Abstract: A capillary pumped heat transfer loop involved an evaporator, a condenser and a reservoir tank for storing a heat transfer fluid, the evaporator being provided with a porous material for generating capillary pumping pressure within the loop; the tank and the evaporator are mutually thermally insulated and interconnected via a channel that has as a first portion a capillary connection for pumping the heat transfer fluid from the tank to the porous material, and has a second portion for conveying gas bubbles and/or steam from the evaporator to the tank, the tank being arranged to be kept at a lower temperature than the evaporator.