The Oxford Laser Heating Facility (OLAHF) has been upgraded with a supercritical carbon dioxide (sCO2) flow circulating loop, capable of providing component level flow rates to test pieces at temperatures and pressures up to 100 C and 100 bar. Near to the critical point, the large variations in sCO2 thermophysical properties can lead to adverse cooling and heat transfer effects. There are currently gaps in the understanding of the effects of larger thermal gradients (i.e. heat fluxes) on the heat transfer performance of sCO2 systems. An initial experimental campaign was performed on an additively manufactured stainless steel test piece containing an array of 11, 1.5 mm square internal channels using the high power laser modules located at OLAHF. Heat fluxes ranging from 116 to 1000 kW/m2 to the upper surface of the horizontally-oriented test piece. Data were obtained for fluid inlet temperatures between 18 and 28C and mass fluxes of 1500 and 3000 kg/m2s at a constant inlet pressure of 80 bar (inlet Reynolds numbers from 3 x 10^4 to 8 x 10^4). Resulting heat transfer coefficients were determined using a combination of ray tracing and finite element (FE) analyses of the test piece at each experimental condition. Average heat transfer coefficients were shown to exhibit a significant dependence on applied heat flux for both mass flux conditions and all inlet temperatures. The heat transfer coefficients were compared to empirical correlations with varied levels of agreement. The data was additionally examined for the potential existence of buoyancy and flow acceleration effects.

Minichannel printed circuit heat exchangers (PCHEs) are commonly utilized in supercritical CO₂ (sCO₂) power cycles. In this study, an experimental single plate PCHE with various header configurations is investigated in terms of flow maldistribution using computational fluid dynamics (CFD) techniques. A series of header optimization simulations with different mass flow rates, inlet/outlet configurations, and header sizes are performed and analyzed. The numerical model of this PCHE is set up and solved with ANSYS-CFX. The variation in thermophysical properties of sCO₂ was incorporated into the numerical model via real gas property data. Additionally, the computed results for temperature distribution i.e., maximum hotspot temperatures and pressure drop were validated with experimental data, showing a good agreement with a maximum discrepancy of 8.6%. The results from this study indicated that optimizing inlet and outlet positions reduced the maldistribution factor (MDF) by 34% at 40 g/sec and 29% at 90 g/sec, as compared to the baseline configuration of the PCHE. Increasing the header area ratio further reduced the MDF by 51% at 40 g/sec and 48% at 90 g/sec, resulting in uniform temperature distribution and effective mitigation of local hotspots. Furthermore, the suggested header configuration also minimized hot and cold stream mixing inside the collector header which can significantly reduce thermal stresses, extending the life span of PCHEs.

The promise of high thermal efficiencies and low initial capital cost make supercritical carbon dioxide (sCO₂) power cycles an attractive technology for concentrated solar power (CSP) applications. As CSP plants are usually located in arid or semi-arid regions, cooling options that minimize water consumption are preferred. Natural draft direct dry cooling systems (NDDDCSs) offer an alternative to traditional dry cooling technologies such as air-cooled condensers (ACCs) or indirect natural draft dry cooling systems. NDDDCSs combine the benefits of low auxiliary power consumption with direct heat rejection, while minimising operational costs and noise.

This study investigates the performance of a NDDDCS when utilised as cooling technology for a precooler and intercooler (combined in one tower) of a partial cooling with reheat (PCRH) sCO₂ cycle for a conceptual 50 MWe CSP plant. Initial work involves the development of a zero-dimensional (0D) lumped parameter sizing model to assess overall system performance and aid in selecting the best-performing geometry between the evaluated cases. Thereafter, a higher fidelity simulation model is developed in Flownex SE software, which employs a thermofluid network approach to solve the governing mass-, momentum-, and energy balance equations for the air- and sCO₂ streams. Empirical heat transfer and pressure drop correlations are also integrated for both air- and sCO₂-sides to capture component characteristics. After verification with the 0D model, the thermofluid network model (TNM) is extended to include discretised finned-tube heat exchangers and a one-dimensional (1D) discretised air-side tower to simulate the natural draft. This model is valuable for providing detailed insights into potential maldistribution effects, particularly under off-design conditions, and is used to evaluate the performance of the cooling system under varying ambient temperature and part-load conditions.

The best-performing system was selected based on minimising both lost work (sCO₂-side pressure drop) and estimated total heat exchanger and tower material costs. The selected configuration features an inlet diameter of 55.79 m, inlet height of 11.14 m, tower height of 55.79 m and outlet diameter of 27.89 m, designed to meet combined heat rejection requirements for the precooler (PC) and intercooler (IC) at design point. At design conditions, the system’s air mass flow rate and total heat rejection rate are 3702 kg/s and 85.03 MW, respectively.

Simulation of the cooling system using the discretised network model shows reductions of 4.7% and 0.8% in overall heat rejection and air mass flow rate at the design point, compared to the lumped model. While significant variations in fluid properties, pressure drops and heat transfer coefficients are observed on the discretised sCO₂-side, the resulting air-side effects dominate differences from the lumped model results.

Further simulations investigate the effects of off-design ambient temperature and crosswind variations. Significant differences in sCO₂-side temperatures and heat transfer coefficients arise from ambient temperature changes. Flow and pressure drop characteristics during partial bypass operation (to avoid overcooling impact on the cycle) of heat exchangers are also illustrated. Simulations highlight the differing impacts of crosswinds on heat rejection, sCO₂ outlet temperature and pressure drop for the PC and IC.

Fluids display sharp, non-linear variations of thermodynamic properties when they are heated at a supercritical pressure. As such, near-pseudo-critical heat transfer is often characterized by large variations in density, leading to sharp near-wall accelerations or strong stratifications when buoyancy becomes dominant. We study the modulation of heat transfer and turbulence by non-negligible buoyancy in such property-variant flows, for the development of near-pseudo-critical heat exchangers for supercritical energy conversion systems. In particular, a liquid-like, horizontal base flow of carbon dioxide at 88.5 bar and 32.6 ◦C is considered, which is subjected to a vertical heat flux of up to 12.0 kW/m2 at Reynolds numbers of up to ReDh ≤ 10.000. Here, optical- and surface temperature measurements are used concurrently to evaluate the flow. Integrated visualizations of the flow field show the onset of strong stratifications with limited heating rates in the near-pseudo-critical region. During unstable stratification, the channel flow is dominated by the upward motion of thermal plumes. When the stratification is stable, any vertical motion and turbulence present in an equivalent neutrally buoyant flow is suppressed. As a result, wall heat is removed more effectively in the unstably stratified configuration than in a forced convective flow, whereas the opposite is true for a stably stratified flow. The difference in the perceived heat transfer between the considered configurations increases as buoyancy becomes more dominant.

Heat exchangers are major components that significantly influence the performance and size of industrial processes. Supercritical CO2 (sCO2) is a highly promising working fluid for various applications due to its potential to reduce equipment size, lower environmental impact, and enhance performance. The properties of CO2 near the critical point exhibit significant variations, making it particularly favorable for heat transfer. As part of the Horizon 2020 DESOLINATION (DEmonstration of concentrated SOLar power coupled wIth advaNced desAlinaTion system in the gulf regION) project, a printed circuit heat exchanger (PCHE) for an sCO2 power cycle was designed, manufactured, and tested. Computational fluid dynamics (CFD) simulations using the SST k-ω turbulence model and real-gas equation of state were used to analyze the heat transfer performance and friction losses of the PCHE in detail. By combining experimental measurements with numerical simulations, this study provides a comprehensive evaluation of the thermo-hydraulic performance of sCO2 near the pseudo-critical region under various operating conditions (73.78 < p < 92.32 bar, 307.02 < T_b < 334.74 K, 1823 < 𝐺 < 4064 kg/m²s). A new friction factor correlation, derived from experimental data, is proposed to enhance the accuracy of friction loss predictions for sCO2 flows in microchannels, with errors within ±10%. The findings highlight the superior heat transfer performance of PCHEs while addressing the challenges associated with their performance prediction due to steep fluid property gradients in the supercritical region.