Desiccant Based Air Conditioning System Research Articles

Water Vapor Adsorption on Desiccant Materials for Rotary Desiccant Air Conditioning Systems

In order to determine the water vapor adsorption performance of a rotary desiccant-based air conditioning system, the behavior of water adsorption on cylindrical pores of different sizes was studied by using classical density functional theory (CDFT) based on perturbated chain statistical associating fluid theory (PC-SAFT). Firstly, the structural parameters of the desiccant material were characterized by scanning electron microscopy (SEM), X-ray Energy Dispersive Spectrum (EDS), and N2 adsorption/desorption isotherms, as well as adsorption equilibrium measurements of water vapor at temperature range 293–308 K. Secondly, the potential energy equation of water molecules in cylindrical pores was determined, and contribution of various terms of PC-SAFT for simulating fluid in cylindrical pores were established. Finally, the pore size distribution (PSD) of the desiccant materials is determined by the PC-SAFT kernel. Moreover, water vapor condensation was investigated with the PC-SAFT model in micropores. The results indicate that the rotary desiccant materials have a large number of micropores with a volume of 0.3669 cm3/g and the amount of water adsorption is about 0.285 g/g. The condensation pressure and the pore width corresponding to the saturated pressure P0 grow with an increase in the temperature, signifying that adjusting the PSD of the material has a significant effect on improving the dehumidification performance. The research concludes that the PSD range of the oxide cylindrical pore between 1.09 and 1.53 nm is particularly beneficial for dehumidification. This study provides valuable theoretical guidance for optimizing dehumidification materials.

Energy retrofit of public office building through liquid desiccant assisted dedicated outdoor air system with fuel cell in Korea

Zero-energy buildings have been mandated in many countries to reduce carbon emissions from buildings. Existing buildings are no exception, and should be retrofitted with renewable energy to be ZEBs. Fuel cells are attracting increasing attention owing to their high efficiency and lack of limitations on the installation area compared to photovoltaic panels. However, a fuel cell simultaneously generates heat and electricity; therefore, it is important to use heat efficiently when using a fuel cell. To achieve this, this study proposes using a liquid desiccant-based air conditioning system when the fuel cell is used for energy self-sufficiency in an existing office building. The configuration of an air conditioning system called thermoelectric and liquid desiccant-assisted dedicated outdoor air system (TELDOAS) is designed, and its annual control logic is established for cooling and heating season operations. To reveal the energy performance of the proposed system, detailed energy simulation models were developed by integrating all mathematical and empirical models of each component using MATLAB. The results revealed that 24.7% of the annual primary energy could be saved by adding a fuel cell to the current air conditioning system consisting of a heat pump and an enthalpy rotary ventilator (ERV). Moreover, the results showed that using TELDOAS instead of ERV could save an additional 32.3% of the annual primary energy by efficiently reclaiming waste heat from the fuel cell. The heat recovery ratios of each system are analyzed to prove the advantage of using TELDOAS, and heat recovery is increased from 37.1% to 51.2% by using heat during the cooling season.

Dehumidification performance improvement by using liquid desiccant blends – An experimental study

To minimize global warming and consumption of electricity, a desiccant-based air conditioning system has been proposed as a better alternative to control air humidity, compared to the conventional air conditioning system. In the present study, the performance of the hybrid desiccant was compared with the pure LiBr and KCOOH solution. Blends of LiBr and KCOOH at different weight ratios were used as desiccants. The performance of the system was compared based on its moisture removal rate and dehumidification effectiveness. The performance of the system increased with increasing desiccant flow rate, the concentration of the solute and the airflow rate. From the tested concentrations, the blend of 40% LiBr and 10% KCOOH concentration showed the highest MRR and dehumidification effectiveness among all blends of LiBr and KCOOH. From the study, it can be concluded that by using an optimized blend of LiBr and KCOOH as a desiccant in place of neat LiBr, the limitations of pure LiBr can be partly resolved without sacrificing much on the dehumidification performance.

New multiple regression and machine learning models of rotary desiccant wheel for unbalanced flow conditions

In this study, five Multiple Linear Regression, three Multilayer Perceptron Regressor, seven Decision Tree and four Support Vector Machine models were constructed to predict outlet temperature and humidity ratio of silica gel desiccant wheels using eight input parameters for unbalanced flow condition. The effect of different kernel functions of Support Vector Machine algorithms, on the modeling of desiccant wheel was investigated for the first time in the open literature. Detailed validation of the developed models showed that the Response Surface model outperformed other Multiple Linear Regression models, and the Support Vector Machine model with Pearson VII Universal kernel was the best among all models. The determination coefficient and root mean square error for temperature were found to be 0.9791 and 1.2832 °C for the Response Surface model and, 0.9984 and 0.3511 °C for the Support Vector Machine model with Pearson VII Universal kernel, respectively. In the case of humidity ratio, the corresponding statistical parameters were 0.9763 and 0.5672 g/kg for the former and, 0.9976 and 0.1810 g/kg for the latter. The proposed models can be used reliably in the analysis of solid desiccant-based air conditioning systems for design and energy analysis.

Experimental investigations on the performance characteristics of plastic surfaces for developing low flow falling film liquid desiccant regenerators

The corrosion problem of liquid desiccants hampers the development of liquid desiccant-based air-conditioning systems. Plastic surfaces can be a promising alternative because of their anti-corrosive property. However, plastic has the inherent problem of low wettability. The present experimental study investigated the regeneration performance of a liquid desiccant over multi-inline vertical polypropylene circular cylinder surfaces in an adiabatic mode, to find a substitute of vertical plate surfaces for developing low flow falling film towers. The mechanical surface modification method was explored to elevate the performance level of the circular cylinder surface. A comparative performance analysis was conducted between the circular cylinder and plate surface. It was found that the regeneration rate of the Conventional circular cylinder surface was superior to the Conventional plate surface by 50.5% for the evaluated conditions. The regeneration rate of the circular cylinder surface can reach up to 0.684 g/s at a 2.5 mass flow rate ratio of the solution to air. A new generalized empirical correlation was proposed to estimate the effectiveness of falling film regenerators by incorporating the driving potential of heat and mass transfer process and wetness parameters. The prediction accuracy of the present correlation was 16.5% (i.e. mean effective percentage error) against nine datasets. The outcome of the present study would be beneficial for developing low-flow solar-assisted plastic falling film liquid desiccant regenerators.

Progressive Development and Challenges Faced by Solar Rotary Desiccant-Based Air-Conditioning Systems: A Review

A rotary desiccant-based air-conditioning system is a heat-driven hybrid system which combines different technologies such as desiccant dehumidification, evaporative cooling, refrigeration, and regeneration. This system has an opportunity to utilize low-grade thermal energy obtained from the sun or other sources. In this paper, the basic principles and recent research developments related to rotary desiccant-based cooling systems are recalled and their applications and importance are summarized. It is shown that with novel system configurations and new desiccant materials, there is great potential for improving the performance and consistency of rotary desiccant systems; at the same time, the use of solar energy for regeneration purposes can minimize the operating cost to a great extent. Some examples are presented to demonstrate how rotary desiccant air conditioning can be a promising solution for replacing traditional vapor-compression air-conditioning systems. Recent advances and ongoing research related to solar-powered hybrid rotary desiccant cooling systems are also summarized. The hybrid systems make use of a vapor-compression system in order to have better operational flexibility. These systems, although they consume electrical energy, use solar energy as the principal source of energy, and hence, significant savings of premium energy can be obtained compared to conventional vapor-compression systems. However, further research and development are required in order to realize the sustainable operation of solar rotary desiccant air-conditioning systems, as solar energy is not steady. Reductions in capital cost and size, along with improvements in efficiency and reliability of the system is still needed for it to become a player in the market of air conditioning.

Development and performance evaluation of a passive solar liquid desiccant regenerator

Vapour Compression Refrigeration (VCR) cycle that consumes electrical energy dominates airconditioning market today. Production of electrical energy is generally associated with fossil fuel consumption, emission of greenhouse gases and pollution. Air conditioning systems that use waste or renewable energy sources should be investigated as an alternative solution. Liquid desiccant-based air conditioning system (LDAC) is one such a potential candidate. The heat from a solar thermal collector can be supplied to the regeneration process of LDAC systems. A novel solar collector with the goal of no electrical power consumption, simplicity, low cost, and high efficiency was developed in this work for the regeneration of LD (aqueous calcium chloride solution). Condensate output in this device used for water desalination was 3.7 kg/m2, which is comparable with the best in the literature. An area of 7.5 m2 of such a device with no parasitic power would be needed per kW of latent cooling.

Mathematical Model Development of a Solar Assisted Liquid Desiccant Air Conditioning System with Fin Coil Dehumidifier

A novel scheme of a solar assisted, liquid desiccant supported comfort air conditioning system has been proposed in this paper. The primary intent of the system is to supply air at 25°C and 52% relative humidity, even during the hot and humid summer months prevailing in the plains of sub-tropical countries like India. A mathematical model has been put forward to predict the output air temperature, humidity ratio and the effectiveness of dehumidifier as well as the regenerator. The predicted air humidity ratio has been validated against data available in the literature. The model has been used to predict the optimum mass flow rates required to achieve the desired output conditions for the selected location (Kolkata). The study thus strengthens the need and feasibility of solar assisted desiccant based air conditioning systems for the sub-tropical climates as a replacement of the traditional VCR (vapor compression refrigeration) systems.

Experimental analysis and performance optimization of a counter-flow enthalpy recovery device using liquid desiccant

The liquid desiccant enthalpy recovery is an efficient way to save energy in air-conditioning systems. In this study, a counter-flow liquid desiccant enthalpy recovery device was proposed and experimentally analyzed. Enthalpy transfer capacity, enthalpy efficiency and pressure drop per height of packing were used as indices to describe its performances. Based on the experiment results, the heat and mass transfer model of a packed tower was used to simulate and optimize the performance of this device. The maximum enthalpy efficiency and enthalpy transfer capacity were achieved when the optimal air velocity (1.9–2.1 m/s in this study) maintained to be slightly below the air velocity at the loading point and the thermal capacity ratio of air to desiccant ( m*) equaled to 1. These conclusions are valuable to both design and operation of such an enthalpy recovery device. Practical application: A counter-flow enthalpy recovery device with liquid desiccant was proposed and experimentally investigated. Based on the experiment results, a numerical model for this device was built and validated. The optimal air and desiccant mass fluxes were analyzed to maximize the enthalpy efficiency of this device, which could be higher than the conventional device with cross-flow pattern. These results could provide guidelines for both design and operation management of counter-flow enthalpy recovery devices in liquid desiccant-based air-conditioning systems.

Demonstration of Liquid Desiccant Based Air Conditioning System using Solar Energy and Waste Heat

Objectives: The objective of the current work is to demonstrate Liquid desiccant based air conditioning (LDAC) systems to reduce energy consumption and cater to latent cooling load more effectively. Methods/Statistical Analysis: This work describes three different projects undertaken to demonstrate efficacy of LDAC systems. One of the project demonstrated regeneration of liquid desiccant in a simple but economical solar thermal collector along with integrated liquid desiccant (LD) heat exchanger. Two other projects demonstrated regeneration using waste heat source and dehumidification of air in an internally cooled LD-air contacting device. Experiments were carried out in city of Vadodara in summer months of year2017 and encouraging results are found. Findings: The passive solar regeneration project showed that a latent cooling rate of 96 W can be provided using an LDAC system per m2 area of solar thermal collector. Another useful utility in form of distilled water could be produced at a rate of 1.7 liters in a day per m2 area in this collector. The internally heated device used for demonstrating use of waste heat for LD regeneration could provide moisture removal rate of around 3.0 kg/h from air using aqueous solution of calcium chloride as LD with an average concentration of 37.7%.In the internally cooled air-LD contacting device, 2.0 kW of latent cooling along with 0.3 kW sensible cooling could be provided for air entering at around 34.7o C temperature and 47.5% relative humidity with an average chilled water temperature of 9.5o C and LD concentration of 40.0%. These experiments have thus demonstrated and established efficacy of LDAC system to provide effective dehumidification with use of energy sources with nominal recurring costs and reduced pollutant emission. Application/ Improvements: Reducing heat losses in solar collector, air bypasses in the contacting device and higher residence time of LD in contacting device can help improve these results further. Keywords: Air Conditioning, Dehumidification, Energy Conservation, Liquid Desiccant, Solar Energy, Waste Heat

Preliminary Investigations on a Novel Rotating Media Liquid-air Contacting Device without Liquid Pool

Abstract Contacting device is a system that bring air and water in contact with each other in evaporative cooling systems while it brings liquid desiccant and air in contact with each other in case of liquid desiccant-based air conditioning systems. They should provide high surface area density for heat and mass transfer and also provide high heat and mass transfer rates in smaller volume with lower electrical power consumption. A novel design of contacting device is presented in this work, where plastic porous cylindrical contact media is used and unlike previous designs, the liquid pool in lower part of the device is eliminated. This would help to utilize the contact surfaces more effectively and reduce power consumption of motor used for rotation of the media as compared to earlier designs in literature. The novel contacting device and the experimental set up were developed and preliminary testing was done. The contact media is 230 mm long with diameter equal to 145 mm, providing surface area density equal to 445 m 2 /m 3 . In a preliminary experiment, the device provided evaporative cooling rate up to 288 W and effectiveness of 33.1%. The performance can be further improved by increasing mass flow rate of water for better wetting, increasing surface density by decreasing porosity and increasing path length of air.

Exergoenvironmental Evaluation of the Desiccant Air-Conditioning System Subjected to Different Regeneration Temperatures

This paper presents the exergoenvironment analysis of the developed desiccant-based air-conditioning system subjected to different regeneration temperatures. Exergoenvironment is a new evaluation tool combining the exergetic and life-cycle analyses to determine the environmental effect of the system’s components and the energy utilized for the system’s operation. The different materials in the manufacture of the system’s components and sources of energy to operate the system have different environmental impacts. The results of the study show the system components exergy efficiency, system exergy destruction, relative environmental impact difference and exergoenvironmental impact factor which are important parameters for this analysis. It shows that some components need exergy performance improvements through replacement with high efficiency components to minimize environmental impact. Based on the results, application of the exergoenvironmental analysis is an important tool for knowing the environmental impact of the system components and its operation. The method is important in desiccant air-conditioning systems in which different materials of the components and energy are utilized for the system design and operation.

Performance Test of Desiccant Heating, Ventilating and Air-Conditioning System by Using Multiple Tracer Gas Dilution Method

The desiccant-based air-conditioning system consists of many heat and mass transfer components operating together with air fans. The performance test using the multiple tracer gas dilution method is important for knowing the internal and external flow rates, including the air leakages. The results of the test show the actual internal air flow rates and the sources of air leakage, and also show the interaction of air in the air-conditioned lecture room and in the mechanical room. The performance test using the multiple tracer gas dilution method in air flow and air leakages is important for a detailed evaluation of any air-conditioning system such as this desiccant-based one.

Performance Evaluation and Parameter Analysis on a Composite Solid Desiccant based Air Conditioning System

Performance Evaluation and Parameter Analysis on a Composite Solid Desiccant based Air Conditioning System

Three years experimental comparative analysis of a desiccant based air conditioning system for a flower greenhouse: Assessment of different desiccants

This paper presents three years experimental comparative analysis of a desiccant-based and a traditional air conditioning system for a flower greenhouse in the winter season. Two identical neighbouring flower greenhouses were equipped with a traditional and an innovative air conditioning system respectively. The innovative air conditioning system is based on the Ventilated Latent Heat Converter (VLHC) AGAM 1020 that consists of a dehumidification and a regeneration unit. Heat recovery is performed on the desiccant regeneration process to warm up the dehumidified air coming back into the greenhouse.Comparative analysis was carried out for three years using three different desiccants in the VLHC: H2O/LiCl in 2010, H2O/KCOOH in 2011 and H2O/LiBr in 2012. The greenhouse equipped with a sorption unit exhibits an energy saving of 9.6% in 2010, 11.7% in 2011, and 15.1% energy saving in 2012. The comparative analysis gives the opportunity for the assessment of the hygroscopic salt solutions currently used as desiccants. Although H2O/LiBr desiccant exhibits the best performance, the solution H2O/KCOOH seems to be very promising as “desiccant of the future”.

Experimental analysis of an unconventional desiccant-based air-conditioning system: the influence of cooling air flow and chiller on the energy and environmental performance

Nowadays, the increasing demand of summer cooling is typically covered by electric chillers, often determining electric peak loads and black-outs. Thus, a wide interest is spreading in small scale natural gas-fired cogenerators driving desiccant-based air-conditioning systems, which represent interesting alternatives to conventional systems based on vapor compression cooling only. In this article, experimental tests performed on an air handling unit (AHU) equipped with a desiccant wheel (DW), coupled to a small scale cogenerator and an electric chiller are described. A new layout of the desiccant-based AHU is investigated, considering a third flow (the cooling air), besides the process air flow and the regeneration one. A cross-flow heat exchanger between process air and cooling air is used; the cooling air, cooled by an adiabatic humidifier, is aimed to precool the process air exiting the DW. The relevant influence of the heat exchanger and of the humidifier, as well as that of the chiller performance, on global primary energy requirements, water consumption and CO2 equivalent emissions of the system is experimentally evaluated.

Experimental performance study of a proposed desiccant based air conditioning system

An experimental investigation on the performance of a proposed hybrid desiccant based air conditioning system referred as HDBAC is introduced in this paper. HDBAC is mainly consisted of a liquid desiccant dehumidification unit integrated with a vapor compression system (VCS). The VCS unit has a cooling capacity of 5.27kW and uses 134a as refrigerant. Calcium chloride (CaCl2) solution is used as the working desiccant material. HDBAC system is used to serve low sensible heat factor applications. The effect of different parameters such as, process air flow rate, desiccant solution flow rate, evaporator box and condenser box solution temperatures, strong solution concentration and regeneration temperature on the performance of the system is studied. The performance of the system is evaluated using some parameters such as: the coefficient of performance (COPa), specific moisture removal and energy saving percentage. A remarkable increase of about 54% in the coefficient of performance of the proposed system over VCS with reheat is achieved. A maximum overall energy saving of about 46% is observed which emphasizes the use of the proposed system as an energy efficient air conditioning system.

Investigation of solar energy utilization in a novel desiccant based air conditioning system

In this study, a novel desiccant based air-conditioning system was considered. This system consisted of a desiccant wheel, heat exchangers, fans, evaporative cooler, electric heater unit to simulate solar energy, and refrigeration unit. A model was developed to investigate the utilization of solar energy in the system. In the model, the temperatures obtained from the experiments over the cooling season of 2008 and solar radiation data measured by the State Meteorological Affairs (DMI) between 1986 and 2006 years for Adana were utilized. The increase in the regeneration air temperature due to solar energy assistance for the days, on which the experiments were carried out, was calculated using the model. The results obtained from the model and the experiments were also compared with each other. It may be concluded that utilization of solar energy in the system increases the coefficient of performance (COP) between 50% and 120%.

Experimental analysis on the dehumidification and thermal performance of a desiccant wheel

The advantages of desiccant-based air conditioning systems, compared to conventional ones based on the dehumidification by cooling, have been highlighted in many research papers. The energy saving and the reduction of the environmental impact are higher when the desiccant material is regenerated by using “free” thermal energy (for example, waste heat from cogenerators or solar energy). Further investigation on the performance of the desiccant wheel is useful: therefore, in this paper, an experimental analysis on this component is presented, with particular attention to the variation of the performance as a function of the process and regeneration air flow rates. The desiccant material is regenerated by means of low-temperature thermal energy (about 65 °C) from a microcogenerator. Both the experimental results obtained by the authors and the data provided by the manufacturer have been used to calculate some performance parameters, and a satisfactory agreement has been obtained. The performance parameters have been evaluated as a function of the regeneration temperature, the inlet process air humidity ratio and temperature and the ratio between the regeneration and process air flow rates, in both the cases of fixed regeneration temperature and fixed regeneration thermal power: the results show that higher influence on the dehumidification process is due to the regeneration temperature rather than to the regeneration air flow rate. Moreover, the process air humidity ratio and regeneration temperature influence the desiccant wheel performance more than the process air temperature.

Experimental investigation of a novel desiccant cooling system

Desiccant based air conditioning systems are a suitable way to improve indoor air quality due to its superior humidity control. In this study, a novel desiccant based air conditioning system is designed and tested experimentally to improve the indoor air quality and reduce energy consumption. In the system studied, a heat exchanger, which is not used in this type of systems, for pre-heating the regeneration air with exhaust air is used. This paper reports results of initial operation and operational procedures. The performance of the system and its components is discussed.