Latent cooling load accounts for around one third of the total load of air-conditioning, and its proportion is even higher in some subtropical and tropical areas. In order to maintain indoor comfort and to prevent microorganism growth, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends that indoor relative humidity should be maintained between 40% and 65%, which is the desired comfort relative humidity range for human being. In the conventional air-conditioning system, air is cooled and dehumidified simultaneously. The latent cooling load is removed by the refrigeration dehumidification process. Air is cooled to the dew point first, and then re-heated to the set-point for indoor environment, which results in wasting a lot of energy. Desiccants help the system to independently control both temperature and humidity and thus contribute to the reduction of the energy used. Silica gels and zeolites are the most commonly used solid desiccants, but they are not very efficient for dehumidification owing to their high regeneration temperature, long cycling time and the fact that a larger part of their water vapor sorption occurs outside the desired comfort relative humidity range or the operating vapor pressure windows (i.e. between 40% and 65% RH). Dehumidification by liquid sorbents uses less electrical energy than refrigeration, but the relevant technology has distinct drawbacks for commercialization, e.g. the system is complex and bulky, and has corrosion problems.
The ideal materials for autonomous regulation of indoor relative humidity should meet the following criteria:
- The material should have an IUPAC-type V S-shape isotherm and exhibit a steep uptake isotherm at a specific relative humidity depending on the targeted application. For indoor humidity control for thermal comfort, the adsorption isotherm must have a steep rise around 65% RH, and the desorption isotherm must have a steep decrease around 40% RH (see Fig. 1);
- High water vapor uptake within the operating vapor pressure window;
- Low regeneration temperature and high reproducible cycling performance;
- High hygrothermal stability, non-toxicity and non-corrosion.
Solid porous material that meets all above four criteria is named as Precise Humidity Control Material (PHCM).
Figure 1. Working principle of the ideal PHCM for autonomous indoor humidity control
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M. QIN*, P. Hou, Z. Wu, J. Huang, Precise humidity control materials for autonomous regulation of indoor moisture, Building and Environment, Vol. 169, 2020.