Membrane-based Energy Recovery Ventilator Research Articles

Experimental study on the net outdoor airflow ratio in supply air of membrane-based energy recovery ventilator

The total energy recovery ventilator for outdoor air handing plays an important role in reducing energy consumption of the ventilation system. At the same time, the cross infection between fresh air and return air is a direct threat to the safety of energy recovery components with the influence of COVID-19. Therefore, how to improve the total exchange effectiveness and net outdoor air flow ratio in supply air of the heat recovery system has become an urgent problem to be solved. In this study, the composite membrane was prepared by non-woven fabric, siloxaneamide and lithium chloride solution, which was used as the membrane for the heat and mass transfer between fresh air and return air. The variation of the selective permeability of the composite membrane was studied experimentally. The experimental results show that the highest permeance of the composite membrane for the water vapor permeability can reach until 32.5×10-8kg/m2·s·Pa. The net outdoor air flow ratio in supply air is 94% when the air volume is 550m3/h The heat exchange efficiency of the heat recovery device is 63.2% under the conditions of the dry and wet bulb temperatures of return air and outdoor air are 21.2°C/12.9°C and 2.3°C/1.2°C, respectively.

Experimental investigation of condensation in energy recovery ventilators

Membrane-based Energy Recovery Ventilators (ERV) have become an important part of modern ventilation systems for commercial and residential buildings due to their high sensible and latent effectiveness. Using ERVs in winter conditions, however, can result in condensation and frost formation. Whereas frost formation requires extremely low temperatures, condensation can occur even in mild winter weather conditions. In this study, a widely used ERV is experimentally tested under various indoor and outdoor operating conditions. Short time tests are used to determine the onset of condensation while long time tests are used to investigate the effect of channel blockage due to condensation on pressure drop and effectiveness of the ERV. The presence of condensation is inferred from visual observation of water in the exchanger, investigation of the sensible and latent effectiveness, and measurements of pressure drop over the exchanger. It is shown that condensation increases the sensible effectiveness of the supply side and decreases the sensible effectiveness of the exhaust side. Additionally, the latent effectiveness of both air streams increases when condensation occurs, although the increase in exhaust side latent effectiveness is more significant. Increasing the relative humidity and temperature of the indoor air increases the possibility of condensation and its rate, while increasing flowrate only increases the rate of condensation and does not affect its onset. Finally, the accumulation of water in the channels significantly increases the pressure drop in the exhaust side while it does not significantly impact the effectiveness.

Energy exchange efficiency prediction from non-linear regression for membrane-based energy-recovery ventilator cores

For the design and optimization of membrane-based energy-recovery ventilators (MERVs) it is essential to know the energy-exchange efficiency. Unfortunately, performing a detailed and comprehensive measurement of the energy-exchange efficiency for all operating conditions is time-consuming and costly. In this paper, a method is proposed that makes it possible to predict the efficiency of MERV cores even if only a limited set of experimental data is available. By using a nonlinear regression model, which was derived from experimentally tested data, the efficiency of cross-flow air-to-air ventilator cores can successfully be predicted for different core-sizes and air-flow rates. As verified by experimental data, the relative error of the sensible- and enthalpy-heat efficiency prediction can be controlled within about ±7.0%, while the relative error of the latent heat efficiency prediction can be controlled within about ±8.0%. The method is suitable for both winter and summer operating conditions, and there is no need to test the characteristics of the membrane materials. Overall, the proposed method is practical and can be used to guide the design and operation of future MERVs.

A manufacturing process design for producing a membrane-based energy recovery ventilator with high aspect ratio support ribs

Heating, ventilation, and air conditioning systems represent nearly half of building energy usage in the United States. Recent building regulations requiring increased air turnover within buildings will result in even greater air conditioning usage. To alleviate these concerns, membrane-based heat-and-moisture exchangers known as energy recovery ventilators have been developed to reduce the energy expenditure of air conditioning systems by pre-conditioning incoming air through sensible and latent heat transfer with building air exhaust. In this work, we propose and validate a manufacturing process design for channel lamination based on surface mount adhesives capable of meeting the process requirements of a membrane-based ventilator. In particular, the device requires rib supports with height-to-width aspect ratios greater than one. The proposed manufacturing process design is capable of meeting this process requirement by stretching the adhesive after initial adhesive tacking. A manufacturing process flow diagram, a machine tool specification and a cost model are proposed for meeting process requirements. A set of design constraints are developed detailing the adhesive requirements necessary for meeting requirements. The cost model is validated by building a sub-scale mesochannel array demonstrating the ability to meet process requirements. Results show that additional work is needed to validate the curing step but that the process is capable of producing a mesochannel array with an acceptable level of variation.

Development of CFD model for membrane-based energy recovery ventilators

One of the methods of energy recovery in air conditioning systems is the membrane-based heat exchanger. This device uses the energy difference between two airflows (supply and exhaust air). The energy of the airflow is exchanged in the energy recovery system and does not require any external energy. This paper analyzes the numerical study of the new membrane shapes including curvature and circular, using COMSOL Multiphysics. These novel duct structures are used to improve the performance of the membrane-based heat exchanger. The model considers the mechanism of heat and mass transfers for turbulent flow. The flows in these geometries with Reynolds numbers ranging from 100 to 6000 are transitional. The low Reynolds k–w model is used. The cross-flow design is investigated under summer conditions. The flow distribution (velocity contours) is obtained. The friction factor, Nusselt numbers and Sherwood numbers in different Reynolds numbers for each cycle are calculated. The model is validated with available experimental data. After comparing and evaluating the four heat exchangers, it has been made clear that the total heat exchanger with curvature duct is more effective than other heat exchangers.

Enhancing the performance of energy recovery ventilators

Thermal performance enhancement of membrane based energy recovery ventilators (ERV) under turbulent flow conditions is investigated utilizing the computational fluid dynamics (CFD) approach. The standard k-ε model was adopted with the enhanced wall treatment option to simulate conjugate heat and mass transfer across the membrane. A user defined function was developed and incorporated into FLUENT to simulate the heat and mass transfer processes across a variable resistance 60 gsm membrane. A mesh sensitivity analysis was conducted and the developed CFD model was validated against an in-house experimental data. The performance of the investigated ERV was tested under different number of: flow channels, flow configurations, weather conditions and air flowrates. Results have shown that face velocity is more significant than flow separator in affecting the thermal performance of the investigated ERVs with a ratio of almost 5 to 1. Furthermore, the layout of the quasi-counter flow might present a preferable overall option over the L-Shape hybrid flow option. The final decision would be dependent on the HVAC system in-use and the higher priority between pressure drop, thermal energy recovered, manufacturability and/or installation.

Performance study of sodium alginate-nonwoven fabric composite membranes for dehumidification

A novel composite membrane is proposed to improve the dehumidification performance of the membrane-based energy recovery ventilator (ERV). In this study, the composite materials based on nonwoven fabrics, sodium alginate and lithium chloride (LiCl) were fabricated. Nonwoven fabrics were adopted as supporting constructions; sodium alginate and LiCl were the corresponding active layer and hydrophilic additive. Membranes with different LiCl mass fraction were tested and analyzed to evaluate practical performance when applied in the ERV. Results show that the highest vapor permeance of the composite membrane is 29.8×10−8kg/m2·s·Pa, 10 times higher than that of composite one without LiCl. Water solubility declines to 20.6%, which demonstrates a certain waterproofness of membranes. Besides, simultaneous thermal analysis shows that LiCl accelerates evaporation rate of bound water. Furthermore, two ERVs with different composite membranes are fabricated and characterized by measuring their sensible, latent and enthalpy efficiencies. It is found that these three efficiencies can reach up to 83.9%, 87.4% and 86.7% under summer condition, and 80.8%, 81.7%, and 81.1% under the winter condition, respectively.

Evaluation of governing heat and mass transfer resistance in membrane-based energy recovery ventilators with internal support structures

Energy recovery ventilators are an effective way to reduce energy consumption in buildings while satisfying ventilation standards. As membrane technology and manufacturing methods improve, membrane-based energy recovery ventilators are seeing increased market penetration. In the present study, the feasibility of membrane energy recovery ventilators designed to fit in a commercial building wall cavity is investigated. A heat and mass transfer resistance and pressure drop model is developed for a low aspect ratio (width/height) exchanger and is used to evaluate the sensible and latent effectiveness of a counter-flow energy recovery ventilator with internal support structures. The performance of strip-fin and pin-fin structures are compared and dominant heat and mass transfer resistances are investigated. It is shown that for all cases the sensible heat transfer is dominated by the convective resistance while the dominant mass transfer resistance shifted to the membrane at smaller hydraulic diameters. The results suggest that as membrane technology improves, enhancements to the airside heat and mass transfer coefficients will be required to continue to realize performance gains.

The energy saving potentials of zone-level membrane-based enthalpy recovery ventilators for VAV systems in commercial buildings

Enthalpy recovery ventilators exchange heat and mass between exhausted air and outdoor ventilation air to reduce mechanical ventilation loads for HVAC systems in buildings. This study demonstrates the energy saving potential of integrating membrane-based energy recovery ventilators at zone-level into the conventional variable air volume (VAV) systems for commercial buildings. Building energy simulations are used to evaluate various operation strategies and potential energy savings for four selected climates including Minneapolis, Atlanta, Baltimore, and Miami. A medium-size DOE reference commercial building model with conventional VAV systems was used as a baseline. Three cases implemented with energy recovery ventilators were simulated using EnergyPlus and their results were compared with baseline cases in compliance with ASHRAE standard 90.1. The annual HVAC energy saving potential can reach up to 18–49% with this technique for various climates. The challenges of implementing and operating the zone-level energy recovery ventilators are also discussed in this paper.

Contaminant transport in membrane based energy recovery ventilators

Ventilation systems are used to exhaust stale air and bring fresh air into sealed buildings. To maintain a comfortable indoor environment, this incoming air must be heated or cooled, which consumes energy. Consequently, membrane-based plate-type energy recovery ventilators are a component used in many energy-efficient ventilation systems. In these air-to-air energy recovery ventilator exchangers, incoming and outgoing air streams are passed over opposing sides of a membrane through which heat and moisture are transferred. This decreases the energy use of buildings by using the exhaust air to heat/cool and humidify/dehumidify the incoming air depending on the season. Ideally, membranes for these devices should have high water vapor permeation rates and be selective for water vapor over the transport of other gases and volatile organic compounds, including formaldehyde, odors, and contaminants, that may be present in the outgoing indoor air stream. Current certification and standards for contaminant crossover in North America focus on the measurement of the exhaust air transfer ratio based on tracer gas tests. This study demonstrates that although this test may be appropriate for measuring defects and leakage in exchangers, it may not account for all sorption and permeation phenomena that may be observed in polymeric membrane systems. Results are reported for the transport of water vapor, carbon dioxide, oxygen, and volatile organic compounds through a number of energy recovery ventilator membranes based on different polymers. Building ventilation systems are modeled using CONTAM software to demonstrate the effect of crossover through the energy recovery ventilator on indoor air quality under intermittent release (i.e., cooking odors, smoking, cleaning, etc.) and consistent release (i.e., off-gassing of volatile organic compounds or carbon dioxide associated with building materials or occupancy) of contaminants. It is shown that moderate crossover leakage in energy recovery ventilators should have minimal overall impact on indoor air quality in ventilated buildings.

Cross-flow membrane-based enthalpy exchanger balanced and unbalanced flow

In order to deepen the understanding of the main phenomena governing the membrane based energy recovery ventilators performance, we present a three dimensional numerical study of the mechanisms of heat and mass transfer in a cross-flow heat exchanger with balanced and unbalanced flow. This work involves the use of a control-volume method and solves the set of Navier–Stokes equations in the air streams and as well as in the membrane core. The results are compared to literature available data and the agreement is seen to be satisfactory. The effect of operating parameters such as the Reynolds number, volume flow rates and temperature on the performance of the heat exchanger are also investigated.

Moisture transfer through the membrane of a cross-flow energy recovery ventilator: Measurement and simple data-driven modeling

The moisture transfer effectiveness (or latent effectiveness) of a cross-flow, membrane-based energy recovery ventilator is measured and modeled. Analysis of in situ measurements for a full year shows that energy recovery ventilator latent effectiveness increases with increasing average relative humidity and surprisingly increases with decreasing average temperature. A simple finite difference heat and moisture transfer model is developed, which can explain these results and predict energy recovery ventilator latent effectiveness based on simplified physics and material properties. The model parameters are discussed and, in the case of the membrane’s moisture sorption curve and moisture permeability, compared to direct laboratory measurements.

Thermal performance enhancement of a periodic total heat exchanger using energy-storage material

A periodic total heat exchanger is a double flow circuit heat exchange device, which is improved by a membrane-based energy recovery ventilator with two additional fans to change the flow direction. Similar to membrane-based energy recovery ventilators, the deficiency of latent heat effectiveness poses a problem. Thus, this paper investigates the usage of energy-material in the periodic total heat exchanger to enhance the device performance. Three types of energy-storage materials, activated carbon, aluminum, and a mixture of activated carbon and aluminum are respectively installed and experimentally investigated. The results show that adding activated carbon to the device increases the latent heat effectiveness by 17.3%. Also, installing aluminum results in an increment of 15.1% on the sensible heat effectiveness. Finally, among the three types of energy-storage materials, the device with activated carbon has the best performance, with an increment of the total heat effectiveness of up to 11.7%.

Numerical analysis of heat and energy recovery ventilators performance based on CFD for detailed design

This paper presents a detailed numerical analysis of heat and membrane-based energy recovery ventilators (HRV/ERV), using computational fluid dynamics (CFD). The CFD model includes conjugate heat and mass transfer mechanisms for laminar flow to investigate the thermal performance of these systems. Both co-current and counter flow designs were investigated under typical summer/winter Canadian conditions. The model was validated with data obtained from the open literature. It was then applied to investigate the effect of a wide range of parameters on ventilators sensible and latent effectivenesses.The numerical results confirmed the superior effectiveness of counter flow over the co-current flow. The results showed a decrease in the HRV/ERV effectiveness with the increase in supply/exhaust air velocity. The ERV effectiveness in the summer was found to be higher than the winter. The effectiveness decreased noticeably and slowly with the increase in membrane spacing and thickness. The latent effectiveness increased significantly with the diffusivity of water in the membrane. The results also indicated that the outdoor temperature and humidity had only minor effects on HRV or ERV performance.CFD simulations proved to be an effective approach for detailed design of HRVs/ERVs.

EXPERIMENTAL AND THEORETICAL INVESTIGATIONS OF MEMBRANE-BASED ENERGY RECOVERY VENTILATOR PERFORMANCE

Experimental and theoretical studies were conducted to investigate the performance of a membrane-based energy recovery ventilator, which is an air-to-air heat exchanger with a water vapor permeable core. Tests were done on a commercially available membrane-based energy recovery ventilator, during which the supply and exhaust air states were recorded continuously over a long period of time. The test results show that as time passes the sensible effectiveness decreases very slightly from 56% to 55%, while the latent effectiveness increases appreciably from 28% to 39%. As a resultant of sensible and latent effectiveness, the enthalpy effectiveness provides an intermediate value between them and increases from 39% to 43%. The reason that the effectiveness changes with time is that the air relative humidity changes with time, which alters the moisture content at the membrane surface. Calculations were then implemented to predict the performance of the membrane-based energy recovery ventilator based on the model we developed previously and the calculation results were compared with the experimental data. The comparison suggests that the calculated effectiveness agrees well with the measured one, supporting the reasonability of the model. Calculations were also made to investigate the effects of various membrane parameters on latent and enthalpy effectiveness and latent-to-sensible heat ratio.

Performance analysis of a membrane-based energy recovery ventilator: Effects of outdoor air state

Abstract A theoretical study has been carried out to investigate the effects of the outdoor air humidity and temperature on the performance of a membrane-based energy recovery ventilator operating in both hot and cold seasons. The results show that the outdoor humidity and temperature affect not only the moisture transfer resistance but also the heat transfer resistance through membrane, resulting in the ventilator performance being a complex function of the outdoor humidity and temperature. In hot weather, the sensible effectiveness changes barely with the outdoor temperature but decreases gently with increasing outdoor humidity. The latent effectiveness decreases slightly with increasing outdoor temperature but increases rapidly with increasing outdoor humidity. As the resultant of sensible and latent effectiveness, the enthalpy effectiveness initially decreases and then increases, with the minimum occurring at a certain outdoor humidity. In cold weather, the sensible effectiveness changes scarcely with the outdoor air temperature and humidity, the latent effectiveness decreases with increasing outdoor temperature but increases with increasing outdoor humidity, with the variation of the enthalpy effectiveness being similar to that of the latent effectiveness.

Performance analysis of a membrane-based energy recovery ventilator: Effects of membrane spacing and thickness on the ventilator performance

A mathematical model was built to predict the thermal–hydraulic performance of a membrane-based energy recovery ventilator. Calculations were conducted to investigate the effects of the membrane spacing (channel height) and membrane thickness on the ventilator performance under equal fan power conditions. The results show that for a fixed fan power, as the channel height increases, the total heat transfer rate initially increases, after reaching a maximum at a certain channel height, turns to decrease. A larger fan power leads to a larger total heat transfer rate, with the maximum total heat transfer rate occurring at a smaller channel height. As the channel height or the fan power increases, the enthalpy effectiveness decreases. Further, as the membrane thickness increases, both the total heat transfer rate and enthalpy effectiveness decrease.

Conjugate heat and mass transfer in membrane-formed channels in all entry regions

Membrane-based energy recovery ventilators (or total heat exchangers) are key equipments to fresh air ventilation, which is helpful for the control of respiratory diseases like Swine flu (H1N1) and SARS. Parallel-plates narrow channels are common structure for membrane-based energy recovery ventilators. In practice, the exchanger channel lengths are limited due to the confinement in pressure drops and noises. In these channels, the hydraulically, thermally and concentrationally entry regions account for a large fraction of the total duct length. However, previous investigations neglected the entry issues for simplicity. Either hydraulically fully developed, or thermally or/and concentrationally fully developed flow were assumed, which would underestimate equipments performances seriously. This study provides a more accurate methodology: fluid flow, heat and mass transport equations were solved directly as they enter into the channel. In other words, both the fluid flow and the heat and mass transport are in simultaneously developing regions. The membrane and the two neighboring flows are considered as a conjugate problem. The conjugate heat transfer problem is solved with a commercial CFD code. Then the conjugate mass transfer problem is solved by transferring it to another conjugate heat transfer problem by heat mass analogy. The Nusselt and Sherwood numbers in the entry regions are calculated. The effects of three typical flow arrangements: cocurrent, counter and cross flow, on the boundary conditions and the consequent Nusselt and Sherwood numbers in the channels are evaluated.

Energy requirements for conditioning fresh air and the long-term savings with a membrane-based energy recovery ventilator in Hong Kong

Energy requirements for conditioning fresh air and the long-term savings with a membrane-based energy recovery ventilator in Hong Kong

Energy requirements for conditioning fresh air and the long-term savings with a membrane-based energy recovery ventilator in Hong Kong

A study of the energy requirements for conditioning ventilation air and the yearly performance of a membrane-based energy recovery ventilator (MERV) in Hong Kong is carried out. The weather data are classified into six process regions in the psychrometric chart and the percentage of annual hours in each region is determined to describe different energy requirements. The variations in the amount of required heating and cooling energy are calculated for different indoor temperature and humidity set points. It is found that the required annual energy is primarily used to remove moisture from fresh air, with only a small fraction used for sensible cooling. The energy for heating in cold weather is negligible. Energy recovery ventilators are employed to study the possible annual energy savings. Hour-by-hour calculations disclose that in hot and humid regions like Hong Kong, about 58% of the energy required for conditioning fresh air could be saved annually with an MERV, which recovers both latent and sensible energy, while only about 10% of the energy could be saved with a traditional sensible-only energy recovery ventilator (SERV). The more humid the weather, the more superior is an MERV in comparison with an SERV.