Advances in Desiccant Dehumidification

Population increase, rapid industrialization, economic growth, and increased demand for thermal comfort have exponentially raised buildings’ air-conditioning requirements. Over 90% of the current air-conditioning market is dominated by mechanical vapor compression (MVC) technology, which is essentially a coupled condensation dehumidification-cooling method. Its energy efficiency is low due to significant overcooling and reheating of the process air. In addition, its high electricity consumption contributes to substantial CO₂-equivalent emissions and significantly impacts climate change. If the cooling demand continues to grow at the present rate, the MVC cooling technology will alone contribute to almost 0.5 °C rise in global temperatures by 2100s. Therefore, energy-efficient alternative cooling technologies are essential to address the sharp rise in building energy. Scientists worldwide are evolving new cooling technologies and have categorized them into solid-state, electrically driven mechanical, and thermally driven alternatives. Among the 20 alternatives that exist in different stages of research, prototyping, and commercialization, desiccant dehumidification technology decouples latent dehumidification and sensible cooling and has demonstrated to be an excellent solution to promote the energy efficiency of the conventional cooling process. This chapter broadly discusses the future of cooling, highlights key sustainable alternative technologies, and comprehensively reviews the merits of solid desiccant dehumidification technologies.

Desiccant coated heat exchangers (DCHEs) yield higher dehumidification and thermal efficiency over other solid desiccant dehumidifiers due to their effective removal of the exothermic heat of sorption and improved heat transfer effectiveness. Accordingly, they offer prospective energy and cost savings to several energy-related applications such as heat pumps, chillers, water harvesters, etc. A comprehensive review of the current state-of-the-art in DCHEs is imperative to understand this technology’s marked impact and its performing strategies and capabilities. This chapter first introduces the different types of isotherm and hysteresis profiles and specifies the adsorption mechanisms. Then it presents a list of conventional pure/composite desiccants employed and highlights their limitations. Next, the detailed steps involved during its binder material selection are presented, and a comparison is made between the different types of coating techniques. Different regeneration techniques are then described, and the relevance of thermal regeneration vis-à-vis microwave and ultrasonic methods is established. Lastly, the ideal characteristics of a desiccant are listed, which would pave the way for performance-enhancing synthesis of advanced desiccant materials.

The performance of desiccant-coated heat exchangers (DCHEs) strongly depends on the sorption and desorption characteristics of the employed desiccant material. While several attempts have been made to synthesize superior desiccants, the DCHE performance has not reached its highest potential due to challenges associated with the limited working capacity of the conventional pure/composite desiccants. This chapter presents two novel advanced desiccants, namely, composite superabsorbent polymers (SAPs) and metal-organic frameworks (MOFs). The composite SAPs offer superior sorption capacity, faster kinetics, and low regeneration possibility while the MOFs providing excellent hydrophilicity and tailorable structures. These desiccants are deemed to be the next generation of advanced materials in thermally driven dehumidifiers. Aside from documenting the characteristics of these new materials, a detailed list of experimental and theoretical material characterization studies is provided. Isotherms and kinetics are identified as key properties that fundamentally govern the desiccant’s performance in dehumidifiers, and relevant experimental and regression techniques to analyze them are also presented. In addition, the transient performance of the superabsorbent polymer DCHEs is compared and benchmarked against silica gel-coated heat exchangers. Lastly, results from a series of parametric experiments are presented by varying different operating parameters, and their sensitivity towards dehumidification capacity and thermal energy efficiency is estimated.

In contrast to experimental testing, which is time-consuming and expensive, theoretical studies provide an economical and judicious methodology to evaluate the performance of desiccant dehumidifiers. Such approaches, developed based on the fundamental laws of thermodynamics, flow physics, and heat transfer, are critical to improving the dehumidifier design. This chapter firstly presents a general mathematical approach to predict the performance of DCHEs based on the governing principles of mass, momentum, energy, and species conservation. Next, the model is validated with the experimental results for different types of desiccants. Further, the validated model is employed to predict the performance of new fin-tube configurations. Detailed energy and economic analysis are then conducted on a hybrid central air-conditioning system comprising a composite superabsorbent polymer and mechanical vapor compression chillers. The hybrid configuration’s electrical power savings and its payback period are computed via this analysis. Lastly, the second law of thermodynamics is applied to study DCHEs and identify the causes of key irreversibility.

The enhanced energy performance of desiccant dehumidifiers through new material synthesis and optimized design have extended their potential to several state-of-the-art energy-related applications including heat transformation, adsorption chilling, energy storage, and water harvesting. A review of the latest research trends in these system-level applications is presented in this chapter. Additionally, controlling methods to yield precise moisture levels for human thermal comfort and various industrial manufacturing processes are described. For human well-being, both extreme low and high humidity levels can cause health issues such as dryness and nausea including critical illnesses due to the airborne spread of bacteria, fungal, and viral infections. In industrial operations, the presence of high moisture levels may damage machines, deteriorate product quality, and ultimately lead to significant revenue losses. For such critical applications, desiccant dehumidifiers offer close and precise moisture control in an energy-efficient and cost-effective manner.