Frequency-domain heat current method for dynamic analysis of thermal systems

Understanding dynamic performance of thermal systems is crucial for enhancing energy system flexibility and integrating renewables, but conventional time-domain simulations face the trade-off between accuracy and efficiency. This paper proposes a Fourier transform-based frequency-domain heat current method to tackle this challenge. The frequency-domain heat current model for heat exchangers is first derived by combining Fourier transform and electro-thermal analogy, while other processes are described by using steady-state models considering their short characteristic times. These component models are then assembled by integrating transfer function matrices to construct a system-level frequency-domain model. An algorithm is developed for the efficient solution of frequency-domain system models, where equations with distinct mathematical properties are solved by using suitable methods. Two case studies of thermal systems are conducted to validate the proposed method and demonstrate its superiority. For the case of pumped thermal energy storage system, the proposed method achieves an acceleration of one to two orders of magnitude over the finite difference method (FDM) simulation with comparable accuracy. For the case of S-CO2 power generation system, the maximum deviations of node temperatures compared to experimental data and FDM simulation are within 1 K, showing the capability of proposed method on more complex cases. Moreover, the proposed method's computational time remains relatively stable regardless of the simulation duration and can be further reduced through parallelization. It offers a powerful tool for dynamic analysis and optimization by favorably balancing accuracy and efficiency for complex thermal systems.