Within the western society, air conditioning is an essential part of a comfortable lifestyle. Common opinion may even suggest it’s a basic necessity tied to shelter. These Heating, Ventilation, and Air Conditioning (HVAC) systems are conventionally driven by the refrigerant vapor compression cycle and are able to reach a wide temperature range (Jie Lin et al. 2017). In this way its effective in providing for heating and cooling needs, but it is not efficient in it processes – more often than not overcooling air while condensing moisture from it. Decoupling the needs, cooling and dehumidifying, is an approach promising more energy efficiency. Therefore desiccant based systems have gained increasing interest as an alternative air conditioning technology (Zheng, Ge, and Wang 2014). This review will explore the differing materials used as desiccant dehumidifiers as well as its relation to the combined system employed.
The first key area of this review deals with the progresses in solid desiccant cooling (SDC) system in regards to desiccant material. The material summaries included are of composite desiccants, nanoporous inorganic materials, polymeric desiccants, and absorption isotherms. Among the various kinds of novel desiccants, composite desiccants are most frequently used in SDC systems (Zheng, Ge, and Wang 2014). This material is formed by impregnating hydroscopic salt to the pores of a porous host material. Conventional host materials such as silica gels, mesoporous silicate, active carbon, natural rock and clay, and so forth are advantageous due to their stable chemical characteristics, low cost. However, the disadvantage of low adsorption capacity leads to the huge size of SDC units (Zheng, Ge, and Wang 2014). More advanced materials such as aluminosilicate zeolites, aluminophosphates based sieves, aerogels, metal-organic frameworks polymers (MOFs), surpass the water adsorption capacities of any conventional desiccant material. The authors importantly note that each reviewed compound have respective shortcomings in regards to the SDC application. For instance, high cost and lab-scale limitations for MOFs, relatively high regeneration temperature for composite desiccants, and low water adsorption capacity quantities for nanoporous inorganic materials – zeolites, sieves, and aerogels. This comparative analysis and equal critique on each material as it relates to practical uses is appreciated because it allows the reader to understand the scope of necessary challenges for further exploration in the field. Additionally, the authors addressed an important factor little discussed in the other collected literature – cost, economies of scale, and long term durability. Silica gel and natural rock/mineral clays have low cost, varying water absorption capacities, and opposite regeneration requirements. In all materials these characteristics together can index the respective performances for comparison. The ability to address this may be due to the extensive research and knowledge on solid desiccants; in the past decades observations and investigations have optimized SDC systems (Zheng, Ge, and Wang 2014). This is extremely longer than that of liquid and membrane based systems within the recent years.
In regards to membrane-based dehumidification technology, Yang et al. (2015) summarizes recent research results, fundamental principles, membrane material composition, module structures, operational conditions, and theoretical models (Yang et al. 2015). Defined in the literature, membrane-based dehumidification is based on the moisture transfer driven by mass transfer potential and sensible heat transfer. The authors reveal that the most important performance characteristics of membranes lie in the materials’ selectivity and permeability. The former meaning that only vapor can transfer while air is obstructed. The latter relates to the degree of porosity. Although the two terns seem incompatible, Lin et al (2015) do a good job in describing exactly how one membrane with both qualities is fabricated. Herein also details a handful of different membrane composite assemblies. It is important to note that the researchers selected PVA-1 (PVA solution with LiCl as an additive) as the best material – having a high moisture permeability and high water selectivity with respect to VOCs.,, Although this is a helpful gesture, the lack of techno-economic factors discussed in relation to each material described leaves more to be desired. What is the price point? At what costs do we sacrifice quality? Are there economies or diseconomies of scale, especially in relation to building type and program? It may be beneficial to our research to compare these advanced membrane materials with common ones through the lens of accessibility and democratization.
Contrast to the broad attention to membrane materiality by Yang et al, “Thermodynamic Analysis…” focuses on an application of membrane technology. The literature proposes a desiccant-enhanced evaporative cooling system typology using the hybrid membrane liquid desiccant to provide for HVAC solutions. As alluded to previously, this type of system involves the decoupling of air humidity control and temperature control – the former relating to latent heat load, the later to sensible heat load. The authors’ claim of this system compared to the conventional that: “(1) the dehumidifier and cooler have less capital cost than the mechanical chiller, although they require larger physical footprint. Low-grade waste heat and solar energy can also be incorporated with the dehumidifier to reduce the electricity consumption and operational cost; (2) the dehumidifier and cooler cost less and have a simpler system layout than the absorption/adsorption chillers” (Jie Lin et al. 2017). Experiments conducted do not prove this but rather demonstrates a limited scope on thermodynamic performance as it relates to influential geometric parameters and properties and high humidity conditions. The experimentation and simulation setup involved developing a cross-flow membrane liquid desiccant dehumidifier and a counter-flow dew point evaporative cooler. The dehumidifier is comprised of a single pair of the air and solution channels, and one membrane sheet is sandwiched between the two channels. The membrane matrix was a porous polyvinylidene fluoride (PVDF) sheet with a layer of silica gel coated on the membrane surface, not only a more affordable assembly but also less advanced (and therefore needing less specialized training). I applaud this simplicity and believe that a very similar physical model could be adapted for our own experimentation. The results and conclusions informed us on the greatest impacts on performance. First, the dehumidification performance, its ability to reduce humidity, improves at longer lengths (affecting surface area) but worsens at greater heights (affecting overall thickness). Here it is important to note the affect only reduces the latent effectiveness not the total latent, sensible or cooling capacity. Secondly, the influence on the operating conditions, the various climatic weather conditions simulated, reveals that the hybrid system can temper air within the thermal comfort zone. The use of can rather than does is deliberate in that the researchers show many outcomes, some seemingly contradictory. The decision to illustrate such a wide range of testing conditions backfired, the information seemed exhaustive – introducing more controlled variables may have illustrated more poignant conclusions. These shortcomings bring awareness to what we may encounter in our own experimentation. The experimentation sought out in Panaras et al (2011) reveals these exact limitations and proposes control strategies to tests effectiveness and efficiency towards desired values and results. Unlike, previous sources the scope Panaras et al details is not one to propose a specific design preference but rather to demonstrate the operation of a proposal as it relates to controlling variables (Panaras, Mathioulakis, and Belessiotis 2011). First it lays out the “basic stages for the formulation of a control strategy” – the decision parameters to set prior to experimenting. The most important being to differentiate the operation mode of each test e.g. only testing cooling performance or dehumidification and cooling. Then one must select the appropriate controls to regulate appropriate values for the given operational parameters (Panaras, Mathioulakis, and Belessiotis 2011). The simplicity in this breakdown of methodology is extremely beneficial to a novice. It helps to frame and understand the sought after research question and hypothesis. Another great approach deduced is related to design parameters or variable processes during experimentation – air flow rate. Here, using a developed mathematical model, a specific air flow rate was calculated. Supplying relatively stable air conditions among different testing groups is important to achieve predictable performance outcome (Jie Lin et al. 2017). Considering we are not interested in the cooling capacity, having an ideal rate is not particularly necessary. However knowing and/or maintaining a specific and constant air flow rate may be beneficial for future considerations and analysis. All in all, these approaches presented suggests the need for repeatability and/or reproducibility among the scientific community.
In conclusion, these resources work together to form a basis of understanding in our research. More specifically it informs our understanding of desiccant material performance implications within a cooling system application. Although the literatures’ focus varies a commonality is shared – much opportunity in material research is ahead to find the optimal desiccant for efficient, low cost mass use in cooling technology. Herein is where our proposal lies.
Endnotes  PVA is polyvinylalcohol, a hydrophilic polymer.