Universitat Rovira i Virgili

New Working Fluids based on Natural Refrigerant and Ionic Liquids for Absorption Refrigeration

Ionic liquids (IL) or low temperature molten salts are considered today to constitute the class of liquids with the highest potential to revolutionize the chemical industry, as they constitute innovative fluids for chemical processing which are generally non-flammable and non-volatile at ambient conditions, and, thus, perceived as "green" solvents. The ionic liquids emerged in recent years as the great alternative in the chemical industry as solvent, reagents, co-solvents, electrolytes, lubricants, etc., and permit the retrofitting of existing equipment with new replacement working fluids, to decrease the environmental impact, or, in our opinion, increase the energetic efficiency of the chemical processes. They have been recently proposed as absorbents in absorption and refrigeration cycles. On the other hand, also recently a great attention has been redirected toward recovery of waste heat looking for working pairs with natural refrigerants (NR) that can be characterized by a well-balanced set of properties.

The technology of absorption heat pumps and refrigeration has received growing attention in the past years from air-conditioning and refrigeration applications especially in connection with energy efficiency. In South Europe with long and hot summer and moderate winter periods the use of absorption chillers driven by solar thermal energy for cooling and air-conditioning applications in buildings can allow for a decrease in the use of primary energy for cooling and for an optimised utilisation of the solar thermal collector system. The development of thermal cooling technologies and others sustainable energy technologies is promoted by the EU Research Policy. In absorption technology a fluid with high volatility is used as refrigerant, whereas a second fluid which less volatility but strong affinity to the former is used as absorbent. So, essentially the processes in the absorption cycle are based on absorption and separation between refrigerant and absorbent. The properties of the working fluids are key factors in the performance of the absorption chillers. Among the commonly used fluid combinations are water/lithium bromide for moderate temperature air conditioning and ammonia/water for industrial refrigeration applications. Problems in using aqueous solutions of lithium bromide are crystallization and corrosion. Ammonia/water systems have drawbacks related with the high pressure and the separation. Various organic chemicals as alternatives to the conventional working fluids have been reported. The main drawbacks of these organic working pairs are related with the thermal instability, the volatility of the absorbent and the low process efficiency. As a new type of fluids with a great solvent character and other interesting properties such as a good thermal stability and very low vapour pressure, Ionic Liquids can be excellent candidates as absorbents for absorption systems. In fact, the present state of art permits to "tailor" an ionic liquid with required characteristic for a given duty, by choosing the cation and the anion from thousands of possibilities. As a drawback, the IL are moderately to highly viscous, especially at low temperature, restricting the flow of the absorbent in the machine. This problem can be overcome by a careful choice of the IL or by the addition of a molecular solvent that can reduce dramatically the viscosity. As a consequence an adequate design of a mixture IL + molecular solvent can lead to more efficient cycles.

A major focus of the network will be the development of new working fluids, composed by natural refrigerants (water, ammonia or carbon dioxide) and an absorbent based in a mixture of IL + molecular solvent (hence IL+NR). The strategic goal of the project is to enlarge the field of applications of absorption refrigeration systems and may be even to substitute the existing working fluids of absorption heat pumps and refrigeration systems to improve the overall process efficiency and make possible new developments of multi-effect cycles with their potential for primary energy savings. The programme implementation activities that will be carried out involve several steps, from selection of adequate systems, by a careful screening of existing data and foreseen measurable thermophysical properties, experimental measurement of thermodynamic and transport properties, development of prediction/estimation methods to avoid excess experimental commitment, heat transfer studies, thermodynamic cycle simulation and analysis and optimization of the working fluids. The competence and research capabilities of the institutions involved in the present proposal guaranty the success of the project and make the seed for a real development in refrigeration industry in a near future in Europe. To fulfil the requisitions of the EC work programme it will be necessary to build-up a tight and well organised exchange network for researchers from both sectors.

Periode: 1/04/2011 a 31/03/2015