Solar-driven thermochemical water splitting cycle is a promising, energy efficient and environmentally friendly approach to produce hydrogen. In this paper, numerical work has been undertaken using a cylindrical solar receiver to investigate fixed and variable aperture sizes to maintain constant steady-state temperature over a day for thermochemical part of a novel hybrid photo-thermochemical sulfur-ammonia cycle. A previously developed and validated optical model in commercial software, TracePro? is used to simulate the light sources of 10, 15, and 28 kW. The sunlight intensity variations for the designated reference day for this study is selected as July 1, 2011, at 39.74 N, 105.18 W and at an elevation of 1829 m. A developed and validated finite volume based coupled Monte Carlo, Heat Transfer model is used to calculate the steady-state temperatures in the receiver by utilizing the output of the optical model. The simulations are performed at different aperture diameters from 2 to 14 cm to quantify the effect of fixed aperture size on the steady-state temperatures of the receiver. Furthermore, simulations to maintain steady-state temperatures of 673, 823, and 1123 K for different sub-cycles of the selected cycle via variable aperture has been performed and compared with selected fixed apertures. It is found that the variable apertures can maintain desired constant temperatures over the day for each thermochemical sub-cycle. The comparison of overall power consumption and savings for fixed and variable apertures has also been investigated and reported.