Regeneration Air Stream Research Articles

Optimization of a rotary desiccant wheel for enthalpy recovery of air-conditioning in a humid hospitality environment

Excessive condensation on cooling coils can be costly energy-wise due to humid process air streams. An innovative desiccant cooling system is introduced into an existing hotel in Taoyuan, Taiwan, where the ambient is humid year-round. Rejected heat from the existing cooling system is used as the energy source to heat the regeneration airstream for dehumidification of the process airstream via a rotary desiccant wheel. The paper develops and validates a numerical model for the heat and mass (moisture) transfer of the rotary desiccant wheel. The validated numerical model is used to simulate the expected performance of the rotary wheel under various conditions and operating strategies. The desiccant wheel’s performance includes the steady-state moisture removal capacity (MRC) and the transient response time to reach steady-state. The examined factors include the ambient temperature and humidity, air flow rate, rotational speed of the wheel, wheel-split, and regeneration airstream temperature. From the simulation results, the paper offers the optimized control strategies for the operation of the rotary desiccant wheel system in the hotel.

Performance of an internally cooled and heated desiccant-coated heat and mass exchanger: Effectiveness criteria and design methodology

Internally cooled and heated desiccant-coated heat and mass exchangers (ICHDHMX) driven by low-grade heat are very attractive owing to their energy-saving potential, especially for applications where substantial moisture removal (such as air-conditioning) is a necessity. In this paper, we derive equations for the performance of an ideal ICHDHMX, allowing us to define humidity-ratio effectiveness (εY) and relative-humidity effectiveness (εRH) such that their values approach 1 as the performance approaches that of an ideal ICHDHMX. Besides an equation-based approach, an easy-to-use psychrometric-chart based approach is presented to determine the performance of an ideal ICHDHMX. We invoke conservation principles to ascertain whether or not it is feasible to use the ICHDHMX for a given set of inlet conditions of air and water streams for dehumidification and regeneration. The dimensions of the ICHDHMX can be determined using this methodology, not even requiring knowledge of a tuning parameter unless a precise outlet specific humidity is required. Simulations are conducted for cases involving three incoming hot water temperatures (38, 44 and 50 °C) and several mixing ratios of room return air (25 °C at 0.011 kg/kg dry air) and outdoor air (32 °C at 0.02 kg/kg dry air), typical of warm and humid weather conditions. For all cases, the cool-water inlet is fixed at 30 °C. The results show that even when the dehumidification air-stream humidity is high, if the regeneration air-stream humidity is low (typical of room-exhaust air), the operation of an ICHDHMX is feasible using a low regeneration temperature of only 38 °C. When the regeneration temperature is 50 °C, the exchanger can operate under the complete range of humidity conditions tested. A cooling coefficient of performance up to 9.8 and effectiveness value up to 0.88 is realized, while the fluid power required is generally very low. These findings substantiate the case for commercial adoption of this technology for air-conditioning.

Solid- and gas-side resistances of a silica-gel desiccant wheel

The operation of a solid desiccant evaporative cooling system largely depends on the dehumidification ability of the desiccant wheel and mathematical models are an effective tool to predict the transport behaviour of this wheel. A complete theoretical model would account for both the gas-side resistance, which is due to the convective heat and mass transfer between the air and the desiccant surface, and the solid-side resistance, which is caused by the diffusion of moisture and conduction of heat through the solid desiccant layer. A mathematical model is developed by deriving the partial differential equations of mass and energy balance of a representative channel the desiccant wheel moving through the supply and regeneration air streams in a 360° circular path. The effect of these resistances at different operating conditions on the performance of the wheel is rarely investigated. In this paper, two heat and mass transfer models are presented, the first one incorporating gas-side resistance and the second one includes both solid-side and gas-side resistances. The models are compared with experimental data. It can be concluded that the gas-side resistance is of considerably higher significance compared to the solid-side resistances.

Numerical simulation of novel trombe wall-assisted desiccant wheel

In the present study, a novel trombe wall-assisted desiccant wheel system has been modeled, in which the trombe wall is divided into three equal parts and it provides the heat energy needed to regenerate the desiccant wheel. The components of the system, including the desiccant wheel, the trombe wall are separately modeled in MATLAB software and then assembled to investigate the surface area of the trombe wall and the output humidity of the desiccant wheel for attaining air conditioning comfort. It has been discussed that the integrated system presented here, can be utilized in all humid climate conditions around the globe. The results of the present study for some special cases have been compared with results available in open literature. The optimal surface area of the trombe wall has been extracted according to the parameters of the desiccant wheel. Results shows that the solar energy received by the trombe wall is 600-740 W/m2 (1May – 15August) in warm and humid climate of Gilan (Iran), the temperature of the wall surface is obtained 77-86 oC, and the outlet temperature of regeneration air stream from trombe wall is obtained 60-70 oC, the output humidity of the desiccant wheel reduces 10-12 gw/kga.

Performance prediction of rotary solid desiccant dehumidifier in hybrid air-conditioning system using artificial neural network

Desiccant air conditioning systems are considered as better alternatives to the conventional air conditioning system because of the independent control of temperature and humidity and being environment friendly. An artificial neural network (ANN) model has been developed to predict the performance of a rotary desiccant dehumidifier for different process air inlet conditions. Dry bulb temperature, humidity ratio and flow rate of the process as well as regeneration air streams of dehumidifier and regeneration temperatures are used as inputs to the model. The outputs of the model are outlet dry bulb temperature and humidity ratio of process as well as regeneration air streams of dehumidifier, the moisture removal rate and the effectiveness of the dehumidifier. Moisture removal rate and effectiveness of the dehumidifier are considered as the performance indicators of the system. Experiments are also conducted to investigate the performance of the desiccant wheel and the test results are used as target data to train the ANN model. Performance predictions through ANN are compared with the experimental test results and a close agreement is observed.

Experimentally validated model for atmospheric water generation using a solar assisted desiccant dehumidification system

This paper examines an alternative solution for emergency situations where freshwater and utilities are often interrupted. Generating freshwater from the atmosphere using a small-scale air-cooled desiccant wheel dehumidifier was experimented. Condensed water was collected and systematically recorded against local meteorological data. A synthetic model simulating the actual lay-out of the experiment was built in TRNSYS. The model validated the experimental results and generated approximately 52 litres in 9 days. The validated model is then run for three different climates; Sydney, Abu Dhabi and London to estimate annual water production. Abu Dhabi showed the best results compared with Sydney and London by generating 18.5 kL of water per year. The model is further developed to evaluate thermodynamic benefits of using dehumidified processed air as a feed stream for a proposed small-scale air-conditioning system. The energy required for the wheel regeneration process is met by thermal gain of modified solar PVT panels where the stagnant heat at the back is used to heat up the regeneration air stream. It was found that the dehumidification process can significantly decrease the latent load of the air-conditioner and easily bring indoor humidity level to the human comfort zone. However, dehumidified process air is also increasing the sensible load because of the higher temperature associated with dehumidification process. Rooftop solar PV panels can easily meet the power demand of appropriate lighting, a computer and mini refrigerator for extended hours if an appropriate set of batteries are fitted, but unable to exclusively meet the air-conditioner power demand to maintain indoor temperature within human comfort zone.

A non-adiabatic desiccant wheel: Modeling and experimental validation

Desiccant/evaporative cooling systems offer an environmentally friendly alternative to conventional vapor compression chillers. The desiccant wheel is the heart of this heat-driven cooling system. The main drawback of conventional desiccant wheels is the excessive heating of the supply air and desiccant material during dehumidification. This increases the partial pressure of water on the desiccant material and reduces the amount of moisture that can be removed from the air. To overcome this problem, a non-adiabatic desiccant wheel design has been investigated. This new wheel has an internal heat transfer structure with alternative channels for dehumidification and for indirect cooling of the dehumidification process. The supply air and regeneration air streams flow axially in opposite directions whereas the cooling air enters axially at the center of the wheel and exits radially, achieving simultaneous cooling and dehumidification of the supply air. A numerical model for this design is developed and a representative channel of the wheel was constructed to validate the model. Predicted modeling results show good agreement with experimental results.

Experimental assessment and thermodynamic analysis of a solar desiccant cooling system

The objectives of this study were to evaluate the performance of a solar desiccant cooling system with LiCl desiccant wheel as well as to obtain useful data and experiences for practical application in Mediterranean areas with higher solar fractions. An experimental set-up was built and used to test the system performance under typical summer environmental conditions. It has been found that the required regeneration temperature of the system is low while the COP th of the system is high. Regeneration temperatures up to 65°C were recommended for each environmental condition. In addition, the effects of some important operating parameters, such as inlet temperature and humidity ratio of process and regeneration air, on system performance were also investigated in this study. Useful experiences from international standards were taken into account for the evaluation of desiccant wheels’ performance. The analysed procedure and the developed performance indicators provide useful information about the sustainability of solar desiccant cooling systems with solid desiccant wheels. This information in combination with several set of experiments was the basis for the development of nomographs, which are very helpful for the sustainability and the design analysis of solid desiccant cooling systems. This novel diagrams (nomographs) can provide information on the outlet conditions of process and regeneration air streams of a solar cooling unit with LiCl desiccant wheel and therefore the dehumidification capacity of these systems as a function of ambient conditions. The model development was followed by a sensitivity analysis performed on these predicted results. The key parameters were the thermodynamic condition of process and regeneration air streams, the thermal coefficient of performance and the mass air flow ratio of the regeneration and process streams.

Thermodynamic analysis of an open cycle solid desiccant cooling system using Artificial Neural Network

This paper examines the performance of an installed open cycle air-conditioning system with a silica gel desiccant wheel which uses a conventional heat pump and heat exchangers for the improvement of the outlet air of the system. A neural network model based on the training of a black box model with experimental data was developed as a method based on experimental results predicting the state conditions of air at the process and regeneration stream. The model development was followed by a Sensitivity Analysis performed on these predicted results. The key parameters were the thermodynamic condition of process and regeneration air streams, the sensible heat factor of the room, and the mass air flow ratio of the regeneration and process streams. The results of this analysis revealed that all investigated parameters influenced the performance of the desiccant unit. Predicted output values of the proposed Neural Network Model for Desiccant Systems are in line with results from other correlation models based on the interpolation of experimental data obtained from industrial air conditioning installations.

Performance of VOC Abatement by Thermal Swing Honeycomb Rotor Adsorbers

Fine powder of hydrophobic high silica zeolite was solidified with silica sol in the void of a 0.2 mm thick ceramic fiber sheet fabricated into a honeycomb structure, typically with 3.2 mm pitch × 1.7 mm height. The honeycomb rotor of adsorbent was applied to the VOC abatement system operating with thermal swing adsorption. Dependence of the removal efficiency of VOC on various variables was investigated for a test honeycomb rotor of 300 mm in diameter. The following operating guide was recommended to reach high performance where more than 95% of VOC in the feed gas is removed: superficial velocity of feed air = 2−4 m/s, flow rate ratio of process to regeneration zone = 5−15, relative humidity < 80%, and regeneration air temperature = 180 °C with rotation speed optimized. The optimal rotation speed of the honeycomb rotor was investigated closely, and the result was interpreted in terms of the heat capacity ratio between the honeycomb rotor and the regeneration air stream.

Evaluation of rotary dehumidifier performance with and without heated purge

In this paper we evaluate the potential benefits from separating process air stream at the exit of rotary dehumidifier into two streams. One air stream, hot and humid, is called purge air stream and other is remaining process air stream. The remaining process air stream has a lower temperature and humidity ratio as result of separation of initial hot process air stream. It is found that as the purge angle increases the exit humidity ratio of remaining process air stream decreases up to a point where it reaches a minimum. The purge angle for which this occurs is named “effective purge angle”. The effective purge angles for different splits between adsorption and desorption side of the rotary dehumidifier, various regeneration temperatures, non-dimensional lengths and their corresponding optimum non-dimensional times are determined. An existing finite-difference model, developed by the authors of this paper, for simulation of desiccant wheel performance is extended to account for the separation of the process air stream at the exit of rotary dehumidifier and later mixing of purge air stream and outside air to form the regeneration air stream. The performance of desiccant wheel with heated “effective purge angle” is evaluated and compared with performance of the same wheel without purge angle at all. It is found, for all cases considered in this study, that having heated “effective purge angle” has overall positive effect on the performance of the rotary dehumidifier.

Experimental Study on a Process Design for Adsorption Desiccant Cooling Driven with a Low-Temperature Heat

Among the desiccant cooling process, 2-rotor process consisting of a honeycomb rotor dehumidifier and a sensible heat exchanger is the mainstream of the cooling processes which are practically applied to supermarket, hospital and so on. Most of them are driven with a higher regeneration temperature around 100–140∘C obtained from gas-engine heat pump or micro gas turbine generator. However, dehumidifying performance of this typical configuration driven with a low temperature heat is not sufficient for cooling at higher ambient humidity. In this study, 4-rotor desiccant cooling process equipped with a double stage dehumidification was proposed and investigated experimentally. In this process, regeneration temperature around 70∘C could produce a sufficient dehumidifying performance at high ambient humidity. Furthermore, the cascade use of hot water inside the cooling cycle was applied and confirmed somewhat lower cooling performance than that operated with parallel supply of hot water. Against this result, COPr of the former was much higher than that of the latter. Effect of water spray evaporative cooling at the inlet of regeneration air stream on the process performance was also investigated. This evaporative cooling was expected to cause humidity increase in regeneration air reducing the dehumidifying performance of the honeycomb absorber, while the evaporative cooling plays an important role to produce a lower temperature in supply air. Experimental results showed that the amount of dehumidified water at the process without water spray evaporative cooler was actually larger than that of process with water spray evaporative cooler. This behavior was due to increase of humidity or relative humidity in the regeneration air as expected. However, temperature of supply air produced by the process with evaporator was rather lower than that of the other, resulting higher COP value. It was concluded that the evaporative cooler effectively worked at higher regeneration temperature and lower ambient humidity.

Absorption/regeneration non-conventional system for water extraction from atmospheric air

The present work suggests a non-conventional method of water production from atmospheric air, on a 24-h basis using a compact system. The operation of the system is described and its efficiency is defined. The system performs under forced convection absorption and regeneration through a packed tower. The packed tower consists of two identical columns, each of them is packed with an identical bed. Each bed consists of vertical multi-layers of cloth material impregnated with calcium chloride solution of different concentrations. A numerical model, based on the experimental results, has been developed to predict the performance of the system under various operating conditions. The system efficiency is found to have peak values at certain cycle times, desiccant final concentration, regeneration temperature and absorption air stream velocity. It is also found that the maximum efficiency increases with initial concentration and decreases with the increase of the regeneration air stream velocity and absorption temperature.

Effect of Water Spray Evaporative Cooling at the Inlet of Regeneration Air Stream on the Performance of an Adsorption Desiccant Cooling Process

Effect of Water Spray Evaporative Cooling at the Inlet of Regeneration Air Stream on the Performance of an Adsorption Desiccant Cooling Process

Process Configurations and Their Performance Estimations of an Adsorptive Desiccant Cooling Cycle for Use in a Damp Climate.

Typical adsorptive desiccant cooling process mainly consisting of a rotary dehumidifier and a sensible heat exchanger can be driven with low-temperature heat energy like solar energy or waste heat, and it has been expected to be alternative air conditioning equipment considering various energy/environmental problems such as global warming. However, the fact that the cooling performance is decreased significantly as ambient humidity increases was also found. This is mainly due to a simultaneous increase of humidity and temperature in the process outlet air of dehumidifier. For the use of the cooling process in high humid condition, two process configurations were proposed and investigated by means of simple calculation. One of the proposed configurations, the 4-wheel cycle with double stage dehumidification, indicated a sufficient cooling performance for the use in factories or laboratories even though the ambient humidity was nearly 20 g/kg. Furthermore, the process performance was improved more with water spray evaporative cooling at the inlet of a regeneration air stream in the pre-dehumidification section. This means that lower humidity and lower temperature of air supplied to the process inlet of the dehumidifier is strongly required to give the cooling process a sufficient performance for actual use. Meanwhile, it was found that lower temperature heat around 50–60°C could drive the 4-wheel desiccant cycle in a moderate ambient humidity. The other, a 3-wheel cycle equipped with a total heat exchanger, gave us a reasonable improvement in the cooling effect and COP with less additional investment than that for the 4-wheel cycle.

Simplified models for the performance evaluation of desiccant wheel dehumidification

In the present communication, simple models have been presented to evaluate the performance of rotary desiccant wheels based on different kind of solid desiccants e.g. silica gel and LiCl. The first part of the paper presents ‘Model 54’ which is developed for silica gel desiccant rotor. The model has been derived from the interpolation of experimental data obtained from the industry and the correlations have been developed for predicting outlet temperature and absolute humidity. The ‘Model 54’ consists of 54 coefficients corresponding to each correlation for outlet absolute humidity and temperature and it is found that the model predicts very well the performance of silica gel desiccant rotor (Type-I). In the second part of the paper, a psychrometric model has been presented to obtain relatively simple correlations for outlet temperature and absolute humidity. The developed psychometric model is based on the correlations between the relative humidity and enthalpy of supply and regeneration air streams. The model is used to predict the performance of three type of desiccant rotors manufactured by using different kind of solid desiccants (Type I, II and III). The model is tested corresponding to a wide range of measurement data. The developed psychometric model is simple in nature and able to predict very well the performance of different kind of desiccant rotors. Copyright © 2002 John Wiley & Sons, Ltd.

An Analytical Examination of Methods for Improving the Performance of Desiccant Cooling Systems

The conventional Pennington Cycle desiccant cooling system offers a clear opportunity for heat-actuated air conditioning. However, efforts to translate this opportunity into commercially viable hardware have not been successful. The performance of the hardware has been inadequate, resulting in excessive solar collector requirements or, in the case of gas-fired equipment, uneconomical use of natural gas. Two methods for improving the coefficient of performance (COP) of these systems are: (1) the addition of inert heat capacity to the desiccant matrix, and (2) “staging” the regeneration air stream. An analysis is presented in this paper which explains the benefits and drawbacks of these methods based upon the wave nature of the heat and mass transfer processes occurring within the desiccant bed. The results indicate that the best overall system performance is obtained by staging the regeneration process while minimizing the amount of inert heat capacity.

Second Law Analysis of Solid Desiccant Rotary Dehumidifiers

This paper presents a second law analysis of solid desiccant rotary dehumidifiers. The equations for entropy generation for adiabatic flow of humid air over a solid desiccant are developed. The generation of entropy during operation of a rotary dehumidifier with infinite transfer coefficients is investigated and the various sources of irreversibility are identified and quantified. As they pass through the dehumidifier, both the process and regeneration air streams acquire nonuniform outlet states, and mixing both of these air streams to deliver homogeneous outlet streams is irreversible. Transfer of mass and energy between the regeneration air stream and the desiccant matrix occurs across finite differences in vapor pressure and temperature and these transfer processes generate entropy. The second law efficiency of the dehumidifier is given as a function of operating conditions and the effect of finite transfer coefficients for an actual dehumidifier is discussed. It is shown that operating the rotary dehumidifier at conditions that minimize regeneration energy also yields a local maximum for the second law efficiency.