Evaluation of frost prevention strategies for membrane energy exchangers

In cold climates, the application of heat recovery is restricted by the issue of frost, which causes potential damage to heat exchangers and degrades their effectiveness. Membrane energy exchangers (MEEs), which enable simultaneous heat and moisture transfer, can reduce and delay frost formation and accumulation in cold climates. MEEs are recognized as the essential component for the new generation of Heating, Ventilation, and Air Conditioning (HVAC) systems. Despite of extensive studies on heat and mass transfer characterising and increased use of MEEs, the evaluation of suitable frost control strategies for the emerging MEEs in cold climates are still missing. This study presents numerical models of a quasi-counter-flow membrane energy exchanger (QCFMEE) and a quasi-counter-flow heat exchanger (QCFHE). Three different frost prevention strategies are examined: preheating outdoor air, heating room air and bypassing outdoor air. These strategies’ threshold values to prevent frost are calculated numerically and validated against experimental measurements. The results show that QCFMEE has lower threshold values and thus better frost tolerance ability compared with QCFHE because of mass transfer through the membranes. Moreover, the frost prevention strategies are evaluated based on annual energy consumption, energy saving ratio (ESR), and complexity of control for real-life applications. The simulated results show that among the discussed frost prevention strategies, preheating outdoor air has the advantage of the lowest energy consumption and highest ESR. Meanwhile, heating room air consumes the most energy and faces the problem of overheating outdoor air. Finally, concerning the bypassing outdoor air strategy, the significant fluctuation of its threshold values increases the complexity of control for real-life applications.publishedVersio

Heat recovery ventilation design limitations due to LHC for different ventilation strategies in ZEB

Today's buildings are becoming more insulated and airtight to reduce transmission heat losses. Energy use for ventilation can represent up to half of these buildings' total energy use. Heat recovery in ventilation and demand-controlled ventilation (DCV) are energy-efficient measures to reduce ventilation energy use, especially when combined. However, this study revealed that the often-overlooked longitudinal heat conduction (LHC) in aluminium rotary heat exchangers might yield less efficient heat exchangers, particularly for intended high-efficiency heat recovery at low ventilation rates in DCV. This study presents a theoretical method to assess the effect of LHC on the amount of energy used to heat ventilation air for several ventilation strategies. The method is demonstrated in a case study for a virtual office building in a cold climate (Oslo, Norway). When neglecting the LHC effect, the energy used to heat the supplied air using DCV with a rotary heat exchanger is about three times smaller than when considering LHC. Unlike earlier studies, we find that DCV may consume more ventilation heating energy than constant air volume (CAV) ventilation when the selected wheel is deep and oversized due to LHC. This study highlights the need to design rotary heat exchangers carefully in order to account for the LHC effect, particularly when targeting zero emission buildings (ZEB).publishedVersio

Global sensitivity analysis and optimal design of heat recovery ventilation for zero emission buildings

Energy-efficient building services are necessary to realise zero-emission buildings while maintaining adequate indoor environmental quality. As the share of ventilation heating needs grow in well-insulated and airtight buildings, heat recovery in mechanical ventilation systems is increasingly common. Ventilation heat recovery is one of the most efficient and viable means to reduce ventilation heat losses and save energy. Highly efficient heat exchangers are being developed or applied to maximise the energy-saving potential of heat recovery ventilation. Nevertheless, the effects of practical operating conditions and the constraints of heat recovery – such as variations in ventilation rates, frost protection, and the prevention of an overheated air supply over a long-term period, which may significantly influence realistic recovery rates – have been less considered in efforts to maximise the energy savings. It is unclear which design parameters for heat recovery devices have the greatest effect on the annual energy savings from ventilation. This study proposes annual efficiency and annual net energy saving models for heat recovery ventilation that consider ventilation rate variations, the longitudinal heat conduction effect and operating controls. We use a global sensitivity analysis to quantify the contributions of various design input parameters to the variation in annual recovery efficiency and annual net energy savings. We identify the most influential parameters and their significant interaction effects for the annual energy performance of heat recovery ventilation. More attention should be paid to these most influential parameters during the design process. Furthermore, the optimal designs for rotary heat exchangers (as identified by a pattern-search optimisation algorithm) can improve annual net energy savings in demand-controlled ventilation by 33–48%, depending on the building areas. In combination with the reference year analysis presented in this study, heat recovery and demand-controlled ventilation can help to meet the need for highly efficient ventilation systems and zero-emission buildings.publishedVersio

Measurements of indoor air quality in four Norwegian schools

Children spend a minimum of six hours per day in Norwegian schools. Their exposure to different indoor air quality it is known to affect their performance. It is very common to use demand-controlled ventilation (DCV) in schools as is estimated to save about SO% of the conventionally used energy for ventilation. CO2 and temperature are the preferred control parameters. Usually, it was expected that these human-centric controls resulted in high indoor air quality as occupants are the largest source of contaminants. This study presents measurements for two months to up to one year in the supply and room air in the four classrooms whose ventilation is CO2-based DCV. Using low-cost sensors formaldehyde, PM1, PM2.s, relative humidity CO2 and temperature were monitored. Even when the CO2 concentration lied below 1000 ppm 1) the concentration of formaldehyde surpassed the recommended WHO thresholds in 30 % of the time and 2) RH is below 20 % during 56 % of the time.publishedVersio

Experimental study on the exposure level of surgical staff to SARS-CoV-2 in operating rooms with mixing ventilation under negative pressure

The purpose of this study was to reveal the exposure level of surgical staff to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from the patient's nose and wound during operations on COVID-19 patients. The tracer gas N2O is used to simulate SARS-CoV-2 from the patient's nose and wound. In this study, concentration levels of tracer gas were measured in the breathing zones of these surgical staff in the operating room under three pressure difference conditions: −5 pa–15 pa and −25 pa compared to the adjunction room. These influencing factors on exposure level are analyzed in terms of ventilation efficiency and the thermal plume distribution characteristics of the patient. The results show that the assistant surgeon faces 4 to 12 times higher levels of exposure to SARS-CoV-2 than other surgical staff. Increasing the pressure difference between the OR lab and adjunction room can reduce the level of exposure for the main surgeon and assistant surgeon. Turning on the cooling fan of the endoscope imager may result in a higher exposure level for the assistant surgeon. Surgical nurses outside of the surgical microenvironment are exposed to similar contaminant concentration levels in the breathing zone as in the exhaust. However, the ventilation efficiency is not constant near the surgical patient or in the rest of the room and will vary with a change in pressure difference. This may suggest that the air may not be fully mixed in the surgical microenvironment

A multi-objective optimisation framework to design membrane-based energy recovery ventilation for low carbon buildings

Membrane energy exchangers (MEEs) are increasingly being studied and utilized to contribute to realising energy-efficient building services and providing satisfactory indoor environments. The performance of MEEs has been extensively studied in terms of heat and mass transfer and pressure drop (PD). However, a model for optimizing the performance of membrane energy exchangers in residential ventilation, which takes into account the influential factors, is lacking in order to support the design of membrane energy exchangers. The purpose of this study was to establish a framework for the multi-objective optimisation design of membrane energy exchanger performance. This framework was demonstrated by considering the competing objectives of maximising thermal recovery effectiveness and minimising pressure drop. One of the constraints used for optimising membrane energy exchangers was the total membrane area, which strongly influences the investment cost of the exchanger. Another constraint was the moisture recovery intensity of the membrane energy exchangers, which affects indoor humidity levels. Pareto optimal solutions were obtained by solving the developed multi-objective optimisation framework using the genetic algorithm in MATLAB. Using multi-objective optimisation, the pressure drop of the MEE was reduced by 41% while the thermal recovery effectiveness remained unchanged. The resulting pressure drops as low as 5 Pa, enables the application of membrane energy exchangers in natural and hybrid ventilation. Factors influencing the Pareto optimal solutions including moisture recovery effectiveness, total membrane area and operating airflows have been investigated. A better understanding of optimal membrane energy exchanger designs considering thermal recovered energy and fan power resulted from this study.publishedVersio

Experimental study on the thermal plume from a surgeon in an operating room with mixing ventilation during COVID-19 pandemic

Following the outbreak of COVID-19 (SARS-CoV-2) in 2019, studies show positive results in protecting the surgical staff from patients infected by COVID-19 in operating rooms (ORs) with negative pressure. A negative pressure environment inside the operating room (OR) reduces the virus's circulation outside the OR (Chen et al., 2020). Nevertheless, it is unclear whether the surgeon's thermal plume can impact the transport of contaminants up to the breathing zone and thus cause infection in ORs with various pressure differences compared to adjacent rooms. The results show that a gap between the surgical manikin and the operating table greatly affects the development of the thermal plume from the head surgeon. A plate between the surgical manikin and the operating table may significantly influence the airflow distribution in front of the head surgeon more than the pressure difference inside the operating room.publishedVersio

Holistic methodology to reduce energy use and improve indoor air quality for demand-controlled ventilation

Ventilation control logics are usually based on the control indicators of occupancy. However, strategies including control of contaminants not linked to occupancy are requested and more feasible with the introduction in the market of low-cost sensors (LCS). In this work, a methodology for the improvement of demand-controlled ventilation (DCV) using measurements of IAQ parameters with LCS, correlation analysis, and co-simulation EnergyPlus/CONTAM is presented. Its goal was reduced annual energy use and the fraction of time with room air concentration of IAQ parameters outside thresholds. The ventilation control sequences of supply airflow rates and recirculation of return air focused on the significant parameters chosen by cross-correlation functions in the de-trended measurements. The results revealed that the methodology successfully developed control sequences that simultaneously reduced annual energy use and the number of hours outside the recommended IAQ guidelines compared to the baselines. In cold cities with excellent outdoor air quality, recirculation could reduce energy use and increase the RH in winter. Further simulations demonstrated that the use of recirculation had a protective effect on the indoor concentrations of PM2.5, assuming low outdoor air quality. However, when using recirculation, it is essential to control the IAQ to avoid excessive pollutants, RH, and temperatures.Holistic methodology to reduce energy use and improve indoor air quality for demand-controlled ventilationpublishedVersio

Assessing the indoor air quality and their predictor variable in 21 home offices during the Covid-19 pandemic in Norway

In this study, concentrations of pollutants: formaldehyde, carbon dioxide (CO2), and total volatile organic compounds (TVOC) and parameters: indoor room temperature and relative humidity (RH) were measured in 21 home offices for at least one week in winter in Trondheim, Norway. Eleven of these were measured again for the same duration in summer. Potentially explanatory variables of these parameters were collected, including building and renovation year, house type, building location, trickle vent status, occupancy, wood stove, floor material, pets, RH, and air temperature. The association between indoor air pollutants and their potential predictor variables was analyzed using generalized estimation equations to determine the significant parameters to control pollutants. Significantly seasonal differences in concentrations were observed for CO2 and formaldehyde, while no significant seasonal difference was observed for TVOC. For TVOC and formaldehyde, trickle vent, RH, and air temperature were among the most important predictor variables. Although higher concentrations of CO2 were measured in cases where the trickle vent was closed, the most important predictor variables for CO2 were season, RH, and indoor air temperature. The formaldehyde concentrations were higher outside working hours but mostly below health thresholds recommendations; for CO2, 11 of the measured cases had indoor concentrations exceeding 1000 ppm in 10% of the measured time. For TVOC, the concentrations were above the recommended values by WHO in 73% of the cases. RH was generally low in winter. The temperature was generally kept over the recommended level of 22–24 ◦C during working hours.publishedVersio

Experimental Study on the Surgical Microenvironment in an Operating Room with Mixing Ventilation under Positive and Negative Pressure

Due to the outbreak of Covid-19, negative pressure operating room (NPOR) are strongly recommended to be applied to prevent spreading virus from infected patients to adjacent rooms during surgery procedures. However, there have been few experimental studies on the effect of OR pressure difference on the surgical microenvironment. This study aims to experimentally investigate the airflow distribution in the surgical microenvironment in an OR under different pressure conditions. All measurements were performed in a fullscale laboratory, which has an area of 62 m2, and a mixing ventilation. The air velocity and temperature in the surgical microenvironment of a lying patient were measured under positive pressure of 5 Pa, 10 Pa, 15 Pa and negative pressure of -5 Pa, -10 Pa and -15 Pa. The effect of heat generated by operating lamps was also considered. The results show that the airflow distribution around the surgical wound is dominated by thermal plume from the patient under the condition of both positive and negative pressure. In other areas of the surgical microenvironment, regardless the pressure difference conditions, the room airflow distribution by ventilation system is the dominant factor on surgical microenvironment. Variations in differential pressure can affect the temperature distribution around the surgical site, with a smaller differential pressure producing a slightly larger vertical temperature gradient.publishedVersio