Search

Highlights • Simulation of a microchannel H 2 O-LiBr desorber using a microporous membrane. • Sensitivity analysis of cooling capacity/desorber volume to various parameters. • Direct diffusion and boiling desorption modes are compared. • Parameters to be optimized at the design stage of the desorber are identified. • Solution inlet concentration is the most relevant operational parameter. Abstract A parametric analysis of a membrane-based microchannel desorber working with H 2 O–LiBr is presented. Mass and heat transfer equations are solved, for different membrane properties and design and operating conditions. Sensitivity of the ratio between chiller cooling capacity and desorber volume (r qV) to changes in the following parameters is evaluated: width and height of solution and hot water channels; solution concentration, temperature, pressure and mass flow rate; hot water temperature and mass flow rate; membrane porosity, pore diameter and thickness; thickness and thermal conductivity of the wall between solution and hot water and vapour pressure. The analysis includes direct diffusion and boiling modes. In both cases, during design, most important parameters to maximize r qV are solution channel height and wall thickness. Additionally, effect of channel length is remarkable during boiling. Good operation requires adequate inlet concentration and hot water temperature. In direct diffusion, vapour pressure is also a relevant parameter. [ABSTRACT FROM AUTHOR]

Highlights • A novel adiabatic membrane-based microchannel absorber is experimentally tested. • Absorber performance is evaluated under variable operating conditions. • Solution mass transfer resistance is dominant in the absorption process. • Higher absorption ratios than in previous adiabatic configurations are obtained. • The ratio between cooling power and absorber volume reaches 559 kW/m3. Abstract In the interest of reducing the size of absorption chillers, a novel adiabatic membrane-based micro-absorber prototype is experimentally studied. Water–lithium bromide solution is used as the working fluid flowing through 50 rectangular microchannels of 0.15 mm height, 3 mm width and 58 mm length. In the present study, a laminated microporous PTFE membrane of 0.45 µm pore diameter, separating the solution from the vapour, is tested. It incorporates a supporting layer of polypropylene. Different operating parameters were tested, including the inlet solution mass flow rate, temperature and concentration and the pressure potential for absorption. The measured concentration and temperature of the solution at the absorber outlet are used to evaluate the mass transfer characteristics of the micro-absorber. It is demonstrated that the process is controlled by the solution mass transfer resistance. Calculated results of the absorption rate and the absorption ratio show the advantages of the proposed design considering its compactness. The cooling power of a hypothetical chiller equipped with the tested micro-absorber of 73.7 cm3 effective volume, for the range of variables considered in this study, is 41 W. The modular configuration of the absorber allows to easily scale-up the cooling capacity. [ABSTRACT FROM AUTHOR]

Abstract: The influence of the pressure drop in Plate Heat Exchangers (PHE) in the boiling temperature of LiBr–H2O and NH3–H2O solutions is studied. For the NH3–H2O solution, the pressure drop-temperature saturation relationship estates that high pressure drops can be allowed in the solution with negligible changes in the saturation temperature, and in the PHE performance. Besides, in the case of the LiBr–H2O solution, as the working pressure is usually very low, the analysis of the pressure drop must be taken as a main limiting parameter for the use of Plate Heat Exchangers as vapour generators. In this case, the pressure drop may considerably change the boiling temperature of the solution entering the heat exchanger and therefore a higher heating fluid temperature may be required. A guideline to design these systems is proposed. [ABSTRACT FROM AUTHOR]

Abstract: This work presents an experimental analysis of the hydrodynamic and thermal performance of micro-heat exchangers. Two micro-heat exchangers, characterized by microchannels of 100×100 and 200×200μm square cross-sections, were designed for that purpose. The fluid used was deionized water and there was no phase change along the fluid circuit. The fluid pressure drop along the heat exchanger and the heat transfer were measured and corrections were made to isolate the contribution of the microchannels. The results were compared with the predictions of the classical viscous flow and heat transfer theory. The main conclusions show that the experimental results fit well with these theories. No effects of heat transfer enhancement or pressure drop increase were observed as a consequence of the small scale of the microchannels. [Copyright &y& Elsevier]

A review on the use of nanoparticles to improve the performance of absorption chillers is presented. The review includes a compilation and discussion of research data on the subject using as base fluids NH 3 -H 2 O, H 2 O-LiBr and other solutions. Topics analysed include: theoretical and experimental studies about mass transfer enhancement in absorbers and desorbers; research to improve performance of complete absorption chillers; evaluation of thermophysical properties and stability of nanofluids, including viscosity, surface tension, thermal conductivity and other properties; and other studies with results useful for implementation in absorption chillers. Trends about the use of nanofluids in absorption systems, type and concentration of nanoparticles that provide the best performances and specific topics that require more research are identified from the review. Strategies are proposed for future research or for decision makers related to absorption chillers. [Display omitted] • Research data about nanofluids including NH 3 -H 2 O and H 2 O-LiBr are presented. • Theory and experiments on devices, stability and properties are reviewed. • Trends about the use of nanofluids in absorption systems are summarized. • Type and fraction of nanoparticles that provide the best performances are found. • Specific topics that require more research are identified. [ABSTRACT FROM AUTHOR]

The effect of different parameters on the pyrolysis of Cynara cardunculus L. was studied through an innovative technique based on a precision scale, capable of measuring the time evolution of the biomass samples mass during their thermochemical conversion process while moving freely inside a fluidized bed. A silica sand bed reactor, operated under different values of excess gas velocity and reactor temperature, was employed to hold the pyrolysis reaction of cardoon particles of three different size ranges. The pyrolysis was accelerated for higher excess gas velocities, obtaining pyrolysis times as short as 17.3 s for experiments conducted under bubbling fluidized bed regimes, compared to 185.9 s required to complete the pyrolysis of the same sample in a fixed bed configuration. Similarly, the effect of increasing the reactor temperature promoted faster heating rates across the fuel samples, especially under fixed bed configurations, for which the pyrolysis time is reduced from 321.7 s to 132.0 s when increasing the bed temperature from 450 to 650 °C. Regarding the biomass particle size, small sizes are preferred to minimize the conduction thermal resistance inside the fuel particles and, thus, reduce pyrolysis times and increase volatile yields for the pyrolysis in a bubbling fluidized bed reactor. The opposite result was found when the pyrolysis took place in non-bubbling beds, where the use of larger particles is beneficial to accelerate the biomass pyrolysis reaction. [ABSTRACT FROM AUTHOR]

In this paper a microporous membrane is used in combination with rectangular microchannels in the absorber of an absorption chiller, working in two different configurations: cooled by a water flow and adiabatically. In the non-adiabatic case, the configuration of the channels allows the heat released during absorption to be extracted using a cooling water flow. The results for solution concentration, pressure potential, absorption coefficient, absorption rate, temperatures and power exchanged/stored by the working fluids along the absorption channels are presented. The ratio between the cooling power of the chiller equipped with the simulated absorber and the absorber volume, r qV , is used to characterise the absorber compactness. A parametric analysis is also performed to evaluate the influence on the ratio r qV of the inlet solution mass flow rate, the solution inlet temperature, and the height and width of the solution channels, for both absorbers. For the base case considered in this study, both absorber configurations offer r qV higher than 1 MW m −3 . This ratio is higher than usual values found in falling film absorbers using conventional circular tubes. Moreover, the new adiabatic configuration presented has significant advantages respect to the non-adiabatic one in terms of higher r qV and fabrication simplicity. [ABSTRACT FROM AUTHOR]

The simulation of the heat and mass transfer in a H 2 O–LiBr microchannel desorber endowed with a microporous membrane is presented. The heat and mass transfer processes are modelled by means of selected correlations gathered from the open literature. The simulation provides the evolution along the channels of the parameters involved in the process: heat and mass transfer coefficients, solution concentration, temperatures of the working fluids and pressure potential. The calculated values of the desorption rate are compared to experimental data gathered from the open literature. For the case considered in this study, the maximum ratio between cooling capacity of the chiller and desorber volume is 2312 kW m −3 . This value is more than one order of magnitude higher than the ones found in conventional small-size absorption cooling chillers. [ABSTRACT FROM AUTHOR]

A microporous membrane is used in combination with rectangular microchannels in the absorber of an absorption chiller with the aim of reducing the size of this cooling technology. The simulation of the heat and mass transfer between the solution and the vapour phase in a H 2 O–LiBr absorber using porous fibres is considered. Heat and mass transfer processes are modelled by means of selected correlations and data gathered from the open literature. This new model is applied for the simulation of the absorber under typical operating conditions of absorption cooling chillers. Absorption rate, heat and mass transfer coefficients, solution concentration, temperatures of the working fluids and pressure potential along the absorption channels are calculated. For the case considered in this study, the absorber channels are of 5 cm length, offering a maximum ratio between cooling capacity of the chiller and absorber volume of 1090 kW/m 3 . This ratio is higher than twice the usual values found in falling film absorbers using conventional circular tubes. The mean absolute error between the model results and the experimental data gathered from the open literature is 8.5%, showing the capability of the model to predict the performance of membrane-based absorbers. [ABSTRACT FROM AUTHOR]

• A new membrane desorber combined with air-cooled condenser is experimentally tested. • Desorption rate in micro-channels varies between 0.002 to 0.004 kg/m2s. • Condensing temperatures range from 37 °C to 47 °C. • Measured pressure drop is acceptable for the LiBr-water absorption requirements. • The ratio between cooling power to volume of the desorber/condenser is 27 kW/m3. A new membrane-based desorber, directly coupled with an air-cooled condenser is experimentally tested. The working fluid is a LiBr-H 2 O solution that flows through the desorber in micro channels 2 mm width and 100 μm thick. To desorb the vapour from the solution, hot water circulates in a counter flow arrangement in channels, with the same geometry. A hydrophobic membrane with a pore diameter of 45 μm separates the confined solution from the cavity where the generated vapour is immediately condensed. Forced air convection is used to cool the finned condenser. The experimental performance of the new compact design is evaluated. Tests have been carried out varying the solution mass flow rates (40 ml/min, 50 ml/min and 60 ml/min) and the heating water temperatures (70 °C, 80 °C and 90 °C). The experiments show the viability of operation of the new design (combination of desorber and air-cooled condenser in a unique compact device) with the production of 0.1 to 0.17 kg/h of water vapour when the power input to the desorber ranges from 150 to 275 W. The heat simultaneously dissipated in the condenser ranges from 60 W to 120 W at condensing temperatures in the range from 37 °C to 47 °C. Desorption rates obtained vary from 0.002 to 0.004 kg/m2s. Measured pressure drops through the micro channels are in the order of 70 Pa/m. The results demonstrated the feasibility of increasing compactness in absorption systems with a performance of cooling power to total volume ratio of a chiller, equal to 10.75 kW/m3, higher than conventional systems. [ABSTRACT FROM AUTHOR]

Abstract: The effect of buoyant forces on the motion of a large object immersed in a bubbling fluidized bed (BFB) was experimentally studied using digital image analysis. The experiments were performed in a 2-D bubbling fluidized bed with glass spheres as bed material and cylindrical objects with different densities and sizes. The object motion was measured using non-intrusive tracking techniques. The effect of gas velocity was also analyzed. The circulation of an object in a BFB is defined by several parameters. The object might be able to circulate homogeneously throughout the bed or stay in preferred regions, such as the splash zone or the bottom zone. While circulating, the object moves back and forth between the surface of the bed and the inner regions, performing a series of cycles. Each cycle is composed by sinking and rising paths, which can be one or several, depending on whether a passing bubble is able to lift the object to the surface or the object is detached from it or its drift at an intermediate depth. Therefore, the number of rising paths or number of jumps that the object undergo in a cycle, interleaved with sinking paths, and the maximum attained depth characterize each cycle, together with the mean sinking and rising velocities of the object. In this work, experimental measurements of the probability distributions of the number of jumps and the maximum attained depth, the axial homogeneity of object motion and rising and sinking object velocities are presented for objects with different sizes and densities. The results show a coherent behavior, independent of density and size, for the probability distributions of the number of jumps. This is also true for the maximum attained depth, but only when a proper circulation throughout the bed is ensured. Such a proper circulation and axial homogeneity is, on the other hand, much affected by object density, size and gas velocity. Rising and sinking velocities are highly dependent on gas velocity, as established in well-known models of bubble and dense phase velocities. Nevertheless, rising velocities are practically unaffected by object density or size, while sinking velocities show a low dependence on density and a steeper one on size. These results suggest that buoyant forces are relevant during the sinking process, and almost neutral during the rising path. [Copyright &y& Elsevier]

Transient and steady state performance of an air-cooled square minichannel heat exchanger was experimentally evaluated. 82 minichannels of square cross section, 11 cm long and 2 mm width, were manufactured on AISI 316 L. Heat sinks made of aluminium were attached to each side of the heat exchanger. Natural and forced air convection were investigated, while the hot fluid was flowing in the downward counterflow direction. Two other variables were changed: inlet water temperature, from 34.8 to 75.4 °C, and water flow rate, from 5.7 to 29.1 kg/h, while in the case of forced air convection its velocity was fixed at 0.75 m/s. Ambient air temperature and water temperature and pressure at the inlet and outlet were measured. Results obtained include the thermal resistances and heat capacities in the different parts of the heat exchanger, the transient time, the residence time of water inside the heat exchanger and the time constant of the device. Correlations were obtained to predict the duration of the transient period in the natural and forced convection cases. Additionally, a lumped mass heat transfer model was developed to estimate the time evolution of the average water temperature in the heat exchanger. [ABSTRACT FROM AUTHOR]

• An air-cooled subcooler for absorption cooling systems is experimentally evaluated. • Square minichannels of 2 mm side and 11 cm length and natural or forced air are used. • Approach temperature, pressure drops and heat transfer parameters are obtained. • Correlations are derived for heat transfer and pressure drop in the subcooler. • A notable increase in the cooling power of the absorption system is obtained. A mini square channel air-cooled heat exchanger, operating as subcooler in the typical temperature range of absorption cooling systems, is experimentally evaluated. Working fluid flows along 82 square minichannels of 2 mm side and 11 cm length, manufactured on stainless-steel. Hot fluid flows downward through the channels and two cases are evaluated: natural and forced air convection. In the last case, ambient air flows in the upward direction. Two additional operating parameters are modified: the inlet fluid temperature, between 34.8 and 75.4 °C and the fluid mass flow rate, in the range from 5.7 to 29.1 kg/h. Inlet and outlet temperature and pressure for the fluid and ambient temperature are measured, while the air velocity is kept constant at 0.75 m/s. Results obtained for the approach temperature, pressure drops, heat transferred and heat transfer coefficients for the air-cooled subcooler are provided. Correlations are derived to predict the heat transfer coefficient and pressure drop of the subcooler. Integration of the air-cooled subcooler in a single-effect absorption cooling system is evaluated, showing a notable increase in the cooling power mainly in the case of using forced air flow. [ABSTRACT FROM AUTHOR]

Abstract: An understanding of electricity consumption due to residential air conditioning (AC) may improve production and environmental impact strategy design. This article reports on a study of peak and seasonal electricity consumption for residential air conditioning in the region of Madrid, Spain. Consumption was assessed by simulating the operation of AC units at the outdoor summer temperature characteristics of central Spain. AC unit performance when operating under part load conditions in keeping with weather conditions was also studied to find cooling demand and energy efficiency. Likewise final electricity consumption was computed and used to calculate energy costs and greenhouse gas emissions (GHGs). Cooling demand, when family holidays outside the region were factored into the calculations, came to 1.46×109 kWh. Associated seasonal electricity demand was 617×106 kWh and seasonal performance of AC units around 2.4. Electricity consumption in the whole region was observed to peak on 30 June 2008 at 5.44×106 kW, being the load attributable to residential AC 1.79×106 kW, resulting about 33% of the total peak consumption. The seasonal cost per household was about €156 and the total equivalent warming impact was 572×103 tCO2. The method proposed can be adapted for use in other regions. [Copyright &y& Elsevier]