Investigation of miniature lithium-bromide/water absorption refrigeration systems

Abstract

Nowadays, due to the global demand for the reduction of energy usage and the use of harmful refrigerants which are associated with vapour compression refrigeration systems, alternative cooling means, like the Lithium-Bromide/Water (LiBr/H2O) absorption refrigeration system, are being investigated and produced in developed countries. However, the majority of these LiBr/H2O absorption refrigeration systems, which are available on the market, have a large cooling capacity. In fact, there is little literature related to miniature LiBr/H2O absorption refrigeration systems with small cooling capacities. This is because of the difficulties encountered during the design and operation of such systems (such as crystallization of the Lithium-Bromide/Water solution, choosing the appropriate equipment, and dimensioning of the components for compactness, etc.). Therefore, this research focuses specifically on the development of mathematical design models and experimental work that provide a successful description of the phenomenon of the absorption rate for a system with a small cooling capacity. For this reason, a miniature LiBr/H2O absorption refrigeration system equipped with an adiabatic absorber was designed and constructed, having a cooling capacity of 45 𝑊 at 10 ℃, which is also the minimum system temperature. A thermal analysis consisting of mass and energy balances was made for a range of generator and adiabatic absorber temperatures. This analysis enabled the optimum coefficient of performance (COP) to be found and recommended components’ temperatures for the absorption refrigeration system without a solution heat exchanger. The decision not to include a solution heat exchanger was taken in order to keep the system as simple as possible. Thereafter, the sizing of the system heat exchangers was made to accommodate this optimum COP and corresponding temperatures. The maximum temperature of the system was set by the generator at 80 ℃. The designed heat input in the generator was 69.2 𝑊. The design of the system and individual components is presented and explained. This includes the design of the helical coil condenser operating at 35 ℃, and the design of the adiabatic absorber operating at 30 ℃. The optimal exposed surface area of the LiBr/H2O solution to the vapour refrigerant (vapour-solution interface area) inside the adiabatic absorber was determined experimentally and found to be equal to 140 𝑐𝑚2. The most challenging part of the design was estimating the heat transfer coefficients present in the various heat exchangers.