1. Introduction
The energy requirements of many countries for refrigeration and air conditioning (AC) systems are dramatically increasing [
1,
2,
3]. AC devices account for 20–30% of the worldwide electricity consumption of buildings [
4,
5], which corresponds to at least 10% of global electricity [
4]. Considering these numbers and thinking about the non-postponable necessity to reduce electricity consumption and obtain wide-scale decarbonisation, the efficiency enhancement of the AC sector is crucial. AC systems (Heating Ventilation and Air Conditioning, HVAC), ensure thermo-hygrometric comfort in an indoor environment and operate a dehumidification process over the airflow that they treat. The latent heat of the building load (i.e., the water vapour content of the air treated by the system) could account for a great part of the total load (around 30–40%) [
6], with greater value in humid climates or in applications where consistent outdoor airflow is required. In traditional HVAC systems, the dehumidification process is driven by a cooling coil (e.g., the evaporator of a Vapour Compression Refrigeration cycle, VCR). To obtain the dehumidification, the coolant of the coil is kept at a temperature much lower than the dew point temperature of the treated airflow, resulting in an over-cooling of the airflow (energy waste) and with a consequent necessity of re-heating. Moreover, these low temperatures of the coolant lead to a low value of the Energy Efficiency Ratio (EER) of the VCR device, with huge electricity consumption. A strategy to minimise the impact of these problems is the utilisation of materials able to remove water vapour contained in the airstream. Between them, an important place is occupied by the adsorbent (or desiccant) materials [
7]. Once put in contact with an airflow, these materials can remove part of the water vapour content from it. In this way, sensible and latent heat could be managed separately [
5]. Depending on thermal load and environmental conditions, it is possible to achieve consistent savings. For example, Mazzei et al. showed energy savings of up to 35% using desiccant materials in the HVAC systems of a reference Italian building [
8].
Ideally, a sorbent material capable of taking up water from the air should show hydrolytic stability and a relevant adsorption capacity, maintaining its efficient performance over a multitude of uptake-release cycles. Moreover, the regeneration process must require low energy consumption. Desiccant materials act through adsorption processes, taking up water molecules on their surface. This behaviour is well described by the adsorption isotherms, graphs defined by a non-linear relationship between the amount of the adsorbate and the relative humidity measured at defined temperatures. The isotherm trend describes the mass transfer between the aqueous vapour and a porous matter. A suitable material thoughtfully designed for the dehumidification process should be able to adsorb water even at low relative humidity (0.05 < RH < 0.40) in order to provide a low energy demanding adsorbent technology. Thereby, a step-function isotherm represents the best adsorption performance.
During the past few decades, the most common adsorbent solids employed for water harvesting have been silica-gels and zeolites [
7,
9,
10,
11]. Concerning the International Union of Pure and Applied Chemistry (IUPAC) classification of physisorption isotherms [
12], microporous silica-gels generally present a type II trend, which reflects slow uptake kinetics. On the other hand, although microporous zeolites exhibit a steep uptake tendency at low relative humidity, due to the presence of numerous and strong binding sites for water molecules in their structure, the recovery of the material is energetically costly. Indeed, high temperatures are required to evacuate zeolite porous frameworks from the adsorbed water molecules. This is the main reason for the incomplete degassing process, which translates into low cycling durability for this kind of material [
13]. Recently, a zeolite material was proposed as a high-performance water adsorbent, exhibiting fast adsorption kinetic hydrothermal stability, and most importantly, a lower temperature regeneration around 65 °C [
14]. Authors in the paper well characterised the adsorption process, describing how water molecules first coordinate the aluminium centres (at RH = 0.01) and then start to fill the 1D pores, forming a dense H-bonding network once the vapour pressure increases; however, defining a mechanism characterized by a low enthalpy value can lead to a low-cost regeneration process. This study explains how the water affinity of the adsorbers drives the adsorption process, maintaining high uptake efficiencies, without increasing the energy consumption for the water desorption. This major challenge can be addressed via a targeted structural design of the material. Consequently, many studies are focusing on the development of tunable materials.
In this regard, very promising adsorbents that are successfully emerging for this purpose have been identified in Metal-Organic Framework (MOF), a class of hybrid organic-inorganic crystalline materials built upon metal-containing units linked to organic ligands through coordination bonds extending within a regular and porous framework. Their robustness, porosity and uncommonly high specific surface area (up to 6000 m
2/g) have made MOFs an attractive proposal in adsorption applications [
6,
15,
16,
17]. Nowadays, theoretical and experimental studies on MOF adsorption abilities have demonstrated great loading and selectivity performances for specific gasses, such as CO
2 and many volatile organic compounds [
18,
19,
20]. In this sense, MOFs have gained great consideration for gas-cleaning applications.
MOFs benefit from the possibility of modulating their final properties by a careful design of their reticular structures, thus broadening their range of applications. This representative feature differentiates MOFs from other materials such as their inorganic analogues (i.e., zeolites). The tunability of MOF structures can be exploited to provide suitable adsorption profiles for a high-efficient water capture system (as HVAC airflows dehumidification) [
21]. Therefore, owing to their large specific surface area, low framework density and wide-reaching tunable porosity, MOFs can exceed potential traditional microporous adsorbent materials in terms of water adsorption capability. The water adsorption process in a MOF is mediated by chemisorption occurring on the open metal sites, physisorption characterized by the formation of weak interactions with the framework and capillary condensation, which occurs when large pores are present. Since the regeneration of material is strongly connected to these events, tuning the structural properties of the MOF is a valuable pathway to control the desired adsorption and desorption qualities, with the aim of achieving exceptional water harvester systems [
22]. Yagi et al. gave efforts to the progress that these functional materials have shown as harvesters for water, mostly in terms of their hydrolytic resilience and their tunable porosity [
23]. The Authors underlined the correlation between water uptake and pore volume, emphasizing the need to overcome the energy barrier during the regeneration process of MOF structures displaying large pores. In these cases, hysteresis phenomena are present in water adsorption isotherms, due to irreversible capillarity condensation, responsible for the high-temperature demand for water release. Thus, the pore size should be designed in terms of both high moisture capture capacity and relatively easy material regeneration, for example, by modulating the length of the organic linker or by introducing chemical functionalization during post-synthetic modification [
24].
It is crucial to understand the water uptake mechanism by evaluating how the water molecules populate their binding sites within the MOF structure. In this sense, X-ray diffraction in combination with Density-Functional Theory calculations can be appropriate tools, as demonstrated by Hanikel et al. with the studies performed on MOF-303, which exhibits water adsorption with a steep trend at very low RH [
25]. Understanding the hierarchically filling of the pores was demonstrated to be essential in order to induce a more adequate water uptake behaviour. In particular, they proposed a multivariate approach for the modification of the architecture of the pores, finding the most appropriate material design in compliance with the water adsorption enthalpy values, the limiting desorption temperature, the stability and the water capacity. In addition, the condition of the adsorption mechanism can be varied to understand the mechanism of water harvesting as was reported by Yanagita et al. [
26]. In detail, they measured the adsorption/desorption isotherms of chromium terephthalate MIL-101 and the time trend of the amount of adsorbed water by stepwise modification of the relative humidity, describing how the porosity and the hydrophobic-hydrophilic structural composition of the MOFs drive the uptake/release kinetic of the water. Based on these all considerations, it is evident that a proper design of the material is a reasonable strategy to allow a successful material development for water uptake applications.
In the literature, it is possible to find few works that deal with the utilisation of MOF materials as desiccants in HVAC applications. Some of them propose using the traditional desiccant wheels coated with MOF materials. Bareschino et al. performed a numerical simulation of a desiccant wheel with MOF MIL-101 [
27], including a gas-side resistance model to define the behaviour of MOF material. Dehumidification effectiveness of 30% better than the silica-gel wheel is demonstrated. The Authors estimated a reduction of 20.5% in CO
2 emissions compared to the HVAC system with silica-gel. Shahvari et al. explained the functioning of a MOF desiccant wheel [
15], proposing a complete first-principle analysis validated with experimental tests. They showed that MOF-based wheels are regenerable at lower temperatures (40–60 °C) compared to silica-gel (80–140 °C), with consistent energy saving for regeneration (10–50%, depending on environmental conditions). Shahvari obtained promising results for a system that integrates MOF-assisted dehumidification with indirect evaporative cooling [
28]. Again, the low regeneration temperatures (40–75 °C) have been confirmed, with consequent gain in the efficiency of the MOF system with respect to the silica-gel system (MOF system has an efficiency 2.7–6 times higher). The idea of using MOF-coated desiccant wheels has been well-reviewed and studied, as in the publication [
29] and in the work of Wang [
30], where the MOF materials are cited as good desiccant options due to their low regeneration temperatures (30–60 °C). Cui et al. proposed to coat the surface of the coils of a VCR cycle with MIL-100 (Fe) (
Figure 1), [
6].
In adsorption mode, the evaporator acts as a cooler and dehumidifier while the waste heat of the condenser regenerates the wet MOF. The MOF-coated evaporator removes at the same time as the sensible and latent heat of the airflow, operating an isothermal-dehumidification. In the paper, the low regeneration temperature of the material (about 50 °C) is highlighted. With a theoretical EER of 7.9 in typical European summer conditions, the estimated energy-saving is 36.0% compared to traditional dehumidification with a cooling coil. The idea of coating a heat exchanger with desiccant material has been reported by review papers of Saeed et al. [
31] and Venegas et al. [
32], which considered MOFs as material to be used for the purpose. Even if this solution is not directly applied in the HVAC system, it is important to cite in the context of the control of thermo-hygrometric parameters of indoor environments and the energy-enhancement of HVACs, which was proposed by Feng et al. [
16]. They proposed, in accordance with the results of a lumped-model simulation, to install a MIL-100 (Fe) in an indoor environment with the role of a moisture buffer; the removal of indoor humidity accounts for 73.4% of latent heat, leading to minor efforts of the HVAC system.
In this paper, the Authors theoretically compare a HVAC system that operates traditional dehumidification with a HVAC system equipped with a MOF-Assisted Dehumidifier (MAD). First of all, the reference psychrometric transformations are shown. Then, the mathematical model used in the simulations is presented. The two kinds of dehumidification are compared by considering a case study, based on real boundary conditions. The benefits, in terms of energy consumption, that characterise MOF-assisted dehumidification are quantified, also highlighting better results obtainable with MOF materials compared to other desiccant materials, in terms of consumption for the regeneration process. The obtained results lead the Authors to consider this technology as a very competitive option in the HVAC sector, and it inspires us to further research this topic.
6. Conclusions
In this paper, the importance of using desiccant materials for the energy enhancement of HVAC systems has been discussed. Desiccant materials can dehumidify the airflows, removing the latent heat, with better performances of cooling devices and consequent energy savings. Among the many available materials, attention has been addressed to Metal-Organic Framework (MOF) materials, which have unique adsorption characteristics (high water uptake, low regeneration temperatures) compared to other known desiccants (i.e., silica-gel).
In the first part of the paper, a general overview of MOFs is presented, in particular emphasising the possibility to control the design of their structures, aimed at the desired properties. Moreover, the application of MOFs in the HVAC context has been reviewed. The literature analysis shows that the big advantage of MOFs over silica-gel is the low regeneration temperature (40–60 °C against 90–100 °C). The proposed solutions are the desiccant wheel and exchangers coated with MOFs and cooled by a refrigerant.
Then, a comparison between a traditional HVAC (with dehumidification operated by a cooling coil) and a HVAC equipped with a MOF-Assisted Dehumidifier (MAD) has been discussed. The mathematical model of the two HVAC alternatives is applied to a real case study. The energy savings achievable with MOF dehumidification are in the range of 30–50%, depending on the outdoor air conditions and the repartition between sensible and latent heat. Comparing the MOF material with other desiccants (silica-gel and a new zeolite), the MOF-driven dehumidification shows the highest water uptake and the lowest regeneration temperature. The obtained results lead us to consider the utilisation of MOF-assisted dehumidification as a very competitive strategy to reduce the energy consumption of the HVAC sector, making decarbonization closer, and inspiring the Authors to make deeper studies on this topic.