Pelletized thermochemical energy storage media has the potential for long duration energy storage. The production of charged solid state energy storage media can be done within a cylindrical cavity chemical reactor that captures concentrated solar radiation from a solar field. Temperature stability of a solar reactor is directly influenced by the solar flux intercepted. This paper presents a low-order physical model to simulate the dynamic response of temperature inside a tubular plug-flow reactor prototype. Solid granular particles are fed to the tube from the top whereas a counter-current flowing gas enters the tube from the bottom. An in-house code was developed to model transient heat transfer of the tube wall, gas, and moving particles. The model was preliminarily validated with packed beds for different temperatures ranges and two gas flow rates. Dynamic response of the reactor temperature is simulated for different input power and gas/particle flow rates. The results show that the system response can be controlled efficiently by utilizing input power (solar flux) as a control parameter. A conventional PI controller is designed to control the temperature inside the reactor and to maintain it during the solar flux intermittency. Controller parameters are tuned using the Ziegler-Nichols method to ensure optimal system response. The results show that the feedback control model is successful in tracking different reference reactor temperatures within reasonable settling time of 30 minutes and eliminated overshoot. This study can be extended to include a hybrid reactor with a multi-input, multi-output variable system.