Cooling systems are often regarded as energy wasters that still use environmentally harmful refrigerants even after the ban on chlorofluorocarbons (CFCs). Systems using water are an environmentally friendly alternative. A research team at the Institute of Inorganic Chemistry at Kiel University, working together with the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, has now developed an extremely porous material that enables these cooling systems to be operated using less electricity than in the past. Previously unused waste heat – for example, from district heating systems, solar thermal collectors or data centers – could now be used. The results of the study were published recently in the journal "Advanced Materials".

Data centers in particular are real "energy factories". High-performance computers produce a considerable amount of heat and therefore need to be cooled on a continuous basis. In other words, they cause high energy and power costs. At the same time they give off unused waste heat to the environment – the temperature is too low for any use. Theoretically, however, this waste heat could be used for running cooling systems that use water as a refrigerant (so-called absorption refrigeration systems). For this purpose the material used must be capable of absorbing a lot of water and able to regenerate at a very low temperature.

Environment-friendly and resource-saving cooling

These two conditions are fulfilled by the porous material developed by Dr. Norbert Stock at the Institute of Inorganic Chemistry and his working group. As a result, a part of the absorption refrigeration system can be operated using only the energy from existing waste heat or solar thermal systems. According to Stock, "This could also represent a major contribution to the use of renewable energy." The material boasts two key benefits for this type of environmentally friendly system. Stock adds: "The systems consume less power and at the same time the production of the material is environmentally friendly."

The cooling effect in these absorption refrigeration systems occurs due to the evaporation of water when ambient heat is extracted. The molecules of water vapor are then absorbed by the porous material, i.e. they are deposited in the cavities of the material. This is followed by a regenerative phase. Thermal energy is applied and the water molecules are released from the material. They liquefy and can be evaporated again in the next cycle. The material can also be re-used.

Metal-organic frameworks ensure ideal interaction

The sorbents used in cooling systems are normally crystalline zeolites or silica gels. By virtue of their porous structure they can easily absorb water. The material used by the Kiel research team exhibits very good sorption properties – it can absorb a lot of water very quickly, and also release it quickly after a slight increase in temperature. As a result the material is "dried" within a short space of time and soon ready to be used again. As Stock explains: "This is made possible by the ideal size of the pores in the material and the perfect interaction with the water molecules." The highly porous crystal structure of "CAU-10-H" (the official name of the material, named after the place where it was developed, the version number and the abbreviation for hydrogen) is an example of a metal-organic framework (MOF). Metal-organic frameworks have been tested intensively during the last few years in very different fields of application.

From basic research to practical application

The Kiel research team has been investigating new MOFs for many years now, but has focused only on basic research to the exclusion of practical application. For the purpose of industrial application the team cooperated with the Fraunhofer ISE to coat commercially available heat exchangers with their material. "The examination of the heat exchanger under more-or-less real conditions demonstrates the enormous potential of this material", reports Dr. Stefan Henninger from Fraunhofer ISE. The material can already be produced in the laboratory in kilogram quantities under mild reaction conditions, i.e. at a temperature of 100° C with water as a solvent ("green synthesis"). Stock says: "In order to produce the material for large-scale industrial use, we now plan to get in touch with the relevant companies." The partners have already applied for a patent for their production method.

From 23 to 27 April 2018, the research team will be presenting the material and its various applications at the Kiel University stand at HANNOVER MESSE (Hall 2, Stand C07). At 2.30 p.m. and 5 p.m. on Wednesday, 25 April, Dr. Norbert Stock will also be giving a presentation entitled "Nanoporous materials for modern and environment-friendly cooling and air conditioning" . Kiel University will be participating in the world’s largest industrial technology show for the second time and is presenting innovative contributions from Schleswig-Holstein, a center of research and innovation.