Solid Desiccant System Research Articles

A novel dual-wheel air-conditioning system operating strategy in different seasons

The application of solid desiccant air-conditioning systems can significantly reduce building energy consumption. Previous studies have focused on innovating the integration of solid desiccant air-conditioning systems (DACS) to improve dehumidification performance in summer. In this paper, a novel DACS is proposed, which can humidify the supply air in winter based on satisfying the dehumidification demands in summer. It can satisfy the year-round humidity requirements of domestic housing with significant savings in humidification costs. Furthermore, the operating strategy of the novel DACS is proposed, particularly for winter humidification conditions. It is adaptable as well as cost-effective compared to traditional operating strategies. The dehumidification/humidification capacity of the core component, polymer desiccant wheel, and the variation of temperature rise/temperature drop are investigated under different variations of parameters, including air temperature, air humidity, air volume flow rate, and wheel speed. The results show that with the increase of supply air mass flow from 0.6 kg/s to 1.9 kg/s, the power consumption of the system only increases by 9.5 kW. In winter, when the supply air temperature decreases from 8 °C to 1 °C, the power consumption increases by 7.2 kW, but in summer, when the temperature of the supply air increases by 7 °C, the power consumption of the system decreases by 7.0 kW. The power consumption of the system varies greatly when the humidity of the supply air is less than 17 g/kg, conversely, the power consumption of the system showed a negative trend. The analysis demonstrates the effective performance of the proposed system under varying climatic conditions.

Experimental study on wavy fin desiccant coated heat exchanger for hybrid air conditioning systems

The desiccant-coated heat exchanger (DCHE) is an energy-efficient alternative dehumidification equipment for humidity control in air conditioning. It exhibits a higher dehumidification performance than other solid desiccant systems and can simultaneously handle both latent and sensible loads. Taking the clue of research outcomes from the alternative heat exchangers to enhance the performance, a wavy fin DCHE is designed, fabricated and tested. Objectives of the study are to (i) analyse the dynamic performance of DCHE, (ii) compare the heat and mass transfer characteristics of plain and wavy fin-type DCHEs, and (iii) examine the influence of different operating parameters on the performance. The experimental investigation shows that the dehumidification capacity and thermal coefficient of performance of the proposed wavy fin DCHE are 0.34 g/s and 1.041, respectively, which are 75% and 60% higher than the conventional plain fin type. In addition, the developed DCHE has an average cooling capacity of 0.85 kW, which is 42% higher. Besides, the total quantity of moisture adsorbed also increased by 35%. Furthermore, the sensitivity analysis of the parametric study identified air velocity, cold water temperature, and inlet humidity ratio as the most influential operating parameters on the performance of wavy fin DCHE.

Household solid desiccant dehumidifier with multiple operating modes: Concept and prototype device

Solid desiccant dehumidification has proven to be more energy-efficient than traditional condensation dehumidification. However, owing to the complexity and large size of existing solid desiccant dehumidifier systems, their application in residential spaces is limited. In this study, we explore the potential of using small solid desiccant dehumidifiers in residential spaces. The dehumidifier uses a variable frequency fan to replace the two fans of a traditional solid desiccant dehumidifier. Meanwhile, the flow direction of the dehumidified air and regeneration air was adjusted via the air valve such that the air in different working modes flows in the same space, thereby reducing the size of the dehumidifier. Moreover, the dehumidifier used an equalizing orifice to ensure the stability and uniformity of the airflow in a small space and used an electric heating tube for regeneration, as opposed to a heat exchanger, to reduce the size of the system. The experimental results of the prototype device show a dehumidification efficiency of more than 85%. The humidity at the inlet of the dehumidifier, air volume, and adsorption heat affected dehumidification performance. The analysis results of the energy coefficient of dehumidification (ECOD) index indicated that through reasonable design and usage patterns, the ECOD of small-sized solid desiccant dehumidifiers can exceed 1.73 kg/(kW·h). Compared to traditional condensing dehumidifiers for home use in the market, the energy efficiency ratio is improved by 8.1%. This indicates that the dehumidifier exhibits a high energy efficiency ratio.

Effect of Geometry and Operational Parameters on the Dehumidification Performance of a Desiccant Coated Heat Exchanger

Solid desiccant dehumidification systems are an alternative to vapor-compression-based air dehumidification systems. They use solid desiccant materials to adsorb air moisture in space cooling. DCHE can integrate the cooling component in the heat exchanger to achieve higher dehumidification capacity. Comprehensive numeric modeling would be necessary to assist in the design and operation of the DCHE under development to achieve maximum performance. This paper provides the details of a heat and mass transfer model developed for this purpose. The model aims to cover a gap in existing models by independently modeling heat transfer on the solid desiccant, fin, and tube. The model results were compared with an experimental reference for validation. The air outlet humidity and temperature results for dehumidification and regeneration showed a deviation lower than 15% from the experiment. The validated model was used to perform a parametric study for a series of design and operating conditions. The parametric analysis showed that an increase of 25% in desiccant layer thickness and thermal contact resistance between solid elements reduces moisture removal by 9.1% and 1%, respectively. These results indicate the need to independently model the desiccant layer.

Experimental investigation of an integrated absorption- solid desiccant air conditioning system

Separate handling of sensible and latent components of building loads can be an energy efficient approach compared to simultaneous management of both. This paper presents a detailed experimental analysis of an energy efficient air conditioning system using solid desiccant integrated with absorption system for separate building thermal load handling. The experimental system consists of silica-gel based solid desiccant for latent load handling, whereas the gas fired air-cooled NH3-H2O absorption chiller is used to handle sensible load of the space. A chilled water-cooling coil heat exchanger is installed on the process side of desiccant cooling system to integrate it with the absorption chiller. The performance of integrated absorption- solid desiccant system is compared with a standalone conventional desiccant air conditioning system with a direct evaporative cooler considering it as baseline system under wide range of operating conditions including air temperature, air humidity, and regeneration temperature. The comparative analysis is performed in terms of supply air temperature, cooling capacity, and coefficient of performance. The experimental results exhibit that supply air temperature of the integrated system is 15.2 °C compared to 24.6 °C achieved by conventional desiccant system. Moreover, COPth of the integrated system is also 50–55% higher than its counterpart and it is almost comparable with double-effect absorption chiller. It is concluded that the integrated system using separate load management approach is more efficient for HVAC applications.

Performance assessment of a desiccant air-conditioning system combined with dew-point indirect evaporative cooler and PV/T

In this study, a new solid desiccant air-conditioning system configuration integrated with direct and dew-point indirect evaporative coolers and water-cooled photovoltaic-thermal solar collectors (PV/T) is proposed and simulated for a building located in Adana, a humid and hot city on Mediterranean coast of Turkey. New multiple linear regression equations are developed for the performance of desiccant wheel and dew-point indirect evaporative cooler, which are the two crucial components of the air-conditioning system and used in hourly simulations. Building set temperature, exhaust air mixing ratio, suitable warm air source for preheating regeneration air and optimum connection strategy of PV/T collectors to each other are examined in detail. Hourly and daily thermal coefficient of performance (COPth), solar fraction, primary energy consumption and specific primary energy consumption are calculated to evaluate the energetic performance of the system. COPth of the system ranges from 0.28 to 0.40 during the day, with a daily COPth of 0.34. 69% of the thermal energy required to regenerate desiccant wheel can be generated by the PV/T water collectors.

Evaporative Cooling Integrated with Solid Desiccant Systems: A Review

Evaporative cooling technology (ECT) has been deemed as an alternative to the conventional vapor-compression air conditioning system for dry climates in recent years due to its simple structure and low operating cost. Generally speaking, the ECT includes two types of different technologies, direct evaporative cooling (DEC) and indirect evaporative cooling (IEC). Both technologies can theoretically reduce the air temperature to the wet-bulb temperature of outdoor air. The major difference between these two technologies is that DEC will introduce extra moisture to the supply air while IEC will not. The enhanced IEC, Maisotsenko-cycle (M-cyle) IEC, can even bring down the air temperature to the dew point temperature. The ECT integrated with solid desiccant systems, i.e., solid desiccant-assisted evaporative cooling technologies (SDECT), could make the technology applicable to a wider range of weather conditions, e.g., weather with high humidity. In this paper, the recent development of various evaporative cooling technologies (ECT), solid desiccant material and the integration of these two technologies, the SDECT, were thoroughly reviewed with respect to their configuration, optimization and desiccant unit improvement. Furthermore, modeling techniques for simulating SDECT with their pros and cons were also reviewed. Potential opportunities and research recommendations were indicated, which include improving the structure and material of M-cycle IEC, developing novel desiccant material and optimizing configuration, water consumption rate and operation strategy of SDECT system. This review paper indicated that the SDECT system could be a potential replacement for the conventional vapor-compressed cooling system and could be applied in hot and humid environments with proper arrangements.

Drying characteristics of mint leaves (Mentha arvensis) dried in a solid desiccant dehumidifier system.

In this present study, solid desiccant-based pressure-swing adsorption (PSA) dehumidifier was developed and the process parameters were optimized to deliver the air continuously at 0.1% relative humidity. Mint (Mentha arvensis) leaves are tested to study the drying characteristics at varied flow rates of dehumidified air in the drying chamber. The initial moisture content of 5.059g water/g dry matter have been reduced to a safe storage level in 360min at 0.160 m3/min volume low rate. The effective moisture diffusivity of the mint leaves was found in the range of 2.07534 × 10-11m2/s to 3.45817 × 10-11m2/s. The percentage of retention of ascorbic acid in dried mint leaves is increased by an increase in the volume flow rate of dry air and a maximum of 70.11% is achieved by 0.160m3/min. The colour measurement and chlorophyll content of the dried samples indicated that the desiccant dehumidified air dryers are suitable for heat sensitive green leafy vegetables.

An overview on use of desiccant dehumidifiers in modern air-conditioning

The solid desiccant based dehumidifier used in conjunction with the conventional HVAC combines the dehumidification of solid desiccant system and with the cooling capacity of the conventional air conditioning system. This hybrid cooling system provides thermal comfort to the occupants of the conditioned space. The hybrid systems main appeal lies in the fact that, it consumes much lesser high grade electrical energy as compared to the dedicated standalone traditional air conditioning systems. The electrical energy usage is possible still lower by use of primary energy sources for to supply the thermal energy needed for the desiccant regeneration. For this purpose freely available renewable solar energy or industrial waste heat can also be used for the regeneration heat source. Sometimes it is also possible to provide condenser waste heat for the part of desiccant reactivation heat supply may increase the overall performance of the system. It was also found that this cooling system with use of air to air waste heat recovery wheel performed better than without it in terms of dehumidification as well as cooling performance. The present study report important literature survey on the dehumidification potentials of desiccant integrated hybrid cooling system operating in hot and humid climates. Keywords: Hybrid air-conditioning; Rotary desiccant dehumidifier; Heat recovery wheel; Regeneration heat; Renewable solar energy; Waste heat

Performance Enhancement of Hybrid Solid Desiccant Cooling Systems by Integrating Solar Water Collectors in Taiwan

A hybrid solid desiccant cooling system (SDCS), which combines a solid desiccant system and a vapor compression system, is considered to be an excellent alternative for commercial and residential air conditioning systems. In this study, a solar-assisted hybrid SDCS system was developed in which solar-heated water is used as an additional heat source for the regeneration process, in addition to recovering heat from the condenser of an integrated heat pump. A solar thermal collector sub-system is used to generate solar regeneration water. Experiments were conducted in the typically hot and humid weather of Taichung, Taiwan, from the spring to fall seasons. The experimental results show that the overall performance of the system in terms of power consumption can be enhanced by approximately 10% by integrating a solar-heated water heat exchanger in comparison to the hybrid SDCS system. The results show that the system performs better when the outdoor humidity ratio is large. In addition, regarding the effect of ambient temperature on the coefficient of performance (COP) of the systems, a critical value of outdoor temperature exists. The COP of the systems gradually rises with the increase in ambient temperature. However, when the ambient temperature is greater than the critical value, the COP gradually decreases with the increase in ambient temperature. The critical outdoor temperature of the hybrid SDCS is from 26 °C to 27 °C, and the critical temperature of the solar-assisted hybrid SDCS is from 27 °C to 30 °C.

Investigation of energy-efficient solid desiccant system for wheat drying

The study investigates the applicability of solid desiccant system for drying of freshly harvested wheat grains in order to reduce the moisture content to an optimum level. Fast and low-temperature drying systems are required by today’s drying industries in order to provide economical and safe drying. Therefore, comparison of desiccant drying has been made with the conventional method in terms of drying kinetics, allowable time for safe storage, the total time for drying cycle, and overall energy consumption. It has been found that the air conditions of proposed desiccant drying system provides a high drying rate and longer allowable storage time for safe drying. As the desiccants possess water adsorbing ability by means of vapor pressure deficit, therefore, the desiccant system successfully provides low-temperature drying which ensures the quality of wheat grains. Overall energy consumption is estimated for both conventional hot air drying and desiccant drying system. It has been found that the desiccant system requires less energy as drying is accomplished at minimum level of air flow and within allowable storage time. In addition, the overall performance index of the desiccant system is higher at all temperatures. The study is useful for developing a low-cost and sustainable drying technology for various agricultural products. Keywords: desiccant, solid desiccant system, drying, grain, wheat, performance evaluation DOI: 10.25165/j.ijabe.20191201.3854 Citation: Hanif S, Sultan M, Miyazaki T, Koyama S. Investigation of energy-efficient solid desiccant system for wheat drying. Int J Agric & Biol Eng, 2019; 12(1): 221–228.

Investigation of energy-efficient solid desiccant system for wheat drying

Investigation of energy-efficient solid desiccant system for wheat drying

Performance prediction of solid desiccant rotary system using artificial neural network

This paper presents an artificial neural network model for solid desiccant rotary system to predict its performance in terms of temperature and relative humidity of process air leaving the desiccant wheel after losing latent heat. Present paper also explains the experimental test setup that is used for taking reading. The experimental readings are all taken at steady state by varying the input conditions such as process air inlet velocity, regeneration air inlet velocity and regeneration temperature and process air inlet temperature and relative humidity. Majority of data taken from experiments is used to train the model (85%) and rest (15%) is used for testing of the model. The performance output predicted by the ANN model have high correlation factor(R>0.98336). The results predicted by the ANN model shows that ANN model can be successfully applied to predict the performance of solid desiccant wheel with sufficient accuracy and reliability.

Advanced Technology in Desiccant Wheel Air Conditioning System: A Review

Desiccant wheel air conditioning have been considered as one of the effective process to control humidity of ambient air. Desiccant wheel air conditioning system do not use any ozone- depleting refrigerant and also intake lesser energy compared to vapour compression system. All the types and kind of desiccants have been discussed here. Such as solid desiccant system, liquid system have been reviewed briefly. Finally, a summary of economic prospect, future scope, environmental friendly technology and future scope of desiccant wheel air conditioning system have been discussed briefly.

Integration of solar assisted solid desiccant cooling system with efficient evaporative cooling technique for separate load handling

Air-conditioning load is generally composed of sensible and latent parts. Currently, various stand-alone electric and heat driven HVAC systems serve the purpose with each having performance limitations while managing cumulative load. However, integration of both electric and heat driven systems can be efficient especially if sensible and latent loads are handled separately. Here an integrated solar assisted cooling system is proposed consisting of a solid desiccant system for handling latent load and a Maisotsenko cycle (MC) based evaporative cooling system for sensible loads. The experimental setup consists of a purposely designed hybrid arrays of solar thermal collectors, a solid desiccant wheel with heat recovery and a coupled indirect MC evaporative cooler in cross flow arrangement. The integrated system is tested for the dehumidification effectiveness, dew point effectiveness, thermal COP, and cooling capacity. The resulted average cooling capacity of the system is around 3.78 kW with average COP of 0.91 at solar fraction of about 70%. The uncertainties for cooling capacity and COP are ±8.6% and ±9.3%, respectively.

Analysis on integrated low grade condensation heat powered desiccant coated vapor compression system

Hybrid solid desiccant and vapor compression system is developed to realize independent control of temperature and humidity. However the large volume has always hindered further development and heat pump system based on desiccant coated heat exchanger (DCHE) is proposed to solve the problem. It’s DCHE instead of conventional sensible heat exchanger is adopted as evaporator/condenser in this system, which also serves as a bridge to link conventional solid desiccant system and vapor compression system together. Simulation model is established in this paper to analyze system performance operating as separate air conditioner. It is found that under new operation condition with higher evaporation temperature and taking COP, pressure and flammability into account, R134a is recommended as refrigerant. Switch time is identified as crucial parameter to avoid condensation. In full fresh air mode, 20–300 s and 30–180 s are the recommended switch time under ARI summer and humid conditions respectively. Also, switch time around 100 s can obtain the highest cooling capacity. For hot and humid summer time in Shanghai, the system can’t meet the indoor requirement with full fresh air mode. However, decreasing the handled load is proved to be effective method. Also the system can obtain the overall COP as high as 5.8 under simulation conditions.

Mathematical modelling of solid desiccant systems

In this study, the mathematical model equations for solid desiccant system integrated with indirect evaporative coolers with Maisotsenko - Cycle are presented. The authors chose the modified ε–NTU method to describe heat and mass transfer processes in regenerative indirect evaporative cooler and desiccant wheel. The models based on the ε–NTU method show satisfactory agreement with experimental results. That is why this method allows to analyze and develop the performance of solid desiccant systems. In this study, the models allowed to prove that solid desiccant system with an additional heat exchanger before the desiccant wheel (System 1) obtains higher thermal COP values, higher humidity ratio drop and lower supply airflow temperatures in comparison to system with only one heat exchanger after the desiccant wheel (System 2).

Mathematical modeling and performance evaluation of a desiccant coated fin-tube heat exchanger

A solid-desiccant system that utilizes low grade heat is potentially a viable add-on to conventional HVAC systems since it can help reduce power consumption significantly, for achieving indoor thermal comfort conditions. In contrast to desiccant wheels which carry out adiabatic dehumidification, isothermal dehumidification process that may be realized by a cross-flow heat exchanger is much more efficient. In this paper, a novel mathematical model is developed to simulate heat and mass exchange phenomena of a desiccant coated fin tube heat exchanger (DCFTHX). This model takes solid side mass transfer resistance as well as fin efficiency into consideration. The model is validated using experimental results in the literature. It is also compared against simplified models to establish its utility. A parametric study is then conducted to investigate the effects of geometrical parameters as well as mass flow rate of water and air velocity on dehumidification and adsorption heat removal performance of the DCFTHX as well as the performance of the augmented air-conditioning system under warm and humid ambient conditions. Under the range of parameters and conditions simulated, if low grade waste heat (50 °C hot water) is available for regeneration, integration of DCFTHX sub-system with a conventional air conditioning system can yield as high as 31% energy savings (even when the additional power consumed by pumps and blower fans is accounted for).

Experimental evaluation of a solid desiccant system integrated with cross flow Maisotsenko cycle evaporative cooler

This paper presents the performance investigation of a solid desiccant dehumidifier integrated with Maisotsenko cycle (M-Cycle) based cross flow heat and mass exchanger (MC-DAC). The experimental test rig consisting of a silica gel based desiccant wheel and a heat recovery wheel is coupled with M-Cycle indirect evaporative cooler. The effect of wide range of inlet air parameters such as ambient temperature, humidity ratio, and regeneration temperature on the performance of integrated system was analyzed and compared with the conventional desiccant air conditioning (DAC) system. Set of experiments were carried out for both systems at constant process as well as return air mass flow rates under different operating conditions. MC-DAC system was observed to be around 60–65% more efficient than the other system in terms of COPth providing same supply air conditions at low regeneration temperatures.

PERFORMANCE ASSESSMENT OF A SOLID DESICCANT AIR DEHUMIDIFIER

The presence of moisture in the air along with temperature has a long term and devastating effect on man and material. One way to create a low humidity environment is by using a solid desiccant wheel system. In the present work, an experimental analysis has been carried out under steady-state conditions to investigate the effects of different operating parameters on a solid desiccant wheel system performances. An experimental rig consists of an FFB300 air dehumidifier system was constructed. A parametric investigation was carried out to examine the effects of the reactivation air inlet temperature and process air outlet velocity on the thermal effectiveness, dehumidification efficiency, and moisture removal rate of the desiccant wheel system. The analysis shows that both thermal effectiveness and dehumidification efficiency decrease with the increase of the reactivation air inlet temperature, by 2.5 % and 43 %, respectively. Likewise, when the process air outlet velocity increases both performances criteria reduce by 10 % and 28 %, respectively. The moisture removal rate increases significantly by 30 % as the reactivation air inlet temperature increases. However, the process air outlet velocity has no significant effect on the moisture removal rate.