## 5 Conclusions  
Fixed-pressure control methods use compressor frequency and flow control of expansion valves to fix evaporative pressure, which limits the response to cooling load fluctuations such as rising and decreasing room temperature. In this study, the cooling-load-estimation control method and the fan-load-control condition of the cooling-load-estimation control method were reviewed to overcome the limitations of cooling load fluctuation control. In the case of the fan-load-control condition of the cooling-load-estimation control method, the heat transfer rate of the heat exchanger can be varied by varying the air volume of the fan, while controlling the temperature difference between the room temperature and the evaporative temperature. These two controls enable sophisticated regulation of heat transfer rates in response to fluctuations in cooling loads in the room. The cooling-load-estimation control method can adjust the heat transfer rate only by the temperature difference between the room temperature and the evaporative temperature. As a result, cooling-load-estimation control has a lower ability to adjust the heat transfer rate due to cooling load fluctuations compared to fan-load control of the cooling-load control method. Finally, fixed-pressure control methods perform the worst in terms of energy efficiency, as they have little control over cooling load fluctuations compared to the two previously mentioned control methods.  
For outdoor temperatures of 27 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, and 35 \({}^{\circ}\)C, the room temperature fluctuations and power consumption according to the control method were confirmed through the experiments. Sequentially, temperature fluctuations were stable in the fixed-pressure control, cooling-load-estimation control, and cooling-load-estimation-with-fan control methods. When the fan air volume and number of compressor rotations in conjunction with changes in the load were controlled, comfort could be improved. Moreover, sequentially, the power consumption was high in the fixed-pressure control, cooling-load-estimation control, and cooling-load-estimation-with-fan control methods. The fan-control technique of the cooling-load-estimation control method is linked to the difference between the indoor room and evaporation temperatures to control the fan air volume to reduce indoor temperature fluctuations to improve indoor-space comfort and system energy efficiency.  
Figure 10: Comparison of power-consumption ratio according to initial indoor temperature and control method.  
The fan-load-control condition of the cooling-load-estimation control method showed an energy savings effect of 16.1-48.7% compared with the fixed-pressure control method and 1.2-37.7% compared with the cooling-load-estimation control method. Increase in environmental temperatures and the effects of energy savings tend to be inversely proportional. When the environmental temperature was 27 \({}^{\circ}\)C, the fan-load-control condition of the cooling-load-estimation control method had an energy savings effect of 48.7% compared with the fixed-pressure control method and 37.7% compared with the cooling-load-estimation control method. When the environmental temperature was 35 \({}^{\circ}\)C, the fan-load-control condition of the cooling-load-estimation control method had an energy savings effect of 16.1% compared with the fixed-pressure control method and 1.2% compared with the cooling-load-estimation control method.  
Conceptualization, T.P. and B.K.; data curation, T.P., G.H. and Y.A.; formal analysis, Y.K. and I.L.; investigation, I.L. and Y.K.; resources, T.P. and G.H.; validation, T.P., B.K. and Y.A.; writing original draft, T.P.; writing review and editing, Y.K. and Y.A. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) through GCRC-SOP (No. 2011-0030013).  
Not applicable.  
The authors declare no conflict of interest.