Abstract
Vapor-compression systems are the most popular method of refrigeration. However, the throttling loss at the expansion valve is one of the “energy parasites” of such systems. This is especially acute in systems with large operating pressure differences like the transcritical CO2 refrigeration systems. In this chapter, a method to solve this issue by using an expander to recover the expansion energy of refrigeration systems is explained. Relevant research works are then discussed to provide a general overview about the state of the art technology. Various types of expander mechanisms, including reciprocating, rolling piston, rotary vane, scroll, screw, turbine, swing piston and revolving vane, are discussed. Works on the various aspects of expanders are also discussed. These include heat transfer, exergy analysis, expansion process, internal leakage, lubrication, integration with refrigeration systems and the economic aspects.
TopIntroduction
Refrigeration systems are integral parts of modern life. It provides comfort for people, thermal management for industrial processes, preserves food, etc. Among all of the available systems, vapor compression system is currently the most popular. It is compact, reliable and can be used for a wide range of applications. A typical vapor compression system has four main components: compressor, condenser, expansion valve and evaporator as illustrated in Figure 1 (a). In some applications, additional components like internal heat exchangers, accumulators, etc. may be installed. These components are linked with pipes. Refrigerant flows through these components in a closed system in the direction shown by the arrows in Figure 1(a). Pressure and temperature of the refrigerant are increased through compression process in the compressor. This hot and high pressure refrigerant then flows to the condenser and rejects heat. This is the useful heat in heat pumps. In cooling applications, this heat is usually rejected to the ambient as waste heat. The refrigerant then enters the expansion valve to drop its pressure and temperature. The expansion process is typically instantaneous, resulting in a near-isenthalpic expansion process. The resulting cold refrigerant then enters the evaporator and absorbs heat. This is the cooling provided for refrigeration applications. The refrigerant finally goes back into the compressor and the cycle continues.
Figure 1. Schematic diagrams of vapor compression systems (a) with and (b) without expanders, and (c) pressure-enthalpy diagrams of the systems
Performance of such refrigeration systems is usually quantified using a parameter called Coefficient of Performance (COP). Depending whether the system is used for cooling or heating applications, the definitions of COP are expressed in Equations 1 and 2.
(1)(2) where
Qcold is the cooling capacity of the system (W),
Qheat is the heating capacity of the system (W) and
Wcomp is the compressor power requirement (W).
Globally, air conditioning and refrigeration accounts for up to 15% of the total electricity consumption (Coulomb, 2006). In automotive applications, air conditioning may account for up to 30% of the overall fuel consumption in a typical city driving cycle (Empa, 2010). Energy consumption itself is actually a natural process. However, high energy consumption is damaging to the environment mainly because of our current over-dependence on energy produced by conventional fossil fuel burning processes. These processes produce greenhouse gases (especially CO2) which in turn causes climate change problems when released into the atmosphere. Until we have finally successfully substituted this fuel burning process with sustainable energy production mechanisms, it is crucial to save whatever amount of energy we can. Therefore, it is important for the air conditioning and refrigeration sector to respond by improving the energy efficiency of their systems.
In this chapter, one effort to increase the energy efficiency of refrigeration systems is presented.
Key Terms in this Chapter
Vapour Compression Refrigeration System: The most popular refrigeration system concept, consisting of at least: a compressor, a condenser, an expansion device and an evaporator.
Compressor: A device to increase the pressure of a fluid by reducing its volume.
Expansion Valve: A device to reduce the pressure of a fluid by increasing its volume instantaneously, expansion power is wasted during the process.
Refrigerant: Working fluid of a refrigeration system.
Coefficient of Performance (COP): A parameter to quantify the efficiency of refrigeration systems, defined as the ratio between cooling capacity to the power consumption.
Efficiency: A general term to quantify the performance of a machine, defined as the ratio between output and input.
Transcritical Cycle: A series of thermodynamics processes that involve sub-critical and supercritical fluid states.
Expander: A device to extract power from high pressure fluid by expanding its volume and hence, reducing its pressure.