Thermodynamic And Transport Performance For A High Temperature Absorption Heat Transformer

The continued rising of energy prices and global warming is remarkably attracting the attention of governments and academic communities around the world. Energy efficiency, reducing energy consumption per unit output has become the focus of world’s energy strategy. The absorption heat transformer (AHT) mainly driven by waste heat source for low grade energy reuse after ascending temperature is an energy conservation and environmental protection technology and has a broad market and application prospect. In order to extend the temperature range of industrial waste heat recycling of AHT technology, and realize the cascade utilization of high temperature heat, in this paper, the thermodynamic and transport performance for high temperature absorption heat transformer (HT-AHT) are investigated theoretically and experimentally.A high precision experiment system for vapor-liquid phase equilibrium of lithium bromide solution at high temperature is established and the results of saturation vapor pressure of water and lithium bromide solution obtained by experiment are in good agreement with the results in literature. The data of saturation vapor pressure of lithium bromide solution at the temperature ranging from156.06-257.84℃, the concentration of43.14%,52.67%,54.26%,59.33%,65.26%are measured. The fitting curves of P-T-X from experimental results for lithium bromide solution at high temperature are obtained. The scope of existing data is extended and these physical property data of lithium bromide solution are significant for the design and operation of high temperature (150-250℃) absorption system in industry.The exergy balance-energy level analysis method is proposed and the thermodynamics model of HT-AHT cycle is established. The energy level difference is applied to investigate the intensity of the irreversible factors in the process of energy transfer. The exergy loss distribution and its mechanism are comprehensively analyzed by separating the number factors and irreversible factors (exergy loss mechanism) of exergy loss in energy transfer process. The results show that the absorber is the element with the highest exergy loss, about40-50%of total system exergy loss. Irreversible factors caused by the temperature difference of input and output fluids are dominated and less affected by the operation temperature. The exergy loss of the generator is about10-40%. Irreversible factors caused by the concentration difference of import and export solution are dominated and significantly influenced by the operating temperature. The COP and ECOP of the AHT system are0.25-0.48and0.4-0.7at low temperature and are0.4-0.46and0.8-0.9at high temperature. This suggests that the performance of HT-AHT is superior to conventional AHT system in the first and second law of thermodynamics aspect. In the proposed combined cycle, a high temperature absorption heat transformer and an absorption heat pump (HAHT-HP) are employed to reuse the high temperature waste heat for higher thermal and exergy efficiencies. The energy distribution results of waste heat in the cycle show that around46%of the total input energy is available as a useful energy output at high temperature (205℃), about98%is available at intermediate temperature (70℃) and around43%waste heat at low temperature (40℃) is recovered by the cycle. The exergy distribution shows about49%is useful exergy output at high temperature.The three-dimensional CFD model is established to simulate falling liquid film flow on functional coating tubes with VOF method. The momentum source term with respect to the surface tension is introduced into the model to examine the influence of the surface wettability on falling liquid film flow. The results show that the effusion ring is formed at the top boundary of coating area. The liquid film flows through the coating area via droplet and rivulet flow after reaching a certain thickness and then liquid film spreads out as thin liquid film at bottom boundary of the coating. The intensified disturbance mixing effect of the axial and radial directions are formed within the liquid film during the effusion, passing and spreading processes. The mixing effect is beneficial to enhance the heat and mass transfer. The falling film flow pattern and internal flow field on the two kinds of designed coatings are analyzed. The results indicate that the liquid film is separated as two streams after the convergence of the liquid film with effusion ring by two diversion areas on#1configuration coating. The thickness of the effusion ring decreases and thus the mixing effect inside the liquid film is enhanced. The liquid film on the#2configuration coating converges into a rivulet flow again after a diversion area. The internal liquid film mixing effect in radial direction is enhanced although the thickness of the liquid film in rivulet flow increases. The forced mixing effect inside the liquid film is obtained by the above optimization configuration coatings. The heat and mass transfer properties of the smooth tube and the optimized design tubes with functional configuration coating are investigated experimentally. The results show that the total heat transfer coefficient of#1and#2coating tube are21.47%and24.25%higher than that of the smooth tube, respectively.The high temperature absorption heat transformer (HT-AHT) system is designed and built. The experimental investigations proved that all the properties of this prototype approach to the designed value. The experimental system is operated at the temperature of150-205℃and high pressure, the gross temperature lift of the system is over40℃. The corrosion phenomenon is insignificant due to the usage of surface anti-corrosion technology. The experimental results show that the COP and Qa increase with the increase of the evaporating temperature, and increase firstly and then decrease with increasing the absorbing temperature, and increase firstly and then decrease lightly with the increase of generating temperature, and decrease with the increase of GTL. The ECOP increases firstly and then decreases with the increase of evaporating temperature and absorbing temperature, and increases with the increase of generating temperature, and increases firstly and then decreases rapidly with the increase of GTL.