Linking Alpine Reservoirs to Ocean (LARO)

The research project Linking Alpine Reservoirs to Ocean (LARO) includes basic investigations for an innovative energystorage conceptcalled BuoyantEnergy (http://www.buoyant-en- ergy.com/english/home.html). This concept, which was invented at the University of Innsbruck, should bring the common pumped-storage hydropower technology to store electric energy directly to the ocean and close to the offshore wind power generation. It bases on big objects floating near the energy generation, which are filled with a lower inner water level. When pumping the water out of the structure by use of the generated energy, the complete system is lifted up. This stored energy can be returned with inverting the flow direction through turbines and providing the electric energy to the grid, when it is needed. This helps to stabilise the electrical grid and to support a further expansion of renewable energies. The difference between the inner and outer water level is nearly constant hence only the position of the floating structure is changed. This allows running the turbines and pumps always at an optimal point and helps to reduce the losses of the storage. As a main outcome of the project LARO, fundamental design criteria for the construction of such water filled floating structures should be found, which can help with the realisation of this storage system. The project is conducted at the University of Innsbruck with the applicant Dr Roman Gabl as well as at two universities in the United Kingdom. The main research institute abroad, the University of Edinburgh, offers, apart from the outstanding competence in coastal engineering and numerical modelling of waves (Prof. David Ingram; Professor of Computational Fluid Dynamic CFD), a special test bed (FloWave, http://www.flowavett. co.uk) to corroborate numerical simulation results. Its circular tank provides a unique opportunity to augment competence in the field of wave modelling and measurements. In addition, Dr Valentin Heller from the University of Nottingham, who is currently working with the SPH-method and shares the research interest of the applicant in the field of impulse waves, is included as an additional advisor for the project. The central research hypothesis for this project is that the use of the meshless SPH-methodology (smoothed particle hydrodynamics) will improve the prediction of the movement of water filled structures, thus helping with the optimisation of their design. This work will include a comparison with common grid-based CFD (computational Fluid Dynamics) techniques. These numerical simulations, for which the movement of the fluid is approximated with the help of computational algorithms, will be assisted by validation experiments to check the results.

The project Linking Alpine Reservoirs to Ocean (LARO) provided an additional step to realise the innovative storage technology Buoyant Energy (http://www.buoyant-energy.com). The concept is to store energy by pumping water out of a floating structure and then to recover this energy with a turbine. Such a pump storage power plant, without the large height difference that's normally needed, helps to buffer the available renewable energy production and balance generation and consumption. It can either be installed close to the offshore renewable energy production or close to the consumers. For coastal cities this has the additional benefit that those floating structures can be used for a wide range of applications including housing as well as industry. The constant water level difference between the outside and inside water level is a significant advantage from a hydraulic point of view. This allows to optimise the pump and turbine to achieve excellent efficiencies. On the other hand, the inside water has the potential to start to interact with the floating structure and result in an unwanted motion response. The LARO project could fill the research gap by providing a validation experiment conducted at FloWave Ocean Energy Research Facility (University of Edinburgh Scotland, UK). It combines the fluid-water-fluid-interaction as a fully coupled system with big motions introduced by waves. A cylindrical basic geometry was chosen to limit the input parameters to wave height and frequency as well as filling level inside of the structure. To allow a good comparability with different numerical approaches, a soft mooring system was chosen to simulate a system as close as possible to free-floating. This resulted in the need to capture the free water surface inside a floating cylinder in the middle of a circular wave tank (with a diameter of 25 m) without adding too much weight. A combination of bespoke made wave run-up gauges as well as floating marker balls, which were captured with the motion capturing system, were introduced. The main tests included the comparison of solid and liquid ballast under different wave conditions. An interesting motion response based on the sloshing of the inside water could be identified and was further investigated in additional experimental investigations. The results were published in multiple papers and can be used as a validation experiment for comparable numerical investigations. A key output from the project was that the assumption of a solid ballast instead of water is acceptable for a wide range of motions. This allowed to focus the subsequent experimental investigations of a hexagonal structure, which is also available as a validation experiment. Therewith the LARO project could provide an important step to the realisation of the development of the floating energy storage solution of the future.