Quenching the Thirst of the Arid World: Compact Fuel-Powered Atmospheric Water Harvesters

As the global population continues to surge and climate change disrupts traditional water sources, the need for innovative approaches to freshwater production has never been more pressing. Over 4 billion people worldwide live in water-stressed regions, and experts predict this number will only grow in the coming decades. Confronting this grand challenge requires decentralized, reliable technologies that can extract water from the air, even in the driest climates.

A promising solution has emerged in the form of sorption-based atmospheric water harvesting (SAWH) devices. These systems leverage porous materials called sorbents to capture water vapor from the air through a cyclic adsorption and desorption process. Recent years have seen rapid progress in SAWH, with novel sorbent materials and innovative device designs pushing the boundaries of what’s possible. However, significant engineering challenges remain before SAWH can be broadly deployed as a viable freshwater solution worldwide.

Researchers at the University of Utah have taken on these challenges head-on, developing a first-of-its-kind SAWH prototype that leverages compact, rapid cycling and fuel-powered desorption to achieve unprecedented water production in a small footprint. Their “compact rapid cycling fuel-fired” (CRCF) device represents a major step forward in making SAWH a practical reality, addressing key limitations that have hindered the widespread adoption of this promising technology.

The Quest for Compact, High-Performance SAWH
SAWH devices work by cycling sorbent materials through adsorption and desorption phases. During adsorption, the sorbent material absorbs water vapor from ambient air. When the sorbent becomes saturated, it is then heated to release the captured water, which is then condensed into liquid form. This cyclic process allows SAWH systems to continuously produce freshwater from the air.

While the basic SAWH concept is straightforward, optimizing the performance and compactness of these devices has proven challenging. A key metric is the daily water productivity (P), which measures the amount of water produced per unit mass of sorbent material. Maximizing P is crucial for minimizing the size and cost of SAWH systems. Another critical metric is the volumetric productivity (Pv), which quantifies the daily water output per unit volume of the adsorbing system. High Pv enables compact, space-efficient SAWH devices.

Recent SAWH innovations have focused heavily on developing novel sorbent materials with high water uptake and rapid adsorption/desorption kinetics. Metal-organic frameworks (MOFs) have emerged as a particularly promising sorbent class, offering exceptional water capture capabilities. However, material advances alone are not enough – system-level engineering plays a vital role in unlocking the full potential of SAWH.

“Contemporary SAWH research has often overlooked the importance of system design and integration,” explains Sameer Rao, lead author of the study and an assistant professor of mechanical engineering at the University of Utah. “Achieving high performance in both productivity metrics – P and Pv – is crucial for making SAWH a practical, widely deployable technology.”

Rao and his team recognized that addressing the system-level challenges of SAWH would require a fresh approach. Their solution was the CRCF device, which tackles two key limitations of existing SAWH systems: the reliance on intermittent solar energy for desorption, and the difficulty of achieving compact, high-density water production.

Fueling the Future of Atmospheric Water Harvesting
Conventional SAWH devices often use solar energy or electricity to power the desorption process, where the captured water is released from the sorbent material. This approach works well in sunny, grid-connected locations, but it introduces significant limitations. Solar-powered systems are dependent on daylight hours and weather conditions, while electrically-driven systems require costly energy storage to operate continuously.

The CRCF prototype developed by Rao and his team takes a different approach, using a compact combustion-based heat source to drive the desorption process. By tapping into the high energy density of liquid fuels, the CRCF device can operate independently of the sun or the electrical grid, enabling reliable, all-day water production.

“Fuel-powered desorption is a game-changer for SAWH,” says Rao. “It allows us to decouple the system from intermittent energy sources, paving the way for truly autonomous, off-grid water harvesting.”

The CRCF device features a custom-designed adsorbent heat exchanger (AHX) that houses the sorbent material – in this case, an aluminum fumarate MOF. The AHX is connected to a fuel-fired heat source via a passive heat pipe assembly, which efficiently transfers the combustion heat to the sorbent for desorption. During the adsorption phase, ambient air is drawn through the AHX, allowing the MOF to capture water vapor. When the sorbent becomes saturated, the heat pipes deliver the necessary thermal energy to release the water, which is then condensed and collected.

This fuel-powered approach not only enables continuous operation, but also allows for a more compact device design compared to solar-driven systems. “By eliminating the need for bulky solar panels and energy storage, we can package the CRCF device in a much smaller footprint,” explains Rao. “This is a crucial advantage for real-world deployment, where space and portability are often at a premium.”

Prototype Testing and Performance Optimization
To validate the CRCF concept and identify opportunities for further improvement, the researchers conducted a series of indoor and outdoor experiments. The prototype was first tested in a controlled laboratory setting, where it achieved a daily water productivity of 0.95 kg per kg of MOF and a volumetric productivity of 38.5 kg per cubic meter of AHX per day.

The team then took the CRCF device outside, subjecting it to the harsh, arid conditions of Salt Lake City, Utah. Over the course of 25 hours, the prototype completed five continuous water harvesting cycles, producing a total of 266 grams of liquid water. While this outdoor performance was slightly lower than the indoor tests, it still represented a significant achievement, given the challenging environmental conditions.

“The outdoor experiments provided valuable insights into the real-world challenges of SAWH, such as the impact of low temperatures and humidity on adsorption kinetics,” says Rao. “These learnings will be crucial as we work to further optimize the CRCF design.”

One of the key limitations identified during the testing was the efficiency of the water condensation process. The team found that a significant portion of the desorbed water vapor was not successfully condensed, reducing the overall water yield of the system. This issue is a common challenge in SAWH devices, as the presence of non-condensable gases (such as air) can impede the condensation process.

To address this, the researchers explored strategies to enhance the condensation efficiency, such as incorporating a closed-loop air circulation system. By recirculating the desorbed air stream, they were able to increase the condensation rate and improve the overall water harvesting performance.

Building on these experimental insights, the research team turned to computational modeling to explore the design space and identify opportunities for further optimization. Using their validated simulation framework, they conducted a parametric study to understand the relationships between key design variables – such as the thickness of the sorbent fins and the extent of adsorption truncation – and the system’s P and Pv metrics.

The results of this optimization study were impressive. By carefully tuning the AHX geometry and cycling patterns, the researchers were able to project a CRCF system capable of achieving a daily water productivity of 3.19 kg per kg of MOF and a volumetric productivity of 718 kg per cubic meter of AHX per day. These figures represent a 1.53-fold and 2.13-fold improvement, respectively, over the current state-of-the-art in MOF-based SAWH devices without refrigeration.

“The key to unlocking this performance boost lies in our ability to rapidly cycle the sorbent material and maximize the water production per unit volume,” explains Rao. “The CRCF design, with its fuel-powered desorption and compact heat exchanger, provides the flexibility to optimize these critical parameters.”

Scaling Up for Global Impact
The CRCF prototype developed by the University of Utah team is not just a proof-of-concept; it represents a significant step towards making SAWH a viable solution for addressing global water scarcity. By combining innovative system design with high-performance materials, the researchers have demonstrated the untapped potential of this technology.

“What’s truly exciting about the CRCF approach is its scalability and adaptability,” says Rao. “The building-block nature of the AHX design means we can readily scale up the system to meet the water needs of communities of all sizes, from individual households to larger municipalities.”

Indeed, the researchers envision a future where CRCF-based SAWH devices are deployed worldwide, providing reliable access to freshwater in even the most arid regions. By decoupling the system from the sun and the grid, these fuel-powered harvesters could become a game-changer for off-grid communities and disaster relief efforts, where access to clean water is often a critical challenge.

“Ultimately, our goal is to unlock the full potential of SAWH and make it a practical reality for those who need it most,” says Rao. “The CRCF prototype is a significant step in that direction, and we’re excited to continue pushing the boundaries of what’s possible in atmospheric water harvesting.”

As the global water crisis intensifies, innovative solutions like the CRCF device will be crucial in quenching the thirst of arid regions around the world. By leveraging the abundance of water vapor in the atmosphere and decoupling from intermittent energy sources, these fuel-powered harvesters hold the promise of providing reliable, decentralized access to freshwater – a vital step towards a more sustainable and equitable future.

Reference(s)

1. DOI:https://doi.org/10.1016/j.xcrp.2024.102115

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