Experimental study on dynamic performance analysis of a flat-plate solar solid-adsorption refrigeration for ice maker

A flat-plate solid-adsorption refrigeration ice maker has been built for demonstration purposes. The working pair consists of methanol used as the refrigerant and activated carbon as the adsorption medium. The adsorbent bed is constructed of two flat-plate collectors, with a total surface area of 1.5 m². Solar radiation can be simulated with quartz lamps and some important parameters such as temperature and pressure of each subsystem can be handled by a computer. The experimental results show that this machine can produce 4–5 kg of ice after receiving 14–16 MJ of radiation energy with a surface area of 0.75 m², while producing 7–10 kg of ice after receiving 28–30 MJ of radiation energy with 1.5 m². These are the most advanced results for a solar ice maker so far. All these successful achievements will speed up the commercial processing of a solar ice maker.     

Performance of an adsorptive solar ice maker operating with a single double function heat exchanger (evaporator/condenser)

With the help of an experimental data base, the overall heat transfer coefficient of the double function heat exchanger (condenser/evaporator) of a novel adsorptive solar ice maker (ASIM) was estimated. The calculation shows that this parameter, which is required for simulation, is not constant during the adsorption–evaporation phase and that it is higher during the water freezing phase. With the help of a predictive mathematical model the operation of the studied ASIM was simulated and the results were analysed. They showed that with a consistent design of the different components of this machine the daily ice production could exceed 5.2 kg, with a COPs of more than 0.14. This value of the COPs is very interesting in comparison with those usually obtained from the ASIMs operating with separate condenser and evaporator, i.e. 0.08–0.2, according to the meteorological conditions or technologies.     

民用型太阳能制冰机的研究

在传统大阳能制冰机实验研究基础上，对1m^2的太阳能制冰机按产业化生产的技术要求进行了样机的设计与研发，所试制样机从外观要求，结构配置，系统操作上均克服了以往样机试验中的缺陷，使太阳能制冰机真正走入了产业化生产的流水线，所试制样机可直接进入市场。     

Experimental study on adsorbent of activated carbon with refrigerant of methanol and ethanol for solar ice maker

Two different working pairs, one of them activated carbon–methanol and the other activated carbon–ethanol, are used in a solar ice maker to determine ice production and refrigerant criteria in desorption and adsorption processes under similar radiation energy and environmental conditions. Experimental results show that ice can be produced with activated carbon–methanol, however, ice could not be obtained with the use of activated carbon–ethanol using the same solar ice maker with similar environments and radiation intensities. Further experiments have shown that the adsorbing and desorbing refrigerant amounts of the two working pairs gave quite different results, the activated carbon–methanol has proven to be the superior one of the two working pairs in ice production of solar adsorption refrigeration.     

Heat and mass transfer in a flat plate solar solid adsorption refrigeration ice maker

A uniform pressure model is presented to describe the heat and mass transfer in an adsorbent bed for a flat plate solar ice maker. This model accounts for heat and mass transfer in a porous bed in a two-dimensional transient process. An experiment has been conducted to validate this model and the calculated results are in good agreement with experiments. With the help of this model, the transient analysis and performance prediction of an intermittent solar powered solid refrigerator can be presented.     

A new solar powered adsorption refrigerator with high performance

An adsorptive solar refrigerator was built in September 2000 in Yverdon-les-Bains, Switzerland. The adsorption pair is silicagel + water. The machine does not contain any moving parts, does not consume any mechanical energy except for experimental purposes and is relatively easy to manufacture. Cylindrical tubes function as both the adsorber system and the solar collector (flat-plate, 2 m² double glazed); the condenser is air-cooled (natural convection) and the evaporator contains 40 l of water that can freeze. This ice functions as a cold storage for the cabinet (320 l).The first tests (September 2000) showed a very promising performance, with a gross solar cooling COP_SR of 0.19. After minor modifications, a second test series was carried out during summer 2001. This test series shows how the external parameters influence the machine with respect to the COP_SR (irradiation and external temperature). The latter varies between 0.10 and 0.25 with a mean value of 0.16. These values are higher than those obtained by earlier solar powered refrigerators (0.10–0.12).This paper describes the principle of the cycle, the different components of the machine, and the test procedure. The test procedure includes a constant daily cooling requirement. The experimental results presented were taken over a period of two months.     

Simulation of operating characteristics of the silica gel-water adsorption chiller powered by solar energy

A lumped parameter model of a silica gel-water adsorption chiller driven by solar energy was introduced for the operating characteristics investigation. Matlab-Simulink, as a high-performance computing and programming tool, was used to simulate the operating characteristics of the chiller. Effects of the hot water tank capacity, the cycle time and the initial hot water temperature on the performance of the chiller were analyzed when the chiller was driven by a stable heat source and solar energy respectively. The simulation results indicated that when the chiller was driven by solar energy, the open circulation of the hot water with a short cycle time and the closed circulation of hot water with a longer cycle time were better. A proposal was also provided for the chiller driven by solar energy to work under the optimum working conditions, such as hot water circulation mode, cycle time and initial temperature.     

Preparation, thermal properties and thermal reliability of palmitic acid/expanded graphite composite as form-stable PCM for thermal energy storage

This study is focused on the preparation and characterization of thermal properties and thermal reliability of palmitic acid (PA)/expanded graphite (EG) composite as form-stable phase change material (PCM). The maximum mass fraction of PA retained in EG was found as 80 wt% without the leakage of PA in melted state even when it is heated over the melting point of PA. Therefore, the PA/EG (80/20 w/w%) composite was characterized as form-stable PCM. From differential scanning calorimetry (DSC) analysis, the melting and freezing temperatures and latent heats of the form-stable PCM were measured as 60.88 and 60.81 °C and 148.36 and 149.66 J/g, respectively. Thermal cycling test showed that the composite PCM has good thermal reliability although it was subjected to 3000 melting/freezing cycles. Fourier transformation infrared (FT-IR) spectroscopic investigation indicated that it has good chemical stability after thermal cycling. Thermal conductivities of PA/EG composites including different mass fractions of EG (5%, 10%, 15% and 20%) were also measured. Thermal conductivity of form-stable PA/EG (80/20 w/w%) composite (0.60 W/mK) was found to be 2.5 times higher than that of pure PA (0.17 W/mK). Moreover, the increase in thermal conductivity of PA was confirmed by comparison of the melting and freezing times of pure PA with that of form-stable composite. Based on all results, it was concluded that the form-stable PA/EG (80/20 w/w%) has considerable latent heat energy storage potential because of its good thermal properties, thermal and chemical reliability and thermal conductivity.     

Thermal property measurement and heat transfer analysis of acetamide and acetamide/expanded graphite composite phase change material for solar heat storage

As a phase change material (PCM), acetamide (AC) can be a potential candidate for energy storage application in the active solar systems. Its utilization is however hampered by poor thermal conductivity. In this work, AC/expanded graphite (EG) composite PCM with 10 wt% (mass fraction) EG as the effective heat transfer promoter was prepared; its thermal properties were studied and compared with those of pure AC. Transient hot-wire tests showed that the addition of 10 wt% EG led to about five-fold increase in thermal conductivity. Investigations using a differential scanning calorimeter revealed that the melting/freezing points shifted from 66.95/42.46 °C for pure AC to 65.91/65.52 °C for AC/EG composite, and the latent heat decreased from 194.92 to 163.71 kJ kg⁻¹. In addition, heat storage and retrieval tests in a latent thermal energy storage unit showed that the heat storage and retrieval durations were reduced by 45% and 78%, respectively. Further numerical investigations demonstrated that the less improvement in heat transfer rate during the storage process could be attributed to the weakened natural convection in liquid (melted) AC because of the presence of EG.     

Thermal characterization of phase change materials based on linear low-density polyethylene,paraffin wax and expanded graphite

Thermal characterization of Phase Change Materials (PCMs) based on linear low-density polyethylene (LLDPE), paraffin wax (W) and expanded graphite (EG) is reported in this paper. Investigated PCMs showed high potential for application in energy storage systems.The latent heat, L_m, sensible heat Q_sens, and the ability of the prepared PCMs to store and release thermal energy were investigated using specific home-made equipment based on the transient guarded hot plane method (TGHPT). The sensible heat of PCM containing 40 wt.% of paraffin wax was investigated in the temperature range 25–35 °C, they exhibited a drop in Q_sens from 31 to 24 J/g depending on the concentration of EG. A similar decrease in sensible heat with increased loading of EG was observed for PCMs containing 50 wt.% of EG.The storage and release of thermal energy during phase change which is associated with the latent heat of the materials were investigated within the temperature range 20–50 °C. PCMs containing 40 wt.% of paraffin wax exhibited latent heat of 36 J/g, whereas the latent heat of PCMs containing 50 wt.% of paraffin wax was 49 J/g. The addition of EG decreased the time needed to melt and solidify PCMs due to increase in thermal conductivity of PCMs with increase in EG content. This behavior was confirmed by the thermal conductivity measurements, where thermal conductivity increased from 0.252 for sample without EG to 1.329 W/m × °C for PCM containing 15 wt.% of EG.The reproducibility of storage and release of thermal energy by PCMs was demonstrated by subjecting them to repeated heating and cooling cycles (over 150 cycles).     

Preparation and properties of thin epoxy/compressed expanded graphite composite bipolar plates for proton exchange membrane fuel cells

Although the composite bipolar plates prepared by the method of the vacuum resin impregnation in compressed expanded graphite (CEG) sheets have been applied in the KW-class stacks, there have been few investigations of the preparation and properties of them so far. In this research, the influences of the microstructure on the physical properties of the thin epoxy/CEG composites (the thickness is 1 mm) are investigated for the first time and the optimum preparation conditions are obtained. Results demonstrated that the mechanical property and the impermeability of the composites increases evidently with the resin content changing from 4% to 30%, while the electrical properties keep nearly constant. It can be attributed to the continuous expanded graphite (EG) conductive network of the raw CEG sheet. The epoxy (30 wt.%)/CEG composite is shown to be the optimum composite, displaying in-plane conductivity of 119.8 S cm⁻¹, through-plane resistance of 17.13 mΩ cm², density of 1.95 g cm⁻³, gas permeability of 1.94 × 10⁻⁶ cm³ cm⁻² s⁻¹ and flexural strength of 45.8 MPa. The alcohol scrubbing is the optimum method of surface post-processing. The performance of a single cell with the optimum composite bipolar plates is tested and demonstrated to be outstanding. Above all, the composite prepared by resin vacuum impregnation in the CEG sheet is a promising candidate for bipolar plate materials in PEMFCs.     

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.     

Design and performance prediction of a novel double heat pipes type adsorption chiller for fishing boats

A novel double heat pipe type adsorber, which uses compound adsorbent of CaCl₂ and expanded graphite to improve the adsorption performance, is designed. The double heat pipes are integrated into the adsorbers in order to solve the problem of the corrosion between seawater and the steel adsorber in ammonia system and improve the heat transfer performance of the adsorber. There are two kinds of heat pipes integrated with the adsorber. One is the split type heat pipe for heating the adsorber in desorption phase, the other one is the two-phase closed thermosyphon heat pipe for cooling the adsorber in adsorption phase. The performance of two-adsorber adsorption chiller integrated with double heat pipes is predicted. The heat transfer performance of the heat pipes can meet the heat demands for adsorption/desorption of the adsorbent when the heating/cooling time is 720 s and mass recovery time is 60 s. When the exhaust gas temperature is 550 °C, the cooling water temperature is 25 °C, the inlet and outlet chilled water is −10 and −15.6 °C, respectively; the simulation results show that the cooling power and COP of this adsorption system are 5.1 kW and 0.38, respectively.     

Novel composite sorbent for resorption systems and for chemisorption air conditioners driven by low generation temperature

The utilization of a composite sorbent (NaBr and expanded graphite) in chemisorption air conditioning systems driven by low-grade heat source, and in resorption systems with simultaneous heating and cooling effects was experimentally investigated using bench-scale prototypes. The mass of ammonia desorbed and adsorbed was measured, and used to calculate the specific cooling capacity. The sorbent produced 219 kJ kg⁻¹ of cooling at 5 °C and 510 kJ kg⁻¹ at 15 °C, when the heat source temperature was 65 °C and the heat sink temperature was 30 °C. The air conditioning system mean specific cooling power (SCP), and mean coefficient of performance (COP) were calculated based on the desorbed and adsorbed masses, and on the variation of temperature in the reactors. For the same heat source and heat sink temperatures mentioned above, the air conditioning system had a SCP of 129 ± 7 W kg⁻¹ and a COP of 0.46 ± 0.01, when cooling occurred at 15 °C. Regarding the utilization of the composite sorbent in resorption machines, the prototype was tested for production of cooling/heating at −5/50 °C, and at 10/70 °C. In the former condition, the COP was only 0.02, but in the latter condition, there was a tenfold increase in the COP, and the combined coefficient of performance and amplification reached 1.11, which indicates the energy saving potential of resorption systems using the studied sorbent.     

Lithium oxalyldifluoroborate/carbonate electrolytes for LiFePO4/artificial graphite lithium-ion cells

The electrolytes based on lithium oxalyldifluoroborate (LiODFB) and carbonates have been systematically investigated for LiFePO₄/artificial graphite (AG) cells, by ionic conductivity test and various electrochemical tests, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge test. The conductivity of nine electrolytes as a function of solvent composition and LiODFB salt concentration has been studied. The coulombic efficiency of LiFePO₄/Li and AG/Li half cells with these electrolytes have also been compared. The results show that 1 M LiODFB EC/PC/DMC (1:1:3, v/v) electrolyte has a relatively higher conductivity (8.25 mS cm⁻¹) at 25 °C, with high coulombic efficiency, good kinetics characteristics and low interface resistance. With 1 M LiODFB EC/PC/DMC (1:1:3, v/v) electrolyte, LiFePO₄/AG cells exhibit excellent capacity retention ∼92% and ∼88% after 100 cycles at 25 °C and at elevated temperatures up to 65 °C, respectively; The LiFePO₄/AG cells also have good rate capability, the discharge capacity is 324.8 mAh at 4 C, which is about 89% of the discharge capacity at 0.5 C. However, at −10 °C, the capacity is relatively lower. Compared with 1 M LiPF₆ EC/PC/DMC (1:1:3, v/v), LiFePO₄/AG cells with 1 M LiODFB EC/PC/DMC (1:1:3, v/v) exhibited better capacity utilization at both room temperature and 65 °C. The capacity retention of the cells with LiODFB-based electrolyte was much higher than that of LiPF₆-based electrolyte at 65 °C, while the capacity retention and the rate capacity of the cells is closed to that of LiPF₆-based electrolyte at 25 °C. In summary, 1 M LiODFB EC/PC/DMC (1:1:3, v/v) is a promising electrolyte for LiFePO₄/AG cells.     

Recent progress about expanded graphite matrix composite phase change material for energy storage

膨胀石墨基复合相变材料具有导热系数高,储能密度大以及相变过程无液体泄漏等优点,是近年来储能科学领域的研究热点.本文探讨了应用于储热系统的相变材料的性能及分类,并对膨胀石墨及其复合相变材料的制备方法进行了简要分析,最后综述了石蜡类,脂肪酸类,共晶混合物类,聚乙二醇以及乙酰胺等膨胀石墨基复合相变材料的国内外研究进展.     

Effects of impregnating variables on dynamic sorption characteristics and storage properties of composite sorbent for solar heat storage

A family of composite sorbents was prepared by impregnating silica gel in the solution of the hygroscopic salt CaCl₂ for solar heat storage. The characteristics of water adsorbed on the composite sorbents prepared under different impregnating conditions were measured by a micromeritics gas adsorption analyzer, a Calvet-type microcalorimeter and an open-type gravimetric method. From the results of these dynamic sorption measurements, the effects of impregnating variables on the characteristics of water adsorbed on the composite sorbents were evaluated. The composite sorbents prepared under different impregnating conditions were also tested on an open-type sorption storage system. The composite sorbent prepared by impregnating in the CaCl₂ solution of 30% showed a high and stable storage capacity of 1020 J g⁻¹ at the charging temperature of about 90 °C. This study demonstrates a great potential in controlling the sorption characteristics as well as the storage properties of the composite sorbents by optimizing the impregnating variables to meet the specific demands of solar heat storage.     

Desorption characteristics of desiccant bed for solar dehumidification/humidification air conditioning systems

Theoretical and experimental investigation on the desorption characteristics of a packed porous bed is presented in this study. The granules of burned clay are applied as a desiccant carrier. Calcium chloride is used as the working desiccant. The theoretical model defines the transient gradient of air stream parameters (humidity and temperature) as well as desiccant concentration in the bed. In the experimental study, transient concentration gradient in the bed is evaluated by weight method. The bed is divided into seven separate layers. Air stream at low temperature and nearly constant inlet parameters are used for desorption purposes. Concentration gradient in the bed is found highly dependent on the mass transfer rate. For the specified operating conditions and stated assumptions, experimental measurements shows acceptable agreement with the analytical solution.