Submitted by Viviane Lima da Cunha (viviane@biblioteca.ufpb.br) on 2017-06-08T13:37:10Z No. of bitstreams: 1 arquivototal.pdf: 5529350 bytes, checksum: 5bf79c2d77151888e66a617ba01f6714 (MD5) / Made available in DSpace on 2017-06-08T13:37:10Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 5529350 bytes, checksum: 5bf79c2d77151888e66a617ba01f6714 (MD5) Previous issue date: 2015-12-04 / The environmental issues and the search for renewable and cheaper resources are encouraging the development of new air conditioner types. One of these models that can substitute the most usual nowadays is the adsorption air conditioner system, which does not need a vapor compressor and demands only a hot and a cold source to work. This hot source can easily be supplied by thermal waste or solar energy. However, the fact that solar energy varies throughout the day implies using a thermal storage system and an auxiliary heat source. Those have to be carefully designed in order to guarantee the maximum performance for the available solar energy. This text presents the development of the software SimAds, which employs numerical routines to solve the heating flow problem that occurs inside the adsorber bed during regeneration. The results given by SimAds are applied as boundary conditions in the thermal storage tank analysis of an adsorption air conditioner system. This study was carried out numerically evolving the Finite Volume Method, with physical and mathematical equations validated by S. Ievers’ (2009) work. The results showed that the storage tank's thermal stratification is higher between 2 p.m. and 5 p.m. in the afternoon, keeping lower levels during the rest of the day. The energy fraction demanded by the air conditioner's hot water circuit supplied by solar energy was find as 70%. The main conclusion shows that changing the tank inlet's height could increase the thermal performance of the storage system, principally in problems with more than one water inlet and transient temperatures. / A preocupação ambiental e a busca por utilização de recursos renováveis e mais econômicos vêm incentivando o desenvolvimento de novos modelos de condicionadores de ar. Um modelo alternativo ao mais empregado atualmente é o ar condicionado por adsorção, que dispensa o uso de compressores de vapor e precisa de uma fonte fria e uma fonte quente para seu funcionamento, podendo esta última ser resíduo térmico ou energia solar. O fato de a energia solar variar ao longo do dia e entre dias, no entanto, implica na adoção de sistemas de armazenamento de calor e na utilização de uma fonte auxiliar de energia, que devem ser devidamente estudados e dimensionados para que o aproveitamento da energia solar seja o maior possível. Esta dissertação apresenta o desenvolvimento do software SimAds, que simula através de rotinas numéricas o aquecimento dos módulos de adsorção durante a fase de regeneração. Os resultados obtidos através do SimAds serviram de condições de contorno para a análise do escoamento dentro do tanque de estocagem de água quente de um sistema de ar condicionado por adsorção. Este estudo foi conduzido numericamente utilizando o método dos volumes finitos, tendo a modelagem físico-matemática do problema validada pelo estudo de tanque de estocagem térmica realizado por S. Ievers (2009). Os resultados da simulação mostraram que o nível de estratificação térmica do tanque apresenta melhores resultados no período das 14 às 17 h, se mantendo em patamares menos favoráveis durante o restante do dia. A fração da energia consumida pelo circuito de água quente do ar condicionado suprida pelos coletores solares foi encontrada de 70%. A principal conclusão aponta que a alteração de altura das entradas do tanque pode melhorar o desempenho térmico do mesmo, principalmente em problemas em que há mais de uma entrada de água quente e com temperatura transiente.

Heat actuated adsorption heat pumps offer the opportunity to improve overall energy efficiency in waste heat applications by eliminating shaft work requirements accompanying vapor compression cycles. The coefficient of performance (COP) in adsorption heat pumps is generally low. The objective of this thesis is to model the adsorption system to gain critical insight into how its performance can be improved. Because adsorption heat pumps are intermittent devices, which induce cooling by adsorbing refrigerant in a sorption bed heat/mass exchanger, transient models must be used to predict performance. In this thesis, such models are developed at the adsorbent particle level, heat/mass exchanger component level and system level. Adsorption heat pump modeling is a coupled heat and mass transfer problem. Intra-particle mass transfer resistance and sorption bed heat transfer resistance are shown to be significant, but for very fine particle sizes, inter-particle resistance may also be important. The diameter of the adsorbent particle in a packed bed is optimized to balance inter- and intra-particle resistances and improve sorption rate. In the literature, the linear driving force (LDF) approximation for intra-particle mass transfer is commonly used in place of the Fickian diffusion equation to reduce computation time; however, it is shown that the error in uptake prediction associated with the LDF depends on the working pair, half-cycle time, adsorbent particle radius, and operating temperatures at hand. Different methods for enhancing sorption bed heat/mass transfer have been proposed in the literature including the use of binders, adsorbent compacting, and complex extended surface geometries. To maintain high reliability, the simple, robust annular-finned-tube geometry with packed adsorbent is specified in this work. The effects of tube diameter, fin pitch and fin height on thermal conductance, metal/adsorbent mass ratio and COP are studied. As one might expect, many closely spaced fins, or high fin density, yields high thermal conductance; however, it is found that the increased inert metal mass associated with the high fin density diminishes COP. It is also found that thin adsorbent layers with low effective conduction resistance lead to high thermal conductance. As adsorbent layer thickness decreases, the relative importance of tube-side convective resistance rises, so mini-channel sized tubes are used. After selecting the proper tube geometry, an overall thermal conductance is calculated for use in a lumped-parameter sorption bed simulation. To evaluate the accuracy of the lumped-parameter approach, a distributed parameter sorption bed simulation is developed for comparison. Using the finite difference method, the distributed parameter model is used to track temperature and refrigerant distributions in the finned tube and adsorbent layer. The distributed-parameter tube model is shown to be in agreement with the lumped-parameter model, thus independently verifying the overall UA calculation and the lumped-parameter sorption bed model. After evaluating the accuracy of the lumped-parameter model, it is used to develop a system-level heat pump simulation. This simulation is used to investigate a non-recuperative two-bed heat pump containing activated carbon fiber-ethanol and silica gel-water working pairs. The two-bed configuration is investigated because it yields a desirable compromise between the number of components (heat exchangers, pumps, valves, etc.) and steady cooling rate. For non-recuperative two-bed adsorption heat pumps, the average COP prediction in the literature is 0.39 for experiments and 0.44 for models. It is important to improve the COP in mobile waste heat applications because without high COP, the available waste heat during startup or idle may be insufficient to deliver the desired cooling duty. In this thesis, a COP of 0.53 is predicted for the non-recuperative, silica gel-water chiller. If thermal energy recovery is incorporated into the cycle, a COP as high as 0.64 is predicted for a 90, 35 and 7.0°C source, ambient and average evaporator temperature, respectively. The improvement in COP over heat pumps appearing in the literature is attributed to the adsorbent particle size optimization and careful selection of sorption bed heat exchanger geometry.

Adsorption refrigeration systems and heat pumps still possess a relatively reduced market share as compared to the traditional compression systems. Despite having the great advantage of being powered by cheap heat (instead of expensive electric work), the implementation of systems based on adsorption principles remains limited to few specific applications. The main drawback that needs to be solved is their reduced specific power due to the low thermal conductivity and low stability of the adsorbents. The current state of the art of commercial adsorption cooling systems rely on adsorbers based on coated finned heat exchangers to optimize the cooling power. It is a well known result, that the reduction of the thickness of the coating derives in a reduction of the mass transport impedance, and that the increment of the ratio surface to volume of conductive structures increases the power without reducing the efficiency. The metallic fibres used in this work can offer a ratio of specific surface in the range of 2500–50,000 m²/ m³.Three methods of preparing very thin but stable salt-hydrate coatings on metallic surfaces, including metallic fibres, for the production of coated heat exchangers with high specific power, are presented for the first time. A surface treatment based on aluminium anodizing was chosen to create a stronger bond between coat and substrate. The microscopic structure of the resulting surface was analysed by Scan Electron Microscopy. To verify the presence of the desired species Attenuated Total Reflectance-Fourier Transformed Infrared and Energy dispersive X-ray spectroscopy were employed in the analysis. Their capacity to form hydrates was verified via simultaneous Thermogravimetric Analysis (TGA)/Differential Thermogravimetry (DTG). Over a mass difference of 0.07 g(water)/g(composite) was detected in the coating of MgSO₄, which showed signs of dehydration at temperatures around 60 °C, and repeatability after rehydration. Also positive results were obtained with SrCl₂ and ZnSO₄ with mass differences around 0.02 g/g below 100 °C. Hydroxyethyl Cellulose was chosen as additive to increase the stability and adherence of the coatings. The adsorption properties of the product were evaluated with simultaneous TGA-DTG, while their adherence was characterized by means of a procedure based on the test described in ISO2409. Coatings of CaCl₂ displayed a much improved consistency and adherence, while retaining its adsorption capacity, showing mass differences of around 0.1 g/g at temperatures below 100 °C. Also MgSO₄ retains the capacity of forming hydrates, showing a mass difference of more than 0.04 g/g below 100 °C. Finally, coated metallic fibres were investigated. Results show that the effective heat conductivity of a fibre structure coated with Al₂(SO₄)₃ can be up to 4.7 times higher as compared to a block of pure Al₂(SO₄)₃ . The coverage of the pursued coatings was visually investigated and the internal structure was evaluated by microscopic imaging of cross-sections. Coatings of around 50 μm of Al₂(SO₄)₃ were generated, but in general the process requires optimization to achieve a more uniform distribution.

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