Experimental Evaluation of a Solar Intermittent Refrigerator Working With the Mixtures NH₃ – LiNO₃ and NH₃ – LiNO₃ – H₂O

Solar energy could help to reduce the request of electric energy which is generated via hydrocarbons, having the possibility to provide energy to rural communities which do not have it through the conventional distribution. The application of solar system on refrigeration has a great range of development, that will have a competitive cost when the hydrocarbons diminish. An intermittent solar refrigerator that works with the absorption cycle is studied in the Center of Research on Energy of UNAM. It was designed to produce 8 kg of ice. The refrigerator consists of a condenser, an expansion valve, an evaporator and a generator/absorber. A compound parabolic concentrator (CPC) receives solar energy and turns into thermal energy. It is concentrated in the cylindrical receiver that works as generator or absorber. Being intermittent, the refrigerator performs two stages to complete a refrigeration cycle: a generation-condensation stage and another of evaporation-absorption stage. The present work focuses on the evaluation of the generation-condensation stage. One of the working mixture is ammonia-lithium nitrate (NH₃ – LiNO₃) that presents some advantages over the common mixtures such as the avoiding of rectification and working with relatively low manometric pressures. A disadvantage is that it presents crystallization at low concentrations. The other working mixture is ammonia-lithium nitrate-water (NH₃ – LiNO₃ – H₂O) with higher conductivity and lower viscosity than the ammonia/lithium nitrate. Several test were made at different solution concentrations for both mixtures, binary mixture (NH₃ – LiNO₃) and ternary mixture (NH₃ – LiNO₃ – H₂O). Comparing the performance of the system operating with the two mixtures, it was found that with the ternary mixture the solar coefficients of performance can be up to 24% higher than those obtained with the binary mixture.