Thermodynamic analysis on direct air capture for building air condition system: Balance between adsorbent and refrigerant

Direct air capture (DAC) is one of the most potential technologies to mitigate CO₂ emission. Adsorption technology is recognized as a promising CO₂ capture method in view of its desirable characteristics including reusability of adsorbents and low capital investment. To further improve thermal performance, evaporation/condensation heat of vapor compression refrigeration (VCR) cycle in air condition system of buildings is adopted for adsorption/desorption process of DAC. Thermal performance of a 4-step temperature swing adsorption process (TSA) is analyzed at various adsorption/desorption temperatures by using different adsorbents. Analysis on Coefficient of Performance (COP) of VCR cycle is also conducted in search for a balance between adsorbent and refrigerant. Taking both real working capacity and COP into consideration, Mg-MOF-74&R134a is the best choice for more amounts of CO₂. Real working capacity of Mg-MOF-74 is up to 0.38 mol•kg⁻¹ at 70 °C, which is twice as much as that of zeolite 13X. While zeolite 13X&R134a shows the best performance of two cycles in view of exergy efficiency and COP, which could reach 81.9% and 7.21, respectively, at 35 °C. These matches will provide some guidelines for the practical application of the combination of DAC with heating, ventilation and air conditioning (HVAC).