The role of thermal storage in nuclear hybrid energy systems

Electricity systems with increasing shares of variable renewable energy sources are characterized by stronger price volatility, more frequent periods of very low or negative prices, and a growing need for operational flexibility. In this context, large light-water nuclear plants, traditionally operated as steady baseload units, may face reduced market value if they cannot adapt output to price signals without incurring efficiency penalties, additional wear, or constraints on safe operation. Thermal Energy Storage offers a pathway to decouple reactor heat production from electricity generation, allowing the nuclear island to remain at high thermal utilization while shifting electrical output in time. This thesis explores Thermal Energy Storage as a flexibility option for nuclear systems and frames its potential both as a market-value enhancer and as a tool to reduce the occurrence of uneconomic operation during oversupply periods. The work first reviews integration concepts and key design choices for coupling Thermal Energy Storage with nuclear plants. It then develops and evaluates a specific hybrid configuration in which a 1GWe nuclear unit charges a two-tank molten-salt Thermal Energy Storage via a steam Compressor Heat Pump, optionally supported by an Electrical Heater, and discharges through a dedicated peaker steam Rankine cycle. The economic performance is quantified with an optimization model that determines the optimal dispatch and sizing of components under representative electricity market conditions, reporting operational indicators (Round Trip Efficiency, Capture prices, Capture rates and Capacity Factors) and economic metrics (Levelized Cost Of Energy). Four scenarios (no storage, integration with Electrical Heater, integration with Heat Pump, integration with both Electrical Heater and Heat Pump) are assessed for three European spot markets (Poland, Germany, France). Results indicate that TES systematically reduces exposure to negative-price periods by diverting part of the reactor output to heat storage and helps generating electricity during higher-price hours. Heat-pump charging delivers high electric round-trip efficiencies, whereas heater-only charging remains lower. Optimal operation requires extracting a limited share of boiler steam for the Compressor Heat Pump and baseload turbine electricity for both the Heat Pump and the Electrical Heater, when deployed. Economically, HP-based configurations can increase the Total Capture Price of the integrated system but also introduce significant additional costs, making profitability sensitive to assumed inputs. Under an energy-only arbitrage framework, the investment case is strongest in markets with higher price volatility and may require complementary revenues as prices spreads narrow.