June 6, 2019 - The traditional means of maintaining indoor air quality (IAQ) is to displace the ‘dirty’ air inside a building with new, ‘fresh’ air from outside. Since outdoor air quality is generally very good across North America, this method has proven effective at both (a) maintaining comfortable indoor temperatures and (b) reducing indoor buildup of pollutants, but at an energy cost that begs the question: is it possible to spend less to get the same results?
June 6, 2019 By Cindy Loosemore
Costs of conditioning
For buildings with central air handling systems, the ideal temperature for IAQ is 15 C (59 F). This setting allows for heat removal from activities within the space. Therefore, before fresh air is disseminated in the space, it must be conditioned to an appropriate temperature to ensure occupant comfort.
The conditioning of this additional flow of air can represent a substantial energy cost to building owners. Under normal conditions, approximately 10% to 20% of the energy consumed by a building is used to condition intake air; but during extreme seasonal temperatures, that figure jumps to more than 30%.
Most indoor spaces require the delivery of new outside air at a rate of 8 to 10 L per second per person, as outlined in ASHRAE’s standard 62.1-2016, Ventilation for Acceptable Air Quality. Given this flow rate and the heat and density of the air, it is possible to calculate the amount of energy required for this task over a range of temperatures typically encountered in North American cities.
This calculation represents only the energy required to heat the air; it does not reflect the additional energy associated with humidity management. Most warm air is also humid, after all, and the removal of excessive moisture represents a substantial energy requirement in itself. Cold air, meanwhile, is inherently dry, leading in winter to the addition of moisture, which is also very energy-intensive.
By way of example, if the outside temperature is -25 C, conditioning intake air to a comfortable 20 C can increase the corresponding energy demand by nearly 50%, with 0.42 KW per person for heat increasing to 0.62 KW per person with humidification.
In an effort to reduce these energy expenses, many building managers limit the amount of outside air delivered to the occupants. This limitation can lead to a decrease in IAQ, which in turn reduces the well-being of the occupants.
To address this common dilemma, building operators seek to strike a balance between energy expenses and occupants’ health.
Addressing the dilemma
Biofiltration provides an alternative means of refreshing indoor air without the expense of bringing in new, fresh air from outside. The technology uses considerably less energy than conventional systems, without sacrificing the benefits of IAQ improvements.
One example of biofiltration is a ‘living wall.’ This is a hydroponic plant whose roots spread between layers of synthetic growth media. A living wall is designed to draw dirty indoor air in through this root zone.
Microbes on the plant’s roots consume pollutants as food. In turn, the plant emits clean air, which is then distributed throughout a building through the heating, ventilation and air-conditioning (HVAC) system. And the emitted air is already nearly the ideal temperature and humidity level.
Researchers have used RETScreen clean energy management software to calculate the cost savings of biofiltration. Developed by Natural Resources Canada (NRCan) with a variety of partners, RETScreen uses advanced algorithms and databases to help the private and public sectors determine if proposed energy efficiency, cogeneration and renewable power projects make financial sense.
Specifically, the software was used to calculate the cost savings of generating 100 L of clean air per second with 1 to 2 m2 of biofilter, compared to bringing in the same volume of air from outside, over an entire year. Data available through RETScreen, including regional temperature ranges and utility rates, allowed the researchers to assign local costs to clean and conditioned air.
The results showed using biofiltration to supply ‘virtual’ outdoor air achieved energy cost savings.
Generation and capacity
Assuming air flows at 0.05 m3 per second per square metre of biofilter, with an average removal efficiency of 50%, this rate is equivalent to 25 L of new outside air.
Further, assuming an occupancy density of one person per 20 m2 of floor space and a recommended ratio of 1 m2 of biofilter per 100 m2 of floor space, the biofilter could provide 50% of the ‘fresh air’ requirements of five occupants. This capacity translates into electricity savings during peak HVAC use—i.e. in the heat of summer for cooling air and the cold of winter for heating it—of 0.3 and 0.58 KW per occupant, respectively.
These calculations are for the absolute amount of cooling and heating required. They do not take into account the use of heat pumps, which would reduce power requirements further. By way of example, an efficient heat pump can generate the equivalent of 4 KW in cooling while consuming only 1 KW of electrical power. If heat pumps are in place, then the biofilter is still generating clean air using less than half the energy of the traditional system.
In terms of the energy required to condition biofiltered air, intake air, cold air, ‘normal’ air and hot air, as the biofilter emits air close to the ideal temperature, it requires the least amount of energy.
Interestingly, conditioning cold air (i.e. warming it) requires the greatest amount of energy, even though doing so uses the cheaper form of energy: natural gas.
Since the electricity used for air conditioning is relatively more expensive than the natural gas used for heating, the researchers’ original assumption was biofiltration would achieve greater savings in warmer climates, where buildings rely more heavily on electricity. Instead, the savings were greater in cities where buildings use more heating throughout the year with natural gas.
The reason for this unexpected result is the degree of difference. In Northern Canada’s colder climes, the absolute temperature gradient is greater. In fact, it is common for such buildings to experience a 40 to 50 C gradient.
The primary byproduct of using biofiltration for air in a building is, of course, a vibrant, living plant wall. Unlike other IAQ systems, there are no dirty filters to dispose of and no off-gassing of collected pollutants. Nature takes care of the volatile organic compounds (VOCs).
Living wall biofilters can also help buildings achieve ‘green’ certifications, including those relating to Leadership in Energy and Environmental Design (LEED), ASHRAE 62.1, the Canada Green Building Council’s (CaGBC’s) Well standard and the Green Infrastructure Foundation’s Living Architecture Performance Tool (LAPT). In many cases, such certifications can help generate higher property values and tenant rents.
Cindy Loosemore is a writer and marketing professional based in Waterloo, Ont. She wrote this article on behalf of Nedlaw Living Walls in Breslau, Ont., which designs, builds and maintains biofiltration systems. For more information, visit www.nedlawlivingwalls.com.
This article originally appeared in the May 2019 issue of Energy Manager Canada magazine.
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