Energy Conservation and Indoor Air Biofilters


Longo’s Maple Leaf Square, Toronto, ON


Introduction

The presented data is based upon theoretical calculations. We are not suggesting a mechanical system be designed outside guidelines set out by ASHRAE. We present this as an indication of the sort of the energy savings could be potentially experienced.

Background

The traditional means of maintaining indoor air quality is to displace the ‘dirty’ air within the building with new outside air. Since the outside air quality in Canada is generally very good, this method can be extremely effective at controlling the build up of pollutants with the space. However, before being disseminated in the space, the additional air must be conditioned to an appropriate temperature to ensure occupant comfort. For buildings with central air handling systems, the ideal temperature of the disseminated air is 15°C. This allows for heat removal from activities within the space. The conditioning of this additional flow of air can represent a substantial cost to building owners. Under normal operating conditions, approximately 10–20% and during temperature extremes, over 30% of the energy consumed by the building is used to condition this additional flow of air. As a consequence of energy conservation concerns, many buildings may not be delivering enough outside air to the occupants. This can lead to a decrease in the air quality which can reduce the well-being of the occupants. Biofiltration of the air circulating within the building is an alternative means of refreshing the indoor air without having to bring new expensive outside air into the building. This could lead to substantial energy savings without sacrificing air quality.


Traditional Energy Requirements to Air Quality

Most indoor spaces require delivery of new outside air to the occupants at a rate of 8 to 10 liters per second per person (ASHRAE 1989). Given these flow rates, the density of air and its specific heat, it is possible to calculate the amount of energy required to heat this amount of new air over a range of temperatures typically encountered in North American cities. This is presented below.




The above figure represents only the energy required to heat the air and does not reflect the energy associated with the humidity management. Most warm air is frequently also humid and the removal of this excessive moisture represents a substantial energy requirement.

Further, cold air is inherently dry, adding the required moisture to make the air acceptable to the occupant is also very energy intensive. For example, adding enough water vapour to the air originally at -25°C to make in comfortable at 20°C increases the energy demand by close to fifty percent (0.42 KW per person to simply heat the air to 0.62 KW per person for properly humidified air).


Considering air at 30°C and 80%RH, to cool an adequate volume of air to 15°C requires 0.16 KW per person and to concurrently cool and dehumidify requires approximately 0.36 KW per person of cooling energy.


Energy requirements of the biofilter


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