In the Fluid Test Laboratory at Università degli Studi di Brescia, innovative working fluids and mixtures for power generation and conditioning systems are experimentally tested to assess their thermal stability limits. The laboratory provides qualified scientific research and technical consulting.
As consequence of the Montreal Protocol on substances that deplete the ozone layer (1987), the production of numerous substances that are responsible for ozone depletion was phased out.
Concerns on global warming provide further constraints in terms of Global Warming Potential (GWP) on the selection of fluids or mixture to be employed in power plants, heat pumps and refrigeration systems.
Other aspects such toxicity, costs, flammability and thermal stability should be also take into account.
Thermal Stability Procedure
The method adopted to assess the thermal stability of a working fluid or mixture, is based on the analysis of the deviations in saturation pressure curves that may occur after subjecting the fluid to thermal stress tests at increasing temperature. The set of activities that define a thermal stability test can be grouped into four steps:
(a) set up of the test circuit filled with the sample fluid
(b) evaluation of the reference vapor pressure of the virgin fluid
(c) thermal stress test in a furnace
(d) measurement of the vapour pressure curve and comparison to the reference value
The experimental apparatus is composed of two main sections:
(i) a stainless steel measurement setup where the fluid is loaded, tested and characterised through temperature and pressure measurements;
(ii) two temperature controlled environments that consist of, respectively, a thermostatic bath for vapour pressure measurements and a muffle furnace for isothermal stress tests.
The measurement setup is composed of a cylinder containing the sample fluid to be tested, a type K thermocouple for fluid temperature measurements, two pressure transducers with the relative maximum operating pressures of 1 bar, and 50 bar.
"The right working fluid - with its thermophysical and thermochemical properties - is the very core of heat engines operating in closed thermodynamic cycles. If it is not well characterised, not only the performances degrade, but also the whole engine could be compromised."