Abstract: A heat exchanger for use with an electronic heat pump including a plurality of narrow channels defined between adjacent walls. The channel heights are between 0 mm and 10 m to ensure thermal resistance is below 0.1° C./W for a 40 mm heat exchange width. The acceptable geometry for the channels is described by one or more variables, including the minimum number of channels and the minimum wall thickness for a given channel width. These variables control the surface area of the boundary of the channels in direct contact with the fluid and the cross-sectional area available for transmission of heat by conduction up the walls of the channels.