## 2 Materials and Methods
### Air Compression Section Coupled with OAR System  
As shown in Figure 2b, solution-air heat exchangers (SAHEXs) and a cooling water-air heat exchangers (CWAHEXs) are present in each compressor. The compressed air is first pre-cooled by solution in the SAHEXs, and then further cooled to the target temperature by chilled water in the CWAHEXs. Solution in the SAHEXs comes from the absorber of the OAR, and chilled water in the CWAHEXs comes from the evaporator of the OAR. Three SAHEXs, an open generator, an absorber, an evaporator, two throttle valves, and a solution pump comprise of the OAR system. This system produces chilled water during the evaporation process, occurring in the evaporator, where water is sprayed externally, evacuates at a low pressure, and undergoes flash evaporation into water vapor. Chilled water produced in the evaporator flows into the CWAHEXs to further cool the compressed air to a relatively low temperature. Water vapor created in the evaporator flows into the absorber and is absorbed by the strong solution. The continuous absorption of water vapor maintains a low pressure in the evaporator and absorber. A cooling water loop takes away the absorption heat. The solution becomes diluted during the absorption process, and is collected at the bottom of the absorber before being pumped into the SAHEXs. Temperature of the diluted solution increases by exchanging heat with the compressed air in the SAHEXs. Then, solution from SAHEXs flows into the open generator, where water vapor in the diluted solution is transferred to the atmospheric air due to the water vapor partial pressure difference between the solution and air. Strong solution is collected at the bottom of the open generator and flows back to the absorber through a throttle valve, thus completing the OAR cycle. To better illustrate the advantage of the proposed cycle, p-T-x diagrams of the OAR cycle and CAR cycle are compared under the same working condition, as shown in Figure 3. The red line represents the OAR system, while the blue line represents the CAR system. The solid line, dashed line, and dot line represent the liquid flow, vapor flow, and compressed air flow, respectively. As can be seen from Figure 3, it takes three steps for the CAR system to cool the compressed air (purple line). An intermediate cooler has to be installed to compensate for the mismatch between the driving heat and cooling capacity, as explained in the introduction section. It takes only two steps for the OAR system, the first of which occurs in the SAHEXs, while the second occurs in the CWAHEXs (orange line). As the OAR can be driven by waste compression heat as low as 55 \({}^{\circ}\)C, the driving heat load can be well matched with the cooling demand of the CWAHEXs, as well as the cooling capacity of the OAR. As a result, no intermediate heat sinks are required; also, an evaporation temperature as low as 15 \({}^{\circ}\)C can be achieved. Waste heat obtained from SAHEXs is used to drive the OAR to produce continuous cold energy. In addition, the low evaporation temperature is also helpful for removing the WCT and ACT2 before the molecular sieve stage. AlthoughACT1 is still required for the absorber of the OAR system, the cooling load of ACT1 for the OAR is much lower than the cooling load of ACT1 for the conventional cooling water loop, as the former deals with the cooling load of compressed air from 45 to 17 \({}^{\circ}\)C, while the latter deals with the cooling load of compressed air from 110 to 40 \({}^{\circ}\)C.  
On the other hand, a lower evaporation temperature means lower suction temperatures, which is beneficial for the energy saving and performance enhancement of compressors. Hence, compared to CAR, the OAR system has the advantages of a simpler structure and less supplementary heat sinks. However, the energy conversion between the ACS and OAR is closely coupled to the waste heat and cold energy in the forms of energy quantity and quality (temperature), forming a complicated constrained energy flow feedback loop.