R. Mastrullo

Energy costs and environmental impact joined to the substitution of CFCs in vapour compression plants

SYNOPSIS At present, the utilization of vapour compression plants in refrigeration and air conditioning fields may have a negative impact on the environment both in direct and indirect terms. In fact, polluting substances are released during the conversion processes which occur to make the necessary mechanical energy available; but it is also acknowledged that a great many of the usual working fluids take part in stratospherical ozone destruction and/or in the so-called “greenhouse effect”. Therefore, such problems are the subject of many studies and meetings in which international research workers, manufacturers and designers are involved. These studies especially aim at: i. an appraisal of actual and potential damage ii. the setting up of plants and/or alternative working fluids iii. a comparative evaluation in terms of energy efficiency and environmental impact (see 2) iv. localization of any potentially dangerous working practices. In this paper, energy costs and environmental impacts associated with ...

Experimental investigation on predictive models for motive flow calculation through ejectors for transcritical CO2 heat pumps

Nowadays, air conditioning systems, especially those used in residential and office buildings, contribute largely to the energy consumptions and to the direct and indirect emissions of greenhouse gases. Carbon dioxide (CO2) is an interesting option to replace traditional HFCs in vapor compression systems, due to its environmentally friendly characteristics: zero ODP and extremely low GWP. In the case of heat pumps, the use of ejection systems for the expansion phase can contribute to recovery a fraction of the mechanical energy otherwise dissipated as friction, bringing to significant benefits in terms of performance. Currently, at the laboratory DTE-PCU-SPCT of the research center ENEA (Casaccia) in cooperation with the Industrial Engineering Department of Federico II University of Naples, a project is in progress, in order to evaluate experimentally the effect of several ejectors geometries on the global performance of a CO2 heat pump working with a transcritical cycle. As a part of this project, measurements of the motive flow mass flow rate have been carried out, in transcritical CO2 conditions. The ejector sizing is a crucial point for the balancing of components and the correct operation of the CO2 heat pump and therefore the availability of reliable calculation methods for the motive flowrate would be useful. This paper presents the results obtained by a comparison between the new experimental data and the predictions of some predictive semi-empirical correlations available in the open literature for transcritical CO2 conditions. Their predictions are analyzed as a function of the main physical parameters of the process to assess their reliability compared to the experimental data. Based on these indications and of the available experimental data, a new semi-empirical correlations and a calculation method based on the hypothesis of isentropic and choked two-phase flow are presented.