Solar heating of grape berries and other spherical fruits

Abstract The components of the energy balance at the surface of spherical fruits are presented and their relevance in the field situation is discussed. Of these, forced convection is the most complicated, varying over the sphere's surface, depending on wind direction, velocity and turbulent intensity, and the presence of adjacent bluff bodies. The flux density of solar radiation and wind velocity are the two most important environmental parameters in determining fruit temperature. Of lesser importance are fruit size and albedo, wind direction, fruit transpiration and thermal exchange by long-wave radiation. Based on an energy balance at the surface, simple relationships are proposed to predict maximum and minimum temperature increases on the surfaces of sunlit fruits. These relationships are functions of flux density of absorbed radiation, fruit size and thermal conductivity, and a convective heat transfer coefficient that may be calculated from wind velocity using equations developed by Nobel (1975). Good agreement is found between the simple model and an infinite series analytical model of the system, and also with laboratory measurements of berry temperatures. Measurements on green grape berries in the field showed maximum skin temperatures of the order of 12°C above air temperature, with a gradient of 3°C across sunlit berries. Berries on tight clusters heated more than those on loose clusters.