Adsorption refrigeration—An efficient way to make good use of waste heat and solar energy

This paper presents the achievements gained in solid sorption refrigeration prototypes since the end of the l970s, when interest in sorption systems was renewed. The applications included are ice making and air conditioning. The latter includes not only cooling and heating, but also dehumidification by desiccant systems. The prototypes presented were designed to use waste heat or solar energy as the main heat source. The waste heat could be from diesel engines or from power plants, in combined cooling, heating and power systems (CCHP). The current technology of adsorption solar-powered icemakers allows a daily ice production of between 4 and 7 kg m⁻² of solar collector, with a solar coefficient of performance (COP) between 0.10 and 0.16. The silica gel–water chillers studied can be powered by hot water warmer than 55 °C. The COP is usually around 0.2–0.6, and in some commercially produced machines, it can be up to 0.7. The utilization of such chillers in CCHP systems, hospitals, buildings and grain depots are discussed. Despite their advantages, solid sorption systems still present some drawbacks such as low specific cooling power (SCP) and COP. Thus, some techniques to overcome these problems are also contemplated, together with the perspectives for their broad commercialisation. Among these techniques, a special attention was devoted to innovative adsorbent materials, to advanced cycles and to heat pipes, which are suitable devices not only to improve the heat transfer but also can help to avoid corrosion in the adsorbers. Recent experiments performed by the research group of the authors with machines that employ composite adsorbent material and heat pipes showed that it is possible to achieve a SCP of 770 W kg⁻¹ of salt and COP of 0.39 at evaporation temperatures of −20 °C and generation temperature of 115 °C.     

Experimental investigation of heat transfer coefficient of R134a during condensation in vertical downward flow at high mass flux in a smooth tube

The two-phase heat transfer coefficients of pure HFC-134a condensing inside a smooth tube-in-tube heat exchanger are experimentally investigated. The test section is a 0.5 m long double tube with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is constructed from smooth copper tubing of 9.52 mm outer diameter and 8.1 mm inner diameter. The test runs are performed at average saturation condensing temperatures between 40–50 °C. The mass fluxes are between 260 and 515 kg m^− 2s^− 1 and the heat fluxes are between 11.3 and 55.3 kW m^− 2. The quality of the refrigerant in the test section is calculated using the temperature and pressure obtained from the experiment. The average heat transfer coefficient of the refrigerant is determined by applying an energy balance based on the energy transferred from the test section. The effects of heat flux, mass flux and condensation temperature on the heat transfer coefficients are also discussed. Eleven well-known correlations for annular flow are compared to each other using a large amount of data obtained from various experimental conditions. A new correlation for the condensation heat transfer coefficient is proposed for practical applications.     

Lithium chloride - Expanded graphite composite sorbent for solar powered ice maker

Consolidated composite material made from expanded graphite (EG) powder impregnated with LiCl salt is proposed for use in solar powered adsorption ice makers. Laboratory experiments were done to test the adsorption and desorption performance of the sorbent under different temperature conditions suitable for solar energy utilization. More than 75% of the reaction between LiCl and ammonia was completed after 30 min of synthesis at evaporation temperatures of −10 and −5 °C and adsorption temperature between 25 and 35 °C. Under the same period, it was possible to obtain 80% conversion in the desorption phase, when the generation temperatures ranged between 75 and 80 °C, and the condensation temperature varied from 25 to 35 °C. The highest average specific cooling power during the synthesis phase was 117 W per kg of the block. The calculated theoretical coefficient of performance (COP) under different cycle conditions was nearly constant at 0.47. Moreover, the new composite sorbent showed higher Specific Cooling Capacity (SCC), compared to activated carbon (AC)/methanol pair. Experiments done with blocks with different proportion of EG, showed that the proportion of EG influence the cooling capacity per unit mass of salt and had almost no influence on the cooling capacity per unit mass of the block. Moreover, the reaction enthalpy (ΔH) and entropy (ΔS) were calculated from experimental data obtained experimentally, and confirmed previous reported values.     

Power-based performance comparison between carbon dioxide and R125 transcritical cycles for a low-grade heat source

In order to compare the power output of the carbon dioxide transcritical cycle and the R125 transcritical cycle for a low-grade heat source of about 100 °C, the two cycles were optimized for power output using a simulation method. In contrast to conventional approaches, each working fluid’s heat transfer and pressure drop characteristics within the heat exchangers were taken into account by using a discretized heat exchanger model. To fairly compare the power output of the cycles by using different working fluids, the inlet temperatures and the flow rates of both the heat source and the heat sink were fixed. The cycle minimum temperature was not given, but was determined by the heat sink conditions and the working fluid’s heat transfer and pressure drop characteristics, as it is in actual practice. The total heat transfer area was fixed, whereas the allocation of the heat-exchanger area between the vapor generator and the condenser was optimized in the simulation. The R125 transcritical cycle produced 14% more power than did the carbon dioxide transcritical cycle. Even though the carbon dioxide cycle shows better heat transfer and pressure drop characteristics in the heat exchangers, the high pumping power required to manage the large pressure head degrades the cycle’s power output. Based on this study, the R125 transcritical cycle is recommended for heat sources of about 100 °C.     

Solubility of hydrogen in ice Ih at pressures up to 8 MPa

Experimental studies of the solubility of hydrogen in ice Ih (usual low-pressure ice) at temperature −1 to −2 °C and pressures up to 8 MPa were carried out. At a pressure equal to 1.90 and 8.04 MPa, hydrogen solubility in the ice was found to be 0.15 and 1.32 cm³/g, respectively (hydrogen volume was reduced to the normal conditions).     

Evaluation of a solar intermittent refrigeration system for ice production operating with ammonia/lithium nitrate

A novel solar intermittent refrigeration system for ice production developed in the Centro de Investigación en Energía of the Universidad Nacional Autónoma de México is presented. The system operates with the ammonia/lithium nitrate mixture. The system developed has a nominal capacity of 8 kg of ice/day. It consists of a cylindrical parabolic collector acting as generator-absorber. Evaporator temperatures as low as −11 °C were obtained for several hours with solar coefficients of performance up to 0.08. It was found that the coefficient of performance increases with the increment of solar radiation and the solution concentration. A dependency of the coefficient of performance was not founded against the cooling water temperature. Also it was found that the maximum operating pressure increases meanwhile the generation temperature decreases with an increase of the solution concentration.     

Water sorption–desorption in Nafion membranes at low temperature,probed by micro X-ray diffraction

Using a micro X-ray beam, the structure of a water swollen Nafion^® membrane, alone or in a membrane electrode assembly (MEA) designed for fuel cells, was studied upon cooling down to −70 °C. By scanning the membranes along their thicknesses, the water sorption–desorption process was investigated as a function of cooling/heating stages. From the scattering curves, it was deduced that the state of the water at a sub-zero temperature is glassy inside the membrane and ice crystals are observed only outside it. In the case of the MEA, this growth can be destructive since this formation is localised inside the active layers.     

Heat transfer model of slurry ice melting on external surface of helical coil

This research studies the heat transfer phenomenon of melting slurry ice on external surface of a copper helical coil. There is water flowing inside the tube coil and exchanging heat with the slurry ice. In this experiment, the coil diameters are 6.35 mm and 9.53 mm each of 4.2 m coil length. The mass flow rate of water in the helical coil is between 0.0149–0.0562 kg/s, while the inlet temperature of water is varied in the range of 23–27 °C. The slurry ice has 60% ice and 40% water by mass at the starting.     

Simulation experiments on the effect of freezing of carbon substrate catalyst

By adiabatic expansion due to the introduction of an evacuation process, the water drop of about 2 mm diameter were cooled to 243 K by forming ice drops at the triple point of water. The sublimation temperature of ice rapidly decreased 243 K within 60 s and remained constant for 80 s. Subsequently, it became room temperature in about 60 s and the water drop disappeared owing to sublimation. By repeating this process, carbon and catalyst were markedly altered. Transmission electron microscopy (TEM) based on in situ observation elucidated the freezing problem in polymer electrolyte fuel cell (PEFC). Structural alterations of carbon due to freezing were observed and discussed as the oxidation phenomenon of carbon.     

A PEM fuel cell model for cold-start simulations

In this paper, a transient multiphase multi-dimensional PEM fuel cell model has been developed in the mixed-domain framework for elucidating the fundamental physics of fuel cell cold start. Cold-start operations of a PEM fuel cell at a subfreezing boundary temperature of −20 °C under both constant current and constant cell voltage conditions have been numerically examined. Numerical results indicate that the water vapor concentration inside the cathode gas channel affects ice formation in the cathode catalyst layer and thus the cold-start process of the fuel cell. This conclusion demonstrates that high gas flow rates in the cathode gas channel could increase fuel cell cold-start time and benefit the cold-start process. It is shown that the membrane plays a significant role during the cold-start process of a PEM fuel cell by absorbing the product water and becoming hydrated. The time evolutions of ice formation, current density and water content distributions during fuel cell cold-start processes have also been discussed in detail.     

Design and development of a refrigeration system energized with hydrogen produced from scrap aluminum

Hydrogen holds out great promise as an energy source whose use does not pollute the environment. In this context, methods of hydrogen production which do not involve formation of carbon dioxide are especially attractive. The present work describes a cheap and versatile prototype of an alkaline hydrolyser which efficiently produces hydrogen from aluminum scrap and aqueous sodium hydroxide, the hydrogen produced being used directly to energize, by combustion, a refrigerator working on the ammonia–water principle, which was also designed and developed in our laboratory. A direct comparison of the system when energized by liquid-propane flame and by hydrogen flame shows a clearly better performance in the latter case, which produces a temperature of −20 °C after about 2 h of operation.     

Silver (Ag) as anode and cathode current collectors in high temperature planar solid oxide fuel cells

Planar electrolyte supported solid oxide fuel cells were operated at 900 °C with humidified H₂ for 200 h using silver mesh and paste for cathode current collection. Continuous potentiostatic tests at 0.7 V appeared to induce migration of Ag towards electrode-electrolyte interphase, while continuous OCV tests caused no mass transport. Similar SOFCs fueled by coal syngas at 850 °C using Ag for both anode and cathode current collection indicated little, if any, Ag migration; providing the possibility of employing Ag for 100 h laboratory scale tests using coal-derived syngas. Use of high temperature steam, carbon dioxide and carbon monoxide did not result in the formation of silver carbonates.     

New manganese dioxides for lithium batteries

Lithium/manganese dioxide primary batteries use heat treated manganese dioxide (HEMD), a defect pyrolusite structure material as the cathode active material. Ion exchange of the structural protons in electrolytic manganese dioxide (EMD) with lithium before heating results in formation of a lithium containing γ-MnO₂. Increased lithium hydroxide concentration and increased temperature lead to increased lithium levels. At 80 °C with a combination of LiOH and LiBr, almost all of the structural protons in MnO₂ are replaced by lithium resulting in a γ-MnO₂ phase substantially free of protons and containing about 1.8% Li. This highly substituted lithium containing MnO₂ is reduced at between 3.5 and 1.8 V and has a capacity of 250 mAh g⁻¹. There are two reduction processes, one at 3.25 and the other at 2.9 V. TGA studies reveal two processes during heat treatment. Heating the lithium substituted MnO₂ to 350–400 °C results in a partially ordered HEMD-like MnO₂ (LiMD) phase with higher running voltage and superior discharge kinetics. Continued heating of the lithiated manganese dioxide to 450–480 °C under oxygen partial pressure can result in formation of a mixed phase containing both HEMD and a new, ordered MnO₂ phase (OMD). The intimately mixed HEMD/OMD composition has a discharge voltage near 2.9 V with a capacity about 220 mAh g⁻¹. Heating exhaustively lithiated MnO₂ to 350–400 °C results in formation of the partially ordered LiMD MnO₂ phase as with the previous partially lithium substituted MnO₂. Additional heating of the highly lithium substituted MnO₂ to 450–480 °C under oxygen results in formation of the new OMD phase in substantially pure form. Discharge of the new OMD phase shows it has a discharge capacity near 200 mAh g⁻¹ between 3.4 and 2.4 V versus lithium in a single, well-defined discharge process. OMD demonstrated good cycling against Li with no indication of formation of LiMn₂O₄ spinel after 80 deep discharge cycles.     

Experimental analysis for the determination of the convective heat transfer coefficient by measuring pressure drop directly during annular condensation flow of R134a in a vertical smooth tube

This study investigated the direct relationship between the measured condensation pressure drop and convective heat transfer coefficient of R134a flowing downward inside a vertical smooth copper tube having an inner diameter of 8.1 mm and a length of 500 mm during annular flow. R134a and water were used as working fluids on the tube side and annular side of a double tube heat exchanger, respectively. Condensation experiments were performed at mass fluxes of 260, 300, 340, 400, 456 and 515 kg m⁻² s⁻¹ in the high mass flux region of R134a. The condensing temperatures were around 40 and 50 °C; the heat fluxes were between 10.16 and 66.61 kW m⁻². Paliwoda’s analysis, which focused mainly on the determination of the two-phase flow factor and two-phase length of evaporators and condensers, was adapted to the in-tube condensation phenomena in the test section to determine the condensation heat transfer coefficient, heat flux, two-phase length and pressure drop experimentally by means of a large number of data points obtained under various experimental conditions.     

Study of hydrogen physisorption on nanoporous carbon materials of different origin

Hydrogen adsorption capabilities of different nanoporous carbon, i.e. amorphous carbons obtained by chemical activation (with KOH) of a sucrose-derived char previously ground by ball milling and carbon replicas of NH₄-Y and mesocellular silica foam (MSU-F) inorganic templates, were measured and correlated to their porous properties. The porous texture of the prepared carbon materials was studied by means of N₂ and CO₂ adsorption isotherms measured at −196 °C and 0 °C, respectively. Comparison with nanoporous carbons obtained without pre-grinding the sucrose-derived char [12] shows that the ball milling procedure favours the formation of highly microporous carbon materials even at low KOH loadings, having a beneficial effect of the interaction between the char particles and the activating agent. Hydrogen adsorption isotherms at −196 °C were measured in the 0.0-1.1 MPa pressure range, and a maximum hydrogen adsorption capacity of 3.4 wt.% was obtained for the amorphous carbon prepared by activation at 900 °C with a KOH/char weight ratio of 2. Finally, a linear dependence was found between the maximum hydrogen uptake at 1.1 MPa and the samples microporous volume, confirming previous results obtained at −196 °C and sub-atmospheric pressure [12].     

Thermal and flow behavior of ice slurries in a vertical rectangular channel—Part II. Forced convective melting heat transfer

The convective heat transfer characteristics of ice slurries flowing vertically upward in a rectangular channel have been experimentally investigated. At steady state, the local heat transfer coefficients were obtained during convective melting, and the effects of ice fraction, Reynolds number and wall heat flux were determined. To gain more insights into the flow properties, local measurements of the axial mixture velocity, temperature and ice fraction distributions were also made near a heated wall. Four main factors were shown to enhance the convective heat transfer coefficient of ice slurries relative to single-phase flow: (1) the ice fraction, (2) thermally and/or hydrodynamically developing flow conditions, (3) mixed-convection and (4) non-Newtonian effects experienced at Re < 4000. Two new and simple heat transfer correlations are also proposed for the practical design of compact heat exchangers involving phase change ice slurries.     

Theoretical and experimental analysis of the vacuum pressure in a vacuum glazing after extreme thermal cycling

Details of theoretical and experimental studies of the change in vacuum pressure within a vacuum glazing after extreme thermal cycling are presented. The vacuum glazing was fabricated at low temperature using an indium-copper-indium edge seal. It comprised two 4 mm thick 0.4 m by 0.4 m glass panes with low-emittance coatings separated by an array of stainless steel support pillars spaced at 25 mm with a diameter of 0.4 mm and a height of 0.15 mm. Thermal cycling tests were undertaken in which the air temperature on one side of the sample was taken from −30 °C to +50 °C and back to −30 °C 15 times while maintaining an air temperature of 22 °C on the other side. After this test procedure, it was found that the glass to glass heat conductance at the centre glazing area had increased by 10.1% from which the vacuum pressure within the evacuated space was determined to have increased from the negligible level of less than 0.1 Pa to 0.16 Pa using the model of Corrucini. Previous research has shown that if the vacuum pressure is less than 0.1 Pa, the effect of conduction through the residual gas on the total glazing heat transfer is negligible. The degradation of vacuum level determined was corroborated by the change in glass surface temperatures.     

Maximum density effect on laminar water pipe flow solidification

This paper investigates experimentally and numerically the effect of maximum density on laminar water pipe flow solidification. To visualize the secondary flow patterns and ice morphology experimentally, the water was seeded with a small amount of mercurochrome solution. The numerical analysis employs the large Prandtl number assumption and considers a gradual variation of solid shell in the streamwise direction. The experimental parameters cover the ranges of the dimensionless axial length from 0.008 to 0.32, the inlet water temperature from 5 to 20°C, the wall temperature from −12 to 0°C and the Rayleigh number from 10⁴ to 3×10⁶. The comparison of the measured and calculated heat transfer rates are in good agreement for wide ranges of operating parameters.     

Effect of carbon fiber cloth with different structure on the performance of low temperature proton exchange membrane fuel cells

This study uses fuel cell gas diffusion layers (GDLs) fabricated in the laboratory from carbon fiber cloth with different structure in proton exchange membrane fuel cells (PEMFCs), and investigates the relationship between the structure of the carbon fiber cloth and fuel cell performance.The paper discusses the relationship between fuel cell performance and structure of the carbon fiber cloth, and also examines the effect of the carbon fiber cloth’s thickness, air permeability, surface resistivity, XRD and elemental analysis. Carbon fiber cloth is carbonized at rates of 190, 220, 250, 280, and 310 °C min⁻¹ respectively, and the resulting carbon fiber cloth is tested in cells. When the test piece area is 25 cm², the test temperature 40 °C, the gasket thickness 0.36 mm, and the carbonization rate 280 °C min⁻¹, a fuel cell using the carbon fiber cloth achieves a current density of 1968 mA cm⁻² and a maximum power density of 633 mW cm⁻² at 0.3 V.     

Modelling airflow,heat transfer and moisture transport around a standing human body by computational fluid dynamics

In this study a combined computational model of a room with virtual thermal manikin with real dimensions and physiological shape was used to determine heat and mass transfer between human body and environment. Three dimensional fluid flow, temperature and moisture distribution, heat transfer (sensible and latent) between human body and ambient, radiation and convection heat transfer rates on human body surfaces, local and average convection coefficients and skin temperatures were calculated. The radiative heat transfer coefficient predicted for the whole-body was 4.6 W m^− 2 K^− 1, closely matching the generally accepted whole-body value of 4.7 W m^− 2 K^− 1. Similarly, the whole-body natural convection coefficient for the manikin fell within the mid-range of previously published values at 3.8 W m^− 2 K^− 1. Results of calculations were in agreement with available experimental and theoretical data in literature.