Performance optimization of an integrated adsorption-absorption cooling system driven by low-grade thermal energy

The integrated adsorption-absorption system is a novel technology to convert low-temperature waste thermal energy into useful cooling, which substantially improves energy utilization efficiency and lowers environmental pollution. This novel system incorporates the adsorption cycle into the absorption cycle so that the generation/adsorption pressure can be adjusted to enhance the system performance at low-temperature heat sources. In this paper, the thermal characteristics of the integrated system are theoretically evaluated. The optimization of the key parameters (e.g. cycle time, switching time, intermediate pressure and solution concentration) are conducted to achieve the best system performance. Furthermore, different configurations of the integrated system and the effect of the heating and cooling flow arrangements on the system performance are also investigated. The results showed that the coefficient of performance of the proposed system was as high as 0.4 at a heat source temperature of 50 degrees C. As the heat source temperature increased from 50 to 85 degrees C, an optimal intermediate (generation/adsorption) pressure varied gently from 2.36 to 2.16 kPa while the optimal solution concentration increased from 52.4 to 65% to achieve the best system coefficient of performance. The results also showed that the maximum specific cooling power is 193.7 W/kg when the heating and cooling water flow arrangement are both in parallel. In contrast, the lowest specific cooling power is 157.9 W/kg for both heating and cooling water flow arrangements in series. When compared the two different configurations, the cooling capacity and coefficient of performance of the configuration with absorption-cycle as the bottom cycle and adsorption-cycle as the top cycle were about 5 and 15% higher, respectively than those of the configuration with adsorption-cycle as the bottom cycle and absorption-cycle as the top cycle.