Analyzing the Opportunities for NIPAAm Dehumidification in Air Conditioning Systems

abstract: When air is supplied to a conditioned space, the temperature and humidity of the air often contribute to the comfort and health of the occupants within the space. However, the vapor compression system, which is the standard air conditioning configuration, requires air to reach the dew point for dehumidification to occur, which can decrease system efficiency and longevity in low temperature applications. To improve performance, some systems dehumidify the air before cooling. One common dehumidifier is the desiccant wheel, in which solid desiccant absorbs moisture out of the air while rotating through circular housing. This system improves performance, especially when the desiccant is regenerated with waste or solar heat; however, the heat of regeneration is very large, as the water absorbed during dehumidification must be evaporated. N-isopropylacrylamide (NIPAAm), a sorbent that oozes water when raised above a certain temperature, could potentially replace traditional desiccants in dehumidifiers. The heat of regeneration for NIPAAm consists of some sensible heat to bring the sorbent to the regeneration temperature, plus some latent heat to offset any liquid water that is evaporated as it is exuded from the NIPAAm. This means the NIPAAm regeneration heat has the potential to be much lower than that of a traditional desiccant. Models were created for a standard vapor compression air conditioning system, two desiccant systems, and two theoretical NIPAAm systems. All components were modeled for simplified steady state operation. For a moderate percent of water evaporated during regeneration, it was found that the NIPAAm systems perform better than standard vapor compression. When compared to the desiccant systems, the NIPAAm systems performed better at almost all percent evaporation values. The regeneration heat was modeled as if supplied by an electric heater. If a cheaper heat source were utilized, the case for NIPAAm would be even stronger. Future work on NIPAAm dehumidification should focus on lowering the percent evaporation from the 67% value found in literature. Additionally, the NIPAAm cannot exceed the lower critical solution temperature during dehumidification, indicating that a NIPAAm dehumidification system should be carefully designed such that the sorbent temperature is kept sufficiently low during dehumidification.Dissertation/ThesisMasters Thesis Mechanical Engineering 201

Performance analysis and design implementation of a novel polymer hollow fiber liquid desiccant dehumidifier with aqueous potassium formate

A novel cross-flow liquid desiccant polymer hollow fiber dehumidifier (PHFD) is investigated numerically in this paper. The main objective of this research is to simulate, and validate the numerical model for future design implementations. The experimentally verified simulation data will be used to develop a set of design and implementation tables and charts as the guidance for selecting the number of fibres and the solution-to-air mass flow ratio of the PHDF under given conditions. A numerical model is developed to simulate the performance of the proposed innovative dehumidifier. This model is validated against three sets of data, i.e. the experimental obtained testing results, analytical correlations and the modelling results from the literature. The influence of various operating conditions such as inlet air properties (i.e. velocity, relative humidity) and inlet solution properties (i.e. temperature, concentration, mass flow rate) on the dehumidification sensible, latent, and total effectiveness, moisture removal rate are numerically analyzed. Dimensionless parameters including the number of heat transfer unit (NTU) and the number of mass transfer unit (NTUm), the solution to air mass flow rate ratio (m*), and the air to solution specific humidity ratio () have been used to evaluate the system performance. The results show that the increase in NTU and NTUm lead to a substantial change in dehumidification effectiveness. When the NTU increases from 0.47 to 7, the sensible effectiveness rises from 0.35 to 0.95. Increasing is another good option for increasing the amount of the absorbed moisture without influencing the latent effectiveness. For an increase of from 1.4 to 2.2, the air inlet and outlet specific humidity difference varies in the range of 0.008 kg/kg and 0.018 kg/kg

Next-generation HVAC: Prospects for and limitations of desiccant and membrane-based dehumidification and cooling

Recently, next-generation HVAC technologies have gained attention as potential alternatives to the conventional vapor-compression system (VCS) for dehumidification and cooling. Previous studies have primarily focused on analyzing a specific technology or its application to a particular climate. A comparison of these technologies is necessary to elucidate the reasons and conditions under which one technology might outperform the rest. In this study, we apply a uniform framework based on fundamental thermodynamic principles to assess and compare different HVAC technologies from an energy conversion standpoint. The thermodynamic least work of dehumidification and cooling is formally defined as a thermodynamic benchmark, while VCS performance is chosen as the industry benchmark against which other technologies, namely desiccant-based cooling system (DCS) and membrane-based cooling system (MCS), are compared. The effect of outdoor temperature and humidity on device performance is investigated, and key insights underlying the dehumidification and cooling process are elucidated. In spite of the great potential of DCS and MCS technologies, our results underscore the need for improved system-level design and integration if DCS or MCS are to compete with VCS. Our findings have significant implications for the design and operation of next-generation HVAC technologies and shed light on potential avenues to achieve higher efficiencies in dehumidification and cooling applications

Hybrid Liquid Desiccant System Design and Operation Under High Latent Load Conditions in Taiwan

Hybrid Liquid Desiccant systems (HLDS) combine the liquid desiccant technology for dehumidification of air with conventional compression cycle technology for cooling. They are an alternative to conventional compression cooling systems, being more efficient and offering the possibility of independently control temperature and humidity. In this paper the design and operation of a HLDS is presented, for the air conditioning of a high latent load application with high ambient humidity levels. An analysis of the daily evolution of the performance of the system under different environmental conditions has been included. The innovative demonstration unit placed in Taiwan, in continuous operation since November 2015, achieved Energy efficiency Ratios (EER) up to 4.6.The authors would like to thank the support of the project “nanoCOOL” funded by the Seventh Framework Programme FP7/2007-2013, Project No. 314701

Investigation of Water Vapor Transport in Membrane Mass Exchangers for HVAC Applications

Vapor compression systems have dominated the HVAC area for close to 100 years. These systems require significant amounts of energy to complete the compression cycle and the refrigerants used are known contributors to global warming. As a result, new innovations are being sought and a membrane-based system, the subject of this thesis, is one such technology. Water selective membrane materials offer a promising alternative to vapor compression for dehumidification of building air. For HVAC systems, process air pressure drop constrains flow path design in a membrane-based approach. The transport resistance for water vapor from air flow channels is experimentally investigated for a plate-type membrane mass exchanger design. Convective and diffusive resistances are measured for polymer membranes with varying flow channel dimensions. A series of experiments and analyses is developed to separate diffusive and convective transport resistance to water vapor removal from supply air. Results are compared with empirical Sherwood number correlations to enable improved mass exchanger design. The validated mass transfer correlations were used to develop a mass transfer model and later implemented for the simulation of a 3 rTon (1 rTon = 3.516kW) membrane heat pump dedicated outdoor air systems. From the various analyses, maintaining process air pressure drop at less than 50 Pa while at a typical HVAC face velocity results in a convective resistance that is 6 times greater than the diffusive resistance to water vapor through an ionic membrane. Furthermore, the tradeoff between required membrane area, system size, pressure-drop and effective latent cooling is explored. Simulation results show that when conforming mass exchanger designs to meet ASHRAE standards, a system electrical COP of 7.5 or greater can be achieved

Experimental study of a membrane-based liquid desiccant dehumidifier based on internal air temperature variation

A membrane-based liquid desiccant dehumidifier with the separated air stream and liquid desiccant channels has the ability to solve its working fluid carryover problem in the traditional direct contact system. The sensible, latent, total effectiveness and air moisture removal rate are adopted for the dehumidifier performance evaluation in this paper, and the dehumidifier main operating parameters are investigated experimentally to identify their influences and internal air temperature variations, including inlet air relative humidity (RH), inlet solution concentration and temperature, heat capacity rate ratio (Cr*) and number of heat transfer units (NTU). It is found that both the inlet air RH and solution temperature have the negative influences on the dehumidifier effectiveness, while the desiccant solution concentration has little positive influence; the air moisture removal rate rises sharply with the inlet air RH and solution concentration. The highest sensible, latent and total effectiveness achieved in this study are 0.823, 0.802 and 0.810 respectively when both Cr* and NTU are equal to 12. However the operating condition with NTU=8 and Cr*=6 is recommended with the corresponding sensible, latent and total effectiveness of 0.758, 0.71 and 0.728 respectively

A techno-economic investigation of conventional and innovative desiccant solutions based on moisture sorption analysis

Liquid desiccant technology is an energy-efficient substitute for technologies that are conventionally applied for temperature and humidity control; however, innovative desiccant solutions have not been extensively explored in terms of their performance and feasibility. This work aimed to investigate desiccant solutions with moisture sorption analysis technically and economically. Various conditions of temperature and humidity were tested in a climatic chamber and the moisture absorption and desorption capacity, thermo-chemical energy storage capacity, and cost of conventional and innovative desiccant solutions were assessed by experiment. Calcium chloride showed the highest moisture desorption capacity (0.3113 gH2O/gsol in the climatic chamber at 50 °C and 25% RH) and the lowest cost, despite its low moisture absorption capacity. Ionic liquids show high moisture absorption capacity (as high as 0.429 gH2O/gsol in the climatic chamber at 25 °C and 90% RH) and could be used as additives (in which a maximum increase of 84.1% was observed for moisture absorption capacity due to the addition of ionic liquids), and thus, they are promising substitutes for conventional desiccant solutions. As solutions for better performance under various conditions were identified, the study will advance liquid desiccant technology

Design and performance testing of a novel ceiling panel for simultaneous heat and moisture transfer to moderate indoor temperature and relative humidity

An important aspect in the design of buildings is the comfort of the occupants inside the building. Although many factors can affect comfort, the factor of particular interest in this research is the indoor humidity level. High indoor humidity levels can make the indoor air feel stuffy at warm temperatures and reduce the amount of moisture that can be removed from a person’s body, making a person feel warm. On the other hand, low indoor humidity levels can cause health problems such as dry eyes and sore throats. In addition to the comfort implications of improperly maintaining indoor humidity levels, the building itself can suffer from mould growth and rotting materials at high humidity levels. This thesis presents the first research on a novel panel that can simultaneously transfer heat and moisture to/from an occupied space. The panel is referred to as a heat and moisture transfer panel (HAMP). A HAMP is similar in design to a radiant ceiling panel, but uses a liquid desiccant as the heat and moisture transfer medium and the surface of the panel is made of a semi-permeable membrane. A HAMP can be installed into a space and heat and moisture will be transferred between the liquid desiccant and the space air, through the membrane. The main objectives of this thesis are to design a prototype HAMP and to measure the performance of the HAMP under different operating conditions. The performance of the HAMP is quantified by the sensible and latent effectivenesses, as well as the total heat and mass flux between the HAMP and the air in the test section. The results of the experiments show that the HAMP is able to simultaneously transfer heat and moisture with the air in the test section under all operating conditions. The sensible and latent effectivenesses of the HAMP are higher when the air in the test section becomes unstable, due to the natural convection in the duct (sensible effectiveness ≈ 15%, latent effectiveness ≈ 40%). These include cases of cooling and/or dehumidification. The sensible and latent effectivenesses of the HAMP are lower when forced convection is dominant in the test section (sensible effectiveness ≈ 5%, latent effectiveness ≈ 25%). This occurs during cases of heating and/or humidification. The presence of natural convection in the test section is confirmed with flow visualization photographs. The photographs show laminar boundary layer flow during the stable airflow cases, and the presence of convection roll cells during the unstable airflow cases. The total heat flux increases with an increase in the temperature difference between the panel and the air and the mass flux increases with an increase in the humidity ratio difference between the panel and the air. For a temperature difference of 10°C, the prototype HAMP can provide ~4 W/m2 of cooling. This is small compared to the actual cooling loads that would be required in a space. The HAMP is able to provide enough moisture transfer to remove the moisture generated by one person (~70 g/hr), with ~2 m2 of panel surface area. Although the rate of heat transfer between the HAMP and the airflow is limited, the moisture transfer rates are very good. An analysis of standard heat exchanger correlations typically used to predict the effectiveness of a heat exchanger shows that these methods are not applicable for a HAMP. Instead, new correlations must be developed to predict the sensible and latent effectivenesses of a HAMP. These correlations are not determined in this thesis, but an analysis of the experimental data is presented to show that the effectivenesses are functions of several design parameters such as the number of heat transfer units (NTUexp), the number of mass transfer units (NTUm,exp), the effective Rayleigh number of the air (Ra+), the operating condition factor (H*) and the ratio of NTUexp/NTUm,exp. The experimental results presented in this thesis show that a HAMP can be used to simultaneously transfer heat and moisture with the air in a space, and may be used as a ceiling panel in a space to simultaneously control the temperature and relative humidity of the air. The performance of a HAMP can be determined using two parameters: NTUexp and NTUm,exp. Determining these two parameters is very complicated and involves analysis of several design parameters, such as NTUtheo, NTUm,theo, H*, Ra+ and the ratio NTUexp/NTUm,exp. The experimental data collected in this thesis was used to analyze the relationships between NTUexp and NTUm,exp and these design parameters and is a starting point for future research on developing a correlation for predicting the performance of a HAMP

OPTIMIZATION OF PROCESS PARAMETERS OF PRESSURE-SWING ADSORPTION CYCLE IN A SILICA GEL DESICCANT DEHUMIDIFICATION SYSTEM

Parametric study on pressure-swing adsorption cycle desiccant dehumidification system is a continuous engineering task with the aim of analyzing its effects and attains target quality of dry air for an industrial process. An experimental setup is developed with a dehumidification tower, regeneration tower,and flow control valve.The effect of process air inlet moisture content, cycle time ratio, and regeneration air flow rate on the adsorption performance is studied to evaluate the potential of the dehumidification system suitable for drying applications. The optimal dehumidification parameters are found, and a regression equation is also developed for the process. It is concluded that, process air inlet moisture is the most influencing parameter compared to regeneration air flow rate and cycle time ratio for the silica gel desiccant dehumidification system.&nbsp

A techno-economic evaluation of low-grade excess heat recovery and liquid desiccant-based temperature and humidity control in automotive paint shops

The paint shop is the most energy-intensive process in an automotive manufacturing plant, with air management systems that supply air to paint booths consuming the most energy. These systems are crucial for temperature and humidity control, in which they ensure the quality of the final product by preventing paint defects and thus avoid the additional cost of reworking. This is especially true for water-based paints, in which evaporation and film formation processes are influenced by the temperature and humidity of the surrounding air. This study aims to investigate the incorporation of liquid desiccant technology into a conventional air management system for paint shops operating in different climates, which presents the novelty of the study. The technology is promising because it can regulate humidity, act as a dehumidifier or humidifier depending on the demand and stores energy in a thermo-chemical form. In addition, waste heat sources available in the paint shop can be used for the regeneration of the liquid desiccant solution. The techno-economic evaluation of this novel process indicates that the proposed system can control the temperature and humidity of the supply air within the range required for optimal painting and achieve significant energy savings in both cold and hot/humid climates, with a reduction of 44.4% and 33.6% of the energy cost compared to the conventional operation and a payback period of 6.15 and 5.74 years respectively, using calcium chloride as the desiccant solution. The sensitivity analysis investigates the effect of the energy and carbon price on the performance of the system. It is concluded that the integration of liquid desiccant technology into conventional air management systems for paint booths has a huge potential to increase the energy-efficiency of automotive painting