Emerging sorption pairs for heat pump applications: an overview

When Polymorphism in Metal-Organic Frameworks Enables Water Sorption Profile Tunability for Enhancing Heat Allocation and Water Harvesting Performance

The development of thermally driven water-sorption-based technologies relies on high-performing water vapor adsorbents. Here, polymorphism in Al-metal-organic frameworks is disclosed as a new strategy to tune the hydrophilicity of MOFs. This involves the formation of MOFs built from chains of either trans- or cis- µ-OH-connected corner-sharing AlO4(OH)2 octahedra. Specifically, [Al(OH)(muc)] or MIP-211, is made of trans, trans-muconate linkers, and cis-µ-OH-connected corner-sharing AlO4(OH)2 octahedra giving a 3D network with sinusoidal channels. The polymorph MIL-53-muc has a tiny change in the chain structure that results in a shift of the step position of the water isotherm from P/P0 ≈ 0.5 in MIL-53-muc, to P/P0 ≈ 0.3 in MIP-211. Solid-state NMR and Grand Canonical Monte Carlo reveal that the adsorption occurs initially between two hydroxyl groups of the chains, favored by the cis-positioning in MIP-211, resulting in a more hydrophilic behavior. Finally, theoretical evaluations show that MIP-211 would allow achieving a coefficient of performance for cooling (COPc) of 0.63 with an ultralow driving temperature of 60 °C, outperforming benchmark sorbents for small temperature lifts. Combined with its high stability, easy regeneration, huge water uptake capacity, green synthesis, MIP-211 is among the best adsorbents for adsorption-driven air conditioning and water harvesting from the air.

High-performance sorbents from ionic liquid activated walnut shell carbon: an investigation of adsorption and regeneration

In this paper, walnut shells were selected to make activated charcoal using ionic activators. Based on the physical/chemical activation process and the properties of activated carbon products, the Fourier Transform Infrared reflection and Brunauer-Emmett-Teller analysis methods were adopted to comparatively analyse activation principles and pore-structure parameters. Also ciprofloxacin adsorption was compared among various activated carbon. Then, an absence of microporous structure in both walnut shells and their carbonized derivatives was found. Moreover, the specific surface area of activated carbon, prepared via KOH wet activation within physical/chemical procedures, attains a noteworthy 1787.06 m² g^-1, underlining its commendable adsorption performance. The specific surface areas of five distinct activated carbons, processed via ionic activation, extend from 1302.01 to 2214.06 m² g^-1. Concurrently, the micropore volumes span from 0.47 to 0.93 cm³ g^-1. Obviously, the adsorption proficiency of ion-activated carbon markedly exceeds that of carbons activated physically or chemically. Of all materials investigated in this paper, ion-activated carbon D consistently exhibits superior performance, maintaining a ciprofloxacin removal rate nearing 100% at 40 °C. Remarkably, the maximum regeneration frequency of ion-activated carbons can reach up to 10 cycles. In conclusion, these five ion-activated carbons, demonstrating superior pore-structure parameters and adsorptive capacities, outperform those prepared through physical/chemical methods. They emerge as promising contenders for new, high-performing adsorbents.

An Industrial Approach for the Optimization of a New Performing Coated Adsorber for Adsorption Heat Pumps

In the present work, the optimization of a new coating formulation was investigated, taking attention to an industrially focused research approach used for the engineering design of the adsorber. The adsorbent was a composite zeolite or silica-gel based coating applied by using new flexible polymer matrices. The FC-80 formulation represents a good compromise between mechanical stability and absorption capacity. Using the developed coating process, a new compact HEX design was developed to reach the AHP target performance with easy and fast manufacturing. The specific cooling power of the coated heat exchanger was estimated to be about 500 W/kg of adsorbent. The new coated HEX was integrated in a new adsorption chiller and has been tested by a laboratory test-rig under realistic operating conditions. Results of preliminary testing demonstrated that the prototype provides a cooling capacity of around 10 kW with a COP of 0.54.

Silver-Nanoparticle-Assisted Modulation of NH3 Desorption on MIL-101

Ammonia has recently emerged as a promising hydrogen carrier for renewable energy conversion. Establishing a better understanding and control of ammonia adsorption and desorption is necessary to improve future energy generation. Metal-organic frameworks (MOFs) have shown improved ammonia capacity and stability over conventional adsorbents such as silica and zeolite. However, ammonia desorption requires high temperature over 150 °C, which is not desirable for energy-efficient ammonia reuse and recycling. Here, we loaded silver nanoparticles from 6.6 to 51.4 wt% in MIL-101 (Ag@MIL-101) using an impregnation method to develop an efficient MOF-based hybrid adsorbent for ammonia uptake. The incorporation of metal nanoparticles into MIL-101 has not been widely explored for ammonia uptake, even though such hybrid nanostructures have significantly enhanced catalytic activities and gas sensing capacities. Structural features of Ag@MIL-101 with different Ag wt% were examined using transmission electron microscopy, X-ray powder diffraction, and infrared spectroscopy, demonstrating successful formation of silver nanoparticles in MIL-101. Ag@MIL-101 (6.6 wt%) showed hysteresis in the N₂ isotherm and an increase in the fraction of larger pores, indicating that mesopores were generated during the impregnation. Temperature-programmed desorption with ammonia was performed to understand the binding affinity of ammonia molecules on Ag@MIL-101. The binding affinity was the lowest with Ag@MIL-101 (6.6 wt%), including the largest relative fraction in the amount of desorbed ammonia molecules. It was presumed that cooperative interaction between the silver nanoparticle and the MIL-101 framework for ammonia molecules could allow such a decrease in the desorption temperature. Our design strategy with metal nanoparticles incorporated into MOFs would contribute to develop hybrid MOFs that reduce energy consumption when reusing ammonia from storage.

Synergistic Effect of MIL-101/Reduced Graphene Oxide Nanocomposites on High-Pressure Ammonia Uptake

Ammonia has emerged as a potential working fluid in adsorption heat pumps (AHPs) for clean energy conversion. It would be necessary to develop an efficient adsorbent with high-density ammonia uptake under high gas pressures in the low-temperature range for waste heat. Herein, a porous nanocomposite with MIL-101(Cr)-NH₂ (MIL-A) and reduced graphene oxide (rGO) was developed to enhance the ammonia adsorption capacity over high ammonia pressures (3-5 bar) and low working temperatures (20-40 °C). A one-pot hydrothermal reaction could form a two-dimensional sheet-like nanocomposite where MIL-A nanoparticles were well deposited on the surface of rGO. The MIL-A nanoparticles were shown to grow on the rGO surface through chemical bonding between chromium metal centers in MIL-A and oxygen species in rGO. We demonstrated that the nanocomposite with 2% GO showed higher ammonia uptake capacity at 5 bar compared with pure MIL-A and rGO. Our strategy to incorporate rGO with MIL-A nanoparticles would further be generalizable to other metal-organic frameworks for improving the ammonia adsorption capacity in AHPs.

Discovery of Colossal Breathing-Caloric Effect under Low Applied Pressure in the Hybrid Organic-Inorganic MIL-53(Al) Material

In this work, "breathing-caloric" effect is introduced as a new term to define very large thermal changes that arise from the combination of structural changes and gas adsorption processes occurring during breathing transitions. In regard to cooling and heating applications, this innovative caloric effect appears under very low working pressures and in a wide operating temperature range. This phenomenon, whose origin is analyzed in depth, is observed and reported here for the first time in the porous hybrid organic-inorganic MIL-53(Al) material. This MOF compound exhibits colossal thermal changes of ΔS ∼ 311 J K^-1 kg^-1 and ΔH ∼ 93 kJ kg^-1 at room temperature (298 K) and under only 16 bar, pressure which is similar to that of common gas refrigerants at the same operating temperature (for instance, p(CO₂) ∼ 64 bar and p(R134a) ∼ 6 bar) and noticeably lower than p > 1000 bar of most solid barocaloric materials. Furthermore, MIL-53(Al) can operate in a very wide temperature range from 333 K down to 254 K, matching the operating requirements of most HVAC systems. Therefore, these findings offer new eco-friendly alternatives to the current refrigeration systems that can be easily adapted to existing technologies and open the door to the innovation of future cooling systems yet to be developed.

Water Vapor Adsorption on CAU-10-X: Effect of Functional Groups on Adsorption Equilibrium and Mechanisms

Metal-organic frameworks (MOFs) possess unique flexibility of structure and properties, which drives them toward applications as water adsorbents in many emerging technologies, such as adsorptive heat transformation, water harvesting from the air, dehumidification, and desalination. A deep understanding of the surface phenomena is a prerequisite for the target-oriented design of MOFs with the required adsorption properties. In this work, we comprehensively study the effect of functional groups on water adsorption on a series CAU-10-X substituted with both hydrophilic (X = NH2) and hydrophobic (X = NO2) groups in the linker. The adsorption equilibrium is measured at P = 7.6-42 mbar and T = 5-100 °C. The study of water adsorption by a set of mutually complementary physicochemical methods (TG, XRD in situ, FTIR, and 1H NMR relaxometry) elucidates the nature of primary adsorption sites and water adsorption mechanisms.

Equilibria and Isosteric Heat of Adsorption of Methane on Activated Carbons Derived from South African Coal Discards

Isosteric heat of adsorption (H _st) is critical for evaluating the thermal effects of adsorption-based storage systems. Poor management of the thermal effects of an adsorptive storage system often alters the overall performance of the storage system. In this study, methane equilibrium uptake on activated carbons derived from coal discards and isosteric heat of adsorption were evaluated. The methane adsorption capacity of the produced activated carbons was measured using a high-pressure volumetric analyzer. The isotherm results in temperature ranges of 0-50 °C and pressure of up to 40 bar are analyzed using the Langmuir, Tóth, and Dubinin-Astakhov (DA) isotherm models. The results showed that, for the two activated carbons, the DA model was the best fit. In addition, we evaluated the isosteric heat of adsorption using two theoretical frameworks, Maxwell's thermodynamic relations and the modified Polanyi potential function. The Tóth potential function and Clausius-Clapeyron equations were applied to the Dubinin-Astakhov adsorption model to obtain an analytical expression of H _st. Both methods were compared, and the result showed an overall error margin between 6 and 12%. The values of H _st obtained are over a range of 10-17 kJ/mol. It was observed that H _st decreases with an increase in methane fractional load. The H _st values obtained are useful in designing an efficient thermodynamic scheme for the ANG storage system.

Recent Developments in Coating Technologies for Adsorption Heat Pumps: A Review

The use of adsorbent heat pumps as heating and cooling systems is particularly relevant thanks to their ability to exploit low-grade heat (e.g., below 90 °C) from renewable energy sources and waste energy streams with prospective applications in several fields, e.g., industrial and residential. Their development began in the 20th century and is still in full evolution. The great interest in their improvement and optimization was determined by some key factors inherent to their sustainability compared to traditional refrigeration systems (e.g., low electricity consumption and the low environmental impact of the employed refrigerants). Recently, strong efforts have been dedicated to increasing the achievable heating/cooling power density of this technology through the development of innovative adsorbent coating technologies. Indeed, the deposition of thin coatings on the surface of the heat exchanger could reduce the heat transfer resistance existing on the adsorbent material side, thus increasing the overall adsorption dynamics. Three main approaches have been assessed, namely a thick consolidated bed, binder-based composite coatings and in situ directly crystallization coatings. This paper provides a brief overview of some of the main achievements related to adsorbent coating technology developments for adsorption heat pump applications.

Advanced Adsorbent Materials for Waste Energy Recovery

Nowadays, waste thermal energy represents a huge quantity of energy that, in most cases, is unfortunately dispersed rather than recovered. Although it is well known that its recovery could result in a considerable impact reduction of human activities on the environment, it is still a challenging issue. In view of this, absorption chillers and heat pumps, based on the use of porous materials capable of reversibly adsorbing and desorbing water vapor, can be considered among the preferred systems to recover waste thermal energy, especially at medium–low temperatures. This study deals with the preparation and performance of a new generation of advanced adsorbent materials specifically produced as coatings for water adsorption systems driven by low temperature heat sources (around 150 °C). The proposed coating consists of hybrid SAPO-34/polyacrilonitrile microfibers directly deposited on the surface to be coated by means of the electrospinning technique. Their zeolite morphology and concentrations, as well as their distribution over the polymeric microfibers, were key variables in achieving the best combination of adsorption properties and hydrothermal stability of the coating.

A Statistical Approach to Determine Optimal Models for IUPAC-Classified Adsorption Isotherms

Adsorption heat transformation (AHT) systems can play a major role in protecting our environment by decreasing the usage of fossil fuels and utilizing natural and alternative working fluids. The adsorption isotherm is the most important feature in characterizing an AHT system. There are eight types of International Union of Pure and Applied Chemistry (IUPAC) classified adsorption isotherms for different “adsorbent-adsorbate” pairs with numerous empirical or semi-empirical mathematical models to fit them. Researchers face difficulties in choosing the best isotherm model to describe their experimental findings as there are several models for a single type of adsorption isotherm. This study presents the optimal models for all eight types of isotherms employing several useful statistical approaches such as average error; confidence interval (CI), information criterion (ICs), and proportion tests using bootstrap sampling. Isotherm data of 13 working pairs (which include all eight types of IUPAC isotherms) for AHT applications are extracted from literature and fitted with appropriate models using two error functions. It was found that modified Brunauer–Emmet–Teller (BET) for Type-I(a) and Type-II; Tóth for Type-I(b); GAB for Type-III; Ng et al. model for Type-IV(a) and Type-IV(b); Sun and Chakraborty model for Type-V; and Yahia et al. model for Type-VI are the most appropriate as they ensure less information loss compared to other models. Moreover; the findings are affirmed using selection probability; overall; and pairwise proportion tests. The present findings are important in the rigorous analysis of isotherm data.

A Review of Heat Pump Systems and Applications in Cold Climates: Evidence from Lithuania

Similar to other cold climate countries, space heating and domestic hot water (DHW) accounts form the largest share of household energy demand in Lithuania. Heat pump technology is considered to be one of the environmentally friendly solutions to increase energy efficiency and reduce the carbon footprint of buildings. Heat pumps have been finding their way into the Lithuanian market since 2002, and currently there are many good practice examples present in the country, especially in the residential and public sectors. Heat pump use is economically advantageous in the Baltic Region, and the market share of these systems is growing. Studies have reported seasonal performance factor (SPF) ranges within 1.8 and 5.6. The lower SPF values are typically attributable to air source heat pumps, whereas the higher efficiency is achieved by ground or water source heat pump applications. While the traditional heat pump techniques are well established in the region, there is a slow uptake of new technologies, such as solar-assisted heat pumps, absorption heat pumps and heat pumps integrated into foundations, tunnels or diaphragm walls. This paper provides a critical review of different heat pump technologies, using Lithuania as a cold climate case study to overview the market trends within the European context. Potential trends for the heat pump technology development in terms of application areas, cost-benefit predictions, as well as environmental aspects, are discussed.

Lithium Hydroxide Reaction for Low Temperature Chemical Heat Storage: Hydration and Dehydration Reaction

As a key parameter of a chemical heat storage material, the hydration and dehydration reaction characteristics of lithium hydroxide (LiOH) at pure vapor condition is unclear. In this study, we focused on the hydration reaction and dehydration process of LiOH at the pure vapor condition. The pressure–temperature diagram of LiOH equilibrium was measured. The hydration and dehydration of LiOH at various conditions have been experimentally investigated. The results show that the steam diffusion can be greatly enhanced at vacuum condition. A thin layer of LiOH is uniformly dispersed in the reactor, which can greatly increase the heat transfer between the LiOH material and reactor, leading to a higher hydration reaction rate of LiOH. Furthermore, the steam pressure, reaction temperature, and the particle size of LiOH can greatly influence the hydration reaction. A maximum hydration reaction rate of 80% is obtained under the conditions of 47 °C, steam pressure of 9 kPa, and particle size of 32–40 μm. LiOH exhibits a different reaction property at the condition of pure steam without air and below atmospheric pressure. A store and reaction condition of LiOH with isolation of air is recommended when apply LiOH as a heat storage material at low temperature.

Manufacturing and Assessment of Electrospun PVP/TEOS Microfibres for Adsorptive Heat Transformers

A new adsorbent coating for the adsorber unit of an adsorption heat pump made of hybrid, organic–inorganic microfibres was prepared and characterized. Different coatings were obtained by the electrospinning of polyvinylpyrrolidone (PVP) solutions added with different quantities of tetraethyl orthosilicate (TEOS). PVP is a polymer with water adsorption capability and the TEOS addition allowed to increase the thermal stability of microfibres. The aim, indeed, was to preserve the polymeric structure of microfibres in order to obtain coatings with high flexibility and mechanical strength. The results demonstrated that TEOS concentrations in the range of 5–13 wt.% produced microfibre coatings of non-woven textile structure with both good water affinity and good thermal stability. SEM images of coatings showed that the deposited microfibre layers have both a high surface area and a high permeability representing a significant advantage in adsorption systems.