**4. Conclusion**

Cho et al. [71] measured the cooling performance of a variable speed CO2

between condensing and evaporation temperature.

than for the examined cold storage plant.

of the CO2

136 Refrigeration

from 40 to 60 Hz.

by varying the refrigerant charge amount, compressor frequency, EEV opening, and length of an internal heat exchanger (IHX). As a result the cooling COP decreased with the increase of compressor frequency at all normalized charges. The optimum EEV opening increased with compressor frequency. The optimal compressor discharge pressure of the modified CO<sup>2</sup> cycle with the IHX was reduced by 0.5 MPa. The IHX increased the cooling capacity and COP

In addition to these studies, the Ekren and Kucuka study [72] has been carried out by a fuzzy logic-controlled chiller system with variable speed compressor. In this study, not only variable capacity modulation has been studied, but also fuzzy logic control effect on chiller system has been investigated. Scroll compressor, designed to study as fixed-speed compressor, was operated as variable speed with a PWM inverter. Also, electronic expansion valve, fuzzy controlled, was used. In this system 33.4% COP increase was obtained according to on-/offcontrolled system. This increase has been obtained because of the less temperature difference

**3. Electrical control research for capacity-modulated refrigeration systems**

Any system, whose outputs are controlled by some inputs of the system, is called a control system. Electrical control techniques of the capacity-modulated refrigeration systems are very important, because of the quick response of the load changes. On/off control, proportionalintegral-derivative (PID), stochastic control, adaptive control, nonlinear methods, and robust control are known as conventional control systems. On/off and PID controls are the most commonly used methods in the heating ventilating and air-conditioning system control. Recent developments in artificial intelligence-based control systems have brought into focus a possibility of replacing control system with fuzzy logic control (FLC) equivalent. Therefore, fuzzy logic control is needed in the investigation on capacity-controlled refrigeration system. A standard PID controller is based on mathematical modeling of the system being controlled, whereas a fuzzy logic controls relies on physical rules rather than mathematical equations. In addition, fuzzy logic control provides a formal methodology for representing, manipulating, and implementing a human's heuristic knowledge about how to control a system.

Comparison of fuzzy logic and conventional controllers for a refrigeration system was investigated by Cheung and Kamal [73]. Fuzzy control application is realized on various compressor types following two authors. Aprea et al. [74] investigated fuzzy logic-based control algorithm which could select the most suitable compressor speed according to function of the cold store room air temperature. They found that this algorithm provides 13% energy saving when the R407C refrigerant was used at fuzzy logic-controlled variable speed reciprocating compressor. Furthermore, regarding the inverter cost, the payback period was more acceptable

Aprea and Mastrullo [75] investigated a vapor compression system which is able to operate as a water chiller and as a heat pump. The main aim of this study is to evaluate energy-saving

cycle by 6.2–11.9% and 7.1–9.1%, respectively, at the tested compressor frequencies

cycle and analyzed

In this paper, the literature review of capacity control methods is completed. The application of capacity control methods to refrigeration systems offers the potential for substantial energy saving or energy efficiency. Also, it is known that the overall energy consumption of a refrigeration, air-conditioning, or heat pump during its service life is a considerable cost factor and frequently is a multiple of the initial investment. Consequently, with a view to indirect environmental impact (CO2 emission due to power generation), optimum capacity control should be aimed for, that is, closely matched to demand. According to previous researches, variable speed capacity control is the most common and efficient methods in the capacity control studies. In addition to this, both of variable speed and digital scroll modulation present close modulation result. Also, it is seen that artificial intelligence control techniques show superiority toward conventional control methods on refrigeration system.
