Abstract: A pump driven by the radiation energy of the sun, wherein the radiation energy is converted into mechanical energy by means of a liquid evaporated by the effect of the radiation heat. The pump has a face that absorbs radiation and a casing part whose inside space is divided into two parts by means of a flexible and elastic membrane that can be moved back and forth. One of the parts constitutes the intake space into which the liquid to be pumped is sucked during the intake phase while the membrane moves so that the intake space becomes larger. The intake space is connected to an intake opening provided with a valve, which valve permits flow only into the intake space, and to an exhaust opening provided with another valve, which other valve permits flow only out of the intake space. A tightly sealed evaporation space, which contains evaporable liquid, is placed between the casing and the membrane against the membrane, whereby the vapor pressure of the evaporable liquid directly moves the membrane.