Joon-Hyuk Lee

This study aims to provide essential data for the optimal design of a transcritical CO₂ water-cooled heat pump system utilizing an internal heat exchanger (IHX). To investigate cycle and application characteristics, a heat pump test rig consisting of a compressor, gas cooler, expansion valve, evaporator, IHX, and liquid receiver was fabricated. All heat exchangers were counterflow, concentric dual copper tubes. System performance was evaluated by varying the IHX and evaporator outlet superheat degrees (10–30 °C and 0–20 °C, respectively) via expansion valve opening control, alongside changes in the secondary fluid inlet temperature. The experimental results indicate that optimal high-pressure conditions exist to maximize both capacity and coefficient of performance (COP) in cooling and heating modes. Furthermore, changes in the secondary fluid inlet temperatures of the evaporator and gas cooler significantly impact the COP, showing trends similar to conventional Freon-based systems. Ultimately, precise control of the expansion valve opening, using the IHX outlet superheat as a control variable, is a key factor in operating the CO₂ heat pump system near its maximum COP.

HFC계 냉매는 기존 냉매에서 ODP를 고려하여 사용되었으나, 높은 GWP로 인하여 2016년 키갈리 개정에 따라 사용이 단계적으로 감축되어야만 한다. 이에 따라 HFC계 냉매에서 대표적인 R410A 냉매를 대체하는 냉매를 찾아야 할 필요성이 커졌다. 기존 다양한 연구는 R410A와 유사한 성능을 가진 냉매를 찾기 위해 진행되었으나, 추가적으로 성능향상에 대한 연구는 부족한 상황이다. 본 연구에서는 R410A의 대체 냉매 특성을 비교하고, 내부 열교환기를 적용한 시뮬레이션을 통해 과냉각도가 성능 향상에 미치는 영향을 분석하였다. 그 결과, HFC계 냉매인 R32를 제외하고 R455A가 R410A와 매우 유사한 냉동용량과 COP 등을 보였다. 따라서 이를 검증하고자 향후에는 실제 냉매를 사용한 실험을 통해 시뮬레이션 결과의 유사성을 검증할 계획이다.

현대사회에서 신재생 에너지원은 환경보전에 매우 필수적인 요소지만 다양한 문제점이 존재한다. 이러한 단점을 보완하기 위해 ESS에 관한 연구가 진행되며, LAES는 다양한 장점으로 활발히 연구되고 있다. 그러나 해당 연구의 경우 주로 발전과 액화에서의 효율만을 연구하며, 버려지는 건조 공기를 활용하는 연구는 미미하다. 본 논문에서는 해당 건조 공기를 냉방에 활용하는 연구를 진행하였다. 건조 공기는 단열 가습을 진행하여 냉방에 사용하며, 이때 1일 중 부분 부하 운전에서의 대체 냉각량에 대해 연구를 진행하였다. 그 결과, 급기 온도가 낮고 외기의 온도와 습도가 높을수록 대체 냉각량의 크기는 커지는 경향을 보였다. 따라서 건조 공기를 활용한 냉방 시스템의 경우 외기의 온도와 습도가 높으며, 급기 온도가 낮은 조건에서 효율적인 운전이 가능하다.

Ultra-low temperature chillers have seen increasing demand with the advancement of semiconductor technology. Mixed refrigerant (MR) cascade refrigeration systems (CRSs) are widely utilized for their stability and high cooling performance at low temperatures. Extensive research has been conducted on optimizing MR, which has a significant impact on CRS performance. However, most previous studies have either fixed the system pressure or used the refrigeration effect as the sole performance indicator. This did not account for the potential of achieving higher performance with an optimal MR composition at the same target temperature. In this study, a detailed parametric analysis was performed to investigate how the mass fractions of high-, mid-, and low boiling point refrigerants affect the coefficient of performance (COP) and exergy in ultra-low temperature CRSs without fixing the suction pressure. The analysis revealed that at the point of maximum COP, the refrigeration effect was relatively low, highlighting the limitations of using the refrigeration effect alone as a performance indicator. Additionally, COP was found to inversely correlate with total exergy destruction. As cascade temperature increases, COP tends to decrease, emphasizing the need for appropriate cascade temperature selection for MR CRS performance. This study introduces a novel approach to optimizing MR composition under various operating conditions, contributing to the advancement of ultra-low temperature CRSs.

This study investigates the implementation of a cryogenic chiller utilizing a mixed-refrigerant cascade refrigeration cycle (MRCRC). In this setup, R-404A is employed in the high-temperature circuit (HTC), while a mixture of refrigerants is utilized in the low-temperature circuit (LTC). Unlike a conventional MRCRC that operates without a receiver to maintain the composition ratio, this research explores the impact of receiver installation on system performance. Experiments were conducted with and without a receiver to assess performance improvements and device behavior. With a fixed refrigerant charge of 4 kg, the suction and discharge pressures of the LTC compressor remained low and stable after the receiver’s installation. The addition of a receiver significantly reduced the cooling time, with further reductions observed as the refrigerant charge increased. The system achieved evaporative heat capacities of 0.59, 1.76, and 2 kW for refrigerant charges of 4, 7, and 9 kg, respectively. Notably, at the maximum refrigerant charge of 11 kg, the evaporative heat capacity peaked at 3.3 kW. These findings indicate that incorporating a receiver is crucial for enhancing the cooling performance of cryogenic coolers using mixed refrigerants and stabilizing device operation. This contrasts with previous studies that omitted receivers due to concerns over potential alterations in the composition ratio of the mixed refrigerant.

Recent advancements in cryogenic etching, characterized by high aspect ratios and etching rates, address the growing demand for enhanced performance and reduced power consumption in electronics. To precisely maintain the temperature under high loads, the cascade mixed-refrigerant cycle (CMRC) is predominantly used. However, most refrigerants currently used in semiconductor cryogenic etching have high global warming potential (GWP). This study introduces a −100 °C chiller using a mixed refrigerant (MR) with a GWP of 150 or less, aiming to comply with stricter environmental standards and contribute to environmental preservation. The optimal configuration for the CMRC was determined based on a previously established methodology for selecting the best MR configuration. Comprehensive analyses—energy, exergy, environmental, and exergoeconomic—were conducted on the data obtained using Matlab simulations to evaluate the feasibility of replacing conventional refrigerants. The results reveal that using eco-friendly MRs increases the coefficient of performance by 52%, enabling a reduction in compressor size due to significantly decreased discharge volumes. The exergy analysis indicated a 16.41% improvement in efficiency and a substantial decrease in exergy destruction. The environmental analysis demonstrated that eco-friendly MRs could reduce carbon emissions by 60%. Economically, the evaporator and condenser accounted for over 70% of the total exergy costs in all cases, with a 52.44% reduction in exergy costs when using eco-friendly MRs. This study highlights the potential for eco-friendly refrigerants to be integrated into semiconductor cryogenic etching processes, responding effectively to environmental regulations in the cryogenic sector.

The cascade refrigeration systems (CRS) used in hypermarkets and supermarkets, which are used by many people, have been employing R744 for the low-temperature cycle (LTC) and R404A for the high-temperature cycle (HTC) due to environmental and public safety issues. However, the use of R404A is limited due to its high GWP, and therefore research on alternative refrigerants is necessary. Nevertheless, there is no detailed study in the literature that compares and analyzes the three refrigerants for practical design by applying R744 for LTC and R404A, R448A, and R449A for HTC in CRS. Therefore, this study aims to provide data for the practical detailed design of an alternative system to R744/R404A CRS. Under standard conditions, we analyzed how the exergy destruction rate (EDR) and exergy efficiency (EE) of the system and the EDR of each component change when the important factors affecting CRS (degree of superheating (DSH), degree of subcooling (DSC), and internal heat exchanger (IHX) efficiency of HTC, DSH of LTC, condensation temperature (CT), evaporation temperature (ET), cascade evaporation temperature (CET), and temperature difference of CHX) are varied over a wide range. The main conclusions are as follows. (1) Under the given constant conditions, the smallest change in system EDR based on R448A is DSH of HTC (decreased by 0.07–0.1 kW), followed by IHX of HTC (decreased by 0.12–0.3 kW), DSH of LTC (increased by 0.19–0.25 kW), DSC of HTC (decreased by 0.59–0.69 kW), temperature difference of CHX (increased by 1.57–1.83 kW), CET (decreased and then increased by 0.67–4.43 kW), CT (increased by 1.49–3.9 kW), ET (decreased by 2.39–4.61 kW). (2) The highest change rate of system EE based on R448A is CET (increased and then decreased by 1.38–8.28%), followed by temperature difference of CHX (decreased by 2.96–3.16%), ET (increased and then decreased by 0.63–2.75%), DSC of HTC (increased by 1.26–1.34%), CT (increased and then decreased by 0.24–1.12%), IHX of HTC (increased by 0.11–1.02%), DSH of LTC (decreased by 0.35–0.49%), and DSH of HTC (increased by 0.14–0.19%).

The R744/R404A cascade refrigeration system (CRS) has been widely used in supermarkets and hypermarkets, but due to the refrigerant regulation of R404A, research on alternative refrigerants is necessary. In addition, although there have been quite a few studies on R448A and R449A, which are well-known alternatives to R404A, few studies have analyzed the performance coefficients of the three refrigerants, and the studies that have analyzed them are not based on enough variables. Therefore, we aimed to understand the performance characteristics of CRS combined with an internal heat exchanger (IHX) by applying R744 for the low-temperature cycle (LTC) and R404A, R448A, and R449A for the high-temperature cycle (HTC). The analysis method was to analyze the coefficient of performance (COP) and mass flow rate (MFR) of the three refrigerants according to the degree of subcooling (DSC) and degree of superheating (DSH), IHX efficiency, temperature difference in the cascade heat exchanger (CHX), condensation temperature (CT), evaporation temperature (ET), and cascade evaporation temperature (CET). The purpose of this study is to compare R448A and R449A, alternative refrigerants to R404A, in an R744/R404A CRS, with R404A to provide sufficient data for optimal CRS design. The comparison results are as follows: (1) Compared with R404A, the MFR of R448A and R449A are 67.27–77.6% and 70.05–80.80%, respectively, under the same conditions. Therefore, R448A and R449A are economically favorable because they have less refrigerant charge than R404A, and R448A is more favorable than R449A. (2) The R744/R448A CRS is stable and performs better than the R744/R449A CRS in places with large changes in the surrounding environment.

Although the R404A indirect refrigeration system (IRS) with an internal heat exchanger (IHX) and R744 as the secondary fluid has potential applications in supermarkets and hypermarkets, the exergy characteristics of this IRS have not been extensively investigated. In this study, the factors affecting the R744 exergy characteristics (degree of subcooling (DSB) and degree of superheating (DSP) of the R404A cycle, DSP of the R744 cycle, condensation temperature (CT) and cascade evaporation temperature (CET), and IHX efficiency) were experimentally evaluated to obtain basic data for the design of R404A IRS with R744 as the optimal secondary fluid. The main results can be summarized as follows: (1) Under given conditions, the smallest change in the system exergy destruction rate (EDR) according to the change in each parameter is the DSP of the R744 cycle (0.3–1%), followed by the DSB of the R404A cycle (6.1–8.8%), the IHX efficiency of the R404A cycle (3.8–14.3%), the DSP of the R404A cycle (11.7–15.9%), the CET (29.4–41.9%), and the CT (35–47%). (2) Also, in terms of the exergy efficiency of system (EES), the largest value was obtained for the DSP of the R404A cycle (2.4–12.7%), followed by the IHX efficiency of the R404A cycle (3–10.2%), the CET (2.2–8.7%), the CT (4–6.9%), the DSB of the R404A cycle (2.7–6.2%), and the DSP of the R744 cycle (0.04–1.2%). (3) In order to lower the system EDR, DSP, DSB, and IHX efficiency of the R404A cycle, the CET must be increased to the maximum, and to lower the DSP of the R744 cycle, the CT must be reduced to the minimum.