Experimental Study of Heat Pump Thermodynamic Cycles Using CO 2 Based Mixtures -Methodology and First Results

TL;DR

This study investigates CO2-based mixtures in heat pump cycles, developing a non-intrusive optical method for real-time composition measurement, and presents initial results comparing pure CO2 with mixtures including propane and R-1234yf.

Contribution

It introduces a novel optical spectroscopy technique for dynamic composition analysis in heat pump cycles using CO2 mixtures, enabling better understanding of their thermodynamic behavior.

Findings

Pure CO2 cycle performance characterized.

Mixtures with propane and R-1234yf analyzed.

Method enables rapid, non-intrusive composition measurement.

Abstract

The aim of this work is to study heat pump cycles, using CO 2 based mixtures as working fluids. Since adding other chemicals to CO 2 moves the critical point and generally equilibrium lines, it is expected that lower operating pressures as well as higher global efficiencies may be reached. A simple stage pure CO 2 cycle is used as reference, with fixed external conditions. Two scenarios are considered: water is heated from 10 {\textdegree}C to 65 {\textdegree}C for Domestic Hot Water scenario and from 30 {\textdegree}C to 35 {\textdegree}C for Central Heating scenario. In both cases, water at the evaporator inlet is set at 7 {\textdegree}C to account for such outdoor temperature conditions. In order to understand the dynamic behaviour of thermodynamic cycles with mixtures, it is essential to measure the fluid circulating composition. To this end, we have developed a non intrusive method. Online optical flow cells allow the recording of infrared spectra by means of a Fourier Transform Infra Red spectrometer. A careful calibration is performed by measuring a statistically significant number of spectra for samples of known composition. Then, a statistical model is constructed to relate spectra to compositions. After calibration, compositions are obtained by recording the spectrum in few seconds, thus allowing for a dynamic analysis. This article will describe the experimental setup and the composition measurement techniques. Then a first account of results with pure CO 2 , and with the addition of propane or R-1234yf will be given.