Addressing solar power plant heat transfer fluid degradation: Experimental measurements of hydrogen transport properties in binary eutectic biphenyl/diphenyl ether

摘要

High-temperature decomposition of receiver field heat transfer fluids (HTFs) can cause hydrogen build-up in the receiver annulus of Concentrating Solar Power (CSP) parabolic trough power plants. This build-up increases the receiver thermal losses and results in a decline in power output. Prior numerical work has shown that removal of hydrogen from the head space gas (HSG) in the plant expansion tanks can be an effective mitigation strategy. This approach requires a matching molar rate of hydrogen coming out of solution from liquid HTF into the HSG. Simulations show that the driving force introduced by removing hydrogen from the HSG results in sufficiently large hydrogen transport rates solely across the surface interface (with no active mixing or bubbling). However, uncertainties in Henry's Law Constant and especially mass transfer coefficient require obtaining experimental data to back up the simulations. This paper presents an experimental procedure and measured data for Henry's Law Constant and interface mass transfer coefficient for hydrogen absorbing in the binary eutectic mixture biphenyl/diphenyl ether as HTF. To the best of our knowledge, the experiment is the first of its kind to derive these two parameters in a relatively inexpensive, straightforward, and rapid manner, with moderate accuracy (about +/- 15% uncertainty). Measurements were taken at different temperatures, pressures, HTF mixing rates, HTF aging, and HSG compositions. The design of the experiment was based on literature review and numerical models simulating the transient and equilibrium behavior of the hydrogen/HTF system at laboratory-scale conditions. Values measured for Henry's Law Constant are around 400 bar/(mol/L), and they decrease slightly with increasing temperature. Both this magnitude and temperature dependency agree with correlations found in literature. The mass transfer coefficient measured for pure hydrogen dissolving in HTF ranges from 5 x 10(-7) mol/(s.mbar.m(2)) (for 100 degrees C) to 4 x 10(-6) mol/(s.mbar.m(2)) (for 293 degrees C). These values and strong temperature dependency agree with theoretical model predictions. HTF aging and mixing rate had negligible impact on the mass transfer coefficient. Lowering the hydrogen HSG pressure slightly increased the mass transfer coefficient (up to 10%). The presence of nitrogen significantly increased the mass transfer coefficient (up to 100%).