The present invention provides a process for absorbing solar energy and, simultaneously having a continuous control of the light admission making it possible to install the device in external openings of buildings and equipment, such as windows, shutters or skylights. The system comprises two frames which support plates of transparent material, between which it is possible to introduce a colourful solution coming from an external reservoir. Both frames are interconnected through a flexible membrane which allows the plates to be pushed against each other, making the window transparent, or pushed away interposing the colourful solution, which will make the window uniform and gradually translucent or opaque. The said liquid comes from a reservoir and the access to the gap between the plates through is obtained by means of a channel that separates the two frames, allowing the liquid to flow evenly into the whole surface, thus being a homogenous and adjustable darkening obtainable. The inflow or outflow of the liquid between the plates is ruled by the vacuum level inside the reservoir containing the colourful solution. When the window is opaque or partially transparent, the solar energy heats the liquid which is then forced to flow by a circulating pump, passing through a heat exchanger which allows the energy absorption.