Enabling Increased Variable Renewable Energy Penetration via Thermal Energy Storage Coupled with Nuclear Power

Abstract

Thermal energy storage (TES) used with baseload nuclear power plants to provide low-carbon flexible electricity and to support the expansion of variable renewable energy sources is analyzed. A thermodynamic model quantifies the impact of options for integration of TES into the Rankine power cycle on cycle behavior and capacity factor. The diurnal energy production ratio, equivalent to a relative capacity factor, is compared in a parametric study of operating conditions, including discharge power (up to 2 times the baseload power, charge duration (2-10 hours), discharge power, discharge duration and the round-trip efficiency of the TES (0.7 to 1), for three configurations. Configuration I charges the TES using high-pressure steam and discharges steam to the low-pressure turbine. Configuration II charges the TES in the same manner and discharges preheated condensate to the steam generator. Configuration III charges the TES using low-pressure steam and discharges the TES to a secondary cycle. Conceptual designs of sensible and latent heat storage devices are discussed with estimates of volume, mass and cost of the storage material. Configuration III has the highest energy production ratio over the entire parameter range. Use of a secondary cycle eliminates any penalty on baseload operation and reduces the penalty on turbines compared to discharge to the primary cycle. At a discharge power of 1.2 times baseload power, charge and discharge durations of 4 and 3 hours, respectively and a TES round-trip efficiency of 0.9, the energy production ratio is 0.99. Discharge powers up to approximately 1.6 times baseload power are achievable for these parameters. Configuration I can also reach high discharge power but at a lower energy production ratio. Configuration II is restricted to a discharge power of 1.1 times the baseload. The energy production ratio of all configurations decreases with increasing discharge duration and discharge power and increases with increasing TES round trip efficiency. Increased discharge power can be achieved through an increase in charging duration and TES round-trip efficiency and a decrease in discharge duration. Sensible heat storage is favorable with estimated costs of material from $5-20 per kWhe compared to $20-40 kWhe for latent heat storage. The narrow temperature range restricts latent heat storage materials to expensive hydroxide-based salts. Configuration II, although limited in discharge power and energy production ratio, requires the lowest thermal capacity, 950 MWhth to provide a discharge power of 1.1 times baseload power with an estimated cost of $4.87 per kWhe for a discharge duration of 3 hours, and round-trip efficiency of 0.9. At the same operating parameters, configurations I and III require a storage capacity of approximately 1500 MWhth at cost $9 and $21 per kWhe. The use of TES can be a transformative technology in the ability to convert baseload nuclear power plants to flexible generation sources for the support of renewable energy. All configuration of TES presented in this work allow for some degree of flexibility. This work demonstrates the importance of how the TES is integrated into the cycle on the energy production of the system, a key indicator of economic viability, the maximum discharge power and the required storage capacity.