A method for utilizing heat in a plant or animal growing device includes circulating a heat transfer fluid through a circuit forming a closed fluid loop, heating the heat transfer fluid by a heat source, supplying heat from the heat transfer fluid to a first heat user which may be a thermal desalination unit, and returning at least part of the heat transfer fluid that has been cooled down. The heat transfer fluid supplies heat to at least one additional heat user serially arranged before or after the thermal desalination unit. The temperature ranges of the heat transfer fluid are within the optimal operating temperature ranges of the respective heat users in the fluid circuit. A corresponding system and greenhouse by which the method of the invention may be implemented is also described.