Supercritical carbon dioxide (S-CO₂) Brayton cycles (BC) are soon to be a competitive and environment friendly power generation technology. Progressive technological developments in turbo-machineries and heat exchangers have boosted the idea of using S-CO₂ in a closed-loop BC. This paper describes and discusses energy and exergy analysis of S-CO₂ BC in cascade arrangement with a secondary cycle using CO₂, R134a, ammonia, or argon as working fluids. Pressure drop in the cycle is considered, and its effect on the overall performance is investigated. No specific heat source is considered, thus any heat source capable of providing temperature in the range from 500 °C to 850 °C can be utilized, such as solar energy, gas turbine exhaust, nuclear waste heat, etc. The commercial software ‘Aspen HYSYS version 9’ (Aspen Technology, Inc., Bedford, MA, USA) is used for simulations. Comparisons with the literature and simulation results are discussed first for the standalone S-CO₂ BC. Energy analysis is done for the combined cycle to inspect the parameters affecting the cycle performance. The second law efficiency is calculated, and exergy losses incurred in different components of the cycle are discussed.

The organic Rankine cycle (ORC) exhibits considerable promise in efficiently utilizing low-to-medium-grade heat. Currently, there is a range of organic fluids available in the market, and selecting the appropriate one for a specific application involves considering factors such as the cycle’s thermodynamic performance, plant size, and compatibility with turbomachinery. The objective of our study is to examine the exergetic performance of the ORC with internal heat regeneration. We analyze 12 different organic fluids to evaluate their suitability based on parameters like exergy efficiency and heat exchange area requirements. Additionally, we investigate the need for internal heat regeneration by comparing the overall exergy performance with a simpler ORC configuration. To ensure broad applicability, we consider source temperatures ranging from 150 to 300 °C, which are relevant to industrial waste heat, geothermal sources, and solar energy. For each case, we calculate specific net power output and the UA value (heat exchanger conductance) to gain insights into selecting the appropriate organic fluid for specific source temperatures. Cyclohexane, benzene, isopropyl alcohol, and hexafluorobenzene show poor exergy efficiency due to their high boiling points. Pentane and cyclopentane provides the highest exergy efficiency of 62.2% at source temperature of 300 °C, whereas pentane is found to be the most suitable at source temperatures of 200 and 150 °C with exergy efficiency of 67.7% and 61.7%, respectively. At 200 °C source temperature, RE347mcc achieves 65.9% exergy efficiency. The choice of organic fluid for a given heat source is highly influenced by its critical properties. Moreover, the normal boiling temperature of the organic fluid significantly impacts exergy destruction during the condensation process within the cycle.

In combined cycle gas turbines, most of the energy loss is usually due to the high temperature of the exhaust gases. Different heat recuperation methods are used. In this study, a novel direct method for heat recovery is investigated. Confidence in the results is established by accounting for all the losses and simulation errors while comparing with the conventional cycle. Aspen HYSYS and MATLAB are the simulation tools used. The General Electric (GE) 9HA.02 combined cycle is taken as a base case. Five gases, air, argon, hydrogen, nitrogen, and carbon dioxide, are studied with the proposed modification. The efficiency maximization function is updated and the pressure and temperature ratios of individual Brayton and Rankine cycles are discussed. The combustor/heat exchanger is modified and simulated according to the known principles of heat and momentum transfer. The whole simulation algorithm is provided. Equation of state (EOS-PR) is used to calculate the properties at every discretized step (for H₂, critical properties are modified/HYSYS inbuilt feature). Different gases are analyzed according to their property profiles over the whole cycle. The effect of fluid properties on efficiency is discussed as a guideline for any tailored fluid.

Cadmium and lead are generally taken as model heavy metal ions in water to scale the detection limit of various electrode sensors, using electrochemical sensing techniques. These ions interact with the electrochemically deposited antimony electrodes depending on the diffusion limitations. The phenomenon acts differently for the in-situ and ex-situ deposition as well as for porous and non-porous electrodes. A method has been adopted in this study to discourage the stripping and deposition of the working ions (antimony) to understand the principle of heavy metal ion detection. X-ray photoelectron spectroscopy (XPS) technique was used to establish the interaction between the working and dissolved ions. In addition to the distinct peaks for each analyte, researchers also observed a shoulder peak. A possible reason for the presence of this peak was provided. Different electrochemical tests were performed to ascertain the theory on the basis of the experimental observations