids emerging just about now. Key advantages of HVDC transmission are the lower loss rates and the ability to run a very high voltages across coaxial underwater and underground cables, making them a suitable solution to transport large amounts of energy across long distances.

The recently inaugurated Viking Link connecting the UK and Denmark across the North Sea is currently the longest operating next-gen HVDC interconnect in Europe at 765km carrying up to 1.4GW of power in either direction. Germany started construction on the SuedLink project with several 4GW underground HVDC links connecting the German North-Sea coast with the industrial and population centers further south. Maybe the most ambitious among the many HVDC projects across Europe is the 3800km 3.8GW Xlinks project connecting solar and wind farms in Marocco to consumers in the UK via a submarine HVDC cable. A similar building boom of long-distances HVDC links can be observed for the last few decades around the world.

In order to see how much interconnection beyond the synoptic scale of weather systems could automatically contribute to the balancing of supply and demand at either end, we are using a joint LP optimization problem across sets of 2 countries similar to the one used for this post.

First we are looking at an interconnection of Germany and Spain. Both among the largest energy markets in Europe with significant geographic dimensions and they are over a thousand kilometers apart from each other and are at least partially in different climate zones. By varying the available interconnection capacity from zero to 32 GW, we can see the import/export flows increase and the unmatched shortfalls decrease accordingly:

Germany-Spain Interconnection

Looking at the shortfall across different timescales of Germany for example, we can see that most of the improvements wold be over relatively short time-scales - hours to weeks.

Germany - imbalance across time-scales

Both countries being still essentially being in a similar climate zone and having similar seasonality, does not seem to provide enough complementarity to significantly improve the imbalance across the very long seasonal time scales (month to quarters).

We can also try a more extreme example, connecting the Netherlands with Greece. Both are geographically smaller and distinctively different - with Netherlands on the shore of the North Sea and Greece in the South-Eastern Mediterranean. Both are also much smaller energy markets, which means that a much smaller interconnection capacity would be needed take advantage of the interchange of matching surplus and shortfalls.

Netherlands-Greece Interconnection

Despite the more extreme scenario, we only see a slightly higher relative import/export flow and corresponding decrease in unmatched load across both markets.

Netherland - imbalance across time-scales

With this experiment we can show that introducing some additional geographic diversity in wind & solar production by naively interconnecting 2 power-grids over distances of more than thousand kilometers could reduce the load & production imbalance by 5-10%.  A less arbitrary choice of interconnection points would probably increase the potential impact.