Accurate thermodynamic property data of dense fluids and supercritical fluids are important in many scientific investigations and engineering applications. A flow calorimeter has been constructed to measure the isobaric heat capacity of pure fluids and fluid mixtures over a range of temperatures, 25-150 °C, and pressures, 1-300 bar. Base measurement accuracy approaches ±1% and is limited by precise equipment calibration and refinement of the measurement technique. Validation of the calorimeter's operation was achieved with supercritical carbon dioxide, methanol, and carbon-dioxide-methanol mixtures, over a range of conditions from sub-critical to supercritical temperatures and densities, demonstrating the calorimeter's robustness.

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R1234yf (2,3,3,3-tetrafluoropropene) is a low global warming potential (GWP) replacement for R134a, a fluid commonly used as a refrigerant in domestic and automotive air conditioning systems and as a working fluid in organic Rankine cycles. The use of R1234yf in energy conversion cycles requires an accurate determination of its thermophysical properties, including the isobaric heat capacity (c P). Using a custom-made flow calorimeter, experimental c P measurements of supercritical R1234yf were made at temperatures T = 373.15 to 413.5 K and absolute pressures P = 3.5 to 10 MPa. The experimental apparatus was calibrated using R134a to increase the measurement accuracy. The heat capacity measurements of R1234yf were compared to available published experimental data and to values obtained from a multiparameter equation of state (EOS). The measured c P values agreed well with the EOS, providing an average absolute deviation (AAD) of 1.6%.

Pumped-thermal energy storage (PTES) systems consume and produce electrical energy using thermal storage media as an intermediate stage. PTES lends itself to long-duration energy storage to facilitate high penetration of intermittent electricity generation. This work presents a model-based comparison of two thermal storage types within a PTES system: a conventional, single-phase, stratified water-glycol sensible storage system (SGS), and an ideal isothermal, two-phase heat exchanger that freezes a water reservoir (IHEX). The SGS thermal storage capacity is based on the liquid’s sensible heat change with temperature, whereas the capacity of the IHEX is based on the latent heat of isothermally freezing and melting water. The idealized IHEX modeled here undergoes steady-state melting and freezing (in contrast to transient rates, as observed with ice-on-coil storage). A computational model of a complete PTES system is presented and used to evaluate the PTES system-level performance with each type of cold storage. Compared to SGS-based PTES, under nominal operating conditions the IHEX-based PTES increased electrical round-trip efficiency from 61% to 82% and increased energy density from 1.13 to 8.09 kWh per cubic meter. The performance of the PTES configured with IHEX storage was also analyzed under varying operating parameters.

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