A method for treating a fluid being salt water, whereby the salt water is divided into at least a first and second fluid part, wherein the first fluid part is desalinated water and the second fluid part is brine, wherein the brine has a larger amount of salt than the salt water, the method comprises the steps of: - flowing a flow of salt water along a solar panel being heated by solar energy towards and into a fluid treatment device; the salt water being cold relative to the solar panel; - at least heating the flow of salt water in the fluid treatment device to evaporate a part of the flow of salt water into a vapour of evaporated desalinated water, whilst brine remains at a bottom of the fluid treatment device, - flowing of a flow of the evaporated desalinated water being the first fluid part of the salt water and a flow of remaining brine being the second fluid part of the salt water from the fluid treatment device in a heat exchanger along the flow of salt water, wherein the direction of the flows of the evaporated desalinated water and brine is opposite to the direction of the flow of salt water; - whereby heat from the relatively warm solar panel, the relatively warm flow of the desalinated water and the relatively warm flow of the brine is being used to preheat the flow of salt water before entering the fluid treatment device, characterized in that the solar panel is a photovoltaic solar panel being located against the heat exchanger, whereby by means of the photovoltaic solar panel solar energy is converted into electrical energy, which electrical energy is at least partly used to operate at least partly the fluid treatment device, wherein the photovoltaic solar panel is being cooled down by the heat transfer to the relatively cold fluid, when the relatively cold flow of salt water is flowing through the heat exchanger. - flowing a flow of salt water along a solar panel being heated by solar energy towards and into a fluid treatment device; the salt water being cold relative to the solar panel; - at least heating the flow of salt water in the fluid treatment device to evaporate a part of the flow of salt water into a vapour of evaporated desalinated water, whilst brine remains at a bottom of the fluid treatment device, - flowing of a flow of the evaporated desalinated water being the first fluid part of the salt water and a flow of remaining brine being the second fluid part of the salt water from the fluid treatment device in a heat exchanger along the flow of salt water, wherein the direction of the flows of the evaporated desalinated water and brine is opposite to the direction of the flow of salt water; - whereby heat from the relatively warm solar panel, the relatively warm flow of the desalinated water and the relatively warm flow of the brine is being used to preheat the flow of salt water before entering the fluid treatment device, characterized in that the solar panel is a photovoltaic solar panel being located against the heat exchanger, whereby by means of the photovoltaic solar panel solar energy is converted into electrical energy, which electrical energy is at least partly used to operate at least partly the fluid treatment device, wherein the photovoltaic solar panel is being cooled down by the heat transfer to the relatively cold fluid, when the relatively cold flow of salt water is flowing through the heat exchanger.