4. The role of interconnection for renewables

One of the challenges of moving to an electricity system running with a large proportion of wind and solar is that electricity output from these sources is variable, because the wind does not always blow and the sun does not always shine at any particular location.  With small proportions of these sources on the system this is not much of a problem.  However as a system becomes more dependent on these variable renewables, production is potentially curtailed in periods of high output, while there is a shortage of power at other times.  I’ll refer in this post to wind and solar photovoltaic (PV) power, which is by far the predominant form of solar electricity.  The issues with concentrated solar thermal power are different because there is some potential for storage in the power plant.

Difficulties with balancing supply and demand typically begin to become more severe as each type of variable renewable power, for example wind, begins to supply more than (very roughly) around 15% of annual electricity.  In practice the extent of the challenge of integration depends on a range of factors.  One influence is the amount of inflexible generating capacity such as nuclear on the system.  Another influence is the correlation between variable supply and demand.  For example, there is some seasonal correlation between electricity demand and wind output in Europe (in winter demand is higher and it’s windier), and there is some daily correlation between electricity demand and solar PV output in Australia (the demand peak is in the daytime).  Nevertheless, in any case there will be challenges in reliably meeting demand while accommodating very large amounts of variable supply.

To address this problem variable supply can be moved to different times, using storage, and to different places via transmission capacityDemand management can help, especially for a demand peaks, and some thermal back-up capacity is likely to be needed anyway in most cases.

The extent to which transmission links can help system balancing depends on the extent to which generation in different places is correlated.  If distant power sources tend to produce power at different times then transmission can greatly help smooth out variations in production, because when output is low in one location electricity can be moved there from another.  But if distant sources tend to produce power at the same time the advantages of long distance transmission will be reduced, because if output is low in one place there will be little electricity in the other place to move to fill the gap.  This is especially so if patterns of demand are also similar in the different places

Typically, correlations of output across regions for solar PV are much higher than for wind, especially as distances increase (see chart).  This is because solar output tends to depend strongly on time of day and season, which is quite similar even across an area as large as Western Europe, whereas wind varies with weather systems, which are less strongly related over distance of the order of 1000 miles or so.  Looking at the correlations between hourly output in different countries in Western Europe shows that the correlation between solar output in Germany and France is 88%, much greater than for wind at 44%.  Moving further away, the correlation between output in Germany and Spain is close to zero for wind, but still as high as 84% for solar.  The match of solar output is is actually greater than these figures imply due to the large number of times when solar output is zero for both countries.

Correlations of hourly output between different Eastern European countries are much greater for solar than for wind …

Correlations chart

These factors mean that increasing interconnection will tend to enable higher proportions of wind more than it will enable higher proportions of solar.  (This is assuming transmission is for load balancing.  Interconnection to transport power from sunnier to less sunny regions may well be valuable.  Also, this conclusion would change if intercontinental of transmission of large amounts of power were possible, but this remains a distant prospect.)

Incidentally, the correlation between solar and wind output is negative (when it’s windy it tends to be less sunny and vice versa), so having some of each on the system tends to increase the combined total that can be accommodated, compared with a system only having either one or the other.

A recent study of the USA by the National Oceanic and Atmospheric Administration (NOAA) confirmed very large benefits from High Voltage Direct Current (HVDC) long distance transmission for systems with large amounts of variable renewables.   The study used very detailed weather and load data, and data on existing power plants.  It concluded that with optimistic projections of wind and solar costs it is possible to reduce CO2 emissions by 82% with somewhat lower electricity costs, provided sufficient transmission capacity is in place.  The study emphasised the importance of the transmission network encompassing a large geographical area, such as the 48 contiguous US states, due to the large geographical scales over which weather is variable.

However, interconnectors are not always quick, cheap or easy to build.  They often link or cross different jurisdictions – US States, Chinese provinces or European countries – and will often link different types of electricity trading arrangement.  This can impose substantial barriers around permitting, and also around operation.  Policy can help the growth of wind by reducing these barriers and recognising the growing role of internconnection, although the precise policy measures necessary will be quite locally specific.  Enabling increased transmission is likely to be an important step in enabling the continuing growth of wind power in particular, and is likely to become increasingly urgent as growth continues.

Updated 7th November 2014

References

Thanks to Mathieu Ecoifier for providing the correlation coefficients between wind and solar in Europe.  Correlation coefficients are a rough and ready indicator of independence.  Actual effects on system operation will depend on many factors.

For analysis of the inverse correlation of wind and solar see for example Correlations Between Large-Scale Solar and Wind Power in a Future Scenario for Sweden, Joakim Widén, IEEE Transactions on Sustainable Energy, Vol. 2, No. 2, April 2011

The US study mentioned is Alexander MacDonald et. al., NOAA Earth System Research Laboratory, Low Cost and Low Carbon Energy Systems Feasible with Large Geographical Size (2014) – Presentation at Imperial College London 27th May 2014

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