Modelling and analysis of the hydraulic energy conversion processes for offshore hydro-pneumatic energy storage systems

Abstract

Energy storage systems are imperative for addressing instability issues in the electricity network arising from a high penetration of intermittent sources of renewable energy, such as wind and solar power. The integration of storage will avoid the curtailment of renewable energy production and revenue loss during periods of low energy demand and grid congestion. This thesis investigates the performance of an offshore Hydro-Pneumatic Energy Storage (HPES) system consisting of a subsea accumulator pre-charged with compressed air. Unlike conventional pumped hydro systems, HPES systems operate under a highly variable head. As a result, designing the hydraulic machinery to be able to maintain high energy conversion efficiencies over a wide pressure range is a major engineering challenge. The present study applies numerical modelling to understand the hydraulic performance of a megawatt-scale, topside Energy Conversion Unit (ECU) of an offshore HPES system when used to smoothen the intermittent supply of energy from offshore wind and solar parks. The ECU comprises a centrifugal pump to store excess energy in the HPES system and a Pelton turbine to convert the stored energy back into electricity during periods of low renewable energy production. Three different numerical models are used to investigate the ECU performance: The first model (Alpha) is a quasi-steady state and computationally efficient model of the storage system implemented in PythonTM. The second model (Beta), modelled in MATLAB® Simulink® and SimscapeTM is a more comprehensive model which provides more realistic operation due to the addition of transients, losses and inertias. The third model (Alpha Plus) is the Alpha model that has been upgraded to integrate Time Series Forecasting to simulate a smoothened power output from an intermittent power input. The simulations have shown that the power absorbing capacity of the large scale centrifugal pump is highly dependent on the state of charge (pressure) of the subsea HPES accumulator. Additionally, a single centrifugal pump is limited in its ability to meet the smoothing requirements of the wind turbine when operating under the HPES system’s variable head constrains. While the Pelton turbine model showed excellent operational flexibility over a wide range of pressure and power levels, maintaining a high efficiency over the same wide range with a centrifugal pump remains challenging given that the pressure and power are hydraulically coupled. The final stage of this study involved a techno-economic feasibility assessment of a floating breakwater integrating the proposed HPES system in deep waters. The proposed hybrid breakwater is modelled to generate multiple revenue streams, including the provision of energy storage services to offshore wind and solar parks as well as sheltered waters for facilitating multi-use of space at sea. The hybridisation is aimed at reducing costs for integrating the HPES offshore. However the technoeconomic assessment showed that the overall revenue generation, including that involving the use of the storage system, is insufficient to cover the high investment costs of the hybrid breakwater.