Mixed Feelings – Australia’s Hot & Cold Relationship with Mixed-Mode Air Conditioning.
Lawrence Yu, Senior Sustainability Engineer.
Although we share the implicit grasp of how comfort should feel, we often diverge in our opinions of how to achieve it. JHA goes back to school for a look at thermal comfort in the classroom.
With air-conditioning having evolved from a luxury item to a commodity, we have seen the contemporary definition of a comfortable building shift towards: a sealed, air-conditioned box. Air conditioning has relegated a building’s architecture – its scale, orientation, materials, glazing and ventilation – to a supporting role in our quest for comfort. While air-con gives us the flexibility to adopt built forms that would otherwise be uninhabitable, it can also allow inefficient structures to populate our landscape.
This answer for controlling temperature is energy-intensive. From a sustainability perspective, it contributes to the climate changes that we’re trying to mitigate against.
The polar opposite to air-con-in-a-box is natural ventilation, where the basic properties of air movement govern the architecture. Visit a public school built last millenium and you’ll see wide, shaded verandahs, operable glazing, high ceilings, radiant heaters or ceiling fans depending on your latitude. While these features give some control over the criteria listed above, the density of our growing cities makes such dimensional extravagance impractical.
Current Traditional Options
One of the challenges for natural ventilation in education facilities suggests that the resulting environment is not always conducive to student learning.
Sealed box air-conditioning work for comfort but are energy intensive.
Simple mixed mode can prove problematic in education. If manually operated it relies on users for proper use. If automated can cause a conflict of expectations and cost implications.
Re-Envisioning Mixed Mode Ventilation
The buildings are designed to have Interconnected but distinct air-conditioned zones, naturally ventilated zones and transitional zones.
For an educational facility the air-conditioned zones could include areas such as classrooms, lecture halls and offices. The hallways, corridors and breakout areas would be in the naturally ventilated areas. Informal learning spaces, study areas and breakout areas are located in the transitional zones.
Between the extremes, there’s mixed-mode ventilation, where both air conditioning and natural ventilation facilities are on offer in the same space, allowing us to choose natural cooling and save energy when desired. The problem with this concept is the ‘desire’ part – many failed attempts have proven that most humans will simply set and forget the air-con thermostat, regardless of the weather outside. Some engineers have tried taking choice out of the equation by adding control systems which only allow the air-con to run when it’s necessary. This too often fails (it turns out that humans like to make their own decisions, even if they’re technically wrong).
So despite its technical feasibility, this binary model of mixed mode ventilation design can fail to meet our comfort expectations.
3D airflow model showing air change in natural wind-driven ventilation.
Perhaps we need to imagine buildings where there’s a purposeful blend of air-conditioned and naturally-ventilated space. By purposeful, we mean that the occupants’ natural modes of use drive both the architecture and the cooling solution together, rather than one reacting to the other.
In a modern teaching space, for example, we may have zones for focused group learning, individual activities, circulation paths and storage. Under a re-envisioned mixed-mode ventilation strategy, we would condition only the focused learning area, where research suggests temperature influences learning efficacy. The class retains control over temperature in this area.
Architecturally, we would encourage proximity of the individual activity area adjacent the learning zone. The air-conditioning would be designed to allow a controlled amount of spill into this zone. The air is tempered just enough for comfort, without disturbing a relaxed sense of separation that encourages quiet absorption and reflection.
Further outward still, the circulation and storage areas are ventilated naturally. This perfectly aligns with their intermittent and undemanding occupancy. On temperate days, the class may elect to break out here for informal instruction in a more natural atmosphere.
The idea above is not groundbreaking. Yet it is energy efficient, cost effective and, most importantly, it is likely to be perceived as comfortable. Freedom of environmental choice is perhaps the ultimate determinant of thermal comfort.