Online, Non-Intrusive Composition Measurements of Circulating CO2 Based Mixtures in an Experimental Heat Pump by Means of Infra-Red Spectroscopy
Résumé
Using CO 2 based mixtures as a working fluid in refrigeration and heat pumps is a potential alternative to the use of
traditional fluids which are environmentally problematic. In order to investigate the efficiency of such systems, we
have designed an experimental heat pump equipped with different measurement devices. In the present article, we
will focus on the mixture composition measurement technique. In order to fully understand the behaviour of such a
heat pump, it is necessary to measure the composition of the mixture at each point of the cycle. In order to do so, we
have used a technique based on infra-red spectroscopy and chemometry. At each point in the loop, we have installed
optical cells equipped with transparent windows and optical fibres. The spectra are recorded by a Fourier transform
infra-red spectrometer. The mixtures composition is then extracted from the spectra by the partial least squares
(PLS) method which is now common in analytical chemistry. But, beforehand, the PLS method has to be calibrated.
This is done by recording a statistically meaningful set of spectra on samples of known composition. Then a model
can be derived to relate the compositions to the characteristics of the spectra.
The calibration stage can be performed independently on separate well controlled samples. In our case, the
calibration has been carried out directly on the experimental loop. To this end, micro sampling devices (ROLSI TM)
have been installed in different places next to the optical cells. During this calibration process, the composition of
the fluid is measured by gas chromatography at varying conditions and compositions along with the recording of the
spectra.
Once this calibration is done, the composition can be measured in short times, typically of the order of the second by
the spectroscopic technique. This allows for a dynamic analysis of the working conditions.
This article will describe in detail the experimental set up and the calibration process for a mixture of CO 2 and
propane on the composition range of interest at temperatures ranging from 263 K (-10 °C) to 393 K (+120 °C) and
pressures from 25 to 100 bar and the expected accuracy will be discussed.
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