Novel desiccant materials for more energy-efficient AC systems

Teams of researchers at MIT and at Texas A&M University have developed novel desiccant materials that could lead to more energy-efficient and economical room air conditioners and dehumidifiers. 

Metal organic frameworks (MOFs) [1] 


Traditional vapour compression air conditioning systems include a refrigeration dehumidification process. In a nutshell, traditional ACs not only cool the air, but they also dry the air they are cooling. Traditional ACs use an evaporator, which is a cold coil that extracts water from the air through condensation. In order to collect moisture, the cold coil must be made much colder than the desired room temperature. This process often requires a great deal of subsequent re-heating, thus lowering the COP of traditional ACs. According to an MIT researcher, extracting moisture from the air takes up about half of the electricity used by traditional ACs. The use of desiccant materials is therefore a promising alternative. 


A start-up co-founded by MIT researchers has reported using a class of materials called metal organic frameworks, or MOFs, which passively extract moisture from the air. The intriguing properties of MOFs stem from their large internal surface area and the ability to fine-tune the size of the tiny chambers that run through them. The MIT researchers had previously developed MOFs with chambers just big enough to trap the water molecules in the air. 


In the system being developed by the start-up, the MOFs passively collect moisture as air enters the system. The waste heat is then used to dry the MOF material for continuous reuse. Furthermore, the system uses R32 as the refrigerant, which has a lower global warming potential than other commonly used HFC refrigerants. According to the start-up, the prototype air conditioner is close to commercialisation.  


Polyimide membranes [2] 


Dehumidifiers remove moisture from the air to a comfortable level of dryness, thereby improving air quality. Most commonly available dehumidifiers use refrigerants. Researchers have thus investigated more environmentally friendly alternatives.


For instance, naturally materials known as zeolites have been widely considered for their drying action. Unlike refrigerants, zeolites are desiccants capable of absorbing moisture into their water-attracting or hydrophilic pores. However, zeolites are expensive to synthesise and have limited mechanical properties.


In a recent study, Texas A&M University researchers improved the dehumidification efficiency of an existing and rather robust polymeric material, called polyimides.


Polyimides are cost-effective organic materials that are well-known for their high stiffness and tolerance to heat and chemicals. In order to enhance the dehumidification properties of the polyimide material, the researchers triggered a chemical process called hydrolysis, which led to the formation of water-attractive percolation channels within the polyimide material. The polyimide membrane they created was capable of extracting excess moisture from the air by trapping it in the percolation channels. The researchers noted that these membranes could operate continuously since in a standard dehumidifier, the trapped water molecules exit on the other side through a pre-installed vacuum pump. 


Although polyimide membranes have shown great potential for dehumidification, the researchers admit that their performance still lags behind zeolite membranes and that further optimization is needed. Nevertheless, they believe that polyimide membranes could help develop a generation of energy-efficient HVAC and dehumidifier technologies with a lower carbon footprint than current systems.