With 33% of the electricity it generated in 2013 coming from wind Denmark is a world leader in wind power – a remarkable achievement considering the difficulty of integrating Denmark’s highly erratic and sometimes overwhelming wind output with the small Danish grid. (Figure 1). The hourly data used to construct the Figures in this post are from the data base compiled by Paul-Frederik Bach:
Figure 1: Total wind generation vs. load, hourly data, Denmark, 2013
The degree of difficulty is illustrated by the comparable plot of 2013 UK wind generation versus load shown in Figure 2:
Figure 2: Total wind generation vs. load, hourly data, National Grid, 2013
How do the Danes do it?
It’s commonly assumed that they do it by exporting the wind power surges the Danish grid can’t handle to the Norwegians and Swedes, who use the power to replace hydro generation and conserve water in their hydro reservoirs (our colleague Hugh Sharman in his 2009 study “Wind Energy, the Case of Denmark” estimated that only about half of the wind power generated in Denmark between 2000 and 2007 was actually consumed in Denmark). But while the Danes undoubtedly export a lot of their wind power to Norway and Sweden that isn’t all they do.
First some background on Denmark’s electricity system. According to ENTSO-E Denmark had at the end of 2013 a total of 14,865MW of installed generating capacity, consisting of 8,887MW of conventional thermal, 4,811MW of wind, 595MW of biomass, 563MW of solar and 9MW of hydro. These numbers provide two interesting insights, always assuming they are correct. The first is that Denmark’s total installed capacity exceeded its ~6,000MW 2013 peak load by a factor of well over two. The second is that if 4,811MW of wind supplied 33% of Denmark’s total 2013 generation, i.e. 11.1TWh, then the remaining 10,054MW generated 22.4TWh, which gives a load factor of only 25%, a percent or two less than the load factor for wind. It seems that Denmark’s electricity system may already be operating at something less than peak efficiency, which may have something to do with why its domestic electricity rates are the highest in Europe.
But the key to Denmark’s high level of wind penetration is its location on the Nordic Grid between its larger neighbors Norway, Sweden and Germany, who between them generated 26 times as much electricity as Denmark in 2013. This gives Denmark access to an additional 5,820MW of interconnector capacity that it makes full use of (Figure 2):
Figure 2: Denmark’s Nordic Grid interconnectors (from Energinet.dk)
And the way it makes full use of it is by cycling interconnector flows to balance its erratic wind output. How Denmark does this under the constraints imposed by the Nordic Grid merit order system is something I haven’t looked into, but do it it does. The year-round correlation between wind generation and interconnector flows both in and out of Denmark, while not exact, is too close to permit any other interpretation. Examples are shown in Figure 3, which plots wind generation against interconnector flows for January, February and March (R=0.81) and June, July and August 2013 (R=0.83). The sine-wave patterns in the blue line show that interconnector flows were also cycled to follow the daily load curve as well as adjusted to balance fluctuations in wind output. The correlation coefficients would be higher if these patterns were not present. Note also that exports are plotted positive so that they move in the same sense as the interconnector flows:
Figure 3: Wind generation vs. interconnector flows (imports negative exports positive), Denmark, winter and summer months, 2013
It would appear that Denmark’s ready access to balancing power from the Nordic Grid allows it generate a lot more wind power than it would otherwise be able to, whether it consumes it or not.
If we assume that Denmark Island’s 8,887MW of conventional thermal capacity has load-following capability it should in theory have been able to consume all of it (peak 2013 hourly load was 6,138MW at 6pm on January 24, a time when of course the wind wasn’t blowing). And had it done so 33% of its electricity demand would have been supplied by wind. But its thermal load-following capacity would have had to track the peaks and troughs shown in Figure 4 to balance the irregularities in wind output (the red lines below the zero line show where wind would have had to be curtailed, but these make up less than 1% of total wind generation):
Figure 4: Backup generation needed to balance wind against load, “Denmark Island” actual data, 2013. The plot is constructed by subtracting hourly 2013 wind generation from 2013 hourly load.
And Denmark Island, like mainland Denmark, isn’t content with the status quo. It targets 50% electricity from wind by 2020. To analog a 50% level of penetration I scaled up 2013 installed wind capacity by 50/33 = 1.5, i.e. from 4,500MW to 6,750MW, factored 2013 wind generation up in proportion and subtracted it from 2013 load. Figure 5 shows the resulting generation curve the load-following plants would have to track. It’s not all that different from the Figure 4 curve, but the spikes are narrower and “spikier”:
Figure 5: Backup generation needed to balance wind against load, “Denmark Island”, 50% wind penetration. The plot is constructed by subtracting hourly 2013 wind generation times 1.5 from 2013 hourly load.
The problem, however, is that Denmark Island wouldn’t get 50% of its electricity from wind with a 50% capacity expansion because 20% of the added wind generation exceeds demand (the red lines go well off the bottom of the graph) and would have to be curtailed, leaving the island about 4% short of its target. To reach 50% it would in fact have to increase total wind generation by a factor of 1.7, i.e. from 4,500 to 7,650MW, whereupon more wind power is curtailed and the spikes get spikier yet.
And should Denmark Island then wish to expand its wind generation further it would find itself on the slippery slope of diminishing returns, because the more wind generation it adds the more it has to curtail until eventually almost all of it is curtailed and no significant amount added. Figure 6 summarizes the impacts of increasing levels of wind penetration, calculated by scaling up 2013 wind generation:
Figure 6: Impacts of increased installed wind capacity on wind penetration, wind curtailment and wind load factor, “Denmark Island”
Taking one point on the graph as an example, to achieve 80% wind penetration Denmark Island would need around 25,000MW of installed wind capacity – over five times as much as it has now – and at this capacity level almost 90% of the generation added by each extra MW would get curtailed along with over half of the total wind power generated. .
At high levels of wind penetration problems also begin to arise with the load-balancing capacity. As shown in Figure 7, it’s heavily underutilized:
Figure 7: Backup generation needed to balance wind against load, “Denmark Island”, 80% wind penetration. The plot is constructed by subtracting hourly 2013 wind generation times 5.4 from 2013 hourly load.
And the spikes have become spiky to the point where it’s questionable whether the thermal load-following capacity would be able to ramp quickly enough to follow them, as shown in Figure 8. (The 100MW/minute upper limit is from Wartsila, who cite an average of 25MW/minute for “industrial frame gas turbine models”.) Ramp rates don’t change appreciably up to 55% wind penetration but above this level they begin to increase rapidly and above ~75% penetration they exceed the 100MW/minute limit. Might this preclude further wind expansion? Comments are solicited from experts:
Figure 8: Ramp rates versus percent wind penetration, “Denmark Island”.
But Denmark Island is of course imaginary. What does the future hold for the real Denmark? Interconnector flows are reportedly not limited by ramp rates, so Denmark has an agile load-following capability that shouldn’t prevent it integrating more wind power into its grid. So if the Nordic Grid can keep Denmark supplied with the electricity it needs to balance increasing wind output then wind power in Denmark, all other things being equal and assuming the Danes are prepared to pay for it, will grow to meet Denmark’s goal of 100% renewable energy. But if it can’t growth will stall, and if the supply starts to dry up Danish wind power will start to dry up with it. And the risk for Denmark, of course, is that supply from the Nordic Grid will start to dry up after it’s gone 100% renewable.