Energy-saving potential for ventilation in buildings when using gas-phase air cleaners

Ventilation accounts for up to 20% to 30% of the global energy use for providing an acceptable indoor environment in buildings. The main purpose of ventilation is to reduce exposures indoors by diluting and removing air pollutants to ensure a healthy and comfortable indoor environment.

The most energy-efficient method for controlling indoor exposures is by source control, i.e., reducing the emissions from indoor sources, which are usually referred to as building sources (building materials, furnishing, and consumer/cleaning products) and the sources associated with humans. However, it is challenging to implement the source control in existing buildings and impossible to control emissions from humans unless trapped locally (no such solutions are present now). This results in high demand for ventilation to ensure high quality of indoor air. High ventilation has two repercussions. It is costly energetically because of the need to treat the air supplied indoors, and it may bring pollutants from outdoors; outdoor pollution becomes the issue because of climate change and the increasing influx of people into the cities (urbanization). The supply air can be treated in the system, but it will be very costly as well.

The alternative to source control and increased ventilation is air cleaning, particularly gas-phase air cleaning, to remove the gas phase pollutants emitted indoors and brought from outdoors. Some of them can also have other benefits depending on the operation principle. For example, they can kill infectious agents. However, these benefits are not discussed in the context of the proposed project. If exposure level is reduced by using air cleaners, the ventilation rates can also be reduced because air cleaners will provide clean air that can, to some extent, substitute the need for outdoor air supplied by the ventilation system and needs to be treated. This is expected to result in considerable energy savings.

Worldwide, there is an increasing number and sales of air cleaning products. Many manufacturers claim that their air cleaners effectively remove pollutants from indoor air that have a hazardous impact on people. However, there are no standardized methods to document that these claims are correct. So, on the one hand, the application of these systems may not bring the expected positive effect on air-cleaning if claims are incorrect and cannot be verified, and on the other hand, they also will not bring benefits in the form of reduced energy use for ventilation of buildings.

Several air cleaning technologies are using different principles, and their efficiency must be verified before their use. Some of them have been documented to be efficient, and some not. Some were shown actually to aggravate air quality. The performance of air cleaners depends on the removal efficiency of air pollutants. They can have excellent performance for the specific pollutants and not at all for some specific pollutants, which can additionally be hazardous.

Additionally, some air cleaners have been shown to become sources of pollutants due to the production of unwanted hazardous pollutants (by-products) during the cleaning process. Many can also generate ozone during operation because of their principle, which is unwanted as well and can later participate in the chemical transformations. In conclusion, there is a need to develop a method to assess the performance of air cleaners to evaluate their potential risk of producing unwanted pollutants and the benefit of reducing the energy for ventilation.

Another aspect that needs to be considered is how much energy is used by the air cleaners themselves. Because they clean the air that has already been conditioned, there is no need to use energy for hygrothermal processes to secure the proper temperature and humidity of air. Still, the air cleaning process (to be efficient) may require energy, and this energy must be compared with the efficiency to remove pollutants and energy that is saved when reducing outdoor air supply for ventilation. There are basically no such comparisons in the existing literature regarding the performance of air cleaners.

Finally, using the gas phase air cleaner can disturb the operation of the existing ventilation systems, particularly those that use CO2, to control the outdoor air supply rate. CO2 is not removed by gas-phase air cleaners available on the market unless specifically designed to do so. In fact, the removal of CO2 is also a very energy-demanding process. So in order to use air cleaners as a supplement or a partial substitute for ventilation, the method must be developed to account for the effect provided by the air cleaner in terms of reduced CO2, i.e., define a “CO2 credit” that can be associated with the operation of an air cleaner. The credit converts the clean air delivered by the air cleaner into the same effect achieved by increasing the outdoor air supply rate. No such “credit” has been developed so far.