By Josh Perry, Editor
[email protected]

Collaboration between IBM Research in Zurich, the University of Applied Sciences Rapperswil (HSR), the Swiss Federal Laboratories for Materials Science and Technology (Empa), the School of Management and Engineering Vaud (HEIG-VD), and the Paul Scherrer Institute has developed a new adsorption heat pump (AdHP) technology using silica gel that will reduce the consumption of fossil fuels for heating or cooling buildings.

Final assembly of the 10 kW prototype.
(Institut für Solartechnik, Hochschule für Technik Rapperswil HSR)

According to a report from IBM Research, the THRIVE (Thermally Driven Heat Pumps for Substitution of Electricity and Fossil Fuels) project is part of the European Commission’s efforts to address climate change by reducing the environmental costs from energy consumption.

The scientists turned to silica gel (commonly used in desiccant packs in shoe boxes or electronics packaging) because it can adsorb as much as 40% of its own weight in moisture.

“In doing so,” the article explained, “it produces a kind of suction effect that can be used to pump heat much like a conventional air conditioner, but without using electricity. One application of this technology is in data centers to harness waste heat from hot-water cooled high-performance servers to produce cool air to cool power supplies and data storage in the same data center, essentially enabling data centers to cool themselves with their own waste heat.”

Empa researchers substituted silica gel for a new monolithic activated carbon adsorbent, similar to charcoal, which can be machined into different shapes to fit heat exchangers for AdHP. According to the article, “The material provided a 3.8x higher cooling power per unit mass compared to silica gel for regeneration by waste heat at 60°C.”

IBM Research and ETH Zurich demonstrated methods for characterizing adsorbents that allowed scientists to predict optimal shapes and coatings that enhanced adsorption rate threefold. “These structured adsorbents are capable of supporting a cooling power of 5 kW for each m² of adsorption heat exchanger area,” the article noted.

The combination of materials science and structural enhancements provided a 10-fold increase in the power density of adsorption heat exchangers and a corresponding reduction in the cost of future AdHP.

“To prove that the approach also works at larger scale, HSR scientists built a test rig equipped with a weighing scale in vacuum, capable of characterizing adsorption heat exchangers that produce a cooling power of up to 1.5 kW,” the article continued. “For even larger systems, they also built a four-chamber adsorption heat pump system that delivers up to 10 kW of cooling power – that would meet the typical air-conditioning requirements of a single-family home in warm climates.”

Scientists are already busy on a follow-up project, which will study the viability of AdHP as heat transformers in thermal grids.

Several reports were recently published, including one in the International Journal of Heat and Mass Transfer. The abstract stated:

“The rate of water adsorption in SAPO-34 zeolite coatings for adsorption heat pumps has been shown to be limited by mass transfer. In the present contribution, uniformly spaced longitudinal channels of width 75?µm were introduced into SAPO-34 coatings and their effect on the heat and mass transport limitations during water sorption was explored.

“Different channel spacings at constant adsorbent mass per unit area were tested by means of temperature-swing adsorption measurements and thermal impedance analysis (TIA). The optimal ratio between the characteristic transport length (CTL) for heat transfer and the CTL for mass transfer was 6, which is in agreement with the TIA predictions and was validated experimentally.

“Geometries with CTL ratios greater or less than 6 exhibited higher thermal impedance and lower rates of water sorption. At the optimal CTL ratio, the water sorption rate was enhanced 2x at the same adsorbent mass per unit area when compared to unstructured SAPO-34 coatings. Compared to other adsorbent structures reported in literature, the structured coatings exhibit the highest power density and energy density at short cycle times.

“These findings may be used to improve efficiency and/or power in temperature-swing processes in which mass transfer in adsorbent coatings is rate-limiting. It is recommended to develop methods to structure coatings at the scales relevant for commercial adsorption heat pump modules, and further investigate the rate-limiting transport mechanisms in large adsorption heat exchanger modules.”