Energy Storage Solutions for Wave Energy in Stand Alone Systems
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- Institutt for elkraftteknikk 
Energy Storage technology is a key to further development of renewable energy sources. The objective of this master thesis is wave energy in a Stand Alone System. Two point absorber systems developed by FOBOX, Fred Olsens wave energy project, are used to illustrate the challenges in the power production from wave energy. There are two main concerns considered in this thesis. The Stand Alone System must be able to deliver power whenever it is needed, not only when there are waves. The second concern is that it can deliver power with sufficient quality. Both concerns can be solved by energy storage systems, respectively long- and short-term. Long term energy storage solutions are for instance pumped hydro, CAES and some types of batteries. Short term energy storage systems could be super capacitors, flywheels, SMES and also some types of batteries. The focus on Stand Alone Systems in an isolated grid is chosen for its` beneficial environmental and economical potential. Alternatives for power supply on islands are uneconomical, such as laying cables out to an island. Use of fossil fuel in diesel aggregates is problematic in an environmental concern. In addition islands often have a unique potential for renewable energy with good natural resources of energy and benefits related to promotion of renewable energy worldwide. Both point absorber cases proposed in this thesis are built up respectively by a three phase generator, a rectifier, an energy storage system with appurtenant DC/DC converter and finally an inverter. The converter proposed, made by Guidi Guiseppe, is a half controlled cascade with bidirectional power flow. This converter is very attractive in terms of cost and efficiency. It is proven that the fluctuations from 7 wave energy platforms with a power production on several MW, require very expensive and huge energy storage solutions. Therefore alternatives to smooth the power is required. It can be done by spatial aggregation of the platforms, so the power is smoothed out to an acceptable level for storing. A super capacitor is used to achieve higher power quality in a short term period. Long term variations are taken care of by a NaS battery. The last case discussed is a smaller system of 3 floating wave energy buoys supplying 12 houses on an island. They produce an average power at 20 kW. The short term fluctuations are in this case also handled by super capacitors, but for the synchronization between consumption and production, cheap Lead-Acid batteries are proposed.