Silica Gel-water Pair Research Articles

Three Dimensional CFD Simulation Model with Experimental Validation of Silica Gel–Water Pair Adsorption Cooling Bed

In this paper, a three-dimensional full simulation model of a solar-driven adsorption cooling system integrated with user-defined functions (UDF) is successfully validated with a manufactured adsorption chamber filled with silica-gel type A. model is validated on the desorption phase with a maximum deviation error of 1.7%. Temperature with adsorption contours are shown for more understanding of the heat flow with adsorption behaviour inside the chamber in all directions. The maximum temperature at the desorption phase is reached after 300 seconds at fins area, and adsorption maximum uptake reached 0.16 kg/kg at fins wall but reached from 0.13 to 0.14 kg/kg around fins area. The average adsorption uptake reached 0.136 at the end of the adsorption process. The resultant uptake between adsorption and desorption is 0.018 kg/kg with a refrigerant mass of 1.8 kg in a complete cycle time.

Knowledge construction and systematization of solar adsorption refrigeration prototypes

Tests of adsorption cooling prototypes with solar energy that use the silica gel–water​ pair have been reported in the literature; however, there is no work that analyzes, compares and integrates them to generate new knowledge from their data and results. In this context, our proposal consists comparing thermodynamic results, determining the characteristics of their parts and functions, and analyzing technical contradictions through the TRIZ methodology, for twelve identified adsorption refrigeration prototypes (including one where a co-author of this paper participated). The results and characteristics of the analyzed prototypes have been systematized, also a diversity of constructive forms was found in the studied prototypes, as well as in the non-standardized results and tests because some aspects were emphasized in each test. The results and simulations are compared using COP and refrigeration capacity as parameters to extrapolate new results consisting of a hybridized conceptual model and projected test results based on those found in the literature.

The Effects of Using Steam to Preheat the Beds of an Adsorption Chiller with Desalination Function

Adsorption chillers are a promising alternative to traditional compressor-based devices. Adsorption chillers can be supplied with low- or medium-temperature waste heat or heat from renewable energy sources. In addition, they can be used for water desalination purposes. Thus, the adsorption chillers are unique devices that meet essential problems of the modern world: a need to limit the negative impact of humankind on the natural environment and growing problems with access to drinking water. However, adsorption chillers also have disadvantages, including ineffective operation and large size. Therefore, in this paper, the influence of steam utilization on the operation of an adsorption chiller with water desalination function was investigated experimentally, which has not been done before. The research was carried out on the adsorption chiller, working on a silica gel–water pair, installed in the AGH UST Center of Energy. The chiller was modified to preheat the sorbent with the use of steam. The results show that the use of steam instead of water for preheating the bed leads to higher temperatures in the heat exchanger and the bed. As a result, heat transfer from the heating medium to the bed is more intense, and a significant shortening of the desorption process is observed. In the case of using steam for preheating, the desorption time was about 30 s, while for water, it was 300 s. Thanks to this solution, it is possible to reduce the size of the device and increase its efficiency. The proposed solution opens a new course of research on adsorption chillers and broadens the horizon of their applications, as steam is a by-product of many industrial processes.

Evaluation on performance of solar adsorption cooling of silica gel and SAPO-34 zeolite

As a technology of the green energy utilization, solar adsorption cooling is a promising way to make use of the solar energy in the future. Aiming to the efficiency improvement of such kind system, the performance of the solar adsorption cooling system that employed the silica gel-water as the working pair was evaluated experimentally. With a mono-axial solar collector of tracking parabolic trough to provide the heat supply to the adsorption bed, the effect of the adsorption time on the system performance was mainly addressed in the study. The experimental results revealed that the performance of the system reached the maximum as the adsorption time was 45 min. Correspondingly, the coefficient of performance of the system reached 0.258. In addition, a comparison of the optimal performance of the silica gel system to the SAPO-34 zeolite system was conducted. It was revealed that both the cooling capacity and the performance coefficient of the silica gel system depended more strongly on the adsorption time than those of the SAPO-34 zeolite system did. At the optimal adsorption time, the coefficient of performance of the silica gel system was 1.93 times as that of the SAPO-34 zeolite system. In general, the silica gel-water pair has shown better performance as compared to the SAPO-34 zeolite-water pair as used in the solar adsorption cooling system.

Method for estimating optimum cycle time based on adsorption chiller parameters

The objective of this study is to construct mathematical expressions to estimate the optimum cycle time which maximizes the specific cooling power (SCP) of adsorption chillers with silica gel-water pair. The operating temperature conditions of hot water, cooling water and chilled water, as well as the design parameters such as heat transfer coefficient and heat capacities of the adsorbent heat exchanger and silica-gel, are taken into account to estimate the optimum cycle time. Totally 729 cases were examined to calculate the optimum cycle times from which mathematical expressions were derived. The empirical method reflecting the six parameters successfully estimates the optimum cycle time of the simulations with the coefficient of determination as high as 0.98. The performance for estimating the maximum SCP through the optimum cycle time is even better.

Utilizing Accessible Heat Enhancing Cooling Effect with Three Bed Solar Adsorption Chiller

ABSTRACTConventional solar heat-driven single stage two bed chillers demand a large area for installation of solar thermal collector to activate the chiller, but in a highly populated tropical country open spaces is insufficient. In the intention to utilize accessible solar energy with better performance, a mathematical investigation is carried out with a three bed adsorption cooling unit working with silica gel-water pair. The studied chiller is powered by direct solar heat collected by a series of compound parabolic concentrator solar thermal collectors without any heat or mass recovery. The working principal of the chiller is, in principle, the same as the conventional two-bed adsorption chiller. However, instead of two half cycles, there are three one third cycles in the proposed chiller in which at every cycle the former desorber is kept in the precooling mode and as an adsorber for the next two one third cycles, respectively. As desorption kinetic is faster than the adsorption kinetics, this longer precooling mode helps the silica gel granules to adsorb more water molecules and increase evaporation rate. Hence, a better cooling effect of at least 1°C can be observed, increases chiller working hour after sunset for almost a further one hour.

Performance evaluation of a solar-driven adsorption desalination-cooling system

Problems of energy and fresh water shortage become hard challenges facing survival of the human beings. Egypt especially is facing increasing of water supply challenges due to population growth. In this study, a new proposed solar adsorption desalination-cooling (ADC) system is designed, built, and tested under Egypt's climate conditions. A commercially available silica gel of about 13.5 kg is used. The Adsorption characteristics of the selected silica gel-water pair are evaluated firstly to design the proposed system. In addition, a theoretical dynamic model is developed to predict the system performance. A good agreement is found between the theoretical and experimental results under different Egypt climate conditions. The results show that, the average specific cooling power is 112 W/kg and average specific daily water production is 4 m3 per ton silica gel with a COP of 0.45. Based on the obtained results, ADC system driven by solar energy proven to be an adequate solution to offer proper alternative to the traditional desalination systems and have promising future in Egypt's climate. However, studies about improving the performance of such system and new designs are still needed.

Study of a two-bed silica gel–water adsorption chiller: performance analysis

ABSTRACTIn this study, a lumped parameter simulation model has been developed for analysis of the thermal performance of a single-stage two-bed adsorption chiller. Since silica gel has low regeneration temperature and water has high latent heat of vaporisation, silica gel–water pair has been chosen as the working pair of the adsorption chiller. Low-grade waste heat or solar heat at around 70–80°C can be used to run this adsorption chiller. In this model, the effects of operating parameters on the performance of the chiller have been studied. The simulated results show that the cooling capacity of the chiller has an optimum value of 5.95 kW for a cycle time of 1600 s with the hot, cooling, and chilled water inlet temperatures at 85°C, 25°C, and 14°C, respectively. The present model can be utilised to investigate and optimise adsorption chillers.

Experimental investigation of a solar adsorption refrigeration system working with silicagel/water pair: A case study for Bou-Ismail solar data

The objective of the present work is to build and test a solar adsorption refrigeration device with an adsorbent–adsorbate silicagel–water pair in the weather conditions of Bou-Ismail, Algeria. The refrigeration system proposed in this study consists of three major components: a solar collector (tubular adsorbent bed, reflecting surface, single glazed cover, and 4.5kg of silicagel material), an air-cooled condenser, an evaporator and supplementary system components. The working operation parameters of the cooling adsorption system were tested successfully. Experimental results indicated that the maximum temperature generated varied from 95°C to 117°C, 33°C of average ambient temperature, the condensing temperature varied from 45°C to 53°C, +5°C of minimum evaporator temperature, and the pressure values recorded in the adsorbent bed during the heating desorption period varied from 80 to 100mbar. With a total energy received of around 19MJ/m2, this solar adsorption refrigeration device can provide a solar COP ranging from 0.083 to 0.09. Such results show the practicability of our solar adsorption refrigeration module and that it has a favorable design for use in medical storage in areas without an electrical network (e.g. the Algeria Sahara region).

Transient behavior of a cylindrical adsorbent bed during the adsorption process

A transient two dimensional local thermal non-equilibrium model is developed to investigate the influences of heat transfer and operating parameters on the dynamic behavior of a cylindrical adsorbent bed during the adsorption process. Local volume averaging method is used to drive the macro scale governing conservation equations from the micro scale ones. In the model, linear driving force model and Darcy’s equation are considered to account for the resistances to internal and external mass transfer, respectively. Silica gel–water pair widely used in the adsorption cooling systems is chosen to be an adsorbent–adsorbate working pair. The parameters of interest are convective heat transfer coefficient, solid phase thermal conductivity, bed thickness, evaporator pressure, condenser pressure, driving heat source temperature and cooling source temperature. It is found that amount of refrigerant circulated through the system increases with increasing evaporator pressure-driving heat source temperature and decreasing condenser pressure-cooling source temperature. The duration of adsorption process is more sensitive to heat transfer resistances than to mass transfer resistances. The conductive and convective resistances need to be reduced to reach the equilibrium conditions in a short period of time and hence to have a better specific cooling power.

Solar Adsorption Cooling and Hot Water Supply for Climatic Condition of Dhaka

For the climatic condition of a tropical country like Bangladesh, solar heat driven adsorption cooling seems to have great prospect. Installation cost is one of the foremost drawbacks of such a system. Although, once installed, system maintenance and operation cost is negligible. Besides, the solar unit can not only support as a cooler for adsorption space cooling purpose during summer season but also as a source of hot water supply for domestic use during the winter season. A conventional two bed basic adsorption chiller, driven by solar heat run by silica gel-water pair as adsorbent and adsorbate respectively, has been investigated for the climatic condition of Dhaka 23°46′N (latitude), and 90°23′E (longitude). 30 enhanced compound parabolic concentrator (CPC) collectors each of area 2.415 m2 along with a hot water storage tank of volume 2.197 liters is optimum for considered climatic condition. With such a unit 30.27°C continuous water supply during the peak hours can be assured, with the storage of 44.77°C of hot water in the month of January.

Dynamic behaviors of adsorption chiller: Effects of the silica gel grain size and layers

This article presents the dynamic behaviour of a single effect two bed adsorption chiller employing adsorbent beds with various layers of loose grain configurations and silica gel particle sizes, which is based on the experimentally confirmed adsorption isotherms and kinetics data. Compared with the experimental data of conventional adsorption chiller based on RD silica gel-water pair, we found that the silica gel configuration in terms of layers and sizes provides an interesting result, that is, the “grain size sensitive” regime is realized for large adsorbent grains with more layers. From numerical simulation, it is found that the specific cooling power and the coefficient of performance are reduced and the peak chilled water temperatures are increased with increasing the grain size and grain layers. We also demonstrate here that the sizes and layers of adsorbents should be considered for the design of adsorption heat exchanger for adsorption cooling applications.

Comparison of Uniform and Non-uniform Pressure Approaches Used to Analyze an Adsorption Process in a Closed Type Adsorbent Bed

Heat and mass transfer in an annular adsorbent bed filled with silica gel particles is numerically analyzed by uniform and non-uniform pressure approaches. The study is performed for silica gel–water pair, particle radius from 0.025 to 1 mm and two bed radii of 10 and 40 mm. For uniform pressure approach, the energy equation for the bed and the mass transfer equation for the particle are solved. For non-uniform pressure approach, the continuity and Darcy equations due to the motion of water vapor in the bed are added, and four coupled partial differential equations are solved. The changes of the adsorbate concentration, pressure, and temperature in the bed throughout the adsorption process for both approaches are obtained and compared. The obtained results showed that the particle size plays an important role on the validity of uniform pressure approach. Due to the interparticle mass transfer resistance, there is a considerable difference between the results of the uniform pressure and non-uniform pressure approaches for the beds with small size of particles such as $$r_\mathrm{{p}} =$$ 0.025 mm.

Effects of porosity on heat and mass transfer in a granular adsorbent bed

In the present study, the mechanism of heat and mass transfer in an annulus adsorbent is handled. The heat and mass transfer equations for the adsorbent bed and the mass balance equation for the adsorbent granules are numerically solved to obtain the distributions of temperature, pressure, adsorptive density and adsorbate concentration in the adsorbent bed. The study is performed for the silica gel–water pair and for three different values of porosity as 0.1, 0.2 and 0.3. The distributions of temperature and adsorbate concentration are considerably influenced from the bed porosity. The adsorption period increases with the increase of the porosity value. The porosity affects the pressure and adsorptive density distributions at the beginning of the process and after a relatively short time, the averages of these dependent variables approach to the final equilibrium state.

The influence of heat exchanger parameters on the optimum cycle time of adsorption chillers

The research investigated the influences of heat exchanger parameters, such as heat capacity and NTU, on the optimum performance of a single-stage adsorption chiller. Silica gel–water pair was chosen as the adsorbent–adsorbate combination so that low temperature heat source under than 100 °C could be utilized as the driving force. The mathematical model of the adsorption chiller using dimensionless parameters was developed and a global optimization method called the particle swarm optimization was applied in the simulation to obtain the optimum cycle time. The results showed that the smaller heat capacity heat exchanger improved both the maximum specific cooling capacity (SCC) and the COP. While, the larger NTU of the adsorbent bed resulted in the decrease of the COP due to the short cycle time although the maximum SCC was enhanced.

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.

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.