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• Thirteen fluids have been considered for blends in ultra-low temperature refrigeration. • Optimum blends are provided for different temperature, flammability and GWP ranges. • The highest energy performance is observed with high flammability blends. • Carbon dioxide is a refrigerant present in most blends and temperature conditions. • Ethane and ethene-based blends present the highest COP and volumetric cooling capacity. The absence of GWP limitation for refrigerants operating in vapour compression systems with a target temperature below –50 °C has caused slower market development. Therefore, while typical refrigeration applications have several mixtures offering different characteristics, a few mixtures already exist for –80 °C refrigeration (R-469A, R-472A, and R-473A). This paper explores the combination of pure fluids with varied characteristics as three-component mixtures for ultra-low temperature refrigeration. Different parameters have been considered in the theoretical screening, such as volumetric cooling capacity, coefficient of performance, global warming potential, and flammability at constant operating conditions. R-32, R-41, R-125, R-134a, R-152a, R-170, R-227ea, R-290, R-744, R-1132a, R-1150, R-1234ze(E) and RE-170 have been combined at steps of 5%. Mixtures with the lowest global warming potential and highest coefficient of performance result in high flammability, particularly at −80 °C. Environmental and energy aspects would require a lower priority in trade-off selections to reduce the flammability classification. [ABSTRACT FROM AUTHOR]

• A high fidelity model for a low -GWP refrigerant supermarket refrigeration system was developed. • The model was validated by manufacture and lab testing data with accuracy within ±4%. • A near-optimal high side pressure was obtained through extensive simulation running of the model. • The model was also used to generate battery equivalent model for using in grid interactive controls. As an environmentally friendly low global warming potential (GWP) refrigerant, Carbon dioxide (CO 2) has continuously gained popularity and research attention as alternative refrigerant for supermarket refrigeration system. In this paper, to fulfill the increasing need of accurate Low-GWP supermarket refrigeration models for development of supervisory level control and optimization strategies, a high fidelity model is developed for CO 2 transcritical supermarket refrigeration system which includes compressor rack of low temperature (LT) compressors and medium temperature (MT) compressors, air-cooled gas cooler, evaporator, expansion valves and other auxiliary equipment. A resistance-capacity model structure is proposed to simulate the display cases. Semi-thermodynamic models are proposed to estimate reciprocating compressors volumetric efficiency and power consumption. The zone modeling approach is used for evaporator simulation, and air-cooled gas cooler is modeled with distributed modeling method. The expansion valve simulation is based on orifice flow model. To calibrate these models, both manufacture performance data and experimental data are used. The experiments are conducted with a full instrumental CO 2 supermarket refrigeration system installed in Oak Ridge National Lab Flexible Research Platform (FRP). The simulation model can predict the system performance, including power consumption, cooling capacity, mass flow rate, temperature, and pressure, with high accuracy (within ±4%) compared to experimental data. In addition, this developed model has been used to create the system optimum high side pressure for high side expansion valve control, and to generate wide operating range simulation data for developing the batter-equivalent commercial refrigeration system model which can be used in grid interactive control development. [ABSTRACT FROM AUTHOR]

Part I showed that Carbon Dioxide (CO 2) is a promising candidate for use in vertical direct expansion ground evaporators, as it requires smaller pipe diameters within the geothermal heat exchanger and lower power for refrigerant circulation compared to conventional refrigerants. Furthermore, pressure change along the geothermal evaporator influences marginally the evaporation temperature leading to quasi-isothermal evaporation of CO 2. However, in single U-pipe configurations, small pipe diameter means lower pipe surface area and therefore higher thermal resistance. In Part II, all these advantages are taken into consideration toward proposing new geothermal evaporator configurations with multiple U-pipes (2 to 4 U-pipes in one borehole). R410A is also compared against CO 2 for the new configurations. Operating parameters of all configurations were chosen so that CO 2 and R410A are both subjected to a complete evaporation for equal total pipe surface area and equal heat extraction rate. Although all configurations have same total pipe surface area, it is shown that adding U-pipes and reducing pipe diameters diminishes borehole thermal resistance significantly. A novel configuration is also proposed to lower pressure drop of CO 2 in the 4-U borehole configuration by insulating a portion of the downward U-pipe legs. Decreasing pipe dimensions by adding U-pipes while keeping total pipe surface area constant offers an opportunity to bundle pipes closer to each other and make the evaporator more compact as seen in the proposed Polymer Block configuration. [ABSTRACT FROM AUTHOR]

Abstract: The condensation heat transfer characteristics of CO2 flowing in a horizontal smooth tube were investigated. The tube diameter was 5.15 mm. The condensation temperature ranged from −10 to 5 °C, and the mass flux was from 600 to 1000 kg m−2 s−1. When the temperature changed from 0 to −10 °C, the increase rate of the heat transfer coefficient was from 9.4 to 14.6%, and the pressure drop increased from 6.2 to 52.9%. When the mass flux increased from 600 to 1000 kg m−2 s−1, the heat transfer coefficient increased from 6 to 35%, and the pressure drop increased from 60 to 165%, which were dependent on the condensation temperature. The increases come from the change of vapor velocity and thermophysical properties with condensation temperature. Considering large variation of mass flux from 200 to 1200 kg m−2 s−1 by including the existing studies, the effect of mass flux on condensation heat transfer coefficient was minor. [Copyright &y& Elsevier]

Abstract: This paper presents recent measurements of heat transfer coefficient obtained during condensation of R32 inside a commercial brazed plate heat exchanger (BPHE). The experimental data show the effect of refrigerant mass velocity, vapor quality, temperature difference (saturation-to-wall) and inlet vapor superheating. In particular, the specific mass velocity is varied between 15 and 40 kg m−2 s−1 and the outlet vapor quality between 0.0 and 0.65, while inlet vapor superheating goes from 5 to 25 K. The saturation temperature is kept constant at around 36.5 °C, which can be considered a usual temperature level for water cooled heat pump applications. The present authors provide a numerical procedure to calculate the condensation heat transfer in the BPHE, accounting also for the superheating effect. This model is assessed by comparisons with the experimental measurements relative to R32, R410A, and R744. [Copyright &y& Elsevier]

Abstract: Using computer simulation modeling this study investigates the performance of a CO2 trans-critical system with heat recovery from the de-superheater. The influence of sub-cooling (or further cooling) in the condenser/gas cooler on system performance is investigated. Following the suggested control strategy in this study, the extra operating energy demand required to recover the needed heating energy from the analyzed CO2 system is smaller than what a typical heat pump would require for the same load. This is the case for almost all ambient temperatures over a full season. When taking the simultaneous heating and cooling loads into account, the CO2 trans-critical system has lower annual energy usage in an average size supermarket in Sweden when compared to a conventional R404A refrigeration system with separate heat pump for heating needs. CO2 trans-critical systems are efficient solutions for simultaneous cooling and heating needs in supermarkets in relatively cold climates. [Copyright &y& Elsevier]

Abstract: The space-based Orbiting Carbon Observatory (OCO) mission will achieve global measurements needed to distinguish spatial and temporal gradients in the CO2 column. Scheduled by NASA to launch in 2008, the instrument will obtain averaged dry air mole fraction with a precision of 1 part per million (0.3%) in order to quantify the variation of CO2 sources and sinks and to improve future climate forecasts. Retrievals of from ground-based measurements require even higher precisions to validate the satellite data and link them accurately and without bias to the World Meteorological Organization (WMO) standard for atmospheric CO2 observations. These retrievals will require CO2 spectroscopic parameters with unprecedented accuracy. Here we present the experimental and data analysis methods implemented in laboratory studies in order to achieve this challenging goal. To cite this article: C.E. Miller et al., C. R. Physique 6 (2005). [Copyright &y& Elsevier]

Global change — including warmer temperatures, higher CO2 concentrations, increased nitrogen deposition, increased frequency of extreme weather events, and land use change — affects soil carbon inputs (plant litter), and carbon outputs (decomposition). Warmer temperatures tend to increase both plant litter inputs and decomposition rates, making the net effect on soil carbon sequestration uncertain. Rising atmospheric carbon dioxide levels may be partly offset by rising soil carbon levels, but this is the subject of considerable interest, controversy, and uncertainty. Current land use changes have a net negative impact on soil carbon. Desertification and erosion associated with overgrazing and excess fuelwood harvesting, conversion of natural ecosystems into cropland and pasture land, and agricultural intensification are causing losses of soil carbon. Losses increase in proportion to the severity and duration of damage to root systems. Strategic landscape-level deployment of plants through agroforestry systems and riparian plantings may represent an efficient way to rebuild total ecosystem carbon, while also stabilizing soils and hydrologic regimes, and enhancing biodiversity. Many options exist for increasing carbon sequestration on croplands while maintaining or increasing production. These include no-till farming, additions of nitrogen fertilizers and manure, and irrigation and paddy culture. Article 3.4 of the Kyoto Protocol has stimulated intense interest in accounting for land use change impacts on soil carbon stocks. Most Annex I parties are attempting to estimate the potential for increased agricultural soil carbon sequestration to partly offset their growing fossil fuel greenhouse gas emissions. However, this will require demonstrating and verifying carbon stock changes, and raises an issue of how stringent a definition of verification will be adopted by parties. Soil carbon levels and carbon sequestration potential vary widely across landscapes. Wetlands contain extremely important reservoirs of soil carbon in the form of peat. Clay and silt soils have higher carbon stocks than sandy soils, and show a greater and more prolonged response to carbon sequestration measures such as afforestation. Increased knowledge of soil organisms and their activities can improve our understanding of how soil carbon will respond to global change. New techniques using soil organic matter fractionation and stable C isotopes are also making major contributions to our understanding of this topic. [ABSTRACT FROM AUTHOR]