Control of adsorption chillers by a gradient descent method for optimal cycle time allocation

Abstract

Adsorption-based energy systems allow environmentally friendly heating and cooling by transforming solar or waste heat. However, solar and waste heat sources are often fluctuating over time. In this case, an optimal operation policy has to be identified. This operation policy has to consider the intrinsic dynamics of adsorption-based energy systems: the cyclic switching between adsorption and desorption phases. This work therefore presents a feedback-control strategy for adsorption-based energy systems. A two-step approach is employed: First, the original nonlinear dynamic process model is approximated by a set of linear ordinary differential equations (ODEs). In a second step, an optimization problem based on the linearized equations is formulated. The latter is solved approximately by a sequence of gradient steps. We apply the method to models for a solar cooling adsorption chiller and a solar collector. The method is shown to reliably provide optimal control and to be robust for a wide range of the investigated parameters.