Energy-Efficient Atmospheric Water Harvesting Using Multicyclic Sorption Systems and Waste Heat Integration

Original Authors: Xiangyu Li, Bachir El Fil, Buxuan Li, Gustav Graeber, Adela C. Li, Yang Zhong, Mohammed Alshrah, Chad T. Wilson, and Emily Lin

Original paper is accessible at: https://doi.org/10.1021/acsenergylett.4c01061

Atmospheric water harvesting (AWH) is increasingly viewed as a promising pathway to address water scarcity, particularly in arid and inland regions where conventional solutions such as desalination or fog harvesting are either infeasible or inefficient. Sorption-based atmospheric water harvesting (SAWH), in particular, has demonstrated the ability to operate under low relative humidity conditions (<30%), making it suitable for some of the most water-stressed environments.

However, despite significant advances in adsorbent materials—including zeolites, metal–organic frameworks (MOFs), and hygroscopic composites—the actual water production of most SAWH systems remains insufficient for practical deployment. The limitation is not purely material-related. Instead, it arises from a deeper engineering challenge: the mismatch between material capabilities and device-level design, especially in terms of heat and mass transfer.

Why cycling and energy source matter

Most existing SAWH systems rely on solar energy, which inherently limits operation to single daily adsorption–desorption cycles. While energy-efficient, this approach leads to slow kinetics and low water productivity. In contrast, systems powered by higher energy density sources—such as electricity or waste heat—enable multicyclic operation, significantly increasing water output.

Yet this introduces another challenge. Adsorption and desorption processes are governed by energy-intensive phase interactions, particularly due to the high adsorption enthalpy of water in sorbents. Therefore, improving performance requires not only faster cycling but also efficient thermal management and system integration.

From flat coatings to fin-array architectures

A central contribution of this work is the transition from conventional flat adsorbent coatings to a fin-array adsorption bed design. Traditional thick coatings provide high sorbent loading but suffer from slow diffusion and long cycle times. Thin coatings, on the other hand, improve kinetics but reduce compactness.

The proposed fin-array structure resolves this trade-off by combining:

• Millimeter-scale thin coatings for fast mass transfer

• Parallel fin geometry for high packing density

• Metal-supported structures for enhanced heat transfer

Humid air flows through narrow channels between fins during adsorption, while heat is supplied from the base during desorption. This configuration enables both rapid adsorption kinetics and compact system design, a combination that is difficult to achieve with conventional approaches.

The importance of air-gap optimization

One of the most insightful findings of the study is the role of air gap thickness between adsorbent fins. Rather than simply maximizing airflow, the system must balance two competing effects: vapor supply and diffusion resistance.

If the air gap is too small, the system becomes vapor-limited, restricting adsorption. If it is too large, diffusion resistance increases, slowing down water transport to the adsorbent surface. As a result, there exists an optimal air gap that maximizes water uptake—a result that challenges the common assumption that larger airflow channels always improve performance.

Fast kinetics enable multicyclic operation

The fin-array design significantly enhances both adsorption and desorption kinetics. Experimental results show that adsorption can reach saturation within approximately 30 minutes, while desorption driven by high-density waste heat (around 90 °C) can be completed in less than 10 minutes.

This rapid cycling enables more than 24 adsorption–desorption cycles per day, representing a major shift from traditional single-cycle systems. The desorption process itself follows three distinct phases: initial heating, active vapor release, and final sensible heating, reflecting efficient thermal utilization throughout the cycle.

Performance metrics: what changes with this design

Using a commercially available zeolite adsorbent (AQSOA-Z02), the system achieves a water productivity of approximately 5.826 L per kg of sorbent per day at 30% relative humidity. In a compact configuration with a 1-liter adsorption bed containing ~230 g of sorbent, this translates to about 1.3 liters of water per day.

Compared to conventional SAWH systems, this represents a 2–5× improvement in daily water production, primarily enabled by multicyclic operation and improved transport dynamics rather than new materials alone.

Material–device coupling: the real design driver

The study highlights that system performance depends on the interplay between material properties and device geometry. Two key material parameters govern performance:

• Equilibrium water uptake

• Intracrystalline diffusivity

However, their impact is strongly modulated by device-level factors such as coating thickness and vapor transport pathways. As coating thickness increases, diffusion limitations dominate, reducing effective water uptake despite favorable material properties.

In fact, the study shows that optimizing geometry alone can lead to order-of-magnitude differences in water production, emphasizing that device engineering is as critical as material innovation.

Where this matters in practice

The integration of waste heat as a driving energy source opens new pathways for real-world deployment. Unlike solar-driven systems, this approach allows continuous operation and can be integrated into:

• Industrial facilities

• Power plants

• Buildings and HVAC systems

• Transportation and mobile systems

This creates an opportunity to convert low-grade waste heat into a valuable water resource, particularly in water-scarce regions.

Takeaway: a shift from materials to systems thinking

The key contribution of this work is not the introduction of a new adsorbent, but the demonstration that system-level design can unlock the full potential of existing materials. By combining optimized geometry, efficient heat transfer, and multicyclic operation, the study establishes a pathway toward scalable and high-performance atmospheric water harvesting.

Ultimately, it reinforces a critical insight for the field:

The future of AWH lies in integrating materials, thermodynamics, and device engineering into a unified system design.