Silica Gel Water Research Articles

Universal relation between the boundary temperatures of a basic cycle of sorption heat machines

For analyzing a basic cycle of an adsorption heat machine (AHM) an empiric rule was suggested, which manifests that adsorption isosters and equilibrium line ln P ( 1 / T ) for the pure sorbate intersect at T approaching infinity. This rule prompts how to plot the cycle, gives a link between the boundary temperatures of the cycle and allows estimation of a minimal temperature T g min of an external heat source that is necessary to drive the cycle. In this paper the validity of the T g min estimation was justified for working pairs which are most commonly used for adsorption units: water–silica gel, water–zeolite 13X, water–zeolite 4A, water–selective water sorbents (SWSs), CO 2 –carbon, methanol–carbons (AC-35, TA90), methanol–hydrophobic zeolite CBV 901 Y and ammonia–carbon PX31. Four main working pairs for absorption heat machines—ammonia–water, water–LiBr, methanol–LiBr and R22–isobutylacetate—are also analyzed. This allowed the formulation of requirements to an optimal adsorbent to be used in a single-effect non-regenerative cycle of an AHM. The accuracy of the T g min estimation was examined for each pair. Moreover, it was shown that Trouton's rule is always valid if sorption equilibrium obeys the Polanyi potential theory, i.e., the equilibrium sorption is a unique function of the sorption potential Δ F = - RT ln ( P / P 0 ) . For chemical reactions between various salts and sorbates this rule is violated because of a large difference between the standard changes of the entropy and enthalpy in the course of reaction and evaporation. In this case T g min can be calculated from the Clausius–Clapeyron and Vant-Hoff equations.

Experimental study on improved two-bed silica gel–water adsorption chiller

A novel silica gel–water adsorption chiller with two chambers has been built in Shanghai Jiao Tong University (SJTU). This chiller combines two single bed systems (basic system) without any vacuum valves. One adsorber, one condenser and one evaporator are housed in the same chamber to constitute one adsorption/desorption unit. In this work, the chiller is developed and improved. The improved chiller is composed of three vacuum chambers: two adsorption/desorption vacuum chambers (the same structure as the former chiller) and one heat pipe working vacuum chamber. The evaporators of these two adsorption/desorption units are combined by a heat pipe. So, no valves are installed in the chilled water sub system and one vacuum valve connects the two adsorption/desorption chambers together to improve its performance. The performance of the chiller is tested. As the results, the refrigerating capacity and the COP of the chiller are, respectively, 8.69 kW and 0.388 for the heat source temperature of 82.5 °C, the cooling water temperature of 30.4 °C and the chilled water outlet temperature of 11.9 °C. For a chilled water outlet temperature of 16.5 °C, the COP reaches 0.432, while the refrigerating capacity is near 11 kW. There is an improvement of at least 12% for the COP compared with the former chillers.

Experimental study of adsorption chiller driven by variable heat source

A silica gel–water adsorption chiller has been developed in recent years and has been applied in an air conditioning system driven by solar energy. The heat source used to drive the adsorption chiller is variable at any moment because the solar radiation intensity or the waste heat from engines varies frequently. An adsorption cooling system may be badly impacted by a variable heat source with temperature variations in a large range. In this work, a silica gel–water adsorption chiller driven by a variable heat source is experimentally studied. The influences of the variable heat source on the performance of the chiller are analyzed, especially for a continuous temperature increase process and a continuous temperature decrease process of the heat source. As an example, the dynamic characteristics of the heat source are also analyzed when solar energy is taken as the heat source of the adsorption chiller. According to the experimental results for the adsorption chiller and the characteristics of the heat source from solar energy, control strategies of the adsorption chiller driven by solar energy are proposed.

Comparative analysis of thermally activated, environmentally friendly cooling systems

This paper compares the relative performances of three different thermally activated, environmentally friendly cooling systems, e.g. a silica-gel–water adsorption system, a LiBr–H 2O absorption system and a desiccant air system. The adsorption and absorption systems in the current study employ water as the refrigerant, while the desiccant system cools atmospheric air directly. Each of these systems can be utilized at relatively low heat source temperatures such as achieved by flat plate solar collectors, but it is unclear which of these systems is best suited to what range of heat source temperature. Our study explores answers to this question by generating quantitative results comparing their relative thermal performance, i.e. COP and refrigeration capacity, and a qualitative comparison based on the size, maturity of technology, safe operation etc. In order to provide a fair comparison between the fundamentally different systems, a UA (overall heat transfer coefficient multiplied by the heat transfer area) value of 1.0 kW °C −1 is considered for the heat exchanger that transfers heat from the supplied hot water. Furthermore, to compare systems of similar size, the mass of silica-gel in the adsorption and desiccant systems and the mass of LiBr–H 2O solution in the absorption system were specified such that each system provides the same amount of refrigeration (8.0 kW) at a source temperature of 90 °C. It is found that the absorption and adsorption cooling systems have a higher refrigeration capacity at heat source temperatures below 90 °C, while the desiccant air system outperforms the others at temperatures above 90 °C.

Thermodynamic limits to thermal regeneration in adsorption cooling cycles

Energy and exergy models for ideal adsorption cycles with isothermal beds and no mass recovery are developed to predict the limits to COP enhancement using thermal regeneration. The models are applied to compare the performance of zeolite–water and silica gel–water adsorbent–refrigerant pairs over a range of maximum bed temperatures. The thermodynamic consistencies of several alternate adsorption property assumptions are quantified. Differences in adsorption characteristics between zeolite–water and silica gel–water result in a significantly larger potential to enhance COP by implementing thermal regeneration for zeolite–water. Based on COP, the zeolite–water pair is preferred when both thermal regeneration and a high temperature thermal energy source (>150 °C) are used, while the silica gel–water pair is preferred when thermal regeneration is not used and/or a low temperature thermal energy source (<100 °C) is used.

Fabrication of bulk-modified carbon paste electrode containing α-PW 12O 403− polyanion supported on modified silica gel: Preparation, electrochemistry and electrocatalysis

α-PW 12O 40 3− (PW 12) supported on the surface of silica gel derivatized by 3-aminopropyl(triethoxy)silane (devoted briefly as SiNH 3PW 12) was synthesized and used as bulk modifier to fabricate a renewable three-dimensional chemically modified electrode. The electrochemical behavior of the modified electrode was characterized by cyclic voltammetry. There is an ionic bonding character between PW 12 and the surface amino groups of modified silica, which greatly improves the stability of SiNH 3PW 12-modified carbon paste electrode due to insolubility of silica gel in water. The SiNH 3PW 12 bulk-modified carbon paste electrode not only maintains the electrochemical activity of PW 12, but also exhibits remarkable advantages of renewability, as well as simple preparation and inexpensive material. The modified electrode offers an excellent and stable electrocatalytic response for the reduction of IO 3 − and hydrogen peroxide. The SiNH 3PW 12-CPE is successfully applied as an electrochemical detector to monitor IO 3 − in flow injection analysis (FIA). The catalytic peak current was found to be linear with the IO 3 − concentration in the range 5 × 10 −6 to 1 × 10 −3 mol L −1. The detection limit of the proposed method was found to be 3.1 × 10 −6 mol L −1 for IO 3 − determination.

Comparison among three adsorption pairs for using as the working substances in a multi-bed adsorption heat pump

A comparison among three adsorption pairs (activated carbon–methanol, silica gel–water and 13X molecular sieves–water) for using as the working substances in a four-bed adsorption heat pump is presented. A solid-side resistance model considering a finite mass flowrate of heating/cooling fluid was adopted. The result shows that, for small mass flowrates of the heating/cooling fluid, assuming a constant temperature for the heating/cooling fluid might give rise to an over-predication of the system performance. For the adsorption heat pump, there exists an optimum mode operating time ( τ) which corresponds to a maximum specific cooling power. For operations at the maximum specific cooling power ( τ = 3–6 min) and with regeneration temperatures in the range 80–120 °C, among the three adsorption pairs, the activated carbon–methanol pair leads to the highest specific cooling power, COP value and second-law efficiency. For the silica gel–water pair, the temperature rise resulting from adsorption heat significantly downgrades the system performance. This causes its corresponding specific cooling power, COP value and second-law efficiency to be lower than those for the activated carbon–methanol pair. The 13X molecular sieves–water pair possesses the smallest cyclic variation of adsorbate content, thus it yields the lowest specific cooling power, COP value and second-law efficiency.

Two bed silica gel–water adsorption chillers: An effectual lumped parameter model

This article develops an improved lump-parameter design model to investigate the water-circulation heat recovery scheme as applied to the two-bed silica gel–water adsorption chillers. We demonstrate that performance predictions stemming from this improved lump-parameter formalism compare favorably with experimental results at various conditions, particularly at the industrial rated conditions. We find that the present lump-parameter formalism adequately elucidates the water-circulation heat recovery scheme as does the distributed-parameter formalism. In the studied working condition of a two-bed silica gel–water adsorption chiller, the differences in cooling capacities and coefficients of performance (or COP) by using the two different formalisms are typically less than 10%. This gives rise to a useful and rapid design tool for the industry.

Theoretical and experimental study on characteristics of a novel silica gel–water chiller under the conditions of variable heat source temperature

In this paper, a transient model of a silica gel–water adsorption chiller, which is developed in Shanghai Jiao Tong University (SJTU), is developed in order to simulate the evaporating, condensing, and adsorption temperature. Furthermore, this model is verified by a series of experiments. The theoretical studies and experimental data show that the coefficient of performance (COP) is influenced significantly by the variation rates of the heat source temperatures. The results also show that when this chiller is driven by solar energy, a buffer tank should be adopted in the system in order to get better performance when solar insolation is low, and should not be utilized when solar insolation is high, otherwise low COP will be gotten for the reason of the consumption of high electric energy.

Characteristics of a Heat Output of an Adsorption Heat Pump Aided with Pumping

An adsorption heat pump (AHP) is proposed to expand the available heat sources and to increase cooling power. The main objective of this research is to carry out the conceptual study of operation range in points of adsorption equilibria and experimental study of desorption and adsorption processes by silica gel–water AHP which is composed of a mechanical booster pump (MBP) and an adsorber of packed bed. This effect is confirmed experimentally. In these experimental ranges, the amount of pump power is effective to increase cooling power. Therefore, it is confirmed that the range of temperature, which is able to use as a heat source, is expanded and cooling power is increased directly by pump power.

An efficient solar-powered adsorption chiller and its application in low-temperature grain storage

A novel solar-powered adsorption cooling system for low-temperature grain storage has been built, which consists of a solar-powered water heating system, a silica gel–water adsorption chiller, a cooling tower and a fan coil unit. The adsorption chiller is composed of two identical adsorption units, each of them containing an adsorber, a condenser, and an evaporator/receiver. The two water evaporators have been incorporated into one methanol evaporator by the use of the concept of a gravity heat pipe. In order to improve the system efficiency and achieve continuous cooling production, the adsorbers are operated out-of-phase, and heat and mass recovery processes have been used. During the period from July to September of 2004, the system was put into experimental operation to cool the headspace (i.e., the air volume above the grain) of a grain bin. Three months of operation showed promising performance. The chiller had a cooling power between 66 and 90 W per m 2 of collector surface, with a daily solar cooling coefficient of performance (COP solar) ranging from 0.096 to 0.13. The electric cooling COP was between 2.6 and 3.4.

A comparative evaluation of two different heat-recovery schemes as applied to a two-bed adsorption chiller

This study investigates the efficiency of two distinct heat-recovery schemes as applied to the two-bed silica gel–water adsorption chiller. Performance predictions stemming from an experimentally verified distributed-parameter model compare favorably with the experimental results at various operating conditions. Our study highlights the pivotal role played by heat-recovery schemes to ameliorate the chiller performance and substantially improve the coefficient of performance (or COP). The two different schemes essentially offer the same cooling capacity and similar COP boosting capability. In the studied working conditions of a two-bed adsorption chiller, the difference in COP by using the two schemes is less than 5%.

Development of a Small-Capacity Adsorption System for Heating and Cooling Applications

This paper describes the prototype of a small adsorption heat pump working on the adsorption pair silica gel-water. The developed system consists of two modules, each containing an adsorption reactor and a second heat exchanger for evaporation and condensation of the working fluid. The two modules are operated periodically in order to obtain a quasi-continuous heating or cooling output. A reliable control unit has been developed and implemented in the machine. Hot water provided at temperatures of 80°C to 95°C is used to drive the machine, and heat from the low-temperature heat source (10°C to 20°C) is upgraded to the intermediate temperature level of about 35°C to 45°C, which is suitable for low-temperature heating systems such as wall or floor heating installations. For cooling applications, chilled water of 10°C to 15°C is produced in an almost continuous mode. The first prototypes have been tested. Heating coefficients of performance (COPs) of more than 1.3 and cooling COPs for air-conditioning purposes (12°C to 15°C) of 0.5 have been achieved. The machine provided a mean heating power of up to 15 kW and a cooling power of up to 8 kW.

06/01215 Energy optimisation model for a CCHP system with available gas turbines: Kong, X. Q. et al. Applied Thermal Engineering, 2005, 25, (2–3), 377–391

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.

Efficient adsorption refrigerators integrated with heat pipes

Several novel ideas to use heat pipes in adsorption water chiller or ice maker are presented in this paper. Experimental results have shown that the adsorption refrigerators are very efficient. The first example of such systems is a small scale silica gel–water adsorption water chiller with cooling power rated as 10 kW; the system could be powered by 60–100 °C hot water, a cooling COP = 0.4 has been achieved when driven by 85 °C hot water. This adsorption chiller has been used for solar powered air conditioner and also as the chiller for CCHP system. The second example is a silica gel–water adsorption room air conditioner powered by 80 °C hot water. The system is very compact and is suggested for potential applications of micro CCHP system based on fuel cells. The system has a COP of over 0.3 and cooling power of about 1 kW. The third example is the use of split heat pipes to heat or cool the adsorber for making ice in fishing boats. The application of these technologies avoids the corrosion of adsorber at the heating phase by exhausted gases and at the cooling phase by seawater, and also has the advantage of high heat transfer performance. With such arrangement and careful considerations of the arrangement of wicks in heat pipes, and also the use of composite adsorbent (calcium chloride and activated carbon)-ammonia adsorption pair, the system test has shown the specific refrigeration power for more than 730 W/kg at −15 °C.

Evaluation of the performance of solid sorption refrigeration systems using carbon dioxide as refrigerant

The refrigerant–adsorbent pairs at present mostly used in research on solid-sorption refrigeration cycles are ammonia–carbon, methanol–carbon, water–silica gel and water–zeolite. Porosity tests have been carried out on an alternative, carbon dioxide, as refrigerant with several types of carbon, zeolite and silica gel as adsorbent. The results of fitting to the Dubinin equation and modelling of cycles based on these pairs are presented. Due to its low latent heat, the performance of adsorption refrigeration systems using CO 2 as refrigerant is poor. However, carbon dioxide may be useful where the toxicity and incompatibility of ammonia and copper is valued.

Experimental investigation of a solar adsorption chiller used for grain depot cooling

The solar cooling technology is attractive since cooling load of building is roughly in phase with solar energy availability. In this study, a solar adsorption chiller was built and tested with aim of developing an alternative refrigeration system used for grain cooling storage. This solar adsorption chiller consists of four subsystems, namely, a solar water heating unit with 49.4 m 2 solar collecting area, a silica gel–water adsorption chiller, a cooling tower and a fan coil unit. In order to achieve continuous refrigeration, two adsorption units are operated out-of-phase with mass recovery cycle in the adsorption chiller. Field test results show that, under the climatic conditions of daily solar radiation being about 16–21 MJ/m 2, this solar adsorption chiller can furnish 14–22 °C chilled air with an average cooling output ranging from about 3.2–4.4 kW, its daily solar cooling COP (coefficient of performance) is about 0.1–0.13.

Study on a dual-mode, multi-stage, multi-bed regenerative adsorption chiller

In this paper, a detailed parametric study on a dual-mode silica gel–water adsorption chiller is performed. This advanced adsorption chiller utilizes effectively low-temperature solar or waste heat sources of temperature between 40 and 95 °C. Two operation modes are possible for the advanced chiller. The first operation mode will be to work as a highly efficient conventional chiller where the driving source temperature is between 60 and 95 °C. The second operation mode will be to work as an advanced three-stage adsorption chiller where the available driving source temperature is very low (between 40 and 60 °C). With this very low driving source temperature in combination with a coolant at 30 °C, no other cycle except an advanced adsorption cycle with staged regeneration will be operational. In this paper, the effect of chilled-water inlet temperature, heat transfer fluid flow rates and adsorption–desorption cycle time effect on cooling capacity and COP of the dual-mode chiller is performed. Simulation results show that both cooling capacity and COP values increase with the increase of chilled water inlet temperature with driving source temperature at 50 and 80 °C in three-stage mode, and single-stage multi-bed mode, respectively. However, the delivered chilled-water temperature increases with chilled-water inlet temperature in both modes.

Development of an adsorption chiller and heat pump for domestic heating and air-conditioning applications

The scope of this paper is to present the development of a prototype of a small adsorption heat pump working on the adsorption pair silica gel–water. The development of this prototype with remarkable high power densities has been carried out during the last year and is a result of continued joint work on adsorption heat transformation systems carried out at SorTech AG and the Fraunhofer Institute.

Experimental investigation of a micro-combined cooling, heating and power system driven by a gas engine

An experimental investigation of the performance of a micro-combined cooling, heating and power (CCHP) system is described. The natural gas and LPG-fired micro-CCHP system uses a small-scale generator set driven by a gas engine and a new small-scale adsorption chiller, which has a rated electricity power of 12 kW, a rated cooling of 9 kW and a rated heating capacity of 28 kW. Silica gel–water is used as working pair in the adsorption cooling system. The refrigeration COP of the adsorption chiller is over 0.3 for 13 °C evaporation temperature. The test facility designed and built is described, which supplies better test-rig platform for cooling, heating and power cogeneration. Experimental methodology of this system is presented and the results are discussed. An energetic analysis of micro-CCHP system is performed as well. The overall thermal and electrical efficiency is over 70%.