Abstract

Increasing ventilation is an effective method to reduce indoor airborne disease transmission. An Energy recovery ventilator (ERV) is a passive energy recovery device used to reduce the energy consumption of heating, ventilation and air-conditioning (HVAC) systems for conditioning the ventilation air. It preconditions the ventilation air by transferring energy from building exhaust air. Therefore, the ventilation and exhaust air streams interact directly/indirectly in the ERV for energy transfer. It is surmised that the ERV may transfer bioaerosols (with pathogens) from the exhaust air to ventilation air, resulting in the spread of infectious diseases. Consequently, many pandemic HVAC guidelines recommend that the use of ERVs be limited. This is a highly unsustainable direction given the increased energy requirements associated with the high ventilation provision advocated for pandemic operation. It must be noted that no validated experimental evidence exists in literature for bioaerosol transfer in ERVs. Hence, it is necessary to conduct extensive bioaerosol transfer research before adopting the unsustainable practice of limiting the utilization of ERVs. The main objective of this review study is to summarize the experimental methods and instrumentation for bioaerosol transfer research in ERVs. This comprehensive article provides a detailed overview of the generation, sampling, and analysis of bioaerosols for conducting the experiments. Further, it explains the possible mechanisms for bioaerosol transfer in various types of ERVs based on which the ERVs that need immediate attention are identified. The main contribution of this research paper is that it provides a novel experimental method which encompasses the biosafety aspects, instrumentation, performance parameters and uncertainties in conducting virus contained bioaerosol transfer study in ERVs. The findings from this review will be helpful in designing bioaerosol transfer experiments and developing future ERV test standards for such experiments.

 

Introduction

The substantial growth in the population, rapid urbanization, and development of international transport infrastructure witnessed at a global level have led to the rapid transmission of infectious diseases both within and across countries. This is evident in the outbreak of the unprecedented COVID-19 pandemic, which has become one of the most dangerous health threats in world history. As per the World Health Organization (WHO), there are 767,984,989 confirmed cases of COVID-19 globally, including 6,943,390 deaths as of June 15, 2023 [1]. Infectious diseases like COVID-19 not only affect public well-being (physical as well as mental health) but also lead to substantial economic loss and significant changes in the functioning of society, such as people’s behavior, lifestyle, etc.

Outbreaks of pandemic diseases are not new to the world. A plague outbreak caused by the flea-borne bacteria Yersinia pestis killed around 100 million people in the Roman Empire between 541 and 543 CE. A recent review reported that the world faced 17 major pandemics before COVID-19 [2]. However, not much importance has been accorded to airborne transmission which is one of the major modes of disease transmission. This is attributed to the difficulty in traceability, lack of research studies and continual misunderstanding. Generally, there are inconsistencies while addressing the different sources of environmental infections. Greater attention has been given to waterborne and foodborne transmission modes compared to the airborne transmission of diseases. This is evident from the large number of well-established standards for food and water processing (including wastewater and sewage) and professional government officials to oversee food and water quality [3]. It is imperative that airborne transmission be accorded attention equal to that of foodborne and waterborne infections to ensure public safety and the well-being of society.

Ventilation plays a crucial role in providing good quality air and mitigating airborne transmission in an enclosed space, as shown in Fig. 1 [4], [5]. It can be an effective engineering control measure when designed properly. There are studies documented in literature establishing the direct relationship between improper ventilation and disease transmission [6], [7]. Hence, to ensure an effective ventilation system to control the airborne transmission of infectious diseases, countries include recommendations about ventilation in all the 17 pandemic guidelines [8]. In the ventilation technique, the outdoor airflow rate is increased to dilute the concentration of infectious aerosols indoors, thereby minimizing disease transmission. However, increasing the supply of outdoor air is not feasible if the prevailing climatic conditions are far away from human comfort conditions. For example, the average monthly temperature in Saskatoon, Canada is −9℃ in winter (January) and in Mumbai, India is 30℃ during summer (May) [9]. Supply of ventilation air at these extreme outdoor conditions will lead to an uncomfortable indoor environment which can cause thermal stress to the occupants. The thermal stress may be life-threatening and can also lower human resistance to infection [10]. Hence, it is essential to condition the outdoor air before supplying it to an indoor environment. However, this increases the energy consumption of heating, ventilation and air conditioning (HVAC) systems.

The most common pandemic guidelines of HVAC systems, in addition to ventilation, are (i) avoiding recirculation which in turn averts the possibility of pathogen transfer across the air-conditioned rooms of a building and (ii) increasing operating time to increase the air change per hour which reduces the concentration of infectious aerosols indoors. However, all these guidelines significantly increase the energy consumption of HVAC systems. The increase depends on the prevailing climatic conditions and operational conditions of the building. A study assessing the climatic conditions of China predicted that the increase in energy consumption of buildings was likely to be as high as 140% [4]. Thus, energy conservation is essential to achieve sustainability (specific to Sustainable Development Goals: 7.3 “Energy Efficiency” and 11.6 “Air Quality”) of HVAC systems while providing adequate ventilation.

There are different passive techniques available to reduce the energy consumption of HVAC systems during pandemic operation which ideally requires 100% fresh air supply. In general, the passive cooling/heating techniques use low/high temperature natural resources as a heat sink/source and thereby condition the ventilation air with least energy input. For example, the earth air heat exchanger uses the low temperature ground as a heat sink during summer in order to lower the temperature of the supply ventilation air. After a certain depth, the ground temperature is almost constant throughout the year. Of late, radiative cooling technique is gaining more popularity; it uses the sky as a heat sink. This technique uses spectrally selective surfaces/films, which have the ability to achieve a sub-atmospheric temperature due to long wave irradiation into the sky and can be used to cool the ventilation air. Evaporative cooling is another passive technique that reduces the temperature of non-saturated air by evaporating the water present in it due to humidity difference. The recent development of dew-point evaporative coolers has significant potential to reduce the temperature of the ventilation air depending on the prevailing climatic condition [11], [12].

Energy recovery ventilator (ERV) is a passive energy recovery device that can be used to reduce the energy consumption of an HVAC system. Table 1 shows the energy savings or HVAC system performance improvement by using different types of ERVs. It also reduces operating costs, especially during pandemic operations when the required ventilation rate is substantially higher than normal operating times. In ERV, the energy from the exhaust air is used to precondition the fresh outdoor ventilation air. While exchanging energy between exhaust and supply airstreams, it is likely that bio-aerosols/contaminants or other indoor pollutants may be transferred due to the cross-contamination (with or without mixing of exhaust and supply air streams). For Volatile Organic Compounds (VOCs) and other pollutants, it is evident from literature that cross-contamination occurs due to carryover, leakage, adsorption/desorption, evaporation/condensation, and absorption. Hence, the operation of ERVs may lead to the transfer of bioaerosols (a type of airborne material that has microorganisms originating from living organisms) from the exhaust air to the fresh ventilation air entering the building, similar to that of VOCs and other pollutants. Consequently, it may lead to transmission of infectious disease-causing pathogens in the building. ERVs components such as desiccant coating (in enthalpy wheels and fixed-bed regenerators) and porous or dense membranes (in membrane exchangers) have crucial role in the contaminant/aerosol transfer.

During the COVID 19 pandemic situation, a majority of standards and certification agencies recommended limiting the use of energy recovery ventilators due to the possibility of cross-contamination of bioaerosols through the ERVs [4], [8]. However, there is a distinct lack of experimental / field evidence available in the open literature. Therefore, it is necessary to conduct the relevant experimentations on ERVs before following the unsustainable practice of bypassing or operating ERV with constraints, as prescribed in many pandemic HVAC guidelines [4], [8]. This identified research gap forms the motivation for the present study.

The main objective of this work is to summarize the available experimental instrumentation for bioaerosol transfer studies and propose a test method to quantify the transfer of bioaerosols in ERVs. A new performance parameter is proposed along with the test methods and uncertainty bounds for the experiments. Unlike conventional heat and mass transfer performance testing, the tests to determine the transfer of aerosols in ERVs are quite different and challenging due to the instrumentation, material preparation, need for accuracy, reliability, and safety requirements. This review article summarizes the available experimental approaches and information to address the aforementioned challenges in the bioaerosol transfer tests in ERVs. To the best of the authors’ knowledge, there is no literature available on this novel and multi-disciplinary research topic except a very preliminary study by Shirey et al. [19]. Hence, the authors believe that the proposed experiments, instrumentation and performance parameters would be useful in designing and conducting bioaerosol transfer experiments in ERVs, and also for framing test standards (such as ASHRAE 84) for ERVs.