Narrow Results

We have investigated metastability and nucleation in the CO₂ system. To facilitate comparison with experiment and analytical theories, we have also calculated the coexistence curve. The metastability properties were computed using three methods to calculate the compressibility: droplet distribution, density fluctuations, and structure factor. These provide three measures for the time-dependent compressibility. If the compressibility showed a window in which it was quasi time-independent, for the observation period we categorized the quench to lead into a metastable state. Our fully atomistic simulation shows that the metastable region is extremely small. Already, very shallow quenches into the two-phase region have given rise to nonclassical behavior. The calculated pseudo spinodal is located far from the mean field expectation.

Presented are initial, S₀, and coverage, Θ, dependent, S(Θ), adsorption probability measurements, respectively, of CO₂ adsorption on a hydrogen precovered, polar, Zn-terminated surface of ZnO, parametric in the impact energy, E_i, and atomic hydrogen precoverage, Θ_H. Furthermore, CO₂ Thermal Desorption Spectroscopy has been used to estimate Θ_H as well as the binding energy of CO₂ on H/Zn-ZnO. The S(Θ) curves are below E_i=0.56 eV, consistent with precursor-mediated adsorption (S~const), and above that impact energy with adsorbate-assisted adsorption (S increases with Θ). Although a decrease in the CO₂ binding energy from 32.5 to 28.8 kJ/mol with Θ_H is present, S(Θ, Θ_H) curves are consistent with a physical site blocking, as demonstrated by Monte Carlo Simulations.

This paper summarizes the approaches to and the implications of bottom–up infrastructure modeling in the framework of the EMF28 model comparison "Europe 2050: The Effects of Technology Choices on EU Climate Policy". It includes models covering all the sectors currently under scrutiny by the European Infrastructure Priorities: Electricity, natural gas, and CO₂. Results suggest that some infrastructure enhancement is required to achieve the decarbonization, and that the network development needs can be attained in a reasonable timeframe. In the electricity sector, additional cross-border interconnection is required, but generation and the development of low-cost renewables is a more challenging task. For natural gas, the falling total consumption could be satisfied by the current infrastructure in place, and even in a high-gas scenario the infrastructure implications remain manageable. Model results on the future role of Carbon Capture, Transport, and Sequestration (CCTS) vary, and suggest that most of the transportation infrastructure might be required in and around the North Sea.

The existing artificial and chemical refrigerants have been phased out due to environmental concerns, and they have been replaced with environmentally friendly refrigerants. Among them, carbon dioxide, ammonia, and hydrocarbons are paid attention as next generation refrigerants, and their application has been widely expanded. Therefore, in this paper, the latest studies of flow boiling and condensation heat transfer characteristics of carbon dioxide, ammonia, and hydrocarbon are reviewed. The heat transfer characteristics of ammonia and hydrocarbon show the relatively similar trends with the conventional refrigerants compared to those of carbon dioxide. The general trends and recommendable models of flow boiling and condensation heat transfer with carbon dioxide, ammonia, and hydrocarbons are summarized.

Energy, environment and economics are considered as very vital parameters for the evaluation of an air conditioning system and associated indoor environment. The cooling performance of an air conditioner has an effect on the thermal comfort of occupants in the room. Transcritical CO₂ air conditioner (System B) with a control for gas cooler pressure has better energy performance than a transcritical CO₂ air conditioner (System A) without any control on the gas cooler pressure. An experimental technique is used for generating performance equations to define transcritical CO₂ air conditioners in the EnergyPlus program. EnergyPlus simulates combined model of a transcritical CO₂ air conditioner and room for known yearly weather data for an effect on thermal comfort in the room. Thermal comfort in the room is evaluated using the Fanger thermal comfort model and the Pierce two node model. The better energy performance of System B results in improved indoor room environment of the room. The total cooling of System B is 15.78–20.2% higher than that of System A. The Fanger thermal comfort model shows that 95% to 133% people are more dissatisfied with an indoor thermal environment during the morning and 85% to 127% people during the afternoon for a room coupled with System A vis-à-vis room with System B.

In this study, we investigated two-phase flow patterns and effects of oil concentration on the heat transfer coefficient and pressure drop of CO₂ undergoing condensation process in a microchannel, and from the data collected, we developed a prediction model of CO₂ condensation heat transfer coefficient in smooth tube and microchannels. The narrow single rectangular channel was utilized to observe flow patterns of CO₂ under the condensation process. Experimental results show that the transition of vapor quality from intermittent flow to annular flow advances with increase of mass flux and with decrease of condensation temperature. The heat transfer coefficient decreased by 50% as compared to that of the pure CO₂ when the oil concentration was increased from 0.7 to 1.2 wt.% for the mass flux of 600 kg ⋅ m^-2 ⋅ s^-1. The pressure drop slightly decreased with oil concentration as compared to that of pure CO₂. We developed the prediction model for the heat transfer coefficient of CO₂ undergoing condensation process in microchannel by considering the effects of the liquid film thickness and of the interface shape between liquid and vapor phases on the heat transfer coefficient. The present prediction model estimated the experimental data within 18.9% of mean deviation. For the pressure drop in microchannel tubes, the existing models developed by Mishima and Hibiki, and Garimella showed the marginal predictability.