This work introduces a new concept for a utility scale combined energy storage and generation system. The proposed design utilizes a pumped thermal energy storage (PTES) system, which also utilizes waste heat leaving a natural gas peaker plant. This system creates a low cost utility-scale energy storage system by leveraging this dual functionality. This Part A of a two-part paper presents a review of previous work in PTES as well as the details of the proposed integrated bottoming and energy storage system. A time domain system model was developed in Mathworks R2016a Simscape and Simulink software to analyze this system. Validation of both the fluid state model and the thermal energy storage model are provided. The experimental results showed the average error in cumulative fluid energy between simulation and measurement was +/- 0.3% per hour. Comparison to a Finite Element Analysis (FEA) model showed <1% error for bottoming mode heat transfer. Part B of this two-part paper gives a detailed characterization of the integrated bottoming with energy storage system. (C) 2016 Elsevier Ltd. All rights reserved.