**2. Capacity control methods**

technologies, higher amount of primary energy will be needed in 2020. Energy from renewable sources will have an important impact on markets during the time period, but will not dominate any market. The use of nonconventional energy decreases in Asia, Latin America,

nificantly between countries. Developing countries with high economic growth have doubled

The inefficient use of electricity in refrigeration and air-conditioning systems is regarded as an indirect contributor to the emission of greenhouse gases to the atmosphere. These emissions can be decreased by more energy conversion, efficient refrigeration systems. In addition to this, it is known that the overall energy consumption of a refrigeration, air-conditioning, or heat pump system during its service life is a considerable cost factor and frequently is a multiple of the initial investment. Generally, refrigeration systems are designed for fixed capacity to achieve cooling capacity based on the maximum demand at the highest ambient temperature. The consequence is that the refrigeration system delivers high cooling capacity, is selected to overcome the worst condition, and needs to be cycled on and off when normal conditions occur. However, refrigeration systems are operated at partial loads in most of their life cycle. For example, chillers typically run 99% of the time at part-load (off-design) conditions [3]. Therefore, to evaluate the refrigeration systems' partial load efficiency is mandatory. The European Commission has published seasonal efficiency standard EN 14,825 taking into account part load conditions. Furthermore, Air-Conditioning, Heating and Refrigeration Institute (AHRI) has published AHRI standard 551–591/2011 which explains water chiller and heat pump performance rating. In this standard, a part-load chapter contains calculation methods and performance rating of partial load heating and cooling loads [3]. Capacity modulation which matches the system capacity to the load improves overall system efficiency at partial loads. In this chapter, the previous published researches on refrigeration system

emissions, while Europe has nearly stabilized its emissions, partly because of cli-

emissions

emissions vary sig-

and Africa because of a lack of effective government engagement [2]. Global CO2

from energy use were 34% higher in 2006 than in 1990. Trends in CO2

their CO2

120 Refrigeration

mate change policies (**Figure 1**).

capacity control methods will be reviewed.

**Figure 1.** Distribution of world CO<sup>2</sup>

emissions from energy use [2].

Refrigeration system applications where the load may vary over a wide range, due to lighting, product loading, ambient weather variations, or other factors during operation, can be optimized by capacity control. Capacity control can be employed either within or outside the compressor, but their basic function of varying the refrigerant flow rate in the cycle remains the same. Depending on the system, the requirements may change; whereby, the following criteria should be assessed carefully: control performance, energy consumption, costs of selected solution, operation reliability, application range of the compressor, minimum compressor running time, and loading of the power supply. The most common methods are on/off control, digital scroll compressor, cylinder unloading, hot gas bypass, slide valve, multiple compressor, and variable speed [4–7]. The review of capacity modulation methods and electrical control techniques is illustrated in **Table 1** as a summary. And, various types of capacity control methods based on compressor are shown in **Figure 2**.




**No Year Author Objective Compressor Control**

analysis

methods comparison

to compressor suction side

modulation comparison with mathematical model

and on/off capacity control

19 2002 Reindl [32] Slide valve capacity control fact Screw Conventional

29 2003 Winandy and Cristian [33] Two-parallel compressor control Scroll Conventional

control for domestic A/C

on the performance

of variable speed

capacity control

compressors

speed capacity control

capacity-controlled system

and two-speed heat pump

application with different

of two-cylinder compressor

30 1988 Shimma et al. [41] Energy saving of inverters Conventional

speed compressor

and mechanical effect of variable

control

Reciprocating Conventional

Reciprocating Conventional

Reciprocating Conventional

Reciprocating Conventional

Rotary Conventional

Rotary Conventional

Reciprocating rotary

Scroll reciprocating

Reciprocating (open type)

Scroll rotary Conventional

Conventional

Conventional

Conventional

Conventional

Conventional

Conventional

Conventional

Conventional

Conventional

14 2001 Yaqub and Zubair [23] Cylinder unloading and other

15 1995 Yaqub et al. [27] Hot gas bypass thermodynamic

16 2000 Yaqub et al. [28] Liquid and gas injection

17 2001 Tso et al. [29] Hot gas bypass and suction

18 2005 Cho et al. [30] Compared hot gas bypass

21 1979 Muir and Griffith [36] Variable speed and on/off

22 1981 Tassou et al. [37] Energy conservation on capacity

23 1982 Tassou et al. [38] Effect of capacity modulation

25 1982 Itami et al. [44] Compressor lubrication problem

26 1983 Tassou et al. [4] Comparison of performance

27 1984 Janssen and Kruse [5] Continuous and discontinuous

28 1984 Tassou et al. [39] Economic comparison of fixed-

29 1985 Senshu et al. [45] Small-capacity heat pump

31 1988 McGovern [47] Variable speed performance

32 1988 Ischii et al. [48] Dynamic behavior of variable

24 1982 Lida et al. [43] Experimental analysis

Hot gas bypass capacity control

122 Refrigeration

Slide valve capacity control

Multiple compressor capacity control

Variable speed capacity control


**Table 1.** Researches on capacity control methods and control applications.
