Compensating losses in an offshore hydraulic windpower transmission pipeline through the exploitation of a natural thermal energy storage medium : deep sea water

Abstract

The use of hydraulic power transmission to transport offshore wind energy from deep offshore sites to shore using sea water pipelines may present a more feasible option for integrating wind farms with land-based hydro-energy storage systems. Yet, power transmission losses resulting from fluid friction are significantly larger than those encountered using electrical power transmission cables. This paper investigates the possibility of compensating for such losses by exploiting the cold deep sea water that is stored naturally below thermocline formations. The temperature of deep sea water is stable and sufficiently lower than ambient temperatures experienced in countries with warm climatic conditions. A numerical study simulating a single large-scale offshore wind turbine-driven pump supplying deep sea water to shore across a pipeline in a Central Mediterranean climate is presented. Sea water leaving the grid-connected hydro-electric power plant is allowed to flow through the condenser unit of a centralised district air-conditioning unit operating on a vapour compression cycle. Any shortfall in the supply of deep sea water due to lack of wind is compensated for by sea surface water to maintain a constant flowrate in the condenser. The performance of the air conditioning unit is compared to that of a conventional one operating on sea surface water only. The analysis is repeated for sea water pipelines having different diameters and lengths. It is shown that although the use of the deep sea water supply from the offshore wind turbine is intermittent, this reduces the energy consumption of the air conditioning system considerably. The resulting energy savings are found to compensate for a significant proportion of the losses encountered in the transmission pipeline. The degree of compensation depends on the pipeline diameter and length.