Experimental bench design for heat pump using CO2 based mixtures.

Efficiency of CO2 (R-744) heat pumps for domestic hot water production is well known, but this technology shows rather low performances when applied to central heating of buildings. Indeed, efficiency of CO2 cycles is, among other parameters, linked to the gas cooler outlet temperature: the coldest it is, the higher is the coefficient of performance (COP). The return temperature of central heating circuits (30 to 60°C depending of the heat emitters) is not cold enough to fit usual conditions for good performances of CO2 transcritical cycles. This study is about modifying characteristics of CO2 cycles, using CO2 based mixtures. The main objective is to obtain higher efficiency for central heating production, without degrading performances of domestic hot water generation. Adding other chemicals to CO2 refrigerant moves up critical point of the fluid, and generally equilibrium lines. This leads to modifications of the cycle which can be used in order to improve the efficiency of central heating as well as domestic hot water generation. This article presents the facility established to study the thermodynamic cycles using CO2 based mixtures. The experimental bench has the particularity of analysing in-line fluid composition in several points of the cycle using infra-red spectrometry. First results are planned for mid 2015.