Separate sensible and latent cooling systems: A critical review of the state-of-the-art and future prospects

Dehumidification is a major contributor to the energy consumed by residential and commercial buildings. The emergence of separate sensible and latent cooling (SSLC) systems have provided energy-efficient solutions to control moisture when compared to traditional vapor compression systems. However, a strong need has emerged to categorize and to characterize the performance of SSLC systems. The current study provides a critical review of major developments including components, systems and processes pertaining to SSLC systems. Additionally, various working media (solid and liquid desiccants and ionic fluids) have been explored to determine the potential merits and demerits. The review effort focuses on highlighting the key features which can be used for classification, performance evaluation and steady-state capacity of the such systems. Along with reviewing the earlier developments, the study also provides guidelines for further research and important performance matrixes for future developments of such technologies

A Critical Literature Review of Defrost Technologies for Heat Pumps and Refrigeration Systems

When the operating conditions are extremely cold and humid and the surface temperature of the heat exchanger well below the freezing point (lower than the dew point temperature of the air) moisture from the air stream will freeze on the surface after condensation and the frost will start growing. The frost growth degrades the performance of the system considerably. It hinders the airflow and increases the pressure drop through the coil which means more fan power is requires for to maintain the desired flow rate. With reduced flow rate due to the increase of pressure drop, system’s capacity drops rapidly. In the case of heat pump the capacity of the evaporator decreases due to the airflow drop, which reduces the overall heating capacity and coefficient of performance of the heat pump. Additionally, the frost layer increases the thermal resistance to the heat transfer between the air and refrigerant. The reduction in airflow and increased thermal resistance reduces the heat energy extracted by the evaporator and decreases the heat pump capacity and efficiency. Similar process is observed for the cooling coils of commercial refrigeration system where the frost growth can dramatically reduce the system capacity. Once the performance reaches its minimum acceptable stage, a defrost process is introduced to remove the frost layer and to achieve the performance at the start of the cycle. The frost defrost process is repeated continuously. Overall the frost growth is highly undesired phenomena which can cause considerable reduction in performance of the system. This study overviews different procedures to counteract the frost growth. Various frost mitigation procedures have been reviewed and compared to access their feasibility. The methods such as air treatment before entering the heat exchanger are used to effectively eliminate or at least minimize the frost growth rate. Such procedures are discussed under two major categories, air treatment processes to mitigate the frost and appropriate system modification to minimize or eliminate the frost growth

Performance Modeling Of A Novel “Smart” Magnetic Particle-Embedded PCM Layer For Thermal Management Systems

Developing stable and environmentally friendly phase change materials (PCM) has been an active area of research due to many applications, such as industrial energy storage system, cooling/heating applications in buildings, and thermal management for electronics equipment as well as for batteries and photovoltaic modules. However, previous studies confirmed that most PCMs suffer a serious disadvantage of low thermal conductivity. A novel design of PCM layer is investigated here for thermal management application. The novel PCM layer is comprised of a PCM and magnetic particles coated on the shell. When a magnetic field is applied in the thermal management system, due to the magnetic of the particles, PCM layer are attached to the heat source to absorb heat, which significantly improves the heat transfer between PCM and the heat source. The melting temperature of the PCM and Curie temperature of the magnetic particles are carefully selected to optimize the performance and to ensure materials compatibility. A numerical study is conducted to reveal the heat transfer and performance improvement of the PCM layer

Genome-wide nucleosome dynamics under heat stress in Arabidopsis thaliana

Heat is a common stress, causing many changes in plant physiology and growth, leading to major economic losses in crops. There is growing evidence that heat stress also affects chromatin architecture. However, genome-wide patterns of these changes and its relationship with transcription of genes are poorly understood. Previously, it was shown that under heat stress nucleosomes are depleted prior to, and strongly enriched at the transcription start site (TSS) of heat shock protein coding genes in Arabidopsis thaliana. Aim of this work is the analysis of changes in nucleosome occupancy under ambient, heat stress and post-stressed (recovery) conditions in Arabidopsis thaliana. Therefore, a genome-wide map of nucleosome positions was generated using the approach of Micrococcal nuclease sequencing (MNase-seq). This revealed that unlike intergenic region, nucleosomes are abundantly present in genic regions and further, are more prominent in exons than introns. Some of these signals were very strong, indicating precise nucleosome positioning at specific genes over many plant tissues in Arabidopsis. Further observations, have shown substantial changes in the nucleosome occupancy including both nucleosome gain and loss in response to heatstress. In particular, loss of nucleosome occupancy has been observed around TSS region of genes, which were highly transcriptionally up-regulated during heat stress response. The opposite, but weaker, trend was observed in strongly down-regulated genes, showing gain in nucleosome occupancy. In conclusion, this study suggests a correlation between the nucleosome occupancy and expression of heat stress responsive genes

Aerogel coated metal foams for dehumidification applications

Separate sensible and latent cooling systems offer significant increases in the overall performance of cooling/dehumidification systems compared to conventional vapor-compression air-conditioning systems. Key to the energy efficiency of such systems is the performance of the heat and mass exchangers that provide sensible cooling and dehumidification. Metal foams have emerged as a potential material for advanced heat exchangers in air-cooling systems. Metal foams have a large surface-area-to-volume ratio and a tortuous structure, which promotes flow mixing in heat exchanger applications. The subject of this thesis is the use of metal foams for air-side heat and mass transfer in air-conditioning heat exchangers. In this work, the thermal-hydraulic performance of metal foams is studied. Experimental data are obtained, leading to new correlations for the friction factor and the Colburn j factor, valid over a wide range of foam geometry and flow rate. Geometrical parameters (pore size, ligament size, etc.), the base metal of the metal foam, and the geometry of the heat exchanger govern its performance. Metal foams are shown to provide very high air-side heat transfer coefficients, but they also induce high pressure drops. Notwithstanding potential increases in the fan power, it is shown that the overall thermal-hydraulic performance of metal foams can surpass the performance of louvered-fin heat exchangers. Hence, metal foams can compete with state-of-the-art heat exchangers in managing the sensible load. In order to manage the latent load, metal foams are studied as substrates for aerogel desiccants. Silica aerogels are excellent desiccants, with much higher moisture adsorption rates and capacities than other solid desiccants, such as carbon sieves or salts. In this work, it is shown that silica aerogel can be deployed over the large surface area of metal foams in the form of a thin film. In this way, the effect of the low thermal conductivity of the desiccant can be mitigated, allowing the heat of adsorption to be removed and regeneration heat to be added via the metal foam substrate. The dehumidification performance of silica aerogels is affected by their micro-structure, which depends on the catalyst used in the sol-gel process to manufacture the desiccants. Dynamic vapor sorption experiments are used to determine mass diffusivity, and the data show that silica aerogel coated on metal foam has the same mass diffusivity in adsorption/desorption as bulk silica aerogel; however, the catalyst used in the sol-gel process significantly affects the mass diffusivity. A silica aerogel coating prepared using hydrofluoric acid as a catalyst (with tetra methyl orthosilicate as a precipitator and methanol as a solvent) results in a mass diffusivity that can be an order of magnitude higher than using other catalysts, such as potassium hydroxide, steric acid etc. Analysis of the simultaneous heat and mass transfer processes in the silica aerogel coating shows that the moisture adsorption rate and the moisture saturation time depend on the type of foam and the thickness of coating, as well as the thermophysical properties of the desiccant coating. Silica aerogel coated on the metal foams provides better moisture removal rate and adsorption capacity per unit volume than does a coated flat plate or louvered-fin substrate. Metal foam heat and mass exchangers have excellent thermal-hydraulic performance and may find application in separate sensible and latent cooling systems for air conditioning. However, questions regarding fouling, manufacturing cost, and heat exchanger geometry constraints remain to be addressed

Functional gastrointestinal disorders and gut-brain axis: What does the future hold?

Despite their high prevalence, lack of understanding of the exact pathophysiology of the functional gastrointestinal disorders has restricted us to symptomatic diagnostic tools and therapies. Complex mechanisms underlying the disturbances in the bidirectional communication between the gastrointestinal tract and the brain have a vital role in the pathogenesis and are key to our understanding of the disease phenomenon. Although we have come a long way in our understanding of these complex disorders with the help of studies on animals especially rodents, there need to be more studies in humans, especially to identify the therapeutic targets. This review study looks at the anatomical features of the gut-brain axis in order to discuss the different factors and underlying molecular mechanisms that may have a role in the pathogenesis of functional gastrointestinal disorders. These molecules and their receptors can be targeted in future for further studies and possible therapeutic interventions. The article also discusses the potential role of artificial intelligence and machine learning and its possible role in our understanding of these scientifically challenging disorders