**1. Introduction**

To meet the energy demand is the most critical problem in many countries because of steadily increasing energy consumption. If technologies and economic structures in the main world regions had remained at their 1990 level, in 2006 the world would have consumed 4.4 Gtoe more energy. The amount of electricity required to generate one unit of value added (electricity intensity) is increasing in most regions, especially in less industrialized regions in which the service sector is expanding rapidly, and in countries with air-conditioning (AC) requirements [1].

By 2050, it is expected that the world's energy supply will double today's demand [2]. Although some regions are moderate at this expected energy need since using more energy-efficient

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, and Africa because of a lack of effective government engagement [2]. Global CO2 emissions from energy use were 34% higher in 2006 than in 1990. Trends in CO2 emissions vary significantly between countries. Developing countries with high economic growth have doubled their CO2 emissions, while Europe has nearly stabilized its emissions, partly because of climate change policies (**Figure 1**).

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 capacity control methods will be reviewed.

**Figure 1.** Distribution of world CO<sup>2</sup> emissions from energy use [2].
