The standard of living and comfort at home in industrial countries strongly depend on available energy. Due to the fact that fossil energy ressources are running low and demand is increasing concurrently, alternative plans for power generation have to be developed.

Renewable energy provides a solution to this. Comparing with gross power generation in Germany in year 2007, their portion is only 15%, wherein wind power (6,2%) and hydro power (4,4%) are the main parts (s. Figure 1, according to sources: Statistisches Bundesamt; Bundesministerium für Wirtschaft und Technologie; BDEW Bundesverband der Energie- und Wasserwirtschaft e.V.; Statistik der Kohlenwirtschaft e.V.; AG Energiebilanzen e.V.). According to a resolution by the Bundestag from 06 of June 2008, this amount of renewables has to top 30% in year 2020 /1/. Within the framework of the sustainability strategy, the Federal Government focus on offshore-windenergy /2/.

In spite of better meteorologic conditions offshore, wind power is still dependent on the weather, which results in a mismatch between energy production and demand (s. Figure 2).

Such a thing might happen when a dead calm exists in times with high system load for instance during morning hours. This has to be compensated with conventional power plants. Conversely, this means that a system overload happens.

Moreover, it is annotated that major investments for the onshore grid connection occur due to the long distances from the offshore windpark to the shoreline. It is required because nature reserves are coastal in Germany and in many cases tourists do not want to see windmills at the horizon.

For this reason, researcher at the Technical University of Clausthal (TUC) have developed a concept to compensate fluctuations in the power generation from offshore windparks and to match better with the actual demand. It enables the integration large offshore wind parks into the power grid on the mainland.

In a research project under the authority of the Federal Environment Ministry it was investigated wether an integration of diverse energy resources into an offshore combination power plant even makes a base load supply possible.

The fundamental idea at the researched concept is a balancing of wind fluctuations in the immediate vicinity of the wind park to utilise fully the transmission capacity (s. Figure 3 and 4).

In case that wind energy is more than the system demands, this surplus can be converted. For this, a compressor is powered which pumps air into a cavern (man-made subground excavations). This process can be compared with a bagpipe enabling the player to play continuous sound for a while. Mainly, these caverns are solution mined in salt. Hence excessive wind energy can be stored temporarily until wind power feed of the grid is falling. Then, the compressed-air is expanded by a turbine which is driving a generator. With this method, heat is produced by compressing the air and it is emitted to the environment. Furthermore, the relaxed air has to be preheated to avoid icing of the aggregate. Therefore, it is usefull to capture also the heat of compression in a storage tank to heat up the air during its discharge of the cavern. This technology is called adiabatic compressed air energy storage and its efficiency is better than the diabatic one.

Owing to the limited storing capacities of a cavern and the slower generation of electrical energy with this principle with reference to changes in wind speed, another component of this combination power plant is the conversion of fallow-gas to electricity. In the North Sea, several gas deposits contain a high proportion of molecular nitrogen (N₂) what reduces the calorific value below 8,5 MJ/Nm³. Due to this high concentration of (N₂), a direct exploitation is not possible and transport via a pipeline to the shore is gainless. But according to proven examples, this gas mixture can be still combusted in a turbine. This would provide an obvious solution to generate electric power on-site with fallow-gas in addition to wind power and compressed air storage.