Estudo experimental de refrigeradores por absorção/difusão usando resistor elétrico e óleo térmico como fontes de energia.

Abstract

The Covid-19 pandemic has brought together several areas of knowledge, including the sciences of immunology and refrigeration. New vaccination programs should take place in the 2020s. Vaccine storage requires modified refrigerators according to WHO (World Health Organization) standards. Considering the logistical difficulties of transportation and the lack of electrification in some more remote communities, a solar refrigerator for vaccine storage represents an alternative to kerosene or gas refrigerators. Absorption/diffusion refrigerators allow the use of solar radiation as an energy source. In this scenario, the objective of this work is to study the prediction of replacing the electrical resistor of an absorption/diffusion refrigerator with a thermal oil heat exchanger with a view to applying the system for storing vaccines in areas without electrification and using a collector solar heating oil. A small commercial NH3/H2O/H2 refrigerator, equipped with an 80W/220V electrical resistor, was chosen for this study. Firstly, to characterize the device, experimental tests indicated a performance coefficient between 0.14 and 0.19, according to the thermal load imposed by an internal electric heater. To replace the electrical resistor with a thermal oil heat exchanger, a mathematical model in EES and Python was developed to assist in device design and oil selection. The model's input variables came from manufacturer data and measurements made with thermocouples at specific points in the refrigerator. From this, the heat exchanger was constructed with a length of 15 cm, an inner tube diameter of 10 cm and an envelope tube diameter of 15 cm. The model was used to determine the oil type, operating temperature of 207°C and pressure loss of 2.23 kPa throughout the oil circuit. This loss implied a minimum operating pressure above the vapor pressure of the hot oil. Finally, two refrigerator start-up tests were carried out, one with an electrical resistor as a power source (test 1), the other using the heat exchanger with the oil heated remotely (test 2). The operability of the thermal oil cooler was proven: first signs of cooling after 15 minutes (tests 1 and 2); time for full cooling of 6 hours (tests 1 and 2); reduction in relative internal temperature of 0.22°C/W in test 1 and 0.23°C/W in test 2; cooling curve time constant of 75 minutes in test 1 and 80 minutes in test 2.