  Dew-harvesting technology radiatively cools a condenser below the dewpoint to
achieve condensation of the water vapor from the atmosphere. Due to its passive
nature, this technology has attracted a broad interest, in particular in the
context of the worldwide drinking-water scarcity. However, the fundamental
limit of its performance has not yet been clarified. Moreover, the existing
applications have been limited to humid areas. Here, we point out the upper
bound of the performance of this technology by carefully considering the
spectral directional atmospheric transmittance in a wide range of parameters
such as the ambient temperature, the relative humidity, and the convection
coefficient. Moreover, we highlight the potential of a condenser consisting of
a selective emitter, which is capable of condensing water vapor under
significantly more arid conditions as compared with the use of a blackbody
emitter. For example, a near-ideal emitter could achieve a dew-harvesting mass
flux of 13 gram per square metre per hour even at ambient temperature of 20
degrees Celsius with relative humidity of 40%, whereas the black emitter cannot
operate. We provide a numerical design of such a selective emitter, consisting
of six layers, optimized for dew-harvesting purposes.
