Concentrated solar energy finds applications for power generation and as a source of heat for solar thermochemical processes. However, solar energy reaching the earth's surface is intermittent and fluctuates with weather conditions, position of the sun throughout the day and other seasonal changes. This causes a major drawback in receiver efficiency as semi-constant temperatures are required for efficient operation of solar thermochemical processes. This paper introduces a new variable aperture mechanism which is coupled with a heat exchanger to collect unused heat during peak times. The paper presents an optical and heat transfer analysis of the concept using Monte-Carlo ray tracing technique via TracePro and an in-house developed heat transfer code. The heat transfer analysis of the proposed concept shows the optimum aperture diameter with a compromise between reactor temperature and reradiation losses. It also predicts the losses incurred by the variable sized aperture mechanism when the incoming solar radiation changes.