<p dir="ltr">Vapor Compression Systems (VCS) are the most common air conditioning technology. However, the VCS process is energy inefficient due to overcooling and reheating. Liquid Desiccant air conditioning (LDAC) is a potentially more energy-efficient air conditioning technology. LDAC removes vapor in the air using the liquid desiccant’s high-water affinity and controls temperature using an additional cooling device. Additionally, LDAC typically requires heating to regenerate the diluted Liquid Desiccant (LD) for repeated use after absorbing moisture.</p><p dir="ltr">Earlier types of the LDAC systems operated at a relative high concentration and temperature during the dehumidification process, resulting in an increased heat source temperatures required for regeneration, which substantially diminished the energy efficiency advantages of LDAC systems. In the past two decades, researchers have explored a new LDAC system configuration that integrates an LDAC system with a heat pump (HP). The HP can deliver sensible cooling to lower the LD operating temperature and cool the process air. Simultaneously, it provides heating at the condenser side to facilitate the regeneration process. Subsequently, membrane-based dehumidifiers were introduced to separate the LD and airflow using a membrane that permits the passage of water vapor. This approach prevents direct contact, which otherwise would result in LD droplet carryover, addressing concerns related to health and the corrosion of air ducts. An internally cooled membrane-based dehumidifier with enhanced performance garners significant attention, as it essentially functions as a three-stream heat exchanger that facilitates both heat and mass transfer processes. Because of the intricate characteristics of the three-stream heat and mass exchanger, the finite difference models used to analyze the internally cooled membrane dehumidifier is highly detailed and comprehensive. These models are well-suited for assisting in the device’s design but are not suitable for system-level simulations. The lack of simple models for internally cooled membrane-based dehumidifiers limits the evaluation of energy performance at the system level. The limitation becomes particularly pronounced when a HP is integrated, as the model hinders our comprehension of the interactions between the HP and LDAC under the transient operating conditions.</p><p dir="ltr">The thesis research aims to bridge the gaps related to system configuration design, limitations of existing dehumidifier models, and the analysis and assessment of transient system level performance. A model of the internally cooled membrane-based dehumidifier, based on artificial neural networks, was created using data generated through the utilization of a published and detailed finite element dehumidifier model. The resulting model was validated by testing it with out-of-sample data and comparing its results with the validated finite difference model. An LDAC system setup using the internally cooled dehumidifier was established in Modelica using the artificial neural network model created. Furthermore, models of a VCS and an LDAC based on adiabatic dehumidifier were also developed to facilitate performance comparison. The different systems underwent simulation for an entire cooling season spanning from May to September. The internally cooled dehumidifier-based system exhibited superior energy performance, achieving seasonal energy performance levels up to 104% and 34% higher than the VCS and adiabatic dehumidifier systems, respectively. The improved performance in comparison to the VCS is due to the higher temperature operation of the HP. The improvement in comparison to the adiabatic dehumidifier system is due to the improved capacity of the internally cooled dehumidifier to deal with the absorption heat released during dehumidification. Depending on the geographical location, the internally cooled dehumidifier system displayed enhanced performance in the applications characterized by moderate sensible cooling, while its efficiency was relatively lower in arid and hot regions. Additionally, the results demonstrated that the adiabatic system performed similarly to the internally cooled dehumidifier system in locations with high sensible and latent cooling loads.</p><p dir="ltr">This work introduces a pioneering data-driven model for internally cooled membrane liquid desiccant dehumidifiers, representing a significant advancement in the field. The model's computational efficiency and accuracy address the challenges posed by sophisticated and computationally expensive physical models, providing a valuable tool for simulating such devices. The creation of the simple ANN-based dehumidifier model opens the possibility for simulation of internally cooled devices as part of dehumidification systems, whereas as of today its study has been mostly limited to single devices simulations. In the study, a model-based comparison of system performance between an HP-coupled internally cooled dehumidifier-liquid desiccant air conditioning system and HP-coupled adiabatic LDAC, as well as Vapor Compression Systems, elucidates the optimal operational configuration and rationale. Furthermore, a climate sensitivity analysis of system simulations guides researchers toward focusing on the development of HP-coupled internally cooled/heated liquid desiccant systems, particularly in climates that offer the greatest potential for energy savings compared to commonly used vapor compression systems. This comprehensive exploration enhances our understanding and paves the way for more efficient and effective developments in liquid desiccant-based dehumidification technologies.</p>

<p>Evaporative cooling is a highly energy efficient alternative to conventional vapor compression cooling system. The sensible cooling effect of evaporative cooling systems is well documented in the literature. Direct evaporative cooling however increases the relative humidity of the air as it cools it. This has made it unsuitable for data centers and other applications where humidity control is important. Desiccant-based dehumidifiers (liquid, solid or composites) absorb moisture from the cooled air to control humidity and is regenerated using waste heat from the data center. This work is an experimental and theoretical investigation of the use of desiccant assisted evaporative cooling for data center cooling according to ASHRAE thermal guidelines, TC 9.9. The thickness (depth) of the cooling pad was varied to study its effect on sensible heat loss and latent heat gain. The velocity of air through the pad was measured to determine its effect on sensible cooling. The flow rate of water over the pad was also varied to find the optimal flow for rate for dry bulb depression. The configuration was such that the rotary desiccant wheel (impregnated with silica gel) comes after the direct evaporative cooler. The rotary desiccant wheel was split in a 1:1 ratio for cooling and reactivation at lower temperatures. The dehumidification effectiveness of a fixed bed desiccant dehumidifier was compared with that of a rotary desiccant wheel and a thermoelectric dehumidifier. A novel condensate recovery system using the Peltier effect was proposed to recover moisture from the return air stream, (by cooling the return air stream below its dew point temperature) thereby optimizing the water consumption of evaporative cooling technology and providing suitable air quality for data center cooling. The moisture recovery unit was found to reduce the mass of water lost through evaporation by an average of fifty percent irrespective of the pad depth.</p> <p> </p>

Les fumées à basse température (<120-150 °C) sortant des procédés industriels pourraient être récupérées pour la production d'électricité et constituent un moyen efficace de réduction de la consommation d'énergie primaire et des émissions de dioxyde de carbone. Cependant, des barrières techniques tels que la faible efficacité de conversion, la nécessité d'une grande zone de transfert de chaleur, et la présence de substances chimiques corrosives liées à une forte teneur en humidité lors du fonctionnement en environnement sévère entravent leur application plus large. Cette thèse porte particulièrement sur les secteurs industriels les plus énergivores rencontrant actuellement des difficultés à récupérer l'énergie des sources de chaleur à basse température dans des environnements hostiles. Des cycles thermodynamiques existants basés sur le Cycle de Rankine Organique (ORC) sont adaptés et optimisés pour ce niveau de température. Deux méthodes de récupération de chaleur classiques sont étudiées plus particulièrement : les déshumidifications à contact direct et indirect. Des méthodes de conception optimisées pour les échangeurs de chaleur sont élaborées et validées expérimentalement. Pour la déshumidification à contact indirect, des matériaux à revêtement anticorrosifs sont proposés et testés. Pour la déshumidification à contact direct, les effets du type et de la géométrie des garnissages sur les performances hydrauliques sont étudiés. Des cycles thermodynamiques innovants basés sur la technologie de déshydratation liquide sont proposés. Un cycle de régénération amélioré (IRC) est développé. Comparé aux technologies de récupération de chaleur classiques, l'IRC proposé améliore à la fois la puissance nette et le taux de détente de la turbine en prévenant par ailleurs les problèmes de corrosion. / Low-temperature waste-gas heat sources (< 120-150°C) exiting several industrial processes could be recovered for electricity production and constitute an effective mean to reduce primary energy consumption and carbon dioxide emissions. However, technical barriers such as low conversion efficiency, large needed heat transfer area, and the presence of chemically corrosive substances associated with high moisture content when operating in harsh environment impede their wider application. This thesis focuses on particularly energy-hungry industrial sectors characterized by presently unsolved challenges in terms of environmentally hostile low-temperature heat sources. Existing thermodynamic cycles based on Organic Rankine Cycle (ORC) are adapted and optimized for this temperature level. Two conventional heat recovery methods are studied more particularly: indirect and direct contact dehumidification. Optimized design methods for heat exchangers are elaborated and experimentally validated. For the indirect contact dehumidification, advanced anti-corrosion coated materials are proposed and laboratory tested. For the direct contact dehumidification, the effects of packing material and geometry on the corresponding hydraulic performances are underlined. Innovative thermodynamic cycles based on the liquid desiccant technology are investigated. An improved regeneration cycle (IRC) is developed. Compared to the conventional heat recovery technologies, the proposed “IRC” improves both net power and turbine expansion ratio besides preventing faced corrosions problems.