Performance modelling of an offshore floating wind turbine-driven deep sea water extraction system for combined power and thermal energy production : a case study in a central Mediterranean context

Abstract

Current research is exploring a new design concept for offshore wind turbines whereby the electrical generator in a conventional wind turbine is replaced by a large positive displacement pump that supplies pressurized sea water to a centralized hydro-electric plant. This paper investigates the potential of applying this concept to concurrently exploit thermocline thermal energy through deep sea water extraction in conjunction with offshore wind energy. A performance analysis is presented for a single wind turbine-driven pump supplying combined power and thermal energy by delivering pressurised deep sea water to a land-based plant consisting of a hydro-electric generator coupled to a heat exchanger. The steady-state power-wind speed characteristics are derived from a numerical thermo-fluid model. The latter integrates the hydraulic characteristics of the wind turbine-pump combination and a numerical code to simulate the heat gained/lost by deep sea water as it flows through a pipeline to shore. The model was applied to a hypothetical megawatt-scale wind turbine installed in a deep offshore site in the vicinity of the Central Mediterranean island of Malta. One year of wind speed and ambient measurements were used in conjunction with marine thermocline data to estimate the time series electricity and thermal energy yields. The total energy yield from the system was found to be significantly higher than that from a conventional offshore wind turbine generator that only produces electricity. It could also be shown that in regions where the offshore wind resource is not as rich, but where the ambient temperature is high as a result of a hotter climate, the cooling energy component that can be delivered is relatively high even at periods of low wind speeds.