Zeolite Materials for Solar Thermal Energy Harvesting and Water Remediation Sustainable Technologies

Sustainable development is declared as “an overarching objective” of the European framework programme Horizon 2020. More in particular, the development of sustainable technologies and materials to exploit renewable energy sources, e.g. solar and geothermal energy, and to safeguard one of the most important primary resource of the Earth planet, namely water, is rated with high priority among the societal challenges. Owing to several factors such as the rising world population and increasing life standards, the demand of energy could effectively grow by as much as 55% by 2030 and potentially double by 2050.1 Although our reliance on fossil fuels is not expected to change significantly between now and 2050, reduction of primary energy consumption is strongly needed to reduce global warming, in line with the Kyoto Protocol (1998), which requires that industrialized nations reduce greenhouse gas emissions to below 1990 levels. Since a large fraction of energy production (e.g. up to 40% in Europe and to 80% in North America) is used for the heating and cooling of residential buildings, research on cheaper and environmentally clean energy sources and technologies is a key priority in many countries. In particular, the application of solar energy instead of electricity for air conditioning/refrigeration appears as a very promising technology owing to the close coincidence of high peak cooling demands with the maximum available solar thermal energy, especially in those countries with high solar irradiation. Moreover, solar-powered refrigeration devices are suitable to meet requirements for the preservation of food, drugs and vaccines in remote areas.2 Different physical phenomena can be exploited for the long-term ‘solar heat storage’ (SHS) including physisorption (based on weak Van der Waals forces and hydrogen bonding) or chemisorption (valency forces). Zeolites and related microporous minerals are among the best suited adsorbing materials owing to their tunable host-guest interaction strength, efficiency, and sustainability. The development of natural zeolite sorption refrigeration systems powered by solar energy emerged in the late 1970s, following the pioneering work of Tchernev3 on mineral chabazite and its water adsorption properties. The exothermic enthalpy of hydration, thermodynamically stabilizing the otherwise metastable anhydrous zeolite structure, explains the endothermic nature of the dehydration phenomenon in these minerals. Thermally activated sorption of zeolites can be used in Solar Coolers (SCs) as a possible alternative to electricity driven vapour compression refrigerator. Basically, in an adsorption cooling cycle the mechanical compressor of a conventional gas (refrigerant) compression system powered by electricity is replaced by a thermal compressor (zeolite) driven by low grade thermal energy like solar or geothermal energy to evaporate water. As a further advantage, the use of water vapour as the refrigerant fully complies with the Montreal protocol (1988) on substance that depletes the ozone layer. Alternatively, the zeolite water ad/desorption cycles can be used to increase efficiency of Adsorption Heat Pumps (AHPs) or in seasonal Solar Heat Storage (SHS) systems. The thermal stability of several zeolite topologies4 makes the ad/desorption cycle fully reversible.