By Roy L Hales
When I first visited Denmark’s real time energy site, the wind turbines seemed to be producing +90% of the nation’s need. That’s an overly simplistic explanation. Yesterday George Taylor, executive director of Palmetto Energy Research, explained it in a way that made it obvious that though this system works for Denmark, it is misleading to portray it as an illustration of what is possible with today’s wind technology.
“Denmark exports its surplus generation to Norway, Sweden and Germany and imports back from them at other times,” Taylor began. “I haven’t looked up the total import/export exchange or how many megawatt-hours Norway can absorb before its reservoirs fill up. However, Denmark is a small country which operates in the context of a much larger grid. Therefore, it is misleading to compare Danish wind generation with Danish consumption, and draw any general conclusions. That would be similar to comparing generation vs consumption for a single county in Iowa which happens to contain a large wind facility.”
Taylor said you can do that with combined-cycle (CC) natural gas plants too, but costs go up if you run them below 40-50% of capacity.
“CC gas plants are not designed to run below a certain threshold, due to both emissions requirements and limitations of the back-end steam turbine. Each shutdown and restart increases maintenance costs and decreases plant lifetime. However, if CC gas plants paired with wind turbines never shut down, that would limit wind’s maximum level of output to 50% of the total demand which the gas plus wind combination services. Multiplying that by wind’s average output relative to it peak output (over a large facility, not just a single turbine) would mean that wind could generate at most about 20% of the combined amount of electricity in any CC gas plus wind combination. If anyone’s proposal is for wind penetration to exceed 20%, then they must be proposing to shutdown and restart CC gas plants (or coal plants.)
“Combustion turbine (CT) gas plants can shutdown and restart with a much lower maintenance penalty than CC gas plants. However, pairing wind with CT gas doesn’t make sense because the combination would consume more fuel that CC gas running alone without wind.”
The live online graph of Denmark’s power production (http://energinet.dk/Flash/Forside/UK/index.html) testifies to the accuracy of much of what Taylor suspected (even without having fully researched the topic).
During one of my early visits, the wind turbines were producing 3.407 MW and the nation’s total consumption was 3.608 MW (left). The turbines were producing more than 94% of national consumption.
We are at the opposite end of the chart this morning, the turbines are producing 7%.
The numbers Taylor is pointing to are Denmark’s two fossil fuel burning sectors. Local CHP plants use natural gas and the central power stations that primarily burn coal or wood chips.
When the turbines were able to supply 94% of domestic need, the Local CHP plants were producing 247 MW. This morning the turbines are only supplying about 7% and the CHP plants are pumping out 520 MW. That is slightly more than twice the amount. Or to put it another away, when the turbines were at peak production the CHP plants were cut back to 47.5% – the range that Taylor indicated.
Right now, the bulk of Denmark’s power is coming from its central power stations, which are producing 2.039 MW or around 51% of the domestic consumption.
When the wind turbines were close to peak capacity, at the beginning of last month, the central power stations were cut back to 681 MW. That is roughly 33% of what they are running at right now.
This is less than the 40%-50% limit Taylor suggested, which raises the question of whether the Danes are being hard on their system.
Perusing descriptions of the nine power stations, I note that Kyndby Power station is described as an “emergency and peak load facility” which “can be started up within minutes if operational irregularities occur in the high voltage electricity grid or problems arise at other power stations.” This facility has “two oil-fired steam power units – peak load facilities each having a capacity of 260MW. Two oil-fired gas turbine units of 63MW each constitute the remaining peak load facilities at Kyndby Power Station.”
Taylor believes that instead of replacing fossil fuels, using wind ensures their usage will continue. This is because the you need a back-up, which up to this point has largely been a fossil fuel powered plant.
The Danes are trying to get around this by increasing their usage of wood pellets in central power stations.
Denmark’s achievements are remarkable. CO2 emissions are currently close to 1990 levels. The Government recently set a goal of going 40% lower than this by 2020 and obtaining 100% of its energy from renewable sources by 2050.
Wind turbines produced 33.2% of the nation’s electricity last year. As there were less the normal amount of winds, they might provide as much as 40% this year.
Denmark is the only net energy exporting nation in Europe, but they have only achieved this status because they are hooked into the Norwegian, Swedish and German grids. When Denmark’s turbines were pumping out 3407 MW, they were exporting 897 MW of excess energy. The turbines are not producing as much right now, so the Danes are making up for the deficiency by importing 862 MW.
Taylor was correct when he said it is misleading to use Danish wind generation as an illustration of what is possible with today’s technology. The Danes are only able to perform at this level because other nations take their excess energy when it is being produced, and provide energy when it is not.
He is also correct in stating that we should not examine wind technology as an isolated entity. We need to evaluate the whole package. In Denmark’s case, it is wind technology + other domestic energy production + imports/exports from neighbouring countries.
(Image at top of page: Danish wind turbines near Copenhagen. Wind often flows briskly and smoothly over water since there are no obstructions. The large and slow turning turbines of this offshore wind farm near Copenhagen take advantage of the moderate yet constant breezes at this location. While the wind at this location is not strong it is very consistent, with the turbines generating substantial power over 97 percent of the time.- Leonard G, cc by 1.0, en wikipedia )