In a previous blog, I detailed the purpose of humidity control in buildings. To summarize, this came down to three, non-mutually exclusive, reasons:
- Health and safety
- Maintenance of building materials
What I did not go into was the fact that proper control of relative humidity inside buildings can be a very energetically costly process. This presents a challenge in the context of a global impetus to develop and adopt sustainable practices. As Laustsen puts it in Energy Efficiency Requirements in Building Codes, Energy Efficiency Policies for New Buildings (2008), “itself an often energy-intensive process, dehumidification can be integrated into air conditioning systems. Building regulations in humid climates should account for the energy involved in humidity control.” This can be especially important in cities located in tropical zones. Essentially, acknowledging that control of relative humidity is of vital importance in buildings, we have to ask how to lessen the energetic demand of doing so: how is efficient dehumidification is realized?
This is a complex issue. Undoubtedly, it is the complexity of achieving efficient dehumidification that has led to a general paucity of information in international codes on sustainable building practices on how to do so. In one of the most comprehensive pieces of international legislation on energy efficiency, ASHRAE Standard 90.1, guidance regarding effectiveness in humidity control is scarce; only a few specific strategies are mentioned:
- By prohibiting the use of fossil fuel and electricity for humidification above 30 % relative humidity (RH) and dehumidification below 60 % RH, except in exceptional circumstances.
- A dead-band of at least 10 % is required when a specific humidity level needs to be maintained for special spaces.
- By requiring at least 75 % of the annual energy used for reheat to be from recovered or site-generated solar energy when specific humidity levels are set, and dehumidification control is needed.
These all serve to create a more efficient dehumidification system. However, they do not speak to the type of system itself. In that regard, research has accumulated over the last twenty years to show that decoupling the handling of sensible (heat) and latent (humidity) energy, such as through the use of a dedicated outdoor air system (DOAS), is the best way achieve efficient dehumidification (DOAS are generally recognized to optimize energy efficiency in all aspects of heating, ventilation, and air conditioning (HVAC)).A DOAS “is a 100% outdoor air constant volume system designed to deliver the volumetric flow rate of ventilation air to each conditioned space in the building” (Mumma 2001). Essentially, DOAS are designed to accurately manage the air ventilation inside a building, using 100% fresh, outdoor air. These systems decouple the air conditioning process, dealing with ventilation and latent energy, and sensible energy separately. The latent energy component of a DOAS system is generally one of two apparatus (or a combination of the two); a cooling coil to lower the air temperature such that moisture condenses out or a desiccant wheel which absorbs moisture from the incurrent air. The sensible energy is handled in a variety of ways, with radiant systems considered the best option for energetic efficiency and thermal comfort.
Several studies have shown the energetic benefits of DOAS, with estimates of savings ranging from 10 – 42 % compared to conventional HVAC systems. These benefits are maximized in hot-humid climates, where conventional systems with AHUs that regulate sensible and latent energy loads at once drastically increase their energy consumption to do so.For efficient dehumidification, in particular, DOAS should optimally be fitted with a desiccant wheel (or enthalpy wheel to conserve sensible energy simultaneously). Compared to cooling coils, desiccant wheels provides continuous dehumidification without having to drastically cool the air. This mitigates the adverse consequences of having to reheat the air post latent energy handling, which can be considerable. This is highlighted by Larrañaga et al. (2008), “to comprehend the advantages of enhanced dehumidification systems, it is vital that owners and HVAC system designers understand this quadruple penalty associated with the use of reheat systems.” The use of desiccant wheels also eliminates problems of condensation associated with traditional cooling coil systems.Finally, DOAS systems achieve efficient dehumidification not only in the energetic sense of the word but also the technical sense. By decoupling sensible and latent energy handling, DOAS control relative humidity much more accurately than traditional HVAC systems.
Controlling relative humidity is mandated in legislation, as I have covered in previous blogs. This is because the relative humidity within buildings has a strong effect on health, safety, and comfort. However, controlling humidity can be very energetically demanding, which creates a problem for building designers attempting to maximize sustainability.
Solving this problem is no easy task. However, progress toward efficient dehumidification can come from modifications to existing HVAC systems, such as those listed by ASHRAE Standard 90.1. Beyond that, there is a large body of evidence to suggest that decoupling the HVAC system to handle latent and sensible energy separately is the best methods for achieving efficient dehumidification in buildings, while still meeting the mandated requirements for indoor air quality (ventilation rates, relative humidity < 65 %, etc.).