The purpose of the present work is the investigation of the effect of the outdoor air temperature and relative humidity on the performance of an air heat pump, when the reverse-cycle defrosting is considered. The frost formation process has been analyzed by developing a simplified model which relates the number of defrost cycles to the outdoor air conditions. Moreover the energy consumption due to the defrosting has been taken into account in the evaluation of the heat pump performance. The results, carried out for many Italian sites, point out that the outdoor air conditions play an important role in determining the amount of defrost cycles; however the frost formation is mainly affected by the relative humidity. The analysis highlights also that the defrosting contribution on the heat pump performance is not negligible when the heat pump that operates in wet weather, although cold; in these conditions the hourly COP may be reduced by up to 20%. However, this effect becomes less relevant, but not negligible, when the seasonal heat pump performance is evaluated; the maximum decrease of SCOP, observed for the all analyzed cases, is less than 13%

This paper outlines a numerical model for determining the dynamic and thermal performances of a rarefied fluid flowing in a microduct with elliptical cross-section. A slip flow is considered, in laminar steady state condition, in fully developed forced convection, with Knudsen number in the range 0.001−0.1, in H1 boundary conditions. The velocity and temperature distributions are determined in the elliptic cross-section, for different values of both aspect ratio γ and Knudsen number, resorting to the Comsol Multiphysics software, to solve the momentum and energy equations. The friction factors (or Poiseuille numbers) and the convective heat transfer coefficients (or Nusselt numbers) are calculated and presented in graphs and tables. The numerical solution is validated resorting to data available in literature for continuum flow in elliptic cross-sections (Kn = 0) and for slip flow in circular ducts ($\gamma =1$ ). A further benchmark is carried out for the velocity profile for slip flow in ellipticalcross-sections, thanks to a recent analytical solution obtained using elliptic cylinder coordinates and the separation of variables method. The Poiseuille and Nusselt numbers for elliptic cross-sections are discussed. The results may be used to predict pressure drop and heat transfer performance in metallic microducts with elliptic cross-section, produced by microfabrication for microelectromechanical systems (MEMS).