• Energy Research

An increased awareness of the impacts of synthetic refrigerants on the environment has prompted the refrigeration industry and researchers worldwide to seek better alternatives in terms of technical, economic and environmental performance. CO2 refrigerant, also known as R744, has re-emerged as a potential alternative to existing refrigerants with its zero ozone depletion potential (ODP) and impressively low global warming potential (GWP). A refrigeration system utilising this refrigerant, however, suffers performance degradation when it operates in warm or hot climatic regions due to its inevitable operation in the supercritical region. In addition, the CO2 refrigerant properties necessitate the need for components designed to withstand very high operating pressures. These challenges have not been let unnoticed; related industries and researchers are actively involved in research and development of various components and systems which in turn encourages increased applications of these systems. In this paper, a comprehensive review of CO2 refrigeration systems and the state of the art of the technology and its applications in various industries is presented. In particular, the paper reviews recent research and developments on various aspects of CO2 systems including cycle modifications, exergy analysis of the systems, system modelling, transcritical operation consideration and various existing and potential applications.

An increased awareness of the impacts of synthetic refrigerants on the environment has prompted the refrigeration industry and researchers worldwide to seek better alternatives in terms of technical, economic and environmental performance. CO2 refrigerant, also known as R744, has re-emerged as a potential alternative to existing refrigerants with its zero ozone depletion potential (ODP) and impressively low global warming potential (GWP). A refrigeration system utilising this refrigerant, however, suffers performance degradation when it operates in warm or hot climatic regions due to its inevitable operation in the supercritical region. In addition, the CO2 refrigerant properties necessitate the need for components designed to withstand very high operating pressures. These challenges have not been let unnoticed; related industries and researchers are actively involved in research and development of various components and systems which in turn encourages increased applications of these systems. In this paper, a comprehensive review of CO2 refrigeration systems and the state of the art of the technology and its applications in various industries is presented. In particular, the paper reviews recent research and developments on various aspects of CO2 systems including cycle modifications, exergy analysis of the systems, system modelling, transcritical operation consideration and various existing and potential applications.

Abstract This paper presents a detailed review of shell materials that have the potential to be used for high temperature thermal energy storage (TES) applications, particularly in conjunction with concentrated solar power (CSP) plants. This paper considers shell materials that are thermally stable at more than 300 °C and have successfully been used to encapsulate a phase change material (PCM). The current review does not consider the thermal performance of the shell material and PCM combinations that have been studied. Using these constraints several feasible materials were identified including: steel (carbon and stainless), nickel (and nickel alloy), sodium silicate, silicon dioxide, calcium carbonate and titanium dioxide. These materials have the potential to encapsulate high temperature PCMs and thus provide a suitable method of high temperature TES.

Abstract This paper presents a detailed review of shell materials that have the potential to be used for high temperature thermal energy storage (TES) applications, particularly in conjunction with concentrated solar power (CSP) plants. This paper considers shell materials that are thermally stable at more than 300 °C and have successfully been used to encapsulate a phase change material (PCM). The current review does not consider the thermal performance of the shell material and PCM combinations that have been studied. Using these constraints several feasible materials were identified including: steel (carbon and stainless), nickel (and nickel alloy), sodium silicate, silicon dioxide, calcium carbonate and titanium dioxide. These materials have the potential to encapsulate high temperature PCMs and thus provide a suitable method of high temperature TES.

Abstract Effective extraction of latent energy is critical in phase change material (PCM) thermal storage applications, including CSP plants. For tube-in-tank arrangements, research to date has not explicitly investigated the impact of the boundary condition applied to the PCM surrounding the tube on the heat transfer process. In Part 1 of this study, the impact of this boundary condition was investigated by applying different tube configurations, defined by the heat transfer fluid either flowing parallel, counterflow or in a serpentine arrangement. The study identified that the critical factor was the loss of heat transfer area experienced once the phase front between parallel tubes meet. This was significant for parallel flow but essentially eliminated for the counterflow arrangement, which delivered a more uniform phase front parallel to the tube wall. As a result it was identified that the amount of redundant PCM when applying the counterflow arrangement was 9%, while this value was 32% for the parallel flow arrangement. This difference has a significant impact on the cost of thermal storage for CSP plants. Part 2 will involve a parametric assessment of the parallel and counterflow configurations.

Abstract Effective extraction of latent energy is critical in phase change material (PCM) thermal storage applications, including CSP plants. For tube-in-tank arrangements, research to date has not explicitly investigated the impact of the boundary condition applied to the PCM surrounding the tube on the heat transfer process. In Part 1 of this study, the impact of this boundary condition was investigated by applying different tube configurations, defined by the heat transfer fluid either flowing parallel, counterflow or in a serpentine arrangement. The study identified that the critical factor was the loss of heat transfer area experienced once the phase front between parallel tubes meet. This was significant for parallel flow but essentially eliminated for the counterflow arrangement, which delivered a more uniform phase front parallel to the tube wall. As a result it was identified that the amount of redundant PCM when applying the counterflow arrangement was 9%, while this value was 32% for the parallel flow arrangement. This difference has a significant impact on the cost of thermal storage for CSP plants. Part 2 will involve a parametric assessment of the parallel and counterflow configurations.

AbstractSolar thermal electricity generation is taking up increasing proportions of future power generation worldwide. Recent research indicates that a closed-loop Brayton cycle using supercritical carbon dioxide (s-CO2) offers the potential of higher power cycle efficiency versus the conventional superheated steam cycle at temperatures relevant for CSP applications. Thermal energy storage solves the time mismatch between the solar energy supply and the electricity peak demand and allows for a more efficient use of the turbine and other power block components. The narrow storage temperature range required for the s-CO2 cycle advantages the use of latent heat storage, which has a higher storage density compared to the conventional two-tank storage. This paper demonstrates a design of a cascaded shell and tube phase change storage system potentially applicable for the s-CO2 cycle. A previously developed effectiveness-number of transfer unit method is employed as a design guide and computational fluid dynamics modelling is performed to examine the sensible energy extraction. The results prove that the effectiveness of the extracted sensible energy can be increased by increasing the number of phase change storage systems in series. Stainless steel (SS) AISI 316 as well as a creep resistant SS AISI 446 is considered as the tube material in the design and results suggest that using AISI 446 can minimize the overall amount of storage and tube materials.

AbstractSolar thermal electricity generation is taking up increasing proportions of future power generation worldwide. Recent research indicates that a closed-loop Brayton cycle using supercritical carbon dioxide (s-CO2) offers the potential of higher power cycle efficiency versus the conventional superheated steam cycle at temperatures relevant for CSP applications. Thermal energy storage solves the time mismatch between the solar energy supply and the electricity peak demand and allows for a more efficient use of the turbine and other power block components. The narrow storage temperature range required for the s-CO2 cycle advantages the use of latent heat storage, which has a higher storage density compared to the conventional two-tank storage. This paper demonstrates a design of a cascaded shell and tube phase change storage system potentially applicable for the s-CO2 cycle. A previously developed effectiveness-number of transfer unit method is employed as a design guide and computational fluid dynamics modelling is performed to examine the sensible energy extraction. The results prove that the effectiveness of the extracted sensible energy can be increased by increasing the number of phase change storage systems in series. Stainless steel (SS) AISI 316 as well as a creep resistant SS AISI 446 is considered as the tube material in the design and results suggest that using AISI 446 can minimize the overall amount of storage and tube materials.