Although this part of the system is now running more smoothly than before, and innovation continues to push down costs, getting the power to the onshore transformer station is not the end of the story.
Inland, the south eastern state of Bavaria is blocking planning progress on the high-voltage direct current links that will carry the offshore (and onshore) wind generation into its state. Bavarian state premier Horst Seehofer's opposition is perplexing as this wind-generated electricity will be increasingly needed to substitute output from nuclear plants that are being phased out to 2022 and fossil fuel power stations being closed as low wholesale electricity prices make profitable operation difficult.
Thrown in the deep end, with mainly far-shore projects in the German exclusive economy zone of the North Sea, the pioneering first-mover offshore developers have dealt with grid link problems that include long delays in completing high-voltage alternate current (AC) to direct current (DC) converter platforms and clearance of World War II ordnance on the sea bed holding up cable installation. The first high-voltage DC grid system was stymied by uncontrolled electricity harmonics (see box, right).
But over time, efforts by the permitting authorities, the federal energy regulator, transmission network operators and the industry itself have resulted in careful and methodical grid-link planning and standardisation of hardware specifications. Even a change in the method for allocating transmission capacity to shore from individual offshore wind projects - through a revision of the Energy Industry Act at the end of 2012, which pushed implementation of some farshore projects way into the future - has been accepted. Projects closer to shore have been prioritised under the assumption that they are more cost effective. Although this has disappointed those with far shore projects in advanced planning, it was accepted as the price paid for a firm, if modest, expansion path for the offshore wind sector as a whole.
The revised Energy Industry Act also introduced a new offshore liability levy, paid by electricity consumers, to raise funds to compensate offshore wind project businesses for lost income due to late commissioning or outages during commercial operation of their grid links to the onshore network.
The German government is using the allocation of transmission links to shore to ensure its expansion targets for offshore wind - 6.5GW of offshore capacity by 2020 and 15GW by 2030 - are not exceeded. The outlook for 2025 is 8.9-10.5GW, of which 7.7-9.2GW will be in the North Sea and 1.2-1.3GW in the Baltic Sea. These figures come from the scenarios in the annually updated ten-year offshore transmission network expansion plan, Offshore-NEP 2025, rubber-stamped last December by federal energy regulator the Bundesnetzagentur.
The Bundesnetzagentur controls the number and capacity of offshore grid links to shore through the permits it grants to transmission system operators Tennet (for the North Sea) and 50Hertz (for the Baltic). An auctioning procedure was introduced in August 2014 to allocate offshore projects to grid links if the transmission capacity is insufficient to serve all the applicants for that link.
In the first round of allocations announced by the Bundesnetzagentur in October 2014, new transmission capacity of 1.7GW was announced, but only 1.5GW of transmission links were sought by eligible projects.
However, two projects competing for capacity on one grid link had to bid against each other. The ensuing legal dispute - over what was argued to be a negative, retroactive effect on offshore wind planning that had been carried out under the previous grid allocation rules - ended in the federal agency deciding in March that EnBW's 450MW Hohe See and 50MW Albatros projects, both reduced in size apparently to fit the grid, be allocated grid capacity on the Borwin 3 link. It also allowed the Gode Wind 1 project to use Borwin 2 connection but reallocate it to Borwin 3 when commissioned in 2019.
All in all, the offshore network development plan prepared in 2014 for the period to 2024 (its second draft published and put up for consultation in February 2015), shows 13 North Sea and five Baltic Sea grid links built or under construction over the period to 2019, as well as another five in the pre-permitting or permitting phase.
With the mechanisms for getting offshore wind generation to land relatively settled, the spotlight has turned to political eruptions in the distant southern German state of Bavaria, which seem to threaten the process of getting offshore wind generation to the consumers who need it.
A Bavarian political blockade on the high-voltage direct-current cable links, in particular the two 2GW links bundled together as the Sudlink project, from the windy north, where Germany's North Sea offshore wind farms are concentrated, to the deep south of the country, has brought uncertainty and irritation. Curiously, Bavaria's governing ultra right-wing Christian Social Union (CSU) party is part of the federal coalition government that is steering Germany's course through the Energiewende, the transition to increasing dependence on renewables in energy supply.
Bavarian premier Horst Seehofer's resistance to the connections grew as local protest groups argued loudly against the cable routes, even though some of them are softening their opposition if cables could be laid underground. Last year, Seehofer also began to promote the idea of his state becoming self sufficient in electricity generation, arguing for new gas power stations despite the huge surplus of generating capacity in the rest of Germany.
The opposition Green party in the Bavarian parliament called for Seehofer to be "brought to reason on energy policy" on 24 February, addressing the appeal to the highest level, to chancellor Angela Merkel and federal economy minister Sigmar Gabriel.
At the E-World energy fair in Essen on 10 February, Hildegard Muller, head of the managing board of energy and water federation Bundesverband Energie-und Wasserwirtschaft (BDEW) appealed to Germany's other state premiers to head off Bavaria's energy plan, which revolved around "autonomy through new gas power stations" rather than embracing the Energiewende and importing offshore and onshore wind energy from the north.
There are various reasons for the Bavarian blockade, according to Germany's offshore wind energy foundation, the Stiftung Offshore Windenergie. But the industry has been reassured by the federal energy regulator confirming in February, once again, that the HVDC north-south links are needed, a spokesman said. The Bavarian government also voted in the official transmission line expansion plan, Bundesfachplan 2013, which outlined the expansion of Germany's transmission networks, he added.
Some critics argue that Bavaria's underlying aim is to undermine the nuclear phase-out law and keep reactors online. Preventing new transmission lines, and thereby blocking imports of wind generation from northern Germany, could help create a local electricity shortage in Bavaria. Building new gas capacity is economically unviable due to low wholesale electricity market prices resulting from some 100GW of generation overcapacity in Europe, which would cause gas stations to operate at a loss. This means Bavaria could be granted a nuclear plant lifetime extension as the instant solution for generating power more cheaply.
At the same time, there is debate on whether a new electricity trading zone will have to be created in Bavaria because the shortage in north-south transmission capacity is causing transmission system operators to intervene in the market and redispatch power stations to keep the network stable. Without a clear plan for transmission network expansion, the additional cost of the redispatch carried by consumers nationwide is expected, at some point, to be rejected in favour of creating a new electricity trading zone in Bavaria.
But the inevitable higher prices reflecting the electricity scarcity created if the transmission grid links are not built will hardly be welcomed by industry in Bavaria, Germany's largest and most industrialised state.
The federal coalition parties now hope to reach an understanding on the new north-south transmission lines this month.
ELECTRICAL DISHARMONY - YEARS OF TRANSMISSION WOES FOR BARD 1 OFFSHORE
The BorWin 1 grid link and BorWin Alpha HVDC converter platform system linking the 400MW Bard Offshore 1 wind farm to the onshore network has been especially problematic.
Feed-in from the first clutch of turbines at the project began in December 2010. But more than four years later the project was still not operating at full capacity. Statkraft, which is marketing the power on the German wholesale energy exchange, EpexSpot, registered problems on as many as 17 days between 1 January and 19 February 2015 alone.
Completion of Bard Offshore 1, with all 80 turbines of 5MW connected, was celebrated in August 2013, but the saga of setbacks was by no means over. An outage of the BorWin Alpha converter platform for maintenance in October 2013 lasted nearly three months. That was followed by sporadic outages that culminated in a smouldering fire in March 2014 that had developed in an electrical component on the converter platform. Repair and cleaning took three months.
After Tennet had noted in June 2014 that the grid connection system could only transmit electricity without shutting down when the quality of the electricity lay "within the usual technical standards", a taskforce was set up to analyse and solve the problems. This joint effort involved Tennet, cable company ABB and Ocean Breeze Energy, the project company for Bard Offshore 1, which is in turn owned by Hypovereinsbank (part of Unicredit bank).
The taskforce identified that the instability of the grid link between Bard Offshore 1 and Tennet's BorWin Alpha was due to electricity harmonics. ABB developed new grid software that was installed by Tennet in early 2015 "which so far has resulted in stabilisation of the grid link system", according to an Ocean Breeze Energy spokesperson. "Whether the stability can be maintained under full load and normal operation will become clear as tests continue. We are cautiously optimistic," she said.
Full operation cannot yet be tested because Bard is not feeding in all its 400MW of power, and will not return to full operation until the summer.
After the long outage, the turbines are being re-commissioned step by step.
Negotiations continue between the parties over compensation for lost generation at Bard Offshore 1. But the company is not entitled to compensation for outages that occurred since October 2014, the point from which the grid link was declared to be functioning again.
OUTPERFORMING EXPECTATIONS - RECORDED OUTPUT FOR GERMAN OFFSHORE PROJECTS
The wind harvest in German waters has generally been better than originally expected, according to Jan Bachmann, head of the North Sea Fino 1 and Baltic Sea Fino 2 wind research platform project at Kiel Technical University's Research and Development Centre.
But the picture is complex. One of the results of long-term offshore measurements is that the seasonal average wind speed may vary for different years, says Andreas Beeken, offshore consultant at DNV GL.
Aside from wind speeds, output is affected by factors ranging from the technical availability of the turbines, substation and the grid to internal and external wake effects, turbine performance when cutting in and out of high winds, and outages due to failure or for repairs and maintenance.
The uncertainties involved perhaps account for the rather convoluted declarations of output from the offshore wind companies. Although only Alpha Ventus and Baltic 1 have been operating long enough to present even the beginnings of a track record, both have performed better than predicted.
Between 2011 to 2014, Alpha Ventus, using 5MW Areva and Senvion turbines, achieved output 7% above forecasts, project company DOTI said. The 60MW project's overall 994.9GWh output amounted to 16,582 full load hours, an average of 248.73GWh per year, equivalent to 4,146 full load hours a year.
In 2014 alone, Alpha Ventus generated 235.6GWh, equivalent to 3,927 full load hours a year. Although this was below the four-year average, due to replacement of several components on the turbines during the course of the year, it was still 1.5% above the original forecast of 232GWh a year, equivalent to 3,867 full load hours.
EnBW's 48.3MW Baltic 1 project performance was 204GWh (4,224 full load hours) in 2012, 191GWh (3,954 full load hours) in 2013, and 196GWh (4,048 full load hours) in 2014. Back in April 2011, when the first of the 2.3MW Siemens turbines was connected, EnBW said it expected the project to generate 185GWh (3,830 full load years) a year. So even the 2013 performance exceeded expectations.
A Bard Engineering presentation in October 2008 predicted Bard Offshore 1 would generate 1.7TWh a year, equivalent to 4,250 full load hours. But the project has not yet been able to demonstrate anything like its full potential.
In April 2013, RWE Innogy predicted its 295MW Nordsee Ost project, with 6.2MW Senvion turbines, will generate 1TWh per year, equivalent to only 3,390 full load hours per year. The Global Tech One developers are forecasting annual output of 1.4TWh (3,500 full load hours) from its 5MW Areva-equipped 400MW project, after taking into account reductions due to transmission losses.
Trianel is forecasting annual output of 8TWh (4,000 full load hours) from its Trianel Windpark Borkum 200MW phase one, which began generating electricity from 40 turbines in February.
Explicitly accounting for wind shadow effects from other wind sites, the MEG1 offshore project firm said in 2013 that 4,329 full load hours per year was expected. This was based on measurements at the Fino 1 research platform in "weak wind years 2011 and 2012", and the performance of Alpha Ventus in 2011.