No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Membrane processes for heating, ventilation, and air conditioning
... 1. Systems that only use vapor compression cycles; for example, a sequential system with only one vapor compression cycle that can switch between low-SHR and sensible only modes using a variable speed compressor [12] and a system that uses multiple vapor compression cycles to handle sensible and latent loads separately [13]. 2. Systems that consist of or are enhanced by non-vapor compression devices, such as liquid desiccant [14][15][16][17], solid desiccants [18][19][20][21], and/or membranes [22][23][24][25][26][27]. ...
... It uses a cooling coil to meet both latent and sensible loads. Using the energy balance shown in Eq. (22), the supply air flow ratė3 can be calculated. It is assumed that the minimum of the supply air flow rate (0.025 3 ∕ , 53 CFM) is large enough, so that no reheat is needed to increase the supply air specific enthalpy. ...
... Since the supply air condition is specified, as the indoor SHR increases, the return air humidity decreases because the latent load decreases. If we fix the outdoor temperature and total building load, the mass flow rate of the supply air must increase with increasing SHR to accommodate the decrease in the change in the enthalpy of the supply air and the return air, as shown in Eq. (22). The increase in the mass flow rate of the supply air leads to an increase in the energy consumption associated with the cooling coil. ...
Dehumidification plays a significant role in space conditioning energy use. Conventional vapor compression cooling systems employ dewpoint condensation to deal with latent loads. In contrast, separate sensible latent cooling (SSLC) and other advanced alternative dehumidification systems can significantly reduce the electricity usage for dehumidification. The second law efficiency can be used as a benchmark to evaluate thermodynamic performance of alternative dehumidification systems. However, limitations exist in previous studies that define the thermodynamic reversible limits and second law efficiency for cooling and dehumidification systems. This work presents a new physics-based definition for the reversible limit and the second law efficiencies for cooling and dehumidification systems with air recirculation. The new framework is then extended to define a novel performance metric, the seasonal second law efficiency, to form a universal benchmark for assessing various cooling and dehumidification systems. Five cooling and dehumidification systems including magnetocaloric cooling, solid desiccant dehumidification, and membrane dehumidification are evaluated using this benchmark. Steady-state thermodynamic models are constructed for each system. Second law efficiency for each system under various outdoor temperatures and indoor sensible heat ratios (SHR) are calculated. The annual electricity usage of the five systems is used to justify the seasonal second law efficiency definition. The results show that compared to conventional vapor compression systems with mechanical dehumidification, the membrane-based AMX-R cycle can reduce annual electricity use by 12.2%-22.2% and increase the seasonal second law efficiency by up to 36%.
... The main membrane HVAC processes are different, including vacuum membrane dehumidification and membrane evaporative cooling and humidification, where each process has different inlets and outlets, and the membranes used are different as well (Woods, 2014). ...
... Membrane evaporative cooling ( Fig. 8.13) is a very promising technology regarding the application of evaporative cooling as a method to cool the environments, maintaining the temperature, and operating as a low-cost and energy-efficient technique with a low environmental impact. Membrane evaporative humidification is a technique that is used to control humidity within rooms through the aid of membranes (Woods, 2014). ...
... Schematic of membrane vacuum drying. Reprint fromWoods, J. (2014). Membrane processes for heating, ventilation, and air conditioning. ...
... Air dehumidification substantially affects building performance and the health of occupants [1]. Representative air dehumidification technologies include dew-point vapor-compression cycle dehumidification [2][3][4], desiccant dehumidification [5,6], and membrane-based dehumidification [7][8][9][10]. Vapor-compression cycle dehumidification and desiccant dehumidification systems must consume a relatively high amount of energy to achieve the desired supply conditions (e.g., reheat energy in the vapor-compression cycle dehumidification and sensible cooling in the case of desiccant dehumidification). ...
... Thus, the membrane-based dehumidification process is preferred because it is less expensive and uses less energy than the aforementioned methods [7,8]. Furthermore, membrane-based exchangers can be used to perform energy recovery besides dehumidification [9,10]. ...
... Membrane-based equipment has been widely applied in water treatment, electrochemistry, gas separation, and other similar applications [11], but its application in building and HVAC systems is still in a very early stage of development. However, this application has attracted substantial interest in the past two decades [1,9]. For example, Zhang et al. [12] investigated a compression heat pump driven with a membrane-based air dehumidification system. ...
Membrane-based dehumidification is a promising solution for building applications because of its low cost and limited energy consumption. Developing an efficient and cost-effective open-source code simulation tool is important for optimizing and evaluating such devices in HVAC applications. This paper describes a physics-based model, which accounts for the fundamental heat and mass transfer between a humid-air vapor stream on the feed side and a flowing stream on the permeate side of a membrane. The developed model comprises two mass transfer submodels—a microstructure model and a performance map model—and adopts a segment-by-segment method for discretizing heat and mass transfer governing equations for flow streams on the feed and permeate sides of a membrane. The model can simulate dehumidifiers and energy recovery ventilators with parallel-flow, cross-flow, and counter-flow configurations, and the predictions compare reasonably well with the measurements. The model was used to evaluate the effect of membrane microstructure parameters and membrane surface deflection factors, as well as to investigate the performance of combined dehumidification and energy recovery exchangers. The model and C++ open-source codes are expected to become a fundamental tool in analyzing future membrane-based dehumidification systems.
... On the other hand, closed systems are not as dependent on the ambient conditions but must spend a considerable amount of energy condensing water 2 out of the air. Conventional air conditioning technologies experience similar energy penalties when dealing with humidity [10]. Generally, closed heat pump dryers are more common in the literature, but even open systems could benefit from pre-dehumidification technologies. ...
... The enthalpy at State 2 (ℎ 2 ) is known based on the set temperature and knowing the relative humidity is 100%, and the enthalpy at State 3 (ℎ 3 ) is known based on the set drying temperature ( 3 ) and knowing that 3 = 2 . As discussed before, excess heat is available on the condenser side of the vapor compression cycle due to the energy balance, therefore, the heat transfer rate in the auxiliary condenser is given by Equation 10. ...
Convective drying processes consume a significant amount of energy within the industrial sector. Heat pump-based drying processes are gaining attention as a potential technology for enabling efficient, electricity-driven drying for various applications. In this work, we propose a new heat pump drying system concept that employs water vapor-selective membranes for active control of the air humidity in drying processes, termed the MemDry system. We developed system-level models for the MemDry and representative baseline systems based on the first and second laws of thermodynamics to explore energy trends and limitations of the concept. It was found that energy savings on the order of 30-40% are possible when high temperature, low humidity conditions are required for the drying process. Furthermore, membrane dehumidification could theoretically reduce required drying temperatures by 10-20℃ while still saving energy. The unique design of the MemDry system and its use of exhaust air condensation may improve heat pump COPs by as much as 2x. This theoretical work shows that the MemDry concept has significant potential to provide efficient, feasible, and controllable conditions for industrial heat pump drying applications. © HPC2023. Selection and/or peer-review under the responsibility of the organizers of the 14 th IEA Heat Pump Conference 2023.
... The mode of water vapor transfer in dense membrane is via molecular adsorption on the polymer surface and diffusion by a concentration gradient. In the case of a porous membrane, water vapor directly diffuses through the pore space [128]. On the other hand, membranes construction typically takes the form of flat sheets and hollow fiber in nature. ...
... A vacuum membrane dehumidification system is thermodynamically more effective because it works isothermally, and no heating or cooling source is required. Flat sheet and hollow fiber tube construction have been adopted within the membrane design as shown in Fig. 20 [122,128,130]. ...
Internal space cooling technology is pushing toward promoting alternatives from conventional vapor compression systems to mitigate global warming. Many innovative cooling systems are currently focusing on Research and development (R&D) and commercialization. An evaporative cooling system is one of the possible solutions for vapor compression systems. Despite showing a promising alternative to vapor compression systems, their market penetration is insignificant in society. This stems from the challenges inherent within its operation in diverse climate conditions. In this critical review, various evaporative cooling systems along with their recent developments, were presented and discussed. In search of best outcome and limitation of evaporative cooling systems, a comparison of the performance of selected evaporative cooling system were discussed further. The current review also encapsulates the fundamentals and working principles of various dehumidification systems used in the industrial, residential, and agricultural sectors. Then, the solid and liquid desiccant dehumidification systems were further explored, and their effects on the evaporative cooling system were analyzed. Finally, the liquid desiccant and membrane dehumidification systems were comprehensibly reviewed with their effects on the evaporative cooling system. Based on the energy-saving potential analysis, it was found that independent dehumidification before cooling is an effective method to improve the efficacy of the evaporative cooling system in comparison to the conventional approach at different outdoor conditions. Also, an enhanced evaporative cooling system was found to be a promising replacement for vapor compression system. To provide thermal comfort in hot and humid conditions, membrane dehumidification with a dew point evaporative cooling system shows promise as a substitute air-cooling technology with reduced power usage.
... Vacuum pump-assisted membrane dehumidification has gained significant scientific interest which utilizes the pressure gradient as a driving force across the membrane to capture water vapor [17,18]. The phenomenon of moisture transport across membranes is predominantly regulated by the mechanism of diffusion that operates under different levels of pressure between the two sides. ...
Achievement of thermal comfort in air conditioning is a vital issue in the verge of global warming, where it is essential to provide a system that is less harmful to the environment, energy-efficient and economical. In this regard, dehumidification plays an important role in achieving thermal comfort of air conditioning system. Most of the dehumidification systems as of now require high energy and induce negative environmental impacts. Recently a vacuum membrane-based dehumidification has been considered for the alternative air-conditioning system. Development of efficient membranes for the dehumidification operation is indispensable for moisture removal. Typically, the membrane is constructed out of a fine and porous twilled Dutch weave stainless steel wire mesh serving as the support structure laminated with a film of active materials that serve to attract and isolate the moisture from the working air. In this work a combination of nanomaterial and hygroscopic polymer was adopted to improve the film functionality on the wire mesh. In particular, titanium dioxide (TiO2) is coated as intermediate layer on the wire mesh. Further, Polyvinyl alcohol (PVA) and various concentration of potassium formate (KCOOH) were used as the active additive material aiming to increase sensitivity of the membrane towards water vapor. The water vapor and air permeability tests were conducted to investigate the water vapor permeation and air selectivity of the membrane under various air conditions. Morphological and permeance analogy of the produced membrane showed that adding KCOOH enhanced hydrophilicity, which also increased water vapor permeability by 720%.
Graphical Abstract
... The importance of extracting water from the air is undeniable, particularly in arid regions where there are no other sources of freshwater (Woods, 2014). This applies to both permanent water supply and water supply in natural disaster-affected areas (Bergmair, 2015;yin et al., 2014;Bui et al., 2016). ...
The increase in the world's demand for water has become one of the most relevant topics on environmental sustainability and the conservation of water resources. As a result of the scarcity of potable water, processes such as the use of water through practices, techniques, and technologies for its efficient use have become essential for future generations. Among the possibilities for using water, air conditioning systems stand out, due to the formation of condensed water during operation. The world's population is increasingly concerned about environmental issues, showing that it is necessary to seek alternatives to recycle water, whether for use in gardening, sewage systems, washing sidewalks, and cars, among others. The present study carried out a literature review to reflect on the theme of the reuse of condensation water from air conditioners as a strategy for the sustainable use of water resources and enhancing the future of planet Earth.
... In addition, dehumidification by condensation will generate a large amount of liquid water, and the existence of liquid water has the risk of breeding bacteria and corroding metals. In recent years, some high-efficiency dehumidification technologies, such as rotary dehumidification (Ge, 2014;Liu, 2022), solution dehumidification (Gao, 2013;Dong, 2022), and membrane dehumidification (Cho, 2022;Woods, 2014) have been widely studied. Chua et al. (2018) proposed a hybrid composite desiccants membrane dehumidification system: the moisture removal of the system could reach 5.9g, and the energy efficiency was improved by 40% compared with silica gel systems. ...
Renewable energy is the key to a sustainable and environmentally friendly world. Hydropower production has grown in popularity as the most rapidly growing renewable energy source due to its ability to provide green energy. The hydrokinetic turbine is a potential technology that generates electricity from flowing river; this contrasts with traditional hydroelectric technology, which needs enormous dams or reservoirs to provide a sufficiently high head to operate the turbine. Numerous villages in distant places are situated near rivers and streams with very high-water velocity at the mountain bed. Savonius hydrokinetic turbine, due to its simplicity in design have been selected for the study. The purpose of this article is to compare the computational results with the experimental data conducted in laboratory and irrigation channels. The experimental analysis was performed in a channel with flow 0.6 m/s at Reynolds number 1.2 x 105. The maximum value of power coefficient (Cp) was found to be 0.24 at tip speed ratio (TSR) value 0.95 with a blockage ratio of 32%. Numerical simulations are performed using the Computational Fluid Dynamics (CFD) programme ANSYS Fluent 19.2. The Unsteady Reynolds averaged Navier–Stokes (URANS) equations are used to solve a three-dimensional SHKT simulation. The findings indicated that the data of computational and the experimental are in good agreement and the error between the values are less than 4%.
... The importance of extracting water from the air is undeniable, particularly in arid regions where there are no other sources of freshwater (Woods, 2014). This applies to both permanent water supply and water supply in natural disaster-affected areas (Bergmair et al., 2014;Yin et al., 2014;Bui et al., 2016). ...
The increase in the world's demand for water has become one of the most relevant topics on environmental sustainability and the conservation of water resources. As a result of the scarcity of potable water, processes such as the use of water through practices, techniques, and technologies for its efficient use have become essential for future generations. Among the possibilities for using water, air conditioning systems stand out, due to the formation of condensed water during operation. The world's population is increasingly concerned about environmental issues, showing that it is necessary to seek alternatives to recycle water, whether for use in gardening, sewage systems, washing sidewalks, and cars, among others. The present study carried out a literature review to reflect on the theme of the reuse of condensation water from air conditioners as a strategy for the sustainable use of water resources and enhancing the future of planet Earth.
... Moreover, membrane separation units are favoured for their low maintenance requirements, and synthetic membranes are (or can be tailored to be) environmentally friendly [6]. The use of membrane separation technologies for carbon dioxide removal applications has demonstrated that they can contribute to significant energy saving compared with standard ventilation systems [7]. When using membranes for carbon dioxide separation applications, it is not necessary to follow the traditional fixed-bed columns arrangement for control of carbon dioxide in an indoor environment. ...
... HVAC: HVAC (Heating, Ventilation, and Air Conditioning) systems play a crucial role in ensuring the comfort and well-being of occupants in buildings. Smart HVAC systems use advanced sensors and algorithms to optimize temperature and air quality while minimizing energy consumption (Woods, 2014). These systems can also be integrated with energy management systems to optimize energy usage and reduce operating costs. ...
... The application of microporous membrane materials can form extremely thin flow films (less than 100 μm), which significantly enhances the desorption/absorption performance while reducing the volume of MBMs [16]. Statistics show that membranes can be categorized into two types according to their structures: dense and porous membranes [26], as shown in Fig. 6. Dense membranes as nonporous (pore size around 0.1 nm) membranes with high selectivity are used for the separation of tiny molecules, which are commonly used in gas separation, pervaporation, and dehumidification [22]. ...
Introducing microporous membranes into absorption refrigeration systems (ARSs) provides a promising technology that can improve heat/mass transfer performance of absorbers/desorbers and reduce the size of ARSs significantly, which facilitates the use of ARSs driven by renewable/waste energy in small-capacity building and transportation applications. In this work, a comprehensive review of existing studies regarding membrane materials, membrane-based absorbers/desorbers, working fluids, and enhancement techniques is carried out. A literature review indicates that membrane-based ARSs have attracted increasing interest in recent decades. Microporous membranes applied in ARSs should have low mass transfer resistance and high thermal resistance, and considerable studies are urgently needed to elucidate membrane wetting and fouling issues for the long-term operation of membrane-based ARSs. Experimental and numerical studies related to plate-and-frame and hollow fiber membrane-based modules prove their higher heat/mass transfer performance, higher compactness, higher stability, and lighter weight than conventional falling film modules that have been commonly used in ARSs. Meanwhile, enhancement technologies, including advanced working fluids and microstructures, are discussed, indicating that ionic liquids and microstructures are promising candidates to further improve the performance of membrane-based modules. Finally, future perspectives on potential developments of membrane-based ARSs are outlined, aiming to promote the wider use of renewable/waste-powered ARSs for small-capacity applications.
... Implementing membranes in conventional dew processes has shown dramatic (nearly 50%) reductions in energy consumption 27 . Similarly, many studies have staged such membranes in HVAC systems and other processes to reduce the energy requirement for cooling and dehumidification of air-water mixtures [28][29][30] . Please do not adjust margins Please do not adjust margins Dew (S3) 20,24,33 Membrane (S11) 24 As described by Table 1, the environmental requirements of each approach refer to typical operating humidity ranges of each process. ...
Atmospheric water harvesting (AWH) is a rapidly emerging approach for decentralized water production, but current technology is limited by trade-offs between energy consumption and yield. The field lacks a common basis to compare different AWH technologies and a robust understanding of the performance impacts of water recovery, desorption humidity (for sorbent systems), and realistic component-level efficiencies. By devising a set of unifying assumptions and consistent parameters across technologies, we provide the first fair thermodynamic comparison over a broad range of environmental conditions. Using 2nd law analysis, or least work, we study the maximum efficiency for common open system AWH methods - fog nets, dew plates, membrane-systems, and sorption processes - to identify the process performance limits. We find that the thermodynamic minimum for any AWH process is anywhere from 0× (relative humidity (RH) ≥ 100%) to upwards of 250× (RH < 10%) the minimum energy requirement of seawater desalination. Sorbents have a particular niche in colder (T < 310 K), arid regions (<6 g kg⁻¹). Membrane-systems are best at low relative humidity and the region of applicability is strongly affected by vacuum pumping efficiency. Dew harvesting is best at higher humidity (RH > 40%) and fog harvesting is optimal when super-saturated conditions exist. Increasing efficiency at the component-level, particularly for vacuum pumps and condensers, may be the most promising avenue for improvement. Enabled by peta-scale computing, our findings use geographical and parametric mapping to provide a framework for technology deployment and energy-optimization.
... However, M-ERV's can only provide partial dehumidification and cooling since they rely solely on temperature and humidity gradients between inlet and exhaust air streams to passively exchange energy, without any external work or heat input. Additionally, the relatively non-selective membranes in M-ERV's can allow pollutants in from the indoor exhaust air stream to enter the fresh inlet air [17,18]. The technology analyzed in this work is fundamentally different from M-ERVs because it employs highly selective membranes and relies on significant pressure gradients but can provide complete cooling and dehumidification. ...
Traditional air conditioning systems use a significant amount of energy on dehumidification by condensing water vapor out from the air. Membrane-based air conditioning systems help overcome this problem by avoiding condensation and treating the sensible and latent loads separately, using membranes that allow water vapor transport, but not air (nitrogen and oxygen). In this work, a computational fluid dynamics (CFD) model has been developed to predict the heat and mass transfer and concentration polarization performance of a novel active membrane-based energy exchanger (AMX). The novel design is the first of its kind to integrate both vapor removal via membranes and air cooling into one device. The heat transfer results from the CFD simulations are compared with common empirical correlations for similar geometries. The performance of the AMX is studied over a broad range of operating conditions using the compared CFD model. The results show that strong tradeoffs result in optimal values for the channel length (0.6-0.8 m) and the ratio of coil diameter to channel height (~0.5). Water vapor transport is best if the flow is just past the turbulence transition around 3000-5000 Reynolds number. These trends hold over a range of conditions and dimensions.
... Therefore, MEEs are suitable for office buildings and residential buildings (especially near zero-energy buildings) in cold and hothumid climate zones [15,16]. However, membrane frosting and clogging may occur in MEEs, which has drawn a lot of attention [17][18][19][20][21]. The frosting problem of the air-to-air energy exchangers was reviewed [21], and it was found that the blockage of the air channels by frost considerably decreased the effectiveness of the exchanger. ...
Air-to-air membrane energy exchangers (MEEs) have become a key component in technological advancements for near-zero-energy building and green building. MEEs recover both heat and moisture from the exhaust air transferring them to the incoming outdoor air through the inner membrane and reduce the energy consumption of the building ventilation. This article reviews the main factors affecting the energy-recovery performance of MEEs, including the membrane, configuration, and operating conditions. It was found that the latent effectiveness of MEEs can be significantly improved with high-permeability membranes. By optimizing the flow arrangement and channel shape, the air-contact membrane area can be enlarged and the convective heat/moisture transfer on each side of the membrane can be enhanced. The curvature channel shape showed good comprehensive performance by improving the resistance, heat, and moisture-transfer rates. The actual operating conditions of MEEs are complex and dynamic processes involving outdoor climate conditions and airflow rates. This study aims to provide a reference for product performance optimization and engineering application energy efficiency improvement of MEEs. The application of MEE technology is an effective way to reduce the energy consumption in buildings in the future.
... Bu tip sistemler etkinliği yüksek, güvenilir, sade ve pratik kullanışlı olmalarına karşın söz konusu sistemlerde havadaki nemin giderilmesi için aşırı soğutma ve yeniden ısıtma işlemleri yapıldığından verimsizdirler (Dai ve ark., 2001). Bu olay, özellikle sıcak ve nemli iklimlerde nem kontrolü, yani iklimlendirme için harcanan enerjiyi daha da arttırmaktadır (Dai ve ark., 2001;Woods, 2014;Qu ve ark., 2018). ...
Bu çalışmada, anülus kanalda hava-hava arasında nem geçişinde membranın etkisi incelenmiş, nemli hava-kuru hava akışkanları için matematiksel modelleme yapılmıştır. Oluşturulan modelin paralel ve karşıt akış tiplerinde COMSOL Multiphysics yazılımı kullanılarak kütle transferi eşitlikleri çıkarılıp analizler gerçekleştirilmiştir. Sistemin MATLAB yazılımında sonlu farklar yöntemi ile aynı şartlarda modellemesi yapılarak COMSOL yazılımında elde edilen sonuçların düşük bağıl hatayla doğrulaması yapılmıştır. Hesaplama sonuçlarına göre paralel ve karşıt akışta en yüksek Sherwood sayısı (Sh) sırasıyla yaklaşık 26 ve 28 olarak belirlenmiştir. Çalışmada ayrıca, COMSOL yazılımında yapılan analizden elde edilen sonuçlar kullanılarak, her iki akış tipi için, belirlenen koşullarda (Reynolds sayısı, Re=250-2000, en-boy oranı, L/D=4-128 ve Schmidt sayısı, Sc=0,68), nemli ve kuru hava taraflarında Sh sayısı için yüksek hassasiyette eşitlikler elde edilmiştir.
Condensation dehumidification in conventional air conditioning technologies is energy-intensive, accounting for up to 50% of building cooling energy used in some climates. Selective vacuum membrane dehumidification (VMD) is one of the leading alternative dehumidification technologies due to its potential for significant energy savings, and the “dual-module humidity pump” is one of the most promising VMD concepts. This work is the first to provide experimental proof-of-concept for the dual-module humidity pump system and provides the first thermodynamic modeling framework that accounts for realistic steady-state operating limitations, both of which are lacking in the current literature. Additionally, this work is the first to provide a system design solution that overcomes practical challenges associated with air accumulation in the vacuum channels. The experimental results in this work show that the current prototype can remove up to 45% of the water vapor in the air stream, and the vapor pressure difference in the vapor rejection module needs to be approximately 2–4 times greater than that of the dehumidification module in order to maintain balanced mass transfer. The thermodynamic model applied to typical air conditioning conditions shows that the ideal dehumidification (latent) COPs can reach up to 40, but practical COPs are limited to approximately 10. Furthermore, the model shows that the overall energy efficiency increases as the membrane air selectivity increases, though this improvement gradually starts to diminish when the membrane selectivity is increased above 10,000.
Green Membrane Technology Towards Environmental Sustainability covers experimental and theoretical aspects of greener membranes and processes. The book fills the gap in current literature and offers a platform that introduces and discusses new routes in fabricating green membranes and processes for developing green membranes. Although membranes and membrane processes have decades of history, rapid development in membranes manufacturing and emerging membrane driven markets is requiring new and more sustainable engagement of manufacturers, membrane operators and scientists.
This book is written for chemical and polymer engineers, materials scientists, professors, graduate students, as well as general readers at universities, research institutions and R R&D departments in industries who are engaged in sustainable engineering and practical strategies in circular economy.
The membrane technology has attracted increasing attention in the field of separation and desalination due to its high efficiency, energy saving, environmental protection, and other advantages. However, related mass transfer theories especially under confined conditions (such as reverse osmosis and nanofiltration) lack the understanding of common mass transfer mechanisms and control methods, thereby seriously restricting the design and development of related membrane materials. This article reviews the research progress of separation membranes with confined mass transfer effects in recent years and analyzes the challenges faced by classical mass transfer models under confined mass transfer conditions. The confined mass transfer mechanism, influencing factors, construction methods, and types of preparation materials are summarized. Furthermore, the development of confined mass transfer separation membranes and the problems to be solved are prospected. Nanomaterial-modified membranes with confined mass transfer function are most likely to break through the trade-off effect of high flux and high selectivity. The confined mass transfer mechanism and preparation strategies of separation membranes are rarely reviewed. Hence, this review can provide guidance for new entrants and existing researchers in the field of membrane separation and has certain reference value for the development of confined mass transfer membranes in the future.
This chapter focuses on electrospun membranes produced by the electrospinning process for air filtration applications. Air pollution, above all in metropolitan and highly populated cities, can be characterized by particulate (PM2.5 and PM0.1) or gaseous pollutants that can be captured by different mechanisms employing electrospun membranes. The filtration efficiency can be influenced by their properties. Electrospun membranes are characterized by different advantages such as uniform and controllable structure, tunable porosity, and high surface area. Electrospinning is the most employed technique for the production of nanofiber membranes used in air filtration. The materials that can be employed for their production have been well investigated and most of them rely on polymers and biopolymers that are fundamental for determining the overall membrane performance. The presence of additives is often crucial for giving specific properties to the final membrane. Individual protection devices and uses for environmental remediation are the two main areas where electrospun membranes find application in the air filtration sector.
Vacuum membrane air dehumidification has gained significant interest in recent years as a highly efficient means of air dehumidification. Prior theoretical modeling work by the authors introduced the Active Membrane Energy Exchanger (AMX) concept, which combines active heat exchange and vacuum membrane dehumidification into one non-isothermal system, and found that it could outperform other air conditioning technologies under many conditions. However, no experimental literature exists on combining active heat exchangers and vacuum membrane dehumidification. The goal of the present study is to evaluate the dehumidification (mass transfer) performance of the AMX concept relative to isothermal membrane dehumidification through three main methodologies: (1) membrane material-level characterization, (2) experimental prototype development and testing, and (3) computational fluid dynamics (CFD) simulations. The dense membranes used in this work showed up to a 40% increase in water vapor permeance at cooler temperatures, and the prototype system showed up to a 6% increase in humidity removal when the air was simultaneously cooled. Furthermore, the membrane module-integrated heat exchange tubes provided additional mixing and turbulence, leading to a 4–8% increase in humidity removal. The upper limit coefficient of performance was equal to approximately 2.2, necessitating advanced system designs to improve efficiency. This study is the first to highlight that combining the cooling and vacuum dehumidification processes can improve dehumidification performance.
Vacuum-based membrane dehumidifiers (VMDs), an emerging air dehumidification technology, use selective membranes to remove moisture from humid air; these dehumidifiers have the potential to save energy and enhance thermodynamic efficiency through decoupled latent and sensible cooling. In this study, a prototype of a vacuum-based membrane dehumidifier at the scale of an air-conditioning system was developed, and its dehumidification characteristics were experimentally evaluated under various test conditions (i.e., air temperature, air humidity, air velocity, and pressure on the permeate side). Four indices were selected to examine the dehumidification performance of the system: the humidity ratio difference, moisture removal rate, dehumidification efficacy, and performance coefficient. The results revealed that the humidity ratio difference ranged 3.38–14.76 g/kg, moisture removal rate 0.16–1.09 kg/h, dehumidification effectiveness 35.4 %–82.7 %, and COP 0.16–0.86. In the parametric analysis of the prototype, the airflow rate and humidity ratio are identified as essential parameters for the dehumidification performance. Moreover, the thermodynamic perspective in the Psychrometric chart and transient operation were utilized to evaluate the operational characteristics of the prototype unit. All experimental results showed that the difference in temperature between the inlet and outlet air was less than 1.5 °C, and the saturation time is shown to be within 5 min.
The proposed hollow fiber membrane-based evaporative cooler (HFMEC) is expected to be an alternative to the conventional direct evaporative cooler because of its advantages such as the isolation of air from liquid water and the large specific surface area. For the common counter-flow HFMEC with many influencing parameters, it is a bit laborious or even incompetent to rely on experiment or numerical simulation for the parametric study and optimization. Therefore, this study aims to develop accurate and rapid performance prediction models for the proposed HFMEC with the statistical method. An experimental test system for a counter-flow HFMEC was set up. 120 sets of simulations were carried out based on the experimentally validated numerical model and the response surface methodology. Five accurate and practical empirical equations were derived using simulated data: the considered eight input factors consisted of four operating parameters and four membrane module design parameters; the five output responses included the outlet air temperature, outlet air relative humidity, saturation effectiveness, cooling capacity per unit volume, and COP. These simplified equations were adopted to facilitate parameter sensitivity analysis and multi-objective optimization. A case study on the regional applicability of the counter-flow HFMEC demonstrated the ability of the derived equations to conveniently make performance predictions. The results indicated that the regression models could contribute to the rapid performance prediction of the counter-flow HFMEC, aiding in optimization and design.
Air pollution is one of the major environmental concerns in most highly populated cities, which is typically caused by particulate (PM2.5 and PM0.1) or gaseous pollutants. In this framework, membranes produced by the electrospinning technique are attracting more and more interest thanks to their peculiar properties such as interconnected pore structure, tunable porosity and fiber dimension, high surface area to volume ratio and controllable morphology. This review aims to provide an exhaustive overview on the electrospun membranes applied in air filtration introducing the key principles and fundamentals of the separation mechanisms and discussing the influence of membrane properties (e.g., morphology and charge) on their filtration efficiency. The materials generally employed for the fabrication of electrospun membranes (polymers, solvents) and their combination with additives with defined properties are reviewed also in light of the new environmentally friendly approaches which are increasingly adopted in membrane fabrication. Finally, the practical use of electrospun membranes in several application fields such as individual protection devices, environmental remediation, recovery of volatile organic compounds (VOCs), and ventilation and climate control aspects is widely discussed providing also an outlook on the upscaling potential of electrospun membranes and future directions.
With the recent development of the independent control of temperature and humidity, the demand for dehumidification has significantly increased, and liquid desiccant dehumidification systems have attracted considerable attention because of a high dehumidification efficiency. This study presents a new design for a liquid desiccant dehumidification system that applies a membrane-assisted dual sump for maintenance the solution concentration with a low solution cooling and heating loads. A detailed heat and mass transfer analysis was conducted to evaluate the feasibility of the membrane for the maintenance the solution concentration according to the mass transfer resistance of the membranes. Moreover, the variation in the solution temperature and concentration was predicted by performing detailed simulations. The simulation results indicated that the low-mass-transfer-resistance membrane requires a large amount of solution load and the high-mass-transfer-resistance membrane cannot be used to maintain the solution concentration. Thus, the mid-mass-transfer-resistance (i.e., 15,000 s/m) membrane is suitable for the liquid desiccant dehumidification system. Compared with the conventional liquid desiccant dehumidification system based on a solution exchange, the membrane-assisted liquid desiccant dehumidification system could save about 19% load for solution cooling and heating owing to the lower heat and mass transfer rate on the absorber and regenerator sump.
In this study, a hollow fiber membrane-based dual-core ventilation system is proposed to reduce space heating and cooling energy requirements. Simple prediction models were developed for the seasonal latent heat exchange performances of the proposed system. A mock-up model of the hollow fiber membrane module that serves as a latent heat exchanger in the proposed system was fabricated. A total of 32 experimental sets for the cooling and heating seasons were designed based on five operating parameters (outdoor air temperature and corresponding relative humidity, room air temperature and corresponding relative humidity, and outdoor air face velocity). The R-squared values of the derived models exceeded 96%, and the models were validated using additional measured data. Empirical correlations to predict the latent and sensible effectiveness were derived based on the response surface methods. A sensitivity analysis of each operating parameter on the latent heat exchange performance of the hollow fiber membrane module was performed. Finally, a guideline for the implementation of the developed models for energy simulation in building applications was presented. The proposed ventilation system improved the energy saving over that of the conventional energy recovery ventilator and reduced the ventilation load by 54.8% and 19.9% in summer and winter, respectively.
In recent years, membrane dehumidification, as a novel dehumidification technology, has received many attentions for energy efficiency in air conditioning systems. Its dehumidification technology is characterized by isothermal dehumidification, which can avoid excess energy consumption. This experimental study focuses on the performance measurement of the planar vacuum membrane dehumidifier. Nafion membrane is used in combination with a planar membrane dehumidifier. A vacuum membrane dehumidification method is also employed to investigate the dehumidification performance. Four serpentine flow channel designs, including three-snake, four-snake, five-snake, and six-snake flow channel designs, are tested at the certain values of temperature and humidity and different inlet air flow rates. Different performance indexes, including water collection, dehumidification rate, pressure loss, and pumping power, are investigated. The results show that the amount of water collected by the cold trap increases with boosting the inlet flow rate in the range of 35 l/min to 55 l/min and it has the largest value when the inlet air flow rate is 55 l/min. However, the amount of water collected by the cold trap decreases with further increase of the inlet flow rate in the range of 55 l/min to 65 l/min.
Crystallization fouling is an important phenomenon that impacts the design and operation of membrane-based separation processes. Recently, membranes have been applied to separate aqueous salt solutions and air streams for moisture control in buildings using liquid-to-air membrane energy exchangers (LAMEEs). LAMEEs are efficient energy exchangers that use a semi-permeable hydrophobic membrane to separate air and aqueous salt solutions, while allowing simultaneous heat and moisture exchange between the fluids. However, in some design and operating conditions, crystallization fouling occurs which hinders the performance of LAMEEs. The main objective of this paper is to develop a model which predicts the fouling rate for membranes in LAMEEs. The semi-empirical model estimates the decline in moisture transfer rate by taking into account the surface blockage of the membrane due to crystallization fouling. The model is validated with experimental data available in the literature. The model can then be used to predict the theoretical crystallization fouling limits and the effect of design and operational parameters on fouling rates. For example, the initial fouling rate is four times higher for a supersaturated solution (3% above saturation) than for a saturated solution. The main contribution of this study is that the developed semi-empirical model can help the designers predict the fouling rate for long-term periods and determine the cleaning intervals of the LAMEE.
This paper purposed to develop a simplified model for a vacuum membrane dehumidifier in an air conditioning system. The proposed model is a gray model that uses mathematical equations that are deducted from the effectiveness and number of the transfer unit of the heat exchanger and a regression equation for the ratio of humidity capacities (X factor) analogous to the heat capacities ratio of heat exchangers. The applicability of the simplified effectiveness and number of transfer unit methods to vacuum-based membrane dehumidifiers in air conditioning systems has been validated via numerical simulation and experimental data. The error bound for the validation of the numerical simulation and experimental data for dehumidification performance (i.e., outlet humidity ratio and dehumidification effectiveness) was 20%. In addition, an operation example using the simplified model was performed to control the vacuum pressure of the vacuum-based membrane dehumidifier in the air-conditioning system. The results show that the proposed algorithm using the simplified model exhibits an energy-saving potential of 42.4% compared to the conventional vacuum-based membrane dehumidification operated in a variable air volume system.
Polytetrafluoroethylene (PTFE) membranes with three-dimensional interconnected porous networks are usually used as air filters. However, they are easily contaminated by oily aerosols in air filtration, resulting in a decrease in filtration efficiency. Herein, a strategy for the preparation of amphiphobic PTFE membranes with a ring-on-string-like micro/nano structure for air purification was proposed. First, the PTFE fibers were locked by OTS polysiloxane rings to form an OTS layer with ring-on-string-like structure by spraying octadecyltrichlorosilane (OTS) solution on the membrane surface. Subsequently, fluorinated silica (F–SiO2) nanoparticles were sprayed to build the micro/nano structure and the surface with low surface energy. The prepared amphiphobic membrane had high liquid repellency to water (～153°), diiodomethane (～136°) and hexadecane (～124°). Moreover, the membrane amphiphobicity exhibited a high stability, which can be ascribed to the OTS layer. The layer has a special ring-on-string-like structure and can act as an anchor to fix the nanoparticles on the membrane surface because of the strong interaction between OTS and the nanoparticles. The filtration test results showed that the amphiphobic membrane developed possessed high removal efficiency for 0.1, 0.16 and 0.2 μm particles (˃ 99.9670%). As for the anti-oil fouling ability, the amphiphobic membrane maintained a pressure drop below 3 kPa in the smoke filtration within 10 min, meaning that the membrane developed had a high anti-oil fouling ability. The developed amphiphobic PTFE membrane in this work has a promising prospect in air purification in oil environments.
Membrane-based dehumidification system is an emerging technology which drives based on the renewable low-grade energy sources. Water vapor selective membrane is able to separate water vapor from the air which can be used to condition air in buildings, more efficiently than conventional air-conditioning equipment. Flow channel designs (especially serpentine-type) and their impacts on the thermal–hydraulic performance of membrane-based dehumidifiers are essential and not well-studied. In the present work, a novel flat-sheet membrane dehumidifier with serpentine flow channels is designed and evaluated experimentally. The serpentine flow channel offers a long flow path that the air in the wet and dry sides of the channel interacts with each other through the membrane, thereby improving the heat and water vapor transfer. The effect of inlet air flow rate, relative humidity, and temperature of both wet and dry sides on the dehumidification rate, approach temperature, pressure drop, and coefficient of performance of the dehumidifier is examined. Results show that passing the humid air with higher flow rate, humidity, and temperature through the wet side channel leads to the higher dehumidification rate, approach temperature, and pressure drop. Increase of wet side air temperature and flow rate enhances the coefficient of performance, and the effect of inlet air relative humidity on the coefficient of performance is observed negligible at high air flow rates. Parametric study of dry side air condition reveals that the air with lower humidity improves the dehumidification rate and coefficient of performance, while its effect on the approach temperature and pressure drop is observed negligible. Elevated air temperature of dry side has positive and negative impact on the approach temperature and coefficient of performance of the dehumidifier, respectively.
The increasing need to cool the air in our built environment is both a cause and an effect of climate change. Air conditioning accounts for a large portion of global greenhouse gas emissions today, which we estimate at 3.9%; however, the role that humidity plays in these emissions is often overlooked. Here, we show that the emissions associated with reducing air humidity (i.e., removing water vapor from air) are larger than emissions associated with reducing air temperature (i.e., cooling air). We calculate these emissions for today and 2050 and show how dramatically humidity-related emissions will increase with rising cooling demand around the world. We also calculate the minimum separation energy for removing water vapor from air and find that this is at least an order of magnitude less than the processes used today.
Compared to the conventional amine adsorption process to separate CO₂ from natural gas, the membrane separation technology has exhibited advantages in easy operation and lower capital cost. However, the high CO₂ partial pressure in natural gas can plasticize the membranes, which can lead to the loss of CH₄ and low CO₂/CH₄ separation efficiency. Crosslinking of polymer membranes have been proven effective to increase the CO₂ induced plasticization resistance by controlling the degree of swelling and segmental chain mobility in the polymer. This thesis focuses on extending the success of crosslinking to more productive asymmetric hollow fibers. In this work, the productivity of asymmetric hollow fibers was optimized by reducing the effective selective skin layer thickness. Thermal crosslinking and catalyst assisted crosslinking were performed on the defect-free thin skin hollow fibers to stabilize the fibers against plasticization. The natural gas separation performance of hollow fibers was evaluated by feeding CO₂/CH₄ gas mixture with high CO₂ content and pressure.
In recent years, indoor humidity levels are gaining greater attention in building design and operation, due to the increasing concern over moisture-related problems, such as mold growth, indoor air quality and discomfort of the occupants. At the same time, building energy consumption, especially at peak electric demand, is also becoming a significant operating cost conern. It has been suggested that besides energy recovery, the inclusion of an energy recovery ventilator (ERV) with a right-sized cooling coil can improve temperature and humidity control in buildings. ERV also has the potential to reduce peak electric demand. This paper presents the methodology and the findings from modeling the effect of an enhanced latent effectiveness ERV on indoor thermal conditions, system sizing and behavior, and system energy consumption under different system scenarios (with ERV and/or economizer and bypass mode), based on a generic building configuration. The applicability and effectiveness of the ERV for different climatic conditions as well as different outside air flow rates are also discussed.
A novel liquid-desiccant air conditioner that dries and cools building supply air has been successfully designed, built and tested. The new air conditioner will transform the use of direct-contact liquid-desiccant systems in HVAC applications, improving comfort and indoor air quality, as well as providing energy-efficient humidity control. Liquid-desiccant conditioners and regenerators are traditionally implemented as adiabatic beds of contact media that are highly flooded with desiccant. The possibility of droplet carryover into the supply air has limited the sale of these systems in most HVAC applications. The characteristic of the new conditioner and regenerator that distinguishes them from conventional ones is their very low flows of liquid desiccant. Whereas a conventional conditioner operates typically at between 10 and 15 gpm (630 and 946 ml/s) of desiccant per 1000 cfm (0.47 m3/s) of process air, the new conditioner operates at 0.5 gpm (32 ml/s) per 1000 cfm (0.47 m3/s). At these low flooding rates, the supply air will not entrain droplets of liquid desiccant. This brings performance and maintenance for the new liquid-desiccant technology in line with HVAC market expectations. Low flooding rates are practical only if the liquid desiccant is continually cooled in the conditioner or continually heated in the regenerator as the mass exchange of water occurs. This simultaneous heat and mass exchange is accomplished by using the walls of a parallel-plate plastic heat exchanger as the air/desiccant contact surface. Compared to existing solid and liquid desiccant systems, the low-flow technology is more compact, has significantly lower pressure drops and does not “dump” heat back onto the building’s central air conditioner. Tests confirm the high sensible and latent effectiveness of the conditioner, the high COP of the regenerator, and the operation of both components without carryover.
This Final Report covers the Cooperative Program carried out to design and optimize an enhanced flat-plate energy recovery ventilator and integrate it into a packaged unitary (rooftop) air conditioning unit. The project objective was to optimize the design of a flat plate energy recovery ventilator (ERV) core that compares favorably to flat plate air-to-air heat exchanger cores on the market and to cost wise to small enthalpy wheel devices. The benefits of an integrated unit incorporating an enhanced ERV core and a downsized heating/cooling unit were characterized and the design of an integrated unit considering performance and cost was optimized. Phase I was to develop and optimize the design of a membrane based heat exchanger core. Phase II was the creation and observation of a system integrated demonstrator unit consisting of the Enhanced Energy Recovery Ventilator (EERV) developed in Phase I coupled to a standard Carrier 50HJ rooftop packaged unitary air conditioning unit. Phase III was the optimization of the system prior to commercialization based on the knowledge gained in Phase II. To assure that the designs chosen have the possibility of meeting cost objectives, a preliminary manufacturability and production cost study was performed by the Center for Automation Technologies at RPI. Phase I also included a preliminary design for the integrated unit to be further developed in Phase II. This was to assure that the physical design of the heat exchanger designed in Phase I would be acceptable for use in Phase II. An extensive modeling program was performed by the Center for Building Performance & Diagnostics of CMU. Using EnergyPlus as the software, a typical office building with multiple system configurations in multiple climatic zones in the US was simulated. The performance of energy recovery technologies in packaged rooftop HVAC equipment was evaluated. The experimental program carried out in Phases II and III consisted of fabricating and testing a demonstrator unit using Carrier Comfort Network (CCN) based controls. Augmenting the control signals, CCN was also used to monitor and record additional performance data that supported modeling and conceptual understanding. The result of the testing showed that the EERV core developed in Phase I recovered energy in the demonstrator unit at the expected levels based on projections. In fact, at near-ARI conditions the core recovered about one ton of cooling enthalpy when operating with a three-ton rooftop packaged unit.
Pacific Northwest National Laboratory (PNNL) has an ongoing program focused on the development of compact chemical heat pumps based on the absorption cycle. Design and laboratory data suggest that, by taking advantage of the high rates of heat and mass transfer available in microstructures, we are able to radically reduce the size of a heat-actuated heat pump based on the absorption cycle. Current estimates suggest a size reduction of a factor of 60 compared with a conventional heat pump. High performance is achieved by using microchannel combustor technology, microchannel heat exchangers and ultra-thin-film micromachined contactors. Currently all of the components of the system have been demonstrated and PNNL is assembling a complete bench-scale version of the device. Successful development of this technology will enable a new class of compact, heat-actuated space conditioning systems that can be used for portable or distributed heating and cooling.
Indoor moisture and temperature conditions and equipment operation were measured and analyzed for 43 homes in warm-humid and mixed-humid climate regions of the United States. A range of house and mechanical system types were evaluated, including standard building enclosures and cooling systems and high-performance building envelopes with enhanced cooling or supplemental dehumidification systems. Conventional cooling systems in standard houses usually provide reasonable humidity control (below 60% RH) in midsummer. However, high humidity levels are observed at times when cooling loads are modest. The addition of continuous mechanical ventilation to standard houses in humid climates did not consistently increase indoor humidity levels. Indoor humidity levels were highest in high-performance, low sensible heat gain homes with mechanical ventilation. In these homes, the temperature balance point is higher, so there are many hours when sensible cooling is not required yet there are still significant moisture loads from internal sources and ventilation. These homes often require a separate dehumidifier to maintain space humidity in the swing seasons and at night when the thermostat is satisfied. The use of supplemental dehumidification in a high-performance house enables the implementation of efficiency improvements that significantly reduce sensible cooling demand while still maintaining proper humidity levels.
In view of the current interest in the theory of gases proposed by Bernoulli (Selection 3), Joule, Krönig, Clausius (Selections 8 and 9) and others, a mathematical investigation of the laws of motion of a large number of small, hard, and perfectly elastic spheres acting on one another only during impact seems desirable.
This paper describes an experimental and theoretical investigation of a non-traditional approach to the air humidification process that uses a hydrophobic membrane contactor acting as a porous barrier between the water and the air to be humidified. The cross-flow contactor consists of a 1.2 m2 total membrane surface of hollow polypropylene capillaries, 200 μm wall thickness, arranged in a staggered array. A set of experimental results obtained with air flow-rates up to 80 m3/h outside the bundle of capillaries is presented and discussed in relation to the theoretical predictions given by a numerical model developed to predict the humidification efficiency of the contactor. Results show a good humidification efficiency of this system. The influence of the various parameters that affect vapour mass flux through the contactor is pointed out.
Owing to the stringent indoor air quality (IAQ) requirements and high cost of desiccants, one of the major concerns in liquid desiccant technology has been the carryover, which can be eliminated through indirect contact between desiccant and air. Membrane contactors using microporous semipermeable hydrophobic membranes have a great potential in this regard. This communication investigates the performance of semipermeable membrane based indirect contactors as dehumidifiers in liquid desiccant cooling applications. Experiments on different types of membrane contactors are carried out using lithium chloride (LiCl) solution as desiccant. The membrane contactors consist of alternate channels of air and liquid desiccant flowing in cross-flow direction. Hydrophobic membranes form a liquid tight, vapor permeable porous barrier between hygroscopic solution and moist air, thus eliminating carryover of desiccant droplets. In order to provide maximum contact area for air-desiccant interaction, a wicking material is sandwiched between two membranes in the liquid channel. It is observed that vapor flux upto 1300 g/m(2) h can be achieved in a membrane contactor with polypropylene (PP) membranes, although the dehumidification effectiveness remains low. The effect of key parameters on the transmembrane vapor transport is presented in the paper. (c) 2013 Elsevier Ltd. All rights reserved.
This paper reviews and surveys the available hybrid liquid-desiccant air-conditioning system technologies. These technologies are proposed as alternative to the traditional vapor-compression systems because of its advantages in removing air latent load, environment-friendly feature, ability to remove pollutants from the processed air, and ability to reduce electrical energy consumption. This paper first introduces the traditional air-conditioning system: vapor compression, vapor absorption, and evaporative cooling. In addition, the principles of liquid desiccants and liquid-desiccant dehumidification systems and the hybrid liquid-desiccant classifications are discussed. Next, combination of the liquid-desiccant systems with vapor compression, vapor absorption, and direct and indirect evaporative cooling units are outlined. Finally, conclusions and some important suggestions are presented based on the collected information.
Advanced membranes-from fundamentals and membrane chemistry to manufacturing and applications. A hands-on reference for practicing professionals, Advanced Membrane Technology and Applications covers the fundamental principles and theories of separation and purification by membranes, the important membrane processes and systems, and major industrial applications. It goes far beyond the basics to address the formulation and industrial manufacture of membranes and applications. This practical guide: Includes coverage of all the major types of membranes: ultrafiltration; microfiltration; nanofiltration; reverse osmosis (including the recent high-flux and low-pressure membranes and anti-fouling membranes); membranes for gas separations; and membranes for fuel cell uses. Addresses six major topics: membranes and applications in water and wastewater; membranes for biotechnology and chemical/biomedical applications; gas separations; membrane contractors and reactors; environmental and energy applications; and membrane materials and characterization. Includes discussions of important strategic issues and the future of membrane technology.
This article uses a numerical model to analyze a concept combining a liquid desiccant dehumidifier with a dew-point indirect evaporative cooler. Each of these components, or stages, consists of an array of channel pairs, where a channel pair is two air channels separated by a thin plastic plate. In the first stage, a liquid desiccant film lining one side of the plates removes moisture from the process (supply-side) air through a membrane. An evaporatively cooled exhaust airstream on the other side of the plastic plate cools the desiccant. The second stage sensibly cools the dried process air with a dew-point evaporative cooler. This article uses a parametric analysis to illustrate the key design tradeoff for this concept: device size (a surrogate for cost) versus energy efficiency. The analysis finds the design parameters with the largest effect on this tradeoff and finds the combinations of design parameters giving near-optimal designs, which are designs with the highest efficiency for a given device size. The results indicate that there are two key parameters contributing to this tradeoff: the supply-side air channel thickness and the exhaust-air flow rate in the evaporative cooler.
This paper concerns an innovative three-fluid membrane contactor able to exchange both sensible heat and latent heat with the process air. A condensing/evaporating refrigerant (first fluid) exchanges heat with a second fluid (water/liquid desiccant) flowing over the outer surface of hydrophobic membrane capillaries, inside which the process air flows (third fluid). In this study, bench tests were carried out with either distilled water or saturated KCOOH solution as the desiccant. Water at a controlled temperature was used to simulate the refrigerant-phase change across the component. A theoretical model of the component yielded predictions that displayed good agreement with the experimental results. Ways of improving the enthalpy effectiveness of the component are discussed, as are applications in refrigeration plants and hybrid air-conditioning systems (vapour-compression cycles plus liquid desiccants). This new component raises very interesting applicative opportunities; though fairly compact, it can improve the energy efficiency of refrigeration and air-handling systems.
This article discusses a novel isothermal system for removing water vapor from gases using water selective membranes. Previous membrane systems for dehumidifying atmospheric pressure gases suffered from low driving forces across the membrane. The approach presented and tested here used a fraction of the dehumidified gas as “a dehumidifying working fluid” that passes through an expansion valve prior to re-entering the membrane unit. The combination of gas expansion and low absolute pressure sweep gas establishes a driving force strong enough to achieve dehumidification efficiencies >200%. The produced gas humidity is significantly reduced compared to the feed gas. It is even possible to produce gases with dew point's <0 °C. Experiments confirm these results using sweep gas pressures obtainable by rotary water-sealed or single stage reciprocating vacuum pumps. These evaluations encourages further research especially, but not limited to, those applications where the data indicated the technology is competitive with existing systems; such as, building ventilation dehumidification and dryers.
A new membrane liquid desiccant air-conditioning (LDAC) system is proposed and investigated in this paper. Liquid-to-air membrane energy exchangers (LAMEEs) are used as a dehumidifier and a regenerator in the proposed membrane LDAC system, which can eliminate the desiccant droplets carryover problem occurring in most direct-contact LDAC systems. A parametric study on steady-state performance of the membrane LDAC system is performed using the TRNSYS energy simulation platform. The impacts of various climatic conditions and key system parameters on the system performance are evaluated. Results show that the proposed membrane LDAC system is capable of achieving recommended supply air conditions for productive, comfort and healthy environments if the key system parameters are effectively controlled. The system coefficient of performance (COP) at the design condition is 0.68, and the sensible heat ratio (SHR) for the dehumidifier lies in the range between 0.3 and 0.5 under different climatic, operating and design conditions. The proposed membrane LDAC system is able to effectively remove latent load in applications that require efficient humidity control.
This paper provides a state-of-the-art review of the separation process known as membrane distillation, MD. An introduction to the terminology and fundamental concepts associated with MD as well as a historical review of the developments in MD are presented. Membrane properties, transport phenomena, and module design are discussed in detail. A critical evaluation of the MD literature is incorporated throughout this review.
Like many separation processes, ultrafiltration and reverse osmosis are often compromised by concentration polarization. Such polarization can be mitigated by static mixers and other flow barriers placed as spacers next to the membrane surface. These spacers can be shaped like ladders, herringbones, and helices. The effect of these spacers can be successfully predicted without adjustable parameters from extensions of the Lévêque equation. The predictions are in agreement with results of computational fluid mechanics and with electrochemical experiments. They supply a tool for optimizing spacer design.
Anovel system for air conditioning is proposed which combines membrane air-drying and an indirect/direct evaporative cooling (M/ID) system. This combination extends the operating range of the evaporative cooler for small differences of dry and wet bulb temperatures. The study investigates the feasibility of operating the proposed system for the cooling of ambient air to an outlet temperature of 19°C and a relative humidity of 90%. The analysis is performed for the summer weather data of Kuwait, which varies from extremely hot and dry conditions (50°C and less than 20% relative humidity) to warm and humid conditions (35°C and more than 60% relative humidity). System analysis shows limitations imposed on air cooling by the direct evaporative cooler (DEC), the indirect evaporative cooler (IEC), and the indirect/direct evaporative cooler (ID). For ambient temperatures above 35°C, operation of the ID system requires relative humidity values below 30%. Operation of the DEC or the IEC systems is limited to temperatures below 30°C and relative humidity below 50%. The ID system operates at temperatures above 45°C and relative humidity below 50%. The M/ID operation covers a relative humidity range between 30–100% and a temperature range between 25–45°C. Energy consumption for various cooling combinations, including mechanical vapour compression (MVC), is evaluated by the energy efficiency rating (EER). The M/ID system shows savings of up to 86.2% of the energy consumed by the stand-alone MVC system. Also, the combined systems of MVC/IEC and the MVC/ID show savings of 49.8 and 58.9% over the conventional MVC.
Fluid flow and convective heat mass transfer in membrane-formed parallel-plates channels are investigated. The membrane-formed channels are used for liquid desiccant air dehumidification. The liquid desiccant and the air stream are separated by the semi-permeable membrane to prevent liquid droplets from crossing over. The two streams, in a cross-flow arrangement, exchange heat and moisture through the membrane, which only selectively permits the transport of water vapor and heat. The two flows are assumed hydrodynamically fully developed while developing thermally and in concentration. Different from traditional method of assuming a uniform temperature (concentration) or a uniform heat flux (mass flux) boundary condition, the real boundary conditions on membrane surfaces are numerically obtained by simultaneous solution of momentum, energy and concentration equations for the two fluids. Equations are then coupled on membrane surfaces. The naturally formed boundary conditions are then used to calculate the local and mean Nusselt and Sherwood numbers along the channels. Experimental work is performed to validate the results. The different features of the channels in comparison to traditional metal-formed parallel-plates channels are disclosed.
Detailed design analysis of an ambient-pressure, membrane-based heat pump serves as a basis for examining the applicability of a variety of correlations used in coupled heat and mass transfer devices that include membranes and aqueous electrolyte streams. The transport phenomena were studied using scaling analysis and finite-volume numerical methods. Commonly accepted mass and heat transfer correlations for developing flow were found to be adequate for the liquid streams in these designs without requiring additional transport phenomena. We found that an air gap-separating a temperature gradient up to 20 degrees C-may be as large as 3 mm (vertical orientation) and 5 mm (horizontal orientation) before natural convection becomes important. But air gaps this wide are unlikely to be used in these membrane devices since radiation dominates the total energy transfer for air gaps larger than 2 mm. We also introduce a selectivity-productivity tradeoff based on the sustained-temperature-gradient per unit vapor pressure gradient (selectivity) and the overall heating capacity (productivity) of a design.
This article presents modeling and experimental results on a recently proposed liquid desiccant air conditioner, which consists of two stages: a liquid desiccant dehumidifier and an indirect evaporative cooler. Each stage is a stack of channel pairs, where a channel pair is a process air channel separated from an exhaust air channel with a thin plastic plate. In the first stage, a liquid desiccant film, which lines the process air channels, removes moisture from the air through a porous hydrophobic membrane. An evaporating water film wets the surface of the exhaust channels and transfers the enthalpy of vaporization from the liquid desiccant into an exhaust airstream, cooling the desiccant and enabling lower outlet humidity. The second stage is a counterflow indirect evaporative cooler that siphons off and uses a portion of the cool-dry air exiting the second stage as the evaporative sink. The objectives of this article are to (1) present fluid-thermal numerical models for each stage, (2) present experimental results of prototypes for each stage, and (3) compare the modeled and experimental results. Several experiments were performed on the prototypes over a range of inlet temperatures and humidities, process and exhaust air flow rates, and desiccant concentrations and flow rates. The model predicts the experiments within ±10%.
This article investigates various support spacers for airflow through membrane-bound channels in energy recovery ventilators (ERVs) to enhance heat and mass transfer. Although liquid flow through membrane-bound channels has been extensively investigated, little work has looked at airflow through these channels. This article presents theoretical pressure drop and heat transfer for an open channel and for simple triangular corrugation (or plain-fin) spacers, which are common in heat exchangers and in some ERVs. It then presents the experimental pressure drop and heat transfer for two new corrugated mesh spacers, with one spacer in three orientations. Results indicate that these can improve heat transfer with little pressure-drop penalty compared to the triangular corrugation spacers. Results also show that unsteady flow occurs in the mesh spacers once a certain flow rate is reached. The optimal spacer depends on the application, which is shown with a cost savings estimate for a hypothetical ERV. Simpler performance metrics that do not require cost estimates can be used to compare two spacers, as long as their limitations are considered.
It is shown how the efficiency of a gas-fired boiler plant can be boosted by as much as 20% with the installation of a condensation heat exchanger to reclaim latent heat otherwise wasted in flue gases. A condensation heat exchanger is similar in operation to a submerged combustion boiler but is distinctly different in that it is not involved in the combustion process itself. The essential operating principle is cooling waste gases and condensing part of the water contained in them.
NREL has developed the novel concept of a desiccant enhanced evaporative air conditioner (DEVap) with the objective of combining the benefits of liquid desiccant and evaporative cooling technologies into an innovative 'cooling core.' Liquid desiccant technologies have extraordinary dehumidification potential, but require an efficient cooling sink. DEVap's thermodynamic potential overcomes many shortcomings of standard refrigeration-based direct expansion cooling. DEVap decouples cooling and dehumidification performance, which results in independent temperature and humidity control. The energy input is largely switched away from electricity to low-grade thermal energy that can be sourced from fuels such as natural gas, waste heat, solar, or biofuels.
The objective of this report is to provide information to help DOE plan its future activities on liquid-desiccant technologies. The report meets this objective by (1) identifying commercial and residential markets where the liquid-desiccant systems will first be most successful and (2) identifying advances in the individual components of a liquid-desiccant system that will allow it to expand into new markets.
This study focuses on fouling while utilizing hollow fiber membranes as wetting media in evaporative cooling applications. A duct-mountable, hollow fiber membrane array is fed tap water or water with a known concentration of calcium sulfate or calcium carbonate to simulate evaporative cooling with scale forming waters. Mass transfer coefficients are experimentally derived from collected data, and concentration polarization effects and fouling mechanisms are quantified from changes in the measured mass and heat transfer coefficients along with visual verification from SEM imaging. Results indicate that membrane fouling can be characterized by a decrease in the effective porosity, the extent of fouling can be related to mineral solution concentrations within the membrane, and that a fouling induction period and wasting flow rates influence fouling rates.
Hollow fiber membrane based air humidification offers great advantages over the traditional methods because the liquid water droplets are prevented from mixing with the process air, while water vapor can permeate through the membranes effectively. The novelty in this research is that the coupled heat and moisture transport in a hollow fiber membrane module for air humidification is investigated, both numerically and experimentally. The air stream and the water stream flow in a counter flow arrangement. It is found that the membranes play a key role in humidification performances. For sensible heat transfer, both the liquid side and the membrane side resistance can be neglected, while the total heat transfer coefficients are determined by the air side heat transfer coefficients. In contrast, in mass transfer, only the liquid side resistance can be neglected, while the total mass transfer coefficients are co-determined by membrane properties and the air side convective mass transfer coefficients.
The steady-state performance of a run-around membrane energy exchanger (RAMEE) for a wide range of outdoor air conditions is presented here. The RAMEE is numerically simulated and the sensible and latent effectiveness values corresponding to the maximum total energy effectiveness for different outdoor air conditions are presented. The effectiveness values are shown to be very dependent on outdoor conditions which results in some effectiveness values exceeding 100% or being less than 0% for several of the outdoor air conditions investigated. The heat and moisture transfers are shown to influence the latent and sensible performances of the RAMEE, respectively.
Recently, there is growing demand for energy saving technologies in buildings due to global warming and environmental impact issue. As a result to this, energy-efficient technologies are becoming more popular amongst researchers and designers. In this regards, to fulfil energy conservation demands, researchers have focused on the development of advance heat or energy recovery with energy-efficient ventilation system. The aim of this paper is to review heat or energy recovery technologies for building applications. The reviews were discussed according to the concept and classification of heat or energy recovery based on types and flow arrangement. The developments of these technologies in integrated energy-efficient system such as mechanical and passive ventilation, air conditioning, dehumidification and photovoltaic panel have also been presented.
In recent years, the attention of researchers has been focused on energy conservation demands due to the environmental impact of energy consumption throughout the built environment and global warming issue. Heat or energy recovery is one of the main energy-efficient systems that has been approved to overcome this problem. However, in conventional heat or energy recovery for building applications, only sensible energy has been recovered and neglecting the latent energy. In this work, enthalpy recovery system has been developed and the performances of sensible and latent energy have been investigated experimentally. The efficiency of close to 66% has been achieved for sensible energy and the latent energy efficiency was nearly 59% gained. Comparison of efficiency with effectiveness-NTU method showed both were in good agreement. Recovered energy was achieved up to 167W at 3.0m/s air velocity with 4.3°C temperature difference.
Does pore-size distribution need to be considered to accurately model vapor transport in membrane distillation (MD)? This paper addresses that question from a theoretical perspective. Although some previous work has discussed pore-size distribution in MD, there has yet to be a comprehensive, general analysis of its effects on MD and its various configurations. In this work, a numerical model is used to calculate the flux through all pore sizes to estimate the effect of pore-size distribution on MD flux. The modeling shows that the error in the calculated flux incurred by neglecting pore-size distribution is largest for a microfiltration process, where viscous flow dominates, somewhat smaller for vacuum MD, where Knudsen flow dominates, smaller still for direct-contact MD, where molecular diffusion usually dominates, and smallest for air-gap MD, where the air gap dominates the overall mass transfer resistance. Considering a membrane with a mean pore size of at least 100nm and a geometric standard deviation of the pore size of 1.2, the error is: 9% for vacuum MD, 3.5% for direct-contact MD, and less than 1% for air-gap MD.
A one-step approach is employed to prepare an asymmetric cellulose acetate (CA) membrane that is used for total heat recovery (heat and moisture recovery). The process only involves a cheap raw material of CA, an environmental friendly solvent acetic acid and an additive deionized water. A porous support layer and a dense skin layer are formed simultaneously during the solvent and coagulant medium exchange. Investigations found that the approach is successful in making membranes for heat and moisture recovery. The new membranes have high moisture permeability. Moreover, the permeation of other unwanted gases like CO2 is effectively prohibited. Analysis found that the optimum solution compositions for casting membranes are: acetic acid to deionized water ratio 70:30. The approach provides an environmental friendly yet economical solution for preparing membranes for heat and moisture recovery.
Liquid desiccant cooling system (LDCS) is a novel air-conditioning system with good energy saving potential. However, the present LDCS has a poor performance, mainly because the conventional thermal regeneration method wastes too much energy during the regeneration process. To improve that, photovoltaic-electrodialysis (PV-ED) regeneration method is introduced: it has a higher performance by using solar photovoltaic panels to drive an electrodialysis regeneration process. To further explore the PV-ED method, both single-stage and double-stage photovoltaic-electrodialysis regeneration systems are presented in this paper. Analysis is made on these two systems and some influential factors are investigated. It reveals that the concentration difference between the desiccant solution before and after regeneration has a strong impact on system performance. Moreover, comparison is conducted between the single-stage and the double-stage systems, the results show that the double-stage system is more energy-efficient and it can save more than 50% energy under optimized working conditions.
Heat mass exchangers are crucial for the prevention of epidemic respiratory diseases such as H1N1 (swine flu). The flow maldistribution affects their performance seriously. The flow maldistribution and the consequent performance deteriorations in heat and mass exchangers are investigated. The focus is on moisture effectiveness deteriorations. As a first step, a computational fluid dynamics (CFD) code is used to calculate the flow distribution, by treating the plate-fin core as a porous medium. Then a coupled heat and moisture transfer model between the two airflows in the plate-fin channels is set up with slug flow assumption in the channels. Using the CFD predicted core face flow distribution data, the sensible heat and moisture exchange effectiveness and the performance deterioration factors are calculated with finite difference scheme. The results indicate that under current core to whole exchanger pressure drop ratio, when the channel pitch is below 2.0 mm, the flow distribution is quite homogeneous and the sensible and latent performance deteriorations due to flow maldistribution can be neglected. However, when the channel pitch is larger than 2 mm, the maldistribution is quite large and a 10-15% thermal deterioration factor and a 20-25% latent deterioration factor could be found. Mass transfer deteriorates much more than heat transfer does due to larger mass transfer resistance through membranes.
Membrane contactors represent an emerging technology in which the membrane is used as a tool for inter phase mass transfer operations: the membrane does not act as a selective barrier, but the separation is based on the phase equilibrium. In principle, all traditional stripping, scrubbing, absorption, evaporation, distillation, crystallization, emulsification, liquid‐liquid extraction, and mass transfer catalysis processes can be carried out according to this configuration. This review, specifically addressed to membrane distillation (MD), osmotic distillation (OD), and membrane crystallization (MCr), illustrates the fundamental concepts related to heat and mass transport phenomena through microporous membranes, appropriate membrane properties, and module design criteria. The most significant applications of these novel membrane operations, concerning pure/fresh water production, wastewater treatment, concentration of agro food solutions, and concentration/crystallization of organic and biological solutions, are also presented and discussed.
The thermal performance of an enthalpy/membrane heat exchanger is experimentally investigated. The heat exchanger utilizes a 60gsm Kraft paper as the heat and moisture transfer surface for HVAC energy recovery. The heat exchanger sensible, latent and total effectiveness have been determined through temperature and moisture content measurements. The annual energy consumption of an air conditioner coupled with an enthalpy/membrane heat exchanger is also studied and compared with a conventional air conditioning cycle using in-house modified HPRate software. The heat exchanger effectiveness are used as thermal performance indicators and incorporated in the modified software. Energy analysis showed that an air conditioning system coupled with a membrane heat exchanger consumes less energy than a conventional air conditioning system in hot and humid climates where the latent load is high. It has been shown that in humid climate a saving of up to 8% in annual energy consumption can be achieved when membrane heat exchanger is used instead of a conventional HVAC system.
Energy recovery ventilators (ERVs) transfer energy between the air exhausted from building and the outdoor supply air to reduce the energy consumption associated with the conditioning of ventilation air. In this paper, the applicability of ERVs with sensible and latent effectiveness values in a practical range is studied using TRNSYS simulation program. The impact of ERV on annual cooling and heating energy consumption is investigated by modeling a 10-storey office building in four American cities as representatives of major climatic conditions. The results show that heat and moisture recovery can lead to a significant reduction in the annual heating energy consumption (i.e., up to 40%, which is 5% higher than heat recovery). Also, an ERV with the capability of moisture recovery may reduce the annual cooling energy consumption by 20% provided the ERV is properly controlled. Since the un-controlled operation of ERVs during the summer may increase the cooling energy consumption, an optimum control strategy is developed and verified in the paper. This optimum control strategy depends on ERV's latent to sensible effectiveness ratio. For instance, an ERV with equal sensible and latent effectiveness should be operated when either the outdoor enthalpy or temperature is greater than that of the indoor air.
Liquid desiccants used for many industrial and domestic applications have to be reconcentrated for reuse. Instead of using thermal energy, a method is proposed in this paper to use mechanical energy for regeneration of weak desiccants. The osmotic pressure required to regenerate two such desiccants, namely calcium chloride and lithium chloride, for given operating conditions is predicted, and correlations are developed. It is found that the pressure required for calcium chloride is much less than that of lithium chloride for the same operating conditions.
In Part І, a numerical model for coupled heat and moisture transfer in a run-around heat and moisture exchanger with a liquid desiccant coupling fluid is developed. The numerical model is two dimensional, transient and is formulated using the finite difference method with an implicit time discretization. The results from the numerical model for the case of only heat transfer for a single heat exchanger are compared to an available analytical solution and good agreement is obtained. For the simultaneous heat and moisture transfer in the run-around membrane energy exchanger (RAMEE), a comparison between numerical model results and experimental measurements obtained from laboratory testing for both sensible and latent effectiveness showed satisfactory agreement at different operating conditions. Part II of this paper applies the model for a range of initial conditions [32].
Part І of this paper [17] developed and verified the numerical model for simultaneous heat and moisture transfer in the run-around membrane energy exchanger (RAMEE) system to determine the transient behavior of the system under different initial and operating conditions.This paper presents the transient response of the RAMEE system for step changes in the inlet supply air temperature and humidity ratio. Also the system quasi-steady state operating conditions are predicted as the system approaches its asymptotic operating condition. The transient responses are predicted with changes in various parameters. These include: the number of heat transfer units, thermal capacity ratio, heat loss/gain ratio, storage volume ratio and the normalized initial salt solution concentration. It is shown that the storage volume ratio and the initial salt solution concentration have significant impacts on the transient response of the system and heat transfer between the RAMEE system and the surrounding environment can change the system quasi-steady conditions substantially.
The latent effectiveness and the latent number of transfer units (NTUs) for mass transfer in membrane humidity exchangers were applied to proton exchange membrane fuel cell (PEMFC) membrane humidifiers. We report on two limitations that cause deviations in the theoretical outlet conditions reported previously: (1) using a constant enthalpy of vaporization derived from the reference temperature in the Clausius–Clapeyron equation; and (2) simplifying the relationship between relative humidity and absolute humidity as linear. These limitations are alleviated by using an effective mass transfer coefficient Ueff. The constitutive equations are solved iteratively to find the flux of water through the membrane. The new procedure was applied to three types of membrane and compared to the curves of εL and NTUL found using Zhang and Niu’s method, which is normally applied to energy recovery ventilators (ERVs).
An analysis of simultaneous exchange of heat and water vapor in a crossflow-type total heat exchanger which is made of a Japanese paper impregnated with some kind of hygroscopic agent is carried out by employing a permeability coefficient based on an analogy between heat and mass transfer. It is shown that the predictions are matched well to experiments for the temperature and humidity efficiency. In respect to heat and mass transfer processes through the total heat exchanger, a particularly interesting fact is also revealed that the rate of heat transfer is dominated by air rather than the paper, while the rate of mass transfer of water vapor is dominated by the paper rather than air.
The heat and mass transfer characteristics of a water permeable membrane were studied to determine appropriate selection criteria for such a membrane in a heat recovery ventilator (HRV). A general physical model was developed to analyze the performance of various types of membrane based HRVs. The theoretical results obtained using the model were validated experimentally. The advantages of such a system are: simultaneous recovery of the sensible and latent heat, high heat and moisture exchange effectiveness, no mechanical components and year-round energy savings in air-conditioning systems. The model and results can be used to develop membrane based heat recovery ventilators.
This study aims at investigating experimentally and analytically the characteristics and properties of a membrane utilized to design compact absorbers for lithium bromide–water absorption chillers. The main focus of this study are the factors that influence the water vapor transfer flux into a lithium bromide–water solution in confined narrow channels under vacuum conditions, as well as the properties limits for utilization in compact absorber design. The results indicate that the desired membrane characteristics for this application are as follows: high permeability to water vapor, hydrophobic to the aqueous solution with high liquid entry pressure (LEP) to avoid wettability of the membrane pores and no capillary condensation of water vapor to avoid blocking of the pores. For practical use, this membrane should have a thin hydrophobic microporous active layer with a thickness up to 60μm, mean pore sizes around 0.45μm and a porosity of up to 80%. The active layer should be attached to a porous support layer to meet the mechanical strength requirements needed for practical use in the absorber of lithium bromide water absorption chillers application.
The performance of an improved ceramic membrane used as a dehumidifier was studied experimentally and the separation mechanism was considered. It is found that the permeability ratio of water/air can be seen to be quite large, larger than 1000. This is due to the separation mechanism proposed in the previous work, or capillary condensation followed by liquid flow in the pores of the membrane. Whether or not a membrane dehumidifier of this kind is applicable to practical dehumidification of gases depends on the power consumption required for the evacuation system of the permeates, which depends largely on the operating inside pressure of the membrane module.
Hollow fibre and flat sheet membranes with an interfacially polymerized coating of polyamide have a permeance for water vapour of about 0.16msec–1. These membranes can serve as a basis for building ventilation which provides fresh air while recovering about 70% of the specific heat and 60% of the latent heat. Because these membranes are selective for water vapour, the air is exhausted with internal pollutants like carbon monoxide, formaldehyde, and radon. The expense of the ventilator should be recovered in reduced heating costs in about three years.