CO2 Power System Research Articles

Special Issue dedicated to the 1st International Conference on Supercritical CO2 Power System (ICSCPS 2018)

Special Issue dedicated to the 1st International Conference on Supercritical CO2 Power System (ICSCPS 2018)

Effect of cross-section geometry on the thermohydraulic characteristics of supercritical CO2 in minichannels

Carbon dioxide (CO2) is becoming an important commercial and industrial working fluid as a potential replacement of the non-environmental friendly refrigerants. For refrigeration and power systems, the minichannel heat exchangers are becoming attractive for transcritical CO2 Rankine cycle and supercritical CO2 Brayton cycle, due to their highly compact construction, high heat transfer coefficient, high pressure capability and lower fluid inventory. This paper employs three-dimensional numerical models to investigate the heat transfer and pressure drop characteristics of supercritical CO2 in minichannels. The models consider real gas thermophysical properties and buoyancy effect and investigate the effect of cross-section geometry on the thermohydraulic characteristics. Six minichannel cross-section geometries with the same hydraulic diameter of 1.22 mm are considered. The geometries include circle, semicircle, square, equilateral triangle, rectangle (aspect ratio = 2) and ellipse (aspect ratio = 2). The inlet temperature, outlet pressure and wall heat flux are 35 °C/75 bar/100 kW/m2 and 35 °C/150 bar/300 kW/m2 for heating conditions and 120 °C/75 bar/-100 kW/m2 and 120 °C/150 bar/-300 kW/m2 for cooling conditions. Comparisons of local Nusselt number and friction factor with those employed empirical correlations are made and useful information and guidelines are provided for the design of compact heat exchangers for supercritical CO2 power system applications.

Diffuser performance of centrifugal compressor in supercritical CO2 power systems

The paper focuses on understanding the performance of a vaned diffuser in a supercritical CO2 compressor using real gas assumptions. The rapidly changing properties of CO2 in the non-ideal thermodynamic region can have a significant impact on the performance of the diffuser. To account for this, the relationship between the flow properties (i.e. fundamental derivative) and the local geometry (i.e. cross-sectional area of the nozzle) has been discussed theoretically. To examine it analytically, the study has considered the influence of real gas properties on the performance of a vaned diffuser using computational fluid dynamics (CFD) modelling. The selected compressor stage geometry is similar to the compressor impeller tested in the Sandia sCO2 compression loop facility. The effect of changes in the number of blades and the corresponding changes in the cross throat area on the flow properties such as density and speed of sound were investigated and discussed. The results illustrate that a diffuser with a higher number of blades (smaller throat area) compared to a diffuser with a wider throat area has a higher probability of creating flow instability in the passage stage.

Preliminary conceptual exploration about performance improvement on supercritical CO2 power system via integrating with different absorption power generation systems

Supercritical CO2 (sCO2) power system has been investigated by many scholars due to its attractive advantages of higher efficiency, compact system structure and eco-friendly working fluid. In this paper, some preliminary conceptual exploration about performance improvement on sCO2 power system by integrating with two types of absorption power generation (APG) systems are conducted. Parameter analysis, genetic algorithm (GA) optimization and exergy analysis are carried out quantitatively for the proposed combined sCO2/APG systems based on the self-built simulation platform from the viewpoints of thermodynamics and economics. Parameter analysis results reveal that there exist optimal compressor pressure ratio to maximize the thermal efficiency or minimize the total product unit cost. Higher turbine inlet temperature and lower absorber temperature could contribute to the overall system performance. In addition, compared with the stand-alone sCO2 system, improvements of 5.98% and 5.07% in thermodynamics as well as promotion of 4.24% and 2.19% in economics can be obtained for sCO2/LiBr-H2O system and sCO2/ammonia water system, respectively. Furthermore, exergy analyses show that the main exergy destructions occur in the reactor and the cooler and the proposed combined sCO2/APG system could effectively reduce around half of the exergy destruction within the cooler of the stand-alone sCO2 system.

Comparison between ORC and CO2 power systems for the exploitation of low-medium temperature heat sources

Low-medium temperature heat sources in the range 5–50 MWth are made available by many industrial fields but they may also be of interest for biomass, geothermal and solar energy applications. ORC has been proposed in the last 20 years as a reliable solution for the exploitation of these energy sources since the alternative represented by steam cycles leads to an inefficient conversion of such small available thermal powers. However, the use of organic fluids involves a number of safety and environmental issues, either related to fluid flammability (for hydrocarbons) or to their high Global Warming Potential (for halogenated fluids), and of limitations to the achievable cycle maximum temperature, due to fluids thermal decomposition. To overcome these limitations, in recent years CO2-based transcritical and supercritical cycles have been proposed as a viable option for this kind of applications. The present work aims to present a comparison between four CO2 cycle configurations and four ORC layouts using a working fluid selected from 47 candidates. The final result is a set of performance maps that allow for an easy selection of the best solution for applications exploiting low-medium temperature heat sources.

Multi criteria optimization of plate heat exchanger for super critical CO2 power systems

Supercritical CO2 power systems are an interesting option to allow the use of low grade thermal sources, especially for small size plants, where compactness and costs are of primary importance in order to spread their use; in such applications innovative plate heat exchangers, able to work at high pressure, could be used.In this paper a model for detailed heat transfer and pressure drop calculations is presented and used to carry-out an analysis oriented to the definition of a map, presented according to the criteria of size of the heat exchanged and total performance index, in order to look for the best trade-off between component cost and energetic performance.